Colloquium: Hanna Nekvasil

"Crustal evolution of planetary bodies: Linking observations and experiments"

Abstract: Martian surface data coupled with petrogenetic studies of meteorites and experimental simulation have provided detailed insight into the evolution of the martian crust over time. These data lay the foundation for a model in which the secondary crust is built by input of both primary and differentiated magmas from the mantle, modified by burial metamorphism from zeolite through granulite facies conditions, and altered by magmatic fluid/wallrock interactions. Over time, local dehydration and dehalogenization of magma source regions result in lower bulk volatile contents of mantle-derived magmas, allowing magmas to ascend to shallower levels before boiling, thereby increasing the likelihood that they exsolve two aqueous phases. Loss of a water-rich, chloride-poor vapor phase can lead to wallrock alteration and the formation of smectites, kaolinite, and chlorite. The highly acidic oxidized aqueous liquid can produce silica-rich fumerolic deposits and hydrothermal jarosite/hematite/goethite assemblages. The high Fe content and relatively low silica contents of differentiated martian magmas rising through a 50-65 km thick crust facilitates this shallow alteration by allowing retention of dissolved chlorine and sulfur to shallow levels, thereby increasing the surface abundance of chloride and sulfate-bearing salts relative to Earth or the Moon.

For the Moon, models of both lunar formation and lunar crust evolution have relied on the low magmatic volatile abundance inferred from Apollo and Luna samples and lunar meteorites. However, new analyses of the volatile contents of lunar apatite have indicated the presence of significant OH in addition to F and Cl. Beyond this, lunar apatites show strong evidence for shallow level magmatic degassing, suggesting even higher source region volatile contents than obtained by simple extrapolation based on crystallization alone. Coupling these data with degassing and phase equilibrium experiments is paving the way for developing a new paradigm for lunar crust formation.

Hanna Nekvasil, Professor
Department of Geosciences, Stonybrook University, New York