Metasomatic mineral titanate complexing in the upper mantle

Abstract

The extent and expression of metasomatism in the upper mantle are contentious issues^1,2, although the process has gained widespread acceptance to account for the subsequent enrichment of previously depleted lithosphere^3,4. Uncertainties exist in the origin and precise compositions of the fluids involved, as well as in the total inventory of introduced metasomatic elements (particularly silicate-incompatible Ti, Zr, Nb, REE (rare-earth elements), Ba, Sr, Rb, K and Na). Metasomatism is readily identified by the presence of phlogopite, and in the most advanced stages by K-richterite⁴. In K-richterite-bearing harzburgites, these characteristic minerals are accompanied by rutile, armalcolite and lindsleyite–mathiasite (LIMA) solid solution members⁵ of the crichtonite mineral series⁶. The LIMA series is one upper mantle repository for silicate-incompatible elements, but the series is also specifically enriched in chromium (12–18 wt% Cr₂O₃). While it has been assumed^5,7 that Cr is immobilized in the depleted lithosphere as a restite element, either in residual garnet, pyroxene, or in chromian spinel, there has been no direct evidence for the source of Cr in LIMA. We show here that LIMA solid-solution members are derived from Cr-spinel through interaction with metasomatic fluids by formation of a magnetoplumbite mineral with up to 14 wt % BaO, and which is inferred to contain Ce⁴⁺. Fluid compositions are heterogeneous and an increase in the variety of repositories for silicate-incompatible trace elements is demonstrated in the subcontinental lithosphere at depths <100 km.