Cryptic metasomatism in the upper mantle beneath Kutch: Evidence from spinel lherzolite xenoliths

Spinel lherzolite xenoliths entrained in the alkaline rocks from central Kutch vary from fertile (10–12% modal clinopyroxene) to depleted (2–3% modal clinopyroxene) types. Low equilibrium temperatures (884–972°C) indicate entrainment of lherzolite xenoliths from shallow depths within the lithosphere. Variations of major oxides and incompatible elemental concentrations in clinopyroxene indicate a primary control by partial melting. In most of the depleted xenoliths however, the light rare earth elements (LREE), thorium (Th) and niobium (Nb) are strongly enriched and cannot be attributed totally to the process of partial melting. The absence of typical 'metasomatic' minerals, low equilibration temperatures and enriched LREE patterns indicate that the upper mantle below Kutch underwent an event of cryptic metasomatic enrichment prior to partial melting. The distinctive chemical features, viz. LREE enrichment, strong Ti depletion relative to Eu, and slight Nb enrichment, fractionation of Zr and Hf and therefore high Zr/Hf ratio, high La/Yb, Nb/La and low Ti/Eu are all results of interaction of refractory peridotite residues with carbonatite melts.