The extent and expression of metasomatism in the upper mantle are contentious issues1,2, although the process has gained widespread acceptance to account for the subsequent enrichment of previously depleted lithosphere3,4. 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 assumed5,7 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.