Quantifying the Effects of Metasomatism in Mantle Xenoliths: Constraints from Secondary Chemistry and Mineralogy in Udachnaya Eclogites, Yakutia

Abstract

In mantle xenoliths, metasomatism is recorded by compositional variations within and between minerals, and by the introduction of secondary minerals. However, metasomatism has not been quantitatively evaluated as a process with respect to the fluid composition involved. Diamondiferous eclogites from the Udachnaya kimberlite provide a unique suite of samples that allow a semiquantitative estimation of metasomatic fluid composition. The basis of our analysis involves comparison of reconstructed whole-rock compositions with measured whole-rock analyses. Primary minerals in these samples are relatively homogeneous, and permit the use of modal analyses and mineral chemistry for reconstruction of “pristine” whole-rock compositions. The metasomatic overprint, which is similiar in all samples studied, has produced depletions in SiO2, Na2O, and FeO and enrichments in TiO2, K2O, MgO, and LREE. Secondary minerals from the samples are interpreted as the direct result of metasomatism (i.e., typical metasomatic minerals such as phlogopite, amphibole, djerfisherite, and sodalite are present in these xenoliths). Enrichment/depletion signatures demonstrate that the major metasomatic source for Udachnaya eclogites was not derived from the host kimberlite. These metasomatic agents appear to have been more enriched in TiO2, K2O, C1, FeO, and LREE than are kimberlites, and may have contained significant amounts of F, CO2, and H2O. The high Ca contents of two samples are interpreted to be the product of metasomatism by a carbonatite-like fluid.