Other
Scientific paper
Jul 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996gecoa..60.2347g&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 60, Issue 13, pp.2347-2363
Other
7
Scientific paper
Zircons enclosed in corundum megacrysts from basalts have a short prismatic habit with the prism slightly more developed than the pyramidal faces. The {110} prism is dominant compared with {100}, indicating crystallisation from alkaline-peralkaline environments. Rare zircon decomposition reactions suggest the possible presence of a primary SiO 2 phase in the source paragenesis for corundum megacrysts. The zircon inclusions are geochemically distinctive: high Y (up to 1.2%), U (UO 2 up to 1.7 wt%), Th (ThO 2 up to more than 2 wt%), Hf (up to 3.4 wt%), and REE ( REE up to 5000 ppm). They form a cogenetic group that is characterised by extreme REE fractionation with preference for the HREE varying from 30-100× chondritic Sm to 5000-20000× chondritic Lu. REE partitioning between zircon and silicate liquid is evaluated. The parental melts calculated for the zircon inclusions and other corundum-related zircons, using zircon/melt partition coefficients derived in the present paper, show concave-down REE patterns with a maximum Sm concentration close to 100× chondritic Sm abundance. Such REE patterns represent highly evolved melts after extensive fractionation of low-Ca feldspar prior to and throughout the crystallisation of zircon. The characteristics of zircon inclusions provide evidence that corundum megacrysts must have crystallised from alkaline and highly evolved melts under very reducing conditions, implying that the corundum is unlikely to have formed as high pressure phenocrysts in the host mafic magmas.
Griffin William L.
Guo Jingfeng
O'Reilly Suzanne Y.
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