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 TiO2
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 Si02
• 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