Highly Silicic Compositions on the Moon Glotch et al. Presented by Mark Popinchalk

The Moon! Red spots – Deep UV w/ respect to NIR, low FeO and TiO 2 concentrations Feldspathic magmas with high silica contents UV/Visible/NIR/GRS directly sensitive to silicic volcanism Most silicic areas Anorthite

Diviner Lunar Radiometer Experiment Lunar Reconnaissance Orbiter 7.8, 8.25, 8.55 µm Compared Mid IR spectra shapes; Red Spots vs surrounding mare and highlands Data covers all Red Spots

Christiansen Function Directly sensitive to silicate mineralogy – Bulk Si0 2 Convoluted with Laboratory Spectra : Concavity, curvature, strong positive slope – Show silicate content I – slope between 3,4 c – concavity from 3-5

Most- silicic strong concave up Concave down, positive I is a mix

Mare and Highlands, negative concave, negative slope CF – Mare = 8.33 – Highlands = 8.19

Concavity Index on Lunar Orbiter IV image More Red, more Silicic

Silicon, or Silicoff? Red Spots with CF shorter than anorthite, positive I and c – Quartz, Si rich glass, alkali feldspars Hansteen Alpha, Lassel Massif, Gruithusien Domes, rim/ejecta Aristarchus Crater – High Si, evolved lithologies Helmet Feature, Montes Riphaeus indistinguishable

Theories Represent both Extrusive and Intrusive igneous processes. Gruithuisen Domes/Hansteen Alpha – Previously proposed silicic volcanic constructs Diviner consistent with extrusive volcanic process Aristarchus/Lassell appear to be silicic lithologies at depth – Craters revealing plutonic or pyroclastic deposits

A), D) Impacts reveal silicic below B), C) Volcanic formation D) Nothing

Extrusive Silicic Volcanism Gruithuisen Domes/Hansteen Alpha Occurred Late Imbrian epoch – Before mare volcanism Crystals in Apollo Rocks – Crystallization ages that span 500 million years Silicate Immiscibility? Granitic Plutons – Slow crystalization late stage magma Prevent large extrusive features -> revealed

Basaltic Underplating Hot basaltic magma intrudes into lunar crust – Melting of crust, generate silicic magmas – Buoyant, rise as rhyolite plumes Thin curst, heat producing elements, basalitic magmatisim in PKT

Conclusions 4 distinctly different geologies. Gruithusien Domes/Hansteen Alpha – Extrusive silicic volancism Aristarchus/Lassell – Impact craters reveal silicic plutons at depth Multiple igneous processes over time – With silicic results