Multiphase Solid Inclusions Research Articles

Experimental constraints on the nature of multiphase solid inclusions and their bearing on mantle wedge metasomatism, Bohemian Massif

This study tests experimentally the hypothesis that calculated bulk compositions of multiphase solid inclusions present in minerals of ultrahigh pressure rocks, can be equated to the composition of the former trapped fluids. We investigated samples from the ultrahigh pressure garnet peridotites of the Bohemian Massif, spatially associated with ultrahigh pressure crustal rocks and representing a former subduction interface environment. Inclusions present in garnets, composed of amphibole + Ba-mica kinoshitalite + carbonates (dolomite + magnesite + norsethite), were taken to their entrapment conditions of c. 4.5 GPa and 1075 ºC. They (re)crystallized into a garnet fringe at the boundary between inclusion and host garnet, kinoshitalite ± olivine, carbonatite melt, and a hydrous fluid. Although the latter may have exsolved from the carbonatite melt upon quenching, microstructures suggest it was present at trapped conditions, and mass balance indicates that it corresponds to a Na-K-Cl-F-rich saline aqueous fluid (brine). Experiments demonstrate the stability of kinoshitalite at 4.5 GPa and 1075 ºC, and suggest that Ba-rich mica + carbonatite melt + brine coexisted at near-peak conditions. Barium is compatible in the carbonatite melt and mica with respect to the brine, with a partition coefficient between carbonatite melt and mica of ≈ 2.5–3. The garnet fringe formed from incongruent reaction of the former inclusion assemblage due to reversing the fluid(s)-host garnet reaction that occurred upon natural cooling/decompression. Loss of H2 or H2O from the inclusions due to volume diffusion through garnet and/or decrepitation, during geological timeframes upon decompression/cooling, may have prevented rehomogenization to a single homogeneous fluid. Our study shows that great care is needed in the interpretation of multiphase solid inclusions present in ultrahigh pressure rocks.

Crystallisation of trapped carbonate–silicate melts terminating at the carbonated solidus ledge: a record of carbon immobilisation mechanism in the lithospheric mantle

Orogenic peridotites in the crystalline basement of the northwestern Bohemian Massif contain multiphase solid inclusions (MSI), which are interpreted to be crystallisation products of trapped former carbonate–silicate melts metasomatizing their host rocks. We applied conventional thermobarometry and forward thermodynamic modelling to constrain the P–T evolution ranging from the peak metamorphic conditions of the investigated harzburgite and lherzolite, through entrapment of the melts in the outer parts of garnets, to the (re)-equilibration of the MSI assemblages. The peak conditions of c. 1100 °C/4.5–5.5 GPa are recorded by garnet cores and large pyroxene porphyroclasts. The melt entrapment, during which garnet outer parts grew, was associated with influx of the metasomatizing liquids and probably took place during the early stage of the exhumation. Thermodynamic model of amphibole-free MSI assemblage comprising kinoshitalite/Ba-rich phlogopite (approximated by phlogopite in the model), dolomite, magnesite, clinopyroxene, orthopyroxene, garnet and chromite provided robust estimate of P and T of its (re)-equilibration, c. 900–1000 °C, 1.8–2.2 GPa. Furthermore, the lack of olivine reflects co-existence of COH fluid with high X(CO2) = CO2/(CO2 + H2O) ≥ 0.6. Models employing identical P–T–X(CO2) parameters successfully reproduced the other two amphibole-bearing assemblages observed. The modelled stability fields show perfect alignment with a characteristic isobaric segment of the solidus curve of carbonated peridotite. This co-incidence implies that the (re)-equilibration corresponds to the termination of the melt crystallisation once the near-isothermal exhumation path intersected the solidus. Decreased solubility of silicates at the carbonated peridotite “solidus ledge”, inferred from the published experimental data, as well as concentric textures of some MSI indicates sequential crystallisation from the early silicates to late dolomite. The carbonated “solidus ledge” is a relatively narrow boundary in the lithospheric mantle capable of an abrupt immobilisation of fluxing or transported carbonated melts. The investigated rocks are estimated to store approximately 0.02 kg C/m3 (or 6 ppm C) occurring as carbonates in the MSI.

To the transition zone and back: Records from diopside exsolution lamellae in chromite from Archean ophiolitic podiform chromitites

Plate tectonics and deep subduction provide the conveyor belt for elemental cycling between Earth's surface and deep interior. Here, we report Archean diopside exsolution lamellae in chromite grains from 2.5 billion years old (Ga) subducted then exhumed Archean oceanic crustal fragments in ophiolitic mélange. Diopside exsolution lamellae are a key indicator of a precursor ultra-high pressure (UHP) phase of chromite, known previously only from Phanerozoic subduction zones, indicating deep subduction and elemental cycling between the surface and deep Earth. These lamellae, along with trapped dolomite, and UHP TiO2(II)-bearing multi-phase solid crustal mineral inclusions in the host chromite, suggest that a precursor chromite polymorph (chenmingite) with a crystal structure stable only at conditions in the mantle transition zone (> 413 to 660 km) preserve the history of subduction, entrapment of UHP crustal mineral inclusions, and return flow to the surface in an Archean subduction/forearc system. These nano-scale diopside lamellae and UHP TiO2(II)-bearing multi-phase solid inclusions are strong evidence for deep and steep subduction in the Neoarchean, and early operation of the deep crustal material recycling, linking surface and deep environments, forging a habitable world.

Omphacite Melting and the Destruction of Early High‐Pressure Rock Records

AbstractThe presence of eclogite marks high‐pressure metamorphism and deep, cold burial of crust, and is therefore a widely accepted indicator for the onset of modern‐style plate tectonics. However, eclogite is rarely preserved in many heavily granulite‐overprinted orogens, which are particularly common across ancient metamorphic terranes (i.e., Archean and Paleoproterozoic terranes). Here, we show that eclogite melting processes may be principally responsible for the poor preservation of high‐pressure records. We demonstrate eclogite melting via detailed petrological, geochronological, and geochemical analyses on eclogites and separated centimetric leucosomes from the central Himalaya. The central Himalayan eclogites were overprinted by strong granulite‐facies metamorphism, such that omphacite is only sparsely preserved. Thermodynamic modeling results indicate the eclogite experienced two types of anatectic reactions: phengite dehydration melting at high‐pressure in the presence of omphacite, and subsequent omphacite dominated melting during exhumation. Diagnostic features for these melting reactions include (a) the occurrence of Kfs‐Pl‐Qz multiphase solid inclusions and Cpx‐Pl‐Qz polymineralic inclusions; (b) the high Na2O/K2O melts and Na‐rich plagioclase in leucosomes; (c) the consistent trace elements patterns between omphacite and leucosomes; and (d) obvious Na2O zonation in clinopyroxene. Omphacite dominated melting is characterized by the jadeite breakdown, releasing Na and Al into the melts. This melting mechanism subsequently forms a less sodic clinopyroxene and high Na2O/K2O melts. These Himalayan findings appear relevant to early Earth explorations because of the high thermal gradients and intense granulite‐facies overprints, implying that partial melting of eclogite dominated by omphacite breakdown could have erased early high‐pressure records of modern‐style plate tectonics.

Pressure–temperature–time path of ultrahigh-pressure metamorphic terranes constrained by zircon petrochronology: A case study of orthogneisses from the northern Sulu belt, China

Reconstruction of pressure–temperature–time (P–T–t) path of volumetrically-dominant orthogneisses in deeply subducted continental crusts is challenging due to their simple amphibolite-facies mineral assemblages. To provide new insights into this issue, a detailed study of mineral inclusions, trace elements and U-Pb ages within robust zircon containers, together with titanite U-Pb dating and petrology were conducted on ultrahigh-pressure (UHP) orthogneisses from the northern Sulu belt. The following P–T–t conditions of three metamorphic stages were constrained: (i) cathodoluminescence (CL)-grey zircon cores (ca. 238–237 Ma) with flat heavy rare earth element (HREE) patterns include garnet, phengite and omphacite that yield estimated P–T conditions of 2.3–2.8 GPa/730–800 °C, corresponding to prograde high-pressure (HP) metamorphism before the UHP peak. (ii) CL-dark zircon mantles (ca. 224 Ma) contain multiphase solid inclusions of plagioclase + K-feldspar + quartz and show extremely low Th/U ratios (≤0.08) and negative Eu anomalies, demonstrating their crystallization from hydrous melt at 2.0–1.8 GPa/890–820 °C during HP eclogite-facies exhumation. In addition, titanite grains are characterized by high LREE contents (1939–5239 ppm), Th/U ratios (0.37–0.81) and crystallization temperatures (830–801 °C), suggesting their peritectic origin. The lower intercept age (220 ± 4 Ma) of titanite is broadly consistent with the ages of the zircon mantles, indicating that they both crystallized from the melt at the same stage. (iii) CL-bright zircon rims (ca. 212–211 Ma) preserve low-pressure mineral inclusions of plagioclase, K-feldspar and quartz, and they show low Th/U ratios (0.03–0.12) and steep HREE patterns with negative Eu anomalies, witnessing amphibolite-facies overprinting at 1.0–0.8 GPa/610–550 °C. Combined with the metamorphic peak conditions of 3.2 GPa/840 °C at ca. 232 Ma constrained by previous studies from the same area, the P–T–t path of the northern Sulu UHP belt is reconstructed. The data reflects two-stage exhumation of this terrane, including an early-stage (5.8 km/m.y.) and a later-stage slower (3.1 km/m.y.) exhumation processes. The faster exhumation during early decompression was probably enhanced by extensive melting activity recorded by the CL-dark zircon mantles from the orthogneisses.

Fingerprinting crustal anatexis with apatite trace element, halogen, and Sr isotope data

The geochemistry of apatite is a valuable tool for granite petrogenetic study. Here we explore the capability of apatite for fingerprinting anatectic reactions during crustal melting. We conducted a detailed petrographic, trace element, volatile, and Sr isotopic study of the apatites from four groups of leucosomes and leucogranites that formed by amphibole dehydration melting (group I), hydration melting (group II), muscovite dehydration melting (group III), and feldspar accumulation (Group IV) from the central Himalayan orogen. The coexistence of primary CO2-rich fluid and multiphase solid inclusions in apatite indicates fluid–melt immiscibility and suggests that C–O–H fluids were present during hydration melting. We observe marked differences in major and trace element, as well as halogen (F and Cl) contents in apatite from different groups, corresponding to different melting reactions. Apatite REE, Y, Mn, F, and F/Cl exhibit a decrease, whereas Sr/Y and Cl increase, from muscovite dehydration melting to hydration melting, and to amphibole dehydration melting. Amphibole dehydration melting results in similar CaO, Sr, and Eu/Eu* in apatite to hydration melting, but the latter leads to lower (La/Yb)N in apatite. These geochemical differences in apatite can be explained by different minerals participating in the melting reactions and varying melt polymerization resulting from the different melting reactions. The degree of melt polymerization decreases in the sequence from muscovite dehydration melting to hydration melting, and to amphibole dehydration melting. A higher degree of melt polymerization results in higher values for trace element partition coefficients between apatite and melt, and therefore the enhanced incorporation of REE, Y and Mn in apatite. Higher Sr and Eu/Eu* in apatite reflect the consumption of a larger amount of plagioclase in the melting reaction. The F and Cl contents in partial melts were calculated based on apatite composition, which exhibit an increasing trend from hydration melting to muscovite hydration melting, and to amphibole hydration melting. Such a trend can be explained by different proportions of hydrous minerals (muscovite and amphibole) participating in the melting reactions, with amphibole responsible for the high Cl content in melt and apatite. In addition, the apatite Sr isotopic compositions record the Sr isotopic disequilibrium during partial melting and exhibit distinct evolutional trends resulting from different anatectic reactions. Muscovite dehydration melting forms melt with higher 87Sr/86Sr than the restite due to radiogenic Sr released by muscovite. Apatite therefore records progressively decreasing 87Sr/86Sr during melt evolution before the isotopic homogenization between the partial melt and the restite. In contrast, apatites crystallized from the melt of hydration melting show an evolutional trend of increasing 87Sr/86Sr. In summary, the elemental and isotopic compositions of apatite can be related to different types of melting reactions, and can therefore be used as an indicator of the mechanisms of melt formation during crustal anatexis. We demonstrate such an application through the case study of group IV apatite from feldspar accumulation, and also explore its utility in granite petrogenetic studies.

Compositional trends in Ba-, Ti-, and Cl-rich micas from metasomatized mantle rocks of the Gföhl Unit, Bohemian Massif, Austria

Abstract Ba-, Ti-, and Cl-rich micas associated with other Ba- and/or Cl-rich minerals in the rock matrix or in garnet and clinopyroxene hosted multiphase solid inclusions (MSI) are observed in mantle-derived garnet pyroxenites. The micas show extremely high variability in chemical composition ranging between Ba-rich phlogopite, chloroferrokinoshitalite, and oxykinoshitalite. Elemental covariation trends in mineral chemical data reveal the principal substitution mechanisms responsible for the observed chemical variability. The substitution Ba2+Al3+ ↔ K1+Si4+ associated with either OH1– ↔ Cl1– or Ti4+2O2– ↔ Mg2+2OH1 links phlogopite to chloroferrokinoshitalite and oxykinoshitalite, respectively, whereas the substitution Ti4+2O2– ↔ Fe2+2Cl1– links chloroferrokinoshitalite to oxykinoshitalite. The preferred incorporation of Cl in Fe-rich mica and of Ti+O in Mg-rich mica indicates that XFe (Fetot/Fetot+Mg) exerts an important control on mica composition. The positive correlation of XFe with Cl led to the formation of possibly the most Cl-rich mica so far described classified as chloroferrokinoshitalite (XFe0.88, Ba0.95K0.03Fe2.68Mg0.37Al1.91Si2.01Cl1.98) with 10.98 wt% Cl. Substantial substitution of OH– by Cl– and O2– in mica, and the presence of Cl-apatite, a rare Cl-rich phosphate goryainovite, and carbonates together with Cl-rich micas indicate high-Cl and -CO2 activity and low-H2O activity in metasomatizing fluids or melts that may be classified as Ba-Cl-rich silicocarbonatitic. The coexistence of two micas with distinct compositions close to chloroferrokinoshitalite (XFe0.57–0.77, K~0.1Ba0.6–0.8Mg0.7–1.3Fe1.7–2.3Ti0.0–0.1 Si2.2–2.3Al1.5–1.7Cl1.2–1.8) and oxykinoshitalite (XFe0.19–0.20, K~0.3Ba~0.5Mg2.0–2.1Fe~0.5Ti0.2–0.4Si2.4–2.6Al~1.8Cl~0.3) suggests that a miscibility gap exists between these two compositions. The exotic mineral assemblage was formed by interaction with metasomatizing fluids or melts whose origin cannot be defined with certainty. They may be derived from crustal or mantle lithologies or from the host garnet pyroxenites. The textural position of the MSI in garnet and their characteristic mineral assemblages indicate that they have been introduced into the garnet crystals under post-peak conditions, possibly during decompression. With this research we document substitution mechanisms in Ba-, Ti-, and Cl-rich micas and shed light on the behavior and composition of fluids or melts at the upper mantle/lower crust interface.

Evolution of fluids and melts in deeply subducted continental crust: Insights from an UHP eclogite–vein system in the Dabie terrane, China

Both slab-derived high-pressure (HP) or ultrahigh-pressure (UHP) aqueous fluids and hydrous melts are important agents for material cycling in subduction zones. Although numerous studies have investigated the genesis of both fluid phases, the temporal–spatial relationship and the chemical difference between aqueous fluids and hydrous melts during subduction-zone metamorphism remain mysterious. In this study, we present detailed petrological and geochemical studies on the UHP eclogites and enclosed epidote-quartz veins in the Dabie terrane (eastern China) to understand the compositions and behaviors of fluids/melts in subduction zones. The eclogites consist mainly of garnet, omphacite, and hydrous minerals (epidote, phengite, and paragonite). Two stages of epidote, including epidote inclusions in omphacite (Ep-in) and porphyroblastic epidote (Ep-p), are recognized in the eclogites. The obviously higher contents of strontium (Sr) and light rare earth elements (LREEs) in Ep-p than in Ep-in demonstrate the previous coexistence of lawsonite and Ep-in. The combination of petrographic analyses, epidote Sr isotopes, and phase equilibria modeling indicates lawsonite in the eclogite breakdown at ~2.2–2.7 GPa and 620–750 °C during the initial exhumation, accompanied by the significant release of H2O-rich fluids. This process led to the growth of Ep-p in eclogite and the precipitation of vein omphacite, quartz, and epidote (Ep-v1). On the other hand, many lines of evidence for partial melting in the eclogites are found, including (1) multiphase solid inclusions (MSIs) of Ky + Kfs ± other phases, which represent the in situ products of phengite partial melting; (2) MSIs of Kfs ± Pl ± Qz, which represent the crystallized products of hydrous melts; and (3) veinlets of Kfs ± Di that crosscut the omphacite. Importantly, the MSIs in texture (1) are exclusively hosted in the Ep-p, which formed by lawsonite breakdown. This distinctive phenomenon indicates that the partial melting of phengite was most likely triggered by the injection of H2O from the lawsonite breakdown (i.e., water-fluxed melting). Phase equilibria modeling suggests that the partial melting of phengite occurred at ~1.5–2.0 GPa and 700–750 °C during the further exhumation stage of the eclogites. The occurrence of Ep (Ep-v2) + Pl + Kfs in the veins indicates that melts derived from the eclogites infiltrated the vein system. The compositions of the fluids and melts are quantitatively constrained using the epidote compositions (Ep-v1 and Ep-v2, respectively) and epidote–fluid/melt partition coefficients. The results indicate that the fluids have trace element patterns similar to those of the melts with high contents of large ion lithophile elements (LILEs), Th, U, Pb, and LREEs. Thus, aqueous fluids have the ability to transfer significant amounts of mobile elements from the slab like the hydrous melts, if the appropriate P–T conditions and chemical characteristics of wallrocks can be satisified.Our study reveals multistage metamorphic-fluid/melt evolution in the UHP eclogites, involving metamorphic dehydration and subsequent partial melting during the exhumation of subducted continental crust. We provide petrological evidence linking fluid liberation to the occurrence of partial melting in eclogites. Moreover, this study indicates that the development of vein systems in HP–UHP rocks may provide high-permeability channels that allow the flow and escape of melts formed during later-stage process. We propose that the occurrence of early-stage dehydration events in deeply subducted slabs facilitates the generation of hydrous melts and their transport in subduction zones.

Multiphase solid inclusions reveal the origin and fate of carbonate-silicate melts in metasomatised peridotite

Orogenic peridotite in ultrahigh-pressure ultrahigh-temperature terranes provides increasingly strong evidence of material transfer in a subduction-zone setting, causing the metasomatism and melting of mantle rocks. Multiphase solid inclusions (MSI) trapped in garnet can provide clues to these processes occurring at great depth. The MSI enclosed in garnet in pyroxenite, lherzolite and harzburgite from a drillcore in the Saxothuringian basement in the Variscan Bohemian Massif are dominated by hornblende, barian mica and carbonate (dolomite and magnesite), and they contain a large variety of other phases, including spinel, clinopyroxene, orthopyroxene, garnet II, apatite, monazite, thorianite, graphite, norsethite and barian feldspar, scheelite and sulfides (e.g., pentlandite). We demonstrate, that the bulk chemical composition of the MSI evolves from the MSI trapped in garnet pyroxenite, which was the source of the metasomatising liquids, to the MSI in the host peridotite. Garnet pyroxenite has a carbonate-silicate composition comparable to kimberlite, and represents a rare example of a low-degree supercritical melt derived from a mixed crust-mantle source frozen in the mantle. Its high-pressure fractional crystallization produced a residual melt, enriched in alkalies (Na, K), highly incompatible elements (LILE – Ba, Sr; Th, U), LREE, Ti, W and volatiles (CO2, Cl, F, P), which is now trapped in the MSI in pyroxenite garnet. The MSI in peridotite reflect changes in the physical and chemical characteristics of the metasomatising medium during its reactive infiltration into peridotite through silicate crystallization and interaction with mantle minerals reflecting the type of rock (lherzolite vs. harzburgite). A change from a more silicic, solute-rich melt to a more diluted carbonate-rich liquid gradually enriched in LILE (K, Ba) and volatiles (CO2, Cl) and LREE fractionation with an increasing degree of fluid evolution coincides with the evolution of kimberlitic melts to carbonatitic melts. The low-viscosity carbonate-rich liquids can migrate far and metasomatise a considerable volume of the rock. Our work demonstrates that the MSI trapped in garnet represent a unique tool to trace the chemical and physical characteristics of the liquids that metasomatise the mantle wedge and their evolution.

Petrogenesis and Lu–Hf Dating of (Ultra)Mafic Rocks from the Kutná Hora Crystalline Complex: Implications for the Devonian Evolution of the Bohemian Massif

Abstract The Lu–Hf isotope system and Sr–Nd–Hf–Os isotope systematics of mantle rocks are capable of unravelling the early processes in collision belts, especially in a hot subduction context where the Sm–Nd and U–Pb systems in crustal rocks are prone to resetting owing to high temperatures and interaction with melts during exhumation. To improve models of the Devonian–Carboniferous evolution of the Bohemian Massif, we investigated in detail mafic and ultramafic rocks (eclogite, pyroxenite, and peridotite) from the ultrahigh-pressure and ultrahigh-temperature Kutná Hora Crystalline Complex (KHCC: Úhrov, Bečváry, Doubrava, and Spačice localities). Petrography, multiphase solid inclusions, major and trace element compositions of rocks and minerals, and radiogenic isotopic data document contrasting sources and protoliths as well as effects of subduction-related processes for these rocks. The Úhrov peridotite has a depleted composition corresponding to the suboceanic asthenospheric mantle, whereas Bečváry and Doubrava peridotites represent lithospheric mantle that underwent melt refertilization by basaltic and SiO2-undersaturated melts, respectively. Multiphase solid inclusions enclosed in garnet from Úhrov and Bečváry peridotites represent trapped H2O ± CO2-bearing metasomatizing agents and Fe–Ti-rich melts. The KHCC eclogites either formed by high-pressure crystal accumulation from mantle-derived basaltic melts (Úhrov) or represent a fragment of mid-ocean ridge basalt-like gabbroic cumulate (Spačice) and crustal-derived material (Doubrava) both metamorphosed at high P–T conditions. The Lu–Hf age of 395 ± 23 Ma obtained for the Úhrov peridotite reflects garnet growth related to burial of the asthenospheric mantle during subduction of the oceanic slab. By contrast, Spačice and Doubrava eclogites yield younger Lu–Hf ages of ∼350 and 330 Ma, respectively, representing mixed ages as demonstrated by the strong granulite-facies overprint and trace element zoning in garnet grains. We propose a refined model for the Early Variscan evolution of the Bohemian Massif starting with the subduction of the oceanic crust (Saxothuringian ocean) and associated oceanic asthenospheric mantle (Úhrov) beneath the Teplá–Barrandian at ≥380 Ma, which was responsible for melt refertilization of the associated mantle wedge (Bečváry, Doubrava). This was followed by continental subduction (∼370–360 Ma?) accompanied by the oceanic slab break-off and incorporation of the upwelling asthenospheric mantle into the Moldanubian lithospheric mantle and subsequent coeval exhumation of mantle and crustal rocks at ∼350–330 Ma.

Commensurate Growth of Magnetite Microinclusions in Olivine under Mantle Conditions

Magnetite-bearing multiphase solid inclusions hosted in metamorphic olivine have been interpreted as final products of the trapping of the aqueous fluid produced by the subduction-zone dehydration ...

Polymineralic inclusions as tracers of multistage metasomatism in a paleo mantle wedge

Paleosubduction zones are key areas where slab-derived fluids interact with the overlying mantle wedge. Slab fluids play therefore an important role in the deep element cycle and can metasomatize ultramafics showing particular mineral assemblages. The nature of slab fluids is usually investigated from fluid inclusions and/or multiphase solid inclusions, as well as from incompatible elements in mantle minerals. We have made a study on a garnet-peridotite from the Variscan Ulten Zone (Eastern Alps, Italy) in which peculiar polymineralic inclusions (PI) in garnet provide a wealth of information on the interaction between mantle wedge and slab fluids. The selected sample shows a fine-grained amphibole-olivine-orthopyroxene-spinel-clinopyroxene matrix and a fractured large garnet porphyroclast, surrounded by a kelyphitic corona. Garnet core and rim host randomly distributed PI, which may be as large as 284,000 μm2. The PI show irregular, fracture-controlled shapes and often contain unusual mineral associations. Many PI are directly associated with Mg-hornblende- or chlorite-sealed micro-fractures crosscutting garnet. The most abundant PI contain spinel and amphibole as the main minerals. Spinel-bearing PI consists of high-Cr spinel (Cr# = 0.31–0.41) surrounded by amphiboles (magnesio-hornblende), locally associated with dolomite, sapphirine, calcite, sulphides and kinoshitalite (i.e. a Ba-rich phlogopite). Amphibole-bearing PI contain zoned amphiboles (tschermakitic cores surrounded by Mg-hornblende, which is rimmed by tremolite and Mg-amphibole), dissakisite (an REE-bearing epidote), apatite, dolomite, calcite, spinel, sapphirine, chlorite, kinoshitalite, and sulfides. The retrieved mineral reaction history and the major and some incompatible elements (Cl, Ba, Sr) of PI minerals provide clues about the composition of metasomatic fluids. The studied PI formed in garnet fractures during exhumation by reactions between garnet and (1) slab-derived COH fluids and (2) late-stage saline brines. Metasomatic fluids/brines were rich in volatile elements and had a crustal signature (enrichment in LILE and LREE) with variable Cl contents and CO2/H2O ratios. Products of early garnet-fluid reactions became reactants of later reactions, thus generating complex textures, including replacement textures of sapphirine over spinel, calcite over dolomite and chlorite over Mg-hornblende. Textures and compositions of the PI testify to a multistage metasomatic overprint at peak P-T conditions above 2 GPa (M1), during the first exhumation stage at 775–1000 °C, below 2 GPa (M2) and finally at 775 °C and 1 GPa (M3). In this study we demonstrate that Gibbs free energy minimization may aid in distinguishing between multiphase solid inclusions and polymineralic inclusions; the latter do provide valuable information on the evolution of retrograde P-T paths in the paleosubduction channel, the relative chronology of metasomatic events and clues on the chemical signature of the fluids.

High resolution X–ray computed tomography and scanning electron microscopy studies of multiphase solid inclusions in Oman podiform chromitite: implications for post–entrapment modification

High resolution X–ray computed tomography and scanning electron microscopy studies of multiphase solid inclusions in Oman podiform chromitite: implications for post–entrapment modification

Separating multiple episodes of partial melting in polyorogenic crust: An example from the Haiyangsuo complex, northern Sulu belt, eastern China

Abstract The exotic Haiyangsuo complex is structurally part of the Sulu belt but its contact relationship with surrounding Sulu gneisses is unexposed and therefore unknown, making its affinity uncertain. It comprises gneisses with in-source leucosomes that host minor metabasite bodies; both are cut by leucogranite dikes. In this study, we determine the timing and petrogenesis of leucosomes and leucogranites and assess the tectonic affinity of the complex based on data from gneisses and metabasites. Most zircon from gneisses and leucosomes has oscillatory-zoned cores with CL-bright overgrowth rims, but some has CL-dark cores or mantles between cores and rims. CL-dark and bright zircon yield weighted mean ages of ca. 1817–1812 Ma. CL-dark zircon has flat heavy rare earth element (HREE) patterns and crystallization temperatures of 829–875 °C, suggesting metamorphic growth, whereas rims have steep HREE patterns but a similar range of crystallization temperatures, suggesting growth from anatectic melt; εHf (t = 1813 Ma) of –18.3 to –10.8 indicates a North China Craton source. Magmatic zircon from metabasites yields ages of ca. 825 Ma, similar to those of scattered metabasite occurrences in the North China Craton. Paleoproterozoic zircon cores were scavenged during magma ascent. By contrast, zircon cores from the leucogranites yield concordant dates of 776–701 Ma, consistent with protolith ages in the Sulu belt, whereas overgrowth mantles and rims yield weighted mean ages of ca. 220 Ma and 162 Ma, respectively. Both mantles and rims host multiphase solid inclusions, representing former melt, suggesting anatexis and crystallization of zircon first during initial decompression and then during orogenic collapse of the Sulu belt; whole-rock Nd and Sr isotope compositions implicate the Sulu belt gneisses as the source of these melts. Our interpretation of these data is that the Haiyangsuo complex has an early geologic history similar to the Jiaobei terrane from the southeastern part of the North China Craton and was incorporated into the Sulu belt during Triassic collision of the Yangtze and North China Cratons. The two stages of melting relate to Upper Triassic early exhumation and Upper Jurassic late-stage orogenic collapse, during which the leucogranite magma was derived from a source similar to one elsewhere in the Sulu belt such as the subducted Yangtze Craton and not the North China Craton. This shows that during continental collisions, crust from the upper plate may be dragged into the subduction channel, deformed, and subsequently exhumed in association with partial melting of the crust.

Composition of barian mica in multiphase solid inclusions from orogenic garnet peridotites as evidence of mantle metasomatism in a subduction zone setting

Multiphase solid inclusions in minerals formed at ultra-high-pressure (UHP) provide evidence for the presence of fluids during deep subduction. This study focuses on barian mica, which is a common phase in multiphase solid inclusions enclosed in garnet from mantle-derived UHP garnet peridotites in the Saxothuringian basement of the northern Bohemian Massif. The documented compositional variability and substitution trends provide constraints on crystallization medium of the barian mica and allow making inferences on its source. Barian mica in the multiphase solid inclusions belongs to trioctahedral micas and represents a solid solution of phlogopite KMg3(Si3Al)O10(OH)2, kinoshitalite BaMg3(Al2Si2)O10(OH)2 and ferrokinoshitalite BaFe3(Al2Si2)O10(OH)2. In addition to Ba (0.24–0.67 apfu), mica is significantly enriched in Mg (XMg ~ 0.85 to 0.95), Cr (0.03–0.43 apfu) and Cl (0.04–0.34 apfu). The substitution vector involving Ba in the I-site which describes the observed chemical variability can be expressed as BaFeIVAlClK−1Mg−1Si−1(OH)−1. A minor amount of Cr and VIAl enters octahedral sites following a substitution vector VI(Cr,Al)2□VI(Mg,Fe)−3 towards chromphyllite and muscovite. As demonstrated by variable Ba and Cl contents positively correlating with Fe, barian mica composition is partly controlled by its crystal structure. Textural evidence shows that barian mica, together with other minerals in multiphase solid inclusions, crystallized from fluids trapped during garnet growth. The unusual chemical composition of mica reflects the mixing of two distinct sources: (1) an internal source, i.e. the host peridotite and its garnet, providing Mg, Fe, Al, Cr, and (2) an external source, represented by crustal-derived subduction-zone fluids supplying Ba, K and Cl. At UHP–UHT conditions recorded by the associated diamond-bearing metasediments (c. 1100 °C and 4.5 GPa) located above the second critical point in the pelitic system, the produced subduction-zone fluids transporting the elements into the overlying mantle wedge had a solute-rich composition with properties of a hydrous melt. The occurrence of barian mica with a specific chemistry in barium-poor mantle rocks demonstrates the importance of its thorough chemical characterization.

Fossilized Melts in Mantle Wedge Peridotites

The shallow oxidized asthenosphere may contain a small fraction of potassic silicate melts that are enriched in incompatible trace elements and volatiles. Here, to determine the chemical composition of such melt, we analysed fossilized melt inclusions, preserved as multiphase solid inclusions, from an orogenic garnet peridotite in the Bohemian Massif. Garnet-poor (2 vol.%) peridotite preserves inclusions of carbonated potassic silicate melt within Zn-poor chromite (<400 ppm) in the clinopyroxene-free harzburgite assemblage that equilibrated within the hot mantle wedge (Stage 1, > 1180 °C at 3 GPa). The carbonated potassic silicate melt, which has a major element oxide chemical composition of K2O = 5.2 wt.%, CaO = 17 wt.%, MgO = 18 wt.%, CO2 = 22 wt.%, and SiO2 = 20 wt.%, contains extremely high concentrations of large ion lithophile elements, similar to kimberlite melts. Peridotites cooled down to ≅800 °C during Stage 2, resulted in the growth of garnet relatively poor in pyrope content, molar Mg/(Mg + Fe + Ca + Mn), (ca. 67 mol.%). This garnet displays a sinusoidal REE pattern that formed in equilibrium with carbonatitic fluid. Subsequently, subduction of the peridotite resulted in the formation of garnet with a slightly higher pyrope content (70 mol.%) during the Variscan subduction Stage 3 (950 °C, 2.9 GPa). These data suggest the following scenario for the generation of melt in the mantle wedge. Primarily, infiltration of sediment-derived potassic carbonatite melt into the deep mantle wedge resulted in the growth of phlogopite and carbonate/diamond. Formation of volatile-bearing minerals lowered the density and strength of the peridotite. Finally, phlogopite-bearing carbonated peridotite rose as diapirs in the mantle wedge to form carbonated potassic silicate melts at the base of the overriding lithosphere.

CO2‐bearing fluid inclusions associated with diamonds in zircon from the UHP Kokchetav gneisses

CO2‐bearing fluid inclusions coexisting with diamonds were identified in zircons from diamondiferous gneiss in the Kokchetav Massif. This discovery provides evidence for the presence of CO2 in UHP fluids and diamond formation in moderately oxidized conditions in the Kokchetav gneiss. Fluid and multiphase solid inclusions coexisting in zircons represent immiscible melt and fluid captured close to the peak metamorphic conditions for the Kokchetav UHP gneiss. Most of CO2‐bearing inclusions are CO2+H2O mixtures except for some cases when they also contain daughter phases (e.g. muscovite, calcite and quartz) tracing the presence of aqueous and solute‐rich fluids at different phases of UHP metamorphism. Decrease of pressure and temperature may have been responsible for the reduction of solutes in the CO2‐bearing fluid. The lack of CO2‐bearing inclusions in garnet porphyroblasts from diamond‐bearing gneiss, as well as the common coexistence of aqueous CO2‐bearing inclusions with calcite, testify that most likely all CO2 in fluid was consumed by the calcite‐forming reaction and hydrous melt was the only remaining growth medium during retrograde metamorphism of the Kokchetav UHPM gneisses. Neither K‐cymrite nor kokchetavite was identified among daughter phases in the hydrous melt inclusions in garnet, which indicates that they hardly could originate in a metapelitic system. Copyright © 2017 John Wiley &amp; Sons, Ltd.

Melting History of an Ultrahigh-pressure Paragneiss Revealed by Multiphase Solid Inclusions in Garnet, Kokchetav Massif, Kazakhstan

Abundant multiphase solid inclusions (MSI) were found in garnet in an ultrahigh-pressure (UHP) paragneiss from the Kokchetav complex, Kazakhstan. The MSI are composed of mineral associations that include rock-forming and accessory minerals, which crystallized during exhumation. We present experimental and analytical protocols for how such inclusions can be homogenized to glass and analysed for major and trace elements. After homogenization we identified two types of glass. One type is present in garnet porphyroblasts in the melanocratic part of the sample and represents a high-pressure melt formed close to peak conditions of >45 kbar, 1000°C. These inclusions are characterized by high concentrations of light rare earth elements (LREE), Th and U. Extraction of these melts resulted in a pronounced depletion of the Kokchetav gneisses in those elements. Measured partition coefficients of large ion lithophile elements (LILE) between phengite inclusions and melt inclusions are D Rb = 1·9–2·5, D Ba = 1·1–6·9 and D Cs = 0·6–0·8, resulting in limited depletion of these elements during partial melting in the presence of phengite. The Nb concentration in melts (27 ppm) is about double that in the restite (15 ppm), indicating slightly incompatible behaviour during UHP anatexis, despite the presence of residual accessory rutile and phengite. A second type of inclusion occurs in garnet from the leucocratic part of the rock and represents a late-stage melt formed during exhumation at 650–750°C and crustal pressures. These inclusions are characterized by low LREE and Nb and high U. Zircon domains formed during high-temperature melting are characterized by high Ti content (100–300 ppm) and unfractionated Th/U (0·4–0·8), whereas the low-temperature domains display low Ti (10 ppm) and Th/U (0·08). The composition of UHP melts with moderate enrichment in LILE, no depletion in Nb and extreme enrichment in LREE and Th is remarkably different from the trace element signature of arc basalts, arguing against involvement of this type of melting in the generation of arc crust. The composition of the UHP melt inclusions is similar to that of melt inclusions from HP crustal xenoliths from Pamir and also to some shoshonites from Tibet. UHP anatexis, as observed in the Kokchetav massif, might be related to the formation of shoshonitic alkaline igneous rocks, which are common in collisional settings.

Multi-stage barite crystallization in partially melted UHP eclogite from the Sulu belt, China

Barite inclusions in rock-forming and accessory minerals and in multiphase solid inclusions (MSI) in ultrahigh pressure (UHP) eclogites have been controversially interpreted to record the presence of high-salinity oxidizing fluids1 close to the peak of metamorphism. A detailed petrographic and microstructural study of barite in partially melted UHP eclogite from Yangkou, in the central Sulu belt of China demonstrates that barite precipitation is a multi-stage process. The different generations of barite provide insight into the evolution of fluid systems as P-T conditions evolved through the late prograde stage of subduction to the peak of UHP metamorphism and subsequently during exhumation. Five microstructural types of barite are recognized in this study. Type I barite (variable high to low Sr/Ba) occurs as equant primary inclusions with rutile in garnet and omphacite within coesite-bearing eclogite. Zr-in-rutile thermometry on the primary inclusions of rutile yields T = 658–699 °C at P = 2.5–4.5 GPa. Thus, barite inclusions were likely precipitated from an internally buffered fluid during the late prograde evolution. Type II barite (low Sr/Ba) occurs in MSI located toward the rims of garnet and omphacite. Since peak pressure was above the second critical endpoint for basaltic compositions, the MSI are inferred to represent aliquots of a silica-rich supercritical fluid trapped during the late prograde evolution close to the metamorphic peak. Type III barite (low Sr/Ba) occurs in pseudomorphs after phengite inclusions in garnet. The replacement phase assemblage formed during exhumation, by in situ melting of phengite. Type IV barite (low Sr/Ba) forms coarse-grained patches associated with sub-solidus replacement of omphacite by hornblende and albite symplectites along grain boundaries. Type V barite (low Sr/Ba) occurs as grain-boundary veinlets, intergranular grains, and closed rings around pyrite that is partly replaced by hematite and goethite. We interpret the types IV and V barite to have precipitated from an internally generated grain-boundary aqueous fluid, which is expected to be a response to H 2 O exsolving from garnet and omphacite during low-pressure amphibolite facies conditions. Therefore, barite precipitated during the late prograde subduction and the retrograde exhumation of UHP eclogite yields information about the mobility of Ba, Sr, and S during the metamorphic evolution.

Using Multiphase Solid Inclusions to Constrain the Origin of the Baima Fe‐Ti‐(V) Oxide Deposit, SW China

Acta Geologica Sinica - English EditionVolume 89, Issue s2 p. 56-56 Meeting Abstracts: IGCP-649 Workshop on Ophiolites and Related High-Pressure Rocks in the Qilian Mountains, Xining, Qinghai, China, 6 to 10, AugustFree Access Using Multiphase Solid Inclusions to Constrain the Origin of the Baima Fe-Ti-(V) Oxide Deposit, SW China Pingping LIU, Pingping LIU Department of Earth Sciences, University of Hong Kong, Hong Kong, ChinaSearch for more papers by this authorMeifu ZHOU, Corresponding Author Meifu ZHOU Department of Earth Sciences, University of Hong Kong, Hong Kong, ChinaCorresponding author. E-mail: mfzhou@hkucc.hku.hkSearch for more papers by this authorWeiterry CHEN, Weiterry CHEN Department of Earth Sciences, University of Hong Kong, Hong Kong, ChinaSearch for more papers by this authorMarijn BOONE, Marijn BOONE Department of Geology and Soil Sciences-UGCT, GHENT University, Krijgslaan 281/S8,900 GHENT, Belgium Unit Sustainable Materials Management, Vito, Mol 2400 BelgiumSearch for more papers by this authorVeerle CNUDDE, Veerle CNUDDE Department of Geology and Soil Sciences-UGCT, GHENT University, Krijgslaan 281/S8,900 GHENT, BelgiumSearch for more papers by this author Pingping LIU, Pingping LIU Department of Earth Sciences, University of Hong Kong, Hong Kong, ChinaSearch for more papers by this authorMeifu ZHOU, Corresponding Author Meifu ZHOU Department of Earth Sciences, University of Hong Kong, Hong Kong, ChinaCorresponding author. E-mail: mfzhou@hkucc.hku.hkSearch for more papers by this authorWeiterry CHEN, Weiterry CHEN Department of Earth Sciences, University of Hong Kong, Hong Kong, ChinaSearch for more papers by this authorMarijn BOONE, Marijn BOONE Department of Geology and Soil Sciences-UGCT, GHENT University, Krijgslaan 281/S8,900 GHENT, Belgium Unit Sustainable Materials Management, Vito, Mol 2400 BelgiumSearch for more papers by this authorVeerle CNUDDE, Veerle CNUDDE Department of Geology and Soil Sciences-UGCT, GHENT University, Krijgslaan 281/S8,900 GHENT, BelgiumSearch for more papers by this author First published: 06 December 2015 https://doi.org/10.1111/1755-6724.12308_35Citations: 1AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article.Citing Literature Volume89, Issues2Special Issue: Special Issue: Meeting Abstracts: IGCP-649 Workshop on Ophiolites and Related High-Pressure Rocks in the Qilian Mountains, Xining, Qinghai, China, 6 to 10, August 2015December 2015Pages 56-56 RelatedInformation