HR: 1340h
AN: P13D-1550    [Abstracts]
TI: Petrogenesis of Alkaline Magmas in Gusev Crater, Mars
AU: * Usui, T
EM: tusui@utk.edu
AF: Dept. Earth & Planet. Sci., Univ. of Tennessee, 1412 Circle Drive, Knoxville, TN 37996- 1410, United States
AU: McSween, H Y
EM: mcsween@utk.edu
AF: Dept. Earth & Planet. Sci., Univ. of Tennessee, 1412 Circle Drive, Knoxville, TN 37996- 1410, United States
AB: The Spirit rover has encountered a variety of volcanic rocks in Gusev crater, including picritic basalts and different types of alkaline basalts. Phase equilibrium experiments [1] suggested that the picritic basalts (Adirondack- class) represent batch melts of a primitive Marian mantle compositionally similar to the Dreibus-Wänke (DW)- mantle [2]. Here we demonstrate that the Gusev series of alkaline basalts also represent products by melting of the DW-mantle. Three melt compositions obtained by partial melting experiments of the DW-mantle [3] were used as parental melt compositions; their degrees of melting range from ~10% to ~30%. Liquid lines of descent for these melts are calculated by the MELTS program for both fractional and equilibrium crystallization over a wide range of parameters (P = 1 bar to 5 kbar, H₂O = 0-1 wt%, fO₂ = QFM-3 to QFM). These parameters affect liquidus temperatures and stabilities of minerals, but do not significantly change the compositional trends of liquids when spinel and olivine are the only crystallizing phases (except for Cr). In the spinel+olivine liquidus fields, the MELTS program yields the compositional trends almost identical to the results from the melting experiments [1]. Ten rock samples from three rock classes (Irvine, Backstay and Barnhill) are compared with the MELTS results, because they represent alkaline basalts that do not contain significant amounts of xenocrystic and/or cumulus materials and have experienced relatively low degree of aqueous alteration. Because most of these alkaline basalts are enriched in incompatible elements (e.g. Ti, Na) but have similar MgO contents compared to those of the Adirondack-class, they could not have formed by fractionation of a parental magma similar to the Adirondack- class rocks. Instead, these alkaline basalts can be explained by fractionation of olivine and spinel from the lower- degree melts of the DW-mantle than the Adirondack-class. Thus, all the Gusev basalts (both picritic and alkaline) could have formed by different degrees of melting of the same DW-mantle. Differences in the degree of melting may reflect temporal or spatial thermal structure of adiabatic mantle upwelling beneath Gusev crater. [1] Monders et al. (2007) MPS 42, 131-148. [2] Dreibus and Wänke (1985) Meteoritics 20, 367-382. [3] Bertka and Holloway (1994) CMP 115, 323-338.
DE: 5464 Remote sensing
DE: 5475 Tectonics (8149)
DE: 5480 Volcanism (6063, 8148, 8450)
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting