The Role of Compositional Buoyancy in Planetary Evolution: Energies of accretion and metallic core segregation are sufficient to melt significant portions of large planetary bodies, like the Earth and Mars. Large impacts during accretion would also promote the development of "magma oceans"; and the existence of a magma ocean has been the long favored explanation for fundamental characteristics of the Moon. Solidification of a magma ocean would lead to chemical fractionations and gravitationally unstable compositional stratification with Fe-rich minerals and incompatible elements concentrated in the late crystallizing mantle at the top of this stratigraphy (see Figure 1). The overturn of this unstable mantle stratification would create a compositional stratification with long-lived radioactive elements (U, Th, and K) concentrated in dense silicates at depth.
Fractional solidification of a magma ocean and subsequent overturn explains important characteristics of the early evolution of the Moon (Hess and Parmentier, 1995) and Mars (Elkins-Tanton, et al. 2005) including the creation of an early crust by partial melting during overturn, and isotopic fractionations dating from the earliest evolution. For Mars, overturn also can explain the existence of an early and very strong but short-lived internally generated magnetic field. In the Earth, as well as in other planets which have been sampled, silicate-liquid fractionations of Sm and Nd isotopes that occurred within the first 100 Myr of planetary evolution are a direct indication of such early chemical differentiation that survive to the present day.
A stably stratified planetary interior with heating concentrated at depth would thus be the expected initial condition for planetary evolution on longer geologic time scales. On these time scales, planetary evolution would thus depend on the opposing effects of compositional buoyancy and thermal buoyancy that develops due to radioactive heating. An example of the convective evolution of such an initial stratigraphy is shown in Figure 4.
E.M. Parmentier, J.F. Mustard, B.L. Ehlmann, and L.H. Roach, Deep hydrothermal circulation and implications for the early crustal compositional and thermal evolution of Mars, Lunar and Planetary Science 39, 1544, 2008.
Stanley, S., L. Elkins-Tanton, M. T. Zuber, E. M., Parmentier, Mars' Paleomagnetic Field as the Result of a Single-Hemisphere Dynamo, Science 321, 1822-1825, 10.1126/science.1161119, 2008.
E. M. Parmentier and M. T. Zuber, Early evolution of Mars with mantle compositional stratification or hydrothermal crustal cooling, J. Geophys. Res., 112, E02007, doi:10.1029/2005JE002626, 2007.
E.M. Parmentier, L.T. Elkins Tanton, and P.C. Hess, Melt solid segregation and fractional magma ocean solidification with implications for Mars, Lunar Planet. Sci. 37, 1995-1996 (2006).
L.T. Elkins Tanton, S.E. Zaranek, E.M. Parmentier, and P.C. Hess, Early magnetic field and crust on Mars from magma ocean cumulate overturn, Earth Planet. Sci. Lett., 236, 1-12 (2005).
S.E. Zaranek and E.M. Parmentier, Convective cooling of an initially stably stratified fluid with temperature-dependent viscosity: Implications for the role of solid-state convection in planetary evolution, J. Geophys. Res. 109, B03409, doi:10.1029/2003JB002462 (2004).
L.T. Elkins Tanton, E.M. Parmentier, and P.C. Hess, Magma ocean fractional crystallization and cumulate overturn in terrestrial planets: implications for Mars, Meteoritics and Planetary Science 38, 1753-1771 (2003).
K.M. Alley and E.M. Parmentier, Thermal convection in an initially stratified fluid heated from below: application the early evolution of planets and particularly the Moon, Phys. Earth Planet. Int., 108, 15-32 (1998).
P.C. Hess and E.M. Parmentier, A model for the thermal and chemical evolution of the Moon's interior: implications for the onset of mare volcanism, Earth Planet. Sci. Lett. 134, 501-514 (1995).
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Figure 1: Schematic of a solidifying magma ocean. Because the solidus is steeper than an adiabat in the convecting magma ocean, solidification occurs from the bottom up. A simple model assumes that composition remains well-mixed in predominantly liquid regions. Double-click on the image to view a full-size PDF.
Figure 2: Density stratifications before and after overturn of a solidified magma ocean.
Figure 3: Comparison of overturn of monotonic (top in each panel) and non-monotonic (bottom in each panel) density distributions. Density and initial depth, a proxy for composition, are shown in the top and bottom panels, respectively, in each case.
Figure 4: Example of the behavior of a stably stratified fluid heated from below at three times from a numerical experiment (from Alley and Parmentier, 1998). Colors represent compositional density with high values in red. Isotherms and velocities are shown as contours and vectors, respectively. Buoyant material with density reduced due to thermal expansion rises into cooler surroundings, cools becoming denser, and sinks.Back to Marc Parmentier's Brown Research Profile