Séminaire - The ungrouped chondrite El Médano 301 and its comparison with other reduced chondrites - Hamed Pourkhorsandi (Site Buffon)

IMPMC, Salle des élèves, 2^e étage, 61 rue Buffon, Paris 5^e

Vendredi4 mai 2018 à 14 h 30

Hamed POURKHORSANDI - Département des Sciences de la Terre et de l’Environnement, Université Libre de Bruxelles)

Abstract

El Médano 301 (EM 301) is an ungrouped chondrite with overall texture and trace element distribution similar to those of ordinary chondrites (OCs), but with silicate (olivine and low-Ca pyroxene) compositions more reduced than those in OCs, with average olivine and low-Ca pyroxene of Fa_3.9±0.3 and Fs_12.8±4.9, respectively. These values are far lower than the values for OCs and even for chondrites designed as “reduced” chondrites. Low-Ca pyroxene is the dominant mineral phase and shows zoning with higher MgO contents along the crystal rims and cracks (reverse zoning). The Co content of kamacite is also much lower than the concentrations observed in OCs (below detection limit of 0.18 wt% versus 0.44–37 wt%). Oxygen isotopic composition is Δ¹⁷O = +0.79,+0.78 ‰ and slightly different from that of OCs. The lower modal olivine/pyroxene ratio, different Infrared (IR) spectra, lower Co content of kamacite, lower mean FeO contents of olivine and pyroxene, different kamacite texture, and different oxygen-isotopic composition show that EM 301 does not belong to a known OC group. EM 301 shows similarities with chondritic clasts in Cumberland Falls aubrite, and with Northwest Africa 7135 (NWA 7135) and Acfer 370 ungrouped chondrites. However, dissimilar to NWA 7135 and the clasts, it does not contain highly reduced mineral phases like daubréelite.

Our observations suggest the formation of EM 301 in a nebular region compositionally similar to OCs but with a different redox state, with oxygen fugacity (ƒO₂) in this region lower than that of OCs and higher than that of enstatite chondrites condensation region. A second, possibly nebular, phase of reduction by the production of reducing gas phases (e.g., C-rich) could be responsible for the subsequent reduction of the primary material and the occurrence of reverse zoning in the low-Ca pyroxene and lower average Fa/Fs ratio. Based on the IR spectra of EM 301 we suggest the possibility that the parent body of this chondrite was a V-type asteroid.