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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