DePaolo, D

Feineman, M

Ryerson, F J

UNT Digital Library

UCRL-CONF-204391Quantitative models of subduction zonefluids: How hydrous phases in the slabdetermine the composition of subductionzone lavasM. Feineman, F. J. Ryerson, D. DePaoloMay 28, 2004Goldschmidt 2004Copenhagen, DenmarkJune 5, 2004 through June 19, 2004This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes.  Updated October 14, 2003 Quantitative models of subduction zone fluids:How hydrous phases in the slab determine the composition of subduction zone lavasMaureen FeinemanUC Berkeley, LLNLFrederick RyersonLLNLDonald DePaoloUC Berkeley, LBNLOutstanding questions:• What is the nature and composition of the slab-derived fluids?• What features of arc volcanism and geochemistry are acquired from fluids?• How do fluids and melts move through the mantle after being released from the slab?• What are the timescales of fluid and melt generation/migration in the slab and mantle?Overview: experiments and models• General characteristics of arc lavas• Mineral/fluid trace element partitioning experiments• Bulk rock/fluid partitioning for warm vs. cold slabs• Geochemical characteristics of the Northeast and Southwest Japan Arcs• Questions concerning fluid migration in the mantle wedgeTrace element variations in arc basalts from Northeast and Southwest JapanIwateDaisenEnriched in LILEDepleted in HFSE Depleted in REEEnriched in LILERELATIVELY depleted in HFSE, LREEDepleted in HREEWater leaves slab in continuous reaction series as hydrous minerals break down in descending slab after Schmidt and Poli 1998Mineral/fluid equilibrium partition coefficientmineral fluidDmineral/fluid =CmineralCfluidEnd-loaded piston cylinder apparatus:can achieve upper mantle pressures (<4GPa)Partitioning experiments: 700-900˚C, 1.0-3.0 GPaExperimental run products:crystals and quenched solutemicafish roeMSM %KSM =Mmica %Kmica +Msolute %KsoluteMSM %NaSM =Mmica %Namica +Msolute %NasoluteMica/solute ratio calculated by K, Na mass balanceTrace element analysis by LA-ICP-MSCSM =Xmica Cmica +X fluid C fluidRun-product micaAnalyses run on VG ExCellat Boston UniversityFluid by mass balanceMica by LA-ICP-MSMineral/fluid partition coefficientsMineral-fluid partitioning review• Th more compatible in cpx, U more compatible in garnet• Many trace elements incompatible in cpx, garnet• Most trace elements, especially LILE (Ba, Sr), compatible in hydrous minerals• Y compatible in all major slab mineralsWarm vs. cold slab trajectoryafter Schmidt and Poli 1998Cold slab retains lawsonite, H2OWarm slab “dries up” by ~100 km depthDetermining bulk partition coefficientsDbulk =XiDiafter Schmidt and Poli 1998Bulk partition coefficients in slabNot fractionated by mantle meltingIndicator of slab meltingVariation in fluid releaseddepends on evolving mineralogyAssuming equilibrium in the slabFluid compositionsC fluid = C0F +(1 F) DJapan ArcPeacock and Wang 1999Northeast (cold) vs. Southwest (warm) JapanPeacock and Wang 1999 after Schmidt and Poli 1998Trace element variations in the Japan ArcVolcanic front occurs at greater slab depth than predicted for cold slabModel fluid Arc basaltsArc volcanism and geochemistry review• Fluid composition models predict trace element fractionation for certain key elemental ratios in arc basalts that cannot be explained by simple melting processes• Appearance of volcanic front is delayed relative to model in cold arcsConclusions• Arc basalt geochemistry is determined to a large extent by residual hydrous phases in the slab• The differing mineralogy in warm and coldsubducting slabs is apparent in the compositions of arc basalts• The location of the volcanic front also appears to be related to thermal conditions in the slab

Quantitative models of subduction zone fluids: How hydrous phases in the slab determine the composition of subduction zone lavas

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Quantitative models of subduction zone fluids: How hydrous phases in the slab determine the composition of subduction zone lavas

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