Multiphase Solid Inclusions Research Articles

The chemical behavior of fluids released during deep subduction based on fluid inclusions

This review combines fluid inclusion data from (HP-)UHP rocks with experimental research and thermodynamic models to investigate the chemical and physical properties of fluids released during deep subduction, their solvent and element transport capacity, and the subsequent implications for the element recycling in the mantle wedge. An impressive number of fluid inclusion studies indicate three main populations of fluid inclusions in HP and UHP metamorphic rocks: (1) aqueous and/or non-polar gaseous fluid inclusions (FI); (2) multiphase solid inclusions (MSI); and (3) melt inclusions (MI). Chemical data from preserved fluid inclusions in rocks match with and implement “model” fluids by experiments and thermodynamics, revealing a continuity behind the extreme variations of physico-chemical properties of subduction-zone fluids. From fore-arc to sub-arc depths, fluids released by progressive devolatilization reactions from slab lithologies change from relatively diluted chloridebearing aqueous solutions (±N2), mainly influenced by halide ligands, to (alkali) aluminosilicate-rich aqueous fluids, in which polymerization probably governs the solubility and transport of major (e.g., Si and Al) and trace elements (including C). Fluid inclusion studies point to a reconsideration of the petrological models explaining deep volatile liberation, and their flux into the mantle wedge.

Accessory priderite and burbankite in multiphase solid inclusions in the orogenic garnet peridotite from the Bohemian Massif, Czech Republic

Accessory priderite and burbankite in multiphase solid inclusions in the orogenic garnet peridotite from the Bohemian Massif, Czech Republic

Dehydration and anatexis of UHP metagranite during continental collision in the Sulu orogen

AbstractDehydration and anatexis of ultrahigh‐pressure (UHP) metamorphic rocks during continental collision are two key processes that have great bearing on the physicochemical properties of deeply subducted continental crust at mantle depths. Determining the time and P–T conditions at which such events take place is needed to understand subduction‐zone tectonism. A combined petrological and zirconological study of UHP metagranite from the Sulu orogen reveals differential behaviours of dehydration and anatexis between two samples from the same UHP slice. The zircon mantle domains in one sample record eclogite facies dehydration metamorphism at 236 ± 5 Ma during subduction, exhibiting low REE contents, steep MREE–HREE patterns without negative Eu anomalies, low Th, Nb and Ta contents, low temperatures of 651–750 °C and inclusions of quartz, apatite and jadeite. A second mantle domain records high‐T anatexis at 223 ± 3 Ma during exhumation, showing high REE contents, steeper MREE–HREE patterns with marked negative Eu anomalies, high Hf, Nb, Ta, Th and U contents, high temperatures of 698–879 °C and multiphase solid inclusions of albite + muscovite + quartz. In contrast, in a second sample, one zircon mantle domain records limited hydration anatexis at 237 ± 3 Ma during subduction, exhibiting high REE contents, steep MREE–HREE patterns with marked negative Eu anomalies, high Hf, Nb, Ta, Th and U contents, medium temperatures of 601–717 °C and multiphase solid inclusions of albite + muscovite + hydrohalite. A second mantle domain in this sample records a low‐T dehydration metamorphism throughout the whole continental collision in the Triassic, showing low REE contents, steep MREE–HREE patterns with weakly negative Eu anomalies, low Th, Nb and Ta contents, low temperatures of 524–669 °C and anhydrite + gas inclusions. Garnet, phengite and allanite/epidote in these two samples also exhibit different variations in texture and major‐trace element compositions, in accordance with the zircon records. The distinct P–T–t paths for these two samples suggest separate processes of dehydration and anatexis, which are ascribed to the different geothermal gradients at different positions inside the same crustal slice during continental subduction‐zone metamorphism. Therefore, the subducting continental crust underwent variable extents of dehydration and anatexis in response to the change in subduction‐zone P–T conditions.

Composite carbonate and silicate multiphase solid inclusions in metamorphic garnet from ultrahigh‐P eclogite in the Dabie orogen

AbstractComposite multiphase solid (MS) inclusions composed of carbonate and silicate minerals have been found for the first time in metamorphic garnet from ultrahigh‐P eclogite from the Dabie orogen. These inclusions are morphologically euhedral to subhedral, and some show relatively regular shapes approaching the negative crystal shape of the host garnet. Radial fractures often occur in garnet hosting the inclusions. The inclusions are primarily composed of variable proportions of carbonate and silicate minerals such as calcite, quartz, K‐feldspar and plagioclase, with occasional occurrences of magnetite, zircon and barite. They are categorized into two groups based on the proportions of carbonate and silicate phases. Group I is carbonate‐dominated with variable proportions of silicate minerals, whereas Group II is silicate‐dominated with small proportions of carbonates. Trace element analysis by LA‐ICPMS for the two groups of MS inclusions yields remarkable differences. Group I inclusions exhibit remarkably lower REE contents than Group II inclusions, with significant LREE enrichment and large fractionation between LREE and HREE in the chondrite‐normalized REE diagram. In contrast, Group II inclusions show rather flat REE patterns with insignificant fractionation between LREE and HREE. In the primitive mantle‐normalized spidergram, Group I inclusions exhibit positive anomalies of Zr and Hf, whereas Group II inclusions show negative anomalies of Zr and Hf. Nevertheless, both groups exhibit positive anomalies of Ba, U, Pb and Sr, but negative anomalies of Nb and Ta, resembling the composition of common continental crust. Group I inclusions have higher Ba and U contents than Group II inclusions. Combined with petrological observations, the two groups of MS inclusions are interpreted as having crystallized from composite silicate and carbonate melts during continental subduction‐zone metamorphism. The differences in trace element composition between the two groups are primarily attributed to the proportions of carbonate and silicate phases in the MS inclusions. The silicate melts were derived from the breakdown of hydrous minerals such as paragonite and phengite, whereas the occurrence of carbonate melts indicates involvement of carbonate minerals in the partial melting and thus has great bearing on recycling of supracrustal carbon into the mantle. The coexistence of silicate and carbonate melts in the eclogitic garnet provides insights into the nature of hydrous melts in the subduction factory.

Multiphase solid inclusions in zoisite-bearing eclogite: evidence for partial melting of ultrahigh-pressure metamorphic rocks during continental collision

Abstract Multiphase solid (MS) inclusions in both garnet and omphacite were investigated for zoisite-bearing eclogite from the ultrahigh-pressure (UHP) metamorphic zone in the Sulu orogen. The results provide petrological evidence for local anatexis during exhumation of deeply subducted continental crust. There are three types of MS inclusions: (1) plagioclase + quartz, (2) plagioclase + quartz + K-feldspar, and (3) barite + plagioclase + K-feldspar ± zoisite/epidote. The host minerals mostly exhibit radial fractures surrounding the MS inclusions. The first and second types of MS inclusions were analyzed for their bulk compositions, yielding high SiO 2 and Na 2 O but very low FeO + MgO + TiO 2 with variable K 2 O for the second type. Trace element analyses of representative MS inclusions yield generally very low concentrations except such large ion lithophile elements as Sr, Ba and Rb. These features suggest different origins for the three types of MS inclusions. The first type of MS inclusions would be primarily derived from dehydration melting of paragonite, whereas the second type of MS inclusions would be derived from dehydration melting of both paragonite and phengite. The third type of MS inclusion may result from the interaction between metamorphic fluid and host mineral in view of the occurrence of barite as a filling phase in the fractures of host minerals. Zoisite breakdown is also indicated by its highly cuspate shape in coexistence with quartz, providing components for growth of garnet during the exhumation. Therefore the anatexis of zoisite-bearing UHP eclogite is primarily driven by the breakdown of hydrous minerals such as phengite and paragonite. The major and trace element compositions of MS inclusions in the eclogites mainly depend on the species of hydrous minerals involved in the mineralogical reactions of dehydration melting in subduction channel.

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

Multiphase solid inclusions within cumulus silicates, particularly olivine, in Fe–Ti oxide ores from the Lower Zone of the Baima intrusion, Emeishan large igneous province, SW China, have been identified for the first time using 2-D scanning electron microscope and 3-D high-resolution X-ray computed tomography. These inclusions are spherical to subspherical and range from 100 to 300 µm in diameter. They are composed dominantly of titanomagnetite and ilmenite with minor apatite, hornblende, phlogopite and pyrrhotite. The titanomagnetite in the inclusions has a low Cr content (<700 ppm) similar to the interstitial titanomagnetite, suggesting that these inclusions cannot be early crystallized mineral aggregates. In contrast, the spherical shape of these inclusions provides evidence of early trapped liquids from which these minerals crystallized. Based on the composition and modal proportions of the daughter mineral phases within the inclusions, the trapped liquids are estimated to have 82·1–59·6 wt % FeOT, 11·4–18·5 wt % TiO2, 2·69–6·12 wt % Al2O3, 1·40–4·47 wt % MgO, 0·87–4·93 wt % SiO2 and ∼1 wt % volatiles including F, S, Cl, P and H2O. Such a liquid composition deviates substantially from that of the slightly evolved ferrobasaltic magmas inferred to be parental to the Fe–Ti–(V) oxide-bearing mafic–ultramafic intrusions of the Emeishan large igneous province. It is thus speculated that these trapped liquids are immiscible Fe–Ti-rich melts formed upon cooling of ferrobasaltic magma. The net-textured and disseminated oxide ores have titanomagnetite compositions similar to those in the inclusions, suggesting that the oxide ores of the Baima intrusion also formed from Fe–Ti-rich melts immiscibly separated from ferrobasaltic magmas. We propose that immiscible Fe–Ti-rich liquids with high density percolated down through crystal-bearing silicate magma and crystallized an interconnected Fe–Ti oxide network interstitial to olivine, plagioclase and clinopyroxene. This study highlights that immiscible separation of Fe–Ti-rich liquids from ferrobasaltic magmas is an important mechanism in the formation of magmatic Fe–Ti–(V) oxide deposits hosted in mafic–ultramafic layered intrusions.

Element mobility in mafic and felsic ultrahigh-pressure metamorphic rocks from the Dabie UHP Orogen, China: insights into supercritical liquids in continental subduction zones

Geochemical analyses of minerals and whole rocks for major and trace elements as well as Sr-Nd-O isotopes on samples along a profile across the boundary between amphibolite retrogressed from ultrahigh pressure (UHP) eclogite and its country rock granitic gneiss from the South Dabie low-T/UHP zone, China, are studied to assess the process that controls element mobility under extreme metamorphic conditions. Directly at the contact with the granitic gneiss, the amphibolite has higher concentrations of K, Al, LILEs, REEs, HFSEs, Th, and U, slightly lower concentrations of SiO2, MgO, and CaO, but very similar FeOt and transitional metal element contents relative to the other amphibolites further away from the boundary. Consistently, δ18O values of the amphibolite display a progressive increase towards the boundary, indicating fluid-assisted O-isotope exchange across the contacts of different lithologies at local scales. These can neither be attributed to amphibolite-facies retrogression of eclogite, which is known to have no significant effect on their major and trace elements, nor to Si-rich metasomatism from partially melted granitic gneiss, which should increase the silica content of the amphibolite at the contact. Therefore, neither of the two processes is viable here. We thus propose that the variations observed here were caused by metasomatism of supercritical liquids that were generated at pressure higher than that of the second critical endpoint in the granitic gneiss–H2O system. This interpretation is supported by multiphase solid inclusions of K-feldspar + quartz + calcite + zircon ± amphibole ± clinozoisite ± garnet ± apatite within garnet in the amphibolite.

Geochemistry of continental subduction-zone fluids

The composition of continental subduction-zone fluids varies dramatically from dilute aqueous solutions at subsolidus conditions to hydrous silicate melts at supersolidus conditions, with variable concentrations of fluid-mobile incompatible trace elements. At ultrahigh-pressure (UHP) metamorphic conditions, supercritical fluids may occur with variable compositions. The water component of these fluids primarily derives from structural hydroxyl and molecular water in hydrous and nominally anhydrous minerals at UHP conditions. While the breakdown of hydrous minerals is the predominant water source for fluid activity in the subduction factory, water released from nominally anhydrous minerals provides an additional water source. These different sources of water may accumulate to induce partial melting of UHP metamorphic rocks on and above their wet solidii. Silica is the dominant solute in the deep fluids, followed by aluminum and alkalis. Trace element abundances are low in metamorphic fluids at subsolidus conditions, but become significantly elevated in anatectic melts at supersolidus conditions. The compositions of dissolved and residual minerals are a function of pressure-temperature and whole-rock composition, which exert a strong control on the trace element signature of liberated fluids. The trace element patterns of migmatic leucosomes in UHP rocks and multiphase solid inclusions in UHP minerals exhibit strong enrichment of large ion lithophile elements (LILE) and moderate enrichment of light rare earth elements (LREE) but depletion of high field strength elements (HFSE) and heavy rare earth elements (HREE), demonstrating their crystallization from anatectic melts of crustal protoliths. Interaction of the anatectic melts with the mantle wedge peridotite leads to modal metasomatism with the generation of new mineral phases as well as cryptic metasomatism that is only manifested by the enrichment of fluid-mobile incompatible trace elements in orogenic peridotites. Partial melting of the metasomatic mantle domains gives rise to a variety of mafic igneous rocks in collisional orogens and their adjacent active continental margins. The study of such metasomatic processes and products is of great importance to understanding of the mass transfer at the slab-mantle interface in subduction channels. Therefore, the property and behavior of subduction-zone fluids are a key for understanding of the crust-mantle interaction at convergent plate margins.

First report on the occurrence of CO2-bearing fluid inclusions in the Meiduk porphyry copper deposit, Iran: implications for mineralisation processes in a continental collision setting

Abstract Hydrothermal alteration of the Meiduk porphyry copper deposit, south of the Kerman Cenozoic magmatic arc and southeast of the central Iranian volcano-plutonic belt has resulted in three stages of mineralisation characterised by veins and veinlets. These are, from early to late: (1) quartz + K-feldspar + biotite + pyrite ± chalcopyrite ± pyrrhotite ± magnetite (early potassic alteration and type-A veins); (2) quartz + chalcopyrite + pyrite + bornite + pyrrhotite + K- -feldspar + biotite + magnetite (potassic-sericitic alteration and type-B veins); and (3) quartz + pyrite + chalcopyrite + sericite (sericitic alteration and type-C veins). Most ores were formed during stages 2 and 3. Three main types of fluid inclusions are distinguished based on petrographical, microthermometrical and laser Raman spectroscopy analyses, i.e. type I (three-phase aqueous inclusions), type II (three-phase liquid-carbonic inclusions) and type III (multi-phase solid inclusions). The fluid inclusions in quartz veins of the stages are mainly homogenised at 340-530°C (stage 1), 270-385°C (stage 2) and 214-350°C (stage 3), respectively, with salinities of 3.1-16 wt.% NaCl equivalent, 2.2-43 wt.% NaCl equivalent and 8.2-22.8 wt.% NaCl equivalent, respectively. The estimated trapping pressures are 97.9-123.6 MPa (3.7-4.6 km) in stage 1 and 62.5-86.1 MPa (2.3-3.1 km) in stage 2, respectively. These fluid inclusions are homogenised in different ways at similar temperatures, suggesting that fluid boiling took place in stages 2 and 3. The fluid system evolved from high-temperature, medium-salinity, high-pressure and CO2-rich to low-temperature, low-pressure, high-salinity and CO2-poor, with fluid boiling being the dominating mechanism, followed by input of meteoric water. CO2 escape may have been a factor in increasing activities of NaCl and S2- in the fluids, diminishing the oxidation of the fluids from stage 1 to 3. The result was precipitation of sulphides and trapping of multi-phase solid inclusions in hydrothermal quartz veins.

Trace element composition of continentally subducted slab‐derived melt: insight from multiphase solid inclusions in ultrahigh‐pressure eclogite in the Dabie orogen

AbstractThe ultrahigh‐pressure (UHP) eclogite in the Dabie orogen preserves petrological evidence for the existence of hydrous silicate melts that formed during continental subduction‐zone metamorphism. This is indicated by occurrence of multiphase solid (MS) inclusions in garnet that primarily consist of K‐feldspar + quartz ± epidote/allanite. All the MS inclusions are euhedral to subhedral in morphology and surrounded with radial cracks in the host garnet. Their trace element compositions were analysed by two different approaches of laser sampling. The mass budget method was used to estimate the trace element abundances of MS inclusions from their mixtures with the host garnet. The results are compared with the direct sampling of MS inclusions, providing a first‐order approximation to the trace element composition of MS inclusions. The MS inclusions exhibit consistent enrichment of LILE, Sr and Pb, but depletion of HFSE in the primitive mantle‐normalized spidergram. Such arc‐like patterns of trace element distribution are common for continental crustal rocks. The melts have variably high K, Rb and Sr abundances, suggesting that breakdown of phengite is a basic cause for partial melting of the UHP eclogite. These MS inclusions also exhibit consistently low HFSE and Y contents, suggesting partial melting of the eclogite in the stability fields of rutile and garnet. Consequently, the trace element composition of MS inclusions provides a proxy for that of hydrous silicate melts derived from dehydration melting of the UHP eclogite during continental collision.

Petrological and zircon evidence for anatexis of UHP quartzite during continental collision in the Sulu orogen

AbstractPetrological evidence is provided for anatexis of ultrahigh‐pressure (UHP) metamorphic quartzite in the Sulu orogen. Some feldspar grains exhibit elongated, highly cuspate shapes or occur as interstitial, cuspate phases constituting interconnected networks along grain boundaries. Elongated veinlets composed of plagioclase + quartz ± K‐feldspar also occur in grain boundaries. These features provide compelling evidence for anatexis of the UHP quartzite. Zircon grains from impure quartzite are all metamorphic growth with highly irregular shape. They contain inclusions of coesite, jadeite, rutile and lower pressure minerals, including multiphase solid inclusions that are composed of two or more phases of muscovite, quartz, K‐feldspar and plagioclase. All zircon grains exhibit steep REE patterns, similar U–Pb ages and Hf isotope compositions with a weighted mean of 218 ± 2 Ma. Most grains have similar δ18O values of −0.6 to 0.1‰, but a few fall in the range −5.2 to −4.3‰. Thus, these grains would have grown from anatectic melts at various pressures. Zircon O isotope differences indicate that anatectic melts were derived from different sources with contrasting O isotopes, but similar Hf isotopes, that is, one from the quartzite itself and the other probably from the country‐rock granitic gneiss. Zircon grains from pure quartzite contain relict magmatic cores and significant metamorphic overgrowths. Domains that contain eclogite facies minerals exhibit flat HREE patterns, no Eu anomalies and concordant U–Pb ages of c. 220 Ma. Similar U–Pb ages are also obtained for domains that contain lower pressure minerals and exhibit steep REE patterns and marked negative Eu anomalies. These observations indicate that zircon records subsolidus overgrowth at eclogite facies conditions but suprasolidus growth at lower pressures. Zircon enclosed by garnet gave consistent U–Pb ages of c. 214 Ma. Such garnet is interpreted as a peritectic product of the anatectic reaction that involves felsic minerals and possibly amphibole and titanite. The REE patterns of epidote and titanite also record multistage growth and metasomatism by anatectic melts. Therefore, the anatexis of UHP metamorphic rocks is evident during continental collision in the Triassic.

Element mobility during continental collision: insights from polymineralic metamorphic vein within UHP eclogite in the Dabie orogen

AbstractMetamorphic dehydration and partial melting are two important processes during continental collision. They have significant bearing on element transport at the slab interface under subduction‐zone P–T conditions. Petrological and geochemical insights into the two processes are provided by a comprehensive study of leucocratic veins in ultrahigh‐pressure (UHP) metamorphic rocks. This is exemplified by this study of a polymineralic vein within phengite‐bearing UHP eclogite in the Dabie orogen. The vein is primarily composed of quartz, kyanite, epidote and phengite, with minor accessory minerals such as garnet, rutile and zircon. Primary multiphase solid inclusions occur in garnet and epidote from the both vein and host eclogite. They are composed of quartz ± K‐feldspar ± plagioclase ± K‐bearing glass and exhibit irregular to negative crystal shapes that are surrounded by weak radial cracks. This suggests their precipitation from solute‐rich metamorphic fluid/melt that involved the reaction of phengite breakdown. Zircon U–Pb dating for the vein gave two groups of concordant ages at 217 ± 2 and 210 ± 2 Ma, indicating two episodes of zircon growth in the Late Triassic. The same minerals from the two rocks give consistent δ18O and δD values, suggesting that the vein‐forming fluid was directly derived from the host UHP eclogite. The vein is much richer in phengite and epidote than the host eclogite, suggesting that the fluid is associated with remarkable concentration of such water‐soluble elements as LILE and LREE migration. Garnet and rutile in the vein exhibit much higher contents of HREE (2.2–5.7 times) and Nb–Ta (1.8–2.0 times) than those in the eclogite, indicating that these normally water‐insoluble elements became mobile and then were sunken in the vein minerals. Thus, the vein‐forming agent would be primarily composed of the UHP aqueous fluid with minor amounts of the hydrous melt, which may even become a supercritical fluid to have a capacity to transport not only LILE and LREE but also HREE and HFSE at subduction‐zone metamorphic conditions. Taken together, significant amounts of trace elements were transported by the vein‐forming fluid due to the phengite breakdown inside the UHP eclogite during exhumation of the deeply subducted continental crust.

Amphibole-bearing Multiphase Solid Inclusions in Olivine and Plagioclase from a Layered Gabbro: Origin of the Trapped Melts

This study presents new data regarding the petrographic and mineralogical characteristics of amphibole-bearing multiphase solid inclusions (MSI) enclosed in olivine and plagioclase from a well-characterized Japanese layered gabbro, the Murotomisaki Gabbroic Intrusion. Bulk compositions of the MSI were obtained using wavelength-dispersive (WDS) and energy-dispersive (EDS) techniques combined with modal analysis. Many MSI in olivines are spherical or polygonal in shape and are mainly composed of pargasitic amphibole, orthopyroxene, and mica (biotite and aspidolite). The MSI in olivine phenocrysts in the chilled margins of the intrusion are composed of plagioclase and hornblende rather than pargasite. Many of them are found to be enriched in olivine components in their bulk compositions, and therefore they cannot be regarded as simple trapped basaltic melts or their derivatives. The MSI in plagioclase are more irregular in shape and occur in the resorbed part of calcic plagioclase cores; they are mainly composed of pargasite and sodic plagioclase. Attempts were made to obtain the original composition of the MSI melts by applying a correction for Fe^Mg exchange reactions with the host olivine. It was found that after the correction, the MSI in the chilled margin were closest to the published initial basaltic melt composition, whereas the olivine-hosted MSI from the olivine gabbros were still fairly high in olivine components despite the correction. Such post-entrapment modification was probably minimal for the plagioclase-hosted MSI because plagioclase is essentially free of Mg and Fe, and therefore plagioclase-hosted MSI are believed to represent the compositions of the original trapped melts. These are characteristically enriched in plagioclase components. We believe that the anomalous compositions of the trapped melts (after the corrections) were generated by local dissolution of olivine and plagioclase caused by the introduction of water from lower horizons of the crystallization boundary layer in a magma chamber. The MSI are therefore considered to provide new evidence for the dissolution of silicates by hydrous fluxing in a lower crystallizing boundary layer of a magma reservoir during solidification of basaltic magma.

Dehydration melting of ultrahigh‐pressure eclogite in the Dabie orogen: evidence from multiphase solid inclusions in garnet

AbstractSeveral types of multiphase solid (MS) inclusions are identified in garnet from ultrahigh‐pressure (UHP) eclogite in the Dabie orogen. The mineralogy of MS inclusions ranges from pure K‐feldspar to pure quartz, with predominance of intermediate types consisting of K‐feldspar + quartz ± silicate (plagioclase or epidote) ± barite. The typical MS inclusions are usually surrounded with radial cracks in the host garnet, similar to where garnet contains relict coesite. Barite aggregates display significant heterogeneity in major element composition, with total contents of only 57–73% and highly variable SiO2 contents of 0.32–25.85% that are positively correlated with BaO and SO3 contents. The occurrence of MS inclusions provides petrographic evidence for partial melting in the UHP metamorphic rock. The occurrence of barite aggregates with variably high SiO2 contents suggests the coexistence of aqueous fluid with hydrous melt under HP eclogite facies conditions. Thus, local dehydration melting is inferred to take place inside the UHP metamorphic slice during continental collision. This is ascribed to phengite breakdown during ‘hot’ exhumation of the deeply subducted continental crust. As a consequence, the aqueous fluid is internally buffered in chemical composition and its local sink is a basic trigger to the partial melting during the continental subduction‐zone metamorphism.

Preiswerkite and högbomite within garnets of Aktyuz eclogite, Northern Tien Shan, Kyrgyzstan

We report the occurrence of preiswerkite and hogbomite as inclusion phases within the garnets of eclogite from the Aktyuz area of Northern Tien Shan, Kyrgyzstan. Preiswerkite and hogbomite occur both as a constituent of multiphase solid inclusions (MSI) and as single discrete grains in the mantle and rim of the garnets. However, they do not occur in the core of the garnet and in the matrix of the eclogite. Preiswerkite is associated with the minerals paragonite ± staurolite ± Mg-taramite ± Na-biotite ± hematite ± hogbomite ± chlorite ± titanite ± phengite ± magnetite, and hogbomite is associated with paragonite ± preiswerkite ± staurolite ± hematite ± chlorite ± Na-biotite ± magnetite in MSI. The average compositions of preiswerkite and hogbomite are (Na0.96K0.02Ca0.01)0.99(Mg1.52Fe2+0.54VIAl0.93)2.99(IVAl1.93Si2.07)4.00O10(OH)2 and (Mg1.47Fe2+3.02Zn0.04Fe3+1.45)5.98(Fe3+0.31Al15.13Ti0.56)16O30(OH)2, respectively. Na-biotite, with an average composition of (Na0.89K0.07Ca0.01)0.97(Mg1.66Fe2+0.69VIAl0.63)2.98(IVAl1.57Si2.43)4.00O10(OH)2, corresponding to the intermediate composition between preiswerkite and aspidolite (i.e., Na-phlogopite), is also observed. The compositions of the newly found preiswerkite and Na-biotite with similar XMg values (0.66-0.78) are arrayed along preiswerkite-aspidolite solid solution series. The mode of occurrence of inclusion phases in garnets may suggest that the activity of Na-Al-rich and Si-undersaturated aqueous fluids played a major role in the formation of preiswerkite during the prograde stage of high-pressure eclogitic metamorphism.

The Pressure–Temperature Path and the Origin of Phlogopite in Spinel–Garnet Peridotites from the Blanský Les Massif of the Moldanubian Zone, Czech Republic

A new lithotype of peridotite, phlogopite- and apatite-bearing spinel–garnet peridotite, associated with leucocratic granulite, has been recognized at the Plešovice quarry in the Gföhl Unit within the Moldanubian Zone of the Bohemian Massif, Czech Republic. There are three equilibrium stages in the Plešovice peridotite. The existence of Stage I, the precursor spinel ± garnet peridotite stage, is supported by the presence of an aluminous (Al2O3 ∼ 3·0 wt %) orthopyroxene megacryst in the matrix. The minimum temperature of Stage I was estimated to be 1020 ± 15°C. Stage II is defined by the cores of relatively large (<3 mm long) grains of olivine, low-Al orthopyroxene (Al2O3 ∼ 1·3–1·7 wt %), clinopyroxene, and chromian spinel [Cr/(Cr + Al) = 0·50–0·57], along with relatively small (<1 mm long) Ba-rich phlogopite (BaO = 1·0–4·0 wt %), Sr-rich apatite (SrO ∼1·7 wt %) and rare potassic (K2O ∼0·9–1·2 wt %) amphibole. Garnet generally occurs as large spheroidal grains (up to 20 mm in diameter). It contains inclusions of olivine, orthopyroxene, chromian spinel, and phlogopite, all of which have similar compositions to their matrix counterparts. Therefore, garnet appears to be in equilibrium with the matrix phases at Stage II. Application of appropriate geothermobarometers to the assemblage at Stage II yielded temperatures of 850–1030°C and pressures of 2·3–3·5 GPa. Stage III is defined by aluminous orthopyroxene (Al2O3 ∼ 2·1–4·0 wt %), aluminous clinopyroxene and aluminous spinel along with pargasitic amphibole and Ba-rich phlogopite in kelyphite; temperature conditions at this stage were estimated to be 730–770 (± 27)°C at 0·8–1·5 GPa. Multiphase solid inclusions, mainly composed of phlogopite, dolomite, apatite and calcite with minor amounts of chlorite and magnesiohornblende, are present only within large grains of chromian spinel, which are surrounded by kelyphites. The idiomorphic outline of the multiphase solid inclusions suggests that frozen remnants of carbonatite melts or supercritical fluids were trapped in the spinel. The mineral assemblage in the multiphase solid inclusions suggests relatively low-P and low-T conditions (T < 750°C; P < 1·6 GPa) for its crystallization. Furthermore, the timing of the crystallization of the multiphase solid inclusions appears to predate Stage II, as most multiphase solid inclusions are completely surrounded by the host chromian spinel. These data suggest that the Plešovice peridotite experienced cooling after Stage I and was then transformed to spinel–garnet peridotite by subsequent subduction processes.

Multiphase solid inclusions in UHP rocks (Su-Lu, China): Remnants of supercritical silicate-rich aqueous fluids released during continental subduction

Primary multiphase solid (MS) inclusions without preserved fluid are found within peak minerals in kyanite quartzite ± topaz and kyanite–phengite–epidote eclogite from Donghai area (Su-Lu terrane). Typical mineral association in inclusions is: paragonite + muscovite + anhydrite ± corundum ± “alunite-type” sulphate ± zircon ± calcite ± chlorite ± SiO 2 ± barite ± pyrite ± apatite in quartzites, and paragonite + rutile + apatite ± amphibole ± Zn-staurolite ± magnetite ± plagioclase ± zircon ± pyrite ± “alunite-type” sulphate ± Zn–Mg–Fe–Al–Ti spinel in eclogites. On the basis of the fluid inclusion textures and of the daughter-phase assemblage, calculated fluid composition is as follows: in quartzites 24 wt.% SiO 2, 30 wt.% Al 2O 3, 9 wt.% CaO, 5 wt.% K 2O, 3 wt.% Na 2O, 11 wt.% SO 3, 18 wt.% H 2O, with traces of TiO 2, Fe 2O 3, FeO, MgO, BaO, P 2O 5, Cl −, F, and (CO 3) 2−, and in eclogites 26 wt.% SiO 2, 21 wt.% TiO 2, 20 wt.% Al 2O 3, 2 wt.% MgO, 4 wt.% FeO, 6 wt.% Fe 2O 3, 7 wt.% CaO, 3 wt.% Na 2O, 3 wt.% P 2O 5, 7 wt.% H 2O, 1 wt.% Cl, and traces of ZnO, MnO, K 2O, SO 3, and F. Originally trapped chloride-poor aqueous fluids contained very high amounts – in the order of tens of wt.% – of Si 4+, Al 3+, and Ti 4+. Solute species reflect the chemical composition of the host rocks: Mg 2+, Fe 2+, Ti 4+, P 5+, and Na +, are abundant in the fluids present in eclogites, while fluids are enriched in Al 3+, K +, Na +, (SO 4) 2−, (CO 3) 2− in quartzites. We propose that multiphase solid inclusions represent remnants of high-density supercritical silicate-rich aqueous fluids that were in equilibrium with peak minerals at UHP conditions. These fluids show characters which are transitional between aqueous fluids and silicate melts, and were probably produced by dehydration reactions of the host rocks during the latest stages of subduction.