The nature and origin of ureilites |
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| Authors: | John L Berkley GJeffrey Taylor Klaus Keil George E Harlow Martin Prinz |
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| Institution: | Department of Geology and Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131, U.S.A.;Department of Mineral Sciences, American Museum of Natural History, New York, NY 10024, U.S.A. |
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| Abstract: | Ureilites are carbon-bearing olivine-pigeonite achondrites which constitute a unique achondrite type. We performed a comprehensive mineralogical and petrological study on eight ureilites (Kenna, Novo Urei, Goalpara, Haverö, Dingo Pup Donga, Dyalpur, North Haig, and ALHA77257) the results of which were used to construct a hypothesis for the origin of ureilites. This hypothesis suggests that ureilites are primarily olivine-pigeonite cumulates which crystallized from a silicate liquid that also contained suspended solid carbon phases, mainly graphite. This carbon caused reduction of the melt and influenced ureilite mineral compositions. Carbonaceous material was trapped by settling cumulus mafic silicates along with other intercumulus material to form the carbon-rich ‘veins’ common to ureilites. Petrofabric analyses show that mafic silicates are oriented in lineated and foliated patterns characteristic of cumulate rocks, specifically adcumulates. Strain state of silicates suggests that ureilites were deformed subsequent to lithification by mild tectonic stress as well as by moderate to severe shock. The latter event caused the formation of micron-sized diamonds and lonsdaleite from graphite and carbon-induced reduction of silicate grain margins during temporal shock-heating. |
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