The role of brucite in water and element cycling during serpentinite subduction – Insights from Erro Tobbio (Liguria, Italy)

Peters, Daniel; Pettke, Thomas; John, Timm; Scambelluri, Marco (2020). The role of brucite in water and element cycling during serpentinite subduction – Insights from Erro Tobbio (Liguria, Italy). Lithos, 360-361, p. 105431. Elsevier 10.1016/j.lithos.2020.105431

Preview	Text Peters-et-al_2020_Lithos.pdf - Accepted Version Available under License Creative Commons: Attribution-Noncommercial-No Derivative Works (CC-BY-NC-ND). Download (2MB) | Preview
	Text 1-s2.0-S0024493720300682-main.pdf - Published Version Restricted to registered users only Available under License Publisher holds Copyright. Download (7MB)

The Erro Tobbio olivine-antigorite serpentinites and associated dehydration veins represent hydrated oceanic mantle rocks that escaped complete dehydration and recycling into the mantle after subduction to ~ 550-600 °C and 2.0-2.5 GPa. These rocks thus offer valuable insights into the petrological evolution of a slice of hydrated oceanic mantle and the geochemical cycling down to intermediate subduction zone depths. Our study emphasises the role of brucite upon rock-buffered hydration and subduction dehydration employing bulk and in situ chemical data sets combined with petrology.
Bulk rock data reveal a coherent mantle peridotite slice affected by variable melt depletion
and refertilisation. Subsequent fluid-rock interaction stages proceeded isochemically with
respect to SiO2, i.e., without significant SiO2 enrichment characteristic for hydrothermal
ocean floor serpentinisation. Relicts of low-T mesh textures after olivine and preservation of precursor mineral and low-T hydration geochemical features indicate a lack of subsequent fluid and metamorphic overprinting, even on scales of tens of micrometres. Fluid-mobile element enrichments are modest with exceptions for B and W. Enrichment signatures of U/Cs << 1 and Rb/Cs of 4-26 are characteristic of shallow forearc hydration within or atop the slab by fluids derived from breakdown of clays or first dehydration of altered oceanic crust with a subordinate sedimentary pore fluid component. Overall, the geochemical and petrological changes of the Erro Tobbio peridotites during fluid-rock interactions were rockbuffered, in contrast to fluid-buffered hydration accompanied with significant SiO2 metasomatism at, e.g., mid ocean ridges.
Silica-neutral rock-buffered serpentinisation resulted in prominent brucite formation upon
olivine hydration. In absence of excess SiO2, subsequent serpentine transformation of
chrysotile/lizardite to antigorite likely produced even more brucite. Rock-buffered fluid-rock
interactions thus provide a mechanism for stabilising brucite in subduction zone
serpentinites, presumably along hydration fronts and within deeper sections of the oceanic
lithospheric mantle. Finally, brucite + antigorite dehydration produced up to 40 vol. % of
metamorphic olivine and prominent olivine + Ti-clinohumite + magnetite vein networks at
temperatures < 550-600 °C, prior to complete antigorite breakdown. Wall rocks released
alkali elements, B, Cr, As, Sb, and Ba into the dehydration fluids, along with substantial Sr,
REE and HFSE redistribution into vein minerals.

Interest & Impact

Downloads

173 since deposited on 24 Mar 2020
75 in the past 12 months

Citations

5 Citations in Web of Science ®
6 Citations in Scopus

Search

in Google Scholar™

Services

Actions (login required)

Edit item

Actions (login required)

Edit item