The Adhi Kot breccia and implications for the origin of chondrules and silica-rich clasts in enstatite chondrites

Earth and Planetary Science Letters, 64 (1983) 201-212 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

201

[61

The Adhi Kot breccia and implications for the origin of chondrules and silica-rich clasts in enstatite chondrites Alan E. Rubin Department of Mineral Sciences, National Museum of Natural Historr', Smithsonian Institution, Washington, DC 20560 (U. S. A.)

Received February 14, 1983 Revised version received April 28, 1983

The Adhi Kot EH4 enstatite chondrite breccia consists of silica-rich clasts (12 ± 5 vol.%), chondrule°rich clasts (55 ± 10 vol.%) and matrix (35 ± 10 vol.%). The silica-rich clasts are a new kind of enstatite chondritic material, which contains more cristobalite (18-28 wt.%) than enstatite (12-14 wt.%), as well as abundant niningerite and troilite. The bulk atomic Mg/Si ratios of the clasts (0.22-0.40) are much lower than the average for enstatite chondrites (0.79). Kamacite and martensite (with 8-11 wt.% Ni and a martens±tic structure) occur in all three breccia components. The clasts have kamacite-rich rims, and kamacite-rich aggregates occur in the matrix. A unidirectional change in the ambient p S J p O 2 ratio in the region of the solar nebula where Adhi Kot agglomerated can explain many of the breccia's petrologic features, if this region initially had a very high pS2/pO 2 ratio in a gas of non-cosmic composition, sulfurization of enstatite and metallic Fe (e.g., MgSiO 3 + 2Fe + C + 3H2S = MgS + SiO 2 + 2FeS + H20 + CH4) may have occurred, producing abundant niningerite, free silica and troilite at the expense of enstatite and metallic Fe. The Ni content of the residual metal would have increased, perhaps to - 8-10 wt.%. The silica-rich clasts agglomerated under these conditions; a significant fraction of the originally produced niningerite was lost (perhaps by aerodynamic size-sorting processes), lowering the clasts' bulk Mg/Si ratios. The pS2/pO ~ ratio then decreased (perhaps because of infusion of additional H20) and sulfurization of metallic Fe and enstatite ceased. The chondrule-rich clasts agglomerated under these conditions, acquiring little free silica and niningerite. An episode of chondrule formation occurred at this time (by melting millimeter-sized agglomerates of this relatively silica-poor enstatite chondrite material and concomitant fractionation of an immiscible liquid of metallic Fe.Ni and sulfide). The chondrule-rich clasts agglomerated many such chondrules. Subsequently, the matrix agglomerated, acquiring the few remaining chondrules. Kamacite-rich aggregates formed, after the cessation of metal sulfurization, and agglomerated with the matrix. The kamacite-rich clast rims were acquired at this time. The components of Adhi Kot accreted to the EH chondrite parent body, where the breccia was assembled, buried beneath additional accreting material, and metamorphosed at temperatures of > 700°C. Impact-excavation of the breccia and deposition onto the surface caused the formation of martensite from taenite inside the clasts and the matrix. At the surface, impact-melting produced an albite glass spherule, which was incorporated into the matrix. However, the absence of solar-wind-implanted rare gases in bulk Adhi Kot indicates that the breccia spent little time in a regolith.

1. Introduction T h e A d h i K o t e n s t a t i t e c h o n d r i t e fell a t local n o o n o n M a y 1, 1919, a b o u t 1.6 k m s o u t h o f t h e v i l l a g e o f A d h i K o t in W e s t P u n j a b , I n d i a ( n o w West Pakistan). A single stone weighing 4238.8 g w a s r e c o v e r e d [1]. V a n S c h m u s a n d W o o d [2] t e n t a t i v e l y c l a s s i f i e d A d h i K o t as p e t r o l o g i c t y p e 3, b u t W a s s o n [3] r e c l a s s i f i e d it as t y p e 4. B i n n s [4] l i s t e d t h e m e t e o r i t e as a b r e c c i a . T h e A d h i K o t 0012-821X/83/$03.00

E 1983 Elsevier Science Publishers B.V.

b r e c c i a c o n s i s t s o f s m a l l s i l i c a - r i c h clasts, l a r g e r c h o n d r u l e - r i c h clasts, a n d m a t r i x . O n l y t w o o t h e r brecciated enstatite chondrites are known: Abee ( E 4 ) [5,6] a n d H v i t t i s ( E 6 ) [7,8].

2. Analytical procedures P o l i s h e d t h i n s e c t i o n s f r o m t h e m e t e o r i t e collect i o n o f t h e S m i t h s o n i a n I n s t i t u t i o n , U S N M 2358-1

202 and USNM 2358-2 of Adhi Kot and USNM 5285 of St. Sauveur (E5), were studied microscopically in transmitted and reflected light. A polished slab of Adhi Kot, prepared without water to avoid alteration of water-soluble minerals, was studied in reflected light. One thin section (USNM 2358-1) was etched with a dilute solution of nitric acid in alcohol to bring out structural details in metallic Fe,Ni. Modal analyses were made microscopically using an automated point counter, grouping the silicates together as a single phase and the metallic Fe,Ni as a single phase. Individual silicate and metallic Fe,Ni minerals were than point-counted on the electron microprobe. Mineral vol.% was converted into wt.% using estimated mineral densities. Electron microprobe mineral analyses were made using crystal spectrometers, following standard Bence-Albee and ZAF correction procedures. Carbon in cohenite was analyzed with a lead stearate (LSD) crystal. Analyses of metallic Fe,Ni, schreibersite and cohenite were also corrected for Co interference from the Kp peak of Fe. Bulk atomic ratios for the different components were calculated from mineral compositions and modal abundances.

3. Results

3.1. Petrography The Adhi Kot breccia (Fig. 1) consists of silicarich clasts, chondrule-rich clasts, and matrix. Orthoenstatite and clinoenstatite (MgSiO3), albite (NaAlSi3Og), cristobalite (SiO 2), troilite (FeS), niningerite (Fe,Mg)S, oldhamite (CaS), schreibersite (Fe,Ni)3P, graphite (C) and kamacite and martensite (Fe,Ni) occur in all three components (Table 1). Cohenite (Fe,Ni)3C occurs in a few kamacite grains in the matrix and in the chondrule-rich clasts. Silica-rich clasts (Fig. 1) are angular, 3-5 mm in size, and constitute 12 + 5 vol.% of the Adhi Kot breccia. Three such clasts (1, 2 and 4) were studied. The clasts have -200-~.m-thick rims composed of kamacite with minor troilite and no martensite. This contrasts with the clast interiors, wherein both kamacite and martensite occur. The

boundaries between the matrix and the clast rims appear moderately recrystallized--i.e., mineral grains have grown across the clast-matrix boundaries, making it difficult to define the exact boundaries microscopically. The clasts contain more free silica, i.e., cristobalite (18-28 wt.%) than enstatite (12-14 wt.%). By weight, clasts 1 and 2 contain twice as much cristobalite as enstatite (Table 1). The clasts also contain abundant niningerite and troilite and no apparent chondrules. Cristobalite grains are typically 80-100 ~tm in size. Clast 2 itself encloses a millimeter-sized, finer-grained (typically 10- to 15-~m-sized grains) silica-rich clast with a moderately recrystallized boundary. Chondrule-rich clasts (Fig. 1) are sub-rounded, 1-2 cm in size, and constitute 55 + 10 vol.% of the breccia. Two such clasts (3 and 5) were studied. They contain 15-20 vol.% radial and porphyritic pyroxene chondrules (0.24-2.0 mm in apparent diameter). There is little cristobalite (~<1 wt.%) and much less niningerite than in the silica-rich clasts. Clast 3 has a 5- to 15-/~m-thick rim of troilite with minor niningerite and accessory kamacite; clast 5 has a discontinuous - 200-~mthick rim of kamacite (Fig. 1). Both clasts contain kamacite veins up to 0.2 × 8 mm in size (Fig. 1). Matrix regions constitute 35 + 10 vol.% of Adhi Kot, contain only 2-5 vol.% radial and porphyritic pyroxene chondrules and very little cristobalite (~< 0.1 wt.%). Two matrix regions (matrix 1 and matrix 2) were studied. The amount of niningerite in the matrix is similar to that of the chondrule-rich clasts and much lower than that of the silica-rich clasts. Round to irregular kamacite-rich aggregates (0.2-2.2 mm in size) (Fig. 1) constitute - 15 vol.% of the matrix. The aggregates characteristically contain anhedral to euhedral albite grains with minor to accessory enstatite and cristobalite; some contain graphite aggregates a n d / o r troilite as well. No martensite is present in any of the kamacite-rich aggregates. Rare kamacite-rich aggregates also occur in the chondrule-rich clasts. Modal analyses indicate that 40-50% of the enstatite grains in different components of Adhi Kot exhibit parallel extinction under crossed polarizers. Because clinoenstatite may exhibit

203

l

Chondrule - rich Clast

\

\

Matrix

ii

I \

\ Fig. 1. Polished slab of the Adhi Kot EH4 enstatite chondrite breccia showing silica-rich clast 4, chondrule-rich clast 5 and matrix region 2 ("matrix 2"). Several large kamacite-rich aggregates occur in the matrix. Kamacite veins occur in clast 5 and kamacite-rich rims occur around clasts 4 and 5.

parallel extinction under certain crystal orientations, the actual abundance of orthorhombic enstatite is estimated to be 30 + 10%. Euhedral en-

statites, ranging in size from 3 x 8 # m to 100 x 300 # m (typically 20 x 100/~m), occur inside kamacite grains, and, to a lesser extent, inside troilite grains,

TABLE I Modal abundances (wt.%) in different areas of Adhi Kot Chondrule-rich clasts

Matrix regions

clast 1

clast 2

clast 4

clast 3

matrix 1

matrix 2

Enstatite Christobalite Plagioclase Oldhamite Niningerite Troilite Kamacite Martensite Sehreibersite Graphite

14 28 10 < 0.1 12 21 6 3 7 < 0.1

12 25 5 < 0.1 3 26 I1 13 5 < 0.1

14 18 7 1 15 14 16 6 9 < 0.1

Total

li-0"]--

~

62 < 0.1 11 < 0.1 0.5 10 11 4 2 0.1 100.6

49 0.1 16 < 0.1 0.2 8 12 10 5 0.1 ~

52 < 0.1 8 2 2 13 10 I1 1 <0.1 99

Points counted Area analyzed (ram2 )

385 10

269 8

267 5

533 56

617 43

650 37

Silica-rich clasts

clast 5 48 I 6 I 6 10 13 11 4 0.3 100.3 1445 99

204

in the matrix and chondrule-rich clasts. In rare instances, euhedral enstatites are enclosed by schreibersite or surrounded by small grains of niningerite. A few euhedral enstatite grains are fractured; the fractures are filled with troilite a n d / or kamacite. In silicate-rich areas in the matrix and chondrule-rich clasts, anhedral to euhedral enstatite grains are intergrown with aibite. An isotropic stoichiometric albite glass spherule (110 ~m in apparent diameter), containing a few 4- to 25-~m-sized blebs of troilite, was discovered in the matrix. The spherule is surrounded by grains of enstatite ( - 80%) and albite ( - 20%).

3.2. Mineralogy Mineral compositions (Table 2) are fairly uniform throughout the breccia and there are no systematic differences in mineral compositions among the major components. In most cases, agreement with the mineral compositions reported by Keil [9] and Leitch and Smith [10] is very good. Enstatite is fairly homogeneous throughout the breccia and averages Fsl.0Wo0.2 (Table 2). Leitch and Smith [10] detected both red- and blueluminescing enstatite in Adhi Kot and found minor compositional differences between them. Most of the enstatite analyzed here is blue-luminescing (by far the more abundant type), but the few analyzed red-luminescing grains are much richer in MnO (e.g., - 0.2 wt.% vs. < 0.08 wt.%). Free silica contains - 9 9 wt% SiO: and - 1 wt.% FeO, AI203, Na20, K 2 0 and MgO (Table 2), indicating that the mineral's crystal structure was sufficiently open to trap foreign cations. Some of the free silica grains in the silica-rich clasts exhibit the distinctive curved fracture that is characteristic of cristobalite. These observations are consistent with Mason's [11] X-ray diffraction identification of cristobalite as the only silica polymorph in Adhi Kot. Plagioclase grains (or glass of plagioclase composition and stoichiometry) are typically ~< 2 ~m in size and decompose readily under electron bombardment; Keil [9] was unable to determine any quantitative analyses of Adhi Kot plagioclase. The average of my 10 analyses (Table 2) is Ab96Or2. The l l0-Fm albite glass spherule in the

matrix is considerably more potassic (AbgiOr,). Both kamacite (6.8 wt.% Ni, 3.7 wt.% Si) and martensite (8-11 wt.% Ni, 3.4 wt.% Si) occur in the matrix, silica-rich clasts and chondrule-rich clasts (Tables 1, 2). Martensite grains display a typical martensitic structure when etched (Fig. 2). Most kamacite and martensite occur as separate grains, but intergrowths are common (Fig. 2). Such intergrowths typically consist of large kamacite grains (100-300 # m in size) with one or more internal angular patches of martensite (20-70 #m in size). In some cases, the kamacite occurs only as a thick ( - 4 0 ~m) partial rim around a larger martensite grain. Graphite occurs as round to somewhat irregular aggregates (10-150 p.m in size) composed of individual radiating to randomly-oriented graphite laths, which range in size from less than a micrometer to 1 × 20 ~tm. None of the aggregates has the cubic morphology of cliftonite. The graphite aggregates occur inside kamacite and martensite grains, kamacite-rich aggregates and the kamaciterich rims on the clasts. A few graphite aggregates are completely surrounded by troilite. There is no tendency for graphite aggregates to preferentially occur near identifiable grain boundaries. One isolated graphite lath, 3 × 65 ~m in size, was located in the matrix, totally surrounded by enstatite. This single graphite lath occurs in the same textural assemblage as all of the graphite identified in Abee by Rubin and Keil [6].

Fig. 2. Typical intergrowth of kamacite ( K ) and martensite ( M ) from silica-rich clast 1.

205 TABLE 2

Average mineral compositions (wt.%) in Adhi Kot and richterite in St. Sauveur En

Crist

Plag

Old

23

10

17

Nin

Troil

Kam

Mart

Schreib

Cohen

71

109

125

81

98

4

St. Sauveur

richterite Number of grains

93

SiO 2 TiO 2 AI203 Cr203 FeO MnO MgO CaO Na20 K20 F S Mg Ca Cr Ti Mn Fe Ni Co Si P C

59.5 < 0.08 0.08 < 0.09 0.7 < 0.08 40. I 0.1 < 0.05 < 0.04

Total

100.48

Endmember

Fs i.oW°o. 2

99.0 < 0.08 0.5 < 0.09 0.7 < 0.08 0.03 < 0.03 0.1 0.07

70.3 < 0.08 19.2 < 0.09 0.2 < 0.08 0.2 0.3 9.7 0.3

6 57.4 < 0.08 < 0.06 < 0.09 0.8 < 0.08 25.0 5.6 6.5 0.4 4.1

100.4.0

100.0

43.8 1.5 52.9 <0.07 <0.08 0.5 1.1

42.3 12.9 1.7 1.7 <0.08 6.9 34.4

36.9 0.05 <0.05 2.5 0.4 0.4 58.6

99.8

99.9

98.85

~.3 6.8 0.4 3.7 0.2

88.0 9.1 0.3 3.4 0.2

78.4 6.4 0.3 0.3 15.8

101.4

101.0

101.2

89.5 2.5 0.1 0.9 0.05 7.3 100.35

99.8

Ab96 Or2

En = enstatite; Crist = cristobalite; Plag = plagioclase; O l d - o l d h a m i t e ; N i n = niningerite; Troil = troilite; Kam = kamacite; Mart = martensite; Schreib = schreibersite; C o h e n = cohenite.

Sulfides (troilite, niningerite and oldhamite) occur as finely-disseminated submicrometer blebs in silicates and as isolated grains up to 600 ~ m in size, a few of which enclose euhedral enstatites. Troilite is typically 150 # m in size and is the most abundant sulfide, except in silica-rich clast 4 where niningerite is more abundant (Table 1). Most troilite grains contain little ( < 5%) or no intergrown niningerite. Niningerite occurs primarily in the silica-rich clasts as grains typically 40 ~tm in size. Virtually all of the niningerite grains are actually assemblages (Fig. 3) of - 7 5 % niningerite and - 25% troilite blades (typically 2 × 15 ~ m in size). Oldhamite occurs as isolated grains or grain clusters, typically - 50 ~ m in size. The reported abundances of oldhamite in clasts 1, 2 and 3 and matrix

Fig. 3. Typical intergrowths of niningerite ( N ; light grey) and troilite (T; white) consisting of - 75% niningerite and ~ 25% troilite blades from silica-rich clast I.

206

1 are much too low, because these areas are from thin sections which had previously been prepared with water, which dissolves oldhamite. Sulfide analyses (Table 2) agree well with those of Keil [9], but only fairly well with those of Leitch and Smith [10]. The most serious discrepancy is in the F e / M g ratio of niningerite: 2.7 in my analyses (Table 2), 3.0 in Keil's [9], and 1.6 in the analyses of Leitch and Smith [10]. This discrepancy is very similar to that reported for the F e / M g ratio in Abee niningerite [6] and must result from a systematic error in standardization or correction procedures. (In my analyses of Adhi Kot niningerite, Mg was analyzed using pure Mg and San Carlos olivine (Fal0) standards; Fe and S were analyzed using pyrite.) Schreibersite occurs as isolated - 25-~m grains, which are in some instances associated with troilite a n d / o r kamacite. This association is different from the schreibersite in Abee clasts and matrix which occurs only along the margins of some kamacite grains [6]. Schreibersite in Adhi Kot averages 6.4 wt.% Ni and 0.3 wt .% Si in solid solution (Table 2).

Cohenite is rare and occurs inside kamacite grains in the matrix and chondrule-rich clasts as 0.5- to 2-~m wide elongated anisotropic laths, a few of which form very squiggly inclusions. This texture appears to differ from the pearlite described in Abee by Herndon and Rudee [12]. Adhi Kot cohenite contains 2.5 wt.% Ni and 0,9 wt.% Si in solid solution (Table 2); however, because the laths are so small, the electron probe beam may have overlapped the surrounding kamacite and detected more Ni and Si than is present in the cohenite.

3.3. Bulk composition The bulk atomic M g / S i ratios of the silica-rich clasts (0.22-0.40) are much lower than the average for enstatite chondrites (0.79), whereas the clasts' bulk atomic S/Si ratios (0.55-0.67) are much higher than the enstatite chondrite average (0.22). Bulk atomic AI/Si, Fe/Si and Ca/Si ratios are typical of enstatite chondrites (Table 3). Bulk atomic Mg/Si, S/Si, AI/Si, F e / S i and Ca/Si ratios of the chondrule-rich clasts and the matrix

TABLE 3 Bulk composition of different components of Adhi Kot a Silica-rich clasts

SiO 2 MgO A1203 CaO Na20 K 20 H20 Fe Ni Co S Cr Mn Ti P C Total

Chondrule-rich clasts

Matrix regions

clast I

clast 2

clast 4

clast 3

clast 5

matrix 1

matrix 2

44 8 2 0.3 1.1 0.06

38 6 1.2 0.1 0.6 0.04 . 41 2 0.1 11 0.7 0.3 0.1 0.8 0.05 ~

33 9 1.3 1.4 0.6 0.07

46 25 2 0.1 1 0.02 . 21 1.3 0.08 4 0.2 0.06 0.04 0.4 0.1 101.30

36 20 1.3 1.5 0.5 0.06

42 20 3 0.1 1.5 0.04

38 22 2 1.5 0.6 0.08

32 2 0. I 6 0.3 0.4 0.04 0.6 0.3 101. I0

29 2 0.1 3 0.2 0.04 0.03 0.9 0.1 102.01

29 1.8 0.08 7 0.4 0.2 0.05 0.2 0.05 102.96

30 1.1 0.06 13 0.8 0.9 0.1 1.1 0.04 10--0-f'f-.-.-56 .-.-.-.~.

.

. 41 2 0. I 12 0.6 1.2 0.06 1.4 0.05 ~

Bulk [ 111

.

35.94 16.67 2.02 1.22 0.81 0.10 0.21 33.31 1.95 0.09 5.65 0.30 0.17 0.08 0.16 0.38 99.06

a The high totals result from expressing all Si, Mg and Ca as oxides. The low Ca() contents of clasts I, 2 and 3 and matrix 1 result from the absence of oldhamite in these areas (which are from thin sections prepared with water).

207 areas are typical of enstatite chondrites (Table 3). In these determinations, only the C a / S i ratios of clasts 4 and 5 and matrix 2 (all from the polished slab) were considered; the other areas are from thin sections, in which oldhamite was dissolved from prior preparation with water, resulting in anomalously low bulk C a / S i ratios.

4. Discussion

4.1. Formation of silica-rich clasts The silica-rich clasts in Adhi Kot are a unique kind of enstatite chondritic material, possessing more free silica than enstatite (Table 1) and more total sulfide (29-33 wt.%) than any other enstatite chondrite (5.5-19.3 wt.% sulfide; [9]). Although dark inclusion 51 in Abee contains even more sulfide (51 wt.%, [6]), the silica-rich clasts in Adhi Kot are not related to the Abee dark inclusions (which are very rich in oldhamite, highly depleted in free silica and contain a few silica-poor chondrules). Some Abee clasts contain significant amounts of cristobalite (up to 16 wt.%), but all such clasts contain even more enstatite (27 wt.%), and all contain chondrules [6]. Adhi Kot's silica-rich clasts either formed on a chondrite parent body or in the solar nebula. Each possibility is considered separately below:

Parent body. The silica-rich clasts may possibly have formed by metamorphic processes on a chondrite parent body. The high abundance of cristobalite, niningerite and troilite and comparatively low abundance of enstatite in the silica-rich clasts might have resulted from sulfurization of enstatite and metallic Fe in normal enstatite chondrite material during metamorphism at high p S 2 / p O 2 ratios. In this case, the low Mg/Si ratios of the clasts indicate that some niningerite may have been lost by some sort of fractionation process. The absence of chondrules in the clasts may reflect an origin as pulverized a n d / o r melted material in which pre-existing chondrules were obliterated. Such conditions may have occurred on portions of a very heterogeneous Adhi Kot (EH) parent body or, more likely, on a separate "en-

statite chondrite-like" parent body (from which the silica-rich clasts were ejected by meteorite impacts). Larimer and Buseck [13] calculated equilibration temperatures for enstatite chondrites based on equilibrium constants for the following chemical reactions: Si + 02 = SIO2; 2CaSiO 3 + S2 = 2CaS + 2SIO2 + 02; and 2 F e + S2 = 2FeS; and microprobe analyses of CaO in enstatite and Si in metal. I used the resulting equilibrium equations to determine pS2/pO 2 ratios for the 15 enstatite chondrites listed in reference 13 (using the temperatures calculated by Larimer and Buseck [13]) plus the recent Antarctic E5 find, RKPA80259 (using my own unpublished microprobe data). My reanalysis of four of the enstatite chondrites listed in reference 13 yields pS2/pO 2 ratios that are systematically - 5% higher than those calculated from the data presented in Table 2 of Larimer and Buseck [13]. These discrepancies do not affect the results discussed below: I found that Adhi Kot, Abee and St. Marks (E5) appear to have formed at pS2/PO 2 ratios three to seven orders of magnitude higher than the other meteorites (i.e., 10 25-27 vs . 102°-22). If sulfurization of enstatite played a significant role in the formation of Adhi Kot, Abee and St. Marks, as suggested by their calculated in high p S J p O 2 ratios, they should contain abundant free silica and niningerite. Abee [6] and St. Marks [i 1] contain numerous chondrules rich in free silica (cristobalite in Abee and quartz in St. Marks); abundant free silica also occurs in the matrix of St. Marks and in some Abee clasts. Abee contains abundant niningerite [6,9]; St. Marks contains very magnesian (albeit underabundant) niningerite [9]. However, the total MgS content of Abee is much higher than that of St. Marks [9]. St. Sauveur (E5) was also calculated to have formed at a high pS2/pO 2 ratio (1028), but I found that this chondrite contains little free silica. However, I did find several - 4 0 x 100 ttm subhedral grains of the amphibole, richterite (Table 2), which may have acquired much of the Ca which would have otherwise entered enstatite. The very low bulk AI in St. Sauveur [14] may have retarded the formation of albite, resulting in excess Na which may then have combined with available

208 Ca to form richterite. (Keil's [9] difficulty in analyzing St. Sauveur plagioclase indicates that few large albite grains occur in this meteorite.) If richterite had not been present and the enstatite possessed - 0 . 5 wt.% CaO, instead of 0.07 wt.% [9], then the calculated pS2/pO 2 ratio would have been much lower (1022)-comparable to that of the majority of the enstatite chondrites. (Richterite was previously found in portions of kamacite pods in the matrix of Abee that apparently formed at lower pS2/pO 2 ratios than the rest of Abee [ 15,16].) For Adhi Kot, the occurrence of silica-rich clasts, the absence of chondrules with abundant free silica (in the matrix and chondrule-rich clasts), the low abundance of free silica in the matrix and chondrule-rich clasts, and the high p S J p O 2 ratios calculated for all of the breccia's major components suggest that: (a) the silica-rich clasts formed at very high p S J p O 2 ratios, (b) the matrix and chondrule-rich clasts formed at relatively low pS2/pO 2 ratios (comparable to those for most of the enstatite chondrites), and (c) post-brecciation metamorphism partially equilibrated the mineral compositions, thereby equalizing the calculated pS2/pO 2 ratios throughout the breccia. Solar nebula. Alternatively, the silica-rich clasts

may have formed in the solar nebula under ambient conditions of extremely high pS2/pO 2 ratios such that sulfurization of enstatite and metallic Fe occurred in a reaction of the following sort: MgSiO3(s ) + 2Fe(s) + C(s) + 3H2S(g) = MgS(s) + SiO2(s ) + 2FeS(s) + H20(g ) + CH4(g). This particular reaction has a AG.... ,on < 0 from 298°K through 1200°K and produces abundant Mgsulfide, troilite and free silica at the expense of enstatite and metallic Fe. (I was unable to find any other reaction with a A G r e a c t . . . . < 0 that produces MgS and SiO 2 at the expense of MgSiO3.) The simultaneous production of Mg-sulfide and troilite may account for the prevalent intergrowths of niningerite and troilite in Adhi Kot (Fig. 3). Sulfurization of metallic Fe would have increased the Ni content of the residual metal, perhaps to 8-10 wt.% Ni. The sulfurization reaction presupposes a gas of non-cosmic composition, wherein CH4 and H2S coexist. Although I do not propose any mechanism

to account for the existence of this gas in the solar nebula, such high pS2/pO 2 ratios could have resuited from anomalously high concentrations of gaseous H 2 0 or low concentrations of H 2 0 in the nebula. If this reaction had proceeded in a closed system, the bulk Mg/Si ratios of the clasts would not have been affected. The very low Mg/Si ratios of the silica-rich clasts (0.22-0.40) compared to average enstatite chondrites (0.79) could have resulted from the loss of 10-25% of the pure-Mg niningerite produced by the enstatite sulfurization reaction. If the niningerite produced by this reaction had the same amount of iron as it does now (Table 2), then 55-95% must have been lost. (These calculations assume normal enstatite chondritic material to start with.) Because the niningerite grains (typically 40 p.m in size) in the silica-rich clasts are significantly smaller than the cristobalite grains (typically 80-100 p.m in size), it is possible that the niningerite loss was due to nebular size-sorting processes. (Although niningerite is denser than cristobalite, troilite is denser than niningerite and the silica-rich clasts have bulk atomic Fe/Si ratios typical of enstatite chondrites. It is thus unlikely that niningerite was lost by density separation.) Nebular size-sorting mechanisms probably involved aerodynamic particle-gas interactions [17]; any of the following mechanisms would suffice: Weidenschilling [18] suggested that particles may have been size-sorted by radial transport (toward or away from the sun) by gas drag. Clayton [19] suggested three other mechanisms for aerodynamic size-sorting of particles in the nebula: (a) transport by shock waves through a gas, (b) deposition of particles from a turbulent gas, wherein particles of different sizes were deposited at different degrees of turbulence, and (c) sedimentation in a gas, wherein larger particles sank faster than smaller ones (for example, to the central plane of the nebula). 1 believe that it is more likely that the silica-rich clasts were formed in the solar nebula because (1) the millimeter-sized, fine-grained silica-rich clast enclosed by silica-rich clast 2 is much easier to explain by nebular agglomerationary processes, (2) the similarities in metallic Fe,Ni occurrences and compositions in all of the Adhi Kot components

209 suggest a common origin, (3) the occurrence of graphite aggregates of similar size and morphology in metallic Fe,Ni in all of the Adhi Kot components also suggests a common origin, (4) the occurrence of martensite-free, kamacite-rich clast rims on the silica-rich and chondrule-rich clasts suggests that the rims were acquired by both types of clasts in the nebula from a low-Ni metal source, (5) the low bulk Mg/Si ratios of the silica-rich clasts can best be explained by loss of niningerite in the nebula rather than by fractionation on a parent body, and (6) the similarities in the bulk A1/Si, Fe/Si and Ca/Si ratios of the silica-rich clasts to the rest of Adhi Kot suggest derivation from a common source. The absence of chondrules in the silica-rich clasts may simply indicate that the clasts formed prior to chondrule formation (see below). The observation that the three enstatite chondrites with the greatest abundance of free silica (Adhi Kot, Abee and St. Marks) are the ones with the highest calculated pS2/P 2 ratios (based on metamorphic equilibration temperatures) can be interpreted as indicating that metamorphism did not completely wipe out the mineralogical evidence for the high pSz/pO z ratios that were initially experienced by these chondrites when they were agglomerating in the nebula. However, the possibility remains that the silica-rich clasts represent unusual material formed on a separate parent body and accreted to the EH body as exotic clasts. Although this possibility cannot presently be excluded, the remaining sections of the discussion presuppose a nebular origin for the silica-rich clasts. 4. 2. Formation of silica-rich chondrules

Scott et al. [20] and Rubin et al. [21] proposed that chondrules in ordinary chondrites were formed by the melting of millimeter-sized dustballs of the Fe-rich fine-grained opaque silicate matrix material ("Huss matrix"; [23]) that constitutes 10-17 vol.% of type 3 ordinary chondrites [24]. This melting was accompanied by reduction of FeO in the matrix material to metallic Fe with subsequent fractionation of an immiscible liquid of metallic Fe,Ni and sulfide from molten silicate. Crystalliza-

tion of the molten silicate droplets formed chondrules. Petrographic observations of the only known E3 chondrite, Qingzhen (A. El Goresy, personal communication, 1983), indicate that only small amounts of fine-grained opaque silicate matrix material are present. It is thus possible that chondrules in enstatite chondrites were formed by the melting of millimeter-sized agglomerates of the coarser-grained silicate-sulfide-metallic Fe,Ni material that forms the matrix in which the chondrules are embedded. Melting of this material would produce two immiscible liquids, one composed of silicate (which later formed chondrules) and the other of metallic Fe,Ni and sulfide (which may be related to the kamacite globules in Abee; [5,6]). The occurrence of numerous chondrules rich in free silica in Abee and St. Marks (both of these meteorites have abundant free silica in the areas outside the chondrules and appear to have formed at high pSz/pO2 ratios) and the occurrence of silica-poor chondrules in other enstatite chondrites (these meteorites have very little free silica outside chondrules and appear to have formed at lower pS2/PO 2 ratios) indicate that either (a) chondrules in enstatite chondrites formed in the same local environment and under the same p S z / p O z conditions as the matrix in which they now reside, or (b) that chondrules were formed directly from millimeter-sized agglomerates of this coarser-grained matrix material (or from Mg-rich finer-grained opaque silicate matrix material) by melting and concomitant fractionation of an immiscible liquid of metallic Fe, Ni and sulfide. I favor alternative (b) because of the analogy to the proposed formation of chondrules in ordinary chondrites [20,21], but the discovery and examination of additional E3 chondrites are necessary before less tentative conclusions can be drawn. It is possible that the comparatively rare SiO2-rich chondrules in ordinary chondrites (e.g. [22]) also formed under high pSz/pO 2 conditions in the nebula directly, or by melting material formed under such conditions. Any Mg-sulfide produced under these conditions may have been lost from molten chondrule droplets by liquid immiscibility, along with metallic Fe,Ni.

210

4.3. Agglomeration of components The occurrence of both kamacite and martensite inside the silica-rich clasts, chondrule-rich clasts and matrix suggests that the metallic Fe,Ni in these components was of similar composition at the time these components agglomerated. However, the absence of martensite in the kamacite-rich aggregates and the clast rims indicates that the metal in these areas was acquired from a compositional reservoir with less Ni than the one that supplied the rest of Adhi Kot. The occurrence of graphite aggregates of similar size and morphology inside kamacite and martensite in the Adhi Kot matrix, chondrule-rich clasts and silica-rich clasts, as well as in the kamacite-rich aggregates and martensite-free clast rims indicates that the graphite did not exsolve from the metal during slow cooling from high temperatures after the breccia was assembled. The solubility of carbon in kamacite is extremely low ( - 0 . 0 2 wt.%; [25]); if, at high temperatures, the carbon had been dissolved in taenite in the kamacite-rich aggregates and clast rims, martensite should have formed from this taenite during subsequent quenching. Because no martensite occurs in the kamacite-rich aggregates or clast rims, the graphite aggregates must have already been present in the metallic Fe,Ni from both the higher- and iower-Ni metal compositional reservoirs during agglomeration. The presence of identical graphite aggregates in metallic Fe,Ni from two compositional reservoirs is consistent with a genetic relationship between these reservoirs. (The single isolated graphite lath surrounded by enstatite in clast 3 is unrelated to the graphite aggregates and must have originally agglomerated with the enstatite, as did all of the graphite in Abee; [6].) A unidirectional change in the ambient pS2/pO2 ratio in the region of the solar nebula where Adhi Kot agglomerated can explain many of the breccia's petrologic features. If this region initially had a very high pS2/pO 2 ratio, sulfurization of metallic Fe,Ni and enstatite may have occurred through a reaction similar to the one proposed above. Some of the Fe in the metal combined with S to form FeS (troilite), increasing the Ni content of the residual metal ( t o - 8 - 1 0 wt.% Ni). Abundant

Mg-sulfide and free silica were also produced, at the expense of enstatite. Subsequent agglomeration of the silica-rich clasts under these high pS2/pO 2 conditions resulted in the acquisition of abundant troilite and niningerite (accounting for the clasts' high bulk S/Si ratios), abundant free silica, and metal with - 8-10 wt.% Ni. Loss of a significant fraction of the niningerite, perhaps by aerodynamic size-sorting processes, lowered the bulk Mg/Si ratios of the silica-rich clasts to the observed values. If the PS2/PO 2 ratio in this nebular region subsequently decreased (possibly by infusion of additional H 2 0 ), sulfurization of enstatite and metallic Fe,Ni would have ceased. At this point, the chondrule-rich clasts agglomerated, acquiring little free silica and niningerite, but a significant amount of previously-unagglomerated higher-Ni metal. An episode of chondrule formation (lightning?? [26,27]) occurring at this time (by melting millimeter-sized agglomerates that had formed under these lower pS2/pO~ conditions), would have furnished these clasts with abundant silica-poor chondrules. After the chondrule-rich clasts had almost finished agglomerating, the greater availability of lower-Ni metal ( - 5-7 wt.% Ni) derived from the cessation of metal sulfurization, resulted in the formation of kamacite-rich aggregates. The greater abundance of albite than enstatite in these aggregates may possibly have resulted from mineralogical sorting in the nebula. Few kamacite-rich aggregates agglomerated to the chondrule-rich clasts, but many joined the matrix, which had just begun to agglomerate. The matrix also acquired the still abundant higher-Ni metal grains and the few chondrules that remained in the region. At this time, some of the lower-Ni metal formed veins in the chondrule-rich clasts and rims around the silica-rich and chondrule-rich clasts (Fig. 1). The Adhi Kot breccia was asembled much later, after its components accreted to the EH enstatite chondrite parent body.

4. 4. Thermal history After assembly on the EH parent body, the Adhi Kot breccia may have been buried by the accretion of additional enstatite chondritic mate-

211 rial. T h e r m a l m e t a m o r p h i s m (caused b y heat gene r a t e d by short-lived r a d i o n u c l i d e s such as 26A1) m o d e r a t e l y recrystallized the breccia's clast-matrix b o u n d a r i e s and p a r t i a l l y equilibrated the mineral c o m p o s i t i o n s . T h e niningerite g e o t h e r m o m e t e r of Skinner a n d Luce [28] indicates that m e t a m o r p h i c t e m p e r a t u r e s must have been >~ 700°C. At these temperatures, the metallic F e , N i in the interiors of the clasts and matrix existed as taenite with - 8 - 1 0 wt.% Ni in e q u i l i b r i u m with the k a m a c i t e in the k a m a c i t e - r i c h aggregates and clast rims. (The FeN i - C phase d i a g r a m [29] indicates that taenite can coexist with k a m a c i t e at these temperatures. It is not yet k n o w n w h a t phase relations exist in the F e - N i - C o - C - P - S i system, a p p r o p r i a t e for enstatite c h o n d r i t e metal.) Slow-cooling caused the precipitation of a d d i t i o n a l k a m a c i t e inside the taenite, f o r m i n g much of the k a m a c i t e in the matrix (outside the k a m a c i t e - r i c h aggregates), as well as in the interiors of the silica-rich and chondrule-rich clasts. S u b s e q u e n t l y , the A d h i K o t breccia was quenched, t r a n s f o r m i n g the r e m a i n i n g taenite into m a r t e n s i t e ( a c c o u n t i n g for the p r e s e n t l y - o c c u r r i n g k a m a c i t e - m a r t e n s i t e intergrowths; Fig. 2). It is likely that r a p i d cooling took place radiatively when the breccia was i m p a c t - e x c a v a t e d from d e p t h a n d d e p o s i t e d o n t o the p a r e n t b o d y surface. The rare cohenite inclusions in k a m a c i t e grains m a y have nucleated at this time, p e r h a p s at t e m p e r a tures as low as 300°C [30]. T h e albitic glass spherule was p r o b a b l y prod u c e d by i m p a c t - m e l t i n g at the p a r e n t b o d y surface, b y a n a l o g y with glass spherules in howardites a n d the lunar regolith (e.g. [31,321). The higher K 2 0 in the spherule (1.1 wt.%) c o m p a r e d with typical A d h i K o t plagioclase (0.3 wt.%) m a y have resulted from N a - K exchange via a v a p o r p h a s e [33] and is a n a l o g o u s to a n o m a l o u s l y potassic i m p a c t - m e l t - r o c k clasts in o r d i n a r y c h o n d r i t e breccias (e.g., [33,34]). The glass spherule was later i n c o r p o r a t e d into an u n c o n s o l i d a t e d p o r t i o n of the A d h i K o t matrix that was subsequently welded to the rest of the breccia b y shock or long-period, l o w - t e m p e r a t u r e annealing. T h e absence of s o l a r - w i n d - i m p l a n t e d rare gases in bulk A d h i K o t [35] indicates, however, that the breccia spent little time in a regolith. Either Adhi K o t resided in the regolith for a very short time

after its excavation from d e p t h or its p a r e n t b o d y was too small to develop a substantial regolith.

Acknowledgements I thank the following p e o p l e for interesting and helpful discussions: D . W . G . Sears, G.J. Taylor, R.S. Clarke, Jr., B. M a s o n , E.R.D. Scott and W . G . Meison. T h e c o m m e n t s of two a n o n y m o u s reviewers were very useful in revising the manuscript.

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