Cr-rich magnesiokatophorite as an indicator of mantle metasomatism by hydrous Na-rich carbonatite

Abstract

Na -and Cr -rich amphibole and clinopyroxene are commonly observed in hydrous mantle xenoliths hosted by Pliocene -Quaternary basalts from Yemen. The amphibole is Cr -rich magnesiokatophorite high in Na 2 O (3.66 -5.65 wt%) and Cr 2 O 3 (2.21 -3.09 wt%), relatively low in K 2 O (0.23 -0.66 wt %) and almost free of Cl and F. The clinopyroxene is characterized by high Cr 2 O 3 (up to 5.64 wt%) and Na 2 O (up to 2.97 wt%), indicating appreciable amounts of kosmochlor. Both minerals show high REE concentrations as well as HFSE depletion, suggesting an involvement of carbonatitic melt in their formation. The magnesiokatophorites and kosmochlor -bearing diopside possibly formed through metasomatism by hydrous Na -rich carbonatite melt. Magnesiokatophorite is an indicator of metasomatism by hydrous Na -rich carbonatite in the upper mantle.

Katophorite from the Jade Mine Tract, Myanmar: mineral description of a rare (grandfathered) endmember of the amphibole supergroup

“…This feature is also present in the only other katophorite sample described by Ali and Arai (2013) from Yemen xenoliths, and might suggest crystal-chemical or geochemical constraints for the crystallization of katophorite. As expected in B (NaCa) amphibole compositions with little or no F, the A cations distribute between the two different subsites A(m) and A(2) within the A cavity (Hawthorne et al, 1996).…”

“…Recently, Ali and Arai (2013) reported analyses of zoned crystals of Cr-rich (up to 3.1 wt.% oxide) ''magnesiokatophorite'' (i.e. katophorite after Hawthorne et al, 2012) in hydrous mantle xenoliths from Yemen.…”