Guillaume Florin, B\u00e9atrice Luais, Tracy Rushmer, Olivier Alard<\/p>\n\n\n\n
Geochimica et Cosmochimica Acta
\nIn Press, Journal Pre-proof, Available online 6 November 2019<\/p>\n\n\n\n
LINK<\/strong><\/a><\/p>\n\n\n\n Ordinary chondrites (OCs) are classified \ninto three groups, according to their oxidation state, which increases \nfrom the H to L to LL groups. This is demonstrated by the decrease in \nmetal content (H=\u223c8 vol%, L=\u223c4 vol%, and LL=\u223c2 vol%), and by a positive \ncorrelation between \u039417<\/sup>O and %Fa through the OC sequence. \nCompared to other chondrites, OCs exhibit the largest variation in \noxidation state but there is an ongoing debate on the processes that \ncontrol this variation. To constrain the causes of the variations in the\n oxidation state with respect to the associated nebular versus parent \nbodies processes, we investigated the elemental and isotopic variations \nof germanium (moderately siderophile and volatile) in the bulk sample, \nas well as in the metal, silicate and sulfide phases, over a range of \npetrographic types for the H, L, and LL ordinary chondrites.<\/p>\n\n\n\n We found that \u03b474\/70<\/sup>Gemetal<\/sub> is a proxy for the \u03b474\/70<\/sup>Gebulk<\/sub> composition and that each OC group is distinguishable by their \u03b474\/70<\/sup>Gemetal<\/sub>,\n which increases from \u20130.51\u00b10.09\u2030 for H chondrites, \u20130.31\u00b10.06\u2030 for L \nchondrites, and, finally, to \u20130.26\u00b10.09\u2030 for LL chondrites (2\u03c3SD). \nAdditionally, the OC sequence exhibited a positive correlation, from H \nto L to LL, between \u03b474\/70<\/sup>Gemetal<\/sub> and %Fa, as well as oxygen isotopes (\u03b417<\/sup>O, \u03b418<\/sup>O and \u039417<\/sup>O),\n that was not a consequence of a \u201csize sorting effect\u201d on chondrules \n(i.e. chondrule mixing) or metamorphic processes in the parent bodies \nbut, rather, was the result of nebular processes. We propose that the \ncorrelation between the \u03b474\/70<\/sup>Ge values and %Fa, \u039417<\/sup>O, \u03b418<\/sup>O can be explained by an increasing proportion of accreted hydrated phyllosilicates, from the H, L to LL groups, with high \u03b474\/70<\/sup>Ge and \u039417<\/sup>O. We found that 10 to 15% of phyllosilicates, with a composition of [Ge]=4\u20137 ppm and \u03b474\/70<\/sup>Ge=3\u20132.5\u2030, is needed to change the \u03b474\/70<\/sup>Ge from H to LL, which corresponds to a \u039417<\/sup>O\u22488-7\u2030. This value agrees with the \u039417<\/sup>O\u22487\u2030 composition of the accreted nebular component reported by Choi et al. (1998)<\/a>.\n During thermal metamorphism, phyllosilicates destabilize, liberating \ngermanium that will be incorporated in the metal, then leading to its \nhigh \u03b474\/70<\/sup>Ge signature.<\/p>\n\n\n\n High-temperature metamorphism can explain the lack of \u03b474\/70<\/sup>Gemetal<\/sub>\n variation with the petrologic type in the OC, even for the type 3 \nchondrites (T\u2248675\u00b0C), implying a complete reaction even at low \npetrologic types. In addition, metal-silicate re-equilibration in \nresponse to thermal metamorphism results in a decrease in \u039474\/70<\/sup>Gemetal-silicate<\/sub> from 0.33\u2030 to 0.06\u2030, within the H chondrite group, which is interpreted as the result of \u03b474\/70<\/sup>Gesilicate<\/sub> variation. The mean positive \u039474\/70<\/sup>Gemetal-silicate<\/sub>\n fractionation factor of +0.22\u00b10.36\u2030 (error propagation on individual \nerror) also displays a remarkable similarity to the direction of \nisotopic fractionation with other germanium isotopic metal-silicate \ndatasets, such as the magmatic iron meteorites, the Earth silicate \nreservoirs. We propose that the \u039474\/70<\/sup>Gemetal-silicate<\/sub> and the negative \u03b474\/70<\/sup>Ge\n values of OCs are inherited from metal-silicate melting and partial \nexchange before planetesimal accretion in a light isotope-enriched gas. \nFinally, the \u03b474\/70<\/sup>Gemetal<\/sub>-\u039417<\/sup>Osilicate<\/sub> correlation between the IIE iron meteorites and OCs, provides new evidence for the existence of a highly reduced HH group.<\/p>\n","protected":false},"excerpt":{"rendered":" Guillaume Florin, B\u00e9atrice Luais, Tracy Rushmer, Olivier Alard Geochimica et Cosmochimica Acta In Press, Journal Pre-proof, Available online 6 November 2019 LINK Ordinary chondrites (OCs) are classified into three groups, according to their oxidation state,…<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[59,6,23,24,7],"tags":[5260,1841,5259,1807],"_links":{"self":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/20666"}],"collection":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=20666"}],"version-history":[{"count":1,"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/20666\/revisions"}],"predecessor-version":[{"id":20667,"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/20666\/revisions\/20667"}],"wp:attachment":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=20666"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=20666"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=20666"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}