Experimental petrology and petrogenesis of mare volcanics

摘要: Mare volcanics consist of basalts and picritic pyroclastic glasses spanning a wide range TiO2 concentration. The more primitive low-Ti have olivine alone on their lowpressure liquidi. Most the chemical variation among is result fractionation in series parental MgO-rich liquids differing With one possible exception (Apollo 17 VLT) none compositions suitable parent for any observed basalts. high-Ti magmas fractionated along low-pressure + armalcolite/ilmenite cotectic. All liquidus, but Volcanics with intermediate concentrations (5 to 10 wt%) are widespread maria, even though they not well represented sample collections; however, there no evidence either samples or from remote sensing studies > 13 wt% that would be expected as differentiates highest concentrations. Controlled-cooling-rate crystallization variety mare provided basis reconstructing size and, some cases, stratigraphy flows. Groundmass textures, crystal size, morphology, nucleation density, zoning patterns all been employed quantify cooling histories A single-stage linear rate may produce porphrytic texture. Rapid also cause plagioclase crystallize after mineral it precedes during slow cooling. Mare highly reduced. Mineral assemblages intrinsic oxygen fugacity measurements indicate ƒO2 below wustite-iron buffer at near iron metal saturation. Accordingly, experiments run high-purity capsules gained lost little iron. undersaturated respect sulphur, so reduction through sulphur volatilization cannot invoked explain presence phenocrysts. low oxidation state most likely melting reduced interior under fluid-absent conditions. progressive Cr3+ → Cr2+ Ti4+ Ti3+ lunar melts permits elimination Fe3+ present iron-saturation simple systems. Crystal-liquid partition coefficients determined used calculations major trace element evolution. coefficient Fe-Mg exchange between liquid apparently varies concentration particularly useful assessing whether fine-grained rocks excess olivine. Nb Ti relative adjacent REE incompatibility diagrams. These excesses positive anomalies consistent ilmenite accumulation light measured imply continuous accumulated source regions. Pressures multiple saturation (olivine pyroxene ± Cr-rich spinel ilmenite) 5 12.5 kbar 18 25 glasses. Low-Ca only liquidus glasses; augite commonly high-pressure liquidi basalts: High-Ti picrites subliquidus region pressures where phase, orthopyroxene phase Because steep depth/pressure gradient outer portion Moon (20 km/kbar), these imply: 1. (a) great depths within Moon. 2. (b) means transporting hundreds kilometers surface without significant modification, if both were left residuum. Modeling elements polybaric partial suggests reproduce composition signature (green) by accumulating small degrees melt extracted an upwelling region. In such case, differentiated must begin 1000 km depth (40 kbar) cease ~ 100 kbar). If this model correct, then experimental determination pressure gives average melting: onset higher actual segregation mantle lower pressure.