Fluid-mobile elements in serpentinites: Constraints on serpentinisation environments and element cycling in subduction zones

Peters, Daniel; Bretscher, Annette; John, Timm; Scambelluri, Marco; Pettke, Thomas (2017). Fluid-mobile elements in serpentinites: Constraints on serpentinisation environments and element cycling in subduction zones. Chemical geology, 466, pp. 654-666. Elsevier 10.1016/j.chemgeo.2017.07.017

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Fluid-mobile element (FME) systematics in serpentinites are key to unravel the environments of mantle rock
hydration, dehydration, and element recycling in subduction zones. Here we compile serpentinite geochemical
data and, for the first time, report discriminative FME enrichment trends for mid ocean ridge vs. forearc serpentinisation
by applying alkali element-U ratios. Characteristic element fractionations are thereby governed by
redox-dependent differential U mobility at mid ocean ridges and in forearcs, and by high Cs input in forearcs due
to fluids equilibrated with sediments. Simple modelling reproduces the observed enrichment trends in serpentinites
that range over several orders of magnitude. From these systematics, first constraints on potentially
discriminative fractionation trends for unconventional fluid tracers such as B, As, and Sb can be deduced.
Prominent W enrichments that correlate with FMEs suggest significant W mobility in low-temperature serpentinising
environments.
Application of the alkali element-U systematics to the subducted metaperidotites of Erro Tobbio (recording
initial brucite +antigorite breakdown during subduction) and Almirez (recording final antigorite breakdown)
reveal that pre-subduction FME enrichment signatures are retained in progressively subducted hydrous mantle
rocks to beyond subarc levels. Associated dehydration veins and fluid inclusions reveal subordinate alkali element-
U fractionation trends during dehydration. Subducted hydrous mantle rocks therefore may introduce
characteristic element signatures and thus contribute towards mantle geochemical heterogeneities.

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