Distribution of niobium, tantalum, and other highly incompatible trace elements in the lithospheric mantle: The spinel paradox

Details Distribution of niobium, tantalum, and other highly incompatible trace elements in the lithospheric mantle: The spinel paradox Distribution of niobium, tantalum, and other highly incompatible trace elements in the lithospheric mantle: The spinel paradox

Feb 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996gecoa..60..545b&link_type=abstract

Geochimica et Cosmochimica Acta, Volume 60, Issue 3, p. 545-550.

Computer Science

19

Scientific paper

Because its crystal structure is unfavorable to large cations, the contribution of chromium-spinel to the trace-element budget of mantle rocks is generally thought to be negligible. Here we present a conflicting picture of trace-element distribution in spinel peridotites, emphasizing the strong capacity of this mineral to trap highly incompatible elements, such as Nb, Ta, Rb, Ba, and U. This paradox is due to the existence of a very thin reactional layer (<10 μm thick) coating the surfaces of spinel crystals and concentrating these elements. Our detailed study of a peridotite from the Ronda massif, Spain, and five xenoliths from the East African Rift, Ethiopia, has revealed that this layer is dominantly composed of titanium oxides and phlogopite, hardly detectable by optical methods. In spite of their extremely low abundance in the rocks (e.g., 0.002-0.015% niobium rutile), these microphases account for 45-90% of the whole-rock budget for Rb, Ba, Nb, and Ta. The spinel rims are attributed to a metasomatic process involving pervasive percolation of silica-rich and K-rich small melt fractions in the lithospheric mantle. This process bears major implications for the genesis of the continental crust and Nb-Ta-depleted magmas such as arc lavas.

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