Fluids in the peridotite-water system up to 6 GPa and 800°C: New experimental constrains on dehydration reactions

O. Dvir, T. Pettke, P. Fumagalli, R. Kessel*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

56 Scopus citations

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.

Original languageAmerican English
Pages (from-to)829-844
Number of pages16
JournalContributions to Mineralogy and Petrology
Volume161
Issue number6
DOIs
StatePublished - Jun 2011

Bibliographical note

Funding Information:
Acknowledgments This work was supported by Israel Science Foundation grants (320/05, 324/05, and 251/09). Vitali Gutkin and Dr. Vladimir Uvarov from the nano-characterization center at the Hebrew University are thanked for their help with the SEM and XRD work. Yaakov Weiss is thanked for helping with the EPMA. TP acknowledges support by the Swiss National Science Foundation (grant Nr. PP002-106569). We thank M. Hirschmann and an anonymous reviewer for their useful comments in the previous version of the manuscript. The helpful and valuable reviews of this version of the manuscript of C.B. Till and A. Pawley are highly appreciated.

Keywords

  • Fluids
  • High-pressure experiments
  • Hydrous phases
  • Mantle
  • Peridotite
  • Subduction zone

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