Composition Of Melt Inclusions Research Articles

Chromite Paleoplacer in the Permian Sediments at the East Edge of the East European Platform: Composition and Potential Sources

A chromite occurrence called the Sabantuy paleoplacer was discovered in the Southern Pre-Ural region, at the east edge of the East-European Platform in the transitional zone to the Ural Foredeep. A ca. 1 m-thick chromite-bearing horizon is traced at a depth of 0.7–1.5 m from the earth’s surface for the area of ca. 15,000 m2. The chromspinel content in sandstones reaches 30–35%, maximum values of Cr2O3 are 16–17 wt.%. The grain size of detrital chromspinel ranges from 0.15 to 0.25 mm. Subangular octahedral crystals dominate; rounded grains and debris are rare. The composition of detrital chromspinel varies widely and is constrained by the substitution of Al3+ and Cr3+, Fe2+ and Mg2+ cations. Chemically, low-Al (Al2O3 = 12 wt.%) and high-Cr (Cr2O3 = 52–56 wt.%) chromspinel prevail. The compositional analysis using discrimination diagrams showed that most chromites correspond to mantle peridotites of subduction settings. Volcanic rocks could be an additional source for detrital chromites. It is confirmed by compositions of monomineralic, polymineralic and melt inclusions in chromspinels. The presented data indicates that ophiolite peridotites and related chromite ore associated with oceanic and island-arc volcanic rocks, widespread in the Ural orogen, could be the main sources of the detrital chromspinel of the Sabantuy paleoplacer.

Potassium-rich granite melt inclusions in zircon from gabbro-hosted felsic stringers, Mid-Atlantic Ridge at 13°34′N: E-MORB connection

We present new data on crystallized micro-inclusions in zircon from clinopyroxene-bearing felsic stringers within gabbronorite of gabbro-peridotite oceanic core complex at Mid-Atlantic Ridge, 13°34′ N. A relatively large (about 49 × 22 μm) crystallized inclusion exposed in situ in thin section consists of the muscovite + biotite + plagioclase + K-feldspar + quartz assemblage. Smaller inclusions (≤20 μm) found in zircon separates consist of silicate minerals, interstitial water solution, and gas bubbles. At temperature close to 800 °C and estimated pressure about 1.5 kbar the inclusions completely homogenize into a K2O-rich (3.9 ± 0.1 wt%), H2O-bearing (about 4 wt%) granite (SiO2 70 ± 2 wt%) melt. Calculations employed DOMINO software show that complete closed-system crystallization of the analyzed melt exactly reproduces mineral assemblage of the large crystallized inclusion (+H2O) at about 1.5 kbar below 600 °C. Granitoid mineralogy of felsic stringers hosting zircon does not correspond to that of the crystallized melt inclusions, with a significantly lower K2O in the former. The K2O loss might result from interaction of the melt with pyroxenes of the host gabbro. This implies that considering felsic granitoid veins, often present in the oceanic lower crustal sections, as simply “frozen melt” is not always justified. Compositional variations of basalt lava glasses from the 13°25′–14° portion of the Mid-Atlantic Ridge show continuous geochemical trends from D-MORB to E-MORB with K2O enrichment up to 0.9 wt%, suggesting mixing from two sources. The most K2O-enriched E-MORB composition (K2O = 0.9 wt%, H2O = 0.7 wt%, K2O/TiO2 = 0.42, (La/Sm)N = 2.45) was assumed as a source of the granitoid inclusions. Computer simulation of equilibrium crystallization of this E-MORB composition returned a granite residual melt closely matching the average composition of the zircon-hosted melt inclusions at 820 °C, 1.5 kbar and melt fraction 5.2 vol%. The result suggests a direct link between the E-MORB magma occurring in the studied area, and the origin of the K-rich granite melt in zircon-hosted inclusions.

Geochemical and lead isotope compositions of olivine-hosted melt inclusions from the Yaeyama Graben in the southern Okinawa Trough: Implications for slab subduction and magmatic processes

Chemical compositions were measured in olivine-hosted melt inclusions in basaltic volcanic rocks from the Yaeyama Graben in the southern Okinawa Trough to investigate the magmatic processes of erupted lavas in an incipient epicontinental back-arc basin in the western Pacific. Melt inclusions have widely variable element and Pb isotope compositions, while erupted host whole-rock compositions are homogeneous, suggesting that melt inclusions preserve more information about magmatic processes than their host whole rocks. Two groups of melt inclusions can be distinguished by their trace elements and Pb isotopes. Group 1 melt inclusions have geochemical compositions similar to those of their host whole rocks, which have low 207Pb/206Pb ratios (<0.850), and may be attributed to the contributions of recycled continental crust and subducted sediments to magma sources during Philippine Sea Plate subduction. Group 2 melt inclusions have higher 207Pb/206Pb ratios (>0.850) and Li, Rb, and U contents and lower Cu, Zn, and Pb contents than their host whole rocks. High Li, Rb, and U contents may be induced by specific melts generated by possible breakdown reactions of some accessory minerals, such as phengite and cordierite, while low Cu, Zn, and Pb contents are attributed to sulfide precipitation, metal flushing and/or metallic element fractionation during magma evolution. The Pb isotopes of the group 2 melt inclusions are similar to those of the Kuanhsi-Chutung basalts, which are correlated with some intraplate volcanism in the Fujian-Taiwan region. We infer that the group 2 melts were preexisting small volumes of magma that were emplaced in the middle-lower continental crust (~11 km), experienced extensive magma evolution and were entrained by upwelling magma. Our melt inclusion data suggest that magma sources are compositionally heterogeneous and that the mixing of magmas from different sources may be widespread beneath the southern Okinawa Trough because of Philippine Sea Plate subduction and preexisting intraplate magmatism.

Features of Mineral Crystallization at Different Stages of the Magmatism Evolution of the Gorely Volcano (Kamchatka): Data on Melt and Fluid Inclusions

Abstract ––Studies of melt and fluid inclusions and minerals as well as computational modeling (based on the data on the composition of melt inclusions, clinopyroxenes, and amphiboles) gave an insight into the physicochemical parameters of magmatic systems during the evolution of the precaldera Pra-Gorely Volcano and during the subsequent formation of rock complexes of the Young Gorely Volcano. The estimated temperatures of crystallization of olivine, clinopyroxene, and plagioclase phenocrysts (1115–1260 °С) and amphibole (740–890 °С) are in agreement with the earlier published data on the magmatism of the Gorely Volcano. Computational modeling based on the compositions and homogenization temperatures of melt inclusions showed that the established depth interval of mineral crystallization (21.0–1.5 km) with pressures of 7.0–0.5 kbar can be divided into two ranges, 21–15 km and 9.0–1.5 km. Both the Pra-Gorely and Young Gorely volcanoes have magma chambers in these depth ranges. The Pra-Gorely Volcano is characterized by higher temperatures of mineral crystallization (1240–1190 °С) as compared with the Young Gorely Volcano (1190–1125 °С). The presence of primary fluid inclusions with low-density CO2 and of syngenetic primary melt inclusions in plagioclase of the Pra-Gorely Volcano indicates that the mineral crystallized from a heterophase melt. At the same time, the cores of plagioclase phenocrysts formed from a homogeneous melt. A drastic drop in pressure led to the phase separation of magma throughout the magma column (upper and lower chambers) and to the growth of zones saturated with CO2 fluid inclusions in the plagioclase crystals formed from a two-phase melt. The subsequent closure of the system and the disappearance of CO2 phase resulted in the growth of plagioclase from a homogeneous melt.

Physico-chemical conditions of crystallization of minerals-phenocrysts in basaltic complexes hosting massive sulfide deposits of the Urals fnd Siberia

he paper discusses the physico-chemical formation conditions of minerals-pheno-crysts in basaltic complexes, which host massive sulfde deposits of the Urals and Siberia. It is found as a result of study of melt inclusions that clinopyroxene from basalts of the Valentorka (North Urals) and Kyzyl-Tashtyg (South Siberia) deposits crystallized from melts with similar temperature (1165–1130 and 1210–1085 °С, respectively) and chemical parameters. In both cases, the composi¬tions of basalt-andesite magmas (with features of igneous island arc and back-arc basin systems) evolved with a decrease in FeO, MgO, and CaO contents and increase in K2O and SiO2 contents. Modeling, which is based on the compositions of inclusions and clinopyroxene, showed that miner-als-phenocrysts crystallized from intermediate magma chambers of diferent depth, the parameters of which are consistent with each other and with data on present-day suprasubduction magmatism: Valentorka deposit – 33–27, 23–13, and 10–3 km, 1185–1090 °C; Kyzyl-Tashtyg deposit – 27–20, 15.0–6.7, and 5.0–1.7 km, 1215–1105 °С. Calculations using compositions of melt inclusions show a consistent change of melts from basalts to rhyolites, indicating that the combination of contrasting volcanic complexes of the deposits studied are a result of the evolution of initial basaltic magmas during their uplift to the upper crustal horizons. Our studies of rocks, clinopyroxene and melt inclu¬sions indicate that the basaltic complexes of the Valentorka and Kyzyl-Tashtyg deposits formed in an ancient suprasubduction island arc – back-arc basin system.

Boron isotopic signatures of melt inclusions from North Iceland reveal recycled material in the Icelandic mantle source

Trace element and volatile heterogeneity in the Earth’s mantle is influenced by the recycling of oceanic lithosphere through subduction. Oceanic island basalts commonly have high concentrations of volatiles compared to mid-ocean ridge basalts, but the extent to which this enrichment is linked to recycled mantle domains remains unclear. Boron is an ideal tracer of recycled subducted material, since only a small percentage of a recycled component is required to modify the bulk δ11B of the source mantle. Boron isotopic compositions of primary melts thus have potential to trace the fate of recycled subducted material in the deep mantle, and to constrain the lengthscales of lithologic and compositional heterogeneities in diverse tectonic settings.We present new measurements of volatiles, light elements and boron isotopic ratios in basaltic glasses and melt inclusions that sample the mantle at two endmember spatial scales. Submarine glasses from the Reykjanes Ridge sample long-wavelength mantle heterogeneity on the broad scale of the Iceland plume. Crystal-hosted melt inclusions from the Askja and Bárðarbunga volcanic systems in North Iceland sample short-wavelength mantle heterogeneity close to the plume centre. The Reykjanes Ridge glasses record only very weak along-ridge enrichment in B content approaching Iceland, and there is no systematic variability in δ11B along the entire ridge segment. These observations constrain ambient Reykjanes Ridge mantle to have a δ11B of −6.1‰ (2SD = 1.5‰, 2SE = 0.3‰). The North Iceland melt inclusions have widely variable δ11B between −20.7 and +0.6‰. We screen melt inclusions against influence from crustal contamination, identifying high [B] and low δ18O as fingerprints of assimilation processes. Only the most primitive melt inclusions with MgO ⩾ 8 wt.% reliably record mantle-derived δ11B. In North Iceland, incompatible trace element (ITE)-depleted primitive melt inclusions from Holuhraun record a δ11B of −10.6‰, a signal that has also been seen in melt inclusions from southwest Iceland (Gurenko and Chaussidon, 1997). In contrast, primitive ITE-enriched melt inclusions from nearby Askja volcano record a δ11B of −5.7‰, overlapping with our new constraint on the δ11B of Reykjanes Ridge mantle. Coupled [B], δ11B and δ18O signatures of more evolved melt inclusions from North Iceland are consistent with primary melts assimilating <5–20% of hydrothermally altered basaltic hyaloclastite as they ascend through the upper crust.Our data reveal the presence of a depleted, low-δ11B and an enriched, higher-δ11B mantle component, both intrinsic to the Icelandic mantle source and distinct from Reykjanes Ridge mantle. Non-modal melting calculations suggest that the enriched and depleted mantle components both contain ∼0.085 μg/g B, slightly lower than the 0.10–0.11 μg/g calculated for Reykjanes Ridge mantle. These data are consistent with the Icelandic mantle containing B-depleted dehydrated recycled oceanic lithosphere, in keeping with the low B/Pr of Icelandic melt inclusions in comparison to Reykjanes Ridge glasses or MORB. Our new data provide strong support for the role of recycled subducted lithosphere in melt generation at ocean islands, and highlight the need for careful screening of melt inclusion compositions in order to study global volatile recycling in ocean island basalts.

Mechanisms of Partial Melting of Metasomatized Mantle Ultramafic Rocks beneath Avacha Volcano, Kamchatka, and the Growth of Minerals from a Gas Phase in Fractures

Data on xenoliths carried by lavas of Avacha volcano and ejected during eruptions are used in discussing the partial melting of metasomatized rocks of the lithospheric mantle (which were tectonically reformed by seismic events beneath the frontal zone of Avacha volcano) and the associated growth of minerals from a gas phase in open fractures above the magma chamber feeding the volcano. Based on a comprehensive study of representative mantle xenoliths from explosions of the volcano, we developed and numerically studied a mathematical model of convective heat and mass transfer in the permeable zone above the feeding magma chamber. The model enables analysis of the area of active seismic destruction of rocks at depths of 30–70 km beneath Avacha volcano. This analysis involves (1) current qualitative genetic hypotheses concerning the nature of the identified textural and structural relationships between the xenoliths and lavas transporting them, (2) data on the composition of mineral-hosted fluid and melt inclusions, (3) estimated homogenization temperatures of the inclusions, and (4) hypothetical melting mechanisms of metasomatized ultramafics and the associated growth mechanism of minerals in open fractures. It is demonstrated that fractured porous seismogenic regions above magma chambers are coupled with a set of convective processes of fluid-mediated heterophase heat and mass transfer in the lithosphere: infiltration metasomatism at the spinel depth facies and the sublimation and condensation of major components from a gas phase. It is hypothesized that local partial decompression melting of highly heated ultramafic and mafic rocks may be followed by major seismic events. It is shown that spinel-facies infiltration metasomatism in fractured mantle rocks may be associated with the growth of crystal crusts and clusters of clinopyroxene and amphibole from a gas phase in open fractures. There are two paths by which decompression partial melting takes place: (1) the melting of spinel aggregates with the host crystals of “metasomatic” orthopyroxene, which replaced olivine in harzburgites, and (2) the development of clinopyroxene, amphibole, and relict orthopyroxene vein fillings. To test these hypotheses, physical experiments were conducted with the use of convective heating of samples of natural xenoliths. The experiments were carried out in a flow-through reactor capable of reproducing the partial melting phenomena and in an electron gun with a power unit for a welding system with a dense electron beam. The latter experiments simulated the sublimation and deposition of major components from a gas phase over the solidus boundary for all petrographic types of mantle ultramafic rocks. Neither structural nor mineralogical indications of the infiltration of magmatic fluid were detected in the experiments, and no traces of diffusion–reaction zoning of the “melt–rock” type were found in the xenoliths.

Formation conditions of the bronze age metallurgical slags of the South Urals and Kazakhstan

Four main mineralogical types are identifed among a wide variety of metallurgical slags from the bronze Age sites of the South Urals and Kazakhstan: chromite-bearing olivine, sulfde-bearing olivine, glassy sulfde-bearing and pyroxene. The slag types signifcantly difer in morphology, major and accessory minerals and composition of melt inclusions. The temperature of melt and the dynamics of its changes are estimated from phase diagrams and experimental studies. Crystallization models are proposed on the basis of ontogeny of newly formed minerals, relict inclusions, and possible temperatures for all slag types. Figures 15. Tables 3. References 46.

Zircon melt inclusions in mafic and felsic rocks of the Bushveld Complex – Constraints for zircon crystallization temperatures and partition coefficients

Melt inclusions in zircon represent time capsules, which provide deep insights into igneous rock formation including timing, physicochemical conditions, and the compositional evolution of cogenetic magmas with respect to major, trace, and volatile elements. However, their full potential as petrogenetic indicators, in particular their usability and consistency as geothermometers, is poorly investigated. Therefore, we present new mineralogical and chemical data for recrystallized and homogenized melt inclusions and host zircon from different mafic and felsic rocks of the Bushveld Complex (BC), South Africa. Samples include rutile-bearing cumulate rocks of the Marginal and Critical zones, as well as rutile-free, magnetite-ilmenite-titanite-bearing diorites and granites of the Upper Zone, Rashoop Granophyre and Lebowa Granite Suite. All melt inclusions, irrespective of rock type, have rhyolitic compositions with SiO2 contents ranging from 65 to 78 wt.%, and H2O from 1.6 to 4.0 wt.%, whereas trace element contents differ systematically between rock types. In rutile-bearing mafic rocks, melt inclusions commonly show higher Ti contents (>800 ppm), higher Th/U ratios (up to 38), and lower REE contents (ƩREE < 150 ppm) compared to those in rutile-free mafic and felsic rocks (Ti < 800 ppm; Th/U < 5; ƩREE > 150 ppm). Liquidus temperatures of melt inclusions obtained from normative Qz-Ab-Or and rhyolite-MELTS modelling indicate melt entrapment mostly at 930–850 °C (at 200 MPa), tailing down in some samples to 700 °C. For rutile-bearing rocks, these temperatures overlap with those obtained by TiO2-in-melt and Ti-in-zircon geothermometry. For all rutile-free mafic and felsic rocks, reduced TiO2 activities of aTiO2 ∼ 0.3 are required for Ti-in-zircon geothermometry, and TiO2rutile ∼ 30 wt.% for TiO2 saturation geothermometry, to match temperatures from other geothermometers. Furthermore, partition coefficients obtained from melt inclusion - host zircon pairs are within error identical for mafic and felsic rocks and also reveal no systematic dependency on melt inclusion size, composition and entrapment temperature. The results of this study demonstrate that melt inclusions in zircon are a powerful tool for geothermometry and to constrain magma compositions, H2O contents, and TiO2 activities, which are critical for the understanding of magmatic processes shaping Earth’s crust.

Physico-chemical parameters of mineral-forming processes in pegmatites of the ilmenogorsk alkaline complex (South Urals) аnd Altai-Sayany Region (Siberia)

The conditions of mineral formation in non-granite pegmatites of the Urals and Siberia are examined. The physico-chemical parameters of the crystallization of minerals in miaskite-pegmatites of the Ilmenogorsk alkaline complex (South Urals) and pyroxenite-pegmatites from ophiolite associations of the Altai-Sayan region (South Siberia) are determined using inclusions of mineral-forming media in minerals and computational modeling based on data on the composition of melt inclusions in COMAGMAT, PETROLOG and PLUTON programs. The study of multiphase (silicate and salt crystals + vapor and liquid) inclusions showed that nepheline miaskite-pegmatites formed from melts-brines at maximum temperatures of 700–750 °C and pressures of 3100–3600 bar. During further decrease in temperature (up to 500–510 °C) and pressure (up to 1200 bar), cancrinite crystallizes from mostly fuid systems. Sodalite forms at the fnal stages of the pegmatite-forming processes (370–440 °C, up to 1000 bar). The analysis of multiphase fuid inclusions with salt phases indicates the involvement of highly saline fuids with temperatures of 445–455 °C during the formation of pyroxenite-pegmatites in serpentinites. Data on melt inclusions in Cr-spinels from ophiolite ultramafc rocks (including clinopyroxenites) indicate the possible crystallization of pyroxenite-pegmatites (hosted in ordinary pyroxenites) in high-temperature (1315–1245 to 1205–1100 °С) magmatic systems.

Composition, crystallization conditions and genesis of sulfide-saturated parental melts of olivine-phyric rocks from Kamchatsky Mys (Kamchatka, Russia)

Sulfide liquids that immiscibly separate from silicate melts in different magmatic processes accumulate chalcophile metals and may represent important sources of the metals in Earth's crust for the formation of ore deposits. Sulfide phases commonly found in some primitive mid-ocean ridge basalts (MORB) may support the occurrence of sulfide immiscibility in the crust without requiring magma contamination and/or extensive fractionation. However, the records of incipient sulfide melts in equilibrium with primitive high-Mg olivine and Cr-spinel are scarce. Sulfide globules in olivine phenocrysts in picritic rocks of MORB-affinity at Kamchatsky Mys (Eastern Kamchatka, Russia) represent a well-documented example of natural immiscibility in primitive oceanic magmas. Our study examines the conditions of silicate-sulfide immiscibility in these magmas by reporting high precision data on the compositions of Cr-spinel and silicate melt inclusions, hosted in Mg-rich olivine (86.9–90 mol% Fo), which also contain globules of magmatic sulfide melt. Major and trace element contents of reconstructed parental silicate melts, redox conditions (ΔQFM = +0.1 ± 0.16 (1σ) log. units) and crystallization temperature (1200–1285 °C), as well as mantle potential temperatures (~1350 °C), correspond to typical MORB values. We show that nearly 50% of sulfur could be captured in daughter sulfide globules even in reheated melt inclusions, which could lead to a significant underestimation of sulfur content in reconstructed silicate melts. The saturation of these melts in sulfur appears to be unrelated to the effects of melt crystallization and crustal assimilation, so we discuss the reasons for the S variations in reconstructed melts and the influence of pressure and other parameters on the SCSS (Sulfur Content at Sulfide Saturation).

The hydrogen isotopic composition of lunar melt inclusions: An interplay of complex magmatic and secondary processes

Since the discovery of water (a term collectively used for the total H, OH and H2O) in samples derived from the lunar interior, heterogeneity in both water concentration and its hydrogen isotopic ratio has been documented for various lunar phases. However, most previous studies have focused on measurements of hydrogen in apatite, which typically forms during the final stages of melt crystallisation. To better constrain the abundance and isotopic composition of water in the lunar interior, we have targeted melt inclusions (MIs), in mare basalts, that are trapped during the earliest stages of melt crystallisation. Melt inclusions are expected to have suffered minimal syn- or post-eruption modification processes, and, therefore, should provide more accurate information about the history of H in the lunar interior. Here, we report H−/18O− measurements as calibrated water concentrations, and hydrogen isotope ratios obtained by secondary ion mass spectrometry (SIMS) in a large set of basaltic MIs from Apollo mare basalts 10020, 10058, 12002, 12004, 12008, 12020, 12040, 14072 and 15016. Our results demonstrate that partially crystallised MIs from lunar basalts and their parental melts were influenced by a variety of processes such as hydrogen diffusion, degassing and assimilation of material affected by solar-wind implantation. Deconvolution of these processes show that lunar basaltic parental magmas were heterogeneous and had a broadly chondritic hydrogen isotopic composition with δD values varying between −200 and +200‰.

The role of early sulfide saturation in the formation of the Yulong porphyry Cu-Mo deposit: Evidence from mineralogy of sulfide melt inclusions and platinum-group element geochemistry

Sulfide melt inclusions enclosed in silicate minerals have been observed in many oxidized Cu-bearing porphyries. The origin of such sulfide melt inclusions could bear critical information of metal enrichment or depletion during magma evolution of porphyry systems. The giant Yulong porphyry Cu-Mo deposit in eastern Tibetan Plateau consists of a series of felsic intrusions, among which the mineralized Yulong intrusion emplaced at a late stage of the magmatic activity. This study reports detailed texture and composition of sulfide melt inclusions from barren and mineralized intrusions, as well as platinum-group elements of these intrusions, to evaluate potential role of early sulfide saturation in the porphyry mineralization system. These sulfide melt inclusions, occurring as globules or droplets, are preserved in zircon, amphibole and quartz grains. Most of them are composed of a major phase of pyrrhotite and minor pyrite, chalcopyrite and pentlandite. Smooth boundaries between these phases suggest the origin of fractionation from sulfide melt upon cooling. Small amounts of Cu and Ni are heterogeneously distributed in the pyrrhotite. The sulfide melt inclusions could be observed in both the pre- and syn-mineralization intrusions at Yulong, indicating that sulfide saturation had commonly occurred during magma evolution. Magnetite mineral inclusions could be observed in zircon and amphibole grains as well. The mineral assemblage of magnetite + pyrrhotite + pyrite of the mineral inclusions constrains the Yulong magmatic system to redox conditions near the magnetite-pyrite/pyrrhotite buffer and above the fayalite/ magnetite-quartz buffer (FMQ). High Eu/Eu* values (0.56–0.78) of the hosting zircons and high calculated ΔNNO values (>0.5) of the hosting amphibole indicate highly oxidized nature of the parental magma.Both pre- and syn-mineralization intrusions have low PGE contents (Pt = 0.04 to 0.17 ppb, Pd = 0.19–0.60 ppb) with compositions plotting in the Au-poor porphyry Cu fields in discriminating diagrams. Early sulfide saturation may have been reached and small amounts of sulfides have been segregated from magma. Most PGEs and Au but little Cu have been removed from the evolved magma because the partition coefficient of Cu between sulfide and silicate melt is about two orders of magnitude lower than PGE or Au. As a consequence, the evolved magma would have high Cu but very low PGE and Au, resulting in the Au-poor porphyry Cu mineralization at Yulong.

Petrogenesis of middle Okinawa Trough volcanic rocks: Constraints from lead isotopes in olivine-hosted melt inclusions

Olivine-hosted melt inclusions provide crucial information about their parental magma composition and evolution and represent ideal targets for determining the nature of the poorly understood and still-debated magma sources in the Okinawa Trough, a back-arc basin in the western Pacific. We present the first analyses of the lead (Pb) isotope compositions of olivine-hosted melt inclusions to evaluate the mantle properties and petrogenesis of middle Okinawa Trough volcanic rocks. The melt inclusions have more variable major and trace elements and Pb isotope compositions than the host whole-rock samples. We report the discovery of both high-207Pb/206Pb (>0.865) and low-207Pb/206Pb (<0.865) isotope compositions in melt inclusions in individual volcanic rocks, even within a single host olivine, indicating a compositionally heterogeneous magma source. The trace element and Pb isotope characteristics of the melt inclusions show that the magma source is affected by enriched components. We modeled the injection of enriched components into a magma source to explain the generation of the magma heterogeneity. The results indicate that the mixing of Pacific Ocean-type mantle (MORB), an EMI-like component from recycled lower continental crust and EMII-like material from subducted sediments can explain the low-207Pb/206Pb isotope values observed in the melt inclusions. The discovery of small proportions of melt inclusions with high 207Pb/206Pb ratios, high K2O, P2O5, Rb and U contents and low Pb and Cu contents in the studied andesites suggests that the andesitic magma may have been formed by the mixing of materials with different elemental and isotopic compositions. Our study results suggest that pervasive magma mixing may have occurred in the magma source prior to eruption in the Okinawa Trough.

What lies beneath? Reconstructing the primitive magmas fueling voluminous silicic volcanism using olivine-hosted melt inclusions

Abstract Understanding the origins of the mantle melts that drive voluminous silicic volcanism is challenging because primitive magmas are generally trapped at depth. The central Taupō Volcanic Zone (TVZ; New Zealand) hosts an extraordinarily productive region of rhyolitic caldera volcanism. Accompanying and interspersed with the rhyolitic products, there are traces of basalt to andesite preserved as enclaves or pyroclasts in caldera eruption products and occurring as small monogenetic eruptive centers between calderas. These mafic materials contain MgO-rich olivines (Fo79–86) that host melt inclusions capturing the most primitive basaltic melts fueling the central TVZ. Olivine-hosted melt inclusion compositions associated with the caldera volcanoes (intracaldera samples) contrast with those from the nearby, mafic intercaldera monogenetic centers. Intracaldera melt inclusions from the modern caldera volcanoes of Taupō and Okataina have lower abundances of incompatible elements, reflecting distinct mantle melts. There is a direct link showing that caldera-related silicic volcanism is fueled by basaltic magmas that have resulted from higher degrees of partial melting of a more depleted mantle source, along with distinct subduction signatures. The locations and vigor of Taupō and Okataina are fundamentally related to the degree of melting and flux of basalt from the mantle, and intercaldera mafic eruptive products are thus not representative of the feeder magmas for the caldera volcanoes. Inherited olivines and their melt inclusions provide a unique “window” into the mantle dynamics that drive the active TVZ silicic magmatic systems and may present a useful approach at other volcanoes that show evidence for mafic recharge.

The relationship between the Taihe Fe-Ti oxide ore-bearing layered intrusion and the adjacent peralkaline A-type granitic pluton in SW China: Constraints from compositions of amphiboles and apatite-hosted melt inclusions

The Taihe igneous complex, which is located in the inner zone of the Emeishan Large Igneous Province (ELIP), consists of an Fe-Ti oxide ore-bearing layered gabbroic unit and a silicic enclave-bearing peralkaline A-type granitic unit. The apatite-hosted melt inclusions in the gabbroic unit and the amphiboles in both the gabbroic unit and the granitic unit were used to investigate the relationship between the two units. The apatite-hosted melt inclusions have 32.3–68.3 wt% SiO2, 0.51–23.9 wt% FeOt, 0–7.19 wt% TiO2, 8.87–19.0 wt% Al2O3, 2.39–19.5 wt% CaO, 1.70–7.27 wt% Na2O, 0.24–5.60 wt% K2O, and 0.13–6.83 wt% P2O5. Such a large compositional range is difficult to be achieved through fractional crystallization but can be accounted for by silicate liquid immiscibility developed during the differentiation of the gabbroic unit. The weighted bulk composition of the Taihe gabbroic unit is extremely rich in Fe and poor in Si (17.2 wt% FeOt and 36.2 wt% SiO2), indicating that some Si-rich components must have been removed. A significant compositional gap in the amphiboles in the gabbroic rocks and the adjacent peralkaline A-type granitoids, i.e., calcic amphibole in the gabbros and apatite-hosted inclusions and sodic-calcic to sodic amphiboles in the granites and their silicic enclaves, was observed. This cannot be explained by the previously suggested differentiation of a common basaltic parental magma from gabbros to silicic enclaves and granites, indicating a sudden change in the physical-chemical conditions of the related magmas. We propose that the silicic enclaves enclosed in the peralkaline A-type granitic pluton represent the missing Si-rich components of the Taihe layered gabbroic unit. The continuous expulsion of immiscible Si-rich melts from the lower crystal mush into the upper main magma body caused the crystallization of large amounts of Fe-Ti oxides in the gabbroic unit. Then, the residual Si-rich magmas in the upper part of the Taihe mafic magma chamber were drawn into the subsequently intruding peralkaline granitic magmas during the magmatic transgression process, forming the silicic enclaves within the adjacent contemporaneous peralkaline A-type granitic pluton.

Temporal Evolution of Proto-Izu–Bonin–Mariana Arc Volcanism over 10 Myr: Constraints from Statistical Analysis of Melt Inclusion Compositions

AbstractInternational Ocean Discovery Program (IODP) Expedition 351 ‘Izu–Bonin–Mariana (IBM) Arc Origins’ drilled Site U1438, situated in the northwestern region of the Philippine Sea. Here volcaniclastic sediments and the igneous basement of the proto-IBM volcanic arc were recovered. To gain a better understanding of the magmatic processes and evolution of the proto-IBM arc, we studied melt inclusions hosted in fresh igneous minerals and sampled from 30–40 Myr old deposits, reflecting the maturation of arc volcanism following subduction initiation at 52 Ma. We performed a novel statistical analysis on the major element composition of 237 representative melt inclusions selected from a previously published dataset, covering the full age range between 30 and 40 Ma. In addition, we analysed volatiles (H2O, S, F and Cl) and P2O5 by secondary ion mass spectrometry for a subset of 47 melt inclusions selected from the dataset. Based on statistical analysis of the major element composition of melt inclusions and by considering their trace and volatile element compositions, we distinguished five main clusters of melt inclusions, which can be further separated into a total of eight subclusters. Among the eight subclusters, we identified three major magma types: (1) enriched medium-K magmas, which form a tholeiitic trend (30–38 Ma); (2) enriched medium-K magmas, which form a calc-alkaline trend (30–39 Ma); (3) depleted low-K magmas, which form a calc-alkaline trend (35–40 Ma). We demonstrate the following: (1) the eruption of depleted low-K calc-alkaline magmas occurred prior to 40 Ma and ceased sharply at 35 Ma; (2) the eruption of depleted low-K calc-alkaline magmas, enriched medium-K calc-alkaline magmas and enriched medium-K tholeiitic magmas overlapped between 35 and 38–39 Ma; (3) the eruption of enriched medium-K tholeiitic and enriched medium-K calc-alkaline magmas became predominant thereafter at the proto-IBM arc. Identification of three major magma types is distinct from the previous work, in which enriched medium-K calc-alkaline magmas and depleted low-K calc-alkaline magmas were not identified. This indicates the usefulness of our statistical analysis as a powerful tool to partition a mixture of multivariable geochemical datasets, such as the composition of melt inclusions in this case. Our data suggest that a depleted mantle source had been replaced by an enriched mantle source owing to convection beneath the proto-IBM arc from &amp;gt;40 to 35 Ma. Finally, thermodynamic modelling indicates that the overall geochemical variation of melt inclusions assigned to each cluster can be broadly reproduced either by crystallization differentiation assuming P = 50 MPa (∼2 km deep) and ∼2 wt% H2O (almost saturated H2O content at 50 MPa) or P = 300 MPa (∼15 km deep) and ∼6 wt% H2O (almost saturated H2O content at 300 MPa). Assuming oxygen fugacity (fO2) of log fO2 equal to +1 relative to the nickel–nickel oxide (NNO) buffer best reproduces the overall geochemical variation of melt inclusions, but assuming more oxidizing conditions (log fO2 = +1 to +2 NNO) probably reproduces the geochemical variation of enriched medium-K and calc-alkaline melt inclusions (30–39 Ma).

Chasing the mantle: Deciphering cryptic mantle signals through Earth's thickest continental magmatic arc

Understanding the magmatic processes occurring in the upper mantle and lowermost crust above subduction zones is paramount if we hope to understand the primary mechanisms driving arc magmatism. In the Central Volcanic Zone of the Andes (CVZ), extreme crustal thicknesses and the prominence of silicic magmatism in the Neogene have prevented a clear understanding of the primitive magma input into what is arguably the Earth's archetypal active continental margin. However, we propose that phase chemistry and equilibria from mafic mineral assemblages in basaltic andesite enclaves from the Purico-Chascon Volcanic Complex reveal rare constraints on the character of primitive mantle-derived magmas feeding the CVZ arc. We interpret olivine, Cr-spinel, and olivine-hosted melt inclusion compositions from these mafic assemblages to record the partial melting of wet, oxidized mantle peridotite and the production of high-Mg basalt in the upper mantle. In turn, clinopyroxene and spinel-free olivine are interpreted to reflect magma modification in the lower crust and the adiabatic ascent of differentiated basaltic magma up to ∼20 km. Interestingly, olivine-hosted Cr-spinel compositions suggest that magmas in the Purico-Chascon system ascend from the upper mantle to surface with no significant change in Fe-oxidation state despite obvious differentiation and crustal interactions. Finally, comparison of high Mg-basalt in melt inclusions from Purico-Chascon (CVZ arc) and CVZ back-arc lavas show that the magmas may share a common peridotite source. However, the peridotite beneath the arc appears to be more hydrous and oxidized than that beneath the back-arc, confirming that back-arc lava compositions may not adequately represent mafic magma input into the arc. We posit that the relationships between H2O-content, oxidation conditions, and mafic phase equilibria discussed in this study are directly linked to subduction zone geometry, and thus may be applicable to arcs globally.

Geochemistry of phosphorus and the behavior of apatite during crustal anatexis: Insights from melt inclusions and nanogranitoids

AbstractThe solubility of apatite in anatectic melt plays an important role in controlling the trace-element compositions and isotopic signatures of granites. The compositions of glassy melt inclusions and nanogranitoids in migmatites and granulites are compared with the results of experimental studies of apatite solubility to evaluate the factors that influence apatite behavior during prograde suprasolidus metamorphism and investigate the mechanisms of anatexis in the continental crust. The concentration of phosphorus in glassy melt inclusions and rehomogenized nanogranitoids suggests a strong control of melt aluminosity on apatite solubility in peraluminous granites, which is consistent with existing experimental studies. However, measured concentrations of phosphorus in melt inclusions and nanogranitoids are generally inconsistent with the concentrations expected from apatite solubility expressions based on experimental studies. Using currently available nanogranitoids and glassy melt inclusion compositions, we identify two main groups of inclusions: those trapped at lower temperature and showing the highest measured phosphorus concentrations, and melt inclusions trapped at the highest temperatures having the lowest phosphorus concentrations. The strong inconsistency between measured and experimentally predicted P concentrations in higher temperature samples may relate to apatite exhaustion during the production of large amounts of peraluminous melt at high temperatures. The inconsistency between measured and predicted phosphorus concentrations for the lower-temperature inclusions, however, cannot be explained by problems with the electron microprobe analyses of rehomogenized nanogranitoids and glassy melt inclusions, sequestration of phosphorus in major minerals and/or monazite, shielding or exhaustion of apatite during high-temperature metamorphism, and apatite–melt disequilibrium. The unsuitability of the currently available solubility equations is probably the main cause for the discrepancy between the measured concentrations of phosphorus in nanogranites and those predicted from current apatite solubility expressions. Syn-entrapment processes such as the generation of diffusive boundary layers at the mineral-melt interface may also be responsible for concentrations of P in nanogranitoids and glassy melt inclusions that are higher than those predicted in apatite-saturated melt.

Temporal evolution of magma and crystal mush storage conditions in the Bárðarbunga-Veiðivötn volcanic system, Iceland

The depth(s) of magma storage reservoirs beneath active volcanic regions may change with time. Determining the rates and causes of millennial-scale changes in magmatic system architecture is critical for the development of realistic time-integrated models of crustal evolution. Here we examine a suite of samples from the exceptionally well-exposed Bárðarbunga-Veiðivötn volcanic system in central Iceland in order to resolve the temporal evolution of magma storage conditions within one of Iceland’s most productive volcanic systems. We have measured the major and minor elemental composition of glass, mineral and melt inclusion from five erupted units that span a full glacial cycle, from a <100 ka subglacial eruption to a historical eruption in 1477 AD. All samples contain macrocrysts (>500 μm), polymineralic clots and high-crystallinity nodules, consistent with derivation from crystal mush bodies. Macrocryst rims are in chemical equilibrium with their respective carrier melts, while macrocrysts cores are too primitive to have crystallized from these melts. Each sample records a distinct range of macrocryst compositions, indicating that the composition and/or eruptibility of stored crystal mush has changed with time. Macrocrysts from the oldest units are the most primitive, and the macrocryst compositional range becomes wider and, on average, more evolved, with time. Clinopyroxene-melt and melt-based (OPAM) geobarometers reveal temporally invariant crystallization conditions of 1.9–2.2 ± 0.7 (1σ) kbar pressure, corresponding to depths around 6.8–7.8 ± 2.5 km. All the samples also contain melt inclusions trapped at mid-crustal pressures of ∼2.6 kbar (9.6 km). In addition, melt inclusions hosted in most primitive olivines and plagioclases from subglacial and early Holocene eruptions preserve evidence of crystallization in a lower-crustal storage level(s) located at 17.5 km (4.9 kbar). This petrological record of deep crystallization may be linked to a surge in eruption rates, tapping of lower-crustal magma reservoirs, consistent with a crustal response associated with postglacial isostatic rebound. In contrast, the absence of a deep crystallization signature in the younger eruptive units may reflect lower magma production rates under steady-state conditions of the crust, and new magma pathways favouring melt storage in the mid-crust.