Host Serpentinite Research Articles

Carbonatization and silicification of ophiolitic ultramafic rocks and formation of gold-bearing listvenites in the Arabian-Nubian shield: A case study from the Al-Barramiya district

Late Neoproterozoic mantle section in the Gabal Al-Barramiya area, the northwestern corner of the Arabian-Nubian Shield (ANS), contains variably serpentinized peridotites that are highly altered along shear zones and thrust planes to form gold-bearing listvenites. They can be categorized into carbonate listvenite, silica-carbonate listvenite and silica listvenite (birbirite). Carbonate listvenite is characterized by the presence of schistosity and deformation fabrics similar to the host serpentinites, but such fabrics are absent in the silica-carbonate and silica listvenites, suggesting that they postdate carbonate listvenite and serpentinization. The presence of listvenites along shears zones and the presence of relics of serpentine and Cr-spinel reflects their formation through metasomatism of ultramafic rocks by hydrothermal fluids circulating along the thrust faults. Silica-carbonate listvenite is characterized by the presence of fuchsite and is enriched in Zn, Pb, Cu, Ag, and Au. Rare earth element (REE) contents differ between the studied listvenites. Silica -carbonate listvenite has the lowest total REE (∑₌0.98–2.56 ppm), whereas the silica listvenite contains the highest total of REE (∑ ₌ 15.70–21.42 ppm). Based on the above, carbonate listvenite is the earliest to form by the infiltration of CO2–bearing fluids released during serpentinization of the original fore-arc peridotite slab, followed by formation of silica-carbonate listvenite due to the activities of SiO2–saturation and CO2-bearing fluids released during ophiolite obduction. Fuchsite in silica-carbonate listvenite formed as a result of metasomatic reactions of Si- and K-rich fluids with Cr-spinel due to hydrothermal alteration of serpentinized peridotite. Silica listvenite formed at the final stage by the silicification of the early formed silica-carbonate listvenite. Listvenitization of the mantle section of the Al-Barramiya ophiolite led to concentration of Au, Pb, Zn, Cu and Ag. The silica-carbonate listvenite contains higher concentration of gold (899–2199 ppb) than the carbonate listvenite (119–191 ppb) and silica listvenite (156–233 ppb).

Gold and sulfide-bearing listvenite in the mantle section of the Tays ophiolite in the Arabian Shield, Saudi Arabia

The mantle section of the Late Neoproterozoic Tays ophiolite in the Arabian Shield consists principally of thoroughly serpentinized peridotite with characteristics typical of depleted mantle protoliths from a fore-arc environment. The serpentinite is altered along shear zones and thrust planes to gold-bearing listvenite bodies of various sizes. These bodies are divided into carbonate listvenite and silica‑carbonate listvenite; they may be dyke-like or lenticular in form, and are yellowish-brown, reddish-brown, or greyish in outcrop. Carbonate listvenite expresses schistose deformation fabrics concordant to fabric in the host serpentinite, whereas silica‑carbonate listvenite is undeformed at field scale and contains a generation of undeformed minerals at thin-section scale. Silica‑carbonate listvenite contains Cr-rich muscovite (fuchsite) and base-metal sulfides and is enriched in Zn, Pb, Cu, Ag, and Au along with SiO2. The transformation of serpentinite along shear zones to different types of listvenite reflects successive episodes of fluid-mediated metasomatism. Carbonate listvenite develops first, driven by infiltration of CO2–bearing fluids during serpentinization of the original fore-arc peridotite. Silica‑carbonate listvenite marks a later episode associated with infiltration of K-bearing, SiO2-saturated fluids released during emplacement of the ophiolite. Listvenitization in the Tays serpentinite concentrated gold in sub-economic to economic extents, with concentrations increasing from host serpentinite (2–4 ng/g) to carbonate listvenite (267–937 ng/g) to silica‑carbonate listvenite (1717–3324 ng/g).

Exhumation of ultrahigh-pressure eclogite and metarodingite within UHP serpentinites from the Chinese southwestern Tianshan

Extensive research over the past two decades suggests that serpentinite loses its ability to effectively facilitate the exhumation of high-density high-pressure (HP) eclogites at depths of 60–80 km under average subduction zone geotherms. However, at Changawuzi, the western termination of Chinese southwestern Tianshan, eclogite and eclogitic metarodingite are commonly enclosed within ultrahigh-pressure Ti-chondrodite serpentinites (510–530 °C, 37 ± 7 kbar). This metamorphic mafic–ultramafic association, which has undergone a complex evolution from oceanic alteration to deep subduction and subsequent exhumation, presents a unique opportunity to study the incompletely understood mechanisms leading to the exhumation of ultrahigh-pressure (UHP) eclogite in UHP serpentinite.The Changawuzi serpentinite wraps three different rock types: retrograded eclogite, Rodingite I and Rodingite II. Field observations and petrological analyses indicate that the precursor of Rodingite I intruded as vein into the serpentinite protolith prior to subduction, sharing the evolution towards ultrahigh-pressure conditions. According to its composition, the serpentinite originated from a depleted oceanic mantle peridotite, with only minor enrichment in fluid-mobile elements (FME) during oceanic serpentinization. The eclogites have N-MORB like bulk compositions, with lower Si but higher Mg content compared to other eclogites in the Tianshan area, and appear to have been relatively unaffected by oceanic alteration processes. The Changawuzi serpentinite and eclogites likely evolved within a slab setting with limited interaction with seawater. Petrological studies and phase equilibria modeling suggest that the eclogites experienced peak UHP metamorphism at 28–34 kbar and around 460–465 °C. Density measurements yielded relatively low densities of 2.66–2.97 g/cm3 for the host serpentinites, but 3.27–3.63 g/cm3 for Rodingite I. The similarity in the low temperature/ultrahigh pressure conditions, as well as the geochemical characteristics shared by the serpentinite and eclogites, further support the connection between the Changawuzi serpentinites and the interior of the subducting slab. According to our data, two types of UHP eclogitic mafic rocks exhumated with UHP serpentinite. (1) High density Rodingite I veins are embedded within serpentinite and share a common evolution and exhumation history with the host serpentinite. (2) The eclogite and serpentinite tectonically detached from the interior of the subducting slab at different depths and ascended to the plate interface above the slab (subduction channel). These two rock types experienced separate exhumation processes within the subduction channel during the early stages of exhumation. Subsequently, the buoyant serpentinite captured the denser retrograded eclogite at a depth of approximately 80 km, and together they underwent further retrograde metamorphism as they ascended towards the surface. During a later exhumation stage, external fluid induced a second serpentinization process, leading to the formation of Rodingite II within the subduction channel. This study presents a novel exhumation mechanism specific to the Tianshan eclogites and highlights the significant role of deeply subducted serpentinite in the exhumation of eclogites and eclogitic metarodingites from sub-arc depths within an oceanic subduction zone.

Variations in the Composition of MORB Chilled Glasses from the Mid-Atlantic Ridge, 12°–31° N: Reflection of Composition Evolution of Parent Melts and the Influence of the Hydrothermal Component

The geochemical peculiarities of sample collection of MORB chilled glasses obtained in six areas of the axial zone of the Mid-Atlantic Ridge (MAR), 12°–31° N have been studied. The results of this study provided information on the composition of the parent melts for these glasses and made it possible to assess probable geochemical effects reflecting the interaction of magmatic melts with hydrothermal systems of the MAR axial zone or with altered oceanic crust (AOC). It is shown that basalts of the E-MORB family which includes most of samples are localized mainly in the “cold” segments of the MAR in the crustal section of which serpentinites predominate. On the other hand, samples with depleted signature (N-MORB) belong to segments of the MAR in which serpentinite outcrops are either absent or play a subordinate role. The E-MORB chilled glasses from “cold” segments of the MAR show signs of contamination of basaltic melts with components borrowed either from the host serpentinites or from aqueous-saline fluids circulating in hydrothermal systems located in serpentinites (“serpentinite hosted”). Judging by the data obtained on the nature of variations in the content of Cl, U, and Sr in the studied chilled glasses belonging to the N-MORB family, there are no signs of intracrustal contamination. It is assumed that relics of the ancient continental lithosphere preserved under axial zone of the MAR and involved in the process of partial melting of the shallow mantle took part in the formation of E-MORB parental melts in some segments of the MAR.

Mineralogy of chromitites of mount Poklonnaya of the Karabash massif, South Urals

Mineral composition of chromitites and host serpentinites from a quarry at Mt. Poklonnaya of the Karabash serpentinite massif is studied using optical and electron microscopy. In addition to previously known native osmium and laurite, eight minerals of platinum group elements (PGE) are found: isoferroplatinum, irarsite, iridium, naldrettite, cuproiridisite, sperrilite, tolovkite and erlichmannite. It is found that magnesioalumochromite is the early magmatic Cr-spinel and magnesiochromite and ferrichromite are late magmatic. The earliest native iridium and native osmium are replaced by PGE sulfdes, arsenides and stibnides. Magnesiochromite is associated with native gold, Ni chalcohenides (gersdorffte, millerite, pentlandite, heaslewoodite) and chalcopyrite. The formation of Cr-magnetite, magnetite, native iron, native nickel, galena and barite is related to serpentinization. Carbonates (calcite and dolomite), brucite, andradite, sepiolite and an unidentifed Ca-silicate formed at the latest stage of serpentinization. Secondary Ni minerals (gaspeite, nepuite, «garnierite») are most likely the products of the latest mineral-forming process.

Formation of gold-bearing listvenite in the mantle section of the Neoproterozoic Bir Umq ophiolite, Western Arabian Shield, Saudi Arabia

Serpentinized peridotite and associated listvenites of Neoproterozoic age outcrop in the Bir Umq area of western Saudi Arabia. The mantle section of the Bir Umq ophiolite is extensively serpentinized. Serpentinite host samples are low in Al2O3 (0.48–0.75 wt%) and CaO (0.28–1.24 wt%) and have high Mg# (0.90–0.92), indicating a strongly depleted mantle protolith, typically associated with supra-subduction zone environments and more specifically with fore-arc settings. Listvenite bodies of various shapes and sizes developed by alteration of serpentinite. Listvenites occupy the hanging walls of a stack of thrust faults, while serpentinite dominates the footwalls. Based on mineralogical composition and whole-rock geochemistry, the listvenites of Bir Umq are distinguished into carbonate listvenite and silica-carbonate listvenite; the latter is further divided into mineralized and non-mineralized samples. Carbonate listvenite is high in MgO, Fe2O3, and CaO, but depleted in SiO2. Silica-carbonate listvenite is characterized by the presence of rhythmic banding of quartz and carbonate minerals and by the presence of fuchsite. The Bir Umq listvenites preserve various stages of the progressive alteration and metasomatic transformation of their ultramafic protoliths due to interaction with hydrothermal fluids enriched in CO2, SiO2, Au, K, and other fluid-mobile elements. The association with thrusting suggests that faults acted as conduits for fluids derived from metamorphism of the underlying units during subduction and obduction. Schistosity and deformation fabrics in carbonate listvenite imply that initial listvenitization took place at conditions similar to the conditions of serpentinization. On the other hand, the absence of deformation fabrics in silica-carbonate listvenite suggests that it postdates serpentinization and therefore represents a separate and later fluid infiltration event. Finally, the mineralized silica-carbonate listvenite is highly enriched in fluid-mobile elements Zn, Pb, Cu, Ag, and most notably Au; this enrichment is not correlated with silica content. This suggests that yet a third fluid infiltration event is responsible for the mineralization and Au enrichment. Au concentrations are 0.84–2.31 ng g−1 in host serpentinite, 26–403 ng g−1 in carbonate listvenite, 152–545 ng g−1 in non-mineralized silica-carbonate listvenite, and 2286–3712 ng g−1 in mineralized silica-carbonate listvenite.

Decoupling of inorganic and organic carbon during slab mantle devolatilisation

Serpentinites are an important sink for both inorganic and organic carbon, and their behavior during subduction is thought to play a fundamental role in the global cycling of carbon. Here we show that fluid-derived veins are preserved within the Zermatt-Saas ultra-high pressure serpentinites providing key evidence for carbonate mobility during serpentinite devolatilisation. We show through the O, C, and Sr isotope analyses of vein minerals and the host serpentinites that about 90% of the meta-serpentinite inorganic carbon is remobilized during slab devolatilisation. In contrast, graphite-like carbonaceous compounds remain trapped within the host rock as inclusions within metamorphic olivine while the bulk elemental and isotope composition of organic carbon remains relatively unchanged during the subduction process. This shows a decoupling behavior of carbon during serpentinite dehydration in subduction zones. This process will therefore facilitate the transfer of inorganic carbon to the mantle wedge and the preferential slab sequestration of organic carbon en route to the deep mantle.

Carbonation of Neoproterozoic mantle section and formation of gold-bearing listvenite in the Northern Nubian Shield

The ultramafic rocks of Gabal Balamhindit are exposed as conspicuous ridges in the south Eastern Desert of Egypt, Northern Nubian Shield. These rocks include extensively to completely serpentinized harzburgites and dunites, which are listvenitized along faults and shear zones. The relict primary minerals of these serpentinized peridotites are represented by olivine, orthopyroxene and Cr-spinel, whereas serpentine minerals, amphiboles and chlorite are secondary phases. The listvenite is composed essentially of quartz and carbonates, with relics of serpentine minerals and Cr-spinel. The carbonate minerals include magnesite, dolomite, breunnerite and calcite. A characteristic feature of listvenite is the presence of rhythmic banding of quartz and carbonate minerals and presence of fuchsite. Mineral chemistry of primary minerals revealed that the Fo content of olivine, Mg# of orthopyroxene and Cr# of Cr-spinel in serpentinized dunite are higher than those of corresponding minerals in the serpentinized harzburgite. The composition of primary minerals in both serpentinized dunite and harzburgite (olivine Fo = 90–92; orthopyroxene Mg# = 0.91–0.93; Cr-spinel with Cr# of 0.61–0.72 and TiO2 content of 0.04–0.2 wt%) and whole-rock geochemical characteristics (Mg# = 0.91–0.92; low TiO2 content of values <0.04 wt%) are consistent with depleted mantle protolith, typically associated with supra-subduction zone environments, and more specifically with fore-arc settings. The examined serpentinized peridotite represents mantle residue that experienced different degrees of partial melting (33–40%). The restriction of listvenization to shear zones and presence of relics of mesh-textured serpentine and Cr-spinel with ferritchromite rims in the listvenite reflect formation through replacement of ultramafic rocks during ophiolite obduction in the Arabian-Nubian Shield. Schistosity and deformation fabrics in the listvenite imply that initial listvenitization took place at conditions similar to those where serpentinization of primary ultramafic precursors occurred. The Balamhindit listvenite has variable contents of fluid mobile elements such as As (127–164 ppm), Sb (8–18 ppm), Sr (88–162 ppm) and Pb (193–276 ppm), with presence of native gold. Compared to listvenites, serpentinites contain less amounts of these elements (4–9 ppm As, 0.26–1.02 ppm Sb, 4–10 ppm Sr and 0.27–1.27 ppm Pb). The wide distribution of listvenites along the suture zones favor a metamorphic source of the fluids during the accretionary tectonics. The listvenitization process concentrated gold in listvenite compared to the low-grade Au concentration in the host serpentinite. The Au content in listvenite samples shows positive correlation with the metals Zn, As, Cu, Ag and Pb, suggesting a common source. The listvenites of Gabal Balamhindit are auriferous, with gold content up to 3 ppm, suggesting that the listvenites of the Arabian-Nubian Shield (ANS) are potential target for gold exploration.

Podiform magnetite ore(s) in the Sabzevar ophiolite (NE Iran): oceanic hydrothermal alteration of a chromite deposit

Serpentinite-hosted massive magnetite ore bodies are reported for the first time in the Late Cretaceous Sabzevar ophiolitic belt, northeastern Iran. They show irregular and discontinuous shapes with variable sizes ranging from 30 to 60 cm. Chromian spinel grains are observed within both magnetite ores and host serpentinite. Magmatic chromian spinels, (Cr,Al)-spinel I, with compositions close to (Mg0.6,Fe0.4)(Cr1.2,Al0.75,Fe3+0.05)O4 are preserved in the host serpentinite where they display a porous alteration rim composed of Cr-bearing chlorite and three different spinel-structure minerals: Cr-spinel (Fe0.6,Mg0.4)(Cr1.4,Al0.4,Fe3+0.2)O4, named Cr-spinel II (second generation), magnetite and ferritchromite, nominally FeCr2O4. In the magnetite ore body, no (Cr,Al)-spinel I is found and Cr-spinel II occurs as relict cores surrounded by ferritchromite and magnetite. Detailed X-ray elemental mapping revealed that the 200 μm-thick magnetite rim is composed of two magnetite types with different minor element compositions: the first rim found at the contact with ferritchromite is thin (20 μm; magnetite-I); the thicker outer rim contains numerous Fe-poor and Mg- and Si-rich silicate inclusions (magnetite-II). Observations at the TEM scale allows to identify ferritchromite which occurs as a micrometer-sized rim between Cr-spinel II and magnetite I. Thermodynamic modelling of the phase relationships in the studied Sabzevar serpentinite suggests that Cr-spinel II is produced along with chlorite during a first alteration stage at temperatures between 725 and 575 °C in the course of peridotite-water interactions. A second hydrothermal alteration stage producing ferritchromite and magnetite is inferred from the thermochemical modelling at temperatures < 400 °C under high H2 fugacity. This latter stage corresponds to the serpentinization of the Sabzevar oceanic peridotite and associated podiform chromitite deposit. The two alteration stages are interpreted as the result of the interaction between seawater and oceanic mantle at two different depth ranges in the course of its exhumation. Our thermodynamic calculations and textural relationships revealed that Cr is immobile and Fe is the main element to be transferred to the magnetite ore during alteration processes. Fe possibly originated from direct transport of the Fe2+ produced during olivine dissolution or from the dissolution of nano-sized magnetite grains initially formed in the host serpentinite during early serpentinization. Mass balance calculation reveals significant iron transport at a scale > 10 m during serpentinization.

Serpentinization in the Archean and Early Phanerozoic – Insights from chromium isotope and REY systematics of the Mg[sbnd]Cr hydroxycarbonate stichtite and associated host serpentinites

Chromium isotopes have emerged as powerful redox proxies for past ocean-atmosphere systems as shifts in Cr isotope ratios towards heavier isotope compositions are indicative of Cr redox cycling on Earth. Knowledge of the Cr cycle in the critical zone and the fate of Cr during serpentinization and other hydrothermal processes is therefore important to assess the geologic interpretation of Cr isotopes in the rock record. An investigation of the trace element geochemistry of stichtite may provide such insights by shedding new light on the conditions of past serpentinizing environments.Stichtite, a magnesium‑chromium-bearing hydroxycarbonate, is a rare mineral that forms during serpentinization by conversion of chromite and other Cr-rich spinels. Its occurrence is restricted to Archean and Early Phanerozoic serpentinite bodies on ancient cratons and to mine waste. In this contribution, Cr isotope and rare earth elements and Y (REY) systematics in stichtites and their respective serpentinite hosts were investigated. This comparative study of Archean and Early Phanerozoic stichtites from different localities in South Africa, Western Australia, and Tasmania revealed variable Cr isotope fractionation with δ53Cr ranging from −0.145 ± 0.052‰ up to +0.24 ± 0.04‰ in the stichtites. The serpentinites are less fractionated with δ53Cr ranging from −0.13 ± 0.04‰ to +0.05 ± 0.02‰ and show a fractionation similar to the terrestrial mantle δ53Cr inventory. Whereas δ53Cr of all stichtites, except of one, are within the chromite array, the deviations from host serpentinites indicate that Cr was subject to Cr cycling during serpentinization. The variable Cr isotope ratios of stichtite reflect a complex interplay of several physicochemical processes. These are difficult to identify solely from isotope compositions as the resulting Cr isotope fractionation represents the net result of all processes that affect the complex Cr cycling in the critical zone.The very first REY concentration data for stichtite further corroborate the notion that stichtite formation is indeed not an isochemical process and that its formation involves intense metal mobility during associated metasomatism. Rare earth elements and yttrium are depleted in the stichtites relative to the serpentinites, but the overall similar REYCN patterns indicate that the REY in both, the serpentinites and stichtites, were sourced from a chemically similar fluid that was dominated by interaction with the serpentinized rocks. Seawater and marine carbonates can be excluded as potential contributors towards REY inventory of stichtite. The presence of superchondritic Y/Ho ratios in stichtite further emphasizes intense water-rock interaction at high CO32− and HCO3− activities. Positive EuCN anomalies in some stichtites and serpentinites point to reducing conditions with regard to the Eu3+/Eu2+ redox couple and temperatures in excess of 250 °C at the site of REY mobilization, prior to stichtite formation. Lack of Ce anomalies in stichtite further confirm reducing conditions at the site of stichtite formation.

Petrology and geochronology of the Yushigou nephrite jade from the North Qilian Orogen, NW China: Implications for subduction-related processes

The North Qilian Orogen in NW China hosts several commercial quality serpentine jade deposits, although nephrite jade has not so far been reported from this region. Here, we investigate a green nephrite (tremolite) jade from Yushigou ophiolite suite in the North Qilian orogen. We present results from petrographic, scanning electron microscopy (SEM), X-ray diffraction, Electron microprobe analyses (EMPA) and zircon U-Pb geochronological studies to characterize the nephrite jade and its genetic setting. Our results suggest that the high-quality green nephrite jade is predominately composed of tremolite and actinolite, with transformation to actinolite with increasing of Fe and Cr, together with minor chlorite, calcite, magnetite, and titanite. In contrast, the serpentine jade consists mainly of antigorite with minor magnesite, talc, calcite, magnetite, chromite, and apatite. Zircon U-Pb dating yields a lower intercept age of 500 ± 12 Ma, representing the formation age of the Yushigou ophiolite. A few young zircon grains yield a weighted mean age of 227.9 ± 5.3 Ma, which is interpreted to represent the crystallization age of the tremolite, in response to the tectonics associated with late Paleo-Tethys to early Neo-Tethys. Accordingly, we propose that the host serpentinites in this region formed within a Late Cambrian supra-subduction zone through water-rock interaction, involving Ca-rich and COH-bearing fluid systems. The Ca- or COH-bearing fluids released from mantle-derived magma, and then migrated to higher levels during late metamorphic-metasomatic processes in the Mesozoic, finally generating the nephrite jade. Moreover, our study also provides insights into the tectonic processes associated with the formation of jade within subduction zone settings.

Hydrothermal alteration of tourmaline from pegmatitic rocks enclosed in serpentinites: Multistage processes with distinct fluid sources

Hydrothermal alteration of primary tourmaline (dravite, oxy-dravite, fluor-dravite,” uvite“, schorl, oxy-schorl, fluor-schorl) and associated plagioclase and quartz is examined in coarse-grained pegmatitic plagioclase-tourmaline rocks, simple granitic pegmatites and zoned beryl-columbite pegmatites, all cutting serpentinite from the Moldanubian Zone, Czech Republic. Tourmaline is replaced by a wide spectrum of minerals—prehnite, pumpellyite-(Al), K-feldspar, chlorite, muscovite, natrolite, arfvedsonite, titanite, epididymite, and hydrous Mg-silicates—and idealized replacement reactions of tourmaline are presented. Secondary mineral assemblages replacing tourmaline and chlorite geothermometry suggest evolution of the PT conditions of tourmaline alteration from T ~ 400–350 °C to 200–100 °C and P < ~1–2 kbar. The replacement processes of tourmaline proceed from the early to late subsolidus stages, and the intensity of tourmaline replacement decreases with decreasing temperature. Alteration typically occurs in small bodies of primitive plagioclase-tourmaline rocks, whereas large, more evolved and locally Li-bearing crosscutting pegmatites exhibit less pronounced tourmaline alteration. Based on the secondary mineral assemblages and types of fluids, four distinct compositional/paragenetic types are defined: Ca,Mg-, Na,K,Mg-, K,Mg- and K,Na-dominant. Sources of fluids include (i) fluids exsolved from pegmatite melt (Na, K, Ca, ± Li), particularly in the more evolved pegmatites, and (ii) fluids infiltrated/diffused from host rocks (Mg, Ca, ± Ni, Cr) at all localities; however, mixing of the fluids is obvious. Boron released from the altered tourmalines into fluids escaped from the system, and the behavior of B in aqueous fluids from granitic pegmatites is discussed. The instability of quartz and tourmaline as well as common late K-feldspar, zeolites, arfvedsonite, and minor smectite suggests alkaline conditions. The alkaline character of fluids promoted by host serpentinite was very likely the main factor affecting tourmaline replacement and the instability of plagioclase, quartz, and other minerals.

Element distribution in the root zone of ultramafic-hosted black smoker-like systems: Constraints from an Alpine analog

Fluid-rock interactions at Mid-Oceanic Ridges lead to metal deposition in the so-called seafloor massive sulfides at ultramafic-hosted systems. Due to restricted access to the seafloor and scarcity of 3D exposures, these systems are poorly understood at-depth. A way to access the vertical dimension is to focus on fossil analogs preserved on-land such as the one preserved in serpentinites from the Platta nappe (SE Switzerland). For this example, we document the element distribution in the mineralized rocks at three distinct levels in the rock column using both whole rock chemical analyses and LA-ICPMS analysis of sulfide (chalcopyrite and pyrrhotite) and magnetite. We bring, for the first time, chemical signatures of the mineralization in the root zone of ultramafic-hosted black smokers. At any given depth, the Co/Ni ratio is maximum in the most mineralized samples indicating that this ratio is linked to the intensity of hydrothermal alteration. Additionally, the Co/Ni ratio decreases in mineralized rocks towards the paleosurface, whereas the Se content increases. An episode of carbonation recorded in the highest structural level of the system was responsible for a slight remobilization of the former Cu stock. We propose a model in which the uprising mineralizing fluid mixed with seawater within the host serpentinites, before venting at the paleoseafloor.

Petrogenesis of gold-bearing listvenites from the carbonatized mantle section of the Neoproterozoic Ess ophiolite, Western Arabian Shield, Saudi Arabia

The variably serpentinized mantle peridotites of the Late Neoproterozoic Ess ophiolite (Western Saudi Arabia) are highly altered along shear zones and thrust planes to form erosion-resistant listvenites. The listvenites are distinguished petrographically and geochemically into three types: carbonate, silica-carbonate and silica (birbirite) listvenites. Geochemical analyses are consistent with expectations from petrography: carbonate listvenite is low in SiO2 content but high in MgO, Fe2O3, and CaO relative to silica-carbonate and birbirite, which is remarkably high in SiO2 at the expense of all other components. The total REE contents are low in silica-carbonate and carbonate listvenites but highly enriched in birbirite, with a large positive Eu anomaly. The host serpentinites have all the characteristics typically associated with highly depleted mantle harzburgite protoliths in supra-subduction fore-arc settings: bulk compositions are low in Al2O3 and CaO with high Mg# [molar Mg/(Mg + Fe)], relict Cr-spinel has high Cr# [molar Cr/(Cr + Al)] and low TiO2, and relict olivine has high Mg# and NiO content. The Cr-spinel relics are also found in the listvenites; those in serpentinite and carbonate listvenites have significantly higher Mg# than those in silica-carbonate and birbirite, suggesting re-equilibration of Cr-spinel in the later phases of listvenitization. The varieties of listvenite capture successive stages of fluid-mediated replacement reactions. The carbonate listvenite appears to have developed syn-contemporaneously with serpentinization, whereas silica-carbonate listvenite and birbirite formed later. The listvenite formation resulted in leaching and removal of some components accompanied by deposition of others in the solid products, notably CO3, SiO2, REE (especially Eu), Au, Zn, As, Sb and K. Our data show that listvenitization concentrated gold at sub-economic to economic grades; measured gold concentrations in the host serpentinite are 0.5–1.7 ng/g, versus 4–2569 ng/g in carbonate listvenite, 43–3117 ng/g in silica-carbonate listvenite and 5–281 ng/g in birbirite. The listvenite deposits in the Jabal Ess area merit further exploration for gold.

Geochemical evolution of rodingites during subduction: insights from Cerro del Almirez (southern Spain)

Metarodingites from Cerro del Almirez (Betic Cordillera, southern Spain) experienced multiple mineralogical and compositional changes during their evolution, from rodingitization on the seafloor to dehydration of the hosting subducted antigorite serpentinites at ~ 1.7 GPa and 650 °C. Basaltic and doleritic protoliths of Grandite-metarodingites lost alkaline metals (Na, K, Cs and Rb), Si, Ba, Pb and Sr, and gained Ca through intense seafloor rodingitization by highly reducing fluids reacting with serpentinites. Rare earth elements in whole-rocks, conversely, preserved their igneous abundances during rodingitization and partitioned into the new metamorphic assemblage dominated by granditic garnet, chlorite and diopside. At the ocean floor and during subduction, the strong gradients of chemical potentials of Mg, Ca and Al at the serpentinite-rodingite interface triggered the transformation of metarodingites to metasomatic chlorite-blackwalls and of host serpentinites to chlorite (± olivine)-diopside rims.Grandite-metarodingites interacted during subduction with fluids released by the brucite breakdown in serpentinites at ~ 1.5 GPa, leading to oxidation of iron, precipitation of new andraditic garnets with high MREE/HREE ratios, and loss of Re. Strong fluxing of fluids produced by the antigorite breakdown in serpentinites at ~ 1.7 GPa promoted the transformation of Grandite- to Epidote-metarodingites. This transformation caused the release of Ca and Mn into serpentinite-derived fluids and the incorporation of Si, Sr, Pb, Ba ± Eu into metarodingites, especially in epidote. Pyralspite-metarodingites formed at peak metamorphic conditions (~ 1.8 GPa and 680 °C) by, among other reactions, epidote breakdown that liberated Ca, Sr and Pb into fluids. Finally, all metarodingite types underwent variable degrees of retrograde amphibolitization induced by external fluids derived from metasedimentary rocks, causing Ca depletion and enrichment in Mg, Ba and alkaline elements (Na, K, Cs and Rb).These geochemical variations reflect the exchange of major and trace elements between different lithologies (serpentinites, metarodingites and metasedimentary rocks) in a subducting slab or slab-mantle interface. Metarodingites may influence the redox state and trace element signature of slab fluids, especially through iron oxidation, fractionation of Sr, Pb and Ba into epidote, and release of Ca and Re in fluids that may induce metasomatism in the sub-arc mantle. Recycling of residual metarodingites in the convective asthenosphere may produce Ca-rich reservoirs in the deep mantle.

Seismically-induced serpentine dehydration as a possible mechanism of water release in subduction zones. Insights from the Alpine Corsica pseudotachylyte-bearing Monte Maggiore ophiolitic unit

The Monte Maggiore ophiolitic unit of Alpine Corsica consists of an intact to variably hydrated spinel lherzolite intruded by Jurassic gabbro dykes. A widespread serpentinization, marked by low-pressure serpentine polymorphs, results from sea water interaction with mantle rocks during Jurassic sea-floor spreading of the Piemonte-Liguria ocean. A second serpentinization event, marked by the presence of antigorite, is related to the Cretaceous to Paleogene subduction of the Piemonte-Liguria oceanic lithosphere. Tectonic pseudotachylyte veins locally crosscutting the unit, display several original characteristics. (1) They are hosted by serpentinite, showing that seismic ruptures can propagate through hydrated mantle rocks. (2) The host serpentinite in contact with pseudotachylyte veins shows a thin (500 μm or less) rim of secondary olivine newly crystallized at the expense of serpentine. The heat necessary for serpentine dehydration is likely provided by the frictional melt. Antigorite-bearing clasts (themselves partly dehydrated along their boundaries) reworked in pseudotachylyte veins on one hand and antigorite veins crossing pseudotachylyte veins on the other hand show that frictional melting took place at pressure and temperature conditions compatible with antigorite stability. This indicates that frictional meting occurred at pressures between 0.7 and 0.85 GPa, that is, at ~20 to ~30 km depths in the subducting Piemonte-Liguria oceanic slab. Since the dehydration of serpentine into olivine is only observed at the host-rock selvages of pseudotachylyte veins, it cannot be related to the crossing, by the subducting slab, of the regional 610 ± 100 °C isotherm of the subduction zone, temperature at which antigorite starts to dehydrate into olivine. An estimate of the water released by the dehydration reaction suggests that for a magnitude 6 earthquake, the average amount is about 1.36 L/m2 or 1.36 × 105 m3 for the entire rupture surface (assumed to be 100 km2). If the released water is not incorporated in the nearby frictional melt, it can flow away from the slip zone and contribute to the various fluid-rock interactions active in subduction zones. Alternatively, if the water cannot escape from the slip zone and if the pore pressure is locally high enough, it could trigger aftershocks following the initial seismic rupture.

Comparative Fe and Sr isotope study of nephrite deposits hosted in dolomitic marbles and serpentinites from the Sudetes, SW Poland: Implications for Fe-As-Au-bearing skarn formation and post-obduction evolution of the oceanic lithosphere

Nephrites belonging to both the dolomite-related and serpentinite-related genetic types, from the Złoty Stok deposit, and Jordanów Śląski and Nasławice prospects in the Ślęża Ophiolite, respectively, were analyzed for Fe and Sr isotope compositions. These deposits in the Central Sudetes (SW Poland), are unusually situated relatively close to each other. This study is a first attempt to perform combined Fe and Sr isotope study of nephrites. Our results show that Fe in the Fe-As-Au-bearing skarn samples from Złoty Stok (2.77–7.00 wt% Fe, δ56Fe +0.07 to +0.14‰, and δ57Fe +0.12 to +0.21‰), to which the dolomite-related nephrite belongs, is not inherited from the host dolomitic marbles (ca. 0.4 wt% Fe; δ56Fe −0.08 to −0.07‰, and δ57Fe −0.12 to −0.09‰), but derived from the nearby, ca. 340 Ma granite intrusion (2.64–5.64 wt% Fe). In contrast, iron in the serpentinite-related nephrites (2.06–3.88 wt% Fe, δ56Fe +0.41 to +0.45‰, and δ57Fe +0.59 to +0.66‰) was likely inherited from host serpentinites (5.27–4.81 wt% Fe), rather than metasomatically derived from adjacent granite intrusions (0.13–0.45 wt% Fe). Moreover, both the marble transformation into a dolomite-related nephrite (and Fe-As-Au-bearing skarn), and the abyssal serpentinite – through ophiolite – towards serpentinite-related nephrite transformation, are accompanied by a progressive increase of δ56Fe and δ57Fe. This increase is regardless of changes in the bulk Fe content, increasing during the dolomite-related nephrite formation, and decreasing during serpentinite-related nephrite formation. This phenomena may be caused by one of the below listed factors, or combinations: a) preferential incorporation of heavy iron by tremolite and diopside; b) preferential incorporation of the heavy iron in the Fe3+-bearing phases, crystallizing during nephrite formation; c) preferential incorporation of the heavy Fe from migrating granite-derived fluids (positive fractionation between aqueous Fe-bearing fluid, and tremolite/diopside); d) positive isotope fractionation between dolomite/antigorite precursor, and newly formed amphibole/clinopyroxene; e) change of the Fe oxidation state. Furthermore, Fe in the rock-forming tremolite from the serpentinite-related nephrites is isotopically heavier than Fe in tremolite from the dolomite-related ones. In contrast to the iron, the isotopic composition of strontium, combined with its content and structural evidence, suggest multiple sources for this element. In the case of the dolomite-related nephrite (ca. 3–4 ppm Sr; initial 87Sr/86Sr = 0.7085), Sr inheritance from the host-dolomitic marble (ca. 90 ppm Sr; initial 87Sr/86Sr = 0.7081), as well as its derivation from intrusions of both the syn- to late-tectonic, ca. 340 Ma granites (ca. 400–440 ppm Sr), and post-tectonic ~305 Ma granites (86.4 ppm Sr), is suggested. In the case of the serpentinite-related nephrite (6.3–61.7 ppm Sr; initial 87Sr/86Sr 0.7037 to 0.7081), the situation is similar, i.e., Sr in the most primitive samples tends to be sourced from ophiolitic rocks (up to 2.4 ppm Sr in serpentinite), whereas in the more evolved types, it was likely delivered from the adjacent, ca. 340 Ma partially rodingitized granites (8.8–102.6 ppm Sr), as well as ~305 Ma granites (ca. 80 ppm Sr). Moreover, these granites caused the fluid-flow, responsible for Sr-bearing clinozoisite vein formation; these veins are likely related with nephrite formation, and with a metallic mineralization.

Multistage CO2 sequestration in the subduction zone: Insights from exhumed carbonated serpentinites, SW Tianshan UHP belt, China

Climate is regulated by the carbonate–silicate cycle in which slab outgassing of C into deep fluids and volcanic degassing of CO2 into the atmosphere are an important part. However, the mechanisms of C mobility in subduction zones remain largely unresolved. Previous research has focused mainly on investigating the upward transfer of slab-derived carbonic fluids for the forearc mantle metasomatism and partial melting. Furthermore, percolation of CO2-bearing fluids parallel to the downgoing plates can potentially drive carbonation of subducted rocks, which influences the global estimates of C fluxes at convergent margins. Nevertheless, the geological conditions and processes leading to the carbonation of subduction-zone lithologies by fluid–rock interactions are still poorly understood. Here, we present new field, petrological, and isotopic results of carbonated serpentinites—high-pressure (HP) ophidolomites and low-pressure (LP) ophimagnesites and listvenites—from the Chinese southwestern Tianshan HP–UHP metamorphic belt. These rocks recorded the carbonation of subduction-zone serpentinites at HP and LP conditions during exhumation, reflecting the multistage transfer and infiltration of carbonic fluids along the plate boundary. The HP ophidolomites are characterized by the growth of carbonates (dolomite, aragonite, and Mg-calcite) at the expense of silicates in the host serpentinites. Integrated Sr–C–O isotopic data and thermodynamic modelling suggest that carbonic fluids (containing a CO2,aq concentration of up to 1.9 molal) emanating from carbonate-bearing metamafic rocks (e.g., eclogites) likely contributed to HP carbonation of serpentinites at about 15–25 kbar and 550–600 °C. The close contact of ophidolomites and carbonated metasedimentary rocks in the field as well as their similarities in Sr isotope compositions suggests that the latter could also have acted as the possible C source. Alternatively, both lithologies may have formed by coupled HP carbonation along the lithological interface between serpentinites and metasedimentary rocks. Subsequent fluid–rock interactions at relatively shallow crustal levels resulted in a second stage of serpentinite carbonation to form LP ophimagnesites and listvenites, during which the metasomatic CO2-bearing fluids may have originated from metasedimentary rocks. The multistage CO2 sequestration in subduction-zone serpentinites implies that hydrated ultramafic rocks in subducted slabs are highly effective reactants to capture and store slab-released C over a wide range of P–T conditions, with the potential to substantially control the C distribution between shallow and deep reservoirs and thus modulate C fluxes in subduction zones.

Fluid-mediated carbon release from serpentinite-hosted carbonates during dehydration of antigorite-serpentinite in subduction zones

Subduction of serpentinite-hosted carbonate rocks formed during the oceanic stage in the oceanic lithosphere (ophicalcite) or by metasomatism at the slab mantle-wedge interface (hybrid carbonate–talc rocks) constitute important fluxes of carbon in subduction zones. If hosted in the hydrated mantle lithosphere of the slab, these carbonate-rocks may be preserved from the infiltration of fluids produced by prograde metamorphism of the overlying meta-sediments and the hydrated oceanic crust to subarc depths, where they are fluxed by fluids derived from dehydration of their host serpentinite. Because the dissolution of carbonate in aqueous fluids is enhanced at high P and T, fluid-mediated carbon release at subarc depth is critical to understand the global carbon balance and magnitude of carbon fluxes from the subducting plate into the deep mantle. This contribution presents thermodynamic modelling —using the implementation of the DEW aqueous database in Perple_X— of prograde devolatilization reactions (solid = fluid + solid) and infiltration-driven devolatilization reactions (external fluid + solid = fluid + solid) of serpentinite-hosted carbonate rocks. In line with previous studies of prograde devolatilization of carbon-bearing oceanic crust, our models show that, in warm and cold subductions zones, the carbon released into fluids produced by prograde devolatilization reactions of meta-ophicalcite and carbonate-talc rocks is limited even if electrolytic complexing in the fluid phase is taken into account. Therefore, serpentinite-hosted meta-carbonate rocks are likely to be preserved to subarc depths, where they undergo infiltration-driven devolatilization by fluids derived from Atg-serpentinite dehydration. At the P–T conditions of Atg-breakdown in cold to warm subduction zones, our models of infiltration-driven devolatilization of meta-ophicalcite and carbonate-talc rocks indicate that carbon release is accompanied by significant Ca loss, particularly at high pressures (P>3.5 GPa), due to the high concentration of CaHCO+3 in the fluid. The carbon solubility in fluids equilibrated with meta-ophicalcite and carbonate–talc rocks is markedly higher than for pure aragonite. Mass balance considerations combined with the parameterization of the P–T conditions of Atg-serpentinite dehydration as a function of the thermal parameter of subduction zones allow us to investigate the carbon loss caused by infiltration of serpentinite dehydration fluids. Unlike the dissolution of pure CaCO3, carbonate dissolution in serpentinite-hosted meta-carbonate rocks is highest at the slab surface of warm subduction zones and lowest at Moho depths of cold subduction zones, where Atg-serpentinite dehydration occurs at greater depths. Carbon in subducted meta-ophicalcite —with thicknesses of up to 70 m typically found in oceanic ophicalcite— will be preserved beyond the subarc depths of cold and hot subduction zones, and recycled into the deep mantle as carbonate-garnet-clinopyroxene-olivine rocks. At the subarc depths of most subduction zone regimes, infiltration-driven devolatilization of subducted carbonate-talc rocks is a highly efficient decarbonation mechanism transforming these rocks into carbon-free orthopyroxenite.

Platinum-group minerals and Fe–Ni minerals in the Sartohay podiform chromitite (west Junggar, China): Implications for T–pH–fO2–fS2 conditions during hydrothermal alteration

Complex assemblages consisting of native iron + pentlandite, pentlandite + heazlewoodite + magnetite, heazlewoodite + magnetite + platinum-group minerals (PGM), and millerite ± heazlewoodite are associated with three types of altered chromites (partly altered chromite, double-rimmed chromite, and porous chromite) discovered in the Sartohay podiform chromitite hosted by the Sartohay ophiolitic mélange in west Junggar of China. We propose a two-stage genetic model to explain the chromite alteration processes based on detailed petrographic observations and thermodynamic calculations. Reactions between primary chromite and olivine formed porous Fe2+-rich chromite and chlorite patches under low-temperature, reducing conditions. Afterwards, oxidizing Fe3+- and CO2-rich fluids derived from host serpentinite were incorporated into the Fe2+-rich chromite, shifting its composition to heterogeneous Fe3+-rich chromite (ferrian chromite). Thermodynamic calculations suggest that the co-evolution of fO2 and fS2 played a crucial role in controlling the formation and/or transformation of the Fe–Ni minerals coevally with the sulfides, antimonides, and alloys of platinum-group elements (PGE). Pd and Pt precipitated as sulfides, while Ir precipitated as an alloy when log fS2 ranged from –16.0 to –14.2 within the heazlewoodite stability field. Compared with the host serpentinite, the fO2 in the Sb-bearing fluid equilibrated with chromitite was low enough to decrease the Pd solubility, leading to co-precipitation of Sb and Pd.