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Author Posting. © American Geophysical Union, 2022. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 49(15), (2022): e2022GL099185, https://doi.org/10.1029/2022gl099185., Several large strike slip faults in central and northern California accommodate plate motions through aseismic creep. Although there is no consensus regarding the underlying cause of aseismic creep, aqueous fluids and mechanically weak, velocity-strengthening minerals appear to play a central role. This study integrates field observations and thermodynamic modeling to examine possible relationships between the occurrence of serpentinite, silica-carbonate rock, and CO2-rich aqueous fluids in creeping faults of California. Our models predict that carbonation of serpentinite leads to the formation of talc and magnesite, followed by silica-carbonate rock. While abundant exposures of silica-carbonate rock indicate complete carbonation, serpentinite-hosted CO2-rich spring fluids are strongly supersaturated with talc at elevated temperatures. Hence, carbonation of serpentinite is likely ongoing in parts of the San Andres Fault system and operates in conjunction with other modes of talc formation that may further enhance the potential for aseismic creep, thereby limiting the potential for large earthquakes., This work was supported by National Science Foundation (NSF) grants NSF-EAR-1220280 to F. K. and J. L., NSF-EAR-1219908 to D. G., and NSF-OCE-2001728 to J. L.

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2023., Subduction zones are important sites of material recycling on Earth, with volatiles playing key roles in mass transfer processes and magma formation. This thesis investigates outstanding questions associated with a continuum of interrelated processes that occur as oceanic plates descend in subduction zones by integrating petrological and geochemical constraints from exhumed high-pressure rocks and erupted arc magmas, high pressure-temperature laboratory experiments, and thermodynamic calculations. Chapters 2 and 3 investigate the fluid-mediated reactions between mafic and ultramafic rocks at conditions relevant to the slab-mantle interface and show that Mg-metasomatism of mafic rocks to form chlorite-rich assemblages is favored and is likely more pervasive in subduction zones than in oceanic settings. Contrary to common belief, talc is unlikely to form in high abundance in ultramafic rocks metasomatized by Si-rich slabderived fluids. This means that talc-rich assemblages formed via Si-metasomatism along the slabmantle interface are less likely to be playing prominent roles in volatile transport, in facilitating slow-slip events, and in controlling the decoupling-coupling transition of the plate interface. Chapter 4 experimentally investigates the phase equilibria, melting, and density evolution of mélange rocks that formed by mixing and fluid-rock interactions. Results show that melting of mélanges is unlikely to occur along slab-tops at pressures ≤ 2.5 GPa. Accordingly, diapirism into the hotter mantle wedge would be required to initiate melting. The density contrast between mélanges and the overlying mantle would allow for buoyancy-driven diapirism at relatively low pressures and melting could subsequently occur in the hotter mantle wedge during ascent. However, diapir buoyancy may be limited at higher pressures due to the formation of abundant garnet especially in mélange rocks with peraluminous composition. Chapter 5 experimentally investigates the compositions, This work has been supported financially by the National Science Foundation-Office of International Science & Engineering, Petrology & Geochemistry (NSF-OISE) PIRE, Award# 1545903 to F.K. and the Geoprisms program Collaborative Research Award# 1852610 to V.L.R. Research support through the WHOI Ocean Ventures Fund, US Science Support Program E-FIRE European Training Fund, WHOI Conference Fund are awarded to E.A.C.

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Codillo, E., Klein, F., Dragovic, B., Marschall, H., Baxter, E., Scambelluri, M., & Schwarzenbach‬, E. Fluid‐mediated mass transfer between mafic and ultramafic rocks in subduction zones. Geochemistry Geophysics Geosystems, 23, (2022): e2021GC010206, https://doi.org/10.1029/2021gc010206., Metasomatic reaction zones between mafic and ultramafic rocks exhumed from subduction zones provide a window into mass-transfer processes at high pressure. However, accurate interpretation of the rock record requires distinguishing high-pressure metasomatic processes from inherited oceanic signatures prior to subduction. We integrated constraints from bulk-rock geochemical compositions and petrophysical properties, mineral chemistry, and thermodynamic modeling to understand the formation of reaction zones between juxtaposed metagabbro and serpentinite as exemplified by the Voltri Massif (Ligurian Alps, Italy). Distinct zones of variably metasomatized metagabbro are dominated by chlorite, amphibole, clinopyroxene, epidote, rutile, ilmenite, and titanite between serpentinite and eclogitic metagabbro. Whereas the precursor serpentinite and oxide gabbro formed and were likely already in contact in an oceanic setting, the reaction zones formed by diffusional Mg-metasomatism between the two rocks from prograde to peak, to retrograde conditions in a subduction zone. Metasomatism of mafic rocks by Mg-rich fluids that previously equilibrated with serpentinite could be widespread along the subduction interface, within the subducted slab, and the mantle wedge. Furthermore, the models predict that talc formation by Si-metasomatism of serpentinite in subduction zones is limited by pressure-dependent increase in the silica activity buffered by the serpentine-talc equilibrium. Elevated activities of aqueous Ca and Al species would also favor the formation of chlorite and garnet. Accordingly, unusual conditions or processes would be required to stabilize abundant talc at high P-T conditions. Alternatively, a different set of mineral assemblages, such as serpentine- or chlorite-rich rocks, may be controlling the coupling-decoupling transition of the plate interface., M. Scambelluri acknowledges the Italian Ministry of Research MUR for granting the PRIN project n. 2017ZE49E7. This research was funded by NSF-OISE (Office of International Science & Engineering, Petrology & Geochemistry) PIRE, Award #1545903, and the WHOI Ocean Ventures Fund.

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Zhang, Y., Gazel, E., Gaetani, G. A., & Klein, F. Serpentinite-derived slab fluids control the oxidation state of the subarc mantle. Science Advances, 7(48), (2021): eabj2515, https://doi.org/10.1126/sciadv.abj2515., Recent geochemical evidence confirms the oxidized nature of arc magmas, but the underlying processes that regulate the redox state of the subarc mantle remain yet to be determined. We established a link between deep subduction-related fluids derived from dehydration of serpentinite ± altered oceanic crust (AOC) using B isotopes and B/Nb as fluid proxies, and the oxidized nature of arc magmas as indicated by Cu enrichment during magma evolution and V/Yb. Our results suggest that arc magmas derived from source regions influenced by a greater serpentinite (±AOC) fluid component record higher oxygen fugacity. The incorporation of this component into the subarc mantle is controlled by the subduction system’s thermodynamic conditions and geometry. Our results suggest that the redox state of the subarc mantle is not homogeneous globally: Primitive arc magmas associated with flat, warm subduction are less oxidized overall than those generated in steep, cold subduction zones., Y.Z. acknowledges funding from the National Science Foundation of China (91958213), the Chinese Academy of Sciences (XDB42020402), and the Shandong Provincial Natural Science Foundation, China (ZR2020QD068). This study was supported in part by the U.S. National Science Foundation NSF EAR 1826673 to E.G. and G.A.G. and OCE 1756349 to E.G.

Talc formation via silica-metasomatism of ultramafic rocks is believed to play key roles in subduction zone processes. Yet, the conditions of talc formation remain poorly constrained. We used thermodynamic reaction-path models to assess the formation of talc at the slab-mantle interface and show that it is restricted to a limited set of pressure–temperature conditions, protolith, and fluid compositions. In contrast, our models predict that chlorite formation is ubiquitous at conditions relevant to the slab-mantle interface of subduction zones. The scarcity of talc and abundance of chlorite is evident in the rock record of exhumed subduction zone terranes. Talc formation during Si-metasomatism may thus play a more limited role in volatile cycling, strain localization, and in controlling the decoupling-coupling transition of the plate interface. Conversely, the observed and predicted ubiquity of chlorite corroborates its prominent role in slab-mantle interface processes that previous studies attributed to talc. Key Points: Limited talc formation by Si-metasomatism of ultramafic rocks in subduction zones Chlorite formation is likely pervasive at the slab-mantle interface Preferential formation of chlorite has wide-ranging chemical and physical implications for subduction zone processes Plain Language Summary: In subduction zones, talc can form during chemical reactions of mantle rocks with silica-enriched fluids at the interface between descending oceanic plates and the overriding mantle. Its formation and distribution in subduction zones are believed to affect the volatile budget, rheological properties, and the down-dip limit of the decoupling of the slab-mantle interface. Therefore, illuminating the conditions that facilitate talc formation at high pressure-temperature conditions is key in assessing its roles in fundamental subduction zone processes. Using thermodynamic reaction-path models, we show that the formation of talc at the slab-mantle interface is restrict