Igneous rims on low-FeO and high-FeO chondrules in ordinary chondrites

Geochimica et Cosmochimica Acta, Vol. 59, No. 23. pp. 495 I-4966, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in the USA. All rights reserved 00 I6-7037M $9.50 + .OO

Pergamon

00167037(95)00337-l

Igneous rims on low-Fe0 and high-Fe0

chondrules in ordinary chondrites

ALEXANDER N. KROT* and JOHN T. WASSON? Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024, USA

(Received

January 25,

1995; accepted in revisedform

August 22, 1995)

Abstract-Many ordinary chondrite (OC) chondrules are surrounded by rims that show evidence of appreciable melting and can be called igneous; these rims formed after the melting and solidification of the host chondrule. We studied thirteen igneous rims on low-Fe0 chondrules (Fa or Fs < 10 mol%) and nine rims on high-Fe0 chondrules (Fa or Fs >lO mol%) in ordinary chondrites of petrographic type. 53.5 by electron-microprobe analysis and scanning-electron microscopy. In both sets, olivine and pyroxene compositions of the rims are similar to or moderately higher in Fe0 than those in the host. Igneous rims around low-Fe0 chondrules show evidence of large degree (>80%) melting including: (1) resorbed chondrule surfaces, (2) rounded metalltroilite nodules and feldspathic mesostasis, (3) euhedral and subhedral morphology of olivine and pyroxene and limited ranges in grain sizes, and (4) a rarity of relict grains. Most low-Fe0 chondrules have olivine as their major phase, but pyroxene is commonly the predominant phase in their rims. Rims on high-Fe0 chondrules generally have smaller grain sizes and exhibit lower degrees of melting than rims around low-Fe0 chondrules; Mg-rich relict grains are observed in highFe0 rims. Most low-Fe0 chondrules have matrix-like rims around the igneous rims, but matrix-like rims are rare around rims on high-Fe0 chondrules. There is a continuum between igneous rims and enveloping secondaries of compound chondrules; the existence of both sets of objects supports the view that, in the OC region, most mm-size grain assemblages experienced more than one melting event. The precursors melted to produce the igneous rims on low-Fe0 chondrules differ from those that comprise the rims on high-Fe0 chondrules. A sizeable fraction of the precursors of low-Fe0 rims are from the host chondrule. Most high-Fe0 rims appear to consist largely of melted matrix-like materials. If the rim precursors are mainly independent, the similar Fa and Fs contents in mafic minerals of rims and host chondrules indicate that ( 1) OC nebular subregions (bounded in space and/or in time) were dominated by well-mixed solids, (2) many chondrules experienced multiple heating events during a short period, and were then withdrawn from the chondrule-forming region, and (3) chondrules formed at different times were mixed before agglomeration to form chondrites. If the host chondrules were the main source of the low-Fe0 rim materials, the fact that pyroxene is generally more abundant than in the host suggests that SiO*-normative matrix was also an important ingredient and/or that kinetics favored pyroxene crystallization in the rim. Rims around high-Fe0 chondrules were heated to lower temperatures than rims around low-Fe0 chondrules. The rarity of matrix-like rims around high-Fe0 igneous rims may indicate that the dust-chondrule ratio was lower where they formed. 1. INTRODUCTION

Rims around chondrules can be divided into two major categories: fine-grained or matrix-like rims (MR), typically opaque and FeO-rich (e.g., Ashworth, 1977; Allen et al., 1980; Huss et al., 1981; Fujimaki et al., 1981; Taylor et al., 1984; Alexander et al., 1989; Nagahara, 1984, Matsunami et al., 1984, 1990; Scott et al., 1984, 1988; Metzleret al., 1992), and coarse-grained (Rubin, 1984; Rubin and Wasson, 1987) or igneous rims (IR) that range from low- to high-Fe0 contents and from transparent to opaque. Matrix-like rims are similar in phase abundances and chemistry and in grain size (generally < 1 pm in LL3.0 Semarkona) to interchondrule matrix material and consist of micrometerto-submicrometer-sized grains of malic silicates, opaque min-

In our study of compound chondrules (Wasson et al., 1995 ) we found evidence that most chondrules have, on average, experienced about two strong heating events, and that independent materials added during the second event tended to be similar in degree of oxidation to the primary chondrule. This unexpected relationship led Wasson et al. ( 1995 ) to conclude that chondrules tended to remain in a chondrule-forming region long enough to experience about two melting events and were subsequently mixed with chondrules having different degrees of oxidation that formed at a different time or place. Because enveloping compound chondrules are closely related to the coarse-grained rims first described by Rubin ( 1984), we studied a set of the latter objects with the goal of learning additional details about nebular conditions and processes during the chondrule-forming epoch.

erals, and amorphous material. Although Alexander et al. ( 1985) suggested that opaque MR derived from the chondrules during regolith formation, the most widely accepted model is that they accreted around chondrules in the solar nebula and were not subsequently heated before aggregation

* Present address: Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA. +Also Department of Earth and Space Sciences and Depprtment of Chemistry and Biochemistry.

into a parent body (e.g., Allen et al., 1980; Taylor et al., 1984; Nagahara, 1984; Scott et al., 1988; Metzler et al., 1992).

Rubin (1984) estimated that coarse-grained rims surround -5O%, -lo%, and < 1% of the chondrules in CV3, H-L-LL3, 495 1

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A. N. Krot and J. T. Wasson

and CO3 chondrites, respectively; they also surround some chondrule fragments and isolated mineral grains. In OC, Rubin ( 1984) observed that such rims consist mainly of olivine and suggested that these rims formed by sintering of matrix material. Rubin and Wasson (1987) studied thirteen IR in CV3 Allende by instrumental neutron activation analysis and inferred that the precursor components were the same as those of chondrules. Prinz et al. (1986) described eleven layered chondrules (10 low-FeO, 1 high-FeO) from carbonaceous chondrites and emphasized their significance as objects that reflected a sequence of conditions during chondrule formation. However, their definition of layered chondrules as chondrules with any number of units, of any grain size or sequence, eliminates the distinction between coarse-grained lR and fine-grained MR. A rare type of rim surrounds about 5% of chondrules in CO3 Kainsaz (Kring, 1991) . These monosilicate or multisilicate rims have clear igneous textures; borders between the chondrule cores and rims are sharp. Based on chemistry and mineralogy, Kring ( 1991) divided them into Fe, Ca-rich, and Mg-rich; he concluded that these rims were added during repeated high temperature processing of the chondrules. In a study of OC compound chondrules, Wasson et al. (1995) divided the secondary (later-molten) members of compound chondrules into two categories: siblings of the primary and independent. The primaries and secondaries of sibling compound chondrules have similar textures and similar compositions of the mafic minerals consistent with these chondrules having formed in the same flash-heating event. Primaries and secondaries of independent compound chondrules typically have different textures and mineral chemistry: generally the ma& minerals in secondaries have somewhat higher Fa or Fs than those in primaries. Independent secondary chondrules appear to have formed by melting an assemblage of grains that included the primary chondrule (Wasson et al., 1995). ln this paper we describe igneous rims around low-Fe0 and high-Fe0 chondrules in ordinary chondrites of type 5 3.5. We classify as igneous those rims which surround the chondrule and show evidence of appreciable ( > 20% ) melting. The properties of these rims record conditions during OC chondrule formation including : ( 1) the local density of solid matter at the moment of chondrule formation, (2) the frequency of flash heating events in the solar nebula, (3) the physical and chemical state of solid matter when chondrule formation was occurring, and (4) temperature and oxidation state of the solar nebula at the time of chondrule formation. Although the rims described by Kring ( 1991) also satisfy our definition of IR, we have not observed such rims in OC and we do not discuss them in this paper. 2. SAMPLESAND ANALYTICAL PROCEDURES The chondrules were divided into two compositional categories ignoring textural properties: FeO-poor (Fs or Fa < 10 mol%) and

FeO-rich (Fs or Fa > 10 mol%). Scott and Taylor ( 1983) divided porphyritic-oliviue (PO) and porphyritic-olivine-pyroxene (POP) OC chondrules into two textural categories: type-1chondrules having small (<30 pm), rounded olivine crystals, abundant metal, and clear glassy mesostases; type-II chondmles having large (>30 pm), faceted, divine crystals, much smaller contents of metal, and more turbid mesostases. In unequilibcated chondrites type-1 textures are gen-

erally associated with olivine having low (58 mol%) Fa and typeII textures with high (212 mol%) Fa. Some researchers have broadened the meaning of type-1 to include all low-Fe0 and type-11

to include ail high-Fe0 chondrules but we restrict our use of these terms to chondmle texmres. We searched for relatively transparent rims with well defined borders relative to the host chondrule. From this set of -50 IR we selected for detailed investigation the listed sets mainly on the basis of completeness of the rim. Because our selection criteria yield potential biases, we were not able to calculate the relative abundances of chondrules with IR or other statistical information such as mean rim thickness. Twenty-two chondrules (thirteen low-FeO, nine high-FeO) with IR were smdied petrographically in transmitted and reflected light. The meteorites, thin-section numbers, chondrule sizes, rim thicknesses, textures, and mineralogy are listed in Table 1. Backscattered electron images were made with the Cameca Camebax-microbeam electron microprobe at UCLA. Mineral compositions of olivine, pyroxene, and feldspathic mesostasis were determined with the UCLA probe using crystal spectrometers,counting times of 10 s for alkalis and 20 s for other elements, a ZAPcorrection procedure,and a beam currentof 15 nA at 15 kV. Standardsused were gross&r for Si, Ca, and Al, forsterite for Mg, magnetite for Fe, spessartine for Mn, sphene for Ti, chromite for Cr, albite for Na, and orthoclase for K. Typical detection limits for individual analyses of pyroxene, olivinc, and feldspathic mesostaais were 0.03 wt% for Na, 0.02 wt% for Mg, Al, and Si, and 0.014 wt% for the remaining elements. Several olivine and pyroxene grains in the host chondmie and IR were analyzed; the number of analyzed grains, average and range of oxide contents, and Fa, Fs, and Wu in divine and pyroxene of the host chondrules and IR studied are listed in Table 2. Compositions of relict grains in FeOrich chondrules and rims are listed separately and were not included in the average compositions. 3. PETROGRAPHIC OBSERVATIONS AND CHEMISTRY

3.1. Degree of Melting of ChondFule Rims We define igneous rims to be those in which the petrographic evidence indicates a degree of silicate melting >20%; the degree of silicate melting of a matrix-like rim appears to be ~10%. This leaves a gap of unclassifiable rims. CompliTable 1. Sizes (A, B and thickness in km) and textures of low-Fe0 and high Fe0 chondmles with igneous rims (IR) and one (Bishunpor 1) that seenw to be a matrix-like rim (MR). chondnte type

sEEe

seZknc”

tex_B

text%ckn.

low-Fe0 chondrul ALHA77034* L3.?MWG ALHA77034’ L3.5 MWG ALHA77034’ L3.5 MWG ALHA L3.2 MWG ALHA77260’ L3.5 MWG Bishunpur LL3.1 SI Bishunpur LL3.1 SI Bishunpur LL3.1 SI Chainpur ULL3. SI LL3.1 SI LL3.1 SI Zw%339 L3.4 MWG LEW86158 L3.2 MWG

8 8 8 15 16 2359-2 2359-2 2359-2 4020-2 2488-2 2488-2 8 6

1 3 5 2 2 2 15 16 3 2 3 3 1

PO PO BO PO PO POP POP PP-f POP PO PP POP BO

1250 620 450 900 800 1320 800 700 1170 2ooO 840 1500 800

950 PP 600 PP 450 PP 800 PP 640 PP 108OPOP 600 POP 50 FGF 1100 PP 1200 PP 600 PG 1500 PP 740 PO

100 30 30-50 1W 140-440 SO-200 O-140 u)-170 60-200 340 80 75-150 260-340

ALHAn L3.2 MWG 15 1 POP ALHA77260* L3.5 MWG 16 1 POP ALHA77260+ L3.5 MWG 16 8 PO ALH83010* L3.3 MWG 24 3 POP* Bishunpur LL3.1 SI 2359-2 1 POP Bishunpur LL3.1 SI 2359-2 4 PO Chainpur LnL3.4 SI 4020-2 5 PO Chainpur UL3.4 SI 4020-2 6 POP LL3.1 SI 2488-2 1 POP ~~~~~h~~~Noof*ti*o;cn=cbonmule”~~r;

1200 640 600 loo0 1100 450 630 1400 2000

1100 PO 100-200 600 PO 200 600 PO 110 700 PO 50-350 960 PO 30-300 350 PG 160-180 630 PO 100 970 POP 100-150 13OOPGF340-MO f=

sources: MWG = Meteorite Working Group, Houston; SI = Smithsonian Institution, Washington. *cho”drole 3 in AL&U010 is a compound chondmle (a BO rim and enveloping POP secondary). +pairedw1thALk47zl1.

Table 2. Average and range of oxide contents (wt.%) and Fa Fs and Wo (moi%) in the 01ivine and wmxene of chondrulesand their ianeous rims (Bishunw _ 1 is an MR). idsit Fakr.Fd) cku&ite nr~minS~~~Cr~Fd)~~csONkpFalsFdsmgWo cim&ite co anminS~~~Cr~~Fto~MgOCld)N~FalsFa/smgWo lc 9 d .38.7 4.03 0.17 17.6 0.78 41.1 0.20 eO.04 19.5 15.4-26.1 A~~A77034 Ic .._...,__.,._-“ 3 d 42.0......-..“ 0.10 ^^..~-“ 8.a. ”-” 3.5 Co.04 -.“ 54.4 2.0-5.0 . - . ALHA -.....-.-....“ .....”0.32 8.8...._.3.5 “.._.__I___._ _“_ lc .7.._.p - .___ 56.0 0.44 7.9 0.53...“29.6 3.3 0.09 12.5 10.9-13.0 _“_ lr 2 px 60.8 0.44 tt.a 1.6 0.11 38.5 0.51 tt.a 2.2 1.8-2.6 0.9 -_“-_“ -_..” 0.95 . . -..“ __“..-._“ _..“” .._ -.” _.I_..._.._ -” _-._. 0.6 “_._ _‘I_ ir 9 01 39.6 0.72 0.16 17.8 0.47 39.2 0.50 0.18 20.4 18.4-25.4 ALHAn 3~ 5 01 42.3 0.14 0.18 2.2 0.13 54.5 0.31 <0.04 2.3 0.7-3.5 _“_ 11 1 px 56.9 0.24 0.62 8.5 0.49 32.5 0.34 cQ.04 12.7 0.7 _“_ 3c ._4_~ .._..“ 58.2 0.54 0.62 0.35 0.50~0.04 2.0 1.2-3.2 0.9 __._” _...“”._.... “_.._ ..1.3 ......._... _-..__37.2 . .“._.__“ ...__________.” _‘I_ lr 2 oirc 42.2 0.43 0.28 2.1 0.30 54.2 0.31 <9.04 2.1 2.0-2.22 _“_ 31 8 px 58.9 0.48 0.59 1.7 0.29 37.6 0.43 8.04 2.7 1.8-4.7 0.8 A~~~77260 lc 4 01 39.3 ~0.03 n.a 18.9 0.45 40.4 0.08 ~.a. 21.1 10.6-34.0 ~~~~77034 k ___._1__3 2 oi 43.4 aofj n.a 1.00.14 57.1 0.19 11.a 1.0 0.94 _....... - .._ -._. _....._.._.._..I__._.___.___..“ ._..___._._.“ II.& 6.0 0.54 _“_ lc .“4_. fi -_.-_._-_.-“ 58.5 0.56 33.2-._”1.0 2.0 _I’_ 51 2 px 60.8 0.61 n.a. 1.2 0.12 38.7 0.59 a.& 1.7 M-2.2 1.1 -__-“._.-__~-__-._.“ --__-__” ..._“. ma. 9.1 4.bll.2 _“_ lr 2 01 37.4 4.03 II.& 29.5 0.43 31.5 0.06 ILO. 34.4 33.7-35.2 A~~A77176 2c 7 oi 42.2 0.04 0.33 1.8 0.09 55.1 0.20&04 1.8 1.3-2.3 txx tt.& II,L _“_ lr 1 58.4 0.26 7.9 0.59 30.9 2.1 12.0 4.1 _“_ 1.2 2c l_~ ._^_.“ 58.5_._._ 0.59 9.51 ___. 2.0 0.07 37.0 0.62~0.04 “I_.._I_ ___“..___.“ .__ 2.9 . . _:-_.__._.. 34.7 0.08 tta. 28.9 27A30.3 _“_ l__“._l--..^_“___. 2t 10 px 58.5 0.45 0.65 2.2 0.19 37.2 0,61 co.oa 3.2 1.65.6 1.1 ALnA77260 8c ..~._..~!__.~~~_~~_~_~~_~.~ II.% _“ _ 8r 3 01 36.3 ~0.03 31.5 0.42 29.4 -SO4 11.a 37.636.2-40.0 ALHA%?~O~~ .“ 2..__“ 01._“ 433 009“““.....~.___~__.._~” n.a 22 012 -_” 552 0.23 ...~” na. ...“ 2.2 ~“_.~....“ .--~._--...“ _...~...22 ALH83010 3c 6 01 39.2 #.03 0.06 17.1 0.46 42.3 0.06 &04 18.5 14.9-29.2 _“_ 2r 1 0142.2 0.12 11.8. 5.8 0.10 51.9 0.26 ~.a. 5.9 ___.---_.-.--_--“ _“_ 3c lU__Ex 56.3 0.22 _-~ 0.89 10.1 0.70 30.1 1.6 a09 15.3 13&17.0 3:.?. _‘I_ _“_ 31 7 01 37.3
4954

A. N. Krot and J. T. Wasson

(9

Igneous

rims on chondrnles in OC

eating the equation is FeS, which often shows evidence of appreciable melting even when the degree of silicate melting is low. 3.2.

vs. Remelted Host Degree of Melting

Chu draft criterion was that igneous rims should consist largely (~50%) of foreign material as opposed to remelted host chondrule. However, our initial studies indicated that the amount of remelted chondrub materials was commonly large and always difficult to estimate. We, therefore, accepted all rims with a clear rim-host borders. In some cases we made rough estimates of the fraction of foreign materials. The majority of the IR around low-Fe0 chondrules show clear evidence in BSE images that they were significantly melted. The degree of melting is commonly less in rims around &O-rich chondrules and more difficult to define. In part because of the presence of relict grains, these rims show large variations in the grain size of (generally anhedral) mafic minerals. Most rims have a moderate-to-high abundance of fine-gramed troilite, which complicates the use of optical techniques. Most of these features are also present in MR that, in our opinion, were melted to a negligible ( < 10% ) degree. We will show that the dominant precursor materials of most igneous rims around high-Fe0 chondrules appear to be matrix-like materials; thus it is not surprising that there is a continuum in properties between largely melted and unmelted rims, with attendant ambiguity in their classification. We ultimately made our assessment of the degree of melting based on BSE images. The melt fraction was taken to consist of the observed glass and euhedral or subhedral minerals; minerals with fragmental shapes were inferred to be relicts and were not included in the estimate. 3.3. Igneous Rbus around Low-Fe0 ChEleven of thirteen low-Fe0 chondrules studied in detail have porphyritic olivine (PO), porphyritic pyroxene (PP) , or porphyritic olivine-pyroxene (POP) textures; two have barred olivine (BO) textures; IR around these thirteen chondrules have microporphyritic or granular textures with mean grain sizes in the range 5-40 w. Rim thicknesses are fairly uniform over the surface of the chondrules (Table 1, Figs. l-3, 4a-d) . The boundary between a chondrule and IR is generally easy to recognize based on differences in grain size and mineralogy: most rims have high abundances of troilite and metallic Fe-Ni that contribute appreciable opacity in transmitted light (Figs. le, 2a, 3a,b). In some cases, the boundary

4955

between the host chondrule and IR is marked by a layer of metallic Fe-Ni + troilite (Figs. 2d,e, 3d,e). One rim entirely surrounds a POP-chondrule fragment (Fig. 4e,d). These rims show clear evidence of high degrees of melting including: resorbed chondr& surfaces along the boundary with IR (Fig. lb,f ), the presence of rounded metal or troilite nodules (Figs. lc,f, 3c,e,f ), and/or feldspathic mesostasis (Figs. 2c,f, 3f, 4b,d), euhedral morphology of olivine and pyroxene grains and their limited size range (Figs. 2c, 3f, 4b,d), and the absence of relict grains. All the IR we studied are surrounded by MR (Figs. l-3, 4a-d); boundaries between these rim types are generally sharp. Most low-Fe0 chondrules have olivine as the dominant mafic mineral, but the main matic mineral in the rims is lowCa pyroxene (Table 1) . Possibly related is the observation by Jones and Scott (1989) that pyroxene is a common minor constituent in the outer shell of low-Fe0 chomhules having PO textures, but the rather thick pyroxene-rich IR (Figs. lb,d,f, 2d,f, 3a,c) in our set of PO chomfrmes seem too voluminous to be explained by melting pyroxene-rich chondrule edges. Pyroxene in IR has significantly smaller grain-sizes than olivine in the host chondrule. These features indicate that the pyroxene-rich layers around PO chondrules are real rims rather than outer parts of the host chondrule. As discussed later in connection with Fig. 7, olivines and pyroxenes in low-Fe0 chondrules and their IR have similar compositions ( Fa,,.5_9and Fa2_9, Fs,-, and Fs,-~, respectively). Relict grains of iron-rich olivine or pyroxene were not observed in these chondrules and IR. A magnesian porphyritic pyroxene chondrule and some chondrule fragments are present in the rim of chondrule 2 in Krymka; the rim is rich in troilite (Fig. 3a). 3.4. Igneous Rims around High-Fe0 Chondrules High-Fe0 chondrules with IR have coarse porphyritic textures (PO, POP, and PP) (Table 1, Figs. 4e,f, 5, 6); the igneous rims have micropoiphyritic textures. Although igneous rims and host chondrules have similar modal mineralogy (Table 1 ), the boundaries between them are distinct because the rims have much smaller grain sizes of m&c minerals and higher modal abundances of troilite and metallic Fe-Ni (Figs. 4e,f, $6). Many IR are largely opaque because of the high abundance of troilite and metallic Fe-Ni, (Fig. 6d); they are thus difficult to distinguish from MR on the basis of optical microscopy. However, BSE images show evidence of appreciable melting including: relatively uniform grain sizes -210 pm, subhedral and euhedral morphology of maiic minerals, and the presence of feldspathic mesostasis (Fig. 5a-f).

FIG. 1. Low-Fe0 chondrules with igneous rims (JR) in the L3.5 ALBA77034. (a,b) Barred-olivine chondtule 5 is surrounded by an IR consisting mainly of 1owCa pyroxene. The boundary between the host chondrule and the IR marked by arrows in b is distinct; as seen in b the surface of host chondrule is deeply corroded. The IR contains abundant droplets of metallic Fe-Ni (white); it is surrounded by illl MR (light-gray) of variable thickness which fills irregularities on the surface. Pyroxene grains are roughly 10 pm in size. (c,d) Porphyritie olivine cbcmdrule 3 is surrou&d by an IR marked by arrows in c and d) and MR (light-gray). The texture of the IR in (c) strongly suggests that the IR formed by of pyroxene requiring an remelting au outer metal and FeS-rich she11 of the host chondrule, but the rim has a much higher modal akndaace additional Sio;l-rich component. The IR has an irregular outer surface; its ma& grains are roughly 10 q in size. (e,f) Porphyritic olivine chond&le 1 is also surrounded by an IR consisting mainly of low-Ca pyroxene. The poorly resolved boundary between the host chondrule and the IR is marked by arrows in (f). Because of abundant Fe-Ni and troilite and the small silicate grain size, the IR is nearly opaque in transmitted light (e). The rim has an irregular surface, and it is surrounded by a MR (light-gray, top left, and right in f) of irregular thickness. Scale bars without legends are 100 pm. (a-c/f) back-scattered electrons (BSE); (e) partly-crossed nicols.

A. N. Krot and J. T. Wasson

Igneous rims on chonrhules in OC As noted above, there is true ambiguity in the classification when the degree of melting becomes very low, 120%. Olivine and pyroxene in the IR are commonly much more FeOrich than the host chondrules and our data generally show smaller ranges in Fa and Fs in the rims (Fig. 7, Table 2). Many of these IR contain relict grains of magnesian olivine (Fa,,_,,,) and pyroxene (Fs,_~) (Fig. 4e,f). In some cases, these relict grains and the surfaces of the host &o&rules are overgrown by thin layers of iron-rich olivine (Fig. 4f, 5c), a clear indication of appreciable degrees of melting. Summarizmg our petrographic observations for both lowand high-Fe0 sets, we conclude that ( 1) IR around low-Fe0 chondrules show evidence of high degrees of melting; there is no doubt that all these rims are igneous. There is a continuity between enveloping secondaries of compound chondrules (Wasson et al., 1995) and these rims. Most of the igneous rims have porphyritic textures rather than radial, cryptocr$taIline, or other textures associated with complete melting; the latter are somewhat more common in the secondaries on low-Fe0 primaries on independent compound chondrules. (2) Some IR around high-Fe0 chondrules show evidence of large-scale (>90%) melting, others show lower fractions although probably not less than 30% in our set; in many cases the evidence of melting is only clear in BSE images. Many high-FeG rims contain relict low-Fe0 grams, an indication that the heat pulse and duration were too low to allow these materials to be completely assimilated. An important constraint on formation models is this difference in the temperature histories of the IR around low-Fe0 and highFe0 chondtules. (3) Many of IR around low-Fe0 chondrules are surrounded by iron-rich MR; the boundary between these rims are typically sharp indicating that the IR were solid when the MR were added. (4) only a small fraction of IR around FeG-rich chondrules are surrounded by resolvable MR, in part because MR appear to be the main source constituents of these rims. (5) Olivine and pyroxene of the IR are compositionally similar to but commonly have higher FeO/(FeO + MgO) than those of the host chondrules; in general, the higher the Fe0 content of the host, the greater the difference between rim and host.

4. FORMATION OF IGNEOUS RIMS 4.1. A §+senee of Nebular Aeeretionary Me&g Events

steps and

There is now general agreement that chondrules are formed by flash melting (heat-pulse duration s 60 s) in the solar nebula (e.g., Grossman, 1988; Wasson, 1993; Boss, 1995).

4957

The MR that enclose the majority of the IR around FeO-poor chondrules and some of the IR around FeO-rich chondrules were not significantly melted before or during aggregation and incorporation into a parent body (e.g., Allen et al., 1980; Taylor et al., 1984; Nagahara, 1984; Scottet al., 1988; Metzler et al., 1992). They formed around the chondrules after the last high-temperature event that appreciably melted their host. As discussed in more detail below, the MR appear to record the last stage of grain-grain adhesion in the nebula although impact events on the parent body may have rearranged these grains and reduced the porosity. Thus, when an entity such as an IR is surrounded by an MR it is an indication that it formed in the nebula. In contrast to the MR and representing an intermediate stage, most IR experienced significant heating and appreciable degmes of melting. This view contrasts with that of Rubin ( 1984), who proposed that the coarse-grained rims studied (mainly in CV chondrites) only reached sintering temperatures, i.e., the silicate solidus. Resorbed chondrule surfaces along the boundaries with the IR, presence of feldspathic mesostasis, overgrowth zones, and euhedral or subhedral morphology of mafic silicates, absence of relict grains, small ranges in grain sizes, and compositional variations of mafic minerals in IR around low-EeO &&rules indicate that the silicate portions of the IR were largely melted. The modal mineralogy and phase compositions of the IR and host chondrules are quite similar. We calculated melt-solid relationships for compositions of low-Fe0 (Jones and Scott, 1989) and high-Fe0 (Jones, 1990) porphyritic chondnrles using the MELTS program of Ghiorso and Sack (1995) and estimate that typical SO%-melting temperatures of low-Fe0 IR are - 1840 K and those for high-Fe0 IR - 1700 K. At our UIQI lower limit for classifying rims on high-Fe0 chondrules as igneous, the typical temperatures were 1400-1600 K. These temperatures are appreciably greater than the minimum temperature of 1250 K required to explain the eutectoid spherical blebs of troilite and metallic Fe-Ni in these assemblages. In contrast to IR around low-Fe0 chondrules, those around high-Fe0 chondrules show a wider range in degree of melting and commonly contain relict grams. Thus, these rims not only have lower SO%-melting temperatures, theii average degree of melting was appreciably lower than those of LR around FeO-poor chondrules. We conclude that they experienced a smaller heat fluence. All evidence is consistent with the view that the heat source responsible for heating rim material to T 2 1400 K is the same one responsible for melting the &o&rules (Rubin, 1984). That the host chondrules were only melted to a minor extent,

FIG.2. Low-Fe0 chondmles with IR. (a,~) Porphyritic olivine-pyroxene chondrule 2 in Bishunpur is surroundedby au IR (black in a). The sharp boon&q between the host chondrule and the IR marked by arrows in (b) results from differences in opacity and gram sizes. The IR has an itmgt&r outer surface and is surroundedby an MR (white in b). In (c) the silicate-rich portion of the rim consists of e&e&al and subhedral olivine and pyroxene grams embedded in plagio~lase mesostasis (Welled mes); the mean size of the larger grains is -6 pm. (de) Porphytitic olivine ekmdmle 2 in ALHA7726B is surrounded by an IR consisting mainly of low-Ca pyroxene. The hreg?darbot@ary between the host chondruk and the IR marked by arrows in (e) is distinct due to the presence of metallic Fe, Ni + troilite layer on the ohond&e surface (black in (d) and white in (e)). The shapes and sixes of metal-troijite modules are similar in the LR suggesting that the latter may mainly consist of remelted chondrule. but the higher pyroxene abundance requires the addition of an SiOr-rich component. (f) Porpbyritic olivine-pyroxene chondrule 3 in Chainpur is surroundedby an IR consisting mainly of low-Ca pyroxene. Note the presence of feldspathic mesostasis (labelled mes) in the rim; the mean sire of the pyroxene grams is -5 pm. Black scale bar in (c) is 10 pm; black scale bar in (b) is 100 pm; white scale bars in (e) and (f) are 100 pm. (a,d) partly-crossed nicols; (b,c,e,f) BSE.

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Igneous rims on chonclmles in OC

the troilite in the rim was not dissociated, and the S evaporated testifies to the brevity of the rim heating event and is consistent with formation in llash-heating events of the sort inferred to be responsible for chondrule formation (e.g., Grossman, 1988; Wasson, 1993). Some of these chondrules contain relict grains which were probably produced by disaggregation of previous generations of chondrules (Nagahara, 1981; Rambaldi and Wasson, 1982); thus these objects experienced at least three flash-heating events that led to silicate melting. 5. TBE ABSENCE OF lUATBJX-LIKE RIMS AROUND HIGH-FE& CHQF45RULESHAVING IGNEOUS

RIMS

Most low-Fe0 chondrules having IR also have MR. All thirteen low-Fe0 pairs in our set have MR with distinctly finer grain size. A number of these MR show evidence of troilite and metallic Fe-Ni melting and a few show evidence of low degrees of silicate melting. In contrast, the high-Fe0 chondrule-rim pairs are generally not surrounded by MR (Figs. 4e, Sa,b,e,f, 6a,e). Only Kryr&a chondrule 1 (Fig. 5d) is known to be surrounded by an MR. We failed to note whether Chainpur chondrule 6 (not figured) has an MR. but there is no evidence of an MR in our extensive set of images on the remaining seven high-Fe0 chondrule-rim pairs. The presence of MR is widely accepted to indicate that the enclosed object formed in the solar nebula, thus we conclude that low-Fe0 choudrule-rim pairs existed as free-floating entities in the solar nebula. The absence of MR does not require extranebular formation, but the absence of MR on most highFe0 &o&rule-rim pairs calls into question our conclusion that the high-Fe0 IR were melted during nebular chondruleforming events. We re-examined the question of whether our standards had changed and whether some high-Fe0 IR were really MR. Our backscattered images of the MR around low-Fe0 chondruleIR pairs show that these are consistently finer-grained and much more opaque than the IR around high-Fe0 chondrules with no appreciable melting. We considered three possible reasons for the absence of MR around high-Fe0 IR: ( 1) MR are more difficult to resolve around the more opaque high-Fe0 IR, (2) high-Fe0 IR formed at a time when the nebular dust/chondrule ratio was lower than at the time the low-Fe0 chondrules formed or (3) high-Fe0 IR were melted by a process other than the nebular chondrule-forming process. There is merit in the first point; it is more difficult to resolve the borders between the relatively opaque, less completely

4959

melted high-Fe0 IR and any MR that might be present. However, it seems unlikely that this could have caused us to miss more than half the MR in our set. Regarding the third point, we considered the possibility that high-Fe0 MR were melted during compressive, p. Au (pressure times the decrease in volume) heating during impact-induced compaction of the parent body. However, J. Paque (pers. commun., 1995) pointed out this would inevitably lead to shear-induced flow of the melt, which is not observed. We are left with the possibility that the dust/chondrule ratio at the formation place/time of the low-Fe0 chondrules was higher than that prevailing when the high-Fe0 &or&rules formed. This is consistent with the view that the low-Fe0 chondrules formed earlier, and that the fraction of the nebular solids present as chondrules increased with time. 6. MULTIPLE HEATING EVENTS

ANB THE RECYCLING

OF NEBULAR SOLHIS

Wasson et al. ( 1995) suggested that independent secondaries of compound chondrules formed by melting of material aggregated around earlier-formed primary chondrules. Wasson et al. ( 1995) designated as enveloping those secondaries that completely surround the primary chondrule; thus these are morphologically closely related to igneous rims. We find that there is a textural continuum between IR and enveloping compound chondrules (e.g., Figs. 2d, 3d), a further indication that chondrule-forming events were experienced by most OC chondrules two or more times. The presence of IR around chondrule fragments (Fig. 4c,d) and individual mineral grains (Rubin, 1984) supports this conclusion. Repeated fragmentation and chondrule formation is also consistent with some other independent observations: (a) Clayton et al. (1991) suggested that the small range in oxygen-isotope composition of OC chondrules resulted from the homogenization of the OC chondrule oxygen isotope composition during several fragmentation and melting episodes. (b) To explain the paucity of 26Al in OC chondrnles, Hutcheon et al. ( 1994) suggested that many chondrules are not primary objects but represent material recycled through multiple episodes of evaporation and recondensation, mixing and partial melting, and gas-dust fractionation. (c) Based on interelement correlations in bulk-chondrule data, Alexander ( 1994) concluded that the random sampling of a previous generation of chondrules could produce the observed range of bulk compositions. However, as Grossman and Wasson ( 1983) stated, the number of generations could not have been large without erasing the distinction between low-FeO, refractory-rich and high-FeO, refractory-poor chondrules.

FIG. 3. Low-Fe0 &o&rules with IR. (a) Porphyritic-olivine chondrule 2 in Krymka is surrounded by an IR consisting mainly of low-Ca pyroxene and tro~lite.The boundary between the host chondrute and the rim (arrows) is assigned on the basis of troilite abundance. Note that rim contains rounded fragments of radial-pyroxene or porphyritic-pyroxene chondrules (labelled as R); their shapes suggest spalls off the host, but their compositions would then require SiOz addition and recrystallixation. (b,c) Porphyritic-olivine chondntle 2 in L3.2 ALHA is sunoumied by an IR consisting mainly of low-Ca pyroxene. Tbe boundary between the host chondrule and the IR is sharp due to the difference in opacity; the rim is nearly opaque in transmitted light (b). Note that the host chondrule and the rim have similar abundances of metal and troilite (white in (c)) and that two of the large metal-troilite nodules in the host extend into the IR (b). The IR is smmunded by a matrix-like rim (the 0rWmoat wbite lityer in (c)). (d-f) Barred-olivine chondrule 1 in L3.2 LFJW86158is surrounded by a thick IR. The boundary between the host chondrtde and the IR (arrows in (e)) is distinct due to the high abundance of metal and troilite and (black in (d) and white in (e)) along the chondrule surface. Note that the rim has an irregular surface and consists of subhedrai olivine grains (mean size -30 pm), partly recrystallized feldspathic mesostasis, and numerous metallic Fe-Ni and troilite droplets (white in (e) and (f)). It is surrounded by an MR (black in (d), white in (e)). Scale bars in (c) and (f) are 100 pm. (a,c,e,f) BSE; (b,d) partly-crossed nicols.

A. N. Krot and J.

T.Wasson

Igneous rims on chondrules in OC

COMPO!SITIONAL EVOLUTIONOF THE NEBULA, AGGLOMERATION, AND CHONDULEFORMATION

7.

7.1.

Hosts

The Wasson et al. ( 1995) data showed the unexpected result that the primary-secondary pairs of independent compound chondrules formed from precursor materials having similar degrees of oxidation. Our study of IR yields similar observations. In Fig. 7 the compositions of olivine and lowCa pyroxene in IR around chondrules are plotted. Our working model of rim formation at the time we initiated this research was that igneous rims largely formed late in the history of the nebula from matrix-like dust; this model would have led to uniformly high olivine-Fa contents in the rims on all chondrules but this did not prove to be the case. Olivine and pyroxene of the IR around low-Fe0 chondrules (Fig. 7) are compositionally similar to or slightly more FeOrich than those of the host chondrules; those around high-Fe0 chondrules are also similar to their hosts but, with two exceptions, have moderately higher Fa or Fs contents and are more similar to the product expected from melting MR. There are two possible scenarios that can explain this tendency of the composition of the rim is to be similar to that of the host and of independent secondaries to resemble the primary chondrules they are attached to. Both are considered in terms of the standard cosmochemical model of the nebula, in which nebular components formed by the condensation of grains in a monotonically cooling nebula having a solar composition. This model yields an initial set of reduced solids, i.e., solids containing negligible amounts of FeO; as the nebula cooled metallic Fe was oxidized to FeO, with most finegrained Fe-Ni oxidized by the time the nebula cooled to 500 K. There is abundant evidence supporting this model including the presence of relict grains and fragments of low-Fe0 chondrules in high-Fe0 chondrules and the rarity of the converse. The two scenarios are: (a) the rims were largely derived from independent nebular materials, and their close resemblance to the host indicates similar precursor materials; or (b ) the materials that melted to form the rim were largely derived from the host chondrule, with minor dilution by other materials, possibly more evolved solids having higher Fe0 contents.

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7.2. Scenario 1: Rim Materials Independent of Host Chondrule If the bulk of the rim consisted of independent materials accreted to the host chondrule and melted there to form the igneous rims, it follows that these materials were closely related to the source of the host, albeit with a lower MgISi ratio to account for the much higher pyroxenelolivine ratio. Similarly, this scenario also requires that primaries and secondaries of independent compound chondrules formed from the same general batch of nebular materials. According to the standard cosmochemical model, low-Fe0 chondrules and their low-Fe0 rims or secondaries (i.e., lowFe0 pairs) formed early in nebular history when most malic silicates were still magnesian. Similarly, high Fe0 pairs formed at a later time when most of the fine metal had been oxidized. (A variant of this model associates differences in Fe0 content with nebular location or with tempo4 differences in the protosolar solids accr&ng to the nebula. These require more complex nebular&fall mixing mechanisms and we will not develop such models here.) In terms of this scenario the absence of mixed pairs, i.e., high-Fe0 igneous rims or compound secondaries on low-Fe0 host chondrules, requires the withdrawal of low-Fe0 chondrules from the chondrule-forming region during the following nebular period when high-Fe0 chondrules and their igneous rims or independent secondaries formed. It is important to assess the ambient temperatmes of the nebula at the times when the chondrule-forming heating events occurred that formed w two kinds of pairs. Because the high-Fe0 pairs contain large amounts of oxidized Fe, they have formed after temperatures had fallen below 500 K. Most low-Fe0 chondrules have FeO/(FeO + MgO) ratios suggesting formation in the range 550-650 K, and thus consistent with the fact that most also confain FeS which condenses at -650 K. At 650 K equilibrium in a canonical nebula yields olivine with a composition of about Fa,, and kinetics constraints probably caused the mean Fe0 content to have been somewhat lower. The high bulk Ca and low Na contents of some low-Fe0 chondrules suggest that these formed largely from precursors formed at relatively high nebular temperatures above the condensation temperature of Na and K, perhaps >lOOO K. The withdrawal of the low-Fe0 chondrule rim pairs can possibly be understood if these were able to settle into a region very close to the nebular midplane. If the turbulence required

FIG. 4. Low-Fe0 (a-d) and high Fe0 (e,f) chondndes with IR in Bishunpur. (a,b) Low-Fe0 porphyritic olivine-pyroxene chondrule 15. The boundary between the host chondmle (on tight in (b)) and IR is marked by arrows; the rim includes large m&c grains very similar to those in the host; this and the general texture suggest that it formed by fragmentation and remelting of the host. The main part of the rim consists of euhedral and subhedral grains of Iow-Ca pyroxene, olivine and feldspathic mesostasis (labelled mes in (b)). (c,d) Low-Fe&I porphyritie pyroxene~olivine &o&rule fragment 16 is surrounded by a thick IR of variable width. The bou&uy between host chondnde and fR is marked by arrows. The rim consists of zoned euhedral and subhedral grains of olivine, low-Ca pyroxene, high-Ca pyroxene+ and feIdspa@c mesostasis (labelled as mes); the @zan size of the malic grains is about 15-20 pm. This low-Fe0 rim is an exception in that it cotins as mxtch olivine as the host. (e,f) High-Fe0 PO chondmle 4 (irregularly shaped dark-gray coarse-grained region in the center of image (e)) is 5urroumW by a thick IR consisting of high&O olivine; metal and troilite (white). Although the IR is distinguished by a higher abu&imce of Ft-Ni + troilite and lower silicate grain size than in the host chondrule, the boundary between them is difficult to define. The rim contains numerou s relict grains of forsteritic olivine (black in (e) and (f)); one relict grain (labelled R in (f)) is overgrown by a layer of high-Fe0 olivine having the same composition as other olivine grains in the rim. Grain sizes in the rim show a large range; a rough mean is 5 pm. Scale bars are 100 pm in (b), (d), and (e), and 10 pm in (f). (a,c) uncrossed polars; (b), (d), (e), and (f) BSE.

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Igneous rims on chondrules in OC

to power the chondrule-forming process was mainly present in regions above the midplane, high-Fe0 chondrules could have still been forming from low-density precursors suspended in these higher regions.

Bims Largely consist of 7.3. Seal@0 2: Reraeltsdkkoat C!bon&ule It is commonly observed among the rimmed low-Fe0 chondrules that the chondrule surfaces are appreciably corroded, and that the rims share key petrographic features (such as size and abundance of metal grains) with the host. Thus, this second, remelting scenario seems somewhat more consistent with the petrographic observations. Irregular grains observed in the rims commonly appear to be detached fragments of the primary. Perhaps the chondrule was heavily fractured before the rim formed, and melt penetration along cracks led to the detachment of these grains. The alternative, that these grains are unrelated to the host, seems less plausible. Recently, Krot et al. ( 1995) found large numbers of microchondrules in matrix-like rims of several low-Fe0 OC chondrules. The composition of the microchondrules also suggests formation from the host or from a compositionally similar igneous rim. The high concentration of these microchondrules requires a mechanism that prevented them from escaping the parent-host. They suggested that a more porous version of the MR was already in place at the time these microchondrules formed. We suggest that, at the time of rim formation, the chondrule may have been enmeshed in a fluffy aggregate of the sort described by DOM and Meakin (1989) and by Wasson et al. ( 1995). 7.4. Pysxnre

Normative Igneous Rims on Low-Fe0

The fact that low-Fe0 olivine normative host chondrules are commonly surrounded by pyroxene-normative igneous rims can also be understood in terms of the standard cosmochemical model by including a plausible additional feature, i.e., reaction with fine nebular materials that were SiO, normative. A plausible explanation for the high abundance of olivine in low-Fe0 chondrules and of normative SiOz in finegrained matrix is the condensation-overshoot model of Grossman and Wasson (1983); partial distillation of molten chondrules would also enhance the SiOz contents of line-grained nebular materials, but our current interpretation of the evidence is that it played at most a minor role. Olivine was the first major ma& mineral to nucleate in the nebula: under equi-

4963

librium conditions, the pyroxene MgSiO? should have begun condensing at a temperature about 30” lower than forsteritic olivine Mg,Si04. However, if the nucleation of pyroxene was inhibited, almost all of nebular Mg may have condensed as relatively coarse crystals of MgzSiOa , in which case the SiO, would have supersaturated and then condensed as line, subpm SiO, grains. There is chemical and petrographic evidence supporting this enhancement of SiOz in the matrix. Grossman and Wasson ( 1983) cited compositional evidence from the literature; more recently, Matsunami et al. ( 1990) observed SQ in matrix including matrix-like rims. Wasson and Krot (1994) reported evidence that SiO, in chondrules is partially converted to Fe$iO, by the oxidation of metal on the parent body; in such an environment the fine SiOl in the matrix would have been the first to react. There is good reason to suspect that this was a ubiquitous situation on the ordinary-chondrite parent bodies. It follows that the Si02 content of matrix in the nebula was appreciably higher (and the Fe0 content lower) than that now preserved, even in Semarkona and other OC of very low petrographic type. Reaction of this fine SiQ with small amounts of material melted from the surface of chondrules can yield a lower Mgl Si ratio in IR. At Z’- 600 K, the metal remaining following FeS formation had largely not yet been converted to FeO, thus reaction of this fine matrix would not have led to an appreciable increase in the FeO/(FeO + MgO) ratio of the IR relative to the host chondrule. It is possible that nucleation also played a role in producing the high content of low-Ca pyroxene in the low-Fe0 IR. Perhaps the melting event largely destroyed olivine nuclei but left pyroxene nuclei (the pyroxene that is enhanced near the chondrule edge) and these, therefore, dominated the crystalgrowth process in the JR. 7.5. ADecreaclehtheI&eas&yoff2&&uk-FonnMg

Independent of the above interpretations, the decrease in the inferred degree of rim melting with increasing Fe0 content is counter-intuitive, since the mehing temperamre of chondritic matter decreases as Fe0 increases. Events of equal intensity would thus yield higher degrees of melting of materials having higher mean Fe0 contents. Thus interpretation of our Observations in terms of the standard cosmochemical model has the important implication ahat the mean energy content of the chondrule-forming-heating events decreased as the temperature of the nebula decreased.

FIG.5. High-Fe0 chondrules with IR. (a-c) ALHA POP chondrule 1 surrounded by an IR consisting of sub&d& and euhedral olivine grains (light-gray in (c)) and a&edral Iow-Ca pyroxene grains (dark-gray in (c)). The outer part of the rim is rich in troi&e (black in (a), white in (b) and (c)). Note that the rim side of the large olivine grain in the chondrule (laWled A) is overgrown by olivine Wmg R&her FW content similar to o&&e in tbe rim. (d) Krymka chondrule 1 consists of two constituents: a relict BO chonrkule fragment (top I&t) and a figment of POP chondr& (top right). These are surroundedby a thick IR having a porphytitic texture (boundary marked by arrows, g&r size - 10 pm) containing some large pyroxene and olivine grains. A possible interpretationis that tbe rim largely formed by melting the host chondrule; compositional data (Table 3) are consistent with this view. The IR is surroundedby a layer mainly consisthsg of troihte (white) followed by a thin and ill-defhted xone consisting of matrix. (e,f) ALHA PO chondrule 8 is sunmmded by an IR cot&t&j mainly of olivine having a significantly higher Fa content than observed in the host chondrule (34 vs. 21 mol%). The boundary between the host chondrule and W is assigned on the basis of the smaller grain size. and higher mean abundance of troilite in the IR. Grain size in the IR is highly variable; a rough mean is 4 pm. Scale bars in (b), (c), (d), (e) are 100 pm; that in (f) is 10 pm. (a) partly-crossed nicols; (b-f) BSE.

A. N. Krot and J. T. Wasson

Igneous rims on chondrules in OC

vironment than those experienced by high-Fe0 chondrules. This implies that the earlier, low-Fe0 chondrules were stored outside the region where the later, high-Fe0 chondrules were formed. Perhaps the most plausible region is the nebular midplane.

.E 20 3 & !z 10 .o, .c 65 $4 a9 B E 2 1 0.5

4965

are most grateful to Alan Rubin for introducing us to this topic and for sound advice throughout the progress of the study. We thank A. Lee, C. Manning, and D. Winter for technical support, and P. H. Benoit, J. M. DeHart, and J. M. Paque for constructive reviews. Samples were kindly provided by G. J. MacPherson, M. Lindstrom, and by the members of the U.S. Antarctic Meteorite Working Group. This work was largely supported by NASA grant NAGW-3535.

Acknowledgments-We

1

2

4 5

10

20

40

mol% Fa/Fs in chondrules

FIG. 7. Mean olivine and pyroxene compositions of mafic minerals in the host chondrules, IR, MR, and ambiguous rims. Closed triangles are low-Fe0 chondrules (Fa or Fs < 10 mol%) with IR, open triangles are high-Fe0 chondrules (Fa/Fs > 10 mol%) with IR. The open square is a matrix-like rim. Lines connect divine and pyroxene pairs. With few exceptions, rim mafic minerals have moderately higher FeO/(FeO + MgO) ratios than those in the host mafics even though, in most low-Fe0 chonclrules, rim pyroxene is plotted against chondrule olivine.

8. NEBULAR PROCESSES THAT PRODUCED OC: A REVISED SEQUENCE OF EVENT!3 DURING CHONDRULE FORMATION AND SOLID AGGLOMERATION

The following sequence of processes seems to have occurred during the evolution of nebular solids in the region where the ordinary chondrites (OC) formed. The processes involving rim formation represent a refinement of the picture based on the present study. Different sets of grain-assemblages formed in the solar nebula and served as the precursors of chondrules; the simplest scenario involves coarse, low-Fe0 grains forming at high nebular temperatures and finer, high-Fe0 materials forming at low temperatures. Some chondrule formation occurred at high nebular temperatures, but vanishingly few of these chondrules survived; most chondrules in the OC contain FeS and formed at nebular temperatures <650 K. Chondrules produced in earlier generations became incotporated into later genemtions; evidence for this includes relict grams and primaries enclosed by secondaries of independent compound chondrules. Heating events that caused silicate melting were experienced two or more times by most nebular materials. Krot et al. ( 1995) observed one chondmle with microchondrules in a matrix-like rim that records four melting events. But, as shown by this study, the multiple melting events experienced by low-Fe0 chondrules occurred in different en-

Editorial handling: C. Koeberl

REFERENCES Alexander C. M. 0. (1994) Trace element distributions within ordinary chondrite chondrules: Implications for chondrule formation conditions and precursors. Geochim. Cosmochim. Acta 58,345 l3467.

Alexander C. M. O., Barber D. J., Davis M., and Hutchison R. ( 1985) Relationship between dark rims, interchondmle matrix, and chondrules in U.0.C.s (abstr.). Mereotitics 20,600. Alexander C. M. O., Hut&son R., and Barber D. J. (1989) Origin of chondrule rims and interchondmle matrices in unequilibrated ordinary chondrites. Earth Planet. Sci. Lea 95, 187-207. Allen J. S., Nor&e S., and Wilkening L. L. (1980) A study of chondrule rims and chondrule irradiation records in unequilibrated ordinary chondrites. Geochim. Cosmochim. Acta 44, 1161-l 175. Ashworth J. R. (1977) Matrix textures in unquilibrated ordinary chondrites. Earth Planet. Sci. Lett. 35, 25-34. Boss A. (1995) A concise guide to chondrule formation models. In Chondrules and the Protoplanetary Disk (ed. R. H. Hewins, et al.), Cambridge Univ. Press (in press). Clayton R. N., Mayeda T. K., Goswami J. N., and Olsen E. J. ( 1991) Oxygen isotope studies of ordinary chondrites. Geochim. Cosmochim. Acta 55,2317-2337.

Donn B. and Meakin P. (1989) Collisions of macroscopic fluffy aggregates in the primordial solar nebula and the formation of planetesimals. Proc. Lunar Planet. Sci. Cof 19,577-580. Fujimaki H., Matsu-Ura M., Sunagawa I., and Aoki K. ( 198 1) Chemical compositions of chondmles and matrices in the ALHAchondrite (L3). Nat. Inst. Polar Res. Spec. Issue 20, 161- 174. Ghiorso M. S. and Sack R. 0. ( 1995) Chemical mass transfer in magmatic processes IV. A revised and internally consistent thermodynamic model for the interpolation and extrapolation of liquidsolid equilibria in magmatic systems at elevated temperatures and pressures. Contrib. Mineral. Pefrol. 119, 197-212. Grossman J. N. ( 1988) Formation of chondrules. In Meteorites and the Early Solar System (ed. J. F. Kerridge and M. S. Matthews), pp. 680-696. Univ. Arizona Press. Grossman J. N. and Wasson J. T. ( 1983) The compositions of chondrules in unequilibrated chondrites: An evaluation of models for the formation of chondrules and their precursor materials. In Chond&es and their Origins (ed. E. A. King), pp. 88- 121. Lunar Planet. Insti.

FIG. 6. High-Fe0 chondrules with IR. (a-c) Chainpur PO chondrule 5 is surrounded by an IR consisting mainly of olivine. The boundary between the host chondrule and the rim (marked arrows) is sharp due to the lower grain size and higher troilite (white) abundance in the rim. Troilite is more abundant in the outer part of the rim and occurs as large anhedral grains. Rare relict grains of magnesian pyroxene and olivine are present in the rim (labelled as R in (c)). Mean grain size in the IR - 10 pm. (d-f) ALHA POP chondrule 1 surrounded by a rim (black in (d)) rich in troihte (white in (e) and (f)) and consisting mainly of olivine. The boundary between the host chondrule and the rim (marked by arrows) is sharp due to small silicate grain size and larger abundance of troilite (greater in the outer part) of the rim. For reference, one pyroxene grain in the chondrule is labelled as A in (d) and (e). Rare relict grains of magnesian pyroxene (labelled as R in (e)) occur in the rim. The igneous texture shown near the center in (f) requires a high degree (>70%) of melting. Scale bars in (a) and (b) are 100 pm. (a-c), (e,f) BSE; (d) partly crossed nicols.

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