Oxygen and iron isotope constraints on near-surface fractionation effects and the composition of lunar mare basalt source regions

Oxygen and iron isotope analyses of low-Ti and high-Timare basalts are presented to constrain their petrogenesis and to assess stable isotope variations within lunar mantle sources. An internally-consistent dataset of oxygen isotope compositions of mare basalts encompasses five types of low-Ti basalts from the Apollo 12 and 15 missions and eight types of high-Ti basalts from the Apollo 11 and 17 missions. High-precision whole-rock dO values (referenced to VSMOW) of low-Ti and high-Ti basalts correlate with major-element compositions (Mg#, TiO2, Al2O3). The observed oxygen isotope variations within low-Ti and high-Ti basalts are consistent with crystal fractionation and match the results of mass-balance models assuming equilibrium crystallization. Whole-rock dFe values (referenced to IRMM-014) of high-Ti and low-Ti basalts range from 0.134& to 0.217& and 0.038& to 0.104&, respectively. Iron isotope compositions of both low-Ti and high-Ti basalts do not correlate with indices of crystal fractionation, possibly owing to smallmineral-melt iron fractionation factors anticipated under lunar reducing conditions. The dO and dFe values of low-Ti and the least differentiated high-Ti mare basalts are negatively correlated, which reflects their different mantle source characteristics (e.g., the presence or absence of ilmenite). The average dFe values of low-Ti basalts (0.073 ± 0.018&, n = 8) and high-Ti basalts (0.191 ± 0.020&, n = 7) may directly record that of their parent mantle sources. Oxygen isotope compositions of mantle sources of low-Ti and high-Ti basalts are calculated using existing models of lunar magma ocean crystallization and mixing, the estimated equilibrium mantle olivine dO value, and equilibrium oxygen-fractionation between olivine and other mineral phases. The differences between the calculated whole-rock dO values for source regions, 5.57& for low-Ti and 5.30& for high-Ti mare basalt mantle source regions, are solely a function of the assumed source mineralogy. The oxygen and iron isotope compositions of lunar upper mantle can be approximated using these mantle source values. The dO and dFe values of the lunar upper mantle are estimated to be 5.5 ± 0.2& (2r) and 0.085 ± 0.040& (2r), respectively. The oxygen isotope composition of lunar upper mantle is identical to the current estimate of Earth’s upper mantle (5.5 ± 0.2&), and the iron isotope composition of the lunar upper mantle overlaps within uncertainty of estimates for the terrestrial upper mantle (0.044 ± 0.030&). 2010 Elsevier Ltd. All rights reserved. 0016-7037/$ see front matter 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2010.08.008 ⇑ Corresponding author. Tel.: +1 865 974 6024; fax: +1 865 974 2368. E-mail address: [email protected] (Y. Liu).