157 Geochemistry (Posters)

Tuesday, 6 November 2012
9:00 AM - 6:00 PM
Hall B

IS THE WORLD'S ONLY CARBONATITE VOLCANO A DRY / ANHYDROUS SYSTEM?

Paper number: 157-2
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

Carbonatites are unusual mantle melts characterised by SiO2-depletion, alkali- and REE –enrichment and are dominated by carbonate minerals. At present, their petrogenetic history remains enigmatic with two main models invoked: carbonatite-silicate immiscibility at crustal level; and low-degree mantle melts. An important yet curious constraint is the role of water in their petrogenesis, or conversely the lack of water. Carbonatite melts can dissolve three-times as much water as silicate melts based upon high-pressure, high-temperature experimental constraints; this naturally leads to the inference that carbonatitic melts should be H2O-rich. Such a wet carbonatitic system has been documented in the fibrous growths on cubic mantle diamonds (coated diamond) that have trapped pristine, mantle-derived carbonatitic, metasomatic fluids. However the hygroscopic nature of natro-carbonatitic lava at the Oldoinyo Lengai volcano, Tanzania, suggests that water is not a large constituent of the melt.

For carbonatite systems in general, it is thought that the water is released upon crystallisation at depth, resulting in a fenitised aureole containing minerals with abundant H2O-rich fluid inclusions. However, recent investigations of the in-situ compositions of fluid inclusions trapped within fenitised rocks from Oldoinyo Lengai demonstrate that the metasomatism is characterised by CO2-rich and H2O-poor fluids, which is an entirely new discovery. The studied inclusions reveal the presence of alkali-rich and H2O-poor fluids that have precipitated secondary carbonatite minerals thereby resulting in a residual high-density CO2 fluid in the inclusions. The fluids and daughter minerals present are derived from a degassing, alkali-rich carbonatitic magma body with a high CO2/H2O value. Combined with trapping pressures equivalent to the lower crust at~30 km, which is at a much greater depth than the proposed magma-storage system for this volcano this would suggest a deeper-mantle origin for the carbonatitic melt.