OS-ISOTOPE AND HIGHLY SIDEROPHILE ELEMENT EVIDENCE FOR THE ORIGIN OF SERPENTINITES FROM THE MOTAGUA (GUATEMALA) SUTURE ZONE

Serpentinite mélanges from either side of the Motagua Fault, Guatemala are associated with a suite of high-pressure metamorphic rocks including jadeitite and eclogite. These mélanges record two separate subduction events during the evolution of the Caribbean-North American plate boundary. Sorensen et al. (2010) documented elemental exchanges between these serpentinites and jadeitites. We continue her pioneering work with new highly siderophile element (HSE) abundance data and Re-Os isotopic data for serpentinite samples. This new data, in combination with existing geochemical data can be used to evaluate the protoliths of the serpentinites and their geologic setting within the paleo-subduction zone. Comparison of the new data with that available for global subduction-related serpentinites demonstrates that these samples are consistent with oceanic lithosphere of a subducted slab (though some are ambiguous and may represent mantle wedge). Furthermore, ¹⁸⁷Os/¹⁸⁸Os ratios are unradiogenic (<0.127) and are typical of oceanic lithosphere; therefore, the data do not provide firm evidence for nor conclusively disprove formation in an arc/back-arc environment or extensive interaction with seawater. There are subtle differences HSE and Os isotopic data between the northern and southern serpentinites indicating these had different melt histories probably prior to serpentinization. Elevated Os abundances in samples from either side of the fault indicate that Os was added by an unradiogenic metasomatic agent (i.e. not a slab-derived fluid). Dramatic differences in Pd between samples record the influence of aqueous and/or sulfide metasomatism, including sulfide metasomatism related to melt-rock reaction. Metasomatism is capable of fractionation between Pt and Pd. The presence of pentlandite and other sulfides is supportive of sulfide metasomatism. So, we are left with a dichotomy of processes: early non-subduction related melt-rock interaction followed by clear hydrothermal interaction with subduction-sourced fluid as indicated by fluid mobile elements as well as their the B isotopic signature.