Spatial variation of subduction zone fluids during progressive subduction: Insights from Serpentinite Mud Volcanoes

Geological processes at subduction zones control seismicity, plutonism and volcanism, and geochemical cycling between the oceans, crust, and mantle. The down-going plate experiences metamorphism, and the associated dehydration and fluid flow alters the physical properties of the plate interface and mantle wedge, as well as controlling the composition of material descending into the mantle. Any direct study of slab evolution during subduction is inhibited by the prohibitive depths at which these processes occur. To examine these processes we use serpentinite mud volcanoes in the Mariana forearc, that permit sampling of serpentinite materials and their pore waters that ascend from the subduction channel. We present new pore water chemical data from the summit and flanks of three serpentinite mud volcanoes that were drilled during International Ocean Discovery Program Expedition 366 which are reflective of reactions within the crust and mantle during the early, shallow (<20 km) stages of subduction. We show, via thermodynamic modelling, that our new data on the evolution of pore water chemical compositions reflect mineralogical characteristics of a predominately basaltic source from the downgoing Pacific Plate. However, a component from sedimentary sources is likely, especially for those mud volcanoes near the trench. Other potential slab-derived constituents, such as lithospheric serpentinite, carbonate-rich sediments, or seamount basalts with an intraplate geochemical character, are not required to explain our results. Our results indicate that with progressive subduction the lawsonite-epidote mineral transformation boundary at ∼250 °C may help drive slab carbonate destabilisation, despite its apparent thermodynamic stability at such temperatures and projected pressures (∼300 °C and ∼0.6 GPa). New dissolved gas data also point to primary thermodynamic controls over methane/ethane production within the subduction channel as depths-to-slab increase. Our findings provide direct evidence for the progressive mineralogical and chemical evolution of a subducting oceanic plate, which liberates a progressively evolving fluid phase into the subduction channel

Towards a better comprehension of reactive transport coupling experimental and numerical approaches

In this work we focus on further understanding reactive transport in carbonate rocks, in particular limestones characterized by a bimodal pore size distribution. To this end, we performed injection experiments with CO2-saturated water on a sample of Euville limestone and monitored the experiments with a medical CT scanner. Microscanner imaging was performed before and after alteration. Experiments showed that permeability increased by nearly two decades due to the alteration process. This increase could be attributed to the formation of a preferential dissolution path visualized on the CT images. Microscanner images show that preferential dissolution areas are characterized by the presence of numerous enlarged macropores. The preferential dissolution path created therefore retains a porous structure and does not correspond to a wormhole-type channel. To provide further knowledge of the small-scale physics of reactive transport, we performed Lattice-Boltzmann simulations of flow in a numerically generated model 2D porous medium having geometrical and topological features designed to approach Euville limestone. We showed that the fluid velocity increased in nearly percolating paths of macropores. Considering the experiments, this means that the CO2-saturated water starts to enter high-velocity zones earlier than low-velocity zones, inducing an earlier onset of the alteration process and a more pronounced local dissolution. However, numerical results showed that the alteration of non-connected macropores leads to an increase of permeability much smaller than the experimentally observed one. To explain this fact we used effective medium modelling that permits predicting the variation in permeability as a function of the fraction of macropores and consequently as a function of alteration. It proved that as long as there is no alteration-induced percolating path consisting of macropores, the increase in permeability is relatively low as shown by the Lattice-Boltzmann simulations. An increase in permeability of several orders of magnitude is only observed when the macroporosity is close to the percolation threshold. This fact is in accordance with the experimentally observed results

Reassessing the role of magnetite during natural hydrogen generation

Interactions between water and ferrous rocks are known to generate natural H2 in oceanic and continental domains via the oxidation of iron. Such generation has been mainly investigated through the alteration of Fe2+-silicate and some Fe2+-carbonates. So far, magnetite (α-Fe3O4) has never been considered as a potential source mineral for natural H2 since it is considered as a by-product of every known chemical reaction leading to the formation of H2, despite it bears 1/3 of Fe2+ in its mineral lattice. This iron oxide is rather seen as a good catalyst for the formation of H2. Recently, hydrogen emissions were observed in the surroundings of banded iron formations (BIF) that are constituted of, among other minerals, magnetite. Thus, this work is an attempt to constrain the true potential of magnetite by means of batch reactor experiments and additional thermodynamic calculations. It explores theoretical and experimental reaction pathways of magnetite during water-rock interactions, focusing on low temperatures (T &lt; 200°C). For the purpose of the experiments, gold capsules filled with magnetite powders were run at 80°C and 200°C. Gas products were analyzed using gas chromatography (GC) while solid products were characterized by X-ray diffraction (XRD), Mössbauer spectroscopy, and scanning electron microscopy (SEM). After experimental alteration, high amounts of H2 were quantified while mineralogical transitions were observed by SEM. It showed self-reorganization of the primary iron oxide resulting in sharp-edge and better crystalized secondary minerals. In parallel, XRD analyses showed tiny changes between the patterns of the initial powder and the solid products of reaction. Finally, Mössbauer spectroscopy revealed that the starting magnetite was partly converted to maghemite (γ-Fe2O3), a metastable Fe-oxide only containing Fe3+. Major implications arise from these results. Concerning H2 exploration, this work provides evidence that natural hydrogen can be generated at near-ambient temperature. It also infers that magnetite-rich lithologies such as BIF should be targeted while looking for H2 source rocks. In addition, these outcomes could be of major interest for mining companies as they provide key elements to understand the formation of BIF-hosted iron ores

Mariana serpentinite mud volcanism exhumes subducted seamount materials: implications for the origin of life

The subduction of seamounts and ridge features at convergent plate boundaries plays an important role in the deformation of the overriding plate and influences geochemical cycling and associated biological processes. Active serpentinization of forearc mantle and serpentinite mud volcanism on the Mariana forearc (between the trench and active volcanic arc) provides windows on subduction processes. Here, we present (1) the first observation of an extensive exposure of an undeformed Cretaceous seamount currently being subducted at the Mariana Trench inner slope; (2) vertical deformation of the forearc region related to subduction of Pacific Plate seamounts and thickened crust; (3) recovered Ocean Drilling Program and International Ocean Discovery Program cores of serpentinite mudflows that confirm exhumation of various Pacific Plate lithologies, including subducted reef limestone; (4) petrologic, geochemical and paleontological data from the cores that show that Pacific Plate seamount exhumation covers greater spatial and temporal extents; (5) the inference that microbial communities associated with serpentinite mud volcanism may also be exhumed from the subducted plate seafloor and/or seamounts; and (6) the implications for effects of these processes with regard to evolution of life.Copyright 2020 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/ by/4.0/, which permits unrestricted use, provided the original author and source are credited

Mineral carbonation of New Caledonian ultramafic mine slag : Effect of glass and secondary silicates on the carbonation yield

International audienceNew Caledonian nickel (Ni) mines produces ∼12 million metric tons of mine slag every year, that are considered undesired and uneconomic waste materials. However, due to the high content of Mg2+ and Fe2+, these slags exhibit a potential of carbon dioxide (CO2) storage by ex-situ aqueous mineral carbonation. We conducted laboratory batch experiments at 200 °C/ 150 bar, 250 °C/ 300 bar, 300 °C/ 300 bar, in CO2-saturated water to better understand the slag dissolution, secondary mineral formation at the fluid-mineral interface, and the effect of the secondary silicates on the carbonation yield. The slag contains olivine (45 wt%) and glass (55 wt%). Olivine in the slag dissolves faster than glass, contributing to carbonate precipitation (Fe-rich magnesite) at 200 °C/150 bar. At 250 °C/ 300 bar, olivine is almost completely dissolved while glass dissolves only partially, both contributing to the highest recorded carbonation yields of the slag of 44 wt%. Although both olivine and glass are completely dissolved at 300 °C/ 300 bar, the carbonate yield drops at a value half of that at 250 °C/ 300 bar. This is due to formation of large quantity of Mg-bearing phyllosilicates that reduces the amount of Mg2+ available for carbonation. These results emphasize that increasing temperature above 250 °C would not increase the carbonate yield of New Caledonian mine slags. However, as glass only partially reacts at T < 250 °C, mobilizing Mg2+ in glass could potentially increase the carbonation yield, for example by adding suitable organic ligands. Mg-bearing phyllosilicates and amorphous silica that are formed in these experiments do not exhibit passivation properties, and therefore do not affect the carbonation rate

CO 2 geological storage: The environmental mineralogy perspective

International audienceGeological storage of carbon dioxide (CO2) is one of the options envisaged for mitigating the environmental consequences of anthropogenic CO2 increases in the atmosphere. The general principle is to capture carbon dioxide at the exhaust of power plants and then to inject the compressed fluid into deep geological formations. Before implementation over large scales, it is necessary to assess the efficiency of the process and its environmental consequences. The goal of this paper is to discuss some environmental mineralogy research perspectives raised by CO2 geological storage

An Attempt to Study Natural H₂ Resources across an Oceanic Ridge Penetrating a Continent: The Asal–Ghoubbet Rift (Republic of Djibouti)

Dihydrogen (H2) is generated by fluid–rock interactions along mid-ocean ridges (MORs) and was not, until recently, considered as a resource. However, in the context of worldwide efforts to decarbonize the energy mix, clean hydrogen is now highly sought after, and the production of natural H2 is considered to be a powerful alternative to electrolysis. The Afar Rift System has many geological features in common with MORs and offers potential in terms of natural H2 resources. Here, we present data acquired during initial exploration in this region. H2 contents in soil and within fumaroles were measured along a 200 km section across the Asal–Ghoubbet rift and the various intervening grabens, extending from Obock to Lake Abhe. These newly acquired data have been synthesized with existing data, including those from the geothermal prospect area of the Asal–Ghoubbet rift zone. Our results demonstrate that basalt alteration with oxidation of iron-rich facies and simultaneous reduction in water is the likely the source of the hydrogen, although H2S reduction cannot be ruled out. However, H2 volumes at the surface within fumaroles were found to be low, reaching only a few percent. These values are considerably lower than those found in MORs. This discrepancy may be attributed to bias introduced by surface sampling; for example, microorganisms may be preferentially consuming H2 near the surface in this environment. However, the low H2 generation rates found in the study area could also be due to a lack of reactants, such as fayalite (i.e., owing to the presence of low-olivine basalts with predominantly magnesian olivines), or to the limited volume and slow circulation of water. In future, access to additional subsurface data acquired through the ongoing geothermal drilling campaign will bring new insight to help answer these questions

An Attempt to Study Natural H2 Resources across an Oceanic Ridge Penetrating a Continent: The Asal&ndash;Ghoubbet Rift (Republic of Djibouti)

Dihydrogen (H2) is generated by fluid&ndash;rock interactions along mid-ocean ridges (MORs) and was not, until recently, considered as a resource. However, in the context of worldwide efforts to decarbonize the energy mix, clean hydrogen is now highly sought after, and the production of natural H2 is considered to be a powerful alternative to electrolysis. The Afar Rift System has many geological features in common with MORs and offers potential in terms of natural H2 resources. Here, we present data acquired during initial exploration in this region. H2 contents in soil and within fumaroles were measured along a 200 km section across the Asal&ndash;Ghoubbet rift and the various intervening grabens, extending from Obock to Lake Abhe. These newly acquired data have been synthesized with existing data, including those from the geothermal prospect area of the Asal&ndash;Ghoubbet rift zone. Our results demonstrate that basalt alteration with oxidation of iron-rich facies and simultaneous reduction in water is the likely the source of the hydrogen, although H2S reduction cannot be ruled out. However, H2 volumes at the surface within fumaroles were found to be low, reaching only a few percent. These values are considerably lower than those found in MORs. This discrepancy may be attributed to bias introduced by surface sampling; for example, microorganisms may be preferentially consuming H2 near the surface in this environment. However, the low H2 generation rates found in the study area could also be due to a lack of reactants, such as fayalite (i.e., owing to the presence of low-olivine basalts with predominantly magnesian olivines), or to the limited volume and slow circulation of water. In future, access to additional subsurface data acquired through the ongoing geothermal drilling campaign will bring new insight to help answer these questions

High-pressure serpentinization and abiotic methane formation in metaperidotite from the Appalachian subduction, northern Vermont

International audienceSerpentinization is the process of hydroxylation of olivine-rich ultramafic rocks to produce minerals such as serpentine, brucite and magnetite. This process is commonly accompanied by Fe oxidation and release of H2, which can be involved in abiotic reaction pathways leading to the genesis of abiotic light hydrocarbons such as methane (CH4). Examples of this phenomenon exist at the seafloor, such as at the serpentinite-hosted Lost City hydrothermal field, and on land in ophiolites at relatively shallow depths. However, the possibility for serpentinization to occur at greater depths, especially in subduction zones, raises new questions on the genesis of abiotic hydrocarbons at convergent margin and its impact on the deep carbon cycle. High-pressure ultramafic bodies exhumed in metamorphic belts can provide insights on the mechanisms of high-pressure serpentinization in subduction zones and on the chemistry of the resulting fluids. This study focuses on the ultramafic Belvidere Mountain complex belonging to the Appalachian belt of northern Vermont, USA. Microstructures show overgrowth of both primary (Mg# 0.91) and metamorphic (Mg# 0.95) olivine by delicate antigorite crystals, pointing to at least one stage of serpentinization at high-temperature conditions and consistent with the high-pressure subduction evolution of the Belvidere Mountain complex. Formation of ubiquitous magnetite and local Fe–Ni alloys testifies to the partial oxidation of Fe2+ into Fe3+ and generation of reduced conditions. Fluid inclusion trails cross-cutting the primary olivine relicts suggest their formation during the antigorite serpentinization event. MicroRaman spectroscopy on the fluid inclusions reveals a CH4-rich gaseous composition, as well as N2, NH3 and H2S. Moreover, the precipitation of daughter minerals such as lizardite and brucite in the fluid inclusions indicate the initial presence of H2O in the fluid. High-pressure serpentinization driven by the infiltration of metasediment-derived aqueous fluids is proposed at the origin of CH4 and other reduced fluid species preserved in the fluid inclusions. This suggests the Belvidere Mountain complex as an example of deep abiotic hydrocarbon genesis related to high-pressure serpentinization in an early Paleozoic subduction zone