Pervasive fluid-rock interaction in subducted oceanic crust revealed by oxygen isotope zoning in garnet

Dehydration reactions in the subducting slab liberate fluids causing major changes in rock density, volume and permeability. Although it is well known that the fluids can migrate and interact with the surrounding rocks, fluid pathways remain challenging to track and the consequences of fluid-rock interaction processes are often overlooked. In this study, we investigate pervasive fluid-rock interaction in a sequence of schists and mafic felses exposed in the Theodul Glacier Unit (TGU), Western Alps. This unit is embedded within metaophiolites of the Zermatt-Saas Zone and reached eclogite-facies conditions during Alpine convergence. Chemical mapping and in situ oxygen isotope analyses of garnet from the schists reveal a sharp chemical zoning between a xenomorphic core and a euhedral rim, associated to a drop of ~ 8‰ in δ18O. Thermodynamic and δ18O models show that the large amount of low δ18O H2O required to change the reactive bulk δ18O composition cannot be produced by dehydration of the mafic fels from the TGU only, and requires a large contribution of the surrounding serpentinites. The calculated time-integrated fluid flux across the TGU rocks is 1.1 × 105 cm3/cm2, which is above the open-system behaviour threshold and argues for pervasive fluid flow at kilometre-scale under high-pressure conditions. The transient rock volume variations caused by lawsonite breakdown is identified as a possible trigger for the pervasive fluid influx. The calculated schist permeability at eclogite-facies conditions (~ 2 × 10–20 m2) is comparable to the permeability determined experimentally for blueschist and serpentinites

OH incorporation and retention in eclogite-facies garnets from the Zermatt–Saas area (Switzerland) and their contribution to the deep water cycle

The incorporation mechanisms of OH groups in garnet were investigated in a suite of high-pressure rocks from the Zermatt–Saas area (Switzerland) using a combination of Fourier transform infrared spectroscopy (FTIR) and electron probe micro-analysis (EPMA). Investigated garnet specimens include grossular–andradite–uvarovite solid solutions in serpentinite and rodingite and almandine–grossular–pyrope–spessartine solid solutions in eclogite, mafic fels and meta-sediment. All rocks experienced the same peak metamorphic conditions corresponding to a burial depth of ∼ 80 km (∼ 540 ∘C, 2.3 GPa), allowing determination of the OH content in garnet as a function of rock type. The capacity for OH incorporation into garnet strongly depends on its composition. Andradite-rich (400–5000 µg g−1 H2O) and grossular-rich garnet (200–1800 µg g−1 H2O) contain at least 1 order of magnitude more H2O than almandine-rich garnet (&lt; 120 µg g−1 H2O). Microscale analyses using FTIR and EPMA profiles and maps reveal the preservation of OH zoning throughout the metamorphic history of the samples. The OH content correlates strongly with Mn, Ca and Ti zoning and produces distinct absorption bands that are characteristic of multiple nano-scale OH environments. The use of 2D diffusion modelling suggests that H diffusion rates in these rocks is as low as log(D[m2 s−1]) = −24.5 at 540 ∘C. Data were collected for the main garnet-bearing rock types of the Zermatt–Saas area allowing a mass balance model of H2O to be calculated. The result shows that ∼ 3360 kg H2O km−1 (section of oceanic crust) yr−1 could be transported by garnet in the subducting slab beyond 80 km depth and contributed to the deep-Earth water cycle during the Eocene subduction of the Piemonte–Liguria Ocean.</p

Pressure–temperature–time evolution of subducted crust revealed by complex garnet zoning (Theodul Glacier Unit, Switzerland)

Collisional orogens commonly include mono-metamorphic and poly-metamorphic units, and their different evolution can be difficult to recognize and reconcile. The Theodul Glacier Unit (TGU) in the Western Alps consists of an association of metasedimentary and metamafic rocks embedded within the Zermatt-Saas tectonic unit. In spite of recent petrological studies, it remains unclear whether these rocks underwent one or multiple metamorphic cycles. In this study, different lithologies from the TGU unit (mafic schist, mafic granofels, and chloritoid schist) were investigated for petrography, quantitative compositional mapping of garnet, thermodynamic modelling, and Lu–Hf garnet dating. The data reveal a coherent mono-metamorphic history with a β-shaped pressure–temperature (P–T) path characteristic of oceanic subduction. Garnet Lu–Hf ages yield a restricted garnet crystallization time window between 50.3 and 48.8 Ma (±0.5%, 2SD). A prograde metamorphic stage recorded in garnet cores yields conditions of 490 ± 15℃ and 1.75 ± 0.05 GPa. Maximum pressure conditions of 2.65 ± 0.10 GPa and 580 ± 15℃ were reached at 50.3 ± 0.3 Ma. Initial exhumation was rapid and led to isothermal decompression to 1.50 ± 0.10 GPa within 1 Ma. This decompression was associated with lawsonite breakdown in mafic schist and in mafic granofels, causing intense fluid–rock interaction within and between different lithologies. This process is recorded in garnet textures and trace element patterns, and in the major element composition of K-white mica. Initial exhumation was followed by re-heating of ~30℃ at a pressure of 1.50 ± 0.10 GPa. Perturbation of the subduction-zone thermal structure may be related to upwelling of hot asthenospheric mantle material and transient storage of the unit at the crust–mantle boundary

Surface Analysis by Secondary Ion Mass Spectrometry (SIMS): Principles and Applications from Swiss laboratories

Secondary Ion Mass Spectrometry (SIMS) extracts chemical, elemental, or isotopic information about a localized area of a solid target by performing mass spectrometry on secondary ions sputtered from its surface by the impact of a beam of charged particles. This primary beam sputters ionized atoms and small molecules (as well as many neutral particles) from the upper few nanometers of the sample surface. The physical basis of SIMS has been applied to a large range of applications utilizing instruments optimized with different types of mass analyzer, either dynamic SIMS with a double focusing mass spectrometer or static SIMS with a Time of Flight (TOF) analyzer. Here, we present a short review of the principles and major applications of three different SIMS instruments located in Switzerland

Quantitative compositional mapping of mineral phases by electron probe micro-analyser

Compositional mapping has greatly impacted mineralogical and petrological studies over the past half-century with increasing use of the electron probe micro-analyser. Many technical and analytical developments have benefited from the synergies of physicists and geologists and they have greatly contributed to the success of this analytical technique. Large-area compositional mapping has become routine practice in many laboratories worldwide, improving our ability to measure the compositional variability of minerals in natural geological samples and reducing the operator bias as to where to locate single spot analyses. This chapter aims to provide an overview of existing quantitative techniques for the evaluation of rock and mineral compositions and to present various examples of applications. A new advanced method for compositional map standardization that relies on internal standards and accurately corrects the X-ray intensities for continuum background is also presented. This technique has been implemented into the computer software XMapTools. The improved workflow defines the appropriate practice of accurate standardization and provides data-reporting standards to help improve petrological interpretations

S3. Quantitative compositional mapping of mineral phases by electron probe micro-analyser

Example of standardization report generated by XMapTool

S1. Quantitative compositional mapping of mineral phases by electron probe micro-analyser

Recommended reporting template for EPMA quantitative mappin

Variations in water saturation states and their impact on eruption size and frequency at the Aso supervolcano, Japan

The exsolution of a magmatic volatile phase in the plumbing systems of volcanoes plays a key role in controlling growth dynamics of subvolcanic reservoirs and eruptive styles. By using common petrological proxies found in volcanic deposits, such as melt inclusions and apatite crystals, the presence of such an exsolved magmatic volatile phase can be traced, specifically the exsolution of water from silicate melt. To monitor variations in the water saturation state of magmas from the Aso volcanic complex (Kyushu, Japan) prior to and during the catastrophic Aso-4 caldera-forming event (at ∼86 ka BP), we investigate a set of pre-Aso-4 and Aso-4 deposits, combining volatile budgets (F, Cl, OH, S) of melt inclusions, matrix glasses, and apatite crystals, with sulfur isotope signatures in apatite. F-Cl-OH partitioning in apatite along with melt inclusion data from pre-Aso-4 units indicate water-undersaturated conditions during magma evolution until around 10 ka prior to the Aso-4 event. In contrast, eruptions occurring within the last ∼10 ka prior to the Aso-4 event indicate the presence of a water-rich exsolved magmatic volatile phase in the eruptible portions of the subvolcanic reservoir. Likewise, sulfur systematics in apatite and melt inclusions from the Aso-4 event suggest the exsolution of a water-rich magmatic volatile phase at some point prior to the caldera-forming event. Recharge of mafic magmas shortly before the Aso-4 eruption induced chemical hybridization in the resident upper crustal mush, bringing the reservoir back to less evolved compositions and water-undersaturated conditions. This hybridization event is recorded by volatile contents of both apatite and matrix glasses from late-erupted, crystal-rich products of the Aso-4 event, all yielding water-undersaturated signatures. During this hybridization event, chemical dilution and partial redissolution of the exsolved volatile phase reduced the magma compressibility significantly, so that additional magma influx from depth might have allowed a sharply increasing overpressurization in the subvolcanic reservoir and served as a potential trigger for the cataclysmic Aso-4 eruption. Drawing from our observations made on Aso, we propose that recharging of large silicic upper-crustal reservoirs with increased volumes of drier and more mafic melts can influence their water saturation states and associated physical properties. Such changes could contribute to the triggering of large-scale caldera-forming events.ISSN:0012-821XISSN:1385-013