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

Abstract

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.

Characterization of Mason Gully (H5): The second recovered fall from the Desert Fireball Network

“…As a comparison, using the same calculations, the H6 meteorite Kernouvé gives similar results for the olivine‐chromite thermometer (717 °C, f (O 2 ) = −23.34) and a slightly higher peak temperature using the 2‐pyroxene method (Ca‐transfer) (998 °C, f (O 2 ) = −16.78) (Ford et al. ).…”

“…; Ford et al. ). A suite of elemental X‐ray maps were obtained for both thin sections prepared: MG1, with an area of approximately 150 mm 2 ; and MG2, with an area of approximately 165 mm 2 .…”

“…Elemental X-ray maps, obtained on the entirety of both sections, were one of several methods used to calculate the bulk modal mineralogy of Mason Gully. This was done via pixel counting using the commercial software program Adobe Photoshop Ò and has been used previously (e.g., Yugami et al 1998;Ford et al 2008). A suite of elemental X-ray maps were obtained for both thin sections prepared: MG1, with an area of approximately 150 mm 2 ; and MG2, with an area of approximately 165 mm 2 .…”

Mg‐rich harzburgites from Vesta: Mantle residua or cumulates from planetary differentiation?

“…We interpret this to be a reflection of the low f O 2 , which is indicated by the occurrence of Cr‐bearing Fe metal (e.g., Ford et al. ). Olivine in QUE 93148 has an average Mg# of ~86 and Fe/Mn of ~40, well within the range of the DOM 10 olivines.…”

Advances in determining asteroid chemistries and mineralogies

“…However due to their fragile nature that does not easily allow passage through the atmosphere, CI chondrites are relatively rare in our meteorite collections. Chondritic groups are generally thought to have compositions similar but not exactly like the precursors for the different achondritic (differentiated) meteorites (e.g., Keil, 1989;Ford et al, 2008). The relatively narrow differences in bulk elemental compositions among the chondritic groups (Table 1) are primarily due to enrichments or depletions in refractory and volatile elements (e.g., Brearley and Jones, 1998;Weisberg et al, 2006).…”