A relatively young samarium-neodymium age of 4.36 Ga for ferroan anorthosite 62236

Borg L., Norman M.1, Nyquist L., Snyder G., Taylor L., Lindstrom M. and Wiesmann H. 1. GEMOC, Macquarie

The earliest lunar crust was composed of two general rock suites: ferroan anorthosites (FANs), and what has been described as the highland plutonic suite (HPS) by [1] consisting of Mg- and alkali-suite mafic and evolved rocks. In the magma ocean model of lunar differentiation, FANs are the earliest lunar crust that formed by floatation of plagioclase cumulates, into which the magmas of the HPS were intruded. The FANs are therefore expected to yield the oldest radiometric crystallization ages of any lunar crustal rocks, and be derived from moderately evolved melts enriched through the crystallization of early formed mafic cumulates. In this paper we present evidence that at least one FAN maybe contemporaneous with HPS magmatism, and has Sm-Nd isotopic systematics that are indicative of derivation from a source that was previously depleted in light rare earth elements (LREE) relative to heavy rare earth elements (HREE).

Lunar rock 62236 is a pristine genomict ferroan noritic anorthosite breccia containing 85-90% plagioclase, with 15-10% orthopyroxene > clinopyroxene+olivine. Three fragments of 62236,21 were selected for mineral separations and isotopic analysis. Plagioclase and mafic minerals were separated using heavy liquids of 2.85 and 3.32 g/cm3. The density fractions were further purified by hand-picking yielding very pure plagioclase, orthopyroxene, and pyroxene+olivine fractions. Sm-Nd isotopic analysis of 62236 whole rocks and mineral fractions yield a precise age of 4.355 +/- 0.030 and an initial eNd of +3.2 +/- 0.5 after correction for neutron fluence effects on Sm isotope compositions. The Rb-Sr isotopic systematics of 62236 are highly disturbed, demonstrating the potential for the Sm-Nd systematics of 62236 to be reset by impact metamorphism. If the Sm-Nd systematics of 62236 have been partially reset, then our best age estimate is the CHUR model age determined from the orthopyroxene fraction of ~4.49 Ga.

If the Sm-Nd systematics of 62236 have not been reset, then three aspects of the data are inconsistent with the lunar magma ocean model of differentiation. First, the age of 62236 is significantly younger than previously reported Sm-Nd ages for FANs 60025 (4.44 +/- 0.02 Ga) and 67016 (4.56 +/- 0.07 Ga) by [2], but is contemporaneous with the earliest Sm-Nd ages reported for some Mg-suite rocks (4.46-4.26 Ga) by [4,5,6,7] and the earliest Pb-Pb zircon ages reported for alkali suite granites (4.37-4.21 Ga) by [8,9]. Second, melts in equilibrium with the 62236 plagioclase and orthopyroxene mineral fractions have 20xC1 chondrite abundances of Sm and Nd compared to liquids calculated to be in equilibrium with 60025 and 67016 plagioclases analyzed by [2,3] which have about 30xC1 chondrite and 150xC1 chondrite abundances respectively. Although 62236 is derived from a less evolved melt than 60025 and (particularly) 67016, it appears to be 80 Ma younger than 60025 and 200 Ma younger than 67016. This would preclude derivation of 67016, 60025, and 62236 from a single progressively differentiating magma ocean. Third, the high initial eNd of 62236 would indicate that it was derived from a source that was depleted in LREE/HREE early in its history. Thus, the 62236 source contrasts with hypothetical late stage magma ocean liquids that are expected to have flat to perhaps slightly LREE-enriched REE patterns, and therefore have eNd that is near the chondritic value.

In summary, the young age, the calculated equilibrium melt composition, and the high initial eNd of 62236 are inconsistent with a lunar magma ocean model in which FANs are expected to predate the HPS and crystallize from a progressively differentiating magma ocean with near chondritic Nd isotopic systematics. These observations suggest that lunar differentiation may be more complicated than the standard lunar magma ocean model.

References [1] Snyder et al. JGR 100, 9365-9388. [2] Carlson and Lugmair (1988) EPSL 90, 119-130. [3] Alibert et al. (1992) GCA 58 2921-2926. [4] Nakamura et al. (1976) Proc 7th Lunar Sci Conf 2309-2333. [5] Nyquist et al. (1981) Proc 12th Lunar Sci Conf B67-B97. [6] Carlson and Lugmair (1981) EPSL 56, 1-7. [7] Shih et al. (1993) GCA 57 915-931. [8] Meyer et al. (1989) LPSC XX 691-692. [9] Compston et al. (1984) Proc 14th Lunar Sci Conf B525-B534

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