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Subduction-zone fluid–rock interactions have a direct impact onto elemental and isotopic homogeneity of progressively buried and exhumed crustal lithologies by providing an interface for local mass-transfer and enhancing metamorphic reactions. In order to assess the scales of fluid mobility, chemical and isotopic inheritance, as well as resulting degrees of isotopic heterogeneity in the exhumed high-pressure lithologies, we performed the detailed mineralogical, in-situ trace-element and Rb–Sr isotope studies, combined with P–T–X thermodynamic modelling of representative eclogites from the Alag Khadny accretionary complex (SW Mongolia). The eclogites (garnet + omphacite + phengite + rutile + quartz + retrograde amphibole and clinozoisite) display records of subduction-related burial to 540–625 °C and 1.7–2.1 GPa, with the enclosed phengite supposed to be in equilibrium at prograde-to-peak conditions. Trace-element signatures, including Cs/Rb (0.03–0.08) and Ba/Rb (7.1–13.8) ratios of phengite, are consistent with moderately to strongly altered protoliths of eclogites, which is supported by elevated δ18O values and in-situ Rb–Sr constraints on the initial (87Sr/86Sr)I ratios of phengite within 0.70549–0.70957. Partial backward rehydration (~ 0.5–1.0 wt% of H2O added) during decompression from 1.6 to 1.2 GPa produced amphibole- and clinozoisite-bearing assemblages, did not significantly affect LILE systematics and variable 87Rb/86Sr ratios of phengite. Limited Rb and Ba loss from phengite during recrystallization is suspected in the evidently deformed eclogites based on the LILE mineral–fluid and phengite–amphibole partitioning data. No exclusive evidence is found in amphibole for LILE-rich metasedimentary fluid with high (87Sr/86Sr)I released into eclogites. Instead, unradiogenic 87Sr/86Sr (0.70279–0.70301) of clinozoisite highlights metasomatic addition from the underlying mafic crust or dehydrated peridotitic mantle. Variable deformation-enhanced fluid-rock interaction during early exhumation was recorded by in-situ phengite Rb-Sr geochronology at 568 ± 9 Ma, which is considered a direct fluid flow snapshot and place a new minimum age constraint for the peak subduction burial. We argue that, except cases of apparent metasomatic origin of phengite, its (87Sr/86Sr)I ratios may be a sensitive tracer for the eclogite precursor alteration due to limited Sr mobility. Sample-scale Rb-Sr isotopic heterogeneities may be preserved in the orogenic eclogites due to multi-stage retrograde hydration and should be taken into account while interpreting the bulk-rock Sr isotope data. [ABSTRACT FROM AUTHOR]

Serpentinisation of mantle peridotites below the seafloor is the most important hydration reaction in the Earth's deep water cycle. This critical step in water–rock interaction occurs over multiple serpentinisation stages and at variable temperatures and fluid compositions. We present the first study using spatially coupled in-situ analysis of oxygen isotopes (secondary ionization mass spectrometry) and trace elements (laser ablation inductively coupled plasma mass spectrometry) to unravel the multistage hydration history of oceanic serpentinites. We study samples from the Newfoundland-Iberia extended passive margins, which represents a magma-poor ocean-continent transition zone (Ocean Drilling Program cores, Leg 173 Site 1070 from Iberia, Leg 210 Site 1277 from Newfoundland). The concentrations of the fluid mobile elements chlorine and boron in serpentine are used as a proxy for the salinity of the serpentinising fluid. The correlation of Cl/B with δ18O serpentine compositions provides new insights to disentangle temperature from fluid composition effects. The transition metal composition (V, Co, Sc, Mn, Zn, Ni, Cr) of dominantly lizardite in mesh after olivine and in bastite after orthopyroxene shows a chemical redistribution between textural sites in the Newfoundland samples, indicating the simultaneous serpentinisation of olivine and orthopyroxene. This feature is not observed in the Iberian samples, for which we propose sequential reactions. Lizardite in samples from both localities varies considerably in oxygen isotope composition at the scale of tens of micrometres depending on texture, with a range in δ18O of 3.3–13.5‰ for Iberia samples and a more restricted range of 5.7–9.3‰ for Newfoundland samples. Temperatures calculated from the δ18O serpentine corresponding to the lowest Cl/B ratio (interpreted as closest to seawater composition) indicate sequential serpentinisation with decreasing temperature from ∼190 to ∼60 °C in the Iberia setting. The Newfoundland samples were serpentinised at a lower temperature (100–130 °C), possibly at a shallower depth. Serpentinisation in the Newfoundland samples probably produced a concentrated amount of H 2 in a shorter period of time, whereas in the Iberian samples a small but prolonged amount of H 2 was produced, possibly creating more favourable conditions for microbial activity. [ABSTRACT FROM AUTHOR]

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. • Fluid-mobile elements (FME) used to calculate mass water-rock ratio. • Closed-system behavior in units from Kodiak and open-system behavior in Shimanto. • In Shimanto, the FME points to the high quantity of fluids traversing the rocks. • External fluids infiltrate due to high internal strain and proximity to a fault zone. • Focused fluid flow for >15 km through the subduction forearc. [ABSTRACT FROM AUTHOR]

Serpentinites, which contain up to 13 wt% of water, are important reservoirs for chemical recycling in subduction zones. In the past two decades, forearc mantle serpentinites were identified in different locations around the world. Here, we present petrology and whole rock chemistry of ultramafic and mafic rocks dredged from the Hahajima Seamount, which is located 24–40 km west to the junction of the Izu-Bonin Trench and the Mariana Trench. Nearly all the collected samples are extensively hydrated, and olivine grains in ultramafic rocks are replaced by serpentine minerals, with only one sample preserving remaining trace of orthopyroxene. Our new results show that the Hahajima serpentinized peridotite samples are all MgO-rich (∼42 wt%), but have low contents in Al2O3, CaO, rare earth and high field strength elements, which is consistent with the overall depleted character of their mantle protoliths. Model calculations indicate that these Hahajima peridotite samples were derived from 10%–25% partial melting of the presumed fertile mantle source, which is generally lower than those of peridotites from Torishima Forearc Seamount, Conical Seamount and South Chamorro Seamount (mostly >25%). All the serpentinites from these four forearc seamounts show strong enrichment in fluid-mobile and lithophile elements (Li, Sr, Pb and U). In details, Hahajima Seamount serpentinites do not have obvious enrichment in Cs and Rb, and display remarkably high abundances of U. These observations indicate that the serpentinization of Hahajima peridotites occurred by addition of seawater or low temperature seawater-derived hydrothermal fluid, without or with little contribution from slab-derived fluids. The geochemical signature of serpentinites from Hahajima Seamount could be interpreted as the result of the combination of extensive partial melting and subsequent percolation of seawater through the mantle wedge. [ABSTRACT FROM AUTHOR]

PYRITE is ubiquitous throughout the massive sulfide deposits of the Bathurst Mining Camp, Canada. Pyrite locally preserves primary, pseudo-primary textures, as well as secondary textures, that formed in response to the hydrothermal, metamorphic, and deformation processes to which the host sequence was exposed. In the Bathurst Mining Camp most forms of pyrite are arsenian (up to 3.13 wt% As), with considerable enrichment in Tl, Sb, Sn, Bi, Ag, Se, and to a lesser extent Cd, Ga, Te, Hg, Ge, In, and Au. LA-ICP-MS analyses of primary pyrite show significant enrichment in fluid-mobile elements, in particular, Tl, Sb, As, and Hg, as well as Bi, Pb, and Sn. In contrast, secondary pyrite (syn-to post-deformation varieties) is not enriched in these elements. Superimposed metamorphism typically causes loss of fluid-mobile elements to different degrees, except for Se. Pyrite chemical variations have the potential to monitor ore petrogenesis at inter- and intra-deposit scales. Pyrite from the Cu-zone is enriched in Se relative to pyrite from Zn-Pb zone, in which Tl is elevated. Throughout the host sequence fluid-mobile element concentrations of pyrite vary by several orders magnitude, with the highest fluid-mobile element abundances in footwall rocks immediately proximal to the deposit. Moreover, elevated fluid-mobile element concentrations of pyrite are recognized in footwall felsic volcanic and sedimentary rocks as much as several hundred metres away from the deposit. Hanging-wall zones contain pyrite with fluid-mobile element contents comparable to pyrite from sulfide horizon. The fluid-mobile element concentrations of pyrite are higher in exhalative sedimentary rocks than pyrite from the sulfide mineralization itself. The spatial variation in fluid-mobile element concentrations in pyrite as identified using LA-ICP-MS can be used to identify otherwise cryptic deposit-related haloes. Ultimately, this technique, when combined with traditional exploration tools, can be used to vector toward VMS mineralization in the Bathurst Mining Camp and possibly a useful indicator in a prospect-scale base metal exploration. [ABSTRACT FROM AUTHOR]

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

Ultramafic portions of ophiolitic fragments in the Arabian–Nubian Shield (ANS) show pervasive carbonate alteration forming various degrees of carbonated serpentinites and listvenitic rocks. Notwithstanding the extent of the alteration, little is known about the processes that caused it, the source of the CO2or the conditions of alteration. This study investigates the mineralogy, stable (O, C) and radiogenic (Sr) isotope composition, and geochemistry of suites of variably carbonate altered ultramafics from the Meatiq area of the Central Eastern Desert (CED) of Egypt. The samples investigated include least-altered lizardite (Lz) serpentinites, antigorite (Atg) serpentinites and listvenitic rocks with associated carbonate and quartz veins. The C, O and Sr isotopes of the vein samples cluster between −8.1‰ and −6.8‰ for δ13C, +6.4‰ and +10.5‰ for δ18O, and87Sr/86Sr of 0.7028–0.70344, and plot within the depleted mantle compositional field. The serpentinites isotopic compositions plot on a mixing trend between the depleted-mantle and sedimentary carbonate fields. The carbonate veins contain abundant carbonic (CO2±CH4±N2) and aqueous-carbonic (H2O-NaCl-CO2±CH4±N2) low salinity fluid, with trapping conditions of 270–300°C and 0.7–1.1 kbar. The serpentinites are enriched in Au, As, S and other fluid-mobile elements relative to primitive and depleted mantle. The extensively carbonated Atg-serpentinites contain significantly lower concentrations of these elements than the Lz-serpentinites suggesting that they were depleted during carbonate alteration. Fluid inclusion and stable isotope compositions of Au deposits in the CED are similar to those from the carbonate veins investigated in the study and we suggest that carbonation of ANS ophiolitic rocks due to influx of mantle-derived CO2-bearing fluids caused break down of Au-bearing minerals such as pentlandite, releasing Au and S to the hydrothermal fluids that later formed the Au-deposits. This is the first time that gold has been observed to be remobilized from rocks during the lizardite–antigorite transition. [ABSTRACT FROM PUBLISHER]

Recent geochemical work shows that subduction-zone serpentinites are repositories for fluid-mobile elements absorbed during interaction with sediment-derived fluids. Unraveling the geochemical fingerprint of these rocks helps to define timing of tectonic accretion of sediments along the subduction interface and the role of serpentinite in element recycling to volcanic arcs. Here we present the trace element and isotopic composition (B–O–H, Sr, Pb) of high-pressure serpentinites from the Voltri Massif (Ligurian Western Alps, Italy), to discuss their role as incompatible element carriers and their contribution to recycling of sediment-derived components in subduction zones. The serpentinites presented here record metamorphic olivine growth during eclogite-facies metamorphism and show undeformed and mylonitic textures. Field relations show that undeformed rocks are enclosed in deformed ones and that no metasedimentary rocks are present nearby. Undeformed serpentinite has very high δ 11 B SRM951 (from +26‰ to +30‰), low Sr and Pb isotope ratios ( 87 Sr/ 86 Sr = 0.7053–0.7069; 206 Pb/ 204 Pb = 18.131–18.205) and low As and Sb contents (0.1 and 0.01 μg/g, respectively). Oxygen and hydrogen isotope compositions are +4.5‰ and −67‰, respectively. In contrast, mylonitic serpentinite shows lower δ 11 B (from +22‰ to +17‰), significant enrichment in radiogenic Sr and Pb isotopes ( 87 Sr/ 86 Sr up to 0.7105; 206 Pb/ 204 Pb up to 18.725), and enrichment in As and Sb (1.3 and 0.39 μg/g, respectively). δ 18 O of the mylonitic serpentinites reaches values of +5.9‰, whereas δD is comparable with that of undeformed rocks (approximately −70‰). In mylonitic serpentinites, the B and Sr isotopic values and the fluid-mobile element (FME) concentrations are near those for the Voltri metasedimentary rocks (calc- and mica-schists). Pb systematics also reveal influx of a crust-derived component. Our dataset shows that undeformed serpentinite still preserves an oceanic geochemical fingerprint, whereas mylonitic serpentinite is reset in its concentrations of FME and its B, Sr and Pb isotope compositions, due to interaction with sediment- and crust-derived fluids. The environment of this interaction is either compatible with (i) an outer-rise zone setting, with percolation of seawater-derived fluids enriched in sedimentary components into bending-related fault structures, or with (ii) subduction channel domains, where ascending sediment-derived slab fluids infiltrate slices of former oceanic serpentinite accreted to the plate interface domain. Influx of sediment-derived subduction fluids along major deformation zones in serpentinite modifies the element budget of the rocks, with important implications for element recycling and the tectonic history of serpentinite. The B, Sr and Pb isotopic systematics, coupled with FME concentration in serpentinites are particularly helpful geochemical tracers of interaction between different reservoirs in subduction-interface environments, and are more sensitive than the traditionally applied stable oxygen and hydrogen isotope compositions. [ABSTRACT FROM AUTHOR]

Continental basalts generally display enrichment of fluid-mobile elements and depletion of high-field-strength elements, similar to those that evolved in the subduction environment, but different from oceanic basalts. Based on the continental flood basalt database for six large igneous provinces, together with rift-related basalt data from the Basin and Range Province, this study aimed to test the validity of geochemical tectonic discrimination diagrams in distinguishing arc-like intra-continental basalts from arc basalts and to further investigate the role of deep-Earth water cycling in producing arc-like signatures in large-scale intra-continental basalts. Our evaluation shows that arc-like intra-continental basalts can be distinguished from arc basalts by integrating the following factors: (1) the FeO, MgO, and Al 2 O 3 concentrations of the primary melt; (2) Ti V, Zr Zr/Y, Zr Ti, and Ti/V Zr/Sm Sr/Nd discrimination diagrams; (3) the coexistence of arc-like and OIB-like subtype basalts within the same province; (4) primitive mantle-normalized trace element distribution patterns. The similarity of enrichment in fluid-mobile elements (Ba, Rb, Sr, U, and K) between arc-like and true arc basalts suggests the importance of water flux melting in producing arc-like signatures in continental basalts. Experimentally determined liquid lines of descent (LLD) imply high magma water concentrations for continental flood basalts (CFBs) and the Basin and Range basalts. Furthermore, estimates based on the Al 2 O 3 –LLD method indicates 4.0–5.0 wt% pre-eruptive magma H 2 O concentration for CFBs and the Basin and Range basalts. The tight relationships between H 2 O/Ce and Ba/La, Ba/Nb and Rb/Nb based on global arc basalt data were further used to estimate the primary H 2 O concentrations. With the exception of the Emeishan CFBs (mainly containing 4.0–5.6 wt% H 2 O), all other CFBs investigated have similar estimated primary H 2 O contents, with values ranging from 1.0 to 2.0 wt%. The estimated primary H 2 O content of the Basin and Range basalts is extremely high and up to 10.0 wt%. Thus, this study demonstrates that water flux melting played an important role in the generation of many intra-continental igneous provinces. This new finding was further employed to investigate the tectonic setting of 320–270 Ma basalts in Inner Mongolia, North China. Most basalts from three key rock units (i.e. Amushan, Benbatu, and Dashizhai formations) from the Central Asian Orogenic belt are classified as non-arc types. The estimated magma H 2 O concentrations suggest a strong link between H 2 O content and arc-like geochemical signatures. Together with established geological evidence, we proposed that these 320–270 Ma basaltic rocks were most likely produced in a post-orogenic extensional environment facilitated by subducted slab-driven deep-Earth fluid cycling. We propose a mantle transition zone water-filtering model that links deep-Earth fluid cycling, large-scale intra-continental basaltic magmatism, and supercontinent cycles into a self-organized system. [ABSTRACT FROM AUTHOR]

Serpentinites release at sub-arc depths volatiles and several fluid-mobile trace elements found in arc magmas. Constraining element uptake in these rocks and defining the trace element composition of fluids released upon serpentinite dehydration can improve our understanding of mass transfer across subduction zones and to volcanic arcs. The eclogite-facies garnet metaperidotite and chlorite harzburgite bodies embedded in paragneiss of the subduction melange from Cima di Gagnone derive from serpentinized peridotite protoliths and are unique examples of ultramafic rocks that experienced subduction metasomatism and devolatilization. In these rocks, metamorphic olivine and garnet trap polyphase inclusions representing the fluid released during high-pressure breakdown of antigorite and chlorite. Combining major element mapping and laser-ablation ICP-MS bulk inclusion analysis, we characterize the mineral content of polyphase inclusions and quantify the fluid composition. Silicates, Cl-bearing phases, sulphides, carbonates, and oxides document post-entrapment mineral growth in the inclusions starting immediately after fluid entrapment. Compositional data reveal the presence of two different fluid types. The first (type A) records a fluid prominently enriched in fluid-mobile elements, with Cl, Cs, Pb, As, Sb concentrations up to 10 3 PM (primitive mantle), ∼ 10 2 PM Tl, Ba, while Rb, B, Sr, Li, U concentrations are of the order of 10 1 PM, and alkalis are ∼2 PM. The second fluid (type B) has considerably lower fluid-mobile element enrichments, but its enrichment patterns are comparable to type A fluid. Our data reveal multistage fluid uptake in these peridotite bodies, including selective element enrichment during seafloor alteration, followed by fluid–rock interaction along with subduction metamorphism in the plate interface melange. Here, infiltration of sediment-equilibrated fluid produced significant enrichment of the serpentinites in As, Sb, B, Pb, an enriched trace element pattern that was then transferred to the fluid released at greater depth upon serpentine dehydration (type A fluid). The type B fluid hosted by garnet may record the composition of the chlorite breakdown fluid released at even greater depth. The Gagnone study-case demonstrates that serpentinized peridotites acquire water and fluid-mobile elements during ocean floor hydration and through exchange with sediment-equilibrated fluids in the early subduction stages. Subsequent antigorite devolatilization at subarc depths delivers aqueous fluids to the mantle wedge that can be prominently enriched in sediment-derived components, potentially triggering arc magmatism without the need of concomitant dehydration/melting of metasediments or altered oceanic crust. [ABSTRACT FROM AUTHOR]

Abstract: We provide new insights into the geochemistry of serpentinites from the Alpine orogenic wedge representing a paleo-subduction zone. These serpentinites are derived from similar oceanic protoliths, but they have experienced different metamorphic conditions related to three different structural levels of the paleo-subduction zone ((1) obducted: Chenaillet ophiolite, (2) accretionary wedge: Queyras Schistes lustrés complex and (3) serpentinite channel: Monviso ophiolite). Metamorphism undergone by these three units is well defined, increasing eastward from sub-greenschist to eclogite facies conditions, and allows us to examine trace element behavior from the oceanic ridge environment to subduction. Serpentinites first record moderate trace element enrichment due to seawater interaction resulting in the replacement of olivine and pyroxene by chrysotile and lizardite below 300°C. In the sediment-dominated accretionary wedge, serpentinites are strongly enriched in fluid-mobile-elements (B, Li, As, Sb, and Cs) and act as a trapping system following the metamorphic gradient (from 300 to 390°C) up to total replacement of the lizardite/chrysotile assemblage by antigorite. Under higher temperature conditions (T>390°C), no enrichment was observed, and some fluid-mobile elements were released (B, Li, Cs, and Sr). Moreover, in the serpentinite channel (T>460°C), most of the fluid-mobile elements are absent due to the scarcity of metasediments which prevent geochemical exchange between metasediments and serpentinites. This is also due to the onset of antigorite breakdown and the release of fluid-mobile elements. Thus, we emphasize that the geochemistry of Alpine serpentinites is strongly dependent on (1) the grade of metamorphism and (2) the ability of metasediments to supply fluid-mobile elements. We conclude that serpentinites act as a trap-and-release system for fluid-mobile elements in a subduction context. [Copyright &y& Elsevier]

Abstract: Serpentinites from subduction environments represent an important sink for fluid-mobile elements. In order to constrain geochemical behavior of fluid-mobile elements hosted by serpentine phases during subduction processes, we carried out a geochemical study (trace elements and Pb isotopes) of a series of serpentinites and cumulates from the accretionary wedge of Greater Caribbean (Cuba and Dominican Republic). The trace element compositions of the primary and alteration-related phases were analyzed in situ using LA–HR-ICP-MS techniques. The studied samples represent parts of the subducted proto-Atlantic oceanic lithosphere, which has experienced low to high grade metamorphism (greenschist to eclogite facies), before being exhumed; a subset of these samples were derived from the mantle wedge. This sampling provides the opportunity to trace the chemical mobility of fluid-mobile elements during prograde metamorphism along a cold geotherm in an oceanic subduction setting. Serpentinites display strong enrichment in fluid-mobile elements indicating extensive fluid–rock interaction. In situ analyses allow distinction of three types of serpentines related to the nature of primary minerals (olivine, ortho- or clinopyroxene). Compositions of subducted samples, especially in fluid-mobile elements, are relatively close to those of abyssal peridotites without noticeable evidence of mobility for trace elements during subduction-related prograde metamorphism, with the exception of B. This confirms that the observed enrichment results from seawater/peridotite interactions during residence time in the ocean. It also suggests that most mobile elements stored in serpentine minerals are immobile during subduction processes. A major consequence of this observation is that serpentine minerals are a good sink for mobile elements in subduction zones, until their dehydration. Additionally, Pb isotopes and over-enrichment in As–Sb in high-grade subducted serpentines (antigorite) suggest the contribution of a sedimentary component during a secondary hydration taking place at the lizardite/antigorite transition. We propose that this new serpentinization event, taking place at greater depth, results from mixing between sediments and serpentinites in the subduction channel. Mantle wedge serpentinites present imprints of hydrothermal fluids: they are B-rich but without strong enrichment in As and Sb, and show evidence for moderate contributions of a radiogenic Pb-component. This suggests that the fluids that produced the mantle wedge serpentinites derived from the dehydration of the oceanic crust, with moderate to no contribution of sediments. We posit that mantle wedge serpentinization took place around 20–25km depth: at such depth and temperature conditions (T>200°C), the subducted sediments still released their B-rich pore fluids while their structural water incorporated in hydrous minerals (phengite, lawsonite) remained stable. The existence of various potential reservoirs for fluid-mobile elements in subduction zone environments (subducted serpentinites, mantle wedge serpentinites, as well as subducted sediments and altered oceanic crust) that potentially release their fluids at different depths has strong implications for arc lava formation. [Copyright &y& Elsevier]

Abstract: Serpentinized oceanic lithosphere may be an important source for boron and other fluid-mobile elements that are anomalously enriched in arc volcanic rocks. However, the integrated water/rock ratios associated with different styles of serpentinization may be variable. For example, large water/rock ratios are involved in the serpentinization of abyssal peridotites exhumed to the seafloor, whereas much lower water/rock ratios are likely to dictate serpentinization along deep faults and fractures. To address how fluid-mobile element enrichments vary with serpentinization at different settings, we investigated serpentinized harzburgites from the Feather River Ophiolite (FRO) in northern California. Major and trace element systematics indicate that serpentinization of the FRO ultramafics involved seawater. However, FRO serpentinites have unradiogenic Os isotopic compositions and near-chondritic platinum group element relative abundances, contrasting with serpentinized abyssal peridotites, which have radiogenic Os isotopic compositions and disturbed platinum group element systematics. These observations indicate that the integrated water/rock ratio involved in FRO serpentinization was smaller than that involved in abyssal peridotite serpentinization. B concentrations in the FRO (5–15 ppm), while substantially higher than primitive mantle (<0.1 ppm), are still lower than in abyssal peridotites (10–170 ppm). These low values are not due to metamorphic loss as there is no petrographic evidence for prograde metamorphism (the serpentine minerals are low temperature forms like chrysotile and lizardite) and there is no consistency between observed fluid-mobile element (B, As, Pb, and Li) contents and depletions predicted from metamorphic dehydration models. Low B and fluid-mobile element contents in the FRO may thus be an intrinsic feature of low water/rock ratio serpentinization. Such values may be more representative of serpentinized oceanic lithospheric mantle rather than abyssal peridotites, which sample only the top veneer of the lithosphere. [Copyright &y& Elsevier]

Serpentinite from the North Qaidam ultrahigh pressure metamorphic belt in northern Tibet is studied to provide insight into petro-geochemical evolution of serpentinite from seafloor hydration to subduction zone metamorphism. The North Qaidam serpentinite can be divided into undeformed lizardite serpentinite that was not severely overprinted during subduction and deformed antigorite serpentinite that was well recrystallized during subduction. Petrological and geochemical analyses demonstrate that the serpentinite was originally melt-percolated refractory abyssal harzburgite. Considering the local geodynamic setting, it is inferred that the serpentinite probably originated from the oceanic lithosphere that subducted before continental subduction. Relatively uniform low δ18O (4.0‰–4.5‰) of the antigorite serpentinite indicates high temperature hydrothermal alteration of protolith harzburgite by seawater. In contrast, much lower δ18O (0.6‰–2.7‰) of the lizardite serpentinite is ascribed to exchange with meteoric water at the Earth's surface. Relict serpentinization textures are well preserved in the lizardite serpentinite, with reactions of olivine to lizardite and magnetite, pyroxenes to talc, tremolite, and lizardite, and spinel to chromite and chlorite. The replacement of lizardite by antigorite during subduction mostly initiates along grain boundaries and interconnecting veinlets, implying fluid-assisted transformation of lizardite into antigorite. Partial decomposition of antigorite produces magnesian secondary olivine in the antigorite serpentinite, while direct breakdown of metastable lizardite generates ferroan secondary olivine in the lizardite serpentinite. The serpentinite is enriched in fluid-mobile elements, with U primarily accumulated during seafloor alteration and alkalis notably introduced by sedimentary fluids at bending faults or in accretionary wedge. Infiltration by fluids equilibrated with sediments is also supported by highly elevated 87Sr/86Sr of the serpentinite. Moreover, 87Sr/86Sr of the antigorite serpentinite (0.710649–0.713996) is higher than that of the lizardite serpentinite (0.707184–0.708502), which implies more intense interaction of sedimentary fluids with the former than the latter. However, the lizardite serpentinite contains more alkalis and less U than the antigorite serpentinite, which indicates that large proportions of alkalis were lost during partial dehydration of serpentinite, while U was not released. • Subducted serpentinite preserves relict textures of hydration and metamorphism. • Antigorite and lizardite breakdown produce Mg- and Fe-rich olivine, respectively. • Fluid-mobile elements in serpentinite are stepwise enriched and released. [ABSTRACT FROM AUTHOR]

The compositions of phyllosilicates, with a focus on fluid-mobile elements, were evaluated as a means to fingerprint the Middle Ordovician metamorphosed (greenschist facies) volcanogenic massive sulfide deposits of the Bathurst Mining Camp (BMC), Canada. Ninety-five drill-core samples from six of the major deposits of the Bathurst Mining Camp (Brunswick No. 12, Heath Steele B zone, Halfmile Lake Deep zone, Key Anacon East zone, Louvicourt, and Restigouche) were analyzed using electron microprobe and laser ablation inductively coupled plasma-mass spectrometry. Typically, phyllosilicates (chlorite, white mica, and to a lesser extent biotite) are ubiquitous phases in the host rocks of the massive sulfide deposits of the BMC. Electron microprobe analysis results show a wide compositional variation in chlorite and white mica. Laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis was performed to measure fluid-mobile elements, showing that Tl is distinctly enriched in all white mica (up to 719 ppm) relative to chlorite (up to 50.1 ppm). Chlorite hosts Sn (up to 4600 ppm), Hg (up to 7.3 ppm), Sb (up to 35.4 ppm), As (up to 1320 ppm), In (up to 307 ppm), Cd (up to 83.2 ppm), and Se (up to 606 ppm). White mica hosts Sn (up to 1316 ppm), Hg (up to 93 ppm), Sb (up to 1630 ppm), As (up to 14,800 ppm), In (up to 1186 ppm), Cd (up to 98 ppm), and Se (up to 38.8 ppm). Limited LA-ICP-MS analysis on biotite indicates a higher overall concentration of Tl (mean = 14.6 ppm) relative to co-existing white mica (mean = 2.18 ppm). On average, biotite is also more enriched in Hg, Sn, and Ba relative to chlorite and white mica. Laser Ablation ICP-MS profiles of chlorite, white mica, and biotite demonstrate smooth time-dependent variations diagnostic of structural substitution of these elements. Compositional variation of chlorite-white mica pairs presented in the current study shows systematic variations as a function of distance from the mineralized horizons. This highlights the potential to use trace-element signatures in these phyllosilicate pairs to identify proximal (chlorite) and distal (white mica) footprints for volcanogenic massive sulfides exploration. [ABSTRACT FROM AUTHOR]