Understanding Structural Properties of Carbonate-Silicate Melts: An EXAFS Study on Y and Sr in the System Na2O-CaO-Al2O3-SiO2-CO2

Monday, 15 December 2014
Julia Pohlenz1, Sakura Pascarelli2, Olivier Mathon2, Stéphanie Belin3, Andrei Shiryaev4, Oleg Safonov5, Vadim Murzin6, Ksenia Shablinskaya7, Tetsuo Irifune8 and Max Wilke1, (1)Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Potsdam, Germany, (2)ESRF European Synchrotron Radiation Facility, Grenoble, France, (3)Synchrotron SOLEIL, GIF-sur-YVETTE CEDEX, France, (4)IPCE, Moscow, Russia, (5)Institute of Experimental Mineralogy RAS, Chernogolovka, Russia, (6)National Research Centre “Kurchatov Institute”, Moscow, Russia, (7)Moscow State University, Department of Chemistry, Russia, (8)Ehime University, Matsuyama, Japan
Carbonatite volcanism generally occurs in intra-plate settings associated with continental rifting. The only active carbonatitic volcano is the Oldoinyo Lengai, Tanzania, which generates sodium-rich carbonatites in close association with phonolites and nephelinites1. The processes of carbonatite genesis are still unresolved, however carbonate-bearing melts evidently play a crucial role during mantle melting, in diamond formation and as metasomatic agents. Carbonate melts show extraordinary properties, especially in regard to their low melt viscosities and densities, high surface tensions and electrical conductivities as well as distinct geochemical affinities to a wide range of trace elements2. Understanding the structural properties of carbonate-bearing melts is fundamental to explaining their chemical and physical behaviour as well as modeling processes operating in the deep Earth. Extended X-ray absorption fine structure (EXAFS) spectroscopy is a versatile tool for element specific investigation of the short to medium range structure of melts and glasses. This study focuses on unraveling the influence of carbonate concentration on the structural incorporation of the geochemically important trace elements Y and Sr in silicate and carbonate melts in the system Na2O-CaO-Al2O3-SiO2-CO2. First, we present structural data of silicate glasses with up to 10 wt% CO2, quenched from melts under high temperature and pressure, which indicate that the local structure of Y and Sr is not or only slightly affected by CO2. Melts with higher CO2 contents could not be quenched to glass, so far. Second, we show results of high pressure, high temperature experiments conducted in the Paris Edinburgh-Press, which provides in-situ insight into carbonate-silicate melts. All EXAFS measurements were performed at the synchrotron facility beamlines SAMBA (SOLEIL) and BM23 (ESRF). Information derived from the trace elements’ local structure is used to develop a structural model for carbonate-silicate melts. The ultimate goal is to gain insight into the structural control on chemical fractionation processes in carbonate-bearing magmatic systems in the deep Earth.

1. Klaudius, J & Keller, J, Lithos 91, 173–190 (2006).

2. Jones, AP, Genge, M & Carmody, L, Rev. Mineral. Geochem. 75, 289–322 (2013).