A critical evaluation of oxidation versus reduction during metamorphism of L and LL group chondrites, and implications for asteroid spectroscopy

Abstract

Abstract-Modal mineralogies of individual, equilibrated (petrologic type 4-6) Land LL chondrites have been measured using an electron microprobe mapping technique, and the chemical compositions of coexisting silicate minerals have been analyzed. Progressive changes in the relative abundances and in the molar Fe/Mn and Fe/Mg ratios of olivine, low-Ca pyroxene, and diopside occur with increasing metamorphic grade. Variations in olivine/low-Ca pyroxene ratios (Ol/Px) and in metal abundances and compositions with petrologic type support the hypothesis that oxidation of metallic iron accompanied thermal metamorphism in ordinary chondrites. Modal Ol/Px ratios are systematically lower than normative Ol/Px ratios for the same meteorites, suggesting that the commonly used C.I.P.W. norm calculation procedure may not adequately estimate silicate mineral abundances in reduced chondrites. Ol/Px ratios calculated from visible and near-infrared (VISNIR) reflectance spectra of the same meteorites are not in agreement with other Ol/Px determinations, possibly because of spectral complexities arising from other minerals in chondrites. Characteristic features in VISNIR spectra are sensitive to the proportions and compositions ofolivine and pyroxenes, the minerals most affected by oxidative metamorphism. This work may allow spectral calibration for the determination ofmineralogy and petrologic type, and thus may be useful for spectroscopic studies of asteroids.

Laboratory simulation of terrestrial meteorite weathering using the Bensour (LL6) ordinary chondrite

“…Most minerals vary in abundance over a relatively narrow range but olivine and OPX show the greatest variation, part of which may reflect occasional difficulties in distinguishing olivine and OPX in X-ray spectra. We recognize the problems in obtaining representative mineral abundances from a relatively small sample of such a heterogeneous material (Keil 1962;Gastineau-Lyons et al 2002), but the mineralogy of Bensour compares reasonably well with the modal mineralogy of Saint-Séverin (LL6) determined by Gastineau-Lyons et al (2002) (Table 1). The chemical compositions of the main Bensour silicate minerals are listed in Table 2.…”

Multiple and Fast: The Accretion of Ordinary Chondrite Parent Bodies

“…This early reduction effect, already noted by Menzies et al (2005), is more pronounced among Hs, which contained less oxygen and more C in their starting material. It results in a rapid increase in metal abundance from type 3 to type 4, which is followed by a slight decrease from type 4 to type 6, due to subsequent oxidation (McSween & Labotka 1993;Gastineau-Lyons et al 2002;Dunn et al 2010;this work). Reduction of Fe from olivine in the least equilibrated chondrites makes Si available to create more pyroxene and thus decreases the ol/(ol + low-Ca px) ratio as shown in the lower panel, a trend that is reversed in Figure 10.…”

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

“…The issue of whether these meteorites were open or closed with respect to oxygen during metamorphism is also unresolved. For example, based on intrinsic fO 2 measurements, Brett and Sato (1984) found that increasing intensity of metamorphism in equilibrated ordinary chondrites was accompanied by decreasing fO 2 relative to the IW buffer, while Walter and Doan (1969), McSween and Labotka (1993), and Gastineau-Lyons et al (2002) used geochemical arguments to support the idea that progressive metamorphism was characterized by increasing fO 2 relative to IW and increasing oxygen contents of the rocks. These proposed shifts in fO 2 and/or oxygen content are, however, subtle, and resolving which are correct requires accurate determinations of fO 2 .…”