Enrichment In Fluid-mobile Elements Research Articles

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.

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.

Metasomatized mantle source of nascent oceanic crust in the Guaymas Basin, Gulf of California

The geochemical characteristics of nascent oceanic crust play a crucial role in unraveling the mantle dynamics of the initial opening of an ocean. The Gulf of California, located at the East Pacific margin, is one of the best examples of an active and obliquely rifted continental margin. However, the mantle source composition of the nascent oceanic crust in the central part of the Gulf has not yet been comprehensively investigated. Here, we report in situ major and trace element contents as well as BSr isotope compositions for basaltic glass samples from off-axis sills drilled by the International Ocean Discovery Program (IODP) Expedition 385 at Sites U1547 and U1548 in the Guaymas Basin in the central part of the Gulf of California. These glassy samples represent tholeiites and predominantly show trace element patterns akin to those of enriched mid-ocean ridge basalts (E-MORBs) but with distinctive enrichments in Ba and K and depletions in Nb, Ta, and Ti. In addition, these samples have high B contents (3.07–3.67 ppm) with enriched Sr isotopes (87Sr/86Sr = 0.7032–0.7037) and heavy B isotopes (δ11B = −5.52‰ to 1.20‰). The mixing model based on B/Nb values and SrB isotopes shows that the nascent oceanic crust in the Guaymas Basin might be generated through partial melting of a depleted MORB mantle (DMM) source metasomatized by melts from subducted slab materials referring to partially dehydrated sediment and altered oceanic crust components. The magmas in the Gulf of California show a systematic decline in their enrichment in fluid-mobile elements (Ba) and depletion in fluid-immobile elements (Nb, Ta, and Ti) from the northern (e.g., Isla San Luis volcanic center) to the central part (Guaymas Basin) and southward to the mouth (e.g., Alarcón Basin) of the Gulf. This suggests that the enriched (recycled) components in their mantle source were gradually extracted and exhausted during Gulf opening and oceanic crustal accretion that advanced in a northward direction. Our results indicate that the Guaymas Basin magmas were derived from a mantle that was fertilized by subduction components. The subduction signature corroborates the Gulf opening as a process that started in response to long-term oblique convergence at the eastern Pacific plate margin without any influence from a mantle plume. This evolution is different from nascent ocean basins that evolved from intraplate rifting, such as the Red Sea, leading to vastly different mantle source characteristics.

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.

Trace elements and Li isotopes distinguish between liquid and solid phases contributing to mafic arc magmas

It is generally accepted that metasomatism of the mantle wedge peridotite by subducting slab-derived fluids plays a key role in dictating the composition of mafic arc magmas. However, this fluid metasomatism model has been challenged recently by the mélange diapirism model. In order to distinguish between liquid and solid phases that would exclusively contribute to mafic arc magmas, we have carried out an integrated study of whole-rock and mineral geochemistry for two types of contemporaneous mafic igneous rocks (gabbroic diorites and diorite dikes) from an accretionary orogen. The results indicate primary contributions from liquid phases composed of not only slab-derived aqueous solution but also seafloor sediment-derived hydrous melt. Although these mafic rocks all exhibit typical arc-like trace element distribution patterns, they show a series of differences in stable Li and radiogenic Sr–Nd–Hf isotope compositions as well as other geochemical variables. In general, the gabbroic diorites are characterized by enrichment in fluid-mobile elements (e.g., Rb, Ba, K, Pb, and Sr), high δ7Li and δ18O values, and depletion in Sr–Nd–Hf isotope compositions. By contrast, the diorite dikes exhibit enrichment in melt-mobile elements (e.g., LILE, LREE and Th), relatively low δ7Li values, and slight enrichment in Sr–Nd–Hf isotope compositions. There are also systematic differences and pertinent variations between Li isotope compositions and slab-derived fluid indicators such as whole-rock Rb/Nd, Sr/Nd, Nb/U, Th/La, Th/Nd and Th/Yb ratios for the two types of mafic igneous rocks. These indicate that the mantle sources of these mafic rocks would possess geochemical signatures that were acquired through chemical metasomatism by slab-derived fluids rather than physical mixing with mélange diapirs. Furthermore, the geochemical signatures allow for distinction between the oceanic slab-derived aqueous solution and the seafloor sediment-derived hydrous melt. This qualitative interpretation is quantitatively confirmed by model calculations of trace elements and Sr–Nd–Li isotopes in the two types of mafic igneous rocks. Therefore, the target mafic igneous rocks provide the geochemical evidence for the formation of their mantle sources through chemical metasomatism by incorporating the two types of metasomatic agent into the mantle wedge. This demonstrates a strong preference for the fluid metasomatism model over the mélange diapirism model for the origin of mafic arc magmas.

Melting and Evolution of Amphibole‐Rich Back‐Arc Abyssal Peridotites at the Mado Megamullion, Shikoku Basin

AbstractThe Mado Megamullion is an oceanic core complex (OCC) in the Shikoku back‐arc basin within the Philippine Sea Plate. Mantle peridotites (serpentinized) recovered by six dredge and submersible cruises exhibit signatures of extensive deformation. Amorphous pseudomorphs after plagioclase in many of the samples, as well as plagioclase‐spinel intergrowths, are clear evidence of melt stagnation and mantle reaction. Spinels show a wide range of compositions in terms of their Cr#, Mg#, and TiO2 content. The presence of apparently magmatic high‐temperature pargasitic amphibole in veins and as replacement of clinopyroxene suggests that it may be a primary or near‐primary mineral crystallized from a hydrous melt which is unusual for abyssal peridotites. Two trace‐element populations of clinopyroxenes are in equilibrium with depleted and enriched basaltic melts, respectively. Rare‐earth element (REE) in the most depleted clinopyroxenes are modeled by 10% fractional melting except for a ubiquitous La‐Ce “kick.” Multiple models of open system melting combined with subsequent mixing of an enriched melt can explain the REE data. Broadly it appears that the peridotites underwent variable degrees of partial melting with moderate influx of enriched melts, which agrees with the other textural and chemical evidence of melt‐rock reaction and re‐fertilization. The compositions of the accumulated melts simulated by the open system models reproduce the enrichments in fluid mobile elements (Ba, U, and Pb) observed in basalts dredged from the Shikoku basin. Back‐arc basin peridotites at Mado Megamullion appear to have a unique petrographic and geochemical character that is distinct from those of peridotites exposed at the seafloor after formation from mid‐ocean ridges.

Crustal fluids cause strong Lu-Hf fractionation and Hf-Nd-Li isotopic provinciality in the mantle of continental subduction zones

Abstract Metasomatized mantle wedge peridotites exhumed within high-pressure terranes of continental collision zones provide unique insights into crust-mantle interaction and attendant mass transfer, which are critical to our understanding of terrestrial element cycles. Such peridotites occur in high-grade gneisses of the Ulten Zone in the European Alps and record metasomatism by crustal fluids at 330 Ma and high-pressure conditions (2.0 GPa, 850 °C) that caused a transition from coarse-grained, garnet-bearing to fine-grained, amphibole-rich rocks. We explored the effects of crustal fluids on canonically robust Lu-Hf peridotite isotope signatures in comparison with fluid-sensitive trace elements and Nd-Li isotopes. Notably, we found that a Lu-Hf pseudo-isochron is created by a decrease in bulk-rock 176Lu/177Hf from coarse- to fine-grained peridotite that is demonstrably caused by heavy rare earth element (HREE) loss during fluid-assisted, garnet-consuming, amphibole-forming reactions accompanied by enrichment in fluid-mobile elements and the addition of unradiogenic Nd. Despite close spatial relationships, some peridotite lenses record more intense fluid activity that causes complete garnet breakdown and high field strength element (HFSE) addition along with the addition of crust-derived unradiogenic Hf, as well as distinct chromatographic light REE (LREE) fractionation. We suggest that the observed geochemical and isotopic provinciality between peridotite lenses reflects different positions relative to the crustal fluid source at depth. This interpretation is supported by Li isotopes: inferred proximal peridotites show light δ7Li due to strong kinetic Li isotope fractionation (−4.7–2.0‰) that accompanies Li enrichment, whereas distal peridotites show Li contents and δ7Li similar to those of the depleted mantle (1.0–7.2‰). Thus, Earth's mantle can acquire significant Hf-Nd-Li-isotopic heterogeneity during locally variable ingress of crustal fluids in continental subduction zones.

Multi-stage melting of enriched mantle components along the eastern Gakkel Ridge

The global endmember ultra-slow spreading Arctic Gakkel ridge is an ideal place to study mantle melting and the contributions of different mantle components to ridge volcanism. We carried out a high-resolution geochemical study of basalts from a seemingly normal section of the ultraslow-spreading Arctic Gakkel Ridge between 40°E and 60°E, which we refer to as EVZ2. While the majority of volcanics sampled from EVZ2 could be characterized as normal Mid-Ocean Ridge Basalts (NMORB), we identified a group of Isotopically Enriched, Incompatible Element Depleted MORB (IEDMORB) with low MREE/HREE ratios (i.e., “ghost garnet” signature). EVZ2 IEDMORB are mostly found near the rift valley walls away from axial volcanic centers. We propose that IEDMORB are the products of two stages of melting. They are shallow secondary melts of incompatible trace element enriched low-solidus mantle components that had lost some initial melt in the presence of residual garnet at depth. While the deep initial melts are more likely to be focused toward axial volcanic centers and subsequently diluted by normal peridotite melts, some of the shallow secondary melts could erupt as IEDMORB via pre-existing crustal weaknesses, such as deep-rooted high-angle normal faults that are ubiquitous along ultra-slow spreading ridges, thereby preserving their enriched isotopic compositions. While most dredges that sampled IEDMORB also recovered other types of MORB, all the samples from Dredge 55 are IEDMORB with distinctive arc-type trace element signatures, including relative depletion in Nb (and Ta), as well as enrichment in Th and fluid mobile elements (e.g., high Th/Nb, La/Nb, Pb/Ce, and H2O/Ce). These signatures suggest that they sampled recycled metasomatized arc-mantle wedge material. Other EVZ2 IEDMORB also show relative enrichment in fluid mobile elements and depletion in Nb (e.g., high La/Nb), but lack enrichment in Th. As Th has extremely low mobility in aqueous fluids, Th enrichment requires metasomatism involving silicate melts. Thus, we propose that the high Th/Nb, high La/Nb IEDMORB contain silicate-melt metasomatized arc mantle wedge material while the low-Th/Nb, high La/Nb IEDMORB only contain aqueous-fluid metasomatized arc mantle wedge material. Globally, IEDMORB with ghost garnet signatures are mostly found along ridges near mantle plumes where low-solidus, incompatible element and isotopically enriched mantle components that suffered initial melt loss at depth could be entrained in the upwelling subridge mantle and undergo further melting. However, most near-plume IEDMORB do not show arc-type geochemical signatures. Therefore, the discovery of IEDMORB from 20% of dredges along EVZ2, where most ridge volcanics are NMORB with depleted isotopic compositions, reflects the sporadic distribution of recycled arc mantle wedge material in the Arctic mantle and the prevalence of pre-existing crustal conduits, such as high-angle normal faults, along ultra-slow spreading ridges that facilitate melt migration with limited melt pooling and mixing.

Mineralogical and geochemical study of serpentinized peridotites from the North-Western Pyrenees: New insights on serpentinization along magma-poor continental passive margins

The peridotite massifs from the North-Western Pyrenean belt are fragments of a short-lived mid-Cretaceous hyper-extended rift inverted by the Pyrenean orogeny. We studied the petrology, mineralogy and geochemistry of 32 hydrothermally altered peridotites from the Montaut, Turon and Urdach massifs. This study aims to bring new constraints on the nature of serpentinization processes from domains having experienced various degrees of crustal hyper-extension and mantle exhumation in a context analogous to non-volcanic ocean-continent transitions.The three massifs studied are mainly composed of spinel-lherzolites and sample the same sub-continental mantle lithosphere as Eastern Pyrenean refertilized-peridotite massifs. They display contrasted degrees of hydrothermal alteration and serpentinization during cooling (e.g., serpentine contents ranging from 1 to 100 wt%) as well as variable reaction paths. The Montaut massif, located below an extremely thinned Paleozoic upper crust, records cooling from high temperature hydrothermalism with incipient serpentinization affecting mostly pyroxenes (serpentine, tremolite, talc, clinochlore assemblage; ~350–450 °C) to the formation of lizardite-chrysotile and magnetite after olivine (~200 °C–350 °C). The Turon peridotites, which remained below the detached Mesozoic pre-rift cover after complete removal of the crust, are little serpentinized and have no magnetite, suggesting serpentinization during the latest stages of cooling (< 200 °C). The Urdach massif comprises magnetite-bearing ophicalcites as well as magnetite-poor serpentinites formed concurrently when mantle was exhumed to the sub-seafloor at the margin toe (down to ~200 °C). Each massif shows selective enrichments in fluid mobile elements, in particular, Cs, Sb and Li, indicative of hydrothermal interactions with fluids deriving from neighboring sediments and continental crust, likely mixed with seawater-derived fluids for the Urdach peridotites. Our results show that the formation of magnetite in the studied samples is not tied solely to temperature, and point to a role of fluid composition in the development of the different serpentinization pathways observed from one massif to the other, and within a same massif in the case of Urdach.The development of complex sequences of temperature-, and composition-controlled reaction paths distinguish serpentinization in the Western Pyrenees peridotites from that commonly observed in the oceanic sub-seafloor. It results in variable redistribution of iron between magnetite and serpentine minerals, with the predominance of Fe-rich lizardite in magnetite-poor serpentinized peridotites. It likely controls local redox conditions and H2 production; suggesting that the impacts of serpentinization on the production of dihydrogen will vary from passive margins to oceanic sub-seafloor.

From peridotite to fuchsite bearing quartzite via carbonation and weathering: with implications for the Pb budget of continental crust

Extensive carbonation of peridotite results in listvenite, a rock composed of magnesite and quartz. At Gråberget, Røros, SE-Norway, a variably serpentinized peridotite body, surrounded by the Røros schists, a former abyssal sediment displays all stages of transformation of peridotite to quartzite. In this paper we record the sequence of steps in this process by combining the observation of mineral assemblages, textural relationships and geochemistry, and variations in Pb isotopic compositions. Initial serpentinization, a stage that also involved an enrichment in fluid-mobile elements (Pb, Sb and As), was followed by carbonation through CO2 fluids that formed soapstone, and eventually listvenite. The listvenite grades by decreasing amounts of carbonates into fuchsite bearing quartzite. The carbonates dissolved during supergene alteration and formed pores coated with oxides of Fe, Mn and Ni resulting in a brown rock color. The quartzite displays porous stylolites enriched in Pb, As and Sb and fuchsite with porous chromite grains as the only relicts of the original mineralogy in the peridotite. The dissolution of the carbonate occurred at oxidizing conditions at temperatures below 150 °C, where the solubility of magnesite is higher than that of quartz. Formation of quartzite from peridotite is supported by low REE contents and lack of zircons in the two rock types. The transformation involved enrichment of Pb, coupled with the elimination of Mg and enrichment of Si. This chemical fractionation and selective transfer of elements to the continents is an important mechanism and needs to be taken into account in models of continental evolution.

Slab-controlled progressive evolution of the Kudi back-arc ophiolite in response to the rollback of the Proto-Tethys oceanic slab, in Western Kunlun, NW Tibetan Plateau

Slab-derived components can significantly diversify subduction-related magmatism, for example, fore-arc ophiolites evolving from normal mid-ocean ridge basalts to fore-arc basalts and then to boninites. Here we show a progressive evolution of back-arc ophiolites controlled by slab-derived components but different from fore-arc ophiolites through studying the Kudi back-arc ophiolite in Western Kunlun, NW Tibetan Plateau. Our results reveal that the Kudi ophiolite was formed at 516–512 Ma in a back-arc setting in response to the rollback of the Proto-Tethys oceanic slab. The back-arc setting is supported by geochemistry of basalts (e.g., low Th/Nb and V/Ti ratios), low melting pressure (<10 kbar) revealed by amphibole geobarometer and low δ11B values (−19.1 to −8.1‰) of the mantle units. In such a scenario, the early-stage depleted Kudi basalts are characterized by depletions in light rare earth elements and enrichments in fluid-mobile elements, probably resulting from decompression melting of the primary back-arc mantle slightly modified by slab-derived fluids. Subsequently, partial melting of the back-arc mantle enriched by progressive addition of slab-derived sediments resulted in the formation of residual harzburgites and corresponding primary magmas that evolved into dunites, gabbros, quartz diorites and enriched pillow lavas. Additionally, both the residual harzburgites and cumulated dunites experienced minor impregnation of melts, without significant interstitial melt-crystal reactions. Compared with the early-stage depleted Kudi basalts, the late-stage enriched pillow lavas are characterized by enrichments in light rare earth elements and depletions in high field strength elements, typical arc affinities, confirming the addition of slab-derived sediments. Compared with typical fore-arc ophiolitic series, it is possible that there is a progressive evolution in back-arc basins in which a “new” enriched mantle source (corresponding arc basalts) modified by slab-derived sediments displaces an “old” depleted mantle source (corresponding basalts characterized by enrichments in fluid-mobile elements) modified by slab-derived fluids.

Molybdenum isotope ratios in Izu arc basalts: The control of subduction zone fluids on compositional variations in arc volcanic systems

Molybdenum isotope variations in mafic arc lavas have mainly been attributed to the influence of slab-derived components, such as subducted sediment melts and aqueous fluids. The latter have been hypothesised to fractionate Mo isotopes through interaction with the oceanic crust and carry an isotopically heavy signal that is transferred to the source of arc magmas. Thus, understanding Mo isotope systematics in subduction zones requires characterising the Mo isotope composition of slab-derived fluids and their influence on the Mo isotope budget of arc magmas. However, Mo isotope data reported to date show a considerable influence from subducted sediments that complicate accurate constraints being placed on the fluid contribution. We present Mo isotope data for mafic lavas from the Izu arc, a highly depleted oceanic island arc whose magma compositions show a dominant control from slab-derived fluids. The lavas from the Izu volcanic front are isotopically heavier than MORB and the depleted mantle. Their δ98/95Mo (the relative difference in measured 98Mo/95Mo to NIST 3134) systematically varies with indicators for fluid-mobile element enrichment, suggesting that slab-derived fluids in the Izu arc have heavy Mo isotope compositions. Additionally, co-variations with radiogenic 143Nd/144Nd and 176Hf/177Hf point to a relationship between the addition of aqueous fluids and compositional heterogeneity of the sub-arc mantle. We present mass balance models that show that the influence of subduction zone fluids on the trace element pattern of arc magmas is more dominant when these are added to a more depleted and refractory sub-arc mantle, which preferentially melts due to a relatively higher fluid flux. The mass balance of Mo in the Izu arc predicts a light Mo isotope composition for the residual oceanic crust as a result of the preferential removal of isotopically heavy Mo during slab dehydration, consistent with previous suggestions for the Mariana arc and isotopically light Mo previously reported for eclogites.

Geochemical fingerprints of seawater in the Late Mesoproterozoic Midcontinent Rift, North America: Life at the marine-land divide

The 1.1 Ga Midcontinent Rift (MCR) is a thick volcanic-sedimentary succession that forms a curvilinear belt through central North America and crops out along its northern apex around Lake Superior. Sedimentary units of the MCR have been long interpreted as fluvial-lacustrine and invited a number of studies on the early evolution of life in non-marine habitats. One of the key units is the siliciclastic Nonesuch Formation, thought to record deposition in a large lake. However, recent sedimentological observations indicate the presence of marine incursions. To further test this interpretation, we analysed trace element abundances in a broad suite of samples from multiple drill cores through the Nonesuch Formation. We aimed to differentiate geochemical influences of sediment provenance from post-depositional hydrothermal overprint and thereby identify authigenic enrichments in fluid-mobile elements that are indicators of primary environmental conditions. Our results reveal discrete enrichments in Mo and U in organic- and sulphide-rich horizons, which are most parsimoniously interpreted as marine signatures. This conclusion is supported by Sr/Ba ratios, which suggest mixing between freshwater and saltwater, and by rare cm-thick gypsum in the upper Copper Harbor Formation immediately below the Nonesuch rocks. The gypsum displays δ34S values of +25.9 ± 0.6‰, consistent with input of marine sulphate at least during parts of the basin's history. Collectively, our geochemical data support the sedimentological interpretation that this portion of the MCR archives a marine-influenced estuarine system. Although this conclusion rules out that microbial life documented from the MCR was living in exclusively freshwater habitats, the Nonesuch Fm and associated rocks still hold important clues about organisms that were capable of withstanding salinity gradients and bridging the gap between the marine and non-marine environments of the mid-Proterozoic.

From Decompression Melting to Mantle-Wedge Refertilization and Metamorphism: Insights from Peridotites of the Alag Khadny Accretionary Complex (SW Mongolia)

This study reports on mineral and bulk rock compositions of metaperidotites from the Alag Khadny accretionary complex in SW Mongolia, to reveal their nature and relationships with associated eclogites. The peridotites preserved original porphyroclastic textures and are composed of olivine, orthopyroxene relics, Cr-spinel, interstitial (not residual) clinopyroxene, and secondary chlorite, tremolite, olivine, Cr-magnetite, clinopyroxene, and antigorite. Cr-spinel has Cr# of 0.3–0.5, and primary olivine shows Mg# of 0.90–0.92. The pyroxenes are high-magnesian with low Al2O3 and Cr2O3. The bulk rocks have U-shaped normalized trace-element patterns with enrichment in LILE, L-MREE relative to HREE, and weak Pb–Sr peaks and Nb–Zr–Hf minima. Interstitial clinopyroxene exhibits V- and U-shaped normalized REE patterns with (La/Yb)N &gt; 1 (Yb = 1.2–3 of chondritic values) and enrichment in fluid-mobile elements and Zr. HREE abundances of clinopyroxene can be simulated by 23–26% partial melting of depleted mantle starting at garnet-facies (6–8%) depths, followed by hydrous or anhydrous melting at spinel-facies depths L-MREE characteristics of clinopyroxenes can be simulated by further interaction of harzburgites with an island-arc basaltic melt in a supra-subduction environment. The association of hydrous secondary minerals in the Alag Khadny peridotites suggests their retrograde metamorphism at 1.6–2.0 GPa and 640–720 °C, similar to P–T conditions reported earlier for the spatially associated eclogites. This supports metamorphism of the Alag Khadny peridotites in a mantle wedge, followed by joint exhumation of peridotites and eclogites. Given the findings above and implying the regional geological background, we advocate for a sequential Neoproterozoic evolution the Alag Khadny harzburgites from (1) their formation by decompression partial melting in an Early Neoproterozoic or older spreading center of a mid-ocean or back-arc setting, and (2) refertilization by supra-subduction melts, followed by (3) Late Neoproterozoic–Early Cambrian hydrous-fluid metamorphism and juxtaposition with eclogites.

The role of brucite in water and element cycling during serpentinite subduction – Insights from Erro Tobbio (Liguria, Italy)

The Erro Tobbio olivine-antigorite serpentinites and associated dehydration veins represent hydrated oceanic mantle rocks that escaped complete dehydration and recycling into the mantle after subduction to ~ 550–600 °C and 2.0–2.5 GPa. These rocks thus offer valuable insights into the petrological evolution of a slice of hydrated oceanic mantle and the geochemical cycling down to intermediate subduction zone depths. Our study emphasises the role of brucite upon rock-buffered hydration and subduction dehydration employing bulk and in situ chemical data sets combined with petrology.Bulk rock data reveal a coherent mantle peridotite slice affected by variable melt depletion and refertilisation. Subsequent fluid-rock interaction stages proceeded isochemically with respect to SiO2, i.e., without significant SiO2 enrichment characteristic for hydrothermal ocean floor serpentinisation. Relicts of low-T mesh textures after olivine and preservation of precursor mineral and low-T hydration geochemical features indicate a lack of subsequent fluid and metamorphic overprinting, even on scales of tens of micrometres. Fluid-mobile element enrichments are modest with exceptions for B and W. Enrichment signatures of U/Cs < 1 and Rb/Cs of 4–26 are characteristic of shallow forearc hydration within or atop the slab by fluids derived from breakdown of clays or first dehydration of altered oceanic crust with a subordinate sedimentary pore fluid component. Overall, the geochemical and petrological changes of the Erro Tobbio peridotites during fluid-rock interactions were rock-buffered, in contrast to fluid-buffered hydration accompanied with significant SiO2 metasomatism at, e.g., mid ocean ridges.Silica-neutral rock-buffered serpentinisation resulted in prominent brucite formation upon olivine hydration. In absence of excess SiO2, subsequent serpentine transformation of chrysotile/lizardite to antigorite likely produced even more brucite. Rock-buffered fluid-rock interactions thus provide a mechanism for stabilising brucite in subduction zone serpentinites, presumably along hydration fronts and within deeper sections of the oceanic lithospheric mantle. Finally, brucite + antigorite dehydration produced up to 40 vol% of metamorphic olivine and prominent olivine + Ti-clinohumite + magnetite vein networks at temperatures <550–600 °C, prior to complete antigorite breakdown. Wall rocks released alkali elements, B, Cr, As, Sb, and Ba into the dehydration fluids, along with substantial Sr, REE and HFSE redistribution into vein minerals.

Enrichment of H2O and fluid-soluble trace elements in the Troodos Ophiolite: Evidence for a near-trench origin

The Troodos Ophiolite on the island of Cyprus is a Cretaceous supra-subduction zone ophiolite. It formed by seafloor spreading in the vicinity of a subduction zone, but the exact tectonic setting in which it formed is debated. We determined H2O contents together with other volatiles (S, Cl and CO2) and Sr, Nd and Pb isotope compositions of fresh Troodos volcanic glasses, previously measured for major and trace element contents. Glass compositions range from boninite to tholeiitic andesite. The least degassed glasses have H2O contents of 1.7 to 2.7 wt%, and H2O/Ce ratios of 4400 to 33,600. The most depleted glasses with low Zr/Yb have the highest fractionation-corrected H2O contents and H2O/Ce, and the highest subduction zone input (highest Th/La, Ba/Zr and 87Sr/86Sr). The high H2O contents, enrichment in fluid-mobile elements, and depletion in Zr, Hf and heavy rare-earth elements are consistent with formation above a subduction zone, very close to a trench in a fore-arc position, likely at water depths of ~5000 m. The enrichment in both fluid-mobile (e.g. Rb, Cs) and melt soluble (e.g. Th) elements indicate that the Troodos Ophiolite formed by propagation of a back-arc rift into the fore-arc region of an active subduction zone.

Element Transfer and Redox Conditions in Continental Subduction Zones: New Insights from Peridotites of the Ulten Zone, North Italy

The orogenic peridotites and pyroxenites of the Ulten Zone (north Italy) record multistage metasomatism by crust-derived melts and fluids within a Late-Variscan mantle wedge. We acquired new major and trace element data as well as garnet and whole-rock iron speciation for a representative suite of samples, with the aim to further constrain element cycling and the redox state attending the development of the major mineralogical and textural rock types that occur within the Ulten Zone. Initially, spinel peridotites were refertilized by mafic melts in the hot and shallow mantle wedge, followed by garnet formation as the peridotites were carried towards a cool, subducting slab of continental crust by corner flow. Upon exhumation, ingress of aqueous, crust-derived fluids provoked amphibole-forming reactions, which caused gradual consumption of garnet and clinopyroxene and transformation from coarse- to fine-grained assemblages. Since Si, Al, Na, Ti, Ca and HREE formerly stored in reactants were not fully accommodated in newly-formed phases, these elements were partially removed from the bulk-rock, generating more depleted compositions resembling residues after partial melting. Unexpectedly, the remaining garnet retains low Fe3+/Sigma Fe (<0046) even after the bulk-rocks became strongly enriched in Fe3+ during metasomatism and retrogression (Fe3+/Sigma Fe = 011-023), which was mostly stored in coexistent amphibole and interstitial serpentine. Low Fe3+/Sigma Fe in garnet is consistent with Delta logfO(2) = FMQ-1.7 to FMQ-0.3 at 2GPa. Combined with previous studies, this is evidence for garnet growth within a heterogeneously oxidized mantle wedge, reflecting a variable extent of percolation by oxidizing aqueous fluids. During metasomatism, concomitant variable enrichment in LILE, LREE and some HFSE, and significant compositional differences between sampling localities, reflect both variable fluid/rock ratios at small spatial scales but also indicate chromatographic effects that likely relate to different positions relative to the subducting crust releasing fluids into the mantle wedge. Hydration by dilute fluids during retrogression did not result in additional enrichment in fluid-mobile elements, but caused further replacement of garnet and clinopyroxene. This study highlights the control that changing mineralogies, developed in response to interaction with various crustal melts and fluids under variable pressure-temperature and redox conditions in a continental subduction zone, exert on the retention or release of major and trace elements in peridotite. In particular, formation and/or persistence of amphibole and dolomite, as documented in the present study, suggest that the subduction-modified mantle wedge is an efficient trap for volatiles and fluid-mobile elements.

Circa 1 Ga sub-seafloor hydrothermal alteration imprinted on the Horoman peridotite massif

The chemical compositions of the residues of the mantle melting that produces mid-ocean ridge basalt can be altered by fluid–rock interactions at spreading ridges and, possibly, during seawater penetration along bending-related faults in plates approaching trenches. This chemically modified rock, if subducted deeply and after long-term residence within the deep Earth, is a potential source of chemical heterogeneity in the mantle. Here, we demonstrate that peridotites from the Horoman massif preserve the chemical signatures of sub-seafloor hydrothermal (SSH) alteration at a mid-ocean ridge approximately one billion years ago. These rocks have evolved chemically subsequent to this SSH alteration; however, they retain the SSH-associated enrichments in fluid mobile elements and H2O despite their long-term residence within the mantle. Our results indicate that ancient SSH alteration resulting in the production of sulfide leads to Pb enrichment that could affect the present-day Pb isotopic evolution of the silicate earth. Evidence from the Horoman massif of the recycling of hydrous refractory domains into the mantle suggests that both the flux of H2O content into the mantle and the size of the mantle H2O reservoir are higher than have been estimated recently.

Assessment of pyrite composition by LA-ICP-MS techniques from massive sulfide deposits of the Bathurst Mining Camp, Canada: From textural and chemical evolution to its application as a vectoring tool for the exploration of VMS deposits

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.

Fluid-mobile elements in serpentinites: Constraints on serpentinisation environments and element cycling in subduction zones

Fluid-mobile element (FME) systematics in serpentinites are key to unravel the environments of mantle rock hydration, dehydration, and element recycling in subduction zones. Here we compile serpentinite geochemical data and, for the first time, report discriminative FME enrichment trends for mid ocean ridge vs. forearc serpentinisation by applying alkali element-U ratios. Characteristic element fractionations are thereby governed by redox-dependent differential U mobility at mid ocean ridges and in forearcs, and by high Cs input in forearcs due to fluids equilibrated with sediments. Simple modelling reproduces the observed enrichment trends in serpentinites that range over several orders of magnitude. From these systematics, first constraints on potentially discriminative fractionation trends for unconventional fluid tracers such as B, As, and Sb can be deduced. Prominent W enrichments that correlate with FMEs suggest significant W mobility in low-temperature serpentinising environments.Application of the alkali element-U systematics to the subducted metaperidotites of Erro Tobbio (recording initial brucite+antigorite breakdown during subduction) and Almirez (recording final antigorite breakdown) reveal that pre-subduction FME enrichment signatures are retained in progressively subducted hydrous mantle rocks to beyond subarc levels. Associated dehydration veins and fluid inclusions reveal subordinate alkali element-U fractionation trends during dehydration. Subducted hydrous mantle rocks therefore may introduce characteristic element signatures and thus contribute towards mantle geochemical heterogeneities.