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The Bholghati howardite: Chemical study
0016-7037/89/$3.00 + .%I
Gwchimica et Cosmochimica Acla Vol. 54, pp. 2 167-2 175 Copyright 0 l!XlO Pergamon Press pk. Printed in U.S.A.
The Bholghati howardite: Chemical study J.-C. LAUL and D. C. GOSSELIN* Chemical Sciences Department, Battelle, Pacific Northwest Laboratory, Richland, WA 99352, USA (Received June 12, 1989; accepted in revisedform May 29, 1990)
Abstract-Bholghati is homogeneous on a cm scale in major element composition, but heterogeneous in trace elements such as the rare earth elements (REEs). The REEs are concentrated in accessory phosphates, and their sampling can influence the REE patterns of eucritic clasts which, in turn, can govern the bulk REE patterns. Bholghati can be reasonably modeled with 56% eucritic, 45% diogenitic, and 3% dark clast components; however, an additional component (feldspathic) is required to match the bulk composition. Dark clasts represent carbonaceous CM type material that was admixed with other components during regolith formation on the eucrite parent body (EPB). One eucritic clast (fine-grained) is light REE (LREE) depleted relative to heavy REE (HREE), which is unusual in normal eucrites. The LREE depletion pattern cannot be modeled from one-stage partial melting of a chondritic source; however, it can be modeled from a non-chondritic source depleted in LREEs or multi-stage melting of a chondritic source, provided that the LREE depletion is not from undersampling of phosphates. Bholghati has a complex history that involved early multiple magmatic events, later annealing, and fragmentation and low-temperature mixing of eucritic, diogenitic, and carbonaceous components on the howardite parent body. is a fairly large asteroid. Vesta has been associated with a eucrite composition (DRAKE, 1979). PAPANASTASSIOU et al. (1974) and PAPANASTASSIOU and WASSERBURG( 1976) reported young Rb-Sr ages of 3.63 and 3.89 Ga for two eucritic clasts from Kapoeta howardite. Later, RAJAN et al. (1979) reported K-Ar ages of 3.48 and 4.5 Ga for the same clasts. These data suggest that the isotopic systematics have probably been disturbed by a post-crystallization thermal event(s) on a parent body. Thus, the HED may have originated from a fairly large parent body(ies) that had undergone extensive early (-4.6 Ga) igneous volcanic activity. Detailed studies of HED samples provide us an opportunity to increase our understanding of the complex history of their formation. Bholghati fell on October 29, 1905, in Maurbhanj, Orissa, India. The original recovered mass was 2.5 kg. Through the courtesy of the Geological Survey of India, about 10 g of Bholghati was obtained for multidisciplinary studies. The sample was processed and distributed to the consortium members by the personnel of the meteorite processing laboratory of the Johnson Space Center. The main focus was on a large 340-mg white eucritic clast (our split # 15) and the bulk sample. The details of sample distribution are provided by LAUL (1990). Previous chemical studies on howardites have been mostly on bulk analyses and mixing models using different eucrites and diogenites (e.g., DREIBUSet al., 1977; FUKUOKA et al., 1977). Chemical data on Bholghati have been reported by CHOU et al. (1976) and MITTLEFEHLDTet al. (1979). In this chemical study, we analyzed 32 major, minor, and trace elements in four bulk samples from different locations of Bholghati to test compositional variation in two eucrite clasts, and in mineral separates (pyroxene and plagioclase). 7Ve also performed leaching experiments on the bulk sample with 1M HCl to estimate the REE content in phosphates. The Bholghati bulk composition is modeled using in situ com-
INTRODUCTION HOWARDITESAREPOLYMICTBRECCIASand represent samples of regolith from the eucrite parent body (EPB). DUKE and SILVER (1967) suggested that howardites are mechanical mixtures of eucrites (pigeonite basalt) and diogenites (orthopyroxenites), based on textures of impact metamorphism and mineralogy. JEROME and GOLES (1971) interpreted the chemistry of howardites as simply mixtures of eucrites and diogenites (Fig. la). Their views have been supported by other studies (MCCARTHY et al., 1972, 1973; DREIBUSet al., 1977; FUKUOKA et al., 1977). CHOU et al. (1976) pointed out that siderophile data from howardites suggest the presence of 2 to 4% chondrite material in addition to the eucritic and diogenitic components (Fig. lb). The detailed petrographic and mineralogical studies by BUNCH (1975) DYMEK et al. (1976), LABOTKAand PAPIKE (1980), and FUHRMANand PAPIKE(198 1) indicate that howardites are parent body surface breccias that have undergone intense and repeated fragmentation and metamorphism by impacts. These authors found other components in the howardites in addition to eucritic, diogenitic, and chondritic components. The compositional range of pyroxenes in howardites is broader than those of eucrites and diogenites, thus precluding a simple mixing process (SIMPSON,1975; TAKEDA et al., 1976; DYMEK et al., 1976). Thus, the mineralogy and chemistry of howardites are quite complex, which has led to different views regarding the origin and interrelations of howardites, eucrites, and diogenites (HED). Based on spectral reflectance data, HED meteorites are believed to be derived from near-surface region of a differentiated V-type asteroid, like 4 Vesta (MCCORD et al., 1970; THOLEN, 1984), which
* Present address: Nebraska Geological Survey, Conservation Survey Division, University of Nebraska, Norfolk, NE 6870 1, USA. 2167
2168
J.-C. Laul and D. C. Goss&n EXPERIMENTAL
BHOLGHATI 14
We received four bulk (matrix) fragments (0.4 g each) of splits #I I, 13, 24, and 25 from different locations of Bholghati (LAUL. 1990). These samples were homogenized in an agate mortar and pestle. Aiiquots of # I I, 24. and 25 were sent to M. E. Lipschutz for analysis of volatile/mobile trace elements and to R. N. Clayton for oxygen isotope study. We used two aliquots of #I I. 13,24, and 25 bulk samples: one for instrumental neutron activation analysis (INAA) for about 30 elements; the second for radiochemical NAA (RNAA) for the REEs using the procedure of LAUL et al. (1982). The SiOZ content of bulk samples was measured by atomic absorption. Eucritic clasts #6 (30 mg) and #I 5 (40 mg) were split into two aliquots. One aliquot was used for 1NAA and the other for RNAA. Dark clast #4, diogenite #45, and plagioclase and pyroxene separates from clast # 15 were analyzed by INAA. Following the completion of our INAA study, eucritic clasts #6B (I 2.1 mg) and #15A (22.3 mg) and dark clast #4 (2.84 mg) were sent to M. E. Lipschutz for volatile/mobile element analysis by RNAA (WANG et al., 1990). The plagioclase (3.4 mg) and pyroxene (4.2 mg) were separated from the large eucritic clast #l by L. E. Nyquist’s group. We used 200 mg of bulk sample # 13 in a leaching experiment to assess the influence of phosphate phases on the trace element chemistry. Phosphates dissolve quantitatively in IM HCI in IO to I5 min (LAUI. et al., 1986; LAUL, 1987). Following irradiation, the sample was treated with 5 ml I M HCI for 10 min. The solution was filtered through a fine filter paper and collected in a IO-ml volumetric flask. A 4-ml aliquot was used for INAA and a 6-ml aliquot for RNAA for the REEs. The residue was dried, reweighed, and split into two aliquots. one each for INAA and RNAA. A 25-mg aliquot of the unleached bulk sample was also analyzed by INAA and RNAA for mass balance. A USGS BCR-I standard was used as a control in the INAA and RNAA. Other standards used in the INAA were BHVO- I, GSP1, PCC- 1, and Allende powder. AI1samples were counted on a normal Ge(Li) detector (25% efficiency, FWHM 1.8 keV for a 1332-keV peak of @Co) and a 4096 channel analyzer, and coincidence-noncoincidence Ge(Li)-NaI(T1) counting systems (LAUL, 1979).
. DIOGENITE . EUCRITE CLAST o
Carb. Clasts
BULK
A CAFiB.-LIKE CLAST
0.1 0.20.30.40.50.60.70.8
molar
FeO/(FeO+MgO)
Dark
Clasls
0
2
Eucr~tic
40
20
Clast
60
80
100
120
FeOlMnO FIG. I. (a) Plot of CaO vs. molar FeO/(FeO + MgO). Our samples fall in or near the fields of eucrites, diogenites, and howardites. (b) Plot of MgO/A1203 vs. FeO/MnO for bulk, eucritic clasts, diogenite, and dark clasts.
RESULTS
ponents in a mixing model. Eucritic clast # 15 is compared with other eucrites and is evaluated with respect to petrogenetic modeling. Our detailed approach has provided a much better understanding of the geochemical evolution of Bholghati.
r
wt.
Table 1. Major Abundances Sample Bulk
Split (matrix)
11 13 :z
#
(%) in Bholghati Samples by INAA
(mg)
SiO2
TiO2
Al203
Fe0
MgO
72.3 53.3 49.0 44.5
49.0
0.44 0.74 0.42 0.58
8.30 8.40 8.29 8.53
18.3 18.3 18.2 17.7
17.6 15.7 17.0 16.2
0.55
8.38
18.1
16.6
I\verage Reported of Bulk(=)
The chemical data for 32 elements for the Bholghati samples, along with the USGS standards BCR- 1 and BHVO- 1. are shown in Tables I and 2 for the major elements (oxide wt%) and trace element abundances (ppm), respectively.
CaO
17.5 19.5
Data
Na20
K20
MIIO
CrzO3
Fe’
6.86 6.34 6.70 7.20
0.281 0.310 0.272 0.291
0.026 0.026 0.022 0.027
6.33
0.289
0.025
0.528 0.506 0.526 0.527 0.522
0.783 0.692 0.690 0.766 0.733
0.37 0.39 0.38 0.37 0.38
5.73
0.288 0.252
0.026
0.508
0.685 0.763
0.369 0.520
0.075
0.430 0.351
0.;27
0.61
3.62 8.95
0.400 0.340
0.037 0.030
0.540 0.644
0.351 -
0.65
16.7 5.70
1.08 0.051
0.075 0.0060
0.034 0.963
0.014 0.234
0.64 0.56
Eucrite (While)
Clast
6A B
12.8 12.1
1.1 1.7
8.18 11.8
14.2
5.6 4.6
Eucrite (While)
Clast
15A B
22.3 12.0
0.51 0.47
12.8 10.9
24.5 -
7.0 7.0
3.37 4.21
<.4
31.5 1.65
1.10 30.0
0.35 13.1
4
2.84 2.64
<.5 <.5
2.35 2.58
35.3 34.2
26.3 22.1
1.64 2.46
0.260 0.550
0.045 0.060
0.311 0.245
0.641 0.547
0.43 0.46
45
0.42
0.20
1.60
16.0
26.2
1.90
0.040
0.0030
0.465
0.819
0.25
!&!I&& 12.2 3.3 54.5 -2.20 13.6 BCR-1 11.3 7.6 2.5 13.7 BHVO-1 .ror* baled on counting 51115,155are X.5-3.% tar s,o*. T@ p.1203, MI, M”O, tia*o and cr2 0,
6.90 11.6
1.70 0.52
0.180 0.170
0.0020 0.044
Pkig. (b’ Pyroxene Dark
Clast
Diogenite
, Mllllelchldt e,8,.(1979) ,mg.and Pyraxene mmEusrlle da51 “15, were provided byriyqwt e,a,(w30) )Fe’ =Fe*,(Feo +M$iO, nl.lm
10.9 12.8
-3.20 2.22
;i5x1.3, Idgo, caoB”dI$0
(C)
Chemical mixing model of Bhol~ati
2169
Table 2. Trace Abundances (ppm) in Bholghati Samples by INAA and RNAA* Sample Bulk(matrix)
Split # 11 13 24 25
SC 21.3 24.8 23.3 21.7 22.8
v 110 107 108 110 109
co 30.7 26.6 26.1 31.3 29.0
@a 70 30 $8 15 21
sr 45 60 50 44 50
24.4 25.8 Eucrite (White)Clest Eucrite (White) Clast
w
BHVO.l
1.30 2.05 1.64 1.46 1.61
Ce
Nd
3.2 4.3 4.2 3.6 4.0
2.6 4.2 3.2 2.9 3.2
1.49 1.38 4.9
Sm 0.82 1.30 1.02 0.90 1.01
Eu
Gd
0.30 1.1 0.38 1.5 0.35 1.4 0.34 1.2 0.34 1.3
0.95 0.04 0.33
Tb 0.20 0.28 0.25 0.24 0.24
Ho
Tm
0.28 0.12 0.46 0.19 0.35 0.15 0.31 0.13 0.35 0.15
0.27
Yb
Lu
0.83 0.13 1.24 0.19 1.02 0.16 0.93 0.14 1.00 0.16
Zr
HI
40 30 30 50 40
0.62 0.88 0.74 0.53 0.69
0.98 0.16
6A 0
69 44.0 54
5.2 30
78
15A 0
28.6 70 57
16.4 30
110 1.65 4.0 (6.0)1.10 0.61 1.6 0.34 0.27 1.66 0.27 20 70 1.50 3.8 3.2 1.00 0.50 1.6 0.33 0.43 0.23 1.60 0.23 -
1.6042.7 -
$.F . ;;
250 0.73 1.6 1.0 0.24 1.40 <50 1.10 3.0 3.0 0.95 0.050-
4
10.0 83 10.0 75
460 ~50 <50 0.50 c2 690 0.39 -
45
14.0 140 12.1 -
Plag.tb) Pyroxene OarkCIast
La
-
-32.0'400 "36.0 670 330 25.5 54 31.0 300 44.0 150 16.0 36
Th
0.11 0.14 0.10 0.10 0.11
0.23410 0.30500 0.27340 0.14410 0.23410
NI
540 670
0.60
2.64 7.7 5.8 2.14 0.49 3.2 0.63 0.92 0.39 2.61 0.43 9.75 25.0 22 6.30 0.66 8.5 1.3 1.7 0.72 4.13 0.62 110 4.0
0.30 -
la
0.64 1.2 8.4
0.90 0.10 0.21 26 -
0.0600.17 c.04 0.17 0.028~40 0.62 c.1 .x.04 0.30 0.51 0.23 1.40 0.33 ~40 0.60 c.2 c.2 ~20
PAdJb d'pt 11.8 11.9 5.3 6.4 8.6
21 23 17 23 21
6.9 7.5
20 20
c5
<5
c5 -
<5 -
2.0
cl c2
<4 -
0.30 0.25 0.24 0.22 -
<.5 c.6 c.3 c.3 -
0.32 0.066~50 0.49 0.31 <.5 9880 106 316 0.26 0.0531.3 0.60 <.5 134OS(660) 616
-
0.18 <.03 -
<.7 <.l -
0.47 0.074-
30 27
6.80 2.00 7.3 1.10 1.2 0.56 3.40 0.54 180 "4.70"0.80 8.2 6.30 1.90 8.0 1.00 2.10 0.32 4.4 1.20 1.1 130
-
-
-
4.6
-
-
-
Estlmaled 8rrors based on counting statistics we: 0.64% for SC, Co, La, Sm, Eu, Yb and Lu; 6-M?& for V, Ba, Cs, Nd, Tb, Ho. Hf Ta, Tb and NI; 16-20% for Sr, Gd, Tm, Zr, Au and Ir. ‘REE data we by RNAA for sample split’s # t 1,24,25,6A and 168. Tbe data for remaining samples are by INAA (a) Data from Chou et al. (1976); Mittlefehldt et ai. (1979) (b) Plag. and Pymxene prom Eucrfie clast #IS) were provided by Nyqulst et at. (1990)
Typical errors based on counting statistics are also shown in Tables 1 and 2. The REEs for bulk samples and eucritic clasts were determined by RNAA to obtain better data for Nd, Gd, Ho, and Tm. In INAA, we used BCR- 1 as control for TiO* , A1203, FeO, Na,O, I&O, MnO, SC,Co, Hf, and Ta. We used GSP-1 as 105 ppm for Th; PCC-I as 45.7% for MgO, 0.41% for Cr203, and 0.25% for Ni; and the Allende standard as 140 ppb for Au and 780 ppb for Ir. Our CaO value for the bulk is -20% higher and our Co and Ce values are 30 to 40% lower than those of MITTLEFEHLDT et al. ( 1979). This difference may be due to sampling. WANG et al. (1990) analyzed the same splits as we did, and Au and Co are common elements between their samples and ours. Their Au values agree with ours, but their Co values by RNAA for splits #4, 6B, 15A, and 25 are systematically lower than our INAA data. Our Sm value for pyroxene is about twice as high as that of NYQUIST et al. (1990). Their pyroxene was washed with lM HCl, while ours was not. This difference is likely due to contamination from trace phosphates, which are extremely high in REEs; 0.56% phosphate could explain this difference. CHEMICAL SYSTEMATIB Major Elements
The four bulk samples, from different locations of Bholghati, are quite homogeneous in major composition. Their mean composition is 0.55% TiOz, 8.4% A1203, 18.1% FeO, 16.6% MgO, 6.9% CaO, 0.29% Na*O, 0.025% I&O, 0.52% MnO, and 0.73% Cr203. Their FeO/(FeO + MgO) molar (Fe’ = 0.37 to 0.39) and FeO/MnO (33 to 36) ratios have a narrow range and are similar to other howardites (MITTLEFJZHLDT et al., 1979; SMITH, 1982). Figure la shows a plot of CaO versus Fe’ for HED. Our bulk samples fall in the howardite
field, which lies between the eucrite and diogenite fields. In a plot of MgO/A120~ versus FeO/MnO (Fig. 1b), bulk samples lie nearly between diogenite and eucritic clasts. The two eucritic clasts, #6 and 15, differ notably in TiOz (1.4, 0.50%) and Fe0 (14.2, 24.5%) These eucrite clasts are similar to noncumulate eucrites in terms of their Fe’ (0.61, 0.65) and CaO (11.8,9.3%). The FeO/MnO ratio (40,45) exceeds that generally observed for eucrites (N 35) and might be explained by accessory troilite. The diogenite split #45 has Fe’ of 0.25 and FeO/MnO ratio of 34 and is intermediate in major composition between Roda and Johnstown (FUKUOKA et al., 1977). The ~~~bution of bulk samples in Fig. 1 supports a simple mixing of diogenite and eucrite components for Bholghati. Pyroxenes in the Bholghati matrix (bulk) show a continuum, with compositions ranging from diogenitic to eucritic (REID et al., 1990), which suggests an additional component(s) in the mixing process. The two dark clasts (#4) seem to be a carbonaceous type, based on CaO vs. Fe’ and MgO/Al203 vs. FeOfMnO plots (Fig. 1). Petrologically, three dark clasts resemble CM2 chondrites and one dark clast a CI type, with low-iron silicates, Fe-Ni sulphides, and Ca-carbonate in a fine-grained dark matrix (REID et al., 1990). Preservation of this low-temperature assemblage precludes any significant heating prior to incorporation into the Bholghati matrix (REID et al., 1990). Dark clasts from Kapoeta howardite have been noted to be CI type material (SMITH, 1982). Trace Elements-REEs
The chondrite normalized REE patterns of the bulk samples are shown in Fig. 2. Bulk samples are heterogeneous with respect to trace elements such as REEs, which vary by about 60%. The bulk REE patterns are nearly flat (e.g., La
2170
J.-C. Laul and D. C. Gosselin 10.0 !!
Rholghatl (Bulk Samples) % Leached in 1M HCI, {lO min)
1.0
AEE ionic Radii
2. Chondrite normalized REE patterns of Bholghati bulk samples. The chondrite values used are: 0.34 La, 0.87 Ce, 0.64 Nd, 0.195 Sm, 0.073 Eu, 0.26 Gd, 0.047 Tb, 0.33 Dy, 0.078 Ho, 0.032 Tm, 0.22 Yb, and 0.034 Lu. REE contents vary by 60% in bulk samples.
La SmTb Yb Lu Eu Na Coca
--4X), with a slight negative Eu anomaly that increases with increasing REE contents. Traces of phosphates have been noted petrologically in the matrix (REID et al., 1990). This suggests that the presence of accessory phosphate (whitlockite, apatite) phases can influence the bulk REE patterns (LAUL et al., 1989). To quantify the REE concentrations in phosphates, we performed the leaching experiment discussed below.
Fe SC Cr
Element
I%.
FIG. 3. Percent element leached in I M HCI (IO min.) relative to the bulk in Bholghati. Soluble REE range from 84% La to 28% Lu. Fe, SC. and Cr are leached below 2% level.
Cr are dissolved below 2%. indicating only a minor contribution from pyroxenes. The REE patterns of the leachate, residue, and bulk are shown in Fig. 4. Accounting for the 0.89% phosphate in the leachate, the light REE (La-Sm) pattern is nearly flat at about 490X (chondrite), with a strong negative Eu anomaly at 72X and Gd and Tb at 480X with
1000
Leaching with IM HCI 500
The data for REEs, SC, Co, Ca, Fe, Na, and Cr for the leachate, residue, and bulk are shown in Table 3. Figure 3 shows the percentage of element leached from the bulk. Relative to the residue, the leachate contains 83% La, 7 1%Sm. 57% Tb, 30% Yb, and 12% Eu and Na. About 7.3% of the calcium is dissolved, which corresponds to a normative wholerock phosphate content of 0.89%. The elements Fe, SC, and
g 100 .E P 5 50 i; .c
Q@&J.
Chemical Data of Leachate with 1M HCl (10 min), Residue and Bulk Sample of Bholghati*
Element (wml
Leachate
i&J&
Bulk (#I31
Y 5 ”
La
1.51
0.30
1.80
j/
Ce
3.78
0.84
4.7
5
Nd
3.0
0.81
3.6
Sm
0.82
0.34
1.10 0.37
Eu
0.047
0.32
Gd
1.1
0.64
1.5
Tb
0.20
0.15
0.30
HO
0.21
0.25
0.45
Tm
0.072
0.13
0.19
Yb
0.38
0.88
1.20
LU
0.053
SC
0.23
24.4
24.3
2.5
23.6
27.0
Co
0.14
0.48
6.1
6.6
Fe0
0.38
18.6
19.0
Na20
0.031
0.24
0.29
crz03
0.0028
0.692
0.71
REE data by RNAA, the remaining Errors are l-7%.
elements
5.0
1.0
0.19
(%) cao
*
g
10
SO
La
Ce
Nd
Sm Eu Gd Tb Dy Ho
TmYb Lu
REE ionic Radii
by INAR.
FIG. 4. Chondrite normalized REE patterns in leachate, residue, average bulk, white clasts #6 and 15, diogenite, and dark clasts. Leachate (phosphate) is extremely high in REEs. Its heterogeneous distribution can explain the variation in eucritic ciasts and bulk samples.
Chemical mixing model of Bholghati
2171 Bholghatl
a sharp decline to Lu at 170X. With such very high REE concentrations, the presence of any trace phosphates can easily influence the REE pattern of the bulk samples, eucritic clasts or mineral separates; e.g., 4.3% more phosphate in eucritic clast 6B than 6A can account for a factor of 3 variation in their REE contents (Fig. 4). Eucritic (White) Clasts
The REE patterns of two eucritic clasts, splits #6 and 15, are shown in Fig. 4. Splits of # 15 are nearly identical in trace elements, suggesting homogeneity in the sample, whereas split #6B has three times more REE content than #6A and follows the leachate pattern, suggesting the presence of trace phosphates and their heterogeneous distribution in clast #6. Clasts #6 and 15 are different in major element composition (TiOz 1.4,0.50%; Fe0 14.2,24.5%) and trace element content (Table 2). Their REE patterns are also different. Clast #6 (A and B) shows a negative Eu anomaly while clast 15 shows a positive Eu anomaly. Clasts # 15 and 6A show a light REE depletion pattern relative to HREEs, which is unique to normal eucrites that typically exhibit a nearly flat REE pattern (CONSOLMAGNO and DRAKE, 1977). Clasts #6 and 15 show no primary cumulate texture (REID et al., 1990). Because of their different major and trace element chemistry, clasts #6 and 15 probably represent samples of different liquids from the HED parent body.
FIG. 5. Siderophiles (Ir, Ni, Co, Au) normalized to Cl in bulk, eucritic, and dark clasts. Dark clasts represent carbonaceous CM or Cl type material. CM and Cl values are from KALLEMEYN and WASSON (1981).
ponent, but represent a range of materials which may explain the Eu anomaly element fractionation.
(REID
et al., 1990),
and volatile/mobile
trace
Diogenite Clast (Orthopyroxenite) Chemical Mixing Models
Clast #45 (0.42 mg) has Fe’ = 0.25, FeO/MgO = 0.6 1, and A1203/Ca0 = 1 ratios, respectively, which are typical of diogenites (orthopyroxenite). This clast has La to Sm values at 0.9X (chondrite), exhibits a negative Eu anomaly at 0.4x, and shows an enrichment of HREEs at 2.1 X. Its REE pattern (Fig. 4) is intermediate between the Roda and Johnstown diogenites (FUKUOKA et al., 1977). Dark Clasts
Two dark clasts from split #4 are similar in major elemental composition with an Fe’ of 0.44. Their FeO/MgO (1.4), MgO/ A&O3 (lo), and FeO/MnO (125) ratios are similar to carbonaceous chondrite type material. The siderophile ratios (Ir, Ni, Co, Au) suggest these dark clasts to be CM or CI type (Fig. 5). This is consistent with petrologic evidence from REID et al. (1990), who studied four dark clasts petrologically, reporting three clasts as CM2 type and one clast as CI type. The REE patterns of dark clasts are flat at 1.5X La, with a strong positive Eu anomaly (Fig. 4). The CM or CI type generally has a flat REE pattern at 1.5X with no Eu anomaly. A positive Eu anomaly in dark clasts may be due to nebular fractionation or sampling. Clast #4 (2.84 mg) was also analyzed by WANG et al. (1990) for volatile/mobile trace elements. Their data do not at all show the signature of either CM2 or CI type. Their observed variations may be due either to sampling heterogeneity on a small size scale (- 3 mg) or to volatile redistribution due to thermal alteration by impact prior to incorporation of the carbonaceous material in the Bholghati parent body. Alternatively, dark inclusions in Bholghati are petrologically noted not to be a single com-
The purpose of a multi-element mixing model is to estimate the proportions of components in the bulk Bholghati, which may relate to the overall mixing proportions of components in the parent body. We used in situ components-eucritic clasts #6 and 15, diogenite (orthopyroxenite) #45, and dark clasts #4. For eucritic clast #6, we averaged the compositions of clasts #6A and 6B. Our sample #45 is only 0.42 mg and thus may not be representative of the diogenitic component. The diogenite #45 lies compositionally between Johnstown and Roda; we therefore also used these two diogenites as members to determine whether any compositional changes would affect our calculations. The linear least-square mixing model used here is that of BOYNTONet al. (1975, 1976). We used 18 elements in the mixing calculations (Ti, Al, Fe, Mg, Ca, Na, K, Mn, Cr, SC, V, La, Sm, Eu, Yb, Lu, Co, and Ni), with 10% uncertainty assigned to each element. The elements Co and Ni were used as indicators of dark (carbonaceous) clast. The mixing calculations are shown in Fig. 6. Bholghati composition can be modeled with about 48% eucritic clast # 15, 8% eucritic clast #6 (56% eucrite), 45% diogenitic clast #45, and 3% dark clast #4 components. There is about 5- 10% uncertainty associated with the components and the total sum is 104%. By using Johnstown and Roda as diogenitic endmembcrs, the mixing results are essentially the same (Fig. 6). The higher total sum reflects a somewhat poor fit and suggests the presence of an additional component. REID et al. ( 1990) noted petrologically a wide range of pyroxene compositions in the matrix, and some pyroxenes are more Mg-rich than diogenite. We used pyroxene data (Tables 1 and 2) as an additional component;
J.-C. Laul and D. C. Go&in
2112
Bholghati Mixing Model
FIG. 6. Mixing model calculations for Bholglratibulk using in situ components. Bulk can he reasonably modeled with 57% eucritic, 45% diogenitic, and 3% dark clast proportions. An additional component (Mg-rich pyroxene) is required to match the bulk composition.
however, the mixing results were not satisfactory. In a plot of MgO/A1z09 vs. FeO/MnO (Fig. 1b), the bulk matrix samples fall nearly outside the triangle defined by the eucritediogenite-dark clasts, suggesting the presence of a feldspathic component. We used plagioclase data (Tables 1 and 2) but the mixing results were not satisfactory. Perhaps an additional component of different feldspathic composition is needed to match the bulk composition. Oxygen Isotopes Three samples of Bholghati bulk (fines splits # 11, 24, and 25) and split #18 from the large eucritic clast were analyzed for oxygen isotopes by MAYEDAand CLAYTON(1989). Their data, shown in Appendix A, indicate that except for small isotopic fractionation effects due to mineralogical differences, the isotopic compositions of the bulk samples and eucrite clast are all very similar. All samples lie on a common fractionation line, as shown by the constancy of (“6-0.52 188), and plot within the field of eucrite-howardite-diogenite-mesosidetite-pallasite groups. Thus, Bholghati is a typical howardite in terms of its oxygen isotopic composition. MODELING
OF EUCRITIC
CLAST
#I5
FROM
mg) are homogeneous in major and trace element composition. SMITH (1982) reported two eucritic clasts, BB2 and BB22, from Kapoeta howardite, which also showed depletion of LREEs compared to HREEs (Fig. 7). SMITH (1982) did not report phosphates petrographically or perform a leaching experiment to ascertain the presence of phosphates. However, both BB2 (7.9 mg) and BB22 (9.3 mg) are fine-grained clasts, and BB22 is more annealed to granulitic texture than BB2. The Fe’ values of BB2 and BB22 are 0.63 and 0.56, while Fe’ of clast # 15 is 0.65. The other major and trace element data for BB2 and BB22 clasts are similar to clast # 15. These examples suggest that the LREE depletion in these eucritic clasts is real. Future studies of eucritic clasts from other howardites can support or disclaim the LREE depletion patterns. Basically, two models are used for the genesis of eucrites: 1) fractional crystallization of a mafic melt(s) that had earlier crystallized diogenites as cumulate pyroxenites (MCCARTHY et al., 1973) and 2) partial melting (P.M.) of a source composition of olivine, pigeonite, plagioclase, spinel, and metal (STOLPER, 1977). In Stolper’s model, eucrites that represent primary liquids, such as Stannem, Ibitira, and Sioux County, can be produced by 5 to 20% partial melting, and other eucrites (Bereba and Nuevo Laredo) can be derived by 10 to 40% fractional crystallization of the primary liquid (e.g., Sioux County). Stolper’s partial melting model is favored by CONSOLMAGNOand DRAKE( 1977) MITTLEFEHLDTet al. (1979) DRAKE(1980) J%DD (198 l), and BVSP (198 1). On the other hand, NEWSOM (1985) IKEDA and TAKEDA (1985) and HEWINS and NEWSOM( 1988) favor model 1. Using a massbalance constraint, WARREN(1985) though preferring model 1 over 2, indicated that neither model is satisfactory for all noncumulate eucrites. For a LREE-depleted pattern, neither model is satisfactory.
REES
A striking feature of eucritic clast # 15 is its two-fold LREE depletion relative to HREEs and its positive Eu anomaly. Normal eucrites usually exhibit nearly flat REE patterns ( 1.7 to 15X La) with no Eu anomalies (CONSOLMAGNOand DRAKE, 1977; FUKUOKA et al., 1977; SMITH, 1982). Figure 7 shows a comparison of eucrite clast # 15 with other selected eucrites-Ibitira, Juvinas, and Sioux County. The LREE depletion in clast #15 could possibly be attributed to under-sampling of phosphates due to small sample size. From the leachate REE data (Fig. 4), 0.4 to 0.5% addition of phosphates to clast # 15 can yield a normal eucrite REE pattern, similar to Sioux County. On the other hand, the original weight of the large clast # 15 was 340 mg, and, petrologically, it is a fine-grained clast, which minimizes the potential for non-representative sampling of the eucrite clast. Our aliquot size is 34 mg, and splits of clast #I5 (22 and 12
1 .o
I
I
I
La
Ce
Nd
I
lllll
Sm Eu Gd Tb
III Dy
Ho
Tm YbLu
REE Ionic Radii
FIG. 7. REE pattern of eucritic clast # 15 is compared with other eucrites. Data for Stannem and Juvinas are from SCHNETZLER and PHILFQTTS (1969), SCHMITT et al. (1963) and JEROME (1970). Data for BB2 and B22 clasts from Kapoeta howardite are from SMITH (1982).
Chemical mixing model of Bholghati CONSOLMAGNOand DRAKE (1977) modeled the REE contents of eucrites, starting with chondritic REE abundances in a source containing 50% olivine, 30% metal, 10% otthopyroxene, 5% clinopyroxene, and 5% plagioclase. The melt proportions were 40% plagioclase and 60% pyroxene. CONSOLMAGNOand DRAKE (1977) produced the REE pattern of Stannem with 4% P.M., Juvinas with 10% P.M., and Sioux County with 15% P.M. The HREE pattern of clast # 15 can be produced by 15% P.M., similar to Sioux County, except for the LREE depletion. This indicates that liquids with LREE depletion patterns (# 15, BB2, BB22) cannot be produced by one-stage P.M. of a chondritic source. These unusual (LREEdepleted) eucrites, however, can be modeled from a nonchondritic source that is depleted in LREEs. The initial ‘43Nd/ ‘44Nd ratio (0.505958) of clast #15 is compatible with the LREE-depleted source region (NYQUIST et al., 1990). A LREE-depleted source can be derived from a chondritic source, but this will require multi-stage melting. SMITH (1982) used Binda eucrite as a source material, which has a LREE-depleted pattern relative to HREE (Yb/ La = 1.7) and exhibits a positive Eu anomaly. Binda is a cumulate eucrite composed of -75% orthopyroxene and 25% plagioclase (DUKE and SILVER, 1967). Smith used melt assemblage of 50% plagioclase and 50% pyroxene to explain the REE pattern of BB2 and BB22. Using a similar scenario and the same partition coefficients for the REEs in plagioclase and pyroxene used by SMITH (1982) and Binda as source material, the REE pattern of clast # 15 can be satisfactorily reproduced (Fig. 8) using a number of melting events. REE patterns for Stannem, Juvinas, and BB2 and BB22 as liquids can also be generated (as shown in Fig. 8) by small degrees of partial melting. In each partial melting step, the residual solid becomes more depleted in LREEs. Clast # 15 could be generated by 3% P.M. of residual solid 2. It can also be generated by a higher degree of P.M. of residual solid 1. However,
20,
I
La
ce
Nd
Sm Eu Gd Tb
YbLu
REE Ionic Mdli
FIG.8. Figure from SMITH (1982), showing that BB22 and other eucrites can he generated from a small degree-ofpartial melting of a non-chondritic source, Binda cumulate. Clast # I5 follows the BB22 pattern.
2173
this process is not satisfactory for major elements such as Fe and Mg, because the Fe’ value does not significantly change with a small degree of partial melting. The Fe’ of Binda is 0.35, while Fe’ values of BB2, BB22, and clast #15 are 0.63, 0.56, and 0.65, respectively. The essence of this discussion is not to promote the specific nature of the source for LREEdepleted eucrites, but to point out that multistage melting of a flat REE chondritic source is required. This requirement inherently increases the complexities of evolutionary models for the HED parent body(ies). Bholghati Evolution The Rb-Sr and Sm-Nd age dating of Bholghati indicates that eucrite clast #15 crystallized from a melt 4.53 & 0.03 Ga ago; however, the Rb-Sr isochron of whole rock and pyroxene gave an age of 2.85 + 0.05 Ga ago (NYQUIST et al., 1990). The “Arf4’Ar dating by B~GARD and GARRISON ( 1989) showed that major Ar loss occurred ~3.1 Ga ago. The Rb-Sr and 39Ar/40Ardata suggest that the system was partially reset by a strong thermal event at about 2.85 Ga ago. This thermal event can explain the irregular patterns (distribution) of volatile/mobile trace elements in Bholghati bulk and eucritic clasts (WANG et al., 1990), and excess fission-derived xenon (SWINDLEet al., 1990). Petrologically, the eucritic clasts have undergone recrystallization and then subsolidus annealing (REID et al., 1990), consistent with a post-crystallization metamorphic event. The meteorite has had a complex history involving early multiple magmatic events (4.53 Ga ago), later prolonged annealing (2.9 Ga ago), and fragmentation and low-temperature mechanical mixing of eucritic, diogenitic, and carbonaceous (dark clasts) components on the Bholghati parent body. CONCLUSIONS Bholghati is homogeneous on a cm scale in major element composition but heterogeneous with respect to REEs. Bulk REE patterns are nearly flat with a slight negative Eu anomaly. REEs are concentrated in accessory phosphates as shown by leaching with 1M HCl. Sampling of phosphates can influence the REE patterns observed in eucritic clasts and the bulk. Dark clasts represent carbonaceous chondrite CM type material, which was later admixed during regolith formation on the HPB. Bholghati can be reasonably modeled with in situ components, 48% eucritic clast #IS, 8% eucritic clast #6 (56% eucritic) components, 45% diogenitic component, and 3% dark clast. However, an additional component (feldspathic) is needed to match the bulk composition. Eucritic clast # 15 has an LREEdepleted pattern compared to HREEs, which is unusual for normal eucrites. The LREE-depleted pattern cannot be modeled by one-stage partial melting of a chondritic source. However, it requires modeling from a non-chondritic source depleted in LREEs or multistage melting from a chondritic source, provided that the LREE depletion is not caused by undersampling of phosphates.
2174
J.-C. Laul and D. C. Gosselin
Acknowledgments-We sincerely thank the Geological Survey of India, Calcutta, for providing 10 g of Bholghati sample for the consortium studies; the curatorial staff at NASA/JSC for sample dissection and distribution to consortium members; R. N. Clayton for the oxygen isotope data; and M. R. Smith for his helpful comments on the paper. We thank the reviewers H. E. Newsom, D. W. Mittlefeldt. R. H. Hewins, and G. A. McKay for their valuable comments that improved the manuscript. We also thank the reactor crew of Oregon State University for irradiating the samples. This research was supported by NASA Contract NAS 9-15357. Editorial handling: G. A. McKay REFERENCES D. D. and GARRISOND. H. (I 989) 39Ar-40Arages of eucrites: Did the HED parent body experience a long period of thermal events due to major impacts? (abstr.). Lunar Planet. Sci. Conf: 2&h, Lunar and Planetary Institute. BOYNTONW. V., BAEDECKER P. A., CHOU C. L., ROBINSONK. W., and WASSONJ. T. (1975) Mixing and transport of lunar surface material: Evidence obtained by the determination of lithophile, siderophile, and volatile elements. Proc. Lunar Sci. ConjY 6th, pp. 2241-2259. BOYNTONW. V., CHOU C. L., ROBINSONK. W., WARRENP. H., and WASSON J. T. (1976) Lithophiles, siderophiles, and volatiles in Apollo 16 soils and rocks. Proc. Lunar Sci. Conf: 7th, pp. 727742: BUNCHT. E. (1975) Petrography and petrology of basaltic achondrite polymict breccias (howardites). Proc. Lunar Sci. Conf: 6th. pp. 469-492. BVSP (Basaltic Volcanism Study Project) ( 198 I) Basaltic Volcanism on the Terrestrial Planets. Pergamon. CHOU C. L., BOYNTONW. V., BILDR. W., K~MBERLIN J., and WASSON J. T. (1976) Trace element evidence regarding a chondritic component in howardite meteorites. Proc. Lunar Planet. Sci. Conf 7th, pp. 3501-3518. CONSOLMAGNO G. J. and DRAKE M. J. (1977) Composition and evolution of the eucrite parent body: Evidence from rare earth elements. Geochim. Cosmochim. Acta 41. 127 I-1282. DODD R. T. (198 1) Meteorites: A Petrologic-Chemical Synthesis. Cambridge Univ. Press. DRAKE M. J. (1979) Geochemical evolution of the eucrite parent body: Possible nature and evolution of Asteroid 4 Vesta. In Asteroids (ed. T. GEHRELS),pp. 765-782. Univ. Arizona Press. DRAKEM. J. (1980) Trace elements as quantitative probes of differentiation processes in planetary interiors. Rev. Geoph_vs.Space Phvr. BOGARD
18, 11-26.
DREIBUSG., KRUSEH., RAMMENSEEW., SPETTELB., and WANKE H. (1977) The bulk composition ofthe Moon and the eucrite parent body. Proc. Lunar Planet. Sci. Conf: 8th, pp. 2 I l-227. DUKE M. B. and SILVERL. (1967) Petrology of eucrites, howardites and mesosiderties. Geochim. Cosmochim. Acta 31, 1637-1665. DYMEK R. F., ALBEE A. L., CHODOS A. A., and WASSERBURG G. J. (1976) Petrography of isotopicallydated clasts in the Kapoeta howardite and petrological constraints on the evolution of its parent body. Geochim. Cosmochim. Acta 40, 1 I 15- 1130. FUHRMANM. and PAPIKE J. J. (1981) Howardites and oolvmict . _ eucrites: regolith samples from the eucrite parent body. Petrology of Bholghati, Benunu, Kapoeta, and ALHA 76005. Proc. Lunar Planet. Sci. Conf: 12B, pp. 1257-1279. FUKUOKAT., BOYNTONW. V., MA M. S., and SCHMITT R. A. (1977) Genesis of howardites, diogenites and eucrites. Proc. Lunar Planet. Sci. Confi Bth, pp. 187-210. HEWINSR. H. and NEWSON H. E. (1988) Igneous activity in the early solar system. In Meteorites and the Early Solar System (eds. J. F. KERRIDGEand M. S. MATTHEWS),pp. 73-10 1. IKEDAY. and TAKEDAH. (1985) A model for the origin of basaltic achondrites based on the Yamato 7308 howardite. Proc. Lunar Planet. Sci. Conf: 15th; J. Geophys. Res. 90, C649-C663. JEROMED. Y. (1970) Composition and origin of some achondritic meteorites. Ph.D. dissertation, University of Oregon.
JEROMED. Y. and GOLESG. G. (197 I ) A re-examination of relationships among pyroxene-plagioclase achondrites. Activation Anal. Geochim. Cosmochim. Universitetsforlaget, Oslo, 26 I-266. KALLEMEYNG. W. and WASSON J. T. (1981) The compositional classification of chondrites- I. The carbonaceous chondrite group. Geochim. Cosmochim. Acta 45, 12 17- 1230. LABOTKAT. C. and PAPIKEJ. J. (1980) Regolith ofthe eucrite parent body: Petrology of the howardite meteorite. Proc. Lunar Planet. Sci. Conf: Ilth, pp. 1103-I 130. LAULJ.-C. ( 1979) Neutron activation of geological materials. Aromtc Energy Review; IAEA, 17,603-695. LAULJ.-C. (1987) Rare earth patterns in shergottite phosphates. Proc. Lunar Planet. Sci. Conf: 17th, Part I; J. Geophys. Res. 92, E633E640. LAULJ.-C. (1990) The Bholghati (howardite) consortium: An overview. Geochim. Cosmochim. Acta 54,2 155-2 159 (this issue). LAULJ.-C., LEPELE. A., WEIMERW. C., and WCK~MAN N. A. (1982) Precise trace REE analysis by radiochemical neutron activation. J. Radioanal. Chem. 69, 18 l-196. LAUL J.-C.. SMITH M. R., WANKE H., JAGOUTZE., DREIBUSG.. PALME H., SPETTELB.. BURGHELEA., LIPSCHUTZM. E., and VERKOUTERENR. M. (1986) Chemical systematics of shergotty meteorite and tl’e compositions of its parent body (Mars). Geochim. Carmochim. A, ;a 50,909-926. LAULJ.-C., GOSSELIND. C., and SMITHM. R. (1989) The Bholghati consortium: Chemical study of the Bholghati howardite (abstr.). Lunar Planet. Sci. Conf: ZOth, Lunar and Planetary Institute. MAYEDAT. K. and CLAYTONR. N. (1989) Oxygen isotope in the Bholghati howardite (abstr.). Lunar Planet. Sci. Conf 2Oth, Lunar Planetary Institute. MCCARTHYT. S., AHRENSL. H., and ERLANKA. J. (1972) Further evidence in support of the mixing model for howardite origin. Earth Planet. Sci. Lett. 15, 86-93. MCCARTHYT. S., ERLANKA. J., and WILLISJ. P. (1973) On the origin of eucrites and diogenites. Earth Planet. Sci. Lett. 18, 433442. MCCORDT. B., ADAMSJ. B., and JOHNSONT. V. (1970) Asteroid Vesta: Spectral reflectivity and compositional implications. Science 168, 1445-1447.
MITTLEFEHLDT D. W., CHOU C. L., and WASSON J. T. (1979) Mesosiderites and howardites: Igneous formation and possible genetic relationships. Geochim. Cosmachim. Acta 43, 667-688. NEWWMH. E. (1985) Molybdenum in eucrites: Evidence for a metal core in the eucrite parent body. Proc. Lunar Planet. Sri. Conf 15th; .I. Gcophys. Re.7. 90, C613-C617. NYQIJISTL. E.. BOGARDD. D.. WIESMANNH., BANSALB. M., SHIH C.-Y., and MORRISR. M. (1990) Age of a eucrite clast from the Bholghati howardite. Geochim. Cosmachim. Acta 54, 2 195-2206 (this issue).
PAPANASTASSIOU D. A. and WASSERBURG G. J. (1976) Early lunar differentiates and lunar initial 87Sr/86Sr. Lunar Sci. VII, 665-667. PAPANASTASSIOU D. A.. RAJAIVR. S., HUNEKEJ. C., and WASSERBURG G. .I. (1974) Rb-Sr ages and lunar analogs in a basaltic achondrite: Implications for early solar system chronologies. Lunar Sci. V, 583-585. RAJANR. S., HUNEKEJ. C.. SMITHS. P., and WASSERBURGG. J. ( 1979) Argon-40/Argon-39 chronology of lithic clasts from the Kapoeta howardite. Geochim. Cosmochim. .4cta 43, 957-97 I, REID A. M.. BUCHANANP., ZOLENSKYM. E., and BARRETTR. A. ( 1990) The Bholghati howardite: Petrography and mineral chemistry. Geochim. Cosmochim. Acta 54, 2 161-2166 (this issue). SCHMITTR. A., SMITH R. H., MOSENA. W., OLEHY D. A., and VASILEVSKIS J. (1963) Abundance of the fourteen rare earth elements, scandium
and yttrium
in meteoritic
and terrestrial
matter.
Geachim. Casmochim. Acta 27, 577-622. SCHNETZLERC. C. and PHILPOTTSJ. A. (1969) Genesis of the calcium-rich
achondrites
in light of rare-earth
and concentrations.
In
Meteorite Re.search (ed. P. M. MILLMAN),pp. 206-2 15. D. Reidel. SIMPSON A. B. (1975) Electron microprobe investigation ardite Malverin (abstr.). Meteoritics 10, 489-491.
of the how-
SMITHM. R. (1982) A Chemical and petrological study of igneous
Chemical mixing model of Bholghati lithic clasts from the Kapoeta howardite. Ph.D. dissertation, Oregon State Univ. STOLPERE. M. ( 1977)Experimental petrology of eucritic meteorites. Geochim. Cosmochim. Acta 41,587-611. SWINDLE T. D., GARRISON D. H., GOSWAMI J. N., HOHENBERG C. M., NICHOLSR., and OUNCER C. T. (1990) Noble gases in the howardites Bholghati and Kapoeta. Ge~hj~. Cosmochim. Acta 54, 2 183-2194 (this issue). TAKEDA H., MIYA~OTO M., ISHIIT., and RE!D A. M. (1976) Characterization of crust formation on parent bodies of achondrites and the Moon by pyroxene crystallography and chemistry. Proc. Lunar Sci. Conf: 7th, pp. 3535-3548. THOLEN D. J. (1984) Asteroid taxonomy from cluster analysis of photometry. Ph.D. Dissertation, Univ. Arizona. WANG M. S., PAUL R. L., and LWSCHUTZM. E. (1990) Volatile/ mobile trace elements in the Bhoighati howardite. Geochim. Cosmochirn. Acta 54,2 177-2 18 I (this issue). WARREN P. H. (1985) Origin of howardites, diogenites and eucrites: A mass-balance constraint. Geochim. Cos~~hi~. Aera 49,577586.
2175
AvoendixA. Oxygen IsotopicCompositionsof Bholghatiand Other Achondrites(Mayedaand Clayton 1989)
Meteorite Bholghati (howardite)
Sample
6180
PO
637-0.52P
t1.40 +I.50
-0.32 -0.33
+3.43
tl.60
-0.18
t3.23
11.25
-0.43
BH 18 rock BH 11 fines
t3.31 t3.52
BH 24 fines BH 25 fines Yurtuk (howardite)
rock
+3.16
t1.28
-0.36
Ibitira (eucrite)
rock
t4.00
tl.87
-0.21
Shalka (diogenite)
pyroxene
t3.36
+1.38
-0.37
Estherville (mesosiderite)
pyroxene
t3.36
+1.51
-0.24
Ilimaes (pallasite)
olivine
+3.25
t1.33
-0.36
Gwchimica et Cosmochimica Acla Vol. 54, pp. 2 167-2 175 Copyright 0 l!XlO Pergamon Press pk. Printed in U.S.A.
The Bholghati howardite: Chemical study J.-C. LAUL and D. C. GOSSELIN* Chemical Sciences Department, Battelle, Pacific Northwest Laboratory, Richland, WA 99352, USA (Received June 12, 1989; accepted in revisedform May 29, 1990)
Abstract-Bholghati is homogeneous on a cm scale in major element composition, but heterogeneous in trace elements such as the rare earth elements (REEs). The REEs are concentrated in accessory phosphates, and their sampling can influence the REE patterns of eucritic clasts which, in turn, can govern the bulk REE patterns. Bholghati can be reasonably modeled with 56% eucritic, 45% diogenitic, and 3% dark clast components; however, an additional component (feldspathic) is required to match the bulk composition. Dark clasts represent carbonaceous CM type material that was admixed with other components during regolith formation on the eucrite parent body (EPB). One eucritic clast (fine-grained) is light REE (LREE) depleted relative to heavy REE (HREE), which is unusual in normal eucrites. The LREE depletion pattern cannot be modeled from one-stage partial melting of a chondritic source; however, it can be modeled from a non-chondritic source depleted in LREEs or multi-stage melting of a chondritic source, provided that the LREE depletion is not from undersampling of phosphates. Bholghati has a complex history that involved early multiple magmatic events, later annealing, and fragmentation and low-temperature mixing of eucritic, diogenitic, and carbonaceous components on the howardite parent body. is a fairly large asteroid. Vesta has been associated with a eucrite composition (DRAKE, 1979). PAPANASTASSIOU et al. (1974) and PAPANASTASSIOU and WASSERBURG( 1976) reported young Rb-Sr ages of 3.63 and 3.89 Ga for two eucritic clasts from Kapoeta howardite. Later, RAJAN et al. (1979) reported K-Ar ages of 3.48 and 4.5 Ga for the same clasts. These data suggest that the isotopic systematics have probably been disturbed by a post-crystallization thermal event(s) on a parent body. Thus, the HED may have originated from a fairly large parent body(ies) that had undergone extensive early (-4.6 Ga) igneous volcanic activity. Detailed studies of HED samples provide us an opportunity to increase our understanding of the complex history of their formation. Bholghati fell on October 29, 1905, in Maurbhanj, Orissa, India. The original recovered mass was 2.5 kg. Through the courtesy of the Geological Survey of India, about 10 g of Bholghati was obtained for multidisciplinary studies. The sample was processed and distributed to the consortium members by the personnel of the meteorite processing laboratory of the Johnson Space Center. The main focus was on a large 340-mg white eucritic clast (our split # 15) and the bulk sample. The details of sample distribution are provided by LAUL (1990). Previous chemical studies on howardites have been mostly on bulk analyses and mixing models using different eucrites and diogenites (e.g., DREIBUSet al., 1977; FUKUOKA et al., 1977). Chemical data on Bholghati have been reported by CHOU et al. (1976) and MITTLEFEHLDTet al. (1979). In this chemical study, we analyzed 32 major, minor, and trace elements in four bulk samples from different locations of Bholghati to test compositional variation in two eucrite clasts, and in mineral separates (pyroxene and plagioclase). 7Ve also performed leaching experiments on the bulk sample with 1M HCl to estimate the REE content in phosphates. The Bholghati bulk composition is modeled using in situ com-
INTRODUCTION HOWARDITESAREPOLYMICTBRECCIASand represent samples of regolith from the eucrite parent body (EPB). DUKE and SILVER (1967) suggested that howardites are mechanical mixtures of eucrites (pigeonite basalt) and diogenites (orthopyroxenites), based on textures of impact metamorphism and mineralogy. JEROME and GOLES (1971) interpreted the chemistry of howardites as simply mixtures of eucrites and diogenites (Fig. la). Their views have been supported by other studies (MCCARTHY et al., 1972, 1973; DREIBUSet al., 1977; FUKUOKA et al., 1977). CHOU et al. (1976) pointed out that siderophile data from howardites suggest the presence of 2 to 4% chondrite material in addition to the eucritic and diogenitic components (Fig. lb). The detailed petrographic and mineralogical studies by BUNCH (1975) DYMEK et al. (1976), LABOTKAand PAPIKE (1980), and FUHRMANand PAPIKE(198 1) indicate that howardites are parent body surface breccias that have undergone intense and repeated fragmentation and metamorphism by impacts. These authors found other components in the howardites in addition to eucritic, diogenitic, and chondritic components. The compositional range of pyroxenes in howardites is broader than those of eucrites and diogenites, thus precluding a simple mixing process (SIMPSON,1975; TAKEDA et al., 1976; DYMEK et al., 1976). Thus, the mineralogy and chemistry of howardites are quite complex, which has led to different views regarding the origin and interrelations of howardites, eucrites, and diogenites (HED). Based on spectral reflectance data, HED meteorites are believed to be derived from near-surface region of a differentiated V-type asteroid, like 4 Vesta (MCCORD et al., 1970; THOLEN, 1984), which
* Present address: Nebraska Geological Survey, Conservation Survey Division, University of Nebraska, Norfolk, NE 6870 1, USA. 2167
2168
J.-C. Laul and D. C. Goss&n EXPERIMENTAL
BHOLGHATI 14
We received four bulk (matrix) fragments (0.4 g each) of splits #I I, 13, 24, and 25 from different locations of Bholghati (LAUL. 1990). These samples were homogenized in an agate mortar and pestle. Aiiquots of # I I, 24. and 25 were sent to M. E. Lipschutz for analysis of volatile/mobile trace elements and to R. N. Clayton for oxygen isotope study. We used two aliquots of #I I. 13,24, and 25 bulk samples: one for instrumental neutron activation analysis (INAA) for about 30 elements; the second for radiochemical NAA (RNAA) for the REEs using the procedure of LAUL et al. (1982). The SiOZ content of bulk samples was measured by atomic absorption. Eucritic clasts #6 (30 mg) and #I 5 (40 mg) were split into two aliquots. One aliquot was used for 1NAA and the other for RNAA. Dark clast #4, diogenite #45, and plagioclase and pyroxene separates from clast # 15 were analyzed by INAA. Following the completion of our INAA study, eucritic clasts #6B (I 2.1 mg) and #15A (22.3 mg) and dark clast #4 (2.84 mg) were sent to M. E. Lipschutz for volatile/mobile element analysis by RNAA (WANG et al., 1990). The plagioclase (3.4 mg) and pyroxene (4.2 mg) were separated from the large eucritic clast #l by L. E. Nyquist’s group. We used 200 mg of bulk sample # 13 in a leaching experiment to assess the influence of phosphate phases on the trace element chemistry. Phosphates dissolve quantitatively in IM HCI in IO to I5 min (LAUI. et al., 1986; LAUL, 1987). Following irradiation, the sample was treated with 5 ml I M HCI for 10 min. The solution was filtered through a fine filter paper and collected in a IO-ml volumetric flask. A 4-ml aliquot was used for INAA and a 6-ml aliquot for RNAA for the REEs. The residue was dried, reweighed, and split into two aliquots. one each for INAA and RNAA. A 25-mg aliquot of the unleached bulk sample was also analyzed by INAA and RNAA for mass balance. A USGS BCR-I standard was used as a control in the INAA and RNAA. Other standards used in the INAA were BHVO- I, GSP1, PCC- 1, and Allende powder. AI1samples were counted on a normal Ge(Li) detector (25% efficiency, FWHM 1.8 keV for a 1332-keV peak of @Co) and a 4096 channel analyzer, and coincidence-noncoincidence Ge(Li)-NaI(T1) counting systems (LAUL, 1979).
. DIOGENITE . EUCRITE CLAST o
Carb. Clasts
BULK
A CAFiB.-LIKE CLAST
0.1 0.20.30.40.50.60.70.8
molar
FeO/(FeO+MgO)
Dark
Clasls
0
2
Eucr~tic
40
20
Clast
60
80
100
120
FeOlMnO FIG. I. (a) Plot of CaO vs. molar FeO/(FeO + MgO). Our samples fall in or near the fields of eucrites, diogenites, and howardites. (b) Plot of MgO/A1203 vs. FeO/MnO for bulk, eucritic clasts, diogenite, and dark clasts.
RESULTS
ponents in a mixing model. Eucritic clast # 15 is compared with other eucrites and is evaluated with respect to petrogenetic modeling. Our detailed approach has provided a much better understanding of the geochemical evolution of Bholghati.
r
wt.
Table 1. Major Abundances Sample Bulk
Split (matrix)
11 13 :z
#
(%) in Bholghati Samples by INAA
(mg)
SiO2
TiO2
Al203
Fe0
MgO
72.3 53.3 49.0 44.5
49.0
0.44 0.74 0.42 0.58
8.30 8.40 8.29 8.53
18.3 18.3 18.2 17.7
17.6 15.7 17.0 16.2
0.55
8.38
18.1
16.6
I\verage Reported of Bulk(=)
The chemical data for 32 elements for the Bholghati samples, along with the USGS standards BCR- 1 and BHVO- 1. are shown in Tables I and 2 for the major elements (oxide wt%) and trace element abundances (ppm), respectively.
CaO
17.5 19.5
Data
Na20
K20
MIIO
CrzO3
Fe’
6.86 6.34 6.70 7.20
0.281 0.310 0.272 0.291
0.026 0.026 0.022 0.027
6.33
0.289
0.025
0.528 0.506 0.526 0.527 0.522
0.783 0.692 0.690 0.766 0.733
0.37 0.39 0.38 0.37 0.38
5.73
0.288 0.252
0.026
0.508
0.685 0.763
0.369 0.520
0.075
0.430 0.351
0.;27
0.61
3.62 8.95
0.400 0.340
0.037 0.030
0.540 0.644
0.351 -
0.65
16.7 5.70
1.08 0.051
0.075 0.0060
0.034 0.963
0.014 0.234
0.64 0.56
Eucrite (While)
Clast
6A B
12.8 12.1
1.1 1.7
8.18 11.8
14.2
5.6 4.6
Eucrite (While)
Clast
15A B
22.3 12.0
0.51 0.47
12.8 10.9
24.5 -
7.0 7.0
3.37 4.21
<.4
31.5 1.65
1.10 30.0
0.35 13.1
4
2.84 2.64
<.5 <.5
2.35 2.58
35.3 34.2
26.3 22.1
1.64 2.46
0.260 0.550
0.045 0.060
0.311 0.245
0.641 0.547
0.43 0.46
45
0.42
0.20
1.60
16.0
26.2
1.90
0.040
0.0030
0.465
0.819
0.25
!&!I&& 12.2 3.3 54.5 -2.20 13.6 BCR-1 11.3 7.6 2.5 13.7 BHVO-1 .ror* baled on counting 51115,155are X.5-3.% tar s,o*. T@ p.1203, MI, M”O, tia*o and cr2 0,
6.90 11.6
1.70 0.52
0.180 0.170
0.0020 0.044
Pkig. (b’ Pyroxene Dark
Clast
Diogenite
, Mllllelchldt e,8,.(1979) ,mg.and Pyraxene mmEusrlle da51 “15, were provided byriyqwt e,a,(w30) )Fe’ =Fe*,(Feo +M$iO, nl.lm
10.9 12.8
-3.20 2.22
;i5x1.3, Idgo, caoB”dI$0
(C)
Chemical mixing model of Bhol~ati
2169
Table 2. Trace Abundances (ppm) in Bholghati Samples by INAA and RNAA* Sample Bulk(matrix)
Split # 11 13 24 25
SC 21.3 24.8 23.3 21.7 22.8
v 110 107 108 110 109
co 30.7 26.6 26.1 31.3 29.0
@a 70 30 $8 15 21
sr 45 60 50 44 50
24.4 25.8 Eucrite (White)Clest Eucrite (White) Clast
w
BHVO.l
1.30 2.05 1.64 1.46 1.61
Ce
Nd
3.2 4.3 4.2 3.6 4.0
2.6 4.2 3.2 2.9 3.2
1.49 1.38 4.9
Sm 0.82 1.30 1.02 0.90 1.01
Eu
Gd
0.30 1.1 0.38 1.5 0.35 1.4 0.34 1.2 0.34 1.3
0.95 0.04 0.33
Tb 0.20 0.28 0.25 0.24 0.24
Ho
Tm
0.28 0.12 0.46 0.19 0.35 0.15 0.31 0.13 0.35 0.15
0.27
Yb
Lu
0.83 0.13 1.24 0.19 1.02 0.16 0.93 0.14 1.00 0.16
Zr
HI
40 30 30 50 40
0.62 0.88 0.74 0.53 0.69
0.98 0.16
6A 0
69 44.0 54
5.2 30
78
15A 0
28.6 70 57
16.4 30
110 1.65 4.0 (6.0)1.10 0.61 1.6 0.34 0.27 1.66 0.27 20 70 1.50 3.8 3.2 1.00 0.50 1.6 0.33 0.43 0.23 1.60 0.23 -
1.6042.7 -
$.F . ;;
250 0.73 1.6 1.0 0.24 1.40 <50 1.10 3.0 3.0 0.95 0.050-
4
10.0 83 10.0 75
460 ~50 <50 0.50 c2 690 0.39 -
45
14.0 140 12.1 -
Plag.tb) Pyroxene OarkCIast
La
-
-32.0'400 "36.0 670 330 25.5 54 31.0 300 44.0 150 16.0 36
Th
0.11 0.14 0.10 0.10 0.11
0.23410 0.30500 0.27340 0.14410 0.23410
NI
540 670
0.60
2.64 7.7 5.8 2.14 0.49 3.2 0.63 0.92 0.39 2.61 0.43 9.75 25.0 22 6.30 0.66 8.5 1.3 1.7 0.72 4.13 0.62 110 4.0
0.30 -
la
0.64 1.2 8.4
0.90 0.10 0.21 26 -
0.0600.17 c.04 0.17 0.028~40 0.62 c.1 .x.04 0.30 0.51 0.23 1.40 0.33 ~40 0.60 c.2 c.2 ~20
PAdJb d'pt 11.8 11.9 5.3 6.4 8.6
21 23 17 23 21
6.9 7.5
20 20
c5
<5
c5 -
<5 -
2.0
cl c2
<4 -
0.30 0.25 0.24 0.22 -
<.5 c.6 c.3 c.3 -
0.32 0.066~50 0.49 0.31 <.5 9880 106 316 0.26 0.0531.3 0.60 <.5 134OS(660) 616
-
0.18 <.03 -
<.7 <.l -
0.47 0.074-
30 27
6.80 2.00 7.3 1.10 1.2 0.56 3.40 0.54 180 "4.70"0.80 8.2 6.30 1.90 8.0 1.00 2.10 0.32 4.4 1.20 1.1 130
-
-
-
4.6
-
-
-
Estlmaled 8rrors based on counting statistics we: 0.64% for SC, Co, La, Sm, Eu, Yb and Lu; 6-M?& for V, Ba, Cs, Nd, Tb, Ho. Hf Ta, Tb and NI; 16-20% for Sr, Gd, Tm, Zr, Au and Ir. ‘REE data we by RNAA for sample split’s # t 1,24,25,6A and 168. Tbe data for remaining samples are by INAA (a) Data from Chou et al. (1976); Mittlefehldt et ai. (1979) (b) Plag. and Pymxene prom Eucrfie clast #IS) were provided by Nyqulst et at. (1990)
Typical errors based on counting statistics are also shown in Tables 1 and 2. The REEs for bulk samples and eucritic clasts were determined by RNAA to obtain better data for Nd, Gd, Ho, and Tm. In INAA, we used BCR- 1 as control for TiO* , A1203, FeO, Na,O, I&O, MnO, SC,Co, Hf, and Ta. We used GSP-1 as 105 ppm for Th; PCC-I as 45.7% for MgO, 0.41% for Cr203, and 0.25% for Ni; and the Allende standard as 140 ppb for Au and 780 ppb for Ir. Our CaO value for the bulk is -20% higher and our Co and Ce values are 30 to 40% lower than those of MITTLEFEHLDT et al. ( 1979). This difference may be due to sampling. WANG et al. (1990) analyzed the same splits as we did, and Au and Co are common elements between their samples and ours. Their Au values agree with ours, but their Co values by RNAA for splits #4, 6B, 15A, and 25 are systematically lower than our INAA data. Our Sm value for pyroxene is about twice as high as that of NYQUIST et al. (1990). Their pyroxene was washed with lM HCl, while ours was not. This difference is likely due to contamination from trace phosphates, which are extremely high in REEs; 0.56% phosphate could explain this difference. CHEMICAL SYSTEMATIB Major Elements
The four bulk samples, from different locations of Bholghati, are quite homogeneous in major composition. Their mean composition is 0.55% TiOz, 8.4% A1203, 18.1% FeO, 16.6% MgO, 6.9% CaO, 0.29% Na*O, 0.025% I&O, 0.52% MnO, and 0.73% Cr203. Their FeO/(FeO + MgO) molar (Fe’ = 0.37 to 0.39) and FeO/MnO (33 to 36) ratios have a narrow range and are similar to other howardites (MITTLEFJZHLDT et al., 1979; SMITH, 1982). Figure la shows a plot of CaO versus Fe’ for HED. Our bulk samples fall in the howardite
field, which lies between the eucrite and diogenite fields. In a plot of MgO/A120~ versus FeO/MnO (Fig. 1b), bulk samples lie nearly between diogenite and eucritic clasts. The two eucritic clasts, #6 and 15, differ notably in TiOz (1.4, 0.50%) and Fe0 (14.2, 24.5%) These eucrite clasts are similar to noncumulate eucrites in terms of their Fe’ (0.61, 0.65) and CaO (11.8,9.3%). The FeO/MnO ratio (40,45) exceeds that generally observed for eucrites (N 35) and might be explained by accessory troilite. The diogenite split #45 has Fe’ of 0.25 and FeO/MnO ratio of 34 and is intermediate in major composition between Roda and Johnstown (FUKUOKA et al., 1977). The ~~~bution of bulk samples in Fig. 1 supports a simple mixing of diogenite and eucrite components for Bholghati. Pyroxenes in the Bholghati matrix (bulk) show a continuum, with compositions ranging from diogenitic to eucritic (REID et al., 1990), which suggests an additional component(s) in the mixing process. The two dark clasts (#4) seem to be a carbonaceous type, based on CaO vs. Fe’ and MgO/Al203 vs. FeOfMnO plots (Fig. 1). Petrologically, three dark clasts resemble CM2 chondrites and one dark clast a CI type, with low-iron silicates, Fe-Ni sulphides, and Ca-carbonate in a fine-grained dark matrix (REID et al., 1990). Preservation of this low-temperature assemblage precludes any significant heating prior to incorporation into the Bholghati matrix (REID et al., 1990). Dark clasts from Kapoeta howardite have been noted to be CI type material (SMITH, 1982). Trace Elements-REEs
The chondrite normalized REE patterns of the bulk samples are shown in Fig. 2. Bulk samples are heterogeneous with respect to trace elements such as REEs, which vary by about 60%. The bulk REE patterns are nearly flat (e.g., La
2170
J.-C. Laul and D. C. Gosselin 10.0 !!
Rholghatl (Bulk Samples) % Leached in 1M HCI, {lO min)
1.0
AEE ionic Radii
2. Chondrite normalized REE patterns of Bholghati bulk samples. The chondrite values used are: 0.34 La, 0.87 Ce, 0.64 Nd, 0.195 Sm, 0.073 Eu, 0.26 Gd, 0.047 Tb, 0.33 Dy, 0.078 Ho, 0.032 Tm, 0.22 Yb, and 0.034 Lu. REE contents vary by 60% in bulk samples.
La SmTb Yb Lu Eu Na Coca
--4X), with a slight negative Eu anomaly that increases with increasing REE contents. Traces of phosphates have been noted petrologically in the matrix (REID et al., 1990). This suggests that the presence of accessory phosphate (whitlockite, apatite) phases can influence the bulk REE patterns (LAUL et al., 1989). To quantify the REE concentrations in phosphates, we performed the leaching experiment discussed below.
Fe SC Cr
Element
I%.
FIG. 3. Percent element leached in I M HCI (IO min.) relative to the bulk in Bholghati. Soluble REE range from 84% La to 28% Lu. Fe, SC. and Cr are leached below 2% level.
Cr are dissolved below 2%. indicating only a minor contribution from pyroxenes. The REE patterns of the leachate, residue, and bulk are shown in Fig. 4. Accounting for the 0.89% phosphate in the leachate, the light REE (La-Sm) pattern is nearly flat at about 490X (chondrite), with a strong negative Eu anomaly at 72X and Gd and Tb at 480X with
1000
Leaching with IM HCI 500
The data for REEs, SC, Co, Ca, Fe, Na, and Cr for the leachate, residue, and bulk are shown in Table 3. Figure 3 shows the percentage of element leached from the bulk. Relative to the residue, the leachate contains 83% La, 7 1%Sm. 57% Tb, 30% Yb, and 12% Eu and Na. About 7.3% of the calcium is dissolved, which corresponds to a normative wholerock phosphate content of 0.89%. The elements Fe, SC, and
g 100 .E P 5 50 i; .c
Q@&J.
Chemical Data of Leachate with 1M HCl (10 min), Residue and Bulk Sample of Bholghati*
Element (wml
Leachate
i&J&
Bulk (#I31
Y 5 ”
La
1.51
0.30
1.80
j/
Ce
3.78
0.84
4.7
5
Nd
3.0
0.81
3.6
Sm
0.82
0.34
1.10 0.37
Eu
0.047
0.32
Gd
1.1
0.64
1.5
Tb
0.20
0.15
0.30
HO
0.21
0.25
0.45
Tm
0.072
0.13
0.19
Yb
0.38
0.88
1.20
LU
0.053
SC
0.23
24.4
24.3
2.5
23.6
27.0
Co
0.14
0.48
6.1
6.6
Fe0
0.38
18.6
19.0
Na20
0.031
0.24
0.29
crz03
0.0028
0.692
0.71
REE data by RNAA, the remaining Errors are l-7%.
elements
5.0
1.0
0.19
(%) cao
*
g
10
SO
La
Ce
Nd
Sm Eu Gd Tb Dy Ho
TmYb Lu
REE ionic Radii
by INAR.
FIG. 4. Chondrite normalized REE patterns in leachate, residue, average bulk, white clasts #6 and 15, diogenite, and dark clasts. Leachate (phosphate) is extremely high in REEs. Its heterogeneous distribution can explain the variation in eucritic ciasts and bulk samples.
Chemical mixing model of Bholghati
2171 Bholghatl
a sharp decline to Lu at 170X. With such very high REE concentrations, the presence of any trace phosphates can easily influence the REE pattern of the bulk samples, eucritic clasts or mineral separates; e.g., 4.3% more phosphate in eucritic clast 6B than 6A can account for a factor of 3 variation in their REE contents (Fig. 4). Eucritic (White) Clasts
The REE patterns of two eucritic clasts, splits #6 and 15, are shown in Fig. 4. Splits of # 15 are nearly identical in trace elements, suggesting homogeneity in the sample, whereas split #6B has three times more REE content than #6A and follows the leachate pattern, suggesting the presence of trace phosphates and their heterogeneous distribution in clast #6. Clasts #6 and 15 are different in major element composition (TiOz 1.4,0.50%; Fe0 14.2,24.5%) and trace element content (Table 2). Their REE patterns are also different. Clast #6 (A and B) shows a negative Eu anomaly while clast 15 shows a positive Eu anomaly. Clasts # 15 and 6A show a light REE depletion pattern relative to HREEs, which is unique to normal eucrites that typically exhibit a nearly flat REE pattern (CONSOLMAGNO and DRAKE, 1977). Clasts #6 and 15 show no primary cumulate texture (REID et al., 1990). Because of their different major and trace element chemistry, clasts #6 and 15 probably represent samples of different liquids from the HED parent body.
FIG. 5. Siderophiles (Ir, Ni, Co, Au) normalized to Cl in bulk, eucritic, and dark clasts. Dark clasts represent carbonaceous CM or Cl type material. CM and Cl values are from KALLEMEYN and WASSON (1981).
ponent, but represent a range of materials which may explain the Eu anomaly element fractionation.
(REID
et al., 1990),
and volatile/mobile
trace
Diogenite Clast (Orthopyroxenite) Chemical Mixing Models
Clast #45 (0.42 mg) has Fe’ = 0.25, FeO/MgO = 0.6 1, and A1203/Ca0 = 1 ratios, respectively, which are typical of diogenites (orthopyroxenite). This clast has La to Sm values at 0.9X (chondrite), exhibits a negative Eu anomaly at 0.4x, and shows an enrichment of HREEs at 2.1 X. Its REE pattern (Fig. 4) is intermediate between the Roda and Johnstown diogenites (FUKUOKA et al., 1977). Dark Clasts
Two dark clasts from split #4 are similar in major elemental composition with an Fe’ of 0.44. Their FeO/MgO (1.4), MgO/ A&O3 (lo), and FeO/MnO (125) ratios are similar to carbonaceous chondrite type material. The siderophile ratios (Ir, Ni, Co, Au) suggest these dark clasts to be CM or CI type (Fig. 5). This is consistent with petrologic evidence from REID et al. (1990), who studied four dark clasts petrologically, reporting three clasts as CM2 type and one clast as CI type. The REE patterns of dark clasts are flat at 1.5X La, with a strong positive Eu anomaly (Fig. 4). The CM or CI type generally has a flat REE pattern at 1.5X with no Eu anomaly. A positive Eu anomaly in dark clasts may be due to nebular fractionation or sampling. Clast #4 (2.84 mg) was also analyzed by WANG et al. (1990) for volatile/mobile trace elements. Their data do not at all show the signature of either CM2 or CI type. Their observed variations may be due either to sampling heterogeneity on a small size scale (- 3 mg) or to volatile redistribution due to thermal alteration by impact prior to incorporation of the carbonaceous material in the Bholghati parent body. Alternatively, dark inclusions in Bholghati are petrologically noted not to be a single com-
The purpose of a multi-element mixing model is to estimate the proportions of components in the bulk Bholghati, which may relate to the overall mixing proportions of components in the parent body. We used in situ components-eucritic clasts #6 and 15, diogenite (orthopyroxenite) #45, and dark clasts #4. For eucritic clast #6, we averaged the compositions of clasts #6A and 6B. Our sample #45 is only 0.42 mg and thus may not be representative of the diogenitic component. The diogenite #45 lies compositionally between Johnstown and Roda; we therefore also used these two diogenites as members to determine whether any compositional changes would affect our calculations. The linear least-square mixing model used here is that of BOYNTONet al. (1975, 1976). We used 18 elements in the mixing calculations (Ti, Al, Fe, Mg, Ca, Na, K, Mn, Cr, SC, V, La, Sm, Eu, Yb, Lu, Co, and Ni), with 10% uncertainty assigned to each element. The elements Co and Ni were used as indicators of dark (carbonaceous) clast. The mixing calculations are shown in Fig. 6. Bholghati composition can be modeled with about 48% eucritic clast # 15, 8% eucritic clast #6 (56% eucrite), 45% diogenitic clast #45, and 3% dark clast #4 components. There is about 5- 10% uncertainty associated with the components and the total sum is 104%. By using Johnstown and Roda as diogenitic endmembcrs, the mixing results are essentially the same (Fig. 6). The higher total sum reflects a somewhat poor fit and suggests the presence of an additional component. REID et al. ( 1990) noted petrologically a wide range of pyroxene compositions in the matrix, and some pyroxenes are more Mg-rich than diogenite. We used pyroxene data (Tables 1 and 2) as an additional component;
J.-C. Laul and D. C. Go&in
2112
Bholghati Mixing Model
FIG. 6. Mixing model calculations for Bholglratibulk using in situ components. Bulk can he reasonably modeled with 57% eucritic, 45% diogenitic, and 3% dark clast proportions. An additional component (Mg-rich pyroxene) is required to match the bulk composition.
however, the mixing results were not satisfactory. In a plot of MgO/A1z09 vs. FeO/MnO (Fig. 1b), the bulk matrix samples fall nearly outside the triangle defined by the eucritediogenite-dark clasts, suggesting the presence of a feldspathic component. We used plagioclase data (Tables 1 and 2) but the mixing results were not satisfactory. Perhaps an additional component of different feldspathic composition is needed to match the bulk composition. Oxygen Isotopes Three samples of Bholghati bulk (fines splits # 11, 24, and 25) and split #18 from the large eucritic clast were analyzed for oxygen isotopes by MAYEDAand CLAYTON(1989). Their data, shown in Appendix A, indicate that except for small isotopic fractionation effects due to mineralogical differences, the isotopic compositions of the bulk samples and eucrite clast are all very similar. All samples lie on a common fractionation line, as shown by the constancy of (“6-0.52 188), and plot within the field of eucrite-howardite-diogenite-mesosidetite-pallasite groups. Thus, Bholghati is a typical howardite in terms of its oxygen isotopic composition. MODELING
OF EUCRITIC
CLAST
#I5
FROM
mg) are homogeneous in major and trace element composition. SMITH (1982) reported two eucritic clasts, BB2 and BB22, from Kapoeta howardite, which also showed depletion of LREEs compared to HREEs (Fig. 7). SMITH (1982) did not report phosphates petrographically or perform a leaching experiment to ascertain the presence of phosphates. However, both BB2 (7.9 mg) and BB22 (9.3 mg) are fine-grained clasts, and BB22 is more annealed to granulitic texture than BB2. The Fe’ values of BB2 and BB22 are 0.63 and 0.56, while Fe’ of clast # 15 is 0.65. The other major and trace element data for BB2 and BB22 clasts are similar to clast # 15. These examples suggest that the LREE depletion in these eucritic clasts is real. Future studies of eucritic clasts from other howardites can support or disclaim the LREE depletion patterns. Basically, two models are used for the genesis of eucrites: 1) fractional crystallization of a mafic melt(s) that had earlier crystallized diogenites as cumulate pyroxenites (MCCARTHY et al., 1973) and 2) partial melting (P.M.) of a source composition of olivine, pigeonite, plagioclase, spinel, and metal (STOLPER, 1977). In Stolper’s model, eucrites that represent primary liquids, such as Stannem, Ibitira, and Sioux County, can be produced by 5 to 20% partial melting, and other eucrites (Bereba and Nuevo Laredo) can be derived by 10 to 40% fractional crystallization of the primary liquid (e.g., Sioux County). Stolper’s partial melting model is favored by CONSOLMAGNOand DRAKE( 1977) MITTLEFEHLDTet al. (1979) DRAKE(1980) J%DD (198 l), and BVSP (198 1). On the other hand, NEWSOM (1985) IKEDA and TAKEDA (1985) and HEWINS and NEWSOM( 1988) favor model 1. Using a massbalance constraint, WARREN(1985) though preferring model 1 over 2, indicated that neither model is satisfactory for all noncumulate eucrites. For a LREE-depleted pattern, neither model is satisfactory.
REES
A striking feature of eucritic clast # 15 is its two-fold LREE depletion relative to HREEs and its positive Eu anomaly. Normal eucrites usually exhibit nearly flat REE patterns ( 1.7 to 15X La) with no Eu anomalies (CONSOLMAGNOand DRAKE, 1977; FUKUOKA et al., 1977; SMITH, 1982). Figure 7 shows a comparison of eucrite clast # 15 with other selected eucrites-Ibitira, Juvinas, and Sioux County. The LREE depletion in clast #15 could possibly be attributed to under-sampling of phosphates due to small sample size. From the leachate REE data (Fig. 4), 0.4 to 0.5% addition of phosphates to clast # 15 can yield a normal eucrite REE pattern, similar to Sioux County. On the other hand, the original weight of the large clast # 15 was 340 mg, and, petrologically, it is a fine-grained clast, which minimizes the potential for non-representative sampling of the eucrite clast. Our aliquot size is 34 mg, and splits of clast #I5 (22 and 12
1 .o
I
I
I
La
Ce
Nd
I
lllll
Sm Eu Gd Tb
III Dy
Ho
Tm YbLu
REE Ionic Radii
FIG. 7. REE pattern of eucritic clast # 15 is compared with other eucrites. Data for Stannem and Juvinas are from SCHNETZLER and PHILFQTTS (1969), SCHMITT et al. (1963) and JEROME (1970). Data for BB2 and B22 clasts from Kapoeta howardite are from SMITH (1982).
Chemical mixing model of Bholghati CONSOLMAGNOand DRAKE (1977) modeled the REE contents of eucrites, starting with chondritic REE abundances in a source containing 50% olivine, 30% metal, 10% otthopyroxene, 5% clinopyroxene, and 5% plagioclase. The melt proportions were 40% plagioclase and 60% pyroxene. CONSOLMAGNOand DRAKE (1977) produced the REE pattern of Stannem with 4% P.M., Juvinas with 10% P.M., and Sioux County with 15% P.M. The HREE pattern of clast # 15 can be produced by 15% P.M., similar to Sioux County, except for the LREE depletion. This indicates that liquids with LREE depletion patterns (# 15, BB2, BB22) cannot be produced by one-stage P.M. of a chondritic source. These unusual (LREEdepleted) eucrites, however, can be modeled from a nonchondritic source that is depleted in LREEs. The initial ‘43Nd/ ‘44Nd ratio (0.505958) of clast #15 is compatible with the LREE-depleted source region (NYQUIST et al., 1990). A LREE-depleted source can be derived from a chondritic source, but this will require multi-stage melting. SMITH (1982) used Binda eucrite as a source material, which has a LREE-depleted pattern relative to HREE (Yb/ La = 1.7) and exhibits a positive Eu anomaly. Binda is a cumulate eucrite composed of -75% orthopyroxene and 25% plagioclase (DUKE and SILVER, 1967). Smith used melt assemblage of 50% plagioclase and 50% pyroxene to explain the REE pattern of BB2 and BB22. Using a similar scenario and the same partition coefficients for the REEs in plagioclase and pyroxene used by SMITH (1982) and Binda as source material, the REE pattern of clast # 15 can be satisfactorily reproduced (Fig. 8) using a number of melting events. REE patterns for Stannem, Juvinas, and BB2 and BB22 as liquids can also be generated (as shown in Fig. 8) by small degrees of partial melting. In each partial melting step, the residual solid becomes more depleted in LREEs. Clast # 15 could be generated by 3% P.M. of residual solid 2. It can also be generated by a higher degree of P.M. of residual solid 1. However,
20,
I
La
ce
Nd
Sm Eu Gd Tb
YbLu
REE Ionic Mdli
FIG.8. Figure from SMITH (1982), showing that BB22 and other eucrites can he generated from a small degree-ofpartial melting of a non-chondritic source, Binda cumulate. Clast # I5 follows the BB22 pattern.
2173
this process is not satisfactory for major elements such as Fe and Mg, because the Fe’ value does not significantly change with a small degree of partial melting. The Fe’ of Binda is 0.35, while Fe’ values of BB2, BB22, and clast #15 are 0.63, 0.56, and 0.65, respectively. The essence of this discussion is not to promote the specific nature of the source for LREEdepleted eucrites, but to point out that multistage melting of a flat REE chondritic source is required. This requirement inherently increases the complexities of evolutionary models for the HED parent body(ies). Bholghati Evolution The Rb-Sr and Sm-Nd age dating of Bholghati indicates that eucrite clast #15 crystallized from a melt 4.53 & 0.03 Ga ago; however, the Rb-Sr isochron of whole rock and pyroxene gave an age of 2.85 + 0.05 Ga ago (NYQUIST et al., 1990). The “Arf4’Ar dating by B~GARD and GARRISON ( 1989) showed that major Ar loss occurred ~3.1 Ga ago. The Rb-Sr and 39Ar/40Ardata suggest that the system was partially reset by a strong thermal event at about 2.85 Ga ago. This thermal event can explain the irregular patterns (distribution) of volatile/mobile trace elements in Bholghati bulk and eucritic clasts (WANG et al., 1990), and excess fission-derived xenon (SWINDLEet al., 1990). Petrologically, the eucritic clasts have undergone recrystallization and then subsolidus annealing (REID et al., 1990), consistent with a post-crystallization metamorphic event. The meteorite has had a complex history involving early multiple magmatic events (4.53 Ga ago), later prolonged annealing (2.9 Ga ago), and fragmentation and low-temperature mechanical mixing of eucritic, diogenitic, and carbonaceous (dark clasts) components on the Bholghati parent body. CONCLUSIONS Bholghati is homogeneous on a cm scale in major element composition but heterogeneous with respect to REEs. Bulk REE patterns are nearly flat with a slight negative Eu anomaly. REEs are concentrated in accessory phosphates as shown by leaching with 1M HCl. Sampling of phosphates can influence the REE patterns observed in eucritic clasts and the bulk. Dark clasts represent carbonaceous chondrite CM type material, which was later admixed during regolith formation on the HPB. Bholghati can be reasonably modeled with in situ components, 48% eucritic clast #IS, 8% eucritic clast #6 (56% eucritic) components, 45% diogenitic component, and 3% dark clast. However, an additional component (feldspathic) is needed to match the bulk composition. Eucritic clast # 15 has an LREEdepleted pattern compared to HREEs, which is unusual for normal eucrites. The LREE-depleted pattern cannot be modeled by one-stage partial melting of a chondritic source. However, it requires modeling from a non-chondritic source depleted in LREEs or multistage melting from a chondritic source, provided that the LREE depletion is not caused by undersampling of phosphates.
2174
J.-C. Laul and D. C. Gosselin
Acknowledgments-We sincerely thank the Geological Survey of India, Calcutta, for providing 10 g of Bholghati sample for the consortium studies; the curatorial staff at NASA/JSC for sample dissection and distribution to consortium members; R. N. Clayton for the oxygen isotope data; and M. R. Smith for his helpful comments on the paper. We thank the reviewers H. E. Newsom, D. W. Mittlefeldt. R. H. Hewins, and G. A. McKay for their valuable comments that improved the manuscript. We also thank the reactor crew of Oregon State University for irradiating the samples. This research was supported by NASA Contract NAS 9-15357. Editorial handling: G. A. McKay REFERENCES D. D. and GARRISOND. H. (I 989) 39Ar-40Arages of eucrites: Did the HED parent body experience a long period of thermal events due to major impacts? (abstr.). Lunar Planet. Sci. Conf: 2&h, Lunar and Planetary Institute. BOYNTONW. V., BAEDECKER P. A., CHOU C. L., ROBINSONK. W., and WASSONJ. T. (1975) Mixing and transport of lunar surface material: Evidence obtained by the determination of lithophile, siderophile, and volatile elements. Proc. Lunar Sci. ConjY 6th, pp. 2241-2259. BOYNTONW. V., CHOU C. L., ROBINSONK. W., WARRENP. H., and WASSON J. T. (1976) Lithophiles, siderophiles, and volatiles in Apollo 16 soils and rocks. Proc. Lunar Sci. Conf: 7th, pp. 727742: BUNCHT. E. (1975) Petrography and petrology of basaltic achondrite polymict breccias (howardites). Proc. Lunar Sci. Conf: 6th. pp. 469-492. BVSP (Basaltic Volcanism Study Project) ( 198 I) Basaltic Volcanism on the Terrestrial Planets. Pergamon. CHOU C. L., BOYNTONW. V., BILDR. W., K~MBERLIN J., and WASSON J. T. (1976) Trace element evidence regarding a chondritic component in howardite meteorites. Proc. Lunar Planet. Sci. Conf 7th, pp. 3501-3518. CONSOLMAGNO G. J. and DRAKE M. J. (1977) Composition and evolution of the eucrite parent body: Evidence from rare earth elements. Geochim. Cosmochim. Acta 41. 127 I-1282. DODD R. T. (198 1) Meteorites: A Petrologic-Chemical Synthesis. Cambridge Univ. Press. DRAKE M. J. (1979) Geochemical evolution of the eucrite parent body: Possible nature and evolution of Asteroid 4 Vesta. In Asteroids (ed. T. GEHRELS),pp. 765-782. Univ. Arizona Press. DRAKEM. J. (1980) Trace elements as quantitative probes of differentiation processes in planetary interiors. Rev. Geoph_vs.Space Phvr. BOGARD
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DREIBUSG., KRUSEH., RAMMENSEEW., SPETTELB., and WANKE H. (1977) The bulk composition ofthe Moon and the eucrite parent body. Proc. Lunar Planet. Sci. Conf: 8th, pp. 2 I l-227. DUKE M. B. and SILVERL. (1967) Petrology of eucrites, howardites and mesosiderties. Geochim. Cosmochim. Acta 31, 1637-1665. DYMEK R. F., ALBEE A. L., CHODOS A. A., and WASSERBURG G. J. (1976) Petrography of isotopicallydated clasts in the Kapoeta howardite and petrological constraints on the evolution of its parent body. Geochim. Cosmochim. Acta 40, 1 I 15- 1130. FUHRMANM. and PAPIKE J. J. (1981) Howardites and oolvmict . _ eucrites: regolith samples from the eucrite parent body. Petrology of Bholghati, Benunu, Kapoeta, and ALHA 76005. Proc. Lunar Planet. Sci. Conf: 12B, pp. 1257-1279. FUKUOKAT., BOYNTONW. V., MA M. S., and SCHMITT R. A. (1977) Genesis of howardites, diogenites and eucrites. Proc. Lunar Planet. Sci. Confi Bth, pp. 187-210. HEWINSR. H. and NEWSON H. E. (1988) Igneous activity in the early solar system. In Meteorites and the Early Solar System (eds. J. F. KERRIDGEand M. S. MATTHEWS),pp. 73-10 1. IKEDAY. and TAKEDAH. (1985) A model for the origin of basaltic achondrites based on the Yamato 7308 howardite. Proc. Lunar Planet. Sci. Conf: 15th; J. Geophys. Res. 90, C649-C663. JEROMED. Y. (1970) Composition and origin of some achondritic meteorites. Ph.D. dissertation, University of Oregon.
JEROMED. Y. and GOLESG. G. (197 I ) A re-examination of relationships among pyroxene-plagioclase achondrites. Activation Anal. Geochim. Cosmochim. Universitetsforlaget, Oslo, 26 I-266. KALLEMEYNG. W. and WASSON J. T. (1981) The compositional classification of chondrites- I. The carbonaceous chondrite group. Geochim. Cosmochim. Acta 45, 12 17- 1230. LABOTKAT. C. and PAPIKEJ. J. (1980) Regolith ofthe eucrite parent body: Petrology of the howardite meteorite. Proc. Lunar Planet. Sci. Conf: Ilth, pp. 1103-I 130. LAULJ.-C. ( 1979) Neutron activation of geological materials. Aromtc Energy Review; IAEA, 17,603-695. LAULJ.-C. (1987) Rare earth patterns in shergottite phosphates. Proc. Lunar Planet. Sci. Conf: 17th, Part I; J. Geophys. Res. 92, E633E640. LAULJ.-C. (1990) The Bholghati (howardite) consortium: An overview. Geochim. Cosmochim. Acta 54,2 155-2 159 (this issue). LAULJ.-C., LEPELE. A., WEIMERW. C., and WCK~MAN N. A. (1982) Precise trace REE analysis by radiochemical neutron activation. J. Radioanal. Chem. 69, 18 l-196. LAUL J.-C.. SMITH M. R., WANKE H., JAGOUTZE., DREIBUSG.. PALME H., SPETTELB.. BURGHELEA., LIPSCHUTZM. E., and VERKOUTERENR. M. (1986) Chemical systematics of shergotty meteorite and tl’e compositions of its parent body (Mars). Geochim. Carmochim. A, ;a 50,909-926. LAULJ.-C., GOSSELIND. C., and SMITHM. R. (1989) The Bholghati consortium: Chemical study of the Bholghati howardite (abstr.). Lunar Planet. Sci. Conf: ZOth, Lunar and Planetary Institute. MAYEDAT. K. and CLAYTONR. N. (1989) Oxygen isotope in the Bholghati howardite (abstr.). Lunar Planet. Sci. Conf 2Oth, Lunar Planetary Institute. MCCARTHYT. S., AHRENSL. H., and ERLANKA. J. (1972) Further evidence in support of the mixing model for howardite origin. Earth Planet. Sci. Lett. 15, 86-93. MCCARTHYT. S., ERLANKA. J., and WILLISJ. P. (1973) On the origin of eucrites and diogenites. Earth Planet. Sci. Lett. 18, 433442. MCCORDT. B., ADAMSJ. B., and JOHNSONT. V. (1970) Asteroid Vesta: Spectral reflectivity and compositional implications. Science 168, 1445-1447.
MITTLEFEHLDT D. W., CHOU C. L., and WASSON J. T. (1979) Mesosiderites and howardites: Igneous formation and possible genetic relationships. Geochim. Cosmachim. Acta 43, 667-688. NEWWMH. E. (1985) Molybdenum in eucrites: Evidence for a metal core in the eucrite parent body. Proc. Lunar Planet. Sri. Conf 15th; .I. Gcophys. Re.7. 90, C613-C617. NYQIJISTL. E.. BOGARDD. D.. WIESMANNH., BANSALB. M., SHIH C.-Y., and MORRISR. M. (1990) Age of a eucrite clast from the Bholghati howardite. Geochim. Cosmachim. Acta 54, 2 195-2206 (this issue).
PAPANASTASSIOU D. A. and WASSERBURG G. J. (1976) Early lunar differentiates and lunar initial 87Sr/86Sr. Lunar Sci. VII, 665-667. PAPANASTASSIOU D. A.. RAJAIVR. S., HUNEKEJ. C., and WASSERBURG G. .I. (1974) Rb-Sr ages and lunar analogs in a basaltic achondrite: Implications for early solar system chronologies. Lunar Sci. V, 583-585. RAJANR. S., HUNEKEJ. C.. SMITHS. P., and WASSERBURGG. J. ( 1979) Argon-40/Argon-39 chronology of lithic clasts from the Kapoeta howardite. Geochim. Cosmochim. .4cta 43, 957-97 I, REID A. M.. BUCHANANP., ZOLENSKYM. E., and BARRETTR. A. ( 1990) The Bholghati howardite: Petrography and mineral chemistry. Geochim. Cosmochim. Acta 54, 2 161-2166 (this issue). SCHMITTR. A., SMITH R. H., MOSENA. W., OLEHY D. A., and VASILEVSKIS J. (1963) Abundance of the fourteen rare earth elements, scandium
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of the how-
SMITHM. R. (1982) A Chemical and petrological study of igneous
Chemical mixing model of Bholghati lithic clasts from the Kapoeta howardite. Ph.D. dissertation, Oregon State Univ. STOLPERE. M. ( 1977)Experimental petrology of eucritic meteorites. Geochim. Cosmochim. Acta 41,587-611. SWINDLE T. D., GARRISON D. H., GOSWAMI J. N., HOHENBERG C. M., NICHOLSR., and OUNCER C. T. (1990) Noble gases in the howardites Bholghati and Kapoeta. Ge~hj~. Cosmochim. Acta 54, 2 183-2194 (this issue). TAKEDA H., MIYA~OTO M., ISHIIT., and RE!D A. M. (1976) Characterization of crust formation on parent bodies of achondrites and the Moon by pyroxene crystallography and chemistry. Proc. Lunar Sci. Conf: 7th, pp. 3535-3548. THOLEN D. J. (1984) Asteroid taxonomy from cluster analysis of photometry. Ph.D. Dissertation, Univ. Arizona. WANG M. S., PAUL R. L., and LWSCHUTZM. E. (1990) Volatile/ mobile trace elements in the Bhoighati howardite. Geochim. Cosmochirn. Acta 54,2 177-2 18 I (this issue). WARREN P. H. (1985) Origin of howardites, diogenites and eucrites: A mass-balance constraint. Geochim. Cos~~hi~. Aera 49,577586.
2175
AvoendixA. Oxygen IsotopicCompositionsof Bholghatiand Other Achondrites(Mayedaand Clayton 1989)
Meteorite Bholghati (howardite)
Sample
6180
PO
637-0.52P
t1.40 +I.50
-0.32 -0.33
+3.43
tl.60
-0.18
t3.23
11.25
-0.43
BH 18 rock BH 11 fines
t3.31 t3.52
BH 24 fines BH 25 fines Yurtuk (howardite)
rock
+3.16
t1.28
-0.36
Ibitira (eucrite)
rock
t4.00
tl.87
-0.21
Shalka (diogenite)
pyroxene
t3.36
+1.38
-0.37
Estherville (mesosiderite)
pyroxene
t3.36
+1.51
-0.24
Ilimaes (pallasite)
olivine
+3.25
t1.33
-0.36