Dacitic Melts Research Articles

Chaotic Mixing Experiments: Unravelling Contamination Processes on Dacitic Melts

Magmas are the ultimate source of volcanism on Earth. At high-temperatures these complex silicate melts may easily chaotically mix, on their way towards the surface of the planet.The Paraná-Etendeka Magmatic Province (PEMP), one of the largest volcanic provinces of our planet, serves as the focus of this experimental study. The investigation simulates the contamination of high-Ti dacites passing through the continental crust. Our final aim is to contribute to the origin of the high-Ti Chapecó dacites from PEMP, with particular attention to the genesis of Chapecó-Ourinhos dacites (COD).The experimental apparatus is based on the Journal Bearing System. All experimental runs have been operated at atmospheric pressure and a temperature of 1500 °C. A dacitic magma chamber under the assimilation of crustal material has been simulated. The resulting experimental products (dry/bubble free glasses) exhibit concentric folded and stretched filaments of contrasting compositions. The calculated fractal dimension for a representative section is Dbox = 1.60(3), confirming the achievement of chaotic dynamics. Electron Microprobe Analysis (EMPA) from the experimental glasses clearly shows SiO2-contamination, while Al2O3-data indicate lower contamination levels. As a consequence, different elements depict dissimilar curves in chemical transects. We used the parameter σn2 (normalized variance) to quantify the mixing efficiency and differential elemental mobilities. In addition, our data reproduced the chemical behaviour of some elements in COD- dacites, especially the network formers such as Fe, Al, Ti and Si. EMPA reveals clear deviations from the linear mixing model. Our experiments and data replicate a selective contamination process. Differential elemental mobilities could be calculated. Comparisons with natural data depict how the mixing involving Chapecó-Guarapuava (CGD) and Chapecó-Ourinhos (COD) dacitic end-members may have contributed to the generation of more evolved COD from the interaction of CGD pre-existing melts with crustal rocks from the PEMP basement via chaotic dynamics.

Paleoproterozoic Dacite Dikes of the Vorontsovka Terrane, Volga–Don Orogen: Geochemistry, Age, and Petrogenesis

Metamorphosed dacitic porphyry dikes were first found in the western part of the Vorontsovka terrane, which is located in the Paleoproterozoic Volga–Don orogen at the margin of Archean Sarmatia and Volga–Ural cratons. The magmatic protolith age for the metadacites is ca. 2.07 Ga. These are ferrous, metaluminous calc-alkali I-type granitoids. The sodium specialization of the rocks and their low concentrations of Mg, Cr, Ni, and incompatible elements, with significant REE fractionation, the absence of Eu* anomalies, high Sr/Y ratio, remarkably high (Gd/Yb)n values (>10), and the radiogenic Nd isotopic composition indicate that the dacitic melts were derived from a juvenile mafic source. According to petrogenetic estimations, such conditions could be caused by the partial melting of depleted N-MORB basites in equilibrium with an eclogitic residue. The dacitic magmas were likely generated by the partial melting of mafic rocks at lower levels of the significantly thickened crust (>60 km) in relation to collision processes.

Confocal μ-XANES as a tool to analyze Fe oxidation state in heterogeneous samples: the case of melt inclusions in olivine from the Hekla volcano

Abstract. Here we present a confocal Fe K-edge μ-XANES method (where XANES stands for X-ray absorption near-edge spectroscopy) for the analysis of Fe oxidation state in heterogeneous and one-side-polished samples. The new technique allows for an analysis of small volumes with high spatial 3D resolution of <100 µm3. The probed volume is restricted to that just beneath the surface of the exposed object. This protocol avoids contamination of the signal by the host material and minimizes self-absorption effects. This technique has been tested on a set of experimental glasses with a wide range of Fe3+ / ΣFe ratios. The method was applied to the analysis of natural melt inclusions trapped in forsteritic to fayalitic olivine crystals of the Hekla volcano, Iceland. Our measurements reveal changes in Fe3+ / ΣFe from 0.17 in basaltic up to 0.45 in dacitic melts, whereas the magnetite–ilmenite equilibrium shows redox conditions with Fe3+ / ΣFe ≤0.20 (close to FMQ, fayalite–magnetite–quartz redox equilibrium) along the entire range of Hekla melt compositions. This discrepancy indicates that the oxidized nature of glasses in the melt inclusions could be related to the post-entrapment process of diffusive hydrogen loss from inclusions and associated oxidation of Fe in the melt. The Fe3+ / ΣFe ratio in silicic melts is particularly susceptible to this process due to their low FeO content, and it should be critically evaluated before petrological interpretation.

Water release from dacitic melts near the glass transition and implications for volcanic eruptions

To get insights in the mechanisms of magma degassing and fragmentation, we performed dehydration experiments at ambient pressure with dacitic glasses containing between 1.42 and 5.28 wt% H2O. The focus was on the degassing behavior in the range of glass transition, i.e., from temperatures 50 K below to 50 K above the glass transition temperature Tg. At each T the duration of the experiments was varied between 5 and 125 h. Water contents and initial water speciation was determined by FTIR spectroscopy. Diffusion depth profiles were measured by Raman spectroscopy. We developed an equation to calculate the total water content along the depth profiles from the ratio of the fundamental OH stretching vibration band at 3550 cm−1 and the T-O-T bending band near 500 cm−1. Glasses with high water content (4.66–5.28 wt%) showed huge alterations, i.e., crack formation below Tg and crystallization at and above Tg, but glasses with water contents up to 2.5 wt% were visually unchanged and suitable for water diffusion analysis.The measured profiles of total water content vs. distance can be fitted well by an error function, assuming that the water content in the glass does not decrease to zero at the surface. However, the apparent water concentration at the surface and the total water diffusivity derived from the error function fitting decrease with increasing duration of the experiment. This clearly shows that degassing of dacitic melts is not a simple out-diffusion mechanism. The model of Coumans et al. (2020) that explicitly considers the kinetics of the interconversion of water species can reproduce the temporal evolution of the total water concentration profiles very well. Our results show the importance of considering explicitly the interconversion kinetics of water species when modelling water diffusion in melts at high viscosities. Due to the slow interconversion of OH groups to much more mobile water molecules, dehydration is much slower in the range of glass transition than predicted by diffusion models based on experiments performed on stable melts at magmatic temperatures.

Sulfur speciation in dacitic melts using X-ray absorption near-edge structure spectroscopy of the S K-edge (S-XANES): Consideration of radiation-induced changes and the implications for sulfur in natural arc systems

Abstract The synchrotron technique of micro X-ray absorption near-edge structure spectroscopy at the sulfur K-edge (S-XANES) provides a unique opportunity to measure the proportion of different oxidation states of sulfur (S) in silicate glasses. Although applied extensively in the analysis of basaltic silicate glasses, few S-XANES studies have investigated variations in S oxidation states with fO2 in felsic silicate glasses. In addition, no study has systematically compared the S-XANES results obtained from the same samples at different photon flux densities to quantify the relationship between exposure time and changes in S speciation in silicate glass, as has been done for Fe and V. This study evaluates observed differences in S speciation measured in experimentally produced H2O-saturated dacitic glasses over a range of reducing to oxidizing conditions (from log fO2 = ΔFMQ-0.7 to ΔFMQ+3.3; FMQ is the fayalite-magnetite-quartz mineral redox buffer) and equilibrated at 1000°C and 300 MPa. S-XANES spectra were collected at three different photon flux densities using three microspectroscopy beamlines. As is observed in S-XANES analyses of basaltic silicate glasses, beam-induced changes to the S6+/ΣS are observed as a function of photon flux density and beam exposure time. Our results demonstrate that silicate glasses of dacitic composition undergo beam-induced photo-reduction in samples equilibrated at ΔFMQ > +1.75 and photo-oxidation if equilibrated at ΔFMQ < +1. The time required to observe beam-induced changes in the spectra varies as a function of flux density, and our study establishes an upper photon density limit at ~1.0 × 1012 photons/μm2. The S6+/ΣS calculated from spectra collected below this absorbed photon limit at intermediate flux densities (~1−4 × 109 photons/sec per μm2) are affected by beam damage, as no conditions were found to be completely free of beam-induced changes. However, the S6+/ΣS ratios calculated below the limit at intermediate flux densities are consistent with thermodynamic constraints, demonstrating that S6+/ΣS ratios calculated from S-XANES spectra can be considered reliable for estimating the oxygen fugacity. Our results carry important implications for the S budget of felsic magmas and dissolution mechanisms in evolved melts. While our results from all three flux densities show the presence of S4+ dissolved in relatively oxidized (ΔFMQ > +1.75) dacitic glass, even in the spectra exposed to the lowest photon densities, we are unable to rule out the possibility that the S4+ signal is the result of instantaneous X-ray irradiation induced beam damage using S-XANES alone. When our spectra are compared to S-XANES spectra from basaltic silicate glasses, important differences exist in the solubility of S2- and S6+ between dacitic silicate melts, pointing to differences in solubility mechanisms as melt composition changes. This study highlights the need for further investigation into beam damage systematics, presence of S4+, and the solubility mechanisms of different oxidation states of S as silicate melt composition changes.

Electrical conductivity of anhydrous and hydrous gabbroic melt under high temperature and high pressure: implications for the high-conductivity anomalies in the mid-ocean ridge region

Abstract. The electrical conductivity of gabbroic melt with four different water contents (i.e., 0 %, 2.59 wt %, 5.92 wt %, and 8.32 wt %) was measured at temperatures of 873–1373 K and pressures of 1.0–3.0 GPa using a YJ-3000t multi-anvil high-pressure apparatus and Solartron-1260 impedance spectroscopy analyzer. At a fixed water content of 2.59 wt %, the electrical conductivity of the sample slightly decreased with increasing pressure in the temperature range of 873–1373 K, and its corresponding activation energy and activation volume were determined as 0.87 ± 0.04 eV and −1.98 ± 0.02 cm3 molec.−1, respectively. Under the certain conditions of 873–1373 K and 1.0 GPa, the electrical conductivity of the gabbroic melts tends to gradually increase with a rise in water content from 0 wt % to 8.32 wt %, and the activation enthalpy decreases from 0.93 to 0.63 eV accordingly. Furthermore, functional relation models for the electrical conductivity of gabbroic melts with variations of temperature, pressure, and water content were constructed at high-temperature and high-pressure conditions. In addition, the dependence relation of the electrical conductivity of melts with the degree of depolymerization was explored under conditions of four different water contents at 1373 K and 1.0 GPa, and three previously available reported results on those of representative calc-alkaline igneous rock melts (i.e., dacitic melt, basaltic melt, and andesitic melt) were compared in detail. In combination with our presently acquired electrical conductivity data on gabbroic melt with four different water contents and the available data on polycrystalline olivine, the electrical conductivity of a gabbroic melt–olivine system with variation of the volume percentage of anhydrous and hydrous melts was successfully constructed by using the typical Hashin–Shtrikman upper-bound model. In light of the electrical conductivity of the gabbroic melt–olivine system with previous magnetotelluric (MT) results, we find that anhydrous and hydrous gabbroic melts can be employed to reasonably interpret the high-conductivity anomalies in the Mohns Ridge of the Arctic Ocean.

Petrology of the 2020–21 effusive to explosive eruption of La Soufrière Volcano, St Vincent: insights into plumbing system architecture and magma assembly mechanism

Abstract The 2020–21 eruption of La Soufrière, St Vincent began with extrusion of a viscous lava dome, which was destroyed upon transition to a major explosive phase. Here we present petrological data to reconstruct the processes leading up to these events. Bulk-rock SiO 2 contents range from 52.8 to 55.4 wt%, classifying the lava and the subsequent scoria as basaltic andesite, the latter being slightly more mafic. Macrocrystal chemistry and modes (plag–cpx–opx–tmt–ol) and crystallinity (45–50 vol%) are largely identical for both phases of the eruption. Pyroxenes are homogenous and precipitated mostly from andesitic melts. Conversely, plagioclase shows strong normal zonation resulting from magma ascent and stalling at multiple crustal levels. Clinopyroxene thermobarometry reveals that crystallization predominantly took place between 8 and 13 km depth at temperatures of 997 − 35 + 18 ∘ C . A lack of evidence for mafic recharge and changes in volatile content and the omnipresence of xenoliths, suggests pre-eruptive destabilization of an andesitic–dacitic melt pocket that disrupted and entrained antecedent mush. Olivine diffusion profiles show that this interaction preceded the onset of eruption. Low dissolved sulfur contents (≤270 ppm S) place constraints on the total SO 2 gas release. Melt–mush disruption appears to be a dominant driver of eruptions at La Soufrière.

Investigation of the Mid-Age (65–34 ka) Period of Taranaki Volcano, New Zealand: Indications for the Effect of Volcano Growth

ABSTRACT Many stratovolcanoes are characterised by cycles of edifice growth interrupted by collapse events. The long-term record of the evolution of such magmatic systems is mainly preserved in the deposits of the volcanic apron surrounding the active cone. Taranaki Volcano in New Zealand provides an unusually detailed example of these processes due to excellent coastal ring-plain and young cone exposures. In this study, we investigate the magmatic system of this volcano through three consecutive growth phases by sampling a detailed, stratigraphically controlled selection of volcanic clasts from volcaniclastic mass-flow deposits in the medial ring-plain. The clasts from three growth phases (GP1, 65–55 ka; GP2, 55–40 ka; GP3, 40–34 ka) differ in bulk composition and form geochemically distinct trends on variation diagrams. These trends can be modelled by mainly dacitic melt mixing with gabbroic and ultramafic xenolith compositions representing the plutonic assemblages beneath the edifice. Within short-term growth cycles (104 years), the geochemical differences between lower and upper sequences of GP units indicate that closer to an edifice collapse, both whole-rock major and trace element compositions display more evolved and scattered trends compared to post-collapse stages. Considering the long-term magmatic evolution of Taranaki Volcano, it is apparent that the pre-collapse compositions are more evolved than bulk rock compositions of the growth phases, indicating active upper-crustal reservoir conditions in pre-collapse states. Furthermore, the volume losses caused by sector collapses prior to GP2 and GP3 could decrease the pressure in the upper-crustal reservoir. Overall, the data obtained from the mid-age Taranaki volcanic system elucidate the mid- to upper-crustal magmatic processes and reservoir conditions throughout growth cycles. Further, it demonstrates the top-down control of volcanic edifice load change on the magmatic plumbing system expressed by the evolvement of whole-rock compositions towards the end of a growth cycle.

Contamination of basalt through silicic melts: The first chaotic dynamics experiments with Paraná-Etendeka starting materials

This work is the first attempt to simulate experimentally the impact of underplating high-Ti basaltic melt from Paraná-Etendeka Magmatic Province (PEMP) on a pre-existing continental crust. Our final aim is to unravel the origin of high volumes of high-Ti Chapecó dacitic melts. A campaign of three new chaotic mixing experiments was performed starting from a natural basaltic melt composition (i.e., high-Ti Pitanga-type) while solely varying the chemical composition of rhyolitic end-member (i.e., the contaminant). The choice of end-members was based on the major oxide composition and isotopic signatures. After experiments, chemical analyses were performed by electron microprobe microscopy along contact zones between the interconnected end-members on the experimental charges produced.The results confirm and quantify elemental homogenization in the melt, which occurs by diffusion. The degree of homogenization varies according to the element, and the differential mobility in the mixing system culminates in non-linear correlation in inter-elemental plots. The mobility of major/minor elements along contact areas was quantified calculating the normalized variance, which consistently indicates the connection between viscosity and mobility. Orbicular structures containing dendritic crystals and remnant portions of glass in the basaltic area were described in detailed for the first time. The observed dendritic areas of the basaltic regions point towards an early crystallization process during the initial quench phase of the experiment, and that the crystallization process does not happen homogeneously.Chaotic mixing experiments confirmed that mixing between melts with vastly different viscosities is physically possible. Chaotic dynamics governs this process initially producing vortex structures, and stretched and folded filaments within regions that appear to correspond to remnants of the unmixed end-members. The created filaments become systematically thinner as continuously stretched and folded process develops. It exponentially increases the contact surfaces and, therefore, the mixing efficiency. In respect to the formation of the PEMP high-Ti Chapecó dacites, our results lead us to infer that basalts commonly melt crustal rocks, partially or totally, on their way to the surface. Initial calculations using the linear mixing model are in disagreement with our experimental results regarding the best contaminant candidate, which reflects the very complex simulated scenario. Nevertheless, the reproduced features such as the variability of basaltic compositions and chemical similarities with PEMP intermediary rare outcrops point towards short interaction times, low convective forces, and a predominance of density driven separation of contrasting melts (i.e., simulated conditions) as possible mechanisms involved in the genesis of Chapecó dacites.

Sulfur dissolution capacity of highly hydrated and fluid-saturated dacitic magmas at the lower crust and implications for porphyry deposit formation

Arc basalts with high H2O content are expected to evolve into highly hydrated felsic melts saturated with aqueous fluids at lower crustal pressures, which are potentially important in providing sulfur (S) and metals for porphyry deposit formation. However, the role of such highly hydrated and fluid-saturated magmas in the transfer of S (and metals) remains rarely studied. To investigate the S dissolution capacity of highly hydrated felsic magmas saturated with fluids at the lower crustal conditions, this study performed experiments with dacite + 10 wt% FeS at H2O contents of 10, 15, and 20 wt%. All the experiments were performed in piston-cylinder apparatus at 1.0 GPa and 950 °C with fO2 conditions imposed by the following solid-state buffers: cobalt-cobalt oxide (CCO), nickel-nickel oxide (NNO), rhenium-rhenium oxide (RRO), MnO-Mn3O4 (MMO), and hematite-magnetite (HM), resulting in a fO2 range of FMQ−1.5 to FMQ+3.3 log units (FMQ = fayalite + magnetite + quartz oxgen buffer). The experimental results show that the H2O content in the felsic melt, in addition to fO2 and melt composition, has a strong effect on the concentration of S in the silicate melt at sulfide ± anhydrite saturation. At a given fO2, the S concentration in the melt increases with increasing H2O content (hydration), resulting in a S concentration of up to ∼8000 ppm in the highly hydrated dacitic melts at fO2 ∼ FMQ+2. We found that high melt hydration at an intermediate fO2 not only dramatically increases sulfide and anhydrite solubilities in the dacitic melts but also promotes the saturation of S-rich fluids. The S concentration in the fluid that corresponds to the highest measured S concentrations in the melt (∼8000 ppm) are ∼5 wt%. These results show that highly hydrated felsic magmas saturated with fluids have a high capacity for S dissolution. Therefore, injection of such fluid-saturated magmas from lower crustal to shallow magma reservoirs is a potential mechanism for the effective transport of S (and perhaps metals) to facilitate subsequent porphyry ore deposit formation.

Sulfide and sulfate saturation of dacitic melts as a function of oxygen fugacity

Sulfur is a key element in terrestrial magmatic processes yet its geochemical behavior remains one of the most difficult to model due to its heterovalent chemistry. The maximum amount of sulfur a silicate melt can dissolve before saturating with sulfide (e.g., pyrrhotite) or sulfate (e.g., anhydrite) changes with the redox state of the system and has important implications for the sulfur budget of a magmatic system. Several empirical models have been developed to predict the sulfur content of a silicate melt at either sulfide (under reducing conditions) or sulfate (under oxidizing conditions) saturation, but only one model existed that systematically assessed how the sulfur content of a basaltic melt changes as a function of oxygen fugacity (fO2) across the transition from sulfide- to sulfate-dominated conditions. The applicability of that model to intermediate and felsic melts rests on the assumption that changes in melt composition do not affect how sulfide or sulfate dissolves in the melt. Here, we report new experimental data that constrain the sulfur concentration at sulfide saturation (SCSS) and the sulfur concentration at anhydrite saturation (SCAS) in a dacitic melt as a function of fO2. The experiments were conducted using a H2O-saturated natural dacitic melt at 1000 °C, 300 MPa, and at log fO2 varying over four orders of magnitude encompassing the sulfide-sulfate transition (log fO2 = ΔFMQ−0.7, ΔFMQ+0, ΔFMQ+0.5, ΔFMQ+1, ΔFMQ+1.48, ΔFMQ+1.54, ΔFMQ+1.75, ΔFMQ+2.08 and ΔFMQ+3.3). New SCSS and SCAS data and modeling for dacitic melts reveals that the sulfide-sulfate transition occurs at ΔFMQ+1.81 ± 0.56, defined by the following equations to predict the sulfur content of intermediate to evolved silicate melts as a function of fO2:SCSSdacitic = [S2−] (1 + 10(2.00ΔFMQ – 3.05))SCASdacitic = [S6+] (1 + e(1.26 – 2.00ΔFMQ))The results presented here demonstrate that the basaltic-derived SCSS-SCAS model is not appropriate for dacitic melts and that the sulfide-sulfate transition is shifted to higher fO2 in more evolved silicate melts. Implications include the stability of sulfides to higher fO2 in more evolved silicate melts and the potential for a narrower transition from a sulfide- to a sulfate-dominated melt than that predicted by thermodynamics.

A thermodynamic model for sulfur content at sulfide saturation (SCSS) in hydrous silicate melts: With implications for arc magma genesis and sulfur recycling

Modeling the effect of H2O on the “sulfur content at sulfide saturation” (SCSS) in silicate melt is essential for the estimation of SCSS in both arc magmas and slab melts. Here we present a thermodynamic approach, in which, SCSS in hydrous silicate melt has been modeled as a combination of S dissolving as S2− and HS−/H2S, based on the sulfide capacity (CS2−) and the hydro-sulfide capacity (CHS), respectively. By adopting the thermodynamic framework of O’Neill and Mavrogenes (2002), S dissolution as HS−/H2S can be modeled in an analogous fashion to that for modeling S2− in anhydrous melt with the relation: lnHSSCSS=-ΔG°Fes-FeO/RT+lnCHS-lnaFeOmelt+lnaFeSsulfide. To obtain an expression for CHS, we compiled published experimental data on SCSS in hydrous silicate melts covering a PT range of 0.15–3 GPa and 785–1600 °C, and melt H2O contents of ∼1–13 wt%. While the contribution of S dissolving as S2− in basaltic and andesitic melts can be calculated based on the updated SCSS model for anhydrous basic melt from O’Neill (2021), S2− is considered negligible in rhyolitic and dacitic melts, i.e., when melt FeO content <∼5 wt%. We propose an expression for CHS as a function of temperature, mole fractions of cations for the normalized anhydrous melt composition, ln(XOH + XH2O), and a Na + K-Al term for alkaline-enriched rhyolitic melts. The coefficients for cation mole fractions are adopted from the expression for CS2− from O’Neill (2021); the coefficients for the 1/T and Na + K-Al terms are obtained from regression. The reproducibility of compiled SCSS values is ±50% for hydrous rhyolitic and dacitic melts, and ±10–30% for hydrous basaltic and andesitic melts, representing a significant improvement in comparison to previous models.Our model produces SCSS values for the primitive arc magmas compiled by Ruscitto et al. (2012), that are in most cases higher than the measured S contents, implying sulfide under-saturated conditions during mantle wedge melting. The contribution of H2S dissolution to the calculated SCSS values varies in a range of 70–1234 ppm, which increases with the increase of H2O content (0.3–6.2 wt%). H2S dissolution therefore contributes to the higher S content in arc basalt compared to MORB, an observation which is further corroborated by the positive correlation between H2O and S contents in primitive arc magmas. Applying our current model to experimentally produced sediment melts spanning a PT range of 690–1050 °C and 2.5–4.5 GPa, demonstrates that sediment melts, especially those of intermediate supercritical character with >25 wt% H2O and peralkaline in composition, can have high SCSS values as a result of H2S dissolution, and act as the transfer medium for S recycling between the slab and mantle wedge under reduced conditions.

Explosive Activity on Kīlauea's Lower East Rift Zone Fueled by a Volatile‐Rich, Dacitic Melt

AbstractMagmas with matrix glass compositions ranging from basalt to dacite erupted from a series of 24 fissures in the first 2 weeks of the 2018 Lower East Rift Zone (LERZ) eruption of Kīlauea Volcano. Eruption styles ranged from low spattering and fountaining to strombolian activity. Major element trajectories in matrix glasses and melt inclusions hosted by olivine, pyroxene and plagioclase are consistent with variable amounts of fractional crystallization, with incompatible elements (e.g., Cl, F, and H2O) becoming enriched by 4–5 times as melt MgO contents evolve from 6 to 0.5 wt%. The high viscosity and high H2O contents (∼2 wt%) of the dacitic melts erupting at Fissure 17 account for the explosive Strombolian behavior exhibited by this fissure, in contrast to the low fountaining and spattering observed at fissures erupting basaltic to basaltic‐andesite melts. Saturation pressures calculated from melt inclusion CO2‐H2O contents indicate that the magma reservoir(s) supplying these fissures was located at ∼2–3 km depth, which is in agreement with the depth of a dacitic magma body intercepted during drilling in 2005 (∼2.5 km) and a seismically imaged low Vp/Vs anomaly (∼2 km depth). Nb/Y ratios in erupted products are similar to lavas erupted between 1955 and 1960, indicating that melts were stored and underwent variable amounts of crystallization in the LERZ for &gt;60 years before being remobilized by a dike intrusion in 2018. We demonstrate that extensive fractional crystallization generates viscous and volatile‐rich magma with potential for hazardous explosive eruptions, which may be lurking undetected at many ocean island volcanoes.

Magma reservoir beneath Azumayama Volcano, NE Japan, as inferred from a three-dimensional electrical resistivity model explored by means of magnetotelluric method

An electrical resistivity model beneath Azumayama Volcano, NE Japan, is explored using magnetotelluric method to probe the magma/hydrothermal fluid distribution. Azumayama is one of the most concerning active volcanoes capable of producing a potential eruption triggered by the 2011 Tohoku-Oki Earthquake. The three-dimensional resistivity model reveals a conductive magma reservoir (< 3 Ωm) at depths of 3–15 km below sea level (bsl). The 67% and 90% confidence intervals of resistivity are 0.2–5 Ωm and 0.02–70 Ωm, respectively, for the magma reservoir. We assumed dacitic melt + rock at a shallow depth of 4 km bsl and andesitic melt + rock at a greater depth of 9 km bsl. The confidence interval of resistivity cannot be explained by using dacitic melt + rock condition at a depth of 4 km bsl. This suggests that very conductive hydrothermal fluids coexist with dacitic melt and rock in the shallow part of the magma reservoir. For the depth of 9 km bsl, the 67% confidence interval of resistivity is interpreted as water-saturated (8.0 weight %) andesitic melt–mafic rock complex with melt volume fractions greater than 4 volume %, while the shear wave velocity requires the fluid and/or melt volume fraction of 6–7 volume % at that depth. Considering the fluid and/or melt volume fraction of 6–7 volume %, the conductive hydrous phase is likewise required to explain the wide range of the 67% confidence interval of resistivity. The Mogi inflation source determined from geodetic data lies on the resistive side near the top boundary of the conductive magma reservoir at a depth of 2.7 or 3.7 km bsl. Assuming that the resistivity of the inflation source region is above the upper bound of the confidence interval of resistivity for the conductive magma reservoir and that the source region is composed of hydrothermal fluid + rock, the resistivity of the source region is explained by a hydrothermal fluid volume fraction below 5 volume %, which is the percolation threshold porosity in an effusive eruption. This indicates that the percolation threshold characterizes the inflation source region.

Magma mixing, degassing and late sulfide saturation: Insights into the 1976–2000 eruptive sequence at White Island, New Zealand

This paper reports new petrological and chemical data of ejecta from the 1976–2000 eruptive sequence of White Island, New Zealand. We provide evidence of mixing between shallow stored (0.7–1 km) andesitic to dacitic melts and deeper primitive mafic magma carrying high-Fo olivine, repeatedly injected throughout the sequence. Melt inclusions hosted in plagioclase, pyroxene and olivine were analyzed for major, trace and volatile elements. The low H2O contents in inclusions suggest it exsolves at depth while SO2 degassing is recorded during the rise and crystallization of the mafic magma. The higher solubility of Cl (as HCl and NaCl) delays its exsolution until shallow depth. Convection degassing is suggested to explain the large imbalance between elevated gas emissions and low amount of lava erupted. Frequent inputs of gas-rich, deeply derived magma are essential drivers for the counterflow of viscous degassed melts. Metal contents (Cu, Zn) of the residual mafic magma increases during olivine crystallization. This behavior changes during shallow differentiation of the andesitic magma, with metals leaving the silicate melt. While some metals may be lost to degassing, Cu, Ni, Ag and Au strongly partition into an immiscible sulfide melt. Saturation of such a sulfide melt phase occurs in the shallow environment, likely related to changes in redox state associated with magnetite crystallization and/or SO2 degassing.

In situ Reaction-replacement Origin of Hornblendites in the Early Cretaceous Laiyuan Complex, North China Craton, and Implications for its Tectono-magmatic Evolution

Abstract Two suites of amphibole-rich mafic–ultramafic rocks associated with the voluminous intermediate to felsic rocks in the Early Cretaceous Laiyuan intrusive–volcanic complex (North China Craton) are studied here by detailed petrography, mineral and melt inclusion chemistry, and thermobarometry to demonstrate an in situ reaction-replacement origin of the hornblendites. Moreover, a large set of compiled and newly obtained geochronological and whole-rock elemental and Sr–Nd isotopic data are used to constrain the tectono-magmatic evolution of the Laiyuan complex. Early mafic–ultramafic rocks occur mainly as amphibole-rich mafic–ultramafic intrusions situated at the edge of the Laiyuan complex. These intrusions comprise complex lithologies of olivine-, pyroxene- and phlogopite-bearing hornblendites and various types of gabbroic rocks, which largely formed by in situ crystallization of hydrous mafic magmas that experienced gravitational settling of early crystallized olivine and clinopyroxene at low pressures of 0·10–0·20 GPa (∼4–8 km crustal depth); the hornblendites formed in cumulate zones by cooling-driven crystallization of 55–75 vol% hornblende, 10–20 vol% orthopyroxene and 3–10 vol% phlogopite at the expense of olivine and clinopyroxene. A later suite of mafic rocks occurs as mafic lamprophyre dikes throughout the Laiyuan complex. These dikes occasionally contain some pure hornblendite xenoliths, which formed by reaction-replacement of clinopyroxene at high pressures of up to 0·97–1·25 GPa (∼37–47 km crustal depth). Mass-balance calculations suggest that the olivine-, pyroxene- and phlogopite-bearing hornblendites in the early mafic–ultramafic intrusions formed almost without melt extraction, whereas the pure hornblendites brought up by lamprophyre dikes required extraction of ≥20–30 wt% residual andesitic to dacitic melts. The latter suggests that fractionation of amphibole in the middle to lower crust through the formation of reaction-replacement hornblendites is a viable way to produce adakite-like magmas. New age constraints suggest that the early mafic–ultramafic intrusions formed during ∼132–138 Ma, which overlaps with the timespan of ∼126–145 Ma recorded by the much more voluminous intermediate to felsic rocks of the Laiyuan complex. By contrast, the late mafic and intermediate lamprophyre dikes were emplaced during ∼110–125 Ma. Therefore, the voluminous early magmatism in the Laiyuan complex was probably triggered by the retreat of the flat-subducting Paleo-Pacific slab, whereas the minor later, mafic to intermediate magmas may have formed in response to further slab sinking-induced mantle thermal perturbations. Whole-rock geochemical data suggest that the early mafic magmas formed by partial melting of subduction-related metasomatized lithospheric mantle, and that the early intermediate to felsic magmas with adakite-like signatures formed from mafic magmas through strong amphibole fractionation without perceptible plagioclase in the lower crust. The late mafic magmas seem to be derived from a slightly different metasomatized lithospheric mantle by lower degrees of partial melting.

Petrology of the April 2015 Eruption of Calbuco Volcano, Southern Chile

Abstract Understanding the origin of intermediate magmas that commonly erupt from subduction zone volcanoes is important to better constraining the mechanisms of continental crust formation. We performed a detailed mineralogical and petrological study of the eruptive products from the April 2015 eruption of Calbuco volcano, Chile, a three-phase sub-Plinian eruption that produced pyroclastic deposits of andesitic composition. The eruptive products comprise a glass phase and a high but variable proportion of minerals dominated by plagioclase, clinopyroxene, and orthopyroxene, with minor olivine, amphibole, and magnetite. Plagioclase is very strongly zoned with highly anorthitic cores surrounded by more albitic rims, and no intermediate compositions between them. Based on thermodynamic calculations and published experimental data, we estimate that the anorthitic cores crystallized from a basaltic andesite melt containing 3·5–4·5 wt% H2O. The bulk-rock major and trace element variability at Calbuco is best explained by the accumulation of a variable amount of minerals (in relative proportion 72 % plagioclase, 28 % pyroxene) in a dacitic melt. These minerals most probably formed in the crystal mush zone of the magma chamber, at 200–300 MPa (8–11 km depth) according to pyroxene and amphibole compositions. A few weeks to months before the eruption, the crystal mush was disaggregated, perhaps owing to magmatic underplating, and a crystal-bearing dacitic melt migrated into a subsurface storage region where the albitic plagioclase rims crystallized. The eruption was probably internally triggered by over-pressurization in the shallow magma chamber.

Chlorine and felsic magma evolution: Modeling the behavior of an under-appreciated volatile component

Research on magmatic Cl has proven useful for understanding melt evolution, degassing, and other igneous processes. Magmatic Cl behavior varies strongly with its solubility in melt, and Cl solubility and its partitioning between melts and fluids vary with the Cl content of the system, melt composition, pressure, and the abundance of oxidized magmatic sulfur. Given these relationships and given that they differ from other magmatic volatiles, Cl can provide unique insights on magma evolution. We present a new method of interpreting melt Cl concentrations that are normalized to their calculated Cl solubilities, and the most accurate results apply to solubilities computed at 200 MPa due to insufficient experimental data for other pressures. We apply the new method to compositional data for more than 3500 dacitic to rhyolitic melt inclusions and matrix glasses, representing 64 felsic igneous systems, to demonstrate how the behavior of Cl during magmatic, magmatic-hydrothermal, and degassing processes can be more accurately interpreted.Variations in Cl for most of the 64 systems agree with modeled Cl behavior indicative of melt evolution dominated by fractional crystallization of fluid(s)-saturated melts, and the majority of these systems were apparently saturated in fluid(s) well before melt inclusion entrapment. Comparison of Cl solubility-normalized Cl concentrations of these glasses with modeled results identifies magmas saturated in hydrosaline liquid ± vapor at pressures as high as 600 MPa. Modeling of pressure reductions from 600 down to 20 MPa distinguishes other magmas that exsolved hydrosaline liquid as magmas ascend to shallower depths. This new approach also supports computation of Cl concentrations of magmatic fluids at 200 MPa, for each of the >3500 glasses, and shows how the Cl contents of fluids vary with the (fluid/melt) mass ratio. For example, Cl contents of magmatic fluids range from 0.3 to nearly 70 wt.% at fluid/melt mass ratios of 2 × 10−3, and from 0.3 to a maximum of only 11 wt.% Cl for fluid/melt mass ratios of 4.2 × 10−2.Compositional trends in the glass data show either increasing Cl contents of residual melt, essentially no change of Cl in melt, or decreasing Cl contents of aliquots of residual melt with progressive melt evolution. This method identifies melts that have assimilated magmatic or externally sourced hydrosaline liquids. Plots of (measured Cl in melt/modeled solubility of Cl in melt; Cl[Me/Mo]) versus the Larsen melt differentiation index show minimal dispersion for the least-evolved dacitic melts and increasing dispersion with melt evolution to rhyolitic compositions. Increased dispersion may be a consequence and indication of equilibrium or non-equilibrium degassing and/or assimilation of hydrosaline liquid.

Late Paleozoic supervolcano-scale eruptions in Maine, USA

AbstractContemporaneous mafic and felsic magmatism occurs in a variety of tectonic settings where continental crust is invaded by mantle-derived basaltic melt. Bimodal magmatism, including supervolcano-scale eruptions, occurred in both the Coastal Maine magmatic province and the Central Maine magmatic belt during two phases of accretion of Avalonia to the margin of North America in the Late Silurian and Early Devonian. The magmatic complexes of both coastal and central Maine provide an opportunity to examine the storage and eruption settings of basaltic and rhyolitic magma from the base of plutons through the top of their volcanic successions. The coastal and central Maine magmatic provinces represent the southwesternmost part of a &gt;70,000 km2 bimodal igneous province that includes volcanic and plutonic rocks that extend into the Gaspé Peninsula of Quebec, Canada. Taken together, these magmatic belts embody a large igneous province–scale locus of igneous activity. Four complexes in Maine are part of the Coastal Maine magmatic province and erupted between ca. 424 and 420 Ma, producing volcanic successions up to 3 km thick. The fifth large silicic eruptive complex examined here, the Katahdin/Traveler complex of central Maine, is part of the Central Maine magmatic belt, and it produced the ca. 407 Ma Traveler rhyolite, of which ∼3200-m-thick sections are preserved. The three coastal Maine magmatic complexes for which an arguably clear genetic relationship exists between pluton and volcanic succession are smaller than the Katahdin/Traveler system (on the basis of area of the remnant magma chamber of Mount Desert Island [∼500 km2] compared to that of the Katahdin batholith [∼1350 km2]), but they produced volcanic successions from 0.65 (Vinalhaven) to at least 2.2 km (Mount Desert/Cranberry Isles and Isle au Haut) thick. Single ignimbrite thicknesses in these complexes range from 350 m to 860 m. The remaining coastal Maine magmatic complex discussed here, the Eastport series, differs from the other four in that the pluton(s) from which the ∼2.4-km-thick succession originated has (have) not been identified. A detailed integration of gravity and magnetic data indicates that strong positive magnetic anomalies and weaker gravity anomalies support a model of thin felsic igneous rocks underlain by a significant volume of mafic to ultramafic rocks. The dominance of gabbro in the coastal Maine crustal column suggests that crustal extension and intrusion of mantle-derived basalt drove the development of bimodal magmatic complexes. The gabbro:granite ratio in the subsurface of the Coastal Maine magmatic province, and the presence of hornblende, rather than pyroxene, as the main ferromagnesian mineral in the gabbros of the province are consistent with a model of flux of hydrous basalt into the crust of ∼10–2 m3/m2/yr, leading to a ratio of crustal melt to mantle-derived basaltic melt of ∼1:0.25, if the invading basaltic melt was wet (∼5.6 wt% water). The absence of andesites in this setting is likely a consequence of the subduction-related origin of the hydrous basalts, resulting in the generation of dacitic rather than andesitic melts in lower-crustal mush zones, and the eventual extraction of interstitial melts from those dacitic melts in the upper crust to produce the granites and rhyolites that dominate the igneous rocks at the present surface in coastal and central Maine.

Comparative partitioning of Re and Mo between sulfide phases and silicate melt and implications for the behavior of Re during magmatic processes

The behavior of Re in magmas is of great significance to understanding the distribution of Re in Earth's different reservoirs. In order to better constrain the behavior of Re in magmas of different tectonic settings, the partition coefficients (D) of Re and Mo between sulfide liquid (SL), monosulfide solid solution (MSS), and basaltic to dacitic melts (SM) were determined using a piston-cylinder apparatus. The experiments were conducted at 1050-1200°C, 0.7-1.5 GPa, and oxygen fugacity (fO2) of ∼FMQ−1.8 to FMQ+1.5. The D values range from 0.11 to 76 for DMoSL/SM, 0.32 to 219 for DMoMSS/SM, 45 to 76000 for DReSL/SM, and 280 to 160000 for DReMSS/SM. All the D values increase with decreasing fO2 and the FeO content in silicate melt. We also find that DReSL/SM and DReMSS/SM are strongly correlated with DMoSL/SM and DMoMSS/SM regardless of the temperature, pressure, silicate melt composition, and fO2. This correlation, which can be explained by the similar dissolution behaviors of Re and Mo in silicate melt, provides an approach to use DMoSL/SM or DMoMSS/SM to predict DReSL/SM or DReMSS/SM in magmas. The newly obtained DReSL/SM, DReMSS/SM, DMoSL/SM, and DMoMSS/SM, in conjunction with DCuSL/SM, were applied to constrain the behavior of Re during magmatic genesis and differentiation. The results show that the observed Re contents in primitive MORBs can be explained only if the Re abundance in the depleted mantle is between 0.12 and 0.28 ppb. To satisfactorily explain both the Cu and Re contents in primitive arc basalts, (1) the fO2 of the subarc mantle during melting must be between FMQ and FMQ+1, and (2) the subducting slab must contribute both Re and S to the subarc mantle, although the extent of slab contribution differs between arcs. During arc magmatic differentiation, a significant fraction of Re is sequestered by sulfide, which results in a heterogeneous distribution of Re in the continental crust and the formation of Re-rich cumulates in the deep crust. The heterogeneous distribution of Re in the continental crust may lead to overestimations or underestimations of the Re abundance in the continental crust. The delamination of such Re-rich cumulates can form Re-rich and radiogenic Os-rich reservoirs in the depleted mantle, which partially explains the “missing Re” problem. Our study illustrates that sulfides play a key role in controlling the behavior of Re in magmas of mid-ocean ridge and arc settings and consequently, the distribution of Re in Earth's crust and mantle.