Experimental Investigation of Fluid-Mobile Element Transport in the Upper Mantle
DOEI Reserach Funded: 2005
Proposed ResearchAt convergent plate boundaries, such as Japan and the Cascade range in the western United States, the ocean floor is forced downward (subducted) into the upper mantle of the Earth. This process produces high heat flow and active volcanism on the overriding plate. It is a widely held view that, in subduction zones, a water-rich “subduction component” released from the oceanic crust infiltrates the upper mantle. This component modifies the composition of the mantle rocks, lowering their melting point and leading directly to active volcanism. Despite the importance of the subduction component as the principal agent of mass transfer between the subducting crust and the mantle, there is no consensus as to whether it is a fluid released by dehydration reactions, a small degree hydrous partial melt of the subducting oceanic crust, or a supercritical phase signifying the disappearance of the amphibolite/eclogite solidus. Results from analyses of accessory minerals in high-pressure rocks (eclogites) suggest that the subduction component may not be a fluid. Hydrous accessory minerals such as zoisite, phengite, and allanite retain “fluid mobile” elements in the subducting oceanic crust even after severe dehydration at eclogite facies conditions, suggesting that dehydration and geochemical fluxes from slab to the mantle wedge are decoupled. Further, new models for the thermal structure of subduction zones that incorporate temperature- and/or stress-dependent viscosity for the mantle wedge predict temperatures at the base of the wedge in excess of the H2O-saturated melting point of the subducted oceanic crust. These lines of evidence favor low-degree partial melting of the subducted crust at H2O-saturated conditions as the primary mechanism for mass transfer. As a test of this hypothesis, we will carry out an experimental determination of how fluid-mobile trace elements such as Rb, Sr, Ba, La, Pb, Th, and U are distributed between accessory minerals and aqueous fluids at pressure and temperature conditions corresponding to dehydration of the subducting oceanic crust (2.0 GPa, 600 °C to 3.0 GPa, 750°C). Experiments will be carried out in the experimental petrology laboratory at WHOI and the experimental run products will be analyzed at the Northeast National Ion Microprobe Facility at WHOI. Results from these experiments will allow a quantitative evaluation of the efficiency with which these minerals retains trace elements during dehydration at eclogite facies conditions and, thereby, provide new insights into the role of fluid flow as an agent of mass transport beneath volcanic arcs. This will lead to a better understanding of the processes that produce volcanism in heavily populated parts of the world. Introduction/overview
It is a widely held view that, in subduction zones, an H2O-rich fluid derived from the subducting oceanic crust infiltrates the lower portions of the mantle wedge and triggers peridotite partial melting [e.g., Gill, 1981; e.g., Davies and Stevenson, 1992]. This “subduction component” is also thought to be responsible for enriching the sub-arc mantle, and thereby arc magmas, in fluid-mobile elements such as K, Rb, Sr, Cs, Ba, light rare earth elements (LREE), Pb, Th, and U [e.g., Perfit, et al., 1980; e.g., McCulloch and Gamble, 1991; Tatsumi, 2003]. That a slab-derived H2O-rich fluidas opposed to a hydrous silicate meltis responsible for the flux of trace elements into the mantle wedge is consistent with the thermal structure predicted by most constant-viscosity models for subduction zones. Such models suggest that, unless the subducting lithosphere is young and warm, temperatures remain below the solidi of subducting materials (i.e. ~700 °C for basalt/gabbro and ~650 ° to 850 °C for sediment) [e.g., Lambert and Wyllie, 1970; Peacock, 1991; Schmidt and Poli, 1998]. Thus, geochemical and petrologic observations as well as results from geophysical models have generally been considered to support a scenario in which an H2O-rich fluid is continuously released from the subducting slab through the gradual breakdown of hydrous phases. The trace element composition of this fluid is determined by mineral/fluid partitioning relationships and, therefore, evolves continuously as the stable mineral assemblages in the subducting lithosphere change with pressure and temperature conditions.
However, there are recent indications, based primarily on in-situ analyses of metamorphic minerals, that this picture of mass transfer in the sub-arc mantle may not be entirely correct. Hydrous accessory minerals such as phengite, zoisite, and allanite have been found to retain fluid-mobile elements in the subducted oceanic lithosphere even after severe dehydration at eclogite facies conditions [e.g., Sassi, et al., 2000; Zack, et al., 2001; Spandler, et al., 2003]. Large partition coefficients for fluid-mobile elements between these accessory phases and the major eclogite minerals (omphacite and garnet) demonstrate that the former overwhelmingly account for whole-rock budgets of these elements. The stability of these minerals at pressures in excess of 2 GPa has been experimentally verified for both simplified (e.g., CaO-Al2O3-SiO2-H2O or CASH) and natural compositions [Poli and Schmidt, 1998; Schmidt and Poli, 1998; Hermann, 2002], and they occur in eclogites in apparent textural equilibrium with omphacite and garnet. This implies that the retention of fluid-mobile elements in natural eclogites represents equilibrium partitioning that is stable to high pressures. That reconstructed and measured whole-rock abundances of fluid-mobile elements in eclogites are within ranges of concentrations for altered oceanic crust indicates that dehydration was not accompanied by depletion of these elements. All of this evidence suggests that dehydration and geochemical fluxes from the slab to the mantle wedge are decoupled.
Recent developments in thermal modeling of subduction zones are also challenging the widely held view that “basalts dehydrate but do not generally melt”. Numerical models for the thermal structure of subduction zones that incorporate temperature- and/or stress-dependent viscosity for the mantle wedge predict higher temperatures in shallow portions of the wedge beneath the volcanic front [e.g., van Keken, et al., 2002; Kelemen, et al., 2003]. This results in significantly elevated slab-top temperatures relative to those previously obtained, such that the H2O-saturated solidi for basalt and sediment could frequently be exceeded. Taken together with the evidence for retention of fluid-mobile elements by hydrous accessory minerals during dehydration of the subducting oceanic crust, the elevated temperatures predicted by the new thermal models appear to favor low-degree melting of the subducted oceanic crust as the primary mechanism for transferring volatiles and incompatible trace elements from the slab to the mantle wedge.
Given that these observations are in direct conflict with the widely held view of subduction zone mass transfer via fluid flow, we propose to carry out an experimental investigation of the mineral-fluid partitioning relationships for fluid mobile elements in hydrous accessory phases (phengite; zoisite; allanite; chloritoid) in a simplified system (CaO-Al2O3-SiO2-H2O or CASH) over a range of pressure-temperature (P-T) conditions corresponding to the advanced stages of amphibolite dehydration along realistic P-T trajectories for subduction zones. The data produced in the proposed study will provide a quantitative basis for evaluating the role of these accessory phases in controlling the flux of fluid mobile elements from the subducted slab to the overlying mantle wedge at convergent margins and, thereby, provide a test of the “dehydration transport” model for subduction zones. The proposed work fits into the Fluid Flow in Geologic Systems theme of the DOEI, and will provide new insights into the role of fluid flow as an agent of mass transport beneath volcanic arcs. Results from this study will be used to support our effort to obtain NSF funding for a larger study in which the phase relations for a natural amphibolite are determined at conditions corresponding to various stages of dehydration during subduction and the trace element composition of fluid evolved by the breakdown of hydrous phases at P-T conditions from 1.5 GPa, 450 °C to 3.0 GPa, 780 °C.Work Statement
Mineral-fluid partition coefficients will be experimentally determined for hydrous assessory minerals such as zoisite, phengite, allanite, and chloritoid. Zoisite is an important host for most fluid mobile elements (except for K and Rb) in natural eclogites, and determination of zoisite-fluid partitioning relationships is one of the key objectives of the proposed work. Using the results of Poli and Schmidt  as a starting point, we propose to carry out experiments on a simplified system (CASH) to investigate the partitioning of trace elements between zoisite and aqueous fluid. For a bulk composition within the subsystem CaAl2Si2O8·2H2O-Ca2Al3Si3O12(OH)-Al2SiO5 (lawsonite-zoisite-kyanite), a simple assemblage of zoisite + kyanite + quartz/coesite + fluid can be obtained over a range of P-T conditions relevant for the advanced stages of dehydration along realistic P-T trajectories for subduction zones. In this assemblage, zoisite is the only phase that can host significant quantities of fluid-mobile elements, presenting an ideal analog for natural eclogites. Over a pressure range from 1.5 to 3.0 GPa, lawsonite becomes unstable at temperatures above ~520 °C at 1.5 GPa, 575 °C at 2.0 GPa, and 675 °C at 3.0 GPa [Poli and Schmidt, 1998]. Dehydration of lawsonite produces zoisite, kyanite, quartz/coesite, and H2O by the reaction: providing a lower temperature limit for the proposed experiments. Specifically, experiments will be conducted at 2.0 GPa, 600 °C; 2.0 GPa, 700°C; 3.0 GPa, 700°C; and 3.0 GPa, 750°C, using a starting composition of an equal-weight mixture of lawsonite, zoisite and kyanite (SiO2: 38.3wt%, Al2O3: 42.9, CaO: 14.3, H2O: 4.5) synthesized on an anhydrous basis from high-purity oxides and carbonates. A mixture of 10 wt% H2O and 90% anhydrous oxides of this composition would produce a system saturated with SiO2 and H2O over the proposed range of P-T conditions. Rb, Sr, Ba, La, Pb, Th, and U will be added to the oxide starting materials as ICPMS standard solutions at approximately 50 ppm levels. All experiments will be carried out using a solid-medium piston cylinder device. The major element composition of the crystalline run products will be determined by electron microprobe, while their trace element content will be determined by ion microprobe (SIMS). The trace element composition of the fluid phase will be determined by dissolving the fluid phase as a whole in HF-HNO3 and performing isotope dilution analysis.
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