Petrogenesis of Apollo 14 high-Al basalts: Constraints of Rb-Sr isotope and trace element data

Details Petrogenesis of Apollo 14 high-Al basalts: Constraints of Rb-Sr isotope and trace element data Petrogenesis of Apollo 14 high-Al basalts: Constraints of Rb-Sr isotope and trace element data

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p11a1593h&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P11A-1593

Mathematics

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[1060] Geochemistry / Planetary Geochemistry, [3640] Mineralogy And Petrology / Igneous Petrology, [6250] Planetary Sciences: Solar System Objects / Moon, [8450] Volcanology / Planetary Volcanism

Scientific paper

Apollo 14 high-Al basalts, the oldest volcanic deposits returned from the Moon, can provide geologic records of the Moon between the formation of lunar crust (~4.4 Ga) and the main basin-filling mare volcanism (< 3.85 Ga). Various models have been proposed for the petrogenesis of the Apollo 14 high-Al basalts [e.g., 1,2]. Whole-rock incompatible trace element ratios define at least 3 compositional groups, and Rb-Sr radiometric age data show that at least three eruption episodes for the Apollo 14 high-Al basalts, each derived from different source regions at discrete times [2,3]. However, the literature Rb-Sr ages were determined by different labs over four decades. Such data may not be comparable due to different 87Rb decay constants and isochron iteration methods used. We have mined literature Rb-Sr isotopic data of Apollo 14 high-Al basalts [3-9] and re-processed them with a consistent 87Rb decay constant [10] using Isoplot 3.70 [11]. Four eruption episodes of Apollo 14 high-Al basalts were confirmed: 4.24±0.10 Ga, 4.03±0.03 Ga, 3.97±0.07 Ga and 3.92±0.03 Ga. Both Rb-Sr isotopic compositions and incompatible trace element ratios of bulk samples show that parental melts of Apollo 14 high-Al basalts formed through mixing of KREEP and other melted mantle cumulate(s). Melt evolution of Apollo 14 high-Al basalts has been investigated by comparing the equilibrium melt compositions (calculated from plagioclase compositions using relevant partition coefficients) to the fractional crystallization (FC) and assimilation and fractional crystallization (AFC) models. Trace-element petrogenetic modeling of high-Al basalts shows that the equilibrium melt compositions do not fall on a single AFC or FC trajectory. This could be indicative of fluctuating degrees of assimilation (i.e., variable r-values) and/or variable assimilant compositions during petrogenesis. The compositional micro-heterogeneity in Apollo 14 high-Al basalts may partially result from the following volcanic scenario. The melts crystallized in lava flows that had open channels connecting the vents to the flow fronts after their emplacement on the lunar surface from the source regions. During lava flowing, the lava crust that was assimilated with KREEP/granite-rich loose regolith continually created and destroyed by entrainment into the lava core. Thus, the assimilant changed in terms of both composition (KREEP and/or granite) and mass (r-value) throughout magma crystallization.

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