The Bholghati howardite: Petrography and mineral chemistry

Geoch!micti et Carmaclrimica Copyright E3 I!790 Fergamon

Acfa Vol. 54, pq. 2161-2166 Press pie. Printed in U.S.A.

The Bholghati howardite: Petrography and mineral chemistry A. M. REID,’ PAUL BUCH~AN,’ M. E. ZOLENSKY,*and R. A. BARRETTE ‘Department of Geosciences, University of Houston, Houston, TX 77204, USA 2NASA,Johnson Space Center, Houston, TX 77058, USA 3LockheeclEngineering and Sciences Co., 2400 Nasa Rd. I, Houston, TX 77058, USA (Received May 25, 1989; accepted in revisedform December 5, 1989) Abstract-A

10 g sample of the Bholghati howardite was disaggregated in order to separate two eucrite clasts, several small carbonaceous clasts, fragments of diogenitic pyroxene, and bulk matrix. The eucrite clasts show evidence of moderately rapid cooling from a melt, followed by prolonged subsolidus annealing. The carbonaceous clasts mostly resemble CM2 carbonaceous chondrites with low-iron silicates and FeNi sulphides in a fine grained dark matrix. One clast, however, is mineralogically, petrographically, and compositionally similar to a CI 1 chondrite. Both carbonaceous and eucritic clasts have a complex history prior to incorporation into the howardite matrix with no evidence of significant metamorphism since assembly. Most clasts in the howardite breccia are monomineralic, with pyroxene and plagioclase predominant. Pyroxenes range from ‘diogenitic’ to ‘eucritic’ with diogenitic compositions most abundant; a significant number of intermediate compositions are present, consistent with derivation from a series of rocks related by fractionation.

‘diogenitic’ but, because separation depends on size and color, may not be wholly representative of the diogenitic component. Matrix samples were selected to avoid large clasts and fusion crust.

THE BHOLGI-~ATI HOWARDKEfell at Maurbhanj, Orissa, India, on October 29, 1905. Previous studies have shown that Bholghati is a fairly typical howardite, a polymict breccia composed of eucritic and diogenitic components plus mineral fragments of intermediate composition (LABOTKAand PAPIKE, 1980; ~HRMAN and PAPIKE, 198 1; PRINz et al., 1980). HEWINS and KLEIN (1978), LABOTKAand PAPIKE (1980), and FUHRMANand PAPIKE( 198 1) noted the presence of small carbonaceous clasts within the breccia. The work reported here is part of a consortium study to extend this research to a sample that offered opportunities to separate pristine clast material. The 10 g sample was obtained from the Geological Survey of India and processed in the Meteorite Laboratory of the Johnson Space Center. Clean samples of specific clast types and bulk matrix were provided to members of the consortium; sample numbers, locations, and distribution are given in LAUL (1990). The largest clasts are eucritic and one (approximately 7.5 X 6 X 4 mm, referred to below as the larger eucrite) proved easy to separate, yielding 340 mg of pristine material. A second, much smaller clast (the smaller eucrite) was separated with much more difficulty, yielding only 33 mg. Contaminated fragments of these cfasts, i.e. with adhering matrix, were allocated for petrographic study. Other eucrite clasts are readily recognized in the meteorite but cannot be separated because of their small size. Carbonaceous clasts are generally less than 2 mm in size but can be separated with some confidence because of their distinctive, and contrasting, nature. Thirty of these dark clasts, each weighing a few mg, were removed from the meteorite. As noted by earlier workers, there is a wide range of pyroxene compositions in the matrix. Many of the diogenitic pyroxenes form larger (a few mm diameter) monomineralic clasts with a distinctive green colour and can be easily separated. The fragments thus obtained (LAUL et al., 1990) are

Eucrite class

The ‘larger eucrite’ is similar to other eucrite clasts examined. It has a fine-grained igneous texture (Fig. la), dominated by anhedral pyroxene and subhedral plagioclase. Feldspats are cloudy and pyroxenes show no zoning and have narrow, regular exsolution lamellae. There is no obvious mesostasis and minor amounts of ilmenite, a silica phase, and troilite are also present. Search for a phosphate phase revealed a single, small calcium phosphate grain (adjacent to the meteorite fusion crust) in an area where troilite is locally abundant. Pyroxene compositions, when plotted on the pyroxene quadrilateral (Fig. 2a), show a Juvinas-like distribution of points between ferrohypersthene and ferroaugite, representing the inability of the microprobe beam to resolve the narrow, parallel exsolution lamellae. Plagioclase grains in the large eucrite range from Anss.s to An93 (Fig. 3a). The smaller eucrite clast is similar in mineral composition (Fig. 2b, 3b) but shows areas (Fig. 1b) where r~~s~liization to a fine equigranular aggregate has pardy replaced the original igneous texture. Other smaller eucrite clasts in the matrix have similar mineral compositions and a range of textures that probably represent variations in the extent of recrystallization. No primary cumulate textures were observed in the eucrite clasts. LAULand GCSSELIN (1990) show that the two larger eucrite clasts have compositions similar to noncumulate eucrites, that the two clasts differ in bulk composition, but that both show a light REE depletion pattern unique to normal eucrites. The textures and mineral compositions in the eucrite clasts appear to indicate a history of direct, rapid crystallization 2161

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A. M. Reid et al

FIG. I. Photomicrographs of the Bholghati howardite. (a) The ‘larger eucrite’, (b) the ‘smaller eucrite’, (c) one of the carbonaceous clasts, and (d) the matrix. Widths of field: (a) 3.9 mm. (b) .9 mm, (c) .6 mm. (d) 3.9 mm.

A

6

Hd ‘t . .- . . . 0. E

FS

En

,

t

I

FS

FIG. 2. Compositions of Bholghati pyroxenes, in terms ofquadrilateral components. in (a) ‘larger eucrite’, (b) ‘smaller eucrite’. (c) carbonaceous clasts, and (d) matrix.

FS

Fs

Petrography and mineral chemistry of Bholghati

80

90

2163

All

FIG. 3. Composition of Bholghati feldspars in (a) ‘larger eucrite’, (b) ‘smaller eucrite’, (c) matrix.

from an iron-rich eucrite melt, followed by prolonged annealing promoting exsolution and homogenization ofthe pyroxene lamellae at subsolidus temperatures. During this annealing there was some local redistribution or introduction of FeS (or S) to produce troilite concentrations. Figure 4 shows the relationship between exsolution in one pyroxene and an included troilite grain; exsolution and formation of the troilite may be contemporaneous. Petrography does not differentiate between annealing as a consequence of flattening of the initial temperature-time cooling curve or as a consequence of later

FIG. 5. Secondary electron images of carbonaceous clasts from Bholghati. (a) A typical CM24ike clast, width 0.8 mm. (b) CI l-like clast, width 0.5 mm.

reheating (metamorphism). However, evidence from the larger eucrite indicates a possible disturbance affecting the eucrite pyroxenes at less than 2.9 Ga (NYQUIST et al., 1989), perhaps indicating that annealing occurred as a metamorphic episode postdating initial crystallization by > 1.6 Ga. Carbonaceous

FIG. 4. Backscattered electron scanning image of exsolution lamellae (light-dark lamellae) in a pyroxene from the ‘larger eucrite’, showing their relationship to an included grain of troilite (central light area).

clasts

The carbonaceous clasts are small (l-2 mm) and most consist of subequal amounts of silicate and matrix (Figs. lc, 5). The silicates form irregular clusters, some of which resemble chondrules, and have rare inclusions of kamacite (- 5 weight percent Ni). Texturally, the majority of these clasts resemble CM2 meteorites (Fig. 5a). The major silicate phases are olivine (forsterite) and pyroxene (much of it twinned clinoenstatite), both with low iron contents (- 1 wt% FeO; Fig. 2~). Minor low iron augite is also present. Representative silicate compositions are shown in Table 1. The matrix is very dark, fine grained, and fairly uniform within a single clast, apart from the irregular distribution of sulphides (pyrrhotite and pentlandite) and Ca-carbonate. Average matrix compositions from four of the carbonaceous clasts are shown in Table 2 (electron microprobe defocused beam analyses). Figure 6a is a plot of Ca/Al versus Mg/Si to compare the Bholghati clasts with matrix data from other carbonaceous

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A. M. Reid et al

Table 1.

Selected Analyses of Olivine in Carbonaceous Clasts

OL-2

OL-1 Si02 Ti02 A1203 Cr203 Fe0 MnO MgO CaO NE@

42.1

42.6

.58 1.26 .31 55.0 .21

.62 1.69 .35 54.7 .19

-

_ 100.2

99.5

and Pyroxene

PX-1 59.5 .19 .87 .51 .72 .06 36.9 .59 99.3

PX-2 58.9 .21 1.21 .62 1.20 .12 35.4 1 .61 Ol_ 99.3

chondrites (MCSWEEN and RICHARDSON, 1977). Plotted in this manner, these data resemble, but do not match, CM data; they are notably lower in Mg than most carbonaceous chondrites. However, MCSWEEN (1987) reports carbonaceous chondrite matrices, including several Antarctic meteorites. that have comparable Mg/Si ratios. On a diagram of matrix S/Si versus Fe/Si (Fig. 6b), the different carbonaceous chondrite groups are better distinguished. The matrix compositions of the Bholghati carbonaceous clasts lie, with one exception discussed below, in or near the CM field. The dominant matrix phase is a ferroan antigorite, which is consistent with the tentative identification of these carbonaceous clasts as CM2like material (ZOLENSKY and MCSWEEN, 1988). One ofthe Bholghati clasts exhibits the following interesting petrographic features: (1) no chondrule-like aggregates of anhydrous silicates (perhaps a sampling artifact due to the clast’s small size - 1 mm), (2) abundant, small (~40 pm diameter) aggregates and separate grains of magnetite with framboidal and spherulitic textures, (3) small (~30 pm) aggregates and loose grains of fine-grained, anhedral pyrrhotite and pentlandite (principally the former), all set within (4) dense. opaque, extremely fine-grained dark matrix (Fig. 5B). All of these features are reminiscent of CIl chondrites (e.g., ZOLENSKY and MCSWEEN, 1988). This clast also contains one

Table

Clast SO2 TiO2 A1203 Cr203 Fe0 MnO MgO CaO :g P205

2.

Matrix (electron

Compositions of Carbonaceous microprobe defocused beam)

Clash

1

2

27.4

25.2

26.9

33.1

.07 2.57 .42 33.8 .25 15.8 .58 .72 14.8.08

.07 2.95 .31 40.3 .22 14.1 .75 .99 13.4.09

.08 2.88 .26 36.5 .22 14.6 .27 .66 9.9 .09

0.12 2.64 0.34 20.2 0.19 19.2 0.61 0.54 11.7 0.19

J2 96.6

Jz 98.6

J-9 92.6

89.0

1-3 are CM2-like clasts 4 is the Cll-like clast

3

4

.;

.8

.9

Mg/Si

0.9

1.1

1.3

1.5

1.7 Fk"/s~"

2.3

2.5

2.7

2.9

FIG. 6. (a) Selected element ratio plot (Ca/Al versus Mg/Si) for average matrix material in Bholghati carbonaceous clasts (electron microprobe analyses, defocused beam), in comparison with the matrices of other carbonaceous chondrites (from MCSWEENand RICHARDSON, 1977). Bholghati clasts indicated by large solid dots. (b) Selected element ratio plot (S/Si versus Fe/Si) for average matrix material in Bholghati carbonaceous clasts (electron microprobe analyses, defocused beam). (Data for other carbonaceous chondrites from MCSWEENand RICHARDSON,1977; MCSWEEN,1987; ZOLENSKYet al.. 1989a,b). Bholghati clasts indicated by large solid dots.

grain of olivine, which we suggest was physically inserted prior to incorporation of this clast into the Bholghati host. The composition of the pyrrhotite in this clast ranges from Fe.szNi.oXS to Fe,ssNi,o,S, while that of the pentlandite could not be determined because of its fine intergrowth with the pyrrhotite. All of the analyzed magnetites are very pure Fe304. Fine-grained (< 10 pm) crystals of a Ca-carbonate are also present. The matrix composition of this clast (Table 2) is similar, albeit S-rich, to that of Cl chondrites (Fig. 6b). The dark clasts in Bholghati are not all the same but represent the admixture of a range of materials. The close association of low-iron silicates, metallic iron, sulphides, Ca-carbonates, and phyllosilicate-rich matrix in these clasts strongly suggests assembly at low temperatures prior to incorporation into the howardite matrix. Preservation of this assemblage precludes significant heating subsequent to mixing with silicate fragments to form the polymict breccia. Matrix

The main mass of the Bholghati meteorite is a breccia of angular, mostly monomineralic fragments (Fig. Id). They

Petro~phy

and mineral chemistry of Bholghati

216.5

there is a continuum of compositions in the pyroxene data (for both major and minor elements, Figs. 2d, 8a, 8b), and no evidence for the existence of two, or more, distinct pop ulations of pyroxene in the matrix. Mineral data indicate a wide, but related range of compositions, consistent with derivation from a series of rocks related by f~c~onation. Matrix feldspar data (Fig. 3c) also show a range of values, extending to more calcic compositions than the eucrite data (Figs. 3a and b). Presumably, this more calcic plagioclase originally coexisted with pyroxenes intermediate in composition between the eucritic and diogenitic components.

10

20

50

60

FIG. 7. Histogram of 100 X Fe/Fe + Mg (atomic) for Bholghati matrix pyroxenes.

range in size from fine dust to several mm, with many of the larger fragments being individual grains of magnesian orthopyroxene. At all sizes the texture is fragmental, we have found no evidence of reaction relationship between grams in the breccia, or of the growth of new phases in the finer grained portions of the breccia. As noted above, the eucrite clasts are annealed but there is no indication of reaction between eucrite and matrix; eucrite fragments must have cooled to low temperatures before their incorporation. Shock features are also generally lacking in the breccia. Many of the pyroxene grains show strain and some have completely recrystallized to polycrystalline aggregates, prior to inco~mtion. The more magnesian pyroxenes are large and chemically homogeneous. This is also generally true of the intermediate composition pyroxenes, some of which, however, show well-developed exsolution of high calcium clinopyroxene from a low calcium host. The iron-rich pyroxenes are also unzoned and generally show exsolution. These iron-rich ‘eucritic’ pyroxenes are more commonly intergrown with other phases, dominantly feldspar, than the intermediate to high Mg pyroxenes, which form isolated fragments. Other phases in the matrix include ilmenite, chromite, SiOz, olivine (For2 to Foss), rare calcium phosphate, troilite, and metal. One very small angular fragment of brown glass, and a few small fragments of devitrified glass, were also noted. Figure 2d shows the range of pyroxene compositions in the matrix. Similar plots have been presented by LABOTKA and PAPIKE (1980) and by PRINZ et al. (1980). There are clearly ‘diogenitic’ and ‘eucritic’ pyroxenes present but a wide range of intermediate compositions also occur, from Ena, orthopyroxene to now-exsolved ferropigeonite (M&/Fe = 0.26). Figure 7 is a histogram of 100 X Fe/Fe + Mg in matrix pyroxenes showing that in a continuum of compositions there is a preferred grouping around the ‘diogenitic’ value of En74 (see also LABOTKAand PAPIKE, 1980). Figure 8a is a plot of Mn versus Fe for matrix pyroxene that shows a strong positive correlation and no ~luste~ng. The same is true for the positive correlation between Cr and Al (Fig. 8b). One of the preferred hypotheses to explain howardite compositions is a simple mixing of diogenite and eucrite (e.g., JEROMESand GOLES, 197 1; DREIIKJSet al., 1977). However,

Sequence of events The car~naceous elasts have a complex history beginning with early formation of low iron silicates, Fe-Ni sulphides, and low temperature carbonaceous matrix. Aggregation of these diverse materials was accompanied by probable aqueous alteration of pre-existing anhydrous materials, and formation of phyllosilicates, carbonates, and probably secondary generations of sulfides and magnetite (ZOLENSKYand MCSWEEN, 1988). Disruption of this carbonaceous material was followed by inco~omtion into the howardite matrix, perhaps as a consequence of low velocity impact onto the parent body surface. In sharp contrast, the evolution of the eucrite fragments involves: (1) formation by separation of a ferrobasalt (eucrite) melt in a very early (4.55 Ga) magmatic episode, with fairly rapid (volcanic) cooling on or near the surface of the parent

a

1

0.04

02

04

0.6

a

08

Fe

0.00: 0.01

.

, 0.02

.

, 0.03

.

, 0.04

.

, 0.05

.

, 0.06

.

, 0.07

Al FIG. 8. Minor element plots for Bholghati matrix pyroxenes. (a) Mn versus Fe, (b) Cr versus Al. Units are atoms per 6 oxygens.

A. M. Reid et al

2166

HED PARENT BODY

CARBONACEOUS

PARENT BODY

Diogenites (cumulate/residue) intermediate Compositions (cumulate)

Prolonged annealing 4.5 Ga” 2 9 Ga7 Later? \ Disruption and Mwng (surface regollth)

Low Velocity Impact and Admtxture.

\

Mixing I” Surface Regollth Low Temperature Lithiflcatlon

1 Fall, 1905

I%. 9.

Schematic

evolution

body; (2) burial and subsolidus annealing to produce both the exsolved unzoned pyroxenes and cloudy feldspars and to allow some redistribution of troilite (an intriguing question is whether this annealing event, of the type that affects many eucrites, was a much later, i.e., metamorphic, event): and (3) disruption of the eucrite lavas at temperatures well below the annealing temperatures, and mixing with more magnesian fragments (diogenite, cumulate eucrite) as a surface regolith on the parent body. The Bholghati howardite is a regolith breccia incorporating a wide range of related igneous fragments, with diogenite and eucrite clasts being the most common. Minor amounts of exotic materials (different types of carbonaceous chondrite) have also been incorporated into the regolith indicating that these diverse materials were at one time in orbital proximity. Some of the major events in the evolution of Bholghati are summarized schematically in Fig. 9. It is apparent that we need more quantitative data on the physical processes, temperature, and timing of these events, which are common to a large number of howardites. .4cknowledgments-We thank Vincent Yang for help with the probe work, Dave Meaux for data processing, and Nghi Hua for word processing. Samples for this study were kindly provided by the Geological Survey of India. Support of the Lunar and Planetary Institute, which is operated by the Universities Space Research Association under contract NASW-4066 with the National Aeronautics and Space Administration, is acknowledged by AMR. ME2 was supported by a grant from the NASA Planetary Materials and Geochemistry Program. This is Lunar and Planetary Institute Contribution 725. Editorial handling: R. J. Bodnar REFERENCES DREIBUSG., DRUSE H., SPETTELB., and WANKE H. (1977) The bulk composition of the moon and the eucrite parent body. Proc. Lunar Planet. Sci. Conf: 8th. 2 1l-221.

1

of the Bholghati howardite FUHRMANM. and PAPIKE J. J. (1981) Howardites and polymict eucrites: regolith samples from the eucrite parent body. Petrology of Bholgati, Bununu, Kapoeta, and ALHA 76005. Proc. Lunar Planet. Sci. ConJ: IZth, 1251-1279. HEWINSR. H. and KLEIN L. C. (1978) Provenance of metal and melt rock textures in the Malvern howardite. Proc. Lunar Planet. Sci. Conf: 9th, 1137-1156. JEROMED. Y. and GOLES G. G. (1971) A reexamination of relationships among pyroxene-plagioclase achondrites. In Activation Analysis in Geochemistry and Cosmochemistry (eds. A. 0. BRUNFELT and E. STEINNES), pp. 26 l-266. Universitetslaget, Oslo, Norway. LABOTKAT. C. and PAPIKEJ. J. (1980) Howardites: samples of the regolith of the eucrite parent-body: petrology of Frankfort, Pavlovka, Yurtuk, Malvem, and ALHA 77302. Proc. Lunar Planet. Sci. ConJ: Filth, 1103-l 130. LAUL J.-C. and GOSSELIND. C. (1990) The Bholghati howardite: Chemical study. Geochim. Cosmochim. Acta 54,2 167-2 175 (this issue). MCSWEENH. Y. JR. (I 987) Aqueous alteration in carbonaceous chondrites: mass balance constraints on matrix mineralogy. Gwchim. Cosmochim. Acta 51, 2469-2411. MCSWEENH. Y. JR. and RICHARDSON S. M. (1977) The composition of carbonaceous chondrite matrix. Geochim. Cosmochim. Acta 41, 1145-l 161. NYQUIST L., WIESMANNH., BANSALB., and SHIH C.-Y. (1989) RbSr age of an eucritic clast in the Bholghati howardite and initial Sr composition of the Lewis Cliff 86010 angrite (abstr.). Lunar Planet. Sci. Conf 20th. 198-799. PRINZM., NEHRUC. E., DELANEYJ. S., HARLOWG. E., and BEDELL R. L. ( 1980) Modal studies of mesosiderites and related achondrites, including the new mesosiderite ALHA 772 19. Proc. Lunar Planet. Sri. Cotzf Ilth, 1055-1071. ZOLENSKYM. E. and MCSWEENH. Y. JR. (1988) Aqueous Alteration. In Meteorites and the Early Solar System (eds. J. F. KERRIDCE and M. S. MATTHEWS),pp. I I4- 143. University of Arizona Press. ZOLENSKYM. E., BARRETTR. A., and GILDING J. C. (1989a) Matrix and rim compositions compared for 13 carbonaceous chondrite meteorites and clasts (abstr.). Lunar Planet. Sci. ConJ 20th, 12491250. ZOLENSKYM. E., BARRETTR. A., and PRINZ M. (1989b) Mineralogy and petrology of Yamato-86720 and Belgica-7904. 14th Symposium on Antarctic Meteorites (in press).