Carbonatite Metasomatism of the Oceanic Upper Mantle: Evidence from Clinopyroxenes and Glasses in Ultramafic Xenoliths of Grande Comore, Indian Ocean

Abstract

Petrological and geochemical data are presented for a suite of ultramafic xenoliths (lherzolites and wehrlites) from La Grille volcano, Grande Comore island. These xenoliths have peculiar textural features characterized by reaction rims or glassy patches, where clinopyroxene grows at the expense of orthopyroxene. The glasses vary from very strongly silica-oversaturated to strongly undersaturated varieties. They are generally homogeneous within a single xenolith, are extremely alkali rich, with Na2O contents (up to 14 wt %) higher than any previously encountered, and are highly evolved (mg-number 34–37). The major element (especially Na) and trace element abundances, together with some remarkable Ti and Zr negative anomalies in chondrite-normalized diagrams, suggest that the metasomatizing agent was an ephemeral alkali-rich carbonatite. These metasomatic reactions appear to have involved an increase in oxygen fugacity from lherzolites (−0.56 to 1.29Δlog units) to wehrlites (up to 2.68Δlog units), with a concomitant decrease in temperature (lherzolites: 908–993°C; wehrlites: 816–893°C). Geobarometric indicators point to equilibration in the range of 7–12 kbar, above the carbonate stability field. An experimentally determined carbonatite composition reacting with a harzburgitic–lherzolitic protolith was successfully balanced with the observed secondary paragenesis allowing calculation of the trace element content of the metasomatizing carbonatitic fluid/s. These modelling procedures were subsequently used to estimate a set of Kd cpx/carb values for mantle depths, which compare well with (scarce) data from both natural occurrences and experimental runs. The origin of the CO2 and H2O in the metasomatizing melts or fluids is problematic. The melts or fluids may have originated from recycled material in or, most probably, below the asthenospheric mantle, brought down in a subducting slab at a former convergent plate margin.