Collaborative Research: Fluid Transport and Fluid-Rock Interactions Preserved in Two Serpentinite Melanges in the Guatemala Suture Zone

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Subduction channel fluids are responsible for modification of metamorphic blocks, crystallization of vein rocks, serpentinization of the overlying mantle wedge and ultimately the flux melting that spawns arc volcanism. We propose to extend our studies of the serpentinite mélanges of the Guatemala Suture Zone (GSZ) to elucidate this record of mass transport by fluid as recorded in the rocks. The GSZ encompasses two serpentinite mélanges that preserve the elements described above. One system on the south side of the Motagua fault records very cold and wet conditions within the subduction channel, represented by lawsonite eclogite blocks formed at ~2.6 GPa and 450-500ºC. The other system north of the fault represents somewhat warmer but still wet conditions preserved in epidote-eclogite blocks, with peak metamorphism at 500-600°C and ~ 2 GPa. Both mélanges carry fragments of vein systems that formed within the overlying serpentinizing mantle peridotites; these include jadeitite, omphacitite, albitite, and mica-dominant rocks that contain a strong signature of fluids from altered oceanic crust and subducted sediment. Detailed petrologic and geochemical characterization of representative rocks is proposed for major, minor, and trace-element geochemistry and Sr-Nd isotopic systematics for this research, with a focus on B, Li, Be concentrations and isotopes. Textures, mineralogy, and chemical composition will be utilized to sort between processes that occurred during subduction as opposed to later exhumation. Intellectual Merit: Subduction is a fundamental process on Earth that recycles surface matter into the interior mantle and contributes to growth of continental crust. However, details of fluid-mediated mass transport and alteration in subduction zones are poorly understood and require further resolution. Careful study of preserved subduction zones, where both fluid precipitates and hydrated mantle wedge rocks are preserved, will contribute important insights on this issue. It is proposed to build upon results of a previously NSF-funded project by detailed chemical analysis of minerals in rocks preserved in serpentinite mélanges in the GSZ with the following objectives: (1) Characterize the mantle wedge protolith by study of the relict phases; (2) Analyze serpentinite minerals for major, minor and trace elements with particular attention to B, Li, and Be (and isotopic signatures) and textural settings to assess fluid modification of protolith and distinction between changes produced during subduction versus during exhumation; (3) Analyze minerals from vein rocks (e.g., jadeitite, omphacitite, mica-rock, etc.) with the same approach to determine the solute load of fluids traversing the subduction channel and entering the mantle wedge, as well as the relative timing based on textures and mineral assemblages. Using fluid-mineral partition coefficients we will estimate fluid composition entering the mantle wedge. (4) Utilize Sr-Nd isotopic data to assess sources of lithologic components (e.g., mantle vs. recycled continental or sedimentary sources). The proposed research will enhance understanding of fluid transport from subduction channels to the mantle wedge. Broader Impacts: These include the collaboration of researchers at universities and museums, involvement of students and scientists in many countries, including those in Central America, support of teacher training and geological training in Guatemala, links with archaeological studies on Caribbean-area jade, connections among research groups, and extensive outreach to both the media and the public via interviews, articles and Museum web-sites aimed both at student education and dissemination of our activities, results, and data.