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Interactions between meteoric water and ultramafic rocks in the Oman Ophiolite generate waters of variable physicochemical characteristics. The discharge of these waters forms complex alkaline pool networks, in which mineral precipitation is triggered by mixing, evaporation, and uptake of atmospheric CO 2. A systematic and co-localized sampling of waters and solids in two individual spring sites allowed us to determine the saturation state of a range of minerals and correlate them to the different water and precipitate types. We subdivided the waters of the spring sites into three distinctive types: i) Mg-type; moderately alkaline (7.911.6), Ca2+–OH−-rich waters, and iii) Mix-type; alkaline to hyperalkaline (9.6 < pH < 11.5) waters with intermediate chemical composition. We first report the occurrence of hydrated magnesium (hydroxy-) carbonate phases in Mg-type waters. Nesquehonite forms in these waters via evaporation and transforms into dypingite and hydromagnesite under CO 2 -rich conditions. In Ca-type waters, the coupling of atmospheric CO 2 uptake with evaporation leads to the formation of a calcitic crystalline crust on the air-water interface. The crusts are aragonite- and brucite-bearing, where Mg-type and Ca-type waters discharge and vigorously mix at the same pool. Unlike the Mg-type and Ca-type waters, the pools of Mix-type waters host massive aragonite-dominated deposits due to high Mg/Ca ratio that favors the growth of aragonite over calcite. The hydrodynamics during mixing spatially control brucite precipitation and restrict its formation and accumulation around specific mixing zones, where a continuous supply of Mg of inflowing Mg-type waters takes place. Crystal morphologies record the effect on the values of supersaturation and supersaturation rates in the pools due to mixing processes, evaporation and CO 2 uptake. In Ca-type waters, CO 2 uptake and evaporation dictate the textural characteristics of calcite both in crystalline crusts and rock coatings. Textural evolution of aragonite from crystalline sheaves to spheroidal shapes underlines the different supersaturation rates of calcium carbonate crystallization in flocculent material of Mix-type waters. Geochemical models of mixing between Mg-type and Ca-type waters revealed the evolution of mineral saturation indices under various mixing proportions, and their relation to the observed mineralogy and geochemistry of the pool waters. The thorough documentation of mineral assemblages and crystal morphologies enabled us to provide a more detailed account of how water composition, mixing, and mineral precipitation co-evolve in the alkaline spring systems, where CO 2 is sequestered. Unlabelled Image • Serpentinization-driven carbonation in alkaline spring systems in ophiolites • Pool-by-pool geochemical-mineralogical-textural analysis coupled to hydrodynamics • Assessment of CO2 uptake, evaporation, mixing as the main precipitation mechanisms • Textures encode information about the formation conditions of the occurring phases. • Insights into hydrated Mg-carbonates in alkaline environments [ABSTRACT FROM AUTHOR]

We present a detailed study of the water geochemistry, mineralogy and textures in serpentinization-related hyperalkaline springs in the Ronda peridotites. Ronda waters can be classified into hyperalkaline fluids and river waters that are broadly similar to Ca2+–OH- and Mg2+–HCO 3 - water types described in serpentinite-hosted alkaline springs elsewhere. At the discharge sites of the fluids (fractures or human made outlets) and ponds along the fluid flow paths, the fluids are hyperalkaline (10.9 < pH < 12) and characterized by low Mg and high Na, K, Ca, and Cl- concentrations. River waters, occurring near the spring sites, are mildly alkaline (8.5 < pH < 8.9) and enriched in Mg and DIC compared to Na, K, Ca and Cl-. The chemistry of Ronda Mg–HCO 3 river waters is likely due to the hydrolysis of ferromagnesian peridotite minerals in equilibrium with the atmosphere by infiltrated meteoric water and shallow groundwater in the serpentinized peridotite. The Ronda Ca–OH hyperalkaline fluids are generated by the combination of low temperature serpentinization reactions from infiltrated surface Mg–HCO 3 river waters —or Ca–HCO 3 waters from near karst aquifers— and deep carbonate precipitation isolated from atmospheric CO 2. Mass balance calculations indicate that the weathering of Ca-bearing peridotite silicates, such as diopside, is a feasible source of Ca in Ronda Ca–OH hyperalkaline fluids; however, it requires steady-state dissolution rates substantially greater than those determined experimentally. Travertine, crystalline crusts and sediment deposits comprise the types of solid precipitates observed in Ronda hyperalkaline spring sites. Calcite, aragonite, dolomite and Mg-Al-rich clays are the main mineral phases identified in the spring sites. As illustrated in the Baños del Puerto spring site, (i) calcite-dominated precipitation is due to uptake of atmospheric CO 2 by Ca–OH hyperalkaline fluids during discharge, and (ii) aragonite-dominated precipitation is due to mixing of Ca–OH hyperalkaline fluids with Mg–HCO 3 river waters. Aragonite and dolomite contents increase away from the springs and toward the river waters that uniquely reflects the effect of Mg ions on the precipitation of aragonite versus calcite. Other potential factors controlling the precipitation of these CaCO 3 polymorphs are the Mg/Ca ratio, the CO 2 content, and the temperature of the fluids. Dolomite forms during lithification of travertine due to periodic flooding of river water combined with subsequent evaporation. • Serpentinization-driven carbonation in sub-continental lithospheric mantle peridotites. • CO 2 uptake, evaporation, and mixing are the main drivers of carbonate precipitation. • Role of Mg ions in the formation of aragonite vs. calcite. • Dolomite precipitation due to flooding events and subsequent evaporation of river waters. • Mineral textures encode information about the formation conditions of the occurring phases. [ABSTRACT FROM AUTHOR]

The Sabzevar ophiolite is the most extensive and best-exposed section of oceanic lithosphere in NE Iran. In this study, we examined the mantle section of the eastern part of the Sabzevar ophiolite, where mantle peridotites are the most widespread rock type and are crosscut by pyroxenite veins and mafic dykes. Major and trace element compositions (minerals and whole-rock) of lherzolites are akin to those of abyssal peridotites from mid-ocean ridges (MOR), whereas those of harzburgites evidence formation in a supra-subduction zone (SSZ) setting. Rare-earth element (REE) patterns of whole-rock and clinopyroxene of lherzolites and harzburgites suggest polybaric melting, with initial melting in the garnet peridotite field (~5–6%), followed by ~5–15% melting for lherzolites and ~10–20% for harzburgites in the spinel peridotite field. Harzburgites show LREE enrichment incompatible with partial melting models, indicating significant interaction with SSZ-related melts, whereas analysed lherzolites experienced limited melt-rock interaction. Pyroxene dissolution and olivine precipitation in residual peridotites resulting from reaction with island arc tholeiite and/or boninite melts led to the formation of dunites. These melts eventually intruded as dykes and veins in the mantle and crustal sections of the ophiolite. In addition, boninite melts are inferred to be responsible for forming olivine pyroxenite veins within the harzburgites. We conclude that the mantle section of the Sabzevar ophiolite formed in an intra-oceanic arc setting and experienced a geochemical evolution from a MOR-like to SSZ setting. The intra-oceanic subduction of the Neotethys oceanic lithosphere and shifting conditions of melting during the development of subduction beneath the incipient island arc were responsible for the geochemical heterogeneity of mantle peridotites and the generation of SSZ-related magmas within the Sabzevar oceanic lithosphere. [ABSTRACT FROM AUTHOR]

Abstract The Almirez ultramafic massif (SE Spain) preserves the transformation of high-P antigorite (Atg-) serpentinite to chlorite (Chl-) harzburgite (1.6–1.9 GPa; 680–710 °C), a metamorphic reaction that is the primary source of water at the intermediate depth of subducting slabs. We present a detailed μ-CT and EBSD study of oriented samples across the Atg-serpentinite dehydration isograd to investigate the textural evolution during serpentinite dehydration to peridotite and its relation to stress orientations and the kinematics of subducting slabs. Above the Atg-out isograd, Atg-serpentinite shows a prograde mylonitic foliation and a weak Shape Preferred Orientation (SPO) of oxide aggregates defining a N—S stretching lineation. The antigorite Crystal Preferred Orientation (CPO) is characterized by [001] Atg perpendicular to the foliation, and the poles to (100) Atg and (010) Atg distributed in a girdle-like symmetry with [100] Atg nearly parallel to the stretching lineation. The antigorite microstructure and CPO are consistent with deformation by dislocation creep, twinning, and dissolution-precipitation creep. These textures record the long-term shear deformation near the slab interface where the main compressive stress, σ 1 , was at an acute angle to the foliation. Below the Atg-out isograd, Atg-serpentinite dehydrated to unfoliated, coarse-grainedChl-harzburgite with granofels or spinifex textures distributed in alternating decameter-sized lenses. Crystallization of granofels and spinifex Chl-harzburgite records, respectively, a sequence of slow and fast fluid draining events during serpentinite dehydration under the same orientation of the principal stresses that resulted in the shear deformation of the Atg-serpentinite. Both textural types exhibit coarse-grained textures with systematic mineral CPOs and SPOs and microstructures without evidence of major ductile deformation. The texture of the granofels Chl-harzburgite formed by a topotactic dehydration reaction after Atg-serpentinite coupled to compaction leading to an olivine layering subparallel to the Atg-serpentinite foliation. The olivines of granofels Chl-harzburgite are rounded and display a weak CPO that can be accounted for by the topotactic reaction 〈100〉 Atg ||〈100〉 Ol and (001) Atg ||(010) Ol after Atg-serpentinite. Similarly, orthopyroxene shows a marked CPO consistent with the topotactic reaction (100) Opx ||(001) Atg and [001] Opx ||[100] Atg. Spinifex Chl-harzburgite displays systematic mineral SPO and CPO. Spinifex olivines are tabular on (100) Ol and elongated along [001] Ol (c > b >> a), and define a ESE–WNW platelet lineation. The average texture is characterized by [001] Ol,Opx subparallel to a strong ESE–WNW oxide aggregate lineation, and [100] Ol,Opx and [001] Ol,Opx within a plane of similar orientation to the Atg-serpentinite foliation. The SPOs and CPOs of spinifex Chl-harzburgites are composed of up to four orientation populations of tabular olivines, where one population is volumetrically dominant in all samples. Relative to the main orientation population, the CPO of olivine shows clustered distribution of [100] Ol and [010] Ol maxima rotated at systematic angles around [001] Ol. These clustered CPOs and SPOs show a remarkable correlation with the orientation of the principal paleostresses. The preponderant orientation population of tabular olivines lies on the plane of maximum compression (σ 2 –σ 3 plane) and has [010] Ol and its (100) Ol tabular faces nearly perpendicular to the Atg-serpentinite foliation. This population can be accounted for by oriented growth of platy crystals perpendicular to σ 1. The other populations lie in the plane perpendicular to the least compressive stress (σ 3) and the planes of maximum shear. The driving force and causes for the oriented crystallization of tabular olivine in these planes is uncertain, but their correlation with paleostresses suggests a cause-effect relationship. Spinifex and granofels Chl-harzburgites show a marked ESE–WNW oxide aggregate lineation that differs in orientation from that of Atg-serpentinite and is approximately parallel to the intermediate compressive stress (σ 2). These lineations and the platelet lineation of spinifex Chl-harzburgite may be due to along-strike fluid flow below the permeability barrier that constituted the Atg-out dehydration isograd. Our study shows that the kinematics of the slab, paleostresses, and fluid flow exert a dynamic control on the textures of Atg-serpentinite dehydrating to peridotite in subducting slabs. Graphical abstract Unlabelled Image Highlights • Detailed textural evolution during Atg-serpentinite dehydration to peridotite. • Stress, fluid draining rate, and fluid flow control the prograde peridotite texture. • Topotactic dehydration reaction governs the peridotite texture during slow draining. • Paleostresses correlate with the SPO-CPO of peridotite formed during fast draining. • Fluid flow may control mineral and aggregate lineations of prograde peridotite. [ABSTRACT FROM AUTHOR]

Fluids released through the dehydration of serpentinite can be rich in Cl−, which enables the significant mobility of Cr in subduction zones. However, the Cr isotope behavior accompanying the mobility of Cr during serpentinite dehydration is still poorly constrained. Here, we report high‐precision Cr isotope data for a unique suite of serpentinites that represent metamorphic products at different depths in oceanic subduction zones. Low‐grade serpentinites affected by significant Cr loss during serpentinization exhibit remarkably higher δ53Cr, while samples with Cr contents >∼1,800 ppm typically preserve mantle‐like δ53Cr. Antigorite serpentinites have an average δ53Cr value of −0.17‰ ± 0.19‰ (n = 12, 2SD), which is statistically lower than those of low‐grade serpentinite (−0.05‰ ± 0.30‰, n = 80, 2SD) and higher‐grade chlorite harzburgite (−0.10‰ ± 0.27‰, n = 22, 2SD). This suggests that resolvable Cr isotope fractionation occurs during serpentinite dehydration, which is explained by the variability of Cr isotope behavior in the presence of Cl‐bearing fluids at different dehydration stages. No obvious Cr isotope fractionation was found during chlorite harzburgite dehydration, probably related to the limited Cr mobility in a Cl‐poor fluid. Other processes, such as melt extraction, external fluid influx and retrograde metamorphism, have negligible effects on the Cr isotope systematics of meta‐serpentinites. Fluids released by serpentinite dehydration may have a great effect on the Cr isotope heterogeneity of mantle wedge peridotites and arc magmas. Plain Language Summary: Serpentinite plays an important role in crust–mantle interaction and arc magmatism in oceanic subduction zones. Serpentinite‐derived fluids potentially mobilize Cr in the form of Cl complexes. However, there is limited information available about the Cr isotope compositions of subducted serpentinites, hindering the understanding of Cr isotope behavior during the dehydration of serpentinite. Here, we first report high‐precision Cr isotope data for a unique suite of (meta‐)serpentinite that encompasses low‐pressure oceanic serpentinite to metamorphic products of complete dehydration at great depth. Our results fill the gap in the Cr isotope composition of meta‐serpentinites metamorphosed under diverse P–T conditions in oceanic subduction zones. We found that there is variable and significant Cr isotope fractionation during prograde dehydration of serpentinites. Such serpentinite‐derived fluids may contribute to the Cr isotope heterogeneity of mantle wedge peridotites and arc magmas. Key Points: First high‐precision Cr isotope compositions on a unique suite of meta‐serpentinitesOceanic serpentinites with significant Cr loss tend to exhibit remarkably higher δ53CrSignificant Cr isotope fractionation occurs during prograde dehydration of serpentinites in subduction zones [ABSTRACT FROM AUTHOR]

Field observations and U–Pb LA–ICP–MS zircon ages of magmatic outcrops in the Goaïda area, south-eastern part of Moroccan Central Massif (Meseta domain), provide firm evidence of Neoproterozoic silicic magmatism in the basement of the Moroccan Variscan belt. This magmatic activity produced three distinct plutons —dark, pink and white granitoids—, which form a Neoproterozoic basement strongly reworked in the Carboniferous and intruded by Hercynian gabbro, dolerite and rhyolite bodies. The Goaïda dark granitoids are syeno–granodiorites, whereas the pink and white granitoids are granites sensu stricto . All the facies of Goaïda granitoids have I-type, high-K and calc-alkaline affinities. Concordant U–Pb zircon data yield Lowermost Ediacaran mean age for white granites (625 ± 9 Ma) and Ediacaran mean ages for dark granitoids (600 ± 10 Ma) and pink granites (552 ± 10 Ma). White granites record old high-K calc-alkaline magmatism that started in the Anti-Atlas belt at ca. 620 Ma, coinciding with the end of the second period of the Pan-African orogenesis. On the other hand, the dark and pink granitoids formed in subduction-related to post-collisional regimes during the final stage of the Pan-African orogenesis, namely the WACadomian orogeny. We combine new field, geochronological and geochemical data with results from the literature to propose a new lithostratigraphic reconstruction of the Precambrian to Early Cambrian times in the Moroccan Meseta front at the NW margin of the West African Craton. The presence of Precambrian outcrops —including Paleoproterozoic and Neoproterozoic rocks— in the Moroccan Variscan belt demonstrates the continuity of the peri-Gondwanan Panafrican basement at least from the eastern Anti-Atlas to the Central Meseta domain. Along with the lack of Paleozoic ophiolitic suture and subduction-related metamorphic rocks, this continuity supports the hypothesis that the Moroccan Variscan mountains were part of the NW margin of Gondwana in the Paleozoic. [ABSTRACT FROM AUTHOR]

Spinel (± plagioclase) peridotite xenoliths from the Tallante and Los Perez volcanic centres in the eastern Betics (SE Spain) range from depleted (clinopyroxene-poor) harzburgites to fertile (clinopyroxene-rich) lherzolites and orthopyroxene-free wehrlites. Significantly, only one harzburgite, which is depleted in heavy rare earth elements (HREE), retains the imprint of ca. 20% ancient melting of an original garnet lherzolite source. In contrast, REE abundances of other harzburgites and lherzolites from the eastern Betics have been increased by melt-rock reaction. The whole-rock and mineral compositions of these mantle rocks are largely controlled by three types of modal metasomatism: 1) common clinopyroxene-orthopyroxene addition and olivine consumption which increased FeOt, SiO 2 and Al 2 O 3 , and decreased MgO compared to the refractory melting products; 2) subordinate orthopyroxene dissolution and precipitation of clinopyroxene and olivine, which led to higher FeOt and MgO and lower SiO 2 than in common (orthopyroxene-rich) lherzolites; and 3) rare orthopyroxene consumption and olivine addition that caused higher FeOt and lower SiO 2 compared to the original melting residues. These mineral modal and major element variations have been produced mostly by interactions with relatively FeOt-rich/SiO 2 -poor melts, likely derived from a peridotite-pyroxenite lithospheric mantle with a highly heterogeneous isotopic composition. Melting of the lithospheric mantle in the western Mediterranean was triggered by upwelling of the asthenosphere induced by back-arc extension in the Late Oligocene-Early Miocene. Trapping of small fractions of exotic melts in whole-rocks — likely the parental magmas of Miocene back-arc dykes that intruded the Betic crust — caused local disequilibrium between the trace element signatures and Pb isotopic compositions of clinopyroxene and whole-rock. Subsequent interaction with SiO 2 -undersaturated magmas, similar to the parental melts of the Pliocene alkali basalts that host the xenoliths, promoted orthopyroxene consumption and clinopyroxene-olivine enrichment at locations close to magma conduits, and finally generated orthopyroxene-free wehrlites. This event constitutes the last episode of the Cenozoic magmatic evolution of the westernmost Mediterranean which is recorded in the mantle xenoliths from the eastern Betics. [ABSTRACT FROM AUTHOR]

The Beni Bousera peridotite contains a diversity of pyroxenite layers. Several studies have postulated that at least some of them represent elongated strips of oceanic lithosphere recycled in the convective mantle. Some pyroxenites were, however, ascribed to igneous crystal segregation or melt–rock reactions. To further constrain the origin of these rocks, we collected 171 samples throughout the massif and examined their variability in relation with the tectono-metamorphic domains. A major finding is that all facies showing clear evidence for a crustal origin are concentrated in a narrow corridor of mylonitized peridotites, along the contact with granulitic country rocks. These peculiar facies were most likely incorporated at the mantle–crust boundary during the orogenic events that culminated in the peridotite exhumation. The other pyroxenites derive from a distinct protolith that was ubiquitous in the massif before its exhumation. They were deeply modified by partial melting and melt–rock reactions associated with lithospheric thinning. [ABSTRACT FROM AUTHOR]

We report microstructural evidence for fluid-assisted ductile strain localization in a ≤ 50 m-wide mylonitic–ultramylonitic shear zone in the Ronda Peridotite massif, Southern Spain. Strain localization occurred at relatively low pressure (< 0.8 GPa) and moderate temperature (750–1000 °C). Initial deformation by dislocation creep resulted in formation of mylonites. Focusing of aqueous fluids in the shear zone favored the activation of dissolution–precipitation creep, resulting in further strain localization. This process is recorded by two generations of ultramylonitic bands composed of fine-grained, well-mixed olivine–orthopyroxene aggregates. Microstructural observations in the ultramylonites suggest alternating dissolution and precipitation of olivine and orthopyroxene, which may be explained by local changes in silica molality of the percolating fluid (disequilibrium and mass transfer at scales > mm). In the mylonites, olivine shows a crystal preferred orientation (CPO) coherent with dominant (001)[100] glide, probably due to the presence of interstitial fluids during deformation. In the ultramylonites, olivine CPO is weak to very weak, consistently with a decreasing contribution of dislocation creep to deformation. In contrast, fine-grained orthopyroxene in both mylonites and ultramylonites displays a clear CPO characterized by a [001] maximum normal to the foliation, which is not consistent with dislocation glide in any known slip system for orthopyroxene. We interpret this CPO as formed by oriented crystallization during dissolution–precipitation. In the present study, dissolution–precipitation creep predominates only in small-scale ultramylonite bands due to limited fluid availability and localized dynamic permeability. However, this process may be important in intermediate temperature domains of subduction zones, where it may lead to a feedback between strain localization and fluid transport. [ABSTRACT FROM AUTHOR]

We report gabbroic dikes in the plagioclase tectonite domains of the Ojén and Ronda massifs (Betic Cordillera, southern Spain), which record crystallization at low-pressure syn-, or slightly postkinematic to the late ductile history of the Betic Peridotite in the westernmost Mediterranean. We present mineral major and trace element compositional data of discordant gabbroic dikes in the Ojén massif and gabbroic patches in the Ronda massif, complemented by the whole rock and electron backscattered diffraction (EBSD) data of the Ojén occurrence. In the Ojén massif, gabbro occurs as 1–3 centimeter wide discordant dikes that crosscut the plagioclase tectonite foliation at high angle. These dikes are composed of cm-scale igneous plagioclase and clinopyroxene crystals that show shape preferred orientations subparallel to the lineation of the host peridotite and oblique to the trend of the dike. Intrusion of Ojén gabbro dikes is coherent with the stress field that formed the high temperature, ductile plagioclase tectonite foliation and then attests for a mantle igneous event prior to the intracrustal emplacement of the massif. In the Ronda massif, gabbroic rocks crystallized in subcentimeter wide anastomozing veins, or as interstitial patches in the host dunite. They are mostly composed of plagioclase and clinopyroxene. Plagioclase composition is bytownitic in the Ojén, and andesinic in the Ronda massif. Clinopyroxene in both places shows identical, light Rare-Earth Element (LREE) depleted trace element patterns. The calculated trace element composition of melts in exchange equilibrium with the studied igneous clinopyroxenes reflects LREE-enriched character coupled with negative Eu anomaly, and indicates that gabbro-forming melts in Ronda and Ojén share a common melt source with an island arc tholeiitic affinity. Geothermobarometric data and liquidus mineralogy indicate that gabbro crystallization occurred at shallow depths (0.2–0.5 GPa) in a 7–16 km thick lithospheric section. These data suggest that gabbro-forming melts in the Betic Peridotite record a mantle igneous event at very shallow depths and provide evidence for the hyperextension of the continental lithosphere compatible with extreme backarc basin extension induced by the slab rollback of the Cenozoic subduction system in the westernmost Mediterranean. [ABSTRACT FROM AUTHOR]

We report a new occurrence of melt inclusions in polymetamorphic granulitic gneisses of the Jubrique unit, a complete though strongly thinned crustal section located above the Ronda peridotite slab (Betic Cordillera, S Spain). The gneissic sequence is composed of mylonitic gneisses at the bottom and in contact with the peridotites, and porphyroblastic gneisses on top. Mylonitic gneisses are strongly deformed rocks with abundant garnet and rare biotite. Except for the presence of melt inclusions, microstructures indicating the former presence of melt are rare or absent. Upwards in the sequence, garnet decreases whereas biotite increases in modal proportion. Melt inclusions are present from cores to rims of garnets throughout the entire sequence. Most of the former melt inclusions are now totally crystallized and correspond to nanogranites, whereas some of them are partially made of glass or, more rarely, are totally glassy. They show negative crystal shapes and range in size from ≈ 5 to 200 μm, with a mean size of ≈ 30–40 μm. Daughter phases in nanogranites and partially crystallized melt inclusions include quartz, feldspars, biotite and muscovite; accidental minerals include kyanite, graphite, zircon, monazite, rutile and ilmenite; glass has a granitic composition. Melt inclusions are mostly similar throughout all the gneissic sequence. Some fluid inclusions, of possible primary origin, are spatially associated with melt inclusions, indicating that at some point during the suprasolidus history of these rocks granitic melt and fluid coexisted. Thermodynamic modeling and conventional thermobarometry of mylonitic gneisses provide peak conditions of ≈ 850 °C and 12–14 kbar, corresponding to cores of large garnets with inclusions of kyanite and rutile. Post-peak conditions of ≈ 800–850 °C and 5–6 kbar are represented by rim regions of large garnets with inclusions of sillimanite and ilmenite, cordierite-quartz-biotite coronas replacing garnet rims, and the matrix with oriented sillimanite. Previous conventional petrologic studies on these strongly deformed rocks have proposed that anatexis started during decompression from peak to post-peak conditions and in the field of sillimanite. The study of melt inclusions shows, however, that melt was already present in the system at peak conditions, and that most garnet grew in the presence of melt. [ABSTRACT FROM AUTHOR]

Abstract: We combined microstructural observations and high-resolution crystallographic preferred orientation (CPO) mapping to unravel the active deformation mechanisms in garnet clinopyroxenites, garnet–spinel websterites, and spinel websterites from the Beni Bousera peridotite massif. All pyroxenites display microstructures recording plastic deformation by dislocation creep. Pyroxene CPOs are consistent with dominant slip on [001]{110} in clinopyroxene and on [001](100) or [001](010) in orthopyroxene. Garnet clinopyroxenites have however high recrystallized fractions and finer grain sizes than spinel websterites. Recrystallization mechanisms also differ: subgrain rotation dominates in garnet clinopyroxenites, whereas in spinel websterites nucleation and growth also contribute. Elongated shapes and strong intracrystalline misorientations suggest plastic deformation of garnet, but CPOs are weak. Clinopyroxene porphyroclasts in spinel websterites show deformation twins underlined by orthopyroxene exsolutions. Thermodynamic calculations indicate that garnet clinopyroxenites deformed at 2.0GPa and 950–1000°C and spinel pyroxenites at 1.8GPa and 1100–1150°C. The lower temperatures may explain the faster work rates implied by the finer grained microstructures in garnet clinopyroxenites. Greater stresses may have also reduced the competence contrast between garnet and pyroxene in the garnet pyroxenites and, at the outcrop scale, lowered the competence contrast between pyroxenites and peridotites, favoring mechanical dispersion of pyroxenites in the cooler lithospheric mantle. [Copyright &y& Elsevier]

Abstract: Veins with titanclinohumite (Ti-Chu) and olivine occur close to clinopyroxenite layers scattered through the antigorite serpentinites from the Cerro del Almirez ultramafic massif (Sierra Nevada, Betic Cordillera, S. Spain). They formed at estimated maximum P-T conditions of 1.3 GPa and 475 °C, due to simultaneous breakdown of former Ti-rich clinopyroxene and brucite in the presence of 6 vol.% of free water. Breakdown of Ti-Chu single crystals to an olivine+ilmenite framework was gradual: small relict patches of Ti-Chu appear even at some distance from the reaction front and ilmenite grains coarsen and become less frequent away from the front. Detailed TEM data support this interpretation: olivine in equilibrium with Ti-Chu is structurally and chemically homogeneous, whereas Ti-Chu shows ubiquitous intermixed (001) olivine lamellae. Product olivine close to the front shows pervasive and continuous (001) Ti-Chu lamellar faults. These become less frequent away from the reaction front, due to the completion of the Ti-Chu breakdown. This textural sequence suggests both progressive transfer of Ti to ilmenite and progressive release of H2O during the breakdown of Ti-Chu. A second hydration breakdown reaction can be observed very locally, in the contact between Ti-Chu and diopside, producing a symplectitic intergrowth of tremolite and ilmenite. Textural features of both reactions point to a very limited availability of water during Ti-Chu breakdown. This suggests that veins operated as closed systems. Complete obliteration of Ti-Chu owing to the antigorite dehydration breakdown can be observed in the Almirez massif rocks. This precludes OH-Ti-Chu from Atg-serpentinite as an efficient water or titanium carrier into deep mantle in hot subduction zones. [Copyright &y& Elsevier]

Spinel peridotite xenoliths (one plagioclase-bearing) hosted in alkaline basalts from Tallante (southeast Spain) record the mineralogical and geochemical fingerprint of the subcontinental lithospheric mantle (SCLM) evolution beneath the southern Iberian margin. Mantle metasomatism in fertile lherzolites caused the crystallization of clinopyroxene + orthopyroxene + spinel clusters through the percolation of Miocene subalkaline melts during the westward migration of the subduction front in the western Mediterranean. In the Pliocene, heat and volatiles provided by alkaline host-magmas triggered very low melting degrees of metasomatic pyroxene-spinel assemblages, producing melt quenched to silicate glass and reactive spongy coronae around clinopyroxene and spinel. Refertilization of the Tallante peridotites induced the precipitation of base-metal sulfides (BMS) included in metasomatic clino- and orthopyroxene. These sulfides consist of pentlandite ± chalcopyrite ± bornite aggregates with homogeneous composition in terms of major elements (Ni, Fe, Cu) and semi-metals (Se, As, Te, Sb, Bi), but with wide variability of platinum-group elements (PGE) fractionation (0.14 < PdN/IrN < 30.74). Heterogeneous PGE signatures, as well as the presence of euhedral Pt-Pd-Sn-rich platinum-group minerals (PGM) and/or Au-particles within BMS, cannot be explained by conventional models of chalcophile partitioning from sulfide melt. Alternatively, we suggest that they reflect the incorporation of distinct populations of BMS, PGM, and metal nanoparticles (especially of Pt, Pd, and Au) during mantle melting and/or melt percolation. Therefore, we conclude that Miocene subalkaline melts released by asthenosphere upwelling upon slab tearing of the Iberian continental margin effectively stored metals in metasomatized domains of this sector of the SCLM. Remarkably high Au concentrations in Tallante BMS (median 1.78 ppm) support that these metasomatized domains provided a fertile source of metals, especially gold, for the ore-productive Miocene magmatism of the westernmost Mediterranean. [ABSTRACT FROM AUTHOR]

Crystal morphologies are essential for deciphering the reaction history of igneous and metamorphic rocks because they often record the interplay between nucleation and growth rates controlled by the departure from equilibrium. Here, we report an exceptional record of the morphological transition of olivine formed during subduction metamorphism and high-pressure dehydration of antigorite-serpentinite to prograde chlorite-harzburgite in the Almirez ultramafic massif (Nevado–Filábride Complex, Betic Cordillera, SE Spain). In this massif, rare varied-textured chlorite-harzburgite (olivine+enstantite+chlorite+oxides) —formed after high–P dehydration of antigorite-serpentinite— exhibits large olivine porphyroblasts made up of rounded cores mantled by coronas of tabular olivine grains, similar to single tabular olivines occurring in the matrix. The correlative X-ray μ-CT and EBSD study of two varied-textured chlorite-harzburgite samples show that tabular olivine in coronas is tabular on (100) Ol with c > b >> a, and grew in nearly the same crystallographic orientation as the rounded olivine cores of the porphyroblast. Quantitative textural analysis and mass balance indicate that varied-textured chlorite-harzburgite is the result of a two-stage nucleation and growth of olivine during the progress of the high–P dehydration of antigorite-serpentinite to chlorite-harzburgite reaction. The first stage occurred under a low affinity (ΔG r) and affinity rate (ΔG r /dt) of the antigorite dehydration reaction that resulted in a low time-integrated nucleation rate and isotropic growth of olivine, forming rounded olivine porphyroblasts. With further progress of the dehydration reaction, a second stage of relatively higher affinity and affinity rate resulted in a higher time-integrated nucleation rate of olivine coeval with a shift from isotropic to anisotropic olivine growth, leading to tabular olivines. The two-stage evolution resulted in olivine porphyroblasts made up of rounded cores mantled by coronas of tabular olivine grains characteristic of varied-texture chlorite-harzburgite. Although a switch to anisotropic tabular olivine in the second stage is consistent with the relative increase in the affinity and affinity rate, these changes cannot solely account for the growth of Almirez olivine tabular on (100). Tabular olivines in komatiites and other igneous rocks are tabular on (010) Ol with either a > c >> b, or a ≈ c > > b, in agreement with experimentally determined growth rates of olivine phenocrysts under moderate to high undercooling and cooling rates. On the other hand, olivine tabular on (100) is expected in the presence of highly polymerized fluids where inhibited growth of the olivine (100) and (010) interfaces occurs, respectively, due to dissociative and molecular adsorption of water monolayers. Rounded and tabular olivines in Almirez varied-textured chlorite-harzburgite show differing trace element compositions that we interpret as due to the infiltration of external fluids during antigorite dehydration. Isothermal infiltration of highly polymerized fluids would explain the shift in the affinity and affinity rate of the antigorite dehydration reaction, as well as the olivine morphology tabular on (100) due to the inhibited growth on the (100) and, to a lesser extent, (010). Our study shows that surface-active molecules may play an essential role in shaping the morphology of growing crystals during fluid-present metamorphic crystallization. Unlabelled Image • Morphological transition of olivine during serpentinite dehydration to peridotite • Rounded olivine morphology records near-equilibrium slow crystallization • Tabular olivine morphology is a result of rapid growth at fast reaction kinetics. • Reported habit, tabular on (100) with c > b> > a, differs from common tabular olivine. • Surfactants in infiltrating fluids selectively inhibited growth on (100) and (010). [ABSTRACT FROM AUTHOR]

Highly acidic phosphogypsum wastes with elevated potential for contaminant leaching are stack-piled near coastal areas worldwide, threatening the adjacent environment. Huge phosphogypsum stacks were disposed directly on the marshes of the Estuary of Huelva (SW Spain) without any impermeable barrier to prevent leaching and thus, contributing to the total contamination of the estuarine environment. According to the previous weathering model, the process water ponded on the surface of the stack, initially used to carry the waste, was thought to be the main washing agent through its infiltration and subsequently the main component of the leachates emerging as the edge outflows. Preliminary restorations have been applied to the site and similar ones are planned for the future considering process water as the only pollution agent. Further investigation to validate the pollution pathway was necessary, thus an evaluation of the relationship between leachates and weathering agents of the stack was carried out using stable isotopes (δ 18 O, δ 2 H, and δ 34 S) as geochemical tracers. Quantification of the contribution of all possible end-members to the phosphogypsum leachates was also conducted using ternary mixing via the stable isotopic tracers. The results ruled out ponded process water as main vector of edge outflow pollution and unveiled a continuous infiltration of estuarine waters to the stack implying that is subjected to an open weathering system. The isotopic tracers revealed a progressive contribution downstream from fluvial to marine signatures in the composition of the edge outflows, depending on the location of each disposal zone within the different estuarine morphodynamic domains. Thus, the current study suggests that the access of intertidal water inside the phosphogypsum stack, for instance through secondary tidal channels, is the main responsible for the weathering of the waste in depth, underlying the necessity for new, more effective restorations plans. [ABSTRACT FROM AUTHOR]

This study aims to explore the impact of chronic metal exposure derived from persistent pollution from mining activity using human gallstones as proxies. The samples were obtained from patients residing in geologically and environmentally contrasting areas in the Province of Huelva, SW Spain, allowing for the evaluation of the regional effect of metal pollution. The study group resides in the Iberian Pyrite Belt characterized by natural and anthropogenic metal pollution from mining activities, whereas the control group resides in the Ossa Morena Zone famous for its natural parks. A total of 68 gallstones were first classified based on their phase composition and structure and subsequently their chemical composition was studied using solution Inductively Coupled Plasma-Mass Spectrometry. The metal concentrations increased in the cholesterol-rich gallstones from pure, to mixed and composite cholesterol stones along with the increasing amount of minor phases, such as bilirubinate, carbonate, and phosphate. These cholesterol stones did not show an evident enrichment tendency. On the contrary, pigment stones, composed of bilirubinate, carbonate, and phosphate phases, were rich in a variety of elements and the regional comparison showed that the pigment stones from the study area were enriched in sulfide-associated metal(loid)s, Mn, Fe, Cu, Zn, Sr, As, Ag, Sb, and Pb with respect to the control group. Inhalation of polluted airborne particulate matter is considered as one of the main exposure routes among the residents of the study area. Additionally, consumption of local water and locally produced food products such as fruit and vegetables and dermal contact may be possible sources of exposure, but no direct connection was observed. [ABSTRACT FROM AUTHOR]

The ultramafic massifs of the Serranía de Ronda (namely Ronda, Ojén and Carratraca) are portions of Proterozoic (∼1.2–1.8 Ga) subcontinental lithospheric mantle (SCLM) affected by partial melting and infiltration of melts. The latter of these events was broadly coeval with the tectonic emplacement of the peridotites into the continental crust in the Early Miocene. This resulted in the formation of chromite and Ni-arsenide ores (Cr-Ni) associated with orthopyroxenites and cordieritites. Six zircons recovered from a massive chromitite sample from the Ronda massif yield both concordant and discordant ages between 2309 ± 37 Ma and 109 ± 15 Ma, and δ 18 O between 8.3‰ and 9.4‰. Two Proterozoic ages obtained for zircons of this population (1815 ± 9 Ma and 1794 ± 17 Ma) are identical, within error, to those of zircons reported previously in the garnet pyroxenites of Ronda (1783 ± 37 Ma). Similarly, concordant Early Jurassic (192 ± 13 Ma) and Cretaceous ages (109 ± 15 Ma) obtained from the core and rim, respectively, of a single zircon from the chromitite are also consistent with the ages (180 ± 5 Ma, 178 ± 6 Ma, and 131 ± 3 Ma) already reported for magmatic zircons from corunudum-bearing garnet pyroxenites in the Ronda massif. The observation that chromitites and garnet-pyroxenites contain similar populations of zircons suggests that the parental melts of chromitites inherited zircons from their protolithic garnet pyroxenites, representing relics of oceanic/arc crust recycled in the mantle. Eleven zircons recovered from a massive cordieritite associated with chromitite in the Ronda massif yield scattered concordant and discordant ages between 568 Ma and 21 Ma, with correspondingly variable δ 18 O (4.8–13.5‰) and unradiogenic Hf-isotope ratios (ε Hf (t) from −12.36 to −4.43). The youngest age is concordant at 21.18 ± 0.4 Ma and matches the ages of zircons from the chromitite (weighted average age of 20.4 ± 0.87 Ma, n = 4) and a plagioclasite dyke (scattering between 20.1 ± 0.2 Ma and 17.9 ± 0.1 Ma; n = 11) associated with the Cr-Ni mineralization in the Ojén massif. These zircons show similar unradiogenic Hf-(ε Hf (t) between −14.5 and −7.6) and heavy O-isotope compositions (δ 18 O = 11.3–12.4‰). A sample of the massive cordieritite hosting the chromitites contains abundant zircons that yield scattered concordant, sub-concordant and discordant U-Pb ages varying from 33.8 ± 1 Ma to 781 ± 10 Ma; these zircons (n = 21) have variable U-contents (105–13900 ppm) and Th/U ratios (0.003–0.8). On the basis of O- and Hf-isotope compositions, these zircons define three populations independently of their ages: (1) grains with consistent high δ 18 O (6.1–12.7‰) and negative ε Hf (t) (from −14.42 to −6.88); (2) grains with high δ 18 O (7.6–11.1‰) and positive ε Hf (t) (3.10–4.84); and (3) grains with δ 18 O < 5.5‰ typical of mantle values. We suggest that zircons from this cordieritite with U-Pb ages older than Miocene are inherited, and were incorporated physically into the SCLM by fluids or melts produced during dehydration-melting of the crustal rocks wrapping the peridotite massifs. The population of Early Miocene zircons found in the chromitites and associated cordieritites and the plagioclasite dyke in the mineralization of the Ojén massif date the crustal emplacement of the peridotites and, therefore, the formation of the Cr-Ni ores. We propose a model in which the unique Cr-Ni mineralizations found in the ultramafic rocks of the Serranía de Ronda were formed as a result of contamination of the SCLM with crustal components. [ABSTRACT FROM AUTHOR]

We report an integrated field, petrographic, and geochemical study of three major Cretaceous carbonatite annular occurrences from the Oulad Dlim massif (Adrar Souttouf, NW margin of the West African Craton, south Morocco): Twihinate, Lamlaga, and Lahjayra. These complexes are commonly considered isolated carbonatite structures partially to completely overlain by silica breccias and iron oxide rocks. The carbonatites include two calcio‑carbonatites facies with distinct textural features and bulk-rock and mineral geochemistry: the "white" facies present characteristics consistent with a primary magmatic stage, while the "grey" facies likely resulted from hydrothermal alteration of the white carbonatites. We provide evidence that the annular structures include ultrapotassic alkaline syenites, probably derived from the same primary carbonated/silicated melt as the carbonatites, after partial melting of a K-rich lithospheric mantle source. The overlying silica breccias retain sandstone/quartzite-like stratifications, whereas their trace element signature is analogous to those of the carbonatites, suggesting that they were subjected to intense hydrothermal activity during carbonatite magmas emplacement. The iron-oxide rocks are chemically comparable to worldwide references of carbonatite-related laterite profiles, indicating a similar origin. All subsolidus (hydrothermal and weathering) alteration stages were accompanied by significant REE-Nb concentration resulting in high-grade deposits of monazite, synchysite, apatite and pyrochlore. Like other peri-Atlantic alkaline occurrences, the investigated Cretaceous alkaline‑carbonatite occurrences are likely related to the Atlantic Ocean opening and reactivation of pre-existing Pan-African tectonic structures. [Display omitted] • The NW segment of West African craton hosts Cretaceous carbonatite structures associated with alkaline silicate rocks. • The Twihinate calcite carbonatites are overprinted by late hydrothermal stage, inducing secondary ore formation. • Hydrothermal/weathering processes formed mineralized silica breccia and iron oxide caps, shaping the carbonatite structures. • Western Oulad Dlim massif holds high-grade ores and may comprise a world-scale metallogenic province of critical elements. [ABSTRACT FROM AUTHOR]

This study explores the environmental impact of metal exposure on humans through detailed phase and structural characterization of gallstones from two environmentally contrasting populations in Huelva Province (SW Spain). A total of 42 gallstone samples, obtained after surgical intervention at the Riotinto Hospital, were studied by powder X-ray diffraction (XRD), Fourier Transformed Infra-Red spectroscopy (FTIR), FTIR-μ-ATR (Attenuated Total Reflection) coupled with an optical microscope, and by Environmental Scanning Electron Microscope with Energy Dispersive X-ray Spectroscopy (ESEM-EDS), and subsequently classified according to their phase composition and structure. Additionally, the patients were enquired for their living habits in order to analyze the source of possible exposure to metal contamination. The gallstones were classified into pure, mixed and composite cholesterol stones, black and brown pigment stones, and carbonate stones. The patients from the study group residing in a region with acknowledged metal contamination of both natural and anthropogenic origin have a higher risk of metal exposure through contaminated soil, particle matter in the air, and consumption of local water and food products. According to our findings, the metal exposure is related to a higher tendency of forming black pigment stones in the study group in comparison to the control group residing in a natural park with nearly pristine environmental conditions. Moreover, the gallstones from the study group showed to contain more abundant metal components, such as Cu, Fe, Ni, and Zn, than those from the control group. To our knowledge this is the first study to examine the regional environmental impact of metal exposure on human gallstones. [ABSTRACT FROM AUTHOR]

The study of the composition of primary melts during anatexis of high-pressure granulitic migmatites is relevant to understand the generation and differentiation of continental crust. Peritectic minerals in migmatites can trap droplets of melt that forms via incongruent melting reactions during crustal anatexis. These melt inclusions commonly crystallize and form nanogranitoids upon slow cooling of the anatectic terrane. To obtain the primary compositions of crustal melts recorded in these nanogranitoids, including volatile concentrations and information on fluid regimes, they must be remelted and rehomogenized before analysis. A new occurrence of nanogranitoids was recently reported in garnets of mylonitic metapelitic gneisses (former high pressure granulitic migmatites) at the bottom of the prograde metamorphic sequence of Jubrique, located on top of the Ronda peridotite slab (Betic Cordillera, S Spain). Nanogranitoids within separated chips of cores and rims of large garnets from these migmatites were remelted at 15 kbar and 850, 825 or 800 °C and dry (without added H2O), during 24 h, using a piston cylinder apparatus. Although all experiments show glass (former melt) within melt inclusions, the extent of rehomogenization depends on the experimental temperature. Experiments at 850–825 °C show abundant disequilibrium microstructures, whereas those at 800 °C show a relatively high proportion of rehomogenized nanogranitoids, indicating that anatexis and entrapment of melt inclusions in these rocks likely occurred at pressures ≤1.5 GPa and temperatures close to 800 °C. Electron microprobe and NanoSIMS analyses show that experimental glasses are leucogranitoid and peraluminous, though define two distinct compositional groups. Type I melt inclusions correspond to K-rich, Ca- and H2O-poor leucogranitic melts, whereas type II melt inclusions represent K-poor, Ca- and H2O-rich granodioritic to tonalitic melts. Type I and II melt inclusions are found in most cases at the cores and rims of large garnets porphyroclasts, respectively. We tentatively interpret these two distinct melt compositions as suggesting that these former migmatites underwent two melting events under contrasting fluid regimes, possibly during two different orogenic periods. This study demonstrates the strong potential of melt inclusions studies in migmatites and granulites in order to unravel their anatectic history, particularly in strongly deformed rocks where most of the classical anatectic microstructures and macrostructures have been erased during deformation. [ABSTRACT FROM AUTHOR]

The Middle Atlas is a region where xenolith-bearing volcanism roughly coincides with the maximum of lithospheric thinning beneath continental Morocco. It is therefore a key area to study the mechanisms of lithospheric thinning and constrain the component of mantle buoyancy that is required to explain the Moroccan topography. Samples from the two main xenolith localities, the Bou Ibalghatene and Tafraoute maars, have been investigated for their mineralogy, microstructures, crystallographic preferred orientation, and whole-rock and mineral compositions. While Bou Ibalghatene belongs to the main Middle Atlas volcanic field, in the ‘tabular’ Middle Atlas, Tafraoute is situated about 45 km away, on the North Middle Atlas Fault that separates the ‘folded’ Middle Atlas, to the South-East, from the ‘tabular’ Middle Atlas, to the North-West. Both xenolith suites record infiltration of sub-lithospheric melts that are akin to the Middle Atlas volcanism but were differentiated to variable degrees as a result of interactions with lithospheric mantle. However, while the Bou Ibalghatene mantle was densely traversed by high melt fractions, mostly focused in melt conduits, the Tafraoute suite records heterogeneous infiltration of smaller melt fractions that migrated diffusively, by intergranular porous flow. As a consequence the lithospheric mantle beneath Bou Ibalghaten was strongly modified by melt–rock interactions in the Cenozoic whereas the Tafraoute mantle preserves the record of extensional lithospheric thinning, most likely related to Mesozoic rifting. The two xenolith suites illustrate distinct mechanisms of lithospheric thinning: extensional thinning in Tafraoute, where hydrous incongruent melting triggered by decompression probably played a key role in favouring strain localisation, vs. thermal erosion in Bou Ibalghatene, favoured and guided by a dense network of melt conduits. Our results lend support to the suggestion that lithospheric thinning beneath the Atlas mountains results from the combination of different mechanisms and occurred in a piecewise fashion at a short wavelength scale. [ABSTRACT FROM AUTHOR]

The Cerro del Almirez massif (Nevado–Filábride Complex, Betic Cordillera, SE Spain) is composed of antigorite (Atg-) serpentinite and chlorite (Chl-) harzburgite separated by a thin reaction front formed in a palaeo-subduction setting. These ultramafic rocks preserve unique prograde structures related to the pre- and peak high-pressure event (1.6–1.9 GPa and 680–710 °C). The oldest subduction-related structures are preserved in Atg-serpentinites: a penetrative S 1 foliation and associated L 1 stretching lineation that formed in a non-coaxial regime with a top-to-the-W sense of shear. This planar–linear fabric is crosscut by olivine ± Ti-clinohumite veins, formed during the prograde breakdown of brucite and pre-metamorphic clinopyroxene, which form a system of veins decimetres long. They record embrittlement processes due to the release of 6 vol.% of H 2 O associated with brucite dehydration. The growth of prograde olivine and/or tremolite porphyroblasts is syn- to post-kinematic in relation to the S 1 foliation. Further reactions at higher temperature related to the complete breakdown of the Atg (i.e. Atg-out) are post-kinematic to S 1 . Prograde Chl-harzburgite is crosscut by sets of conjugate zones associated with grain-size reduction of olivine grains. These grain-size reduction zones are interpreted as brittle structures generated by hydrofracturing due to overpressure fluids in a compressional setting at low-differential stresses. Structures related to the exhumation process are mainly preserved in the metasedimentary host rock, where an S 2 /L 2 planar–linear fabric developed within a shear zone dominated by a non-coaxial regime with a top-to-the-W sense of movement in a transpressional regime. Peak metamorphic conditions deduced for the schists are similar in pressure (1.3–1.9 GPa) but lower in temperature (560–590 °C) compared to the ultramafic rocks in contact with them, suggesting a major shear zone at the base of the ultramafic massif during D 2 deformation in a right-handed transpressional regime. [ABSTRACT FROM AUTHOR]

Field observations within the Atg-serpentinite domain of the subducted ultramafic massif from Cerro del Almirez (SE Spain) reveal the existence of two generations of abundant olivine-rich veins formed as open, mixed mode and shear fractures during prograde metamorphism. Type I veins were synchronous with the development of the serpentinite main foliation (S 1) and shearing, whereas Type II veins post-date the S 1 surfaces. These structural relationships indicate that, while the Atg-serpentinites underwent ductile plastic deformation at temperatures of 450°-600 °C and pressures of 0.7–1.7 GPa, they also experienced punctuated brittle behaviour events. The brittle fractures were most likely due to fluid overpressures formed by release of H 2 O during the brucite breakdown reaction for the case of Type I veins (2 vol % H 2 O) and due to a combination of minor dehydration reactions related to continuous compositional and structural changes in antigorite (0.3 vol % H 2 O) for Type II veins. Type II olivine-rich veins were formed by brittle failure in a well-defined paleo-stress field and were not significantly deformed after their formation. Comparison of the principal paleo-stress orientation inferred from Type II veins with those formed at peak metamorphic conditions in the ultramafic rocks at Cerro del Almirez shows a relative switch in the orientation of the maximum and minimum principal paleo-stress axes. These relative changes can be attributed to the cyclic evolution of shear stress, fluid pressure and fault-fracture permeability allowing for stress reversal. • Two types of Ol-rich veins formed as open, mixed mode and shear fractures. • Type I veins formed coetaneous with the main foliation by shearing, while Type II veins post-date the S 1. • Di veins and faults post-date the Ol-rich veins. • Brittle fractures formed by fluid overpressures during the dehydration reactions. [ABSTRACT FROM AUTHOR]

Abstract: A model for the incorporation of alumina in FeO–MgO–Al2O3–SiO2–H2O (FMASH) serpentinites has been developed by considering ideal Tschermak (Al2Mg−1Si−1) solid solution in antigorite. The antigorite model has been calibrated by fitting the experimental conditions for the decomposition of antigorite to chlorite+olivine+orthopyroxene+fluid in the FMASH system. The antigorite Al-contents predicted with this model are in agreement with natural observations and suggest a maximum alumina solubility in antigorite of 3.6wt.% Al2O3 at 20kbar–650°C and of 4.5wt.% Al2O3 at 3kbar–560°C. In the assemblage antigorite–olivine–chlorite–fluid, the Al-content of antigorite is buffered and temperature sensitive. This temperature sensitivity is the basis for a serpentinite geothermometer at greenschist, amphibolite and eclogite facies conditions. The buffered assemblage is stable in harzburgite compositions for relatively moderate amounts of Al2O3 (>1.8wt.%) and is widespread in lherzolites, where it occurs together with diopside or, in a narrow temperature field, with tremolite. [Copyright &y& Elsevier]

We summarize the uptake of carbon and sulfur during serpentinization of seafloor peridotites, and discuss the fate of these volatiles during subduction of serpentinite. We use a simplified classification to divide seafloor serpentinization into high-temperature and low-temperature processes. High-temperature serpentinization typically involves heat and mass transfer from gabbro intrusions, leading to addition of hydrothermal sulfide sulfur (up to >1wt.%) having high δ34S values (+5 to +10‰). Total carbon contents of bulk rocks are elevated (0.008–0.603wt.%) compared to mantle values and δ13CTotal C values of −3‰ to −17.5‰ result from mixtures of organic carbon and seawater-derived carbonate. Low-temperature serpentinization is generally characterized by microbial reduction of seawater sulfate, which leads to addition of sulfide sulfur (up to 1.4wt.%) having negative δ34S values (down to −45‰), although local closed-system conditions can lead to reservoir effects and positive δ34S values (up to +27‰). Extensive circulation of cold seawater can cause oxidation, loss of sulfide, and addition of seawater sulfate resulting in high δ34STotal-S values. High total carbon contents (0.006–7.2wt.%) and δ13C values of −26 to +2.2‰ result from addition of variable proportions of organic carbon and seawater-derived carbonate to serpentinite. We estimate that serpentinization at mid ocean ridges is a sink for 0.35–0.64×1011 molCy−1 and 0.13–1.46×1011 molSy−1, comparable to the sinks of these elements per unit volume of mafic oceanic crust. Serpentinization in the subducting plate at subduction zones may further affect chemical budgets for serpentinization. During subduction metamorphism, sulfur and carbon contents remain unaffected by recrystallization of seafloor lizardite and chrysotile to antigorite, and formation of minor olivine. Dehydration of antigorite-serpentinites to chlorite–harzburgites at higher pressure and temperature results in loss of 5wt.% water, and an average of 260ppm sulfur is lost as sulfate having δ34S=14.5‰, whereas carbon is unaffected. These volatiles can induce melting and contribute to 34S enrichments and oxidation of the sub-arc mantle wedge. Serpentinized oceanic peridotites carry isotopically fractionated water, carbon and sulfur into subduction zones. Up to 0.49×1011 molsulfury−1 and 1.7×1011 molcarbony−1 are subducted in serpentinites, less than 3% of the total subduction budgets for each of these elements. Isotopically fractionated carbon, sulfur, and water remain in serpentinite dehydration products, however, and can be recycled deeper into the mantle where they may be significant for volatile budgets of the deep Earth. [ABSTRACT FROM AUTHOR]

Abstract: Small (<5m) chromitite bodies in the Ojén peridotite massif (southern Spain) were formed when small fractions of volatile-rich melts, residual after kilometric‐scale infiltration through the subcontinental mantle, reacted with Cr-rich pyroxenite enclosed in dunite. The Os isotopic compositions of accessory platinum-group minerals (PGM) and base-metal minerals (BMM, including sulfides and arsenides) in chromitite and the host peridotite have similar values and ranges of heterogeneity. The formation of chromitite, which involved partial melting, melt transport/pooling and melt–rock reactions, did not erase the original 187Os/188Os heterogeneity of the peridotite source. The distribution of Os model ages calculated for chromitite-hosted PGM–BMM mainly clusters at ~0.3Ga; minor peaks extend back to ~1.4Ga. Base-metal sulfides (BMS) hosted in peridotite show a relatively minor age peak at ~0.2Ga and two prominent peaks at ~0.6 and 1.2Ga. Independent geochronological data constrain the formation of chromitites at 21Ma ago, suggesting that these older model ages reflect the sourcing of Os from volumes of older subcontinental lithospheric mantle that underwent several episodes of melting and metasomatism since the Mesoproterozoic. We interpret the different distributions of 187Os/188Os and model ages in chromitite and peridotite as due to the combined control that mantle source heterogeneity, melting and melt/rock interaction processes exert on the Os-isotope composition of the parental melts of chromitite. [Copyright &y& Elsevier]

Abstract: We present new data on spinel peridotite xenoliths hosted in Agardag alkaline lamprophyres from the Sangilen Plateau (Tuva, South Siberia, Russia), sampling at ~450Ma the subcontinental lithospheric mantle of the Tuva-Mongolian micro-continent that belongs to the accretionary Central Asian orogenic belt at the southern edge of the Siberian craton. Xenoliths are spinel lherzolites principally showing poikilitic and subordinately coarse granular and coarse equigranular textures. Geothermobarometric calculations for pyroxene yield a narrow range of equilibration temperature (ca. 1000–1100°C) that corresponds to lithospheric depths from 43 to 53km (1.3–1.6GPa) along a hot intracontinental geotherm. Variation of mean Mg# [100*Mg/(Mg+Fe)] of olivine (87.9–90.9) with mean Cr# [100*Cr/(Cr+Al)] of spinel (9.5–45.7) indicates that spinel lherzolites are mostly residues of up to 10% melting of a fertile peridotite source. In terms of normalized REE (Rare Earth Element) and incompatible trace element patterns of clinopyroxene, the Sangilen xenoliths can be classified into three types: Type I characterized by convex-upward REE patterns depleted in LREE (0.10≤La/YbN ≤0.49), and with relative negative anomalies of Rb, Pb, Hf, Zr and Ti and positive spikes of U and Sr; Type II displaying variable LREE/HREE ratios (0.53

Abstract: IODP Hole U1309D (Atlantis Massif, Mid-Atlantic Ridge 30°N) is the second deepest hole drilled into slow spread gabbroic lithosphere. It comprises 5.4% of olivine-rich troctolites (~>70% olivine), possibly the most primitive gabbroic rocks ever drilled at mid-ocean ridges. We present the result of an in situ trace element study carried out on a series of olivine-rich troctolites, and neighbouring troctolites and gabbros, from olivine-rich intervals in Hole U1309D. Olivine-rich troctolites display poikilitic textures; coarse-grained subhedral to medium-grained rounded olivine crystals are included into large undeformed clinopyroxene and plagioclase poikiloblasts. In contrast, gabbros and troctolites have irregularly seriate textures, with highly variable grain sizes, and locally poikilitic clinopyroxene oikocrysts in troctolites. Clinopyroxene is high Mg# augite (Mg# 87 in olivine-rich troctolites to 82 in gabbros), and plagioclase has anorthite contents ranging from 77 in olivine-rich troctolites to 68 in gabbros. Olivine has high forsterite contents (82–88 in olivine-rich troctolites, to 78–83 in gabbros) and is in Mg–Fe equilibrium with clinopyroxene. Clinopyroxene cores and plagioclase are depleted in trace elements (e.g., Ybcpx ~5–11×Chondrite), they are in equilibrium with the same MORB-type melt in all studied rock-types. These compositions are not consistent with the progressively more trace element enriched (evolved) compositions expected from olivine rich primitive products to gabbros in a MORB cumulate sequence. They indicate that clinopyroxene and plagioclase crystallized concurrently, after melts having the same trace element composition, consistent with crystallization in an open system with a buffered magma composition. The slight trace element enrichments and lower Cr contents observed in clinopyroxene rims and interstitial grains results from crystallization of late-stage differentiated melts, probably indicating the closure of the magmatic system. In contrast to clinopyroxene and plagioclase, olivine is not in equilibrium with MORB, but with a highly fractionated depleted melt, similar to that in equilibrium with refractory oceanic peridotites, thus possibly indicating a mantle origin. In addition, textural relationships suggest that olivine was in part assimilated by the basaltic melts after which clinopyroxene and plagioclase crystallized (impregnation). These observations suggest a complex crystallization history in an open system involving impregnation by MORB-type melt(s) of an olivine-rich rock or mush. The documented magmatic processes suggest that olivine-rich troctolites were formed in a zone with large magmatic transfer and accumulation, similar to the mantle-crust transition zone documented in ophiolites and at fast spreading ridges. [Copyright &y& Elsevier]

[Display omitted] • The Central High-Atlas Mesozoic magmatism and related apatite ores have been studied. • Vein-type apatite ores specially hosted in differentiated alkaline rocks (syenites). • Mineral analyses of apatite veins were used to study differences in apatite deposits. • Hydrothermal fluids percolating through country sedimentary rocks have been recorded. Five main apatite deposits from the Moroccan Central High-Atlas —including Anemzi, Tirrhist-Inouzane, Tassent, Ait-Daoud-Toumliline and Tasraft— have been investigated through integrated field, petrographic, mineralogical, and geochemical studies. The apatite ore occurs as veins, few mm to several cm thick, spatially associated with syenite-quartz monzonite (dominant) to monzodiorite-gabbrodiorite bodies emplaced during the Atlas Jurassic-Cretaceous alkaline magmatic event. Gem-quality apatite crystals, up to 15 cm in size, are associated with albite, clinopyroxene (hedenbergite–augite–diopside), amphibole (hornblende–edenite–actinolite), K-feldspar, quartz, magnetite, titanite, epidote, prehnite, and calcite. Two types of apatite have been distinguished according to their halogen contents: F-rich and Cl-rich apatites. Both types are enriched in REE (up to 2 wt% ΣREE), and have major and trace element contents consistent with a magmatic origin, corroborated by the chemical similarity with accessory apatite in the host alkaline intrusions, as well as with other reference igneous apatites. However, hydrothermal fluids percolating through country sedimentary rocks were also involved, as indicated by the striking Na enrichment observed in the Cl-rich apatites, likely inherited from Triassic evaporites, showing close spatial relationships with the alkaline intrusions. These fluids would also account for pervasive Na-metasomatism of alkaline magmatic rocks, as observed in the wall rocks of the apatite veins. [ABSTRACT FROM AUTHOR]

The Styrian Basin is located at the westernmost part of the Carpathian-Pannonian region, in the transition zone between the Pannonian Basin and the Eastern Alps. The lithospheric mantle beneath the Styrian Basin (SB) was sampled by Plio-Pleistocene alkali basalts, which brought mantle xenoliths to the surface. Mantle xenoliths from the SB are mostly coarse granular, amphibole-bearing spinel lherzolites with microstructures indicating extensive annealing. Three geochemical events were recorded in the SB xenoliths. The initial, ancient partial melting event was followed by an old metasomatism, which formed lithological heterogeneities (e.g. websterite, dunite) occurring as veins, bands and layers. The most recent geochemical event recorded in the SB xenoliths is the migration of a hydrous alkaline melt originating from a nephelinitic melt source. This melt migrated from the asthenosphere, forming melt channels in the lithospheric mantle. Close to the channels, the metasomatic agent caused extensive amphibole and sparse phlogopite formation. This particular pargasite-phlogopite equilibrium mineral assemblage in the spinel facies was not described before in the literature based on our knowledge. The reaction caused significant enrichment in basaltic elements (such as Ti, Fe), K, light rare earth elements and incompatible trace elements such as Zr and Hf. Further from the channels, the metasomatic melt became enriched in volatiles (mainly H 2 O and CO 2) and fluid mobile elements (e.g. U, Pb, Cl and P), but depleted in basaltic elements, K and LREE, which resulted in modal decrease of amphiboles. Previous studies suggested a dehydrating subducted slab beneath the Styrian Basin, however geochemical fingerprints of subducted slab derived fluids were recognized only in one xenolith. Mantle portions with different geochemical characteristics developed in the lithosphere laterally, however metasomatic fingerprints were erased later due to annealing and chemical re-equilibration at ambient mantle conditions. The Neogene evolution of the lithosphere overwrote the previous state of the upper mantle beneath the region, thus the signs of the most recent metasomatism by nephelinitic melts could have been only preserved due to this extensive annealing event. • Mantle beneath the Styrian Basin was affected by multiple metasomatic enrichments. • Neogene annealing in the mantle overwrote the signs of older metasomatic processes. • Only the recent metasomatism by nephelinitic melts is preserved in the xenoliths. • Mantle portions with different geochemical characteristics developed laterally. [ABSTRACT FROM AUTHOR]

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

Ti-rich amphibole, Mg-rich ilmenite, baddeleyite, zirconolite, srilankite, and zircon are important high-field-strength elements (HFSE) bearing phases in the Potosí chromitite bodies located in the Moho Transition Zone of the Cretaceous Moa-Baracoa suprasubduction zone ophiolite (eastern Cuba). Such HFSE-bearing phases were found in the interaction zone between gabbroic intrusions and chromitite pods. In addition to HFSE-bearing minerals, the studied samples are composed of Fe3+ and Ti-rich chromite, olivine (Fo 86-90), clinopyroxene (En 44-49), plagioclase (An 51-56), orthopyroxene (En 84-94), F-rich apatite, and Fe-Cu-Ni sulfides. The studied ilmenite hosting Zr oxides (baddeleyite, zirconolite, and srilankite) contains up to 13 wt.% MgO. The Potosí zirconolite is the first record of this mineral in ophiolitic chromitites and non-metamorphic ophiolite units, and it has relatively high REE contents (up to 10 wt.% of REE 2 O 3) and the highest concentrations in Y 2 O 3 (up to 11 wt.%) reported so far in zirconolite from terrestrial occurrences. Zircon is observed forming coronas surrounding ilmenite grains in contact with silicate minerals, and is characterized by very low U and Pb contents. The zircons formed after high temperature Zr diffusion in ilmenite (exsolution) and a subsequent reaction along grain boundaries following crystallization. Finally, U-Pb dating of baddeleyite exsolutions within ilmenite yielded an average age of 134.4 ± 14 Ma, which provides the first ever dating for a metasomatic event in Potosí that matches well (within uncertainty) the formation age of the oceanic crust of the eastern Cuba ophiolite. We propose that the occurrence of HFSE- and REE-bearing minerals in the Potosí chromite deposit is the result of a two stage process: first, water-rich and HFSE-rich residual melts are produced by intercumulus crystal fractionation after an evolved MORB (BABB)-like melt; and secondly, these residual melt fractions escaped the solidifying mush and extensively reacted and metasomatized the surrounding chromitites, crystallizing HFSE- and REE-bearing minerals and Fe-Cu-Ni sulfides. • HFSE-bearing minerals found in metasomatized chromitites from eastern Cuba ophiolite • Zr diffusion and reaction along ilmenite grain boundaries formed zircon coronas • Modal metasomatism was triggered by infiltration of evolved MORB (BABB)-like melts • The metasomatism age matches the formation age of the ophiolite oceanic crust [ABSTRACT FROM AUTHOR]

Salt marshes are natural deposits of heavy metals in estuarine systems, where sulphide precipitation associated with redox changes often results in a natural attenuation of contamination. In the present study, we focus on the effects of variable redox conditions imposed to a highly-polluted phosphogypsum stack that is directly piled over the salt marsh soil in the Tinto River estuary (Huelva, Spain). The behaviour of contaminants is evaluated in the phosphogypsum waste and in the marsh basement, separately, in controlled, experimentally-induced oscillating redox conditions. The results revealed that Fe, and to a lesser extent S, control most precipitation/dissolution processes. Ferric iron precipitates in the form of phosphates and oxyhydroxides, while metal sulphide precipitation is insignificant and appears to be prevented by the abundant formation of Fe phosphates. An antagonistic evolution with changing redox conditions was observed for the remaining contaminants such as Zn, As, Cd and U, which remained mobile in solution during most of experimental run. Therefore, these findings revealed that high concentrations of phosphates inhibit the typical processes of immobilisation of pollutants in salt-marshes which highlights the elevated contaminant potential of phosphogypsum wastes on coastal environments. • Estuarine salt-marsh systems are geochemically controlled by redox conditions. • In the Huelva Estuary, phosphogypsum wastes are deposited directly on salt marshes. • The waste and the marsh soil geochemistry was simulated under redox oscillations. • Fe-phosphate abundance hinders sulphide formation and contaminants' immobilisation. • Natural attenuation is impeded, leading to high pollution risk to the environment. [ABSTRACT FROM AUTHOR]

Crustal recycling in the Earth's mantle is fingerprinted by trace-element and isotopic proxies in oceanic basalts. Positive Eu and Sr anomalies in primitive lavas and melt inclusions that are not otherwise enriched in Al 2 O 3 are often interpreted as reflecting the presence of recycled, plagioclase-rich oceanic crust in their mantle source - referred to as "ghost plagioclase" signatures. Here, we report natural evidence of Eu anomalies and extreme crystal-scale heterogeneity developed kinetically in mantle peridotite clinopyroxene. Numerical modelling shows that diffusional fractionation between clinopyroxene and melts can account for this intra-crystal heterogeneity and generate Eu anomalies without requiring plagioclase. We demonstrate that kinetically induced Eu anomalies are likely to develop at temperatures, redox conditions and transport timescales compatible with the genesis of mid-ocean ridge and ocean island basalts. Our results show that, in the absence of converging lines of evidence such as radiogenic isotope data, ghost plagioclase signatures are not an unequivocal proxy for the presence of recycled crust in oceanic basalt sources., (© 2023. The Author(s).)

Huelva is a highly industrialized city in SW Spain hosting, among others, a Cu smelter, a phosphate fertilizer plant, a power plant, and oil refineries. This study aims to evaluate metal concentrations in lichens as bioindicators of atmospheric pollution in the impacted urban areas. Xanthoria parietina species from Huelva and nearby villages, as well as reference samples from remote, non-contaminated urban areas, were analyzed for trace elements (V, Cr, Mn, Co, Ni, Cu, Zn, Sr, As, Cd, Sb, Cs, Ba, La, Ce, Pr, Nd, Sm, Er, Tm, Yb, Lu, Pb, Th, U) using Inductively Coupled Plasma-Mass Spectrometry; and for major elements (Ca, K, Mg, P, and S) by Inductively Coupled Plasma-Optical Emission Spectrometry after acid digestion. The metal composition of X. parietina exhibits spatial distribution patterns with extremely elevated concentrations (Co, Ni, Cu, Zn, As, Cd, Sb, Ba, Pb, U, and S) in the surroundings of the industrial estates to <1 km distance. Mean concentrations were significantly lower in the urban areas >1 km from the pollution sources. However, air pollution persists in the urban areas up to 4 km away, as the mean concentrations of Cu, Zn, As, Cd, Sb and S remained considerably elevated in comparison to the reference samples. Though rigorous source apportionment analysis was not the aim of this study, a good positive correlation of our results with metal abundances in ambient particulate matter and in pollution sources points to the Cu smelter as the main source of pollution. Hence, the severe air pollution affecting Huelva and nearby urban areas may be considered a serious health risk to local residents., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

This research reports the effects of pH increase on contaminant mobility in phosphogypsum leachates by seawater mixing, as occurs with dumpings on marine environments. Acid leachates from a phosphogypsum stack located in the Estuary of Huelva (Spain) were mixed with seawater to achieve gradually pH7. Concentrations of Al, Fe, Cr, Pb and U in mixed solutions significantly decreased with increasing pH by sorption and/or precipitation processes. Nevertheless, this study provides insight into the high contribution of the phosphogypsum stack to the release of other toxic elements (Co, Ni, Cu, Zn, As, Cd and Sb) to the coastal areas, as 80-100% of their initial concentrations behaved conservatively in mixing solutions with no participation in sorption processes. Stable isotopes ruled out connexion between different phosphogypsum-related wastewaters and unveiled possible weathering inputs of estuarine waters to the stack. The urgency of adopting effective restoration measures in the study area is also stressed., (Copyright © 2018 Elsevier Ltd. All rights reserved.)