Available online xxxx Editor: M. Bickle Keywords: subduction metamorphic petrology geotherm eclogiteThe maximum-pressure P-T conditions (P max -T ) and prograde P-T paths of exhumed subduction-related metamorphic rocks are compared to predictions of P-T conditions from computational thermal models of subduction systems. While the range of proposed models encompasses most estimated P max -T conditions, models predict temperatures that are on average colder than those recorded by exhumed rocks. In general, discrepancies are greatest for P max < 2 GPa, where only a few of the highest-T model paths overlap petrologic observations and model averages are 100-300 • C colder than average conditions recorded by rocks. Prograde P-T paths similarly indicate warmer subduction than typical models. Both petrologic estimates and models have inherent biases. Petrologic analysis may overestimate temperatures at P max where overprinting occurs during exhumation, although P-T paths suggest that relatively warm conditions are experienced by rocks on the prograde subduction path. Models may underestimate temperatures at depth by neglecting shear heating, hydration reactions and fluid and rock advection. Our compilation and comparison suggest that exhumed high-P rocks provide a more accurate constraint on P-T conditions within subduction zones, and that those conditions may closely represent the subduction geotherm. While exhumation processes in subduction zones require closer petrologic scrutiny, the next generation of models should more comprehensively incorporate all sources of heat. Subduction-zone thermal structures from currently available models appear to be inaccurate, and this mismatch has widereaching implications for our understanding of global geochemical cycles, the petrologic structure of subduction zones, and fluid-rock interactions and seismicity within subduction zones.

The interface between subducting oceanic slabs and the hanging wall is a structurally and lithologically complex region. Chemically disparate lithologies (sedimentary, mafic and ultramafic rocks) and mechanical mixtures thereof show heterogeneous deformation. These lithologies are tectonically juxtaposed at mm to km scales, particularly in more intensely sheared regions (mélange zones, which act as fluid channelways). This juxtaposition, commonly in the presence of a mobile fluid phase, offers up huge potential for mass transfer and related metasomatic alteration. Fluids in this setting appear capable of transporting mass over scales of kms, along flow paths with widely varying geometries and P-T trajectories. Current models of arc magmatism require km-scale migration of fluids from the interface into mantle wedge magma source regions and implicit in these models is the transport of any fluids generated in the subducting slab along and ultimately through the subduction interface. Field and geochemical studies of high-and ultrahigh-pressure metamorphic rocks elucidate the sources and compositions of fluids in subduction interfaces and the interplay between deformation and fluid and mass transfer in this region. Recent geophysical studies of the subduction interface-its thickness, mineralogy, density, and H 2 O content-indicate that its rheology greatly influences the ways in which the subducting plate is coupled with the hanging wall. Field investigation of the magnitude and styles of fluid-rock interaction in metamorphic rocks representing "seismogenic zone" depths (and greater) yields insight regarding the roles of fluids and elevated fluid pore pressure in the weakening of plate interface rocks and the deformation leading to seismic events. From a geochemical perspective, the plate interface contributes to shaping the "slab signature" observed in studies of the composition of arc volcanic rocks. Understanding the production of fluids with hybridized chemical/isotopic compositions could improve models aimed at identifying the relative contributions of end-member rock reservoirs through analyses of arc volcanic rocks. Production of rocks rich in hydrous minerals, along the subduction interface, could stabilize H 2 O to great depths in subduction zones and influence deep-Earth H 2 O cycling. Enhancement of decarbonation reactions and dissolution by fluid infiltration facilitated by deformation at the interface could influence the C flux from subducting slabs entering the sub-arc mantle wedge and various forearc reservoirs. 3 In this paper, we consider records of fluid and mass transfer at localities representing various depths and structural expressions of evolving paleo-interfaces, ranging widely in structural character, the rock types involved (ultramafic, mafic, sedimentary), and the rheology of these rocks. We stress commonalities in styles of fluid and mass transfer as related to deformation style and the associated geometries of fluid mobility at subduction interfaces. Variations in thermal structure among i...

Abstract-The newly found meteorite Northwest Africa 6234 (NWA 6234) is an olivine (ol)-phyric shergottite that is thought, based on texture and mineralogy, to be paired with Martian shergottite meteorites NWA 2990, 5960, and 6710. We report bulk-rock major-and trace-element abundances (including Li), abundances of highly siderophile elements, Re-Os isotope systematics, oxygen isotope ratios, and the lithium isotope ratio for NWA 6234. NWA 6234 is classified as a Martian shergottite, based on its oxygen isotope ratios, bulk composition, and bulk element abundance ratios, Fe ⁄ Mn, Al ⁄ Ti, and Na ⁄ Al. The Li concentration and d 7 Li value of NWA 6234 are similar to that of basaltic shergottites Zagami and Shergotty. The rare earth element (REE) pattern for NWA 6234 shows a depletion in the light REE (La-Nd) compared with the heavy REE (Sm-Lu), but not as extreme as the known ''depleted'' shergottites. Thus, NWA 6234 is suggested to belong to a new category of shergottite that is geochemically ''intermediate'' in incompatible elements. The only other basaltic or ol-phyric shergottite with a similar ''intermediate'' character is the basaltic shergottite NWA 480. Rhenium-osmium isotope systematics are consistent with this intermediate character, assuming a crystallization age of 180 Ma. We conclude that NWA 6234 represents an intermediate compositional group between enriched and depleted shergottites and offers new insights into the nature of mantle differentiation and mixing among mantle reservoirs in Mars.

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