Serpentinization Of Peridotites Research Articles

Compositional variation of olivine related to high–temperature serpentinization of peridotites: Evidence from the Oeyama ophiolite

Compositional variation of olivine related to high–temperature serpentinization of peridotites: Evidence from the Oeyama ophiolite

Compositional variability of spinel-group minerals from the shergol serpentinized peridotites along indus suture zone, ladakh himalaya (India): constraints on tectonomagmatic history

The Shergol ophiolitic peridotites along ISZ, Ladakh Himalaya are serpentinized to various degrees and are harzburgite in composition. Electron microprobe analyses of spinels from Shergol Serpentinized Peridotites (SSPs) were carried out in order to evaluate their compositional variation with alteration. Chemical discontinuity was observed from core to rim in analyzed spinel grains with Cr-rich cores rimmed by Cr-poor compositions. From unaltered cores to rims it was observed that Cr3+# and Fe3+# increases while Mg2+# decreases due to Mg2+ − Fe2+ and Al3+ (Cr3+) − Fe3+ exchange with surrounding silicates during alteration. These peridotites contain Al-rich spinels forming subhedral to anhedral grains with lobate and corroded grain boundaries; altered to ferritchromite or magnetite along cracks and boundaries by later metamorphism episode. The unaltered Cr-spinel cores are identified as Al-rich and are characterized by lower values of Cr3+# (0.34–0.40), high Al3+# (0.58–0.68) and Mg2+# (0.52–0.70). Mineral chemistry of these Al-rich Cr-spinels suggest that host peridotites have an affinity to abyssal and alpine-type peridotites. High TiO2 concentration of magmatic Cr-spinel cores are in agreement with MORB melt-residual peridotite interaction. Presence of unaltered magmatic Cr-spinel cores suggest that they do not have re-equilibrated completely with metamorphic spinel rims and surrounding silicates. Cr-spinel core compositions of SSPs suggest an ophiolitic origin derivation by low degrees of melting of a less-moderate depleted peridotite in a mid-ocean ridge tectonic setting. Based on textural and chemical observations the alteration conditions of studied spinel-group minerals match those of transitional greenschist-amphibolite facies metamorphism consistent with estimated metamorphic equilibration temperature of∼500–600°C.

Nature of serpentinization and carbonation of ophiolitic peridotites (Eastern Desert, Egypt): constrains from stable isotopes and whole-rock geochemistry

Serpentinites are widespread in the Arabian-Nubian Shield (ANS) of the Eastern Desert of Egypt and usually enclose a tremendous carbonate alteration. Combined investigation of the stable isotope compositions of both O-H in serpentines and O-C in the whole-rock and the chemistry of the fluid-mobile elements (FMEs) in whole-rock serpentinites from Wadi (W.) Alam, Gabal (G.) El-Maiyit, and W. Atalla (Eastern Desert of Egypt) allowed to better understand the subsequent fluid sources of serpentinization and carbonation, as well as impact of these processes on the geochemistry of protolith ultramafic rocks. δ 18O values of W. Alam and W. Atalla serpentine minerals are close to the unaltered mantle and propose a lower temperature serpentinization if compared with those of G. El-Maiyit rocks. Moreover, δD values of W. Alam and W. Atalla serpentines (− 94 to − 65‰) correspond to an igneous source that might be hydrothermal solutions mixed with the seawater in the mid-ocean ridge-arc transition setting. On the other hand, G. El-Maiyit serpentine is more depleted in 18O (with lower δ 18O values = 4.08–4.85‰), and its δD values (− 73 to 56 ‰) are most probably caused by an interaction with metamorphic fluids, acquired during on-land emplacement of oceanic peridotites or during burial in fore-arc setting. In addition, the oceanic oxygen isotope composition of most studied ophiolitic serpentinites points to the preservation of the pre-obduction δ 18O signatures and thus local-scale fluid flow at low water/rock ratios. Serpentinization fluids were CO2-poor and the carbonation of the serpentinites resulted from infiltration of externally derived fluids. δ 18OVSMOW values of carbonates in the studied serpentinites vary between heavier oxygen isotope composition in G. El-Maiyit samples (av. = 25.32‰) to lighter composition in W. Alam samples (av. = 19.43‰). However, δ 13C values of all serpentinites point mantle source of carbon. This source might have been evolved in mid-ocean ridge (W. Atalla) and subduction zone (W. Alam and G. El-Maiyit) settings. The studied serpentinites are usually enriched in FMEs, particularly Pb, Sr, Cs, and U. These enrichments were most probably the result of serpentinization and/or carbonation.

Influence of pyroxene and spinel on the kinetics of peridotite serpentinization

AbstractHydrothermal experiments were performed at 311°C and 3.0 kbar on natural olivine and peridotite to investigate the kinetics of serpentinization. The results show that the rates of reaction strongly depend on grain sizes of solid reactants, with smaller grain sizes resulting in faster kinetics. After 27 days of reaction, the reaction extent was 99% for peridotite with grain sizes of <30 μm, and the reaction extent was 28% for grain sizes of 100–177 μm. Compared to peridotite, olivine is serpentinized at much slower rates, e.g., 5.3% of reaction extent was achieved for olivine with grain sizes of 100–177 μm after 27 days, approximately five times lower than that reached during peridotite serpentinization. Such contrasting results are due to the presence of pyroxene and spinel, an interpretation which is supported by a marked increase in reaction extents for experiments with the addition of pyroxene and spinel. The reaction extent achieved in experiments with 3 wt % spinel greatly increased to 98% after 27 days, much higher than that achieved during olivine serpentinization. These results appear to be related to pyroxene and spinel releasing Al and Cr during serpentinization. As indicated by compositions of serpentine, orthopyroxene lost ~60% of Al at a reaction extent of 59%. Influence of Al and Cr is suggested by a dramatic increase in reaction extents with the addition of Al2O3 and Cr2O3 powders. Olivine in natural geological settings is commonly associated with pyroxene and spinel; consequently, serpentinization kinetics may be much faster than previously thought.

Public domain satellite gravity inversion offshore Somalia combining layered-Earth and voxel based modelling

At the end of 2016, Spectrum Geo released two long offset seismic reflection profiles across the Somalian rifted margin, in the Juba Lamu and Obbia basins respectively – part of two larger 2D surveys acquired in 2014 and 2015/16 (Stanca et al., 2016; Figure 1). In frontier areas, potential field data interpretation, in particular public domain satellite-derived gravity, can play a key role in the early stages of exploration by identifying basement structure, sediment thickness, and the continental-oceanic crust transition (COT) or continental-oceanic boundary (COB), and hence indirectly contribute to the understanding of thermal history and the hydrocarbon system. This is particularly relevant over the Somalia margin, where geophysical surveys have been extremely sparse until very recent years. A high-gradient in the gravity is often used as a marker of the COB along passive margins (see Pawlowski (2008) for a review), expressing laterally contrasting physical properties between the continental and oceanic crusts. However, at magma-poor rifted margins, such as the Somalia margin, the lithosphere undergoes a progressive thinning/stretching process and the transition between unaltered continental crust and oceanic crust becomes gradual (Manatschal, 2004). Crustal hyper-extension, serpentinization, embrittlement, and exhumation of mantle peridotites can precede accretion of true oceanic crust, which in turn can blur the simple transitional model between a dense and highly magnetized oceanic crust and a weakly magnetized and lighter continental crust. Along a magma-poor margin, analysis of potential field data can distinguish the different rifting domains and quantify crustal thinning (Stanton et al., 2016; Cowie et al., 2015). In offshore Kenya and Somalia, the image is further complicated by thick sedimentary cover.

Water chemistry and abiogenic methane content of a hyperalkaline spring related to serpentinization in the Argolida ophiolite (Ermioni, Greece)

Hyperalkaline mineral springs related to active continental serpentinization are a theme of growing interest since they may contain significant amounts of abiotic gas and have important implications for energy resource exploration, subsurface microbiology and astrobiology. We report the discovery of a new hyperalkaline (pH∼12) spring issuing in the Agioi Anargyroi monastery at Ermioni (Greece), connected to serpentinization of peridotites of the Argolis ophiolite. Two water samples have been collected from separated springs and analysed for the chemical composition of major, minor and trace elements, and isotopic composition (2H and 18O) of water by IC, ICP-OES, ICP-MS and IRMS, and for the chemical (H2, O2, N2, CH4, CO2 and C2H6) and isotopic (He, δ2H-CH4 and δ13C-CH4) composition of dissolved gases. The Iliokastron Mélange Unit, comprising abundant serpentinized harzburgite, represent the aquifer feeding the hyperalkaline springs. The isotopic composition of water indicates a recent meteoric recharge probably through the close by and stratigraphically higher limestones of the Faniskos Unit. The Ca-OH water composition resulted to be similar to other hyperalkaline waters of Greece and worldwide. Although the concentrations of dissolved H2 are very low (tens of nmol/L) compared to other gases collected in similar manifestations, the concentrations of CH4 are considerable (38–314 μmol/L) and display isotopic compositions indicating a substantial if not exclusive abiogenic origin. Methane oxidation is also hypothesized in one of the two springs.

The production of iron oxide during peridotite serpentinization: Influence of pyroxene

Serpentinization produces molecular hydrogen (H2) that can support communities of microorganisms in hydrothermal fields; H2 results from the oxidation of ferrous iron in olivine and pyroxene into ferric iron, and consequently iron oxide (magnetite or hematite) forms. However, the mechanisms that control H2 and iron oxide formation are poorly constrained. In this study, we performed serpentinization experiments at 311 °C and 3.0 kbar on olivine (with <5% pyroxene), orthopyroxene, and peridotite. The results show that serpentine and iron oxide formed when olivine and orthopyroxene individually reacted with a saline starting solution. Olivine-derived serpentine had a significantly lower FeO content (6.57 ± 1.30 wt.%) than primary olivine (9.86 wt.%), whereas orthopyroxene-derived serpentine had a comparable FeO content (6.26 ± 0.58 wt.%) to that of primary orthopyroxene (6.24 wt.%). In experiments on peridotite, olivine was replaced by serpentine and iron oxide. However, pyroxene transformed solely to serpentine. After 20 days, olivine-derived serpentine had a FeO content of 8.18 ± 1.56 wt.%, which was significantly higher than that of serpentine produced in olivine-only experiments. By contrast, serpentine after orthopyroxene had a slightly higher FeO content (6.53 ± 1.01 wt.%) than primary orthopyroxene. Clinopyroxene-derived serpentine contained a significantly higher FeO content than its parent mineral. After 120 days, the FeO content of olivine-derived serpentine decreased significantly (5.71 ± 0.35 wt.%), whereas the FeO content of orthopyroxene-derived serpentine increased (6.85 ± 0.63 wt.%) over the same period. This suggests that iron oxide preferentially formed after olivine serpentinization. Pyroxene in peridotite gained some Fe from olivine during the serpentinization process, which may have led to a decrease in iron oxide production. The correlation between FeO content and SiO2 or Al2O3 content in olivine- and orthopyroxene-derived serpentine indicates that aluminum and silica greatly control the production of iron oxide. Based on our results and data from natural serpentinites reported by other workers, we propose that aluminum may be more influential at the early stages of peridotite serpentinization when the production of iron oxide is very low, whereas silica may have a greater control on iron oxide production during the late stages instead.

Variation in magnetic properties of serpentinized peridotites exposed on the Yokoniwa Rise, Central Indian Ridge: Insights into the role of magnetite in serpentinization

AbstractMagnetic properties in serpentinized peridotites are of increasing interest in seafloor mapping and petrologic studies because such data can promote the understanding of serpentinization reactions and hydrogen creation in ultramafic rocks. In order to reveal the magnetic properties and magnetite growth in serpentinized peridotites, we analyzed 30 serpentinized peridotite samples from a nontransform offset massif called the Yokoniwa Rise in the Central Indian Ridge. The results from multiple rock magnetic analyses and petrological observations illustrate the details of the creation and growth of magnetite in serpentinized peridotites that have undergone 17–100% serpentinzation. The magnetic carrier of these samples is pure magnetite, which did not suffer from maghemitization (low‐temperature oxidation). The magnetic susceptibility ranged from 0.002 to 0.087 SI and increased nonlinearly with the progression of the serpentinization reaction. The natural remanent magnetization intensities of 0.2–8.4 A/m are comparable to those of basalts, which suggests that the remanence as well as induced magnetization of highly serpentinized peridotite can contribute to magnetization of the oceanic lithosphere. The amount of magnetite estimated from saturation magnetization increased nonlinearly from 0.1 to 5.5 wt % with the progression of the serpentinization. Highly serpentinized peridotites have a well‐developed serpentine mesh texture. Pseudo‐single‐domain (PSD) and multidomain (MD) grains were formed during igneous processes in the mantle and/or during the initial stages of serpentinization. Superparamagnetic (SP) particles were formed during the initial stages of serpentinization. Single‐domain (SD) magnetite was formed during the later stage of serpentinization, and it is assembled inside of mesh structures with strong magnetostatic interactions.

Platinum-Group Minerals and Other Accessory Phases in Chromite Deposits of the Alapaevsk Ophiolite, Central Urals, Russia

An electron microprobe study has been carried out on platinum-group minerals, accessory phases, and chromite in several chromite deposits of the Alapaevsk ophiolite (Central Urals, Russia) namely the Bakanov Kluch, Kurmanovskoe, Lesnoe, 3-d Podyony Rudnik, Bol’shaya Kruglyshka, and Krest deposits. These deposits occur in partially to totally serpentinized peridotites. The microprobe data shows that the chromite composition varies from Cr-rich to Al-rich. Tiny platinum-group minerals (PGM), 1–10 µm in size, have been found in the chromitites. The most abundant PGM is laurite, accompanied by minor cuproiridsite and alloys in the system Os–Ir–Ru. A small grain (about 20 μm) was found in the interstitial serpentine of the Bakanov Kluch chromitite, and its calculated stoichiometry corresponds to (Ni,Fe)5P. Olivine, occurring in the silicate matrix or included in fresh chromite, has a mantle-compatible composition in terms of major and minor elements. Several inclusions of amphibole, Na-rich phlogopite, and clinopyroxene have been identified. The bimodal Cr–Al composition of chromite probably corresponds to a vertical distribution in the ophiolite sequence, implying formation of Cr-rich chromitites in the deep mantle, and Al-rich chromitites close to the Moho-transition zone, in a supra-subduction setting. The presence of abundant hydrous silicate inclusions, such as amphibole and phlogopite, suggests that the Alapaevsk chromitites crystallized as a result of the interaction between a melt enriched in fluids and peridotites. Laurite and cuproiridsite are considered to be magmatic in origin, i.e., entrapped as solid phases during the crystallization of chromite at high temperatures. The sulfur fugacity was relatively high to allow the precipitation of Ir-bearing sulfides, but below the Os–OsS2 buffer. The alloys in the system Os–Ir–Ru are classified as secondary PGM, i.e., formed at low temperature during the serpentinization process. The (Ni,Fe)5P phase is the first occurrence of a Ni-phosphide in terrestrial samples. Its composition indicates that it may be a new mineral. However, the small size has, so far, prevented a crystallographic study to support this conclusion.

Chemical Variation of Chromian Spinel Compositions in a Serpentinized Peridotites: Implications for Evolution of the Neoproterozoic Ophiolites

Acta Geologica Sinica - English EditionVolume 90, Issue s1 p. 210-210 IGCP-649 Workshop II: Probing the Oceanic Mantle: Troodos Ophiolite as a Case Study Chemical Variation of Chromian Spinel Compositions in a Serpentinized Peridotites: Implications for Evolution of the Neoproterozoic Ophiolites Hamed Gamal EL DIEN, Corresponding Author Hamed Gamal EL DIEN hgamaleldin@science.tant.edu.eg Department of earth science, school of Natural System, Kanazawa University, Kanazawa 920-1192 Japan Geology Department, Faculty of Science, Tanta University, 31527 Tanta EgyptCorresponding author. E-mail: hgamaleldin@science.tant.edu.egSearch for more papers by this authorTomoaki MORISHITA, Tomoaki MORISHITA Department of earth science, school of Natural System, Kanazawa University, Kanazawa 920-1192 JapanSearch for more papers by this authorMohamed HAMDY, Mohamed HAMDY Geology Department, Faculty of Science, Tanta University, 31527 Tanta EgyptSearch for more papers by this authorAbdel Salam ABU EL-ELA, Abdel Salam ABU EL-ELA Geology Department, Faculty of Science, Tanta University, 31527 Tanta EgyptSearch for more papers by this authorAdel HASSAN, Adel HASSAN Geology Department, Faculty of Science, Tanta University, 31527 Tanta EgyptSearch for more papers by this authorYusuke SODA, Yusuke SODA Department of earth science, school of Natural System, Kanazawa University, Kanazawa 920-1192 JapanSearch for more papers by this author Hamed Gamal EL DIEN, Corresponding Author Hamed Gamal EL DIEN hgamaleldin@science.tant.edu.eg Department of earth science, school of Natural System, Kanazawa University, Kanazawa 920-1192 Japan Geology Department, Faculty of Science, Tanta University, 31527 Tanta EgyptCorresponding author. E-mail: hgamaleldin@science.tant.edu.egSearch for more papers by this authorTomoaki MORISHITA, Tomoaki MORISHITA Department of earth science, school of Natural System, Kanazawa University, Kanazawa 920-1192 JapanSearch for more papers by this authorMohamed HAMDY, Mohamed HAMDY Geology Department, Faculty of Science, Tanta University, 31527 Tanta EgyptSearch for more papers by this authorAbdel Salam ABU EL-ELA, Abdel Salam ABU EL-ELA Geology Department, Faculty of Science, Tanta University, 31527 Tanta EgyptSearch for more papers by this authorAdel HASSAN, Adel HASSAN Geology Department, Faculty of Science, Tanta University, 31527 Tanta EgyptSearch for more papers by this authorYusuke SODA, Yusuke SODA Department of earth science, school of Natural System, Kanazawa University, Kanazawa 920-1192 JapanSearch for more papers by this author First published: 31 October 2016 https://doi.org/10.1111/1755-6724.12979Citations: 1AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article.Citing Literature Volume90, Issues1Special Issue: The Seventh International Dyke Conference: Dyke Swarms: Keys to Paleogeographic Reconstruction IDC7, 18–20 August, BeijingOctober 2016Pages 210-210 RelatedInformation

Process of serpentinization in the ultramafic massif of Beni Bousera (internal Rif, Morocco)

The Beni Bousera massif forms part of the Sebtide units in the internal Rif Mountain (Morocco). It is mainly composed of mantle peridotites surrounded by crustal metamorphic rocks (kinzigites, micaschists, and schists). The serpentinization affects all of peridotite massif to various degrees. Serpentinization is concentrated at the top of the peridotites, along the mylonitized zone, and in the NE part of the massif. It is manifested by the formation of mesh and hourglass textures along the tectonic foliation in the highly serpentinized peridotites; and brecciated texture in the least serpentinized peridotites. Pyroxene minerals are still intact hosting few serpentine veins. These petrographic features are consistent with the geochemical data, marked by the increasing of LOI and decreasing of MgO and FeO toward the top of the massif and Aaraben fault. The Raman characterization of serpentine with the brecciated mesh and hourglass textures correspond to lizardite type whereas the serpentine with the vein texture is formed by lizardite + chrysotile.

Continental hyperextension, mantle exhumation, and thin oceanic crust at the continent‐ocean transition, West Iberia: New insights from wide‐angle seismic

AbstractHyperextension of continental crust at the Deep Galicia rifted margin in the North Atlantic has been accommodated by the rotation of continental fault blocks, which are underlain by the S reflector, an interpreted detachment fault, along which exhumed and serpentinized mantle peridotite is observed. West of these features, the enigmatic Peridotite Ridge has been inferred to delimit the western extent of the continent‐ocean transition. An outstanding question at this margin is where oceanic crust begins, with little existing data to constrain this boundary and a lack of clear seafloor spreading magnetic anomalies. Here we present results from a 160 km long wide‐angle seismic profile (Western Extension 1). Travel time tomography models of the crustal compressional velocity structure reveal highly thinned and rotated crustal blocks separated from the underlying mantle by the S reflector. The S reflector correlates with the 6.0–7.0 km s−1 velocity contours, corresponding to peridotite serpentinization of 60–30%, respectively. West of the Peridotite Ridge, shallow and sparse Moho reflections indicate the earliest formation of an anomalously thin oceanic crustal layer, which increases in thickness from ~0.5 km at ~20 km west of the Peridotite Ridge to ~1.5 km, 35 km further west. P wave velocities increase smoothly and rapidly below top basement, to a depth of 2.8–3.5 km, with an average velocity gradient of 1.0 s−1. Below this, velocities slowly increase toward typical mantle velocities. Such a downward increase into mantle velocities is interpreted as decreasing serpentinization of mantle rock with depth.

Origin of magnetic highs at ultramafic hosted hydrothermal systems: Insights from the Yokoniwa site of Central Indian Ridge

High-resolution vector magnetic measurements were performed on an inactive ultramafic-hosted hydrothermal vent field, called Yokoniwa Hydrothermal Field (YHF), using a deep-sea manned submersible Shinkai6500 and an autonomous underwater vehicle r2D4. The YHF has developed at a non-transform offset massif of the Central Indian Ridge. Dead chimneys were widely observed around the YHF along with a very weak venting of low-temperature fluids so that hydrothermal activity of the YHF was almost finished. The distribution of crustal magnetization from the magnetic anomaly revealed that the YHF is associated with enhanced magnetization, as seen at the ultramafic-hosted Rainbow and Ashadze-1 hydrothermal sites of the Mid-Atlantic Ridge. The results of rock magnetic analysis on seafloor rock samples (including basalt, dolerite, gabbro, serpentinized peridotite, and hydrothermal sulfide) showed that only highly serpentinized peridotite carries high magnetic susceptibility and that the natural remanent magnetization intensity can explain the high magnetization of Yokoniwa. These observations reflect abundant and strongly magnetized magnetite grains within the highly serpentinized peridotite. Comparisons with the Rainbow and Ashadze-1 suggest that in ultramafic-hosted hydrothermal systems, strongly magnetized magnetite and pyrrhotite form during the progression of hydrothermal alteration of peridotite. After the completion of serpentinization and production of hydrogen, pyrrhotites convert into pyrite or nonmagnetic iron sulfides, which considerably reduces their levels of magnetization. Our results revealed origins of the magnetic high and the development of subsurface chemical processes in ultramafic-hosted hydrothermal systems. Furthermore, the results highlight the use of near-seafloor magnetic field measurements as a powerful tool for detecting and characterizing seafloor hydrothermal systems.

Chromium isotope fractionation during subduction-related metamorphism, black shale weathering, and hydrothermal alteration

Chromium (Cr) isotopes are an emerging proxy for redox processes at Earth's surface. However, many geological reservoirs and isotope fractionation processes are still not well understood. The purpose of this contribution is to move forward our understanding of (1) the Earth's high temperature Cr isotope inventory and (2) Cr isotope fractionations during subduction-related metamorphism, black shale weathering and hydrothermal alteration. The examined basalts and their metamorphosed equivalents yielded δ53Cr values falling within a narrow range of −0.12±0.13‰ (2SD, n=30), consistent with the previously reported range for the bulk silicate Earth (BSE). Compilations of currently available data for fresh silicate rocks (43 samples), metamorphosed silicate rocks (50 samples), and mantle chromites (39 samples) give δ53Cr values of −0.13±0.13‰, −0.11±0.13‰, and −0.07±0.13‰, respectively. Although the number of high-temperature samples analyzed has tripled, the originally proposed BSE range appears robust. This suggests very limited Cr isotope fractionation under high temperature conditions. Additionally, in a highly altered metacarbonate transect that is representative of fluid-rich regional metamorphism, we did not find resolvable variations in δ53Cr, despite significant loss of Cr. This work suggests that primary Cr isotope signatures may be preserved even in instances of intense metamorphic alteration at relatively high fluid–rock ratios. Oxidative weathering of black shale at low pH creates isotopically heavy mobile Cr(VI). However, a significant proportion of the Cr(VI) is apparently immobilized near the weathering surface, leading to local enrichment of isotopically heavy Cr (δ53Cr values up to ~0.5‰). The observed large Cr isotope variation in the black shale weathering profile provides indirect evidence for active manganese oxide formation, which is primarily controlled by microbial activity. Lastly, we found widely variable δ53Cr (−0.2‰ to 0.6‰) values in highly serpentinized peridotites from ocean drilling program drill cores and outcropping ophiolite sequences. The isotopically heavy serpentinites are most easily explained through a multi-stage alteration processes: Cr loss from the host rock under oxidizing conditions, followed by Cr enrichment under sulfate reducing conditions. In contrast, Cr isotope variability is limited in mildly altered mafic oceanic crust.

Thallium as a tracer of fluid–rock interaction in the shallow Mariana forearc

Fluids driven off the subducting Pacific plate infiltrate the shallow Mariana forearc and lead to extensive serpentinization of mantle peridotite. However, the sources, pathways, and chemical modifications of ascending, slab-derived fluids remain poorly constrained and controversial. In this study, we use thallium (Tl) concentrations and isotopic ratios of serpentinized peridotite and rodingitized diabase from the South Chamorro and Conical Seamounts to discriminate between potential fluid sources with distinct Tl isotope compositions. Serpentinite samples from the Mariana forearc all display ε205Tl>−0.5 (where ε205Tl=10,000×(Tl205/Tlsample203−Tl205/TlSRM997203)/(Tl205/TlSRM997203)), which is significantly enriched in 205Tl compared to the normal mantle (ε205Tl=−2). Given that high temperature hydrothermal processes do not impart significant Tl isotope fractionation, the isotope compositions of the serpentinites must reflect that of the serpentinizing fluid. Pelagic sediments are the only known slab component that consistently displays ε205Tl>−0.5 and, therefore, we interpret the heavy Tl isotope signatures as signifying that the serpentinizing fluids were derived from subducting pelagic sediments. A rodingitized diabase from Conical Seamount was found to have an ε205Tl of 0.8, suggesting that sediment-sourced serpentinization fluids could also affect diabase and other mafic lithologies in the shallow Mariana forearc. Forearc rodingitization of diabase led to a strong depletion in Tl content and a virtually complete loss of K, Na and Rb. The chemical composition of hybrid fluids resulting from serpentinization of harzburgite with concomitant rodingitization of diabase can be highly alkaline, depleted in Si, yet enriched in Ca, Na, K, and Rb, which is consistent with the composition of fluids emanating from mud volcanoes in the Mariana forearc. Our study suggests that fluid–rock interactions between sedimentary, mafic, and ultramafic lithologies are strongly interconnected even in the shallowest parts of subduction zones. We conclude that transfer of fluids and dissolved elements at temperatures and pressures below 400 °C and 1 GPa, respectively, must be taken into account when elemental budgets and mass transfer between the subducting plate, the forearc, the deep mantle and the ocean are evaluated.

Occurrence of abiotic methane in the eastern United Arab Emirates ophiolite aquifer

Methane with carbon and hydrogen isotope composition diagnostic of abiotic gas related to serpentinization of peridotites has been detected for the first time in the ophiolitic aquifer in the United Arab Emirates (UAE), along the Wadi Ham fault. This methane is isotopically similar to that previously reported in serpentinization-related springs in Oman, in the same Semail ophiolite nappe. Abiotic gas may be widespread in these ophiolitic rocks. Conventional thermogenic gas fields in the sedimentary basin overthrusted by the ophiolite could be “contaminated” to some extent by this abiotic gas. This may confound the accurate characterization of these thermogenic gases (origin, type of source rock type and maturity) and thus the associated petroleum system model.

Weathering crusts on peridotite

Chemical weathering of dark-green massive peridotite, including partly serpentinized peridotite, produces a distinct and remarkable brown weathering rind when exposed to the atmosphere long enough. The structure and mineral composition of crusts on rocks from the Ronda peridotite, Spain, have been studied in some detail. The generic overall weathering reaction serpentinized peridotite + rainwater = weathering rind + runoff water describes the crust-forming process. This hydration reaction depends on water supply from the outcrop surface to the reaction front separating green peridotite from the brown crust. The reaction pauses after drying and resumes at the front after wetting. The overall net reaction transforms olivine to serpentine in a volume-conserving replacement reaction. The crust formation can be viewed as secondary serpentinization of peridotite that has been strongly altered by primary hydrothermal serpentinization. The reaction stoichiometry of the crust-related serpentinization is preserved and reflected by the composition of runoff waters in the peridotite massif. The brown color of the rind is caused by amorphous Fe(III) hydroxide, a side product from the oxidation of Fe(II) released by the dissolution of fayalite component in olivine.

Redox controls on Ni–Fe–PGE mineralization and Re/Os fractionation during serpentinization of abyssal peridotite

Awaruite (Ni2Fe to Ni3Fe) is often used to infer fugacity and redox gradients in hydrothermally altered peridotites. However, discrepant petrological and thermodynamic data suggest that the fO2–fS2 stability field of awaruite is not well constrained. In this study, we assess the thermodynamic properties of awaruite and re-evaluate the Fe–Ni–S systematics of hydrothermally altered peridotites. New experimental data indicate that awaruite is stable at higher fO2 than previously thought, supporting the common occurrence of awaruite in the reaction zone of modern and ancient ultramafic-hosted hydrothermal vent systems. Awaruite is known to catalyze the abiogenic synthesis of methane during active serpentinization, contributing to methanogenesis at modern oceanic hydrothermal systems and potentially on early Earth. The enhanced stability field of awaruite determined here suggests that abiogenic methanogenesis may be active at a broad range of redox conditions.Serpentinized peridotites also contain platinum-group element (PGE)–Re sulfides and metal alloys that can be derived from desulfurization of primary mantle sulfides under low fS2. Thermodynamic calculations suggest that Os will exist as metal and Re as ReS2 in the redox stability field of awaruite. Under a relatively broad range of fS2–fO2 conditions, hydrothermal alteration and desulfurization of primary mantle may produce open system behavior in the Re/Os system and variable Re–PGE ratios and contribute over time to the Os isotopic heterogeneity of the mantle, through subduction and recycling of altered oceanic lithosphere. PGE-metal alloys occur at such low abundances in peridotite that they often are nearly impossible to find. The stability of awaruite at higher fO2 suggests that it can coexist with Os–Ir–Pt metal alloys, making it an important indicator mineral for their presence in hydrothermally altered peridotites.

Tectonic structure, lithology, and hydrothermal signature of the Rainbow massif (Mid-Atlantic Ridge 36°14′N)

Rainbow is a dome-shaped massif at the 36°14′N nontransform offset along the Mid-Atlantic Ridge. It hosts three ultramafic-hosted hydrothermal sites: Rainbow is active and high temperature; Clamstone and Ghost City are fossil and low temperature. The MoMARDREAM cruises (2007, 2008) presented here provided extensive rock sampling throughout the massif that constrains the geological setting of hydrothermal activity. The lithology is heterogeneous with abundant serpentinites surrounding gabbros, troctolites, chromitites, plagiogranites, and basalts. We propose that a W dipping detachment fault, now inactive, uplifted the massif and exhumed these deep-seated rocks. Present-day deformation is accommodated by SSW-NNE faults and fissures, consistent with oblique teleseismic focal mechanisms and stress rotation across the discontinuity. Faults localize fluid flow and control the location of fossil and active hydrothermal fields that appear to be ephemeral and lacking in spatiotemporal progression. Markers of high-temperature hydrothermal activity (∼350°C) are restricted to some samples from the active field while a more diffuse, lower temperature hydrothermal activity (<220°C) is inferred at various locations through anomalously high As, Sb, and Pb contents, attributed to element incorporation in serpentines or microscale-sulfide precipitation. Petrographic and geochemical analyses show that the dominant basement alteration is pervasive peridotite serpentinization at ∼160–260°C, attributed to fluids chemically similar to those venting at Rainbow, and controlled by concomitant alteration of mafic-ultramafic units at depth. Rainbow provides a model for fluid circulation, possibly applicable to hydrothermalism at oceanic detachments elsewhere, where both low-temperature serpentinization and magmatic-driven high-temperature outflow develop contemporaneously, channeled by faults in the footwall and not along the detachment fault.

Role of silica for the progress of serpentinization reactions: Constraints from successive changes in mineralogical textures of serpentinites from Iwanaidake ultramafic body, Japan

Serpentinization of peridotite in subduction zones and mid-ocean ridges is a key process that controls not only the geodynamic behavior of the mantle but also the activity of modern (and probably primordial) microbial systems on the seafloor. However, there is still controversy about what factors promote the mineralogical reactions of serpentinization in natural ultramafic rocks. Here we report textures, chemistry, and magnetic susceptibility of variably serpentinized harzburgite and dunite samples from Iwanaidake ultramafic body, Japan, which originated from the forearc mantle of the Northeast Japan arc. Successive changes in textures, mineral chemistry, and magnetic susceptibility during serpentinization of harzburgite involved two stages: replacement of olivine by serpentine and brucite, and subsequent formation of magnetite along with more-magnesian serpentine and brucite. The later reactions occurred concurrently with serpentinization of orthopyroxene, which supplied the silica component. In serpentinized dunite, which does not contain orthopyroxene, serpentinization involved replacement of olivine by serpentine and brucite, and the fraction of magnetite did not increase with the progress of serpentinization. These observations suggest that the silica supply from serpentinization of orthopyroxene is an essential factor for the formation of magnetite during serpentinization. Magnetite formation facilitated by addition of silica has often been reported for many serpentinite systems, suggesting that the magnetite formation triggered by silica addition is one of the key reactions for the progress of serpentinization in natural ultramafic rocks.