LINK<\/strong><\/a><\/p>\n\n\n\n“The many unique characteristics of CB chondrites have resulted in the impact hypothesis becoming the favoured model for their formation. Here, we further investigate the formation mechanisms of CB chondrites by analysing the elemental and K isotope compositions of chondrules and bulk fractions from the CBa chondrite Gujba. Similar to previous work, the refractory element ratios in the Gujba chondrules show evidence of a differentiated precursor, with the Nb\/Ta, Zr\/Hf, Sc\/Th and Zr\/Th ratios showing fractionation relative to other chondrites. In addition, the bulk fractions, and to a lesser extent the chondrules with attached matrix and metals, display significantly more refractory element fractionation and a large enrichment in light REEs. Based on EDS elemental mapping and comparisons with previous studies, the most likely source of this highly fractionated material appears to be the small amount of heterogeneously distributed interstitial fine-grained material within Gujba. These large refractory element fractionations (i.e., Nb\/Ta, Zr\/Hf, Sc\/Th Zr\/Th, and LREE\/HREE) are best explained by a significant partial melting process such as crustal formation. Nevertheless, the mechanism of patrial melting cannot be conclusively determined with the data available here. The K isotopic compositions of the Gujba chondrules analyzed here range from \u22122.24\u2030 to \u22120.41\u2030 in \u03b441K, whereas the bulk analyses show \u03b441K values of \u22120.81\u2030 to \u22120.72\u2030. This range of chondrule K isotope compositions is significantly larger, and extends to much lighter compositions, compared to all other chondrites measured so far by bulk ICP-MS. In addition, the Gujba chondrules display a clear negative correlation of K isotopic composition with K concentration, with the chondrules showing the lightest K isotope compositions having the highest K concentrations. This distinctive correlation indicates that evaporation was likely the dominant process affecting the K isotopic variation observed in the Gujba chondrules. Nevertheless, the extremely light \u03b441K values seen in the most K-rich chondrules (which are lighter than any other early solar system material so far measured) indicate that incomplete condensation likely took place before evaporation. As such, we propose a two-stage model to explain the formation of chondrules in Gujba, with Stage 1 characterized by incomplete condensation of vaporized material with an average isotopic fractionation factor (\u03b1) of 0.9984 (when using the most K enriched chondrule to constrain the model), and Stage 2 representing partial evaporation in a vapor plume with an average \u03b1 range of 0.9976 to 0.9990. Using these \u03b1 values we calculate an approximate vapor saturation index value of 0.935 for condensation and between 0.903 and 0.960 for evaporation. This formation process requiring both condensation and evaporation for CB chondrules is consistent with an impact generated vapor plume and further expands our understanding of CB chondrite formation.”<\/p>\n","protected":false},"excerpt":{"rendered":"
Piers Koefoed, Olga Pravdivtseva, Ryan Ogliore, Yun Jiang, Katharina Lodders, Mason Neuman, Kun Wang (\u738b\u6606) Geochimica et Cosmochimica ActaIn Press, Journal Pre-proof, Available online 23 June 2022 LINK “The many unique characteristics of CB chondrites…<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[9,584,56,148],"tags":[3273,1426,1644,1839,1864,1643,1430,1645,5103],"_links":{"self":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/30786"}],"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=30786"}],"version-history":[{"count":1,"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/30786\/revisions"}],"predecessor-version":[{"id":30787,"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/30786\/revisions\/30787"}],"wp:attachment":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=30786"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=30786"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=30786"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}