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

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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.

Item Type:

Journal Article (Original Article)

Division/Institute:

08 Faculty of Science > Institute of Geological Sciences
08 Faculty of Science > Institute of Geological Sciences > Rock-Water Interaction

UniBE Contributor:

Peters, Daniel and Pettke, Thomas

Subjects:

500 Science > 550 Earth sciences & geology

ISSN:

0024-4937

Publisher:

Elsevier

Funders:

[UNSPECIFIED] People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme FP7/2017- 2013/under REA grant agreement no. 604713, ‘Zooming in between Plates (ZIP)’

Language:

English

Submitter:

Thomas Pettke

Date Deposited:

24 Mar 2020 11:59

Last Modified:

12 May 2020 07:51

Publisher DOI:

10.1016/j.lithos.2020.105431

BORIS DOI:

10.7892/boris.141100

URI:

https://boris.unibe.ch/id/eprint/141100

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