Search

Quaternary volcanoes from the southeastern Tibetan Plateau occur at the Tengchong volcanic field (TVF). The Daliuchong volcano is the largest volcano in the TVF, which has the most felsic compositions with explosive eruptions. The eruption history and origin of the Daliuchong volcano are a matter of debate. In the present paper, we report the groundmass K-Ar ages, whole-rock Sr-Nd-Pb-Hf isotopes, zircon U-Pb ages, and Hf-O isotopic compositions for the Daliuchong volcano to constrain its eruption history and petrogenesis. The groundmass K-Ar ages and zircon U-Pb ages indicate mid-Pleistocene (0.6 Ma to 0.3 Ma) eruptions. The presence of zircon phenocrysts with enriched mantle-like O-Hf isotopes (δ18O < 6‰, and εHf about −2) suggests the involvement of mantle-derived basaltic magmas. The whole-rock Pb isotope compositions and Sr-Nd isotope modeling reveal the involvement of magma from the lower crust. The zircon xenocrysts reveal previously unrecognized 20-Ma magmatic activity at the TVF and contamination of late Cretaceous (66–80 Ma) S-type granites during the formation of the Daliuchong dacites. The dacite magma at Daliuchong was formed by mixing of the mantle-derived magma and lower-crust-derived magma and subsequently contaminated by upper crustal materials, including late Cretaceous S-type granitic rocks. [ABSTRACT FROM AUTHOR]

The late Permian Emeishan large igneous province (ELIP) in SW China is a melting product of the Emeishan mantle plume. Recently, it has been debated whether peridotite or pyroxenite is the dominant lithology of the mantle source in the ELIP. To address this, systematic analyses of bulk-rock and coexisting spinel and olivine compositions were conducted on picrites from Lijiang–Yongsheng, Dali–Binchuan, Yumen, Muli, and Ertan. The ELIP picrites exhibit positive TiO2–CaO and negative MgO–CaO correlations, as well as low FC3MS values (−0.24–0.1), supporting a peridotite-dominated mantle source. This lithology of the mantle source is also supported by the high 100 × Mn–Fe (1.43–1.73) and Mn–Zn (13.6–18.4) values but low 10,000 × Zn–Fe (8.0–12.7) ratios of the olivine phenocrysts. The estimated mantle potential temperature for Lijiang, Yongsheng, Yumen–Ertan, Muli, and Dali–Binchuan picrites decreased away from Lijiang and Yongsheng, suggesting that the Lijiang and Yongsheng areas were the center of the ELIP. The Lijiang–Yongsheng primary magma shows similar SiO2 content but lower Al2O3 contents (average of 8.24 wt.%) and higher MgO contents (average of 21.42 wt.%) than those of Dali–Binchuan primary magma (Al2O3: 9.86 wt.%; MgO: 19.02 wt.%). Also considering the high Gd–Yb (average of 3.05) and La–Yb (average of 14.61) ratios and mantle potential temperature (average of 1599 °C), we proposed that Lijiang–Yongsheng lavas are produced via the melting of a garnet–peridotitic mantle. In contrast, the Dali–Binchuan lavas with low Gd–Yb (average of 1.91) and La–Yb (average of 5.88) ratios can be explained by their formation in the garnet–spinel transition zone of a peridotitic mantle. The Yumen–Ertan primary magma displays similar mantle potential temperature (average of 1600 °C), Al2O3 and FeO content, and Gd–Yb ratios to those of Lijiang–Yongsheng lavas, indicating that YumenvErtan primary magma may be attributed to the partial melting of garnet with minor peridotite. Therefore, heterogeneous plume-head mantle sources lead to the evaluation of melting conditions of the late Permian ELIP picrites. [ABSTRACT FROM AUTHOR]

The Suwolbong tuff ring is a basaltic monogenetic volcano in the Quaternary intraplate volcanic field of Jeju Island, Korea. The tuff ring was formerly interpreted to have had a congested magma plumbing system consisting of multiply-sourced dike complexes, based on stepped and mixed chemical trends of alkaline to sub-alkaline glassy pyroclasts. Microscopic observations, petrological analysis, and componentry analysis of the glassy pyroclasts reveal, however, that some of the glassy pyroclasts in the tuff ring are accidental and inappropriate for interpreting magmatic processes. Juvenile particles are vesicular, alkaline in composition, mainly contain olivine, clinopyroxene, and plagioclase phenocrysts, and comprise about 35 vol% of the deposits. In contrast, accidental particles are non-vesicular, alkaline to subalkaline in composition, less abundant (avg. 8 vol%), and show alteration rims. The accidental particles are interpreted to have been derived from the volcaniclastic layers deposited before the eruption of the Suwolbong tuff ring. When removing the effects of the accidental particles and considering only the geochemical characteristics of the newly defined juvenile particles, the Suwolbong tuff ring is interpreted to have had a rather simple, not necessarily congested, plumbing system fed by independently ascending multiple magma batches. This study shows that the interpretation of the properties of the source magma and the magma plumbing system in monogenetic volcanoes must be performed after clearly distinguishing between juvenile and accidental particles based on rigorous microscopic analysis of pyroclastic materials. [ABSTRACT FROM AUTHOR]

Mafic magma recharge of crustal reservoirs and subsequent magma mixing has been considered a direct trigger of volcanic eruptions. However, although recharge frequently occurs in many active volcanoes, it rarely leads to an eruption immediately, making its role as a trigger ambiguous. Sakurajima volcano, Japan, has vigorously erupted three times since the 15th century following a common process; mixed magmas after recharge were once stored in a shallow, thick conduit before each eruption (conduit pre‐charge). We reconstructed the magma migration with a high time resolution by diffusion modeling on orthopyroxene and magnetite. Orthopyroxene phenocrysts recorded prolonged diffusive re‐equilibration timescales of years or more after recharge‐and‐mixing. Magnetite, which has the fastest elemental diffusivity among the phenocrysts examined, predominantly lacks zoning. This demonstrates that the mineral phase was re‐equilibrated with surrounding magma and homogenized via elemental diffusion after the final magmatic perturbation, implying the final repose of the shallow pre‐charged magma body for more than several tens of days. After this shallow stagnation period, the Plinian magmas began to ascend and reached the surface within 55 hr. Mass balance calculations show that crystallization‐driven vesiculation upon pre‐charge can produce overpressure sufficient to cause an eruption. The Sakurajima cases demonstrate the hierarchical timescales of trigger processes leading to the explosive eruptions. Plain Language Summary: Understanding how volcanic eruptions are triggered is an important scientific challenge that directly contributes to studying volcanic hazard. Volcanic ejecta often shows evidence of the injection of hot magma into magma reservoirs, with the injection often considered an eruption trigger. However, such injection is frequently observed in many active volcanoes, while rarely leads to an eruption immediately. Sakurajima volcano, Japan, is an excellent test field to understand the trigger mechanism because three large‐scale eruptions occurred in historical time following a similar sequence. A previous study has revealed that the Sakurajima magmas once stagnated in a shallow, thick conduit before each of the eruptions. In this study, we conducted elemental diffusion simulation of phenocrysts and constrained the duration of the shallow stagnation to be more than tens of days. We also found that magma injection occurred repeatedly but did not immediately lead to the eruptions. Crystallization within the shallow conduit, which causes vesiculation (over‐pressurization) of magma, could have finally led to the eruptions. To prepare to the next large eruption at Sakurajima, one should consider the multiple cascading trigger process, which includes a shallow stagnation period followed by fast magma ascent to the surface. Key Points: We investigated the timescales of hierarchical trigger processes leading to three historical eruptions at Sakurajima volcanoDiffusion modeling reveals that magma reservoir recharges occurred repeatedly but did not immediately lead to eruptionsAfter a shallow stagnation period of more than several tens of days, magma began to ascend and reached the surface within 55 hr [ABSTRACT FROM AUTHOR]

The development of sector zoning in clinopyroxene is attributed to the influence of crystallisation kinetics imposed by magma undercooling (ΔT) and may reflect variations in magma cooling histories. Yet, the degree of compositional variations between sectors has not been explored as a potential recorder of crystallisation dynamics. Here, we investigate the distribution of major, minor, and trace elements between hourglass {−1 1 1} and prism { h k 0} sectors in clinopyroxene with distinct pre-eruptive histories at Mt. Etna, Italy. We analyse sector-zoned clinopyroxene crystals ranging in size from sub-mm to cm (i.e., microphenocrysts, phenocrysts, and megacrysts), from eruptions fed by the central conduits of the volcano (1669 and 2002–03 flank eruptions) and eruptions fed by eccentric dykes which bypass the central conduits, tapping deeper magma storage regions (1974 and 2002–03 flank eruptions). We focus on Cr-rich mantle zones, which crystallised upon eruption triggering mafic rejuvenation and are ubiquitous across our sample set. With decreasing crystal size (i.e., increasing ΔT), tetrahedral aluminium is more strongly partitioned between prism and hourglass sectors. This promotes the uptake of rare earth elements (REE) and high field strength elements (HFSE) into prism relative to hourglass sectors. Combining relative degrees of sector enrichment with ΔT estimates, we propose magma recharge, mush remobilisation and the onset of magma ascent imposed slightly higher ΔT in 1974 than in 2002–03 eruptions at Mt. Etna. Enhanced ΔT in 1974 could be related to vigorous mixing and rapid transport of magma with limited storage, resulting in crystals of smaller sizes. Crystal size populations vary across eruptions, but crystals within a given population (e.g., phenocrysts) return similar calculated ΔT and REE + HFSE sector enrichments, implying connectivity between magmatic environments in the mush system. We show that the magnitude of sector zoning in clinopyroxene can be employed to explore subtle differences in pre-eruptive dynamics in volcanic systems. As an example, we explore sector enrichment in clinopyroxene phenocrysts from the 2021 eruption at La Palma (Canary Islands) and megacrysts from Roman era activity at Stromboli (Italy). Results highlight the role of dynamic mixing and mush remobilisation before eruption in mafic alkaline settings and suggest changes in magma composition across alkaline systems influence clinopyroxene chemistry but do not influence sector enrichment. [ABSTRACT FROM AUTHOR]

Eruptive dynamics is influenced by gas escape from the ascending magma. Gas pathways form in the magma via bubble coalescence, leading to gas channeling. Magmatic crystals play a key role in gas channel formation. This work constrains experimentally decompression‐induced coalescence in high‐crystallinity silicic magmas without external deformation, focusing on low gas content and bimodal crystal size (microlites and phenocrysts). All percolating samples have permeabilities of 10−14 m2 at bulk porosities of 7–10 vol% and bulk crystallinities up to 75 vol%. Our results demonstrate the possibility of coalescence‐related outgassing at high pressure (120–350 MPa) and without external strain, which corresponds to magma stagnating deep in a volcanic conduit. Channeling at such low gas content implies that bimodal crystallinity favors effusive over explosive volcanic behavior. It may also be the missing physical mechanism explaining gas transfer across magmatic systems despite high melt viscosity and low or absent magma extrusion. Plain Language Summary: The way volcanoes erupt is mainly controlled by the ability of gases to escape from the magma (outgassing). An efficient way to outgas is to connect bubbles together (coalescence) up to the point where channels form in the magma (channeling) in which gas can circulate toward the surface. We investigate experimentally how coalescence and channeling happen in immobile, crystal‐rich viscous magma in conditions (pressure and temperature) similar to those found in deep volcanic conduit, where only a small amount of gas (<10%) is present. Our experiments demonstrate that bubble connections are possible thanks to a large amount of crystals on which bubbles can lean, deform, and join each other. That gas can escape from magma at depth could favor effusive eruption over volcanic explosions and even bring new insights on degassing in immobile magma. Key Points: Gas permeability in silicic magma is possible with gas volumes below 10% thanks to high crystallinityOutgassing at low gas content can happen without external deformation nor fracturation of the magmaVolcanic outgassing in stagnant magmas is explained by deep conduit bubble channeling that may be a key mechanism in eruptive transition [ABSTRACT FROM AUTHOR]

Taupō volcanic zone, the site of the 26 ka Oruanui supereruption, produced ~70 km3 of new rhyolites since 11 ka, culminating in 50 km3 Taupō eruption 1.8 ka. Major phenocrysts decrease from 4 to 1 vol%, and Oruanui and post-Oruanui ignimbrites all have identical high-δ18Omelt values of 7.39 ± 0.1‰ and lack low-δ18O values despite overlapping calderas. The Δ'17O values are −0.07‰, lower than the mantle and indicate source contamination of high-δ18O, low-Δ'17O metasediments, and limited interaction with high-Δ'17O hydrothermally altered crust. Previously published U-Th-Pb zircon ages demonstrate their diversity spanning 104–105 years for each unit. Zircon crystal size distribution shows a decrease in abundance and the mean size, and some units lack small (<~10 um) zircons suggesting that zircons were both growing and dissolving in the coexisting magma generation areas. Isotope thermometry indicates heating of the system from ~812 ± 35°C to 874 ± 36°C past zircon saturation in 1.8 ka eruption. We advocate that a deep vertically continuous and laterally discontinuous silicic magma system at the base of the Taupō rift, rather than a shallow batholith or an evolving mush, drives volcanism at Taupō. To explain the post-Oruanui magma production, rift-base silicic magma origin and moderate (~2 km3/1000 years) rhyodacitic magma flux from a growing and heating liquid magma body creates a sufficient solution for the most recent magmatism. [ABSTRACT FROM AUTHOR]

The major- and trace-element compositions of amphiboles in andesite from Quaternary Yufu Volcano, northeastern Kyushu, Japan were analysed to investigate the generation processes of andesitic magma from Yufu Volcano. The amphiboles in andesite from Yufu volcano can be divided into two groups based on major-element composition: pargasite and magnesio-hornblende. To estimate temperature, pressure, and major- and trace-element compositions of melts in equilibrium with amphiboles, we used the recently proposed methods that can calculate temperature, pressure, major element compositions, and partition coefficients of trace-element between amphibole and melt using only the major-element compositions of amphibole. The estimated temperature, pressure, and major-element composition of melt in equilibrium with the amphibole phenocrysts indicate that each group crystallised under different conditions. These differences suggest that two magma chambers at different depths existed beneath Yufu Volcano and that the andesitic magma of Yufu Volcano was formed by mixing of the two magmas. The trace-element compositions of melts in equilibrium with the pargasite and magnesio-hornblende, estimated by applying the partition coefficients calculated from major-element compositions of amphibole to trace-element compositions of amphiboles, indicate magma derived from slab melt and the partial melting of crustal material, respectively. Because magma is a mixture of minerals and melt, we estimate the chemical compositional ranges of the two end-member magmas on the Y versus SiO2 diagram from the mixing relationship between amphibole and estimated melt, as well as phenocrysts of plagioclase, clinopyroxene, and orthopyroxene. The overlap of the estimated compositional range with the trend of whole-rock composition represents the chemical compositions of the end-members of magma mixing, yielding estimates of the mafic (SiO2 ≈ 45 wt %) and felsic (SiO2 ≈ 68 wt %) end-member magmas. Furthermore, we estimate the concentrations of other elements in the end-member magmas by substituting the estimated SiO2 concentrations of the magmas into linear regression equations between the whole-rock contents of other elements and SiO2. The trace-element compositions of the mafic and felsic end-member magmas, as estimated in this study, have similar features to those of gabbroids and Cretaceous granitic rocks, respectively, that are presumed to lie beneath Yufu Volcano. These similarities could be explained by the possibility that the compositions of the end-member magmas were influenced by basement rocks. [ABSTRACT FROM AUTHOR]

Modelling plumbing systems dynamics of active volcanoes through textural and chemical studies of mineral phases is crucial to unravel their eruptive behaviour, but it is rarely applied in ancient volcanic and volcano-plutonic systems. Here, we present an investigation of the architecture, magma dynamics and pre-eruptive timing of Middle Triassic plumbing systems in the Dolomites area (Southern Alps) through a detailed investigation of textures and compositional zoning of clinopyroxenes in lavas and dykes from Predazzo, Mt. Monzoni, Cima Pape and Sciliar volcano-plutonic complexes. The clinopyroxene composition varies between low-Mg# (67–78), low-Cr augite and high-Mg# (77–91), Cr-rich diopside. Diopside is less frequent and appears as variably thick single or multiple bands between augitic cores and rims or as resorbed homogeneous, or patchy-zoned and mottled cores. Rims are homogeneous or oscillatory zoned, with augitic composition. The mid- to low-crustal plumbing systems of all volcanic centres were characterised by the presence of a mildly evolved trachyandesitic magma (Mg# 45; T = 1044–1118 °C), where augitic clinopyroxene formed. Periodic mafic injections of more primitive and hotter trachybasaltic magma (Mg# 56; T = 1056–1170 °C) caused frequent crystallisation of diopsidic bands around augitic cores. The presence of resorbed or patchy-zoned mottled diopsidic cores in clinopyroxene phenocrysts, as well as of rare clinopyroxenitic xenoliths with analogous diopsidic composition, indicates the recycling and remobilisation of antecrysts from the deeper part of the plumbing system, which was probably located at a depth of 10–17 km. Diffusion chronometry models based on Fe–Mg interdiffusion in clinopyroxene revealed that the time elapsed from the mafic injection into the shallow portion of the plumbing systems to the eruption ranges from decades to < 1 year. Our findings enabled us to resolve the different plumbing system dynamics acting at the local scale beneath each volcanic centre. Our data indicate a striking similarity with magma dynamics and timing of pre-eruptive processes at active volcanoes: therefore, we posit that this work sets a starting point to deepen our knowledge of the volcano-plutonic links and, in turn improves our ability to interpret the main processes acting in active plumbing systems. [ABSTRACT FROM AUTHOR]

Processes taking place within the magma plumbing system can exert an important control on the composition of mid-ocean ridge basalts (MORB). Plagioclase ultraphyric basalts (PUBs) found at magma-poor mid-ocean ridges exhibit diverse disequilibrium characteristics, which can provide vital insights for distinguishing the complex effects of melt transport from those of source heterogeneity on the compositions of MORBs. Here, we present new insights into magmatic processes using integrated petrologic and geochemical studies of the PUBs from two zones (~ 50° and ~ 64°E longitude) along the ultraslow-spreading southwest Indian ridge (SWIR). The studied PUBs have complex mineral morphologies, including skeletal and acicular crystals, glomerocrysts with open and closed structure, reverse and normally zoned crystals and external and internal resorption even in single samples. Both low- and high-Fo olivine and An plagioclase crystals are in disequilibrium with their matrix glasses. Some plagioclase phenocrysts have repeated oscillatory zoning (An77–86) going from their core to rim and an abrupt decrease in An content toward the rim. Disequilibrium Sr isotopic compositions are present at several scales: between cores and rims of plagioclase crystals, between different plagioclase crystals and between plagioclase and their host lavas. Inferred pressures of magma storage range from 0.3 to 11.3 kbar. The textural and compositional diversity of crystals together with the variability in melt compositions reflect the combined influences of source heterogeneity and magmatic processes (e.g. crystallization, assimilation and magma mixing processes) taking place within crystal mushes. Our data combined with previous studies suggest that the magmatic processes within the SWIR magma plumbing system involve formation, disaggregation and juxtaposition of crystal-rich mush zones. [ABSTRACT FROM AUTHOR]

Understanding the origin, assembly, and evolution of voluminous magma that erupts in catastrophic caldera-forming eruptions (CCFEs) is a community imperative. A CCFE of the Aira caldera at 30 ka discharged over 350 km³ of magma, which can be grouped into petrographically and geochemically distinct types: voluminous rhyolite, small amounts of rhyodacite, and andesite magmas. To further understand the magma plumbing system of the Aira CCFE, we examined the geochemical characteristics of whole rock and plagioclase from its eruptive deposits. The trace element and 87Sr/86Sr signatures recorded in the plagioclase phenocrysts of these magmas indicate that the three magmas were originally produced by partially melting an identical source rock, which was estimated to be a mafic amphibolite with an 87Sr/86Sr signature of ~0.7055 that comprised the lower crust. Melting of mafic amphibolite produced both felsic and mafic magmas by low and high degrees of partial melting, respectively. The mafic magma assimilated uppermost crustal materials and crystallized to produce an andesite magma type. The andesitic magma consists of phenocrysts (~39 vol%) and melt with a dacitic (~70 wt% SiO2) composition. The felsic magma mixed with ~10% of the andesite magma and crystallized, forming the rhyolite magma. The mixing between the andesite and rhyolite magmas before the Aira CCFE produced the rhyodacite magma. The 30-ka Aira CCFE magmas were generated only by melting two kinds of crustal materials with different geochemical characteristics and had geochemical variations due to different conditions of partial melting and mixing between various crustal melts. The lack of definitive evidence of the mantle component mixing with the Aira CCFE magmas suggests that the mantle-derived magmas worked only as a heat source for crustal melting. [ABSTRACT FROM AUTHOR]

Processes of magma generation and transportation in global mid‐ocean ridges are key to understanding lithospheric architecture at divergent plate boundaries. These magma dynamics are dependent on spreading rate and melt flux, where the SW Indian Ridge represents an end‐member. The vertical extent of ridge magmatic systems and the depth of axial magma chambers (AMCs) are greatly debated, in particular at ultraslow‐spreading ridges. Here we present detailed mineralogical studies of high‐Mg and low‐Mg basalts from a single dredge on Southwest Indian Ridge (SWIR) at 45°E. High‐Mg basalts (MgO = ∼7.1 wt.%) contain high Mg# olivine (Ol, Fo = 85–89) and high‐An plagioclase (Pl, An = 66–83) as phenocrysts, whereas low‐Mg basalts contain low‐Mg# Ol and low‐An Pl (Fo = 75–78, An = 50–62) as phenocrysts or glomerocrysts. One low‐Mg basalt also contains normally zoned Ol and Pl, the core and rim of which are compositionally similar to those in high‐Mg and low‐Mg basalts, respectively. Mineral barometers and MELTS simulation indicate that the high‐Mg melts started to crystallize at ∼32 ± 7.8 km, close to the base of the lithosphere. The low‐Mg melts may have evolved from the high‐Mg melts in an AMC at a depth of ∼13 ± 7.8 km. Such great depths of magma crystallization and the AMC are likely the result of enhanced conductive cooling at ultraslow‐spreading ridges. Combined with diffusion chronometers, the basaltic melts could have ascended from the AMC to seafloor within 2 weeks to 3 months at average rates of ∼0.002–0.01 m/s, which are the slowest reported to date among global ridge systems and may characterize mantle melt transport at the slow end of the ridge spreading spectrum. Plain Language Summary: Mid‐ocean ridges are divergent plate boundaries where asthenospheric mantle undergoes decompressional melting to form basaltic melts and lithospheric mantle. The southwest Indian ridge (SWIR) represents the slowest spreading class of ocean ridges. It is characterized by intermittent volcanism resulting in thin or missing oceanic crust. To better understand the magma dynamics of such specific ridges and their correlation with the spreading rates, we calculated the depth of mineral crystallization in high‐Mg and low‐Mg basalts collected together and constrained the residence time of individual crystals in the melt through Fe‐Mg diffusion profiles. We show that the high‐Mg melts started crystallization close to the base of the tectonic plate, and likely partially crystallized in a deep magma chamber in the mantle to form low‐Mg magmas prior to eruption to the seafloor. This study determined that the rate at which the melts flowed up through the mantle to the crust was the slowest yet found for any ocean ridges. The mineral dynamics of the ultraslow‐spreading SWIR therefore illustrate the critical role of the velocity at which the tectonic plates separate on the formation of magmas at mid‐ocean ridges. Key Points: Deep crystallization depths (∼32 ± 7.8 km) and a deep axial magma chamber (AMC) (∼13 ± 7.8 km) may exist under the ultraslow‐spreading Southwest Indian Ridge (SWIR)The ascent rate from the AMC to seafloor of the SWIR is ∼0.002–0.01 m/s, which is the slowest in global mid‐ocean ridge systemsMagma ascent rates correlate positively with the spreading rates in the global mid‐ocean ridge systems [ABSTRACT FROM AUTHOR]

To better understand the behavior of Ti isotope fractionation during alkaline magma differentiation, we studied well characterized alkaline lavas from St. Helena Island (South Atlantic), as well as their titanomagnetite separates. The lavas are classified into three groups according to petrographic observations and major element composition. Group 1 and Group 2 samples (5 to > 13 wt.% MgO) have a narrow δ49/47Ti range (− 0.02 to 0.05‰), suggesting that Ti isotopic fractionation is insignificant in less evolved basaltic lavas. Conversely, Group 3 samples (MgO < 5 wt.%) are saturated with titanomagnetite and display a wide range in δ49/47Ti (− 0.02 to 1.96‰). The δ49/47Ti values for Group 3 samples show significant correlation with TiO2 and SiO2 content, as well as with Mg and Fe isotopic values. Moreover, titanomagnetite phenocrysts from Group 3 rocks have remarkably lighter δ49/47Ti values (− 0.54 to 0.01‰) relative to the corresponding whole rock (− 0.02 to 0.21‰), indicating that titanomagnetite crystallization exerts significant control over the δ49/47Ti of Group 3 samples. These observations are further supported by modeling calculations. Together with published Ti isotope data, the results demonstrate that the range in Ti isotopic evolution in alkaline, calc-alkaline and tholeiitic magmatic systems is controlled by fractional crystallization of diverse Fe-Ti oxides with contrasting Ti isotopic compositions. This makes Ti stable isotopes an important geochemical tracer for magma evolution. [ABSTRACT FROM AUTHOR]

Eruptions from Mauna Loa's Southwest Rift Zone (SWRZ) pose a significant threat to nearby communities due to high eruption rates and steep slopes resulting in little time for evacuation. Despite the large body of research done on Mauna Loa, knowledge of the timing and duration of magma residence and transfer through its internal plumbing system is still poorly constrained. This study presents a first quantitative look at thermochemical conditions and timescales of potentially deep storage and disaggregation of magmatic mush during the run-up to the voluminous 1950 AD SWRZ eruption. Details of heterogeneous compositions and textures of the macrocryst and glomerocryst cargo in 1950 AD lavas suggest magma mixing and crystal recycling along the entire plumbing system. Furthermore, the crystal cargo contains evidence for the direct interaction between primitive, deeply stored magma and pockets of more evolved magma stored at shallow to intermediate depths. An enigmatic attribute of 1950 near-vent lava is the near-ubiquitous presence of subhedral, unreacted Mg-rich orthopyroxene phenocrysts (Mg#>80). Phase relations of Mauna Loa olivine-tholeiite indicate that orthopyroxene joins olivine as a primary phase at pressures higher than 0.6 GPa. Coexisting Mg-rich olivine and orthopyroxene and the occurrence of harzburgitic (olivine-orthopyroxene) glomerocrysts provide evidence for cognate crystallisation at near-Moho (~ 18 km) depths (Thornber and Trusdell 2008). Petrogenetically diverse populations of glomerocrysts and macrocrysts alongside evidence of multilevel magma storage indicate a network of ephemeral and possibly interconnected magma pockets from near-Moho depths to the upper/mid-crust. Fe-Mg diffusion chronometry applied to 1950 AD olivine populations implies rapid mobilisation and transport of large volumes of magma (376×106 m3) from near-Moho storage to the surface within less than 8 months, with little residence time (~ 2 weeks) in the shallow (3–5 km) plumbing system. [ABSTRACT FROM AUTHOR]

The mafic microgranular enclaves (MMEs) from Mesozoic intermediate-acid magmatic rocks, widely developed along the Fujian coast, are considered to be the results of large-scale crust–mantle interaction by magma mixing. This paper is based on zircon U-Pb chronology, along with zircon Hf isotope and mineral analyses for the host granite and MMEs from Langqi Island, in order to investigate the magma mixing mechanism of the Langqi pluton in Fuzhou, Southeast China. The results indicate that the MMEs were emplaced during the late Early Cretaceous (98.9 ± 2.2 Ma), identical to the age of the granite (100.1 ± 1.1 Ma) within the error range. The zircon εHf(t) values for the granite and MMEs are in the ranges of −2.1~0.0 and −1.7~+1.1. The zircon Hf isotope data indicate that both the granite and MMEs were predominantly derived from the ancient crustal basement of Cathaysia, with a partial mantle-derived contribution. The An values of plagioclase phenocrysts with oscillatory zoning patterns in the MMEs show oscillatory changes from the core to the rim, indicating multiple mixing events between the two magmas with different compositions. Amphiboles in the MMEs show characteristics of crust–mantle contamination, and the Ti migrated from the mafic magma with high concentration to the felsic magma with low concentration during the magma mixing process. Biotites in the host rock and MMEs belong to primary biotite, and they have relatively high MgO contents (ave. 12.78 wt.%) and relatively low FeOT/(MgO + FeOT) ratios (ave. 0.56), showing characteristics of crust–mantle contamination. The crust–mantle magma interaction in a crystal, mushy state played a significant role in controlling the formation and evolution of the Langqi pluton. The magmatism was predominantly sourced from mixing between the mantle-derived mafic magma and the crust-derived felsic magma during the subduction of the Paleo-Pacific Plate, resulting in the formation of the Langqi doleritic veins, granites, and MMEs. [ABSTRACT FROM AUTHOR]

Post-collisional potassic magmatic rocks are widely distributed in the northwestern Tibetan Plateau, yet their magmatic processes remain poorly understood. Here, we present a comprehensive analysis of the whole-rock major and trace elements, as well as the mineral textures and chemistry of the Dahongliutan volcanic rocks in the NW Tibetan Plateau, aiming to reveal the magmatic processes prior to eruption and speculate on the triggering mechanism. The results show that the Dahongliutan volcanic rocks are potassic trachyandesites, which undergo polybaric crystallization during magma ascension. The phenocrysts in these potassic rocks exhibit various textural and compositional zoning styles. The green cores of green-core clinopyroxenes show textural (e.g., resorption texture) and chemical (Fe-rich) disequilibrium with the host rock compositions, suggesting that they may be antecrysts and crystallized from early batches of more evolved magmas. Additionally, alkali feldspar phenocrysts also display disequilibrium characteristics (e.g., overgrowth rim and sieve texture), indicating hot mafic magma recharge and mixing in the magma plumbing system. Therefore, we conclude that the disequilibrium textural and compositional features of green-core clinopyroxene and alkali feldspar phenocrysts provide evidence of magma recharge and mixing prior to eruption. Furthermore, it is likely that the eruption of the Dahongliutan volcano was triggered by magma recharge. [ABSTRACT FROM AUTHOR]

The current debate on volcanic-plutonic connection is centered on whether eficient 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 (<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 (>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 efect 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. [ABSTRACT FROM AUTHOR]

The Dar al Gani (DaG) olivine‐phyric shergottites share mineralogical and geochemical characteristics, which confirm that these meteorites are derived from a single source. Bulk trace elements (La/Yb—0.12), in situ maskelynite 87Sr/86Sr (~0.7014) and redox estimates (FMQ ~ −2) indicate derivation from a depleted, reduced mantle reservoir; identical to all ~470 Ma shergottites ejected at 1.1 Ma. The DaG shergottites have been variably affected by terrestrial alteration, which precipitated carbonate along fractures and modified bulk‐rock fluid mobile (e.g., Ba) elements. Nonetheless, sufficient data are available to construct a multi‐stage formation model for the DaG shergottites and other 1.1 Ma ejection‐paired shergottites that erupted at ~470 Ma. First, partial melting of a depleted mantle source occurred at 1540 ± 20°C and 1.2 ± 0.1 GPa, equivalent to > ~100 km depth. Then, initial crystallization in a staging chamber at ~85 km depth at the crust–mantle boundary took place, followed by magma evolution and variable incorporation of antecrystic olivine ± orthopyroxene. Subsequently, crystallization of olivine phenocrysts and re‐equilibration of olivine antecrysts occurred within an ascending magma. Finally, magmas with variable crystal loads erupted at the surface, where varied cooling rates produced a range of groundmass textures. This model is similar to picritic flood basalt magmas erupted on Earth. [ABSTRACT FROM AUTHOR]

Ca-bearing nepheline found in the Kajishiyama basanite, Tsuyama Basin, southwest Japan, was investigated to clarify its genesis in silica-undersaturated magmas. The basanite contains olivine and augite as phenocrysts and microphenocrysts, with Ca-bearing nepheline, olivine, augite, ulvöspinel, plagioclase, alkali feldspar, apatite and zeolites in the groundmass. Zeolites are more abundant in coarser-grained samples. The whole-rock composition of the basanite is characterised by low SiO2 and P2O5 contents and high total Fe, MgO, Na2O, K2O, Ba and Sr contents. The Ca-bearing nepheline, ~20 μm in size, occurs in the mesostasis of the Kajishiyama basanite and contains up to 2.31 wt.% CaO and 16.75 wt.% Na2O, in contrast to nepheline from the Hamada nephelinite, southwest Japan. The approximate compositional formula of the Kajishiyama nepheline with the highest Ca content is (Ca0.467Ba0.013Na5.286K0.919□Total1.385)Σ8.070(Si0.912Al6.980Cr3+0.003Fe3+0.067 Mg0.017)Σ7.979Si8.000O32; i.e. Ne65.50Ks11.39Qxs11.22CaNe11.89. Basanites are defined as being nepheline-normative, however they are high in normative plagioclase, the amount of which increases with fractionation of the magma. Nepheline crystallised after plagioclase, at the last stage of magmatic solidification is enriched in Ca. Such Ca-rich nepheline only forms from a magma which is high in normative plagioclase, as is the case in the Kajishiyama basanite. In contrast, Ca-poor nepheline is precipitated from nephelinitic magmas that crystallise melilite instead of plagioclase, even when Ca contents are high. [ABSTRACT FROM AUTHOR]

Plagioclase phenocrysts in pre-historic andesites provide insight to the dynamics of magma formation and eruption at Taranaki volcano, New Zealand. The phenocryst population has a diversity of relic cores and a total in situ 87Sr/86Sr range of 0.70440–0.70486. Within-sample 87Sr/86Sr variations of 0.00018 to 0.00043 indicate that many phenocrysts are antecrysts and/or xenocrysts, derived from multiple crystal mush bodies. The Sr-isotopic differences in the phenocrysts of consecutive eruptions indicate that different magmas were tapped or formed on a centennial timescale. Most phenocrysts have multiple resorption/calcic regrowth zone(s) with elevated FeO* but invariant MgO zonation profiles. They likely record mafic melt inputs, and subsequent storage at elevated temperature caused re-equilibration of the Mg gradient. However, distinct rim types record different final pre/syn-eruptive magmatic conditions. Those in magmas erupted at 1030–1157 CE, 1290–1399 CE and 1780–1800 CE are characterised by resorption and calcic regrowth with sharp MgO and FeO* gradients. They record the entry of mafic melt into the system a few days or less before eruption based on Mg diffusion chronometry. In contrast, most phenocrysts erupted at 1755 CE, 1655 CE, and a few pre-1 ka events, have texturally uniform rims, compositionally consistent with closed-system crystallisation. This suggests alternating external and internal eruption triggers. Alternatively, the rate of magma reactivation via intrusion may dictate whether there was sufficient time for a mineralogical response to be recorded in part or all of the system. With respect to anticipating future eruptions, the plagioclase phenocrysts suggest multi-stage magma priming but rapid onset of the final trigger. [ABSTRACT FROM AUTHOR]

Between 38.5 ka cal BP and 32.4 ka cal BP, a dacitic Volcanic Explosivity Index 5 eruption at Misti volcano emplaced the Sacarosa tephra-fall deposit. Its biotite phenocrysts, fine grain size, scarce lithics, and abundant loose crystals characterize the deposit at locations sampled. The eruption's ~ 800 °C magma rose rapidly from ~ 10 km depth, culminating in a Plinian eruption which reached a mass eruption rate of 7.7 × 106–4.1 × 107 kg/s and emplaced about 3 km3 of tephra within tens of hours. The unit comprises two layers of subequal thickness separated by a diffuse contact with the upper distinguished by being slightly coarser and less well sorted than the lower. The deposit's coarser upper layer indicates either climactic conditions or a lesser degree of fragmentation during the latter half of the eruption. Strong winds distributed the deposit southwest of Misti, where it crops out over at least 800 km2 and drapes the present site of Arequipa with up to 100 cm of tephra. The Sacarosa deposit is the first among the Cayma stage deposits, a distinctive group of felsic, biotite-bearing units, to be carefully described and its eruption characterized. Several Cayma stage deposits were emplaced by voluminous explosive eruptions similar to the Sacarosa eruption, representing a ~ 8.9–15.5 ky interval of powerful eruptions. Such an explosive eruption today would threaten Arequipa's over 1,100,000 residents, many of whom live within the Sacarosa deposit's distribution. [ABSTRACT FROM AUTHOR]

Magmas with low δ18O values, which require a contribution of isotopically light oxygen from hydrothermally altered rocks, are globally rare despite the common association of large-scale magmatism with intense hydrothermal activity. Here we consider this paradox in the central Taupō Volcanic Zone (TVZ, New Zealand), one of the most intense regions of Quaternary silicic magmatism and hydrothermal activity on Earth. We present new data on phenocryst δ18O values from 62 magma batches erupted since ∼1.6 Ma, ranging from basalt to rhyolite, and match these against existing and new Sr and Nd isotopic data. Basalt δ18O melt values (typically ≥+6.3 to +6.5‰) calculated from phenocrysts are mostly high relative to mid-ocean ridge basalts (MORB), whereas basaltic andesites extend to +7.1‰. Rhyolite δ18O melt values are mainly +7.0 to +8.5‰, which is considered high-δ18O in most settings. However, assimilation and fractional crystallisation (AFC) models show a mismatch between the amount of greywacke basement assimilation required to explain the Sr-Nd isotopic ratios and the δ18O values of many TVZ rhyolites. We attribute this mismatch to shallow assimilation of low-δ18O altered rocks after prior assimilation of unaltered greywacke at deeper levels. We infer that interactions between large silicic magmatic systems and shallow altered crustal rocks are common and widespread in the TVZ, despite the absence of silicic magmas with low δ18O values (<∼+6.0‰). This absence reflects the combined influence of high δ18O values inherited from the slab in parental basalts, and the limited isotopic leverage available in the TVZ to drive large 18O depletions via hydrothermal alteration. The TVZ example suggests that large-scale interactions between magmas and altered host rocks are likely to be important in many silicic magmatic provinces and not restricted to the sparse occurrences of low-δ18O magmas. Spatial clustering of the lowest TVZ rhyolite δ18O melt values (+6.3 to +7.0‰) occurs along a western corridor of presently weak hydrothermal activity, implying that hydrothermal activity was more intense there in the past, and may have represented a focus of heat flow similar to the present-day eastern TVZ margin. Magma δ18O values can thus be useful for tracing areas of past alteration even in settings where conventionally normal to high δ18O values are retained in erupted products. [ABSTRACT FROM AUTHOR]

The Jurassic Chon Aike Silicic Large Igneous Province (Patagonia and the Antarctic Peninsula) is dominated by voluminous, crust-derived magmas (235,000 km³) that erupted as predominately explosive silicic material over ~40 m.y. In this study, we combine petrological descriptions and bulk-rock major- and trace-element compositions with quartz oxygen-isotope measurements from multiple silicic units (primarily ignimbrites and some rhyolitic flows) from two of the five silicic formations in Patagonia. We have identified that quartz oxygen-isotope values are high (>9°-12°). Quartz phenocrysts analyzed by secondary ion mass spectroscopy (SIMS) are also homogeneous at the microscale with no measurable change in isotope value with respect to internal and often complex zoning textures. The ubiquity of widespread high d18O rhyolites and their trace-element compositions support their origin from melting of a metasedimentary source with a similarly high d18O value. Mass balance calculations require that an average of >75% melt derived from partial melting of the dominant basement lithology is needed to explain the isotopic and chemical composition of the rhyolites. The ideal P-T environment was identified by thermodynamic models for fluid-absent melting of graywackes at 900 °C and 5 kbar. Regional-scale crustal melting occurred during a widespread, high heat-flux environment within an extensional setting during the breakup of the Gondwanan supercontinent. The overlap of a unique tectonic and igneous environment, combined with a fertile crust dominated by graywacke and pelitic compositions in southern Patagonia, generated large volumes of some of the highest d18O silicic magmas documented in the geologic record. [ABSTRACT FROM AUTHOR]

The Azufre volcano (21°47′S; 68°14′W) is emplaced above the western boundary of the Altiplano Puna Magma Body. The youngest lava of the Azufre volcano (50–331 ka) was selected for detailed studies because of the relevant petrological information that could emerge to understand its reservoir as a potential heat source of the neighboring Cerro Pabellón geothermal system. The studied lava corresponds to an andesite-dacite (61–63 SiO2 wt%) with phenocrysts of plagioclase, amphibole (Group 1), biotite, pyroxenes, quartz, and olivine. The lava also contains aphanitic enclaves (58–60 SiO2 wt%), whose groundmass have the same mineralogy of lava samples groundmass consisting of amphibole microphenocryts (Group 2) and microlites of plagioclase, pyroxenes, and Fe–Ti oxides. Disequilibrium textures are commonly observed in the studied samples such as partially resorbed plagioclase phenocrysts, amphibole breakdown, and reverse zoning in pyroxene and plagioclase phenocrysts. Thermobarometry calculations indicated pressures of ~ 2 kbar for Group 1 amphiboles, temperatures of 905–1097 °C for Mg-rich pyroxene phenocrysts and Ca-rich plagioclase (An ≥ 66), and near-solidus temperatures of 712–788 °C for Group 1 amphibole-plagioclase pairs. Group 1 amphiboles also indicate crystallization from an evolved liquid (63–79 wt% of SiO2). Oxidation conditions of QFM + 0.9–2.5 log units were recorded in amphibole and Fe–Ti oxides. Rhyolite-MELTS models reproduce the composition of the high-temperature phases from a melt composition (andesitic enclave) at similar P–T-fO2. The arrival of new hot andesite magma into a crystal-rich shallow reservoir, thermally and compositionally zoned, would have triggered the studied eruption. Diffusion models in Fe–Ti oxides microlites indicated cooling temperatures of 742–866 °C during the sub-aerial emplacement of the lava. [ABSTRACT FROM AUTHOR]

Titanium diffusion profiles in quartz crystals are widely applied to constrain the duration of magmatic processes. However, experimentally determined Ti diffusion coefficients in quartz diverge by three orders of magnitude. To rectify this problem we derive Ti diffusion coefficients from natural quartz phenocrysts from the 1991 eruption at Mt. Pinatubo, by combining U-Th ages of small (15–40 µm long) zircon inclusions with Ti diffusion profiles measured at nearby growth zone contacts in the same quartz crystals. Application of the obtained data to quartz crystals with Ti-rich rims from thirteen silicic volcanic tuffs worldwide suggests that the magmas erupted years to thousands of years after magma chamber rejuvenation, with the priming time increasing with magma volume and decreasing temperature. Here we show that the time scales involved in the generation of silicic volcanic eruptions are much longer than originally thought. Titanium diffusion profiles in quartz record the duration of magmatic processes. Here, the authors use a novel way to constrain Ti diffusion coefficients and apply them to determine the time scales involved in the priming of volcanic eruptions. [ABSTRACT FROM AUTHOR]

The Huachichocana Subvolcanic Complex (HSC) is one of the easternmost Miocene magmatic complexes from the Central Andes back-arc region. This complex comprises three major sheet-like igneous bodies and contiguous minor intrusions of andesitic to dacitic rocks distinguished by their adakite-like high Sr and Sr/Y values. Based on new detailed data of this system, we explore the genesis of intermediate magmas in the far back-arc region of the Central Andes and complement work on similar rocks from the Eastern Cordillera. We add petrography and mineral chemistry studies and we report new whole rock geochemical and Sr–Nd isotopic data, and a U–Pb zircon age for the HSC. The results suggest that these intrusive rocks were derived from melting of a hydrated mantle source and that the adakite-like composition was acquired through amphibole-dominated fractionation at deep to moderate crustal pressures and by the suppression of early plagioclase crystallization in precursor magmas. The presence of mafic microgranular enclaves is frequent and, along with variations in the composition of phenocrysts and common disequilibrium textures, suggests a significant role of mixing between mafic and intermediate magmas during their ascent and storage in the crust. U–Pb radiometric dating yielded a concordia age of 8.343 ± 0.042 Ma for the andesites of the HSC, coinciding with the period of maximum shortening of the eastern border of the Puna plateau. Nevertheless, this igneous complex is located at the southern margin of the Lípez fault system, a zone of lithospheric weakness that should favor magma emplacement so far east of the trench. [ABSTRACT FROM AUTHOR]

Textural and compositional zoning within plagioclase phenocrysts records the magma chamber processes, such as magma differentiation, magma recharge and mixing, and crustal contamination. The plagioclase phenocrysts in the Daqiao and Qiaojia plagioclase-phyric basalts from the Emeishan Large Igneous Province (LIP) show complex textural and compositional zoning patterns, e.g., normal, reverse, oscillatory, and patchy zoning patterns. Most plagioclase phenocrysts exhibit a core–rim normal zoning pattern (Pl-A) with euhedral high-An cores (An = 76–78%, in mole fraction) and low-An rims (An = 68–72%), indicative of the crystal regrowth processes caused by recharge of relatively evolved magmas after the formation of high-An cores. Some phenocrysts have a core–rim reverse zoning pattern (Pl-B) with irregular ovaloid cores, characterized by extremely low An (60–61 mol%) and Ba (84–88 ppm) contents and extremely high 87Sr/86Sr ratios (0.7120–0.7130). The rims of the Pl-B have relatively high An (69–72 %), Ba (~160 ppm) contents, and low 87Sr/86Sri (~0.7056). These Pl-B plagioclase phenocrysts preserve the information about the interaction between the crustal xenocrysts and the transporting magmas. Some plagioclase phenocrysts show a core–mantle–rim oscillatory zoning pattern (Pl-C) with multiple oscillations of An (70–80 %), Ba (88–147ppm) from core to rim, revealing replenishment and mixing of multiple batches of basaltic melts with diverse compositions. 87Sr/86Sr ratios of the Pl-C do not vary significantly (0.7050–0.7054). A small portion of phenocrysts has patchy patterns in the cores (Pl-D), where the low-An patches (72–75 %) in form of elliptical or irregular elongated shapes were enclosed by the high-An domains (80–87 %). These features can be attributed to crystal dissolution and regrowth processes during the reaction between early-formed low-Cumulates and recharged hot primitive melts. The cores, mantles, and rims of different types of plagioclase phenocrysts (except the core of Pl-B) commonly display nearly constant Sr isotopic compositions, implying insignificant wall-rock assimilation at shallow-level magma reservoir(s) during the growth of these plagioclase phenocrysts. In conclusion, the massive crystallization of plagioclase in the late stage was an important controlling factor for the formation of iron-rich basalts in the Emeishan LIP. [ABSTRACT FROM AUTHOR]

We integrate textural and in situ compositional information from plagioclase and clinopyroxene (Cpx) phenocrysts together with groundmass compositions in early Cretaceous andesite dykes within the Sulu belt of China to propose a new petrogenetic model for andesite. Plagioclase phenocrysts are mostly andesine; they are depleted in high field strength elements (HFSE). However, Cpx phenocrysts are either reversely zoned (type I) or homogeneous (type II), with the zoned Cpx divided into subtypes IA and IB. All Cpx has high Mg#, low Na2O and generally low Al2O3, with depletions in HFSE and variably high 87Sr/86Sr ratios, suggesting crystallization above the Moho from magmas derived from enriched lithospheric mantle. The cores of type IA/IB and type II Cpx have normal major‐ and trace‐element compositional variations and similar 87Sr/86Sr ratios to each other and to plagioclase, consistent with fractional crystallization from a common magma (magma 1). The rims of type IA and IB Cpx also have normal major‐ and trace‐element compositional variations, but these are not as evolved as the cores, and the rims have lower 87Sr/86Sr ratios, demonstrating crystallization from an isotopically distinct magma (magma 2). Based on modeled major and rare earth element compositions of magmas inferred to have been in equilibrium with different Cpx (±plagioclase) domains, the measured groundmass compositions can be reproduced by variable mixing between the two magmas. Our study demonstrates for the first time that andesite magma can be made through fractionation and shallow hybridization of magmas derived from variably enriched lithospheric mantle. Plain Language Summary: Andesite is the characteristic volcanic rock that is formed where Earth's tectonic plates are thrust one under another, as occurs along the Andes, the Cordillera of Central and North America, and the western margin of the Pacific Ocean, but how the magmas form remains controversial. In the mantle model, an enriched source is thought to form by reaction between mantle peridotite and metamorphic fluids derived from seafloor sediments and/or altered oceanic lithosphere during under‐thrusting of an ocean plate beneath a continental margin. By contrast, the shallow model proposes that andesite magmas are formed by differentiation of basaltic magmas or mixing of magmas from mantle and crustal sources in the lithosphere of an overriding continental plate. In either case, the composition of the magma may evolve due to crystal fractionation or be modified by crustal assimilation. Here, we use petrographic observations and in situ major‐ and trace‐element, and Sr isotope compositions of plagioclase and clinopyroxene phenocrysts and groundmass in early Cretaceous andesite dykes from the Sulu belt of China, to show for the first time that andesite can be made through fractionation and shallow hybridization of two different magmas derived from variably enriched lithospheric mantle of the underlying continental plate. Key Points: Using compositions of plagioclase/clinopyroxene phenocrysts and the groundmass in andesite we develop a new model of andesite petrogenesisThis model requires polybaric crystallization and hybridization between 2 enriched lithospheric mantle‐derived magmas at crustal depthWe demonstrate that andesite of similar composition to continental crust can be made through mixing magmas derived from the enriched subcontinental lithospheric mantle [ABSTRACT FROM AUTHOR]

Kharchinsky and Zarechny volcanoes and the Kharchinsky Lake zone of monogenic cones are unique eruptive centers of magnesian lavas located above the northern margin of the Pacific plate subducting beneath Kamchatka. This paper presents new geochemical data on the composition of rocks (55 samples) and minerals (over 900 analyses of olivine, pyroxenes, amphibole, and plagioclase) of these centers analyzed by XRF and LA-ICP-MS (rocks) and electron microprobe (minerals). Most of the studied rocks are magnesian (Mg# = 60–75 mol %) medium-K basalts and basaltic andesites. Moderate-magnesian (Mg# = 52–59 mol %) basaltic andesites are present among the monogenic cones of Kharchinsky Lake. The rare rock varieties include the high-K basalts–basaltic andesites of dikes in the center of Kharchinsky volcano and the magnesian andesites (Mg# = 58–61 mol %) of the extrusions of Zarechnу volcano. The distribution of trace-element contents in these samples demonstrates enrichment in large-ion lithophile elements and light REEs at depletion in high field strength elements and heavy REEs, as is typical of arc rocks. The high-K basalts and basaltic andesites show anomalous enrichment in Ba (>1000 ppm), Th (>3.8 ppm), U (>1.8 ppm), Sr (> 800 ppm, Sr/Y > 50), and light REE (La > 20 ppm), and their compositions are close to those of low-Si adakites. The basalts and basaltic andesites contain phenocrysts of high-Mg olivine (up to Fo92.6) and clinopyroxene (Mg # up to 91 mol %). The rocks show petrographic and geochemical evidence of fractional crystallization, along with the processes of mineral accumulation and magma mixing. Some of the olivine phenocrysts show high NiO contents (up to 5000 ppm) and an elevated Fe/Mn ratio (up to 80), which were interpreted as evidence of the participation of a pyroxenite source in the magma generation processes. The use of the Ca/Fe and Ni/Mg ratios allowed us to distinguish the composition fields and evolution trends of olivine associated with different sources: peridotite and pyroxenite, which were formed by a reaction between mantle-wedge peridotites and high-Si melts of the subducted oceanic crust. The new data are consistent with other lines of evidence of melting of the subducted Pacific plate edge beneath the northern part of the Central Kamchatka Depression at the Kurile–Kamchatka and Aleutian subduction zone junction and testify to a significant heterogeneity of the mantle in this area. [ABSTRACT FROM AUTHOR]

Water content plays a significant role in magma genesis, ascent rate, and, ultimately, in the style and intensity of volcanic eruptions, due to its control on the density, viscosity and melting behaviour of silicate melts. A reliable method for determining the pre-eruptive magmatic water content is to use phenocrysts of nominally anhydrous minerals (NAMs) which can preserve water as hydrogen configurations in structural defects. The advantage of this method is that eruptive changes such as water loss during magma degassing may be experimentally reconstructed and analysed by infrared spectroscopy. Applying this to clinopyroxene crystals (n=17) from lava samples (n=7) from April 2021 of the Geldingadalir eruption, SW-Iceland, reveals parental water contents of 0.69 ± 0.07 to 0.86 ± 0.09 wt. % H2O. These values are higher than those expected for typical mid-ocean ridge basalts (MORB 0.3–0.5 wt. % on average) indicating a significant plume (OIB) contribution to the magma source. Moreover, such water concentrations would imply that water saturation in the ascending Geldingadalir magmas was attained only at very shallow levels within the plumbing system. This could explain the at times pulsating behaviour within the uppermost conduit system as being the result of shallow episodic water vapour exsolution in addition to the deep-sourced CO2 flux. [ABSTRACT FROM AUTHOR]

The Karoo large igneous province has been divided into rift zone and basin-related groups, with picrites from the Luenha river, Mozambique, representing an end-member of the latter. New O isotope, major and trace element data for olivine have been combined with MELTS crystallisation modelling to deconvolve compositional diversity associated with magma differentiation from source-derived heterogeneity. Three olivines populations have been discerned as follows: (1) the 'main trend', which records crystallisation from a variety of magma compositions; (2) the 'low Cu trend', which is inferred to represent xenocrysts or antecrysts; and (3) the 'high CaO' olivines, which record polybaric crystallisation of a primitive, little fractionated magma. The trace element variability in olivine phenocrysts relates partially to sampling of different parts of the same overall magma transport and storage systems, and partly to heterogeneity of parental magmas and their mantle sources. When the measured δ18Oolivine values have been converted into δ18Omelt values, the mean δ18Omelt values for the 'main trend' and 'low Cu' groups are indistinguishable from each other (5.7 ± 0.1‰, 2σ); however, the mean δ18Omelt value of 6.1 ± 0.1‰ for the 'high CaO' group is distinctly enriched. These data record source heterogeneity and suggest contributions from two mantle sources, one with elevated δ18O, and another with more 'typical' mantle δ18O. Combining these data with previously reported trace element and Nd and Sr isotope data support derivation from a mantle source similar to non-chondritic bulk silicate earth, but with minor contributions (1–2% for the enriched magmas) from a recycled sedimentary component. This points to the importance of a primitive mantle source for the basin-related successions in Karoo. [ABSTRACT FROM AUTHOR]

In the past few years, zinc isotope systematics of basaltic magmas have been widely used as novel proxies for terrestrial mantle heterogeneity induced by recycled crustal materials and for planet formation and evolution. The influence of crystal-melt isotope disequilibrium during magma differentiation on the zinc isotopic composition of basaltic melts, nonetheless, has received little attention. In addition, no quantitative constraint has yet been given for the diffusion-driven Zn isotope fractionation between olivine crystals and melts. Here we present high-precision zinc isotope data (δ66Zn JMC-Lyon) for a series of olivine phenocrysts separated from intra-plate alkali basalts from the Jiaodong Peninsula in Eastern China, together with in-situ chemical analysis. Olivine phenocrysts have Zn isotopic compositions which are too light in comparison with the host basaltic melts (Δ66Zn ol-melt = −0.41‰ to −0.10‰; n = 16) to be explained by equilibrium isotope fractionation at magmatic temperatures. Instead, the decrease of δ66Zn values with decreasing Mg# (i.e., Mg/[Mg + Fe2+] × 100) and increasing Zn + Fe2+ contents in olivine phenocrysts suggests diffusion-driven kinetic fractionation during olivine crystallization. The data is well-fitted with a diffusion model in which Zn together with Fe2+ diffuse from surrounding melt into olivine crystals and approximately equal flux of Mg diffuses into melt due to the large chemical gradient, yielding a kinetic Zn isotope fractionation factor β Zn of 0.07. By utilizing the fractional crystallization model under equilibrium or disequilibrium conditions, it is suggested that diffusion-induced isotope disequilibrium during olivine crystallization may drive Zn isotopic composition of the residual melt toward heavier values. When using zinc isotope systematics of any basaltic magma to probe the source heterogeneity, this impact must be taken into account if the Zn isotope disequilibrium widely exists in olivine phenocrysts. Based on the time-related quantitative diffusion model, our results show that the magma residence time for strongly alkali basaltic lavas (∼20 days) is 10–60 times shorter than that for weakly alkali basaltic lavas (6 months to 4 years). This supports the generation of weakly alkali basalts via interaction between silica-undersaturated melts and the surrounding lithospheric mantle, implying a probable common mechanism in intra-plate basalts that could be comparable globally. Thus, Zn isotope disequilibrium between olivine crystals and melts could provide valuable information on the evolution history of intra-plate basaltic magmas. [ABSTRACT FROM AUTHOR]

This paper presents the chemical composition (including H2O and CO2) of matrix glass, melt inclusions in phenocrysts, and their host minerals in eruptive products from Aso caldera. We found a group of melt inclusions with clearly lower potassium (0.6–2 wt % K2O at 50–70 wt % SiO2) than previously reported high-K2O whole-rock compositions (3–5 wt % K2O at 55–70 wt % SiO2). While most of the high-K2O intermediate to felsic melt inclusions are vapor undersaturated and show the features of H2O, CO2, and K2O accumulation, the low-K2O basaltic melt already has higher H2O and CO2. We reconcile this discrepancy with a model in which (1) the volatile-rich basalt magmas degas near the surface, (2) migrate back to depths of |$\ge$| 12 km in the crust, and (3) crystallize feldspar and quartz to produce high-K2O felsic melt, (4) magma mixing among the undegassed, degassed, and evolved magmas. By crystallization, about five times as much low-K2O basaltic magma is required to produce high-K2O felsic magma. The quantity of felsic magma ejected from 270 ka (Aso-1) to 90 ka (Aso-4) requires a basalt magma supply rate of 18 to 31 km3/ka. This magma supply rate is comparable to or less than the present-day production rate of degassed magma (73 km3/ka) at an active center of Aso, Nakadake. These findings suggest volcanic gas flux monitoring has the potential to be a 'basalt usage meter' during the dormant period of caldera volcanoes. [ABSTRACT FROM AUTHOR]

The Cima volcanic field, in the southern Basin and Range province (California, USA), includes >70 eruptive units over the last 8 m.y. The youngest (=1 Ma) are low Mg# (=56) hawaiites derived from an asthenospheric mantle source. The Cima hawaiites, and adjacent Dish Hill basanites, are known for carrying large mantle xenoliths, which precludes stalling in a crustal reservoir. This raises the question of how low Mg# hawaiites, which cannot be in equilibrium with peridotite mantle, formed and differentiated while carrying dense, mantle xenoliths. Several hypotheses are evaluated and the only one shown to be viable is mixing between high- MgO basanite (with entrained mantle xenoliths and sparse olivine phenocrysts) and low-MgO mugearite liquids, which formed by partial melting of mafic lower crust under relatively dry and reducing conditions. Multiple lines of evidence, including the presence of mantle xenoliths in hawaiites, diffusion-limited growth textures in olivine and clinopyroxene, and notably thin Fe-rich rims on high-MgO olivine crystals (inherited), indicate magma mixing must have occurred rapidly (days or less) during ascent to the surface along intersecting fractures, and not in a stalled crustal reservoir. Abundant evidence points to clinopyroxene growth immediately after mixing, and application of clinopyroxene-melt barometry constrains the depth of mixing to the lower and middle crust (0.8-0.4 GPa). Results from olivinemelt thermometry/hygrometry (~1196 °C and ~1.4 wt% H2O) applied to a basanite from Dish Hill carrying 5-20 cm mantle xenoliths leads to calculated ascent velocities =0.3-4.9 km/h, enabling ascent through the 36 km thick crust in =7-119 h. [ABSTRACT FROM AUTHOR]

In 1999–2001, Guagua Pichincha volcano, Ecuador, produced a series of cyclic explosive and effusive eruptions. Rock samples, including dense blocks and pumiceous clasts collected during the eruption sequence, and ballistic bombs later collected from the crater floor, provide information about magma storage, ascent, decompression, degassing, repressurization, and regassing prior to eruption. Pairs of Fe-Ti oxides indicate equilibrium within 1.2–1.5 log units above the NNO oxidation buffer and equilibrium temperatures from 805 to 905 °C. Melt inclusions record H2O contents of 2.7–4.6 wt% and CO2 contents (uncorrected for CO2 segregation into bubbles) from 19 to 310 ppm. Minimum melt inclusion saturation pressures fall between 69 and 168 MPa, or equilibration depths of 2.8 and 6.8 km, the lower end of which is coincident with the maximum inferred equilibration depths for the most vesicular breadcrust bombs sampled. Amphibole phenocrysts lack breakdown rims (except for one sample) and plagioclase phenocrysts have abundant oscillatory compositional zones. Plagioclase areal microlite number densities (Na) range over less than one order of magnitude (8.9×103–8.7×104 mm-2) among all samples, with the exception of a dense, low crystallinity sample (Na = 3.0×103 mm−2) and a pumiceous sample erupted on 17 December 1999 (Na = 1.7×103 mm−2). Plagioclase microlite shapes include tabular, hopper, and swallowtail forms. Taken together, the relatively high plagioclase microlite number densities, the high number of oscillatory zones in plagioclase phenocrysts, the presence of CO2 in groundmass glass, seismicity, and time-varying tilt cycles provide a picture of sudden evacuation of magma residing at different levels in the shallow conduit. Explosive eruptions punctuate inter-eruptive repose periods marked by time-varying rates of degassing (volatile fluxing) and re-pressurization. Shallow residence time in the conduit was sufficient to allow precipitation of silica-phase in the groundmass, but insufficient to allow breakdown of hornblende phenocrysts, with the one exception of the final dome sample from 2000, which has the longest preceding repose time. These results support a model of cyclic pressure cycling, volatile exsolution and regassing, and magma decompression decoupled from ascent. [ABSTRACT FROM AUTHOR]

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 >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 (<~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. [ABSTRACT FROM AUTHOR]

Even though no basalts have erupted at Asama volcano, its large, felsic magma chamber standing beneath the summit is frequently replenished by cryptic injections of mafic magma of basalt‐basaltic andesite composition. This mafic magma is preserved within melt inclusions trapped in olivine phenocrysts collected from the old Itabana pumice of the Kurofu stage and the Tenmei pumice of the current Maekake stage. The trapped mafic melts provide a detailed and vivid record of cryptic mafic magma injection into the felsic chamber where they admix to erupt andesite magma. The main phenocrysts of plagioclase, orthopyroxene, and clinopyroxene, commonly seen in the erupted products, are derived from the felsic magmas and trapped felsic melts of dacite‐rhyolite compositions, containing low sulfur contents (0.3 wt% of SO3 or less; most are lower than 0.10 wt%). In contrast, the mafic melts, entrapped by olivine phenocrysts, show high concentrations of SO3, up to 1.06 wt% for Itabana and SO3 0.65 wt% for Tenmei. In addition, the olivine phenocryst commonly encloses early crystalline phases precipitated from the sulfur‐rich mafic magma, such as Cr‐spinel, Mg‐rich orthopyroxene, Fe, Cu, and Ni‐bearing sulfides, and often Al‐rich clinopyroxene. The olivine‐hosted mafic melt inclusions always contain numerous vesicles. Furthermore, they are often included as hourglass‐shaped trapped melts, exhibiting snapshots of intense foaming and gas phase exsolution during the olivine growth and probably in the process of the cryptic injection into the felsic magma beneath the Asama summit. Our new data contributes to a better understanding of the magmatic system of the Asama volcano but also highlight the importance of the "Excess sulfur problem", especially concerning the 1783 Tenmai eruption, which was contemporary to the Laki Fires. [ABSTRACT FROM AUTHOR]

The Kyrdem Granitoids (KG) pluton covers an area of about 230 sq. km. in the Ri-Bhoi district of Meghalaya. The KG (479±26 Ma) which intrudes the Shillong Group of metasediments represents one of the granite plutonism episodes of Meghalaya plateau. Mineralogically, KG can be grouped into coarse-grained porphyritic granite with grey and pink K-feldspar phenocrysts and aplite without phenocrysts. The aplite intrudes the porphyritic one as small pulses at several places. KG exhibits textural evidence for magma mixing and mingling during its evolution. Two types of enclaves are hosted in the KG viz. the country rock xenoliths and the microgranular enclaves (ME). Xenoliths which include amphibolites, granite gneiss, and quartzite of the Shillong Group are noticed in the porphyritic granite of KG. Microgranular enclaves (ME) hosted in KG are confined to the porphyritic granites and they are absent in the aplite. They are mesocratic to melanocratic, fine to medium grained and range in size from a few centimeters to a few meters. The ME shows different shapes ranging from subrounded, ovoid, and discoidal to ellipsoidal. The contacts between the host rock KG and the ME are sharp but occasionally diffused contacts are also noticed. In some cases, the ME shows finger-like projections with a convex face towards the host rock. Elongated, tabular, and sub-rounded K-feldspar megacrysts with a corroded margin are observed in the ME. These K-feldspar megacrysts commonlyresemble those that of the host rock. The above features suggest the origin of KG by magma mixing. Furthermore, textures including quartz ocellli, acicular apatite morphology, inclusion zones in feldspars and mafic clots also indicate that magma mingling and mixing plays a significant role in the evolution of KG. [ABSTRACT FROM AUTHOR]

Oxygen isotopes are useful for tracing interactions between magmas, crustal rocks and surface-derived waters. We use them here to consider links between voluminous silicic magmatism and large-scale hydrothermal circulation in New Zealand's central Taupō Volcanic Zone (TVZ). We present >350 measurements of plagioclase, quartz, hornblende and groundmass glass δ18O values from 40 eruptions from three discrete magmatic systems: Ōkataina and Taupō calderas, and the smaller Northeast Dome system. For each mineral, mean δ18O values vary by ~1% (δ18Oplag = +6.7-7.8%, δ18Oqtz = +7.7-+8.7%, δ18Ohbl = +5.4-+6.4%, δ18Oglass = +7.1-+8.0%), and inter-mineral fractionations mostly reflect high-temperature equilibria. Outliers (e.g., ~+6% or >+10% plagioclase) represent contaminants incorporated on short-enough timescales to preserve disequilibrium (~102 yrs for plagioclase). Melt δ18O values calculated from phenocrysts are ~+7.3-+8.0%. Where multiple magmas were involved in the same eruption their δ18Omelt values are indistinguishable, implying that their parental mushes were isotopically well-mixed. However, small (≤0.5%) but consistent δ18Omelt value gradients occur over millennial timescales at Ōkataina and Taupō, with short-term ~0.4-0.5% decreases in δ18Omelt values over successive post-caldera eruptions correlating with increases in 87Sr/86Sr. These changes reflect tens of percent assimilation of a mixture of hydrothermally altered silicic plutonic material and higher-87Sr/86Sr greywacke. These examples represent the first evidence for assimilation of altered crust into TVZ magmas. The subtle and short-lived isotopic signals of these interactions are only recognized through the high temporal resolution of the TVZ eruptive record and complementary radiogenic isotope data. Similar interactions may have been obscured in other nominally highor normal-δ18O magmatic systems. [ABSTRACT FROM AUTHOR]

The Hongniu-Hongshan porphyry-skarn deposit is located in the southern Yidun terrane in the Sanjiang Tethyan Metallogenic Domain (STMD). Although its metallogenesis has been well constrained in the past decade, the magmatic processes for granitoids in the Hongniu-Hongshan deposit are still poorly understood. Herein, we provide new geochemical data on magmatic minerals (plagioclase, amphibole, and clinopyroxene) in the Hongniu-Hongshan granitoids to get a better insight into these processes. The complex zoning patterns of plagioclase phenocrysts indicate magma recharge and mixing. Physiochemical estimations indicate that clinopyroxenes were crystallized in hotter (919 ± 11 °C) and more mafic (FeO: 2.8–4.6 wt.%, MgO: 0.8–1.8 wt.%) magmas in a deep chamber (18.6 ± 0.9 km) compared with the colder (819 ± 29 °C), more felsic (FeO: 0.9–2.2 wt.%, MgO: 0.3–0.6 wt.%) and shallow magma chamber (13.4 ± 1.6 km) in which amphiboles crystallized. Therefore, we suggest that magmatic minerals in the Hongniu-Hongshan granitoids were produced by multistage magmatic processes within the upper–middle crust range. In this model, the deep-seated magmas recharged into the shallow reservoir and mixed with the shallow magmas therein. The recharged hot magmas may provide heat sources and rejuvenate the shallow magma reservoirs. On this basis, we further infer that ore-forming materials could be pre-concentrated in the crustal range and mobilized by the Late Cretaceous magmatism in the southern Yidun terrane. [ABSTRACT FROM AUTHOR]

Alkaline magmatic systems formed in intraplate settings have a low Au endowment, contrasting with the high Au fertility of alkaline magmatic systems in arc and rift settings, which host numerous Au-rich porphyries and Cu-Au and Au-Ag epithermal deposits. Among other factors, the Au fertility of a magmatic system is determined by the Au concentration of the ore-forming magma, which is strongly controlled by the amount and chemistry of magmatic sulfides that fractionated during magma differentiation. To better explore the factors controlling the low Au endowment of alkaline magmatic systems formed in intraplate settings, we studied magmatic sulfide and silicate melt inclusions hosted in various phenocrysts from the Mont-Dore Massif (France). The magmatic system was saturated with Cu-poor, Au-rich sulfide liquid during its differentiation from basanite to trachyandesite, leading to a strong depletion of both Cu and Au in the residual melt. This presumably reduced the capacity of the magmatic system to form Au-rich magmatic-hydrothermal deposits. Such evolution contrasts with previous studies conducted in sulfide-saturated magmatic systems formed in arc settings, where an Au-poor monosulfide solid solution was the main sulfide phase to crystallize, only marginally affecting the Au budget of those systems. We conclude that the observed saturation of Au-rich sulfide liquid during evolution of the studied volcanic products could explain the low Au endowment of intraplate alkaline magmatic systems. [ABSTRACT FROM AUTHOR]

The sub-continental lithosphere mantle (SCLM) represents a mechanical barrier that separate the converting mantle from the exosphere, and possesses the ability to sequester and release volatiles, making it an integral part in deep volatile cycling. Here we explore the SCLM in modulating F and Cl cycling through a case study from the Early Cretaceous Dabeigou basalts in the North China Craton (NCC). These lavas are associated with decratonization of the NCC, a process involving significant removal and replacement of the ancient SCLM. The Dabeigou basalts, characterized by arc magma-like trace element signatures, evolved initial 87Sr/86Sr (0.70574–0.70578), εNd(t) (−11.2–−11.1), and εHf(t) (−13.7–−13.4) values, along with high Fe/Mn ratios, Ni contents, and δ18O values (5.8‰–6.8ε) of olivine phenocrysts, are compatible with their origination from a veined, pyroxene-rich source formed through the infiltration and interaction of hydrous silicate melts from the subducting Pacific slabs with ambient SCLM. Olivine-hosted melt inclusions of the Dabeigou basalts have compositions similar to the whole rocks, indicating that they record pre-eruptive melt compositions forming the whole rocks. The melt inclusions exhibit high F (1371–2316 ppm) and moderate Cl (688–900 ppm) contents, with high F/Cl ratios of 2.1±0.3 (1σ). The F and Cl signatures of the Dabeigou basalts are different from those of typical arc magmas, which are commonly characterized by high Cl contents (median of ∼1200 ppm) and low F/Cl ratios (<1). The observed differences can be attributed to chromatographic reaction that fractionates elements of different compatibility in the metasomatic agents, with amphibole as the main mineral phase for fractionating and preserving F and Cl. That is, amphibole crystallization prefers to retain F rather than Cl, making the metasomes in vein enriched in F and exhibiting elevated F/Cl ratios compared to original metasomatic agents. It is further suggested that the scenario inferred from the Dabeigou basalts might be common for crystallization of hydrous minerals during chromatographic metasomatism, which causes significant heterogeneity with respect to F and Cl in the SCLM, and might have played a crucial role in regulating F and Cl cycling within the SCLM environments. [ABSTRACT FROM AUTHOR]

The Rio Jacaré Batholith (RJB; 617 ± 4 Ma) is inserted in the Poço Redondo Domain, Sergipano Orogenic System. This batholith is formed by monzodiorite, quartz monzodiorite, monzonite, and quartz monzonite, with abundant microgranular enclaves (MEs). The MEs vary from black to light gray and exhibit globular to slightly elongated shapes with clear-cut, crenulated, and cuspate, or, more rarely, diffuse contacts. They correspond to diorites, monzodiorites, quartz monzodiorites, and monzonites, and textural features indicate mixing of magmas, such as compositional zoning in plagioclase, inclusion zones in plagioclase phenocrysts, poikilitic alkali feldspar, acicular apatite, and ocellar quartz. Calculations of linear correlations of major elements showed that the smallest fraction of mafic magma involved in the mixing was 0.43. MEs represent the breakdown and cooling of a mafic magma that was injected into a cooler felsic magmatic chamber. Emplacement of this mafic magma occurred at different stages of crystallization of the RJB magmatic chamber. The MEs are magnesian and metaluminous, with affinity to the shoshonitic series. Ratios for Ba/Nb (> 23), Ba/La (> 15), and Nb/La (0.22--0.69) are characteristic of magmas generated from partial melting of an enriched lithospheric mantle source. Batch melting modeling suggests that source melting rates of less than 3% are necessary to generate magmas similar to those of the RJB MEs. [ABSTRACT FROM AUTHOR]

The Higashi–Izu Monogenetic Volcano Field (HIMVF) in the northern Izu–Bonin volcanic arc, Japan consists mainly of mafic with minor intermediate and felsic monogenetic volcanoes. The Iwanoyama–Iyuzan (Ioyama) volcanic chain, which formed approximately 2.7 ka, is an unique example of bimodal volcanism involving intermediate (andesite) and felsic (dacite and rhyolite) rocks. Both rock types in the chain are characterized by disequilibrium assemblages and bimodal chemistry of phenocrysts. These results and the mineral chemistry of three types of inclusion (from cumulate–crystal-mush-like rocks to mafic rocks) in the felsic rocks suggest that parental mafic and felsic magmas underwent magma mixing and/or mingling. Whole-rock major- and trace-element (including rare-earth-element) data are consistent with magma mixing process. The 87Sr/86Sr ratios of the rocks have a restricted range (0.70329–0.70349) but differ slightly among different volcanoes. The parental mafic magmas are inferred to have been basaltic to basaltic andesite in composition and were slightly modified by fractional crystallization, or assimilation and fractional crystallization process with lower–upper crustal rocks, whereas the parental felsic magmas (rhyolitic in composition) could have been generated by partial melting of upper crustal rocks. The mafic and felsic magma types mixed and/or mingled to some degree, and the resultant magmas were erupted independently to form different volcanoes with a linear distribution. The magma supply structure of the studied volcanic chain, which shows similarity to that of polygenetic volcanic systems, formed in the central part of the HIMVF, which has an outer zone dominated by basaltic volcanism. [ABSTRACT FROM AUTHOR]

We explore crystal growth and magma recharge during the formation of intermediate lavas using bulk rock compositions and zoning patterns and textural variation in plagioclase feldspars from Hasandağ volcano in south-central Turkey. Hasandağ intermediate lavas formed primarily through fractionation of the observed mineral phases, and also show abundant evidence for magma mixing and thermochemical disequilibrium. Sparse basaltic andesites originated through mixing of observed mafic and felsic lavas rather than fractionation. Plagioclase phenocrysts from all lava types have uniform core (An~39) and variable rim (An39–64) compositions, suggesting plagioclase feldspars from a common magma reservoir are present in all erupted lavas. An-rich crystal rims are regularly enriched in Mg, Fe and Ti, indicating mafic recharge and magma mixing. Mixing timescales determined by diffusion modeling of Mg profiles across core-rim boundaries suggest recharge to eruption occurs on the order of up to ~ 45 days. Thermobarometric calculations constrain shallow crustal storage of dacite to 1.5–2 kbar (~ 5 km) and 800–890 °C, with deeper reservoirs for more mafic magmas (up to 35–40 km for basalts). Abundant disequilibrium textures (sieve-textured zones, oscillatory zoning, resorbed and patchy-zoned cores) indicate mobilization of a homogeneous crystal-rich dacitic reservoir by injection of mafic magma. We suggest that incomplete and dynamic physical mixing at shallow crustal levels results in distinct crystal morphologies, some of which record punctuated ascent and storage, while others are erupted rapidly after the influx of new magma. [ABSTRACT FROM AUTHOR]

The geochemistry of lavas combined with textural constraints can provide important information on magmatic processes occurring at inaccessible depths. Here we carry out a detailed textural and geochemical study on 0.23 Ma contemporaneous and adjacent Qiulin, Longbao, and Dayukou volcanoes in the intraplate Quaternary Datong volcanic field (DVF) in the North China Craton. Lavas from the three tholeiitic volcanoes have similar major and trace element compositions with their trace element patterns resembling enriched mid-ocean ridge basalts (E-MORB). However, distinct textures are observed between the samples from the three volcanoes. Samples from Qiulin are generally poorly-phyric (<10 vol. %) containing orthopyroxene, plagioclase, and olivine phenocrysts and monomineralic orthopyroxene glomerocrysts set in a groundmass of plagioclase, clinopyroxene, orthopyroxene, and Fe–Ti oxides. In contrast, samples from both Longbao and Dayukou are highly phyric (up to ~50 vol. %) and consist mainly of plagioclase, orthopyroxene, clinopyroxene, and olivine phenocrysts with mm- to cm-sized polymineralic glomerocrysts enclosed in a groundmass comprising plagioclase, clinopyroxene, olivine, and Fe–Ti oxides. Crystal types and proportions vary significantly between the glomerocrysts of Longbao and Dayukou even on a thin section scale. The phenocrysts and glomerocrysts at the two volcanoes commonly show resorbed and embayed margins and sieve textures. Moreover, pyroxenes present as phenocrysts and glomerocrysts are always characterised by corona textures containing orthopyroxene cores mantled by clinopyroxene. Notably, orthopyroxene is never observed in contact with groundmass. The relatively low Mg# (75–85) and Ca content in the cores indicate that the orthopyroxenes from all three volcanoes are crystallised early from the magma rather than being mantle xenocrysts. Thermobarometry and kernel density estimation indicate that orthopyroxenes in all three volcanoes initially crystallised at ~7 kbar (23 km depth), whereas clinopyroxenes crystallised at ~4 kbar (13 km) and the final crystallisation pressure is ~3 kbar (10 km). Thermodynamic modelling by COMAGMAT suggests early near-liquidus crystallisation of the orthopyroxene at high pressures, followed by partial dissolution likely associated with decompression. The relatively larger proportions of phenocrysts compared with glomerocrysts in Qiulin samples implies that the magma reservoir was only partially turned into mushes at the periphery. Magma ascending via the melt-rich interior of the magma reservoir formed the crystal-poor Qiulin lava. In contrast, those ascending through the mushes at the peripheries of the deeper reservoir and discrete shallower storage regions formed the highly phyric Longbao and Dayukou lavas. The pervasive occurrence of sieve textures and complex mineral composition profiles in both phenocrysts and glomerocrysts in Longbao and Dayukou samples belie complicated open system crystallisation histories of these magmas. We conclude that open system processes involving mush disaggregation and magma recharge in multi-level plumbing systems played an important role in the petrogenesis of these intracontinental tholeiites. Moreover, the important role of orthopyroxene in the petrogenesis of these tholeiites distinguishes them from many other tholeiite suites like MORB, as well as those erupted through thickened oceanic crust like in Iceland and the Shatsky Rise. This study highlights the importance of combining textural constraints with geochemical analyses. [ABSTRACT FROM AUTHOR]

Characterising equilibrium and disequilibrium crystal-melt processes is critical in determining the extent of magma mixing and crystallization conditions in the roots of volcanoes. However, these processes remain poorly investigated in most Pacific intraplate ocean settings that are difficult to access and study. To help address this issue, we investigated crystallization conditions of clinopyroxene phenocrysts in an accreted Palaeogene oceanic island in Panama. Petrographic and geochemical observations, petrological modelling of major and trace elements, and liquid-mineral multicomponent equilibrium tests were carried out using basalts, picrites, and hawaiites of the transitional tholeiitic shield to alkaline post-shield volcanic stages of the island. Five types of clinopyroxene crystals were identified, including (1) microphenocrysts with micron-scale oscillatory zoning, (2) primitive, yet resorbed picrite-hosted phenocrysts, (3) chemically homogeneous, anhedral crystals found in the remaining basalts, (4) Ti–rich euhedral hawaiite-hosted phenocrysts, and (5) evolved sector-zoned phenocrysts. Liquid-clinopyroxene multicomponent equilibrium tests in combination with textural analysis show that ~ 74% of the studied clinopyroxenes are in possible major element equilibrium with one of the available whole rock magma compositions, of which only 21% are equilibrated with their carrier liquid. To deconvolute clinopyroxene-melt pairings and determine plumbing system conditions, we combine rhyolite-MELTS modelling, geothermobarometry, and major- and trace-element equilibrium evaluations, limiting crystallization conditions to crustal levels (< 23 km depth). No migration of magmatic reservoirs to deeper levels is observed during the shield- to post-shield transition. These results suggest the occurrence of an extensive crystal mush system during the late shield to post-shield volcanic stages of this intraplate volcanic system, with both primitive and evolved crystallization domains sampled during eruptions. [ABSTRACT FROM AUTHOR]

Petrographic and chemical analysis of scoria samples collected during the 1959 Kīlauea summit eruption illustrates the progress of thermal and chemical homogenization of the melts, and the gradual growth and/or re-equilibration of olivine phenocrysts, over the course of the eruption. Glass compositions show that thermal equilibration was largely complete within the span of the eruption, whereas chemical homogenization was a work in progress. The olivine phenocryst population, known to contain conspicuous antecrystic components, is also hybrid within the euhedral population. The bulk of the olivine reached the level of the erupting magma on November 18–19, 1959. Zoning patterns in olivine phenocrysts show that initially unzoned grains developed normal zoning by the end of the eruption. Reverse zoning in relatively Fe-rich olivine phenocrysts (interpreted as cognate to the stored magma) was progressively eliminated from November 21 to December 19, 1959, by diffusive re-equilibration between crystals and melt. Toward the end of the eruption, the only olivine composition in direct contact with the melt was Fo84–86, with the original rim compositional heterogeneity gone in 4–5 weeks' time. Activity in December 1959 differed from that in November, as high fountaining events were more closely spaced and almost all samples were picritic, with bulk MgO ≥16·5 wt%. Three different levels were in play during the 1959 eruption: a deep source for high-MgO melts and forsteritic (Fo87–89) olivines, an intermediate source for the bulk of the stored magma, and a shallower source for the most differentiated magma. This model is consistent with geophysical, petrological and chemical observations. Comparison of the 1959 eruption with results from older explosive deposits suggests that stored and recharge melts and olivine from the deeper parts of Kīlauea's plumbing are similar in composition to those observed or inferred in the 1959 eruption, so they behave similarly during extrusive and explosive periods alike. [ABSTRACT FROM AUTHOR]

Primitive arc magmas provide our closest glimpse of the original mantle-derived magmas that produce the more ubiquitous andesites and dacites found in subduction zones and that ultimately construct Earth's continental crust. This study examines the crustal storage and ascent history of the Mt. Shasta primitive magnesian andesite (PMA), a demonstrated parent magma for the voluminous mixed andesites erupted at Mt. Shasta. Our petrographic and geochemical observations of the PMA identify a mid-crustal magma mixing event recorded in multiple populations of reversely zoned clinopyroxene and orthopyroxene phenocrysts. Thermobarometric calculations conducted as part of this study and prior phase equilibrium experiments (Grove et al., Contrib Miner Petrol 145:515–533, 2003; Krawczynski et al., Contrib Miner Petrol 164:317–339, 2012) suggest the PMA experienced storage, mixing, and subsequent crystallization at ~ 500 MPa and ~ 975 °C. Modeling of Fe–Mg interdiffusion between the rims and cores of the reversely zoned pyroxenes suggests this mixing event and the resulting crystal rim growth occurred less than 10 years prior to eruption ( 2.9 - 2.2 + 6.4 ). Ascent from 500 MPa (~ 15 km) during the calculated diffusion timescales suggests minimum crustal transit rates of ~ 170 MPa (~ 5 km)/year and cooling rates of ~ 5–7 °C/km, consistent with conductive cooling models. This ascent rate is slower than the handful of previously documented trans-crustal magmatic ascent rates and significantly slower than syn-eruptive decompression rates. If this behavior is representative, ~ the 10% mafic magmas erupted as part of the modern Mt. Shasta edifice fluxed through the crust within decades. Coupled with a review of the U–Th–Ra residence times for Shasta andesites to dacites, we suggest that crustal magma flux and assembly beneath modern Mt. Shasta occurred in discrete pulses that occupy a minority of the 700 k.y. period of edifice construction. The results of this study thus constrain the pre-eruptive history and ascent characteristics of a hydrous primitive arc magmas in the upper crust between their shallowest storage region in the mid-crust and volatile exsolution and provide constraints on crustal magma flux beneath continental arc volcanoes. Should future earthquake swarms indicative of magma movement in the middle to upper crust occur beneath Shasta, the results presented here also provide the first estimates of the possible magma ascent rates and the time intervals that could accompany related magma ascent to eruption at Mt. Shasta. [ABSTRACT FROM AUTHOR]

Oxygen isotopes are used to examine the well-documented magmatic evolution of alkaline volcanic rocks at The Pleiades in northern Victoria Land (NVL), Antarctica. Oxygen isotopes were measured in olivine, clinopyroxene, and plagioclase phenocrysts to better understand the origin and evolution of the sodic and potassic differentiation lineages. The volcanic rocks at The Pleiades evolved from a parental basanite by fractional crystallization in crustal magma chambers. Olivine which crystallizes first in the mafic magmas provides the initial oxygen isotope composition of the magmatic lineages. The δ18OOL values for the mafic lavas in the sodic lineage are lower than the potassic lineage. The primary melt derived from the metasomatized lithospheric mantle may have consumed a low-δ18O amphibole metasome. Subsequently, the melt would have evolved to the normal-δ18O potassic lineage magma by a large contribution from surrounding peridotite. In contrast, the sodic lineage magma might preserve low-δ18O characteristics because of insufficient reaction with the surrounding mantle peridotite. Intermediate rocks of the potassic lineage exhibit a wide variation in their oxygen isotope compositions and it deviates from the theoretical normal-δ18O trend. Hence, variable δ18OOL values of the intermediate rocks suggest that high-δ18O recorded in olivine could be reconciled with an assimilation of crustal rocks in NVL, and hydrothermally altered material may have contributed to low-δ18O signature of the olivines. [ABSTRACT FROM AUTHOR]