Keywords:lawsonite, in-situ Sr-Pb isotopes, LA-MC-ICPMS, secular change, subduction zone, external fluid
Lawsonite is a hydrous Ca-Al silicate mineral which is broadly stable in a typical subduction-zone geotherm since late Neoproterozoic. Because lawsonite behaves as a reservoir of trace elements, especially of LREEs, Sr, Pb, Th and U, lawsonite can be a proxy of these elements of a bulk subducting slab, especially of crustal lithologies. In order to understand the elemental behavior of subducting crustal materials, lawsonites in lawsonite-eclogite facies metabasalts and metachert from the South Motagua Mélange, Guatemala, were investigated by a new technique of in-situ LA-MC-ICPMS spot isotope analyses. In-situ Sr-Pb isotope determination for eclogite facies lawsonite revealed isotope variations reflecting their protoliths and isotope zoning in some lawsonite crystals. Lawsonite in a phengite-rich metabasaltic eclogite have relatively `low' 87Sr/86Sr ratios (0.70335-0.70355) with some Pb isotope variations 207Pb/206Pb = 0.8401-0.8512 and 208Pb/206Pb = 2.0757-2.0911. On the other hand, lawsonite from another metabasaltic eclogite is zoned. The cores have 87Sr/86Sr = 0.70558-0.70601 and the ratios increase at the rims (87Sr/86Sr = 0.70636-0.70662); the cores with a MORB-like affinity (207Pb/206Pb = ~0.843-0.844) are overgrown by the rims with 207Pb/206Pb = ~0.839-0.841. Lawsonite in a metachert has `higher' Sr isotope ratios (0.70697-0.70757) with 208Pb/206Pb = ~2.07. In isotope spaces 208Pb/204Pb versus 206Pb/204Pb and 87Sr/86Sr versus 206Pb/204Pb, lawsonite isotopes are scattered mainly in DMM (depleted MORB mantle) and some EMI and EMII fields, respectively. All the age-corrected isotope ratios were not significantly different from non-corrected values. The bulk-rock compositions and lawsonite Sr-Pb isotope suggest that sediment-like `external' fluids have incorporated into the metabasaltic system during the second stage eclogite-facies recrystallization in a mélange of slab-mantle interface. Isotope-zoned lawsonite also supports the presence of sediment-like `external' fluids.