The phase assemblages and compositions in a K-free lherzolite + H2O system were determined between 4 and 6 GPa and 700-800°C, and the dehydration reactions occurring at subarc depth in subduction zones were constrained. Experiments were performed on a rocking multi-anvil apparatus using a diamond-trap setting. The composition of the fluid phase was measured using the recently developed cryogenic LA-ICP-MS technique. Results show that, at 4 GPa, the aqueous fluid coexisting with residual lherzolite (~85 wt% H2O) doubles its solute load when chlorite transforms to the 10-Å phase between 700 and 750°C. The 10-Å phase breaks down at 4 and 5 GPa between 750 and 800°C and at 6 GPa between 700 and 750°C, leaving a dry lherzolite coexisting with a fluid phase containing 58-67 wt% H2O, again doubling the total dissolved solute load. The fluid fraction in the system increases from 0.2 when a hydrous mineral is present to 0.4 when coexisting with a dry lherzolite. Our data do not reveal the presence of a hydrous peridotite solidus below 800°C. The directly measured fluid compositions demonstrate a fundamental change in the (MgO + FeO) to SiO2 mass ratio of fluid solutes occurring at a depth of ca. 120-150 km (in the temperature window of 700-800°C), from (MgO-FeO)-dominated at 4 GPa [with (MgO + FeO)/SiO2 ratio of 1.41-1.56] to SiO2-dominated at 5-6 GPa (ratios of 0.61-0.82). The mobility of Al2O3 increases by more than one order of magnitude across this P-T interval and demonstrates that Al2O3 is compatible in an aqueous fluid coexisting with the anhydrous ol-opx-cpx ± grt assemblage. This shift in the fluid composition correlates with changes in the phase assemblage of the residual silicates. The hitherto unknown fundamental change in (MgO + FeO)/SiO2 ratio and prominent increase in Al2O3 of the aqueous fluid with progressive subduction will likely inspire novel concepts on mantle wedge metasomatism by slab fluids.

Fluids in the peridotite-water system up to 6 GPa and new experimental constrains on dehydration reactions / O. Dvir, T. Pettke, P. Fumagalli, R. Kessel. - In: CONTRIBUTIONS TO MINERALOGY AND PETROLOGY. - ISSN 0010-7999. - 161:6(2011), pp. 829-844. [10.1007/s00410-010-0567-2]

Fluids in the peridotite-water system up to 6 GPa and new experimental constrains on dehydration reactions

P. Fumagalli
Penultimo
;
2011

Abstract

The phase assemblages and compositions in a K-free lherzolite + H2O system were determined between 4 and 6 GPa and 700-800°C, and the dehydration reactions occurring at subarc depth in subduction zones were constrained. Experiments were performed on a rocking multi-anvil apparatus using a diamond-trap setting. The composition of the fluid phase was measured using the recently developed cryogenic LA-ICP-MS technique. Results show that, at 4 GPa, the aqueous fluid coexisting with residual lherzolite (~85 wt% H2O) doubles its solute load when chlorite transforms to the 10-Å phase between 700 and 750°C. The 10-Å phase breaks down at 4 and 5 GPa between 750 and 800°C and at 6 GPa between 700 and 750°C, leaving a dry lherzolite coexisting with a fluid phase containing 58-67 wt% H2O, again doubling the total dissolved solute load. The fluid fraction in the system increases from 0.2 when a hydrous mineral is present to 0.4 when coexisting with a dry lherzolite. Our data do not reveal the presence of a hydrous peridotite solidus below 800°C. The directly measured fluid compositions demonstrate a fundamental change in the (MgO + FeO) to SiO2 mass ratio of fluid solutes occurring at a depth of ca. 120-150 km (in the temperature window of 700-800°C), from (MgO-FeO)-dominated at 4 GPa [with (MgO + FeO)/SiO2 ratio of 1.41-1.56] to SiO2-dominated at 5-6 GPa (ratios of 0.61-0.82). The mobility of Al2O3 increases by more than one order of magnitude across this P-T interval and demonstrates that Al2O3 is compatible in an aqueous fluid coexisting with the anhydrous ol-opx-cpx ± grt assemblage. This shift in the fluid composition correlates with changes in the phase assemblage of the residual silicates. The hitherto unknown fundamental change in (MgO + FeO)/SiO2 ratio and prominent increase in Al2O3 of the aqueous fluid with progressive subduction will likely inspire novel concepts on mantle wedge metasomatism by slab fluids.
Fluids; High-pressure experiments; Hydrous phases; Mantle; Peridotite; Subduction zone
Settore GEO/07 - Petrologia e Petrografia
22-ago-2010
Article (author)
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/2434/165969
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