LINK<\/strong><\/a><\/p>\n“Highlights<\/p>\n
\u2022 Muonionalusta troilite exhibits a range of measured 238U\/235U values.
\n\u2022 Variable 238U\/235U greatly complicates chronologic investigations.
\n\u2022 The core of the IVA parent body must be younger than \u223c4563 Ma.
\n\u2022 Formation, differentiation, and cooling of planetesimal cores took at least \u223c5 Myr.
\n\u2022 Uranium isotope fractionation likely occurs during sulfide\u2013silicate segregation.”<\/p>\n
“The crystallization ages of planetary crustal material (given by basaltic meteorites) and planetary cores (given by iron meteorites) provide fiducial marks for the progress of planetary formation, and thus, the absolute ages of these objects fundamentally direct our knowledge and understanding of planet formation and evolution. The lone precise absolute age of planetary core material was previously obtained on troilite inclusions from the IVA iron meteorite Muonionalusta. This previously reported Pb\u2013Pb age of 4565.3 \u00b1 0.1 Ma\u2014assuming a 238U\/235U =137.88\u2014only post-dated the start of the Solar System by approximately 2\u20133 million years, and mandated fast cooling of planetary core material. Since an accurate Pb\u2013Pb age requires a known 238U\/235U of the sample, we have measured both 238U\/235U and Pb isotopic compositions of troilite inclusions from Muonionalusta. The measured 238U\/235U of the samples range from \u223c137.84 to as low as \u223c137.22, however based on Pb and U systematics, terrestrial contamination appears pervasive and has affected samples to various extents for Pb and U. The cause of the relative 235U excess in one sample does not appear to be from terrestrial contamination or the decay of short-lived 247Cm, but is more likely from fractionation of U isotopes during metal\u2013silicate separation during core formation, exacerbated by the extreme U depletion in the planetary core. Due to limited Pb isotopic variation and terrestrial disturbance, no samples of this study produced useful age information; however the clear divergence from the previously assumed 238U\/235U of any troilite in Muonionalusta introduces substantial uncertainty to the previously reported absolute age of the sample without knowledge of the 238U\/235U of the sample.<\/p>\n
Uncertainties associated with U isotope heterogeneity do not allow for definition of a robust age of solidification and cooling for the IVA core. However, one sample of this work\u2014paired with previous work using short-lived radionuclides\u2014suggests that the cooling age of the IVA core may be significantly younger than previously thought. This work indicates the metallic cores of protoplanetary bodies solidified no earlier than the first \u223c5\u201310 million years of the Solar System.”<\/p>\n","protected":false},"excerpt":{"rendered":"
Gregory A. Brennecka, Yuri Amelin, Thorsten Kleine Earth and Planetary Science Letters Volume 490, 15 May 2018, Pages 1\u201310 LINK “Highlights \u2022 Muonionalusta troilite exhibits a range of measured 238U\/235U values. \u2022 Variable 238U\/235U greatly…<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[77,72,214,255],"tags":[144,4413,2297,1874,1877,3908,3911,2919],"_links":{"self":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/14418"}],"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=14418"}],"version-history":[{"count":1,"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/14418\/revisions"}],"predecessor-version":[{"id":14419,"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/14418\/revisions\/14419"}],"wp:attachment":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=14418"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=14418"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=14418"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}