HR: 0800h
AN: T41D-1249    [Abstracts]
TI: The High P-T Densities of Melt and Residual Mantle in the Archean and Implications for Models of Komatiite Genesis
AU: * Ghosh, A
EM: ayati@tamu.edu
AF: Texas A&M University, Dept. of Geology and Geophysics, College Station, TX 77843-3115 United States
AU: Sparks, D W
EM: sparks@seaver.tamu.edu
AF: Texas A&M University, Dept. of Geology and Geophysics, College Station, TX 77843-3115 United States
AU: Cheadle, M
EM: cheadle@uwyo.edu
AF: University of Wyoming, Dept. of Geology and Geophysics, Laramie, WY 82071-3006 United States
AB: An open question concerning the Archean Earth is the origin of ultramafic komatiites with MgO $>$ 25 wt.% and melting extents $>$ 35%. One endmember model that has been proposed is anhydrous decompression melting of an ultra-hot mantle plume. A key physical constraint for this model is whether the deep melts are less dense than the surrounding mantle, so that they can rise buoyantly out of the mantle. In this study, we use the results of high pressure melting experiments to predict the compositions and densities of both melt and residual mantle in a self-consistent decompression melting column, to determine the cross-over depth at which melt becomes buoyant. We calculate a model 1-D adiabat and the composition of incremental melts formed at each depth (Sparks and Cheadle, GCA, 2002). The parameterized melt composition is based on the melting experiments of Herzberg and Zhang (JGR, 1996). A typical model adiabat that generates a good match to an Archean komatiite has a potential temperature of 1875\deg C beneath a 90 km thick lithosphere. The uniqueness of this model is its ability to yield melt compositions which are variable and cumulative over the height of the melting column. This model assumes fractional melting where the partial melt is instantaneously separated from the rock matrix as each increment of melt is generated. The measured densities of the Komatiitic composition of Courtial et al., (GCA, 1997) were adjusted for the temperatures on our typical model adiabat, using reported values and uncertainties in molar volume, thermal expansion coefficient, isothermal compression coefficient and its pressure derivative. However, the initial near-solidus melt compositions that we predict at a depth are not the same as the composition of erupted komatiite. Our preliminary estimate, based on partial molar volumes for the individual major element oxides indicate that these deep melts are 0.5-1.0% less dense than what a typical erupted komatiite would be at 12 GPa. The modal composition of the rock can be determined from the bulk chemistry assuming simple elemental partitioning among the four most abundant mantle phases, and appropriate phase changes, particularly the conversion to majorite. We will also calculate a range of possible densities for the residual mantle, assuming a range of thermodynamic parameters and extent of phase changes.
DE: 9619 Precambrian
DE: 8125 Evolution of the Earth
DE: 8145 Physics of magma and magma bodies
DE: 3640 Igneous petrology
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting