Petrogenesis of primitive and evolved basalts in a cooling Moon: Experimental constraints from the youngest known lunar magmas

摘要

We have conducted high-temperature experiments over a range of pressures to constrain the petrogenesis of the youngest sampled lunar magmas, which have contrasting primitive and evolved compositions. Our results indicate that at similar to 3 Ga, melting still occurred within the same mantle depth range that produced crystalline mare basalts for the previous similar to 1 Ga, although our data cannot support or confirm that the shallowest extents of melting moved deeper into the mantle by similar to 3 Ga, as is predicted by most thermal evolution models. Furthermore, melting still occurred in regions with low abundances of heat-producing elements. Basaltic lunar meteorite NEA 003A has some of the lowest abundances of incompatible trace elements among all mare basalts and no negative Eu anomaly. Our experiments show that NEA 003A is multiply saturated with olivine and low-Ca pyroxene on its liquidus at similar to 1.1 GPa (similar to 215 km) and similar to 1330 degrees C. If the primitive NEA 003A liquid composition is a minimally-modified melt, the relatively low Mg# of its source region (73-75), its lack of a Eu anomaly, and its chondritic initial Nd isotopic composition indicate its source region likely escaped mixing during mantle overturn with later-stage magma ocean cumulates that formed after plagioclase saturation. This condition would require the sources of the ultramafic glasses to have experienced cumulate mixing, or for assimilation of later-stage magma ocean cumulates by the ultramafic glass parental magmas before eruption in order to account for their higher Mg4t's and deeper negative Eu anomalies. Alternatively, NEA 003A may have undergone some fractional crystallization, in which case its more primitive source region would be deeper than 215 km and may approach the depth range of the ultramafic glass source regions. Iron- and incompatible trace element-rich basaltic lunar meteorites LAP 02205, NWA 032/479, and NWA 4734 have nearly identical bulk compositions and have a multiple saturation point on their high pressure liquidus at <0.6 GPa (<105 km) and 1150-1200 degrees C. The P-T conditions of this multiple saturation point are inconsistent with the expectation from thermal modeling of a thick, cold lunar elastic lithosphere at 3 Ga, and likely indicate these melts underwent fractional crystallization before eruption, thus increasing their abundances of incompatible trace elements. However, these LREE-enriched basalts possess a very deep negative Eu-anomaly but do not show other chemical fingerprints of KREEP, indicating that a component of Fe-rich, low-Ti late-stage magma ocean cumulates are required in their hybridized source region. (C) 2015 Elsevier B.V. All rights reserved.