Polyphase Inclusions Research Articles

Polyphase Inclusions in Chromium Spinels from Upper Triassic Gravelites from the Northeastern Part of the Siberian Platform

A comparative study of the chemical compositions of chromium spinel and polyphase inclusions has been carried out. Chromium spinels were separated from the concentrate of the Carnian Stage (T3) rock sample collected at the northeastern fringe of the Siberian platform. The chromium spinels show a wide variety of compositions, most of which are of the “Kurung” type, which is considered to be a deceptive indicator of a kimberlite. They contain inclusions in the size range of 10–100 μm and contain several mineral phases. The most frequent inclusions are olivine and clinopyroxene; additional phases are nepheline, K–Fsp, Ti–Mg–Ca–amphibole, Ti–biotite (phlogopite), apatite and perovskite. Ilmenite and iron sulfides are less common. The data obtained on the composition of these inclusions indicate differences in the conditions of their formation and possibly different types of parental sources. A possible type of provenance source could be potassium alkali basites.

Trace-element compositions and Br/Cl ratios of fluid inclusions in the Tsushima granite, Japan: Significance for formation of granite-derived fluids

Fluid inclusions in quartz samples from a miarolitic cavity, two quartz veins, and a hydrothermal ore vein in the Tsushima granite, Japan, were analyzed by particle-induced X-ray emission to examine the chemistry and process of formation of hydrothermal fluids in an island-arc granite. Most of the inclusions were polyphase or vapor, and there were smaller numbers of two-phase aqueous inclusions. The inclusions contained Cl, K, Ca, Ti, Mn, Fe, Cu, Zn, Ge, Br, Rb, Sr, Ba, and Pb. For each inclusion, there was a strong positive correlation between Cl content and contents of other elements identified. Concentration ranges for most elements (other than Rb and Ge) in polyphase inclusions from the miarolitic cavity were comparable to those from cavities in alkaline granites; those from the ore vein were comparable to large-scale continental hydrothermal ore deposits. The lower Rb and higher Ge contents in the polyphase inclusions of the Tsushima granite may be characteristic of hydrothermal fluids from calc-alkaline granites in an island-arc setting. Br/Cl ratios (by weight) for the vapor and two-phase inclusions were 0.0013–0.0030 and differed among the three geological settings. Br/Cl ratios of polyphase inclusions increased with increasing Cl content in single-crystal and polycrystalline quartz, and high values of more than 0.0100 were found. The high Br/Cl ratios and the differences among the geological settings sampled may be due to pressure dependences of partitioning of Cl and Br between fluid and magma during fluid segregation and between liquid and vapor during boiling. Using a simple model based on these dependences, we calculated Br/Cl ratios greater than 0.01 in brine generated at pressures <0.89kbar. Differences in Br/Cl ratios in polyphase and vapor inclusions from each geological setting were attributed to mixing between two end-member fluids: a high Br/Cl fluid generated at low pressure and a low Br/Cl fluid generated at high pressure. Br/Cl ratios of polyphase inclusions in quartz may be a key for understanding the conditions under which boiling fluids are generated in simple granite systems.

Behavior of Pt, Pd, and Au During Crystallization of Cu-Rich Magmatic Sulfide Minerals

Abstract The method of quasi-equilibrium directed crystallization was used for experimental modeling of the behavior of Pt, Pd, and Au in the presence of As, Te, Bi, and Sn during the fractional solidification of Cu-rich sulfide magma. Our experimental melt contained (in mol.%): Fe 33.20, Cu 16.55, S 50.03, Pt 0.03, Pd 0.02, Au 0.02, As 0.02, Bi 0.03, Te 0.02, and Sn 0.08, which is similar in composition to the massive cubanite ores from the platinum-copper-nickel deposits of the Noril9sk group. During crystallization, base metal sulfides [pyrrhotite solid solution ( Po ss ) and high-temperature cubanite (isocubanite, Icb )] crystallized successively from the melt. The constructed curves show variations in the concentrations of Fe, Cu, and S along the ingot. Based on these data, we calculated melt composition during crystallization and determined the distribution coefficients of components between Po ss or Icb and the melt. The behavior of minor elements in this process was studied. The data show that the content of minor elements in Po ss is below the detection limit of electron microprobe analysis. Tin dissolves in isocubanite, whereas Pt, Pd, Au, As, Bi, and Te are present in the matrix as microinclusions. These elements form their own microphases: kotulskite–sobolevskite (PdBi x Te 1– x ) and moncheite–insizwaite Pt(Te x Bi 1– x ) 2 solid solutions, cooperite (Pt,Pd)S, sperrylite Pt(As,S) 2 , froodite PdBi 2– x , bismuthinite Bi 2 S 3– x , Au, lisiguangite CuPtBiS 3 , and unnamed PtBiS 3– x . Also present are drop-shaped eutectic-like inclusions in which PdBi x Te 1– x is the matrix containing micron-sized inclusions of Au and PdBi 2– x . The constructed crystallization history of the experimental sample demonstrates the behavior of both major and minor elements in each of the zones. A probable mechanism of formation of monophase and polyphase inclusions is proposed. The mineral forms of Pt, Pd, and Au phases in the samples and their morphology are comparable to the typical natural types in massive Cu-rich sulfide ores from the Noril9sk deposits.

Origin and evolution of ore fluids in the late Mesozoic Naozhi epithermal Au–Cu deposit, Yanbian area, Northeast China: evidence from fluid inclusion and isotopic geochemistry

The Naozhi Au–Cu deposit is located on the continental margin of Northeast China, forming part of the West Pacific porphyry–epithermal gold–copper metallogenic belt. In this paper, we systematically analyzed the compositions, homogenization temperatures, and salinity of fluid inclusions as well as their noble gas isotopic and Pb isotopic compositions from the deposit. These new data show that (1) five types of fluid inclusions were identified as pure gas inclusions (V-type), pure liquid inclusions (L-type), gas–liquid two-phase inclusions (W-type, as the main fluid inclusions (FIs)), CO2-bearing inclusions (C-type), and daughter-mineral-bearing polyphase inclusions (S-type); (2) W-type FIs in quartz crystals of early, main, and late stage are homogenized at temperatures of 324.7–406.7, 230–338.8, and 154.6–308 °C, with salinities of 2.40–7.01 wt% NaCleq, 1.73–9.47 wt% NaCleq, and 6.29 wt% NaCleq, respectively. S-type FIs in quartz crystals of early stage are homogenized at temperatures of 328.6–400 °C, with salinities of 39.96–46.00 wt% NaCleq; (3) Raman analysis results reveal that the vapor compositions of early ore-forming fluids consisted of CO2 and H2O, with H2O gradually increasing and CO2 being absent at the late mineralization stage; (4) fluid inclusions in pyrite and chalcopyrite have 3He/4He ratios of 0.03–0.104 Ra, 20Ne/22Ne ratios of 9.817–9.960, and 40Ar/36Ar ratios of 324–349. These results indicate that the percentage of radiogenic 40Ar* in fluid inclusions varies from 8.8 to 15.5 %, containing 84.5–91.2 % atmospheric 40Ar; (5) the 206Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb ratios of sulfides are 18.1822–18.3979, 15.5215–15.5998, and 38.1313–38.3786, respectively. These data combined with stable isotope data and the chronology of diagenesis and metallogenesis enable us suppose that the ore-forming fluids originated from the melting of the lower crust, caused by the subduction of an oceanic slab, whereas the mineralized fluids were exsolved from the late crystallization stage and subsequently contaminated by crustal materials/fluids during ascent, including meteoric water, and the mineral precipitation occurred at a shallow crustal level.

Fluid-related inclusions in Alpine high-pressure peridotite reveal trace element recycling during subduction-zone dehydration of serpentinized mantle (Cima di Gagnone, Swiss Alps)

Serpentinites release at sub-arc depths volatiles and several fluid-mobile trace elements found in arc magmas. Constraining element uptake in these rocks and defining the trace element composition of fluids released upon serpentinite dehydration can improve our understanding of mass transfer across subduction zones and to volcanic arcs. The eclogite-facies garnet metaperidotite and chlorite harzburgite bodies embedded in paragneiss of the subduction melange from Cima di Gagnone derive from serpentinized peridotite protoliths and are unique examples of ultramafic rocks that experienced subduction metasomatism and devolatilization. In these rocks, metamorphic olivine and garnet trap polyphase inclusions representing the fluid released during high-pressure breakdown of antigorite and chlorite. Combining major element mapping and laser-ablation ICP-MS bulk inclusion analysis, we characterize the mineral content of polyphase inclusions and quantify the fluid composition. Silicates, Cl-bearing phases, sulphides, carbonates, and oxides document post-entrapment mineral growth in the inclusions starting immediately after fluid entrapment.Compositional data reveal the presence of two different fluid types. The first (type A) records a fluid prominently enriched in fluid-mobile elements, with Cl, Cs, Pb, As, Sb concentrations up to 103 PM (primitive mantle), ∼102 PM Tl, Ba, while Rb, B, Sr, Li, U concentrations are of the order of 101 PM, and alkalis are ∼2 PM. The second fluid (type B) has considerably lower fluid-mobile element enrichments, but its enrichment patterns are comparable to type A fluid.Our data reveal multistage fluid uptake in these peridotite bodies, including selective element enrichment during seafloor alteration, followed by fluid–rock interaction along with subduction metamorphism in the plate interface melange. Here, infiltration of sediment-equilibrated fluid produced significant enrichment of the serpentinites in As, Sb, B, Pb, an enriched trace element pattern that was then transferred to the fluid released at greater depth upon serpentine dehydration (type A fluid). The type B fluid hosted by garnet may record the composition of the chlorite breakdown fluid released at even greater depth.The Gagnone study-case demonstrates that serpentinized peridotites acquire water and fluid-mobile elements during ocean floor hydration and through exchange with sediment-equilibrated fluids in the early subduction stages. Subsequent antigorite devolatilization at subarc depths delivers aqueous fluids to the mantle wedge that can be prominently enriched in sediment-derived components, potentially triggering arc magmatism without the need of concomitant dehydration/melting of metasediments or altered oceanic crust.

Diamond in metasedimentary crustal rocks from Pohorje, Eastern Alps: a window to deep continental subduction

AbstractWe report the first finding of diamond and moissanite in metasedimentary crustal rocks of Pohorje Mountains (Slovenia) in the Austroalpine ultrahigh‐pressure (UHP) metamorphic terrane of the Eastern Alps. Microscopic observations and Raman spectroscopy show that diamond occurs in situ as inclusions in garnet, being heterogeneously distributed. Under the optical microscope, diamond‐bearing inclusions are of cuboidal to rounded shape and of pinkish, yellow to brownish colour. The Raman spectra of the investigated diamond show a sharp, first order peak of sp3‐bonded carbon, in most cases centred between 1332 and 1330 cm−1, with a full width at half maximum between 3 and 5 cm−1. Several spectra show Raman bands typical for disordered graphitic (sp2‐bonded) carbon. Detailed observations show that diamond occurs either as a monomineralic, single‐crystal inclusion or it is associated with SiC (moissanite), CO2 and CH4 in polyphase inclusions. This rare record of diamond occurring with moissanite as fluid‐inclusion daughter minerals implies the crystallization of diamond and moissanite from a supercritical fluid at reducing conditions. Thermodynamic modelling suggests that diamond‐bearing gneisses attained P–T conditions of ≥3.5 GPa and 800–850 °C, similar to eclogites and garnet peridotites. We argue that diamond formed when carbonaceous sediment underwent UHP metamorphism at mantle depth exceeding 100 km during continental subduction in the Late Cretaceous (c. 95–92 Ma). The finding of diamond confirms UHP metamorphism in the Pohorje Mountains, the most deeply subducted part of Austroalpine units.

Carbonate, silicate, and sulfide melts: heterogeneity of the UHP mineral-forming media in calc-silicate rocks from the Kokchetav massif

We present data on carbonatite, silicate, and sulfide melts and their immiscibility at different stages of ultrahigh-pressure metamorphism of rocks of the Kokchetav Massif (northern Kazakhstan). The identified silicate, silicate-carbonate, and sulfide inclusions are regarded as crystallization products of high-pressure melts. The detected reactionary garnet-K-feldspar-allanite-calcite symplectite structures as inclusions in garnet and as identical structures around it evidence that they resulted from carbonatite melt crystallization. Carbonate melting was probably triggered by the present free fluid phase (mostly H2O) and/or a high content of alkalies in the system. The coexistence of carbonate and silicate inclusions testifies to the immiscibility of carbonatite and silicate melts. The presence of K-cymrite in the polyphase inclusions indicates that the minimum pressure of silicate melt intake is ~4.5GPa. The maximum pressure of this intake is 6–7GPa at 1000–1100°C and corresponds to the peak of metamorphism of the Kokchetav Massif rocks. Most likely, the field of immiscibility of carbonatite and silicate melts lies within 4.5-7GPa and 950–1100°C. The carbonatite melt can dissolve up to 18wt.% SiO2, and the silicate melt, up to 4.5vol.% CaCO3.

Partial melting of UHP calc-gneiss from the Dabie Mountains

Exhumation melting has been proposed for the ultra-high pressure (UHP) metamorphic rocks in the Dabie Mountains based on melting experiments. We document here the first petrological and mineralogical evidence demonstrating that the UHP calc-gneisses from the Ganjialing area in the Dabie Mountains experienced partial melting during early exhumation. The assemblage of garnet, phengite (Si=3.65pfu), coesite, rutile and carbonate preserved in the calc-gneisses indicates a peak metamorphic condition of 692–757°C and 4.0–4.8GPa. Partial melting is indicated by several lines of evidence: the melting textures of phengite, the feldspar-dominated films, bands, branches, blebs and veins, the euhedral K-feldspars, the intergrowth film of plagioclase and K-feldspar, the plagioclase+biotite intergrowth after garnet and the epidote poikiloblasts. Polyphase inclusions in garnet are characterized with wedge-like offshoots and serrate outlines whereas those in epidote display negative crystal shapes, which can be best interpreted by entrapment of former melts. We propose a wet melting reaction of Phn+Q±Na-Cpx+H2O=Bt+Pl+Grt+felsic melts, which likely took place at ca.650–800°C and ca.1.0–2.0GPa, to interpret the melting event in the calc-gneisses. Chemical exchanges between garnet and melts produced new garnet domains with higher almandine, spessartine, MREE, HREE and Y but lower grossular, pyrope, P, Sc, Ti, V and Zr contents. Zr-in-rutile thermometer reveals a low temperature of 620–643°C at 5GPa, indicating a later reset for Zr in rutile. Healed fractures are suggested to be responsible for the formation of some polyphase inclusions in garnet.

Dehydration melting of UHP eclogite and paragneiss in the Dabie orogen: Evidence from laboratory experiment to natural observation

Dehydration melting of subducted continental crust is significant during exhumation, and its study from both experimental and petrological observations is of great importance to our understanding of continental geodynamics. Dehydration melting experiments were carried out on ultrahigh-pressure (UHP) eclogite from Bixiling in the Dabie orogen using a piston cylinder at 1.5–3.0 GPa and 800–950°C to investigate partial melting of eclogite induced by phengite breakdown. The phengite-bearing eclogite started to melt at T⩽800–850°C and P=1.5–2.0 GPa and produced about 3% granitic melt. The products of dehydration melting vary with temperature and pressure. Such results provide valuable constraints on the micro-texture related to partial melting of UHP rocks in the Dabie-Sulu orogenic belt. Three types of polyphase inclusions were identified in garnet from the Shuanghe UHP eclogite. K-feldspar and quartz inclusions are interpreted to represent the products of segregation and crystallization of minor amounts of melt that formed during dehydration melting of phengite by the inferred reaction Phengite+Omphacite±Quartz→Amphibole±Garnet+Melt (K-feldspar+Quartz±Plagioclase). Polyphase inclusions of phengite and K-feldspar+Quartz inclusions were also found in zoisite/clinozoisite and garnet from the Shuanghe garnet-bearing paragneiss. These polyphase inclusions provide evidence for a continuous process from sub-solidus dehydration to partial melting within the UHP gneissic rocks. The compositional variation of garnets demonstrates that breakdown of epidote-group minerals may have played a crucial role during dehydration melting reaction of phengite. The Ti-in-zircon thermometry and Si content of phengite in zircon suggest that partial melting would occur at 783–839°C and 2.0–2.5 GPa. Therefore, both experimental results and petrological observations indicate that dehydration melting and fluid activity within the Dabie UHP rocks at micro-scale are controlled by the breakdown of phengite.

Polyphase inclusions in the Shuanghe UHP eclogites formed by subsolidus transformation and incipient melting during exhumation of deeply subducted crust

Ultrahigh-pressure (UHP) layered eclogites from the Shuanghe area, Dabie Mountains display the peak assemblage garnet, Na-rich omphacite (Jd60), and minor amounts of coesite, rutile, Si-rich phengite (Si=3.58p.f.u) and apatite. Zr-in-rutile thermometry provides evidence for peak metamorphic conditions of ~650–700°C, which combined with garnet–clinopyroxene–phengite barometry yields a peak pressure of 3.5–4GPa. Three different types of polyphase inclusions that formed after peak conditions are identified in garnet. The first type consists of a symplectitic intergrowth of zoisite+quartz±amphibole associated with epidote and exhibits a rectangular shape. These inclusions are interpreted to have formed during decompression of the UHP rocks as pseudomorphs after prograde to peak lawsonite. The second type of polyphase inclusions contains approximately equal amounts of K-feldspar and quartz. These inclusions are usually surrounded by radial fractures and exhibit regular negative crystal shapes. The observed gradual breakdown of peak metamorphic phengite resulting in these types of inclusions suggests that phengite melting at an advanced stage of exhumation at ~1.5–2GPa and ~750–800°C occurred. The third type of polyphase inclusions displays irregular shapes and preserves the retrograde mineral assemblage amphibole+plagioclase+quartz+biotite+K-feldspar, which likely formed during late stage fluid influx that is also responsible for the replacement of phengite by biotite as well as omphacite by amphibole+plagioclase.The trace element composition of K-feldspar+quartz inclusions has been determined in-situ by LA-ICP-MS and is characterized by a strong enrichment in LILE and a moderate enrichment in LREE with respect to HREE and a depletion in HFSE. The presence of melts assisted in the recrystallization of the eclogites and resulted in the segregation of garnet+quartz-rich and omphacite-rich domains. Field observations show that the melting is very limited and no interconnected network of partial melts formed. Thus, the low degree of partial melting did not result in a chemical differentiation of the subducted rocks but likely influenced the rheology of the eclogites during exhumation.

Hydration and dehydration in the lower margin of a cold mantle wedge: implications for crust–mantle interactions and petrogeneses of arc magmas

Garnet orthopyroxenites from Maowu (Dabieshan orogen, eastern China) were formed from a refractory harzburgite/dunite protolith. They preserve mineralogical and geochemical evidence of hydration/metasomatism and dehydration at the lower edge of a cold mantle wedge. Abundant polyphase inclusions in the cores of garnet porphyroblasts record the earliest metamorphism and metasomatism in garnet orthopyroxenites. They are mainly composed of pargasitic amphibole, gedrite, chlorite, talc, phlogopite, and Cl-apatite, with minor anhydrous minerals such as orthopyroxene, sapphirine, spinel, and rutile. Most of these phases have high XMg, NiO, and Ni/Mg values, implying that they probably inherited the chemistry of pre-existing olivine. Trace element analyses indicate that polyphase inclusions are enriched in large ion lithophile elements (LILE), light rare earth elements (LREE), and high field strength elements (HFSE), with spikes of Ba, Pb, U, and high U/Th. Based on the P–T conditions of formation for the polyphase inclusions (˜1.4 GPa, 720–850°C), we suggest that the protolith likely underwent significant hydration/metasomatism by slab-derived fluid under shallow–wet–cold mantle wedge corner conditions beneath the forearc. When the hydrated rocks were subducted into a deep–cold mantle wedge zone and underwent high-pressure–ultrahigh-pressure (HP–UHP) metamorphism, amphibole, talc, and chlorite dehydrated and garnet, orthopyroxene, Ti-chondrodite, and Ti-clinohumite formed during prograde metamorphism. The majority of LILE (e.g. Ba, U, Pb, Sr, and Th) and LREE were released into the fluid formed by dehydration reactions, whereas HFSE (e.g. Ti, Nb, and Ta) remained in the cold mantle wedge lower margin. Such fluid resembling the trace element characteristics of arc magmas evidently migrates into the overlying, internal, hotter part of the mantle wedge, thus resulting in a high degree of partial melting and the formation of arc magmas.

Metamorphic solid salt (KCl-NaCl) in quartzo-feldspathic polyphase inclusions in the Sulu ultrahigh-pressure eclogite

Unusual polyphase inclusions of K-feldspar + quartz + titanite + solid salt and K-feldspar + albite + quartz + epidote with textures similar to the other K-feldspar + quartz inclusions were found in omphacite grains from the Sulu ultrahigh pressure (UHP) eclogites. One of these inclusions contain square to round solid salt inclusions of KCl-NaCl composition. Such a mineral assemblage within K-feldspar-bearing inclusions hosted by UHP metamorphic phases suggests that (1) potassium granitic melts enriched in Cl components were presented during UHP metamorphism or at the early stage of rapid exhumation of deeply subducted continental slab; (2) they were resulted from reactions between the incoming granitic melts and quartz (or coesite); and (3) solid salt inclusions of NaCl-KCl were derived from dehydration and desiccation of Cl-bearing melts. Our new observations further demonstrate that during the tectonic evolution of UHP rocks, fertile components within deeply subducted continental materials could undergo partial melting, leading to the formation of Cl-bearing potassium granitic melts and substantial migration of fluid-conservative elements (e.g. Ti, Hf) within the UHP slab.

The role of C-O-H and oxygen fugacity in subduction-zone garnet peridotites

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. 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 þ magnesite in between) and hydrous phases (amphibole, phlogopite) equilibrated at some stages in the garnet stability field. The f O2 values, estimated by analysing the Fe 3þ content (skiagite mole fraction) in garnet, indicate that the Ulten and Sulu peridotites record high oxygen fugacities (FMQ to FMQþ2) and a retrograde path with decreasing P and T. The f O2 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 f O2 with increasing pressure. The Bardane websterites contain C-polymorphs in polyphase inclusions, which precipitated from entrapped metasomatic fluids at ultrahigh pressures. The calculated C-O-H fluid phase in equilibrium with the solid phases consists of mixtures of H2O and CO2. Semi-quantitative estimates for the Ulten and Sulu peridotites, in which C-polymorphs have not been found, and petrographic constraints for the Ulten peridotites indicate that the C-O-H component of the fluid could consist of H2OþCO2.

THREE-DIMENSIONAL TEXTURAL AND CHEMICAL CHARACTERIZATION OF POLYPHASE INCLUSIONS IN SPODUMENE USING A DUAL FOCUSED ION BEAM - SCANNING ELECTRON MICROSCOPE (FIB-SEM)

A dual-beam focused ion-beam – scanning electron microscope (FIB–SEM) instrument was used for detailed textural and chemical analysis of polyphase inclusions in spodumene. Our 3D representation of an inclusion (GB–38) was constructed from SEM images of serial cross-sections exposed by milling laterally through a selected inclusion with a 30 keV beam of Ga ions. The volume of the inclusion and daughter phases was determined from the sum of the volume of parallel slices; the volume of each slice was determined by multiplying the measured slice area by its thickness. Semiquantitative energy-dispersive X-ray (EDX) analysis and element mapping were performed on each cross-section to facilitate the identification of all solids within the inclusion. FIB–SEM analysis of spodumene-hosted inclusions from the Tanco and Greenbushes occurrences of granitic pegmatite are presented to demonstrate the advantages of 3D analysis for petrographic investigations of polyphase inclusions. The high-resolution 3D reconstructions afforded by the FIB–SEM method significantly extend our ability to study and interpret complex solid-rich fluid and melt inclusions in igneous and in high-pressure metamorphic rocks.

The oxidation state of mantle wedge majoritic garnet websterites metasomatised by C-bearing subduction fluids

The majoritic garnet-bearing websterites from Bardane (Western Gneiss Region, Norway) are unique examples of metasomatised mantle wedge that interacted with C-saturated COH subduction fluid phases at 200 km depth. These samples represent slices of former Archean transition zone mantle that upwelled, melted and accreted to a thick cratonic lithosphere, where it cooled until the Middle Proterozoic (stages M1–M2). During the subsequent Caledonian to Scandian subduction cycle (stage M3) these depleted mantle rocks were dragged into deep portions of the supra-subduction mantle wedge, where the episodic/progressive infiltration of crustal fluids at increasing depth enhanced crystallisation of diamond and of majoritic garnet. In these rocks microdiamond occurs as daughter phase in polyphase, fluid-related, inclusions and majoritic garnet is present as vein mineral. We studied the peak (stage M3) mineral assemblage garnet + orthopyroxene + olivine ± clinopyroxene ± phlogopite, where garnet composition becomes majoritic with pressure increase from 3 to 6.5 GPa and 800–1000 °C temperature. Majoritic garnet contains polyphase inclusions with daughter Cr-spinel + phlogopite/K-amphibole + dolomite/magnesite + graphite/diamond. They witness COH fluid/mineral interaction closely related to the oxidation state of the rock and responsible for diamond formation. We determined the fO 2 in the M3 assemblage starting from Fe 3+ analyses in majoritic garnet, which is characterised by the progressive enrichment in Fe 3+/ΣFe up to 0.15. The fO 2 values obtained for the Bardane mantle wedge garnet websterites are up to −2 log units lower than the fayalite–quartz–magnetite (FMQ) buffer along a trend from arc lavas (FMQ + 1.5–FMQ + 3) to mantle wedge garnet peridotites equilibrated at 4–5 GPa (FMQ–FMQ + 2). The determination of oxygen fugacity of the hydrate–carbonate-bearing M3 mineral association enabled us to estimate the speciation of slab-derived metasomatic COH fluids responsible for polyphase inclusions precipitation. Such fluids are mixtures of silicates and H 2O + CO 2, where water is the dominant species of the COH component. The peculiar composition of majorite-hosted diamond-bearing polyphase inclusions from Bardane and the speciation of its COH component point to an “oxidised” silicate-rich aqueous fluid contaminant from the subducted slab to the mantle wedge.

Evidence for multiple burial–partial exhumation cycles from the Onodani eclogites in the Sambagawa metamorphic belt, central Shikoku, Japan

AbstractEclogites from the Onodani area in the Sambagawa metamorphic belt of central Shikoku occur as layers or lenticular bodies within basic schists. These eclogites experienced three different metamorphic episodes during multiple burial and exhumation cycles. The early prograde stage of the first metamorphic event is recorded by relict eclogite facies inclusions within garnet cores (XSps 0.80–0.24, XAlm 0–0.47). These inclusions consist of relatively almandine‐rich garnet (XSps 0.13–0.24, XAlm 0.36–0.45), aegirine‐augite/omphacite (XJd 0.08–0.28), epidote, amphiboles (e.g. actinolite, winchite, barroisite and taramite), albite, phengite, chlorite, calcite, titanite, hematite and quartz. The garnet cores also contain polyphase inclusions consisting of almandine‐rich garnet, omphacite (XJd 0.27–0.28), amphiboles (e.g. actinolite, winchite, barroisite, taramite and katophorite) and phengite. The peak P–T conditions of the first eclogite facies metamorphism are estimated to be 530–590 °C and 19–21 kbar succeeded by retrogression into greenschist facies. The second prograde metamorphism began at greenschist facies conditions. The peak metamorphic conditions are defined by schistosity‐forming omphacites (XJd ≤ 49) and garnet rims containing inclusions of barroisitic amphibole, phengite, rutile and quartz. The estimated peak metamorphic conditions are 630–680 °C and 20–22 kbar followed by a clockwise retrograde P–T path with nearly isothermal decompression to 8–12 kbar. In veins cross‐cutting the eclogite schistosity, resorbed barroisite/Mg‐katophorite occurs as inclusions in glaucophane which is zoned to barroisite, suggesting a prograde metamorphism of the third metamorphic event. The peak P–T conditions of this metamorphic event are estimated to be 540–600 °C and 6.5–8 kbar. These metamorphic conditions are correlated with those of the surrounding non‐eclogitic Sambagawa schists. The Onodani eclogites were formed by subduction of an oceanic plate, and metamorphism occurred beneath an accretionary prism. These high‐P/T type metamorphic events took place in a very short time span between 100 and 90 Ma. Plate reconstructions indicate highly oblique subduction of the Izanagi plate beneath the Eurasian continent at a high spreading rate. This probably resulted in multiple burial and exhumation movements of eclogite bodies, causing plural metamorphic events. The eclogite body was juxtaposed with non‐eclogitic Sambagawa schists at glaucophane stability field conditions. The amalgamated metamorphic sequence including the Onodani eclogites were exhumed to shallow crustal/surface levels in early Eocene times (c. 50 Ma).

Mantle wedge peridotites: Fossil reservoirs of deep subduction zone processes: Inferences from high and ultrahigh-pressure rocks from Bardane (Western Norway) and Ulten (Italian Alps)

The garnet websterites from Bardane (Western Gneiss Region, Norway) derive from cold Archean subcontinental lithosphere involved in Scandian continental subduction to ultrahigh-pressures. Subduction zone metamorphism was promoted by slab fluid infiltration into the cold overlying mantle wedge. The earliest subduction transformation (M3-1) consists of garnet/clinopyroxene exsolution from old pre-subduction orthopyroxene. This stage was likely coeval with fluid input and formation of phlogopite and dolomite rods in the exsolution structures. Magnesite formation after dolomite and entrapment of fluid-related diamond-bearing polyphase inclusions in corona structures around the exsolved orthopyroxenes point to pressure increase to 4.5 GPa (M3-2). Peak pressures of 6.5–7 GPa (c.a. 200 km depth) are witnessed by crystallization of majoritic garnet (M3-3), mostly in veins cutting all the above microstructures. When such veins infiltrate the corona domains, formation of majoritic garnet in coronas is enhanced. This multistage evolution thus envisages episodic fluid influx, favouring rock recrystallization and formation of microdiamond-bearing inclusions and of majoritic garnet veins. These mantle rocks thus record fluid circulation along grain boundaries and microfractures down to 200 km depth in subduction environments. The Ulten Zone peridotites are slices of Variscan mantle wedge. Infiltration of metasomatic subduction fluids favoured transition from spinel-facies to garnet + amphibole ± dolomite parageneses at pressures below 3 GPa. Formation of metasomatized garnet-bearing peridotite mylonites suggest channelled influx of subduction fluids. The incompatible element-enriched signature of all subduction minerals in Bardane indicate that previously depleted websterites have been refertilized by COH subduction fluids. Comparison with the Ulten Zone garnet + amphibole ± dolomite peridotites outlines striking similarities in the metasomatic style and in the COH fluid phase involved. Comparable geochemical fingerprints observed in HP and UHP Ulten and Bardane mantle rocks call for concomitant subduction of the continental crust, that would provide the incompatible element-enriched fluids. For Bardane this implies subduction of crustal slabs to 200 km depth, to release the fluid phases necessary for UHP metasomatism and for majoritic garnet crystallization. Mantle refertilization by crust-derived COH subduction fluids thus operates over a large depth range, down to 200 km depth. The textural features of the Bardane and Ulten Zone mantle rocks indicate that rock recrystallization and refertilization is concomitant with fluid infiltration along channelways, outside which the rocks preserve pre-subduction, long-lasting stories and pristine geochemical characteristics. Comparably with uprising magmas, the subducted continental crust appears to be a quite efficient carrier to the surface of ultradeep mantle tectonic ‘xenoliths’ representing major observatories of diverse metamorphic and geodynamic histories recorded by the Earth's mantle at variable depths.

Metamorphic history of eclogites and country rock gneisses in the Aktyuz area, Northern Tien‐Shan, Kyrgyzstan: a record from initiation of subduction through to oceanic closure by continent–continent collision

AbstractEclogites and related high‐P metamorphic rocks occur in the Zaili Range of the Northern Kyrgyz Tien‐Shan (Tianshan) Mountains, which are located in the south‐western segment of the Central Asian Orogenic Belt. Eclogites are preserved in the cores of garnet amphibolites and amphibolites that occur in the Aktyuz area as boudins and layers (up to 2000 m in length) within country rock gneisses. The textures and mineral chemistry of the Aktyuz eclogites, garnet amphibolites and country rock gneisses record three distinct metamorphic events (M1–M3). In the eclogites, the first MP–HT metamorphic event (M1) of amphibolite/epidote‐amphibolite facies conditions (560–650 °C, 4–10 kbar) is established from relict mineral assemblages of polyphase inclusions in the cores and mantles of garnet, i.e. Mg‐taramite + Fe‐staurolite + paragonite ± oligoclase (An&lt;16) ± hematite. The eclogites also record the second HP‐LT metamorphism (M2) with a prograde stage passing through epidote‐blueschist facies conditions (330–570 °C, 8–16 kbar) to peak metamorphism in the eclogite facies (550–660 °C, 21–23 kbar) and subsequent retrograde metamorphism to epidote‐amphibolite facies conditions (545–565 °C and 10–11 kbar) that defines a clockwise P–T path. thermocalc (average P–T mode) calculations and other geothermobarometers have been applied for the estimation of P–T conditions. M3 is inferred from the garnet amphibolites and country rock gneisses. Garnet amphibolites that underwent this pervasive HP–HT metamorphism after the eclogite facies equilibrium have a peak metamorphic assemblage of garnet and pargasite. The prograde and peak metamorphic conditions of the garnet amphibolites are estimated to be 600–640 °C; 11–12 kbar and 675–735 °C and 14–15 kbar, respectively. Inclusion phases in porphyroblastic plagioclase in the country rock gneisses suggest a prograde stage of the epidote‐amphibolite facies (477 °C and 10 kbar). The peak mineral assemblage of the country rock gneisses of garnet, plagioclase (An11–16), phengite, biotite, quartz and rutile indicate 635–745 °C and 13–15 kbar. The P–T conditions estimated for the prograde, peak and retrograde stages in garnet amphibolite and country rock are similar, implying that the third metamorphic event in the garnet amphibolites was correlated with the metamorphism in the country rock gneisses. The eclogites also show evidence of the third metamorphic event with development of the prograde mineral assemblage pargasite, oligoclase and biotite after the retrograde epidote‐amphibolite facies metamorphism. The three metamorphic events occurred in distinct tectonic settings: (i) metamorphism along the hot hangingwall at the inception of subduction, (ii) subsequent subduction zone metamorphism of the oceanic plate and exhumation, and (iii) continent–continent collision and exhumation of the entire metamorphic sequences. These tectonic processes document the initial stage of closure of a palaeo‐ocean subduction to its completion by continent–continent collision.

Trace‐element compositions of single fluid inclusions in the Kofu granite, Japan: Implications for compositions of granite‐derived fluids

AbstractFluid inclusions in quartz from miarolitic cavities, pegmatites, and quartz veins in Miocene biotite‐granite plutons, Kofu, Japan, were analyzed by particle‐induced X‐ray emission to examine chemistries and behaviors of granite‐derived fluids in island‐arc granite. Most inclusions are aqueous two‐phase inclusions, and halite‐bearing polyphase inclusions are also observed in quartz veins in the upper part of the plutons. From element contents of fluid inclusions in the miarolitic cavities, the original fluid released from the granite plutons during solidification is inferred to have concentrations of Mn, Fe, Cu, Zn, Ge, Br, Rb, Pb, and Ba of several tens to hundreds of parts per million by weight (ppm) and a salinity of about 10 wt% NaCl equivalent. We estimated the formation conditions of the fluid to have been at about 1.3–1.9 kb and 530–600°C on the basis of the homogenization temperatures of the inclusions and the solidification conditions of the plutons. The polyphase inclusions probably originated from hypersaline fluid by boiling of part of the released fluid during its ascent in the plutons. The polyphase inclusions contain several hundreds to tens of thousands of ppm of Fe and Mn, and tens to several hundreds of ppm of Cu, Zn, Br, Rb, and Pb. The salinities are about 35 wt% NaCl equivalent. Compositional variations in two‐phase inclusions from the miarolitic cavities and quartz veins are primarily explained by mineral precipitation with dilution by surface water exerting a secondary influence. Thus, chemistries and behaviors of the granite‐derived fluids in the plutons can be explained by mineral precipitation, boiling, and dilution of the originally released fluid.

An alternative model for ultra-high pressure in the Svartberget Fe-Ti garnet-peridotite, Western Gneiss Region, Norway

Thepreviouslyreported''Fe-Titype''garnet-peridotiteislocatedinthenorthernpartofthewellknownultra-highpressure (UHP) area of the Western Gneiss Region (WGR) in Norway. Primary spinel stable up to only 2.0 GPa at 800 ! C coexists with Caledonian ultra-high pressure (4.0 GPa at 800 ! C) grt-cpx-opx-ol assemblages in the Svartberget garnet-peridotite. The body is cut byaconjugatesetofmetasomaticfracturesfilleddominantlywithdiamond-bearinggarnet-phlogopite-websterite (5.5GPaat800 ! C) and garnetite. Single zircon U-Pb dating suggests metamorphic growth of zircon in the garnetite at 397.2 1.2 Ma, either coinciding or predating an initial phase of leucosomes formation at 397-391 Ma. Field observations, major and trace elements, mineral- chemistry, polyphase inclusions including microdiamond, coupled with 87 Sr/ 86 Sr ratios in clinopyroxene and whole rock ranging from 0.73 to 0.74, suggest that the Svartberget garnet-peridotite was infiltrated by melts/fluids from the host-rock gneiss during the CaledonianUHPevent.PresentobservationsintheWGR documentaregionalmetamorphicgradientincreasingtowardstheNW,and structures in the field can account for the exhumation of the (U) HP rocks from # 2.5 to 3 GPa. Assuming lithostatic pressures the diamond-bearing Svartberget peridotite body must have come from a burial depth of more than 150 km. However, there is a lack of observable structures in the field to explain exhumation from extreme UHP conditions (5.5 GPa or more) to normal HP-UHP conditions (2.5-3 GPa), which are common pressures calculated from eclogites in western parts of the WGR. Because of the regional and mostly coherent metamorphic gradient across the WGR terrain, it is difficult to account for local extreme pressure excursions such as documented from within the Svartberget peridotite. We introduce here a conceptual model to explain the main features of the Svartberget body. During burial and heating, rocks surrounding the peridotite start to melt but surrounding non-molten rocks confine the space and pressure builds up. When pressure is high enough, conjugate brittle shear fractures develop in the peridotite. Melt (or supercritical fluid) that has the same pressure (5.5 GPa) as the surrounding gneiss can flow in as soon as fractures propagate into the peridotite. This supercritical fluid is now highly reactive and metasomatism takes place at UHP conditions along the fractures capturing micro inclusions of diamond while growing. Finally the lithosphere holding the overpressurised gneiss con- strained breaks duetoformationoflarge-scale fracturesinthe crustanddecompression meltingstarts.Modelling usingfinite-element method (FEM) shows that melting of the gneiss results in pressure variations when gneiss is ten to hundred times weaker than surroundings and peridotite enclave. These pressure variations can be up to several GPa and are qualitatively similar to observations in the field.