Sm-Nd, Lu-Hf and Nb-Zr Constraints on the Early Differentiation of the Moon

Details Sm-Nd, Lu-Hf and Nb-Zr Constraints on the Early Differentiation of the Moon Sm-Nd, Lu-Hf and Nb-Zr Constraints on the Early Differentiation of the Moon

Dec 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p11a0354m&link_type=abstract

American Geophysical Union, Fall Meeting 2002, abstract #P11A-0354

Mathematics

Logic

5455 Origin And Evolution, 6207 Comparative Planetology, 6250 Moon (1221)

Scientific paper

The dominant chemical and mineralogical differentiation process on the Moon is most likely associated with the formation and subsequent crystallisation of a magma ocean. The major evidence in favour of the existence of a magma ocean is the ancient anorthositic crust that once covered the lunar surface. The complementary heavier minerals that crystallised as cumulates from the magma ocean must have included olivine, orthopyroxene, clinopyroxene, spinel, ilmenite, and possibly garnet. Partial melts from these differentiated silicate and oxide layers gave rise to younger mare basalts that now cover parts of the Moon's surface. Radioactive parent-daughter systems such as 147Sm-143Nd, 176Lu-176Hf and 92Nb-92Zr can provide important time constraints on early lunar differentiation processes, particularly if parent-daughter pairs are sufficiently fractionated during magmatic processes. Therefore we obtained Sm-Nd, Lu-Hf and Nb-Zr isotope data for a variety of well dated lunar samples including KREEP basalts, low and high-Ti mare basalts and lunar soils. 92Zr/90Zr and 96Zr/90Zr in mare basalts agree with the chondritic value within the analytical errors of +/-0.5 and +/-1.5 ɛ-units (2σ ), respectively. The absence of 92Zr-isotope anomalies and the presence of small 182W anomalies in mare basalts [1] confine the crystallisation age of the magma ocean to ca. 4.52 - 4.51 Ga (assuming initial 182Hf/180Hf of 1x10-4 and 92Nb/93Nb of 1x10-3). The initial ɛHf (ɛNd) range from -1.9 (-2.7) in the KREEP rocks to +16.6 (6.6) in high-Ti mare basalts. The initial Hf and Nd isotope ratios define two trends that deviate significantly from the terrestrial Hf-Nd array. The initial 176Hf-177Hf values for mare basalts (+7.3 to +16.6 ɛ) are consistent with the data from Beard et al. [2] (0 to +8 ɛ) using the revised 176Lu decay constant [3], but expand the known compositional range of mare basalts. The high ɛHf compared to the ɛNd in the low-Ti basalts requires that garnet played an important role in the early differentiation of the Moon and the development of the low-Ti basalt source rocks. Variable degrees of ilmenite assimilation (low ɛHf) during magma ascent may have enhanced the observed decoupling of Hf and Nd isotope compositions. [1] Lee et al, (2001) Meteoritics and Planetary Science 36(9), A111; [2] Beard et al, (1998), Geochim Cosmochim Acta, 62, 525-524; [3] Scherer et al, (2001), Science, 293, 683-687.

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