CHEMISTRY OF FLUID PRODUCTION IN THE SUBDUCTION FACTORY

LA-ICPMS allows for the accurate analysis of fluid composition (major to trace elements) equilibrated with rock at high P, from primary multiphase fluid inclusions and fluids trapped interstitially to solid materials from high-P experiments and now present as unquenchable phase mixtures.

Experiments equilibrating water with basaltic eclogite (Kessel et al., 2005, EPSL 237, 873-892) demonstrate that progressively more silicate material is dissolved with increasing pressure as the chemical system approaches its second critical endpoint. This includes “insoluble” elements such as Al, Ti or Th (Kessel et al., 2005, Nature 437, 724-727). During subduction, serpentinite dehydration is arguably the most important water production down to subarc depths. Prograde olivine may host relics of antigorite dehydration fluid as polyphase inclusions. The chemical composition of such inclusions from the Betic Cordillera, Spain, shows an incompatible trace element budget enriched in LILE, Li and B that closely resembles arc magma patterns. Upon rise, such fluids may interact with basaltic or sedimentary slab material and eventually invade the mantle wedge. Evidence for invasion of COH-silicate fluids with a crustal signature at mantle depths is found in UHP garnet pyroxenites of Western Norway. Veins of majoritic garnet, clinopyroxene, orthopyroxene, phlogopite, ±carbonate and ±spinel cut strongly depleted garnet pyroxenites equilibrated at 100-150km depth. Equilibrium trace element signatures of vein garnet, clinopyroxene and phlogopite are enriched notably in LILE and LREE and testify to mantle metasomatism by fluid with a crustal source to depths of at least 200km.

Collectively, the growing data base on high to UHP fluid composition reveals mobilities of silicate components that are much more elevated than predicted based on solubilities of simple species known from low-pressure fluid systems. Spectroscopic investigations and solubility experiments performed at high P identify progressively more polymerized silicate species in aqueous solutions, species that may considerably increase the solubility also of mutually insoluble trace elements like Al or HFSE. These combined data advance our understanding of crust to mantle element recycling and mass transfer in the Earth's interior.