Appraising the Late-Stage Magmatic fO2 of Diabasic Angrites with a Novel Phase Equilibrium Approach. Implications for New and Existing Models of Angrite Petrogenesis

1Aaron S. Bell,2Charles Shearer,1Lydia Pinkham,3Anthony J. Irving
Geochimica et Cosmochimica Acta (in Press) Link to Article []
1Department of Geological Sciences, University of Colorado Boulder
2Institute of Meteoritics and Department of Earth and Planetary Sciences, University of New Mexico
3Department of Earth and Space Sciences, Seattle, WA 98195
Copyright Elsevier

One of the enduring problems in angrite petrogenesis is how to reconcile the extraordinary Fe-rich compositions of many angritic liquids – which seemingly require modestly oxidized fO2 conditions that fall outside the stability field of Fe-Ni alloys during primordial melting – with the geochemical and paleomagnetic evidence that the angrite parent body contains a small metallic Fe-Ni-S-C core. One of the major impediments to resolving these contradictions stems from a distinct lack of rigorous fO2 data for the angrite meteorite clan. To begin addressing this issue, we have developed a new approach for calculating magmatic fO2 values directly from the late-stage olivine-ulvöspinel- silicate liquid assemblages that are common in the volcanic or hypabyssal angrites. The adaptation of the olivine-spinel-oxybarometer for use in angrites requires a careful assessment of
of angritic liquids. We apply the results of metal saturated angrite crystallization experiments and an analysis of the Ca-Tschermak’s-anorthite-equilibrium to obtain estimates of thevalues angritic magmas. We then use the
estimates derived from the experiments and thermodynamic analysis in conjunction with the olivine-spinel-
oxybarometer to calculate the magmatic fO2 of Sahara 99555, D’Orbigny, and Northwest Africa 12004. With this approach we estimate oxygen fugacity values that range from IW+0 to IW+0.25. The relatively reducing fO2 values obtained from our analysis are inconsistent with redox conditions required by the oxidized melting hypothesis. Some caution is warranted in extrapolating these late-stage magmatic fO2 values to higher temperatures; however, we stress that fractionated silicate liquids typically become more oxidized with increasing crystallization via auto-oxidation (i.e., the accumulation of incompatible ferric iron in the liquid). Therefore, we suggest that late stage magmatic fO2 values from our calculations may represent the most oxidized conditions along the angrite liquid line of descent. If this interpretation is correct, a large-scale oxidation event on the APB need not be invoked to reconcile mildly reducing conditions (ΔIW-1.35) thought to have prevailed during core formation with the magmatic fO2 record preserved in angritic meteorites. Future redox studies of compositionally primitive angrites (e.g., Northwest Africa 12774) may shed new light on the redox relationships among primitive angrite magmas, evolved angrite magmas, and core forming alloys.


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