From oceanic to continental subduction: Implications for the geochemical and redox evolution of the supra-subduction mantle

Abstract

Processes taking place in subduction zones are the main controller of the chemical cycle of volatile and incompatible elements in the Earth system. Meta morphic devolatilization reactions occurring during slab burial play a key role in the transfer of elements to the supra-subduction mantle, from forearc to sub-arc depth. Here, we discuss the elements released in fluids and melts from oceanic (i.e., sediments, altered oceanic crust, and hydrated lithospheric mantle) and continental slab materials during prograde subduction and the consequent implications in the chemical evolution of the supra-subduction mantle. We use bulk data and fluid and melt inclusions analyses to show and to constrain the mobility of elements from top to bottom in the subduction zone setting. The development of mélange domains at the slab-mantle interface and its influence in the element cycle are also taken into account. Coupled with trace-element mobility, we review the redox evolution of slab materials during subduction and its implication in the redox conditions of the supra-subduction mantle due to fluid and melt infiltration down to sub-arc depth.

Along-dip variations of subduction fluids: The 30–80 km depth traverse of the Schistes Lustrés complex (Queyras-Monviso, W. Alps)

“…In the SL calcschists, CH 4 and CO 2 may be locally-derived, from the reduced carbonaceous J o u r n a l P r e -p r o o f material of pelitic layers and from carbonate-rich horizons, respectively. CO 2 may in part form by oxidation of CH 4 (Tarantola et al, 2007(Tarantola et al, , 2009, so that variations in the CO 2 /CH 4 ratio among different veins may also reflect local changes in redox conditions depending on lithology (Tarantola et al, 2009;Cannaò and Malaspina, 2018).…”

The water and fluid-mobile element cycles during serpentinite subduction. A review

“…11; see also Scambelluri & Tonarini, 2012), which commonly display Fe 3+ /Fe tot higher than MORB and OIB basalts because of addition to their mantle sources of fluids carrying oxidised species, like H 2 O, CO 2 , Fe 2 O 3 and sulfate (Arculus, 1994;Kelley & Cottrell, 2009;Malaspina et al, 2009Malaspina et al, , 2010Evans & Tomkins, 2011;Evans & Powell, 2015;Evans et al, 2017). In turn, the speciation of subduction fluids is controlled by the variable oxidation state of the source rocks (Tumiati et al, 2015;Debret & Sverjensky, 2017;Evans et al, 2017;Cannaò & Malaspina, 2018;Tumiati & Malaspina, 2019). Because serpentinite is the rock that primarily controls the release of slab fluids, efforts have been recently done to determine the redox state of serpentinized rocks and fluids by means of (i) direct measurements of the Fe 3+ content of bulk rocks and serpentine (Andreani 420 M. Scambelluri et al et al, 2013;Debret et al, 2014bDebret et al, , 2015Evans et al, 2017) and (ii) theoretical calculations (Debret & Sverjensky, 2017).…”

Subducting serpentinites release reduced, not oxidized, aqueous fluids

“…1). Hydrothermally altered oceanic lithosphere contains oxidized species (Fe 3+ , C 4+ , S 6+ ) and these elements are present in different concentrations in sediments, mafic crust and mantle lithologies that are introduced into subduction zones 7–9 . The potential release of such oxidized species during dehydration reactions may be coupled with a change in the rock redox state and thus explain the oxidation of the mantle wedge by infiltrating fluids 10 .

Figure 1Cartoon of a subduction zone illustrating the subduction of hydrated, relatively oxidized, oceanic lithosphere and where fluids releasing reactions from ultramafic rocks occur at the slab surface, along with calculated fluid f O 2 expressed as ∆logQFM (in square bracket) and composition.

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