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

Whattam SA, Frueh-Green GL, Cannat M, De Hoog JCM, Schwarzenbach EM, Escartin J, John BE, Leybourne M, Williams MJ, Roumejon S, Akizawa N, Boschi C, Harris M, Wenzel K, Mccaig A, Weis D, Bilenker L (2022)


Publication Type: Journal article

Publication year: 2022

Journal

Book Volume: 594

DOI: 10.1016/j.chemgeo.2021.120681

Abstract

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

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APA:

Whattam, S.A., Frueh-Green, G.L., Cannat, M., De Hoog, J.C.M., Schwarzenbach, E.M., Escartin, J.,... Bilenker, L. (2022). Geochemistry of serpentinized and multiphase altered Atlantis Massif peridotites (IODP Expedition 357): Petrogenesis and discrimination of melt-rock vs. fluid-rock processes. Chemical Geology, 594. https://dx.doi.org/10.1016/j.chemgeo.2021.120681

MLA:

Whattam, Scott A., et al. "Geochemistry of serpentinized and multiphase altered Atlantis Massif peridotites (IODP Expedition 357): Petrogenesis and discrimination of melt-rock vs. fluid-rock processes." Chemical Geology 594 (2022).

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