Origin of Lunar Very High Potassium (VHK) basalts: A combination of endogenous and exogenous processes

1Sarah E.Roberts,1Clive R.Neal
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.01.023]
1Department of Civil & Env. Eng. & Earth Sciences, University of Notre Dame, Notre Dame, IN 46556, USA
Copyright Elsevier

Very high potassium or VHK basalts have been described from the Apollo 14 landing site at Fra Mauro on the lunar near side. Many of the known samples are clasts in polymict breccias with a few as <1 cm rocklets in the regolith samples. VHK basalts are distinct from the high-Al basalts that are more common at the Apollo 14 site in that they are enriched in the alkali/alkaline earth elements (e.g., K, Rb, Ba, etc.). The source of this enrichment was initially proposed to be through assimilation of granite by a crystallizing high-Al basaltic magma. However, with the discovery of more VHK basalt clasts from three Apollo 14 polymict breccias, the number of different AFC episodes and granite assimilants had to increase in order to explain the whole rock compositional diversity within the VHK basalt suite.
New work completed on fourteen VHK basalts include Crystal Size Distributions (CSDs) and in-situ chemical analyses and element maps collected by electron probe microanalysis (EPMA). CSDs completed on three VHK basalt samples indicate that these basalts are not impact melts and represent endogenous melts of the lunar interior. Potassium Kα element maps reveal the spatial relationship between the K-rich material and the basalt clasts with attached breccia matrix if present. In-situ EPMA analyses have identified two distinct types of K-rich material: K-feldspar and K-rich glass. With these new data, an additional mechanism for an exogenic petrogenesis of VHK basalts is proposed that involves the impact process. VHK basalts can be divided into two groups based on density and viscosity. VHK-1 basalts formed when a hot impact ejecta covered the granite/felsite-rich matrix material containing high-Al basalt clasts. Heat from the impact ejecta partially melted and possibly evaporated nearby K-bearing materials, which then infiltrated and contaminated the high-Al clasts. VHK-2 basalts are consistent with the hypothesis of granite assimilation by a high-Al magma, as seen in the higher abundances of network-forming elements.

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