An experimental investigation of C–O–H fluid-driven carbonation of serpentinites under forearc conditions

Abstract

The carbonation of serpentinites in the forearc region of the mantle wedge in subduction zones and of serpentinites within the subducting slab by fluids derived from prograde dehydration and decarbonation have important implications for the deep Earth carbon cycle. This study shows that the carbonation of serpentinites under the forearc can establish, over time, a significant reservoir for carbon within a partially hydrated mantle wedge and that carbonation of (ultra-) mafic rocks within the subducting slab contributes to C-transfer to greater depths and might supply carbon for arc volcanism or the deep mantle. We report a new high pressure experimental investigation of the interactions between oxidised CO -H fluids and serpentinite and model the reaction progress with time series experiments. The CO 2 , H 2 O and alkane (C n H 2n+2 , n=1-6; e.g. methane, ethane) contents in the fluid phase from quenched experimental run products have been analysed by gas chromatography and the results are compared with thermodynamic calculations. With progressive carbonation, the formation of magnesite + chlorite together with quartz, quartz + talc or talc at 1-2 GPa and 500-650 °C was observed. At temperatures above antigorite stability (T≳700 ºC and 2 GPa) magnesite + chlorite is stable together with talc, talc + enstatite, enstatite or enstatite + forsterite for decreasing CO 2-content in the fluid. Highlights Quantitative CO -H analyses of fluids after piston cylinder experiments Carbonation of serpentinites in the forearc region of the mantle wedge Carbonation is rapid and occurs within the first hour of the experiments Magnesite is a good monitor for Ca, Ba, Sr and Pb in subduction zone fluids

Subduction metamorphism of serpentinite‐hosted carbonates beyond antigorite‐serpentinite dehydration (Nevado‐Filábride Complex, Spain)

“…Metamorphic devolatilization reactions of altered oceanic crust and sediments in the slab generate C‐ and Mg/Ca‐rich fluids (Caciagli & Manning, ; Facq et al., ; Tiraboschi et al., ). At different arc depths, reaction of these fluids with slab and mantle wedge serpentinites may produce ophimagnesite (magnesite–antigorite rocks), talc–magnesite rocks and listvenites (Falk & Kelemen, ; Menzel et al., ; Sieber, Hermann, & Yaxley, ), HP‐ophicarbonate (e.g. Scambelluri et al., ) and carbonate–enstatite rocks (Tumiati et al., ).…”

Carbonation and decarbonation reactions: Implications for planetary habitability

“…This fluid may be dominantly H 2 O, but subduction zone decarbonation reactions may supply CO 2 as well (see section on decarbonation). The ultramafic rocks of the mantle are highly reactive with CO 2 fluids, so when the slab-derived fluid rises into the mantle, carbonation reactions are fast (Sieber et al 2018). Because the degree of carbonation increases with lower temperatures (Sieber et al 2018), mantle wedge carbonation probably dominates in the cooler (but still hot at < ~700 °C) fore-arc region and is less pronounced in the hotter mantle directly below the volcanic arc.…”

“…Whatever the decarbonation mechanism, CO 2 released by subducting rocks may follow several paths. It could flow through the many kilometers of overriding lithosphere (or perhaps along the subduction interface) to escape to the atmosphere as part of a diffuse (i.e., spatially widespread) metamorphic flux (e.g., Sakai et al 1990;Sano and Williams 1996;Campbell et al 2002), or it could become trapped in the overlying mantle wedge by a carbonation reaction (e.g., Piccoli et al 2016Piccoli et al , 2018Scambelluri et al 2016;Sieber et al 2018). Once in the mantle wedge, that carbon could be stored for millions of years.…”

Carbonation and Decarbonation Reactions: Implications for Planetary Habitability

“…The ultramafic rocks of the mantle are highly reactive with CO2 fluids, so when the slab-derived fluid rises into the mantle, carbonation reactions are fast (Sieber et al 2018). Because the degree of carbonation increases with lower temperatures (Sieber et al 2018), mantle wedge carbonation probably dominates in the cooler (but still hot at < ~700 °C) fore-arc region and is less pronounced in the hotter mantle directly below the volcanic arc.…”

“…Whatever the decarbonation mechanism, CO2 released by subducting rocks may follow several paths. It could flow through the many kilometers of overriding lithosphere (or perhaps along the subduction interface) to escape to the atmosphere as part of a diffuse (i.e., spatially widespread) metamorphic flux (e.g., Sakai et al 1990;Sano & Williams 1996;Campbell et al 2002), or it could become trapped in the overlying mantle wedge by a carbonation reaction (e.g., Piccoli et al 2016Piccoli et al , 2018Scambelluri et al 2016;Sieber et al 2018). Once in the mantle wedge, that carbon could be stored for millions of years.…”