Jay. J. Ague

Water availability in the lithosphere has been crucial to the geological evolution of Earth as well as the emergence and persistence of life. The global geological water cycle associated with plate tectonics has been understood as a system controlled by the presence of oxygen and hydrogen, either in fluids and melts or bound within mineral structures. However, recent work on H ₂ production in the lithosphere indicates that a water mass equivalent to about 25 to 50% of global annual water inputs into subduction is converted to H ₂ every year. This H ₂ can be decoupled from the water cycle and potentially lost to space over geological timescales. Here, we show that the interaction of H ₂ -rich fluids with oxygen-bearing minerals results in the formation of unconventional redox water. This influences the residence time of hydrogen in Earth’s interior and offers previously unidentified perspectives on how hydrous fluids, minerals, and melts may form in initially dry geological reservoirs.

Crystallographically oriented lamellar to acicular rutile ± ilmenite inclusions in garnet may record important information about the pressure (P), temperature (T), and petrotectonic evolution of the lithosphere. However, reading this record requires a robust understanding of titanium (Ti) substitution mechanisms and solubility in garnet. To this end, we examine the Ti substitution systematics revealed by 87 garnet analyses from previously published experimental studies relevant for granulites, high-pressure granulites, and deep crustal magmatic rocks. The experimental P-T conditions range mostly between 850–1230 °C and 1.0–2.7 GPa; four 3.2 GPa, 1075–1150 °C ultrahigh-P, ultrahigh-T examples are also included. We find that normalizing garnet formulas based on two octahedral sites per 12 O atoms provides better Fe3+ estimates than the standard 8-total-cation scheme. The Fe3+ must be constrained as both it and Ti substitute on octahedral sites. Graphical analysis reveals that the main substitutions are Tschermak-Ti (VITi4+ + IVAl3+ ↔ VIAl3+ + IVSi4+) and andradite (VIAl3+ ↔ VIFe3+), with lesser but nonetheless important contributions by andradite-Ti (VITi4+ + IVAl3+ ↔ VIFe3+ + IVSi4+) and alkali-Ti (VITi4+ + VIII(Na+,K+) ↔ VIAl3+ + VIIIM2+). Here, VIIIM2+ denotes dodecahedrally coordinated divalent cations (Ca, Fe2+, Mg, Mn). We also posit a small role for Ti substitution coupled to dodecahedral vacancies. These inferences were independently tested using a Monte Carlo modeling procedure developed herein that quantitatively reproduces both the nature and extent of the observed substitutions. We also compared the experimental data with a representative suite of natural garnet analyses. In general, the natural garnets have far lower Ti and alkali (Na ± K) contents than the experimental ones. We attribute this to Ti and alkali loss in nature during exhumation and cooling by processes including diffusion and interface-coupled dissolution-reprecipitation. Some fraction of the lost Ti can be hosted by exsolved precipitates of crystallographically oriented rutile ± ilmenite. However, in many cases, these are absent, and the Ti must have left the garnet structure entirely. The lost alkalis may be hosted by exsolved amphibole or mica precipitates, but these are relatively uncommon, suggesting that alkalis are typically transferred from garnet to the matrix or to non-precipitate inclusions.

Metamorphic devolatilization of subducted slabs generates aqueous fluids that ascend into the mantle wedge, driving the partial melting that produces arc magmas. These magmas have oxygen fugacities some 10–1,000 times higher than magmas generated at mid-ocean ridges. Whether this oxidized magmatic character is imparted by slab fluids or is acquired during ascent and interaction with the surrounding mantle or crust is debated. Here we study the petrology of metasedimentary rocks from two Tertiary Aegean subduction complexes in combination with reactive transport modelling to investigate the oxidative potential of the sedimentary rocks that cover slabs. We find that the metasedimentary rocks preserve evidence for fluid-mediated redox reactions and could be highly oxidized. Furthermore, the modelling demonstrates that layers of these oxidized rocks less than about 200 m thick have the capacity to oxidize the ascending slab dehydration flux via redox reactions that remove H₂, CH₄and/or H₂S from the fluids. These fluids can then oxidize the overlying mantle wedge at rates comparable to arc magma generation rates, primarily via reactions involving sulfur species. Oxidized metasedimentary rocks need not generate large amounts of fluid themselves but could instead oxidize slab dehydration fluids ascending through them. Proposed Phanerozoic increases in arc magma oxygen fugacity may reflect the recycling of oxidative weathering products following Neoproterozoic–Palaeozoic marine and atmospheric oxygenation.