Age of a eucrite clast from the Bholghati howardite

GPackimica et Cosmochimrca Acta Vol. 54. pp. 2195-2206 Copyright D 1990 Pergamon Press Printed in U.S.A.

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Age of a eucrite clast from the Bholghati howardite L. E. NYQUIST,’ D. D. BOGARD,' H. WIESMANN,’B. M. BANSAL,’C-Y. SHIH,’ and R. M. MORRIS’ ‘SN2/Planetary Sciences Branch, NASA Johnson Space Center, Houston, TX 77058, USA ‘Lockheed Engineering and Sciences Co., 2400 NASA Rd. 1, Houston, TX 77058, USA (Received August 15, 1989;accepted in rev~sed.~r~ February 7, 1990)

Abstract- 3gAr-40Ar, Rb-Sr, and Sm-Nd investigations of the “large” eucrite clast (,I) studied by the Bholghati Consortium show that it crystallized from a melt - 4.51 Ga ago. The crystallization age is given most precisely by a Sm-Nd internal isochron of 4.510 + 0.034 Ga (20 error limits) for X(14’Sm) = 0.00654 (Ga)- *. 142Nd/144Ndmeasurements show that the clast contained live 14”Sm at the time of c~st~lization. A solar system initial (‘~Sm/‘~Srn)~ = 0.~43*“:~3g -ooo26at 4.555 Ga ago can be calculated from a i46Sm-‘42Nd isochron and the age of the clast. Alternatively, using (‘~Sm/‘~Srn~ = 0.0045 as previously reported in the literature, a ‘46Sm-‘42Nd formation interval of O.OS$:~~Ga can be calculated, which is concordant with the r47Sm-‘43Nd isochron age and in agreement with Pu-U-Xe and Pu-Nd-Xe formation intervals reported in a companion Consortium paper. The initial ‘43Nd/‘44Nd (ZN,) of the clast was ZNd= 0.505958 f 50 (2a error limits), normalized to ‘46Nd/‘44Nd = 0.724140 and is consistent with the CHUR reference value for evolution in a source having chondritic relative REE abundances prior to magma generation. However, when interlaborato~ bias is considered, the data are also permissive of a LREE depleted source as su8gested in a companion Consortium paper. A primary Rb-Sr isochron yields an age of 4.54 f 0.06 Ga (2g error limits) for X(87Rb) = 0.01402 (Ga))‘, in agreement with the more precise Sm-Nd age. The initial 87Sr/86Sr(Zsr) of the clast was Zsr = 0.699006 ? 2 1 and was the same within error limits as Zsrpreviously determined for a pristine eucrite clast from polymict eucrite Y750 11; compatible with a common parent body. The 3gAr-40Ardata and some of the Rb-Sr data show that Bholghati was affected by a major thermal event - 2-3 Ga ago and also provide weak evidence for a minor thermal event I 1 Ga ago. The age and isotopic systematics of the clast are consistent with those of other eucrites and support the widely held assumption of a common parent body for howardites and eucrites. The isotopic investigations of polymict eucrites and howardites point to a prolonged cratering record on the howardite-eucrite-diogenite (HED) parent body and suggest that it survived the “cataclysm” affecting the Moon - 3.8-4.0 Ga ago.

HOWARDITESARE WIDELYCONSIDERED to have originated on the same meteorite parent body as eucrites (DUKE and SILVER, 1967) and probably also diogenites (JEROME and GOLES, 197 1; MCCARTHY et al., 1972, 1973) and, indeed, to be mechanical mixtures of eucritic and diogenitic materials. Although both eucrites and diogenites have been reliably dated as -4.4-4.5 Ga old, howardites have often yielded ambiguous radiometric ages. For example, PAPANASTASSIOU et al. (1974) reported comparatively young Rb-Sr ages of 3.89 It 0.05 Ga and 3.63 + 0.08 Ga for two basaltic clasts (A and B) from the Kapoeta howardite whereas PAPANASTASSIOU and WASSERBURG(1976) reported an age of 4.54 + 0.12 Ga, more typical of eucrites, for a third clast (C). RAJAN et al. (1979) reported 39Ar-40Arages of 3.48 t 0.04 Ga, 14.57 Ga, and 24.6 Ga, for the same clasts A, B, and C, respectively, and an age 2 4.59 Ga for a fourth clast, p. RAJANet al. (1975) had previously reported 3gAr-40Ar ages of 4.42 + 0.04 Ga and 4.29 + 0.05 Ga for plagioclase separates of the Bununu and Malvern howardites, respectively. However, glass separated from the same meteorites yielded 39Ar-40Arages of 4.24 rlz0.05 and 3.73 + 0.05 Ga, respectively. LEIGH and MONIOT (1976) measured the isotopic composition of Ar extracted from a neutron-irradiated bulk sample of the Bholghati howardite and interpreted their results as showing strong outgassing 3.4 i 0.2 Ga ago. These results show that the how-

ardite parent body was subjected to a number of strong “thermal events”, i.e., events capable of causing significant to total Ar loss from basaltic materials between -3.5-4.5 Ga ago. In addition, there is a suggestion of “young” (~4.5 Ga) igneous events on the howardite parent body, but this suggestion is obscured by the occurrence of clearly discordant ages from differing radiometric dating methods. Fu~he~o~, some of the 3qAr-40Arstudies were either done on bufk samples of these polymict breccias or on minerals separated from bulk samples and, thus, even apparently “good” plateaus may be misleading because of the superposition of Ar from differing origins. The discovery in Bholghati of the comparatively large eucritic clast described in the companion papers (LAUL and GOSSELIN, 1990; REID et al., 1990) presented the opportunity to measure the radiometric age of an unbrecciated igneous component ofthis howardite. Furthermore, our earlier studies of eucritic clasts from polymict eucrites collected in the Antarctic (TAKEDAet al., 1983, 1988; NYQUISTet al., 1986, 1988) had revealed systematic relationships in the Rb, Sr, Sm, and Nd elemental and isotopic compositions of the various mineral phases. Preliminary mineralogical and chemical studies of the Bholghati clast (A. REID and J. C. LAUL, pers. comm.) suggested it would conform to the same systematics. It was thus possible to structure the Sr, Nd, and Ar isotopic investigations to maximize the info~ation obtained from the small (in absolute terms) sample available.

2195

2196

L. E. Nyquist et al. ANALYTICAL

PR~EDURES

Sample preparation

A 247 mg portion of the “larger eucrite” described by REID et al.

( 1990) was allocated for isotopic analysis. This sample was coarsely crushed in a boron carbide mortar. After crushing, 30 mg was used for 39Ar-40Aranalyses, 19.4 mg was used for whole rock Rb-Sr and Sm-Nd analyses, and 13.5 mg was held in reserve. The remainder was used to separate minerals for combined Rb-Sr and Sm-Nd analyses. It was alternately crushed and sieved to pass a 200 mesh (74 lm) sieve. Finally, material finer than 325 mesh (44 hm) was removed by sieving to yield I53 mg of sized material for mineral separations. A flow chart of subsequent sample processing is shown in Figure 1. Mineral separations were guided by prior experience with the pristine eucrite ciast Y75011,84 (NYQUISTet al., 1986) and the monom~~t me~mo~ho~ clast Y7925 10 (NYQUISTet at., 1988). The mesostasis often present in basalts is ofparticular importance to isotopic studies because it usually contains high abundances of trace elements. The pristine clast Y750 1184contained abundant mesostasis and, although mesostasis was absent from the metamorphosed monomict clast Y792510, TAKEDAet al. (1988) argued that dark brown glassy mesostasis such as that in Y7501 I,84 had been replaced in Y7925 10 by fine-grained rectystaltized material. Specifically, a dark brown glassy matrix, subcalcic ferroaugite, fayalite, silica, ilmenite, and Ca phosphate occupy areas of Y750 II,84 which are texturally equivalent to areas now occupied by dominant silica, high Ca augite plus minor low-Ca pyroxene, plagioclase, troilite, Ca phosphate, and ilmenite in Y792510. Like Y7925 10, the Bholgbati clast also lacks mesostasis (REID et al., 1990) but contains those phases present in Y7925 10 except for Fe oxide and hy~oxide weathering products. The pyroxene compositions of the Bhol~ati clast (REID et al., 1990) were nearly identical to those of Y792510 (TAKEDAet al., 1988). Because the amount of sample was limited and it was unclear whether adequate quantities of the recrystallized mesostasis could be separated from the clast, an attempt was made to separate low- and high-Ca pyroxenes by density and thus to increase the probability of analyzing phases of sufficiently high Rb/Sr and/or Sm/Nd ratios for a precise age dete~ination. However. ~~mtion of the pyroxenes according to composition was difficult hecause the high-Ca pyroxene is present primarily in narrow exsolution lamellae (REID et al., 1990). Six density fractions were generated using heavy liquids. A mixture of bromoform and acetone was used to obtain a fraction of density < 2.65 g/cm’. Bromoform was used to obtain a 2.65-2.85 g/cm’ fraction-and mixtures of Clerici’s solution plus water were used to obtain fractions of 2.85-3.4 a/cm’. 3.4-3.55 a/cm3. 3.55-3.7 a/cm-‘. and >3.7 g/cm3, respectively.~The three high2 densities were chosen to separate pyroxene into high- and low-Ca fractions corresponding to compositions measured by REID et al. (1990). Mossbauer spectra for the two pyroxene samples subsequently isotopically analyzed showed that they contained no major Fe-beating phases other than pyroxene. The denser pyroxene (3.55-3.7 g/cm3) did show the pres-

B~OLGHAT~

EUCRITE

Mineral

CLAST

Separation

200-325 m.*il 153mg

i,_i I

I

t

I I

I

1

FIG. 1. Mineral separation scheme for the large eucrite clast (, 1) from the Bholghati howardite.

ence of a trace amount of itmenite. There was no evidence of troilite in the spectra of either of the pyroxene samples and troilite present in the clast (REID et al.. 1990) is assumed to have been included with ilmenite in the fraction denser than 3.7 g/cm3. The usefulness of the Mossbauer spectra as a monitor of sample purity was limited by the insensitivity of this technique to the presence of non-Fe-bearing phases such as plagioclase or phosphates. However, the primary purpose in obtaining Mossbauer spectra for these samples was to monitor the degree of separation of pyroxenes of differing compositions. The spectra of both pyroxene samples analyzed were identical except for the presence of a trace of ilmenite in the 3.553.7 g/cm3 fraction as noted above, suggesting that the attempt to separate low- and high-Ca pyroxene was largely unsuccessful, as might be expected hecause highCa pyroxene predominantly occurs in narrow exsolution lamellae (REID et al., 1990). Distinct differences were nevertheless detected in the isotopic com~sition of the two pyroxene samples, suggesting that either they contained trace impurities not detectable in the Mossbatter spectra, or that some separation of pyroxenes was achieved, perhaps of trace-element rich pyroxenes from the recrystallized mesostasis. REID (pers. comm.) noted the presence of trace phosphate in the eucrite clast and LAUL (pers. comm.) reported that phosphates dissolved quantitatively in I N HCl in 10 to 15 min from a bulk sample of Bholghati. Thus, following measurement of Mosshauer spectra for the pyroxenes, they were “cleaned” of phosphates by leaching with I N HCI at room temperature for 10 min (3.4-3.55 g/cm3) and 30 min (3.55-3.7 g/cm’), respectively; the amount of material dissolved in these HCl leaches was not determined. The leaches and residues were separately analyzed for Rh-Sr and Sm-Nd. The 2.85-3.4 g/cm3 density fraction was considered the most likely repository of phosphates and was teached in 1 N HCl for 10 min during which 1.19 mg of the sample dissolved, corresponding to a “phosphate” content of -0.8% of the total sample. Both the leach and residue of this fraction were analyzed. The other density fractions were analyzed without leaching. Unfortunately, the possible importance of troilite in recording tbe recrystallization history of the clast (REID et al., 1990) was not appreciated when the mineral separations were made. Fu~hermore, troilite is a trace phase and, thus, no attempt was made to separate it from other phases present in the clast. However, since troilite has a density of -4.77 g/cm’, it should occur in the densest fractions. Although troilite is soluble in HCI and thus in principle it could also contribute to the HCI leaches, its absence from the Mossbauer spectra taken prior to leaching of the pyroxenes precludes a significant component in either the pyroxene leaches or residues. Thus, most and probably all ofthe troilite present in the mineral separates was included with ilmenite in the fraction of density > 3.7 g/cm3. Rb-Sr und Sm-Nd chemical procedures

Three types of ion exchange columns were used for separation of Rb, Sr, and ZREEs depending on sample size and type: (1) large (24 mi volume) columns eluted thrum the Sr peak with 2 N HCI; (2) small (5 ml volume) columns from which Rb was first eluted with I N HCI followed by Sr with 2 N HCI (“1 N-2 N” columns); and (3) small columns (also 5 ml volume) eluted through the Sr peak with 3 N HCI (“3 N” columns). For all three types of columns, XREEs were stripped off with 4 N-6 N HCl following the Sr elution. Blanks on these columns were all similar and averaged 21 t 6 p& 22 -C 10 pg. and 14 + 3 pg for Rb and 144 C 55 pg, 260 t 95 ng, and 123 t 68 ua for Sr for the large, I N-2 N. and 3 N columns. resnectivelv. The choice of column t;pe was made according to sample size and whether the sample to be analyzed was Ca-rich (1 N-2 N), Mg-rich (3 N) or neither (large column). The data have been corrected for blanks and error limits include a 50% uncertainty in the blank corrections. Blanks were significant (a few to several per cent) for the leachates, ilmenite, and pyroxene separates, but were ~1% for the plagioclase, whole rock, and ~2.65 g/cm3 samples from which a primary isochron was determined. The Sm-Nd chemical procedures used in this laboratory were described previously (NYQUISTet al., 1979). Sm blanks were - 10 pg and -5 pg for the large and small columns, respectively: and Nd blanks were -80 pg and -25 pg, respectively. The data have been

Age of eucrite clasts corrected for blanks and error limits include a 50% uncertainty in the blank corrections. Nd blank corrections for those samples used to determine the Sm-Nd isochron were ~0.2% except for the plagioclase (2.65-2.85 g/cm3) for which it was 1.3%. A new mixed ‘@Sm‘wd spike was made prior to the analyses reported here. This spike was calibrated against the same standard solutions of Sm and Nd metal (obtained from the Ames laboratory) used to calibrate the i4’Sm‘*Nd spike previously used in our laboratory. The ‘49Sm-‘46Ndspike also had been calibrated in 1982 against the CIT mixed Sm-Nd standard solution n(Sm/Nd)@ (WASSE~BURGet al., 1981) with the result that the ‘@Sm/‘“Nd ratio obtained usine the JSC and CIT standard solutions differed by -0.15% which was c&sidemd to be in~~ificant. A second aliquot of the CIT standard, kindly supptied through the courtesy of G. J. Wasserburg, was used in this investigation to calibrate the new “?Sm-‘5r’Nd snike as well as re-calibrate the ‘49Sm-‘46Nd spike. During these calibrations it was discovered that the 149Sm/ l”Nd ratio as measured using the JSC standards was 0.54 + 0.10% lower than now obtained using the mixed CIT standard, apparently because the con~ntmtion of the JSC Sm standard solution had changed by -0.4%. Similarly, the ‘4qSm/iMNd ratio as measured using the JSC standards was 0.46% lower than now obtained using the mixed CIT standard. We attribute the change in concentration of the JSC Sm standard to evaporation loss. The ages reported here utilize the calibration against the CIT standard and, thus, are -0.5% lower than previously reported for the Bholghati data (NYQUISTet al., 1989a,b). Systematic errors in ‘47Sm/i44Nd ratios and Sm and Nd abundances of the Bholghati samples are estimated to be 10.1% and -0.25% respectively. Random errors in measurement of Sm/ Nd ratios are also estimated as 50.1%. The two spikes were also intercalibrated by analyzing homogenized samples of two lunar basalts and USGS standard sample BCR- 1. No biases were detectable within a limit of about +20 ppm (Table 3).

2197

We have noted typical values of 140Ce/‘44Ndof -0.00003-0.0008 at the beginning of Nd analyses, including those of the isotopic standard, which cause errors of -4-100 ppm in the r42Nd/‘“Nd ratio. 140Ce was not monito~d for the first Bholghati analyses and, thus, the 14*Nd/lqdNddata in those analyses were not useful to search for small variations in ‘42Nd/i44Nd. Moreover, because of the configuration used for the mass spectrometer multi-collector for these analyses, both ‘43Nd/‘44Ndand 14*Nd/lblNd could not be measured simultaneously with ‘40Ce/‘“Nd. The last four samples were analyzed twice, once for ‘43Nd/‘44Ndand again for ‘“Nd/‘“Nd. For the latter analyses, the 14’Ce was measured simultaneously with 14*Nd,allowing subtraction of the j4*Cecont~bution from the mass- 142 peak. The “‘*Nd/ ‘&Nd data were normalized to ‘46Nd/‘44Nd= 0.724140 using a modified power law which allows the functional relationship governing mass fractionation to be empirically determined for each type of analysis from the fractionation observed for shelf standards and standard samples; this procedure will be described in more detail elsewhere. For ‘43Nd/‘44Nd,the difference between the values obtained using this procedure and those obtained using the standard procedure included in the Finn&an-MAT 261 software is insignificant and the ‘43Nd/*44Ndvalues reported here are those obtained from the standard software. Normalization to ‘46Nd/‘44Nd = 0.724140 was chosen to correspond to our previous normalization to ‘48Nd/‘44Nd= 0.24308 as reported by PAPANASTAS~IOU et al. (1977). A 26.7 mg sample of the eucritic clast was irradiated with fast neutrons to convert a portion of the 39Kto 39Arand a portion of the #Ca to “Ar. Samples of terrestrial hornblende of known age were irradiated with Bholghati to characterize the irradiation parameters. The sample was degassed in a high vacuum furnace in a series of known temperature steps, and the isotopic composition of the released argon was measured with a mass spectrometer. The 39Ar-40Arage was calculated for each temperature release using the K decay parameters given by STEICERand JAEGER(1977).

ANALYTICAL RESULTS All Sr and Nd isotopic data were obtained on a Finnigan-MAT Model 26 I mass spectrometer equipped with six movable Faraday cups, one fixed Faraday cup and a secondary electron multiplier. The data were acquired by static multi-collection without peak-jumping. The linearities of the electrometer amplifiers were checked by application of an external signal and no non-linearities in excess of -20 ppm were observed over the range (2 X 10-i’ A to 6 X 10-r’ A) in which data are normally acquired; consequently, no corrections for non-linearities were made. The net response of the individual Faraday cups and associated detector channels has been calibrated to a precision of about 20 ppm relative to that of the two outermost Faraday cups using a Nd ion beam. This calibration is necessary to eliminate po-ssible biases of up to - 150 ppm (worst case) in the response of the individu~ channels. Values of s7Sr/86Sr = 0.7 1025 1 + 0.~28 (2~~) and ‘43Nd/‘44Nd= 0.511101 f i).OOOO14 (2up,, normalized to ‘46Nd/‘“Nd = 0.724 140) were measured for the NBS 987 and Ames Nd metal at the time of the analyses reported here, i.e., FebruaryMarch, 1989. Here *p is the standard deviation of the population of measurements, i.e., flp = [C(mj - r)*/(iV - I)]“’ for Nmeasurements mj with mean value p. The best estimate of the uncertainty of an individual analysis is perhaps 2~ although internal precision is usually somewhat better than this. A limited number of analyses of the Ames Nd standard by dynamic multi-collection gave ‘43Nd/‘44Nd = 0.511102 f 0.000012 (20~) for this normalization. Recently, we have begun measurements with different cup assignments for the Nd isotopes to allow for simultaneous measurement of ‘43Nd/‘“Nd and 14?ld/‘“Nd in&dine. Ce corrections. Measurements of the JSC Ames Nd metal standard a;d CIT n(Nd)@with this configuration are given in Table 3 labelled “December, 1989” and show that the isotopic composition of these two standards is the same within a few ppm. The value of ‘43Nd/‘MNd for this configuration is -30 ppm lower than with the previous configuration, which we believe is close to the practical limit of the accuracy of this method of measurement. All other data in the table are relative to the March 1989, value of the Ames standard. Acquisition of reliable i4’Nd/‘“Nd data needed to search for possible anomalies due to decay of short-lived ‘*Sm requires monito~ng the ‘@Cebackground peak because of possible interference from “*Ce.

3YAr-40Arage data Argon isotopic data are given in Table 1, and the calculated 39Ar-40Ar apparent ages and K/Ca ratios are shown in Fig. 2

as a function of cumtdative fraction of “Ar released. The potassium concentration determined for our sample was 157 ppm. The width of each data “box” shows the fraction of 39Ar released at a given temperature; the height of each data box represents the combined analytical errors in calculated age. Age uncertainties increase at high extraction temperatures primarily because of uncertainties in corrections that are applied for 39Ar made from calcium during irradiation. The Ar release profile is not simple and does not uniquely define an age. The continuous decrease in the K/Ca ratio during the extraction and the complex shape of the apparent age curve suggest that argon is being released from two or more phases with different K/Ca and with different sensitivities to diffusive loss of Ar. The minimum “age” of the most retentive phase can be estimated by averaging the three highest temperature releases to give 4.2 + 0.5 Ga. The considerable variation in ages of these three releases is outside analytical uncertainties, however, and suggests some re-equilibration of argon even in the most retentive phase. This averaged “age” is consistent with the Bhol~ati clast being a fragment of an old basalt with a formation age typical of those for eucrites. Relativ:: to Sm-Nd and Rb-Sr formation ages discussed later, the Bholghati clast has been strongly degassed of radiogenic Ar. For a hypothetical case of partial loss of Ar from a single phase, the 39Ar-40Ar age of the lowest temperature releases can define the time of degassing or an upper limit to this time. The change in K/Ca ratio suggests, however, that

2198

L. E. Nyquist

et al

39Ar ‘I’he Table 1. ArgCln isotopic data for Bholghati eucritic clast (,I). concentrations for stepwise temperature extJ$cticns are given in units of 10+ by 100, cm3/g and are uncertain to &IO%. The 36Ar/ Ar ratios have been multiplied Corrections were made for system blanks. reactor the K/Ca ratios by 1000. were in isotopic ratios interferences, and radioactive Uncertainties decay. derived from uncertainties in measured ratios, blanks, and reactor corrections.

40Ar/39Ar

K/Ca

38Ar/39Ar

37Ar/39Ar

300 4.50 600 750 900

1000 1075 11.50 1250 1400 1550

0.02 0.12 0.4 I 0.78 2.41 2.66 1.69 1.11 0.37 0.33 0.21

4.30~0.26 0.58&O. 15 0.73kO.08

1.449.03

2.09kO.02 2.479.03 2.929.04 3.10+0.05 3.819.20 4.829.21 4.18+0.12

15.4t2.6 15.1+4.6 14.6+2. I r4.5*0.5 9.86*0.2 6.359.1 4.779.1 3.09kO. 1 0.88*0.1 0.84+0. I 2.19+0.2

I I9.+_20 4.01+1.23 5.3120.74 l3.3kO.45 24.4kO.32 33.1*0.61 46.1+1.11 52.?+1.95 85.9+1 1.1 165.~22.3 l10.+8.49

whereas p~a~oclase is probably the mineral releasing most of the Ar, a minor phase with higher KfCa (possibiy “tridymite” or associated phases which also contain high Rbf Sr) is releasing Ar in at least the first - 15% of the 3yArrelease. Thus, a thermal event or continuous thermal environment more recently than -0.7 Ga ago likely explains the low temperature data. This thermal event was probably mild, however, and is unlikely to have been the cause of major Ar loss shown by the plagioclase between -20% and -90% of the 39Arrelease. The time of degassing of the plagioclase probably lies between 2 and 3 Ga ago, and the weighed average age of those four extractions releasing 73% of the 39Ar is 2.5 Ga. Two extreme explanations can be offered for the time of degassing of the plagioclase in this time interval, depending on how one interprets the data in Fig. 2. One alternative is that an event - 2 Ga ago degassed most, but not all, of the Ar present in the plagioclase, and that additiona Ar loss shown by low tem~mture extractions occurred from tridymite more recently than -0.7 Ga ago. A second alternative is that an event - 3 Ga ago degassed most, but not all, of the Ar in

0

0

a.2

CUMULATIVE

0.4

0.6

39Ar

08

36Ar/39Ar (x100)

(x1000)

1

RELEASE

FIG. 2. Apparent 39Ar-40Arages and K/Ca ratios as a function of fractional release of 39Ar during stepwisetemperature degassingof a

eucritic clast from Bholghati. The [K] for this sample was 157 ppm. The mid-point of the Ar release occurred in the 1000°C extraction.

5.57+1.09 2.129.93 0.4 1~0.08 0.06~0.01 0.03~0.~1 0.044.01 0.09+0.01 o.r9&0.01 0.749.10 0.79+0.1 I 0.30+0.04

34.225.8 35.0+1 I 36.0+X 1 36.3k1.3 53.54.9 83.lt1.7 I ll.+2.9 l?l.&6 602.+78 626.285 241.+19

62&l O.&S O&3 1.222.3

1.09.9

2.321 .O 6.8+ 1.4 I I .8;2.2 51.1k9.4 75.4+12.2 28.1+8.5

the plagioclase, that tridymite was degassed more recently than -0.7 Ga ago, but that these two degassing profiles overlap in Fig. 2 in the range of 20-60% release of 3gAr. By this explanation the apparent ages of -2-3 Ga shown by the temperature releases would be due to mixing, during laboratory extraction of Ar, of plagioclase reset by an event - 3 Ga ago and “tridymite” reset by a milder event or environment less than -0.7 Ga ago. We mention these possible degassing models, not because the Ar data alone can differentiate among them, but because the Rb-Sr data discussed below also suggest thermal disturbances at times of -2-3 Ga and < 1 Ga ago, respectively. Rh-Sr ug~’ veldonship.t

Rb-Sr analytical results for the Bholghati clast (,I) and a matrix sample (,23) are given in Table 2. Rb and Sr concentrations in whole rock and mineral separates of the clast are similar to those in clasts Y750 I I,84 (NYQUIST et al., 1986) and Y9725 IO,62 (NYQUIST et al., 1988) from Antarctic polymitt eucrites (Fig. 3). The data from each clast form a “‘eucrite triangle” defined by values for plagioclase, silica (probably tridymite) and pyroxene. The data for ilmenite plot close to those for pyroxene. In each case, the whole rock compositions lie within the triangle, suggesting that the principal Rb-Sr phases have been analyzed. Rb-Sr isotopic data are shown in Fig. 4. Those phases containing the highest concentrations of Rb and Sr, i.e., plagioclase, “tridymite”, and whole rock, define a line of slope corresponding to an age of 4.54 4 0.06 Ga for X(87Rb)= 0.01402 (Ga)-’ (MINSTER et al., 1982). We interpret the line as an isochron analogous to that previously reported for the pristine eucrite clast Y75011,84 (NYQUIST et al., 1986). The enrichment in *7Sr/a6Srachieved (0.710) is somewhat less than for Y75O 1I ,84 (0.7 t7), but distinctly greater than for Y7925 lo,62 (0.702). However, disturbances of the Rb-Sr isotopic systematics are evident for the pyroxenes (both leachates and residues) and ilmenite (Figs. 4 and 5). Thus, those phases have been excluded from the least squares fit (YORK, 1966) used to obtain the primary isochron age. Additionally, the 2.853.4 g/cm3 fraction contained a significant pyroxene compo-

Age of eucrite clasts Table 2. Rb-Sr analytical

2199

results for the Bholghati Howardite

Sample =Rb,=Sr’

F+ja6Srb

1. Eucritic clast (,I)

WR x2.& 2.65-2.85 2.85-3.41. 2.85-3.4R 2.85-3.4Te 3.4-3.55L 3.4-3.55R 3.4-3.55Te 3.55-3.7L 3.55-3.7R 3.55-3.7Te >3.7

19.4 1.23 11.19 1.19 25.3 26.7

0.2529 0.2565 0.1939 0.1879 0.1881

72.14 24.69 198.9 48.60 25.10 26.14

25.9 23.5

0.0294 0.0326

5.66 6.11

23.5 25.9 2.22

0.0186 0.0279 0.1869

1.622 3.025

2. Matrix f,23) WR 17.4

1.494

0.2164

1.308

45.03

4.269.29 4.54*0.05

0.01015+5 0.1751+14 0.00373+3 0.01155564 0.02166&12 0.02082~12 0.02082+12 0.01503+19 0.01545+_21 0.0851+51 0.04118+84 0.0497+12 0.1788~52

0.699660+11 0.7 10524+22 0.699265+12 0.699595+23 0.700352+15 0.700289~16 0.699922+73 0.699876+29 0.699879+31 0.700375+50 0.700925+25 0.7008 I8+30 0.7043+14

2.95kO.60 4.2020.14 4.159.15 2.7020.69 3.76iO.32 3.649.33 0.97io.07 3.09+0.15 2.374.13 2.029.58

O.O1391t_8

0.699853+15

4.01+0.22

3. NBS 987 (N=13)

0.710251+28’

a Error limits apply to the last digits and include a minimum uncertainty of 0.5% plus 50% of the blank correction for Rb and Sr, added quadratically. Uncertainties refer to last digits and are b Normalized to 88sr,&r = 8.37521. 20. where o;n= [~(mi-~)2/(N(N-l))]1’2 for N me urements mj with mean F. y z Model ages relative to plagioclase (2.6?$-2.85 g/cm f. Densities of mineral separates in g/cm . : Calculated from Rb andSr measuredin the $achate (t_hand residue (R). Error limits are 2up where up = [C(mi-r) /(N-l)] for N measurements mi with mean value p.

nent, as shown by the Sm-Nd analyses, and so was also omitted from the regression. The primary Rb-Sr age of the Bholghati clast, as thus determined from the plagioclase, whole rock, and “tridymite” analyses, is identical within error limits with Rb-Sr ages determined for the Y750 11,84 pristine clast (4.56 r?: 0.05 Ga) and also Y75011,73 matrix (4.53 + 0.06 Ga) as well as with the average 2osPb/zo7Pbage of eucrites of 4.54 & 0.02 Ga (BVSP, 1981). As will be shown below, it is also in agreement with the Sm/Nd age of the clast. Thus, we conclude that the whole rock, plagioclase, and “tridymite’” phases have retained the original crystallization age of the clast in spite of extensive Ar loss and disturbances in the Rb-

Sr system of other phases. It is pro~bly significant that the disturbed phases have low Sr concentrations because they would be most sensitive to the addition of ‘“foreign” Sr during partial isotopic re-equilibration. In comparison to earlier work on bulk eucrites (PAPANASTASSIOU and WASSERBURG, 1969; BIRCK and ALL~?GRE, 1978), the whole rock *‘Rb,%r for the Bholghati clast is similar to that of Pasamonte, Bereba, Sioux County, Macibini, and Nuevo Laredo and the enrichment achieved for *‘St-/ 86Sr in the lowest density (~2.65 g/cm’) “tridymite” phase of the Bholghati clast is also comparable to that achieved in the lowest density “quintessence” phase of those meteorites. clad

Bholgh~ti Polymict

Eucrite Clasts in Eucrites and Bholghati

r

Howardite

(X(Rb)=0.01402

T=4.54f0.06 l Y792510.62

Go-‘)

Go

L

+ Y75Ot 1.848. 0 Bhoighati

$

0.706

> m 6

0.702

I(Sr)=0.699367729 0.05

0.10

0.15

0.20

87Rb/86Sr 0.0

I .o

0.5 Rb

1.5

2.0

2.5

(ppm)

FIG. 3. Rb and Sr concentrations in mineral separates and whole rock sampies from the farge eucrite clast (, 1) in Bhoighati compared to those in clasts from the Antarctic pdymict eucrites Y7501 I and Y7925 10.

FIG. 4. RbSr data define a 4.54 t 0.06 Ga primary isochron for the Bholghati clast for the plagioclase, whole rock and tridymite (~2.65 g/cm’) data for X(“Rb) = 0.01402 (Ga)-’ (MINSTER et al., 1982). Recombined data for the pyroxenes plus the ilmenite and whole rock data yield an apparent 2.06 + 0. I 1Ga secondary isochron. Pyroxene data are shown by triangles with solid symbols for the recombined data and open symbols for the directly measured leachates and residues.

L. E. Nyquist et al.

2200

Bholghati

clad

(X(Rb)=0.01402

Go-l)

i

T=2.85tO.O6Go

0.702

l(Sr)=O.69g248~~4

_.__ 2.65

5.00

0.01

0.02

0.03

0.04

0.05

0.06

87Rb/86Sr

RG. 5. Low Rb-Sr data for the Bholghati clast (see inset in Fig. 4). Triangles: 3.55-3.7 g/cm3 pyroxene; inverted triangles: 3.4-3.55 g/

cm3 pyroxene; diamonds: 2.85-3.4 g/cm3 mixed phases. Open symbols for residues (low Rb/Sr) and phosphate leachates (higher Rb/ Sr) are original data; solid symbols are for recombined totals. Secondary isochrons of -2.85 Ga are defined by plagioclase plus phosphates (excluding the 3.55-3.7 &cm3 leachate) and pyroxene residues plus whole rock, respectively.The 2.85-3.4 g&m3sample is a mixture of phasesand does not lie completely on either a primary or secondary isochron.

The Rb-Sr age defined by the “quintessence” of the Bholghati clast is comparable to that obtained by BIRCK and ALL&GRE (1978) for the combined data of Juvinas and Ibitira. Thus, the Rb-Sr systematics of the Bhol~ati clast are generally typical of eucrites. However, the Rb-Sr ages defined by the ‘“quintessence” phases of Bereba and Sioux County. 4.17 + 0.26 Ga and 4.19 + 0.14 Ga, respectively, are distinctly younger than that of the Bholghati clast. The Rb-Sr systematics of Pasamonte are very disturbed (BIRCK and ALL~?GRE. 1978), but the enrichment of “Sr/s%r in the quintessence separates is similar to that of the <2.65 g/cm3 fraction of the ~ho~ghati clast. Those samples for which the Rb-Sr system is disturbed tell a more complicated story (Figs. 4 and 5). As discussed above, although the pyroxene samples were quite pure, they were nevertheless leached in 1 N HCl to remove phosphates after it was learned that bulk Bholghati contained easily leachable phosphates (J. C. LAUL, pers. comm). This leaching step effectively removed phosphates and with them significant amounts of REEs as shown by Sm concentrations in our 3.43.55 g/cm3 sample which were --% those in an unleached aliquot (LAUL and GOSSELIN, 1990). The leaching step introduced an element of uncertainty into the Rb-Sr analyses, however. The Rb-Sr data of the pyroxene leaches are highly disturbed (Figs. 4 and 5). This could be because suficial contamination was removed in the leachate; but this explanation seems unlikely because Bholghati is a fall and the clast was an interior one, processed under clean conditions. Another possibility is that Rb and Sr were differenti~ly leached. This possibility was tested by combining the data for leachates and residues to obtain calculated totals for the density separates (Table 2). The resultant data points are collinear with those for the whole rock and “ilmenite” data. The secondary isochron thus formed corresponds to an apparent age of 2.06 ? 0.11 Ga (Figs. 4 and 5). Thus, one interpretation of the Rb-Sr data is that the clast is a chip of an -4.5 Ga basalt

that was partially reset -2 Ga ago. This scenario is also compatible with the 39Ar-40Ardata as discussed above. There are objections to this interpretation, however, chief of which is that trace elements in the leachates should be derived from phosphates which would be totally dissolved in the 1 N HCl whereas the pyroxene would be little affected: i.e., that differential leaching is unlikely. This viewpoint leads to separate consideration of the leach and residue data. Table 2 lists the model ages of each phase relative to the plagioclase (2.65-2.85 g/cm3) separate, the major repository of Sr in the clast, and thus the most likely Sr donor during r~quil~bration events. The model ages of the three leachates are 0.97 t 0.05, 2.70 I: 0.26, and 2.95 f .24 Ga, respectively. This spectrum of ages corresponds to another possible interpretation of the 39Ar-40Ardata: that the clast was partially isotopically equilibrated - 3 Ga ago with a possible second event I 1 Ga ago. A secondary isochron for plagioclase and the 2.85-3.4 g/cm3 and 3.4-3.55 g/cm3 leachates gives an apparent age of 2.83 + 0.20 Ga (Fig. 5). As previously noted (NYQUIST et al., 1989a), the pyroxene residues plus the whole rock also define a secondary isochron corresponding to an age of 2.85 it 0.05 Ga. Thus, one interpretation is that pyroxenes re-equilibrated with the whole rock s7Sr/86Srcomposition -2.85 Ga ago, at the same time as phosphates were re-equilibrated with plagioclase. This second interpretation, although attractive in several ways, does not account for the Rb-Sr data of the “ilmenite” and the 3.55-3.7 g/cm3 leachate. It is possible that troilite, observed by REID et al. (1990) in the clast, is irn~~nt in this regard. Troilite has a density of -4.8 g/cm3 and, thus, most of the troilite should have been in the >3.7 g/cm3 “ilmenite” fraction. Troilite is soluble in HCI and also may have contributed to the Rb-Sr inventory of the leachates. The amount of troilite present in the pyroxene samples prior to leaching is limited by the detection limits of the Mossbauer analysis since no evidence of troilite was observed in the Mossbauer spectra. However, a small ~ont~bution of troihte to the 3.55-3.7 g/cm3 leachate probably cannot be ruled out. The Mossbauer spectra of the 3.55-3.7 g/cm3 pyroxene shows a trace presence of ilmenite and, since ilmenite and troilite have similar densities and occur in similar textural relationships in the clast, it is reasonable to assume that there may have been undetected troilite in this sample prior to leaching. The amounts of Rb and Sr measured in the leachate of the 3.55-3.7 g/cm3 pyroxene were only 0.22 and 7.4 ng, respectively, which may be compared to Rb and Sr contents of0.08 and 11.5 ng, respectively, measured for the leachate of the 3.4-3.55 g/cm3 pyroxene fraction which was similar in sample size, Thus, there is little evidence that contamination significantly influenced the Rb-Sr data for the leachates. Although the data for “ilmenite” and the 3.55-3.7 g/cm3 leachate could be ascribed to a poor quality mass spectrometer analysis and contamination, respectively, it seems more likely that both are influenced by the presence of troilite which may have heen partially isotopically re-equilibrated in a recent event or, possibly, affected by weathering. Thus, as for the 39Ar40Ar data, alternative explanations of those Rb-Sr data which suggest ages younger than -4.5 Ga are possible. (I) A single event occurred -2 Ga ago which was followed by later minor “events” which caused disturbances in the isotopic data due

Age of eucrite clasts to gas loss (Ar) or contamination (Rb-Sr). (2) Two events occurred, one at - 3 Ga ago which degassed plagioclase and reequilibrated pyroxene to the whole rock average 87Sr/S6Sr composition but phosphates dominantly to the 87Sr/86Sr composition of plagioclase; and another at - I Ga ago which degassed high K/Ca tridymite and re-~uilibrat~ S&earing phases in the ilmenite separate with lower Rb/Sr phases. (3) Residence of Bholghati in an environment of elevated temperatures over most of geologic time, leading to continual Ar loss and differential Rb-Sr isotopic equilibration at minera&specific rates. We prefer the second option as being most compatible both with the isotopic data and with the evolution of Bholghati suggested by REID et al. (1990) from their petrographic study of the meteorite. Initial 87Sr/86Sr

Accepting the validity of the primary isochron (Fig. 4), the age (T) and initial “Sr/%r (Is,) parameters are the same for the Bholghati clast and the pristine clast Y7501184 after correction for instrument bias. That is, Isr = 0.69894 + 2 (uncertainty refers to last digit) reported by NYQUIST et al. ( i 986) for Y750 11,X4 becomes 0.69898 t 3 when corrected for the difference between 87Sr/86Sr = 0.7 1021 + 2 (20,) reported by them for NBS 987 as measured with the JSC NBS 6” mass spectrometer and “Sr/‘%r = 0.71025 1 zt 7 (2~~) currently measured on the JSC Finnigan-MAT 261. Similarly, Zsr = 0.69900 + 4 for the matrix of Y75011 after correction for instrument bias. These values are the same within analytical uncertainty as Zsr= 0.699006 ?I 21 obtained here for the Bhol~ati clast and are consistent with derivation of the howardites and at least the polymict eucrites from the same parent body. This result is expected from the common association of howardites and eucrites on mineralogical grounds, but was not strongly supported by the previously reported Sr isotopic data. For example, PAPANASTASSIOU and WASSERBURG(1976) reported Zsr= 0.69888 + 5 and 0.69885 + 4 for clasts A and C from Kapoeta and higher values of 0.69905 + 6 and 0.69903 +: 14 for clasts B and p, respectively. The Zsr-values for clasts A and C are below the BABI reference value 0.69898 (PAPANASTASSIOU and WASSERBURG,1969), but within error of Zsrfor the Moore County cumulate eucrite. &-values for clasts A and C are also comparable to those for Bholghati and Y75011 after adding 0.00010 to the Kapoeta data to account for inter-laboratory bias as determined from “Sr/‘%r = 0.7 10 15 +- 5 measured by PAPANASTASSIOU and WASSERBURG(1973) for NBS 987. More Z,,values determined from internal Rb-Sr isochrons for individual eucritic clasts from howardites and eucrites would be useful to address the question of whether the Sr-isotopic data support the hypothesis of a single parent body for these meteorites. 147Sm-‘43Nd crystallization age

Sm-Nd data for the clast (, 1) and matrix (,23) are given in Table 3. Figure 6 presents the clast data in an isochron diagram. Some of the data depart slightly from a best-fit isochron which is not surprising in view of the complications in the Ar and Rb-Sr data. The >3.7 g/cm3 and 3.55-3.7 g/cm3 phases have been omitted from the least-squares fit because they are not collinear with the other data and their Rb-Sr

2201

data are severely disturbed. In contrast to the Rb-Sr data, however, data for the leachates and residues were not recombined because Sm and Nd must reside dominantly either in the acid resistant pyroxene or the easily soluble phosphate phases, respectively. Furthermore, of all the pyroxene and leachate data, only the 3.55-3.7 g&m3 pyroxene residue ap pears to be disturbed. The ~2.65 g/cm3 phase was also omitted from the best-fit line primarily because it is not collinear with the other data and because of a possible analytical difficulty; i.e., because of the small amount of Nd available for analysis for this sample, data were acquired at an unusually low ion beam intensity which may have biased the results. Furthermore, tridymite is mesostasis-derived and may have been involved in ~c~st~lization processes. Finally, althou~ the tridymite phase has apparently held its Rb-Sr age, it has suffered the most severe Ar loss. Because of the demonstrated severe disturbances in the 3gAr-*Ar and Rb-Sr systems, it seemed prudent to be conservative concerning the data base accepted as the basis for the primary isochron and to omit this datum from the isochron as well. The remaining seven data points vary significantly in Sm/Nd and give a well defined age of 4.510 + 0.034 Ga and ZN, = 0.505958 +- 0.~50 when regressed with the YORK ( 1966) least-squares program. Regression with the WILLIAMSON(1968) program yields the same age and ZNdparameters but reduced error limits of +O.O15 Ga and +0.000024, respectively. In this case we prefer the error limits from the YORK (1966) program since “geologic error” is included. In any event, the Sm-Nd age is in excellent agreement with the Rb-Sr primary isochron age. ~~Sm-‘4zNd~ormation intewal

The old age and wide variation in Sm/Nd-ratio achieved for the Bholghati mineral separates suggested the possibility of observing a variation in the ‘42Nd/1”Nd ratio due to production of 14*Ndfrom the decay of short-lived 14%rn.previous work (LUGMAIR et al., 1975; LUGMAIR and MARTI, 1977; JACOBSENand WASSERBURG,1984; PRINZHOFERet al., 1989) has shown that a precision in m~surement of -20 ppm is required. As previously discussed, “@Ce, used to correct for ‘42Ce interference on mass-142, was measured for only four of the samples. 14*Nd/‘“Nd data for those analyses are given in Table 3 and shown in Figure 7. A small but distinct variation of 14*Nd/lMNd with L47Sm/ ‘“Nd is observed in Fig. 7. As discussed by LUGMAIRet al. ( 1975) and JACOBSENand WA~SERBURG(1984), the initial ‘*Smf’“Sm can be obtained from the slope of the correlation line. For the Bhol~ati clast we find ‘~Sm/*~Sm = 0.0032 + 0.0016 and cA4== -0.8 + 0.6. This value of ‘46Sm/‘44Sm is similar to values found for Juvinas, Angra DOS Reis (ADOR), and an acid-insoluble residue of Allende by the La Jolla group (LUGMAIR et al. 1975, 1983; LUGMAIR and MARTI, 1977) and also to that found for Moama by JACOBSEN and WASSERBURG(1984) but less than values found by the Caltech group for Angra Dos Reis (JACOBSENand WASSERBURG, 1984) as well as Ibitira and the ~o~stown mesosiderite (PRINZHOFERet al., 1989). The apparent interlaboratory bias in determination of the initial ‘46Sm/‘44Sm ratio creates considerable uncertainty for the calculation of a formation interval. For example, using

L. E. Nyquist

2202 Table

3. Sm-Nd

analytical

resuits for the Bholghati

et al.

Howardite

_.____.__ Sample

Wt. (mg)

I. Eucritic WR <2.6Sd 2.65-2.85 2.85-3.41 2.85-3.4R 3.4-3.551 3.4-3.55R 3.55-3.7L 3.55-3.7R >3.7 2. Matrix WR

Sm (ppm)

clast (,I) 19.4

Nd @pm)

147~m,144Nda

0.2553 16.16 0.9077

4.150 5.676 1.010 54.50 1.665

25.9

0.4207

0.7764

23.5 2.22

0.2 132 I .060

0.4064 3.40 I

0.20092+2 I 0.17821+61 0.1529+10 0.17937+20 0.32969238 0.18962+22 0.32766&5 I 0.18980+_28 0.3173+11 0.18842+10

17.4

1.109

3.437

0.19512+22

1.379

1.23

11.19 1.19 25.5

1.673

r43Nd,‘44Ndc

14zNd,,44Nd~

0.511959~l0 0.511412+23 0.510578+16 0.511340+12 0.515849+13 0.5 Il632+14 0.515745~12 0.511633+14 0.5 15606+44 0.51 la50+29

1.138288+27 1.138397+44 i.138329&42 t.l38320+45

(,23)

0.511791+10

3. Ames Nd Standard March, 1989 (N=ap December, 1989 (N=16)

0.51 I 101+14e 0.5 I 1087+12e

l.138277+17e I. 138264+28e

4. CIT n(Nd)P December, 1989 (N=9)

5. BCR-If BCR- 1g BCR-

1g

9.7 9.9

0.51 1083+lSe

6.38 1

10.3

6.334

28.03 27.97 27.70

0.511835+10 0.511833+10 0.5) 1843+10

0.1380+2 0.1383+2

a Error limits apply to the last digits and include a minimum uncertainty of 0.1% plus 50% of the blank correcpn for Sm and Nd, added quadratically. b Normalized to 146Nd/’ ‘Nd = 0.724140 using a modified power law which corrects for mass fractionation as observed for the Ames metal Nd standard. Error limits apply 2an where Go = [4mi-~f2/(N(N-I))jt” for N measurements Nd = 0.724140 using the supplied with the Finnigan-MAT 261 software. and are 20 d Densities ofmineral separates in g/cm3. e Error limits are 20~ where oL, = [C(m,-~)2/(N-l)]“2 value j.b. ‘ Spiked with 149Sm_‘SoNd spike. g Spiked with r49Sm-‘46Nd spike. h Appropriate for all data in table except the December,

‘46Sm/‘%m = 0.0118 for the solar system initial ‘46Sm/“%m ratio, as determined by JACOBSEN and WASSERBURG ( 1984) from measurements on the Angra DOS Reis an&e, yields a

Bholghati

(sxciuding 0

0.14

0.18

modified linear mass fractionation Error limits apply to the last for

N

mi

measurements

1989 standard

with

mean

analyses.

formation interval of 0. t9$& Ga for the Bholghati clast. This implies an absolute age of 4.36$z Ga, which is discordant with the “‘7Sm-‘43Nd age, assuming an age of 4.555

Bholghati

clast

law digits

Clad

(2.65, >3.7 Q X55-3.7R)

0.22

0.26

0.30

0.34

0.1

0.2 ’

FOG. 6. Sm-Nd isochron for the ciast (,lf. Open diamonds were omitted from the least-squares fit (see text). The remaining data define an isochron corresponding to an age of 4.5 10 t 0.034 Ga and I,, = 0.505958 & 0.000050 for h(‘%m) = 0.00654 (Ga)-’ using the YORK (I 966) least-squares program. Error limits are 20 from the YORK (1966) program and would be decreased to +0.015 Ga and ~tO.OOtXl24 for the WILLIAMSON (1968) program which yields the Same slope and intercept parameters.

47Sm/‘1

0.3

0.4

44Nd

FIG. 7. “‘Nd/‘44Nd for leaches of the 2.85-3.4, 3.4-3.55, and 3.5% 3.7 g/cm3 fractions (phosphates) of the Bholghati clast (,I) and the residue of the 2.85-3.4 g/cm’ fraction. The slope and intercept of the linear correlation obtained from the WILLIAMSON (1968) program correspond to ‘%rn/‘?jrn = 0.0032 _t 0.0016 and &‘* = -0.8 it: 0.6, respectively. Error limits are 2~rand would be decreased to +0.0007 and +0.5 c-units, respectively, for the YORK (1966) program which yields the same slope and intercept parameters.

Age of eucrite clasts Ga for Angra DOSReis. Similarly, using a solar system initial ‘46Sm/‘“Sm = 0.015, as recently reported by PRINZHOFER et al. (1989) from measurements on the Ibitira eucrite and the Morristown mesosiderite, yields a formation interval of 0.23$& Ga for an absolute age of 4.32$$ Ga which is also discordant with the ‘47Sm-‘43Nd age. However, using initial ‘46Sm/‘44Sm = 0.0045 as determined by LUGMAIR et al. ( 1983) for an acid resistant phase of the Allende carbonaceous chondrite, results in a formation interval of O.OS?~:~~ Ga, or an absolute age of 4.5 1?I:?:, concordant with the ‘47Sm-‘43Nd age as well as the Pu-U-Xe and Pu-Nd-Xe ages of 4.47 and 4.52 Ga, respectively (SWINDLEet al., 1990). The formation intervals calculated for the differing assumptions for the initial ‘46Sm/‘44Sm ratios are compared to the Pu-U-Xe and PuNd-Xe formation ages in the inset in Fig. 8. Initial ‘43Nd/144Nd Figure 8 also shows the (T, ZNd)parameters of the Bholghati clast compared to those of the Y750 11,84 clast (NYQUIST et al., 1986) and the CHUR reference values (DEPAOLO and WASSERBURG, 1976; JACOBSENand WASSERBURG, 1980; WASSERBURGet al., 198 1). Both the mass spectrometer used for Nd-isotopic analyses and the mixed Sm-Nd spike used for the concentration measurements were changed between analyses of the two meteorite clasts. The Y750 11,84analyses were done with the mixed ‘49Sm-‘46Nd spike previously in use in our laboratory (NYQUIST et al., 1979) whereas the Bholghati analyses were done with a new mixed ‘49Sm-‘50Nd spike calibrated against the same Sm and Nd shelf standards. Nevertheless, the difference between the (T, ZNd)parameters of the Bholghati clast and Y75011,84 is less than 1 t-unit. The close agreement between data obtained with the old and new spikes is gratifying because biases in either the measured Sm/Nd ratios or the Nd-isotopic composition of spiked samples could in principle exist because ‘43Nd/‘44Nd is measured on spiked samples and corrections made for the spike con-

0.5061

0.5055 4.2

4.4 Age

4.5 (Ga)

FIG. 8. Ages and initial ‘43Nd/‘“Nd ratios for the Bholghati clast (. 1) and the Y75011 clast, respectively. Absolute ages inferred from ‘46Sm-‘42Nd(this investigation), Pu-U-Xe, and Pu-Nd-Xe (SWINDLE et al., 1990) formation intervals are also shown for Bholghati relative to an assumed 4.555 Ga age for the solar system. The ‘46Sm-‘42Nd formation interval is concordant with the Pu-Xe formation intervals for initial ‘46Sm/‘44Sm= 0.0045 as measured for an acid resistant phase of Allende (LUGMAIRet al., 1983) but discordant for 14%m/

‘“Sm = 0.0118 as measured for Angra DOSReis (JACOBSEN and WASSERBURG, 1984) or ‘46Sm/‘44Sm= 0.0 I5 as inferred by PRINZ HOFERet al. (1989) from measurements on the lbitira eucrite and the Morristown mesosiderite.

2203

tributions. The close agreement between the data for samples spiked with the two different spikes places stringent limits on the magnitude of the possible biases which may arise from this source. Both the Bholghati and Y75011,84 data also agree with the CHUR value to within - 1 e-unit. Some ambiguity exists in this comparison because the agreement is worsened if the Bholghati data are adjusted for an apparent interlab bias of -0.8 t-units as inferred from analyses of the Ames metal standard (equivalent to the CIT nNd/3 standard, see Table 3). However, analyses of standard sample BCR-1 indicates a smaller bias of only -0.2 e-units between the data from the two laboratories. In view of this ambiguity, we have chosen not to correct for interlab bias because it is unclear which standard should be used for the primary comparison and BCR- 1, which indicates little apparent bias, may provide the preferred comparison because this standard is processed through all the same procedures as normal samples and thus provides comparison under actual analysis conditions. This may be especially important when the Nd isotopic composition is measured on spiked samples. We note that, although most of our Nd standard analyses are unspiked, we have also analyzed standards spiked with both the ‘46Nd and “‘Nd spikes and found no differences exceeding -20 ppm between unspiked standards and those spiked with either spike. HISTORY OF THE HED PARENT BODY Initial ‘46Sm/‘44Sm ratio The present uncertainty in the solar system initial 14%rn/ ‘“Sm ratio and the difficulty of measuring “*Nd/‘44Nd to the necessary precision limits the usefulness of the ‘46Sm‘42Nd chronometer. Comparatively rapid advances on both these problems can be expected and inclusion of high precision ‘42Nd/‘44Nd measurements will become a valuable adjunct to Sm-Nd studies of ancient (24.4 Ga) meteorites. Combining the ‘46Sm/144Sm ratio found for the Bholghati clast with the measured ‘47Sm-‘43Nd age of 4.510 + 0.034 Ga leads to an estimate of the solar system initial (14%rn/ ‘44Sm)o = 0.0043$:~$~ Ga. This value agrees with initial ‘46Sm/“‘%m = 0.0045 f 0.0005 reported by LUGMAIRet al. (1983). The experiment Of LUGMAIRet al. (1983) should have allowed the most precise determination of the initial 14%rn/ ‘44Sm ratio because of the comparatively large enrichment of ‘42Nd/‘44Nd from recoil of 14’Nd into an acid-resistant carbon-chromite fraction of Allende from neighboring Smbearing silicates. The agreement between Pu-Xe and 14’%rn‘42Nd formation intervals calculated for the Bholghati clast using the Allende initial ‘46Sm/‘44Sm ratio is gratifying. Nevertheless the disturbances observed in several isotopic systems in the Bholghati clast suggests caution in interpreting these results. Growth of ‘43Nd/‘44Nd in the mantle of the HED parent body Figure 9 shows the initial Nd isotopic composition of the Bholghati clast compared to that of other eucrites, Angra DOS Reis, and the reference values CHUR and JUV in c-notation. All the data in the figure agree to within - 1 C-

2204

L. E. Nyquist et al.

Eucrites Smith(1982)

4.3

Source

4.4 Age

the derivation of the howardites and eucrites from the same parent body. However, earlier measurements showing a dif-

: ‘47Srn/144Nd=0.328

4.5

4.6

4.7

@a)

FIG. 9. Comparison of (T, end) for the Bholghati clast (,I), tlast Y7501 I,84 (NYQUISTet al., 1986), Ibitira (PRINZHOFER et al., 1989), and Moama (JACOBSEN and WASSERBURG, 1984) to the reference values CHUR (DEPAOLOand WASSERBURG, 1976; JACOBSEN and WASSERBURG, 1980; WA~~ERBURG et ai., 198I) and JUV (LUGMAIR, 1974;CARL~~N and LUGMAIR,1988), respectively. The growth curve for the depleted mantle source proposed by LAULand GOSSELIN (1990) for the Bholghati ciast by analogy to an earlier study of ciasts in Kapoeta by SMITH (I 982) is also shown. The directly measured (r, +,) values of Bholghati (solid parallelogram) are consistent with a source having chondritic relative REE element abundances; however, adjustment for apparent bias in measurement of 14’Nd/lMNd for the isotopic standard (dotted parallelogram) suggests that the LREE depleted source may he more consistent with the isotopic data.

unit, although comparisons may be obscured by interlab biases. LAUL and GO~~ELIN (1990) using a petrogenetic

model due to SMITH (1982), concluded that the Bholghati clast must have been derived from a very LREE-depleted source. The evolution of 143Nd/‘44Ndin the source they propose is shown by the solid line and would produce -0.9 cunit evolution in ‘43Nd/‘44Nd between 4.56 and 4.5 1Ga ago. The directly measured (T, +d) values of Bholghati (solid parallelogram) are consistent with a source having chondritic relative REE abundances. However, the slightly elevated value of tN6 which would result if a correction were made for in-

terlaboratory bias in measurement of ‘43Nd/‘44Nd based on analyses of the Ames metal and CIT n(Nd)P Nd standards (dotted parallelogram) is compatible with evolution in a LREE-depleted source region such as proposed by LAUL and GOSSELIN(1990) and a solar system initial ‘43Nd/‘44Nd ratio of either CHUR (JACOBSENand WASSERBURG,1980; WASSERBURG et al., 198 I) or JUV, the Juvinas initial ‘43Nd/‘“Nd (LUGMAIR, 1974; CARLSONand LUGMAIR, 1988). Thus, the present Nd-isotopic data do not strongly imply that the Bholghati source was depleted in LREEs, although such a source is most consistent with the data when probable interlaboratory bias is considered. Earl-v history

ofthe

large eucrite dust

Both long- (K-Ar, Rb-Sr, Sm-Nd) and short-lived (i4%m‘42Nd, Pu-Xe) chronometers are consistent with an igneous c~stalli~tion age of 4.5 IO f 0.034 Ga for the “large” eucrite clast from the Bholghati howardite. As shown above, the initial s7Sr/86Sr composition of the clast was the same as that for the pristine eucritic basalt clast Y7501 I,84 (TAKEDA et al., 1983; NYQUIST et al., 1986) and thus is consistent with

ference between initial *‘Sr/%r for Kapoeta clasts and the BABI (Basaltic Achondtite Best Initial) value must be kept in mind (PAPANASTASSIOU and WASSERBURG,1969, 1976). Additional petrographic, geochemical, and isotopic studies of the howardite-euc~te~iogenite afhliation are necessary to firmly establish a common origin for these achondritic meteorites. Although the initial ‘43Nd/‘44Nd ratio is permissive of a severely LREE-depleted source for the parent basalt, the old age of the clast prevents a strong constraint on the Sm/Nd ratio in the mantle of the parent body. However, the precision in measurement of ‘43Nd/‘44Nd currently achievable should allow constraints on mantle Sm/Nd ratios for eucrites as old as -4.4 Ga. Such comparatively young ages have been reported for several eucrites. Thus, Sm/Nd isotopic studies may yet rule on whether the HED parent body was strongly differentiat~ early in its history, a possibility previously thought to be beyond the resolution of Nd-isotopic studies. A major thermal event 2-3 Ga ago The strong, late (- 2-3 Ga ago) degassing and partial Srisotopic ~uilibration of the clast may be coincident with the proionged annealing inferred by REID et al. (1990) from the presence of cloudy plagioclases, lack of zoning in the pyroxene, and the presence of pyroxene exsolution lamellae. As discussed above, the interpretation ofthe isotopic data differs somewhat depending on whether one or two isotopic equilibration events is assumed. If there was only a single event, it probably occurred -2 Ga ago. It is reasonable to associate this event with the prolonged subsolidus annealing noted by REID et al. (1990). It is noteworthy that this “event” apparently applies to the bulk sample as well as to the large eucrite clast, as shown by the comparison of the 39Ar-40Ardata reported here to that of LEIGHand MONIOT (1976) for a bulk sampte of Bhol~ati. They interpreted their data as suggesting a strong outgassing event at 3.4 + 0.2 Ga. Although their release curve was very complex and the assignment of an age appears somewhat problematical, there seems little doubt that the entire breccia was affected by this event. The total 3gAr40Ar age obtained by LEIGHand MONIOT(1976) was 2.9 Ga. in the range of possible ages which we assign to this event. The -2-3 Ga event apparently severely affected the RbSr system of Fe-bearing phases (pyroxenes, ilmenite). Some of these phases may be formed during the recrystallization of mesostasis as suggested for polymict eucrites by TAKEDA et al. (1988). Textural relations suggest that redistribution of troilite was contem~raneous with pyroxene exsolutjon in the Bholghati clast (REID et al, 1990). Also, the most Fe-rich pyroxenes are severely disturbed in the Rb-Sr system and suggest that the disturbance occurred either -2.9 Ga ago (if the leach data are ignored) or -2.1 Ga ago (if the leach data are recombined with the residue). Thus, the Sr-isotopic data are consistent with a picture in which rec~st~li~tion of mesostasis and pyroxene homogeni~tion and exsolution was contemporaneous and comparatively late. Those phases which do not contain Fe (plag, tridymite) appear not to have participated in that event, at least as it affected the Rb-Sr

Age of eucrite clasts system. However, such phases probably lost Ar as shown by low Ar ages for the highest K/Ca phases (probably tridymite) and for most of the Ar released (probably plagioclase). A minor thermal event < I Ga ago?

As discussed above, there are some reasons to favor an interpretation of the isotopic data in which a major thermal event - 3 Ga ago was followed by a later, less severe event I 1 Ga ago. Such an event would have outgassed the highest K/Ca phase (tridymite?) and caused partial &-isotopic equilibration of at least one unidentified phase. The most likely candidate phase is troilite, since it would have been present in the mineral separate with density > 3.7 g/cm3. Troilite is soluble in HCI, and it may also have cont~buted to the 3.553.7 g/cm3 leach along with phosphates shown to be present by the Sm-Nd analyses. Although some important pieces of evidence are missing, we prefer this interpretation because it seems to provide the most consistent overall interpretation of the isotopic data. GEOLOGIC

PROCESSES AND SETTING

REID et al. (1990) interpret the textural relations in the clast as the result of direct, rapid crystallization of a melt followed by prolonged annealing, promoting exsolution and homogenization of pyroxene lamellae at subsolidus temperatures. They note that the feldspars in the clast are cloudy and the pyroxenes lack zoning. They suggest that during annealing there was a local redistribution or introduction of troilite and that pyroxene exsolution and formation of the troilite may have been contem~raneous. Thus, their suggested scenario for the clast’s history includes burial and subsolidus annealing to produce both the exsolved unzoned pyroxenes and cloudy feldspars. They propose a sequence of events for the Bholghati howardite which, for events restricted to the HED parent body, includes (1) early magmatism in which the eucrites are produced by partial melting or fractional crystallization; (2) prolonged subsolidus annealing; (3) low velocity impact and mixing in a regolith which is then lithified in a “low temperature” event: (4) ejection of the meteorite from the regolith; and (5) fall to earth in 1905. The most obvious identification ofthe isotopically defined events with this scenario is (1) magmatism at 4.5 1 k 0.03 Ga ago; (2) prolonged subsolidus annealing at -2-3 Ga ago; and (3) “low temperature” lithification s 1 Ga ago. Such a history for the Bholghati howardite is consistent with the geologic setting and events proposed for the HED parent body by NYQUIST et al. ( 1986, 1988) and, especially, TAKEDA et al. ( 1988). TAKEDA et al. (1988) have compared the textural relationships between a monomict, metamorphosed clast, Y7925 10,62, and a pristine clast, Y75011,84, from the corresponding polymict eucrites. They concluded that interstitial areas in Y792510,62 now containing finegrained aggregates of ilmenite, silica, augite, low-Ca pyroxene. Ca phosphate, plagioclase, and Fe oxides and sulfides were analogous to interstitial mesostasis areas in Y7501 I,84 consisting of subcatcic ferroaugite, fayalite, silica, ilmenite, Ca phosphate, and minor phases. They also noted that the augite and low-Ca pyroxene compositions in the interstitial phases of Y7925 10 were identical to those ofthe host-lamellae exso-

2205

lution pairs in the pyroxenes. To the extent that separation between low- and high-Ca pyroxenes was achieved in the present investigation, it is probable that it was achieved for such mesostasis-derived pyroxenes, which would account for the observation of pronounced differences in the trace element abundances of the 3.4-3.55 g/cm3 and 3.55-3.7 g/cm3 mineral separates, respectively, in spite of lack of a significant difference in the Mossbauer spectra of the two mineral separates. TAKEDAet al. (1988) identify the well-defined 3.2 Ga 39Ar-40Arage of the Y7925 10 clast with the recrystallization of mesostasis, decomposition of men&able Fe-rich pyroxenes, and other associated textural changes and suggest that those events were related to the brecciation of polymict eucrites and howardites. The isotopic investigations of the polymict eucrites and howardites point to a long geologic history for the HED parent body, at least in terms of its cratering record. There is strong evidence in the isotopic data that this parent body survived the “cataclysmic” events which affected the moon -3.8-4.0 Ga ago, and weak evidence that it survived to 11 Ga ago. Further mineralogic, petrologic, chemical, and isotopic studies of howardites, polymict eucrites, and other rocks which may be identified as coming from this interesting if unknown place will fill in the details of the emerging image which we now have of it. Acknowledgments-We thank the Indian Geological Society for providing the Bholghati sample used for the Bholghati Consortium study, J. C. Laul for inviting us to participate in the consortium, M. LindStrom and the curatorial staff of the Meteorite ProcessingLaboratory at JSC for their careful processing of the sample prior to its allocation to us, and Dan Garrison for assistance in the Ar analysis. This work was financially supported by NASA’s Planetary Materials and Geochemistry Program via RTOP No. 152- 14-40-2 1. Editorial handling: J. C. Laul REFERENCES

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