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Processes taking place in subduction zones are the main controller of the chemical cycle of volatile and incompatible elements in the Earth system. Metamorphic devolatilization reactions occurring during slab burial play a key role in the transfer of elements to the supra-subduction mantle, from forearc to sub-arc depth. Here, we discuss the elements released in fluids and melts from oceanic (i.e., sediments, altered oceanic crust, and hydrated lithospheric mantle) and continental slab materials during prograde subduction and the consequent implications in the chemical evolution of the supra-subduction mantle. We use bulk data and fluid and melt inclusions analyses to show and to constrain the mobility of elements from top to bottom in the subduction zone setting. The development of mélange domains at the slab-mantle interface and its influence in the element cycle are also taken into account. Coupled with trace-element mobility, we review the redox evolution of slab materials during subduction and its implication in the redox conditions of the supra-subduction mantle due to fluid and melt infiltration down to sub-arc depth.

Peridotites and different types of eclogites occurring in the Monte Duria area (Adula-Cima Lunga unit, Central Alps, Italy) share a common eclogite-facies peak at P=2·6-3·0GPa and T=710-750°C, constrained by conventional thermobarometry and thermodynamic modelling. High-pressure minerals are replaced both in peridotites and in eclogites by lower-P and high-T assemblages. In peridotites, the zirconium titanate srilankite occurs asmicrometre-sized crystals in textural equilibriumwith spinel, clinopyroxene and orthopyroxene in kelyphites developed between garnet and olivine. By using a new ZrO2-TiO2 solid-solution model, we provide evidence that srilankite is stable in peridotites relative to zircon+rutile for T > 810°C at an assumed P=0·9GPa, consistent with estimates of T≈850°C (at assumed P=0·9GPa) determined for symplectites made of sapphirine+spinel+Al-rich orthopyroxene+amphibole found in fractures within garnet. In eclogites, kyanite is replaced by symplectites made of anorthite-rich plagioclase+spinel6sapphirine6corundum, formed at T≈850°C and P=0·8-1·0GPa, conditions that are coincident with the high-T overprint observed in the peridotites. Thermodynamic modelling coupled with a material-transfer study provides constraints for these sapphirine-bearing symplectites. In thesemicro-domains, the 'inert' components could not fully equilibrate with the surrounding rock, and the locally high Al content promoted the stability of the Al-rich phases (i.e. mosaic equilibrium). This is the first report from the Alps of eclogite-facies rocks of supposed Alpine age showing a granulite-facies metamorphic overprint, which is, in contrast, well documented in the Variscan belt. On these grounds, although the age of the high-pressure and hightemperature stages in the Monte Duria rocks is still not constrained, the possibility that they reached eclogitic and granulitic conditions in pre-Alpine times should be taken into account.

The Anarak Metamorphic Complex, localized in Central Iran, is a fossil accretionary wedge composed of several tectonometamorphic units. Some of these, the Chah Gorbeh, the Morghab and the Ophiolitic complexes, contain mafic rocks that have been metamorphosed at high-pressure-low-temperature conditions. Such units have been stacked together and later refolded during the final stages of exhumation. Structural analysis at the mesoscale recognized at least three deformation events. Microstructural analyses, mineral chemistry and thermodynamic modelling reveal that the mafic schists followed contrasting P-T paths during their tectonometamorphic evolutions. In the schists of the Chah Gorbeh and Ophiolitic complexes an early greenschist-facies stage was later overprinted by blueschist-facies phase assemblages with suggested peak conditions of 390-440 °C at 0.6-0.9 GPa for the meta-basalt within the Ophiolitic Complex and 320-380 °C at 0.6-0.9 GPa for the blueschists of the Chah Gorbeh Complex. P-T conditions at metamorphic peak were 410-450 °C at 0.78-0.9 GPa for the Morghab blueschists, but they are reached before a greenschist-facies re-equilibration. Compositional zoning of amphiboles and epidotes of this greenschist-facies stage suggests a renewed pressure increase at the end of this metamorphic stage. Based on these data we reconstructed a clockwise P-T path for the Morghab mafic schists and a counter-clockwise path for the Chah Gorbeh blueschists and ophiolitic meta-basalts. Such contrasting metamorphic evolutions of tectonic units that were later accreted to the same wedge are indicative of the complex tectonic dynamics that occur within accretionary-subduction complexes.

On page 76, line 13 of Section 4.1.2. and caption of Figure 6, the sentence “chemographic representation for the system Fe3O4-CaMgO2-KAlSiO4 at fixed SiO2 chemical potential, projected through the FeMg−1 exchange vector” must be completed adding “and projected from CaMgO2”. The correction does not change any fundamental conclusions of the paper. It only clarifies the relative positions of the plotted mineral phases.

The Pamir belts, located NW of the Himalaya, formed in response to the Cenozoic collision and indentation of India with Eurasia resulting in strong deformation and bending of pre-existing Paleozoic to Mesozoic orogens. Several crustal blocks separated by complex and scarcely studied suture zones form the present day tectonic architecture of the Pamirs. The extreme crustal shortening experienced by the Pamirs during the indentation of India hinders straightforward correlations between tectonic terranes of the Himalaya-Tibetan area with their potential continuations through the Pamirs into Afghanistan and Iran. Existing stratigraphic and paleontological data suggest that Central and SE Pamir, together with Karakoram and Qiangtang blocks separated from the Gondwana margin in the Early Permian, drifted northward, following the closure of the Paleotethys and other minor oceanic branches, and diachronously collided with Northern Pamir and Tian Shan. The Central and South Pamir are now separated by the Rushan-Pshart Suture Zone that extends E-W from the E-Pamir Fault ??? to the valley of the Panj River. South Pamir is separated into two units (SE and SW Pamirs) by a crustal scale detachment system that brought in contact the Permian to Cenozoic sedimentary succession of the SE block with the gigantic basement domes of the SW block, made of Precambrian metamorphic rocks with an high-T overprint of Miocene age. In SE Pamir the occurrence of Cimmerian tectonics is proved by lowermost Jurassic deposits resting in unconformity on intensively folded and faulted Permian and Triassic units. On the base of available data the suggested collision age of SE Pamir with Central Pamir is roughly bracketed between Middle Triassic to Early Jurassic. Along the contact between the SE and SW Pamir, a small (a few tens of km2 wide) ophiolitic unit occurs: the Bashgumbaz ophiolites. They consist of a low-grade metamorphic association of serpentinized harzburgites and gabbros with minor bodies o

This study investigates the deformation mechanisms of a metagabbroic dyke experiencing syn-kinematic melt-rock interaction in a continental lower-crustal shear zone in the Seiland Igneous Province (northern Norway). Solid state shearing occurred at T=750-820 °C, P=0.75-0.95 GPa and was coeval with melt infiltration from dehydration melting of adjacent metasediments, as evident from thin leucosome veinlets within the dykes. The mylonite consists of cpx [Ca0.47,Mg0.35,Fe0.18]SiO3 + opx [Ca0.1,Mg0.5,Fe0.4]SiO3 + pl (An77Ab22Or1) porphyroclasts with localized grt and ilm coronas, embedded in a fine grained matrix of cpx + opx + pl + qtz + ilm +/- kfs. Porphyroclasts range in size (diameter) between 25 to 650 m, whereas the fine grain matrix is consistently below 10 m (average 4-7 m). Porphyroclasts show varying degrees of elongation, with the opx reaching aspect ratios of 1:16 and the cpx reaching rare maxima of 1:7. Cpx and pl porphyroclasts are micro-fractured and micro-boudinaged with fine-grained material infill. Texturally, opx porphyroclasts display a marked crystallographic preferred orientation (CPO) and activity of the {100}<001> and minor {100}<010> slip systems, whereas cpx and pl porphyroclasts are randomly oriented. All porphyroclasts have strong internal misorientations (undulatory and sweeping extinction) and lack recovery features (subgrains). The fine-grained polyphase matrix wrapping the porphyroclasts displays weak to absent CPO, with the exception of opx that shows a {100} poles-to-planes maxima perpendicular to the foliation. Based on the microstructure, we argue that a large part of the matrix is the product of metamorphic reactions in the presence of melt. To test this hypothesis, the interaction between the studied mafic dyke and an adjacent felsic leucosome was modelled using Perple_X for P-T conditions ranging between 7-9 kbar and 700-1000 °C. Results show that the syn-kinematic mineral assemblage (opx + cpx + pl + qtz + ilm +/-kfs) can be the

Here, we describe the Permian-Lower Triassic sedimentary succession of South Pamir and the associated biota of conodonts, foraminifers and brachiopods. The studied succession comprises the Carboniferous-Lower Permian siliciclastic Uruzbulak and Tashkazyk formations (Bazar Dara Group), which are unconformably covered by upper Lower to Upper Permian units, deposited both in platform settings (Kurteke Formation), and on the slope and basin (Kochusu Formation, Shindy Formation, Kubergandy Formation, Gan Formation, and Takhtabulak Formation). These formations comprise bioclastic limestones, cherty limestones, shales, volcaniclastic rocks, basalts, sandstones and conglomerates, and are locally very rich in fossils (fusulinids, ammonoids, brachiopods, corals and conodonts). The Permian succession is then overlain by shallow water carbonates of the Induan to Anisian Karatash Group. Subsidence analysis and volcanics of the Permian and overlying Triassic successions constrains the timing of rifting of South Pamir from Gondwana in the Early Permian (=Cisuralian), and its docking to Central Pamir, the Eurasian margin and the interposed volcanic arcs at the end of the Triassic. The sedimentary successions of the Pamirs represent a key-point to refine the correlations between the Tethyan regional scale and the International Time Scale. The analyses of the fusulinids and conodonts of the Kubergandian and Murgabian stratotypes of SE Pamir suggest that: (1) the upper Bolorian and the lower part of the Kubergandian correlate to the upper Kungurian; (2) the upper Kubergandian and the lower Murgabian correlate to the Roadian; (3) the mid-upper Murgabian correlates to the Wordian; (4) possibly the uppermost Murgabian and the lower Midian correlate to the lower Capitanian.The Kubergandian is thus a defined regional stage, based on fusulinids, ammonoids and conodonts and can be correlated to the Kungurian and the Roadian; still problematic remains the Murgabian correlation which needs to

The manganese ore of Praborna (Italian Western Alps) is embedded within a metasedimentary sequence belonging to a subduction mélange equilibrated at high-pressure (HP) conditions (ca. 2GPa) during the Alpine orogenesis. The pervasive veining of the ore and the growth of "pegmatoid" HP minerals suggest that these Mn-rich rocks strongly interacted with slab-derived fluids during HP metamorphism. These rocks are in textural and chemical equilibrium with the veins and in contact with sulphide- and magnetite-bearing metabasites at the bottom of the sequence. They contain braunite (Mn2+Mn3+6SiO12), quartz, pyroxmangite (Mn2+SiO3), and minor hematite, omphacite, piemontite and spessartine-rich garnet. Sulphides are absent in the Mn-rich rocks, whereas sulphates (barite, celestine) occur together with As- and Sb-oxides and silicates. This rock association provides an excellent natural laboratory to constrain the redox conditions in subducting oceanic slab mélanges at HP and fluid-present conditions. Similarly to Fe-bearing minerals, Mn oxides and silicates can be regarded as natural redox-sensors. A thermodynamic dataset for these Mn-bearing minerals is built, using literature data as well as new thermal expansion parameters for braunite aud pyrolusite, derived from experiments. Based on this dataset and the observed assemblages at Praborna, thermodynamic calculations show that these mélange rocks are characterised by ultra-oxidized conditions ({increment}FMQ up to +12.7) if the chemical potential of oxygen (or the oxygen fugacity fO2) is accounted for. On the other hand, if the molar quantity of oxygen is used as the independent state variable to quantify the bulk oxidation state, the ore appears only moderately oxidized and comparable to typical subduction-slab mafic eclogites. Such an apparent contradiction may happen in rock systems whenever oxygen is improperly considered as a perfectly mobile component. In the Earth's mantle, redox reactions take place mainly between

Fluid inclusions are known to be formed at pressures reaching some tens of kilobars. The solid matrix encompassing the fluid filled cavity experiences decompression as a consequence of uplift processes such as eruptions. This event may prompt the mechanical failure of the host-mineral matrix through either stretch or decrepitation, depending on a ductile or brittle mechanism of matrix failure, respectively. Laboratory experiments performed on synthetic inclusions show that the decrepitation temperature is strongly size dependent, with smaller cavities observed to decrepitate at higher temperatures. On the other hand, natural inclusions which undergo migration through a pressure gradient are always found intact below a critical size. In this paper, we model fluid inclusions as spherical cavities in a continuous elastic medium. Under these conditions, the tangential stress applied in the matrix has a cubic dependence on 1/r. This means that the maximum tensile stress concentrates at the cavity/matrix interface, and, as can be demonstrated, this stress is independent on cavity size. This means that if a fracture criterion based on the maximum stress concentration is adopted, there is no way for accounting for the size dependence of fracturing. Here, we address this problem by adopting a non-local stress approach to fracturing. Our mechanical model is based on two parameters: d, a characteristic distance which is material-dependent and accounts for the brittleness of the matrix, and T0, the uniaxial tensile strength of the matrix. We show that the model calculation well approximates experimental datasets relating internal pressure to cavity size and we demonstrate the fundamental prediction that the decrepitation phenomenon is characterized by a threshold size, Dth, and a threshold internal pressure of the cavity, below which decrepitation would not be allowed. The order of magnitude of the decrepitation threshold size is 1 µm for the analysed datasets of quartz and oli

Fluid inclusions in igneous and metamorphic rocks are equilibrated at pressures reaching a few tens of kilobars at mantle depths. These microsystems experience decompression as a consequence of uplift processes, such as subaerial volcanic eruptions. On decompression, inclusion fluid overpressure is known to bring about the mechanical failure of the crystal matrix through either stretch or decrepitation, depending on ductile or brittle failure mechanism of the matrix, respectively. On the one hand, laboratory experiments performed on synthetic inclusions at one atmosphere show that the decrepitation temperature is strongly size dependent, with the smaller cavities observed to decrepitate at higher temperatures. On the other hand, natural fluid inclusions, which undergo migration through a pressure gradient, are often found intact below a critical size. Here by modeling fluid inclusions as spherical cavities in a continuous elastic medium and by adopting a nonlocal stress approach to fracturing, we demonstrate that the decrepitation phenomenon is predicted to be characterized by a threshold size of the cavity, below which decrepitation would not be allowed. In order to validate our model, two independent experimental data sets relating internal pressure to cavity size are utilized to calculate pertinent model parameters and to evaluate their consistency.

Remnants of the fluid phase at ultrahigh pressure (UHP) in subduction environments may be preserved as primary multiphase inclusions in UHP minerals. The mode of crystallization of daughter minerals during precipitation within the inclusion and/or the mechanism of interaction between the fluid at supercritical conditions and the host mineral are still poorly understood from a crystallographic point of view. A case study is represented by garnet–orthopyroxenites from the Maowu Ultramafic Complex (China) deriving from harzburgite precursors metasomatized at ~4 GPa, 750 °C by a silica- and incompatible trace element-rich fluid phase. This metasomatism produced poikilitic orthopyroxene and inclusion-rich garnet porphyroblasts. Solid multiphase primary inclusions in garnet display a size within a few tens of micrometres and negative crystal shapes. Infilling minerals (spinel: 10–20 vol%; amphibole, chlorite, talc, mica: 80–90 vol%) occur with constant volume proportions and derive from trapped solute-rich aqueous fluids. To constrain the possible mode of precipitation of daughter minerals, we performed for the first time a single-crystal X-ray diffraction experiment by synchrotron radiation at Diamond Light Source. In combination with electron probe microanalyses, this measurement allowed the unique identification of each mineral phase and reciprocal orientations. We demonstrated the epitaxial relationship between spinel and garnet and between some hydrous minerals. Such information is discussed in relation to the physico-chemical aspects of nucleation and growth, shedding light on the mode of mineral crystallization from a fluid phase trapped at supercritical conditions.

The occurrence in Iran of several ophiolite belts dating between Late Palaeozoic to Triassic poses several questions on the possible existence of various sutures marking the closure of the Palaeotethys ocean between Eurasia and this Gondwana-derived microplate. In this scenario, the Anarak region in Central Iran still represents a conundrum. Contrasting geochronological, paleontological, paleomagnetic data and reported field evidence suggest different origins for the Anarak Metamorphic Complex (AMC). The AMC is either interpreted, as: (1) relict of an accretionary wedge developed at the Eurasia margin during the Palaeotethys subduction as part of the Cimmerian suture zone of NE Iran, displaced to Central Iran by a large counter-clockwise rotation of the central Iranian blocks; (2) autochthonous unit forming a secondary branch of the main suture zone.Our structural, petrographic and geochemical data indicate that the AMC consists of several metamorphic units also including dismembered "ophiolites" which display different tectono-metamorphic evolutions. Three main ductile deformational events can be distinguished in the AMC. The Morghab and Chah Gorbeh complexes preserve a different M1 metamorphism, characterized by blueschist relics in the S1 foliation of the former unit, and greenschist assemblages in the latter. They share a subsequent similar D2 deformational and M2 metamorphic history, showing a prograde metamorphism with syn- to post-deformation growth of blueschist facies mineral assemblages on pre-existing greenschist facies associations. High pressure, low temperature (HP/LT) metamorphism responsible for the growth of sodic amphibole has been recognized also within marble lenses at the contact between the Chah Gorbeh Complex and serpentinites. Evidence of HP/LT metamorphism also occurs in glaucophane-bearing meta-pillow lavas and serpentinites, which contain antigorite and form most of the "ophiolites" within the AMC. Structural relationships show that the Ch

The Iran microplate is part of the Cimmerian blocks which rifted off Gondwana in the Early Permian and collided with the Southern Eurasian margin in the Late Triassic due to the closure of the Palaeotethys ocean. The presence of several ophiolite belts in Iran dating between Late Palaeozoic to Triassic poses several questions on the possible existence of various sutures marking the closure of the Palaeotethys ocean between Eurasia and Iran. Late Palaeozoic complexes with a Carboniferous “Variscan” age have been also described in N Iran, along the Alborz range, and in NE Iran, along the Late Triassic Palaeotethys suture, and they are generally related to the Late Palaeozoic evolution of the southern Eurasian margin, consisting of a mosaic of small continental blocks. In this general frame, the Anarak Metamorphic Complex (AMC), located in Central Iran, presently far from the pre-Late Triassic suture zones, is still an open problem. Detailed structural, petrographic and geochemical analyses have been carried out in these last years through the DARIUS PROGRAMME and an Italian PRIN. The AMC, which consists of several metamorphic units including dismembered “ophiolites”, displays different tectono-metamorphic evolutions. The Morghab and Chah Gorbeh complexes, mainly consisting of metapelites associated with metabasites and thick marble layers display a M1 metamorphism characterized by blueschist relicts along the S1 foliation in the former, and greenschist assemblages in the latter. They share a similar D2 deformational and M2 metamorphic history showing a prograde metamorphism with syn- to post-kinematic growth of blueschist facies phase assemblages. Prograde metamorphism characterized by the syn- to post-D2 growth of sodic amphibole has been recognized also in marbles at the contact between this unit and serpentinites. Evidence of HP/LT metamorphism also occur in Mg-riebeckite-bearing metapillow lavas and serpentinites, which are characterized by the coexistence of liza

C-O-H fluids are released by dehydration, partial melting and/or decarbonation of the slab and transferred to the mantle, where they interact with the surrounding rocks, prompting the growth of carbonates, hydrous minerals and C polymorphs. In the pure C-O-H system, C-saturated fluid speciation is a function of the oxygen chemical potential. Therefore, in natural systems, the fluid speciation can be imposed by the redox state of the rock-forming phases. Alternatively, C-O-H fluids may control the bulk oxidation state of the rock system by redox reactions with the mineral phases. Complexities in the speciation of such fluids are demonstrated by discoveries of hydrates, carbonates and carbon polymorphs in polyphase inclusions, and also by high-pressure H2- or CH4-bearing fluid inclusions in mantle harzburgites, suggesting that the composition of C-bearing hydrous fluids closely relates to the oxidation state of the system. We selected three case studies of garnet-bearing ultramafic rocks (Ulten zone, Italy; Sulu, China; Bardane, Norway), which record metasomatic processes driven by C-O-H fluids at the interface between a subducting slab and the overlying mantle wedge. All these rocks contain carbonates (dolomite-only at P < 1.9 GPa at 900 °C, magnesite- only at P > 2.4 GPa at 900 °C, dolomite + magnesite in between) and hydrous phases (amphibole, phlogopite) equilibrated at some stages in the garnet stability field. The fO2 values, estimated by analysing the Fe3+ content (skiagite mole fraction) in garnet, indicate that the Ulten and Sulu peridotites record high oxygen fugacities (FMQ to FMQ+2) and a retrograde path with decreasing P and T. The fO2 values obtained for the Bardane garnet websterites, which record a prograde path with increasing T and P, are up to -2 log units lower than the FMQ. When combined with data for subduction-zone systems (arc lavas and their mantle sources), the studied ultramafic rocks define a trend of decreasing fO2 with increasing pressure

Remnants of the fluid phase at ultrahigh pressure (UHP) in subduction environments may be preserved as primary polyphase inclusions in UHP minerals. These inclusions show the same textural features as fluid inclusions and locally may enclose UHP daughter minerals. Detailed textural studies, as well as experimental attempts to homogenise such inclusions, demonstrate that polyphase inclusions derive from solute-rich aqueous fluids or hydrous melts. However, the mode of precipitation of daughter minerals from these solute-rich hydrous fluids has never been investigated yet. A case study is represented by garnet orthopyroxenites from the Maowu Ultramafic Complex (China). They derive from harzburgite precursors metasomatised at ~ 4 GPa, 750 °C by a silica- and incompatible trace element-rich fluid phase, sourced from the associated crustal rocks (Malaspina et al., 2006). This metasomatism produced poikilitic orthopyroxene and inclusion-rich garnet porphyroblasts. Solid polyphase primary inclusions in garnet display negative crystal shapes and constant volume ratios of infilling minerals (spinel: 10–20 vol.%; amphibole, chlorite, talc, mica: 80– 90 vol.%). Experimental homogenisation of polyphase inclusions demonstrated that derive from trapped solute-rich aqueous fluids (Malaspina et al., 2006). To constrain the possible mode of precipitation of daughter minerals, significant contribution could be given by the analysis of the growth relationships between host and infillings. As demonstrated by Nestola et al. (2013) for the case of diamond and their olivine inclusions, the use of ‘state of the art‘ single-crystal X-ray diffraction to collect data symoultaneously on both host and inclusion phases, allows to obtain “orientation matrixes” of both phases thus retrieving their reciprocal crystallographic orientation. We performed a single-crystal X-ray diffraction experiment by Synchrotron Radiation at DLS-Diamond Light Source. Combination of the EMPA analyses, Raman Spctrosco