Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p31b1713b&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P31B-1713
Other
[1009] Geochemistry / Geochemical Modeling, [5480] Planetary Sciences: Solid Surface Planets / Volcanism, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
Eruption of silicate lava, whether on Earth or another planet, requires that at some depth the melt has lower density than the surrounding rocks. As the densities of silicate liquids change during crystallization, whether a particular silicate liquid will erupt or be trapped at a level of neutral buoyancy is a complex yet fundamental issue for planetary dynamics. In general, 3 factors drive surface eruptions: inherent buoyancy relative to mantle phases, compositional evolution, and volatile contents. These factors manifest on Earth as terrestrial basalts commonly have compositions close to a density minimum [1]. Recent work has produced estimates of Martian parental magma compositions [2-5] based on shergottite meteorites and from Gusev crater. Using the MELTS algorithm [6] and other density calibrations, we simulated evolution of these liquids, focusing on density changes. For much of the crystallization path, density is controlled by FeO. All of the liquids begin with ρ ~ 2.8 g/cc at 1 bar, and the evolution of liquid density is controlled by the liquidus phases. At low pressures, olivine is the liquidus phase for each melt, and as FeO is not incompatible in olivine, olivine crystallization decreases liquid density, increasing buoyancy with crystallization. However, FeO is incompatible in pyroxene, and thus liquids crystallizing pyroxene become denser and less buoyant with crystallization, producing liquids with densities up to and above 3.0 g/cc. As the olivine-pyroxene saturation relationship is affected by pressure and chemistry, the identity of the liquidus phase and density evolution will vary between magmas. Without spreading centers, Mars has no location where the mantle approaches the surface, and it is likely that any magma which is denser than the crust will stall below or within that crust. The crystallization path of a liquid is a function of pressure, with pyroxene crystallizing first at P > 10 kbar (~80 km depth), close to the base of the Martian crust [7]. If these melts were to stall at this depth, they would begin crystallizing pyroxene and become denser with crystallization, possibly trapping them at the base of the crust. However, as Martian magmas do erupt, some liquids must be able to bypass this crustal filter. One mechanism for driving eruption is magmatic water. Addition of 1 wt.% H2O to a melt reduces the magnitude of the density increase by stabilizing olivine relative to pyroxene (increasing the crossover pressure by 1-2 kb depending on liquid composition) and decreasing the liquid density by 0.08 g/cc. Alternatively, other volatiles, such as chlorine and CO2, have different effects, stabilizing pyroxene relative to olivine, with ~0.8 wt.% Cl causing pyroxene to remain the liquidus phase down to pressures of 7-8 kb [8,9], within the Martian crust. Therefore, melts rich in volatiles other than water may be more likely to stall within the Martian crust leading to cumulate formation, while water rich magmas crystallize olivine and pass through the crust, erupting as the basalts on the surface. [1] Stolper and Walker, 1980. [2] Greshake et al., 2004. [3] Monders et al., 2007. [4] Filiberto et al., 2010. [5] Balta et al., 2011, in prep. [6] Ghiorso and Sack, 1995. [7] Wieczorek and Zuber, 2004 [8] Filiberto and Treiman, 2009 [9] Balta et al., 2011, J. Petrol, in press.
Balta Brian J.
McSween Harry Y.
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