We have studied cloudy taenite, metallographic cooling rates, and shock effects in 30 H3\u20136 chondrites to elucidate the thermal and early impact history of the H chondrite parent body. We focused on H chondrites with old Ar\u2013Ar ages (>4.4 Gyr) and unshocked and mildly shocked H chondrites, as strongly shocked chondrites with such old ages are very rare. Cooling rates for most H chondrites at 500 \u00b0C are 10\u201350 \u00b0C\/Myr and do not decrease systematically with increasing petrologic type as predicted by the onion-shell model in which types 3\u20135 are arranged in concentric layers around a type 6 core. Some type 4 chondrites cooled slower than some type 6 chondrites and type 3 chondrites did not cool faster than other types, contrary to the onion-shell model. Cloudy taenite particle sizes, which range from 40 to 120 nm, are inversely correlated with metallographic cooling rates and show that the latter were not compromised by shock heating. The three H4 chondrites that were used to develop the onion-shell model, Ste. Marguerite, Beaver Creek, and Forest Vale, cooled through 500 \u00b0C at \u2a7e5000 \u00b0C\/Myr. Our thermal modeling shows that these rates are 50\u00d7 higher than could be achieved in a body that was heated by 26Al and cooled without disturbance by impact. Published Ar\u2013Ar ages do not decrease systematically with increasing petrologic type but do correlate inversely with cloudy taenite particle size suggesting that impact mixing decreased during metamorphism. Metal and silicate compositions in regolith breccias show that impacts mixed material after metamorphism without causing significant heating. Impacts during metamorphism created Portales Valley and two other H6 chondrites with large metallic veins, excavated the fast-cooled H4 chondrites around 3\u20134 Myr after accretion, and mixed petrologic types. Metallographic data do not require catastrophic disruption by impact during cooling.<\/div><\/div>\n","protected":false},"excerpt":{"rendered":"
Scott ERD, Krota TV, Goldstein JI and Shigeru Wakita S. Geochimica et Cosmochimica Acta Volume 136, 1 July 2014, Pages 13\u201337 LINK<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[6,7,127,63,62],"tags":[1806,1859,566,370],"_links":{"self":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/1896"}],"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=1896"}],"version-history":[{"count":1,"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/1896\/revisions"}],"predecessor-version":[{"id":1897,"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/1896\/revisions\/1897"}],"wp:attachment":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1896"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1896"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1896"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}