Oxygen isotope constraints on the origin and differentiation of the Moon

Spicuzza, MJ, Day JMD, Taylor LA, Valley JW.  2007.  Oxygen isotope constraints on the origin and differentiation of the Moon. Earth and Planetary Science Letters. 253:254-265.

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geochemistry, giant impact, LaPaz, laser fluorination, lunar magma ocean, lunar mare basalt, mantle, mare basalts, meteorites, moon, oxygen isotopes, petrogenesis, petrology, ratios, solar-system, terrestrial planets, TFL


We report new high-precision laser fluorination three-isotope oxygen data for lunar materials. Terrestrial silicates with a range of delta O-18 values (-0.5 to 22.9 parts per thousand) were analyzed to independently determine the slope of the terrestrial fractionation line (TFL; lambda = 0.5259 +/- 0.0008; 95% confidence level). This new TFL determination allows direct comparison of lunar oxygen isotope systematics with those of Earth. Values of Delta O-17 for Apollo 12, 15, and 17 basalts and Luna 24 soil samples average 0.01 parts per thousand and are indistinguishable from the TFL. The delta O-18 values of high- and low-Ti lunar basalts are distinct. Average whole-rock delta O-18 values for low-Ti lunar basalts from the Apollo 12 (5.72 +/- 0.06 parts per thousand) and Apollo 15 landing sites (5.65 +/- 0.12 parts per thousand) are identical within error and are markedly higher than Apollo 17 high-Ti basalts (5.46 +/- 0.11 parts per thousand). Evolved low-Ti LaPaz mare-basalt meteorite delta O-18 values (5.67 +/- 0.05 parts per thousand) are in close agreement with more primitive low-Ti Apollo 12 and 15 mare basalts. Modeling of lunar mare-basalt source composition indicates that the high- and low-Ti mare-basalt mantle reservoirs were in oxygen isotope equilibrium and that variations in delta O-18 do not result from fractional crystallization. Instead, these differences are consistent with mineralogically heterogeneous mantle sources for mare basalts, and with lunar magma ocean differentiation models that result in a thick feldspathic crust, an olivine-pyroxene-rich mantle, and late-stage ilmenite-rich zones that were convectively mixed into deeper portions of the lunar mantle. Higher average delta O-18 (WR) values of low-Ti basalts compared to terrestrial mid ocean ridge basalts (Delta=0.18 parts per thousand) suggest a possible oxygen isotopic difference between the terrestrial and lunar mantles. However, calculations of the delta O-18 of lunar mantle olivine in this study are only 0.05 parts per thousand higher than terrestrial mantle olivine. These observations may have important implications for understanding the formation of the Earth-Moon system. (c) 2006 Elsevier B.V. All rights reserved.