科研之友
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摘要
We report here an occurrence of cuspidine as part of a complex symplectitic intergrowth formed from the breakdown of akermanite (X(Mg) = 0.97, Na-mel(12)Gh(3)) in a calc-silicate xenolith in the Kiglapait Intrusion, Labrador, Canada. The symplectite is dominated by aluminian diopside, cuspidine, and monticellite, but also contains a calcic garnet (Grs(70)) and serpentine. Cuspidine approaches closely the ideal F-bearing composition. Average proportions of phases in the intergrowth are 33% clinopyroxene, 27% cuspidine, 21% monticellite, 9% garnet, and 9% serpentine. A reconstructed bulk-composition of the intergrowth approximately matches the composition of the adjacent akermanite, with the exception of Na and Fe, which were probably mobile (or retained in residual akermanite in the case of Na). The dominant phases in the intergrowth probably resulted from progress of the reaction: akermanite + F = diopside + cuspidine + monticellite, with additional grossular forming from the gehlenite component of melilite. Serpentine formed in a later stage of retrogression, once sufficient H(2)O was available. The peak T experienced by the xenolith is well constrained at >= 875 degrees C, but the retrograde, symplectite-forming reaction is less certain (probably <= 700 degrees C), with serpentine forming at an even lower T. Fluorine-rich cuspidine requires interaction with a F-bearing fluid, probably derived from the surrounding magma. This interpretation is consistent with the known volatile content of the Kiglapait Intrusion, which contained F far in excess of H(2)O and Cl.
- 出版日期2010-8
