Chromite symplectites in Mg-suite troctolite 76535 as evidence for infiltration metasomatism of a lunar layered intrusion

摘要: Abstract Despite the very low chromium concentrations in its cumulus olivine (∼140 ppm), lunar troctolite 76535 contains large amounts of Cr sporadically, but highly concentrated, symplectite assemblages consisting Mg–Al-chromite and two pyroxenes. Previously proposed formation mechanisms include crystallization trapped interstitial melt, diffusion from olivine, and/or remobilization chromite grains. These would imply that Cr-depleted nature Mg-suite parental magmas their source materials inferred may be illusory. We have conducted a detailed petrologic textural study symplectites, as well veins, intercumulus assemblages, olivine-hosted melt inclusions clinopyroxene-troilite veins with goals constraining origin degree Cr-depletion relative to other basalts. Orthopyroxene clinopyroxene are depleted counterparts, averaging 900 1200 ppm vs. 7400 8100 ppm 2 O 3 , respectively. Olivine contact exhibit profile going into whereas boundaries away symplectites show no profile. There is also distinct lack primary phases inclusions. Multiple observations, inclusion chemistry, modeling chromite–olivine equilibrium rule out previously mechanisms, strongly suggest was not product magma. Accordingly, post-cumulus addition Fe required produce symplectites. After considering multiple models, via infiltration metasomatism by an exogenous chromite-saturated model most consistent geochemical observations. Failure models call upon grains observed many lithologies feature materials. This substantial depletion magma mare basalt still requires satisfactory explanation order petrogenetic currently accepted bulk-Moon compositions. Additionally, if intimate interaction migrating melts early crustal widespread phenomenon after LMO solidification, it provides another mechanism which reset or delay closure radiogenic isotopic systems explain Mg-suite-ferroan anorthosite age overlap.