Chromite composition in relation to chemistry and texture of ordinary chondrites

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.

Section: Starting Materialssupporting

confidence: 55%

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

Kessel¹,

Beckett²,

Huss³

et al. 2004

“…Schmitz et al (2001) analyzed chromites in 33 chondrite falls and finds and showed by comparison that the chromites in the fossil meteorites most probably are from L, or possibly LL, chondrites. Moreover, Schmitz et al (2001) and Wlotzka (2005), following Bunch et al (1967) and Ramdohr (1967), demonstrated that chromite compositions can be used to distinguish between chondrite groups and to reveal information on the heating and cooling histories of their parent bodies. Chromite textures and chemistry are thus of particular importance in identifying the components in the flux of meteorite material preserved within the fossil record.…”

Section: Introductionmentioning

confidence: 94%

Petrographic classification of Middle Ordovician fossil meteorites from Sweden

Bridges

Schmitz

Hutchison

et al. 2007

Abstract-The maximum diameter of chromite (FeCr 2 O 4 ) grains within L chondrites reflects the petrographic type of the sample. On the basis of our measurements of nine recent L chondrites, L3 chromite D max = 34-50 μm, L4 = 87-150 μm, L5 = 76-158 μm, and L6 = 253-638 μm. This variation reflects the crystallization of the chromite grains during parent body thermal metamorphism.We use this calibration to classify six fossil meteorites from the Middle Ordovician in Sweden as type 3 (or 4) to 6. The high flux of L chondrites at 470 Ma contained a range of petrographic types and may have had a higher proportion of lower petrographic type meteorites than are found in recent L chondrite falls. The fossil meteorites have in places preserved recognizable chondrule textures, including porphyritic olivine, barred olivine, and radiating pyroxene. A large relict clast and fusion crust have also been tentatively identified in one fossil meteorite. Apart from chromite, all of the original meteorite minerals have been replaced by carbonate (and sheet silicate and sulfate) during diagenesis within the limestone host. The preservation of chondrule definition has allowed us to measure the mean diameters of relict chondrules. The range (0.4-0.6 mm) is consistent with measurements made in the same way on recent L chondrites.

“…For the formation of primitive achondrites, the main feature that is inconsistent with the partial melting of an oxidized precursor under reducing conditions is chromite composition, specifically the trivalent cation ratio. In ordinary chondrites, chromite Cr/Cr + Al ranges from 1.0 in type 3 chondrites to 0.85 in type 6 chondrites like Kernouvé (Bunch et al 1967). Chromite compositions in winonaites and IAB irons have Cr/Cr + Al that ranges from 0.95 to 1.0, while Fe/ Fe + Mg ranges from 0.3 to 0.6 (Bunch et al 1970;Benedix et al 2005).…”

Section: Implications For Primitive Achondritesmentioning

confidence: 99%

Partial melting of H6 ordinary chondrite Kernouvé: Constraints on the effects of reducing conditions on oxidized compositions

Ford

Benedix

McCoy

et al. 2008

Abstract-Partial melting experiments at temperatures of 950-1300 °C were conducted on the H6 chondrite Kernouvé under reducing conditions using CO-CO 2 gas mixing and graphite-buffered sealed silica tubes to examine the effect of reducing conditions during melting of starting materials that are more oxidized relative to the oxygen fugacity conditions of the experiments. The experiments produced a range of mineralogical and compositional changes. Olivine exhibits significant reduction to compositions of Fa 2-5 at temperatures of 1300 °C. In contrast, orthopyroxene exhibits only slight reduction until the highest temperatures. Chromite is sometimes consumed by intruding sulfides, and displays increasingly magnesian compositions ranging as low as Fe/Fe + Mg of 0.1 at a constant Cr/Cr + Al ratio. The compositional changes with increasing temperature reflect a complex set of reactions, including oxidation-reduction. One application of these experiments address whether primitive achondrites could have formed from ordinary chondrite-like precursors by partial melting under reducing conditions. While changes observed in olivine and troilite compositions might support such an idea, differences in oxygen isotopic composition, Cr/Cr + Al in chromite, orthopyroxene compositions, and thermodynamic evidence against reduction during melting of primitive achondrites (Benedix et al. 2005) firmly refute such an idea.