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1. Coupled Brittle and Viscous Micromechanisms Produce Semibrittle Flow, Grain‐Boundary Sliding, and Anelasticity in Salt‐Rock

   https://doi.org/10.1029/2020JB021261  

   Ding, J.; Chester, F_M; Chester, J_S; Shen, X.; Arson, C. (February 2021, Journal of Geophysical Research: Solid Earth)

   Abstract The operation of fracture, diffusion, and intracrystalline‐plastic micromechanisms during semibrittle deformation of rock is directly relevant to understanding mechanical behavior across the brittle‐plastic transition in the crust. An outstanding question is whether (1) the micromechanisms of semibrittle flow can be considered to operate independently, as represented in typical crustal strength profiles across the brittle to plastic transition, or (2) the micromechanisms are coupled such that the transition is represented by a distinct rheology with dependency on effective pressure, temperature, and strain rate. We employ triaxial stress‐cycling experiments to investigate elastic‐plastic and viscoelastic behaviors during semibrittle flow in two distinctly different monomineralic, polycrystalline, synthetic salt‐rocks. During semibrittle flow at high differential stress, granular, low‐porosity, work‐hardened salt‐rocks deform predominantly by grain‐boundary sliding and wing‐crack opening accompanied by minor intragranular dislocation glide. In contrast, fully annealed, near‐zero porosity salt‐rocks flow at lower differential stress by intragranular dislocation glide accompanied by grain‐boundary sliding and opening. Grain‐boundary sliding is frictional during semibrittle flow at higher strain rates, but the associated dispersal of water from fluid inclusions along boundaries can activate fluid‐assisted diffusional sliding at lower strain rates. Changes in elastic properties with semibrittle flow largely reflect activation of sliding along closed grain boundaries. Observed microstructures, pronounced hysteresis and anelasticity during cyclic stressing after semibrittle flow, and stress relaxation behaviors indicate coupled operation of micromechanisms leading to a distinct rheology (hypothesis 2 above). 

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2. Lawsonite and Garnet Oxygen Isotope Record of Fluid‐Rock Interaction During Subduction

   https://doi.org/10.1029/2023GC011389  

   Kang, Patricia; Martin, Laure_A_J; Vitale_Brovarone, Alberto; Whitney, Donna_L (April 2024, Geochemistry, Geophysics, Geosystems)

   Abstract During the subduction of an oceanic plate, fluids are released from metabasaltic crust, metasediment, and serpentinite under high‐pressure/low‐temperature conditions. Although some fluids may eventually leave the slab, some participate in metamorphic reactions within the slab during subduction and exhumation. To identify fluid sources and other controls influencing mineral composition, we report the in situ‐measured δ¹⁸O of lawsonite and garnet in blueschist‐ to eclogite‐facies rocks from 10 subduction zones that represent various field settings, including mélanges, structurally coherent terranes, and an eclogite xenolith derived from a subducted plate. Lawsonite records distinct δ¹⁸O depending on the host rock type and other rock types that were fluid sources during lawsonite growth. In general, lawsonite in metabasalt (7.6 ± 0.2–14.8 ± 1.1‰) is isotopically lighter than in metasediment (20.6 ± 1.4–24.1 ± 1.3‰) but heavier than in metagabbro (4.0 ± 0.4–7.9 ± 0.3‰). The extent of δ¹⁸O fractionation was evaluated for lawsonite–fluid and lawsonite–garnet pairs as a function of temperature (T). Results demonstrate that variations of >1.7‰ in lawsonite and >0.9‰ in garnet are not related to changingT. More likely, the relative contributions of fluids derived from isotopically heavier lithologies (e.g., sediments) versus lighter lithologies (e.g., ultramafic rocks) are the major control. Monte Carlo simulations were performed to investigate the sources of metasomatic fluids and the water/rock ratio that formed lawsonite‐bearing metasomatite. Results indicate that δ¹⁸O_Lwsand δ¹⁸O_Grtrecord interactions with fluids sourced from diverse lithologies (sediment, serpentinite), further supporting that δ¹⁸O_Lwsis a useful indicator of subduction fluid‐rock interactions. 

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3. The formation of high-Sr/Y plutons in cordilleran-arc crust by crystal accumulation and melt loss

   https://doi.org/10.1130/GES02400.1  

   Brackman, Adam J.; Schwartz, Joshua J. (February 2022, Geosphere)

   Bulk-rock data are commonly used in geochemical studies as a proxy for melt compositions in order to understand the evolution of crustal melts. However, processes of crystal accumulation and melt migration out of deep-crustal, crystal-rich mush zones to shallower storage regions raise questions about how faithfully bulk-rock compositions in plutons approximate melt compositions. This problem is particularly acute in the lower crust of arcs, where melt reservoirs are subject to periodic melt extraction that leaves behind a cumulate residue. Here, we examine bulk-rock data from the perspective of high-Sr/Y plutonic rocks in the lower crust of a well-exposed Early Cretaceous cordilleran-arc system in Fiordland, New Zealand. We test the validity of using high-Sr/Y bulk-rock compositions as proxies for melts by comparing bulk-rock compositions to melts modeled from >100 major- and trace-element analyses of 23 magmatic clinopyroxene grains from the same samples. The sampling locations of the igneous clinopyroxenes and encompassing bulk rocks are distributed across ~550 km2 of exhumed lower crust and are representative of Mesozoic lower-crustal arc rocks in the Median batholith. We confirm that bulk-rock data have characteristics of high-Sr/Y plutons (Sr/Y >50, Na2O >3.5 wt%, Sr >1000 ppm, and Y <20 ppm), features that have been previously interpreted to indicate the presence of garnet as a residual or fractionating phase. In contrast to bulk rocks, igneous clinopyroxenes have low Sr (<100 ppm), high Y (25–100 ppm), and low molar Mg# [100 × Mg/(Mg + Fe)] values (60–70), which are consistent with derivation from fractionated, low-Sr/Y melts. Chondrite-normalized rare-earth-element patterns and Sm/Yb values in clinopyroxenes also show little to no evidence for involvement of garnet in the source or in differentiation processes. Fe-Mg partitioning relationships indicate that clinopyroxenes are not in equilibrium with their encompassing bulk rocks but could have been in equilibrium with melt compositions determined from chemometry of coexisting igneous hornblendes. Moho-depth calculations based on bulk-rock Sr/Y values also yield Moho depths (average = 69 km) that are inconsistent with Moho depths based on bulk-rock Ce/Y, contact aureole studies, Al-in-horn- blende crystallization pressures, and our modeled clinopyroxene crystallization pressures. These data indicate that most Mesozoic high-Sr/Y bulk rocks in the lower crust of Fiordland are cumulates formed by plagioclase + amphibole + clinopyroxene accumulation and interstitial melt loss from crystal-rich mush zones. Our data do not support widespread fractionation of igneous garnet nor partial melting of a garnet-bearing source in the petrogenesis of these melts. We speculate that melt extraction and the production of voluminous cumulates in the lower crust were aided by unusually high heat flow and high magma addition rates associated with an Early Cretaceous arc flareup. We conclude that bulk-rock compositions are poor proxies for melt compositions in the lower crust of the Median batholith, and geochemical modeling of these high-Sr/Y bulk rocks would overemphasize the role of garnet in their petrogenesis. 

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4. The hydrothermal solubility of monazite-(Ce) and xenotime-(Y)

   Gysi, A.P.; Harlov, D.; Miron, G.D. (January 2018, Goldschmidt conference)

   CePO4 and YPO4 are major components in monazite and xenotime, respectively, which are common hydrothermal phases in REE mineral deposits. Both minerals also occur as secondary minerals in iron-oxide-apatite deposits [1,2], and as accessory phases in high-grade metamorphic rocks where they display varying degrees of metasomatism. Studying the cause of their compositional variations using thermodynamic modeling may provide geochemical signals for interpreting P- T-x of crustal fluid-rock interaction. The thermodynamic properties of monazite and xenotime have been determined using several calorimetric methods [3], but only a few solubility studies have been undertaken, which test the reliability of both the calorimetric data and thermodynamic properties of associated REE aqueous species [4]. Combining available calorimetric data with the REE aqueous species from Haas et al. [5], implemented in the Supcrt92 database [6], yields several orders of magnitude differences when compared with our solubility measurements. To reconcile these discrepancies, we have used the GEMS code package [7,8] and GEMSFITS [9] for parameter optimization, and re- evaluated the standard Gibbs energies for aqueous REE species, while maintaining consistency with available calorimetric measurement of the REE phosphates. This study points to a need to revise the thermodynamic properties of the REE hydroxyl species, which will have an impact on the calculated solubilities of the REE phosphates and our understanding of the mobility of REE in hydrothermal fluids. Our new experimental data will be implemented in the MINES thermodynamic database (http:// tdb.mines.edu) [10] for modeling the chemistry of crustal fluid-rock equilibria. 

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5. Abiotic methane synthesis and serpentinization in olivine-hosted fluid inclusions

   https://doi.org/10.1073/pnas.1907871116  

   Klein, Frieder; Grozeva, Niya G.; Seewald, Jeffrey S. (September 2019, Proceedings of the National Academy of Sciences)

   The conditions of methane (CH 4 ) formation in olivine-hosted secondary fluid inclusions and their prevalence in peridotite and gabbroic rocks from a wide range of geological settings were assessed using confocal Raman spectroscopy, optical and scanning electron microscopy, electron microprobe analysis, and thermodynamic modeling. Detailed examination of 160 samples from ultraslow- to fast-spreading midocean ridges, subduction zones, and ophiolites revealed that hydrogen (H 2 ) and CH 4 formation linked to serpentinization within olivine-hosted secondary fluid inclusions is a widespread process. Fluid inclusion contents are dominated by serpentine, brucite, and magnetite, as well as CH 4( g ) and H 2( g ) in varying proportions, consistent with serpentinization under strongly reducing, closed-system conditions. Thermodynamic constraints indicate that aqueous fluids entering the upper mantle or lower oceanic crust are trapped in olivine as secondary fluid inclusions at temperatures higher than ∼400 °C. When temperatures decrease below ∼340 °C, serpentinization of olivine lining the walls of the fluid inclusions leads to a near-quantitative consumption of trapped liquid H 2 O. The generation of molecular H 2 through precipitation of Fe(III)-rich daughter minerals results in conditions that are conducive to the reduction of inorganic carbon and the formation of CH 4 . Once formed, CH 4( g ) and H 2( g ) can be stored over geological timescales until extracted by dissolution or fracturing of the olivine host. Fluid inclusions represent a widespread and significant source of abiotic CH 4 and H 2 in submarine and subaerial vent systems on Earth, and possibly elsewhere in the solar system. 

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