Isotopic and REE studies of lunar basalt 12038: implications for petrogenesis of aluminous mare basalts

Abstract

We report Sr, Nd, and Sm isotopic studies of lunar basalt 12038, one of the so-called aluminous mare basalts. A precise internal Rb-Sr isochron yields a crystallization age of 3.35±0.09 AE and initial 87Sr/ 86Sr=0.69922∓2 (2σ error limits, 1AE=10 9 years, λ( 87Rb)=0.0139AE −1 ). An internal Sm-Nd isochron yields an age of 3.28±0.23AE and initial 143Nd/ 144Nd=0.50764∓28. Present-day 143Nd/ 144Nd is less than the “chondritic” value, i.e. ɛ(Nd, 0)=−2.3±0.4 where ɛ(Nd) is the deviation of 143Nd/ 144Nd from chondritic evolution, expressed as parts in 10 4. At the time of crystallization ɛ(Nd, 3.2AE)=1.5±0.6. We have successfully modeled the evolution of the Sr and Nd isotopic compositions and the REE abundances within the framework of our earlier model for Apollo 12 olivine-pigeonite and ilmenite basalts. The isotopic and trace element features of 12038 can be modeled as produced by partial melting of a cumulate mantle source which crystallized from a lunar magma ocean with a chondrite-normalized REE pattern of constant negative slope. Chondrite-normalized La/Yb=2.2 for this hypothetical magma ocean pattern. A plot of I(Sr) versus ɛ(Nd) for the Apollo 12 basalts clearly shows the influence of varying proportions of olivine, clinopyroxene, orthopyroxene, and plagioclase in the basalt source regions. A small percentage of plagioclase (∼5%) in the 12038 source apparently is responsible for low I(Sr) and ɛ(Nd) in this basalt. Aluminous mare basalts from Mare Crisium (Luna 24) and by inference Mare Fecunditatis (Luna 16) occupy locations on the I(Sr)-ɛ(Nd) plot similar to that of 12038, implying that some basalts from three widely separated lunar regions came from plagioclase-bearing source regions. A summary of model calculations for mare basalts shows a record of lunar mantle solidification during the period when REE abundances in the lunar magma ocean increased from ∼20× chondritic to >100× chondritic. Although there is a general trend from olivine to clinopyroxene-dominated source regions with progressive magma ocean evolution, significant mineralogical heterogeneities in mantle composition apparently formed at any given stage of evolution, as evidenced in particular by the three Apollo 12 magma types.