Constraining the rates and timescales of garnet growth and associated dehydration during metamorphism

This study incorporates high precision zoned garnet samarium-neodymium geochronology and thermodynamic analysis of garnet forming dehydration reactions to determine the amount of water release during both subduction and mountain building. Garnet grows during rock dehydration, providing both a temporal and geodynamic record of not only its growth, but of associated dehydration. Laboratory experiments and geodynamic models have been used to predict amounts of dehydration during metamorphism based on equilibrium assumptions. If equilibrium is not maintained, or if aspects of the geodynamic modeling are incorrect, these model-based predictions will prove inaccurate. Field-based evidence is necessary to test such model predictions and to elucidate both the timing and duration of dehydration and the role of kinetics during metamorphism. Localities that have undergone dehydration and associated fluid flow provide natural laboratories in which to study these geologic processes. This study focuses on two geologic settings: regional orogenesis (Townshend Dam, Vermont) and subduction zone metamorphism (Sifnos, Greece). Regional metamorphism of the pelitic schists of Townshend Dam occurred during the Acadian orogeny peaking at ~381 Ma. Garnet growth lasted for 4.2 ± 2.4 million years. Thermodynamic forward modeling from this study has shown that an early stage of burial of the rocks without significant heating first occurred, followed then by a period of intense heating at depth, during which, roughly 2 vol.% water was lost from the rock. In contrast, metamorphism, and thus dehydration, during subduction of a continental margin in Sifnos, Greece was found to have occurred in as brief a timespan as tens to hundreds of thousands of years, releasing 2-3 vol.% water during a period of intense heating at ~75 km depth between ~47-44 million years ago. This short time interval represents a discrete pulse of dehydration and heating within the context of the process of subduction, which probably occurred over timescales of 10 to 20 million years in this location. This is the first study to provide a field-based constraint on the magnitude, timing, and rate of dehydration during subduction, a process that causes intermediate-depth earthquakes, mantle melting and volcanism, and large scale changes to the global water cycle

Fluid‐mediated mass transfer between mafic and ultramafic rocks in subduction zones

© 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

Fluid-mediated mass transfer between mafic and ultramafic rocks in subduction zones: Insights from the high-pressure Voltri Massif (Ligurian Alps, Italy)

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Release of oxidizing fluids in subduction zones recorded by iron isotope zonation in garnet

Subduction zones are key regions of chemical and mass transfer between the Earth’s surface and mantle. During subduction, oxidized material is carried into the mantle and large amounts of water are released due to the breakdown of hydrous minerals such as lawsonite. Dehydration accompanied by the release of oxidizing species may play a key role in controlling redox changes in the subducting slab and overlying mantle wedge. Here we present measurements of oxygen fugacity, using garnet–epidote oxybarometry, together with analyses of the stable iron isotope composition of zoned garnets from Sifnos, Greece. We find that the garnet interiors grew under relatively oxidized conditions whereas garnet rims record more reduced conditions. Garnet δ56Fe increases from core to rim as the system becomes more reduced. Thermodynamic analysis shows that this change from relatively oxidized to more reduced conditions occurred during lawsonite dehydration. We conclude that the garnets maintain a record of progressive dehydration and that the residual mineral assemblages within the slab became more reduced during progressive subduction-zone dehydration. This is consistent with the hypothesis that lawsonite dehydration accompanied by the release of oxidizing species, such as sulfate, plays an important and measurable role in the global redox budget and contributes to sub-arc mantle oxidation in subduction zones

Fluid Production Rate During the Regional Metamorphism of a Pelitic Schist

Phase equilibria modeling of the pressure–temperature (P–T) path of regional metamorphism and associated fluid expulsion, combined with constraints on the timescale of garnet growth by Sm–Nd geochronology, elucidates the fluid production rate and fluid flux during Barrovian metamorphism of pelitic rocks from Townshend Dam, VT, USA. This modeling builds on a published companion study that utilized Sm–Nd geochronology of concentric growth zones in multiple garnet grains, to constrain the duration of garnet growth in a large sample of schist at Townshend Dam to 3.8±2.2 million years (Gatewood et al., Chem Geol 401:151–168, 2015). P–T pseudosections combined with observed mineral compositions constrain garnet growth conditions, and were utilized to construct P–T path-dependent thermodynamic forward models. These models determine that garnet growth was initiated at ~0.6 GPa and ~525 °C, with a roughly linear loading and heating P–T trajectory to \u3e0.8 GPa and ~610 °C. Loading and heating rates of 2.4 km·Myear−1 (with a range of 1.6 to 5.8 km·million year−1) and 23 °C·million year−1 (with a range of 14 to 54 °C·million year−1), respectively, are consistent with model estimates and chronologic constraints for tectono-metamorphic rates during orogenesis. Phase equilibria modeling also constrains the amount of water release during garnet growth to be ~ 0.7 wt% (or \u3e2 vol%), largely resulting from the complete consumption of chlorite. Coupling this estimate with calculated garnet growth durations provides a fluid production rate of 5.2 kg·m−3·million year−1 (with a range of 3.2 to 12.2 kg·m−3·million year−1) and when integrated over the overlying crustal column, a regional-scale fluid flux of 0.07–0.37 kg·m−2·million year−1. This range of values is consistent with those derived by numerical models and theory for regional-scale, pervasive fluid flow. This study signifies the first derivation of a fluid production rate and fluid flux in regional metamorphism using a direct chronology of water-producing (garnet-forming) reactions and can provide a framework for future studies on elucidating the nature and timescales of fluid release

Meta-Rodingite Dikes as Recorders of Subduction Zone Metamorphism and Serpentinite Dehydration: Voltri Ophiolite, Italy

The metamorphic devolatilization of serpentinites during subduction represents the largest potential source of fluids from the subducting slab and influences a range of important processes from subduction zone seismicity to arc magmatism. Obtaining a record of metamorphic dehydration directly from serpentinites, however, is challenging, as serpentinite mineral assemblages are less conducive to extracting a detailed pressure-temperature-time (P-T-t) history. In this study, we use meta-rodingites (metasomatized gabbros) from the central portion of the Voltri Ophiolite in the Ligurian Alps (Italy), to investigate the P-T and geochronological record of subduction metamorphism and dehydration within the adjacent serpentinites. These meta-rodingites underwent eclogite-facies metamorphic recrystallization to a garnet-clinopyroxene-chlorite-ilmenite assemblage, and are cross-cut by several generations of garnet-bearing veins. A comparison of the Voltri meta-rodingites with un-subducted seafloor rodingites from the Apennines reveals textural and geochemical evidence of complete metamorphic recrystallization of the original seafloor metasomatic assemblage during Alpine subduction. Phase equilibria modeling of the Voltri meta-rodingite sample suggests that the peak metamorphic assemblage recorded was stable in the range of ~450–600 °C, which is consistent with previous P-T estimates for gabbroic and ultramafic lithologies from the Voltri Ophiolite. A late prograde to peak metamorphic age of 40.6 ± 2.9 Ma was obtained from the meta-rodingite using Sm–Nd garnet geochronology, which closely matches the ages of late prograde to peak metamorphism estimated by companion studies of associated eclogites from the central part of the Voltri Ophiolite. The overlap in P-T conditions and timing of peak metamorphism for samples across the area suggests subduction and exhumation of the central part of the Voltri Ophiolite as a coherent lithospheric slice. Dating of a garnetite vein cross-cutting the main meta-rodingite body suggests an influx of fluid at 37.7 ± 1.4 Ma, at or near peak eclogite-facies conditions. Enrichment in the vein selvage of both compatible transition metals (Cr, Ni, Cu) and fluid-mobile elements (Li, Rb, Cs, Ba) suggests the fluid carried both serpentinite and sedimentary geochemical signatures and interacted with the main meta-rodingite body. We suggest that the most probable explanation for this fluid composition, and thus the source of the fluid, is the dehydration of the Voltri serpentinites. These serpentinites likely experienced an influx of sedimentary-derived, fluid-mobile-element-bearing fluids earlier in their history, either on the seafloor or most likely during the early stages of subduction. The presence of a sedimentary signature in this serpentinite-derived fluid emphasizes the importance of subducted serpentinites and their dehydration to the geochemical cycling of fluid-mobile elements in subduction zones. The widespread development of such vein systems within multiple meta-rodingites across the Voltri Ophiolite points to a large-scale fluid release event, likely resulting from the partial dehydration of brucite and antigorite within the surrounding serpentinite hosts. By utilizing direct dating of vein mineralogy formed during serpentinite dehydration, we provide one of the first geochronological records of serpentinite dehydration and fluid release occurring during subduction

Petrochronological Close-Up on the Thermal Structure of a Paleo-Subduction Zone (W. Alps)

International audienceWe present the results from a study combining zoned geochronology, mineral trace element geochemistry and phase equilibria modeling of a mafic lithology from the Zermatt Saas Ophiolite in order to elucidate the tectonic rates and thermal structure of the Western Alps paleo-subduction interface. The Zermatt-Saas Ophiolite represents dismembered slices derived from an eclogitized, 60-km wide coherent fragment of Tethyan oceanic lithosphere. Two cm-sized garnet crystals from a pyrite-rich chlorite-talcschist (representative of sub-seafloor hydrothermally-altered basalts) from the Servette mine locality (St. Marcel Valley, Italy) were microdrilled to separate core and rim growth generations and dated using Sm-Nd geochronology to determine the overall duration of garnet growth. Garnet cores were dated to 46.9 ± 1.6Ma, signifying the timing of the initiation of garnet growth. Garnet rims were dated to 43.5 ± 1.3Ma, signifying the timing of burial to peak depths (constrained to ∼75 km). Major and trace element zoning in garnet suggest two distinct generations of garnet growth. Garnet crystal cores display evidence for rapid nucleation and growth, while garnet crystal rims suggest growth at relatively slow (tectonic) rates. The duration of garnet growth (3.4 ± 2.1Ma), when coupled to constraints from phase equilibria modeling and Zr-in-rutile thermometry, provides estimates on the burial and heating rate of 4.7 (+ 7.

Rapid High-\u3cem\u3eT\u3c/em\u3e Decompression Recorded by Archean Granulites in the Northern Wyoming Province: Insights from Petrological Modelling

This study places new constraints on the pressure–temperature (P–T) path and duration of high-temperature (HT) metamorphism recorded by Archean granulite facies metasedimentary rocks from the northern Wyoming Province in the eastern Beartooth Mountains, MT and WY, USA. These rocks exist as m- to km-scale xenoliths within a c. 2.8 Ga calc-alkaline granitoid batholith. Different interpretations of the timing of HT metamorphism relative to batholith intrusion in previous works have led to ambiguity over the mechanism by which these rocks were heated (i.e. batholith intrusion v. a later, cryptic event). The P–T path recorded by these rocks and the duration of this path may be indicative of the heating mechanism but are not currently well constrained. Here, we combine phase equilibria thermobarometry and diffusion modelling of major element zonation in garnet in order to constrain the P–T path of HT metamorphism and the durations of different parts of this path. It is shown that these rocks record a tight, clockwise P–T path characterized by near-isobaric heating at ~6.5–7 kbar to ˜770–800°C, HT decompression to ~6 kbar, 780–800°C, followed by limited decompression while cooling. Diffusion modelling of major element zonation in garnet suggests that HT decompression was brief (likely \u3c1 \u3eMa), and that cooling rates following this decompression were on the order of 10–100°C/Ma. Substantial changes in apparent thermal gradient along this P–T path indicate that the rocks record a significant but short-lived thermal anomaly that occurred in the Wyoming mid-crust in the Late Archean

Coupled Lu–Hf and Sm–Nd Geochronology on a Single Eclogitic Garnet from the Huwan Shear Zone, China

We present coupled Lu–Hf and Sm–Nd dates from multiple growth zones within a single large garnet porphyroblast in an eclogite from the Huwan shear zone in the Hong\u27an orogen, central China. The eclogite sample contains large garnet porphyroblasts up to several centimetres in diameter and a second major population of smaller, millimetre-sized garnet grains. Elemental compositions and mineral inclusions in garnet suggest that the large garnet crystal formed during an early episode of metamorphism, with the abundant small-sized garnets in the matrix growing during a late metamorphic period contemporaneous with the overgrowth of the rim of the large garnet crystal. Ten coupled Lu–Hf and Sm–Nd dates determined from five micro-sawed garnet sections of a ~2 cm garnet coherently decrease from core to rim. Lu–Hf dates from each section are consistently older than the corresponding Sm–Nd dates. Petrographic and chemical observations of the eclogite show that three generations of garnet growth occurred, with the latter two generations coincident with growth of the rim of the large garnet, and that of the smaller matrix garnets. Five micro-sawed growth zones were analysed for Lu–Hf and Sm–Nd isotopes. The calculated Lu–Hf dates from the mega garnet record garnet growth initiation at about 397 Ma, during prograde to peak metamorphic conditions of the first orogenic episode, lasting about 11 Ma. Sm–Nd garnet dates are systematically younger than corresponding Lu–Hf dates, with the Sm–Nd garnet rim date \u3e 25 Ma younger than the Lu–Hf date. We suggest that the Sm–Nd dates represent a combination of 1) growth and simultaneous diffusion at elevated temperatures during the first metamorphic episode, 2) diffusive age resetting as a result of continued elevated temperatures during residence at mid-crustal depths and/or slow cooling between the two orogenic episodes, and 3) mixing of growth generations. U–Pb dates on three zircon inclusions from the garnet separates range from 386 to 351 Ma, possibly suggesting zircon crystallization during exhumation and cooling. This study highlights that coupled microsampling Lu–Hf and Sm–Nd chronology on a single garnet (down to ~ cm in diameter) can help decipher the timescales of burial and heating (for Lu–Hf) and cooling from elevated temperatures (for Sm–Nd) from a single orogenic cycle, and that zoned Lu–Hf and Sm–Nd garnet can be utilized as a geo (thermo) -chronometer. Lastly, these Lu–Hf garnet, Sm–Nd garnet and U–Pb zircon dates confirm an early high-pressure metamorphic episode during the Devonian before the final convergence of the South China and North China Blocks in the Triassic

Li Isotope Zoning in Garnet from Franciscan Eclogite and Amphibolite: The Role of Subduction-Related Fluids

Fluids released within subduction zones affect many fundamental Earth processes, but very little is understood about processes of fluid transport within subduction zones and the chemistry of the fluids. Centimeter-scale variations in bulk-rock Li isotope compositions in subduction-related metamorphic rocks are observed associated with fluid-related features. The small spatial scales of these variations suggest very short duration fluid infiltration events – on the order of months to centuries. However, bulk-rock measurements record a time-integrated record, while in situ measurements in metamorphic porphyroblasts such as garnet have the potential to record individual events experienced by the rock. In this study, traverses across garnet in Franciscan eclogite and amphibolite were analyzed for Li isotopes in situ using SIMS to determine the potential for inferring the duration of metamorphic fluid flow and for deciphering fluid chemistry. The Li concentrations of these garnet crystals are very low, ∼5 µg/g. The measured range of δ7Li within some Franciscan garnets is 19‰, which falls well outside the uncertainty of the measurement (7Li occur within crystals over a scale of a few hundred microns, with crystal core-to-rim traverses exhibiting troughs of very low δ7Li measurements in the mantle region of the garnet crystals, surrounded by higher values in the garnet crystal rims and cores. Garnet crystal rims have lower δ18O values, which have previously been interpreted as a product of the infiltration of serpentinite-derived fluids. These results suggest a role for fluids fluxing through the slab to create the observed variability, and the unusual trough pattern suggests a role for intracrystalline diffusion. The low δ7Li troughs are observed only in garnet from one of two Franciscan localities. Nevertheless, garnets are similar in composition from the two localities. This contrast suggests that some aspect of the chemistry of the fluids may be responsible for the differences between the two localities; likely candidates are either pH or Eh of the fluid