Carbonate melts in the hydrous upper mantle and their role in H2O controlled metasomatic mantle transformation
Abstract
Experimental studies on simplified carbonate-peridotite systems have proven essential to understand the origins of carbonatite magmas in the upper mantle. Nevertheless, disagreement persists on melting temperatures and melt compositions in anhydrous and H2O-bearing carbonate model systems. Here we experimentally determine carbonate melt compositions along the forsterite-calcite join as a function of temperature and H2O at 1.0-2.0 GPa. A H-trapping, double-capsule technique was used to control H2O contents from nominally anhydrous to 4 wt.% H2O; the range where metasomatic transformation of carbonated dunite to wehrlite is anticipated during hydrous carbonate mantle melting. Results on nominally anhydrous compositions yield solidus temperatures of 1250-1275°C at 1.0-1.5 GPa, which are up to 160°C higher than in previous studies. Near-solidus anhydrous melts have 9 wt% SiO2 with a molar Ca/(Ca+Mg) ratio of 0.77 (1.5 GPa). This differs from previously reported melt compositions of 11-18 wt% SiO2 at 1100°C and 1.0 GPa (Lee & Wyllie 2000, CMP). It is likely that the difference in composition and solidus temperature is due to the presence of H2O in nominally anhydrous melting experiments resulting from hydrogen diffusion into the experimental charge. Melting experiments over a range of temperatures (1275-1350°C) indicate that Ca/(Ca+Mg) in nominally anhydrous melts remains nearly constant with increasing temperature. In contrast, in presence of H2O a continuous compositional spectrum, from SiO2-poor carbonate melts to Mg-rich silico-carbonatites is observed, demonstrating that a two-liquid field is absent in the alkali-free system. We demonstrate that upper mantle metasomatism is restricted to low H2O concentrations (≤2 wt%) resulting in the formation of wehrlite in equilibrium with strongly depolymerized carbonate melts with up to 0.95 Ca/(Ca+Mg). A general petrogenetic model is advocated for hydrous silico-carbonatite melts originating from the interaction between reduced serpentinization fluids and subducted carbonate sediments at 50 km depth and temperatures ≤1100°C.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.V24A..03W
- Keywords:
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- 1012 Reactions and phase equilibria;
- GEOCHEMISTRYDE: 1025 Composition of the mantle;
- GEOCHEMISTRYDE: 1031 Subduction zone processes;
- GEOCHEMISTRYDE: 8430 Volcanic gases;
- VOLCANOLOGY