Paper No. 2
Presentation Time: 9:00 AM-6:00 PM


CARMODY, Laura1, JONES, Adrian P.2, MIKHAIL, Sami3, BOWER, Dina M.3, STEELE, Andrew3, LAWRENCE, David M.4, VERCHOVSKY, A.B5, BUIKIN, A.6 and TAYLOR, Lawrence A.1, (1)Planetary Geosciences Institute, Department of Earth & Planetary Sciences, The University of Tennessee, Knoxville, TN 37996, (2)Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, United Kingdom, (3)Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, Washington, DC 20015, (4)School of Geography, Geology and the Environment, Kingston University of London, Penrhyn Road, Kingston upon Thames, KT1 2EE, United Kingdom, (5)Department of Physical Sciences, PSSRI, The Open University, Milton Keynes, MK7 6AA, United Kingdom, (6)Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 19 Kosygin Str, Moscow, 119991, Russia,

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.