The intrinsic nature of antigorite breakdown at 3 GPa: Experimental constraints on redox conditions of serpentinite dehydration in subduction zones

Abstract

Antigorite is considered as the most important source of water in subduction zones, playing a key role during arc magma genesis. Although, these magmas seem more oxidized than midoceanic ridge basalts (MORB), the possible inherent link between the oxidation state of arc magmas and serpentinite-derived hydrous fluids is still not well established. Here, we have performed dehydration experiments of natural antigorite serpentinite containing 5 weight percent (wt%) magnetite at 3 GPa and in a temperature range from 600 to 900 °C using a multianvil apparatus. These experiments aim to reproduce the different stages of H2O release, forming chlorite, olivine and orthopyroxene and water. Our experimental set up permits to preserve the intrinsic oxygen fugacity (fO2) of serpentinites during their dehydration. The new olivine and orthopyroxene which formed in equilibrium with antigorite, chlorite and magnetite have high XMg numbers setting up the oxygen fugacity to high values, between 3.0 and 4.1 log units above QFM (Quartz-Fayalite-Magnetite buffer). Hematite is observed concomitantly with high XMg in olivine, of 0.94-0.97, generally at low temperatures, below 800 °C, in coexistence with chlorite. Once the magnetite is destabilized, upon chlorite breakdown, above 800°C, the oxygen fugacity decreases to 3.7 due to the decrease of the XMg of silicates. This study demonstrates the highly oxidizing nature of the fluids released from antigorite dehydration. Thus, at high pressure and high temperature conditions, fO2-sensitive elements such as carbon and sulfur are expected to be mobilized under their oxidized form, providing an oxidizing context for arc magmas genesis and assuming that they are not completely reduced by their percolation through meta-gabbro, meta-basalts and meta-sediments.

The stability of antigorite in subduction zones revisited: the effect of F on antigorite stability and its breakdown reactions at high pressures and high temperatures, with implications for the geochemical cycles of halogens

“…1A). These results agree well with published literature values (Ulmer and Trommsdorff 1995;Wunder and Schreyer 1997;Bromiley and Pawley 2003;Maurice et al 2020), and in this system antigorite breaks down to form olivine, orthopyroxene, and fluid.…”

Serpentinite-derived slab fluids control the oxidation state of the subarc mantle

“…If metamorphism and deserpentinization does result in a decrease of Fe(III)/ΣFe, the reduction of Fe(III) to Fe(II) would require the formation of an oxidized component such as SO x or CO x . This interpretation, while still debated, would suggest that serpentinite dehydration has the potential to release oxidizing fluids with an isotopically heavy  11 B signature (15)(16)(17)23).…”

“…Evans and Frost (18) elaborated on a variety of processes that may affect the redox budget of subducted serpentinites and, thereby, the capacity of serpentinite-derived fluids to oxidize subarc mantle. While some studies conclude that fluids released by serpentine dehydration are sufficiently oxidized to affect the redox budget of arc magmas (15)(16)(17), others suggest that these fluids may be notably less oxidized (42). Serpentinite within the subducting plate has isotopically heavy B ( 11 B up to +40‰) and is a possible source of 11 B in arc magmas (37).…”

“…However, the dehydration of subducted serpentinite has been increasingly recognized as an important source of volatiles (e.g., water, sulfur, and halogens) in the subarc mantle (10)(11)(12). Like AOC (7,13), breakdown of oxidized serpentinites can produce highly oxidizing fluids (14)(15)(16)(17)(18), which may have an important influence on the f O2 of arc magmas. Boron (B) isotopes ( 11 B = [( 11 B/ 10 B) Sample /( 11 B/ 10 B) SRM951 -1]*1000) are a powerful tool for tracing the presence of serpentinite-derived fluids, as 11 B is depleted in the upper mantle [B < 0.1 parts per million (ppm);  11 B = −10 to −7‰] but is extremely enriched in serpentinite (up to 100 ppm B;  11 B up to +40‰) (19).…”

Sea-level stability over geologic time owing to limited deep subduction of hydrated mantle

“…Info). Furthermore, the poorly constrained effect of some minor elements (Al, Fe) and the unknown prevailing oxidation state of ultramafic rocks under high-PT conditions [20,21] preclude proper petrological modeling of subducted ultramafic rocks. Consequently, experimental and modeled phase diagrams often differ at pressures above 6 GPa ( Fig.…”