Geochemistry of serpentinized and multiphase altered Atlantis Massif peridotites (IODP Expedition 357): Petrogenesis and discrimination of melt-rock vs. fluid-rock processes

International Ocean Discovery Program (IODP) Expedition 357 drilled 17 shallow sites distributed ~10 km in the spreading direction (from west to east) across Atlantis Massif oceanic core complex (Mid-Atlantic Ridge, 30°N). Mantle exposed footwall of is predominantly nearly wholly serpentinized harzburgite with subordinate dunite. Altered peridotites are subdivided into three types: (I) serpentinites, (II) melt-impregnated and (III) metasomatic serpentinites. Type I serpentinites show no evidence melt-impregnation or metasomatism apart from serpentinization local oxidation. II have been intruded by gabbroic melts distinguishable some cases on basis macroscopic microscopic observations, e.g., mm-cm scale mafic-melt veinlets, rare plagioclase (?0.5 modal % one sample) presence secondary (replacive) olivine after orthopyroxene; other cases, ‘cryptic’ inferred incompatible element enrichments. III characterized silica manifest alteration orthopyroxene talc amphibole, anomalously high anhydrous SiO2 concentrations (59–61 wt%) low MgO/SiO2 values (0.48–0.52). Although many chondrite-normalized earth (REE) primitive mantle-normalized trace anomalies, negative Ce-anomalies, attributable serpentinization, compositional heterogeneities due melt-impregnation. On whole rock elements, a dominant mechanism distinguished central eastern fluid-rock (mostly serpentinization) western increasing as east increase enrichment high-field strength elements light REE. High degrees melt extraction evident whole-rock Al2O3/SiO2 Al2O3, CaO elements. Estimates degree based REE patterns suggest maximum ~20% non-modal fractional melting, little variation between sites. As serpentinite samples ex situ rubble, magmatic histories observed at each site consistent source fault zone) rather than rafted rubble that would be expected more heterogeneity spatial pattern. In this case, studied may provide record enhanced melt-rock interactions time, proposed geological models. Alternatively, signify these processes scales few km.