LINK<\/a><\/p>\n“Northwest Africa (NWA) 6704\/6693 are medium- to coarse-grained achondrites with unique petrologic and geochemical traits that are distinct from the currently established meteorite groups. Here, we report on the extinct 26Al-26Mg and 53Mn-53Cr systems to establish fine-scale chronology of its formation and Cr and Ti isotopic anomalies to constrain the composition of the source reservoir of NWA 6704\/6693. Excesses in the neutron-rich 54Cr and 50Ti isotopes, due to nucleosynthetic anomalies, separate NWA 6704\/6693 from the vast majority of established achondrites and instead resemble the excesses seen among the carbonaceous chondrites; specifically, the CR-type chondrites. The excesses in these isotopes indicate a common feeding zone during accretion in the protoplanetary disk between the source of NWA 6704\/6693 and that of the carbonaceous chondrites. The 26Al-26Mg data for pyroxene and plagioclase from NWA 6704 yield a (26Al\/27Al)0 = (3.15 \u00b1 0.38)\u00d710-7 (MSWD = 0.49) and \u03b426Mg\u2217 = -0.004 \u00b1 0.005 at the time of isotopic closure. This initial (26Al\/27Al)0 translates to an absolute age of 4563.14 \u00b1 0.38 Ma, relative to the D\u2019Orbigny angrite. However, given the potential heterogeneity of 26Al, the D\u2019Orbigny angrite might not be a good age anchor for the purpose of calculating 26Al-26Mg ages. The 26Al-26Mg age relative to another carbonaceous achondrite NWA 2976 is 4562.66 \u00b1 0.60 Ma. The 53Mn-53Cr systematics of NWA 6704\/6693 indicate a (53Mn\/55Mn)0 of (2.59 \u00b1 0.34)\u00d710-6 (MSWD = 1.2) with an evolved initial \u03b553Cr of +0.14 \u00b1 0.03. The (53Mn\/55Mn)0 yields an 53Mn-53Cr age of 4562.17 \u00b1 0.76 Ma relative to the D\u2019Orbigny angrite. Concordant ages determined using the short-lived 26Al-26Mg and 53Mn-53Cr systems and extant 207Pb-206Pb system (4562.60\u00b10.30 Ma for NWA 6704\/6693; Amelin et al., 2018) indicate rapid cooling and nearly contemporaneous closing of multiple isotope systems. The ancient crystallization ages and positive 54Cr and 50Ti anomalies of NWA 6704\/6693 indicate widespread melting and differentiation processes occurring in both non-carbonaceous (NC) and carbonaceous chondrite (CC) regions of the protoplanetary disk. Additionally, we report the Cr and Ti isotopic composition for a petrologic range of CR-type materials (CR2, CR6, and achondrites). The additional Cr and Ti isotopic data for these CR-type materials indicates a range in isotopic composition not previously observed based on CR2 chondrites alone.”<\/p>\n","protected":false},"excerpt":{"rendered":"
Matthew E. Sanborn, Josh Wimpenny, Curtis D. Williams, Akane Yamakawa, Yuri Amelin, Anthony J.Irving, Qing-Zhu Yin Geochimica et Cosmochimica Acta In Press, Accepted Manuscript, Available online 14 October 2018 LINK “Northwest Africa (NWA) 6704\/6693 are…<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[28,12,507],"tags":[332,671,2934,1554,761,397,2285],"_links":{"self":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/16626"}],"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=16626"}],"version-history":[{"count":1,"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/16626\/revisions"}],"predecessor-version":[{"id":16627,"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/16626\/revisions\/16627"}],"wp:attachment":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=16626"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=16626"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=16626"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}