Fluid-mobile Elements Research Articles

Geochemical signatures of forearc serpentinites from oceanic to continental subduction

In subduction systems, the forearc (FA) serpentinized mantle plays a key role as a reservoir for fluid-mobile elements, especially those prone to early mobilisation from the slab. FA serpentinites therefore provide an indication of the fluid characteristics that have been steamed out of the slab, and allow better quantification of elemental recycling and fluid flow. FA serpentinites outcropping along the Indus Suture Zone are formed by hydration of mantle peridotites by fluids derived from the Indian slab, providing a geochemical record of fluid-rock interactions during the India-Eurasia convergence. Systematic trace and multi-isotope (Sr and Pb) analyses of serpentinites from three major sites, namely Shergol-Tingdo, Kargil and Tso Morari, along the Indus Suture Zone in Ladakh (NW Himalaya) show that these rocks record a major change in the origin of metasomatic agents, from oceanic to continental subduction. The Shergol-Tingdo and Kargil FA serpentinites, formed under low temperature and pressure conditions, are characterised by high B enrichment and As and Sb depletion, high alkali/U ratios and relatively unradiogenic Sr and Pb isotope ratios. These geochemical characteristics are identical to those reported for modern oceanic subduction zones (e.g., Mariana forearc) and are consistent with input by fluids derived from subducted oceanic crust. In contrast, FA serpentinites from the Tso Morari ultra-high pressure unit show enrichment in fluid mobile elements such as As, Sb and U, low alkali/U ratios and radiogenic Sr and Pb isotopic compositions. These features suggest a change in the metasomatizing agent with a strong influence from subducted continental material. Consistent with this scenario, the Sr-Pb isotopic composition of the Kargil FA serpentinites can be reproduced by mixing between an Indian MORB-depleted mantle source and fluids derived from dewatering of blueschist facies oceanic metasediments, whereas in the case of Tso Morari a fluid end-member derived from various eclogitic continental metasediments is required. We propose that the observed geochemical signature of the Indus Suture Zone FA serpentinites can be used to reconstruct the geochemical exchanges between the slab and the overlying mantle from intra-oceanic to continental subduction. This successful systematic approach could be applied to other collisional contexts

The impact of exhumation onto fluid-mobile element budget and Rb-Sr isotope heterogeneity of the subducted eclogitic crust (Alag-Khadny, SW Mongolia)

The impact of exhumation onto fluid-mobile element budget and Rb-Sr isotope heterogeneity of the subducted eclogitic crust (Alag-Khadny, SW Mongolia)

Unravelling different mechanisms of metasomatism and geodynamic evolution of pyroxenite-veined subcontinental lithospheric mantle beneath the Central European Variscides

Abstract Pyroxenite-veined garnet peridotites from the Gföhl Unit of the Moldanubian Zone in the Bohemian Massif provide direct constraints on diverse mechanisms of mantle metasomatism and refertilization driven by a single pulse of melt beneath the Central European Variscides. Here, we provide a detailed study on an intriguing example of this rock association where the garnet peridotites show a fertile character (high Al2O3, CaO, TiO2), corresponding to the subcontinental lithospheric mantle (SCLM). By contrast, their conspicuous LREE depletion and Sr–Nd isotopic signatures (87Sr/86Sr338 ≤ 0.7028; εNd338 ~7.3) are typical of depleted mantle residue after melt extraction. Such signatures reflect transformation of an original refractory protolith (likely harzburgite) to fertile lherzolite through percolation of primitive tholeiitic melts, parental to garnet pyroxenite in veins. The SCLM refertilization is further documented by the whole-rock positive correlation between incompatible elements (Zr, Yb, Sc, V), and trace element composition of clinopyroxene (high Ti/Eu and Ti/Nb) and garnet (elevated ∑REE, Zr, Ti). Trace element and Sr–Nd isotopic systematics of pyroxenites (87Sr/86Sr338 ~0.7025–0.7029; εNd338 ≤ 7.9) correspond to a source of melt similar to the depleted MORB mantle (DMM). Three mechanisms of metasomatism related to the interaction of this melt with the host peridotites were distinguished: (i) stealth metasomatism, reflected by extensive clinopyroxene and garnet crystallization in lherzolite adjacent to pyroxenite veins, (ii) cryptic metasomatism, recorded by lower Mg# values of orthopyroxene and olivine in lherzolite, and (iii) modal metasomatism, resulting in crystallization of amphibole and phlogopite in lherzolite close to the veins. The percolating basaltic melt was hydrous, moderately enriched in fluid-mobile elements (Cs, Rb, Ba, Pb, U, Li). Immiscible liquids, dense Ti-Mg-Fe-rich oxide melt and C-O-H fluid, trapped and crystallized as mono/multiphase solid inclusions in garnet, likely separated from a basaltic melt upon cooling. The lherzolite-pyroxenite interface reveals strong micro-scale element fractionation due to differentiation of a basaltic melt within the percolation channel. Volatile-bearing liquids that segregated from the melts migrating through wall-rock peridotites most likely caused chromatographic enrichment in highly incompatible elements (e.g. LREE) in distal peridotites relative to the LREE-depleted lherzolites adjacent to the veins. The DMM-like affinity of pyroxenites and pressure-temperature estimates for lherzolite (3.9–5.4 GPa/1010–1200 °C) and pyroxenites (2.8–4.2 GPa/860–1020 °C) point towards exhumation-driven SCLM refertilization. This was linked to decompression-induced partial melting of upwelling asthenosphere producing basaltic melts penetrating through and metasomatizing the SCLM beneath the Variscan orogenic belt in Central Europe.

Li enrichment in peridotites and chromitites tracks mantle-crust interaction

The ultramafic massifs of the Serranía de Ronda in southern Spain are the Earth's largest exposures of subcontinental lithospheric mantle (SCLM) peridotites (∼450 km2). These ultramafic massifs experienced asthenosphere melt percolation during their crustal emplacement. Mixing of these mafic melts with anatectic melts and fluids led to the formation of a world's unique Ni-arsenide-rich chromitite ores (hereafter CrNi ores) associated with orthopyroxenite and/or cordieritite (i.e., > 90 % volume of cordierite) hosted within the peridotites. This study uses laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to investigate the Li in rock-forming minerals of peridotite and CrNi ores to evaluate the role of Li as crustal tracer. Clinopyroxene crystallized from asthenospheric melts exhibits high Li contents (up to 8.5 ppm), exceeding the average values of the upper mantle (∼ 0.7 ppm), whereas orthopyroxene, olivine, and Cr-spinel from peridotite are mostly Li-depleted. In contrast, all rock-forming minerals of CrNi ores have abnormally high Li contents, displaying an overall Li enrichment trend toward the external parts of ultramafic massifs, on the way to the crustal rocks. This trend is evident in Cr-spinel from the CrNi ores, which display 6.9–7.9 ppm Li in the deepest portions of the massif (Arroyo de la Cala CrNi ore) up to 1.4–8.5 ppm in the shallowest part (La Gallega CrNi ore), as well as in orthopyroxenes that have 31.3–44.7 ppm Li in Arroyo de la Cala, and 45.1–51.4 ppm Li in La Gallega. Cordierite is present only in the CrNi ores situated in the external part of the ultramafic massifs, exhibiting 113.15–160.82 ppm Li in the Barranco de las Acedías CrNi ore and 36.5–60.5 ppm Li in La Gallega CrNi ore. Similarly to Li, LREE, fluid-mobile elements (K, Rb, Ba), and Sr in orthopyroxenes from the CrNi ores display enrichment from the inner to the outer parts of the ultramafic massif. These geochemical variations suggest that Li enrichment in CrNi ores and host peridotites was a twofold process: (1) asthenospheric melt percolation slightly increased Li abundances in the SCLM peridotites by modal and cryptic metasomatism involving clinopyroxene; (2) additional infiltration of Li-bearing crustally-derived fluids during the intracrustal emplacement of the mantle section boosted the Li contents of minerals in the CrNi ores. Our results highlight that Li may effectively track the interaction of the SCLM with crustal components.

The fate of ultramafic-rich mélanges in cold to hot subduction zones: Implications for diapirism (or not) and chemical geodynamics

Buoyant ultramafic-rich (serpentine- or chlorite-rich) mélange diapirs in sediment-starved subduction zones can transport slab material to arc sources. While the buoyancy of chlorite-rich mélanges was previously investigated, serpentine-rich mélanges were never explored. Thus, the overall contribution of ultramafic-rich mélanges to buoyancy, the conditions for diapir formation, and their fate in subduction zones are not well constrained. Here, we investigate the partial melting behavior and the associated density transformations of a serpentine-rich matrix (5–10 wt.% H2O) with minor sediments (9:1 ratio) at fore-arc (∼65 km) to sub-arc (∼95 km) depths (2–3 GPa and 800–1250 °C) and compare to that of chlorite-rich mélanges from the literature. Our results show that the solidus of serpentine-rich matrices is between 1050 and 1100 °C and requires either diapiric rise of the mélange into the hotter mantle wedge or interactions with a hotter asthenosphere through slab tears to partially melt and produce basaltic melts, whether in hot or cold slab channels. Chlorite-rich mélanges may account for the sources of some arc lavas, but partial melting of serpentine-rich mélanges produce melts depleted in CaO, TiO2, alkalis, and are highly enriched in MgO compared to basaltic arc lavas. Both serpentine-rich and chlorite-rich matrices dehydrate to form denser peridotite and lose buoyancy at ∼800 °C and ≥1000 °C, respectively. Even if diapirism initiates in such mélanges near the slab-mantle interface, they would likely lose buoyancy upon ascent into the hotter mantle wedge resulting in stalled or failed diapirs. Diapir growth (τa) is controlled by the interplay of density, thickness and viscosity of the mélange, as well as the timescale of slab subduction (τs) and thermal structure of the subduction zone. We observe that the onset of diapirs in cold subduction zones requires mélanges that may sometimes be thicker than that observed by field and geophysical studies, while hot subduction zones overall require thinner mélanges. Thus, ultramafic-rich mélange diapirs may occur but only under specific conditions and when the diapiric ascent timescale is faster than the thermal equilibration barrier of ∼800–1000 °C (especially at the core of the mélange). Dehydration or partial melting of ultramafic-rich mélanges can affect the large ion lithophile element (LILE), volatiles, and high-field strength element (HFSE) budgets in the mantle wedge. Partial melting (caused by a diapiric rise or slab tear) does not fractionate LILEs from HFSEs at T ≥ 1100 °C and if the mélange has a lower LILE/HFSE to begin with, that signature is transferred to arc sources. Dehydration releases aqueous fluids rich in fluid-mobile elements (LILE and volatiles) relative to HFSE. Thus, the characteristic high LILE/HFSE signature of aqueous fluids is transferred to arc magma sources. Given high LILE/HFSE ratio is a ubiquitous arc magma signature, but slab tears are not, and diapirism in ultramafic-rich mélanges is highly conditional, this study corroborates that aqueous fluids released from sediment-starved mélanges are the predominant mass transfer agents rather than diapirs.

From an accretionary margin to a sediment-rich collision: Spatiotemporal evolution of the magmatism during the closure of the Mongol-Okhotsk Ocean

The closure of the Mongol-Okhotsk Ocean (MOO) marks the final suturing of the Central Asian Orogenic Belt, one of the largest accretionary orogens on Earth and a region that is considered an archetype for crustal growth during the Phanerozoic. Abundant Permian to Triassic magmatism in Mongolia extended into the Jurassic on the eastern side of the Mongol-Okhotsk Belt (MOB), the orogenic belt produced by the closure of the MOO. Magmatic belts formed north and south of the suture along the MOB provide insight into the dynamics of the subduction system and the magmatic, crustal, and mantle processes pre-, syn- and post- collision within this accretionary margin. One of the main questions regarding the magmatism in the region is: Was the magmatism formed during active subduction or during the collision and closure of the basin? Here we compile geochemical data (major and trace elements, and isotopes) from the Permian to Jurassic magmatic rocks in the MOB and analyze their spatiotemporal characteristics. Our goal is to assess how magmatism changed in time and space during the closure of the Mongol-Okhotsk Ocean and how those changes relate to first-order tectono-magmatic processes right before or during the collisional event that closed the basin. Our results show a general enrichment in fluid mobile elements, LILE, and LREE and depletion in HFSE, and HREE in mafic and felsic rocks, which indicates a mantle metasomatized by subduction-related fluids regardless of crustal contamination. Our analysis supports higher enrichment in sediment melts, especially along its western and older extent, and the assimilation of juvenile crustal components without producing abundant S-type peraluminous magmatism which indicates mantle and crustal contributions. Thus, we conclude that magmatism formed above a sediment-rich retreating margin was able to recycle and stabilize young and compositionally evolved crustal material in the Central Asian Orogenic Belt.

Deep recycling of volatile elements in the mantle: Evidence from the heterogeneous B isotope in intra-plate basalts

Volatiles in the mantle are crucial for Earth’s geodynamic and geochemical evolution. Understanding the deep recycling of volatiles is key for grasping mantle chemical heterogeneity, plate tectonics, and long-term planetary evolution. While subduction transfers abundant volatile elements from the Earth’s surface into the mantle, the fate of hydrous portions within subducted slabs during intensive dehydration processes remains uncertain. Boron isotopes, only efficiently fractionating near the Earth’s surface, are valuable for tracing volatile recycling signals. In this study, we document a notably large variation in δ11B values (−14.3‰ to +8.2‰) in Cenozoic basalts from the South China Block. These basalts, associated with a high-velocity zone beneath East China, are suggested to originate from the mantle transition zone. While the majority exhibit δ11B values (−10‰ to −5‰) resembling the normal mantle, their enriched Sr-Nd-Pb isotope compositions and fluid-mobile elements imply hydrous components in their source, including altered oceanic crust and sediments. The normal δ11B values are attributed to the dehydration processes. Remarkably high δ11B values in the basalts indicate the presence of subducted serpentinites in their mantle source. A small subset of samples with low δ11B values and radiogenic isotope enrichments suggests a contribution from recycled detrital sediments, though retaining minimal volatile elements after extensive dehydration. These findings provide compelling evidence that serpentinites within subducted slabs predominantly maintain their hydrous nature during dehydration processes in subduction zones. They may transport a considerable amount of water into deep mantle reservoirs, such as the mantle transition zone.

Evidence for isotopically light Fe mobility under oxidising conditions in subduction zone fluids: A record of Monviso meta-serpentinites, Western Alps

The isotopically light Fe signature of arc magmas relative to mid-oceanic ridge basalts (MORB) is expected to be related to the redox state of their source. This implies a link between Fe mobility and redox variations across subduction zones. Here we address slab chemical variations during prograde metamorphism through a comprehensive geochemical study, including major, trace elements and Fe stable isotope (δ56Fe) analyses, of meta-serpentinites composing the eclogitic meta-ophiolite of the Monviso massif (Western Alps, Italy). These rocks partly preserve mantle related geochemical heterogeneities, as supported by negative correlations between fluid immobile elements (High Field Strong Elements) and LREE/HREE (Light – Heavy Rare Earth Elements) ratios. They are however characterized by large Cs enrichments relative to U or alkali elements suggesting that abyssal fluid mobile elements signature was overprinted by subduction related processes. Meta-serpentinites display large δ56Fe variations, from -0.20 ± 0.05 ‰ to +0.09 ± 0.03 ‰. Mineral separate analyses of olivine bearing veins show low Δ56Fe between olivine and magnetite (∼ 0.4 ‰), which according to Fe isotope thermometry correspond to high temperature equilibration, estimated between 530 and 550 °C, compatible with Monviso metamorphic climax (520–570 °C and 2.6–2.7 GPa). These results, in combination with geochemical modelling, confirm a reset of δ56Fe signature of abyssal serpentinites during prograde metamorphism in subduction zones. The bulk rock δ56Fe values of Monviso meta-serpentinites are negatively correlated with their Fe3+/∑Fe and As concentrations (a redox-sensitive and fluid mobile element). These correlations can be attributed to a metasomatism by external fluids derived from slab serpentinites during subduction. It also concords with the oxygen fugacity (fO2) record in meta-serpentinites, with meta-serpentinites equilibrated at high fO2 displaying the lowest δ56Fe values while samples equilibrated at low fO2 display mantle like δ56Fe signatures. These results highlight that oxidising conditions during serpentinite dehydration are likely to boost isotopically light Fe mobility in slab derived fluids.

Slab melting boosts the mantle wedge contribution to Li-rich magmas

The lithium cycling in the supra-subduction mantle wedge is crucial for understanding the generation of Li-rich magmas that may potentially source ore deposition in continental arcs. Here, we look from the mantle source perspective at the geological processes controlling the Li mobility in convergent margins, by characterizing a set of sub-arc mantle xenoliths from the southern Andes (Coyhaique, western Patagonia). The mineral trace element signatures and oxygen fugacity estimates (FMQ > + 3) in some of these peridotite xenoliths record the interaction with arc magmas enriched in fluid-mobile elements originally scavenged by slab dehydration. This subduction-related metasomatism was poorly effective on enhancing the Li inventory of the sub-arc lithospheric mantle, underpinning the inefficiency of slab-derived fluids on mobilizing Li through the mantle wedge. However, major and trace element compositions of mantle minerals in other xenoliths also record transient thermal and chemical anomalies associated with the percolation of slab window-related magmas, which exhibit an “adakite”-type geochemical fingerprint inherited by slab-derived melts produced during ridge subduction and slab window opening event. As these melts percolated through the shallow (7.2–16.8 kbar) and hot (952–1054 °C) lithospheric mantle wedge, they promoted the crystallization of metasomatic clinopyroxene having exceptionally high Li abundances (6–15 ppm). Numerical modeling shows that low degrees (< 10%) of partial melting of this Li-rich and fertile sub-arc lithospheric mantle generates primitive melts having two-fold Li enrichment (~13 ppm) compared with average subduction-zone basalts. Prolonged fractional crystallization of these melts produces extremely Li-enriched silicic rocks, which may stoke the Li inventory of mineralizing fluids in the shallow crust.

Origin and multi-episodic melt/fluid interactions revealed by the subducted serpentinites in the North Qilian Belt, NW China: Implications for the compositional heterogeneity of the fossil oceanic mantle

In nature, serpentinites act as reservoirs of water and fluid-mobile elements and play an essential role in arc magmatism, fluid metasomatism, and elemental circulation in subduction zones. This paper presents new mineral and whole-rock geochemical analyses of the Qingshuigou serpentinite massif and discusses its petrogenesis, tectonic setting, and mantle metasomatism. The chemical composition (e.g., REE, Y, Mg, and Al) and petrological observations (e.g., pseudomorph and hourglass textures) of serpentines demonstrate that the serpentinites’ protoliths belong to the peridotites. Their degrees of partial melting were estimated to be 15–20 % using Yb, Cr, Ti, and HREEs, indicating that these peridotite protoliths were residual in origin after high degrees of mantle melting. They were relics of the subducted lithospheric mantle in the subduction zone, based on their geochemical similarities with subducted serpentinites. Notably, the Qingshuigou serpentinites show several geochemical fingerprints of melt-rock interactions, such as the U-shaped REE patterns, elevated Zr/Ti, Zr/Hf, and Nb/Ta ratios; this led us to propose that the peridotite protoliths of the serpentinites were not merely melting remnants but were significantly modified by melt metasomatism. Also, they experienced two stages of fluid/rock interactions in the subduction zone: early-stage serpentinization at shallow depths and lizardite/chrysotile to antigorite transition at an elevated depth. Combined with the published data of ophiolitic peridotites from the North Qilian Belt, the compositional heterogeneity of these peridotites and associated serpentinites might be associated with the nature of melt/fluid metasomatism under different geodynamic backgrounds.

Late Neoproterozoic tholeiitic plutonism in far-east Avalonia (the Dirgine Batholith, Istanbul Zone): Exposed mid-crustal section of an oceanic arc

Avalonia, a large composite terrane, extends from the northeastern regions of America and Canada to southern Britain and Belgium/Netherlands, continuing on to Poland. It was accreted to Laurentia and Baltica during early Paleozoic. The late Neoproterozoic basement of the Istanbul Zone (NW Turkey), which is considered the far-east extension of Avalonia, is poorly known. This study deals with the petrology and age of the Dirgine Batholith, the largest component of the late Neoproterozoic basement. The batholith, ∼70 km long and ∼12 km wide, was emplaced into Neoproterozoic low-grade mafic to felsic metavolcanic and metapyroclastic rocks. Tonalite and quartz diorite form the bulk of the batholith. Trondhjemite, granodiorite, and anorthositic gabbro occur in minor amounts. Overall, rock types resemble low-Al tonalite-trondhjemite-granodiorite series and trondhjemites in ophiolite sequences. The batholith formed over a narrow time interval (562–574 Ma, late Ediacaran), as inferred by U–Pb zircon geochronology. In-situ Lu-Hf analyses of igneous zircons reveal a limited range of significantly positive initial εHf values (9.47 ± 1.46, 2σ) and young Hf depleted mantle model ages (0.78 ± 0.06 Ga), indicating the relatively juvenile character of original magmas. All rock types have typical subduction-related trace-element signatures such as depletion in high-field-strength elements and enrichment in several fluid-mobile elements as well as nearly flat chondrite-normalized REE patterns. Geochemical characteristics indicate that crustal assimilation was insignificant, and the batholith formed by fractional crystallization of mantle-derived low-K tholeiitic magmas involving mainly clinopyroxene, hornblende and ± plagioclase. The Dirgine Batholith and metamorphic country rocks probably represent the mid-crustal section of a late Neoproterozoic oceanic arc. The interpretation of an oceanic rather than continental arc is based on the significantly lower concentrations of incompatible elements relative to continental arcs, nearly flat REE patterns of the rock types, and the juvenile nature of the primary magmas. These data, in concert with those in the literature, reveal that late Neoproterozoic oceanic arcs constitute an important component of far-east Avalonian terranes.

Cycling of fluid-mobile elements through the forearc: Insights from the Cl, B, and Li isotope composition of Costa Rican spring fluids

Loss of slab-derived volatile and fluid-mobile elements (FME) through the forearc may represent a large, unaccounted for flux out of convergent margins. To address volatile recycling through the forearc and potential changes in fluid source from the progressively dehydrating subducting slab, we collected water samples from 44 different cold and thermal (>40 °C) springs on a transect across the outer forearc (∼20–40 km to the subducting slab), forearc (∼40–80 km to the slab), and arc (∼80–120 km to the slab) in Costa Rica. We focused on the heavy halogens (Cl, Br, I), as well as B and Li, given their high fluid mobility and thus limited potential for modification by subsequent fluid-rock interaction. Chlorine, boron, and lithium stable isotope ratios show dramatic ranges from −1.7 to +1.0 ‰ (n = 43), −12.0 to +30.9 ‰ (n = 35), and −2.4 to + 27.5 ‰ (n = 38), respectively, with distinct changes across the transect consistent with a sedimentary pore fluid component in the outer forearc, fluids sourced from dehydration of the slab (sediment and altered oceanic crust) in the forearc, and minimal slab input near the arc. Variations in elemental ratios (Br/Cl, I/Cl, B/Cl, Li/B) across the transect are also largely consistent with evolving FME sources during subduction. Boron isotope ratios in some higher-temperature fluids are modified due to phase separation, whereas, lithium and chlorine isotope ratios are minimally modified. Springs located in the outer forearc along a major fault (Falla Morote) deviate from the consistent across-arc trend and record almost the full range of observed isotopic values. This extreme range in values along the fault likely represents a combination of adsorption/desorption effects, weathering, and mixing with crustal fluids. Mass balance calculations show that up to ∼ 70 % of Cl, ∼50 % of Br, ∼30 % of B, ∼15 % of I, and minimal Li may be recycled through the outer forearc and forearc springs of the Costa Rican margin, therefore fluid emission via springs in global forearc regions are a potentially significant output for FME at convergent margins.

Trace metal and organic biosignatures in digitate stromatolites from terrestrial siliceous hot spring deposits: Implications for the exploration of martian life

Novel biosignatures of laminated, microbial, digitate sedimentary structures – stromatolites – from modern geothermal fields of the Taupō Volcanic Zone, New Zealand, and from El Tatio, Chile, provide an opportunity to investigate evidence of extremophile life preserved in siliceous hot spring deposits, or sinters, interpreted as analogs for early life on Earth and possibly Mars. Synchrotron-μXRF, electron microprobe analysis, Raman spectroscopy, and optical microscopy are used in a coordinated approach to identify corroborating textural and chemical (organic, inorganic) evidence of life in these modern, opaline (amorphous) siliceous materials. Fluid mobile elements, such as As and Sr, track the growth history of the digitate structures. Trace element enrichments of Ca, Al, Ga, +/− Fe, Mn, As, Rb, Cs, and Sr, are identified in silicified sheaths of microbial filaments embedded within the sinter. In contrast, silicified diatoms in some sinter samples show no trace element enrichment. Gallium enrichments have also been observed in other 16 ka and Jurassic (150 Ma) microbial palisade sinter textures, suggesting the potential for preservation through geologic time, even after recrystallization to quartz. Raman analysis reveals spectra of organics, consistent with pigments for UV protection in cyanobacteria, in silicified sheaths around microbial filaments and are co-located with trace metal enrichments in digitate structures. Due to spectral bands, the location of these molecules (i.e., in the sheaths), and the sampling locations, we ascribe the spectra to scytonemin and carotenoid class molecules. The combined analytical approach outlined here provides a robust means to assess the validity of novel biosignatures, with application to the exploration of Mars, where preservation of opaline silica in >3.6 Ga deposits has the potential to preserve a range of microbial biosignatures.

Mafic veins in the mantle section of the Oman ophiolite: Petrological and geochronological insights from the Oman Drilling Project cores

The magmatic and tectonic evolution of the Oman ophiolite was investigated using the core samples obtained in the Oman Drilling Project. The core samples were recovered from three drilling sites (BA1B, BA3A, and BA4A) in the mantle section composed mainly of harzburgite and discordant dunite and wehrlite. A large number of mafic veins crosscut the structures of both mantle lithologies, indicating that they intruded after the formation of discordant dunites. The wehrlites include Fe3+-rich and TiO2-poor chromian spinel, suggesting that the discordant dunites and wehrlites were involved in hydrous, Ti-poor arc magmatism. The composition of plagioclase and clinopyroxene and rare-earth element (REE) patterns of clinopyroxene for the mafic veins suggest that the mafic veins were produced from mid-ocean ridge basalt (MORB)-like parental melts. On the other hand, early crystallization of clinopyroxene and Fe3+-rich and TiO2-poor chromian spinel in the mafic veins imply that the parental melts were also hydrous. Zircon UPb dating of the mafic veins yields the igneous age of about 91 Ma, which is younger than the V1 and V2 volcanic sequences. Depending on the distance from the mafic veins, the various REE patterns of clinopyroxene and Ca amphibole in the Oman harzburgites indicate local peridotite metasomatism during intrusion and percolation of late-stage melts that formed the mafic veins. Ca amphibole in the harzburgites and mafic veins lack significant enrichment in fluid-mobile elements, indicating that the hydrous phases were formed by fluids associated with hydrothermal activities in a spreading environment instead of slab-derived fluids. The mafic veins were likely produced in a spreading environment on a supra-subduction zone. As their formation took place after the arc magmatism that formed the Oman V2 lava sequence and discordant dunites, as a result of slab roll-back. This seafloor re-spreading on a supra-subduction zone after the V2 magmatism recorded in the mantle section might have been involved in the formation of the plume-related V3 lava sequence overlying the V2 lavas.

Tracing the Scale of Fluid Flow in Subduction Zone Forearcs: Implications from Fluid-Mobile elements

Despite the importance of fluids in subduction zone processes, the extent of mass transfer and fluid circulation from the subducting plate through the forearc region remain unclear. To estimate the fluid budget and scale of fluid circulation in subducted sediments, we assess the distribution and retention of fluid-mobile elements (FME) in metamorphically equivalent metapelites from the Kodiak complex (Alaska) and the Shimanto Belt (Japan). The temperature range of interest is 230–350 °C, i.e., encompasses the base of the seismogenic zone. We examine the Li, B, Rb, Sr, Cs, and Ba concentrations in the bulk rock, as well as in fluid inclusions and individual minerals by (LA-) ICP-MS.The whole-rock composition in metapelites from Kodiak shows no significant loss of FME, which is in contrast to Shimanto where between 10% and 55% of FME (particularly Li, B and Cs) are leached out of rocks. The FME budget in Kodiak is consistent with a redistribution of elements between metamorphic illite and chlorite, whereas in Shimanto the loss of Li, B and Cs as temperature increases is the result of decreasing concentrations in illite and chlorite. The semi-quantitative analysis of fluid inclusions is consistent with a significant enrichment in all analyzed trace elements relative to seawater and interstitial pore fluids of seafloor sediments.Combining the fluid compositions with the whole-rock compositions, mass balance calculations were performed for B, Cs, and Ba. In the Kodiak complex the mass balance calculations are consistent with closed-system behavior, wherein the fluid is an insignificant reservoir for FME. Conversely, in the Shimanto Belt the mass balance calculations are consistent with open-system behavior, wherein large amounts of fluid percolated through rocks and the mass water-rock ratios correspond to 0.5–2.2. We infer that such an open system behavior was promoted by a larger amount of internal strain and the proximity to a large-scale fault zone. Moreover, fluid compositions observed in this study exhibit similarities to the composition of mud volcano fluids. This similarity is consistent with extensive, focused fluid circulation originating from depths of at least 15 km and ascending to the surface through a substantial damage zone associated with an out-of-sequence thrust.

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 (&lt;640 °C), and olivine + chrysotile ± secondary clinopyroxene (&lt;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.

Mineral precipitation sequence from multi-stage fluids released by eclogite during high-pressure metamorphism

Abstract Arc magmas above subduction zones hold abundant fluid-mobile elements attributed to fluids released from the dehydrating subducted oceanic crust. However, the quantity of trace elements in the fluids and their evolution with the metamorphic processes during subduction and exhumation are still unclear. The precipitation sequence of vein minerals preserves the nature of multi-stage high-pressure (HP) metamorphic fluids and the fingerprint of mass exchange in deep subduction zones. In this contribution, we conducted detailed petrological studies and phase equilibria modeling on a unique HP omphacite-rich vein and its host eclogite from the Chinese southwestern Tianshan. The host eclogite consists mainly of garnet, omphacite, epidote, glaucophane, phengite, quartz, and rutile. Garnet in the eclogite records prograde subduction and early exhumation characterized by decompression heating at P-T conditions of ~2.4–2.6 GPa and 460–540 °C. The embedded omphacite-rich vein has similar mineral assemblage to the host eclogite. Garnet grains in this vein are predominantly distributed along or intersect the vein wall, which record similar eclogite-facies metamorphic conditions to that of the host eclogite. Omphacite is dominant in the vein, while epidote and glaucophane occur interstitially. Phase equilibria modeling reveals a sequential growth of garnet-dominated, omphacite-dominated, and epidote-dominated assemblages from fluids originating from the breakdown of different hydrous minerals. These lines of evidence suggest that the formation of multi-stage HP fluids are a continuous long-term process with a spontaneously short-distance transport and sequential mineral precipitation. Calculated fluid compositions demonstrate that the fluids released by lawsonite breakdown during exhumation have great potential to modify the trace element systematics of arc magmas. Our findings reveal the nature and evolution of multi-stage HP metamorphic fluids from internal sources during subduction and exhumation of oceanic crust, providing valuable insights into the chemical compositions of arc magmas.

Arc Foundations and Subduction Initiation: Insights into the Magmatic Evolution of the Lower Crust/Upper Mantle of the Izu–Bonin Forearc during Subduction Initiation

Abstract Our understanding of the processes at work in the lower crust/upper mantle transition zone during subduction initiation and early arc development has suffered from a general lack of in situ samples. Here, we present the results of petrographic and geochemical analysis of 34 samples (9 harzburgites, 13 dunites, 2 orthopyroxenites, 3 olivine-gabbros, and 7 wehrlites) collected from the inner trench wall of the Bonin Ridge, Izu–Bonin forearc. The sample suite records three main melt–rock reaction events involving melts with forearc basalt (FAB)-like, boninitic, and transitional compositions. The wehrlitic and gabbroic rocks trend towards more transitional to FAB compositions and the rest towards more boninitic compositions. The crosscutting occurrence of all three events in a single sample (wehrlite D31–106) establishes a relative timing of the events like that reported for the volcanic edifice of the Bonin Ridge, which transitioned from forearc basalt volcanism at subduction initiation (c.a., 51–52 Ma) to boninitic volcanism (c.a., 50–51 Ma) as the subduction system matured. We therefore suggest that the lower crust/upper mantle transition of the Bonin Ridge preserves a record of the transition from FAB melts created by decompression melting at subduction initiation to arc-type flux melting and boninitic volcanism thereafter. Orthopyroxenites and two anomalously fresh harzburgites from the sample suite are suggested to represent the later boninitic melts and possibly the result of hybridization between such melts and residual peridotites, respectively. Diffuse melt–rock reaction between the later boninites and/or subduction-related fluids and the earlier-formed FAB-related crust is recorded by enrichments in fluid mobile elements and depletions in first row transition metals in clinopyroxenes from a metasomatic vein in wehrlite sample D31–106. The chemistry of the wehrlitic and gabbroic clinopyroxenes suggests that they crystallized from hydrous, highly depleted melts which lack a slab fluid signature. We thus suggest that highly depleted melt fractions might be created early on during subduction initiation by the introduction of seawater into the proto-mantle wedge. The overall FAB-like nature of the crustal wehrlites and gabbros would suggest that most of the lower arc crust was created by forearc extension during/following subduction initiation and that later, mature arc volcanism may have contributed little or no material to the lower crust/upper mantle record in the outer forearc.

Investigating the Influence of Crustal Contamination on the Stillwater Complex, Montana Using Sr, Nd, and Pb Isotopes

Abstract The presence of pegmatoid bodies in the Stillwater Complex is poorly understood, but they have been suggested to have resulted from the presence of fluids in the complex. To better understand the origin of the pegmatoids and to trace the possible influence of country-rock-derived fluid in the Stillwater Complex, bulk rock Rb-Sr, Sm-Nd, and Pb-Pb isotopes for samples from the Archean Stillwater Complex and its metamorphic aureole are reported. Pegmatoid bodies are compared to spatially associated host rock and the underlying hornfels facies country rocks. Evidence of resetting of radiogenic isotopes during regional metamorphism at 1700 Ma is not observed, and the initial radiogenic isotopic ratios in Stillwater Complex rocks overlap those of the underlying hornfels. Despite the isotopic similarity of the country rock to the Stillwater Complex, the intrusion is modestly isotopically heterogeneous. In Stillwater samples, the average εNd,2710Ma = −1.1 ± 6.9, 206Pb/204Pb2710 Ma = 15.24 ± 2.26, and 87Sr/86Sr2710Ma = 0.703043 ± 0.002747 (1σ). The similarity between country rock and intrusive rock isotopic compositions at Stillwater contrasts with the data reported for the Bushveld Complex, South Africa, where the country rock is isotopically distinct from the intrusion. The variability in radiogenic isotope signatures in Stillwater rocks show a noisy but decreasing influence of country rock up through the Lower Banded series interpreted to reflect variable crustal contamination, in part from &amp;lt;1.0 wt % country rock fluids released during intrusion of the Stillwater Complex. The influence of crustal fluid contamination as compared to more traditional crustal assimilation models or simple magmatic heterogeneity suggests that hydrothermal fluids modified the isotopic compositions of more fluid-mobile elements and can explain aspects of isotopic heterogeneity in layered intrusions.

Mineralogy and Geochemistry of the Mupane and Shashe Gold Deposits of the Tati Greenstone Belt (NE Botswana): Implications for Mineral Deposit Models and Exploration

Abstract The Archean Tati greenstone belt is located at the southwestern margin of the Zimbabwe craton (northeast Botswana) and hosts numerous Cu-Ni ± platinum group element (PGE) and Au ± Ag occurrences and deposits. Gold occurrences/deposits are poorly studied, and key questions pertaining to their genesis remain unclear, including the mode of occurrence(s) of gold, the relative timing of gold introduction with respect to the evolution of the greenstone belt, the number of gold mineralization events, the alteration patterns, and the relationships between each alteration pattern and gold mineralization. A detailed study that includes sulfide and gold grain chemistry using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and electron probe microanalysis (EPMA), respectively, and X-ray diffraction (XRD) of mineralized rock samples of three Tati greenstone belt gold deposits was carried out to constrain the genesis of gold mineralization and formulate exploration guidelines in the Tati greenstone belt. Gold in the Tati greenstone belt is the result of multiple events, is mainly associated with arsenopyrite, pyrite, and sphalerite, and occurs as (1) microinclusions within sulfides, (2) intergrowth with sulfides, (3) minute particles (&amp;lt;2–10 µm) within the silicate matrix, (4) microfractures and microvug infills, and (5) lattice-bound and sulfide-hosted refractory gold. The first gold event (as electrum) is premetamorphic and associated with sphalerite-quartz veins. The second stage is a postmetamorphic high-grade gold event that is accompanied by extensive carbonatization and propylitization of host rocks. The third stage of gold mineralization is marked by the dissolution of gold in the early formed stages 1 to 2 and subsequent reprecipitation within cracks, fractures, and vugs. Auriferous pyrite composition suggests that Au-bearing mineralizing fluids are predominantly of magmatic origin and that their physicochemical compositions changed during the mineralization process, as supported by chemically zoned Au-bearing arsenopyrite, various alteration types containing gold, and variation in gold fineness across the several gold deposits in the Tati greenstone belt. Gold deposition in the Tati greenstone belt mainly occurred through sulfidation, as indicated by the closest spatial association between gold and Fe-bearing sulfides and ferromagnesian silicates. Gold in the Tati greenstone belt is closely correlated with As, Sb, Pb, Bi, Ni, Hg, Tl, Cd, In, Mo, W, Zn, and Te and moderately to weakly associated with Sn, Se, Cr, Co, Ge, Cd, Mn, V, Ga, and Ag. The correlation between Au and fluid mobile elements, i.e., Te, Sb, Se, As, Hg, and Bi, can be used as a vectoring tool during the exploration of gold within the Tati greenstone belt, as these elements likely form halos that are much broader than the primary footprint of gold mineralization.