Physics
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.u21d..06e&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #U21D-06
Physics
5410 Composition (1060, 3672), 5430 Interiors (8147), 5455 Origin And Evolution, 5470 Surface Materials And Properties, 5480 Volcanism (6063, 8148, 8450)
Scientific paper
Early in terrestrial planet evolution energetic impact, radiodecay, and core formation may have created one or more whole or partial silicate mantle magma oceans. The time to mantle solidification and then to clement surface conditions allowing liquid water is highly dependent upon heat flux from the planetary surface through a growing primitive atmosphere. Here we model the time to clement conditions for 500, 1,000, and 2,000 km-deep magma oceans on Earth. Included in our calculations are partitioning of water and carbon dioxide between solidifying mantle cumulate mineral assemblages, evolving liquid compositions, and a growing atmosphere. Magma ocean solidification and subsequent planetary evolution proceeds through three major phases. First, the magma ocean solidifies, partitioning volatiles between solid cumulates, evolving liquids, and a growing primordial atmosphere. In these simplified models the timescale of solidification is limited by the emissivity of the atmosphere. This step produces cumulates with density that non-monotonically increases with radius, and are therefore gravitationally unstable to overturn. In step two, the unstable solidified silicate mantle cumulates overturn to a stable configuration. The overturn process creates a mantle that is gravitationally stable and therefore resistant to the onset of thermal convection, but that is laterally heterogeneous in composition and temperature. Hot cumulates that formed deep in the magma ocean rise to shallower depths during overturn and may melt adiabatically, producing the earliest planetary crusts. The surface of the planet is therefore heated. In step three, the planet conducts heat through its solidified mantle and radiates it to space through the primordial atmosphere formed in step one. We find that small initial volatile contents (0.05 wt% H2O, 0.01 wt% CO2) can produce atmospheres in excess of 100 bars, and that mantle solidification is 99% complete in less than 100,000 years. Subsequent cooling to clement surface conditions occurs in 10 to 50 Ma, not considering impact erosion of the atmosphere. Though the great majority of volatiles are degassed into the atmosphere, a geodynamically significant quantity is sequestered in the solid cumulates, as much as 750 ppm by weight OH in models beginning with 0.5 wt% water, and a minimum of 10 ppm by weight in the driest cumulates of models beginning with just 0.05 wt% water. Even these small water contents significantly lower the melting temperature of mantle materials, facilitating later volcanism.
Elkins-Tanton Linda
Parmentier E.
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