Geochronology and petrogenesis of Apollo 14 very high potassium mare basalts

Details Geochronology and petrogenesis of Apollo 14 very high potassium mare basalts Geochronology and petrogenesis of Apollo 14 very high potassium mare basalts

Sep 1986

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986jgr....91d.214s&link_type=abstract

Journal of Geophysical Research, Volume 91, Issue B4, p. D214-D228

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Scientific paper

Rb-Sr, K-Ar, and Sm-Nd isotopic studies were undertaken for two Apollo 14 very high potassium (VHK) mare basaltic clasts from breccias 14305 and 14168. The samples are distinctly more radiogenic than other lunar mare basalts. Rb-Sr data for the whole rock and mineral separates determine internal isochrons corresponding to essentially identical ages of 3.83+/-0.08 b.y. and 3.82+/-0.12 b.y. for samples 14305 and 14168, respectively, for λ(87Rb)=0.0139 (b.y.)-1. Their corresponding initial 87Sr/86Sr ratios are indistinguishable and are 0.6995+/-0.0005 and 0.6997+/-0.0004, respectively. The 39Ar-40Ar age spectra for the two samples are very similar and give identical plateau ages of 3.85+/-0.02 b.y. for 14305 and 3.85+/-0.05 b.y. for 14168. The identical Rb-Sr and K-Ar isotopic results for the two rocks indicate that they could be derived from the same flow. The Sm-Nd isotopic data of whole rock and mineral separates for the two VHK basalts define an internal isochron age of 3.94+/-0.16 b.y. for λ(147Sm)=0.00654 (b.y.)-1 and an initial 143Nd/144Nd of 0.50673+/-21. The averaged model age, relative to the chondritic initial 143Nd/144Nd of 0.505893 for the basalts is 4.56+/-0.04 b.y. The concordance of Rb-Sr, K-Ar, and Sm-Nd ages strongly suggests that VHK basalts crystallized from a melt ~3.85 b.y. ago. The Rb-Sr and Sm-Nd isotopic data indicate that VHK basalts show very large Rb/Sr fractionation but no significant Sm/Nd fractionation at the time of crystallization. The source material for these basalts had a Rb/Sr ratio similar to those of Apollo 14 high-AL mare basalts and a nearly chondritic Sm/Nd ratio. Extreme enrichments of Rb/Sr and K/La during the formation of VHK basalts can be adequately explained by an assimilation model that involves basalt/granite interaction. K, Rb-rich components of granitic wall rocks in highland crust were selectively introduced into ascending hot high-Al mare basaltic magma upon contact. A similar crustal assimilation process has been frequently observed in terrestrial, mantle-derived magma that ascended through the continental crust.

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