We have performed in situ analyses of C and O isotopic compositions, trace element concentrations, and cathodoluminescence (CL) intensities on calcite in Murchison, a weakly altered CM chondrite. We found that the trace element (Mg, Mn, and Fe) concentrations are heterogeneous within single calcite grains. Grain to grain heterogeneity is even more pronounced. The analyzed calcite grains can be separated into two distinct types with respect to their C isotopic ratios, trace element concentrations, and CL characteristics: Calcite grains with higher \u03b413CPDB values (\u223c75 \u2030) have low trace element concentrations and uniformly dark CL, while grains with lower \u03b413C values (\u223c35 \u2030) have higher trace element concentrations and CL zoning. In contrast to the C isotopic ratios, O isotopic ratios are similar for both types of calcites (\u03b418OSMOW \u223c 34 \u2030).<\/p>\n
The O isotopic ratios, trace element concentrations, and CL characteristics provide no evidence for C-isotope evolution in fluids from a single C reservoir by Rayleigh-type isotope fractionation (i.e., removal of C-bearing gaseous species). Also, it seems difficult to explain the O and C isotopic compositions of the two types of calcites by their formation at different temperatures from a single fluid. Instead, the \u03b413C variation suggests the presence of at least two C reservoirs with different isotopic ratios in the aqueous fluids from which the calcites precipitated. The C reservoirs with lower \u03b413C values are likely to be organic matter. The same holds for the C reservoirs with higher \u03b413C values which might have significant contributions from the 13C-enriched grains identified in meteoritic insoluble organic matter. Thermodynamic calculations show that calcite with lower Fe concentrations formed under more reduced conditions than calcite with higher Fe concentrations. If this is the case, the 13C-rich organic grains may have been destroyed and dissolved in the fluids under more reduced conditions than other organic components. The fact that the two types of calcites were found in different domains in the same thin section suggests that microenvironments with diverse physicochemical conditions such as redox states were present at scales of 100\u2019s \u03bcm.<\/div><\/div>\n","protected":false},"excerpt":{"rendered":"
Wataru Fujiya, Naoji Sugiura, Yves Marrocchi, Naoto Takahata, Peter Hoppe, Kotaro Shirai, Yuji Sano, Hajime Hiyagon Geochimica et Cosmochimica Acta doi:10.1016\/j.gca.2015.04.010 in Press, available online 13 April 2015 LINK<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[9,18,8,576,65],"tags":[1241,250,1242,107,1845],"_links":{"self":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/4326"}],"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=4326"}],"version-history":[{"count":1,"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/4326\/revisions"}],"predecessor-version":[{"id":4327,"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/4326\/revisions\/4327"}],"wp:attachment":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4326"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4326"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4326"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}