Silicate Melt Inclusions Research Articles

Olivine-hosted melt inclusions in Pliocene–Quaternary lavas from the Qorveh–Bijar volcanic belt, western Iran: implications for source lithology and cooling history

ABSTRACT Silicate melt inclusions (SMIs) are small droplets of magma that become trapped in minerals during crystal growth. SMIs in olivine crystals can provide critical information on the range of melt compositions and processes that occur during melt generation, evolution, transport, and eruption. The Pliocene–Quaternary volcanic rocks in the Qorveh–Bijar volcanic belt of western Iran show porphyritic and microlithic textures, with olivine and clinopyroxene being the dominant minerals. Magnesian olivines in these volcanic rocks contain primary SMIs. The composition and characteristic of olivine-hosted SMI of these rocks are investigated to constrain the source lithology for mafic volcanism. Bulk compositions of the SMIs overlap those of their host rocks and extend to higher CaO/Al2O3 values. The estimated entrapment pressures and temperatures of the studied SMIs are 9.1–10.3 kbar and 1220–1355°C. The calculated mafic parental melt contains 42.36 wt.% SiO2, low total alkalis (3.22 wt.%), and high MgO (16.1 wt.%). Exploratory calculations using pMELTS show that this parental composition underwent variable degrees of fractional crystallization, as reflected by the variable compositions of the SMIs. Several lines of evidence including pyroxene xenocrysts and high FeO/MnO, FC3MS (FeO/CaO – 3*MgO/SiO2), and Zn/Fe ratios (14–21), suggest that a metasomatized pyroxenitic source contributed to the genesis of the parental melt. Amphibole in the SMIs indicates a high volatile content in the parental melt, which we conclude was generated from a metasomatized lithospheric mantle source. The pyroxenite source also contained garnet. Our geochemical results lead us to propose a new petrogenetic model. Specifically, we infer that a dense and unstable portion of the lithosphere underwent localized laminar detachment and downward flow, i.e. lithospheric drip. This drip underwent volatile-enhanced partial melting during descent through the underlying hot asthenosphere and generated the studied volcanic rocks.

The First Discovery of Low-temperature Rhyolite Melts in Cenozoic Long-lived Bazman Volcano, East Iran; Some Problems and Discussion

The igneous rocks, some geological specifics, and also the very melt inclusions in Late Cenozoic rhyolites were studied in the long-lived Miocene (N1, ca. 20 Ma) - Late Quaternary (Q3?) Bazman volcano, eastern Iran. Unusual low temperature (ca. 690o C) silicate melt inclusions in quartz (Qtz) with a High/very High water content (from average 6.9 to 8.2 (up to 9.0 as a limit) wt% H2O) in the acid rocks were defined by independent methods for the first time. Whole-rock chemistry of the melts studied is similar to ones in subduction-related acid melts from different regions. Rhyolites studied sometimes have higher concentration of ore elements, similar to ones in even basites. Ore inheritance from economic porphyry – PCD (Cu-Au +- Mo) Paleogene (mainly Eocene - Pg2) mineralization and deposits in the region is proposed. Geological anomaly of the whole region is proposed by a complex analysis. This anomaly is maybe responsible for anomalous magmatism due to a known tomography data (since Paleocene (Pg1), as minimum), tectonics, metallogeny (including economic one), and maybe - hydrocarbons (HC, oil - gas) activity.

Anatexis and fluid regime of the deep continental crust: New clues from melt and fluid inclusions in metapelitic migmatites from Ivrea Zone (NW Italy)

AbstractWe investigate the inclusions hosted in peritectic garnet from metapelitic migmatites of the Kinzigite Formation (Ivrea Zone, NW Italy) to evaluate the starting composition of the anatectic melt and fluid regime during anatexis throughout the upper amphibolite facies, transition, and granulite facies zones. Inclusions have negative crystal shapes, sizes from 2 to 10 μm and are regularly distributed in the core of the garnet. Microstructural and micro‐Raman investigations indicate the presence of two types of inclusions: crystallized silicate melt inclusions (i.e., nanogranitoids, NI), and fluid inclusions (FI). Microstructural evidence suggests that FI and NI coexist in the same cluster and are primary (i.e., were trapped simultaneously during garnet growth). FI have similar compositions in the three zones and comprise variable proportions of CO2, CH4, and N2, commonly with siderite, pyrophyllite, and kaolinite, suggesting a COHN composition of the trapped fluid. The mineral assemblage in the NI contains K‐feldspar, plagioclase, quartz, biotite, muscovite, chlorite, graphite and, rarely, calcite. Polymorphs such as kumdykolite, cristobalite, tridymite, and less commonly kokchetavite, were also found. Rehomogenized NI from the different zones show that all the melts are leucogranitic but have slightly different compositions. In samples from the upper amphibolite facies, melts are less mafic (FeO + MgO = 2.0–3.4 wt%), contain 860–1700 ppm CO2 and reach the highest H2O contents (6.5–10 wt%). In the transition zone melts have intermediate H2O (4.8–8.5 wt%), CO2 (457–1534 ppm) and maficity (FeO + MgO = 2.3–3.9 wt%). In contrast, melts at granulite facies reach highest CaO, FeO + MgO (3.2–4.7 wt%), and CO2 (up to 2,400 ppm), with H2O contents comparable (5.4–8.3 wt%) to the other two zones. Our results represent the first clear evidence for carbonic fluid‐present melting in the Ivrea Zone. Anatexis of metapelites occurred through muscovite and biotite breakdown melting in the presence of a COH fluid, in a situation of fluid–melt immiscibility. The fluid is assumed to have been internally derived, produced initially by devolatilization of hydrous silicates in the graphitic protolith, then as a result of oxidation of carbon by consumption of Fe3+‐bearing biotite during melting. Variations in the compositions of the melts are interpreted to result from higher T of melting. The H2O contents of the melts throughout the three zones are higher than usually assumed for initial H2O contents of anatectic melts. The CO2 contents are highest at granulite facies, and show that carbon‐contents of crustal magmas are not negligible at high T. The activity of H2O of the fluid dissolved in granitic melts decreases with increasing metamorphic grade. Carbonic fluid‐present melting of the deep continental crust represents, together with hydrate‐breakdown melting reactions, an important process in the origin of crustal anatectic granitoids.

In-situ quantification of chlorine and sulfur in glasses, minerals and melt inclusions by LA-ICP-MS

In-situ analysis of sulfur and chlorine in geological materials is typically conducted by electron probe micro-analysis or secondary-ion mass spectrometry. Both of these methods require a homogeneous material that is exposed at the surface, which is commonly not the case for silicate melt inclusions. To overcome this issue the accuracy and precision of sulfur and chlorine quantification by laser-ablation inductively-coupled-plasma (quadrupole) mass spectrometry (LA-ICP-MS) were investigated for melt inclusions and also for glasses and minerals. Samples with known sulfur and/or chlorine concentrations were analyzed using suitable external standards. The main limitation for chlorine and sulfur quantification in geological materials by LA-ICP-MS is a contamination effect resulting in excess Cl and S signals during ablation of any material. This contamination results from the mobilization of chlorine and sulfur impurities attached to the walls of the sample chamber and the gas transport tubes. In the case of Cl accurate results can be obtained for Cl concentrations of few hundred of ppm after applying a simple correction formula that relates the magnitude of the excess Cl signal to the quantity of produced aerosol, which is approximated by sensitivity. In the case of S the application of a similar correction formula does not suffice, and a second correction formula that relates the calculated sulfur content to the actual sulfur content needs to be applied. The combined approach allows accurate quantification of S in minerals and glasses down to a concentration of ~600 μg/g. For unexposed silicate melt inclusions the numerical host correction automatically removes the excess Cl related to the amount of produced aerosol, hence Cl concentrations turn out accurate without any correction. Due to counting statistics and uncertainties in the host correction a minimum melt inclusion size of 30 μm is required to accurately quantify Cl in melt inclusions containing ~1500 μg/g Cl. Sulfur concentrations, on the other hand, can be accurately quantified down to 700 μg/g even in small (15 μm) melt inclusions. In summary, chlorine and sulfur can be accurately and precisely quantified by LA-ICP-MS in glasses, minerals, and melt inclusions down to a concentrations of several hundreds of μg/g, provided that an ICP-MS instrument with relatively low background on Cl and S is available and that instrument-specific correction equations are established on a set of samples with known Cl and S contents.

Degassing-induced fractionation of multiple sulphur isotopes unveils post-Archaean recycled oceanic crust signal in hotspot lava

Mantle source regions feeding hotspot volcanoes likely contain recycled subducted material. Anomalous sulphur (S) isotope signatures in hotspot lavas have tied ancient surface S to this deep geological cycle, but their potential modification by shallow magmatic processes has generally been overlooked. Here we present S isotope measurements in magmatic sulphides, silicate melt inclusions and matrix glasses from the recent eruption of a hotspot volcano at El Hierro, Canary Islands, which show that degassing induces strongly negative δ34S fractionation in both silicate and sulphide melts. Our results reflect the complex interplay among redox conditions, S speciation and degassing. The isotopic fractionation is mass dependent (Δ33S = 0‰), thus lacking evidence for the recycled Archaean crust signal recently identified at other hotspot volcanoes. However, the source has an enriched signature (δ34S ~ + 3‰), which supports the presence of younger 34S-rich recycled oceanic material in the Canary Island mantle plume.

Significance of high temperature fluids and melts in the Grasberg porphyry copper‑gold deposit

A study of quartz-sulfide veins from the Grasberg Cu-Au deposit has shown that the quartz crystals initially grew from the walls of the veins into open fractures. Cathodoluminescence (CL) imaging shows concentric growth zones in the larger crystals and changes in the orientation of the growth zones with occasional truncation of the zoning. Towards the centre of veins the crystals become smaller and irregularly shaped but exhibit CL banding parallel to vein walls, which is normally associated with crack-seal processes. The vein quartz contains silicate and sulfide-rich melt inclusions, virtually water-free salt-melt inclusions, and coexisting hypersaline and vapour-rich inclusions. Late stage, secondary aqueous inclusions are also present in some veins. Most fluid inclusions homogenised at temperatures below 620 °C. However, hypersaline (B2) inclusions had a very complex behaviour and exhibited partial homogenisation to salt melt and clear immiscible fluid at temperatures ranging from 820 °C to 1300 °C. These inclusions contain the highest Cu concentrations (up to 6.3 wt%), suggesting they play a role in metal transport.The contemporaneous trapping of the K-feldspar-rich melt inclusions, hypersaline and vapour-rich inclusions suggests a common source for these fluids and a process involving heterogeneous entrapment of immiscible silicate melt, hypersaline fluid and vapour as previously proposed for other porphyry copper deposits. Furthermore, the Grasberg deposit is estimated to have formed at pressures below 400 bar, which may explain the presence of salt-melt inclusions as the vapour + halite field of the H2O-NaCl system is dominant below 800 °C at these pressures.The unrealistically high temperatures reported herein may have resulted from entrapment of either immiscible silicate melt, hypersaline fluid and vapour or cumulates of an evolving silicate melt saturated in K-spar, brine and vapour. Although these temperatures may not be directly useful, these inclusions give important clues to metal-enrichment processes. These melt and fluid inclusions record cycles of transitory, high-temperature (>700 °C) hydrofracturing, melt and fluid release, and vein formation in a cooler (500–600 °C) background host-rock thermal regime. High sulfur and Cu contents of sulfide-melt and hypersaline B2 inclusions are interpreted to be the fluid composition before the main sulfide precipitation event, and, therefore, may be used as tracers of the magmatic body from which they exsolved at depth.

Evidence of a shallow magma reservoir below Los Humeros volcanic complex: Insights from the geochemistry of silicate melt inclusions

Los Humeros volcanic complex (Eastern Trans Mexican Volcanic belt) is one of the three most important geothermal fields in Mexico and is currently producing electricity (ca. 90 MW). It is therefore pertinent to constrain the magma chamber characteristics, such as its depth, and the magmatic evolution of the complex in order to better define the superficial geothermal activity. In this paper we present the first geochemical data (major and trace elements) for 15 silicate melt inclusions and their five host rocks from Los Humeros volcanic complex.We constrained the evolution of the volcanic complex by coupling X-ray spectrometry, ICP-MS, microprobe, X-ray microtomography and NanoSIMS analyses together with thermodynamical modeling on both whole rocks and silicate melt inclusions.A ponding reservoir located within an interval betwwen 5 and 13 km at depth is evidenced by silicate melt inclusions entrapment pressures. This ponding depth's interval is in agreement with previous studies by thermal modeling, resistivity and geobaromatry done in the same area. The geothermal activity of Los Humeros volcanic area is expected to be the result of the cooling of the magma in the shallow chamber. A large geochemical variability, in less than 1 Ma, is demonstrated by trace and volatile element compositions evolution through time. Subduction contamination is expected into the mantle source with the highest contamination event at 69 Ka, during the second caldera event of Los Humeros volcanic complex.

Controls on lithium concentration and diffusion in zircon

Thermal annealing of zircons prior to uranium-lead dating by laser ablation inductively coupled plasma mass spectrometry is a commonly-implemented procedure which improves data accuracy and precision by partially repairing radiation damage from the decay of uranium and thorium. However, it also leads to significantly higher concentrations of lithium in the zircon lattice, which become positively correlated with trivalent yttrium and rare earth elements. Prior to such treatments, zircons typically contain lithium below detection limits (typically <0.55 μg g−1), unless correlated with lanthanum and aluminum (i.e., melt/mineral inclusion tracer elements). This suggests that lithium in zircon is primarily sequestered within inclusions, and is able to permeate the crystal lattice to couple with yttrium and rare earth elements during the thermal annealing procedure. This process occurs 2–3 orders of magnitude faster than diffusion experiments have previously determined, indicating that another diffusion mechanism may apply. A model is proposed, whereby: (i) charge compensation of stoichiometrically over-abundant trivalent cations under water-rich magmatic conditions is likely accomplished by hydrogen, given the incompatibility of lithium in zircon and the abundance of hydrogen. However, (ii) conditions that are high temperature and low pressure (characteristic of both thermal annealing and the syn-eruptive environment), drive silicate melt inclusions to exsolve water, generating a motive force for both hydrogen and lithium from inclusions to permeate the lattice in order to reestablish electrochemical equilibrium between the interior and exterior of the zircon. To test the pressure dependency of lithium migration in the zircon lattice, thermal annealing experiments were performed at 850 °C and 1 bar, 2 kbar and 6 kbar using zircons from the Fish Canyon Tuff. The experiments demonstrate that thermal annealing at 2 and 6 kbar inhibits lithium mobility, with zircons registering lithium concentrations below detection limits similar to controls. The experimental results suggest that lithium concentrations in zircon are vulnerable to rapid perturbation by decompression (concurrent with high temperatures), which further indicate that lithium-in-zircon diffusion data should be interpreted with caution.

Copper and Li diffusion in plagioclase, pyroxenes, olivine and apatite, and consequences for the composition of melt inclusions

Silicate melt inclusions are an important tool to reconstruct original concentrations of volatiles and metals in silicate melts, as these constituents are partly lost during magma solidification. However, melt inclusions analyzed in co-precipitated minerals in volcanic rocks commonly show strongly divergent Cu contents, raising doubts about the validity of their composition. To understand the origin of this divergence a multiple approach was followed. First, the phenomenon was documented by recording petrographic details of five volcanic samples and analyzing 189 melt inclusions by laser-ablation ICP-MS. Second, diffusion experiments on gem-quality plagioclase, clinopyroxene, orthopyroxene, apatite and olivine were performed in a gas mixing furnace to constrain diffusion coefficients of Cu and to a lesser extent of Li. Third, re-equilibration experiments were performed on melt inclusions within plagioclase crystals to induce changes in their Cu and Li contents.The LA-ICP-MS analyses reveal that plagioclase-hosted and orthopyroxene-hosted melt inclusions in volcanic rocks commonly contain an order of magnitude more Cu than melt inclusions in co-precipitated clinopyroxene and olivine. Plagioclase-hosted melt inclusions in intrusive rocks, on the other hand, do not show this divergence. The diffusion experiments conducted on minerals find that Cu and Li diffusion in plagioclase is extremely fast (log D = −13.0 to −11.5 m2 s−1 at 1000 °C) and in both cases occurs via two separate diffusion mechanisms. Copper diffusion coefficients in apatite, clinopyroxene, orthopyroxene and olivine are 2–3 orders of magnitude lower but are still high compared to most other elements. Both the re-equilibration experiments on melt inclusions and quantitative modeling based on the measured diffusion coefficients demonstrate that at 1000 °C plagioclase-hosted melt inclusions can re-equilibrate their Cu and Li content with that of the surrounding magma within a few hours to a few weeks, whereas apatite-, clinopyroxene-, orthopyroxene- and olivine-hosted melt inclusions require tens of years to hundreds of years to do so. Abnormally high Cu contents of plagioclase- and orthopyroxene-hosted melt inclusions appear to result from postentrapmental Cu gain. In the case of orthopyroxene-hosted melt inclusions Cu seems to have been gained as a consequence of postentrapmental sidewall crystallization, which triggered the formation of sulfide globules that acted as a Cu sink. This process likely occurred at depth in magma chambers. Plagioclase-hosted melt inclusions, on the other hand, seem to have gained Cu in exchange for hydrogen that diffused out of the melt inclusions during or after volcanic eruption. Our results suggest that Cu (and also Li) concentrations measured in melt inclusions have to be generally treated with caution, and that particular prudence has to be used in the case of plagioclase and orthopyroxene hosts.

Advances in 3D imaging and volumetric reconstruction of fluid and melt inclusions by high resolution X-ray computed tomography

Fluid and melt inclusions are tiny pockets of fluid and melt trapped in natural and synthetic minerals. Characterizing the 3D distribution of fluid and melt inclusions within minerals, their shape and the volume fraction of their different phases is crucial for determining the conditions of crystal growth and paleostress analysis. However, their relatively small size (typically 5 to 100 μm), complex shape, heterogeneous content, the opaque nature of some host minerals and projection bias frequently hamper accurate imaging and volumetric reconstruction using conventional microscopic techniques. High resolution X-ray computed tomography (HRXCT) is a non-destructive method which uses contrasts of X-ray attenuation in a series of contiguous radiographs with different view angles to reconstruct the 3D distribution of areas of different densities within a large variety of materials. In this work, we show the capabilities of HRXCT for: (i) imaging the 3D distribution of aqueous and hydrocarbon-bearing fluid inclusions and silicate melt inclusions in a crystal; (ii) characterizing the shape of fluid and melt inclusions and (iii) reconstructing the total volume and the volume of the different phases (liquid, glass, crystal, vapor) of fluid and melt inclusions. We have used a variety of hand specimens and chips of transparent and opaque minerals (olivine, quartz, feldspar, garnet, emerald, wolframite), that we analyzed using three different HRXCT setups. When a resolution of ~1 μm3/voxel is achieved, HRXCT allows identifying >5 μm fluid inclusions, and the identification and volumetric reconstruction of the different phases can be carried out with reasonable confidence for relatively large (>25 μm) inclusions. Density contrasts are high enough to properly identify: (i) a silicate melt inclusion, and its different phases (glass, vapor and crystals such as clinopyroxene and spinel) in an olivine crystal; (ii) aqueous monophase (liquid) and two-phase (liquid + vapor) fluid inclusions in transparent and opaque minerals (quartz, garnet, emerald, wolframite). In the case of hydrocarbon-bearing fluid inclusions containing a vapor phase and two liquid phases (oil and aqueous solution), the two liquid phases could not be distinguished from each other. Volumetric reconstruction of liquid and vapor phases of aqueous and hydrocarbon-bearing fluid inclusions show compatible results with independent calculations using known pressure, temperature, molar volume and composition (P-T-V-x) conditions of trapping or imaging using confocal laser scanning microscopy respectively. Collectively, our results show that HRXCT is a promising tool for non-destructive characterization of fluid and melt inclusions.

Petrogenesis of martian sulfides in the Chassigny meteorite

The Chassigny meteorite, a martian dunite, contains trace amounts (0.005 vol%) of Fe-Ni sulfides, which were studied from two polished mounts in reflected light microscopy, scanning electron microscope (SEM), and electron microprobe (EMP). The sulfide phases are, by decreasing order of abundance, nickeliferous (0–3 wt% Ni) pyrrhotite with an average composition M0.88±0.01S (M = Fe+Ni+Co+Cu+Mn), nickeliferous pyrite (0–2.5 wt% Ni), pentlandite, millerite, and unidentified Cu sulfides. Pyrrhotite is enclosed inside silicate melt inclusions in olivine and disseminated as polyhedral or near spherical blebs in intergranular spaces between cumulus and postcumulus silicates and oxides. This sulfide is considered to be a solidification product of magmatic sulfide melt. The pyrrhotite Ni/Fe ratios lie within the range expected for equilibration with the coexisting olivine at igneous temperatures. Pyrite occurs only as intergranular grains, heterogeneously distributed between the different pieces of the Chassigny meteorite. Pyrite is interpreted as a by-product of the low-T (200 °C) hydrothermal alteration events on Mars that deposited Ca sulfates + carbonates well after complete cooling. The shock that ejected the meteorite from Mars generated post-shock temperatures high (300 °C) enough to anneal and rehomogenize Ni inside pyrrhotite while pyrite blebs were fractured and disrupted into subgrains by shock metamorphism. The negligible amount of intergranular sulfides and the lack of solitary sulfide inclusions in cumulus phases (olivine, chromite) indicate that, like other martian basalts so far studied for sulfur, the parental melt of Chassigny achieved sulfide-saturation at a late stage of its crystallization history. Once segregated, the pyrrhotite experienced a late-magmatic oxidation event that reequilibrated its metal-to-sulfur ratios.

Immiscible sulfide melts in primitive oceanic magmas: Evidence and implications from picrite lavas (Eastern Kamchatka, Russia)

Silicate-sulfide liquid immiscibility in mantle-derived magmas has important control on the budget of siderophile and chalcophile metals, and is considered to be instrumental in the origin orthomagmatic sulfide deposits. Data on primitive sulfide melts in natural samples, even those representing most voluminous magmatism in oceanic rifts, are very scarce due to the small size and poor preservation of incipient sulfide melt globules. Here we present the first detailed report of the crystallized sulfides melts in the oceanic picrites of the (presumably) Cretaceous age Kamchatsky Mys ophiolite complex in Eastern Kamchatka (Far East Russia). Sulfide melts are present in three forms; (1) as inclusions in olivine (87.1–89.6 mol% Fo), (2) interstitial to the groundmass minerals (clinopyroxene, plagioclase, and Ti-magnetite) of studied picrites, and (3) as daughter phases in silicate melt inclusions hosted by olivine and Cr-spinel phenocrysts. The sulfide melt inclusions in olivine and the groundmass of studied rocks are composed of several sulfide phases that correspond to the monosulfide (Fe–Ni; Mss) and intermediate (Fe–Cu–Ni; Iss) solid solutions. Several 0.5], continuously evolve with crystallization of (e.g., increasing Cu/Ni and Au/PGE) and demonstrate metal fractionation between Mss and Iss. Although the compositional systematics found in this study are consistent with those previously recorded, the compositions of individual sulfide phases are strongly affected by the noble metal (PGE, Au) “nuggets” that exsolve at subsolidus temperatures and form during serpentinization of the rocks. We conclude that the budget of noble metals in the studied picrites is controlled by sulfides, but the abundances of Pt and Au are influenced by mobility in post-magmatic alteration. Our data can be also used for modeling sulfide saturation at crustal pressures and understanding behavior of the noble metals in primitive oceanic magmas.

Geochemistry of silicate melt inclusions in middle and southern Okinawa Trough rocks: Implications for petrogenesis and variable subducted sediment component injection

Silicate melt inclusions (SMIs) in olivine, clinopyroxene, and orthopyroxene in basalt and trachyandesite of the middle Okinawa Trough (MOT) and in basaltic andesites of the southern Okinawa Trough (SOT) were investigated in detail to understand the evolution of magma and geochemical characteristics of the basaltic magma source. Both the SMIs and host bulk rocks are more heavily enriched with Pb and large ion lithophile elements than with high field strength elements and rare earth elements. The major element compositions of SMIs in the MOT trachyandesite show a scattered variation trend of medium‐K to shoshonitic series and suggest that a K‐rich magma source may exist in the Okinawa Trough. Two groups with distinct element distribution patterns observed in SMIs of the MOT trachyandesite may represent magmas with different evolutionary histories. Compared with fractional crystallization and partial melting processes, magma mixing may serve as a reasonable process for the production of trachyandesite in the MOT. Fractional crystallization has played an important role in basaltic magma evolution in the MOT and SOT. Simulations conducted using MELTS show that the fractional crystallization evolution trends observed correspond well to the presence of a magma chamber at 300 MPa with an H2O content level of 0.2 wt% in the MOT and to the presence of a magma chamber at 300 MPa with an H2O content level of 0.4 wt% in the SOT. Ratios of incompatible trace elements (e.g., Th/Ce, Th/Sm, Ba/Th, and Th/Nb); positive Pb anomalies; negative Nb, Ta, and Ti anomalies; and the existence of organic hydrocarbons (CnHm) in the SMIs can be attributed to the involvement of subducted sediment from the subducting slab. Modelling results suggest that more primitive magma represented by SMIs from the MOT has formed through the melting of less than 6% of a metasomatized depleted mantle with contributions of 1.2% sediment melt and 0.8% sediment fluid. In contrast, more primitive magma from the SOT formed through the melting of 4–10% of the depleted mantle with contributions of 3% sediment melt and 1% sediment fluid. The different features of the SMIs together with different tectonic settings of the MOT and SOT have formed in response to the westward subduction of the Philippine Sea Plate.

New method for initial composition determination of crystallized silicate melt inclusions

Abstract Silicate melt inclusions (SMI) trapped in minerals and carried up to the surface by volcanism are routinely studied in order to determine the pre-eruptive volatile budgets of volcanic systems. The volatile contents of SMI that are affected by post-entrapment processes, such as crystallization (PEC) and/or bubble nucleation during cooling, are generally difficult to interpret. Therefore, there is a general preference to select the melt inclusions that experienced minimal post-entrapment effects. Conversely, SMI can be homogenized at high temperature and quickly quenched. This method is controversial because heating may induce leakage of water from the silicate melt during experiments in addition to loss by diffusion. Here, we propose a new multi-disciplinary method to study crystallized SMI. This method involves the following processes: (1) X-ray microtomography for measuring phase proportions; (2) NanoSIMS for measuring C and H concentrations in glass as a proxy for CO2 and H2O; (3) EPMA (electron probe microanalyzer) for major and volatile element concentrations in all phases; and (4) thermodynamical modeling using Rhyolite-MELTS for volatile saturation determination. To test this approach, we studied the Los Humeros (East Trans Mexican Volcanic Belt) volcanic complex SMI. Three-dimensional (3D) petrographic observations of the studied SMI demonstrate that minerals inside the SMI were crystallized from the trapped silicate melt and were not trapped with the silicate melt (protogenetic). Accordingly, the mathematical reconstruction of the SMI is performed with the following equation: XSMI = Σ[VMineral(i) ∗ XMineral(i)] + [VGlass ∗ XGlass] + [VBubble ∗ XBubble]. Bubbles are associated with a heterogeneous trapping of a melt + fluid mixed phase. The PEC of large minerals inside SMI is consistent with slow cooling rates of the SMI, meaning that exsolved volatiles diffused toward the bubble in equilibrium with the melt before the glass transition. The volatile pressure determined in XSMI and using Rhyolite-MELTS is therefore equal to the pressure of the bubble. The volatile mass in the bubble is determined and added to the bulk volatiles of the silicate melt. This method can be used for a wide range of samples and may significantly improve our knowledge of volatile degassing processes.

A common origin for Thai/Cambodian rubies and blue and violet sapphires from Yogo Gulch, Montana, U.S.A.?

A wide number of genetic models have been proposed for volcanically transported ruby and sapphire deposits around the world. In this contribution we compare the trace element chemistry, mineral and melt inclusions, and oxygen isotope ratios in blue to reddish-violet sapphires from Yogo Gulch, Montana, U.S.A., with rubies from the Chantaburi-Trat region of Thailand and the Pailin region of Cambodia. The similarities between Thai/Cambodian rubies and Yogo sapphires suggest a common origin for gem corundum from both deposits. Specifically, we advance a model whereby sapphires and rubies formed through a peritectic melting reaction when the lamprophyre or basalts that transported the gem corundum to the surface partially melted Al-rich lower crustal rocks. Furthermore, we suggest the protolith of the rubies and sapphires was an anorthosite or, in the case of Thai/Cambodian rubies, an anorthosite subjected to higher pressures and converted into a garnet-clinopyroxenite. In this model the rubies and sapphires are rightfully considered to be xenocrysts in their host basalts or lamprophyre; however, in this scenario they are not accidental xenocrysts but their formation is intimately and directly linked to the magmas that transported them to the surface. The similarities in these gem corundum deposits suggests that the partial melting, non-accidental xenocryst model may be more wide-reaching and globally important than previously realized. Importantly, in both cases the gem corundum has an ostensibly metamorphic trace element signature, whereas the presence of silicate melt (or magma) inclusions shows they ought to be considered to be magmatic rubies and sapphires. This discrepancy suggests that existing trace element discriminant diagrams intended to separate metamorphic from magmatic gem corundum ought to be used with caution.

Petrogenesis of basalt from the middle Okinawa Trough: New insights from olivine‐hosted melt inclusions

Silicate melt inclusions (SMIs) in basalt from the middle Okinawa Trough are investigated in detail to understand the source geochemical characteristics and petrogenesis of the basalt. The middle Okinawa Trough basalts contain abundant SMIs within olivine phenocrysts. The major element compositions of the SMIs are analysed using an electron microprobe. After correcting for olivine postentrapment crystallization, the major element compositions of the SMIs are more variable in terms of SiO2 (49.7–54.8 wt%) and MgO (5.1–7.2 wt%) than those of the host basalt (SiO2, 52.4 wt%; MgO, 5.7 wt%), which could indicate the existence of more primitive melts than erupted bulk rock. The major element compositions of the SMIs indicate that the SMI‐recorded melt evolution process was mainly dominated by the fractional crystallization of olivine. According to laser ablation inductively coupled mass spectrometer analyses, the trace element compositions of the SMIs are similar to those of the host bulk rock and feature obvious enrichment in Pb and large‐ion lithophile elements relative to high field strength elements, rare earth elements, and highly negative Nb and Ta anomalies. The low Ce/Pb ratios, high Pb contents, and trace element distribution patterns in the SMIs are related to contributions from subducted sedimentary components in the magma source. Based on the electron microprobe analyses, the chemical composition of the olivine suggests that the lithology of the basalt source may be pyroxenite. The basalts may have been generated by the partial melting of pyroxenite during interactions between mantle‐wedge peridotite and Si‐rich melts that were released from the subducted slab.

Mineralized breccia clasts: a window into hidden porphyry-type mineralization underlying the epithermal polymetallic deposit of Cerro de Pasco (Peru)

Cerro de Pasco (Peru) is known for its large epithermal polymetallic (Zn-Pb-Ag-Cu-Bi) mineralization emplaced at shallow level, a few hundred meters below the paleo-surface, at the border of a large diatreme–dome complex. Porphyry-style veins crosscutting hornfels and magmatic rock clasts are found in the diatreme breccia and in quartz–monzonite porphyry dikes. Such mineralized veins in clasts allow investigation of high-temperature porphyry-style mineralization developed in the deep portions of magmatic–hydrothermal systems. Quartz in porphyry-style veins contains silicate melt inclusions as well as fluid and solid mineral inclusions. Two types of high-temperature (> 600 °C) quartz–molybdenite–(chalcopyrite)–(pyrite) veins are found in the clasts. Early, thin (1–2 mm), and sinuous HT1 veins are crosscut by slightly thicker (up to 2 cm) and more regular HT2 veins. The HT1 vein quartz hosts CO2- and sulfur-rich high-density vapor inclusions. Two subtypes of the HT1 veins have been defined, based on the nature of mineral inclusions hosted in quartz: (i) HT1bt veins with inclusions of K-feldspar, biotite, rutile, and minor titanite and (ii) HT1px veins with inclusions of actinolite, augite, titanite, apatite, and minor rutile. Using an emplacement depth of the veins of between 2 and 3 km (500 to 800 bar), derived from the diatreme breccia architecture and the supposed erosion preceding the diatreme formation, multiple mineral thermobarometers are applied. The data indicate that HT1 veins were formed at temperatures > 700 °C. HT2 veins host assemblages of polyphase brine inclusions, generally coexisting with low-density vapor-rich inclusions, trapped at temperatures around 600 °C. Rhyolitic silicate melt inclusions found in both HT1 and HT2 veins represent melt droplets transported by the ascending hydrothermal fluids. LA-ICP-MS analyses reveal a chemical evolution coherent with the crystallization of an evolved rhyolitic melt. Quartz from both HT1 and HT2 veins also contains secondary, low-temperature (~ 300 °C) brine and aqueous fluid inclusions that record the cooling of the system. Both vein types are locally crosscut and/or reopened by a pre-diatreme polymetallic event consisting of pyrite, sphalerite with “chalcopyrite disease,” galena, chalcopyrite, tetrahedrite–tennantite, and minor quartz. LA-ICP-MS analyses of mineral and high-temperature fluid inclusions hosted in HT1 and HT2 veins and in situ secondary-ion mass spectrometry oxygen isotope analyses of vein quartz indicate a magmatic signature for the mineralizing fluids with no major meteoric water input and allow reconstruction of the source and chemical evolution of fluids that formed these porphyry-style veins as snapshots of the early and deep mineralizations at Cerro de Pasco. This detailed study of the porphyry-type mineralization hosted in clasts offers a unique opportunity to reconstruct the late magmatic and early hydrothermal evolutions of porphyry mineralization underlying the world-class Cerro de Pasco epithermal polymetallic (Zn-Pb-Ag-Cu-Bi) deposit.

Reprint of Silicate-sulfide liquid immiscibility in modern arc basalt (Tolbachik volcano, Kamchatka): Part II. Composition, liquidus assemblage and fractionation of the silicate melt

Olivine-hosted inclusions of silicate and sulfide melts, Cr-spinel and pyroxene were studied to estimate magma composition, temperature, pressure, and fO2 at the onset and during the silicate-sulfide immiscibility in modern arc basalt from Tolbachik volcano, Kamchatka arc. We demonstrate that the olivine phenocrysts hosting sulfide and silicate melt inclusions belong to the same population. The compositions of the silicate melt inclusions in most primitive olivine (88–91mol% Fo) represent moderately oxidized (~QFM+1.1) high-MgO (up to 12–12.6wt%) and high CaO/Al2O3 (0.8–1.2) melt that has abundances and ratios of the lithophile trace elements typical of island arc magmas. The initial volatile contents in parental Tolbachik magma are estimated from the melt inclusions and mass-balance considerations to be at least 4.9wt% H2O, 2600ppm S, 1100ppm Cl, 550ppm F, and 1200ppm CO2. These data are used to calculate the temperature (~1220°C) and minimum pressure (3kbar) at which the beginning of crystallization and exsolution of sulfide melt took place. The presence of anhydrite, especially ubiquitous in the crystallized silicate melt associated with sulfide globules, suggest that much higher sulfur abundances prior to degassing and sulfate immiscibility and/or crystallization should be expected. We tentatively considered hydrothermal accumulations of sulfur (elemental, sulfate and sulfide) in the volcanic conduit responsible for local contamination and oversaturation of the Tolbachik magma in sulfur and related sulfide immiscibility. Coexisting sulfide and sulfate can be also interpreted in favor of the magmatic sulfide oxidation and related generation of S-rich fluids. Such fluids are expected to accumulate metals released from decomposed sulfide melts and supply significant epithermal mineralization, including native gold.

Magmatic controls on the genesis of porphyry Cu–Mo–Au deposits: The Bingham Canyon example

Bingham Canyon is one of the world's largest porphyry Cu–Mo–Au deposits and was previously used as an example to emphasize the role of magma mixing and magmatic sulphide saturation in the enhancement of ore fertility of magmatic systems. We analyzed whole rocks, minerals, and silicate melt inclusions (SMI) from the co-genetic, ore-contemporaneous volcanic package (∼38 Ma). As opposed to previous propositions, whole-rock trace element signatures preclude shoshonite–latite genesis via mixing of melanephelinite and trachyte or rhyolite, whereas core to rim compositional profiles of large clinopyroxene phenocrysts suggests the amalgamation of the ore-related magma reservoir by episodic recharge of shoshonitic to latitic magmas with various degrees of differentiation. Major and trace element and Sr and Nd isotopic signatures indicate that the ore-related shoshonite–latite series were generated by low-degree partial melting of an ancient metasomatized mantle source yielding volatile and ore metal rich magmas. Latite and SMI compositions can be reproduced by MELTS modeling assuming 2-step lower and upper crustal fractionation of a primary shoshonite with minimal country rock assimilation. High oxygen fugacities (≈NNO+1) are prevalent as evidenced by olivine-spinel oxybarometry, high SO3 in apatite, and anhydrite saturation. The magma could therefore carry significantly more S than would have been possible at more reducing conditions, and the extent of ore metal sequestration by magmatic sulphide saturation was minimal. The SMI data show that the latites were Cu rich, with Cu concentrations in the silicate melt reaching up to 300–400 ppm at about 60 wt% SiO2. The Au and Ag concentrations are also high (1.5–4 and 50–200 ppb, respectively), but show less variation with SiO2. A sudden drop in Cu and S concentrations in the silicate melt at around 65 wt% SiO2 in the presence of high Cl, Mo, Ag, and Au shows that the onset of effective metal extraction by fluid exsolution occurred at a relatively late stage of magma evolution. Overall, our results show that fluid exsolution during simple magmatic differentiation of oxidized alkaline magmas is capable of producing giant porphyry Cu deposits.

Magma Mixing as a Trigger for Sulphide Saturation in the UG2 Chromitite (Bushveld): Evidence from the Silicate and Sulphide Melt Inclusions in Chromite

AbstractIt is of great importance to understand the origin of UG2 chromitite reefs and reasons why some chromitite reefs contain relatively high contents of platinum group elements (PGEs: Os, Ir, Ru, Rh, Pt, Pd) or highly siderophile elements (HSEs: Au, Re, PGE). This paper documents sulphide‐silicate assemblages enclosed in chromite grains from the UG2 chromitite. These are formed as a result of crystallisation of sulphide and silicate melts that are trapped during chromite crystallisation. The inclusions display negative crystal shapes ranging from several micrometres to 100 μm in size. Interstitial sulphide assemblages lack pyrrhotite and consist of chalcopyrite, pentlandite and some pyrite. The electron microprobe data of these sulphides show that the pentlandite grains present in some of the sulphide inclusions have a significantly higher iron (Fe) and lower nickel (Ni) content than the pentlandite in the rock matrix. Pyrite and chalcopyrite show no difference. The contrast in composition between inter‐cumulus plagioclase (An68) and plagioclase enclosed in chromite (An13), as well as the presence of quartz, is consistent with the existence of a felsic melt at the time of chromite saturation. Detailed studies of HSE distribution in the sulphides and chromite were conducted by LA‐ICP‐MS (laser ablation‐inductively coupled plasma‐mass spectrometry), which showed the following. (I) Chromite contained no detectable HSE in solid solution. (II) HSE distribution in sulphide assemblages interstitial to chromite was variable. In general, Pd, Rh, Ru and Ir occurred dominantly in pentlandite, whereas Os, Pt and Au were detected only in matrix sulphide grains and were clearly associated with Bi and Te. (III) In the sulphide inclusions, (a) pyrrhotite did not contain any significant amount of HSE, (b) chalcopyrite contained only some Rh compared to the other sulphides, (c) pentlandite was the main host for Pd, (d) pyrite contained most of the Ru, Os, Ir and Re, (e) Pt and Rh were closely associated with Bi forming a continuous rim between pyrite and pentlandite and (f) no Au was detected. These results show that the use of ArF excimer laser to produce high‐resolution trace element maps provides information that cannot be obtained by conventional (spot) LA‐ICP‐MS analysis or trace element maps that use relatively large beam diameters.