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Structure and formation of the San Cristobal meteorite, other IB irons and group IIICD
C+eocbimica et
Cosmochimica Acts, 19'74,Vol. 38,pp. 1379to 1891. PergamonPress. Printedin Northern Ireland
Structure and formation of the San Cristobal meteorite, other IB irons and group IIICD Department
EDWARD R. D. SCOTT and RICHARD W. BILD of Chemistry and Institute of Geophysics and Planetary Physics, University California, Los Angeles, CA 90024, U.S.A.
of
(Received 26 November 1973; accepted in revised form 11 March 1974) Abstract-San Cristobal is an unusual group IB atexite with 25 per cent Ni, composed of taenite grains 2-3 cm in diameter and silicate-troilite-graphite nodules concentrated on the grain Silicate compositions are typical of group IAB: olivine Far.s, orthopyroxene boundaries. Plagioclase shows peristerite unmixing, previously unrecorded in Fs,., and feldspar Ab,,. meteorites, and occasional K-rich feldspar grams have an unusual antiperthite exsolution. Brianite Ne,CaMg(PO,), and haxonite (Fe, Ni),,C, are common in nodules and matrix, respectively, while cohenite is rare. Part of the matrix contains a pearlitic kamecite precipitate instead of the usual oriented platelets. San Cristobal has extreme concentrations of many elements; e.g. the highest published Ag, Cu, In and Sb contents and the lowest MO and Pt in irons. These data and the mineralogy show that San Cristobal has many characteristics of both groups IB and IIID, but that it fits group IB trends better. Ratios of refractory element abundances to those in Cl chondrites (both normalized to Ni) decrease through IB from 1 in IA to 0.03 in San Cristobal, but the other siderophilic elements have a small range of abundance ratios, 0.5-2, throughout IAB. We suggest that IB grains either formed in a part of the solar nebula where refractories had been previously removed, or else failed to equilibrate with a refractory-rich, high-temperature condensate. After condensation of the volatiles, Fe was partially removed, perhaps by oxidation. Group IIICD seems to have experienced similar fraction&ions. Unlike other iron meteorite groups, neither IAB nor IIICD appears to have been fully molten. INTRODUCTION (1944) called San Cristobal an “extraordinary meteorite” after viewing its structure. The mineralogy and chemistry also show many unusual features, which will be described below. It is a silicate-bearing ataxite with 25 per cent Ni, a value exceeded by only five anomalous irons (WASSON, in press), and has recently been identified as the high-Ni end-member of the group IAB sequence (SCOTT et al., 1973). WASSON (1970) interpreted the properties of group IAB to indicate that these irons were never molten. The absence in IAB of the severe element fractionations observed within other major groups (SCOTT, 1972) emphasized that IAB had formed under quite different conditions. Because of its extreme position in the IAB sequence, it appeared especially important to study San Cristobal in some detail. Through the kind cooperation of Rudolf Schaudy and Wolfgang Kiesl of the University of Vienna we obtained a specimen containing a cm-sized silicate nodule. We report the results of a study by reflected-light microscopy and electron-microprobe analysis of this nodule and other samples of the matrix. A discussion of these and published chemical data show that despite its many unusual properties, San Cristobal fits well-established trends in group IAB and suggests the existence of others. We offer an interpretation of IAB trends and those in group IIICD which appear to have been produced by similar processes. PERRY
1359
1380
EDWAIZD
R. D. SCOTTand
RICHARD W. BILD
MINEFGALOGIY
Matrix
San Cristobal is composed of taenite grains 2-3 cm in diameter with cm-sized troilite-graphite-silicate nod&s concentrated along the grain boundaries. Most of the matrix is typical of a normal ataxite; 10 pm wide kamacite platelets and grains with associated schreibersite grains are set in taenite partly decomposing to martensite (AXONand SMITH,1972; OWENS and BURNS,1939). Some taenite grains contain a zone of pearlitic kamacite precipitation which has a high kamacite content, perhaps ten times higher than the rest of the matrix. PERRY(1944, Plates 29 and 30) correctly identified the phases and termed the pattern ‘pearlitic’ because of its similarity to the morphology of a-Fe and Fe& in pearlite observed in cast irons. The area at the edge of a 5 x 15 mm pearlitic zone is shown in Fig. 1. This structure is not unique to the San Cristobal matrix: that of Dayton (IIID, 17 per cent Ni)* is partly pearlitic and partly that of a finest octahedrite (GOLDSTEIN and O~IL~IE, 1965, Fig. 2). Pearlitic plessite can also be seen in some group IA and IIIC irons. In San Cristobal and Dayton it appears that pea&tic colonies can form and spread rapidly when convenand tional kamacite nucleation is just beginning. With abundant C (BRENTNALL AXON, 1962), pearlitic growth seems to be an effective alternative for the breakdo~ of high Ni taenite. Carbides
Haxonite, (Fe, Ni)ZBC6is fairly common, replacing kamacite in both octahedral and pearlitic areas of San Cristobal. The former occurrence is similar to that observed in Freda (IIID, 23 per cent Ni) though both cohenite and haxonite were observed in the matrix of Freda (SCOTT,1971a). The latter replacement (Fig. 1) is similar to that in group IA pearlitic plessite (SCOTT,1971b). Some haxonite and a few tiny grains of cohenite were found in the swathing kamacite around nodules in San Cristobal. electron-microprobe analyses of 10 grains of matrix haxonite showed a relatively small range of Ni (5-11f 0.11per cent) and Co (0.27j, 0.06per cent), contents typical of grains in most octahedrites. The cohenite, however, showed a low Ni content of 1.2 per cent; a value observed only in hexahedrites (SCOTT,1971a).
San Cristobal’s nodules contain chondritic silicates (olivine, orthopyroxene and feldspar), troilite, graphite and the phosphate brianite, Na~~aMg(PO*)~. Successive but incomplete layers of troilite, scribers and kamacite partially coat the nodules. Brianite has previously been found in only two meteorites, Dayton (FUCHSet al., 1967) and Youndegin (FUCHS,1969), members of groups IIID and IA, respectively. The olivine and orthopyroxene grains are anhedral to subhedral, about 50-500 pm across and have a granular texture (Fig. 2). Similar sized feldspar grains are interstitial, occasionally enclosing small olivine or pyroxene grains. * The Ni contents and chemical groups of irons which are quoted without referenceswere determined by Wasson and coworkers; see SCOTTetal.(1973) and papers listed therein.
Fig. 1. Pcarlitio type of kamaoite oxsolulion in Sari Cristobal. Irregulnr karnacite lamellae set in ligbtor Ltlunitc;in the lower half of the picture have been occasionally and the replaced by hraxonite (l?e, Ni)&& (h). Between these COR~SR lamellee usual ataxite matrix (ahove) is a, hleck atnhing zone of finer subprtraliel pearlitic kamacite. Scale bar: 200 ,am. Fig, 2. Photomicrograph of part of a silioate nodule in San Cristobal. Some representative grains are labeled: feldspar (f), dark gray; brianite (b); &vine (o) nnd pyroxene (p), both light gray. Metal and troilite appear white and cracks and holoo black. Tbo rectangular box- contains the area with K-rich alkali feldspar and antipertbito cxsolution shown in Fig. 3c. Scale bar: 50 pm.
13Ro
Fig. 3. X-Ray scanning pictures showing two different types of exsolution in feldspar grains from San Cristobal. (a) Na K, and (b) Ca K, pictures from the same area show peristerite unmixing in plagioclase feldspar. The Na-rich blebs have a composition -Ans, and the matrix Anr,. (c) K K, picture of K-rich laths in the alkali feldspar grain in Fig. 2. An antiperthite structure has been observed in Kodaikanal (BUNCH and OLSEN, 1968) but this appears to be the first meteoritic occurrence of peristerite unmixing. Grid mesh: 10 ,um.
1381
Structnre and formation of the San Cristobd meteorite
Silicate compositions measured in one silicate-rich nodule are summarized in Table 1; the electron-probe technique and brianite analysis are described by BILD (1974). Olivine compositions in 18 grains ranged from Fa,., to Fa$.,. (All compositions are given in mole per cent.) A similar number of orthopyroxene grains averaged Fs,., (Fs,.&E’e,.,), with Ca contents of Wo,.,. Bulk analyses of 15 feldspar grains averaged Ab,,An,Or,. The similarity of these silicate compositions to those in group IAB (Table 1) is discussed in a later section. Although the bulk feldspar composition is typical of LAB irons, two types of unmixing have been seen which have not been previously reported in these irons. X-Ray scanning pictures (Figs. 3a, b) of a grain averaging Ab,, show peristerite exsolution ; albite blebs up to 3 x 20 pm are roughly oriented in a more Ca-rich matrix. Terrestrial peristerites form by low temperature exsolution of two phases with compositions An,_, and An,,_,, from plagio~l~ with a bulk ~rnpo~tio~ An,_,, (DEER et at., 1963; FLEETand RIBBE, 1966, and references therein). Compositions of blebs and matrix in San Cristobal are about An, and An14, approaching the terrestrial values. Although the values of An, determined for the Ca-poor phase may be high because of the small size of the albite blebs, the An,, value is signiflctantly lower than the corresponding terrestrial values. This could be a kinetic effect due to the lack of an aqueous phase during exsolution. Because of the small amount of material available, we were unable to confirm that both phases were in the low temperature structnral states reported for terrestrial peristerites (DEERet at., 1963). Exsolution lame&e in the plagioclase of other IAB irons were not reported by Bor;ic~ et al. (1970), but it is possible that they were present but not resolved by their electron probe. X-Ray studies of plagioclase in other meteorites (e.g. VAN SCHMUS and RIBBE, 1968, and FUCHSet at., 1967; on chondrites and Dayton, respectively) found a single phase in the high or ~gh-interme~ate structural state. Some of the plagioclase has adjacent alkali feldspar grains which show another unmixing effect. A K K, scanning picture (Fig. 30) from a grain in Fig. 2 shows an Teble 1. Average silicate compositions (mole %) in San Cristobal and Dayton and composition ranges for IAB and IIE irons
Meteorite
OrthoOlivine pyroxene Fa Fs
San Cristobal (IB)
3.3
IAB Irons
l-8
Dayton (IIID) IIE Irons (=~~eekeroo-tee)
14-32
6.9 4-9
Ab
Plagioclaee An Or
88 & 2
9 & 2 2 f 1
76-87
9-22
12&l
96f2
2+22&l
14-25
82-93
2-14
K-rich feldspar Ab An Or -50
<2 -507
1+-&l
$ -
4-6
65-82 1-9 16-44 9-10 O-I 89-915
a. This work d. BUNCHet al. (1972) b. BUNCHand OLSEN(1968) e. RAMBALDIet al. (1974) c. BUNCHet al. (1970) f. WA~SEX%BURQ et al. (1968). t Maximum Or content observed. 2 K-feldspar is repoti in Odessa (IA) by EL GORESY (1967). $ Colomera contains two K-rich feldspsrs.
* Sources:
Source* a o,d,e a b,o,f b,f
1382
EDWARD
R. D. SCOTT end
RICHARD W. BILD
antiperthite structure of K-rich lamellae (about Ab,,,An,,Or,,) up to 3 x 50 pm in a more Na-rich matrix. Such K-rich grains are sufficiently small and rare that they barely affect the average feldspar composition of Ab,, noted above. Potassium-rich feldspar is rare in meteorites, observed only in several Weekeroo-type irons (recently designated IIE by WASSON, in press) : Colomera, Kodaikanal and Weekeroo Station (BUNCH and OLSEN,1968; WA~SERBERQ et al., 1968). In Kodaikanal, Bunch and Olsen observed a similar antiperthite structure. There is also an unconfirmed report of K-feldspar in a IA member, Odessa (EL GORESY, 1967). COMPOSITION There are published analyses for over 20 trace and minor elements in San Cristobal (Table 2). In addition we determined a Co content from electron-probe analyses of the taenite matrix. Elements are often present in extreme concentrations: the Ag, Cu, In and Sb contents are the highest measured in any iron meteorite, and the contents of As, Ni and Pd are exceeded by less than 2 per cent of analyzed irons. Molybdenum and Pt contents are the lowest recorded in irons, and Ru the third lowest. Of the remaining elements, contents of Ir, Ke and OS are fairly low, and Au, Ga, Ge and Zn fairly high. These analytical data are discussed below. Table 2. Element concentrations* in San Cristobal (ppm except Ni and Co) and their ratio to the group IA mean. Three groupings are suggested: Sb-Co with ratios 1.3-6.5; Ga and Ge; and Mo-Pt with ratios 0.3-<0.06 Element Sb cu In Pd Ni Ag Sn As Au co Ge Ge MO OS Ir Re RLI Pt
San Cristobal 2.1 1000 0.041 136 25.0 % 0.11 13 26.5 2.2 0.61% 11 21 2.2 0.44 0.33 0.024 0.47 <0.5
Source
San Cristobal/IA mean
d,f e,f f f f” d df e,c g
6.5 6 4.1 3.6 3.6 2.7 2.3 2.3 1.4 1.3
e,f e,f
0.15 0.09
f b b,e bd b b
0.30 0.18 0.16 0.12 0.081 <0.06
* Mean values calculated from the following papers: a. BAUERand SCHAUDY(1970) e. SCOTT eta2. (1973) b. CROCHET(1972) f. &VALESet al. (1967) c. Foucti and SXALES (1966) g. This work. d. KIESL et al. (1967) Analyses of Ni, Ga and Ge by (a) and Au and Ir by (d) show greater variability and were excluded.
Structureand formationof the San Ctitobal meteorite
1383
DISCUSSION Groups IAB and IIICD Group I, which originally terminated at Persimmon Creek with 14 per cent Ni, was extended to include San Cristobal by SCOTTet al. (1973), who also revised the group I terminology of WASSON(1970). Those meteorites originally called group I plus those members of category I-An2 with Ge concentrations over 190 ppm now comprise group IA. The remainder of I-An2 form group IB. Although there is no major discontinuity at the boundary, this does divide the IAB sequence into the well populated and fairly homogeneous IA section with Ni contents from 6.1 to 8.8 per cent, and the much rarer IB irons with more diverse compositions and structures forming the high Ni tail. Although assigned to IB, San Cristobal also shares many properties with group IIICD. WASSONand SC~UDY (1971) defined groups IIIC (lo-13 per cent Ni) and IIID (16-23 per cent Ni) and concluded that they were probably related. Supporting evidence came from a survey of analytical data by SCOTT(1972). (His group IIIC included two meteorites with 17 per cent Ni which were assigned to IIID by Wasson and Schaudy. We follow the latter authors’ assignment.) Thus the letters C and D merely designate opposite ends of the sequence IIICD. Both groups IB and IIICD are small, with only eight and 12 members, respectively (WASSON,in press). In this section we compare the mineralogy and chemistry of San Cristobal with that of groups IAB and IIICD; firstly to explain the classification of San Cristobal and secondly to show that IIICD shares many of the unusual features of IAB. (a) Silicates. The silicate-graphite-troilite nodules found in San Cristobal are almost the hallmark of IAB irons, and the average compositions of the silicates (Table 1) lie within the ranges measured in this group by other authors. Although diopside was also commonly found by BUNCHet al. (1970) in their IAB inclusions, we did not observe it in the San Cristobal nodule. The presence of K-rich feldspar might suggest that San Cristobal is related to group IIE, but its growth may have been caused by the incorporation of much Na (and Ca) into brianite. Clearly San Cristobal’s olivine and orthopyroxene compositions are outside the IIE range (Table 1). Group IIICD lacks similar nodules though silicates have been observed in one IIID member, Dayton (FUCHSet al., 1967). Our examination of nodules in a section from the National Museum of Natural History (No. 1592) revealed euhedral or subhedral orthopyroxene and albite grains 0.1-0.5 mm in diameter, in a phosphate matrix. Table 1 shows that the compositions of these silicates, En,,Fs,,Wo,., and Ab,,, respectively, lie outside the IAB and IIE ranges. (The value of En, reported by Fuchs et aZ. could not be reproduced.) A further distinction of the Dayton nodules is the presence of silica grains and, as observed by Fuchs et al., the absence of olivine. (b) High C content. The occurrence of graphite, cohenite, haxonite, and indirectly the presence of pearlitie kamacite exsolution all indicate a high C content in San Ckistobal. Group IAB irons almost invariably show most of these characteristics. Groups IIICD and IIIE are the only other groups that can match the high abundance of carbides (SCOTTand AQRELL,1971; SCOTTet ab., 1973), but group IIIE (which has
1384
R. D. SC-
EDWARD
and RICHARDw. BrLI)
a narrow range of Ni con~n~S.2-8.9 per cent) shows no other ~rn~a~~~ to San Cristobal. Graphite has been seen in two IIID members; Dayton (FUCHSet c&, 1967) which also shows pearlitio kamacite exsolution, and Freda (SCOTT,1971a). (c) B&z&e. As noted above, this phosphate has only been reported previously in Dayton (DID) and Youndegin (IA). However, the other phosphates seen in Dayton by FUCHSet al. (1967), whitlockite and panethite, were not observed in San Gristobal. (d) Taenite grain size. The small, 2-3 cm, taenite grain size in San Gristobal is typical of the precursory grain size observed in some other silicate-rioh IB irons such as Four Corners (PERRY, 1944, Plate 3). Grain boundaries in the parent taenite are generally absent in groups other than IAB and IIE, though BUCH~ALDin press has observed some in two group IIIC members, Carlton and Edmonton (KY). (e) ~u~~~~~. Figure 4 shows a loga~thmic plot of kamacite band~dth (measurements by Buchwald taken from WASSOW,1970, in press) against Ni (analyses by Wasson and coworkers; SCOTTet aE., 1973, and references therein). Group IA oan be extrapolated through IB to include San Cristobal without difficulty. However, IIICD members also lie on this trend, so that this plot does not distinguish a preference for one of these two groups, though it does stress yet another similarity between them. The trace and minor element concentrations in San Cristobal (Table 2) were compared with those of the chemical groups using the graphs and data compilation
6
8
IO 12
16 20
25
Nickel, #
Fig. 4. Logarithmic plot of bandwidth against bulk Ni oontent for members of groups IAB and IIICD. San Crietobal, with a bandwidth of 0.01 mm, can be included in an extrctpolationof IA through IB. Group IIICD members also fall inside or close to this extrapolation, emphasizing the similarities between these two groups. Diagonal dashed lines are isocooling curvea (SEORTand GOLDSTEM, 1907). Groups IA.B and IIICD both plot withinexperimenttderrorofa2’K/Myrline, consistent with burial at comparable depths, if not in cores, in their respective parent bodies. Other evidence (see text) indicates that members of these groups formed isolated iron raisins rather than COW.
1386
Structure and formation of the San Cristobal meteorite
o I Scott (1972).
References can be found in that paper. Like the mineralogical evidence discussed above, the compositional data show that San Cristobal might be related to either group IAR or IIICD. However, on balance, the former is detitely preferred. Although group IAB must be extrapolated further than IIICD to reach San Cristobal, the latter group has only three well analyzed members, all with less than 17 per cent Ni, so that its trends are often more poorly defined than those in IAB. Analytical data for San Cristobal can be divided into three sections according to genetic significance. (a) Elementsfavoring both IAB and IIICD. In plots of As, Co, C-u, Pd and Sb against Ni, group IAB shows a positive correlation. Both San Cristobal and the limited IIICD data lie on or close to extrapolations of these trends to higher Ni contents. The high Au and Zn contents would match the abundances shown by IAB and the few IIICD values. The fractionation of all these elements against Ni in groups IAB and IIICD tends to be less severe than in other groups. (b) Elemelztsfavoring IIICD. Ruthenium and Pt have especially low concentrations (~0.5 ppm) in both San Cristobal and the three analyzed IIICD members, whilst group IA contents are ten times higher. The few available MO analyses indicate that IIICD is closer than IAB to the low abundance in San Cristobal. (c) Elements favoring IAB. The steady decrease in Ga, Ge (Fig. 5) and Ir (SCOTT et al., 1973, Fig. 6) with increasing Ni through the IAB sequence can easily be extended to include San Cristobal. On these plots, group IIICD would have to be grossly enlarged to include San Cristobal. Unlike the elements favoring IIICD, Ga, Ge and Ir have been measured in all known members of IAB and IIICD (WASSON, ’
1
1
Nickel, %
’
’
’
I
‘-
Wicka\ K
Fig. 6. Logarithmic plots of (a) Ga and (b) Ge against Ni for members of groups IAB and IIICD. San Cristobal, with 25 per cent Ni, fits extrapolations of the IAl3 sequenceto higherNi and lower Ga and Ge contents. Group IIICD, like IAB, shows negative oorrelationein both diagrams, which could be extrapolated back to the cosmic Ga/Ni and Ge/Ni ratios (shown by the straight dashed lines) in group IA. Lying on this extrapolationare two possiblelow Ni IIICD candidates: Mundrabilla and Waterville (X).
SCOTT and RICHAZD W. BILD
EDWARD R. Il.
1386
1
‘
t
I
,
I
1
‘
I.‘,
I
I
I
I
0
”
0
0
‘g: c *
R
I-
x
8
c!
x
’
”
x 0
O
0
0
0
0
x
0
x
%
x
5
-s
-
8
u 0
x
u
P
x ‘San C~i~:o~l
%
g <
0.01
0
x
x
x
x
0 Group IA 0 Group IIICO I I I I I I I I, I I ,t,PP As Sb Sn Go Au Pd Cu Ge Co Rh Pt Ru Ir Ho Re OS
Fig. 6. Ratio of element ooncentration/Ni relative to Cl chondrites for San Cristobal and groups IA and IIICD. The elements are arranged in increasing order of pure metal condensationtemperatures. Nearly all elements in Group IA are present in their cosmic abundwices, but the refractory elements @h-OS) have ab~d&nce ratios of 0.03 + O-02 in San Cristobal and similarly low values in IIICD. With the exceptions of Ga and Ge, the remainingelementsin San Cristobal and IIICD fall fairly close to Cl abundances. Some elements with apparently low s;bundanoeratios, e.g. Zn (not plotted) and Cu, may be concentreted in the sulfide phase.
1970, in press; WASSON and SOHAUDY,1971). This provides strong evidence for assigning San Cristobal to group IB. lrrctctionatiortprocesses ~e~ha~srns for fractionating elements in iron meteorites were reviewed by SCOTT (1972). It was sugges~d that ~actionation ~~~~~ groups other than IAB was caused by fraotional crystallization in molten cores, whereas the fractionation that eatablished the differences between.groups took place in the solar nebula during or before the accretion of parent bodies. Group IAB shows none of the gross fractionations seen in other major groups, and various other pieces of evidence (e.g. the presence of silicates-WAssoN, 1970) indicate that they have not been (fully) molten. WASSON (1970) and SCOTT (1972) proposed that the metal-silicate fractionation essential for formation of these irons occurred in the solar nebula, and not in the parent bodies. In Table 2 are listed the ratios of element concentrations in San C~stobal to the mean values in group IA. There appears to be a hiatus between one group of elements (Sb-Co) with ratios 645-1~3 and another (Mo-Pt) with ratios O-3--+06. Gallium and Ge values fall in the second range but are separated in Table 2 because, as will be discussed later, their fractionation mechanism differed from that of the other elements. The ratios of San Cristobal’s element abundances to those in Cl ohondrites (both normalized to Ni) are plotted in Fig. 6 with the elements arranged in reverse order of their nebular condensation as pure metals (LARXBZER,1967 ; SCOTT, 1972). Cl abundances are averages of values from MASOX (1971), KRXHENBUHL et aE.
Structureand form&ion of the San Cristobalmeteorite
1387
(1973) and P. A. Baedeoker (private comm~cation}. It can be seen that the refractory elements on the right, plus Ga and Ge, have abnndance ratios of 0.03 f 0,02.The remainder of the elements in San Cristobal, with a few exceptions discussed below, have abundance ratios fairly close to 1. In group IA (Fig. 6) the abundances of most elements are roughly the same as Cl (i.e. mean solar system) abundances (ANDERS,1964; W~SON, 1970; SCOTT,1972). Exceptions are Cu and Sb (both plotted), and Ag, Cr, In and Zn which may be concentrated in sulfide nodules which were avoided during analytical sampling. The first fractionation event experienced by the grains that eventua~y formed San Cristobal would appear to be one which lowered the abundance ratios of the refractories. The inverse correlation of Ni and Ir in group IB, and the more limited data for other refractories, show that the abundance ratios of these elements decrease from about unity in IA to about 0.03 in San Cristobal. This might be understood in terms of a model suggested by STJESS (1969) in which the first metal condensed from the nebula was Ni-poor and refractory-rich, resulting in an inverse correlation between Ni and the refractory metals. Early formed IA grains would have to be removed from this portion of the nebula prior to condensation of IB grains to prevent grain eq~~bration and mixing. But since the vapor pressure (HULT~REXet al., 1963) of Fe exceeds that of Ni [data from NESMEY~OV (1963)show the opposite relationship], the first Fe alloy to form will be enriched in Ni (GROSSMBN, 1972), assuming an ideal solid solution. Although rapid cooling might reduce the enrichment it is still not possible for Ni contents to increase with falling temperatures. We offer two alternative mechanisms. (1) Metal grains that formed IB irons may have condensed at the same time as IA grains but in a different part of the solar nebula, where the refractory elements had already been removed, perhaps into C&Al rich inclusions like those found in Allende (GROSSMAX, 1973). (2) All refractory metals in the nebula may have condensed into high temperature oxides, and group IA metal obtained large amount of ~fractories by later eq~~brating with the nonmetallic fraction in the nebula or parent body, whilst IB metal equilibrated by a much smaller degree. In San Cristobal the remaining elements, aside from Ga and Ge and some with possible ohalcophilic tendencies, have abundance ratios close to 1. It appears that within a factor of ~4, all IAB irons have similar ratios for these elements. In fact, least-squares lines for elements which are positively correlated with Ni in group IA (As, Pd and Sb) have slopes near the Cl ratios (SCOTT,1972). The few well-analyzed IB irons also plot close to these lines. If we attempt to explain these trends with the SUESS(1969) model or any other which condenses grains with diverse Ni contents, the volatile elements must be constrained to condense in amounts propo~ional to the Ni content of the grains. Since this seems unlikely, we prefer the two alternative schemes discussed above. We then require a mechanism for reducing the Fe/Ni ratio from a IA (or Cl) value of 16 to 3 in San Cristobal, following oondensation of the more volatile elements in their Cl proportions. What we need is a mechanism for selective removal of Fe which leaves the abundances of non-refractory elements unchanged. The amount of Fe removed could either be correlated with the amount of refractory depletion (e.g. both might be a function of heliocentric distance in the nebula) or else IB members
1388
EDWARD R. D. SCOTTand RICELARD W. BILD
could be made by mixing proportions of grains having IA and San Cristobal compositions (a two-component model). The removal of Fe could also have occurred either in the nebula after condensation of the more volatile elements, or later in the parent body. In both cases, Fe could have been removed by oxidation and transferral from the metal to a silicate or sulfide phase. Current amounts of silicate and sulfide phases in San Cristobal are far too small to account for the necessary removal of 80 per cent of the metallic Fe from grains with IA Fe/Ni ratios. Furthermore, in group IB there is no obvious correlation between the abundance of these phases and the bulk Ni content of the metal. If the removal was accomplished by transfer of major amounts into the silicates, there is no record preserved in the IAB silicates; Fe content of ferromagnesian minerals (BUNCHet al., 1970) are uniformly low and uncorrelated with bulk Ni contents. Group IIICD, like 10, does not show the large fractionations observed within the other major groups which SCOTT (1972) attributed to fractional crystallization. In Fig. 6, IIICD, like San Cristobal, has approximately cosmic levels of volatile siderophiles and low abundances of refractories. In IIICD, the decrease in Ir with Ni content is more marked than in IB (SCOTTet al., 1973, Fig. 6); the value plotted in Fig. 6 is the geometrical mean of values differing by a factor of 40. Thus, although IIICD seems to have been subjected to the same two processes that operated in IAB, the refractory depletion was much more effective in IIID than in San Cristobal. Earlier sections of this paper have emphasized the mineralogical and structural similarities between members of groups IAB and IIICD which tend to confirm that they formed in neighboring parts of the solar nebula. The distribution of Ga and Ge in IAB and IIICD (Fig. 5) is very different from that in other groups. First, there is a much wider range in IAB and IIICD : opposite ends differ by factors of about 10 and 20 for Ga and Ge, respectively, compared with factors of 1.2 f O-2 and l-7 -+ O-6 in the other groups. Second, no other groups except IIB and IIIB, the high Ni portions of IIAB and IIIAB, respectively, have negative correlations of Ni with Ga or Ge. This is consistent with the theory that the trends in IA.B and IIICD were produced in a nebular process, whereas those in the other groups were produced by a planetary process-fractional crystallization. It would be convenient to associate the Ga and Ge fractionation in IAB and IIICD with the removal of the refractories. However, the much lower condensation temperatures of Ga and Ge and the absence of any Ir-Ge correlation in IA suggest otherwise. The above two-component model for LAB is not favored by the 20 per cent decrease in Ge/Ga ratio through IA alone, since mixing IA material containing 6.5 per cent Ni with San Cristobal metal would produce a variation of about 1 per cent. Figure 5 suggests that the negative correlation of Ni with Ga and Ge in IIICD, like that in IAB, might be extrapolated back to the cosmic Ga/Ni and Ge/Ni ratios present in IA. Possible low Ni IIIC members are Mundrabilla and Waterville (Fig. 5), analyzed by DELAETER(1972) and WASSON(1970, in press). Parent bodies There is conflicting evidence concerning the distribution of the IAB and IIICD irons in their respective parent bodies. Both groups show a narrow range of cooling
Structure and formation of the San Cristobd meteorite
1389
rates caloulated by the SHORT-G•LIMTEIN(1967) method with the aid of WMSON’S (1971) equation (Fig. 4). These indicate a similar depth of burial for all the group members, and in view of the associated errors, are consistent with storage in ‘cores’ in both parent bodies. However, the presence of silicates (admittedly few in IIICD) rules out a fully molten period. Perhaps such cores might be formed without melting by heterogeneous accumulation (TUREKLUand CLARK,1969). It seems more likely that preferential accumulation of iron grains in the solar nebula would produce m-sized iron masses distributed within the parent bodies instead of producing cores. Five cosmic-ray exposure ages for IIICD members (VOS~AUE,1967) give a negative correlation with Ni content which is significant at the 95 per cent level, suggesting that the members least depleted in refractories and poorest in Ni were closest to the surface. This trend is not supported by Fig. 4, where there is no evidence for a cooling-rate decrease with increasing Ni in IIICD. Group IA members show no such trend in cosmic-ray exposure ages, but no data on IB irons were reported by Voshage. Although IA bandwidth-Ni data would be consistent with an inverse correlation of Ni and cooling rate, kamacite plate impingement in low Ni irons (SHORTand GOLDSTEIN,1967) might also be responsible for the observed trend. Aek~ow~g~~s-We are indebted to J. T. WASSON for valuable ~so~sio~ and criticisms of the manuscript, and to V. F. BUCRW~, who first suggested that San Cristobal was &IBmember. We warmly thank R. SCHAUDY and W. KIESL for supplying the silicate nodule, and R. S. CLARKE and R. HUTCHISONfor the loan of samples. We are grateful to R. E. JONES, J. F. KAWMAN and N. SVETICHfor technical assistance, UCLA Department of Geology and Cambridge University Department of Mineralogy and Petrology for the use of electron-probe analy~ zers. This work was supported by NASA grant NGR 05-007-320 and NSF grant GA-32084. A part of the first author’s early work for this paper was supported from a Natural Environment Research Council grant to S. 0. Agrell at Cambridge. REFERENCES Axnnn~ E. (1064) Origin, age and composition of meteorites. Space Std. Rset. &583-714. Axoa II. J. and SMITE P. L. (1972) Met~o~aphio study of some iron meteorites of high nickel content. Illineral. &lag. 88, 736-755. BAUER R. and SCEAUDYR. (1970) Activation analytical determination of elements in meteorites, 3. Determination of manganese, sodium, gallium, germanium, copper, and gold in 21 iron meteorites and 2 mesosiderites. C&em. Ueol. 6, 119-131. BILD R. W. (1974) New occurrences of phosphates in iron meteorites. Contrib. Mineral. Petrol. 46, 91-98. BRENTNA~LW. D. and AXON H. J. (1962) The response of Canyon Diablo meteorite to heat treatment. J. IFOYX Steel Inst. 200,947-955. BUOH~A.L~ V. F. (in press) Iron Metewitm. Arizona State University. BUNCH T. 1. and OLS~ E. (1968) Potassium feldspar in Weekeroo Station, Kodaikanal, and Colomera iron meteo~t~. Se&es 180,1223-1225. Bnncs T. E., KEIL K. and OLSEN E. (1970) Mineralogy and petrology of silicate inclusions in iron meteorites. Chttib. Mined. Fe&&. 25,297-340. BUNCH T. E., KEIL K. and Huss G. I. (1972) Landes meteorite. Meteor&s 7,31-38. CROCK J. H. (1972) Some aspects of the geochemistry of Ru, OS, Ir and Pt in iron meteorites. Beo&m. Oosmochim. Acta 86,617~635. DEER W. A., HOWIE R. A. and ZTSSSMMA.N J. (1963) Rock-B’ovming MineTab Vol. 4. Framework Silica&a, 435 pp Wiley. DELAETE~ J. R. (1972) The Mundrabilla meteorite shower. Metit& 7,285-294. EL GOREBYA. (1967) Quantitative electron mioroprobe analyses of K-feldspar grains Erom the Odessa iron meteorite. (Abstract) 30th Meeting of the Meteoritical Society, October 25-27.
1390
EDWARD R. D. Scow
and RICIZARD W. BILD
FLEET S. G. and RIBBE P. H. (1965) An electron-microscope study of peristerite plagioclases. Mineral. Mug. 85, 165-176. FOUCH&K. F. and SMALESA. A. (1966) The distribution of gold and rhenium in iron meteorites. Chem. Geol. 1, 329-339. FUCHS L. H. (1969) The phosphate mineralogy of meteorites. In Meteorite Research, (editor P. M. Millman), pp. 683-695. Reidel. FUCHS L. H., OLSEN E. and HENDERSONE. P. (1967) On the occurrence of brianite and panethite, two new phosphate minerals from the Dayton meteorite. aeochim. Cosmochim. Acta 31, 1711-1719. GOLDSTEINJ. I. and OQILVIER. E. (1965) The growth of the Widmanstiitten pattern in metallic meteorites. Geochim. Cosmochim. Acta 29, 893-920. GROSSMANL. (1972) Condensation in the primitive solar nebula. Geochim. Cosmochim. Acta 36, 597-619. GROSSMANL. (1973) Refractory trace elements in Ca-Al-rich inclusions in the Allende meteorite. Ueochim. Cosmochim. Acta 37, 1119-l 140. HULT~REN R., ORR R. L., ANDERSON P. D. and KELLEY K. K. (1963) Selected Values of Thermodynamic Properties of Metals and Alloys, 963 pp. Wiley. KIESL W., SEITNER H., KLUUER F. and HECHT F. (1967) Determination of trace elements by chemical anrdysis and neutron activation in meteorites of the collection of the Viennese Museum of Natural History. Moraatsch Chem. 98, 972-992. KR~HENFJ~HLU., MORUAN J. W., GANAPATHY R. end ANDERS E. (1973) Abundance of 17 trace elements in carbonaceous chondrites. Geochim. Cosmochim. Acta 37, 1353-1370. LARIMER J. W. (1967) Chemical fraction&ions in meteorites-I. Condensation of the elements. Beochim. Oosmochim. Acta 31, 1215-1238. MASON B. (1971) Handbook of Elemental Abundances in Meteorites (edited volume), 555 pp. Gordon & Breach. NESMEYANOVAN. N. (1963) Vqour Pressure of the Elements, (transl. J. I. Carssso), 469 pp. Academic Press. OWNS E. A. and BURNS B. D. (1939) X-Ray study of some meteoric irons. Phil. Mag. (7) 28,497-519. PERRY S. H. (1944) The metallography of meteoric iron. Bull. U.S. Nut. Mu8.184, 206 pp. RAM~ALDI E., JAQOUTZ E. and WASSON J. T. (1974) Bitburg-A group IB iron meteorite with silicate inclusions. Miraeral. Mug. 39, 695-600 SCOTTE. R. D. (1971a) Studies of the structure and composition of iron meteorites. Ph.D. Thesis, Cambridge University. SCOTT E. R. D. (1971b) New carbide (Fe, Ni),,C,, found in iron meteorites. Nature Phys. Sci. 229, 61-62. SCOTTE. R. D. (1972) Chemical fractionation in iron meteorites and its interpretation. Geochim. Cosmochim. Acta 86, 1205-1236. SCOTTE. R. D. and AURELLS. 0. (1971) The occurrence of carbides in iron meteorites. (Abstract) Meteoritice 6, 312-313. SCOTT E. R. D., WASSON J. T. and BUCHWALDV. F. (1973) The chemical classitication of iron meteorites-VII. A reinvestigation of irons with Ge concentrations between 25 and 80 ppm. Beochim. Cosmochim. Acta 37, 1957-1983. &TORT J. M. and GOLDSTEINJ. L. (1967) Rapid methods of determining cooling rates of iron and stony iron meteorites. Science 156, 59-61. &ALES A. A., MUPER D. and FOUCECI? K. F. (1967) The distribution of trace elements in iron meteorites, as determined by neutron activation. Geochim. Cosmochim. Acta 31, 673-720. SUESSH. E. (1969) Nuclear abundance rules and the composition of meteorites. In Meteorite Research, (editor P. M. Millman), pp. 3-6. Reidel. TVREKIAN K. K. and CLARK S. P. (1969) Inhomogeneous accumulation of the earth from the primitive solar nebula. Earth Planet. Sci. Lett. 6, 346-348. VAN SCHMUSW. R. and RIBBE P. H. (1968) The composition and structural state of feldspar from chondritic meteorites. Geochim. Cosmochim. Acta 32, 1327-1342.
1391
Structure and formation of the San Cristobal meteorite VOSHAGE H. (1967) Bestrahlungsalter
und Herkunft der Eisenmeteorite.
2. Nuturforsch. %a,
477-506. WASSERBURG G. J.,
SANZ H. G. and BENCE A. E. (1968) Potassium-feldspar the surface of Colomera, an iron meteorite. Science 161,684-687.
phenocrysts
in
Irons with Ge concenWASSON J. T. (1970) The chemical classification of iron meteorites-IV. trations greater than 190 ppm and other meteorites associated with group I. Icarus 12, 407-423. WASSON J. T. (1971) An equation for the determination of iron-meteorite cooling rates.
Meteoritics 6, 139-147. WASSON J. T. (in press) Metemites-Classification and Properties. Springer. Groups WASSON J. T. and SCHAUDY R. (1971) The chemical classification of iron meteorites-V. IIIC and IIID and other irons with germanium concentrations between 1 and 25 ppm. Icarus 14, 59-70.
Cosmochimica Acts, 19'74,Vol. 38,pp. 1379to 1891. PergamonPress. Printedin Northern Ireland
Structure and formation of the San Cristobal meteorite, other IB irons and group IIICD Department
EDWARD R. D. SCOTT and RICHARD W. BILD of Chemistry and Institute of Geophysics and Planetary Physics, University California, Los Angeles, CA 90024, U.S.A.
of
(Received 26 November 1973; accepted in revised form 11 March 1974) Abstract-San Cristobal is an unusual group IB atexite with 25 per cent Ni, composed of taenite grains 2-3 cm in diameter and silicate-troilite-graphite nodules concentrated on the grain Silicate compositions are typical of group IAB: olivine Far.s, orthopyroxene boundaries. Plagioclase shows peristerite unmixing, previously unrecorded in Fs,., and feldspar Ab,,. meteorites, and occasional K-rich feldspar grams have an unusual antiperthite exsolution. Brianite Ne,CaMg(PO,), and haxonite (Fe, Ni),,C, are common in nodules and matrix, respectively, while cohenite is rare. Part of the matrix contains a pearlitic kamecite precipitate instead of the usual oriented platelets. San Cristobal has extreme concentrations of many elements; e.g. the highest published Ag, Cu, In and Sb contents and the lowest MO and Pt in irons. These data and the mineralogy show that San Cristobal has many characteristics of both groups IB and IIID, but that it fits group IB trends better. Ratios of refractory element abundances to those in Cl chondrites (both normalized to Ni) decrease through IB from 1 in IA to 0.03 in San Cristobal, but the other siderophilic elements have a small range of abundance ratios, 0.5-2, throughout IAB. We suggest that IB grains either formed in a part of the solar nebula where refractories had been previously removed, or else failed to equilibrate with a refractory-rich, high-temperature condensate. After condensation of the volatiles, Fe was partially removed, perhaps by oxidation. Group IIICD seems to have experienced similar fraction&ions. Unlike other iron meteorite groups, neither IAB nor IIICD appears to have been fully molten. INTRODUCTION (1944) called San Cristobal an “extraordinary meteorite” after viewing its structure. The mineralogy and chemistry also show many unusual features, which will be described below. It is a silicate-bearing ataxite with 25 per cent Ni, a value exceeded by only five anomalous irons (WASSON, in press), and has recently been identified as the high-Ni end-member of the group IAB sequence (SCOTT et al., 1973). WASSON (1970) interpreted the properties of group IAB to indicate that these irons were never molten. The absence in IAB of the severe element fractionations observed within other major groups (SCOTT, 1972) emphasized that IAB had formed under quite different conditions. Because of its extreme position in the IAB sequence, it appeared especially important to study San Cristobal in some detail. Through the kind cooperation of Rudolf Schaudy and Wolfgang Kiesl of the University of Vienna we obtained a specimen containing a cm-sized silicate nodule. We report the results of a study by reflected-light microscopy and electron-microprobe analysis of this nodule and other samples of the matrix. A discussion of these and published chemical data show that despite its many unusual properties, San Cristobal fits well-established trends in group IAB and suggests the existence of others. We offer an interpretation of IAB trends and those in group IIICD which appear to have been produced by similar processes. PERRY
1359
1380
EDWAIZD
R. D. SCOTTand
RICHARD W. BILD
MINEFGALOGIY
Matrix
San Cristobal is composed of taenite grains 2-3 cm in diameter with cm-sized troilite-graphite-silicate nod&s concentrated along the grain boundaries. Most of the matrix is typical of a normal ataxite; 10 pm wide kamacite platelets and grains with associated schreibersite grains are set in taenite partly decomposing to martensite (AXONand SMITH,1972; OWENS and BURNS,1939). Some taenite grains contain a zone of pearlitic kamacite precipitation which has a high kamacite content, perhaps ten times higher than the rest of the matrix. PERRY(1944, Plates 29 and 30) correctly identified the phases and termed the pattern ‘pearlitic’ because of its similarity to the morphology of a-Fe and Fe& in pearlite observed in cast irons. The area at the edge of a 5 x 15 mm pearlitic zone is shown in Fig. 1. This structure is not unique to the San Cristobal matrix: that of Dayton (IIID, 17 per cent Ni)* is partly pearlitic and partly that of a finest octahedrite (GOLDSTEIN and O~IL~IE, 1965, Fig. 2). Pearlitic plessite can also be seen in some group IA and IIIC irons. In San Cristobal and Dayton it appears that pea&tic colonies can form and spread rapidly when convenand tional kamacite nucleation is just beginning. With abundant C (BRENTNALL AXON, 1962), pearlitic growth seems to be an effective alternative for the breakdo~ of high Ni taenite. Carbides
Haxonite, (Fe, Ni)ZBC6is fairly common, replacing kamacite in both octahedral and pearlitic areas of San Cristobal. The former occurrence is similar to that observed in Freda (IIID, 23 per cent Ni) though both cohenite and haxonite were observed in the matrix of Freda (SCOTT,1971a). The latter replacement (Fig. 1) is similar to that in group IA pearlitic plessite (SCOTT,1971b). Some haxonite and a few tiny grains of cohenite were found in the swathing kamacite around nodules in San Cristobal. electron-microprobe analyses of 10 grains of matrix haxonite showed a relatively small range of Ni (5-11f 0.11per cent) and Co (0.27j, 0.06per cent), contents typical of grains in most octahedrites. The cohenite, however, showed a low Ni content of 1.2 per cent; a value observed only in hexahedrites (SCOTT,1971a).
San Cristobal’s nodules contain chondritic silicates (olivine, orthopyroxene and feldspar), troilite, graphite and the phosphate brianite, Na~~aMg(PO*)~. Successive but incomplete layers of troilite, scribers and kamacite partially coat the nodules. Brianite has previously been found in only two meteorites, Dayton (FUCHSet al., 1967) and Youndegin (FUCHS,1969), members of groups IIID and IA, respectively. The olivine and orthopyroxene grains are anhedral to subhedral, about 50-500 pm across and have a granular texture (Fig. 2). Similar sized feldspar grains are interstitial, occasionally enclosing small olivine or pyroxene grains. * The Ni contents and chemical groups of irons which are quoted without referenceswere determined by Wasson and coworkers; see SCOTTetal.(1973) and papers listed therein.
Fig. 1. Pcarlitio type of kamaoite oxsolulion in Sari Cristobal. Irregulnr karnacite lamellae set in ligbtor Ltlunitc;in the lower half of the picture have been occasionally and the replaced by hraxonite (l?e, Ni)&& (h). Between these COR~SR lamellee usual ataxite matrix (ahove) is a, hleck atnhing zone of finer subprtraliel pearlitic kamacite. Scale bar: 200 ,am. Fig, 2. Photomicrograph of part of a silioate nodule in San Cristobal. Some representative grains are labeled: feldspar (f), dark gray; brianite (b); &vine (o) nnd pyroxene (p), both light gray. Metal and troilite appear white and cracks and holoo black. Tbo rectangular box- contains the area with K-rich alkali feldspar and antipertbito cxsolution shown in Fig. 3c. Scale bar: 50 pm.
13Ro
Fig. 3. X-Ray scanning pictures showing two different types of exsolution in feldspar grains from San Cristobal. (a) Na K, and (b) Ca K, pictures from the same area show peristerite unmixing in plagioclase feldspar. The Na-rich blebs have a composition -Ans, and the matrix Anr,. (c) K K, picture of K-rich laths in the alkali feldspar grain in Fig. 2. An antiperthite structure has been observed in Kodaikanal (BUNCH and OLSEN, 1968) but this appears to be the first meteoritic occurrence of peristerite unmixing. Grid mesh: 10 ,um.
1381
Structnre and formation of the San Cristobd meteorite
Silicate compositions measured in one silicate-rich nodule are summarized in Table 1; the electron-probe technique and brianite analysis are described by BILD (1974). Olivine compositions in 18 grains ranged from Fa,., to Fa$.,. (All compositions are given in mole per cent.) A similar number of orthopyroxene grains averaged Fs,., (Fs,.&E’e,.,), with Ca contents of Wo,.,. Bulk analyses of 15 feldspar grains averaged Ab,,An,Or,. The similarity of these silicate compositions to those in group IAB (Table 1) is discussed in a later section. Although the bulk feldspar composition is typical of LAB irons, two types of unmixing have been seen which have not been previously reported in these irons. X-Ray scanning pictures (Figs. 3a, b) of a grain averaging Ab,, show peristerite exsolution ; albite blebs up to 3 x 20 pm are roughly oriented in a more Ca-rich matrix. Terrestrial peristerites form by low temperature exsolution of two phases with compositions An,_, and An,,_,, from plagio~l~ with a bulk ~rnpo~tio~ An,_,, (DEER et at., 1963; FLEETand RIBBE, 1966, and references therein). Compositions of blebs and matrix in San Cristobal are about An, and An14, approaching the terrestrial values. Although the values of An, determined for the Ca-poor phase may be high because of the small size of the albite blebs, the An,, value is signiflctantly lower than the corresponding terrestrial values. This could be a kinetic effect due to the lack of an aqueous phase during exsolution. Because of the small amount of material available, we were unable to confirm that both phases were in the low temperature structnral states reported for terrestrial peristerites (DEERet at., 1963). Exsolution lame&e in the plagioclase of other IAB irons were not reported by Bor;ic~ et al. (1970), but it is possible that they were present but not resolved by their electron probe. X-Ray studies of plagioclase in other meteorites (e.g. VAN SCHMUS and RIBBE, 1968, and FUCHSet at., 1967; on chondrites and Dayton, respectively) found a single phase in the high or ~gh-interme~ate structural state. Some of the plagioclase has adjacent alkali feldspar grains which show another unmixing effect. A K K, scanning picture (Fig. 30) from a grain in Fig. 2 shows an Teble 1. Average silicate compositions (mole %) in San Cristobal and Dayton and composition ranges for IAB and IIE irons
Meteorite
OrthoOlivine pyroxene Fa Fs
San Cristobal (IB)
3.3
IAB Irons
l-8
Dayton (IIID) IIE Irons (=~~eekeroo-tee)
14-32
6.9 4-9
Ab
Plagioclaee An Or
88 & 2
9 & 2 2 f 1
76-87
9-22
12&l
96f2
2+22&l
14-25
82-93
2-14
K-rich feldspar Ab An Or -50
<2 -507
1+-&l
$ -
4-6
65-82 1-9 16-44 9-10 O-I 89-915
a. This work d. BUNCHet al. (1972) b. BUNCHand OLSEN(1968) e. RAMBALDIet al. (1974) c. BUNCHet al. (1970) f. WA~SEX%BURQ et al. (1968). t Maximum Or content observed. 2 K-feldspar is repoti in Odessa (IA) by EL GORESY (1967). $ Colomera contains two K-rich feldspsrs.
* Sources:
Source* a o,d,e a b,o,f b,f
1382
EDWARD
R. D. SCOTT end
RICHARD W. BILD
antiperthite structure of K-rich lamellae (about Ab,,,An,,Or,,) up to 3 x 50 pm in a more Na-rich matrix. Such K-rich grains are sufficiently small and rare that they barely affect the average feldspar composition of Ab,, noted above. Potassium-rich feldspar is rare in meteorites, observed only in several Weekeroo-type irons (recently designated IIE by WASSON, in press) : Colomera, Kodaikanal and Weekeroo Station (BUNCH and OLSEN,1968; WA~SERBERQ et al., 1968). In Kodaikanal, Bunch and Olsen observed a similar antiperthite structure. There is also an unconfirmed report of K-feldspar in a IA member, Odessa (EL GORESY, 1967). COMPOSITION There are published analyses for over 20 trace and minor elements in San Cristobal (Table 2). In addition we determined a Co content from electron-probe analyses of the taenite matrix. Elements are often present in extreme concentrations: the Ag, Cu, In and Sb contents are the highest measured in any iron meteorite, and the contents of As, Ni and Pd are exceeded by less than 2 per cent of analyzed irons. Molybdenum and Pt contents are the lowest recorded in irons, and Ru the third lowest. Of the remaining elements, contents of Ir, Ke and OS are fairly low, and Au, Ga, Ge and Zn fairly high. These analytical data are discussed below. Table 2. Element concentrations* in San Cristobal (ppm except Ni and Co) and their ratio to the group IA mean. Three groupings are suggested: Sb-Co with ratios 1.3-6.5; Ga and Ge; and Mo-Pt with ratios 0.3-<0.06 Element Sb cu In Pd Ni Ag Sn As Au co Ge Ge MO OS Ir Re RLI Pt
San Cristobal 2.1 1000 0.041 136 25.0 % 0.11 13 26.5 2.2 0.61% 11 21 2.2 0.44 0.33 0.024 0.47 <0.5
Source
San Cristobal/IA mean
d,f e,f f f f” d df e,c g
6.5 6 4.1 3.6 3.6 2.7 2.3 2.3 1.4 1.3
e,f e,f
0.15 0.09
f b b,e bd b b
0.30 0.18 0.16 0.12 0.081 <0.06
* Mean values calculated from the following papers: a. BAUERand SCHAUDY(1970) e. SCOTT eta2. (1973) b. CROCHET(1972) f. &VALESet al. (1967) c. Foucti and SXALES (1966) g. This work. d. KIESL et al. (1967) Analyses of Ni, Ga and Ge by (a) and Au and Ir by (d) show greater variability and were excluded.
Structureand formationof the San Ctitobal meteorite
1383
DISCUSSION Groups IAB and IIICD Group I, which originally terminated at Persimmon Creek with 14 per cent Ni, was extended to include San Cristobal by SCOTTet al. (1973), who also revised the group I terminology of WASSON(1970). Those meteorites originally called group I plus those members of category I-An2 with Ge concentrations over 190 ppm now comprise group IA. The remainder of I-An2 form group IB. Although there is no major discontinuity at the boundary, this does divide the IAB sequence into the well populated and fairly homogeneous IA section with Ni contents from 6.1 to 8.8 per cent, and the much rarer IB irons with more diverse compositions and structures forming the high Ni tail. Although assigned to IB, San Cristobal also shares many properties with group IIICD. WASSONand SC~UDY (1971) defined groups IIIC (lo-13 per cent Ni) and IIID (16-23 per cent Ni) and concluded that they were probably related. Supporting evidence came from a survey of analytical data by SCOTT(1972). (His group IIIC included two meteorites with 17 per cent Ni which were assigned to IIID by Wasson and Schaudy. We follow the latter authors’ assignment.) Thus the letters C and D merely designate opposite ends of the sequence IIICD. Both groups IB and IIICD are small, with only eight and 12 members, respectively (WASSON,in press). In this section we compare the mineralogy and chemistry of San Cristobal with that of groups IAB and IIICD; firstly to explain the classification of San Cristobal and secondly to show that IIICD shares many of the unusual features of IAB. (a) Silicates. The silicate-graphite-troilite nodules found in San Cristobal are almost the hallmark of IAB irons, and the average compositions of the silicates (Table 1) lie within the ranges measured in this group by other authors. Although diopside was also commonly found by BUNCHet al. (1970) in their IAB inclusions, we did not observe it in the San Cristobal nodule. The presence of K-rich feldspar might suggest that San Cristobal is related to group IIE, but its growth may have been caused by the incorporation of much Na (and Ca) into brianite. Clearly San Cristobal’s olivine and orthopyroxene compositions are outside the IIE range (Table 1). Group IIICD lacks similar nodules though silicates have been observed in one IIID member, Dayton (FUCHSet al., 1967). Our examination of nodules in a section from the National Museum of Natural History (No. 1592) revealed euhedral or subhedral orthopyroxene and albite grains 0.1-0.5 mm in diameter, in a phosphate matrix. Table 1 shows that the compositions of these silicates, En,,Fs,,Wo,., and Ab,,, respectively, lie outside the IAB and IIE ranges. (The value of En, reported by Fuchs et aZ. could not be reproduced.) A further distinction of the Dayton nodules is the presence of silica grains and, as observed by Fuchs et al., the absence of olivine. (b) High C content. The occurrence of graphite, cohenite, haxonite, and indirectly the presence of pearlitie kamacite exsolution all indicate a high C content in San Ckistobal. Group IAB irons almost invariably show most of these characteristics. Groups IIICD and IIIE are the only other groups that can match the high abundance of carbides (SCOTTand AQRELL,1971; SCOTTet ab., 1973), but group IIIE (which has
1384
R. D. SC-
EDWARD
and RICHARDw. BrLI)
a narrow range of Ni con~n~S.2-8.9 per cent) shows no other ~rn~a~~~ to San Cristobal. Graphite has been seen in two IIID members; Dayton (FUCHSet c&, 1967) which also shows pearlitio kamacite exsolution, and Freda (SCOTT,1971a). (c) B&z&e. As noted above, this phosphate has only been reported previously in Dayton (DID) and Youndegin (IA). However, the other phosphates seen in Dayton by FUCHSet al. (1967), whitlockite and panethite, were not observed in San Gristobal. (d) Taenite grain size. The small, 2-3 cm, taenite grain size in San Gristobal is typical of the precursory grain size observed in some other silicate-rioh IB irons such as Four Corners (PERRY, 1944, Plate 3). Grain boundaries in the parent taenite are generally absent in groups other than IAB and IIE, though BUCH~ALDin press has observed some in two group IIIC members, Carlton and Edmonton (KY). (e) ~u~~~~~. Figure 4 shows a loga~thmic plot of kamacite band~dth (measurements by Buchwald taken from WASSOW,1970, in press) against Ni (analyses by Wasson and coworkers; SCOTTet aE., 1973, and references therein). Group IA oan be extrapolated through IB to include San Cristobal without difficulty. However, IIICD members also lie on this trend, so that this plot does not distinguish a preference for one of these two groups, though it does stress yet another similarity between them. The trace and minor element concentrations in San Cristobal (Table 2) were compared with those of the chemical groups using the graphs and data compilation
6
8
IO 12
16 20
25
Nickel, #
Fig. 4. Logarithmic plot of bandwidth against bulk Ni oontent for members of groups IAB and IIICD. San Crietobal, with a bandwidth of 0.01 mm, can be included in an extrctpolationof IA through IB. Group IIICD members also fall inside or close to this extrapolation, emphasizing the similarities between these two groups. Diagonal dashed lines are isocooling curvea (SEORTand GOLDSTEM, 1907). Groups IA.B and IIICD both plot withinexperimenttderrorofa2’K/Myrline, consistent with burial at comparable depths, if not in cores, in their respective parent bodies. Other evidence (see text) indicates that members of these groups formed isolated iron raisins rather than COW.
1386
Structure and formation of the San Cristobal meteorite
o I Scott (1972).
References can be found in that paper. Like the mineralogical evidence discussed above, the compositional data show that San Cristobal might be related to either group IAR or IIICD. However, on balance, the former is detitely preferred. Although group IAB must be extrapolated further than IIICD to reach San Cristobal, the latter group has only three well analyzed members, all with less than 17 per cent Ni, so that its trends are often more poorly defined than those in IAB. Analytical data for San Cristobal can be divided into three sections according to genetic significance. (a) Elementsfavoring both IAB and IIICD. In plots of As, Co, C-u, Pd and Sb against Ni, group IAB shows a positive correlation. Both San Cristobal and the limited IIICD data lie on or close to extrapolations of these trends to higher Ni contents. The high Au and Zn contents would match the abundances shown by IAB and the few IIICD values. The fractionation of all these elements against Ni in groups IAB and IIICD tends to be less severe than in other groups. (b) Elemelztsfavoring IIICD. Ruthenium and Pt have especially low concentrations (~0.5 ppm) in both San Cristobal and the three analyzed IIICD members, whilst group IA contents are ten times higher. The few available MO analyses indicate that IIICD is closer than IAB to the low abundance in San Cristobal. (c) Elements favoring IAB. The steady decrease in Ga, Ge (Fig. 5) and Ir (SCOTT et al., 1973, Fig. 6) with increasing Ni through the IAB sequence can easily be extended to include San Cristobal. On these plots, group IIICD would have to be grossly enlarged to include San Cristobal. Unlike the elements favoring IIICD, Ga, Ge and Ir have been measured in all known members of IAB and IIICD (WASSON, ’
1
1
Nickel, %
’
’
’
I
‘-
Wicka\ K
Fig. 6. Logarithmic plots of (a) Ga and (b) Ge against Ni for members of groups IAB and IIICD. San Cristobal, with 25 per cent Ni, fits extrapolations of the IAl3 sequenceto higherNi and lower Ga and Ge contents. Group IIICD, like IAB, shows negative oorrelationein both diagrams, which could be extrapolated back to the cosmic Ga/Ni and Ge/Ni ratios (shown by the straight dashed lines) in group IA. Lying on this extrapolationare two possiblelow Ni IIICD candidates: Mundrabilla and Waterville (X).
SCOTT and RICHAZD W. BILD
EDWARD R. Il.
1386
1
‘
t
I
,
I
1
‘
I.‘,
I
I
I
I
0
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0
0
‘g: c *
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8
c!
x
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x 0
O
0
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x
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x
%
x
5
-s
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8
u 0
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0
x
x
x
x
0 Group IA 0 Group IIICO I I I I I I I I, I I ,t,PP As Sb Sn Go Au Pd Cu Ge Co Rh Pt Ru Ir Ho Re OS
Fig. 6. Ratio of element ooncentration/Ni relative to Cl chondrites for San Cristobal and groups IA and IIICD. The elements are arranged in increasing order of pure metal condensationtemperatures. Nearly all elements in Group IA are present in their cosmic abundwices, but the refractory elements @h-OS) have ab~d&nce ratios of 0.03 + O-02 in San Cristobal and similarly low values in IIICD. With the exceptions of Ga and Ge, the remainingelementsin San Cristobal and IIICD fall fairly close to Cl abundances. Some elements with apparently low s;bundanoeratios, e.g. Zn (not plotted) and Cu, may be concentreted in the sulfide phase.
1970, in press; WASSON and SOHAUDY,1971). This provides strong evidence for assigning San Cristobal to group IB. lrrctctionatiortprocesses ~e~ha~srns for fractionating elements in iron meteorites were reviewed by SCOTT (1972). It was sugges~d that ~actionation ~~~~~ groups other than IAB was caused by fraotional crystallization in molten cores, whereas the fractionation that eatablished the differences between.groups took place in the solar nebula during or before the accretion of parent bodies. Group IAB shows none of the gross fractionations seen in other major groups, and various other pieces of evidence (e.g. the presence of silicates-WAssoN, 1970) indicate that they have not been (fully) molten. WASSON (1970) and SCOTT (1972) proposed that the metal-silicate fractionation essential for formation of these irons occurred in the solar nebula, and not in the parent bodies. In Table 2 are listed the ratios of element concentrations in San C~stobal to the mean values in group IA. There appears to be a hiatus between one group of elements (Sb-Co) with ratios 645-1~3 and another (Mo-Pt) with ratios O-3--+06. Gallium and Ge values fall in the second range but are separated in Table 2 because, as will be discussed later, their fractionation mechanism differed from that of the other elements. The ratios of San Cristobal’s element abundances to those in Cl ohondrites (both normalized to Ni) are plotted in Fig. 6 with the elements arranged in reverse order of their nebular condensation as pure metals (LARXBZER,1967 ; SCOTT, 1972). Cl abundances are averages of values from MASOX (1971), KRXHENBUHL et aE.
Structureand form&ion of the San Cristobalmeteorite
1387
(1973) and P. A. Baedeoker (private comm~cation}. It can be seen that the refractory elements on the right, plus Ga and Ge, have abnndance ratios of 0.03 f 0,02.The remainder of the elements in San Cristobal, with a few exceptions discussed below, have abundance ratios fairly close to 1. In group IA (Fig. 6) the abundances of most elements are roughly the same as Cl (i.e. mean solar system) abundances (ANDERS,1964; W~SON, 1970; SCOTT,1972). Exceptions are Cu and Sb (both plotted), and Ag, Cr, In and Zn which may be concentrated in sulfide nodules which were avoided during analytical sampling. The first fractionation event experienced by the grains that eventua~y formed San Cristobal would appear to be one which lowered the abundance ratios of the refractories. The inverse correlation of Ni and Ir in group IB, and the more limited data for other refractories, show that the abundance ratios of these elements decrease from about unity in IA to about 0.03 in San Cristobal. This might be understood in terms of a model suggested by STJESS (1969) in which the first metal condensed from the nebula was Ni-poor and refractory-rich, resulting in an inverse correlation between Ni and the refractory metals. Early formed IA grains would have to be removed from this portion of the nebula prior to condensation of IB grains to prevent grain eq~~bration and mixing. But since the vapor pressure (HULT~REXet al., 1963) of Fe exceeds that of Ni [data from NESMEY~OV (1963)show the opposite relationship], the first Fe alloy to form will be enriched in Ni (GROSSMBN, 1972), assuming an ideal solid solution. Although rapid cooling might reduce the enrichment it is still not possible for Ni contents to increase with falling temperatures. We offer two alternative mechanisms. (1) Metal grains that formed IB irons may have condensed at the same time as IA grains but in a different part of the solar nebula, where the refractory elements had already been removed, perhaps into C&Al rich inclusions like those found in Allende (GROSSMAX, 1973). (2) All refractory metals in the nebula may have condensed into high temperature oxides, and group IA metal obtained large amount of ~fractories by later eq~~brating with the nonmetallic fraction in the nebula or parent body, whilst IB metal equilibrated by a much smaller degree. In San Cristobal the remaining elements, aside from Ga and Ge and some with possible ohalcophilic tendencies, have abundance ratios close to 1. It appears that within a factor of ~4, all IAB irons have similar ratios for these elements. In fact, least-squares lines for elements which are positively correlated with Ni in group IA (As, Pd and Sb) have slopes near the Cl ratios (SCOTT,1972). The few well-analyzed IB irons also plot close to these lines. If we attempt to explain these trends with the SUESS(1969) model or any other which condenses grains with diverse Ni contents, the volatile elements must be constrained to condense in amounts propo~ional to the Ni content of the grains. Since this seems unlikely, we prefer the two alternative schemes discussed above. We then require a mechanism for reducing the Fe/Ni ratio from a IA (or Cl) value of 16 to 3 in San Cristobal, following oondensation of the more volatile elements in their Cl proportions. What we need is a mechanism for selective removal of Fe which leaves the abundances of non-refractory elements unchanged. The amount of Fe removed could either be correlated with the amount of refractory depletion (e.g. both might be a function of heliocentric distance in the nebula) or else IB members
1388
EDWARD R. D. SCOTTand RICELARD W. BILD
could be made by mixing proportions of grains having IA and San Cristobal compositions (a two-component model). The removal of Fe could also have occurred either in the nebula after condensation of the more volatile elements, or later in the parent body. In both cases, Fe could have been removed by oxidation and transferral from the metal to a silicate or sulfide phase. Current amounts of silicate and sulfide phases in San Cristobal are far too small to account for the necessary removal of 80 per cent of the metallic Fe from grains with IA Fe/Ni ratios. Furthermore, in group IB there is no obvious correlation between the abundance of these phases and the bulk Ni content of the metal. If the removal was accomplished by transfer of major amounts into the silicates, there is no record preserved in the IAB silicates; Fe content of ferromagnesian minerals (BUNCHet al., 1970) are uniformly low and uncorrelated with bulk Ni contents. Group IIICD, like 10, does not show the large fractionations observed within the other major groups which SCOTT (1972) attributed to fractional crystallization. In Fig. 6, IIICD, like San Cristobal, has approximately cosmic levels of volatile siderophiles and low abundances of refractories. In IIICD, the decrease in Ir with Ni content is more marked than in IB (SCOTTet al., 1973, Fig. 6); the value plotted in Fig. 6 is the geometrical mean of values differing by a factor of 40. Thus, although IIICD seems to have been subjected to the same two processes that operated in IAB, the refractory depletion was much more effective in IIID than in San Cristobal. Earlier sections of this paper have emphasized the mineralogical and structural similarities between members of groups IAB and IIICD which tend to confirm that they formed in neighboring parts of the solar nebula. The distribution of Ga and Ge in IAB and IIICD (Fig. 5) is very different from that in other groups. First, there is a much wider range in IAB and IIICD : opposite ends differ by factors of about 10 and 20 for Ga and Ge, respectively, compared with factors of 1.2 f O-2 and l-7 -+ O-6 in the other groups. Second, no other groups except IIB and IIIB, the high Ni portions of IIAB and IIIAB, respectively, have negative correlations of Ni with Ga or Ge. This is consistent with the theory that the trends in IA.B and IIICD were produced in a nebular process, whereas those in the other groups were produced by a planetary process-fractional crystallization. It would be convenient to associate the Ga and Ge fractionation in IAB and IIICD with the removal of the refractories. However, the much lower condensation temperatures of Ga and Ge and the absence of any Ir-Ge correlation in IA suggest otherwise. The above two-component model for LAB is not favored by the 20 per cent decrease in Ge/Ga ratio through IA alone, since mixing IA material containing 6.5 per cent Ni with San Cristobal metal would produce a variation of about 1 per cent. Figure 5 suggests that the negative correlation of Ni with Ga and Ge in IIICD, like that in IAB, might be extrapolated back to the cosmic Ga/Ni and Ge/Ni ratios present in IA. Possible low Ni IIIC members are Mundrabilla and Waterville (Fig. 5), analyzed by DELAETER(1972) and WASSON(1970, in press). Parent bodies There is conflicting evidence concerning the distribution of the IAB and IIICD irons in their respective parent bodies. Both groups show a narrow range of cooling
Structure and formation of the San Cristobd meteorite
1389
rates caloulated by the SHORT-G•LIMTEIN(1967) method with the aid of WMSON’S (1971) equation (Fig. 4). These indicate a similar depth of burial for all the group members, and in view of the associated errors, are consistent with storage in ‘cores’ in both parent bodies. However, the presence of silicates (admittedly few in IIICD) rules out a fully molten period. Perhaps such cores might be formed without melting by heterogeneous accumulation (TUREKLUand CLARK,1969). It seems more likely that preferential accumulation of iron grains in the solar nebula would produce m-sized iron masses distributed within the parent bodies instead of producing cores. Five cosmic-ray exposure ages for IIICD members (VOS~AUE,1967) give a negative correlation with Ni content which is significant at the 95 per cent level, suggesting that the members least depleted in refractories and poorest in Ni were closest to the surface. This trend is not supported by Fig. 4, where there is no evidence for a cooling-rate decrease with increasing Ni in IIICD. Group IA members show no such trend in cosmic-ray exposure ages, but no data on IB irons were reported by Voshage. Although IA bandwidth-Ni data would be consistent with an inverse correlation of Ni and cooling rate, kamacite plate impingement in low Ni irons (SHORTand GOLDSTEIN,1967) might also be responsible for the observed trend. Aek~ow~g~~s-We are indebted to J. T. WASSON for valuable ~so~sio~ and criticisms of the manuscript, and to V. F. BUCRW~, who first suggested that San Cristobal was &IBmember. We warmly thank R. SCHAUDY and W. KIESL for supplying the silicate nodule, and R. S. CLARKE and R. HUTCHISONfor the loan of samples. We are grateful to R. E. JONES, J. F. KAWMAN and N. SVETICHfor technical assistance, UCLA Department of Geology and Cambridge University Department of Mineralogy and Petrology for the use of electron-probe analy~ zers. This work was supported by NASA grant NGR 05-007-320 and NSF grant GA-32084. A part of the first author’s early work for this paper was supported from a Natural Environment Research Council grant to S. 0. Agrell at Cambridge. REFERENCES Axnnn~ E. (1064) Origin, age and composition of meteorites. Space Std. Rset. &583-714. Axoa II. J. and SMITE P. L. (1972) Met~o~aphio study of some iron meteorites of high nickel content. Illineral. &lag. 88, 736-755. BAUER R. and SCEAUDYR. (1970) Activation analytical determination of elements in meteorites, 3. Determination of manganese, sodium, gallium, germanium, copper, and gold in 21 iron meteorites and 2 mesosiderites. C&em. Ueol. 6, 119-131. BILD R. W. (1974) New occurrences of phosphates in iron meteorites. Contrib. Mineral. Petrol. 46, 91-98. BRENTNA~LW. D. and AXON H. J. (1962) The response of Canyon Diablo meteorite to heat treatment. J. IFOYX Steel Inst. 200,947-955. BUOH~A.L~ V. F. (in press) Iron Metewitm. Arizona State University. BUNCH T. 1. and OLS~ E. (1968) Potassium feldspar in Weekeroo Station, Kodaikanal, and Colomera iron meteo~t~. Se&es 180,1223-1225. Bnncs T. E., KEIL K. and OLSEN E. (1970) Mineralogy and petrology of silicate inclusions in iron meteorites. Chttib. Mined. Fe&&. 25,297-340. BUNCH T. E., KEIL K. and Huss G. I. (1972) Landes meteorite. Meteor&s 7,31-38. CROCK J. H. (1972) Some aspects of the geochemistry of Ru, OS, Ir and Pt in iron meteorites. Beo&m. Oosmochim. Acta 86,617~635. DEER W. A., HOWIE R. A. and ZTSSSMMA.N J. (1963) Rock-B’ovming MineTab Vol. 4. Framework Silica&a, 435 pp Wiley. DELAETE~ J. R. (1972) The Mundrabilla meteorite shower. Metit& 7,285-294. EL GOREBYA. (1967) Quantitative electron mioroprobe analyses of K-feldspar grains Erom the Odessa iron meteorite. (Abstract) 30th Meeting of the Meteoritical Society, October 25-27.
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Meteoritics 6, 139-147. WASSON J. T. (in press) Metemites-Classification and Properties. Springer. Groups WASSON J. T. and SCHAUDY R. (1971) The chemical classification of iron meteorites-V. IIIC and IIID and other irons with germanium concentrations between 1 and 25 ppm. Icarus 14, 59-70.