Composition Of Melt Inclusions Research Articles

Nanoscale hydrous silicate melt inclusions at the clinopyroxene-amphibole interface in a mantle xenolith from the Perșani Mountains Volcanic Field

We studied amphibole lamellae and associated hydrous nano-silicate melt inclusions (NSMI) in upper mantle xenoliths from the Perșani Mountains Volcanic Field, southeastern Transylvania (Romania). Transmission Electron Microscope (TEM) results revealed submicron-scale pieces of evidence of escaped fluids in the form of nano-silicate melt inclusions along clinopyroxene-amphibole interfaces. These NSMIs consist of ∼80 vol% silicate glass and ∼ 20 vol% volatile bubble from which the former has a high SiO2 (>60 wt%) and Al2O3 (>20 wt%) and low CaO, FeO and MgO (<8 wt%) content determined by SDD-EDS analyses. We calculated the original bulk composition of the nano-silicate melt inclusions with Monte Carlo simulation, using hydrated fluid molecules and suggest that originally the nano-silicate melt inclusions had most probably low SiO2 (∼ 43.6 wt%) and high Al2O3 (∼ 15.5 wt%), Na2O (∼11.9 wt%) and H2O (∼ 30.3 wt%) contents. Using petrographic evidence, we propose that the amphibole lamellae formed as a result of post-entrapment reaction between clinopyroxene and the trapped fluid of the fluid inclusion. The presence of the hydrous nano-silicate melt inclusions suggest fluid migration along the clinopyroxene-amphibole interface, supporting the continuous growth of the studied amphibole. The association of the studied amphibole with hydrous nano-silicate melt inclusions suggests that H2O consumption from the fluid found in the micrometer-scale fluid inclusion continued at the nanoscale after a metasomatic event in the lithospheric mantle. This nanoscale process may also give information about the behavior of H2O globally in the lithospheric mantle where amphibole is stable, and along the lithosphere-asthenosphere boundary in younger oceanic and continental plates.

The origin and compositions of melt inclusions in an Al2SiO5‐free paragneiss from the Namche Barwa Complex in the Eastern Himalayan Syntaxis

AbstractMelt inclusions (MIs) in high‐temperature metamorphic rocks provide a unique window into crustal anatexis in collisional orogenic belts and have been widely used to characterize compositions of anatectic melts as well as melting mechanisms. In this study, MIs hosted by peritectic garnet were for the first time identified in an Al2SiO5‐free graywacke‐type paragneiss from the Namche Barwa Complex, the Eastern Himalaya, Southeast Tibet. These MIs occur as nanogranites in the rims of porphyroblastic garnet, exhibit negative crystal shapes with an average diameter of ~12 μm and consist of a mineral assemblage of biotite + quartz + plagioclase + K‐feldspar ± muscovite. Re‐homogenization experiments of these nanogranites were conducted at a pressure of 1.5 GPa and temperatures of 800°C, 850°C and 900°C and produced homogeneous glasses at 850°C. The homogenized glasses are strongly peraluminous and calc‐alkalic in composition, with 66.43–71.31 wt.% SiO2, 12.64–15.06 wt.% Al2O3, high alkaline (5.41–7.22 wt.%) and low ferromagnesian (2.72–4.46 wt.%) contents. They are lower in silica and CaO but higher in K2O compared with MI produced by fluid‐present melting of metasedimentary rocks, thus indicating fluid‐absent melting. These glasses are also characterized by enrichment of large ion lithophile elements (particularly Cs and Rb), depletion of Ba and Sr, low contents of light rare earth elements (3.6 to 33.7 ppm), high Rb/Sr ratios (6.19–37.3) and low Nb/Ta ratios (2.55–18.7). In combination with phase equilibrium modelling, these compositional features suggest that a sequential dehydration melting of muscovite and biotite was responsible for the production of MI during prograde metamorphism of the studied paragneiss. By compiling MI data published in the literature, we show that dehydration melting of metasedimentary rocks from the Himalayan orogen can produce initial melts with various peraluminous and granitic compositions.

Tungsten and tin deposits in South China: Temporal and spatial distribution, metallogenic models and prospecting directions

South China is one of the most important tungsten-tin metallogenic areas in the world. Based on the up-to-date geological and chronological data of more than 130 deposits of economic significance, this paper preliminarily summarized the main types and temporal-spatial distribution characteristics of the large-scale tungsten and tin mineralization related to granite in South China. The statistical results show that skarn and quartz-vein types are the most important tungsten metallogenic styles, while skarn and cassiterite-sulfide types are the most important tin metallogenic styles in South China. Tungsten and tin mineralization related to granite in South China are multi-aged, but the largest scale of mineralization is developed within the Yanshanian period. Tungsten mineralization, the majority of which represented by the Nanling and Ganbei metallogenic belts, mainly formed from the Late Jurassic to Early Cretaceous (160 ∼ 120 Ma), while the most important tin mineralization formed in the Late Cretaceous (110 ∼ 80 Ma, peaking at 90 ∼ 80 Ma) and Late Jurassic (160 ∼ 150 Ma) as represented by the Youjiang Basin and Nanling metallogenic belt, respectively. The combined occurrence of different mineralization styles or metal assemblages in individual deposit and sometimes within the same orefield is the most striking feature of granite-related tungsten-tin deposits in South China, and a comprehensive understanding of their patterns are of great significance to guide ore prospecting. To address this issue, this paper proposed several combined metallogenic models and prospecting directions for tungsten-tin mineralization in South China based on recent researches and exploration progresses from typical examples including the Yaogangxian, Chuankou, Maoping, Shizhuyuan, Dachang and Gejiu deposits or orefields. In addition, conventional petrological and mineralogical characteristics that differentiates tungsten and tin granites are summarized, and on this basis the chemical composition of both melt inclusion and early stage fluid inclusion from granitic magmatic-hydrothermal systems were tested as new index to evaluate the tungsten and tin mineralization potential of granitic system.

Architecture of the Lesser Antilles Arc Illustrated by Melt Inclusions

Abstract Volatiles are an essential aspect of subduction zones and constraining their cycling through subduction zones is of prime importance to better understand the genesis, transport, storage and eruption of arc magmas. Here we performed an along-arc investigation of the chemical composition of melt inclusions trapped in minerals representative of ten volcanic centers and 23 key explosive eruptions along the presently active Lesser Antilles arc, from Montserrat in the North to St. Vincent in the South. We use the melt inclusion compositions to reconstruct pre-eruptive conditions, especially pre-eruptive magma storage and degassing levels that highlight how the magma plumbing system is organized and works and to discuss magma source characteristics. All major and selected trace elements and volatiles (H2O, CO2, S, halogens (F, Cl, Br)) have been measured on the same melt inclusions when possible. Eruptions dominantly involved andesitic to dacitic magmas (Montserrat, Guadeloupe, Dominica, Martinique, St. Lucia) and basaltic andesite magmas from St. Vincent. Melt inclusions have been used as pressure probes for magmas, for inferring crustal equilibration pressures. We shed light on the systematic occurrence and lateral complexity of a vertical transcrustal magmatic systems feeding active volcanoes. The geochemical view of the architecture of the plumbing system and in particular the Moho's depth is more variable than the view obtained by seismic data along the Lesser Antilles arc. We propose that the discontinuity between the upper and the middle crust is a major magma ponding zone beneath most of the Lesser Antilles islands and that the crustal structure thus imparts a control on the geochemical signature of arc lavas. Melt inclusions are highly differentiated (dacitic to rhyolitic in composition), so they are distant in composition from the primary, mantle-derived magmas, but they provide indirect information about the magmatic sources. The along-arc variability in Y and heavy rare earth element contents of melt inclusions is consistent with the presence of garnet in the mantle source. Our results also indicate an important contribution of oxidized and saline slab-derived fluids to the magma source predominantly in the southern Lesser Antilles that may have implication on the accretionary system behavior. In addition, a high geographical gradient between sediment melt and slab-derived fluid contributions, illustrating high variability in magmas erupted in close spatial association is reported for some islands, such as Martinique and Dominica. Volatile contents are variable for MIs across the arc: the highest H2O (&amp;lt;8 wt%), Cl (up to 3800 ppm) and Br (up to 20 ppm) concentrations occur in MIs from Dominica. However, there is no systematic correlation between MI volatile content and position along the arc. Halogen Cl/F and Cl/Br ratios vary from one island to another, even between the different eruptions, but without any along arc zoning, indicating that halogen fractionation occurred by fluid transfer (variable assimilation rate of fluids derived from seawater) or by heterogeneities of mantle origin inherited from the initial differentiation of the mantle.

The obscuring effect of magma recharge on the connection of volcanic-plutonic rocks

Abstract The current debate on volcanic-plutonic connection is centered on whether efficient liquid-crystal segregation dominates the evolution of a mushy reservoir to produce evolved, crystal-poor rhyolite and cumulate leftover. However, magma recharge may remarkably influence the evolution of a mushy reservoir and obscure the evidence of liquid-crystal segregation. This complexity poses a challenge to exploring the connection of volcanic-plutonic rocks. This study investigates the Qinzhou Bay granitic complex (~250–248 Ma) from South China, which contains crystal-poor (&amp;lt;19 vol%) peraluminous rhyolites and subsequent crystal-rich (28–54 vol%) porphyries. Although the rhyolite and porphyry units have a close spatio-temporal link, they do not share a fractionation trend and similar whole-rock Sr-Nd-O isotopic compositions; thus, a direct connection is not evidenced. We further present textural analyses, mineral and melt inclusion compositions, thermobarometry (the combination of Ti-in-zircon thermometer and Ti-in-quartz thermobarometer), and thermodynamic modeling to examine the alternative interpretations, i.e., the two units may have intrinsically independent origins or the connection of the two units has been obscured. For the rhyolite unit, thermobarometric results reveal a polybaric storage system consisting of middle (&amp;gt;600 ± 80 MPa) and upper (~150 ± 40 and ~60 ± 20 MPa) crustal reservoirs. Variations in quartz Fe content and chlorine-rich, metaluminous melt inclusions suggest that magma hybridization with less-evolved metaluminous magmas occurred at both crustal levels. In particular, the elevated Fe contents in the quartz population that crystallized at the shallowest level (~60 ± 20 MPa) suggest that recharge magmas were directly injected into the shallowest reservoir. Deviation of the whole-rock composition from the liquid evolution trend recorded in melt inclusions suggests a combined effect of magma mixing and crystal-melt segregation processes in upper crustal reservoirs. Thermodynamic modeling and mass balance calculations suggest that the whole-rock composition of the rhyolite could be reproduced by mixing between regionally exposed dacites and segregated melts at crystallinities of 50–60% (using parental magma represented by the least-evolved melt inclusion). For the porphyry unit, thermobarometric results reveal magma storage at middle (more than 450 ± 40 to 550 ± 40 MPa) and upper (110 ± 20 to 140 ± 20 MPa) crustal levels. The small-scale oscillatory zonation of plagioclase, the pervasive resorption of quartz and alkali feldspar, and the presence of peraluminous microgranular enclaves in the porphyries suggest a recharge event of metasediment-sourced magmas, triggering reactivation and convection of the reservoir. Autoclastic and overgrowth textures of quartz, plagioclase, and alkali feldspar phenocrysts and development of columnar jointing suggest that the reactivated porphyritic magmas ascended and emplaced at ultrashallow levels (~30 ± 10 MPa). Because of the similar storage pressures, the porphyries may represent remobilized cumulates of rhyolitic magmas, whereas the texture and geochemistry of the cumulate-liquid pair were modified, a key factor rendering a cryptic connection between the rhyolite and porphyry. Alternatively, the plumbing systems feeding the rhyolite and porphyry units are horizontally independent or vertically discrete, but this circumstance is inconsistent with the same evolution trend of quartz Fe and Al contents of the rhyolite and porphyry. Our study highlights that whole-rock composition may record blended information of complex processes, and caution should be taken when whole-rock composition is used to extract information of a single process. Multi-method constraints are required to evaluate the influence of recharge processes on the modification of liquid-cumulate records, and big data analysis on the basis of geochemistry should be conducted with caution to avoid biased understanding.

Origin of nitrogen on Mars: First in situ N isotope analyses of martian meteorites

Martian meteorites are key for assessing the isotopic characteristics of nitrogen in different martian reservoirs (i.e., mantle, crust, and atmosphere), and, ultimately, for constraining the source(s) of nitrogen trapped during the earliest stages of planetary accretion in the terrestrial planet-forming region. In this study, we analysed, for the first time, the nitrogen content and isotopic composition of glassy melt inclusions of Chassigny and of the mesostasis of five nakhlites (MIL 03346, Nakhla, NWA 6148, NWA 998, and Y 000593) by in situ secondary ion mass spectrometry. The nitrogen content of Chassigny melt inclusions, corrected for olivine overgrowth on the inclusion walls, varies from 4 ± 1 to 860 ± 45 ppm N, and the majority of δ15N values range from –35 ± 41 to +73 ± 36‰. The estimated nitrogen isotopic signature of the primitive melt, prior to degassing of N2 or NH3, is 0 ± 32‰. The mesostasis of nakhlites contains 2.7 ± 0.2 to 943 ± 156 ppm N, with δ15N values from –30 ± 37 to +348 ± 43‰. Whereas degassing of N2 or NH3 can explain the lowest nitrogen isotopic ratios measured in the nakhlite mesostasis, the 15N-enriched isotopic composition (δ15N > 150‰) of four nakhlites (MIL 03346, Nakhla, NWA 6148, and Y 000593) likely results from interaction of the mesostasis melt with the martian atmosphere during ejection. The δ15N values (+25 ± 42 and +77 ± 19‰) of two melt inclusions in Y 000593 are comparable to those of Chassigny, further confirming that these meteorites likely sample a common volatile reservoir in the martian interior. Overall, the new results indicate that the chassignite-nakhlite reservoir did not inherit nitrogen from the solar nebula but, instead, from chondritic-like materials. These findings further confirm that planetary bodies in the inner solar system accreted (isotopically) chondritic nitrogen during the first few million years of solar system history.

H2O-Controlled Eruptive Filtering on the Bimodality of Continental Volcanism Across Tectonic Settings

Abstract The contemporary basaltic and rhyolitic volcanism with a dearth of intermediate compositions creates a compositional gap (‘Daly gap’), which is known to be pronounced in continental rifts/hotspots but absent in continental arcs. The intrinsic mechanism(s) producing the Daly gap remain poorly understood. Similar bimodal spectra recorded in melt inclusions from continental rifts/hotspots and arcs suggest that similar processes modulate the bulk composition of erupted magmas. A compiled data set of melt inclusion composition, phenocryst fraction, pre-eruptive temperature and melt H2O content enables precise calculations of melt viscosities and bulk magma viscosities for &amp;gt;100 eruptions of global volcanoes in various tectonic settings. Analyses of the data set suggest lower viscosities of rhyolitic melts in arcs than in rifts/hotspots (~104.5 ± 0.2 versus ~105.5 ± 0.2 Pa·s, respectively), at comparable temperatures, because of ~1.5 wt % higher melt H2O contents in arcs. Combining numerical constraints on ascent dynamics and data set observations, our study defines an eruption threshold of ~106.0 ± 0.3 Pa·s at the base of a volcanic conduit rooted in the subvolcanic reservoir. The lower viscosities of rhyolitic melts in arcs facilitate stirring/re-homogenization of upper crustal reservoirs upon recharge and accommodation of more phenocrysts in erupted products, leading to the common appearance of crystal-rich (30–50%) andesites and dacites (with rhyolitic interstitial melts) with mixing signatures in magmatic provinces of continental arcs. In contrast, the lower mixing efficiency and eruptibility of the more viscous magmas present in continental rifts/hotspots lead to preferential eruption of crystal-poor (&amp;lt;~30%) rhyolites, enhancing compositional bimodality in the erupted records. This H2O-controlled eruptive filter modulates the presence versus absence of the Daly gap and is key to the distinct differentiation patterns across tectonic settings.

Volcano–Plutonic Complex of the Tumrok Range (Eastern Kamchatka): An Example of the Ural-Alaskan Type Intrusion and Related Volcanic Series

Zoned plutons, composed of dunites, pyroxenites, and gabbroic rocks, have been referred to as the Ural-Alaskan type complexes (UA-complexes) and occur in numerous paleo-arc settings worldwide. Many of these complexes are source rocks for economic placers of platinum-group metals. Thus, it is important to understand how UA-complexes form and the origin and behavior of platinum-group elements (PGEs). It is widely assumed that the UA-complexes result from differentiation of supra-subduction high-Ca high-Mg sub-alkaline magmas. However, there is a lack of direct evidence for the existence and differentiation of such magmas, mainly because cases of UA-complexes being spatially and temporally linked to co-genetic volcanics are unknown. We studied an UA-complex from the Tumrok range (Eastern Kamchatka) where a dunite-clinopyroxenite-gabbro assemblage is spatially and temporary related to high-Ca volcanics (i.e., picrites and basalts). Based on the mineral and chemical composition of the rocks, mineral chemistry, and composition of melt inclusions hosted within rock-forming minerals, we conclude that the intrusive assemblage and the volcanics are co-genetic and share the same parental magma of ankaramitic composition. Furthermore, the compositions of the plutonic rocks are typical of UA-complexes worldwide. Finally, the rocks studied exhibit a full differentiation sequence from olivine-only liquidus in picrites and dunites to eutectic crystallization of diopside or hornblende, plagioclase, and K-Na feldspar in plagio-wehrlites and gabbroic rocks. All these results make the considered volcano–plutonic complex a promising case for petrological studies and modelling of UA-complex formation.

Granitic‐melt and carbonic‐fluid inclusions in diopside megacrysts from ankaramitic basalt dikes at Kamisano, Yamanashi prefecture, northeastern Japan

AbstractLarge phenocrysts, known as megacrysts, are focal points for research due to their ability to encapsulate large inclusions suitable for precise chemical analyses. Ankaramite, a distinctive type of undifferentiated volcanic rock, stands out due to its high MgO and CaO contents and the presence of abundant Ca‐rich clinopyroxene (diopside) and less common Mg‐rich olivine phenocrysts. In this study, granitic melt inclusions together with carbonic fluid inclusions were identified within diopside megacrysts of ankaramitic basalt dikes in the Kamisano region, Yamanashi Prefecture, Japan. The identified melt inclusions are completely crystallized and primarily composed of quartz, alkali feldspar, and plagioclase, with smaller amounts of pargasite, augite, apatite, and sulfides. Small amounts of residual glass were also occasionally observed in the inclusions. The average chemical composition of these granitic melts within the inclusions corresponds to that of calc‐alkaline granodiorite and the melts are characterized by low water content (0.38 wt%) and high concentrations of sulfur (7000 ppm), copper, and iron. The findings suggested that the composition of granitic melt inclusions may provide insights into the characteristics of near‐surface hydrothermal metal ore deposits. The diopside megacrysts also contain CO2H2O fluid inclusions, which are completely crystallized and mainly comprised of calcite and chlorite, along with small amounts of quartz. The crystals are interpreted to have formed by the reaction of original CO2H2O fluids and host diopside. The diopside megacrysts are estimated to have started crystallization from tholeiitic basalt at a depth of ~30 km in the lower crust, and trapped fluids and granitic melts as inclusions at a shallower depth when the tholeiitic magma ascended.

Silica-rich melts originating from melt-hydrated peridotite reactions

Interactions between melts and hydrated peridotite may control production of silica-rich melts, fluids and magmas in the upper mantle and may be predominant mechanism for producing felsic crust in shallow ultramafic settings of rocky planets. This research investigated composition of olivine-hosted mineral, melt and multi-phase inclusions in dunite xenoliths from the 1800–1801 Kaupulehu flow of Hualalai Volcano. The host olivine (forsterite Fo 82–89 , mol. %) appears with mineral inclusions of crystalline chromite having compositions similar to those found in alkaline series rocks. Abundant silica-rich glasses (SiO 2 = 63 ± 2.9 wt%, Al 2 O 3 = 21 ± 1.2 wt%; K 2 O = 4.3 ± 0.5 wt%, Na 2 O = 3.6 ± 0.7 wt%, 38 analyses) found in the olivine-hosted melt and multiphase inclusions are interpreted as quenched silicate liquids entrapped due to nephelinite melt percolation in hydrated harzburgite in the presence of free aqueous fluid at pressures of 0.7 to 0.1 GPa and a melt-to-peridotite mass ratio of 0.2 to 0.3. This work also re-interprets the metasomatic origin of global examples of silica-rich glass inclusions in peridotite xenolith minerals according to results from recent experiments on melt-hydrated peridotite interaction processes. As a planetary process, production of silica-rich melts and fluids within series of associated magmas and their crystallized differentiates can occur due to reaction of primitive mafic (e.g., ocean island basalt, OIB) melts with hydrated peridotite (or serpentinite) in the presence of aqueous fluids at pressures below 1 GPa. • Si-rich glasses are produced due to reaction of mafic melts with hydrated peridotite.

Physicochemical Parameters of Ultrabasic Magmatic Systems of the Northeastern Siberian Platform (Data on Melt Inclusions in Chromian Spinel)

Abstract —Experimental studies and analysis of silicate inclusions testify to the magmatic genesis of part of chromian spinel from the Triassic deposits of the northeastern Siberian Platform. The compositions of melt inclusions in the chromian spinel show the participation of alkaline (potassic) magmas in their crystallization. Most data indicate the presence of magmatic systems similar to melts in the Guli ultrabasic massif in the northern Siberian Platform. Studies of the distribution of trace and rare-earth elements in the melt inclusions show the existence of several magmatic systems. These are, first of all, magmas that formed Guli-type ultrabasic massifs and gave rise to meimechites and picrites in the Maimecha–Kotui province. There are also plume-related magmatic systems with kimberlites, lamprophyres, and/or other continental “hot spots”. The composition of the melt inclusions suggests the existence of several types of the primary sources of chromian spinel in the northeastern Siberian Platform, which confirms the earlier data on the heterogeneous composition of the deposits of the diamondiferous Carnian (Upper Triassic) Stage. Applying computer modeling with the well-known COMAGMAT, PETROLOG, and WinPLtb programs as well as the Ol–Sp geothermometers based on the melt inclusions in chromian spinel from the Triassic deposits of the northeastern Siberian Platform, we have determined the P–T conditions of crystallization of minerals in the igneous rocks being the sources of the examined chromites. The temperature of liquidus crystallization of chromian spinel is 1324–1275 ºC. The P–T conditions of formation of olivine and clinopyroxene inclusions in it are estimated at ca. 4.5–4.1 kbar, 1510–1150 ºC and 3.2–1.0 kbar, 1285–1200 ºC, respectively.

The systems KCl–CaCO3 and KCl–MgCO3 at 6 GPa

ABSTRACT Continuous variations in the composition of melt inclusions in diamonds from K-rich saline to carbonatitic indicate their possible genetic relationship. This causes interest in the study of chloride-carbonate systems at high pressure. Here we present experimental data on phase relations in the KCl–CaCO3 and KCl–MgCO3 systems at 6 GPa and 1000–1600 °C. The studied systems have the eutectic type of T-X diagrams. Subsolidus phases are represented by aragonite + KCl and magnesite + KCl. The KCl–CaCO3 eutectic is situated at 1200 °C and K2# 20, while the KCl–MgCO3 eutectic is located at 1400 °C and K2# 43, where K2# = 2KCl/(2KCl + CaCO3 + MgCO3)×100 mol%. The concentration of K and Cl in water-soluble chloride, and Ca, Mg, and CO2 in carbonate suggests that K-rich saline fluid entrapped by ‘fibrous’ diamonds could be a low-temperature derivative of a carbonatite melt containing KCl and H2O.

Chromian Spinels from Kazanian-Stage Placers in the Southern Pre-Urals, Bashkiria, Russia: Morphological and Chemical Features and Evidence for Provenance

Six minor alluvial chromite placers (Kolkhoznyi Prud, Verkhne-Yaushevo, Sukhoy Izyak, Bazilevo, Novomikhaylovka, Kiryushkino) and one major littoral placer (Sabantuy) were found in sandy sediments of the Kazanian stage of the Permian System (Late Roadian and Wordian Stages) in the Southern Pre-Urals. It is shown that the morphological features of chromian spinels are diverse, which is not evidence of the heterogeneity of the source. The bulk chemical composition of chromian spinels from all placers is similar and generally correlates with compositions of chromian spinels from the Kraka ophiolitic complex in the Southern Urals. The morphological diversity of grains, varied chemical composition and presence of melt inclusions in Ti-high octahedral grains of chromian spinels comply with the ophiolitic nature of the source. Thus, there is no need to refer to other sources for chromite ores but ophiolitic. The new placers expand the dissemination area of chromite-bearing deposits on the east edge of the East-European Platform and offer a prospect to discover new placers.

A Petrologic Insight into Transitioning Eruption Styles from the Devil’s Rock Region, Ambae, Vanuatu

AbstractAmbae Island is the largest volcano in the New Hebrides Arc with recent eruptive activity occurring primarily at the summit and along the island’s rift zone. The Devil’s Rock area forms a prominent outcrop on the SW coast. Eruptive deposits here are derived from both strombolian and phreatomagmatic eruptions that contain a similar olivine- and clinopyroxene-rich juvenile basaltic component. This study focuses on a particular transition from strombolian to phreatomagmatic activity to understand if the change in eruption style is a function of magmatic processes or properties (e.g. different composition, ascent rate, degassing history) or if it is driven purely by external factors (e.g. magma–water interaction and/or vent migration). Melts from the strombolian to phreatomagmatic phase record the same melt compositions and volatile contents, suggesting the same magma batch is involved throughout the eruption. More broadly, similarities in H2O, CO2 and S concentrations between olivine- and pyroxene-hosted melt inclusions from Devil’s Rock melt inclusions and those erupted during the 2017–2018 summit eruptions may indicate that a long-term shared magmatic reservoir exists beneath Ambae. Physical characteristics of juvenile tephra including groundmass crystallinity and porosity are combined with melt inclusion compositions to better understand the degassing and crystallisation history and melt evolution of this volcanic system across the transitioning eruptive sequence. Groundmass crystallisation is variable and negatively correlated with connected porosity of erupted scoria reflecting mixing of materials at the vent and inclusion of dense clasts from conduit margins. A direct comparison of crystallinities between strombolian and phreatomagmatic phases reveals higher crystallinity in the strombolian deposits, which is reflective of post-fragmentation crystallisation of clasts. This is particularly evident in the proximal strombolian materials. Qualitative crystallisation textures of melt inclusions are used in a similar fashion to groundmass crystallinities to assess the relative timing of cooling. These trends mirror those of the groundmass and suggest longer cooling times and more effective degassing for samples of the transitional materials. Based on our analysis of deposits at Devil’s Rock, the transition from a strombolian to a phreatomagmatic eruption style was likely driven by groundwater or seawater incursion into the shallow conduit, close to modern-day sea level. Overall, these results suggest a dynamic system where different magmatic cooling histories for strombolian versus phreatomagmatic eruptive phases are reflected in changing groundmass crystallinity. This highlights the propensity for transitions in eruption style over seemingly short time intervals and significantly enhancing eruption explosivity.

Mineralogy and genesis of ultramafic rocks from the Kurtushiba ophiolite belt (Western Sayan)

The studies of minerals and melt inclusions in ultramafc rocks of the Kurtushiba ophiolite belt (Western Sayan) allowed us to establish the conditions of mineral-forming processes during their formation. . The features of the compositions of minerals of ultramafc rocks of Idzhim, Kalny and Ergaki massifs are considered. The presence of melt inclusions directly indicates a magmatic origin of some Cr-spinels from dunites of Idzhim massif. A close assemblage of Cr-spinels from chromitites with magmatic Cr-spinels is important because of the possible formation of chromite ore from melts. The study of melt inclusions revealed the evolution of melts (picrites – picrobasalts – basalts – basaltic andesites) during the consecutive crystallization of minerals from the Western Sayan ophiolites. Computational modeling based on the compositions of melt inclusions and olivine indicates that olivine of ultramafc rocks of the Kurtushiba ophiolites crystallized at a decreasing pressure from 9.2 to 6.2 kbar and decreasing temperature from 1550 to 1430 °С from picritic melts, which evolved to picrobasaltic melts. With a decrease pressure to 3.8–2.3 kbar and temperature of picrobasaltic melts (1240–1230 °C), the formation of olivine terminated following by the crystallization of clinopyroxene from basaltic magmas at 2.7–1.9 kbar and about 1215 °C. The postmagmatic stage was characterized by plastic deformations of the Western Sayan ultramafc rocks at a decreasing temperature from 850 to 640 °C, which resulted in a successive change from protogranular to mesogranular structures and then to porphyroclastic and porphyroleist structures.

Magmatic and Hydrothermal Evolution at Qian’echong, Central-Eastern China: Insights into Dabie-Type Porphyry Mo Mineralization

Abstract Dabie-type porphyry Mo deposits have recently been identified as a new subtype of porphyry Mo deposits, but several questions remain about the role of ore-related magmas in the formation of this type of deposit, as well as distinctions in genetic processes with the well-studied Climax-type porphyry Mo deposits. Here, mineral and melt inclusions from the giant Qian’echong deposit, Dabie orogen, central-eastern China, were studied in order to improve our understanding of the nature and the role of ore-related magmas in the genesis of Dabie-type porphyry Mo deposits. The magmatic and hydrothermal evolution of the system was reconstructed based primarily on the analysis of quartz-hosted silicate melt and mineral inclusions, in concert with field and petrographic relations as well as previously published U–Pb and Re-Os geochronology. Ore-related magmas at Qian’echong include, from early to late, quartz porphyry dikes, rhyolite porphyry dikes, granite porphyries (stock and dikes) and a newly discovered, deep-seated monzogranite porphyry stock. Based on TitaniQ thermobarometry, these lithologies were sourced from a ~16–19-km deep (500–600 MPa) magma chamber and underwent nearly isothermal decompression to ~7 km (200 MPa), at temperatures ranging from 720°C to 690°C. According to the trace element composition of melt inclusions, in combination with published whole-rock Nd isotopic compositions, the early magma evolved from quartz porphyry to rhyolite porphyry through fractionation crystallization and thus became a crystal mush. This viscous crystal mush was subsequently re-melted to produce the granite porphyries and was injected with melts from a different source to generate the monzogranite porphyry. At Qian’echong, all melt inclusions have low concentrations of Mo (2–8 ppm), &amp;gt;5 wt. % H2O, and little to no F (≤0.26 wt. %), arguing against the requirement for ore-related magmas to be enriched in Mo to form large porphyry Mo deposits. Rayleigh fractionation modeling shows that the concentration of Mo in the quartz and granite porphyries increased through fractionation of quartz, feldspars, biotite, magnetite, and ilmenite and that Mo was subsequently depleted in the melt through fluid exsolution, exclusively in the rhyolite and monzogranite porphyries, as a result of magma ascent and decompression. This suggests that, in addition to ore-related granite porphyry, both the rhyolite and the monzogranite porphyries also contributed to the hydrothermal mineralization. This study confirms that the formation of porphyry Mo deposits does not rely on abnormally high concentrations of Mo in ore-related magmas but instead requires efficient extraction of Mo from large volumes of magmas, with normal concentrations of Mo. Unlike the Climax-type deposits where multiple pulses of ore-forming fluids are delivered from convecting shallow magma chambers, Mo mineralization in Dabie-type deposits was achieved by the assembling of ore-forming fluids from successively emplaced, relatively deep intrusions. Although the Dabie- and Climax-type Mo deposits are respectively associated with I-type and A-type granitoids in the Dabie orogen, it is suggested that both types of magmas are derived from the partial melting of subducted Yangtze continental crust and that it is the tectonic transition from compressional to extensional settings that controlled the different styles of porphyry Mo mineralization in this orogen.

The Missing Magmas of MOR: Insights From Phase Equilibrium Experiments on Plagioclase Ultraphyric Basalts

AbstractPlagioclase ultraphyric basalts (PUBs) are a class of mid‐ocean ridge (MOR) lavas found in a variety of ocean floor environments, are characterized by abundant (15–40 volume %) plagioclase megacrysts and a diverse trace element and isotopic signature. Paradoxically, we never see lavas erupted on the seafloor that are in equilibrium with these PUB megacrysts. Based on petrographic evidence, melt inclusion composition, and new data on depth of entrapment calculated from CO2 contents in plagioclase‐hosted inclusions, many of the megacrysts formed at upper mantle pressures (∼3–7 kbars). To constrain the composition of the parent magmas of the plagioclase megacrysts, we conducted a series of experiments at 5 and 10 kbars using mid‐ocean ridge basalts glasses as starting materials. The experimental results were consistent with the presence of a pseudoazeotrope in the anorthitic segment of the plagioclase + basalt pseudobinary. This has the effect of dropping the anorthitic end of the feldspar loop, lowering the solidus for upper mantle conditions, and driving evolving magmas toward higher Ca. As magmas rise and pressure drops, the pseudoazeotrope disappears, and the feldspar loop at the high‐An end rises, causing those magmas to undergo decompression crystallization of plagioclase and resorption of olivine. Therefore, the conditions which generated the magmas from which the megacrysts form disappear as the magmas rise and magmas evolve toward lower Ca, Mg (as we normally assume during plagioclase + olivine crystallization). In effect, the phase equilibria conditions that allow for the generation of such liquids also prevent them from being erupted as lavas.

A dry lunar mantle reservoir for young mare basalts of Chang\u2019e-5

The distribution of water in the Moon’s interior carries implications for the origin of the Moon1, the crystallization of the lunar magma ocean2 and the duration of lunar volcanism2. The Chang’e-5 mission returned some of the youngest mare basalt samples reported so far, dated at 2.0 billion years ago (Ga)3, from the northwestern Procellarum KREEP Terrane, providing a probe into the spatiotemporal evolution of lunar water. Here we report the water abundances and hydrogen isotope compositions of apatite and ilmenite-hosted melt inclusions from the Chang’e-5 basalts. We derive a maximum water abundance of 283 ± 22 μg g−1 and a deuterium/hydrogen ratio of (1.06 ± 0.25) × 10–4 for the parent magma. Accounting for low-degree partial melting of the depleted mantle followed by extensive magma fractional crystallization4, we estimate a maximum mantle water abundance of 1–5 μg g−1, suggesting that the Moon’s youngest volcanism was not driven by abundant water in its mantle source. Such a modest water content for the Chang’e-5 basalt mantle source region is at the low end of the range estimated from mare basalts that erupted from around 4.0 Ga to 2.8 Ga (refs. 5,6), suggesting that the mantle source of the Chang’e-5 basalts had become dehydrated by 2.0 Ga through previous melt extraction from the Procellarum KREEP Terrane mantle during prolonged volcanic activity.

The origin of the Late Quaternary back-arc volcanic rocks from Kamchatka: evidence from the compositions of olivine and olivine-hosted melt inclusions

The origin of the Late Quaternary back-arc volcanic rocks from Kamchatka: evidence from the compositions of olivine and olivine-hosted melt inclusions

Periodically Released Magmatic Fluids Create a Texture of Unidirectional Solidification (UST) in Ore-Forming Granite: A Fluid and Melt Inclusion Study of W-Mo Forming Sannae-Eonyang Granite, Korea

The Upper Cretaceous Sannae-Eonyang granite crystallized approximately 73 Ma and hosted the Sannae W-Mo deposit in the west and the Eonyang amethyst deposit in the east. The granite contained textural zones of miarolitic cavities and unidirectional solidification texture (UST) quartz. The UST rock sampled in the Eonyang amethyst mine consisted of (1) early cavity-bearing aplitic granite, (2) co-crystallization of feldspars and quartz in a granophyric granite, and (3) the latest unidirectional growth of larger quartz crystals with clear zonation patterns. After the UST quartz was deposited, aplite or porphyritic granite was formed, repeating the prior sequence. Fluid and melt inclusions occurring in the UST quartz and quartz phenocrysts were sampled and studied to understand the magmatic-hydrothermal processes controlling UST formation and W-Mo mineralization in the granite. The composition of melt inclusions in the quartz phenocrysts suggested that the UST was formed by fractionated late-stage granite. Some of the melt inclusions occurring in the early-stage UST quartz were associated with aqueous inclusions, indicating fluid exsolution from a granitic melt. Hypersaline brine inclusions allowed the calculation of the minimum trapping pressure of 80–2300 bars. Such a highly fluctuating fluid pressure might be potentially due to a lithostatic-hydrostatic transition of pressure-attending fluid loss during UST formation. Highly fluctuating lithostatic-hydrostatic pressures created by fluid exsolution allowed shifting of the stability field from a quartz-feldspar cotectic to a single-phase quartz. The compositions of brine fluid assemblages hosted in the quartz phenocrysts deviated from the fluids trapped in the UST quartz, especially regarding the Rb/Sr and Fe/Mn ratios and W and Mo concentrations. The study of melt and fluid inclusions in the Eonyang UST sample showed that the exsolution of magmatic fluid was highly periodic. A single pulse of magmatic fluids of variable salinities/densities might have created a single UST sequence, and a new batch of magmatic fluid exsolution would be required to create the next UST sequence.