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