Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p42a..03m&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P42A-03
Physics
3630 Experimental Mineralogy And Petrology, 3640 Igneous Petrology, 5410 Composition (1060, 3672), 5430 Interiors (8147), 5455 Origin And Evolution
Scientific paper
The Martian meteorites, rocks measured by the Mars Exploration Rovers (MER) and lithologies detected by orbital assets represent a diversity of igneous rocks that collectively provide insight into the formation and evolution of Mars. Experimental studies aimed at reproducing the observed igneous lithologies have met with varying degrees of success [e.g., 1,2,3], No study has yet been able to reproduce both Martian meteorite parent magmas and the basalts measured by MER at Gusev Crater [e.g., 1,3]. We attempted a different approach to successfully reproducing Martian igneous lithologies by using geophysical constraints on Martian bulk Fe (wt.%), Fe/Si and mantle Mg# [4,5] to identify mixtures of chondrite compositions that formed plausible Martian mantle compositions. We identified two candidate chondrite mixtures for Mars, CM+L and H+L. We synthesized the CM+L and H+L compositions from oxide, carbonate and phosphate powders and fixed them at an oxygen fugacity below the magnetite-wüstite buffer (MW-1). We conducted experiments at 2 GPa (corresponding to ~150 km in the Martian mantle) between 1300-1600 °C for 4-48 hours in the end-loaded piston cylinder apparatus at the Geophysical Laboratory. Thusfar, we have also conducted experiments at 4 GPa (corresponding to ~320 km in the Martian mantle) between 1425-1475 °C for 210-240 minutes in a Walker-type multi-anvil apparatus at the Geophysical Laboratory. We utilized an 18/11 (octahedron edge length/truncated edge length, in mm) assembly. In both assembly types, the sample was contained within a graphite capsule welded into a Pt tube. We analyzed the experiment products in electron probes at either the Geophysical Laboratory or Arizona State University. Fe and Mg contents of olivine, orthopyroxene and melt were used to assess the attainment of equilibrium for each run product. No significant difference exists between the CM+L and H+L experiment products. The near-solidus phase assemblage of the 2-GPa experiments is ol+opx+cpx. Melts at 2 GPa have MgO, FeO, and Mg# values that either overlap those of Martian meteorite parent melts or are capable of reproducing Martian meteorite parent melt compositions through low-pressure olivine fractionation. The 2- GPa melts do not, however, have CaO/Al2O3 values that intersect those of the Martian meteorite parent magmas. This finding mirrors the inability of previous studies [e.g., 1] to form the Martian meteorites. However, the 2-GPa products can lead to Gusev-like basalts via a two-step process. 20-25% melting yields basalt compositions from which subsequent low pressure olivine fractionation leads to basalts with MgO, FeO, CaO and Al2O3 contents and Mg# and CaO/Al2O3 values like those of the Gusev basalts. The near-solidus phase assemblage of the 4-GPa experiments is ol+opx+cpx+garnet. The melt composition resulting from ~20% melting of the CM+L mantle composition has MgO, FeO, CaO and Al2O3 contents and Mg# and CaO/Al2O3 values that fall among Martian meteorite parent magma compositions. Thus, the geophysically-constrained mantle compositions are capable of producing melts with Gusev and Martian meteorite parent magma affinities by simply shifting the pressure of melting. [1] Bertka C.M. and Holloway J.R. (1994) CMP 115, 313-322. [2] Agee C.B. and Draper D.S. (2005) LPSC XXXVI, #1434. [3] Monders A. et al. (2007) MaPS, 42, 131-148. [4] Bertka C.M. and Fei Y. (1998) Science, 281, 1838-1840. [5] Bertka C.M. and Fei Y. (1998) EPSL, 157:79-88.
Bertka Constance M.
Fei Yingwei
Minitti Michelle E.
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