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An ancient Sm-Nd age for a ferroan noritic anorthosite clast from lunar breccia 67016
Geochimica et Cosmochimica Acta. Vol. 58, No. 13, pp. 2921-2926, 1994 Copyright 0 1994 Else&r Science Ltd Printedin the USA. All rights reserved 00 I6-7037/94 $6.00+ .OO
Pergamon
0016-7037(94)00089-l
An ancient Sm-Nd age for a ferroan noritic anorthosite clast from lunar breccia 67016 CHANTAL
ALIBERT,‘,’MARC D. NORMAN,‘,’and MALCOLMT. MCCULLOCH’
‘Research School of Earth Sciences, Australian National University, Canberra A.C.T. 0200, Australia %entre de Recherches PCtrographiques et Gbchimiques (CRPG), BP 20, 54500 Vandoeuvre-l&-Nancy, France 3Planetary Geosciences, Dept. of Geology and Geophysics, School of Ocean and Earth Sciences and Technology,
University of Hawaii, Honolulu HI 96822, USA (Received September 2 1, 1993; accepted in revised form February 10,
1994)
Abstract-Strontium and neodymium systematics have been examined in a clast of ferroan noritic anorthosite from Apollo 16 breccia 67016. Two splits (,328 and ,326) of the same clast give different SmNd results. Split ,328 gives a well defined internal isochron age of 4.562 + 0.068 Ga and an initial ‘43Nd/ l”Nd ratio of0.50673 f 10 corresponding to tNd= 0. I f 0.2 (2u optimized error) relative to the Murchinson carbonaceous chondrite. The pyroxene separate from split ,326 lies on the same isochron. In contrast, the plagioclase and whole-rock from split ,326 fall below this line, indicating a small-scale disturbance of the Sm-Nd system. This may reflect either an isotopic exchange between the plagioclase and a low Sm/Nd mineral or a loss of radiogenic ‘43Nd from the plagioclase, possibly during the period of major impacts at -3.9 Ga. The preservation of an extremely old age for the noritic ferroan anorthosite 670 16,328 suggests a rapid cooling of this rock at an early stage in the evolution of the lunar magma ocean. This old age is also consistent with giant impact models for the formation of the Moon but implies a relatively early event (pre 4.50 Ga) and, therefore, rapid accretion and differentiation of the terrestrial planets. INTRODUCTION ALTHOUGH SOME CONSENSUS has been reached about the growth of planetesimals by runaway accretion, many aspects concerning the initial conditions, such as the role of nebular gases and size range of planetesimals, are still controversial (e.g., WETHERILLand STEWART, 1989; BARGEand PELLAT, 1991; KOLVOORDand GREENBERG,1992; SAFRONOV,1991). The possible range in these initial conditions corresponds to accretionary timescales which can vary by an order of magnitude, leading to very different implications for planetary differentiation. The relatively short formation interval of less than lo7 years which is expected for hierarchical accretion of massive parent bodies would result in rapid release of gravitational energy and global melting. Alternatively, for the Earth, a more extended formation interval (- 10’ years) would imply cooler conditions and, therefore, less extensive melting of the planet during its initial differentiation. The highlands crust of the Moon contains the oldest preserved record of crustal formation in the Earth-Moon system. The age of the Moon’s primary crust can, therefore, provide an upper limit for the duration of planet formation in the inner Solar System. Remnants of the early lunar crust are represented by the ferroan anorthosites and the Mg-suite of dunites, troctolites, norites, and gabbronorites. Mg-suite rocks have provided Sm-Nd, U-Pb, and 40Ar-39Arplateau ages in the range 4.2-4.43 Ga (e.g., CARLSONand LUGMAIR, 1981; NYQUIS~ et al., 1981; COMPSTONet al., 1984; JESSBERGER et al., 1977; MAURER et al., 1978). More recently, CARLSON and LUCMAIR (1988) reported a precise Sm-Nd age of 4.44 k 0.02 Ga for ferroan anorthosite 60025, and SHIH et al. (1993) reported a Sm-Nd age of 4.46 + 0.07 Ga for a noritic clast from breccia 15445, suggesting a similar antiquity for both suites of rock. These relatively young ages are consistent 292 1
with an origin of the Moon by a giant impact between a Mars-sized planetesimal and the proto-Earth, - 100 Ma after To, and are compatible with models of WETHERILL(1992) suggesting that accretion continues for -100 Ma before a giant impact becomes highly probable. In contrast, an older Rb-Sr age of 4.5 1 f 0.07 Ga has been obtained for the troctolite 76535 (PAPANASTASSIOU and WASSERBURG,1976) and a U-Pb model age of 4.5 1 + 0.0 1 Ga for the ferroan anorthosite 60025 (HANAN and TILTON, 1987). These latter ages are significantly older than the Sm-Nd ages obtained from the same rocks and have, therefore, been considered as unreliable (LUGMAIR et al., 1976; CARLSON and LUGMAIR, 1988; PREMOand TATSUMO~O, 1992). For example, PREMO and TATSUMO~O (1992) call upon a late loss of Rb from olivine in the troctolitic cumulate 76535 and also show that using the Canyon Diablo Pb values as initial Pb gives a model age which is too old for this sample. This conclusion is also likely to be applicable to the ferroan anorthosite 60025. The ferroan noritic anorthosite clast analyzed here for strontium and neodymium isotopes is from feldspathic fragmental breccia 67016, which was collected from the rim of North Ray Crater. Petrographic and geochemical accounts of breccia 670 16 have been given previously (NORMAN, 198 1; LINDSTROMand SALPAS,1983; NORMANet al., 1991; NORMAN and TAYLOR, 1992; NORMAN et al., 1993). The clast which we analyzed belongs to the mafic subgroup of the ferroan anorthosite suite (JAMESet al., 1989). Clasts of similar composition have been described in other feldspathic fragmental breccias from North Ray Crater (LINDSTROMand SALPAS, 1983; MCGEE, 1988). 40Ar-39Ar ages around 3.95 Ga have been reported for a dark clast of melt breccia and a plagioclase separate from breccia 67016 (TURNER and CADOGAN, 1975).
C. Alibert, M. D. Norman, and M. T. McCulloch
2922
SAMPLE DE~RI~~ON AND ANALYTICAL TECHNIQDES Ferroan noritic anorthosite clasts in 670 16 have crystalline textures which indicate a complex petrogenesis involving igneous crystallization, brecciation, subsolidus recrystallization, and sulfide metasomatism. These clasts are composed of 68-69% plagiocfase, 2728% pyroxene (both o~hop~oxene and clinopyroxene in the proportion 2: I f, 2-3% troilite, and minor amounts of ilmenite and chromite. Relatively large piagioclase grains (up to 2 mm long) with irregular shapes are surrounded by a matrix of finer-grained anhedral plagioclase and granular to tabular pyroxenes. We interpret this texture as indicating a coarse-grained, probably plutonic, protolith which was brecciated and subsequently annealed so that the formerly cataclastic matrix recrystallized. Pyroxene compositions indicate equilibration temperatures (KRETZ,1982) of g50-9~*C, although it is not clear whether this represents the temperature of post-brecciation annealing or inheritance of primary, pre-brecciation mineral compositions. Olivine is now very rare in these clasts, occurring only as relicts within troihte-pyroxene intergrowths. Troilite also occurs along cleavage traces and shear planes. The sulfur-rich nature of these clasts and associated enrichments of volatile elements have been attributed to a fumarolic com~nent (NORMAN et al., 1993;COLSON, 1992). In places, the troihte-pyroxene intergrowths closely follow observed grain boundaries, hence indicating that sulfide metasomatism postdates the recrystallization referred to above. However, it predates incorporation of these clasts into the host breccia because other olivine-bearing clasts in 67016 are not affected by the metasomatism. Mineral compositions in these clasts are homogeneous and identical to those of pristine ferroan anorthosites (NORMAN et al., I99 I). The low Ni and Co contents (37 and 7 ppm, respectively) and Ni/Co ratios of these clasts (NORMAN and TAYLOR, 1992) suggest negligible meteoritic contamination. These features indicate these clasts are monomict, noritic members ofthe ferroan anorthosite suite of lunar highlands rocks. Clast ,326/8 was originally allocated to S. R. Taylor as two separate splits weighing 0.641 g (,326) and 0.936 g (,328). Whole rock powders of both splits were prepared and processed for major and trace element analyses, as outlined by NORMAN and TAYLOR f1992). The remaining material was processed for mineral separation. A combination of heavy-liquids and handpicking was used for ,328 whereas, for split ,326 (a small -3 mm long chip), only handpicking was used. Pyroxene separates contain some ilmenite inclusions. For split ,328. a composite grain of ilmenite with minor plagioclase and pyroxene was also analyzed. The large plagioclase separate from ,328 (100 mg) contained some pyroxene. Small amounts of plagiociase from ,328 were teached in dilute acid to assess the role of possible sulfide and
phosphate inciusions. Mineral separates were rinsed in uhrapure watet (Milliporee) and distilled acetone before dissolution by HF-HNO? acids in closed Teflon beakers. Concentrations and isotopic ratios were measured using a Finnigan MAT 261 mass spectrometer using mixed spikes 84Sr-85Rband “‘Nd-““Sm. During this study, the NdSm spike was twice calibrated against the Cahech standard solution and gave the recommended SmiNd ratio to better than 0.1%. Sama&m concent~tions were determined using two procedures: first using the ratio ‘49Sm/‘52Smfor mass fractionation correction with concentrations calculated from the 148Sm/‘47Smand ‘S’Sm/‘4’Sm ratios. The second procedure used ‘48Sm/‘52Smfor mass fracttonation correction and concentrations were calculated from the ‘49Sm/1J7Sm and ‘52Sm/‘47Smratios. Samarium concentrations were identical regardless of the method used. For the ,326 whole-rock and ,328 iimen&e-rich chip, *49Sm/‘s2Smratios (corrected for spike contributio~l) are 0.5 1673 f 7 and 0.5 1687 & 6. respectively. These values are close to the average 0.51684 F 2 obtained for a Sm standard solution. using the same cup configuration. We, therefore, conclude that neu tron effects on the abundance of r4?5rn, if present, are extremely small and not resolvable in this lunar clast. Total chemistry blanks were 5 pg for Rb, 66 pg for Sr, and 6-25 pg for Nd. Neodymium was loaded on double Re-Ta filaments, measured as metal ions and mass fractionation was corrected relative to ‘~Nd~‘~Nd ; 0.72 1’) Isotopic data including whole rock analyses for the Murchinson carbonaceous chondrite and the Angra DOS Reis angrite, and Sr and Nd standards, are reported in Tables I and 2. The Sm-Nd age calculations use 20 errors of 0.2% for Sm/Nd ratios and 0.00002 for 143Nd/‘44Ndratios (external reproducibility). SR AND ND ISOTOPIC
RESULTS
Splits ,328 and ,326 (Table 1) have whole-rock Nd contents -3ppm, which are relatively high compared with typical fer”, roan anorthosites. These whole-rock Nd contents are higher
than expected from a simple mass balance calculation using the measured mineral separates, suggesting the presence of an additional phase with high light REE abundances (Fig. I). A leaching experiment was performed on a plagioclasr separate from split ,328 which contained some minute inclusions of sulfides (Table 1). The leachate contains - 50% of the total Nd, which may reflect the presence of a soluble phase, possibly phosphate inclusions in the plagioclase, although none was identified petrogra~hicall~
Tablel- Nd isotopicdatafor lunarbreccia67016,328 and,326 andfor ADOR
andMwchinson meteorites.
Sample
weight Sm (=@ @pm)
“7Sm Nd (ppJ@ (Wg)
(n”vilg)
whole-rock ,328
18 12 100 17.6
1.O? 1.31 0.681 0.679
3.31 3.20 2.43 1.99
1.065 1.309 0.6791 0.6771
5.463 5.278 4.008 3.285
0.1949 0.2480 0.1694 0.2061
- _.____. _-. 143NdJ’+‘Nd &Nd &Nd at456Ga a :$ ----.-l.__-_ 0.91 0.512654 (13) 0.556751 0.5 I 0.45 0.514239 (8) 0.506728 0.05 0.25 0.511849 (10) 0.506718 -0.15 0 117 0.512951 (13) 0.506709 -0.33
4.20 0.446 1.85 0.844
1.73 3.86
0.4443 0.8413
2.854 6.361
0.1557 0.1323
0.511392 (150) 0.510731 (120)
0.9638 1.142 0.5951
4.831 5.037 4.127
0.1995
0.2268 0.1442
0.512696 (6) 0.513564 (IO) 0.510999 (8)
0.506654 ~~~~ -
-1.40 -0.61 -1.84
-i.w -0.21 -1.44
5.744 28.97 5.991 31.14 0.2105 1.074
0.1982 0.1924 0.1960
0.512735 (8) 0.512546 (7) 0.512662 (10)
0.50673 1 0.506719 0.506725
0.12 -0.12 0.00
(1.52 0.28 0.40
0.1384 0.1381
0.511878 (13) 0.512661 (6) 0.512660 (6)
,328 pyroxenc ,328 plagioclase ,328 iltnenite
,328 plagioclasc residue’ ,328 plagkxlase leachate* ,326 whoie-rock ,326 pyroxene ,326 piagk&se
49 5.1 15.5
0.966 I.f46 0.597
2.93 3.05 2.50
Angrs DOSReis (powder)
56.8
5.76 6.01 0.211
17.6 18.9 0.651
Angm Dos Reis (chip) Murchimson(chip)
La Jolla(31 runs, 1990/91) ECRl(1 nm, 1990) BCRi (t run, 1991)
100 80
‘“Nd
‘47Sn+Nd
t43Nd,‘t”Nd measured
Quoted rrrors for t43Nd/I@Nd ratios (in parentheses)are 2a SE of the mean except for the La Jolla Nd stand& (lo SD of the population). Initial qQj values relative to CHUB (Jacobsen and Wasserburg, 1984) have been caiculefed after notmalization of the 143Nd/t40Ndratios to 0.511854 (La Jolla Nd standard measured by Carlscn and Lugmair, 1988). ENd values referred as a and b are calculated relative to Murchinson and to CHUR, nspectively. * is for leaching in HCl 1N and HN@ 1N.
2923
Ancient lunar anorthosite Table Z- Sr isotopic data for lunar hwcia
67016,328 and ,326 and for ADOR meteorite.
weight (mg) 31.1 ,328 plagioclasc 4.2 ,328 plagioclase res.’ ,328 plagioclasc res.** 3.4 49.0 15.5 5.07
,326 whole mk ,326 plagioclasc ,326 pymxene
16.3 ADOR (8 runs) E&A (10 runs) NEIS 987 (42 runs, 1990,91)
87Rb/=Sr
*7sr/%r at 4.56 Ga
0.132
155
0.002459
0.699397 (10) 0.699175 (13) 0.699247 (19)
0.699011
0.385 0.244 0.1808
153 183 129
0.007274 0.003853 0.004043
0.699499 (18) 0.699308 (8) 0.699330 (8)
0.699012 0.699050 0.699060
0.0357
127
O.ooO813
0.698972 (17) 0.707982 (13) 0.710215 (10)
0.698920
* residue after leaching in HCI and HN@ the 87Sr/%r
87SrPSr measured
1N: ** Icached in HNQ
1N. Quoted errors for
ratios (in parentheses) are 20 SE of the mean cxcepl for the Angra DOS Reis
meteorite (ADOR) and the E&A and NBS 987 Sr standards (lo SD of the population).
0.16
Sm-Nd isotopic data for the two splits ,328 and ,326 are significantly different. However, as isotopic measurements of samples from both splits were undertaken during the same session, together with the La Jolla Nd standard and meteoritic samples, it is not possible to ascribe these differences to analytical problems. The nonlinear distribution of Nd and Sm concentrations in the plagioclase, pyroxene, and ilmeniterich separates from ,328 (Fig. 1) satisfies the prerequisite of enough independent mineral components to form a meaningful isochron. The mineral separates and whole-rock for split ,328 give a good alignment in the ‘43Nd/‘44Nd vs. ‘47Sm/ ‘44Nd diagram (Fig. 2). The best fit line, calculated using a modified version of the Williamson method (MINSTER et al., 1979) corresponds to an age of 4.562 f 0.068 Ga (MSWD = 0.9) and an initial ratio of 0.506728 f 96. Translating the 147Sm/‘44Nd axis to the chondritic value (method of FLETCHER and ROSMAN, 1982) an optimized initial EN,,
n
1.3
0
1.1 -
o.9
s
0.7
,326~~
n 0
p
,328~~
,328wr
,326wr
q n
,328ilm
n
iza2
,328 plag
q ,326plag 0.5 -
0.3 2
3
4
5
6
7
8
“14 Nd @M/g) FIG. 1. ‘44Nd vs. ‘?Srn diagram for ferroan noritic anorthosite 67016,328 (solid squares) and ,326 (open squares). The nonlinear distribution of Nd and Sm concentrations in plagioclase (plag), pyroxene (px) and an ilmenite-rich chip (ilm) from split ,328 indicates the multicomponent nature of this sample and shows that any isochron defined by these three mineral separates cannot be reduced to a two-point isochron. The relatively high Nd and Sm contents of the whole-rock samples ,328 and ,326 point to the presence of an additional minor phase with high 144Nd.A leaching experiment on a plagioclase separate from ,328 shows that the Nd in the leachate (soluble part) amounts to 50% of the total Nd; the position of the leachate in this diagram also satisfies the requirement that the wholerock should be inside the polygon defined by the mineral components.
0.510 0.13
0.15
0.17
0.19
0.2 1
0.20 0.23
0.24 0.25
‘47Sm/‘44Nd FIG. 2. ‘47Sm-‘43Ndevolution diagram for ferroan noritic anorthosite 67016,328 (solid squares) and ,326 (open squares). The isochron calculated for ,328 corresponds to an age of 4.562 f 0.068 Ga with an initial of 0.506728 f 96. Using the optimization procedure of FLETCHER and ROSMAN (1982), this latter error (2~) corresponds to 0.2 t-units. The initial cNd = +O.l, relative to Murchinson, or +0.45. relative to CHUR, indicates a source unfractionated with respect to chondrites. The inset figure shows initial cNdvalues at 4.56 Ga, relative to Murchinson (2~ error bars = 0.4 c-units). The pyroxene separate from ,326 lies within combined uncertainties (including error on the isochron) on the 4.56 Ga isochron. However, the plagioclase and whole-rock lie off this line. Isotopic analyses of the meteorites Angra DOSReis (ADOR) and Murchinson are also reported for comparison. The meteorites and the mineral separates and whole-rock for ,328 are, within analytical uncertainties, part of the same linear array.
= +0.5 + 0.2 can be calculated, using the recommended CHUR values of JACOBSEN and WASSERBURG (1984) and after normalization relative to ‘43Nd/‘44Nd = 0.5 11854 for the La Jolla Nd standard. Using, instead of CHUR, the isotopic composition measured in this study for the Murchinson carbonaceous chondrite (Table 1), an initial tNd = +0.06 f 0.2 is obtained. However, this difference is marginal since JACOBSEN and WASSERBURG (1984) give the ‘43Nd/‘44Nd ratio of CHUR to only within 0.5 t-unit, and there is also a similar uncertainty for calibration of interlaboratory biases. For split ,326, the pyroxene separate lies, within combined uncertainties (including error on the isochron), on the best fit line of split ,328, while the plagioclase and the whole rock fall below this line (Fig. 2 and inset). The isochron parameters calculated from ,328 do not change significantly if the pyroxene separate from ,326 is included (T = 4,536 +- 64 Ma). On the other hand, the alignment shown by samples from ,326 alone corresponds to an age of 4.66 + 0.06 Ga with an initial ratio of 0.50653 f 7 (eNd = -1.3 relative to Murchinson). This anomalously old age and negative tNd clearly indicate a disturbance of the Sm-Nd system which has predominantly affected the plagioclase and hence also the whole-rock sample of split ,326. Strontium isotopic data are reported in Table 2. For comparative purposes, we also report Sr isotopic analyses for the angrite Angra DOS Reis (ADOR) as well as Sr standards. Our measurements of ADOR give an average present-day ratio of 0.698972 t- 17. A significant uncertainty in calculating initial ratios results from the variable Rb content in ADOR
C. Alihert, M. D. Norman, and M. T. McCulloch
2924
(G. W. Lugmair, pers. commun., 1991). The initial s7Sr/86Sr ratio of0.698920 f 30 obtained in this study (relative to NBS 987 = 0.7 102 15 ? 10) agrees with that reported by LUCMAIR and GALER (1992). These authors also conclude that the initial 87Sr/86Srratios for BABI and for Angra DOS Reis are not different from one another. The clast analyzed here is relatively enriched in Rb as well as other incompatible elements, compared to ferroan anorthosites generally (NORMANand TAYLOR, 1992). The whole rock (,326) contains -0.4 ppm Rb. which is higher than that obtained for either the plagioclase or pyroxene separates (0.18-0.29 ppm, Table 2). This could reflect partial leaching of Rb from mineral separates during their preparation (water rinse), or the presence of an additional Rb-rich phase. Two plagiociase separates were leached with dilute acid in order to remove any soluble phase (sulfides, phosphates). The residues have lower Rb/Sr and also 87Sr/86Sr ratios: 0.699170.69925 compared to 0.6993-0.6995 for unleached mineral separates and whole-rock samples. The soluble Rb amounts to -50% of the total Rb. A regression through the leached plagioclase from ,328 and the three data points for ,326 gives a Rb-Sr isochron age of 4.00 + 0.47 Ga (h8’Rb = 0.0142 Ga-‘), which is imprecise but consistent with the period of major impacts indicated by 40Ar-39Arages. The best estimate of the initial “Sr/*Sr ratio is probably given by the lowest value of 0.69901 (Table Z), calculated at 4.56 Ga for the leached plagioclase from ,328. This is also the same as that given by the whole-rock ,326. This estimate is similar, within uncertainties, to the initial “Sr/‘?Sr = 0.69902 + 4 (relative to a value of 0.7 10215 for NBS 987) reported by CARLSON and LUCMAIR( 1988) for ferroan anorthosite 60025. It is also in agreement with the initial Sr ratios of -0.69900 compiled for lunar anorthosites by NYQUIST (1977 1.
The disturbance of the Sm-Nd system which has affected split ,326 is not due to an uncorrected neutron effect on the ‘49Sm. The 147Sm/‘44Nd ratio of whole rock ,326 must be lower by - 1% in order to fit the 4.56 Ga line in Fig. 2: however, as already mentioned, the measured ‘4sSm/‘52Sm ratio in this sample is normal and cannot account for such a large shift. A partial loss of radiogenic ‘43Ndfrom plagioclase is a possible mechanism to explain the observed disturbance in split ,326. The low 143Nd/‘44Ndratio observed in plagioclase ,326 requires a loss of - 17% of the radiogenic 143Nd produced in the interval from 4.56 Ga to 4.0 Ga. Considering that radiogenic ‘43Nd is produced by m-decay from “‘Sm. such preferential loss from radiation-damped sites in plagioclase is not unreasonable. Petrographic observations show that while the smallest plagioclase grains are free of shock effects, some of the large relict plagioclase grains show wavy extinction and development of subgrain domains which are shock related. The pyroxene separate from ,326 has an initial neodymium isotopic composition marginally lower by 0.7 tunit compared to the pyroxene from ,328, indicating that this mineral is only minimally effected by the disturbance. An alternate mechanism which could account for the observed open-system behavior, as well as the high Nd content of the whole rock and mineral separates, is the presence, in trace amounts, of a phosphate phase either primary or Lately introduced along with the fumarolic component. In Fig. 3. we illustrate how the 143Nd/‘44Ndratio of plagiociase can, in principle, be lowered during a partial isotopic exchange, and possibly Sm/Nd fractionation, with another phase (P). This model assumes that the age of the disturbance is around 3.0 Ga (40Ar-39Ar age of melt breccia, TURNER and CADOGAN, 1975). It also requires that P has a lower ‘47Sm/‘44Nd ratio than the plagioclase, and is an early product of lunar differentiation. Although no precise neodymium isotopic ratio was
DISCUSSION The age of 4.56 I 0.07 Ga obtained for 67016,328 and the initial Nd composition agree within uncertainties with those obtained for differentiated meteorites (Fig. 2) such as Angra DOS Reis (Sm-Nd ages of 4.55 r 0.04 Ga, LUGMAIR and MARTI, 1977; 4.564 + 0.04 Ga, JACOBSENand WASSERBURG, 1984; Pb-Pb age of4.5577 + 0.0004 Ga, LUGMAIR and GALER, 1992). This chondrite-like initial neodymium isotopic ratio strengthens the interpretation of this age as the time of closure of the Sm-Nd isotopic system for clast 67016,328. The Sm-Nd isotopic results obtained for split ,326, however, imply an open-system behavior at a small-scale, raising the question of the credibility of the isochron obtained from ,328. If the age of ,328 was overestimated in the same way as for ,326, it would imply that the initial Nd ~om~sition of the Moon is non-chond~ti~. Calculated at 4.44 Ga, the age of ferroan anorthosite 60025 (CARLSONand LUGMAIR, 1988), the pyroxene separate from ,328 would have an +d value of f0.9 (relative to Murchinson), suggesting a LREEdepleted source. Implications of a light REE-depleted source for the parental material of the Moon (mantle of the protoEarth or impactor in the giant impact model) have been considered by CARL~ON and LUCMAIR (1988) and SHIH et al. ( 1993) but further precise neodymium isotopic data for lunar highlands rocks clearly are needed to resolve this issue.
(‘5079
r-_---r____‘----------_
1
z
2 2
_..__
ADOR meteon’tes
o.mt
._.
..~
”
._ ,
--I-
!’
/
0.5075
__
, 326~
IA-j___L~-____i0.12
0.14
0.16
0.18
a.20
0.22
0.24
026
'47S&'44Nd
FIG. 3. ‘43Nd/‘“Nd vs. ‘47Sm/‘44Nd ratios calculated at 3.9 Ga and inset schematic model for the Sm-Nd disturbance observed for split ,326. During an impact event, this part of the clast behaved as an open-system. Pyroxene is assumed to remain undisturbed. A partial isotopic re-equilibration (a) and possibly also Sm/Nd exchange (0) with a low Sm/Nd phase could shiR the piagioclase composition downward from the isochron. Alternatively, the vertical downward shift (a) could result from a partial loss of radiogenic t4’Nd from radiation-damaged sites in the plagioclase.
Ancient lunar anorthosite obtained for the leachate and residue for plagioclase ,328 (Fig. 3), the low ‘47Sm/‘44Nd = 0.132 of the leachate is consistent with the above hypothesis. The slower diffusion of REE in clinopyroxene relative to plagioclase (SNEERINGER et al., 1984) has been called upon by PRINZHOFER et al. (1992) to explain why pyroxene is less disturbed during such an isotopic exchange. Although neither apatite or whitlockite have been observed in the 670 16 clast, the presence of metasomatic sulfides suggests a possible link. However, the very low Sm/Nd ratio required for the exchanging phase seems at variance with the rather flat REE patterns reported for lunar whitlockite and apatite (LINDSTROM et al., 1991; NEAL and TAYLOR, 1991; JOLLIFF et al.. 1993). Ferroan anorthosites are generally thought to represent early cumulates that crystallized from the lunar magma ocean (WARREN, 1985). The initial strontium isotopic composition estimated for the 67016 noritic anorthosite clast analyzed here is indistinguishable from that of other ferroan anorthosites, implying a similar origin. Using an initial strontium isotopic ratio of 0.69901 (NBS 987 = 0.7 10215), a lower limit for the “Rb/%r ratio -0.09 (close to the Bulk-Earth value) can be estimated for the proto-Moon (corresponding to T = 4.50 Ga and using the initial “Sr/%r ratio of ADOR at 4.56 Ga). An upper limit around “Rb/*‘Sr = 0.5 (average chondritic value after BRANNON et al., 1987) corresponds to a maximum age of -4.54 Ga for the Moon. Assuming that 4.56 f 0.07 Ga represents the Sm-Nd closure age of clast ,328, what is its significance compared to the range 4.20-4.46 Ga previously reported for other pristine lunar plutonic rocks? (e.g., recent compilation by NYQUIST and SHIH, 1992). This range of ages may reflect variations in the depth of origin of these rocks and hence the cooling rate rather than a protracted period of crystallization of the lunar magma ocean, in agreement with NYQUIST et al. (198 1) and WARREN et al. (199 1). Petrographic evidence for slow cooling (e.g., coarse exsolution of pyroxenes) has been reported for lunar troctolite 76535 (GOOLEY et al., 1974) and ferroan anorthosite 60025 (WARREN et al., 1991). The latter authors calculated that under the insulation of 2 km megaregolith. the neodymium isotopic closure, at depths greater than 18 km, would take at least 20 Ma. It is perhaps somewhat surprising that a very old Sm-Nd age for clast 67016,328 is preserved, given the complex sequence of events that affected this clast. including brecciation and recrystallization of the pyroxenes. The Sm-Nd age reflects the last isotopic equilibration between the pyroxenes and plagioclase in this rock, suggesting that the recrystallization occurred below the SmNd closure temperature, and did not involve significant chemical or isotopic exchange between mineral phases. In this case, the Sm-Nd age would date the equilibration of the original rock at depth in the lunar crust. Alternatively, the brecciation and recrystallization occurred very early in lunar history. Nevertheless, the older age of 67016,328 compared to 60025 suggests a relatively shallow origin and consequently more rapid cooling, even though both rocks may have formed as rafted cumulates in a growing ferroan anorthosite crust (WARREN, 1990). We propose that ferroan noritic anorthosite clasts, such as found in 67016 and other breccias from the Descartes Formation (JOLLIFF, 1992), in spite of their generally small size,
2925
may represent a significant component of the shallow lunar crust. Their chemical composition is comparable to that of the average lunar crust (NORMAN and TAYLOR, 1992). Mass balance models for polymict samples (KOROTEV and HASKIN, 1988) and crystallization models of an FeO-enriched magma ocean (WARREN, 1990) also predict a mafic component in the ferroan anorthositic crust. Telescopic and orbital remote sensing studies also have shown that noritic anorthosite is widely exposed across the lunar surface (PIETERS, 1986). Detailed telescopic studies of multi-ring basins in the southern lunar hemisphere have discovered that a thick section of pure anorthosite occurs preferentially within the inner rings, hence relatively deep in the section, and tends to be overlain by shallower noritic crust (HAWKE et al., 1993). CONCLUSION The extremely old Sm-Nd age of 4.56 + 0.07 Ga obtained for 670 16,328 suggests that noritic ferroan anorthosites represent early cumulates from the presumed lunar magma ocean which cooled rapidly, probably in the upper part of the lunar crust. The initial “Sr/‘% ratio -0.69901 is consistent with previous estimates for the lunar initial strontium isotopic composition. These results suggest that the formation of the Moon and, by implication, accretion and differentiation ofthe terrestrial planets occurred rapidly, in less than 70 Ma. Acknowledgments-R. Rudowski provided high quality mineral separations for 670 16,328. It is our pleasure to acknowledge S. R. Taylor for interesting discussions. The encouragement of the late Professor A. E. Ringwood in pursuing this work is gratefully acknowledged. P.
Warren, R. Carlson, and R. Korotev are thanked for their constructive comments. M. Norman is supported by NASA grant NAG 9-454 and NAGW 3281 (K. Keil, P. I.). C. Alibert is researcher with the CNRS and carried on this study as a Visitor at RSES in 1990/9 1. Ed/drloriul handling:
D. W. Mittlefehldt
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LII(;MAIRG. W., MARTIK.. KURTZ3. P.. and SCt+l’iYIN%. B. ( lY76) History and genesis of lunar troctolite 76535 or: Ho\+ old is old’! /‘1.0<..,‘//I J!_lU?U!. PlUNCl.Sr? C’ow/..1009-203 7. MA~JRERP., E~ERHARDTP., GEISSJ.. GR&L.ER N.. S? I. I I I 1‘~ A.. BROWN G. M., PECKETT A., and KR~I~E~B[~~~I_ C. I 1978) Prelmbrian craters and basins: ages, compositions and excavation depths of Apollo 16 breccias. Ge0chirn. C~~smothin~. It’~tr42. I6871730. MCGEEJ. J. (I 988) Petrology of brecciated ferroan noritic anorthositc 673 15. Pm. 18th Lunar Plunc~t Sci. C’or~/,, 2 i -3 1. MINSTER J.-F., RICARDL.. and Ai. KXE C. J. (lY79) *‘Rh-%r geochronology of Enstatite meteorites. )Irrrtll p/crnc,r .SV/ I (‘II 42. 333-347. N&XI.C. R. and TAYLOR L. A. (1991)Evidence of met~lsomatism in the lunar highlands and the origin of whitlockite. ~~f,~)~,~~~~? ( ‘cismochi,n. :tcfa 55, 2965-2980.
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M. D. (198 1) Petrology of suevitic lunar hrcccia 6?016.
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Geochemistry of lunar crustal rocks from breccia 670 16. and the composition ofthe Moon L;<&tim. ~‘~~s~z~~~~m. =fcfia52, IO13.-1024. NORMAN M. D.. TAYLOR G. J., and KEU K. ( I W 1i Additional corn. plexity in the lunar crust: Petrology of sodic anorthosites and sulfurrich, ferroan noritic anorthositc\, C;c~qh!.\ Kc*\ i.c’ii 18, 7OJ(I 2084. NORMAN M. D.. GRIFFIN W. L., and R?‘m C’.G. ( 1YY3)Voiatilit, in the lunar crust: Trace element analyses of lunar mine& h\ PIXE proton microprobe. Luncr~ Planet. .G,i. XXIV. 109 I NYQUIST L, E. ( 1977) Lunar Rb-Sr chronology, WIIY ~‘!w~r i:‘oui; 10, 103- 142.
! iY76) Rb-Sr ago oitroctolite 76535. Prot. 7th I.rtrw Plarwt. SC, C‘on/..3035-.1054 PEERS C. M. (1986) Composition of the lunar &ust from neai infrared spectroscopy. Rev Gt$zJ:\-. 24, 557.-5%. PREM~ W. R. and TAT~UMOTO M. (1992) LJ-Th-Ph. Rb-Sr, and SmNd isotopic systematics of lunar rroctolitic cumulate ‘7635: lm plications on the age and origin of this early lunar, deep-seated cumulate. Proc~. 2.?th Lunar Plwwt. SCI. C‘d. 3X i-397. PRINZ~~OFER A.. PAPANASTASSIOI, D. A., and WASS~RBL~K(;Cf. J. ( 1992) Samarium-Ne~yrniunl evolution ofmeteorires. (;~~c~c~hi/;l (iwmochim. .dcta 56, 797-8 15~ SAFRONOV V. S. (1991) Kuiper Prirc Lecture: Somu problem ~(1rhc formation of the planets. 1cml.s 94, -160-27 I SI~IH C.-Y., NYQUIST L. E., DA%?+ E. J., BO(iZKi> I>. L., lfczxst! B. M.. and WIESMANN H. ( 1993) Age ofpristine noritic clasts from lunar breccias 1544.5and 15455. Gc~&irn C’(~\rl!!~/ti~l I(,~u57. 915-931. SNEERINC;ER M., HART S. R.. and SH~MIS Ic. (Ic)X4) Strontrum and Samarium diffusion in diopside (~~~)~~r~/~? ( ~~.\~~7/~~,~~~1 Ir~r. 48, 1589-1608. I‘IJRNERG. and CADOGAN P. H. i 1975) The hislur! ol‘lunar htmbardment inferred from 4”Ar-34Ardating of highland rock?. I’ve. 6rh l,unur Planet. Sci. Cor$. 1.509-153X. WARREN P. H. (1985) The magma ocean concept and lunar evolution ,&n. Ret. 12irtk Planet. 21. 13, 101-240. PAPANASTASSIOU ‘D. A. and WASS~RBC~R~;G. .l
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i ITi I) Mcga-
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Pergamon
0016-7037(94)00089-l
An ancient Sm-Nd age for a ferroan noritic anorthosite clast from lunar breccia 67016 CHANTAL
ALIBERT,‘,’MARC D. NORMAN,‘,’and MALCOLMT. MCCULLOCH’
‘Research School of Earth Sciences, Australian National University, Canberra A.C.T. 0200, Australia %entre de Recherches PCtrographiques et Gbchimiques (CRPG), BP 20, 54500 Vandoeuvre-l&-Nancy, France 3Planetary Geosciences, Dept. of Geology and Geophysics, School of Ocean and Earth Sciences and Technology,
University of Hawaii, Honolulu HI 96822, USA (Received September 2 1, 1993; accepted in revised form February 10,
1994)
Abstract-Strontium and neodymium systematics have been examined in a clast of ferroan noritic anorthosite from Apollo 16 breccia 67016. Two splits (,328 and ,326) of the same clast give different SmNd results. Split ,328 gives a well defined internal isochron age of 4.562 + 0.068 Ga and an initial ‘43Nd/ l”Nd ratio of0.50673 f 10 corresponding to tNd= 0. I f 0.2 (2u optimized error) relative to the Murchinson carbonaceous chondrite. The pyroxene separate from split ,326 lies on the same isochron. In contrast, the plagioclase and whole-rock from split ,326 fall below this line, indicating a small-scale disturbance of the Sm-Nd system. This may reflect either an isotopic exchange between the plagioclase and a low Sm/Nd mineral or a loss of radiogenic ‘43Nd from the plagioclase, possibly during the period of major impacts at -3.9 Ga. The preservation of an extremely old age for the noritic ferroan anorthosite 670 16,328 suggests a rapid cooling of this rock at an early stage in the evolution of the lunar magma ocean. This old age is also consistent with giant impact models for the formation of the Moon but implies a relatively early event (pre 4.50 Ga) and, therefore, rapid accretion and differentiation of the terrestrial planets. INTRODUCTION ALTHOUGH SOME CONSENSUS has been reached about the growth of planetesimals by runaway accretion, many aspects concerning the initial conditions, such as the role of nebular gases and size range of planetesimals, are still controversial (e.g., WETHERILLand STEWART, 1989; BARGEand PELLAT, 1991; KOLVOORDand GREENBERG,1992; SAFRONOV,1991). The possible range in these initial conditions corresponds to accretionary timescales which can vary by an order of magnitude, leading to very different implications for planetary differentiation. The relatively short formation interval of less than lo7 years which is expected for hierarchical accretion of massive parent bodies would result in rapid release of gravitational energy and global melting. Alternatively, for the Earth, a more extended formation interval (- 10’ years) would imply cooler conditions and, therefore, less extensive melting of the planet during its initial differentiation. The highlands crust of the Moon contains the oldest preserved record of crustal formation in the Earth-Moon system. The age of the Moon’s primary crust can, therefore, provide an upper limit for the duration of planet formation in the inner Solar System. Remnants of the early lunar crust are represented by the ferroan anorthosites and the Mg-suite of dunites, troctolites, norites, and gabbronorites. Mg-suite rocks have provided Sm-Nd, U-Pb, and 40Ar-39Arplateau ages in the range 4.2-4.43 Ga (e.g., CARLSONand LUGMAIR, 1981; NYQUIS~ et al., 1981; COMPSTONet al., 1984; JESSBERGER et al., 1977; MAURER et al., 1978). More recently, CARLSON and LUCMAIR (1988) reported a precise Sm-Nd age of 4.44 k 0.02 Ga for ferroan anorthosite 60025, and SHIH et al. (1993) reported a Sm-Nd age of 4.46 + 0.07 Ga for a noritic clast from breccia 15445, suggesting a similar antiquity for both suites of rock. These relatively young ages are consistent 292 1
with an origin of the Moon by a giant impact between a Mars-sized planetesimal and the proto-Earth, - 100 Ma after To, and are compatible with models of WETHERILL(1992) suggesting that accretion continues for -100 Ma before a giant impact becomes highly probable. In contrast, an older Rb-Sr age of 4.5 1 f 0.07 Ga has been obtained for the troctolite 76535 (PAPANASTASSIOU and WASSERBURG,1976) and a U-Pb model age of 4.5 1 + 0.0 1 Ga for the ferroan anorthosite 60025 (HANAN and TILTON, 1987). These latter ages are significantly older than the Sm-Nd ages obtained from the same rocks and have, therefore, been considered as unreliable (LUGMAIR et al., 1976; CARLSON and LUGMAIR, 1988; PREMOand TATSUMO~O, 1992). For example, PREMO and TATSUMO~O (1992) call upon a late loss of Rb from olivine in the troctolitic cumulate 76535 and also show that using the Canyon Diablo Pb values as initial Pb gives a model age which is too old for this sample. This conclusion is also likely to be applicable to the ferroan anorthosite 60025. The ferroan noritic anorthosite clast analyzed here for strontium and neodymium isotopes is from feldspathic fragmental breccia 67016, which was collected from the rim of North Ray Crater. Petrographic and geochemical accounts of breccia 670 16 have been given previously (NORMAN, 198 1; LINDSTROMand SALPAS,1983; NORMANet al., 1991; NORMAN and TAYLOR, 1992; NORMAN et al., 1993). The clast which we analyzed belongs to the mafic subgroup of the ferroan anorthosite suite (JAMESet al., 1989). Clasts of similar composition have been described in other feldspathic fragmental breccias from North Ray Crater (LINDSTROMand SALPAS, 1983; MCGEE, 1988). 40Ar-39Ar ages around 3.95 Ga have been reported for a dark clast of melt breccia and a plagioclase separate from breccia 67016 (TURNER and CADOGAN, 1975).
C. Alibert, M. D. Norman, and M. T. McCulloch
2922
SAMPLE DE~RI~~ON AND ANALYTICAL TECHNIQDES Ferroan noritic anorthosite clasts in 670 16 have crystalline textures which indicate a complex petrogenesis involving igneous crystallization, brecciation, subsolidus recrystallization, and sulfide metasomatism. These clasts are composed of 68-69% plagiocfase, 2728% pyroxene (both o~hop~oxene and clinopyroxene in the proportion 2: I f, 2-3% troilite, and minor amounts of ilmenite and chromite. Relatively large piagioclase grains (up to 2 mm long) with irregular shapes are surrounded by a matrix of finer-grained anhedral plagioclase and granular to tabular pyroxenes. We interpret this texture as indicating a coarse-grained, probably plutonic, protolith which was brecciated and subsequently annealed so that the formerly cataclastic matrix recrystallized. Pyroxene compositions indicate equilibration temperatures (KRETZ,1982) of g50-9~*C, although it is not clear whether this represents the temperature of post-brecciation annealing or inheritance of primary, pre-brecciation mineral compositions. Olivine is now very rare in these clasts, occurring only as relicts within troihte-pyroxene intergrowths. Troilite also occurs along cleavage traces and shear planes. The sulfur-rich nature of these clasts and associated enrichments of volatile elements have been attributed to a fumarolic com~nent (NORMAN et al., 1993;COLSON, 1992). In places, the troihte-pyroxene intergrowths closely follow observed grain boundaries, hence indicating that sulfide metasomatism postdates the recrystallization referred to above. However, it predates incorporation of these clasts into the host breccia because other olivine-bearing clasts in 67016 are not affected by the metasomatism. Mineral compositions in these clasts are homogeneous and identical to those of pristine ferroan anorthosites (NORMAN et al., I99 I). The low Ni and Co contents (37 and 7 ppm, respectively) and Ni/Co ratios of these clasts (NORMAN and TAYLOR, 1992) suggest negligible meteoritic contamination. These features indicate these clasts are monomict, noritic members ofthe ferroan anorthosite suite of lunar highlands rocks. Clast ,326/8 was originally allocated to S. R. Taylor as two separate splits weighing 0.641 g (,326) and 0.936 g (,328). Whole rock powders of both splits were prepared and processed for major and trace element analyses, as outlined by NORMAN and TAYLOR f1992). The remaining material was processed for mineral separation. A combination of heavy-liquids and handpicking was used for ,328 whereas, for split ,326 (a small -3 mm long chip), only handpicking was used. Pyroxene separates contain some ilmenite inclusions. For split ,328. a composite grain of ilmenite with minor plagioclase and pyroxene was also analyzed. The large plagioclase separate from ,328 (100 mg) contained some pyroxene. Small amounts of plagiociase from ,328 were teached in dilute acid to assess the role of possible sulfide and
phosphate inciusions. Mineral separates were rinsed in uhrapure watet (Milliporee) and distilled acetone before dissolution by HF-HNO? acids in closed Teflon beakers. Concentrations and isotopic ratios were measured using a Finnigan MAT 261 mass spectrometer using mixed spikes 84Sr-85Rband “‘Nd-““Sm. During this study, the NdSm spike was twice calibrated against the Cahech standard solution and gave the recommended SmiNd ratio to better than 0.1%. Sama&m concent~tions were determined using two procedures: first using the ratio ‘49Sm/‘52Smfor mass fractionation correction with concentrations calculated from the 148Sm/‘47Smand ‘S’Sm/‘4’Sm ratios. The second procedure used ‘48Sm/‘52Smfor mass fracttonation correction and concentrations were calculated from the ‘49Sm/1J7Sm and ‘52Sm/‘47Smratios. Samarium concentrations were identical regardless of the method used. For the ,326 whole-rock and ,328 iimen&e-rich chip, *49Sm/‘s2Smratios (corrected for spike contributio~l) are 0.5 1673 f 7 and 0.5 1687 & 6. respectively. These values are close to the average 0.51684 F 2 obtained for a Sm standard solution. using the same cup configuration. We, therefore, conclude that neu tron effects on the abundance of r4?5rn, if present, are extremely small and not resolvable in this lunar clast. Total chemistry blanks were 5 pg for Rb, 66 pg for Sr, and 6-25 pg for Nd. Neodymium was loaded on double Re-Ta filaments, measured as metal ions and mass fractionation was corrected relative to ‘~Nd~‘~Nd ; 0.72 1’) Isotopic data including whole rock analyses for the Murchinson carbonaceous chondrite and the Angra DOS Reis angrite, and Sr and Nd standards, are reported in Tables I and 2. The Sm-Nd age calculations use 20 errors of 0.2% for Sm/Nd ratios and 0.00002 for 143Nd/‘44Ndratios (external reproducibility). SR AND ND ISOTOPIC
RESULTS
Splits ,328 and ,326 (Table 1) have whole-rock Nd contents -3ppm, which are relatively high compared with typical fer”, roan anorthosites. These whole-rock Nd contents are higher
than expected from a simple mass balance calculation using the measured mineral separates, suggesting the presence of an additional phase with high light REE abundances (Fig. I). A leaching experiment was performed on a plagioclasr separate from split ,328 which contained some minute inclusions of sulfides (Table 1). The leachate contains - 50% of the total Nd, which may reflect the presence of a soluble phase, possibly phosphate inclusions in the plagioclase, although none was identified petrogra~hicall~
Tablel- Nd isotopicdatafor lunarbreccia67016,328 and,326 andfor ADOR
andMwchinson meteorites.
Sample
weight Sm (=@ @pm)
“7Sm Nd (ppJ@ (Wg)
(n”vilg)
whole-rock ,328
18 12 100 17.6
1.O? 1.31 0.681 0.679
3.31 3.20 2.43 1.99
1.065 1.309 0.6791 0.6771
5.463 5.278 4.008 3.285
0.1949 0.2480 0.1694 0.2061
- _.____. _-. 143NdJ’+‘Nd &Nd &Nd at456Ga a :$ ----.-l.__-_ 0.91 0.512654 (13) 0.556751 0.5 I 0.45 0.514239 (8) 0.506728 0.05 0.25 0.511849 (10) 0.506718 -0.15 0 117 0.512951 (13) 0.506709 -0.33
4.20 0.446 1.85 0.844
1.73 3.86
0.4443 0.8413
2.854 6.361
0.1557 0.1323
0.511392 (150) 0.510731 (120)
0.9638 1.142 0.5951
4.831 5.037 4.127
0.1995
0.2268 0.1442
0.512696 (6) 0.513564 (IO) 0.510999 (8)
0.506654 ~~~~ -
-1.40 -0.61 -1.84
-i.w -0.21 -1.44
5.744 28.97 5.991 31.14 0.2105 1.074
0.1982 0.1924 0.1960
0.512735 (8) 0.512546 (7) 0.512662 (10)
0.50673 1 0.506719 0.506725
0.12 -0.12 0.00
(1.52 0.28 0.40
0.1384 0.1381
0.511878 (13) 0.512661 (6) 0.512660 (6)
,328 pyroxenc ,328 plagioclase ,328 iltnenite
,328 plagioclasc residue’ ,328 plagkxlase leachate* ,326 whoie-rock ,326 pyroxene ,326 piagk&se
49 5.1 15.5
0.966 I.f46 0.597
2.93 3.05 2.50
Angrs DOSReis (powder)
56.8
5.76 6.01 0.211
17.6 18.9 0.651
Angm Dos Reis (chip) Murchimson(chip)
La Jolla(31 runs, 1990/91) ECRl(1 nm, 1990) BCRi (t run, 1991)
100 80
‘“Nd
‘47Sn+Nd
t43Nd,‘t”Nd measured
Quoted rrrors for t43Nd/I@Nd ratios (in parentheses)are 2a SE of the mean except for the La Jolla Nd stand& (lo SD of the population). Initial qQj values relative to CHUB (Jacobsen and Wasserburg, 1984) have been caiculefed after notmalization of the 143Nd/t40Ndratios to 0.511854 (La Jolla Nd standard measured by Carlscn and Lugmair, 1988). ENd values referred as a and b are calculated relative to Murchinson and to CHUR, nspectively. * is for leaching in HCl 1N and HN@ 1N.
2923
Ancient lunar anorthosite Table Z- Sr isotopic data for lunar hwcia
67016,328 and ,326 and for ADOR meteorite.
weight (mg) 31.1 ,328 plagioclasc 4.2 ,328 plagioclase res.’ ,328 plagioclasc res.** 3.4 49.0 15.5 5.07
,326 whole mk ,326 plagioclasc ,326 pymxene
16.3 ADOR (8 runs) E&A (10 runs) NEIS 987 (42 runs, 1990,91)
87Rb/=Sr
*7sr/%r at 4.56 Ga
0.132
155
0.002459
0.699397 (10) 0.699175 (13) 0.699247 (19)
0.699011
0.385 0.244 0.1808
153 183 129
0.007274 0.003853 0.004043
0.699499 (18) 0.699308 (8) 0.699330 (8)
0.699012 0.699050 0.699060
0.0357
127
O.ooO813
0.698972 (17) 0.707982 (13) 0.710215 (10)
0.698920
* residue after leaching in HCI and HN@ the 87Sr/%r
87SrPSr measured
1N: ** Icached in HNQ
1N. Quoted errors for
ratios (in parentheses) are 20 SE of the mean cxcepl for the Angra DOS Reis
meteorite (ADOR) and the E&A and NBS 987 Sr standards (lo SD of the population).
0.16
Sm-Nd isotopic data for the two splits ,328 and ,326 are significantly different. However, as isotopic measurements of samples from both splits were undertaken during the same session, together with the La Jolla Nd standard and meteoritic samples, it is not possible to ascribe these differences to analytical problems. The nonlinear distribution of Nd and Sm concentrations in the plagioclase, pyroxene, and ilmeniterich separates from ,328 (Fig. 1) satisfies the prerequisite of enough independent mineral components to form a meaningful isochron. The mineral separates and whole-rock for split ,328 give a good alignment in the ‘43Nd/‘44Nd vs. ‘47Sm/ ‘44Nd diagram (Fig. 2). The best fit line, calculated using a modified version of the Williamson method (MINSTER et al., 1979) corresponds to an age of 4.562 f 0.068 Ga (MSWD = 0.9) and an initial ratio of 0.506728 f 96. Translating the 147Sm/‘44Nd axis to the chondritic value (method of FLETCHER and ROSMAN, 1982) an optimized initial EN,,
n
1.3
0
1.1 -
o.9
s
0.7
,326~~
n 0
p
,328~~
,328wr
,326wr
q n
,328ilm
n
iza2
,328 plag
q ,326plag 0.5 -
0.3 2
3
4
5
6
7
8
“14 Nd @M/g) FIG. 1. ‘44Nd vs. ‘?Srn diagram for ferroan noritic anorthosite 67016,328 (solid squares) and ,326 (open squares). The nonlinear distribution of Nd and Sm concentrations in plagioclase (plag), pyroxene (px) and an ilmenite-rich chip (ilm) from split ,328 indicates the multicomponent nature of this sample and shows that any isochron defined by these three mineral separates cannot be reduced to a two-point isochron. The relatively high Nd and Sm contents of the whole-rock samples ,328 and ,326 point to the presence of an additional minor phase with high 144Nd.A leaching experiment on a plagioclase separate from ,328 shows that the Nd in the leachate (soluble part) amounts to 50% of the total Nd; the position of the leachate in this diagram also satisfies the requirement that the wholerock should be inside the polygon defined by the mineral components.
0.510 0.13
0.15
0.17
0.19
0.2 1
0.20 0.23
0.24 0.25
‘47Sm/‘44Nd FIG. 2. ‘47Sm-‘43Ndevolution diagram for ferroan noritic anorthosite 67016,328 (solid squares) and ,326 (open squares). The isochron calculated for ,328 corresponds to an age of 4.562 f 0.068 Ga with an initial of 0.506728 f 96. Using the optimization procedure of FLETCHER and ROSMAN (1982), this latter error (2~) corresponds to 0.2 t-units. The initial cNd = +O.l, relative to Murchinson, or +0.45. relative to CHUR, indicates a source unfractionated with respect to chondrites. The inset figure shows initial cNdvalues at 4.56 Ga, relative to Murchinson (2~ error bars = 0.4 c-units). The pyroxene separate from ,326 lies within combined uncertainties (including error on the isochron) on the 4.56 Ga isochron. However, the plagioclase and whole-rock lie off this line. Isotopic analyses of the meteorites Angra DOSReis (ADOR) and Murchinson are also reported for comparison. The meteorites and the mineral separates and whole-rock for ,328 are, within analytical uncertainties, part of the same linear array.
= +0.5 + 0.2 can be calculated, using the recommended CHUR values of JACOBSEN and WASSERBURG (1984) and after normalization relative to ‘43Nd/‘44Nd = 0.5 11854 for the La Jolla Nd standard. Using, instead of CHUR, the isotopic composition measured in this study for the Murchinson carbonaceous chondrite (Table 1), an initial tNd = +0.06 f 0.2 is obtained. However, this difference is marginal since JACOBSEN and WASSERBURG (1984) give the ‘43Nd/‘44Nd ratio of CHUR to only within 0.5 t-unit, and there is also a similar uncertainty for calibration of interlaboratory biases. For split ,326, the pyroxene separate lies, within combined uncertainties (including error on the isochron), on the best fit line of split ,328, while the plagioclase and the whole rock fall below this line (Fig. 2 and inset). The isochron parameters calculated from ,328 do not change significantly if the pyroxene separate from ,326 is included (T = 4,536 +- 64 Ma). On the other hand, the alignment shown by samples from ,326 alone corresponds to an age of 4.66 + 0.06 Ga with an initial ratio of 0.50653 f 7 (eNd = -1.3 relative to Murchinson). This anomalously old age and negative tNd clearly indicate a disturbance of the Sm-Nd system which has predominantly affected the plagioclase and hence also the whole-rock sample of split ,326. Strontium isotopic data are reported in Table 2. For comparative purposes, we also report Sr isotopic analyses for the angrite Angra DOS Reis (ADOR) as well as Sr standards. Our measurements of ADOR give an average present-day ratio of 0.698972 t- 17. A significant uncertainty in calculating initial ratios results from the variable Rb content in ADOR
C. Alihert, M. D. Norman, and M. T. McCulloch
2924
(G. W. Lugmair, pers. commun., 1991). The initial s7Sr/86Sr ratio of0.698920 f 30 obtained in this study (relative to NBS 987 = 0.7 102 15 ? 10) agrees with that reported by LUCMAIR and GALER (1992). These authors also conclude that the initial 87Sr/86Srratios for BABI and for Angra DOS Reis are not different from one another. The clast analyzed here is relatively enriched in Rb as well as other incompatible elements, compared to ferroan anorthosites generally (NORMANand TAYLOR, 1992). The whole rock (,326) contains -0.4 ppm Rb. which is higher than that obtained for either the plagioclase or pyroxene separates (0.18-0.29 ppm, Table 2). This could reflect partial leaching of Rb from mineral separates during their preparation (water rinse), or the presence of an additional Rb-rich phase. Two plagiociase separates were leached with dilute acid in order to remove any soluble phase (sulfides, phosphates). The residues have lower Rb/Sr and also 87Sr/86Sr ratios: 0.699170.69925 compared to 0.6993-0.6995 for unleached mineral separates and whole-rock samples. The soluble Rb amounts to -50% of the total Rb. A regression through the leached plagioclase from ,328 and the three data points for ,326 gives a Rb-Sr isochron age of 4.00 + 0.47 Ga (h8’Rb = 0.0142 Ga-‘), which is imprecise but consistent with the period of major impacts indicated by 40Ar-39Arages. The best estimate of the initial “Sr/*Sr ratio is probably given by the lowest value of 0.69901 (Table Z), calculated at 4.56 Ga for the leached plagioclase from ,328. This is also the same as that given by the whole-rock ,326. This estimate is similar, within uncertainties, to the initial “Sr/‘?Sr = 0.69902 + 4 (relative to a value of 0.7 10215 for NBS 987) reported by CARLSON and LUCMAIR( 1988) for ferroan anorthosite 60025. It is also in agreement with the initial Sr ratios of -0.69900 compiled for lunar anorthosites by NYQUIST (1977 1.
The disturbance of the Sm-Nd system which has affected split ,326 is not due to an uncorrected neutron effect on the ‘49Sm. The 147Sm/‘44Nd ratio of whole rock ,326 must be lower by - 1% in order to fit the 4.56 Ga line in Fig. 2: however, as already mentioned, the measured ‘4sSm/‘52Sm ratio in this sample is normal and cannot account for such a large shift. A partial loss of radiogenic ‘43Ndfrom plagioclase is a possible mechanism to explain the observed disturbance in split ,326. The low 143Nd/‘44Ndratio observed in plagioclase ,326 requires a loss of - 17% of the radiogenic 143Nd produced in the interval from 4.56 Ga to 4.0 Ga. Considering that radiogenic ‘43Nd is produced by m-decay from “‘Sm. such preferential loss from radiation-damped sites in plagioclase is not unreasonable. Petrographic observations show that while the smallest plagioclase grains are free of shock effects, some of the large relict plagioclase grains show wavy extinction and development of subgrain domains which are shock related. The pyroxene separate from ,326 has an initial neodymium isotopic composition marginally lower by 0.7 tunit compared to the pyroxene from ,328, indicating that this mineral is only minimally effected by the disturbance. An alternate mechanism which could account for the observed open-system behavior, as well as the high Nd content of the whole rock and mineral separates, is the presence, in trace amounts, of a phosphate phase either primary or Lately introduced along with the fumarolic component. In Fig. 3. we illustrate how the 143Nd/‘44Ndratio of plagiociase can, in principle, be lowered during a partial isotopic exchange, and possibly Sm/Nd fractionation, with another phase (P). This model assumes that the age of the disturbance is around 3.0 Ga (40Ar-39Ar age of melt breccia, TURNER and CADOGAN, 1975). It also requires that P has a lower ‘47Sm/‘44Nd ratio than the plagioclase, and is an early product of lunar differentiation. Although no precise neodymium isotopic ratio was
DISCUSSION The age of 4.56 I 0.07 Ga obtained for 67016,328 and the initial Nd composition agree within uncertainties with those obtained for differentiated meteorites (Fig. 2) such as Angra DOS Reis (Sm-Nd ages of 4.55 r 0.04 Ga, LUGMAIR and MARTI, 1977; 4.564 + 0.04 Ga, JACOBSENand WASSERBURG, 1984; Pb-Pb age of4.5577 + 0.0004 Ga, LUGMAIR and GALER, 1992). This chondrite-like initial neodymium isotopic ratio strengthens the interpretation of this age as the time of closure of the Sm-Nd isotopic system for clast 67016,328. The Sm-Nd isotopic results obtained for split ,326, however, imply an open-system behavior at a small-scale, raising the question of the credibility of the isochron obtained from ,328. If the age of ,328 was overestimated in the same way as for ,326, it would imply that the initial Nd ~om~sition of the Moon is non-chond~ti~. Calculated at 4.44 Ga, the age of ferroan anorthosite 60025 (CARLSONand LUGMAIR, 1988), the pyroxene separate from ,328 would have an +d value of f0.9 (relative to Murchinson), suggesting a LREEdepleted source. Implications of a light REE-depleted source for the parental material of the Moon (mantle of the protoEarth or impactor in the giant impact model) have been considered by CARL~ON and LUCMAIR (1988) and SHIH et al. ( 1993) but further precise neodymium isotopic data for lunar highlands rocks clearly are needed to resolve this issue.
(‘5079
r-_---r____‘----------_
1
z
2 2
_..__
ADOR meteon’tes
o.mt
._.
..~
”
._ ,
--I-
!’
/
0.5075
__
, 326~
IA-j___L~-____i0.12
0.14
0.16
0.18
a.20
0.22
0.24
026
'47S&'44Nd
FIG. 3. ‘43Nd/‘“Nd vs. ‘47Sm/‘44Nd ratios calculated at 3.9 Ga and inset schematic model for the Sm-Nd disturbance observed for split ,326. During an impact event, this part of the clast behaved as an open-system. Pyroxene is assumed to remain undisturbed. A partial isotopic re-equilibration (a) and possibly also Sm/Nd exchange (0) with a low Sm/Nd phase could shiR the piagioclase composition downward from the isochron. Alternatively, the vertical downward shift (a) could result from a partial loss of radiogenic t4’Nd from radiation-damaged sites in the plagioclase.
Ancient lunar anorthosite obtained for the leachate and residue for plagioclase ,328 (Fig. 3), the low ‘47Sm/‘44Nd = 0.132 of the leachate is consistent with the above hypothesis. The slower diffusion of REE in clinopyroxene relative to plagioclase (SNEERINGER et al., 1984) has been called upon by PRINZHOFER et al. (1992) to explain why pyroxene is less disturbed during such an isotopic exchange. Although neither apatite or whitlockite have been observed in the 670 16 clast, the presence of metasomatic sulfides suggests a possible link. However, the very low Sm/Nd ratio required for the exchanging phase seems at variance with the rather flat REE patterns reported for lunar whitlockite and apatite (LINDSTROM et al., 1991; NEAL and TAYLOR, 1991; JOLLIFF et al.. 1993). Ferroan anorthosites are generally thought to represent early cumulates that crystallized from the lunar magma ocean (WARREN, 1985). The initial strontium isotopic composition estimated for the 67016 noritic anorthosite clast analyzed here is indistinguishable from that of other ferroan anorthosites, implying a similar origin. Using an initial strontium isotopic ratio of 0.69901 (NBS 987 = 0.7 10215), a lower limit for the “Rb/%r ratio -0.09 (close to the Bulk-Earth value) can be estimated for the proto-Moon (corresponding to T = 4.50 Ga and using the initial “Sr/%r ratio of ADOR at 4.56 Ga). An upper limit around “Rb/*‘Sr = 0.5 (average chondritic value after BRANNON et al., 1987) corresponds to a maximum age of -4.54 Ga for the Moon. Assuming that 4.56 f 0.07 Ga represents the Sm-Nd closure age of clast ,328, what is its significance compared to the range 4.20-4.46 Ga previously reported for other pristine lunar plutonic rocks? (e.g., recent compilation by NYQUIST and SHIH, 1992). This range of ages may reflect variations in the depth of origin of these rocks and hence the cooling rate rather than a protracted period of crystallization of the lunar magma ocean, in agreement with NYQUIST et al. (198 1) and WARREN et al. (199 1). Petrographic evidence for slow cooling (e.g., coarse exsolution of pyroxenes) has been reported for lunar troctolite 76535 (GOOLEY et al., 1974) and ferroan anorthosite 60025 (WARREN et al., 1991). The latter authors calculated that under the insulation of 2 km megaregolith. the neodymium isotopic closure, at depths greater than 18 km, would take at least 20 Ma. It is perhaps somewhat surprising that a very old Sm-Nd age for clast 67016,328 is preserved, given the complex sequence of events that affected this clast. including brecciation and recrystallization of the pyroxenes. The Sm-Nd age reflects the last isotopic equilibration between the pyroxenes and plagioclase in this rock, suggesting that the recrystallization occurred below the SmNd closure temperature, and did not involve significant chemical or isotopic exchange between mineral phases. In this case, the Sm-Nd age would date the equilibration of the original rock at depth in the lunar crust. Alternatively, the brecciation and recrystallization occurred very early in lunar history. Nevertheless, the older age of 67016,328 compared to 60025 suggests a relatively shallow origin and consequently more rapid cooling, even though both rocks may have formed as rafted cumulates in a growing ferroan anorthosite crust (WARREN, 1990). We propose that ferroan noritic anorthosite clasts, such as found in 67016 and other breccias from the Descartes Formation (JOLLIFF, 1992), in spite of their generally small size,
2925
may represent a significant component of the shallow lunar crust. Their chemical composition is comparable to that of the average lunar crust (NORMAN and TAYLOR, 1992). Mass balance models for polymict samples (KOROTEV and HASKIN, 1988) and crystallization models of an FeO-enriched magma ocean (WARREN, 1990) also predict a mafic component in the ferroan anorthositic crust. Telescopic and orbital remote sensing studies also have shown that noritic anorthosite is widely exposed across the lunar surface (PIETERS, 1986). Detailed telescopic studies of multi-ring basins in the southern lunar hemisphere have discovered that a thick section of pure anorthosite occurs preferentially within the inner rings, hence relatively deep in the section, and tends to be overlain by shallower noritic crust (HAWKE et al., 1993). CONCLUSION The extremely old Sm-Nd age of 4.56 + 0.07 Ga obtained for 670 16,328 suggests that noritic ferroan anorthosites represent early cumulates from the presumed lunar magma ocean which cooled rapidly, probably in the upper part of the lunar crust. The initial “Sr/‘% ratio -0.69901 is consistent with previous estimates for the lunar initial strontium isotopic composition. These results suggest that the formation of the Moon and, by implication, accretion and differentiation ofthe terrestrial planets occurred rapidly, in less than 70 Ma. Acknowledgments-R. Rudowski provided high quality mineral separations for 670 16,328. It is our pleasure to acknowledge S. R. Taylor for interesting discussions. The encouragement of the late Professor A. E. Ringwood in pursuing this work is gratefully acknowledged. P.
Warren, R. Carlson, and R. Korotev are thanked for their constructive comments. M. Norman is supported by NASA grant NAG 9-454 and NAGW 3281 (K. Keil, P. I.). C. Alibert is researcher with the CNRS and carried on this study as a Visitor at RSES in 1990/9 1. Ed/drloriul handling:
D. W. Mittlefehldt
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i ITi I) Mcga-
regolith insulation and the duration ofcoolingto isotopic siosur~ within differentiated asteroids and the Moon. .I rfvcg~h~~\Hcs. 96. 5909-5923. %'Ef'tfERIl_t. Ci.W. (1992) An alternative model ltrr ihe Ibrmatiot: of the asteroids. Icunts 100, 307 325. ~V~~~~~R~LLG. W. and STEWART G. R. ( 198’)) Accumulation VI _i swarm of small planetesimals. /(~cir-l~.~ 77. 330. Ji y