The activity of chromite in multicomponent spinels: Implications for T‐fO₂ conditions of equilibrated H chondrites

Abstract

Abstract-Activities of chromite in multicomponent spinels with compositions similar to those of H chondrites were experimentally determined by equilibrating Pt-alloys with spinel at known temperature and fO 2 . Our results are consistent with predictions based on the spinel solid solution model incorporated into the MELTS program. Therefore, we combined literature formulations for the activities of components in spinel, the ferromagnesian silicates, and alloys with measured and literature (bulk alloy) compositions of the meteoritic phases to constrain T-fO 2 conditions for the Hgroup chondrites Avanhandava (H4), Allegan (H5), and Guareña (H6).Log 10 fO 2 values based on the assemblage of olivine + orthopyroxene + metal are 2.19-2.56 log units below the iron-wüstite (IW) buffer for any equilibration temperature between 740 and 990 °C, regardless of petrographic type. Only lower limits on fO 2 could be determined from spinel + metal equilibria because of the extremely low concentrations of Cr in the alloys of equilibrated H chondrites (≤3 ppb). Log 10 fO 2 values required by spinel + metal equilibria are inconsistent with those for olivine + orthopyroxene + metal if equilibration temperatures were at or above those inferred from olivine-spinel thermometry. This probably indicates that the closure for spinel + metal equilibria occurred under retrograde conditions at temperatures below ~625 °C for Allegan and Guareña and below ~660 °C for Avanhandava.

Petrogenesis of acapulcoites and lodranites: A shock-melting model

“…Benedix and Lauretta (2006) calculated an oxygen fugacity for acapulcoites of $2.3 log units below the ironwü stite (IW) buffer. This is similar to that of equilibrated H chondrites, i.e., 2.10 to 2.56 log units below the IW buffer (Kessel et al, 2004), although these values are appreciably lower than that of OC determined earlier by Brett and Sato (1984), i.e., 0.4 to 1.1 log units below IW.…”

The thermal history of equilibrated ordinary chondrites and the relationship between textural maturity and temperature

“…The general petrographic features of individual thin sections were consistent with the nominal classification of the meteorites according to the scheme of Van Schmus and Wood (1967). The spinel grains selected for analysis were always in contact with metal in order to facilitate work on redox conditions (Kessel et al, 2004). Olivine was invariably in contact with the spinel, consistent with its high modal abundance (Dodd, 1981).…”

“…All of the samples are falls except for Greenwell Springs, which is a fresh find, and except for Siena (Kurat et al, 1969) and St.-Séverin, all of the samples are unbrecciated. Six of the thin sections (three Hs and three LLs including St.-Séverin) are the same specimens used by Kessel et al (2004), and, according to H.Y. McSween (personal communication, 2001), by Gastineau-Lyons et al (2002) for studying variations in phase chemistry and modes as a function of metamorphic type and redox conditions during metamorphism.…”

“…We also considered only spinel analyses with apparent Fe 2 O 3 contents less than 3 wt% based on an assumed stoichiometry of three cations and four oxygens. This upper limit corresponds to a mole fraction of magnetite of $0.032 (more than three times the highest magnetite mole fraction in Table 2d) and is reasonable given that equilibrated ordinary chondrites were metamorphosed under reducing, metal-saturated conditions (e.g., Mueller, 1964;Williams, 1971;Brett and Sato, 1984;McSween and Labotka, 1993;Kessel et al, 2004). Note that our approach to the literature analyses is potentially problematic in that in contrast to our measurements, the analyses of olivines and spinels were not in general from adjacent grains or even from the same thin section.…”

Cr spinel and chromite as petrogenetic indicators in ordinary chondrites: Equilibration temperatures of petrologic types 3.7 to 6

“…As shown above, however, the use of chromites gives lower values than those derived from Cr spinel, and not the real equilibration temperature. The Sack and Ghiorso (1991a) thermometer was also applied by Kessel et al (2002Kessel et al ( , 2004 to chromite analyses of H and L chondrites. They got also similar equilibration temperatures for type H4, 5 and 6, 680 to 770 °C, average 724 °C, in agreement with the results of this work.…”