LINK<\/strong><\/a><\/p>\n\n\n\n“Al-Mg isotope systematics of twelve FeO-poor (type I) chondrules from CR chondrites Queen Alexandra Range 99177 and Meteorite Hills 00426 were investigated by secondary ion mass spectrometry (SIMS). Five chondrules with Mg#\u2019s of 99.0 to 99.2 and \u039417O of \u22124.2\u2030 to \u22125.3\u2030 have resolvable excess 26Mg. Their inferred (26Al\/27Al)0 values range from (3.5 \u00b1 1.3) \u00d7 10\u22126 to (6.0 \u00b1 3.9) \u00d7 10\u22126. This corresponds to formation times of 2.2 (\u20130.5\/+1.1) Myr to 2.8 (\u22120.3\/+0.5) Myr after CAIs, using a canonical (26Al\/27Al)0 of 5.23 \u00d7 10-5, and assuming homogeneously distributed 26Al that yielded a uniform initial 26Al\/27Al in the Solar System. Seven chondrules lack resolvable excess 26Mg. They have lower Mg#\u2019s (94.2 to 98.7) and generally higher \u039417O (\u22120.9\u2030 to \u22124.9\u2030) than chondrules with resolvable excess 26Mg. Their inferred (26Al\/27Al)0 upper limits range from 1.3 \u00d7 10\u22126 to 3.2 \u00d7 10\u22126, corresponding to formation >2.9 to >3.7 Myr after CAIs. Al-Mg isochrons depend critically on chondrule plagioclase, and several characteristics indicate the chondrule plagioclase is unaltered: (1) SIMS 27Al\/24Mg depth profile patterns match those from anorthite standards, and SEM\/EDS of chondrule SIMS pits show no foreign inclusions; (2) transmission electron microscopy (TEM) reveals no nanometer-scale micro-inclusions and no alteration due to thermal metamorphism; (3) oxygen isotopes of chondrule plagioclase match those of coexisting olivine and pyroxene, indicating a low extent of thermal metamorphism; and (4) electron microprobe data show chondrule plagioclase is anorthite-rich, with excess structural silica and high MgO, consistent with such plagioclase from other petrologic type 3.00-3.05 chondrites. We conclude that the resolvable (26Al\/27Al)0 variabilities among chondrules studied are robust, corresponding to a formation interval of at least 1.1 Myr.<\/p>\n\n\n\n
Using relationships between chondrule (26Al\/27Al)0, Mg#, and \u039417O, we interpret spatial and temporal features of dust, gas, and H2O ice in the FeO-poor chondrule-forming environment. Mg# \u2265 99, \u039417O \u223c\u22125\u2030 chondrules with resolvable excess 26Mg initially formed in an environment that was relatively anhydrous, with a dust-to-gas ratio of \u223c100\u00d7. After these chondrules formed, we interpret a later influx of 16O-poor H2O ice into the environment, and that dust-to-gas ratios expanded (100\u00d7 to 300\u00d7). This led to the later formation of more oxidized Mg# 94-99 chondrules with higher \u039417O (\u22125\u2030 to \u20131\u2030), with low (26Al\/27Al)0, and hence no resolvable excess 26Mg.<\/p>\n\n\n\n
We refine the mean CR chondrite chondrule formation age via mass balance, by considering that Mg# \u2265 99 chondrules generally have resolved positive (26Al\/27Al)0 and that Mg# < 99 chondrules generally have no resolvable excess 26Mg, implying lower (26Al\/27Al)0. We obtain a mean chondrule formation age of 3.8 \u00b1 0.3 Myr after CAIs, which is consistent with Pb-Pb and Hf-W model ages of CR chondrite chondrule aggregates. Overall, this suggests most CR chondrite chondrules formed immediately before parent body accretion.”<\/p>\n","protected":false},"excerpt":{"rendered":"
Travis J. Tenner, Daisuke Nakashima, Takayuki Ushikubo, Naotaka Tomioka, Makoto Kimura, Michael K. Weisberg, Noriko T.Kita Geochimica et Cosmochimica Acta In Press, Accepted Manuscript, Available online 22 June 2019 LINK “Al-Mg isotope systematics of twelve…<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[9,56,148,12],"tags":[3273,1839,1864,5057,1810,5056,2498],"_links":{"self":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/19861"}],"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=19861"}],"version-history":[{"count":2,"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/19861\/revisions"}],"predecessor-version":[{"id":19863,"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/19861\/revisions\/19863"}],"wp:attachment":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=19861"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=19861"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=19861"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}