Trace element contents of chondrule rims and interchondrule matrix in ordinary chondrites

Geochimica

el Cosmochimica

Acta. Vol. 59, No. IS, pp. 3247-3266, Copyright

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Trace element contents of chondrule rims and interchondrule C. M. O’D. McDonnell

Center for the Space Sciences, (Received

December

matrix in ordinary chondrites

ALEXANDER*

Washington

9, 1994; accepted

University,

St. Louis, MO 63130, USA

in revised form May

1I, 1995)

Abstract-The abundances of forty elements have been measured in the rims and matrices of the unequilibrated ordinary chondrites Semarkona, Bishunpur, Chainpur, and Krymka. The rims appear to be mixtures of matrix-like material and material that was volatilized during chondrule formation. Iron, silicon, manganese, and copper all appear to have behaved as volatiles during chondrule formation and their enrichments are generally confined to the rims, suggesting recondensation of these elements was rapid and localized. This is supported by petrologic observations that show many chondrules were still partially molten or plastic when the rims accreted. Some rims also exhibit a decrease in both their Fe0 content and the degree of mineral re-equilibration away from the chondrule. The alkali metals are enriched in both the rims and matrices. The alkalis may have recondensed onto the matrix as well as the rims, but their mobility is such that they have probably undergone extensive redistribution after accretion. Some Na may even have been able to diffuse back into chondrules either during chondrule cooling or during metamorphism/alteration. Most refractory lithophiles are depleted with respect to Al in the rims and matrices in all four UOCs. These depletions are probably due to postaccretional redistribution of these elements into phosphates and other secondary minerals that form in and around metal/sulfide grains. The matrix is probably composed of chondrule fragments and processed nebular dust, but the presence of carbonaceous presolar grains in the matrix indicates that there must be unprocessed presolar silicates present too. On average, the presolar silicates presumably have a CI-like composition but a CI-like signature is not seen in the matrix compositions. Simple estimates suggest that the abundances of carbonaceous presolar grains are consistent with there being at most 10 wt% presolar silicates present in UOC matrix, which would be hard to detect in this study. 1.

INTRODUCTION

sizes greater than a few micrometers have compositions and microstructures that are consistent with a chondrule origin (Fujimaki et al., 1981; Alexander et al., 1989b; Brearley et al., 1989). The lack of a universally agreed upon definition of matrix is one of the causes of confusion/debate in the origin of matrix. Often matrix has been taken to encompass all material that is neither chondrules nor inclusions, but this includes the fine-grained accretionary rims around chondrules and inclusions. These rims are clearly petrographically distinct from, though probably related to, the interchondrule matrix (e.g., Allen et al., 1980) and will be treated as such in this paper. The dominance of elastic material in the coarser fractions of the interchondrule matrix has led some authors (see Scott et al., 1988) to confine the term matrix to the <5 pm interchondrule material, which is dominated by the nonelastic component. While the size distribution studies suggest this size cutoff is somewhat arbitrary, this definition will also be adopted here. Even confining the discussion of matrix to the <5 pm fraction has not resolved the debate over its origin. This material tends to be enriched in Na and Al (e.g., Nagahara, 1984; Scott et al., 1984) amongst other elements. If this material is made from equilibrium or near equilibrium condensates its composition suggests it should contain Al-rich minerals such as plagioclase feldspar (e.g., Grossman, 1972; Wood and Hashimoto, 1993). The correlation between refractory elements, such as Al and Ti, in chondrules suggests more refractory condensates may have survived in the OC formation region (e.g., Grossman and Wasson, 1982, 1983), in which case corundum, hibonite, melilite, and spine1 might also be expected to be present in the matrix. TEM observations have failed to

In developing their two component model of chondrite trace element abundances, Anders (1964) and Larimer and Anders (1967) suggested that chondrites were a mixture of volatile depleted chondrules and inclusions, and matrix with a more or less CI-chondrite-like composition. The two-component model has since been questioned (Wasson and Chou, 1974; Wai and Wasson, 1977; Wilkening et al., 1984). Certainly, chondrules are not as volatile depleted (e.g., Gooding et al., 1980; Grossman and Wasson, 1982, 1983) and matrix is not as volatile rich (e.g., Wilkening et al., 1984; Taylor et al., 1984; Grossman, 1985; Brearley et al., 1989) as the two component model requires. Nevertheless, the idea that the matrix is made of nebular condensates, perhaps modified to some extent by parent body processes, has endured and remains the dominant paradigm, even as improvements in microanalytical techniques have reduced the scale at which the matrix can be studied (e.g., Huss et al., 1981; Nagahara, 1984; Brearley et al., 1989). Others have suggested that the matrices of ordinary chondrites (OCs) are largely composed of chondrule fragments (e.g., Alexander, 1987; Alexander et al., 1989b). Ashworth (1977) and Alexander et al. (1989b) showed that the matrices of OCs contain both elastic and nonelastic components, and that the elastic material forms a single power-law distribution from 100 pm chondrule fragments to submicrometer sized mineral grains. In addition, almost all mineral grains with * Present address: The Department of Terrestrial Magnetism, The Carnegie Institute of Washington, 5241 Broad Branch Road, Washington, DC 20015, USA. 3247

3248

C. M. O’D. Alexander

find significant quantities of any of these minerals in the matrices of the most unequilibrated ordinary chondrites (UOCs) (Ashworth, 1977, 1981; Alexander, 1987; Alexander et al., 198913;Brearley et al., 1989). Instead in UOCs that have not been aqueously altered, the fine-grained matrix is dominated by amorphous feldspathic material and olivine that formed in the solid state (Ashworth, 1977; Alexander et al., 1989b; Brearley et al., 1989), prompting Brearley et al. (1989) to propose that the matrix condensed as an amorphous material that subsequently devitrified. On the other hand, Alexander et al. (198913) suggested that the fine-grained matrix was originally enriched in the feldspathic glass that must have accompanied the chondrule-derived elastic minerals in the coarser interchondrule material. Addition of Fe0 from oxidized metal to the silica and/or pyroxene normative chondrule glass in the matrix would have enabled the solid state growth of olivine during metamorphically driven devitrification. The origin of matrix has been further complicated by two new observations. Firstly, presolar grains are present in the rims and/or matrix of OCs (Alexander et al., 1990; Huss, 1990; Alexander et al., 1992; Gao et al., 1994; Nittler et al., 1994b; Huss and Lewis, 1995). Clearly then, at least some matrix material escaped both evaporation-condensation and chondrule formation. Indeed, Huss (1990) and Huss and Lewis (1995) suggest that, prior to metamorphism, the abundances of presolar grains in OC matrices and rims were similar to those observed in bulk CI chondrites. These presolar grains, which are predominantly carbonaceous, were almost certainly accompanied by more abundant silicates and these may be a significant component of matrix. Secondly, it has been suggested by numerous authors that volatile loss from chondrules was an important process. These volatiles, which may have included Fe, Si, and Mn (e.g., Lu et al., 1990; Huang et al., 1993a; Alexander, 1994; Scott, 1994), would presumably have recondensed, at least partially, on the rim/ matrix and may also constitute a significant fraction of the rim/matrix. Here the abundances of forty elements in rims and matrices from Semarkona (LL3.0), Bishunpur (LL/L3.1), Krymka (LL3.1), and Chainpur (LnL3.4) were determined in an attempt to resolve the debate over their origin(s). The meteorites chosen for this study are all falls with low petrologic types, thus minimizing the effects of weathering and metamorphism. However, all of them have undergone aqueous alteration and/ or metamorphism to some degree (Sears and Hasan, 1987; Alexander, 1987; Hutchison et al., 1987; Alexander et al., 1989a,b; Sears et al., 1995). 2. Techniques Ion probe analyses of rims and matrices require beam currents of 5- 10 nA and beam sizes of IO-20 pm. prior to ion probe analysis, the rims and matrix areas, selected from polished sections of the four meteorites, were photographed and analyzed using a JEOL 840 scanning electron microscope (SEM) equipped with a Tracer Northern energy dispersive (EDS) X-ray analysis system. Each chondrule associated with an analyzed rim was also classified according to textural and chemical type (see Appendix). In the absence of bulk chondrule analyses, chondrules were assigned to chemical types I or II depending on whether the most Fe-rich olivine or low-Ca pyroxene compositions had Mg-numbers, atomic Mg/(Mg + Fe), greater than 0.9 (type I) or less than 0.9 (type II). It should be borne in mind that for ease of ion probe analysis the rims that were chosen are comparatively thick. The rims were also chosen from large backscatter SEM

mosaics of the sections and the more Fe-rich rims tended to show up better in these images. Thus, there is a selection bias in the rim sample. The only meteorite in which no distinction was made between rims and matrix was Semarkona. Apparently aqueous alteration has degraded the outlines of many rims in the section used here. The ion probe analyses were carried out on the Washington University Cameca IMS3f ion microprobe. The experimental procedure, as outlined by Zinner and Crozaz (1986), uses an O- primary ion beam to sputter secondary ions from the surface of the sample, The elemental concentrations are determined by counting the secondary ions of individual isotopes, at low mass resolution with energy filtering, and comparing these to the counts from a reference element, in this case Si. The Si02 content of the sample is known from prior EDS analysis. Unfortunately, the secondary ions include molecular ions that can interfere with the atomic ions used to determine elemental concentrations. Complex molecular ions tend to have relatively low energies and can be effectively eliminated by energy filtering. However, simple molecular ions, such as SiO+, cannot be entirely filtered out in this way. Details of how these molecular ions can be corrected for are given by Zinner and Crozaz (1986) and Alexander ( 1994). Alexander (1994) measured the abundances of thirty-six elements in UOC chondrule silicate minerals and glasses. By combining the mean olivine, low-Ca pyroxene, high-Ca pyroxene, and glass compositions in the appropriate proportions, Alexander (1994) was able to reproduce, within error, the bulk L-group composition for almost all the elements measured, thereby providing confidence in the sensitivity factors being used. The sensitivity factor of only one of these elements, Fe, has been changed here. The Fe sensitivity factor for silicates appears to be a function of Fe concentration (unpubl. data). In the FeO-rich rims and matrices, the Fe sensitivity factor is about 0.64 times that in the more magnesian chondrule silicates. Four new elements, S, Co, Ni, and Cu, have been added to the list of elements being reported here. Unfortunately, the accuracy of their sensitivity factors could not be checked in the same way as the chondrule lithophiles since the metal and sulfide, which are the main hosts for these elements, were avoided in this and the previous study. The appropriate masses for these four elements were monitored by Alexander (1994), but because of their low abundances (Co < 50 ppm; Ni < 80 ppm; Cu < 20 ppm) in chondrule silicates and potential interferences that could not be corrected for, their concentrations were not reported. Even though large areas of metal and sulfide were avoided, the abundances of the four elements are often more than an order of magnitude higher than in the chondrnle minerals. These higher concentrations reduce the potential for significant contributions from interferences and, therefore, their abundances are reported, but in those analyses where the abundances approach those seen in chondrule silicates the results should be viewed with caution. The sensitivity factors for all four new elements were measured on the same silicate glasses as the elements reported by Alexander (1994), but much of the rim/matrix S, Co, Ni, and Cu may be in small metal and sulfide grains. The sensitivity factors for elements in these phases are almost certainly different than in silicates. Nevertheless, S and Ni abundances determined by SEM/EDS and ion probe in most rim/matrix areas are in good agreement. Cobalt and Ni are highly correlated in the rim/matrix analyses and the Co/Ni ratio is only slightly higher than bulk L-chondrite. Thus, the S, Ni, and Co sensitivity factors seem to be reasonable. Copper is perhaps the most uncertain element since its sensitivity factor could not be internally checked. 3. RESULTS

3.1. Petrology There is a considerable body of literature on the petrology of rims and matrix (see Scott et al., 1988). Here the aim is to briefly emphasize some important features and report some relevant new observations. In a previous study, Alexander et al. (1989b) loosely divided the rims into elastic (Fig. la) and fine-grained rims (Fig. lb-d) on the basis of the abundance of mineral fragments in them that are resolvable in the SEM.

Geochemistry

of chondrules

and their matrices

3249

FIG. 1. Backscatter SEM photographs of three Bishunpur rims. All host chondrules are marked Ch. (a) An extensive elastic rim (B-R23) with numerous chondrule fragments around a granular chondrule. (b) An FeO-rich rim (B-R14) grading into a more elastic rim as one moves away from the chondrule. Olivine grains (e.g., OL, upper right) with Ferich rims suggesting partial re-equilibration are common in the outer margins of the rim. The FeO-rich portion of the rim is relatively featureless in large part because the magnesian olivine fragments, which are evident in the outer portions, appear to have completely reequilibrated. There is also an enstatite grain (EN) with cusp shaped margins that suggests it may have reacted with the FeO-rich rim and may have partially melted in the process. The line shows the path of the traverse illustrated in Fig. 2. (c) Silicate inclusions, that appear to have been plastically deformed, trapped in another part of the B-R15 rim. The inclusions have magnesian, low-Ca pyroxene like compositions that are similar to the chondrule pyroxenes and, therefore, they may have been derived from the chondrule. Their FeO-rich margins indicate the inclusions have reacted with the rim material in places. (d) A finger of plastically deformed chondrule extending from the chondrule into the rim (B-Rl5).

The elastic rims, which contain many mineral fragments and, in some cases, obvious chondrule fragments (Fig. la), are texturally very similar to matrix. Fine-grained rims, on the other hand, are typically relatively smooth and featureless in SEM images. Both types of rim appear to have accreted onto chondrules. In general, there is also a chemical difference between these two types of rim. Clastic rims have compositions that, like their textures, are similar to typical matrix, while the most obvious feature of fine-grained rims are their much higher Fe0 contents (Alexander et al., 1989b). In some rims, like the one in Fig. lb, there is a clear decrease in the degree of Fe0 enrichment away from the chondrule (Fig. 2 and Table 1). The compositions of olivine grains in these rims also tend to change from Fe-rich to Mg-rich compositions outwards from the chondrule. Some of the oli-

vine grains, like those in the outer parts of the rim in Fig. lb, are compositionally zoned with Fe-rich rims and Mg-rich cores suggesting incomplete reequilibration. In the rim in Fig. lb there is also a large enstatite grain with cusp-shaped embayments of FeO-rich material (Table l), suggestive of reaction with the FeO-rich rim and perhaps even melting. The fact that rims like that in Fig. lb are clearly grading from FeO-rich towards more elastic-like rims indicates that the two rim types should not be thought of as discreet entities but rather as endmembers of a continuum reflecting accretion onto chondrules under a range of different conditions. Rims often contain silicate and metal/sulfide inclusions that appear from their shapes to have been deformed while either molten or plastic (Fig. lc,d). The compositions of the silicate objects are often similar to that of the rimmed chondrule and

C. M. O’D. Alexander

3250

coherent fragments and be mistaken for matrix. Sectioning through a thick elastic rim or rim fragment may explain some but not necessarily all so called matrix lumps (Ikeda et al., 1981; Scott et al., 1984).

G

E

+ d

3.2. Major and Trace Element Chemistry 0.6

c

&

0.4

o.2-o Distance

80 (pm)

FIG. 2. The variation in Fe-number (solid) and WA1 ratio (open) in rim B-R14 (see Fig. lb) as a function of distance from the chondrule. The Fe-number of the rim drops from 0.85-0.9 near the chondrule to about 0.68 at the edge and the WA1 ratio more or less parallels it (see also Table 2). The rather low Fe-number and WA1 ratio of the first rim point may be due to fluorescence of the near by chondrule.

in a few instances they are still attached to the chondrule (Fig.

Id). The silicate inclusions often have FeO-rich margins and appear to be reacting with FeO-rich rim material (Fig. lc,d). Some of these inclusions may be accreted microchondrules (e.g., Krot and Rubin, 1995) but in those cases where they are clearly still attached to the chondrule, the textures indicate that the chondrules were still partially molten or, at least, plastic at the time the rims accreted. Therefore, some fine-grained material must have survived chondrule formation or was introduced into the formation region shortly afterwards. These petrologic observations suggest that many of the FeO-rich rims may have been sintered, or even partially molten, and that the mineral grains in them reequilibrated to varying degrees (Fig. lb,c). Several other authors (Rubin, 1984; Bunch et al., 1991; Kring, 1991) have also pointed out that some rims must have undergone sintering and partial melting. In retrospect, the term fine grained used by Alexander et al. (1989b) for these rims, which was based mainly on SEM observations, may have been something of a misnomer. Much of the contrast in the SEM images comes from variations in average atomic number and surface relief. As is illustrated by the inner portion of the rim in Fig. lb, the sintering and partial reequilibration removes much of the chemical variation, principally in Fe and Mg contents, and reduces the surface relief that develops during polishing. As a result, these rims give the possibly false impression that mineral grains a few micrometers across are absent. Although, these rims do lack the larger mineral grains and chondrule fragments present in the elastic or matrix-like rims (Fig. la). In the absence of many other distinguishing features these rims will be termed FeOrich rims in what follows. Finally, the identification of a rim is generally unambiguous. However, because thin sections are only two dimensional the identification of true matrix is less certain. An area of finegrained material that is apparently unrelated to a chondrule may, nevertheless, be part of a rim if the chondrule was above or below the section. Also, rims may break off chondrules as

As is the case for their petrology, there is an extensive literature devoted to the major and minor element chemistry of rims and matrix (see Scott et al., 1988). Consequently, only those features of rim/matrix chemistry which are the focus of the detailed discussions to follow will be described in this section. It has been known for some time that, in terms of its absolute abundance, Al is enriched in the matrices of many UOCs compared to the bulk meteorites (e.g., Huss et al., 1981; Ikeda et al., 1981; Matsunami, 1984, Nagahara, 1984; Scott et al., 1984). These enrichments could be explained by the concentration of either Al-rich condensates or fragments of chondrule glass in the matrix. Both explanations predict that all refractory elements should have essentially chondritic relative abundances. However as is evident from Fig. 3, their actual abundances in matrix and rims are rather more complicated. In Semarkona, Bishunpur, and Chainpur matrices the distribution of refractory elements are, with a few exceptions, quite similar, but the most striking feature is the depletion of many of these elements relative to Al by about a factor of two (Fig. 3a). The rims behave in a similar manner (Fig. 3b). Volatility does not seem to have played a role in producing these fractionations. Aluminum is one of the most refractory lithophiles, comparable to Y and SC which are depleted, while Sr, Ba, and Eu, which are generally less fractionated relative to Al, are three of the most volatile of the refractory lithophiles. Magnesium and Si, which are normally classified as common (less refractory) litbophiles, are as depleted as the most refractory elements. Whatever the cause, the depletions of so many refractory elements relative to Al are inconsistent with all the previously proposed origins of rims and matrix. The only exception to the general refractory lithophile depletion relative to Al is the Krymka matrix (but not the Krymka rims). There are only four Krymka matrix analyses and the compositions of two of these (K-M2 and K-M8) varied considerably as the primary ion bean sputtered into the sample during the coarse of the analyses. These two matrix areas also have unusual compositions. K-M2 has very high V Table 1. Representative electron microprobe analyses of rim B-R14 (see Fig. lb). The elemental concentrations are all in mg/g. The analyses from the inner and outer portions of the rim show the large changes in composition that occur away from the chondmle. The enstatite grain has a composition that is not unlike those in the chondrule and its cusp shaped margins seems to be the result of reaction, and possibly melting, with the FeO-rich rim material. cusp 2 Inner rim Outer rim En&the cusp 1

gcmdg)25.0 2.3

46.2 4.0

189.1 1.8

30.9 0.4

38.2 1.6

z

136.9 3.0

187.8 11.1

269.1 6.2

147.8 0.5

141.5 2.8

FeS K Ca Ti

11.4 0.0 6.9 0.2 1.7 1.9 394.3 26.9 90.4

16.0 2.7 12.2 0.2 1.7 3.3 260.9 13.0 89.7

0.0 1.8 7.5 0.8 7.6 5.4 15.1

2.9 0.0 :::

3.2 1.9 7.1 0.0 1.3 3.3 415.7 0.0 93.0

Cf Mn

Fe Ni Total (wt%)

1.3 2.8 424.3 0.0 92.8

3251

Geochemistry of chondrules and their matrices

--OBishunpur ..__._.. A Chainphr ----O---Krymka ----III---- Semarkona

(A) Matrix

O.l’, , MgAl

(

,

,

Si CaSc

,

,

,

I

I

‘I

I

I,

I

I

I

I

I

I

I

I

-O.

(B) Rims

o.l’,,,,,,,,,,,,,,,,,,,,,,,,,,( MgAl Si CaSc

I

I

I

I

I

Ti V Sr Y ZrNbBaLaCePrNdSmEuGdTbDyHoErTmYbLuHf

........& .......

Bishunpur Chainpnr

----O----

Krymka

Ti V Sr Y ZrNbBaLaCePrNdSmEuGdTbDyHoErTmYbLuHf

FIG. 3. The Al and L-chondrite normalized mean refractory lithophile element abundances in (a) the matrices and (b) the rims of Bishunpur, Chainpur, Krymka, and Semarkona. Contrary to previous models for the origin of matrix, all of which predict chondritic relative abundances, most refractory lithophiles are depleted with respect to Al. Only Sr, Ba and Eu have Al normalized abundances that are often close to chondritic.

and Cr abundances, perhaps because it included a cbromite grain, and K-M8 has high (2 x OC) refractory element to Al

ratios. The remaining two matrix areas have compositions that are closer to those of the other meteorites’ matrices. The matrix and rims are not only enriched in Al but also in most volatile elements, including the alkali metals (Fig. 4 and Table 2). As can be seen in Fig. 4, the degree of Na enrichment in a given meteorite generally correlates with enrichments in Al. However, the mean atomic Na/Al ratios vary from about 1.5 in Semarkona, to about 1 in Bishunpur and Chainpur, to about 0.3 in Krymka (Table 2). The variations in the Na/Al ratios of rim/matrix from meteorite to meteorite seems to largely reflect differences in the abundance of Na, as the range of Al contents in rim/matrix are more or less comparable in all four meteorites. Also, while rims and matrix are enriched in all the alkalis (Fig. 4), Na tends to be less enriched than the others (Table 2). A complementary relationship exists in chondrules, where Na tends to be enriched in chondrule glasses relative to the other alkalis (e.g., Alexander, 1994). Another feature of Fig. 4 is that the abundances of both Na and Al are lower in rims compared to matrix. In fact, lower

concentrations in rims compared to matrix are a general feature of most lithophile abundances (Table 2). The obvious exception to this is Fe, which is, on average, about twice as abundant in rims as in matrix (Table 2). In the rims analyzed here, the Fe appears to be oxidized rather than present as metal or sulfide. The high Fe0 contents of rims are also evident in the high atomic number contrast in the backscatter electron images of Fig. 1. FeO-rich silicates are anticipated to be characteristic of low temperature condensates (e.g., Grossman, 1972) but Fe and, to a lesser extent, Si may also be relatively volatile elements during chondrule formation (e.g., Lu et al., 1990; Huang et al., 1993a; Alexander, 1994; Scott, 1994). Normalizing Fe and Si abundances to a refractory element, such as Al, indicates that Fe and Si are correlated and that their enrichment is largest in the rims (Fig. 5). Indeed, it appears from Fig. 5 that rims are a mixture of matrix, or matrix-like material, and a more Fe- and Si-rich component. Manganese and Cu are also enriched in the rims (Fig. 5). This mixing of components is evident even within individual rims (Figs. 2 and 6). The alkali/Al ratios are generally comparable in both rims and matrix, and do not correlate with Fe or Si.

3252

C. M. O’D. Alexander

4.1. Whence the Refractory Element Fractionations in Rim/Matrix?

Bish.

mat

Chain.

rim

Sem.

Al (L-group

rims

Chain.

mat.

norm.)

FIG. 4. The L-chondrite normalized Na and Al abundances in rims and matrices from Semarkona, Bishunpur, Chainpur, and Krymka. Almost all matrices and many rims are enriched in both Na and Al. Part of the reason for the lower Na and Al abundances in the rims are their higher Fe contents. With the exception of Krymka, the rims and matrix are enriched in Na relative to Al and bulk OC. This enrichment probably reflects volatile enrichment during chondrule formation. However, mobilization of Na, driven by changes in the mineralogy of the rims, matrices and chondrules during aqueous alteration and/or metamorphism, may also have played an important role in determining its present distribution.

Figures 5 and 6 are normalized to Al. The abundance patterns in Fig. 3 suggest that the refractory element behaved as two groups, those with more or less chondritic abundances relative to Al and those that are depleted relative to Al. If the cause of the fractionations relative to Al is the addition or mobilization of Al, the correlations in Figs. 5 and 6 may be artifacts. Titanium and most other refractory elements are depleted relative to Al, but normalization to Ti rather than Al (Fig. 7) produces a similar result. Thus, whatever the cause of the refractory element fractionations, Fe, Si, Mn, and Cu all appear to be enriched, relative to refractory elements, in many rims compared to matrix and bulk OC.

4. DISCUSSION In the brief review of the rim and matrix compositions, three important features were emphasized: ( 1) the enrichment of Al in the matrix and the fractionation of Al from most, but not all, other refractory elements in both rims and matrix; (2) the correlation of the alkalis with Al within a meteorite but variable alkali/Al ratios between meteorites; and (3) the marked enrichment of Fe, Si, and other elements, relative to refractory elements, in rims compared to matrix. In trying to explain these observations the two major problems that must be resolved are whether these features of matrix and rims are primary (pre-accretionary ) or secondary ( post-accretionary ) , and if they are primary what were the processes that produced them. Possible processes include: preferential concentration of various nebular condensates, chondrule fragmentation, and recondensation of material volatilized during chondrule formation.

There are two ways the refractory element depletions relative to Al could be achieved-addition of Al-rich but otherwise refractory-poor material to the rim/matrix or partial removal from the rim/matrix of most refractories. The matrices of many UOCs are enriched in Al and the most mundane explanation for this enrichment is that the rims and matrices have been contaminated by the submicrometer corundum which is commonly used in either the final polishing step of section preparation or used to remove a carbon coat. This corundum does collect in cracks and, presumably, in other porous areas like the matrix. Nevertheless, this explanation seems unlikely. Matrix analyses of the same meteorite made on different sections by different authors tend to be fairly consistent, including the study by Huss et al. ( 1981) which used many sections that were prepared without the use of corundum (G. R. Huss, pers. commun.). In Semarkona, smectite dominates the matrix (Hutchison et al., 1987; Alexander et al., 1989a). The composition of Semarkona smectite (measured by TEM) and bulk matrix are similar, and both are enriched in Al relative to bulk meteorite. Therefore, corundum cannot have made a significant contribution to the Semarkona matrix analyses. Also if contamination by corundum were a significant problem, the molar Na/Al = 1 observed in Chainpur and Bishunpur rims and matrices would be a coincidence. SEM and TEM analyses of the matrices in these two meteorites have found amorphous feldspathic material with Na/Al ratios of about one, consistent with the bulk matrix analyses, but no corundum (Nagahara, 1984; Alexander et al., 1989b). There is indigenous corundum, hibonite, and spine1 in UOC matrices (e.g., Nittler et al., 1994a) but their combined concentration of roughly 100 ppm is far too low to account for the variations in matrix compositions. In Tieschitz, and possibly Sharps, there has been considerable redistribution of Na, Al, and Si during the formation of white matrix (Christophe Michel-Levy, 1976; Ashworth, 1981; Nagahara, 1984; Alexander et al., 1989b), which apparently resulted from the passage of Cl- and F-bearing fluids through the meteorite (Christophe Michel-Levy, 1976; Alexander et al., 1994). Both rim/matrix and chondrule glasses display evidence of alteration (Alexander et al., 1994; Hutchison et al., 1994). The white matrix has relatively low trace element abundances (Hutchison et al., 1994) and may contain some feldspar (Nagahara, 1984) and nepheline (Ashworth, 1981). Chainpur and Bishunpur may also have seen Cl- and F-rich fluids (Alexander et al., 1987, 1994; Hutchison et al., 1994) but not to the extent experienced by Tieschitz. Certainly, nothing similar to white matrix has been reported in any of the meteorites studied here. Nor is there evidence for the extensive alteration of chondrule glasses seen in Tieschitz. Thus, it seems unlikely that the rims and matrices of the meteorites studied here have gained aluminosilicate material in the same way Tieschitz white matrix did. 4.1.1. Was albite added to the rim/matrix? The 1: 1 molar ratio of Na and Al in Bishunpur and Chainpur suggests another possibility-the addition prior to accretion of an aluminosilicate such as albite or nepheline. It is an

3253

Geochemistry of chondrules and their matrices Table 2. The mean rim and matrix compositions from Bishunpur, Chainpur, kymka and Semarkona. In parentheses are the one sigma standard errors in the means in the last significant figures. The individual rim and matrix analyses are given in the appendix.

Nos.

anal.

Li (ppm) Na (m&A?) Mg (m&d Al (mdg) Si (m&d P (mp/g) S (m&z) K (m&d Ca (mid) SC (Ppm) Ti (m&z) V (Ppm) Cr (m&d Mn (m&z) Fe (m&d Co (m&d Ni (m&z) Cu (epm) Rb (PPm) Sr (PPm) Y @pm) zr (Ppm) Nb (ppm) cs (ppm) Ba @Pm) h (ppm) Ce (Ppm) Pr (ppm) Nd @pm) Sm (Ppm) Eu (pPm) Gd @pm) Tb (Ppm) DY @pm) Ho (pem) Er (Ppm) Tm (ppm)

Yb @pm) Lu (rwm) Hf (ppm) # Some

Bish. Mat.

Bish. Rims

Chain. Mat.

Chain. Rims

Krym. Mat.

Krym. Rims

Sem. Mat.

6

9

6

7

4

9

8

1S(2) 18(l) 96(6) 24(l) 2 14(9) 2.3( 1.3) 3.6(2) 4.4(3) 17(4) 5.7(S) 0.56(9) 60(5) 2.5(2) 2.9(2) 124(13) 0.4(2) 5(l) 4 13(50) 14(l) 25(6) 2.0(4) 8.4(3.5) 0.5( 1) 1.2(3) 5.3(6) 0.52(25) 1.4(6) 0.17(7) 0.7(2) 0.22(7) 0.16(3) 0.23(5) 0.05(l) 0.32(S) 0.08(2) 0.23(5) 0.030(6) 0.21(4) 0.034(4) 0.28(S)

4(2) 9(3) 89(12) 13(4) 177(7) 0.53(9) 5(2) 2.3(6) 14(2) 5.7(S) 0.33(5) 52(11) 2.8(7) 3.7(4) 338(41) 0.37(S) 5.7(9) 424(39) 6(2) 15(3) 1.1(2) 3.5(5) 0.33(6) 0.5( 1)

2.0(5) 25(2) 97(12) 31(2) 186(9) 0.9(2) 2(l) 3.6(5) 21(5) 80) 1.1(5) 77( 11) 2.9(5) 4.1(7) 187(30) 0.19(6) 2.6(7) 188(31) lO(2) 13(5) 2(l) 9(5) 0.7( 1) 1.2(l)

4..5(6) 24(l) 87(2) 19(2) 182(4) 0.89(5) 5.5(5) 6.1(7) 5.1(3) 5.4(l) 0.42(2) 62(3) 3.1(2) 2.8(l) 210(5) 0.89(4) 12.0(7) 538(59) 28(2) 27(3) 1.28(4) 4.2(3) ma.

8(l) 0.7(4) 1.7(9) 0.2( 1) 1.1(6) 0.3(2) 0.15(2) 0.3(2) 0.07(3) 0.5(3) O.ll(5) 0.4(2) 0.05(2) 0.3(l) 0.04(2) 0.3(2)

8(2) 5(2) I54(3) 13(3) 153(3) 1.3(2) 0.8(3) 0.5(3) 13(4) 11(2) 1.0(4) 207(129) 11(8) 3.1(2) 253(S) 1.0(2) 11.9(6) 170(33) 2(l) 15(2) 2.7(S) 8(2) 0.7( 1) 0.17(3) 5.6(7) 0.4( 1) 1.1(3) 0.14(4) 0.7(2) 0.25(S) 0.050(6) 0.4(l) 0.05(2) OS(l) O.lO(3) 0.29(S) 0.04(l) 0.30(6) 0.05(l) 0.140)

1.22(9) 5(2) 107( 14) 19(3) 157(5) 0.5( I ) 0.6(I) 1.8(6) 10(3) 6.9(S) 0.40(6) 49(8) 2.3(4) 6(2) 345(22) 1.3(5) 17(3) 263(35) 5(l) 12(l) 1.4(3) 4.3(6) 0.60(5) 0.34(S)

60) 0.1 S(2) 0.46(6) 0.07( 1) 0.32(4) O.lO(2) 0.13(7) O.lO(2) 0.024(3) 0.16(2) 0.041(6) 0.13(2) 0.017(2) 0.13(l) 0.019(3) 0.12(l)

0.8(2) 9(3) 81(4) 14(3) 16W6) 0.43(7) 4(2) I .3(7) 9(2) 4.5(3) 0.37(5) 41(3) 2.5(3) 9.3(S) 421(53) 0.7(3) 5(2) 336(97) 5(3) 7(l) 0.9(I) 2.8(3) 0.30(4) W2) 3.7(4) 0.19(3) 0.5( 1) 0.07( 1) 0.35(6) 0.09(2) 0.12(3) 0.12(2) 0.023(3) 0.17(2) 0.039(5) 0.12(2) 0.019(2) 0.12(2) 0.019(3) 0.11(2)

9(3) 0.1 S(3) 0.5(l) 0.08(2) 0.36(S) 0.1 l(2) 0.09(l) 0.15(4) 0.028(6) 0.20(4) 0.044(8) 0.14(2) 0.022(5) 0.15(2) 0.023(4) O.O6(2)

6(2)# 8(l) 0.30(2) 0.90(S) 0.093(7) 0.40(2) 0.112(S) 0.055(S) 0.13(l) 0.025(2) 0.189(4) O&7(2) 0.145(5) 0.020( 1) 0.148(5) 0.024( 1) 0.151(Y)

Semarkona analyses may have been contaminated by Cs.

attractive explanation in that removal of an “excess” albitic component would produce more nearly chondritic abundances for most elements. An albite condensate has also been suggested as a precursor for chondrules with Na/Al ratios near one (Hewins, 1991) but these chondrules exhibit no fractionation of their refractory elements relative to Al (Alexander, 1994). The different Na/Al ratios in Semarkona and Krymka rim/matrix would require an additional process(s) , such as alteration of the albite, that has subsequently modified the distribution of the Na in these two meteorites. The addition of albite or any other aluminosilicate ought to result in a systematic variation in the Al/Si ratio as a function of Al enrichment in the matrix. This is the case, with the Al/ Si ratio in the matrix increasing more or less linearly with the Al content (Fig. 8). Also plotted in Fig. 8 are the compositions of five areas of amorphous aluminosilicate matrix groundmass, mean chondrule mineral and glass compositions, and albite. All but a few of the matrix points are consistent with mixing between materials with albitic (chondrule glass and/or albite) and L-chondrite-like compositions. A mixing line using nepheline rather than feldspar does not produce as good a fit to the matrix data. Nor do any points extend beyond the feldspar composition as one might expect if nepheline

were present since it has an Al abundance that is almost twice that of albite. Only a few trace elements, such as Sr, Ba, or Eu, tend to be compatible in terrestrial alkali feldspar. These three elements are often enriched in the rims and matrices compared to other refractories, but they do not correlate with either Al enrichment (Fig. 9a) or with one another as one would expect if they were being introduced along with albite. Also, if the addition of albite was responsible for the depletions in Fig. 3, there should be an inverse relationship between Al and most other element abundances. However, as is evident from Fig. 9b and c, no such relationship exists. Rather, most refractory element abundances are independent of the Al content. Chemical arguments are not the only objections to the addition of albite to the rims and matrix. If albite were present, it would have to be at levels of 10 wt% or more to explain the depletions relative to Al. At these concentrations, SEM, TEM, or thermoluminescence (Ashworth, 1977; Nagahara, 1984; Alexander et al., 1989b; Sears et al., 1995) studies ought to have been able to detect its presence but they have not. While it is true that all the meteorites studied have undergone some degree of aqueous and/or metamorphic alteration, the preservation of amorphous feldspathic material in

3254

C. M. O’D. Alexander

10 ?? 8

All

Bishunpur, Chainpur, and Krymka matrices (Alexander et al., 1989b) would argue against this as a means of destroying the feldspar. Invoking some other unidentified destruction mechanism is not appealing, since it is ad hoc and inherently untestable, but it cannot be ruled out. One possible origin for the albite is that it was a nebular condensate. Although, calculations suggest feldspar produced by condensation should always have a significant (lo-20%) anorthite content (Wood and Hashimoto, 1993). It has also been suggested that some condensates were amorphous rather than crystalline (e.g., Rietmeyer and McKay, 1986; Brearley et al., 1989; Brearley, 1993). The addition of amorphous material with a feldspathic composition would explain the absence of crystalline feldspar in the matrix but, even if it could be produced, without the mineralogical controls on elemental abundances it is hard to see how amorphous material could exclude all but a few refractory elements from it during condensation. Finally, if one takes a less restricted definition of matrix and includes larger mineral fragments, the bulk lithophile element composition of matrix in Bishunpur, as determined by electron microprobe, is very similar to that of the bulk mete-

matrix

0

Bish.

A

Chain.

rims

??

Krym.

rims

rims

6

0

AA A

??

8 6

l-

1

2

3

4 ,-

Si/Al

(L-group

norm.) ,-

IFIG. 5. The Al and L-chondrite normalized Fe, Si, Mn, and Cu abundances in rims and matrix. The matrix compositions are fairly uniform and close to chondritic. On the other hand, the rims exhibit correlated enrichments between all four elements, which is consistent both with prior suggestions that they were often volatilized from chondrules and recondensed onto the rims, and with the petrologic observations in Fig. lb. The rim trend extends away from the matrix compositions indicating that, prior to recondensation of Fe and Si, the rims also had matrix-like compositions. The volatility of Mn, relative to Si, may have varied from meteorite to meteorite or its abundance has been altered by aqueous alteration/metamorphism. The few Cu-rich matrix points are all from Semarkona where Cu has been concentrated in secondary Fe, Ni sulfides.

,_ I-0

i

Si/Al

i

(L-group

i

norm.)

FIG. 6. Very similar Fe, Si, and Mn enrichment trends to those in Fig. 5 but in this case in a single rim (B-R14, see Fig lb and 2). The abundances were determined by electron microprobe. In general, the most enriched points are found nearest the chondrule (Fig. 2).

Geochemistry

of chondrules

3255

and their matrices

et al., 1992)) as well as Y and variable Ba in merrillite, and Sr and variable Zr in apatite (G. Crozaz, pers. commun.) . In both phosphate minerals Eu is typically significantly depleted relative to the other REEs, presumably because it also partitions into plagioclase. In the least equilibrated UOCs, most of the REEs are in the chondrule glasses (Alexander, 1994) but in the more metamorphosed OCs (> type 3.8) most of the REEs are in the phosphate (e.g., Ebihara and Honda, 1983; Crozaz et al., 1989). In petrologic types 3.8 and above, extensive recrystallization of chondrules as well as matrix has begun, and REE abundances in phosphate grains from these meteorites are already fairly high (Crozaz et al., 1989; Perron et al., 1992). This is consistent with the REEs being driven out of the chondrules and into the phosphates upon devitrification of the glasses (Murrell and Burnett, 1983). Plutonium is lost from chondrules to the phosphates even in Sharps (H3.4) and Tieschitz (H/L3.6) (Murrell and Burnett, 1983). The redistribution of Pu and the REEs into phosphate does not apparently require the presence of a fluid as OCs were metamorphosed under dry conditions. Presumably grain boundary diffusion was sufficient. Unlike the REEs, Al is not lost from chondrules during recrystallization of the mesostasis. Thus, metamorphism is able to affect a significant fractionation of Al from many other refractory elements. Mobilization of the refractory elements would have been much easier in the fine grained UOC rim/ matrix than in chondrules. The refractory elements were prob-

Perron

h $

8

B

0 0

1

Si/Ti

2

(L-group

3

4

norm.)

FIG. 7. An equivalent plot to Fig. 5a but here normalized to Ti rather than Al. Despite both being refractory elements, Al and Ti are fractionated from one another suggesting one or other of them may have been added or removed from the rims and matrices. However, this does not seem to be the cause of the general enrichment of Fe, Si, and Mn in rims relative to both matrix and bulk OCs.

9 in Alexander et al., 1989b). No similar study has been carried out for other UOCs. There is a very limited set of INAA rim (Wilkening et al., 1984; Grossman and Wasson, 1987) and matrix (Grossman, 1985; Brearley et al., 1989) trace element analyses. The INAA analyses are of comparatively large samples and they also tend to have relatively unfractionated lithophile abundances compared to bulk OC or CI. Although, almost all INAA studies find small Eu excesses over the other rare earth elements ( REEs), consistent with the data presented here. If the bulk interchondrule material is essentially chondritic, the chemical signature of the feldspathic material added to the fine-grained matrix and rims must be compensated for by the coarser material. This coarser material is predominantly made up of chondrule mineral fragments and given their generally low refractory element abundances (Alexander, 1994) it is not clear that they could compensate for the albite.

orite (Fig.

0.50

0.10

/

4.1.2. Have some refractories been internally redistributed? 0.01

the above, it seems that the addition of albite to the rims and matrix is not a very satisfactory explanation for the refractory elements depletions illustrated in Fig. 3. The apparently less fractionated compositions of larger matrix samples supports the suggestion by Alexander ( 199 1) that there has been an internal redistribution of the refractory lithophile elements on a scale that is at least comparable to electron or ion probe analyses. Alexander ( 1991) proposed that the host for these mobilized refractory elements are the phosphates and other secondary minerals which grew primarily in and around metal/sulfide grains during aqueous alteration and metamorphism (e.g., Murrell and Burnett, 1983; Bevan, 1985; Perron et al., 1990). Phosphates in more metamorphosed OCs (types 3.8-6) are highly enriched (typically 10-100 X OC) in REEs (e.g., Ebihara and Honda, 1983; Crozaz et al., 1989; From

.

T;;;;y,s ..,,,,,,

1

10 AI

100

(mg/g)

FIG. 8. The Al content of UOC matrices as a function of the Al/Si ratio. All compositions have been recalculated on an Fe-free basis to remove the variable influence of metal, sulfide and Fe0 in the analyses. The CI composition was calculated on an anhydrous basis, with all S as troilite, all remaining Fe as Fe0 and no organic material or carbonates. Also shown are the compositions of albite, mean chondrule minerals (01 + Opx + Cpx) and glass (Alexander, 1994), and amorphous feldspathic material from Bishunpur, Krymka, and Chainpur matrices (Nagahara, 1984; Alexander et al., 1989b). Two possible explanations of the matrix compositions are the addition of albite to L-chondrite-like material (solid line) or mixing of chondrule fragments (dashed line). Although rims are related to the matrix, they are not plotted because volatilized Si has probably recondensed onto them.

C.

M. O’D. Alexander

5.0‘-Ih

g

4.0

E 2

3.0

E ;

2.0

iij 2 CA

1.0

(a) 0.0 ?E

2.0

2 a

1.5

E sy

1.0

I

I

I

I

d 0

.H $

0.5

(b) 0.0 -E

2.0

2 2

1.5

0

2 t! 40

A

A

1.0 A

.m CA 2

0.5

A

1%

A

(c)

0.0 t

I 1.0

I 1.5

Al/Si

I 2.0

(L-group

I 2.5

I 3.0

3.5

norm.)

FIG. 9. The Si and L-group nomakd abundances of (a) Sr, (b) Y, and (c) Ti vs. Al in UOC makes. Sr (or Ba and Eu) is generally fairly compatible in albite and is often enriched, relative to most other &actory elements, in the rimhnatrix. If some Sr (or Ba and Eu) were being intmduced into the matrix along with albite one would expected them to be a correlation between Sr and Al abut&me, which there is not. Conversely, iftheadditionofalbitewasthecauseofthedepletionsofmostother ~fractoq lithophile elements, relative to Al, there should be an inveme ~~~betweenAlandY(ormostother~~es).Thelinein(b) is the calculated path the matrices should follow if albite were being added to an initially L&mdrite4ike matrix. Clearly, the matrices do not conform tothiSliW.RatherthekY abmdames ale essentiauy independent of the Al abmdame. (c) A comparable plot to (b) but for Ti. Rim compositions could not be plotted because volatiked Si has probably recondensed onto them. Two points (K-M8 and C-M3) are off scale in (b) and (c).

ably concentrated in the Al-rich amorphous groundmass of rim/matrix and their mobilization would have been facilitated by the formation of smectite during aqueous alteration or recrystallization of the groundmass during metamorphism. Since metal/sulfide-rich areas, where most of the phosphates are found, were avoided in this study, such a redistribution provides a very natural explanation for the measured refractory element depletions. Those refractory elements that have not been taken up by phosphates and other secondary phases should retain near chondritic Al-normalized abundances. Judging from the type 4-6 phosphate data, Zr, SC, Ti, and Nb should not partition strongly into phosphates. Yet, these elements are often as depleted in the rim/matrix as those elements which are known to partition into phosphates. One possible explanation for this is that the refractory elements are incompatible in the products of both alteration and metamorphism, and, as a result, their expulsion from the rim/matrix was more or less indiscriminate. If this is the case, those elements not normally found in phosphates may have found hosts in other secondary phases. Also, the degree to which an element can be redistributed into phosphates is at least partially controlled by competition between the phosphates and any other phases present. For instance, the often large negative Eu anomalies in type 4-6 OC phosphates are probably due to Eu being distributed between the phosphates and feldspar that form during metamorphism. Since the matrices of UOCs have distinct mineralogies from the more metamorphosed meteorites, it is possible that some elements, such as Eu, behaved rather differently in the UOCs. The analysis of one matrix chlorapatite grain, which included considerable silicate material, in Bishunpur would seem to support this (Fig. 10). The apatite is enriched not only in Ca but also by about 40 X OC in Sr, 20 X OC in Ba and 14 x OC in Eu. The abundances of most other refractory trace elements are two to four times lower than Eu. Some fraction of the refractory trace elements must have been contributed by the silicate matrix included in the analysis. Nevertheless, almost all refractories are enriched by about a factor of two over Al, indicating that the apatite grain is enriched in most refractory elements, including Ti and SC, by between 3 x OC and 6 x OC. Such a composition is quite different from apatites in type 4-6 OCs. The rim and matrix compositions do seem to mimic the abundance patterns of the apatite (Fig. 10). If this pattern is representative of most UOC matrix apatites, it suggests a simple explanation for the range of abundances the refractory elements exhibit. In type 3.8-4s merrilite, which is not generally enriched in Sr, Ba, and Eu, is the dominant phosphate associated with metal/sulfide (Perron et al., 1990). During aqueous alteration or metamorphism most refractories are expelled from the rim/matrix and go into merrilite as well as other secondary phases, but in those rim/matrix regions where apatites can grow in situ much of the Sr, Ba, and Eu is retained. However, given the typical P contents of the rim/matrix (Table 2), apatites like the one measured here could only account for about lo-SO% of the rim/matrix Ca, lo-25% of the Sr, 520% of the Ba, and 2-20% of the Eu. Nor do Sr, Ba, and Eu obviously correlate with P in the matrix, except in Semarkona. Thus, either the phosphates vary considerably in composition and/or the silicates still retain a considerable fraction of the refractory lithophiles.

Geochemistry of chondrules and their matrices

MgAI Si CaSc

3257

Ti V St Y ZrNbBaLaCePtNdSmEuGdTbDyHoErTmYbLuHf

of the Al normalized refractory lithophile element abundances in a small apatite grain (open symbols) in the Bishunpur matrix, along with the mean rim and matrix compositions from all four UOCs studied here (filled symbols). The apatite grain was small and the analysis included considerable silicate matrix. The similar patterns of relative abundance between the apatite grain and the rim/matrices suggest that while most refractory elements were partially lost from the rim/matrix, some elements, principally Sr, Ba and Eu, were retained in tiny apatite grains that grew in the rim/matrix. FIG. IO. Comparison

From the above, it is clear that neither albite addition nor internal redistribution are entirely satisfactory explanations for the fractionation of most refractory elements from Al. Nevertheless, given that albite is not now present in the rim/ matrix of the UOCs and that the geochemical evidence for albite addition is weak, the internal redistribution of elements is the explanation favored here. However, its confirmation must await a careful survey of phosphate and other secondary phase compositions in UGCs. 4.2. Volatile Element Enrichment and Redistribution 4.2.1.

The alkalis

Most chondrules have lower alkali metal and other volatile element contents compared to the rim/matrix and bulk OCs (e.g., Gooding et al., 1980; Grossman and Wasson, 1982, 1983; Swindle et al., 1991a,b). There has been an enduring debate over whether this relative enrichment in the rims and matrix reflects volatile loss from chondrules during their formation or the greater abundance of low temperature condensates, like albite, in rim/matrix (see Grossman et al., 1988). The OCs have CI-like bulk alkali/Al ratios (Wasson and Kallemeyn, 1988). If the alkali enrichment in rims and matrix does reflect a greater abundance of low temperature condensates, the nebula was able to achieve two remarkable feats. First, the chondrule precursors and rim/matrix were kept physically and chemically separate but in close enough proximity for some rims to accrete onto still hot and plastic chondrules. Then during accretion of the parent body, these components were brought together in the correct proportions to produce the CI-like bulk GC alkali/Al ratios. On the other hand, there is no simple correlation between the abundances of Na or K and chondrule size as volatile loss would predict (Grossman et al., 1988). Unless each of the meteorites experienced a unique set of nebular conditions, neither volatile loss from chondrules nor the concentration of low temperature condensates alone can explain the variation in the Na/Al ratios of rim/matrix from

meteorite to meteorite (Fig. 4). Since all four meteorites are falls, terrestrial weathering is an unlikely explanation for the Na/Al variations. From the compilations of Dodd et al. (1967), Kallemeyn et al. (1989), and Jarosewich (1990) the bulk atomic Na/Al ratios for these four meteorites are Semarkona 0.70, Bishunpur 0.70, Chainpur 0.47, and Krymka 0.59. There is apparently no simple correlation between rim/ matrix and bulk NalAl ratios. However, there does seem to be some inconsistency in the Chainpur data. INAA analysis of Semarkona and Chainpur chondmles (Gooding, 1979; Swindle et al., 1991a,b) give average chondrule atomic Na/ Al ratios of 0.58 and 0.70, respectively. The bulk Na/Al ratio of Semarkona is, as expected, intermediate between chondrules and matrix, but both chondrules and matrix in Chainpur have higher ratios than the bulk meteorite. There is only one analysis of bulk Chainpur and it seems likely the bulk ratio is in error. Like Chainpur, Krymka also has a low Na/Al ratio but it has been analyzed twice by different techniques and both analyses give similar results, so the low bulk ratio is almost certainly real. The Na/Al ratios of Semarkona and Bishunpur are comparable to the mean L and LL chondrite ratios and it seems likely the bulk ratio of Chainpur is also similar to other L or LL chondrites. If correct, the variations in chondmle and rim/ matrix NalAl ratios from meteorite to meteorite but similar bulk ratios suggests chondrules and rim/matrix are complimentary. It follows that if the meteorites had similar initial Na and Al distributions, rim/matrix and chondrules must have undergone some exchange to produce the variations. Rubidium and K should be more refractory than Na during condensation (e.g., Larimer, 1988) and probably during chondrule formation. Therefore, both explanations for the higher alkali/Al ratios of rim/matrix compared to chondrules predict that K and Rb should be more abundant in chondrules, relative to Na, than in rim/matrix. Yet, just the opposite is the case. However, Na is not only the most volatile but also the most mobile of the alkalis. If there has been exchange between an alkali-rich rim/matrix and alkali-poor chondrules, Na would

3258

C. M. O’D. Alexander

be the first to diffuse into the chondrules and hence its relative enrichment in chondrules and depletion in rim/matrix. If, as seems possible, the primary alkali distribution in these meteorites has been modified, two things, diffusion rates and mineralogy, are likely to have ultimately controlled the secondary redistribution. All four meteorites have experienced rather different postaccretionary (secondary) conditions, from aqueous alteration in Semarkona to moderate thermal metamorphism in Chainpur. These different secondary conditions are reflected in the mineralogies and microstructures of their rim/matrix. As emphasized above, despite the molar Na/AI ratio of about one in Bishunpur and Chainpur rim/matrix, TEM studies have not found any feldspar (Ashworth, 1977; Alexander et al., 1989b). Rather, the feldspathic material seems to be concentrated in an amorphous groundmass. Alexander ( 1994) pointed out that for Na, as well as the other alkalis, to be easily incorporated into the structure of a feldspathic melt or glass it must take part in the coupled substitution NaAlO*-GO*. This substitution gives a natural upper limit to the amount of Na that an aluminosilicate glass can comfortably incorporate. Thus, whether the rim/matrix was initially composed of chondrule glass enriched in Na by volatile loss from chondrules or was composed of Na-rich amorphous condensates, any Na or other alkalis that are in excess of the Al available would probably be very mobile. The excess Na would have tended to diffuse into alkali depleted regions of rim/matrix and into the depleted chondrules. Semarkona and Krymka do not fit quite so easily into this picture. In Semarkona, the molar Na/Al ratio in the matrix is significantly greater than 1 (Fig. 4). The rim/matrix of Semarkona is dominated by an Fe-rich smectite which has a Na/ Al ratio of about 1.8 (Hutchison et al., 1987; Alexander et al., 1989a). Such a high ratio results from a significant fraction of the Fe in the smectite being trivalent and tetrahedrally co-ordinated. As is the case of Al, Fe”+ requires Na, or some other monovalent cation, for charge compensation when it substitutes for Si. Again, assuming that all UOCs had the same initial distribution, the higher Na/Al ratio in Semarkona matrix requires that either ( 1) the initial ratio in the rim/matrix of Bishunpur and Chainpur was higher than now observed and that some Na has diffused into their chondrules, or (2) that Na diffused out of Semarkona chondrules during formation of the smectite to compensate for the Fe”. Thus, Semarkona is a good illustration of how the mineralogy may have influenced alkali distributions. The low alkali/Al ratios in Krymka rim/matrix are less easily explained. A limited survey of Krymka chondrules suggests that they have similar or even slightly enriched alkali contents compared to the chondrules in the other three chondrites analyzed here (Table 3). Huang et al. ( 1993a) suggest that exchange of Na between rim/matrix and chondrules was more extensive in Krymka, but the alkali depletion of the bulk meteorite and undepleted chondrules suggests that much of the alkalis originally in Krymka’s rim/matrix have been lost from the meteorite. Other volatiles are not depleted in Krymka (Keays et al., 1971; Ikramuddin et al., 1977; Kallemeyn et al., 1989), so the alkalis were probably not lost in some thermal event. Rather, they may have been lost during the passage of a fluid through the rim/matrix. There is evidence for alter-

Table 3. The L-erouo normalized alkali to Al ratios in six Krymka chondr&gl&ses determined by ion microprobe. K-l K-2 K-3 K-4 K-5 K-6 VP0 TYPE Mg nos. f&?S5 g8? ;% $20 0.911 5:6 NEilAl K/Al RWAl CSfAl

2.01

1.I5

1.33 0.70

2.61 2.78 2.65 1.32

2.11 2.75 2.13 1.69

1.38 1.59 1.40 0.9 1

1.91 2.9 1 2.91 2.03

0.87 0.09 0.03 0.25

ation of fayalitic olivine in parts of Krymka’s rim/matrix (Alexander, 1987). If, as proposed above, chondrules and matrix have exchanged alkalis, there are two possible scenarios for where and when the exchange could take place; either during chondrule cooling (Matsunami et al., 1993; Sears and Lipschutz, 1994) and/or during metamorphism/alteration (Hewins, 1991; DeHart et al., 1992; Huang et al., 1993a). As will be discussed below, it seems from the results of simple calculations (Fig. 11) that Na diffusion through chondrule mesostasis is fast enough to allow a significant exchange between chondrules and rim/matrix under both circumstances. There are two major uncertainties in assessing the degree of exchange possible between a rim and a chondrule during chondrule cooling-the cooling rate and the temperature at which the rim accreted or, if the temperature during rim accretion was too high for Na retention, the temperature at which Na recondensed. Here it was assumed that the cooling rate was 1OOO”C/h, which is at the upper end of most cooling rate estimates (e.g., Jones, 1990; Alexander, 1994) and will, therefore, give a conservative estimate of the possible exchange. Appropriate diffusion coefficients are also a concern but Freer ( 198 1) has compiled diffusion data for several glasses whose compositions span those of typical chondrule mesostasis. In Fig. 1 la the calculated Na diffusion distance, given by the approximation x = fi, is plotted as a function of the Na retention/recondensation temperature using Freer ( 1981) glass data. The range of results is large, but what is clear is that provided the diffusion coefficients used are appropriate for chondrule mesostasis and that Na retentionirecondensation took place above 600 K, significant exchange is possible even during fairly rapid chondrule cooling. Note that a cooling rate of lOO”C/h will increase the diffusion distance by v%. Weinbruch and Muller ( 1994) have suggested that chondrule cooling rates may have been only 1-lO“C/h between 1300-1100°C and if these cooling rates persisted to lower temperatures diffusion distances would be one to two orders of magnitude greater than in Fig. 1 la. Estimating diffusion distances during metamorphism is more straight forward. Figure llb shows the Na diffusion distances as a function of time at 500 K for the same glass compositions as before. Again, it seems that at even very modest temperatures Na can diffuse tens to hundreds of micrometers within only a few years. Consequently, it is possible or even likely that the distribution of Na, and even the other alkalis, has been modified in most, if not all, UOCs since their accretion. Potassium and Rb diffuse more slowly than Na, with diffusion distance ratios at 500 K that range from 2-300 for Na/K and 17- 11400 for Na/Rb, depending on the glass composition. The slower diffusivities of K and Rb would explain why they are less enriched in chondrule glasses than Na (e.g., Alexander, 1994) and why they tend to be more en-

Geochemistry of chondrules and their matrices

3259

rification of the feldspathic material in the rim/matrix than in the chondrule mesostasis. Given the speed of Na diffusion in feldspathic glasses, Na exchange may have ceased only when all amorphous material in chondrules and rim/matrix had completely devitrified. If, as seems likely, the primary Na distribution, which almost certainly included an alkali-rich rim/ matrix, has been modified in all UOCs, this will have important implications for models of chondrule formation since the alkali abundances are often used to try and constrain the formation conditions (e.g., Grossman, 1988; Yu et al., 1995). 4.2.2. Were iron and silicon volatiles during chondrule formation?

lE+O

lE+l

lE+2

lE+3

Distance

8

lE+4

lE+5

(pm)

__--___--___---

9 .!2

________..----‘--

Q

Albite

. . .............___ O,igoe,ase --- .._____ Basalt -- ---_____ Obsidian

lo(b)

1

i

_--__-_

io

Rbyolite

100

Years FIG. Il. (a) The distance Na is able to diffuse into a chondrule cooling at lOOO”C/h as a function of the temperature at which Narich material accretes onto a chondrule’s surface without the Na being volatilized. The Na-rich material could be volatiles lost from the chondrule that condense directly back onto it and/or accreted dust that is enriched in volatiles. The various curves were calculated using measured diffusion coefficients in glasses with a range of compositions (Freer, 1981). The results suggest that significant exchange between a chondrule and material on its surface could have occurred as long as Na-rich material condensed/accreted onto it at or above 600 K. (b) The distance Na can diffuse in the same glasses at 500 K as a function of time. It seems that Na can diffuse tens to hundreds of microns in only a few years even at these very modest temperatures. Hence, it is unlikely that the distribution of Na in many or any UOCs has remained undisturbed since accretion.

riched than Na in the rim/matrix. It is important to note that because chondrules make up about 80% of the silicate material in 00, even a limited exchange would have a significant effect on rim/matrix alkali contents but the change would be much more subdued in chondrules. Of course, these calculations only establish that exchange is possible but for it to occur there must be a gradient in the chemical potential between chondrules and rim/matrix. Although the chondrules are depleted in Na, it is still conceivable, under certain circumstances, that the chemical gradient be from chondrules to matrix-e.g., the formation of Na-rich smectite in the matrix or more rapid restructuring and devit-

While the partial redistribution of the alkalis can explain some features of the alkali abundances, it does not help resolve whether the generally higher volatile contents of the rim/matrix is due to an abundance of low temperature condensates or the recondensation of volatiles lost from chondrules. The alkalis have always been thought of as potentially volatile elements during chondrule formation. More recently it has been suggested Fe and Si, amongst others, also behaved as volatile elements during chondrule formation (e.g., Lu et al., 1990; Huang et al., 1993a,b; Alexander, 1994; Scott, 1994). Iron and Si are the two most volatile major elements in chondritic melts, followed by Mg (Hashimoto, 1983; Wang et al., 1993, 1994). These elements should be less susceptible to remobilization than the alkalis and, therefore, leave a less ambiguous record of chondrule volatile loss, if it occurred. Unlike the alkalis, the enrichments in Fe0 and Si are most obvious in the rims (Figs. 2, 5 and 6) and in some instances these enrichments decrease away from the chondrule (Figs, lb, 2, and Table 1) . Enrichments in Fe0 and Si that increase away from the chondrule were never observed in rims in this study. As with the alkalis, the Fe and Si enrichments in the rims could be explained by the presence of low temperature condensates. However, the nebula condensate explanation faces two problems. ( 1) Why when Fe and Si abundances in a rim vary systematically away from the chondrule do they invariably decrease in abundance? During nebular condensation, Fe0 contents should increase as the temperature falls. Yet, despite the incomplete reequilibration of olivines in the outer parts of rims (Fig. lb), which is consistent with falling temperature, the Fe0 contents decrease. (2) The timescales, indicated by incomplete reequilibration in some rims and the lack of reequilibration of the chondrules, would seem too short for rims to directly record nebular-wide condensation. Either the rims record more local phenomena, such as chondrule formation, or the FeO- and Si-rich condensates are preferentially concentrated in rims compared to matrix. The latter explanation would require that three components (chondrule precursors, FeO-rich condensates, and matrix) were present in the OC formation region. It would also require the seemingly unlikely situation in which these components were kept more or less separate, despite their apparently close proximity to one another, and that the FeO-rich material was only accreted when it formed part of a chondrule rim. For these reasons, the recondensation of Fe and Si volatilized during chondrule formation would seem a more preferable explanation for the FeO-rich rims observed here, as well

3260

C. M. O’D Alexander

as a similar Fe and Si enrichment trend observed in a larger suite of Bishunpur rims (Alexander, 1987). TEM observations of amorphous areas in Bishunpur rims that are primarily composed of Fe and Si (Alexander, 1987) are also consistent with recondensation of these elements and the greater enrichment of Fe, relative to Si, follows their relative volatilities (Hashimoto, 1983; Wang et al., 1993, 1994). The correlation between Fe and Si is strongest in Bishunpur rims, and its preservation there may have been facilitated by the comparatively minor alteration and recrystallization experienced by rims and matrix in this meteorite (Alexander et al., 1989b). Copper, a volatile element which can exhibit both siderophile and chalcophile tendencies, is also enriched in many rims, relative to matrix, and its enrichment roughly correlates with Fe and Si (Fig. 5 ). Again, the enrichment is most pronounced in Bishunpur. The other siderophiles measured, Ni and Co, do not obviously correlate with Fe in the rims. Despite its chalcophile tendencies, Cu only correlates with S in Semarkona, where it also correlates with Ni, indicating its host phase is the secondary Ni-rich sulfide present in Semarkona matrix (Alexander et al., 1989a). The Ni/S ratio given by the correlation in Semarkona matrix is about twice L-chondrite, consistent with the ratios measured by TEM/EDS in the Ni-rich sulfides. The Cu/S ratio in these sulfides implied by the correlation is about 27 times L-chondrite but, as discussed earlier, the Cu sensitivity factor is somewhat uncertain. One of the striking features of both the compositions in Figs. 2, 5, and 6 and the textural observations in Fig. 1 is that the matrix appears to have experienced comparatively little recondensation of Fe, Si, or Cu, while the rims appear to be mixtures of compositionally and texturally matrix-like material and recondensates. Matrix and the matrix-like precursors of rims are probably one and the same. The fact that only rims were significantly affected by recondensation of Fe and Si, and even then not always uniformly, suggests that recondensation was very localized and rapid. Rapid recondensation would quickly deplete the gas surrounding chondrules, so that rim material that accreted later would tend to be poorer in Fe and Si and richer in matrix-like dust, as is observed. Chondrule-derived condensates may have even helped rim formation by binding accreting material. Indeed, the compositions of the most Fe-, Si-rich regions of rims would require the dilution of matrix by more than 5 times as much Fe-, Si-rich material. If recondensation was very local, one might expect to see a correlation between the properties of chondrules and their rims. The degree of volatile loss from a chondrule is a function both of its thermal history and of its size. However, there is no correlation between chondrule size and rim chemistry. There is a suggestion from the rims examined here of a correlation between the chondrule size and maximum rim thickness (rim thickness/chondrule radius = 0.2). Such correlations have been previously reported for CM chondrites (Metzler et al., 1992) and UOCs (Huang et al., 1993b). On the other hand, Matsunami ( 1984) did not observe such a correlation in a similar suite of UOCs. In conclusion, the general volatile enrichment of rims and depletion of chondrules is suggestive of volatile loss from chondrules and recondensation on rims. Matrix and matrixlike rims do not appear to have been the main sites for recondensation, at least for Fe, Si, Mn, and Cu. As has been noted

in many previous studies, no strong correlation has yet been found between volatile contents of chondrules and their rims. This may be because the time after chondrule formation at which rims accreted varied considerably. Alternatively, rims may not just include material lost from the host chondrule but also material lost from other chondrules or even material from completely vaporized fine-grained dust. Whether or not the rims retain a record of volatile loss from a single chondrule or a chondrule forming event, once the rims had accreted heat from the cooling chondrule and the surrounding medium appears to have driven partial to complete reequilibration of ferromagnesian minerals in the FeO-rich rims. Matrix-like rims were less affected presumably because they accreted when temperatures were cooler and because their lower Fe0 contents increased their melting points and reduced diffusion rates, particularly in olivine (Buening and Buseck, 1973 ) . 4.3. Origin of Rim/Matrix Even though an internal redistribution seems the more likely explanation for the refractory lithophile fractionations, this still does not explain why there is an enrichment of feldspar normative material in the <5 pm fraction but, apparently, a nearly chondritic composition in larger samples (INAA and very broad electron probe analyses). Two suggestions have been put forward to account for this. The first is that the matrix was largely amorphous and nearly chondritic in composition (Brearley et al., 1989). The amorphous material could have been either condensates or unprocessed interstellar dust. Subsequently, this amorphous material began to devitrify to t’inegrained (
Geochemistry of chondrules and their matrices A rather different explanation for the origin of matrix is that the bulk of it, including the >5 pm material, is dominated by chondrule fragments and that chondrule glass tends to be enriched in the finest ( <5 pm) grained fraction (Alexander et al., 1989b). Just such a grain-size fractionation of feldspar and glassy materials from other minerals is known to occur in experimental and lunar regoliths (e.g., Horz and Cintala, 1984). Even if the fractionation did not occur, glass fragments, by their very nature, would look fine grained in the SEM. That there are chondrule mineral fragments in the matrix and rims is well established (e.g., Alexander et al., 1989b; Brearley et al., 1989) and in Fig. 8 the matrix analyses form an almost linear array between the mean compositions for chondrule minerals and glass. In Bishunpur, elastic silicates and metal/sulfide greater than about 0.5 pm across make up about 65 ~01% of the interchondrule material, the remainder being amorphous material and voids (Alexander et al., 1989b). In this fragmentation model, the fine-grained nonelastic olivines observed by TEM in the matrices of most UOCs result from the addition of Fe0 to, and subsequent devitrification of, the silica and/or pyroxene normative glasses. Those meteorites which have rim/matrix compositions that are close to the bulk meteorite may have a much finer grain size distribution of fragments than in the meteorites studied here, but it is also possible that these meteorites have a larger proportion of condensates or other fine-grained nebular dust. The abundance of elastic material in many rims and matrices in UOCs makes a chondrule origin for much of the amorphous matrix material seem plausable. Although, if an amorphous condensate were nonchondritic in composition then the absence of the complimentary fine-grained olivine would no longer be a valid objection to this explanation. In Bishunpur, the composition of interchondrule material does seem to be close to bulk OC. Extensive surveys of the interchondrule material in other UOCs have yet to be done. The rims and matrix of the CO3 chondrite ALHA have compositions that in many ways are similar to the UOC rims and matrices, including enrichments in Al and an even more exaggerated depletion of the refractory elements Ca and Ti from Al (Brearley, 1993). Although, the Fe0 content of ALHA matrix are higher than in the UOCs and the Mg, Si, and Na contents are lower in both its rims and matrix than in the UOCs. TEM observations of ALHA shows that both the rims and matrix are largely amorphous with far fewer chondrule fragments than in UOC matrices (Brearley, 1993). Brearley ( 1993 ) concluded that the rims and matrix of ALHA were probably made largely of amorphous condensates, in which case, given some of the compositional similarities between UOC and ALHA rim/matrices, it remains possible that some fraction of the UOC rim/matrix was composed of nonchondritic amorphous condensates. In summary, the fractionation of Al from most other refractory lithophiles is problematic for all current ideas for the origin of matrix. Of the two explanations explored, the one considered more likely here is that there has been a partial internal redistribution of most refractory elements into secondary phases. This redistribution would have to have been relatively indiscriminate and it remains to be seen if the compositions of the secondary minerals in the UOCs are consistent with this. It has been suggested by various authors that

3261

the bulk of the matrix was either amorphous presolar material, amorphous condensates, or fragmented chondrules. One might expect that the preso1a.r material or amorphous condensates would be relatively homogenous at scales of more than a few micrometers. If the presolar material and/or condensates were more or less chondritic in composition, they must produce feldspathic-rich areas on the scale of typical electron and ion probe analyses (5-20 pm) by devitrification. This would seem to be problematic if only because the complementary olivine-rich areas are not present in the meteorites studied here (Fig. 8). In addition, a presolar precursor would imply a solar or Cl-like bulk matrix composition that does not seem consistent with the trend in Fig. 8. The chondrule fragmentation hypothesis does not face these problems since the feldspathic glass, due to its physical properties, is naturally concentrated in the fine-grained material. The amorphous and chondrule precursor models need not be mutually exclusive since some UOCs have rim/matrix compositions that are closer to bulk chondrite than in the meteorites studied here. On the other hand, some presolar material survives in ordinary chondrite rim/matrix, so they cannot be entirely composed of condensates and/or chondrule fragments. 4.4. Abundance of CI-Like Material in UOC Matrix The presence of presolar grains clearly shows that there is material in UOC matrix that has not been processed either through evaporation-condensation or chondrule formation. The presolar grains so far identified are predominantly carbonaceous but must surely have been accompanied by silicates with, on average, near Cl-like compositions. Indeed, Huss ( 1990) and Huss and Lewis ( 1995) have suggested that initially UOC matrices had essentially the same abundances of presolar grains as in bulk Cls. Yet, the bulk compositions of UOC matrices appear to be different from bulk Cl. Can the bulk compositions and presolar grain abundances be reconciled in the light of the various explanations for the bulk UOC matrix proposed above? Huss and Lewis ( 1995) suggest that processing in the nebula may explain both the bulk compositions and presolar grain abundances of the various meteorite classes. However, a mechanism capable of producing the UOC rim/matrix or bulk meteorite compositions from Cl-like material without also altering the presolar grain absolute and relative abundances has yet to be demonstrated. Obviously, if the matrix formed from unprocessed interstellar dust, the observed similarity in abundances of identified presolar grains is explained. However, the composition of the matrix does not appear consistent with a Cl-like bulk composition-the trend in Fig. 8 goes towards bulk L-chondrite not Cl. Even if there has been loss of Mg from the normative-feldspar-rich matrix areas, which would shift the matrix analyses to higher Al abundances, there does not seem to be the complementary fine-grained olivine-rich areas necessary to maintain a bulk Cl-like composition. An amorphous condensate, with either a L-chondrite- or Cl-like composition, faces similar objections, namely how to produce the normative-feldspar-rich areas without the complementary olivine-rich ones. Although chondrule fragmentation can produce a normative-feldspar-rich matrix quite straight forwardly, both it and the condensation hypothesis must explain the abundance of presolar grains in the matrix

3262

C. M. O’D Alexander

without there being a clear CI-like signature in its chemistry. In either case, can enough unprocessed material be present to account for the presolar grains but its influence not be obvious in the bulk compositional data? We do not know what the silicate to carbonaceous presolar grain ratio was. A likely minimum estimate is to assume a solar (MgO + SiO*)/C ratio (0.71 by weight) and let essentially all the C in presolar material be in the most abundant presolar grain type, diamond. Estimates of the presolar diamond abundances in UOC matrices are as high as 800- loo0 ppm (Huss, 1990; Russell et al., 1992; Russell, 1993; Huss and Lewis, 1995), which would require only 700 ppm of unprocessed silicates to be present. Clearly this is an absolute lower limit. In reality, much of the presolar C would have been in CO and organic material but even if the silicate/diamond ratio is only 1% of the solar value, the silicate contribution to the matrix would only be about 5-7 wt%, which would still be difficult to detect. From infrared observations, Allamandola et al. (1993) infer that at least 5% and perhaps as much as 20% of all the carbon in molecular clouds is present as diamond, in which case only between 0.4 and 1.5 wt% of the matrix would be unprocessed silicates. CI abundances of this order would be very difficult to detect in this study. If these estimated CI-like presolar silicate abundances (0.4-7 wt%) in UOC matrices are correct, the similar concentrations of known carbonaceous presolar grains in UOC matrices and CI-chondrites (Huss, 1990; Russell, 1993; Huss and Lewis, 1995) require that the proportion of processed and unprocessed material be approximately the same in both of them. Whether this is simply a coincidence or has more profound implications remains to be seen. 5. CONCLUSIONS The abundances of some forty elements have been determined in the rims and matrices of the UOCs Semarkona, Bishunpur, Chainpur, and Krymka. The rims appear to be a mix of matrix-like material and FeO- and Si-rich material. Other elements including the alkali metals, Mn and Cu, are also enriched, relative to refractory elements, in the Fe-, Si-rich rims and it is suggested that this material was volatilized during chondrule formation and recondensed onto the rims. Recondensation, at least of Fe, Si, Mn, and Cu, appears to have been largely confined to the rims. Many rims accreted onto their chondrules while the chondrules were still hot, suggesting recondensation was a rapid and localized process. The alkali metals are enriched, relative to refractory elements, in the matrix as well as the rims. The alkalis may have recondensed onto matrix as well as rims but they are likely to have been very mobile during metamorphism/alteration and their present distribution is probably secondary. The matrices in all four meteorites are enriched in normative feldspar and in all cases most refractory lithophiles are fractionated from Al. One explanation for the matrix compositions, the addition of refractory-lithophile-poor albite to bulk OC-like material, seems unlikely because albite is not found in the matrices and all refractory lithophile contents are independent of Al concentration. The REEs in more metamorphosed OCs (Xype 3.8) appear to have been expelled from chondrules during the recrystallization of their mesostases and are now concentrated

in phosphates. A similar redistribution of the refractory lithophile elements could have occurred in the fine-grained rim/ matrix in UOCs, with their partial expulsion from the silicate matrix during aqueous alteration or metamorphism into secondary phases such as phosphates. Phosphates and other secondary phases typically form in and around metal and sulfide grains which were avoided in this study. Obviously, the test of the redistribution hypothesis is a comprehensive survey of trace element abundances in secondary phases, but initial attempts to do this in Bishunpur have been hampered by the small sizes of the grains. Even if the refractory lithophiles have been partially redistributed, this still leaves the enrichment of matrix in normative feldspar to be explained. Two explanations, which are not necessarily mutually exclusive, have been explored. The first is that the matrix was originally amorphous and had an essentially bulk OC composition. This amorphous material may either have been unprocessed presolar silicates or nebular condensates. In either case, it seems likely that they would be quite homogeneous at the typical scale of electron and ion probe analyses (5-20 pm). Therefore, to produce the normative-feldspar-rich matrix compositions both sources would require some sort of segregation process that can separate olivine-normative-rich from feldspar-normative-rich material. Even if this could be achieved, the fine-grained olivine-normative areas do not seem to be present in the matrix. The second alternative is that the matrix is largely composed of chondrule fragments and that because of its mechanical properties the feldspathic glass is enriched in the <5 pm material. Such grain-size segregation is seen in experimental and lunar regoliths. However, not all matrix can be fragmented chondrules or, indeed, condensates. Presolar grains are present in UOC matrices and at concentrations that are comparable to those in bulk CIs. The known presolar grains are predominantly carbonaceous but must have been accompanied by silicates. The similarity of presolar grain abundances in UOC matrices and CIs suggests that the matrix should be CI-like in composition but this is not the case. Diamond is the dominant known presolar grain and provided the silicate/diamond ratio in the presolar material was more than 1% of the solar (MgO + Si02)/ C ratio, which would be consistent with some astronomical observations, the abundance of CI-like material in the matrix would be less than 10 wt%. Such a concentration of CI-like material, when mixed in with material with an L-chondritelike composition (condensates or chondrule fragments), would be difficult to detect in this study. Acknowledgments-The author would like to thank E. Zinner, W. Hsu, G. Crozaz, and R. Walker for their help and encouragement, and P. Swan for help with the SEM analyses. A. Brearley, J. Grossman, and G. Huss provided very helpful reviews. This work was supported by NASA grant NAGW-337 1.

Edi?orial handling: R. A. Schmitt

RBFERBNCRS Alexander C. M. O’D. ( 1987) The matrices and rims of unequilibrated ordinary chondrites: Origins, metamorphism and alteration. Ph.D. thesis, Univ. Essex.

Geochemistry

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Alexander C. M. O’D. ( 1991) The origin of matrix and rims in Bishunpur (LILL3.1); an ion probe trace element study. Mereorirics 26,312-313. Alexander C. M. O’D. (1994) Trace element distributions within ordinary chondrite chondrules: Implications for chondrule formation conditions and precursors. Geochim. Cosmochim. Actu 58, 3451-3467. Alexander C. M. O’D., Hutchison R., Graham A. L., and Yabuli H. ( 1987) Discovery of scapolite in the Bishunpur (LL3) chondritic meteorite. Mineral. Mag. S&733-735. Alexander C. M. O’D., Barber D. J., and Hutchison R. (1989a) The microstructure of Semarkona and Bishunpur. Geochim. Cosmochim. Acta 53, 3045-3057. Alexander C. M. O., Hutchison R., and Barber D. J. (1989b) Origin of chondrule rims and interchondrule matrices in unequilibrated ordinary chondrites. Earth Planet. Sci. Lett. 95, 187-207. Alexander C. M. O’D., Arden 3. W., Ash R. D., and Pillinger C. T. (1990) Presolar components in the ordinary chondrites. Earrh Planet. Sci. Let?. 99,220-229. Alexander C. M. O’D., Swan P. D., and Walker R. M. (1992) Continued in situ studies of interstellar grains in primitive meteorites. Lunar Planet. Sci. XXIII, 9- 10. Alexander C. M. O’D., Bridges J. C., and Hutchison R. (1994) Cl and alkali metasomatism in unequilibrated ordinary chondrites. Lunnr Planet. Sci. Con$ XXV 11 - 12. Allamandola L. J., Sandford S. A., Tielens A. G. G. M., and Herbst T. M. ( 1993) Diamonds in dense molecular clouds: A challenge to the standard interstellar medium paradigm. Science 260,64-66. Allen J. S., Nozette S., and Wilkening L. L. (1980) A study of chondrule rims and chondrule irradiation records in unequilibrated ordinary chondrites. Geochim. Cosmochim. Acta 44, 1161- 1175. Anders E. ( 1964) Origin, age and composition of meteorites. Space Sci. Rev. 3, 583-714. Ashworth J. R. ( 1977) Matrix textures in unequilibrated ordinary chondrites. Earth Planet. Sci. Lett. 35, 25-34. Ashworth J. R. ( 1981) Fine structure in H-group chondrites. Proc. Roy. Sot. London A374, 179- 194. Bevan A. W. R. ( 1985) Chemical and mineralogical studies of metal in meteorites. Ph.D. thesis, Univ. London. Brearley A. J. ( 1993) Matrix and fine-grained rims in the unequilibrated CO3 chondrite, ALHA77307: Origins and evidence for diverse, primitive nebular components. Geochim. Cosmochim. Acta 57, 1521-1550. Brearley A. J., Scott E. R. D., Keil K., Clayton R. N., Mayeda T. K., Boynton W. V., and Hill D. H. (1989) Chemical, isotopic and mineralogical evidence for the origin of matrix in ordinary chondrites. Geochim. Cosmochim. Actu 53,2081-2094. Buening D. K. and Buseck P. R. ( 1973) Fe-Mg lattice diffusion in olivine. J. Geophys. Res. 78,68.52-6862. Bunch T. E., Schultz P., Cassen P., Brownlee D., Podolak M., Lissauer J., Reynolds R., and Chang S. ( 1991) Are some rims formed by impact processes? Observations and experiments. Icarus 91, 76-92. Christophe Michel-Levy M. ( 1976) La matrice noir et blanche de la chondrite Tieschitz (H3). Earth Planet. Sci. Lett. 30, 143-150. Clayton R. N., Mayeda T. A., Goswami J. N., and Olsen E. J. ( 1991) Oxygen isotope studies of ordinary chondrites. Geochim. Cosmochim. Acta S&2317-2338. Crozaz G., Pellas P., Bourot-Denise M., dechazal S. M., Fieni C., Lundberg L. L., and Zinner E. ( 1989) Plutonium, uranium and rare earths in the phosphates of ordinary chondrites - the quest for a chronometer. Eunh Planet. Sci. L.&f. 93, 157- 169. DeHart J. M., Lofgren G. E., Jie L., Benoit P. H., and Sears D. W. G. ( 1992) Chemical and physical studies of chondrites: X. Cathodoluminescence and phase composition studies of metamorphism and nebular processes in chondrules of type 3 ordinary chondrites. Geochim. Cosmochim. Actu 56, 3791-3801. Dodd R. T., Van Schmus W. R., and Koffman D. M. ( 1967) A survey of the unequilibrated ordinary chondrites. Geochim. Cosmochim. Actu 31, 921-951. Ebihara M. and Honda M. ( 1983) Rare earth abundances in chondritic phosphates and their implications for early stage chronologies. Earth Planet. Sci. Len. 63,433-445.

and their matrices

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Geochemistry

of chondrules

3265

and their matrices

Appe"dixA.The compositions of rims (R) and matrix(M)in Btshunpur(B).Semarkona (S),Chainpur(C)a.ndKrymka (K)detetminedby ion microprobeanalysis.The precismnerrorsin the lastsignificant figuresare in parenthases and are allone standarddeviation. For the rims,the chondrulesthey surroundhave been classified accordingI" chemicaltype and textural class. Chondruleswere assignedt" chemical type I If the M&number (atomicMg/Mg+Fe) in theirphenocrysts arealways above 0.9 and t" typeIIifthe s"me phenocrjsts had Mg-numbers lessthan0.9.I" a few the components in the chondruleshave alsobeen analyzedin the ion probe (seeAlexander,1994) and have the same identification numbers,i.e. B-R17 isthe nmaround Bishunpurchondrule B-17

cases

Chondrule

B-MI

B-M2

B-M3

B-M4

1.95(6) 16.8(2) 99.6(7) 25.0(l) 218(l) 0.80(5) 4.2(2) 4.48(3) 20.1(3) 9 l(3) Ol(3) 76.3(8) 3 37(5) 3 56(6) 101(l) 0 106(3) 2 Ol(6) 307(5) 8(l) 21.4(3) 3.8(2) 26(4) I Ol(7) 2.3cl.4) 4.4(3) l.8(3) 4.4(7) 0.53(7) 1.8(l) 0.58(6) 0.17(l) 0.5(l) O.ll(2) 0.70(5) 0.15(l) 045(3) 0.060(7) 0.37(3) 0.05(l) 0.64(8)

I

1.70(5) 15.7(2) 77(l) 22.0(2) 185(2) l.61(7) 3.2(l) 3.24(4) l3.7(4) 4.46(6) 0.374(4) 46.0(8) 1.92(4) 2.87(5) 126(2) 0.198(6) 4.3(l) 445(6) 13(l) 20.0(5) l.27(6) 3.6(l) 0.28(2) 0.74(l) 4.3(2) 0.20(2) 0.47(4) 0.076(7) 0.37(Z) 0.09(l) 0.23(2) O.ll(2) 0.026(3) 0.17(l) 0.052(5) 0.138(9) O.OlP(6) 0 II(l) 0.023(4) 0.17(2)

14(l) 19.1(4) l.58(6) 4.7(2) ".a. ".a. 5 l(2) 0.21(Z) 0.65(5) 0.089(P) 0.43(3) 0.14(2) 0.10(l) 0.15(2) 0.028(5) 0.23(2) 0.053(6) 0.15(l) 0.025(5) 0.18(2) 0.039(7) 0.19(Z)

67(l) 2.48(4) 2.41(3) 107(l) 0.346(8) 4.6(l) 25X5) 15(l) 15.7(3) 1.39(7) 4.6(2) 0.40(3) 0.9(l) 3.7(2) 0.25(2) 0.65(4) 0.096(8) 0.48(2) 0.12(Z) 0.064(7) 0.15(2) 0.027(5) 0.25(2) 0.051(6) 0.19(l) 0.019(3) 0.17(2) 0.030(4) 0.16(21

C-M3

C-M4

C-M5

B-R3b(ii)

C-MI

Type/Text. Mg nos.

IIFOP 0.98-0.97

Lump

1.4(l) 4.9(l)

1.35(6) 25.2(4) 86.1(7) 34.0(7) 180(l) 0.55(l) 1.35(l) 5.12(5) 16 3(4) 3.09(5) 0.348(4) 51.3(9) 1.69(4) 3.76(5) 178(2) 0.150(6) 2.8(2) 225(4) 16.3(P) 4.6(2) 0.64(3) 2.1(l) 0.21(2) 1.4(2) 4.8(3) 0 19(Z) 0.48(4) 0.057(6) 0.30(2) 0.08(l) 0.23(2) O.ll(2) 0.024(4) 0.17(l) 0.033(6) 0 12(l) O.OlP(4) 0.08(l) 0.015(4) 0.096(2)

Na Wg) Mg (mg/g) Al ("@g) SI ("Jg/g) P@Wg) S (mg/g) K (mg/g) Ca(nvk) SC (ppm) Ti (mg/g) V(ppm) Cr(mp/g) M" (mg/g) Fe (mpig) C" @g/g) i-11 (mg/g) Cu(ppm) Rb (ppm) Sr(ppm) Y (ppm) Zr(ppm) Nb(epm) Cs @pm) Ba(ppm) La(ppm) Ce (ppml pr @em) Nd(ppnv Sm(pem) Eu(ppm) Gd(ppmI 7b(ppm) DY (eem) H" (ppm) Er(ppm) Tm (ppm) Yb (ppm) Lu(epm) Hf@pm)

7ow 6.6(l) l92(2) 0.50(l) 0 65(9) l.l7(2) 18.4(5) 4.79(8) 0.27(l) 37(Z) 1.72(3) 2.88(6) 336(6) 0433(8) 6.49(l) 391(P) 9(l) 8.7(3) I Ol(7) 3.1(3) n.a. 0.34(4) lO.5(7) O.l6(2) 0.44(5) 0 051(7) 0.31(2) 0.12(2) 005(l) 0 IO(Z) 0.017(7) 0.15(Z) 0 047(8) 0.13(l) 0.024(4) 0.16(2) 0.025(6) 0.11(Z)

0.75(4) 20.3(3) 109(l) 23.7(2) 242(2) 0.43(l) 3.7(2) 4.64(4) ll.5(3) 5.7(3) 0.476(7)

1.38(5) 21.7(4) 84.0(8) 27.6(3) 206(2) 9.2(l) 3.3(2) 4.82(7) 35.9(6) 3.4(6) 0 548(6) 46.9(8) 1.76(3) 2.05(4) lO3(3) I 28(6) II(l) 449(11) 17(Z) 58(l) 1.71(8) 5.3(3) 0.51(4) 1.21(l) 6.3(3) 0.40(4) 1.13(8) O.l2(2) 0.62(4) O.lP(3) 0.24(2) 0.28(3) 0.047(5) 0.25(2) 0.072(7) 0.21(2) 0 024(5) 0.20(2) 0 032(5) 0.21(2)

Chondmle

Li @pm)

B-M6

n.a. 15.4(2) 93.7(8) 20.5(l) 202(l) 0.84(Z) 4.5(2) 5.4(l) 8.2(6) 5.5(7) 0.428(4) 58.1(7) 2.82(6) 3.42(P) 184(3) 0.41(l) 5.7(2) 605(ll)

B-R23

B-RI4

1.54(8) 17.6(5) ll3(2) 23.7(5) 234(4) 0.74(6) 2.9(2) 3.9(l) 14.2(6) 6.3(7) 0.55(l) 66(l) 2.80(6) 3.32(7) 121(3) 0.26(2) 4.1(2) 414(13) 14w 18.2(6) 2.0(l) 6.3(4) ".a 0.73(8) 7.8(5) 0.300) 1.03(8) 0.11(l) 0.59(4) 0.20(3) 0.14(2) 0.26(4) 0.05(l) 0.33(Z) 0.080(P) 0.24(2) 0.035(P) 0.22(3) 0.030(P) 0.29(4)

C-M6

3.71(P) 3.31(8) 123(l) 13.3(l) 156(l) 0.51(3) 12.3(P) O.PP(2) 8.4(2) 5.7(3) 0.387(5) 55.1(P) 3.2(l) 4.93(4) 359(8) O.Ill(2) 7.6(3) 510(7)
C-M7

0.77(5) 3.65(4) 60.0(6) 4.43(6) 154(l) 0.43(l) 4.8(2) 0.93(2) 14.1(2) 5.6(3) 0.228(3) 25.9(5) 1.41(3) 3.72(5) 445(5) 0.51(2) 5.7(2) 365(7) 1.60(7) 8.3(3) 0.42(4) 2.0(2) 0.16(2) 0.17(3) 4.4(3) O.ll(2) 0.23(3) 0.030(4) 0.18(Z) 0.04(l) 0.021(7) 0.06(2) 0.007(3) 0.08(l) 0.023(4) 0.060(P) 0.014(4) 0.08(l) 0.003(3) 0.07(2)

C-RI

13(l) 3.60(6) 41.4(5) l.26(2) 1.45(2) 57.4(7) 0.022(i) 0.235(P) 94(3) 6(l) 38.5(7) 7.6(2) 33(l) 1.22(P) 0.45(5) 12.4(4) 2.48(l) 5.9(Z) 0.79(5) 3.9(l) l.l5(9) 0.24(2) 1.2(l) 0.22(2) 1.80(8) 0.37(3) 1.24(6) 0.1X2) 0.88(7) 0.14(Z) l.l6(8)

4.1(l) 22.1(5) 127(9) 28(2) 220(l) l.ll(5) 0.77(7) 3.15(4) 20.0(7) II(l) 0.88(l) lO5(4) 3.68(l) 5.56(4) 200(4) 0.06X4) 1.7(l) 206(5) lO.l(6) 8.1(3) 2.04(P) 6.3(3) 0.54(4) 1.3(l) 10.1(4) 0.42(3) 1.32(9) O.lP(2) 0.83(4) 0.26(3) 0.11(l) 0.25(4) 0.066(7) 0.42(3) 0.097(S) 0.33(2) 0.038(4) 0.28(2) 0.033(6) 0.28(2)

1.24(4) 24.6(6) 91.5(7) 31.1(7) 182(l) 0.68(8) 7.0(3) 4.57(6) 27.9(8) 5.5Cl.l) 0.56(2) 97(3) 4.3(3) 4.97(4) 270(3) 0.15(l) 2.4(l) 29X8) 8.5(P) ll.9(3) l.4(2) 3.3(3) 0.61(5) 1.4(l) 5.9(2) 0.32(4) 0.84(9) O.ll(2) 0.57(5) 0.22(3) 0.15(i) 0.24(4) 0.045(8) 0.38(3) 0.07(l) 0.23(2) 0.030(6) 0.17(2) 0.036(P) 0.19(3)

1.69(6) 30.7(6) 130(l) 35.1(6) 198(Z) 0.79(6) 0.42(8) 4.17(6) 12.5(6) 7.8(7) 0.83(l) 97(2) 3.89(6) 5.50(7) 239(3) 0.40(l) 5.5(3) 208(5) 13.6(P) 5.0(3) 1.45(7) 4.4(3) 0.83(8) 1.3(l) 8.4(3) 0.36(3) 0.98(8) 0.135(9) 0.61(3) 0.16(2) 0.11(l) O.lP(4) 0.050(7) 0.33(2) 0.071(P) 0.22(2) 0.033(4) 0.23(2) 0.029(8) 0.23(2)

2.57(7) 16.8(2) 101(l) 22(2) 157(l) 0.43(3) 3.6(2) 3.25(4) 9.4(3) 4.9(4) 0.46(l) 68(2) 2.65(7) 3.13(5) 175(2) 0.37(2) 3.0(l) lO2(4) 6.2(8) 7.6(5) 0.79(4) 2.9(2) 0.67(5) l.l(l) 7.7(6) 0.20(2) 0.57(3) 0.066(8) 0.35(2) 0.10(Z) 0.07(l) 0.13(2) 0.031(4) 0.19(l) 0.043(5) 0.124(P) 0.016(3) 0.12(l) 0.014(4) 0.14(2)

0.71(6) 3.8(l) 43(l) 3.2(4) 165(l) 0.155(5) 0.82(7) 1.53(2) 27.0(3) 2.6(5) 0.140(3) 18(l) 0.96(2) 3.81(6) 464(4) 0.219(5) 4.08(P) 337(6) 3.3(7) 12.9(3) 0.48(3) 1.61(8) 0.15(l) 0.41(6) 4.9(2) 0.10(l) 0.23(2) 0.031(5) 0.16(l) 0.044(P) 0.027(7) 0.05(l) 0.014(3) 0.079(7) 0.018(2) 0.057(5) O.OlO(2) 0.066(7) 0.013(3) 0.08(l)

C-R4

B-R17

B-RI8

I/POP 1.0-0.99

UFO 0.98

0.86(5) 10.7(Z) 59.3(5) 10.6(l) 194(l) 0.40(3) l.l5(7) 4.79(4) 12.8(2) 5.0(3) 0.214(6) 28(l) I 86(P) 4.33(4) 389(2) 0.55(2) 7.5(4) 664(10) l1.2(8) 14.9(4) 0 82(5) 2.7(2) O.lP(2) 1.0(l) 9.8(4) 0.13(l) 0.38(4) 0.047(5) 0.22(Z) 0.063(8) 0.054(P) 0.11(2) 0.022(4) 0.12(l) 0.026(3) 0.094(7) 0.013(2) O.l(l) 0.021(3) 0 10(l)

1.32(7) 12.5(P) 87.1(P) 14.4(2) 203(2) 0.52(i) 4.4(2) 4.43(6) 14.0(4) 7.2(6) 0.358(9) 44(l) 2.84(5) 4.96(P) 309(4) 0 58(3) 7.1(3) 419(10) P(l) 9.3(3) 0 88(6) 2 7~21 0.30(3) 0.46(6) 6.2(3) O.lP(3) 0.43(4) 0.063(7) 0.29(Z) 0.07(l) 0.07(l) <0.21 0.024(5) 0.14(l) 0.028(4) 0.12(l) 0.016(5) 0.14(l) 0.008(5) 0.13(2)

C-RS(i)

0.993

WPOP 0.87-0.80

I/PO 0.992

0.52(3) 3.6(l) 86.4(6) 7.11(8) 149(l) 0.37(3) 2.3(2) 0.297(6) 2.3(l) 3.5(2) 0.213(3) 26.8(5) l.86(8) 9.7(l) 460(5) 2.25(5) 14.5(2) 345(5) 2.6(5) 4.9(3) 0.55(5) 1.70(P) O.lP(2) 0.8(l) 2.9(2) 0.085(P) 0.22(2) 0.039(4) 0.19(l) 0.029(7) 0.071(7) 0.06(l) 0.013(3) 0.107(9) 0.028(4) 0.057(6) 0.011(3) 0.055(P) 0.009(3) 0.05(l)

1.24(8) 11.8(2) 78.9(6) 14.0(Z) 163(l) 0.385(S) 0.22(4) l.43(2) 9.2(5) 5.5(7) 0.417(9) 42(i) 2.17(6) 7.93(4) 318(3) 0.107(5) 3.2(l) 131(4) 4.8(5) 2.9(2) 0.85(4) 3.2(2) 0.48(4) 0.45(5) 3.3(3) 0.22(2) 0.61(6) 0.087(8) 0.43(3) 0.11(l) 0.12(l) 0.14(Z) 0.026(5) 0.22(l) 0.049(5) 0.16(l) 0.018(3) 0.12(l) 0.019(4) 0.15(2)

1.71(8) 8.9(3) 78.9(8) 11.8(6) 156(l) 0.84(2) 3.4(3) 1.02(2) 15.5(4) 4 04(5) 0.477(6) 41(l) 3.25(P) 9.3(l) 331(2) 1.39(5) 4.8(l) 271(5) 7.9(9) lO.6(3) 1.67(7) 4.5(3) 0.34(4) 0.57(7) 5.5(4) 0.30(2) 0.85(6) 0.12(l) 0.54(3) 0.18(2) 0 17(l) 0.18(3) 0.036(4) 0.27(2) 0.060(6) 0.18(2) 0.020(5) 0.20(2) 0.034(6) 0.17(2)

UFIO 1.31(6) 32.9(3) 48.2(5) 37.6(6) 180(6) 1.95(7) 0.21(5) 1.47(6) 41.3(8)

B-RI5

IlpP uGo UP0 0.97-0.90 0.98-0.97 0.98-0.94

Lump

Type/Text Mg nos. Li (ppm) Na(mg/g) Mg (mp/gI Al (mg/g) Si (mg/g) P(mg/g) S(mg/g) KOWg) Ca(mp/g) Sc(ppm) Ti (mg/g) V(PP"x Cr (mg/g) M"(mg/gJ Fe (mpig) Co (mg/g) Ni (mp/g) Cu (PPN Rb (PPW Sr (PPN Y (Ppm) Zr (Ppm) Nb (ppm) cs (PPW Ba (Pem) b (Ppm) Ce CpPm) pr @pm) Nd @pm) Sm (PP~) Eu CpPm) Gd @pm) Tb (ppm) DY (PP~) Ho (ppm) Er (ppm) Tm (Ppm) Yb @pm) Lu (ppm) Hf (PPW

B-MS

C-RS(ii) UP0 0.992 0.47(3) 24.0(6) 75.0(7) 33(l) 190(l) 0.300(6) 0 71(7) 5.1(l) ll.5((7) 3.8(7) 0.283(3) 41.7(7) 1.79(6) 6.42(8) 273(3) 0.129(4) 3.5(l) 180(4) 19.5(l) 3.63(4) 0.77(4) 2.28(l) 0.33(4) 1.4(2) 3.9(4) 0.17(2) 0.38(3) 0.048(4) 0.28(2) O.lO(2) 0.29(2) O.lO(2) 0.017(4) 0.14(l) 0.033(4) 0.11(l) 0.019(5) 0.09(i) 0.023(5) 0.09(2)

B-R22

B-R2b

B-R3b(l)

l/PO II/FOP IlpoP 0.98-0.97 0.87-0.86 0.98-0.97 15.5(5) 4.5(3) 148(2) 10.0(2) 152(2) 1.00(6) 12.0(3) 1.23(6) lO.2(3) 9.4(4) 0.61(l) 140(14) 7.7(7) 3.31(5) 231(4) 0.054(5) 1.9(l) 277(8)
C-RIO

18(l) 3(i) 0.36(l) 61(2) 2.11(8) 1.16(3) 88(5) 0.33(3) 4.3(3) 476(18) 21(Z) 42(l) 0 97(6) 2.9(2) ".a. 1.2(2) 10.1(4) 0.22(3) 0.46(4) 0.059(9) 0.35(3) O.lO(2) 0.75(5) O.l3(2) 0.023(6) 0.17(l) 0.046(7) 0.13(l) 0.022(8) 0.15(21 0.021(6) 0 II(Z)

1.3(2) 7.3(2) 75.3(8) 6.81(8) 192(l) 0.42(3) 0.5(l) 1.21(l) 12.1(6) 4(l) 0.240(7) 31.3(8) 166(3) 5 l(2) 523(16) 084(Z) lO.6(3) 543(20) 4(I) 10.915) 0 9(l) 2.9(4) ".a 0.17(6) 4.1(4) 0.11(2) 0 33(7) 0 05(2) 0.29(4) 0.05(3) O.l8(3) a041 0.02(l) O.lO(3) 0.032(P) 0.09(2) <0.008 0.09(3) 001(l) 0.16(5)

C-R35

C-R3e

O.PP(6) 32(l) 80(l) 48.6(7) 223(2) 0.39(2) 2.2(Z) 5.5(Z)

I/PO 0.96

Met/Sulf.

0.95(4) 9.5(2) 99.2(9) 13.8(6) 151(l) 0.42(l) 0.33(6) 0.82(2) 6.9(2) 5.2(3) 0.60(5) 46.2(9) 2.05(4) 8.0(l) 349(4) 0.077(6) 4.0(4) 182(6) 3.4(6) 3.7(2) 0.79(4) 2.3(l) 0.22(3) 1.4(2) 4.7(3) 0.31(4) 0.76(8) 0.11(l) 0.56(3) 0.12(l) 0 060(P) 0 l9(3) 0.026(4) O.l9(2) 0.042(6) 0.12(l) 0.028(4) 0.13(l) 0.019(4) 0.13(2)

0.37(3) 3.8(2) 69(l) 8.2(5) 142(l) 0.289(5) 8 P(6) 0.18(l) 2 2(l) 4.5(3) 0.270(6) 47W 3 7~2) 12.0(2) 597(14) 0.57(6) 4.5(3) 881(22) <8 10.4(4) 0.77(5) 2.6(l) 0.30(3) 1 l(2) 3.3(Z) 0.14(2) 0.23(2) 0.036(5) 0.21(2) 0.06(l) 0 063(6) 006(l) 0 019(3) 0.12(l) 0.023(5) 0.108(9) 0.018(3) 0.11(l) 0.012(3) 008(2)

0.40(2) 3.54(8) 80.6(8) 7.9(2) 168(l) 0.4011) 11.6(6) 0.143(2) 1X8(3) 4.9(5) 0.308(5) 40(l) 2.78(6) 11.7(2) 616(8) 0.037(2) l.OO(8) 364(6) I,',:,, O.PP(5) 3.1(2) 0.25(2) 1.4(l) 2.4(2) 0.14(l) 0.37(3) 0.042(5) 0 26(2) 0 07(2) 0.057(h) ".a n ii ".B. ".a. n a n a n a. ".a. ".a.

3266

C. M. O’D Alexander

Appendix A continued. Chondlule

S-M4

S-M8

S-Ml3

S-Ml4

S-MIS

S-Ml6

S-Ml7c

S-MIS

K-M2

K-M3

K-h47

K-MS

Type/Text. Mg nos. 6.4(l) 20.7(5)

6.8(l) 21.1(5)

99(l) 16.4(Z) 178(l) 1.04(3) 7.35(Z) 4.97(6) 3.72(5) 5.66(6) 0.482(5) 70.8(g) 3.15(5) 2.89(3) 210(l) 0.94(l) 11.8(l) 609(10)

89(l) 15.9(4) 176(2) 0.87(4) 6.7(3) 10.2(2) 6.2(7) 5.5(7) 0.430(6)

2.7(l) 5.5(l) 3.2(l) 25.0(3) 21.8(5) 31(l) 82.8(S) 83(2) 87(l) 21.1(3) 9.4(l) 26.4(S) 182(l) 183(4) 165(l) 0.63(l) 0.97(4) 0.836(S) 3.2(2) 6.4(3) 4.6(3) 5.23(4) 7.1(2) 4.27(4) 4.3(3) 4.2(3) 6.4(S) 5.8(4) 5.0(9) 5.6(4) 0.389(3) 0.390(5) 0.40( I 54.3(5) 69(l) 70(3) 60(l) 3.09(9) 2.56(4) 2.57(S) 2.58(3) 2.60(5) 2.21(2) 3.08(S) 2.37(4) 206(4) 227(3) 198(5) 214(l) 0.91(2) 0.85(l) 0.73(2) 0.855(S) 12.3(4) 10.4(l) 11.2(3) 10.2(l) 747(13) 289(6) 696(10) Cu (ppm) 428(11) Rb @pm) 27(2) 32(l) 34(3) 17(l) 25(2) 42.3(7) 37.3(7) 21.3(3) 16.4(3) Sr (ppm) 24(i) 1.53(7) 1.31(6) 1.22(9) 1.17(6) 1.22(6) Y (PPW 5.2(3) 4.1(2) 3.6(l) 3.9(3) 4.21(2) Zr (ppm) n.a. “.a. “.a. n.a. “.a. Nb (ppm) 2.5(4)# 2.7(3)* 2.8(3)’ 3.4(4)X 8.9(7)# Cs (ppm) 13.8(4) 10.2(4) 5.5(Z) 7.4(4) 6.9(2) Ba (PPN 0.31(3) 0.21(2) 0.3’5(3) 0.33(3) La (ppm) 1.0(2) :::;I:; 1.4(2) 0.60(7) 0.87(g) Ce (ppm) 0. O(2) 0.070(S) 0.11(l) O.l3(2) 0.09(l) Pr (ppm) 0.43(3) 0.43(3) 0.48(3) Nd (ppm) 0.12(2) ::::I:; 0.12(2) 0.12(2) Sm (PPW 0.05(l) 0.04(l) 0.099(9) 0.05 l(8) 0.033(j) Eu (ppm) 0.17(3) 0.15(3) 0.12(3) 0.13(2) 0.09(2) Gd (ppm) 0.034(6) 0.020(6) 0.030(7) 0.026(5) 0.020(4) n (PPW 0.19(2) 0.19(2) 0.20(2) 0.20(l) 0.18(2) DY (ppm) 0.055(6) 0.047(9) 0.049(S) 0.041(5) 0.047(4) Ho (ppm) 0.15(l) 0.15(l) 0.14(2) 0.16(2) 0.13(l) Er (ppm) 0.023(3) 0.014(4) 0.023(3) 0.019(4) 0.020(3) Tm (ppm) 0. S(2) 0.14(l) 0.14(l) 0.15(l) 0.15(l) Yb (PPW 0.021(7) 0.029(5) 0.021(4) 0.021(5) 0.024(4) LU (PPW 0.20(2) 0.15(2) 0.15(2) 0.16(2) 0.14(2) Hf @pm) 8 Some or all areas of Semarkona matrix may have been contaminated Li (PP~) Na (Wg) MS (w@ Al (w/g) Si (mp/p) p @g/g) S (w/g) K (Wg) Ca (m&) SC (PPW Ti (m&z) V(ppm) Cr (Wg) Mn (Wg) Fe (w&I co (w/g) Nt (WS)

4.4(l) 27.5(5) 82(l) 23.0(3) lSl(2) 0.86(2) 5.0(2) 6.32(7) 5.4(4) 5.5(5) 0.46(l) 60(Z) 2.77(9) 2.74(4) 218(3) 0.81(l) 10.212) 407(9) 30(2) 27.6(6) 1.28(6) 5.0(Z) n.a. 17(1)X 6.9(3) 0.38(6) 0.9(l) 0.08(l) 0.39(3) 0.09(l) 0.04( 1) O.l3(3) 0.027(6) 0.20( I) 0.054(4) 0.16(l) 0.019(4) 0.15(2) 0.024(4) 0.12(2)

)

I

I

34(2) 3.8(9) 274(3) 0.68(l) 12.5(5) 255(16) <0.9 21.3(6) 3.8(2) 10.7(3) I .04(S) 0.13(2) 4.8(3) 0.52(6) 1.6(l) 0.20(2) 0.98(6) 0.34(6) 0.07(l) 0.55(S) 0.0613) 0.65(5) 0.12(2) 0.39(3) 0.050(g) 0.38(4) 0.07(l) 0.17(11)

10.8(3) 3.9(2) 159(2) 8.0(2) 146(2) 1.10(4) 1.0(l) 0.176(7) 8.4(6) 9.8(7) 0.51(l)

2.40(9) 3.58(7) 150(3) 13.6(3) 154(l) 0.74(3) 0.09(5) 1.35(4) 6.6(4) 5.8(5) 0.38(l)

74(4) 3.3(3) 2.8(4) 25 l(3) 0.57(l) 10.1(4) 106(4) <0.78 15.5(5) 1.8(l) 6.4(2) 0.54(5) O.ll(2) 5.3(2) 0.20(2) 0.55(4) 0.084(7) 0.44(3) 0.12(2) 0.041(6) 0.12(3) 0.024(5) 0.29(2) 0.066(6) 0.19(2) 0.031(5) 0.22(2) 0.039(6) ~0.06

40(2) 1.22(2) 3.1 l(6) 249(5) 1.30(3) 12.8(9) 182(13) 5.2(9) 10.8(3) 0.97(7) 3.1(l) 0.57(6) 0.21(5) 7.6(3) 0.17(3) 0.47(5) 0.056(7) 0.33(3) 0.09(2) 0.05(l) O.lO(3) 0.021(5) 0.18(2) 0.042(6) 0.1 l(l) 0.022(3) 0.16(2) 0.019(5)

K-RI3

K-Rl4(i)

K-Rl4(ii)

K-RI7

WOP 0.96-0.93

l/POP 0.96-0.93

VP0 0.95-0.94

1.20(7) 14.8(2) 83.6(7) 31.3(2) 175(l) 0.7(3) 0.45(S) 5.05(S) 17.2(2) 6.7(3) 0.303(3) 41.8(4) 2.3(2) 4.07(6) 326(4) 0.79(2) 15.8(4) 388(15) 12.6(S) 11.4(2) 0.97(5) 3.2(l) 0.62(5) 0.50(4) 6.6(3) 0.14(l) 0.33(3) 0.053(5) 0.27(2) 0.08(l) 0.18(2) 0.1 l(2) 0;:;;;;;

1.37(7) 4.55(9) 78.7(5) 21.8(l) 153(l) l.l(2) 0.39(7) I .78(4) 8.3(5) 6.9(7) 0.41(l)

I/F-P 0.99-0.98

IUBOP 0.77-0.61

II/PO 0.86

Li (ppm) Na (mg/g)

1.52(4) 0.723(l) 81.3(l) 10.4(l) 149(l) 0.459(3) 0.15(4) 0.416(l) 4.76(l) 5.0(2) 0.275(Z) 29.2(3) 1.87(l) 22.5(l) 407(l) 1.07(l) 15.6(l) 223(4) 1.9(6) 8.8(2) 0.93(5) 3.3(l) 0.49(4) 0.14(2) 2.5(l) 0.13(l) 0 33(3) 0.036(4) 0.22(Z) 0.08(l) 0.057(6) 0.07( 0.018(4) 0. I16(9) 0.023(4) 0.090(S) 0.015(3) 0.11(l) 0.019(4) 0.12(2)

0.92(4) 9.3(3) 99(l) 27.6(3) 173(2) 0.34(l) 0.18(5) 3.38(S) 15.8(3) 6.1(3) 0.451(5) 46.7(S) l.97(6)

1.51(5) 4.2(2) 123(l) 22.1(l) 159(l) 0.6(3) 0.79(S) 1.76(6) 4.8(l) 7.9(2) 0.461(5) 56.9(9) 2.58(5) 3.76(3) 317(2) 0.572(6) 11.5(Z) 409(10) 2.4(7) 18.4(3) 1.46(6) 4.9(l) 0.60(7) 0.25(3) 27.1(6) 0.20(2) 0.56(5) 0.07(I) 0.44(Z) 0.11(l) 0.06(Z) O.ll(3) 0.023(7) 0.22(2) 0.042(5) 0.17(l) 0.020(4) 0.18(2) 0.024(5) 0.1 l(4)

~

32(21 25.5(7) 1.33(6) 4.2(2) “.a. 5.1(5)” 6.1(3) 0.28(4) 0.9(l) 0.09(2) 0.40(3) 0.13(2) 0.042(S) O.lO(3) 0.019(S) 0.19(l) 0.050(6) 0.15(l) 0.024(3) 0.14(l) 0.030(4) 0.16(2)

10.9(2) 10.2(2) 146(l) 20.4(3) 159(l) 1.33(2) 1.7(2) 0.174(6) 11.9(4) 14.0(6) 2.04(7) 590(40)

K-RI2

Type/Text Mg nos.

F?($$) Si (mug) p (mg/g) S (m&s) K @p/g) Ca (we SC (ppm) Ti (m&I V(PPN Cr (m%g) Mn (m&Y Fe (m&z) Co (m&z) Ni (m%g) Cu (PPW Rb (ppm) Sr (PPW y (PPN Zr (PPN Nb (ppm) Cs (PPW Ba (ppm) La (ppm) Ce (ppm) Pr (ppm) Nd (PPW Sm (ppm) Eu (ppm) Gd (PPW Tb (PPW DY (PPW Ho (PP”) Er (ppm) Tm (PPW Yb ., (ppm) u tPPmJ Hf @pm)

2.71(g) 24.6(6) 94.6(7) 21.7(3) 205(l) 0.82(3) 5.09(2) 5.60(5) 5.6(5) 5.3(6) 0.438(5) 63.5(9) 3.40(5) 2.73(2) 183(2) 0.99(2) 13.5(2) 435(5)

with Cs.

K-R9

Chondrule

4.36(7) 22.5(Z) 80.7(7) 21.6(l) 184(l) l.lO(2) 6.0(2) 5.00(3) 4.8(3) 4.8(4) 0.354(4) 48.2(5) 4.5(l) 3.54(3) 227(2) 1.05(3) 16.2(5) 690(37) 24(i) 23.6(4) l.l6(4) 3.0(2) n.a. 3.6(4)” 4.7(2) 0.28(3) 0.80(6) 0.07(l) 0.35(3) 0.12(l) 0.08(l) 0.17(2) 0.027(5) 0.17(l) 0.034(4) 0.12(l) 0.019(3) 0.13(l) 0.023(4) 0.12(2)

I)

‘2;‘59:4’ 0.60(l) 10.7(2) 177(4) 8.1(7) 17.4(4) 0.99(5) 3.3(l) 0.77(6) 0.49(6) 15.1(6) 0.14(l) 0.31(4) 0.059(7) 0.27(2) 0.09( I) O.ll(2) 0.12(2) 0.021(4) 0.15(l) 0.032(5) 0.106(S) 0.022(4) 0.12(l) 0.028(7) 0.043(38)

l.l9(5) 2.87(9) 81.5(5) 11.5(l) 133(l) 0.43(23) 0.7(l) 0.427(7) 2.78(3) 4.7(l) 0.260(6) 34(l) 1.65(7) 4.90(2) 387(2) 0.71(l) 7.6(2) 186(3) 1.5(J) 6.5(2) 0.84(5) 3.3(l) 0.37(3) 0.14(2) 2.9(Z) 0.10(l) 0.26(2) 0.043(6) 0.17(2) 0.09(I) 0.066(7) 0.08(2) 0.017(4) 0.12(l) 0.034(7) O.lOO(9) 0.016(2) 0.10(l) 0.021(3) 0.07(3)

0.033(4) 0.10(l) 0.012(3) O.ll(2) U.U16(4) CO.031

46(2) 1.96(3) 4.84(4) 396(5) 1.05(2) 25.7(3) 254(6) 3.3(S) 13.1(3) 1.53(7) 4.5(l) 0.72(4) 0.26(3) 10.4(3) 0.18(2) 0.48(3) 0.077(7) 0.37(2) 0.11(l) 0.066(9) 0.15(2) 0.pZ17$) 0.058(6) 0.14(l) 0.020(4) 0.15(l) U.fJW4J

0.1 l(3)

“.“Y(4)

8.8(5) 1.84(4) 159(l) 9.8(S) 152(l) 1.89(6) 0.6(2) 0.29(2) 26.0(5) 14.3(7) 0.98(2) 124(6) r 44-1 3.4(3,

2.74(3) 239(5) 1.5(l) 12.1(9) 137(S) 2.7(1.1) 14.1(7) 4.1(2) 10.8(4) 0.8(l) 0.22(4) 4.9(6) 0.63(6) 1.6(l) 0.24(2) 1.12(7) 0.43(5) 0.05(l) 0.66(E) O.llf11 “.

I”,cl,

0.16(2) _ ._,.I

“.4,(4,

0.072(S) 0.42(5) 0.06(l) ^ ^... V.,(1)

K-R4

K-R5

IUFO 0.74-0.72

Crypt.

0.85(4) 1.9(2) 108(l) 13.9(l) 151(l) 0.48(26) 0.75(7)

1.24(7) 1.7(l) 201(4) 13.7(5) 164(3) 0.26(2) 1.3(2) 0.78(4) 23(l) 12(l) 0.77(2) 105(3) 4.6(l) 3.00(5) 274(5) 4.6(2) 16.5(7) 132(11) 4(11 12.0(5) 3.1(Z) 8.X4) 0.76(9) 0.77(5) 9.4(7) 0.40(9) 1.2(2) 0.1 S(3) 0.90(S) 0.23(5) 0. O(2) 0.43(S) 0.07(2) 0.48(6) 0. I O(2) 0.27(4) 0.06(2) 0.30(6) 0.05(2)
“; :gg) 6.4(2) 0.289(3) 36.1(6) 1.57(3) 4.50(2) 408(2) 0.717(5) 30.0(2) 329(6) 3.5(6) 7.1(2) 1.25(6) 3.4(l) 0.52(4) O.l9(3) 3.8(2) 0.13(l) 0.34(3) 0.066(5) 0.26(l) 0.072(9) 0.087(g) O.lO(2) 0.020(4) 0.16(l) 0.032(3) 0.108(8) 0.01 S(4) 0.11(l) 0.012(4) 0.05(3)

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