Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p33d..05l&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P33D-05
Physics
[1026] Geochemistry / Composition Of The Moon, [1041] Geochemistry / Stable Isotope Geochemistry, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
Mare basalts, derived from partial melts from the lunar mantle, provide information on the early differentiation and evolution of the Moon. Highly precise and accurate oxygen isotope ratios were obtained on mg-size samples of low-Ti and high-Ti mare basalts from the Apollo 11, 12, 15 and 17 missions. Low-Ti basalts studied include Apollo 12 pigeonite and ilmenite basalts, Apollo 15 olivine- and quartz-norm basalts. High-Ti mare basalts studied include seven chemical groups (Apollo 11 Types A, B1, B3, and Apollo 17 Types A, B1, B2 and C) largely defined on the basis of trace elements, indicative of different mantle sources. High-Ti basalts display significant variation in δ18O, correlating with major elements. Values of δ18O in these high-Ti basalts increase by ~0.3‰ from Mg# = 53 to Mg# = 34, suggesting a fractional crystallization control. The variation within a given chemical group can be successfully modeled by mass-balance involving ~32% fractionation of olivine+ armalcolite + pyroxene + ilmenite+ plagioclase. This model demonstrates that high-Ti basalts with >12 wt% TiO2 and high Mg# are more primitive than those with 7-12 wt% TiO2. A weaker correlation of oxygen isotopes with major elements in low-Ti basalts is also observed. From Mg# of 48 to 38, values of δ18O increase by up to 0.1‰, consistent with removal of a minor quantity of olivine. Low-Ti mare basalts with the highest Mg# (55) have slightly lower 18O values than those with Mg# of 48, consistent with possible olivine accumulation. More primitive samples in low-Ti and high-Ti groups still display distinct δ18O values.18O versus major-element plots. This implies that low- and high-Ti basalts are derived from separate sources, each with homogeneous oxygen isotopic compositions. The hypothesis that high-Ti basalts were generated from the assimilation of ilmenite-bearing rocks by low-Ti basalts is not supported by oxygen isotope compositions. The major-element and δ18O variations of low- and high-Ti basalts do not follow a binary mixing trend, further supporting their derivation from different source regions. Such heterogeneity of the lunar mantle is consistent with sinking of Ti-rich differentiates of the lunar magma ocean.
Day J. M.
Liu Ya-Ying
Spicuzza Mike
Taylor Lawrence A.
Valley John W.
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