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

Highlights

• Olivine antigorite serpentinites record rock-buffered (de-)hydration histories.

• Brucite is central to water and element cycling during forearc dehydration.

• Rock-buffered serpentinisation limits SiO₂ metasomatism, favouring high brucite modes.

Abstract

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 rock-buffered, 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.

Introduction

Subduction zones are sites of major geological activity including hazardous seismicity, volcanism and large-scale element cycling between exogenous and endogenous reservoirs. Progressive dehydration of hydrated oceanic lithosphere during subduction releases large amounts of fluid-mobile element (FME)-rich fluids (e.g., Becker et al., 1999; John et al., 2004; Kessel et al., 2005; Spandler and Pirard, 2013; Scambelluri et al., 2014) that can metasomatise the overlying mantle wedge (Bostock et al., 2002; Guillot et al., 2001; Zack and John, 2007; Deschamps et al., 2010) and play a crucial role in feeding arc magmatism (Brown et al., 1982; Hattori and Guillot, 2003; Marschall and Schumacher, 2012; Bali et al., 2012; John et al., 2012; Scambelluri and Tonarini, 2012). Moreover, dehydration reactions and escaping fluids are suspected to be linked to certain types of subduction zone seismicity (Hacker et al., 2003; Incel et al., 2017; Bloch et al., 2018; Taetz et al., 2018).

Serpentinites are thereby major water and fluid-mobile element carriers, containing up to 13 wt% of structurally-bound H₂O combined with FME enrichments of up to 10⁵ times primitive mantle concentrations (Hyndman and Peacock, 2003; Hattori and Guillot, 2007; Vils et al., 2008; Kodolányi et al., 2012). The stabilities of serpentine (Ulmer and Trommsdorff, 1995; Wunder and Schreyer, 1997) and chlorite (Fumagalli and Poli, 2004) from the ocean floor to subarc levels and beyond facilitate the preservation of chemical and structural records from fluid infiltration (Cannaò et al., 2015, Cannaò et al., 2016) and in situ dehydration fluid production (Plümper et al., 2017; Bloch et al., 2018) during subduction, and enable the transport of large amounts water and FMEs to arc source regions (e.g., Scambelluri and Philippot, 2001; John et al., 2004; Hattori and Guillot, 2003; Spandler and Pirard, 2013; Chen et al., 2019). Investigations of geochemical signatures recorded in serpentinites subducted to different depths hence allow for constraining dehydration-related element loss (Scambelluri et al., 2004; John et al., 2011; Kendrick et al., 2011, Kendrick et al., 2013; Lafay et al., 2013) and revealing potential interactions with externally derived fluids during subduction (Deschamps et al., 2010; Scambelluri et al., 2014; Cannaò et al., 2015, Cannaò et al., 2016; Schwarzenbach et al., 2018). Together with structural and textural relations in the field these geochemical data enable obtaining a detailed image of a fossil plate interface, including its physical properties (Hermann et al., 2000; Angiboust et al., 2011, Angiboust et al., 2015; Agard et al., 2016), and fluid transport mechanisms (John et al., 2012; Plümper et al., 2017; Bloch et al., 2018; Taetz et al., 2018; Chen et al., 2019).

A variably important dehydration reaction in serpentinites at intermediate subduction zone depths is the olivine-forming brucite + antigorite consumption at 400–550 °C (Padrón-Navarta et al., 2013; Plümper et al., 2017; Bloch et al., 2018; Bretscher et al., 2018; Kempf and Hermann, 2018), expelling up to a few wt% of structurally-bound water and thus also fluid-mobile elements (FMEs) into dehydration pathways (Scambelluri et al., 2001; De Hoog et al., 2014; Cannaò et al., 2016; Plümper et al., 2017; Bloch et al., 2018; Gilio et al., 2019). Effective loss into fluids or retention in the host serpentinites is not well-constrained for many FMEs, however. Extensive formation of metamorphic olivine by this reaction further requires large modes of brucite in subducting serpentinites, which would be produced following SiO₂-neutral (“isochemical”) serpentinisation (Malvoisin, 2015; Schwarzenbach et al., 2016). Present-day ocean floor serpentinites are characterised by scarcity of brucite due to SiO₂ metasomatism prevailing in fluid-dominated serpentinisation environments; i.e., when serpentinisation is non-isochemical with respect to SiO₂ (Paulick et al., 2006; Boschi et al., 2008; Harvey et al., 2014; Malvoisin, 2015).

The Erro Tobbio (Ligurian Alps, Italy) unit represent variably serpentinized oceanic mantle rocks subducted to ~550–600 °C and 2.0–2.5 GPa (~70 km depth; Scambelluri et al., 1991, Scambelluri et al., 1995, Scambelluri et al., 1997). Cross-cut by olivine + Ti-clinohumite + magnetite dehydration veins linked to brucite + antigorite consumption (Scambelluri et al., 1995; Plümper et al., 2017). The antigorite serpentinites within this unit thus offer a unique opportunity to study fluid-mediated element cycling from serpentinisation to partial dehydration at intermediate subduction zones depth (Früh-Green et al., 2001; John et al., 2011; Scambelluri and Tonarini, 2012). We present a detailed study of a suite of Erro Tobbio antigorite serpentinites addressing the role of brucite and of SiO₂ metasomatism during hydration and subduction dehydration employing bulk and in situ chemical data sets. Major to trace element signatures are used to constrain fluid imprints, to derive implications on serpentinisation environments, and to reconstruct the petrological evolution. Ultimately, vein and wall rock geochemistry are used to qualitatively estimate the compositions of dehydration-related fluids.