Serpentinization Of Peridotites Research Articles

Coupled Detachment Faulting and Hydrothermal Circulation at 49.7°E Southwest Indian Ridge Revealed by Seafloor Magnetism

AbstractDetachment faults at slow‐ and ultraslow‐spreading mid‐ocean ridges are essential for lithosphere‐hydrosphere interactions. However, their coupling with subseafloor hydrothermal circulation is poorly understood. Here, we investigated shallow hydrothermal fluid circulations in the active Dragon Horn detachment fault system of the Southwest Indian Ridge using combined near‐bottom magnetic anomaly data and rock magnetic analyses. We observed enhanced magnetic anomalies related to the high magnetization of basaltic country rocks and highly serpentinized (>80%) peridotite, whereas the hydrothermally altered basalts and sulfide deposits had weak magnetization and reduced magnetic anomalies. Therefore, the low‐magnetization zone was interpreted as a proxy for the hydrothermal fluid channel. The spatial distribution of the low‐magnetization zone revealed that the inactive Suye field expanded more than 500 m in diameter horizontally and ∼100 m vertically, and the active Longqi hydrothermal field had a >1 km fluid channel, both centered around the detachment fault fracture zone. Our results establish a robust link between detachment faulting and shallow hydrothermal circulation in which the detachment fault system provides critical fluid paths. Therefore, detachment tectonics are important elements for evolving seafloor hydrothermal fields along the Southwest Indian Ridge and possibly other slow‐ and ultraslow‐spreading mid‐ocean ridges globally.

In Situ Geochemical Evaluation of Retrograde Hydration Effects in the Peri-Siberian Forearc Mantle (Khara-Nur and Alag-Khadny Peridotite Complexes)

In order to assess the geochemical effects of retrograde metamorphic rehydration, fluid metasomatism, and the fluid-mobile elements (FMEs) budget in the case of oceanic and continental subduction, we report the petrography, bulk, and in situ LA-ICP-MS trace-element data for the two poorly studied ophiolites in the northern (Khara-Nur, Eastern Sayan, Russia) and central (Alag-Khadny accretionary wedge, SW Mongolia) parts of the peri-Siberian orogenic framing. Both complexes are relics of the ancient oceanic mantle, which was subjected to processes of partial melting, metasomatism, and retrograde metamorphism. Typical mineral assemblages include olivine + orthopyroxene + chlorite + tremolite ± secondary olivine (640–800 °C), olivine + antigorite ± secondary clinopyroxene (<640 °C), and olivine + chrysotile ± secondary clinopyroxene (<250 °C) and are stable at pressures up to 2 GPa. Hydration and partial serpentinization of mantle peridotites lead to tremolite formation after orthopyroxene, followed by olivine replacement by antigorite. Serpentine-group minerals (antigorite and chrysotile) were distinguished by Raman spectroscopy, and the contents of incompatible elements (mobile and immobile in fluids) in metamorphic minerals (tremolite, antigorite, and chrysotile) were examined in situ by LA-ICP-MS. The behavior of conservative HFSE (Zr, Nb, Ta, and Ti) and—in part—HREE does not distinguish between the two types (oceanic and continental) of subduction environments. Different patterns of FMEs (Cs, Rb, Ba, U, Sb, Pb, Sr, and LREE) enrichment in metaperidotites reflect variations in the slab fluid composition, which was primarily governed by the contrasting nature of subducted lithologies. The affinity of Alag-Khadny to the subduction of a continental margin is recorded by increased FME contents and selective enrichment by some moderately mobile elements, such as U, Th, and LREE, with respect to the oceanic-type subduction environment of Khara-Nur. Distinct patterns of FME enrichment in tremolite and antigorite from two complexes indicate different sequences of fluid-induced replacement, which was controlled by Opx composition. We demonstrate that evaluation of the initial composition of precursor minerals affected by multi-stage melting and melt metasomatism should be considered with care to estimate the differential fluid overprint and associated elemental uptake from subduction fluids.

Experimental evidence of pressure effects on spinel dissolution and peridotite serpentinization kinetics under shallow hydrothermal conditions

Serpentinization reactions are paramount to understand hydro-geothermal activity near plate boundaries and mafic–ultramafic massifs, as well as fluid and element transfer between the Earth’s mantle and crust. However, fluid-rock element exchange and serpentinization kinetics under shallow hydrothermal conditions is still largely unconstrained. Here we present two constant temperature (230 °C) time-series of natural peridotite (77.5% olivine; 13.7% enstatite; 6.8% diopside; 2% spinel) serpentinization experiments: at 13.4 MPa; and 20.7 MPa. Al-enriched lizardite was the main secondary mineral in all runs after olivine (olv) and orthopyroxene (opx) serpentinization (without any detectable brucite, talc or magnetite), while primary spinel and diopside partially dissolved during the experiments. Initial serpentinization stages comprises intrinsically coupled reactions between olivine and enstatite, as Al and Si are progressively transferred from orthopyroxene-derived to olivine-derived serpentine, while the opposite is true for Mg and Fe, with homogenization of serpentines compositions after 40 days. The Ni/Cr ratios of serpentines, however, remain diagnostic of the respective primary mineral. Estimated average serpentine content indicates fast serpentinization rates of 0.55 wt.%·day−1 (0.26 mmol·day−1) and 0.26 wt.%·day−1 (0.13 mmol·day−1) at 13.4 and 20.7 MPa, respectively. Approximately 2x faster serpentinization kinetics at lower pressure is likely linked to enhanced spinel dissolution leading to one order of magnitude higher available Al, which accelerates olivine serpentinization while delays orthopyroxene dissolution. Additionally, time-dependent increase in solid products masses suggests rock volume expands linearly 0.37% ± 0.01% per serpentine wt.% independently of pressure. Mass balance constrains suggests olv:opx react at ∼5:2 and ∼3:2 M ratios, resulting in Si-deficient and Si-saturated serpentines at the end of the low-pressure series (13.4 MPa) and high-pressure series (20.7 MPa), respectively. Elevated starting peridotite olv:opx ratio (7.94:1) therefore indicates orthopyroxene serpentinization is ∼3.3x and ∼5.4x faster than olivine at 13.4 MPa and 20.7 MPa, respectively. This contradicts previous assumptions that olivine should dissolve faster than orthopyroxene at experimental conditions. Finally, serpentinization-derived fluids develop pH > 10 and become enriched in H2, CH4, Ca2+ and Si within 6 weeks. Aqueous silica concentrations are highest after 5 days (265.75 and 194.79 µmol/kg) and progressively decrease, reaching 13.84 and 91.54 µmol/kg at 13.4 and 20.7 MPa after 40 days, respectively. These concentrations are very similar to the low-silica (M6) and high-silica (Beehive) endmembers of the Lost City Hydrothermal Field (LCHF). Beyond fluid characteristics, serpentinization products and conditions analogous to the LCHF suggest similar mechanisms between our experiments and natural processes. Our results demonstrate constant temperature serpentinization of a common protolith leads to distinct serpentine and fluid compositions at different pressures. Although additional data is necessary, recent studies and our experiments suggest peridotite serpentinization rates at 230 °C rapidly decrease with increasing pressures at least up to 35 MPa. Whether pressure directly influences olivine and orthopyroxene serpentinization kinetics or indirectly controls reaction rates due to spinel dissolution under hydrothermal conditions deserves further investigation.

Magnesium isotopic fractionation during post-serpentinization alteration: Implications for arc and oceanic Mg cycles

In recent years, numerous geodynamic studies have concentrated on serpentinites and the serpentinization processes that occur within subduction zones. However, as an important host phase for Mg, whether Mg isotopic fractionation occurs during peridotite serpentinization is still unclear. In our study, we conducted high-precision Mg isotope analyses on serpentinized peridotites and forearc serpentinites from the Mariana subduction zone. Except for one sample affected by strong late weathering with clay formation, which had an anomalously high δ26Mg value (0.01 ± 0.04‰), the moderately serpentinized peridotites (−0.24 ± 0.04‰, 2SD, n = 3) and deep (>40 mbsf) serpentinites (−0.24 ± 0.02‰, 2SD, n = 3) have Mg isotopic compositions consistent with mantle peridotites (−0.25 ± 0.04‰). Shallow serpentinites display slightly higher δ26Mg values (−0.19 ± 0.04‰, 2SD, n = 15) than mantle peridotites, and these values tend to increase toward the seafloor. These high δ26Mg values of forearc serpentinites cannot be attributed to serpentinizing fluid metasomatism and mantle wedge serpentinization processes, while post-serpentinization alteration involving brucite dissolution played a dominant role. Rayleigh fractionation simulations suggest that the heavy Mg isotopes are preferentially allocated to serpentinites during the post-serpentinization alteration process, where the calculated ∆26Mgbrucite-fluid varies from 0.1‰ to 0.8‰. This study deduces that the process of serpentinite dehydration in the subarc mantle cannot significantly change the Mg isotopic composition of arc lavas because serpentinite itself has a mantle-like Mg isotopic composition. Moreover, the fluids released by the brucite dissolution process have a low δ26Mg value, making an important contribution to the Mg budget of modern ocean seawater.

Alteration of chromite during serpentinization of peridotites

Trace element (Zn, Co, Mn, Ni, Ga, V, Ti, and Sc) compositions of chromite in mafic-ultramafic rocks are widely used for addressing their petrogenesis and discriminating tectonic environments, but they can be altered by subsolidus re-equilibration and hydrothermal alteration, and the detailed dynamic processes are not clear. The Jinchang ophiolite in SW China is part of the Tethyan ophiolite, and consists mainly of harzburgite with minor lherzolite. These peridotites are strongly serpentinized, and contain variably modified chromite and magnetite. Two types of chromite are identified in harzburgite, including magmatic and altered chromite. The magmatic chromite occurs as granular or amoeboid minerals, or as cores surrounded by the altered chromite, and they have homogeneous major elemental compositions (Mg0.65Fe0.35(Cr0.46Al0.54)2O4), but their trace elements, such as Ni, Ga, V, Zn, Co, and Mn, are variably changed during subsolidus re-equilibration. The altered chromite consists of ferrous chromite (Mg0.15Fe0.85(Cr0.46Al0.11Fe0.43)2O4) and chromian magnetite (Fe(Cr0.15Fe0.85)2O4), which were formed by two stages of hydrothermal alteration. They are depleted in Ga and Sc, and enriched in Zn, Co and Mn during the alteration, but their Ti, Ni and V are increased and decreased in the first and second stage, respectively. Thermodynamic modeling reveals that: (1) Reaction between magmatic chromite and olivine at the first stage of alteration produces ferrous chromite and chlorite under high temperature (610–470 °C) and increasing oxygen fugacity (ΔFMQ increasing from −1 to 3) and water-rich conditions; (2) Chromian magnetite is generated by further alteration of the ferrous chromite in the second stage alteration under low temperature (< 470 °C) and decreasing oxygen fugacity conditions (ΔFMQ decreasing from 3 to −4). In addition, the first stage alteration significantly changed trace-element compositions of the magmatic chromite compared with the second stage alteration that slightly modified the ferrous chromite. This study clarifies trace element migration of chromite in the silicate mineral-hydrothermal fluid system, indicating that chromite microstructures and trace elemental compositions can record the detailed serpentinization processes.

Origin of Erzgebirge ultrahigh‐pressure garnetite: Formation from a basaltic protolith by serpentinization‐assisted metasomatism?

AbstractErzgebirge ultrahigh‐pressure (UHP) garnet peridotite includes scarce layers of garnet pyroxenite, nodules of garnetite and, very rarely, of eclogite. Peridotite‐hosted eclogite shows the same subalkali‐basaltic bulk rock composition, mineral assemblage and peak conditions as gneiss‐hosted eclogite present in the same UHP unit. Garnetite has considerably more Mg, moderately enhanced Ca and Fe and significantly lower contents of Na, Ti, P, K and Si than eclogite, whereas Al is very similar. In addition, the compatible trace elements (Ni, Co, Cr, V) are elevated and most incompatible elements (Zr, Hf, Y, Sr, Rb and rare Earth elements [REE]) are depleted in garnetite relative to eclogite. In contrast to other large ion lithophile elements (LILEs), Pb (+121%) and Ba (+83%) are strongly enriched. The REE patterns of garnetite are characterized by depletion of light and heavy REE and a medium REE hump indicative of metasomatism, features being absent in eclogite. An exceptional garnetite sample shows an REE distribution similar to that of eclogite. Garnetite is interpreted to have formed from the same, but metasomatically altered, igneous protolith as eclogite. Except for Ba and Pb, the chemical signature of garnetite is explained best by metasomatic changes of its basaltic protolith caused by serpentinization of the host peridotite. Garnetite is chemically similar to basaltic rodingite/metarodingite. Although rodingite is commonly more enriched in Ca, there are also examples with moderately enhanced Ca matching the composition of Erzgebirge garnetite. Limited Ca metasomatism is attributed to the preservation of Ca in peridotite during hydrous alteration. This can be explained by incomplete serpentinization favouring metastable survival of the original clinopyroxene. In this case, most Ca is retained in peridotite and not available for infiltration and metasomatism of the garnetite protolith. This inescapable consequence is supported by the fact that clinopyroxene is part of the garnet peridotite UHP assemblage, which would not be the case if Ca had been removed from the protolith prior to high‐pressure metamorphism. The enrichment of compatible elements in garnetite is attributed to decomposition of peridotitic olivine (Ni, Co) and spinel (Cr, V) during serpentinization. Enrichment of Ba and Pb contrasts the behaviour of other LILEs and is ascribed to dehydration of the serpentinized peridotite (deserpentinization). This requires two separate stages of metasomatism: (1) intense chemical alteration of the basaltic garnetite precursor, together with serpentinization of peridotite at the ocean floor or during incipient subduction; and (2) prograde metamorphism and dehydration of serpentinite during continued subduction, thereby releasing Pb–Ba‐rich fluids that reacted with associated metabasalt. Finally, subduction to &gt;100 km and UHP metamorphism of all lithologies led to formation of garnetite, eclogite and garnet pyroxenite hosted by co‐facial garnet peridotite as observed in the Erzgebirge.

Assessing micrometer-scale contamination from organic materials in serpentinite analysis

Serpentinization of peridotite provides a significant source of energy for the subseafloor biosphere and abiotic organic synthesis. The presence of diverse micrometer-scale organic matter in serpentinites offers insights into deep carbon cycling and the origin of life on Earth. It is critical to maintain stringent lab protocols in analyzing serpentinite samples, limiting the contact with organic materials that could contaminate serpentinites and cause misinterpretations. However, the extent to which these organic materials (e.g. latex gloves or nylon polishing disc) can introduce contamination remains unclear. Here we subject serpentinite samples from the Yap Trench in the western Pacific Ocean to multi-stage cutting and polishing procedures prior to analysis. Our findings from electron microscopy reveal that micrometer-scale organic matter in serpentinites is randomly distributed either on the sample surface or within Cr-spinel fractures. Further analysis using Raman spectroscopy indicates that the organic matter contains several hydrogen bonding moieties, similar to those found in the latex gloves or nylon polishing disc used during the treatment of serpentinite samples. Our results suggest that the detected organic matter is likely due to contamination from the organic materials involved during sample processing. Thus, future studies need to carefully assess micrometer-scale organic contamination and limit the use of organic materials when analyzing organic compounds hosted in serpentinites, not only on Earth but also on other rocky planets.

Structure and dynamics of the Ecuador Fracture Zone, Panama Basin

SUMMARYIn this study, multiple geophysical data types are used to investigate the structure and dynamics of the Ecuador Fracture Zone—a complex multistranded strike-slip fault system located in the Panama Basin. Gravity modelling reveals a 25–30-km-wide region of ∼3-km-thick, low-density crust beneath this system and an anomalously low-density region in the uppermost mantle. Along both edges, the transition to the ‘normal’ structure and thickness oceanic crust formed at both the Ecuador and Costa Rica Rifts is abrupt. Within the Ecuador Fracture Zone itself, normal faults bound the median ridges. These faults traverse the entire thickness of accumulated sediment and offset the seabed, while sediment layer geometries document multiple phases of relative uplift, with the most recent phase still ongoing. Active extensional faulting, with an approximately spreading ridge-parallel strike, is also observed in 6–7 Ma Costa Rica Rift crust. The median ridges and the transverse ridge at the eastern edge of the Ecuador Fracture Zone also have contrasting crustal density structures. Both median ridges have a lower density crust than between the intervening valleys, while the transverse ridge crust has an equivalent thickness and density structure to that formed at the Costa Rica Rift. The active median valley basement-cutting normal faults allow seawater ingress and alternation of the crustal footwall, and also flow to mantle depth where, based on gravity modelling, 30–50 per cent serpentinization of mantle peridotite occurs. The resulting serpentinite-driven buoyancy acts as the primary control on the observed median ridge relative vertical tectonism. In contrast, the relative uplift of the transverse ridge results from lithospheric flexure in response to a change in spreading direction between the Ecuador and Costa Rica Rifts. Contrary to the widely accepted assumption that fracture zones are tectonically inactive systems, the Ecuador Fracture Zone provides evidence of extension, serpentinization due to ongoing hydrothermal circulation and relative uplift.

Multistage hydration during oceanic serpentinisation revealed by in situ oxygen isotope and trace element analyses

Serpentinisation of mantle peridotites below the seafloor is the most important hydration reaction in the Earth's deep water cycle. This critical step in water–rock interaction occurs over multiple serpentinisation stages and at variable temperatures and fluid compositions. We present the first study using spatially coupled in-situ analysis of oxygen isotopes (secondary ionization mass spectrometry) and trace elements (laser ablation inductively coupled plasma mass spectrometry) to unravel the multistage hydration history of oceanic serpentinites. We study samples from the Newfoundland-Iberia extended passive margins, which represents a magma-poor ocean-continent transition zone (Ocean Drilling Program cores, Leg 173 Site 1070 from Iberia, Leg 210 Site 1277 from Newfoundland). The concentrations of the fluid mobile elements chlorine and boron in serpentine are used as a proxy for the salinity of the serpentinising fluid. The correlation of Cl/B with δ18Oserpentine compositions provides new insights to disentangle temperature from fluid composition effects. The transition metal composition (V, Co, Sc, Mn, Zn, Ni, Cr) of dominantly lizardite in mesh after olivine and in bastite after orthopyroxene shows a chemical redistribution between textural sites in the Newfoundland samples, indicating the simultaneous serpentinisation of olivine and orthopyroxene. This feature is not observed in the Iberian samples, for which we propose sequential reactions. Lizardite in samples from both localities varies considerably in oxygen isotope composition at the scale of tens of micrometres depending on texture, with a range in δ18O of 3.3–13.5‰ for Iberia samples and a more restricted range of 5.7–9.3‰ for Newfoundland samples. Temperatures calculated from the δ18Oserpentine corresponding to the lowest Cl/B ratio (interpreted as closest to seawater composition) indicate sequential serpentinisation with decreasing temperature from ∼190 to ∼60 °C in the Iberia setting. The Newfoundland samples were serpentinised at a lower temperature (100–130 °C), possibly at a shallower depth. Serpentinisation in the Newfoundland samples probably produced a concentrated amount of H2 in a shorter period of time, whereas in the Iberian samples a small but prolonged amount of H2 was produced, possibly creating more favourable conditions for microbial activity.

High-resolution seismic imaging of shallow structure at proposed IODP drilling sites, Kane oceanic core complex, Mid-Atlantic Ridge

Two 500-m-deep holes are proposed to be drilled at the Kane oceanic core complex (OCC), one aimed at a serpentinized peridotite massif and the other at a gabbroic body. Ocean drilling is vital to validate lithological interpretation derived from seismic structures. Utilizing a long marine streamer, seismic imaging can effectively delineate the dominant lithologies within the OCCs. This study applied a suite of techniques, including downward-continued multi-channel seismic data, full waveform inversion, and reverse time migration, to obtain detailed fine-scale shallow structures beneath the Kane OCC. Through the downward continuation method, refracted seismic data at near offsets were utilized, thus doubling the resolution at shallow depths. Compared to previous findings, our results greatly enhance the understanding of shallow subseafloor structures, revealing a more precise morphology of the gabbroic bodies and well-imaged very shallow low velocities caused by seawater percolation in the shallow fissure due to footwall rotation extension. We present reference drilling hole velocities and link them to the likely composition, considering potential alteration processes such as serpentinization of mantle peridotite. Our investigation suggests that the lower crustal melt fluxes of the Kane OCC represent a transitional phase from low to medium accretion, providing valuable insights for future scientific ocean drilling projects in this region.

Looking for natural hydrogen in Albania and Kosova

A regional geochemistry field study was conducted in Albania and Kosova to spot natural H2 occurrences related to ophiolite massifs. A total of 21 sites, mainly consisting of natural springs, were studied, and nine were sampled for analyzing associated free gas and C and H isotopes of CH4 and H2 when possible. Four springs showed gas with H2 occurrence, one particularly reaching 16% of H2 in the north of Kosova in a location named Vuçe, which makes it the fifth gas seep most enriched in H2 in the Dinarides, after H2-rich gas seeps in Serbia and Bosnia and Herzegovina. This gas seep is associated with hyperalkaline water having a pH of about 10.7. This would favor the assumption that H2 is derived from the serpentinization of peridotites, a process which is likely still ongoing. H2 is associated mainly with N2 and CH4, like the other H2-rich gas springs in the Dinarides. Based on C and H isotopes, CH4 is abiotic or microbial. H isotopes suggest a formation of H2 at about a 2-km depth. Another hyperalkaline spring was found in the south of Albania, at the border of the Korça Basin, with less than 200 ppm of H2. No relation between H2 and He was identified at the scale of Albania and Kosova, nor at the scale of the whole Dinarides. This work provides a completed map of the H2 occurrences in the Dinarides and allows to highlight some hot spots for H2 exploration, mainly located inside the ophiolite massifs like in other ophiolites (such as Oman, New Caledonia, and The Philippines), and not on major faults like in the Pyrenees.

Ultramafic-Hosted Ni-Cu-Co-(As) Mineralization from an Ancient Oceanic Transform Fault Zone in the Troodos Ophiolite, Cyprus: An Analogue for Ultramafic Sea-Floor Massive Sulfide Mineralization?

Abstract Accumulations of sulfide minerals that are enriched in Ni-Cu-Co-(As) occur as sea-floor massive sulfide (SMS) deposits associated with ultramafic rock types on the sea floor and in ophiolite terranes as Outokumpu-type mineralization. In this study we focus on similar mineralization at Lakxia tou Mavrou in the Limassol Forest Complex of Cyprus, which represents the on-land exposure of an oceanic transform fault zone preserved within the Troodos ophiolite. Mineralization here consists of massive lenses of pyrrhotite associated with veins of isocubanite, chalcopyrite, Co pentlandite, and chrome spinel hosted in serpentinized mantle peridotite. We reexamine the field context of mineral occurrences and use in situ mineral chemistry, element mapping, and sulfur isotope ratios (δ34S) to constrain metal sources and provide an updated paragenetic model for Lakxia tou Mavrou. Highly variable S/Se ratios (304–108,571), a depletion in platinum group elements relative to mantle values, and an average δ34S value of –3.7 ± 2.4‰ (1σ, n = 17) in sulfide minerals support a hybrid hydrothermal and magmatic origin for the mineralization. Metals at Lakxia tou Mavrou were sourced from both the serpentinization of peridotites and from crosscutting intrusions, with later intrusions into the already serpentinized mantle lithosphere host providing a heat source to drive prolonged hydrothermal circulation. The reexamination of the field context of mineralization shows that the Ni-Cu-Co-(As) mineralization at Lakxia tou Mavrou originally formed because of the fault-guided intrusion of hot primitive magma bodies into serpentinized shallow mantle lithosphere in the active domain of an ocean-floor transform fault zone. The mineralization was subsequently partially disrupted by structures related to emplacement of the Troodos ophiolite. We show that the relationship between serpentinization, magmatism, and hydrothermal circulation at Lakxia tou Mavrou can be used to understand the formation of ultramafic-hosted SMS deposits in transform fault and other ultramaficdominated slow- and ultraslow-spreading mid-ocean ridge settings.

A Case Study on Distribution of Gemstone in Bela Ophiolite, District Khuzdar, Balochistan

Bela Ophiolite and suture zone host several types of gemstones in the underlain and overlain rocks such as garnet (demantoid and almandine), quartz, brucite, malachite, chrysocolla, azurite, calcite, natrolite, green chert, jasper, serpentine, agate, epidote, augite and prehnite. However, the rare gemstones including demantoid garnet are found as dodecahedron green crystals associated with the altered peridotite rocks such as serpentinite. Brucite is found in the botryoidal habit in the ultramafic rocks of Bela Ophiolite associated with the chromite mines in the Baran Lak and Amorzai area of Wadh in the host rock dunite. The serpentine is found in several locations associated with the altered ultramafic rocks formed after the serpentinization of peridotite. Furthermore, one of the rare gemstones such as petroleum and diamond quartz is formed due to the hydrothermal intrusion in the sedimentary rocks such as limestone; calcite also occurs in limestone as dogtooth crystals. While azurite, malachite and chrysocolla are formed as secondary deposits in the oxidized zone of copper and sulphide mines in the Manjawi, Nal and Sonaro associated adjacent to basalt. Moreover, epidote is found as green crystals having porphyritic texture in the host rock granite mainly in the Bakhalo area, Wadh. Agate, jasper, green chert and several other kinds of chert are found associated with the basalt chert unit in the mélange and gossan. In addition to that natrolite is found in the Nal ophiolite mainly in the Hazarghanji town found associated with the cracks and vugs of dolerite dykes as fibrous and cylindrical crystals. In metamorphic sole rocks of Bela Ophiolite in the Sonaro area, marble and quartzite are associated with the metamorphic rocks such as greenschist and amphibolite facies that are formed during the e volution period of the ophiolite. In pegmatite gabbro, the elongated and prismatic black crystals of augite are found in the host rock gabbro near the Ornach cross area, and the prehnite gemstone is found associated with the basalt and gabbroic rocks transition zone in the Ornach area.

Effect of Fluid Salinity on Reaction Rate and Molecular Hydrogen (H2) Formation During Peridotite Serpentinization at 300°C

AbstractWe conducted hydrothermal experiments at 300°C and at pressure varying from 2.2 to 3.4 kbar to study the effect of fluid salinity on the coupling between molecular hydrogen (H2) formation and olivine serpentinization, where peridotite and olivine with 25–50 μm of starting grain sizes were reacted with pure H2O and saline solutions (0.5, 1.5, and 3.3 M NaCl). Serpentine, the main hydrous mineral in most experiments, was quantified according to calibration curves based on Fourier‐transformed infrared spectroscopy and X‐ray diffraction analyses. Compared to pure H2O, saline solutions promote the hydrothermal alteration of olivine and peridotite. For experiments with peridotite and pure H2O, 67% of reaction extent was achieved after 14 days, which increased to 89% in experiments with medium‐salinity solutions (1.5 M NaCl) over the same period. Medium‐ and high‐salinity solutions inhibit H2 formation during serpentinization, which is associated with the serpentinization of pyroxene especially clinopyroxene. The redox conditions were constrained according to the equilibrium H2,aq = H2,g, and very reducing conditions were achieved during the serpentinization of olivine and peridotite. This study is the first to show iowaite formation directly from peridotite serpentinization, indicating alkaline solutions. Thermodynamic calculations suggest that the hydrolysis of NaCl (NaCl + H2O = HCl + NaOH) may yield alkaline solutions, due to higher dissociation constants of NaOH compared to HCl. This study suggests that chlorine greatly influences the serpentinization of olivine and peridotite in natural geological settings. It also indicates that iowaite formation may not require oxidizing conditions as previously thought.

Application of Satellite Remote Sensing, UAV-Geological Mapping, and Machine Learning Methods in the Exploration of Podiform Chromite Deposits

The irregular and sporadic occurrence of chromite pods in podiform chromite deposits (PCD), especially in mountainous terranes with rough topography, necessitates finding innovative methods for reconnaissance and prospecting. This research combines several remote sensing methods to discriminate the highly serpentinized peridotites hosting chromite pods from the other barren ultramafic and mafic cumulates. The case study is the area of the Sabzevar Ophiolite (NE Iran), which hosts several known chromite and other mineral deposits. The integration of satellite images [e.g., Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite sensor, Landsat series, and Sentinel-2] coupled with change detection, band rationing, and target detection algorithms [including the Spectral Angle Mapper (SAM)] were used to distinguish potential lithological units hosting chromites. Results have been verified by an initial on-field checking and compared with the high-resolution (GSD ~6 cm) orthomosaic images obtained by the processing of photographs taken from an Unmanned Aerial Vehicle (UAV) at a promising area of 35 km2. The combination of visual interpretation and supervised classification by machine learning methods [Support Vector Machine (SVM)] yielded the production of a geological map, in which the lithological units and structures are outlined, including the crust-mantle transition zone units, mafic cumulates, crosscutting dykes, and mantle sequences. The validation of the results was performed through a second phase, made up of field mapping, sampling, chemical analysis, and microscopic studies, leading to the discovery of new chromite occurrences and mineralized zones. All ultramafic units were classified into four groups based on the degree of serpentinization, represented by the intensity of their average spectral reflectance. Based on their presumed protolith, the highly serpentinized ultramafics and serpentinites were classified into two main categories (dunite or harzburgite). The serpentinite with probable dunitic protolith, discriminated for a peculiar Fe-rich Ni-bearing lateritic crust, is more productive for chromite prospecting. This is particularly true at the contact with mafic dykes, akin to some worldwide chromite deposits. The results of our work highlight the potential of multi-scale satellite and UAV-based remote sensing to find footprints of some chromite mineral deposits.

The Formation of Magnesite Ores by Reactivation of Dunite Channels as a Key to Their Spatial Association to Chromite Ores in Ophiolites: An Example from Northern Evia, Greece

Ophiolite magnesite deposits are among the main sources of magnesite, a raw material critical for the EU. The present work focuses on magnesite occurrences at Kymasi (Evia Island, Greece), in close spatial association with chromitite within the same peridotite massif, and on the relationship between ultramafic rocks and late magnesite veins. Chromitite lenses are hosted within dunite, in contact with a partially serpentinized peridotite cut by magnesite veins. Close to the veins, the peridotite shows evidence of carbonation (forming dolomitized peridotite) and brecciation (forming a serpentinite–magnesite hydraulic breccia, in contact with the magnesite veins). Spinel mineral chemistry proved to be crucial for understanding the relationships between different lithologies. Spinels within partially serpentinized peridotite (Cr# 0.55–0.62) are similar to spinels within dolomitized peridotite (Cr# 0.58–0.66). Spinels within serpentinite–magnesite hydraulic breccia (Cr# 0.83–0.86) are comparable to spinels within dunite and chromitite (Cr# 0.79–0.84). This suggests that older weak zones, such as dunite channels, were reactivated as fluid pathways for the precipitation of magnesite. Magnesite stable isotope composition, moreover, points towards a meteoric origin of the oxygen, and to an organic source of carbon. The acquired data suggest the following evolution of Kymasi ultramafic rocks: (i) percolation of Cr-bearing melts in a supra-subduction mantle wedge within dunite channels; (ii) obduction of the ophiolitic sequence and peridotite serpentinization; (iii) uplift and erosion of mantle rocks to a shallow crustal level; (iv) percolation of carbon-rich meteoric waters rich at shallow depth, reactivating the dunite channels as preferential weak zones; and (v) precipitation of magnesite in veins and partial brecciation and carbonation of the peridotite host rock.

Relation between Magmatic and Tectonic Processes in the Formation of the Oceanic Crust to the South of the Charlie Gibbs Fracture Zone (North Atlantic)

The article presents new data on the structure and relationship of tectonic and magmatic processes during the formation of the Mid-Atlantic Ridge between the Charlie Gibbs and Maxwell fracture zones in the North Atlantic. It is shown that this region is characterized by significant reduction in volcanism, which leads to the excavation of low crustal and upper mantle rocks to the surface. Both individual inland oceanic complexes of the most varied configurations and extended sublatitudinal ridges composed of plutonic rocks are formed. Our analysis showed that this formation setting existed for at least 14‒16 Ma. The formation of most internal oceanic complexes is associated not only with tectonic factors, but also with the serpentinization of peridotites, which leads to a decrease in density, an increase in volume, and, as a result, to the emergence of large ultrabasic massifs, including separated blocks of gabbroids, dolerites, and basalts. Numerous zones of sliding, crushing, abrasion and deformation of rocks indicate tectonic movements. The study region is characterized by numerous non-transform displacements of different amplitudes, which are formed under conditions when relative displacements of oceanic lithosphere segments are realized in wide areas undergoing shear and extension deformations. The morphology of the emerging tectonic-magmatic structures of the region is determined by tectonic factors. The exceptions are cases when the volumes of melts entering the surface in a short period of time are significantly higher than the average for a certain segment of the rift valley. The analysis performed shows the presence within the region of sources of magnetic anomalies that are heterogeneous in nature, both of volcanic origin and associated with manifestations of superimposed tectonic activity.

Element mobility and Mg isotope fractionation during peridotite serpentinization

Serpentinite plays a crucial role in element mobility during seawater hydrothermal alteration of mid-ocean ridge peridotite on the seafloor and fluid metasomatism of peridotite in the subduction zone. However, element mobility and Mg isotope fractionation during serpentinization of peridotites are still uncertain. In order to address this issue, we present a geochemical study of serpentinites and their associated peridotites in the Xigaze ophiolite from southern Tibet. The serpentinites have lower MgO contents and Mg/Si ratios than the peridotites. The low Mg/Si ratios in serpentinites, along with the Isocon calculation results, indicate the Mg loss during serpentinization. In terms of trace elements, the serpentinites have higher contents of Ba, U, Sr and Pb than the peridotites, though both types of ultramafic rocks exhibit similar REE distribution patterns and HFSE contents. The serpentine-magnetite O isotope thermometry yields a relatively low serpentinization temperatures of 200–300 °C. The trace element composition of the serpentinites is distinct from that of forearc serpentinites but consistent with that of mid-ocean ridge ones. The peridotites show mantle-like δ26Mg values of –0.27 to –0.23‰ with an average of –0.25 ± 0.03‰ (2SD, n = 6). In contrast, the serpentinites have systematically higher δ26Mg values of –0.21 to –0.09‰ with an average of –0.16 ± 0.09‰ (2SD, n = 8). The Mg isotope composition of serpentinites shows no correlations with either the chemical weathering proxy or the talc modal content, indicating that the Mg isotope fractionation between serpentinite and peridotite is not caused by chemical weathering of the serpentinites. Instead, it probably resulted from the serpentinization of peridotite on the seafloor. Thus, there is the loss of not only element Mg but also isotopically light Mg during the serpentinization of peridotites. The isotopically heavy Mg serpentinites can be carried to oceanic subduction zones, giving rise to higher δ26Mg mantle sources for various basalts. The isotopically light Mg can be released into the seawater, regulating the Mg isotope composition of seawater in geological history. Taking into account the sources and sinks of Mg in subduction zone fluids, the formation of isotopically heavy Mg serpentinites during seafloor serpentinization can be probably an important process in changing the Mg isotope composition of mantle sources.

Dating fluid infiltration and deformation in the subducted ultramafic oceanic lithosphere by perovskite geochronology

Carbonated ultramafic rocks (i.e., ophicarbonates) form by peridotite serpentinization and carbonation at the seafloor. Their metamorphic evolution is linked to that of serpentinites, hence dating deformation and metamorphism in ophicarbonates will add constraints to the P-T-t history of the subducted ultramafic lithosphere. A potential, yet underexplored, suitable mineral for geochronology in ophicarbonates is perovskite (CaTiO3). We present trace element and geochronological data (LA-ICPMS and ID-TIMS) from perovskite in ophicarbonate rocks and calcite veins from two metamorphic terranes, Lanzo Massif and Val Malenco (Alps). Metamorphic perovskite in ophicarbonates is characterized by enrichment in heavy REE, and strong negative anomalies in Zr and Hf, as well as Sr and Pb, which are attributed to coexistence of a Zr-rich phase and mobilization of these elements by fluids, respectively. Other trace elements broadly reflect the composition of the protolith and allow the distinction of perovskite from mafic and ultramafic rocks.In the Lanzo Massif, the 49.6 ± 1.0 Ma age of perovskite in shear bands constrains H2-mediated carbonate reduction and abiotic methane and graphite formation at near peak metamorphic conditions during deformation. Perovskite from Val Malenco is found in calcite veins cutting across serpentinite shear foliation and its age of 48.9 ± 0.5 Ma coincides with a major thrusting and extension event that is distinct in age from the initial nappe stacking. The Val Malenco perovskite age can be reproduced by LA-ICPMS and ID-TIMS and is proposed as a suitable secondary reference material for metamorphic, low U perovskite. These results show that metamorphic perovskite geochronology yields geologically meaningful UPb ages and is a viable tool to date deformation and fluid flow in the subducted oceanic lithosphere.

Chromite Oxidation Patterns Associated to Serpentinization: Case Studies from the Mid-Atlantic Ridge, the Alter do Chão Massif (NE Alentejo, Portugal) and the Ronda Massif (Spain)

Chromite oxidation during serpentinization of host peridotites is a well-documented process. Detailed compositional characterization of chromites and Cr-rich spinels from three geotectonic settings provided the basis for this study, focused on the comparison of their oxidation patterns as a means to evaluate the relative mobility of spinel components during serpentinization-related oxidation in those different contexts, namely: (i) tectonic exposures of serpentinized oceanic upper-mantle in the Azores sector of the Mid-Atlantic Ridge (MAR); (ii) serpentinized peridotites from Cabeço de Vide (CV, Alter do Chão Massif, Portugal); and (iii) serpentinized subcontinental mantle peridotites from the Ronda Massif (Spain). Electron microprobe data show that: (i) irrespective of geotectonic setting and original composition, Cr-spinels follow similar oxidation trends; (ii) early Cr-spinel oxidation, involving significant Mg- and Al-depletion leading to the formation of ferritchromit rims, has been more intense in the CV serpentinites than in the more recent MAR and Ronda serpentinites; (iii) with the possible exception of Zn, trace components (Mn, Ti, V, Ni and Co) seem to be relatively immobile during spinel oxidation, thus becoming slightly enriched in ferritchromit rims; (iv) extreme Cr-spinel oxidation is responsible for incomplete outer rims of magnetite on several grains. Time, fluid pH, and fluid/rock ratios seem to be the main factors controlling the intensity and extension of chromite oxidation.