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1. Trace and Rare Earth Element Compositions of Lawsonite as a Chemical Tracer of Metamorphic Processes in Subduction Zones

   https://doi.org/10.1093/petrology/egac065  

   Kang, Patricia; Whitney, Donna L; Martin, Laure A; Fornash, Katherine F (August 2022, Journal of Petrology)

   Abstract Lawsonite is a major host mineral of trace elements (TEs; e.g. REE, Sr, Pb, U, Th) and H2O in various rock types (metabasite, metasediment, metasomatite) over a wide range of depths in subduction zones. Consequently, the composition of lawsonite is a useful archive to track chemical exchanges that occurred during subduction and/or exhumation, as recorded in high-pressure/low-temperature (HP/LT) terranes. This study provides an extensive dataset of major element and TE compositions of lawsonite in HP/LT rocks from two mélanges (Franciscan/USA; Rio San Juan/Dominican Republic), two structurally coherent terranes (Tavşanlı/Turkey; Alpine Corsica/France), and the eclogite blocks of the Pinchi Lake/Canada complex. Bulk major and TE compositions were also determined for lawsonite-bearing host rocks to understand petrogenesis and assess compositional evolution. Most analyzed mélange and coherent-terrane metabasalts have normal mid-ocean ridge/back-arc basin basalt signatures and they preserve compositional evidence supporting interactions with (meta)sediment ± metagabbro/serpentinite (e.g. LILE/LREE enrichments; Ni/Cr enrichments). Most lawsonite grains analyzed are compositionally zoned in transition-metal elements (Fe, Ti, Cr), other TEs (e.g. Sr, Pb), and/or REE, with some grains showing compositional variations that correlate with zoning patterns (e.g. Ti-sector zoning, core-to-rim zoning in Fe, Cr-oscillatory zoning). Our results suggest that compositional variations in lawsonite formed in response to crystallographic control (in Ti-sector zoning), fluid–host rock interactions, modal changes in minerals, and/or element fractionation with coexisting minerals that compete for TEs (e.g. epidote, titanite). The Cr/V and Sr/Pb ratios of lawsonite are useful to track the compositional influence of serpentinite/metagabbro (high Cr/V) and quartz-rich (meta)sediment (low Sr/Pb). Therefore, lawsonite trace and rare earth element compositions effectively record element redistribution driven by metamorphic reactions and fluid–rock interactions that occurred in subduction systems. 

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2. Distinct P-T histories in a subduction mélange reveal underplating/mixing processes at the plate interface (North Motagua Mélange, Guatemala)

   Bonnet, G; Flores, K; Martin, C; Harlow, G (December 2020, American Geophysical Union, Fall Meeting 2019)

   Relicts of subducted and exhumed ocean floor preserved in suture zones record the events occurring at the plate interface. In particular, underplating and exhumation are the two main processes required to recover rocks from mantle depths. High-grade blocks exposed in serpentinite mélanges of the Motagua Valley record evidence of past subduction events between the North American Plate and the Caribbean Plate. Previous works suggest the existence of two subduction zones during Cretaceous, with cold metamorphism (lawsonite eclogite and blueschists) in the South (South Motagua Mélange), and warmer eclogites and amphibolites in the North (North Motagua Mélange, NMM). Although little work as been done so far to characterize the P-T paths and variability of the metabasite blocks embedded within serpentinite matrix in the NMM. Here we present new thermobarometric estimates using conventional thermobarometry, pseudosection modeling and thermometry of carbonaceous matter on a set of metabasites of different grades. There a minimum of four kinds of P-T paths: (1) (lawsonite-bearing) garnet-blueschists with peak P-T around 2.1 GPa and 480°C, (2) "cold eclogites" at ~2.2 GPa and 550°C experiencing isothermal decompression and epidote-amphibolite overprints, (3) "warm eclogites" at ~2.3 GPa and 600°C exhumed in cold environments and affected by blueschist-facies overprints, and (4) garnet-bearing epidote-amphibolites that may represent either retrogression of some eclogites, or prograde metamorphism under warm conditions. We find that garnet fractionation has a limited impact on isopleth-derived P-T estimates and that lawsonite breakdown may drive retrograde metamorphism and rheological switches at the plate interface. These new P-T estimates suggest that high-pressure rocks of the NMM may be recovered from different depths of a unique subduction zone, between 65 and 80 km, and exhumed in a relatively cold (and serpentinized) environment. This suggests a more complicated story than previously described, and calls for additional geochronological evaluation (in process). 

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3. Oxidized Sulfur Species in Slab Fluids as a Source of Enriched Sulfur Isotope Signatures in Arcs

   https://doi.org/10.1029/2024GC011542  

   Beaudry, Patrick; Sverjensky, Dimitri A (July 2024, Geochemistry, Geophysics, Geosystems)

   Abstract Recycling of oxidized sulfur from subducting slabs to the mantle wedge provides simultaneous explanations for the elevated oxygen fugacity (fO2) in subduction zones, their high hydrothermal and magmatic sulfur outputs, and the enriched sulfur isotopic signatures (i.e., δ34S > 0‰) of these outputs. However, a quantitative understanding of the abundance and speciation of sulfur in slab fluids consistent with high pressure experiments is lacking. Here we analyze published experimental data for anhydrite solubility in H₂O‐NaCl solutions to calibrate a high‐pressure aqueous speciation model of sulfur within the framework of the deep earth water model. We characterize aqueous complexes, required to account for the high experimental anhydrite solubilities. We then use this framework to predict the speciation and solubility of sulfur in chemically complex fluids in equilibrium with model subducting mafic and ultramafic lithologies, from 2 to 3 GPa and 400 to 800°C at logfO₂from FMQ‐2 to FMQ+4. We show that sulfate complexes of calcium and sodium markedly enhance the stability of sulfate in moderately oxidized fluids in equilibrium with pyrite atfO₂conditions of FMQ+1 to +2, causing large sulfur isotope fractionations up to 10‰ in the fluid relative to the slab. Such fluids could impart oxidized, sulfur‐rich and high δ³⁴S signatures to the mantle wedge that are ultimately transferred to arc magmas, without the need to invoke³⁴S‐rich subducted lithologies. 

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4. Metasomatism and Slow Slip: Talc Production Along the Flat Subduction Plate Interface Beneath Mexico (Guerrero)

   https://doi.org/10.1029/2023GC010981  

   Lindquist, P. C.; Condit, C. B.; Hoover, W. F.; Hernández‐Uribe, D.; Guevara, V. E. (August 2023, Geochemistry, Geophysics, Geosystems)

   Talc‐rich rocks are common in exhumed subduction zone terranes and may explain geophysical observations of the subduction zone interface, particularly beneath Guerrero, Mexico, where the Cocos plate subducts horizontally beneath North America and episodic tremor and slow slip (ETS) occurs. We present petrologic models exploring (a) the degree of silica metasomatism required to produce talc in serpentinized peridotites at the pressure‐temperature conditions of the plate interface beneath Guerrero and (b) the amount of silica‐bearing water produced by rocks from the subducting Cocos plate and the location of fluid pulses. We estimate the volumes of talc produced by the advection of silica‐rich fluids into serpentinized peridotites at the plate interface over the history of the flat‐slab system. In the ETS‐hosting region, serpentinites must achieve ∼43 wt. % SiO₂to stabilize talc, but minor additions of silica beyond this produce large volumes of talc. Our models of Cocos plate dehydration predict that water flux into the interface averages 3.9 × 10⁴ kg m⁻² Myr⁻¹but suggest that only where subducting basalts undergo major dehydration reactions will sufficient amounts of silica‐rich fluids be produced to drive significant metasomatism. We suggest that talc produced by advective transport of aqueous silica alone cannot account for geophysical interpretations of km‐thick zones of talc‐rich rocks beneath Guerrero, although silica‐bearing fluids that migrate along the plate interface may promote broader metasomatism. Regions of predicted talc production do, however, overlap with the spatial occurrence of ETS, consistent with models of slow slip based on the frictional deformation of metasomatic lithologies. 

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5. Reaction-Induced Porosity in an Eclogite-Facies Vein Selvage (Monviso Ophiolite, W. Alps): Textural Evidence and In Situ Trace Elements and Sr Isotopes in Apatite.

   https://doi.org/10.1093/petrology/egac128  

   Hoover, William F; Penniston-Dorland, Sarah; Piccoli, Philip; Kylander-Clark, Andrew (January 2023, Journal of Petrology)

   Porosity generated during fluid–rock reaction can facilitate fluid transport and metasomatism in low permeability high-pressure metamorphic rocks. Evidence for reaction-induced porosity is found in an eclogite-facies clinopyroxene + apatite vein in an undeformed eclogitized Fe–Ti metagabbro from the Monviso Ophiolite (W. Alps) with a distinct garnet-rich selvage. Vein-forming fluids were sourced from adjacent metagabbros and reaction with the host rock removed Ca and P from the selvage and added Fe, REE, Pb and Cr. Textures at the selvage–host rock interface and in the host rock record local heterogeneity in reactivity and porosity during metasomatism linked to variable initial lawsonite abundance. These features reflect a hierarchy of pervasive-to-channelized porosity structures that facilitated widespread metasomatism of the host rock. Development of this metasomatic system in response to locally derived fluids suggests large-scale externally derived fluid transport is not required to drive extensive fluid–rock exchange. The production of porosity during metasomatic reactions could be important in facilitating further fluid–rock reaction and fluid transport in subducting slabs where permeability is low. 

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