Partial Melting of Ordinary Chondrite Under Reducing Conditions

Details Partial Melting of Ordinary Chondrite Under Reducing Conditions Partial Melting of Ordinary Chondrite Under Reducing Conditions

Dec 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.v41d..02f&link_type=abstract

American Geophysical Union, Fall Meeting 2004, abstract #V41D-02

Other

5455 Origin And Evolution, 3630 Experimental Mineralogy And Petrology, 3662 Meteorites, 3672 Planetary Mineralogy And Petrology (5410), 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008)

Scientific paper

A critical parameter in determining the nature and processes of differentiation of planetary materials in the early solar system is oxygen fugacity. Chondrites record a range of oxygen fugacities from approximately 5 log units below the iron-wustite (Fe-FeO) buffer (enstatite chondrites) to close to QFM (some carbonaceous chondrites). Among the equilibrated chondrites, an "oxidation gap" appears to exist between ordinary chondrites and enstatite chondrites, although several groups of unequilibrated carbonaceous chondrites appear to occupy this "gap". Some primitive achondrites fill this gap (e.g. pallasites, acapulcoites, lodranites, winonaites, and silicate-bearing IAB and IIE irons), although the precursors to these groups are poorly known. In this experimental study, we have determined the modification in mineral compositions during partial melting under reducing conditions and explore the idea that the primitive achondrites may be formed through differentiation under reducing conditions of a more oxidized precursor. Partial melting experiments were conducted on an H6 chondrite (Kernouve) under reducing conditions at 1 atm and at 1.3 GPa pressure in a solid media deformation apparatus. In the 1 atm experiments, fO2 was buffered by gas mixing and sealed silica tube techniques to values determined from thermodynamic calculations of primitive achondrites; in the deformation experiments, aluminum jackets were used. The experiments suggest that partial melting of an oxidized precursor under reducing conditions can produce some of the reduced features observed in primitive achondrites such as magnesian olivine, pyroxene and chromite compositions typical of primitive achondrites at temperatures of 1200-1300 ° C, as well as chalcophilic behavior of previously lithophillic ions (e.g., Cr in sulfide) at temperatures at 1000° C. Some features of primitive achondrites (e.g. oxygen isotopic compositions and Cr/(Cr+Al) ratios of chromites) appear to be intrinsic to the precursor chondrite. Further, mafic silicate and chromite reduction (increased Mg/(Mg+Fe)) required higher temperatures than those inferred for primitive achondrite formation. We suggest that the precurser chondrite for many primitive achondrites could have been somewhat more oxidized and subsequent melting under reducing conditions (e.g. in the presence of graphite) produced the reduction of mafic silicates and chromites in addition to chalcophilic behavior in some elements. Melt migration, solid-melt reactions and removal of key elements (e.g., S, Al) during melting might be enhanced by deformation and/or open system conditions, producing more dramatic changes in the residual solid. Other features however, must have been inherited from the precursor chondrite and therefore do not reflect changes produced during melting under reducing conditions.

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