Search

Lithium and hydrogen are volatile elements which diffuse rapidly in crystals and melt, making them powerful geochemical tools to reconstruct geological processes that take place on short time scales, such as syn- and post-eruptive degassing. Although the dynamics of hydrogen are fairly well understood to better constrain such processes, the assessment of Li behaviour within the magma reservoir relevant for ascent-related degassing still lacks detailed evaluation. Here, the first in situ Li concentrations and isotopic compositions (using SIMS analysis) of rhyolitic quartz-hosted, naturally glassy and crystallised melt inclusions (MIs) and groundmass glass (Mesa Falls Tuff, Yellowstone) are used to reconstruct Li elemental and isotopic evolution in the magma reservoir. Lithium concentrations in quartz-hosted glassy MIs (10–61 ppm) from a fallout deposit overlap with their groundmass glass (32–46 ppm) and their host quartz (8–15 ppm). Crystallised MIs from a later erupted flow pumice clast sample have higher Li concentrations (8–190 ppm) compared to the groundmass glass (32–51 ppm) and their host quartz (15–24 ppm). Lithium content in quartz from the early erupted sample is relatively homogenous, whereas it is up to a factor of two higher and heterogeneous in the later erupted sample, with a simultaneous increase in Li versus a decrease in H towards crystal rims. The δ7Li difference (expressed as Δ7LiMI–glass) between MIs (−8.0‰ to + 12.3‰) and groundmass glass (+9.0‰ to + 20.5‰) of two pyroclastic deposits reaches up to 29‰. Glassy MIs are internally heterogeneous in δ7Li and Li abundance. The cores of the glassy MIs record the δ7Li of the least modified melt during entrapment and the data distribution can be modelled by equilibrium fractionation between the melt and vapour phase during early open-system degassing in the magma reservoir. Late degassing during eruption triggers Li–H diffusional exchange between quartz and melt, as the degassing of H2O and the accompanying pressure change trigger H diffusion out of the host quartz and the MIs, which is charge balanced by inward Li diffusion. This results in the modification of Li contents in quartz and δ7Li values in the rims of the glassy MIs. Crystallised MIs reflect the loss of H2O from the MIs and the resulting enrichment of Li during the crystallisation. Additionally, the variations of δ7Li in the groundmass glass can be explained through modelling by kinetic fractionation between the melt and vapour during late stage open-system degassing linked with magma ascent. ISSN:0016-7037 ISSN:1872-9533

Pre- to syn-eruptive fluids released by the magmatic system forming the non-welded rhyolitic Kos Plateau Tuff ignimbrite, Greece, were characterized using fluid inclusions entrapped in quartz from granitic clasts entrained from the roof or walls of the feeding magma reservoir. This fluid was initially of intermediate density and relatively low salinity (3–11 wt% NaClequivalent). Following significant pressure drop(s), it separated into immiscible brine and CO2-bearing vapor occurring as texturally less mature inclusions. The transition from single- to two-phase-fluid occurred at magmatic conditions, as supported by high fluid inclusion homogenization temperatures (ca. 700 °C) and simultaneous trapping of fluid inclusions with silicate melt inclusions. The large recorded pressure drop together with texturally immature fluid inclusion shapes suggests a possible direct relation between the fluid phase change and the incipient 161 ka caldera-forming eruption. All inclusion types occur predominantly along (pseudo-) secondary trails in the granitic clasts, indicating efficient entrapment in microfractures formed at high crystallinity, as attested to by elevated concentrations of incompatible elements in the silicate melt inclusions. Fast quenching of the inclusions due to eruption prevented significant post-entrapment chemical modification, providing pristine samples of magmatic fluid compositions. Element concentrations in co-existing fluids and silicate melt were analyzed using laser ablation inductively-coupled plasma mass spectrometry and compared in view of element transporting capabilities and magmatic-hydrothermal ore formation. Economically important metals such as Cu, Zn, Mo, and W were extracted from the silicate melt by the fluid. In particular, Cu concentrations in the single-phase fluid in the range of 100–500 ppm agree well with previous experimental and modeling studies and, therefore, represent values expected for primary magmatic fluids. Most elements got further enriched in the brine upon phase separation, except Li, B, and As, which may have contributed significantly also to the vapor phase. Considering the similarity between vapor and intermediate-density fluid in terms of salinity, however, the metal-enriched brine phase only comprised a minor fraction of the bulk fluid.

The Ordovician rocks of Sierra Las Planchadas are an exceptionally preserved natural example of a plutonic-volcanic connection. Located in northwestern Argentina, the Las Planchadas Formation consists of a monzogranitic batholith that displays a window into the source of large subvolcanic rhyolitic rocks, cryptodomes and volcanic feeder-dykes. Mafic intrusions and felsic dykes occur cutting the plutonic rocks. Grain-scale relationships, whole-rock compositions and mineral chemistry indicate that monzogranites and rhyolites are two comagmatic members which differentiated within upper crust magma reservoirs. Monzogranites accumulated some plagioclase and trapped a large volume of melt, denoted by a high modal percentage (~64–71%) of near-solidus minerals, comprised of alkali feldspar, albite-rich plagioclase (An

The diffuse emission of CO2 from the south east sector of Nisyros caldera (Lakki plain) has been measured during a detailed survey (~1400 soil CO2 flux measurements) performed in October 2018. The gas emissions are fed by hydrothermal sources and, in minor part, by the soil biogenic activity whose mean CO2 flux (4 g m−2 d−1) is here estimated for the first time. The total amount of hydrothermal CO2 reaches 92 ± 8 t/d, a value that is slightly higher than that estimated with the same method between 1999 and 2001 (74 ± 7 t/d). The gas is emitted by different diffuse degassing structures (DDSs), including volcanic-hydrothermal structures (craters and domes) and NE-SW and NW-SE-trending tectonic lineaments. Even if the total CO2 emission is not particularly high at Nisyros (close to the median of CO2 emissions measured in volcanoes worldwide), the process is very energetic. The thermal energy associated with the shallow condensation of the steam in the DDSs reaches ~60 MW, while we estimate at 134–270 MW the total amount of thermal energy involved in the convective rising of the deep geothermal liquids that transport the gas from the depth toward the surface. This large flux of energy could dramatically increase during future earthquakes by addition of heat and mass from a deep hydrothermal reservoir, potentially triggering hydrothermal explosions, as it happened several times in the past few centuries.

Radioisotopic dates for Late Pleistocene to Holocene silica-undersaturated volcanic rocks are often imprecise, limiting our ability to assess the frequency of eruptions in alkaline volcanic provinces, evaluate related volcanic risk and date associated archaeological sites. Here, we present a new approach to dating alkaline volcanic rocks employing 238U-230Th disequilibrium dating of Ca-rich garnet phenocrysts by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). We document analytical protocols and apply this technique to date magmatic garnets from the 472 AD Pollena eruption of Vesuvius (Southern Italy) and from the Engare Sero Footprint Tuff, a volcaniclastic unit attributed to Oldoinyo Lengai (Northern Tanzania). Garnet phenocrysts from the Pollena phonolite yield a well-defined U-Th isochron with a date of 2.28 ± 0.71 ka (1σ) that is indistinguishable from the historical date and suggests that garnet crystallized close to eruption with a pre-eruption residence time of less than 1.6 kyr. Garnets from the Engare Sero Footprint Tuff yield a U-Th isochron date of 4.91 ± 0.58 ka (1σ). This date is compatible with 14C and 40Ar/39Ar dates that bracket tuff deposition to between ∼5 and ∼19 ka but confidently constrains the age of the Engare Sero Footprint Tuff to the mid-Holocene. These examples demonstrate the potential of this new approach for dating Late Pleistocene to Holocene silica-undersaturated volcanic rocks at millennial-scale precision and for investigating magma chamber processes beneath active alkaline volcanoes.

Studying the inventory and distribution of lithium (Li) in rhyolitic magmas, and the potential Li isotope fractionation during progressive crystallisation is crucial to understand the formation of economic Li deposits. Lithium concentrations and isotopic compositions of co-existing groundmass glass, quartz, sanidine, plagioclase, clinopyroxene, fayalite, and orthopyroxene from the Mesa Falls Tuff (Yellowstone) were determined using LA-ICP-MS and MC-ICP-MS. The highest Li contents were obtained from quartz-hosted melt inclusions (avg. 244 ppm) and groundmass glass (avg. 43 ppm). Lithium concentrations are lower in mineral phases, decreasing from quartz (avg. 18 ppm), plagioclase (avg. 17 ppm), fayalite (avg. 16 ppm), clinopyroxene (avg. 11 ppm), orthopyroxene (avg. 8 ppm) to sanidine (avg. 6 ppm). Lithium isotopic compositions of bulk tuff samples and major mineral phases reveal a range in δ7Li values (per mil deviation of 7Li/6Li ratio relative to L-SVEC reference material), spanning ~10‰ between the co-existing phases and decreasing from groundmass glass (6.5–7.5‰) to feldspars (plagioclase from −2.3 to −1.4‰; sanidine from −0.6 to −0.1‰). Lithium concentration profiles across sanidine and plagioclase crystals show a decrease towards the crystal rims, which is not observed in the other mineral phases of the Mesa Falls Tuff. The variations identified in feldspar are decoupled from both the major and all other trace element compositions, precluding an origin related to mafic recharge and magma mixing. We propose that they result from Li loss from the melt to a vapour or fluid phase taking place immediately prior to, or synchronously with, eruption. This is supported by the observation that Li contents in quartz-hosted melt inclusions are a factor of six greater than those found in groundmass glass while the concentrations of immobile trace elements are identical. The low-Li rims on sanidine and plagioclase crystals observed in the high-silica Mesa Falls Tuff rhyolite are potential candidates to preserve Li isotopic disequilibrium as has been observed in less evolved systems. The variation in Li abundances within feldspar crystals hints at the potential for kinetic isotopic fractionation to play a role in the reported isotopic compositions of bulk separates. The large elemental database of Li concentrations from the MFT allows us to calculate apparent partition coefficients between groundmass glass and co-existing minerals. These data are compared to fifteen other ignimbrites from the Yellowstone-Snake River Plain province to provide a coherent picture of Li partitioning in hot and dry rhyolites with Li most compatible in quartz and olivine, and least compatible in sanidine.

The partitioning of REE,Y and Sc (R3+) between olivine and melt has been investigated experimentally during basalt-carbonate interaction. Three synthetic basalts (meltMg#72, meltMg#75 and meltMg#78) were doped with 0, 10 and 20 wt% CaCO3 and then equilibrated for 72 h at 1 atm, 1,150, 1,200 and 1,250 °C, and the QFM oxygen buffer. The thermal decomposition of CaCO3 produced CaO contents in the melt up to ~22 wt%. Regular relationships are found between the ionic radius and the partition coefficient (DR3+), showing typical near-parabolic patterns. DR3+ is weakly dependent on temperature, but decreases with increasing CaCO3 in the starting material (e.g., DSc decreases from 0.20 to 0.13). From the point of view of the lattice strain theory, DR3+ is described in terms of the radius of the crystal site (r0), the Young Modulus (E) due to the elastic response of that site to lattice strain caused by cation insertion, and the strain-free partition coefficient ( D 0 3+ ). The value of r0 decreases as Ca cations are accommodated into the more distorted M2 site of olivine via progressive Ca Fe substitutions. This mechanism is accompanied by a higher proportion of Mg cations entering into the smaller M1 site, making the optimum ionic radius smaller and favoring the crystallization of more forsteritic olivines from decarbonated melts. The enrichment of Ca in the crystal lattice is also proportional to the number of Si and Ca cations available in the melt. This causes E to be anticorrelated either with Ca in olivine or the activity of CaO in the melt. R3+ cations behave as network modifiers and, during basalt-carbonate interaction, the increasing abundance of non-bridging oxygens enhances the solubility of REE, Y and Sc in the melt. As a consequence, D 0 3+ is negatively correlated with the degree of melt depolymerization. Additionally, the strain of the crystal lattice dominates the DR3+ parabolic patterns and D 0 3+ is strongly controlled by forsterite and aluminium concentrations in olivine. The accommodation of REE, Y and Sc in the crystal lattice requires maintenance of local charge-balance by the generation of vacancies, in accord with a paired substitution of R3+ and a vacancy for Mg in octahedral sites.

Chemical Geology, 501, ISSN:0009-2541, ISSN:1872-6836

The geochemical evolution of olivine from primitive magmatic skarn environments has been studied by atmospheric pressure experiments carried out at 1250, 1200, and 1150 °C under QFM oxygen buffering conditions. The starting materials were three synthetic basalts (i.e., meltMg#78, meltMg#75, and meltMg#72) doped with variable amounts of CaCO3, in order to reproduce the natural concentration levels of CaO-rich magmas interacting with the skarn rock shells. Results from decarbonation experiments evidence that the crystallization of Fo-CaO-rich, NiO-poor olivines is more favored at higher temperatures when primitive basaltic magmas assimilate increasing amounts of carbonate materials. The number of large size Ca cations entering olivine crystal lattice is proportional to the amount of Ca-O-Si bonds available in the melt. Due to differences between Fe2+ and Mg cation radii, the Ca-Fe2+ substitutions into M2 crystallographic site are more facilitated than Ca Mg ones, thus enhancing the forsterite component in olivine. The partitioning behavior of Ni, Mg, Fe2+, Mn, and Ca between olivine and melt has been also investigated to better understand cation redistribution mechanisms at the magma‑carbonate reaction zone. In this context, some partitioning models from the literature have been refined to more accurately quantify the geochemical evolution of primitive skarn systems. Under the effect of CaCO3 assimilation, the partitioning of divalent cations, can be parameterized as a function of temperature, bulk composition (mostly, CaO and MgO contents in both olivine and melt) and melt structure (expressed as the number of non-bridging oxygens per tetrahedrally coordinated cations). Conversely, the exchange partition coefficients between Fe2+/Ca/Mn/Ni and Mg do not vary significantly as a function of temperature and meltMg#, due to the limited influence of these parameters on the melt structure. In turn, cation exchange reactions are primarily controlled by the strong depolymerizing effect of CaCO3 assimilation that increases the number of structural sites critically important to accommodating network-modifying cations in the melt phase. The comparison between cumulates and magmatic skarns from the Colli Albani Volcanic District (Italy) and experiments from this study provides quantitative constraints on the geochemical evolution of olivine phenocrysts and their melt inclusions as a function of carbonate assimilation.

The diffusivity of S in a hydrous dacitic melt (4.5–6.0 wt.% H2O) has been investigated in the temperature (T) and pressure (P) range of 950 °C to 1100 °C and 200 to 250 MPa, respectively. Three series of experiments were conducted at relatively low oxygen fugacity (fO2) conditions [0.8 log units below fayalite-magnetite-quartz equilibrium (FMQ −0.8); referred to as “low fO2”] and high fO2 conditions (FMQ +2.5; referred to as “high fO2”) to determine if the diffusivity of S is affected by its oxidation state and speciation. Sulfur concentration profiles were measured by electron microprobe and the diffusion coefficient (D) was calculated by fitting these profiles. Sulfur diffusion is approximately one order of magnitude faster when S is dominantly present as sulfide species (low fO2) in comparison to the sulfate dominated experiments (high fO2). The following Arrhenian equations were obtained for high and low fO2 conditions at 200 MPa: high fO 2 : D = 10 - 5.92 ± 0.86 ∗ exp - 137.3 ± 21.5 kJ / mol RT low fO 2 : D = 10 - 5.18 ± 1.39 ∗ exp - 125.7 ± 34.4 kJ / mol RT where D is the average diffusion coefficient in m2 s−1, R is the gas constant in 8.3144 J mol−1 K−1 and T is the temperature in K. Our results demonstrate for the first time in natural melts that S diffusion is strongly sensitive to fO2. Our S diffusivities under low fO2 conditions are only slightly slower of those found for H2O, suggesting that S can be rather efficiently purged from reduced dacitic melts during volcanic eruptions. However, for more oxidized systems (e.g. subduction zones), S diffusion will be much slower and will hinder equilibrium syn-eruptive degassing during rapid decompression. Therefore, we conclude that the “excess S” measured during many explosive volcanic eruptions in arcs is dominantly derived from S-rich bubble accumulation in the eruptible portion of the magma reservoir.

Formation and evolution of the Pannonian Basin as part of the Mediterranean region was accompanied by eruptions of compositionally diverse magmas during the Neogene to Quaternary. The long-lasting magmatic activity began with some of the most voluminous silicic eruptions in Europe for the last 20 Myr. This paper describes the eruption chronology of this volcanic activity using new, high-quality zircon U-Pb dates, and provides the first estimates on the volume and areal distribution of the volcanic products, characterizes the magma composition and discusses the silicic magmatism in a region, where the continental lithosphere underwent significant extension. A thorough zircon geochronological study was conducted on samples collected from ignimbrites and pyroclastic fall deposits exposed in the Bukkalja Volcanic Field. In-situ LA-ICP-MS analysis on zircon grains provided a fast, cheap and accurate method for such detailed geochronological work, where the volcanic products occur in scattered outcrops that often have poor stratigraphic constraints. The interpreted eruption ages were determined from the youngest zircon age population within the samples and this methodology was validated by new single zircon CA-ID-TIMS dates and sanidine Ar-Ar ages. The volcanism covers about 4 Myrs, from 18.2 Ma to 14.4 Ma and involved at least eight eruptive phases. Within this, four large eruption events were recognized at 14.358 ± 0.015 Ma (Harsany ignimbrite), 14.880 ± 0.014 Ma (Demjen ignimbrite), 16.816 ± 0.059 Ma (Bogacs unit) and 17.055 ± 0.024 Ma (Mango ignimbrite), which are found in areas across the Pannonian Basin and elsewhere in central Europe. Considering all the potential sources of silicic ash found in the Paratethys sub-basins around the Pannonian Basin and along the northern Alps and in central Italy, we suggest that they were probably derived almost exclusively from the Pannonian Basin as shown by zircon U-Pb dates presented in this paper and published comparable age data from several localities. The new eruption ages considerably refine the Early to Mid-Miocene chronostratigraphy of the Pannonian basin, where the extensive volcanoclastic horizons are used as important marker layers. The cumulative volume of the volcanic material formed during this 4 Myr long silicic volcanism is estimated to be >4000 km3, consistent with a significant ignimbrite flare-up event. Zircon crystallization ages indicate magma intrusions and formations of magma reservoirs in the continental crust for prolonged period, likely >1 Myr prior to the onset of the silicic volcanism accompanied with sporadic andesitic to dacitic volcanic activities. Mafic magmas were formed by melting of the thinned lithospheric mantle metasomatized previously by subduction-related fluids and emplaced at the crust-mantle boundary. They evolved further by assimilation and fractional crystallization to generate silicic magmas, which ascended into the pre-warmed upper crust and formed extended magma storage regions. Zircon Hf isotope and bulk rock Sr-Nd isotopic data indicate a sharp decrease of crustal and/or increase of asthenospheric mantle input after 16.2 Ma, suggesting that by this time the crust, and the lithospheric mantle was considerably thinned. This magmatism appears to have had a structural relationship to tectonic movements characterized by strike-slip and normal faults within the Mid-Hungarian Shear Zone as well as vertical axis block rotations, when the two microplates were juxtaposed. Our new zircon ages helped to refine the age of two major block-rotation phases associated with faulting. This volcanism shows many similarities with other rift-related silicic volcanic activities such as the Taupo Volcanic Zone (New Zealand) and the Basin and Range Province (USA).

Constraining the contribution of crustal lithologies to silicic magmas has important implications for understanding the dynamics of these potentially highly explosive systems. Low- δ 18 O rhyolite lavas erupted after caldera-forming events in Yellowstone have been interpreted as the products of bulk crustal melting of previously deposited and hydrothermally altered rhyolitic material in the down-dropped caldera roof. For lack of compositional data, the “self-cannibalisation bulk melting”-theory relies on the assumption that hydrothermally altered materials are near-cotectic and hydrous (>3 wt% H2O) and will therefore readily melt at temperatures below 850 °C. In this study, we examine the drillcores Y2, Y9 and Y13 from a USGS drilling campaign in Yellowstone in order to characterise the hydrothermally altered material in terms of major and trace elements, oxygen isotopes and water contents. Rhyolite δ 18 O values can decrease from “normal” (+5.8 to +6.1‰) on the surface to as low as −5‰ at depths of 100–160 m and probably lower as a function of increasing temperature with depth. While material in the drillcores is variably altered and silicified, oxygen isotope exchange in these samples is not accompanied by systematic changes in major and trace element composition and is independent of uptake of water. More than 75% of the drillcore samples have 1100 °C. Therefore, large-scale bulk melting is unrealistic and low- δ 18 O rhyolite magmas more likely result from assimilation of δ 18 O into a normal- δ 18 O rhyolite magma from the main reservoir. This mechanism is supported by isotopic mass-balance models as well as thermal and volumetric constraints, and may be similarly applicable to other low- δ 18 O settings worldwide.

Here we document how the different growth features and intracrystalline distributions of both major and trace cations in clinopyroxene phenocrysts are important recorders of the intricate magma dynamics at Vulcano Island (Aeolian Arc, Italy). The compositions of clinopyroxene phenocrysts from products erupted over the last ~54 ka cluster at different degrees of evolution, paralleling the polybaric-polythermal differentiation of mantle-derived mafic magmas into more evolved silicic melts. The hotter lower crust is the most favorable location for the storage of mafic magmas and the early crystallization of diopsidic (Mg#91) clinopyroxene (Pmax ≈ 750 MPa and Tmax ≈ 1220 °C). Diopsidic phenocrysts are depleted in both rare earth elements (REE) and high field strength elements (HFSE) but are enriched in transition elements (TE). The transfer and accumulation of primitive magmas in the colder upper crustal regions lead to the formation of an interconnected series of more differentiated magmatic reservoirs (P ≈ 100–450 MPa and T ≈ 1100–1180 °C) hosting discrete populations of clinopyroxene (Mg#84–85) with a broad spectrum of zonations and dissolution features. Recharge bands in clinopyroxene are markers of multiple inputs of primitive REE-HFSE-poor, TE-rich magmas from depth. Augitic phenocrysts (Mg#82) with strong negative Eu anomaly and REE + HFSE enrichments crystallizes from highly differentiated trachytic and rhyolitic melts stored at very shallow crustal conditions (P ≤ 50 MPa and T ≤ 1100 °C). These silicic reservoirs represent residual melts trapped-extracted from crystal-dominated mush regions in the uppermost part of the plumbing system. The residence time of clinopyroxene increases from ∼0.1 to ∼44 years from basalt to rhyolite, together with an increasing number of recharge bands. The mineral assemblage in more silicic and viscous mush melts is sufficiently resilient to record numerous mafic injections and high degrees of magma mixing, hybridization, and crystallization before eruption. Overall, the compositional zoning pattern of clinopyroxene presents a picture of plumbing system that extends through the crust and is characterized by distributions of melts and crystals which are progressively more evolved and heterogeneous in both space and time.

Granitoids are a major component of the continental crust. They play a pivotal role in its evolution, either by adding new material (continental growth), or by reworking older continental crust. These two roles correspond to two main ways of forming granitic magmas, either by partial melting of pre-existing crustal rocks yielding granitic melts directly, or by fractionation of mantle-derived mafic to intermediate magmas. Both models represent endmembers, or paradigms that have shaped the way the geological community envisions granitoids, their occurrence, features, formation and meaning for crustal evolution and differentiation of the whole planet. In this paper, we expose the two competing paradigms and their implications. We explore the evidence on which each model is based, and how each school of thought articulates a comprehensive view of granitic magmatism based on field geological, petrological, geochemical (including isotopes) and physical constraints; and how, in turn, each view shapes the thinking on crustal growth and evolution, and the interpretation of proxies such as age and Hf isotopic patterns in detrital zircon databases. We emphasize that both schools of thought build a different, but internally consistent view based on a large body of evidence, and we propose that each of them is, or has been, relevant to some portions of the Earth. Thus, the key question is not so much “which” model applies, but “where, when and to which extent”.

While Laser Ablation Inductively-Coupled-Plasma Mass Spectrometry (LA-ICP-MS) is the method of choice for U-Pb dating of zircons in provenance analysis, its application to young (

U-Pb detrital zircon age and trace element data from a Devonian sandstone sample of the Beacon Supergroup provide new evidence for 1450 Ma zircon sources in Antarctica. These grains yield a dominant 1450 Ma (Mesoproterozoic, Calymmian) age probability peak with U/Th ratios suggesting they primarily formed from magmatic processes, also consistent with the presence of grains with oscillatory zonation. Determination of zircon parent rock types using trace element proxies reveals that the zircon grains are likely predominantly derived from granitoid rocks, with subsidiary, yet significant contributions from mafic and alkaline igneous rocks. These results are consistent with a ca. 1440 Ma (Mesoproterozoic, Calymmian) granitoid glacial erratic and similar aged detrital zircon found elsewhere in the Transantarctic Mountains that suggest a continuation of the trans-Laurentian A-type granitoid belt into Antarctica and, therefore, a 1400 Ma SWEAT-like reconstruction of the continental landmasses.

International audience; The origin of highly evolved magmas (e.g. rhyolites) has been a long-standing controversy in earth sciences. They are commonly thought to be generated in the upper crust by melt extraction from mush zones, but due to the rapid cooling of magma reservoirs in such shallow and typically cold environments, high magma emplacement rates of intermediate magmas are thought to be necessary to maintain large silicic mushes above the solidus long enough for the high-SiO2 melts extraction to occur. Late-stage redistribution of interstitial melts (i.e. heat and mass) by channels/dikes within those mushes has been invoked as a mechanism to preserve silicic mushes above their solidi for longer periods (i.e. delaying their final crystallization), but the nature of this process and its implications on plutons zonation are still poorly understood.Here, using time-dependent numerical modeling, we study the feasibility of late-stage interstitial melt extraction/redistribution by channels/dikes from a crystalline mush. Our model accounts for magma fluid dynamics, extraction of residual melts and thermal evolution of the crystallizing magma system and its hosting rocks, considering the thermal effect of the redistributing material. The model was applied to explain the anatomy of the well-documented La Gloria pluton (LGP, Central Chile), which exhibits increasing contents of SiO2 and abundant leucocratic dikes toward the margins, interpreted as trapped residual melts generated elsewhere in the magma chamber.Our results suggest that favorable conditions for extracting late-stage residual melts are reached at temperatures of ~ 750 °C (60 vol% crystallinity), at least for compositions similar to LGP dikes. Simulations correspond to 30 kyr of reservoir cooling, when the concentric compositional zonation of LGP is reproduced after a short period of extraction (< 15 kyr) and outward redistribution of silicic melt, with an extracted mass fraction of up to 0.17. An inward growing crystal-dominated domain is the main source of interstitial melts. A total extracted melt volume of ~ 7.5 km3 and a mean extraction rate of ~ 0.5 km3/kyr were calculated for LGP conditions. For small magma reservoirs, the extracted rhyolitic melt could reach the upper levels of the magma chamber and the overlying host rock, but it seems unlikely that it can reach the surface to feed large rhyolitic eruptions, at least once recharges have stopped.

In the Salton Sea region of southern California (USA), concurrent magmatism, extension, subsidence, and sedimentation over the past 0.5 to 1.0 Ma have led to the creation of the Salton Sea Geothermal Field (SSGF)—the second largest and hottest geothermal system in the continental United States—and the small-volume rhyolite eruptions that created the Salton Buttes. In this study, we determine the flux of mantle-derived basaltic magma that would be required to produce the elevated average heat flow and sustain the magmatic roots of rhyolite volcanism observed at the surface of the Salton Sea region. We use a 2D thermal model to show that a lower-crustal, partially molten mush containing 20 – 40 % interstitial melt develops over a ∼ 10 5 -yr timescale for basalt fluxes of 0.008 to 0.010 m 3 / m 2 / yr (∼0.0008 to ∼0.001 km / 3 yr injection rate) given extension rates at or below the current value of ∼0.01 m/yr ( Brothers et al., 2009 ). These regions of partial melt are a natural consequence of a thermal regime that scales with average surface heat flow in the Salton Trough, and are consistent with seismic observations. Our results indicate limited melting and assimilation of pre-existing rocks in the lower crust. Instead, we find that basalt fractionation in the lower crust produces derivative melts of andesitic to dacitic composition. Such melts are then expected to ascend and accumulate in the upper crust, where they further evolve to give rise to small-volume rhyolite eruptions (Salton Buttes) and fuel local spikes in surface heat flux as currently seen in the SSGF. Such upper crustal magma evolution, with limited assimilation of hydrothermally altered material, is required to explain the slight decrease in δ 18 O values of zircons (and melts) that have been measured in these rhyolites.

Geophysical and petrological probes are key to understanding the structure and the thermochemical state of active magmatic systems. Recent advances in laboratory analyses, field investigations and numerical methods have allowed increasingly complex data-constraint models with new insights into magma plumbing systems and melt evolution. However, there is still a need for methods to quantitatively link geophysical and petrological observables for a more consistent description of magmatic processes at both micro- and macro-scales. Whilst modern geophysical studies provide detailed 3-D subsurface images that help to characterize magma reservoirs by relating state variables with physical material properties, constraints from on-site petrological analyses and thermodynamic modelling of melt evolution are at best incorporated qualitatively. Here, we combine modelling of phase equilibria in cooling magma and laboratory measurements of electrical properties of melt to derive the evolution of electrical conductivity in a crystallizing silicic magmatic system. We apply this framework to 3-D electrical conductivity images from magnetotelluric studies of two volcanoes in the Ethiopian Rift. The presented approach enables us to constrain key variables such as melt content, temperature and magmatic volatile abundance at depth. Our study shows that accounting for magmatic volatiles as an independent phase is crucial for understanding electrical conductivity structures in magma reservoirs at an advanced state of crystallization. Furthermore, our results deepen the understanding of the mechanisms behind volcanic unrest and help assess the long-term potential of hydrothermal reservoirs for geothermal energy production.

We present new U–Pb isotopic age data for detrital zircons from 16 deformed sandstones of the Ross Supergroup in north Victoria Land, Antarctica. Zircon U/Th ratios primarily point to dominantly igneous parent rocks with subordinate contributions from metamorphic sources. Comparative analysis of detrital zircon age populations indicates that inboard stratigraphic successions (Wilson Terrane) and those located outboard of the East Antarctic craton (the Bowers and Robertson Bay terranes) have similar ~ 1200–950 Ma (Mesoproterozoic–Neoproterozoic) and ~ 700–490 Ma (late Neoproterozoic–Cambrian, Furongian) age populations. The affinity of the age populations of the sandstones to each other, as well as Gondwana sources and Pacific-Gondwana marginal stratigraphic belts, challenges the notion that the outboard successions form exotic terranes that docked with Gondwana during the Ross orogeny and instead places the terranes in proximity to each other and within the peri-Gondwana realm during the late Neoproterozoic to Cambrian. The cumulative zircon age suite from north Victoria Land yields a polymodal age spectra with a younger, primary 700–480 Ma age population that peaks at ~ 580 Ma. Cumulative analysis of zircons with elevated U/Th ratios (> 20) indicating metamorphic heritage yield ~ 657–532 Ma age probability peaks, which overlap with the younger dominantly igneous zircon population. The data are interpreted to give important new evidence that is consistent with ongoing convergent arc magmatism by ~ 626 Ma, which provided the dominant zircon-rich igneous rocks and subordinate metamorphic rocks. Maximum depositional ages as young as ~ 493–481 Ma yielded by deformed sequences in the outboard Bowers and Robertson Bay terrane samples provide new support for late Cambrian to Ordovician deformation in outboard sectors of the orogen, consistent with tectonic models that call for cyclic phases of contraction along the north Victoria Land sector of the Ross–Delamerian orogen.

The partitioning of trace elements between crystals and melts provides an important petrogenetic tool for understanding magmatic processes. We present trace element partition coefficients measured between clinopyroxene phenocrysts and trachy-phonolitic magmas at the Campi Flegrei (Italy), whose late Quaternary volcanism has been characterized by two major caldera-forming events (Campanian Ignimbrite at ~ 39 ka, and Neapolitan Yellow Tuff at ~ 15 ka). Our data indicate that the increase of trivalent rare earth elements and yttrium into the crystal lattice M2 site is facilitated by the charge-balancing substitution of Si 4 + with Al 3 + on the tetrahedral site. Higher concentrations of tetravalent and pentavalent high field strength elements on the M1 site are also measured when the average charge on this site is increased by the substitution of divalent cations by Al vi . In contrast, due to these charge balance requirements, divalent transitional elements become less compatible within the crystal lattice. On the basis of the lattice strain theory, we document that the incorporation of rare earth elements and yttrium in clinopyroxene is influenced by both compositional and physical parameters. Data from this study allow to update existing partitioning equations for rare earth elements in order to construct a self-consistent model for trachy-phonolitic magmas based on the lattice strain theory. The application of this model to natural products from the Campanian Ignimbrite, the largest caldera-forming eruption at the Campi Flegrei, reveals that the complex rare earth element pattern recorded by the eruptive products can be successfully described by the stepwise fractional crystallization of clinopyroxene and feldspar where the clinopyroxene–melt partition coefficient changes progressively as a function of the physicochemical conditions of the system.

Water-rich silicic magmas are capable of erupting effusively and explosively, and this drastic change in eruptive styles, termed effusive-explosive transition, has important implications in managing volcanic hazards. Some volcanoes exhibit effusive-explosive transitions during the same eruptive event, while others show this behavior between different eruptions. In the latter case, magma chamber processes induce physical-chemical changes in the magma, which can favor either effusivity or explosivity. This is the case for the Nisyros-Yali volcanic center, from the South Aegean Sea. In the recent stages of activity (past 120 ky), the volcanic area generated eight rhyolitic effusive and explosive events (five on the island of Nisyros and three on the island of Yali), including two caldera-forming eruptions. Changes of water content, temperature and pre-eruptive water-saturation between effusive and explosive deposits point to a potential time-dependency between the two eruptive styles. We investigate this time-dependency by applying U Th disequilibrium dating to zircon crystals. Our eruptive age estimates of the investigated units range from 118.7 ± 10 ka to 19.9 ± 1.5 ka for Nisyros, and from 40 ± 5.2 ka to 22.7 ± 1.6 ka for Yali. Yali volcano has developed after the two caldera-forming events on Nisyros, which occurred at 63.1 ± 4.7 ka and 58.4 ± 2.7 ka. Yali marks the transition to a more geometrically complex system, where the upper-crustal silicic mush hosts at least two eruptible magma chambers (one under Yali, and one under Nisyros). The eruptive styles at both volcanoes seem to be correlated with the length of the repose periods. Effusive events occur after longer periods of volcanic quiescence, while explosive events are generated after shorter periods of repose of ~5–10 ky, which can be extended based on eruption age uncertainty to

Silicic magmas play an important role in the formation of continental crust and are responsible for some of the most hazardous volcanic eruptions on the planet. Low-δ18O silicic magmas (δ18O 800 °C) and dry silicic magmas. The relative scarcity of low-δ18O silicic magmas on Earth compared to normal- and high-δ18O magmas implies that coincidence of these factors is rare, and is most likely encountered in hotspot and rift settings characterized by bimodal basaltic-rhyolitic volcanism. Low-δ18O silicic magmas are usually generated by bulk assimilation of rocks that were hydrothermally altered at high temperatures (>300 °C) by isotopically light meteoric water, prevalent at mid to high latitudes and altitudes and/or linked to global glaciation episodes in Earth’s history. We estimate that

Lake City Caldera (22.93 ± 0.02 Ma) is the youngest of 25 Tertiary calderas within the Southern Rocky Mountain Volcanic Field (SRMVF) and offers an opportunity to study the relationship between plutonic rocks and their volcanic equivalents. Extreme topographical relief of the area reveals the three-dimensional exposure of a complex, high-K calc-alkaline, magmatic system. Lake City caldera is comprised of two principal units: 1) a resurgent quartz syenite intrusion, and 2) the Sunshine Peak Tuff (Lower, Middle, and Upper). The Lower and Middle Sunshine Peak Tuff (LSPT and MSPT) are crystal-poor rhyolite, while the Upper Sunshine Peak Tuff (USPT) is a crystal-rich trachyte. Rhyolite-MELTS modeling and geochemical analyses show that Lake City rhyolites were formed by melt extraction from a long-lived quartz syenitic magma reservoir at intermediate crystallinity (~50–70% crystals). A slight compositional zoning from the Lower Sunshine Peak Tuff (~76 wt% SiO2) to the Middle Sunshine Peak Tuff (~74 wt% SiO2) ignimbrites indicates that further modification by either mixing or additional crystal fractionation might have occurred following melt extraction from the mushy syenitic reservoir. Geochemical and textural analyses show that the Upper Sunshine Peak trachyte was formed by later re-melting of a portion of the left-over syenite cumulate. Quartz syenite crystal size distributions of potassium feldspar show the characteristic shape of crystal accumulation, and both volcanic and plutonic units contain abundant glomerocrysts. The presence of mafic enclaves within both the syenite and USPT trachyte, coupled with large increases of Ba content in rims of the K-feldspars from the trachyte suggest that the injection of less evolved magma into the host reservoir caused the re-melting of the syenite cumulate mush, allowing for the eruption of the full SPT sequence. Titanium-in-quartz thermobarometry shows that the majority of grains in all units formed below ~800 °C, further providing evidence for a petrogenic relationship for all the rocks comprising Lake City Caldera.

We review petrological and geochemical features of silicic pyroclastic deposits of dominantly low to moderate (0–25%) crystallinity, and volumes in the range of 5–1000 km3, erupted from caldera volcanoes. Chemical gradients in zoned deposits with compositions near the water-saturated granite minimum, for example the Bishop and Bandelier Tuffs, are consistent with mineral/melt partitioning predicted from the observed phenocryst assemblages, and are inconsistent with mixing with more mafic magma. Smaller volume alkaline (phonolite and pantellerite) systems show similar behavior. In contrast, high-temperature ignimbrites of the ‘Snake River’-type typically lack compositional zoning. Internal isotopic variations are weak or absent from whole rocks in both types of rhyolite, even in systems where associated volcanic rocks exhibit wide isotopic variation and strong contrasts exist between the isotopic compositions of mantle and crust. An exception to this is 87Sr/86Sr variations in high-silica rhyolite systems, where Sr has been depleted to subchondritic concentrations and is exceptionally sensitive to open-system processes. Both types of ignimbrite commonly contain crystal aggregates, interpreted as fragments of cumulate mush. In zoned systems, these aggregates exhibit evidence for partial resorption of early-formed crystals. We infer that chemical zoning is a near closed-system process and propose that it arises through melting of cognate cumulate mush beneath a crystal-poor body of melt due to heating by invading mafic or intermediate magma with little mass transfer to the eruptible magma. If the crystal mush is fusible (e.g. dominated by sanidine + quartz), part of it melts to yield mobile, water-poor rhyolite that pools at the interface between the mush and overlying rhyolitic liquid. This new, eruptible melt has a cumulate composition and is thus less evolved than the original supernatant melt lens. The result is a chemically zoned crystal-poor rhyolitic magma produced with little mass contribution from the invading magma. This model reconciles evidence for thermal rejuvenation, preserved in crystals, with evidence for the production of zoning by crystallization-differentiation, apparent in whole-rock chemistry. Fusibility of the cumulate is key to the process; high-temperature ‘Snake River’-type rhyolites are not zoned because their cumulates are dominated by a refractory assemblage of pyroxene, plagioclase, and Fe–Ti oxides. Previous models of compositional zoning have envisaged a pot of silicic magma undergoing slow cooling towards thermal senescence. In contrast, we contend that zoning records a history of thermal rejuvenation in which any one recharge event has the potential to trigger a caldera-forming eruption.

The Heiðarsporður ridge, located in the Northern Volcanic Zone of Iceland, was formed approximately 9000 years ago by a volcanic episode known as Ludent Fires. The episode produced a broad spectrum of different magma types, forming approximately 50 small scoria cones and two larger craters (Ludent and Hraunbunga). The bulk compositions cluster in five distinct groups: (1) olivine basalts, (2) Fe-Ti basalts, (3) basaltic icelandites, (4) icelandites, and (5) dacites. Major and trace element trends, together with mineral chemistry and isotopic ratios, suggest that the dominant process involved in generating the evolved magmas was crystal fractionation occurring at variable depth. An origin by polybaric differentiation is confirmed by MELTS modeling. Magma mixing played a dominant role in the formation of the basaltic icelandites. Additionally, the Fe-Ti basalts, which erupted shortly after the dacites and used approximately the same vent area, display unusually high concentrations of Fe, Ti, P, and Sr. Their composition is best explained by some pyroxene-dominated fractionation (prior to Fe-Ti oxide stability), and by entrainment of some crystal cumulate material at shallow depth, mostly left over from the silicic differentiation stage. Textural and chemical features of the minerals (e.g., presence of glomerocrysts, two populations of plagioclase in these basalts) support this interpretation of evolved cumulate remobilization. Fe-Ti basalts with the same field, compositional and textural characteristics have also been erupted in the nearby but magmatically independent Krafla Volcanic System, suggesting that a similar differentiation trend occurs also in this larger central volcano.

Many magmatic provinces produce strongly bimodal volcanism with abundant mafic and silicic magmas yet a scarcity of intermediate (55–65 wt.% SiO2) compositions. In such bimodal settings, much debate revolves around whether the basaltic magmas act as heat sources to melt pre-existing crust, or whether they are the parents to the silicic magmas (a fractionation-dominated evolution). Until now, this lack of coeval intermediate compositions has commonly been used to support models involving large degrees of crustal melting. Detailed analysis of mineral cargoes of ignimbrites from the 6.6–4 Ma Heise volcanic field in the famously bimodal Yellowstone–Snake River Plain (YSRP) volcanic province has revealed the existence of intermediate liquids associated with main stage rhyolitic volcanism. Two closely spaced rhyolitic ignimbrites, the Wolverine Creek Tuff and the Conant Creek Tuff, contain pyroxene crystals with major and trace elemental compositions in equilibrium with intermediate melts prior to significant plagioclase fractionation. Hosted within these crystals are glassy melt inclusions that have compositions (57–67 wt.% SiO2) directly recording the intermediate liquids. The combined mineral and melt inclusion data provide the first evidence for the occurrence of intermediate melts, typically erased in the high temperature YSRP ignimbrites by crystal resorption or diffusive re-equilibration. The results suggest the existence of mostly unerupted mid-crustal reservoirs that drive magma compositions towards the erupted rhyolites via assimilation-fractional crystallisation (AFC).

We present a K-feldspar–liquid hygrometer specific to alkaline differentiated magmas that is calibrated through the regression analysis of sanidine and anorthoclase crystals coexisting with trachyte and phonolite melts. Partial-regression leverage plots were used to determine the minimum number of regression parameters that closely describe the variance of the dataset. The derived model was tested using K-feldspar–liquid pairs not included into the calibration dataset in order to address issues of systematic errors. When K-feldspar and plagioclase crystals coprecipitate from the same alkaline liquid under identical P-T-X-fO2-H2O conditions, the ability prediction of the new hygrometer is comparable to that of previous plagioclase–liquid models. To minimize the error of H2O estimate caused by the inadvertent use of disequilibrium data in natural samples, we have also calibrated a new test for equilibrium based on Or–Ab exchange between K-feldspar and coexisting melt. As an immediate application for both equilibrium and hygrometer models, we used as input data K-feldspar–liquid pairs from alkaline explosive eruptions at the Phlegrean Fields. The estimates of H2O dissolved in natural trachyte and phonolite magmas closely match those determined by melt inclusion analysis and H2O solubility modeling. This leads to the conclusion that our new models can significantly contribute to a better quantitative characterization of the H2O content in differentiated alkaline magmas feeding large-volume explosive eruptions.

This contribution illustrates a case study of a pluton (La Gloria pluton; LGP) where magnetic and magmatic fabrics are locally decoupled. We compare the magmatic fabric with the available magnetic fabric data to explore their abilities and elucidate the magma flow record of LGP. Results indicate that magnetic (controlled by multi-domain magnetite) and magmatic fabrics are generally consistent throughout LGP. Foliations define an axisymmetric pattern that gradually changes from vertical near lateral margins to less steep in the pluton interior, whereas lineations are subhorizontal following the elongation direction of the pluton. However, samples at the pluton center show marked differences between both fabrics: magnetic fabrics indicate subhorizontal magnetic lineations and foliations, and magmatic fabrics indicate subvertical lineations and foliations. Both magnetic and magmatic fabrics are interpreted to record strain caused by magma flow during thermal convection and lateral magma propagation at the transition between low and high crystallinity stages. We suggest that fabrics acquisition and consistency were determined by shear conditions (pure/simple shear rates ratio) and the orientation of the magma flow direction with respect to a rigid boundary (critical crystalline region) of the pluton. Magmatic fabric differs at the center of the pluton because pure shear is dominant and ascendant flows are orthogonal to the horizontal rigid boundary. LGP represents a whole-scale partly molten magma reservoir, where both thermal convection and lateral propagation of the magma are recorded simultaneously. This study highlights the importance of characterizing both fabrics to properly interpret magma flow recorded in plutons.

The formation of crystal-poor high-silica rhyolite via extraction of interstitial melt from an upper crustal mush predicts the complementary formation of large amounts of (typically unerupted) silicic cumulates. However, identification of these cumulates remains controversial. One hindrance to our ability to identify them is a lack of clear predictions for complementary chemical signatures between extracted melts and their residues. To address this discrepancy, we present a generalized geochemical model tracking the evolution of trace elements in a magma reservoir concurrently experiencing crystallization and extraction of interstitial melt. Our method uses a numerical solution rather than analytical, thereby allowing for various dependencies between crystallinity, partition coefficients for variably compatible and/or incompatible elements, and melt extraction efficiency. Results reveal unambiguous fractionation signatures for the extracted melts, while those signatures are muted for their cumulate counterparts. Our model is first applied to a well-constrained example (Searchlight pluton, USA), and provides a good fit to geochemical data. We then extrapolate our results to understanding the relationship between volcanic and plutonic silicic suites on a global scale. Utilizing the NAVDAT database to identify crystal accumulation or depletion signatures for each suite, we suggest that many large granitoids are indeed silicic cumulates, although their crystal accumulation signature is expected to be subtle.

Article history: Most magmatism on Earth forms by direct melting of the mantle, generating basalts at the low silica end of the terrestrial compositional spectrum. However, most subduction zone magmas erupted or sampled at the surface are basalt-andesitic to andesitic and hence have higher Si contents. Endmember hypotheses for the origin of andesites are: (1) direct melting of the mantle at water-saturated conditions, (2) partial re-melting of altered basaltic crust, (3) crystal fractionation of arc basalts in crustal magma chambers, and (4) mixing of mafic magmas with high Si crust or magmas, e.g., dacite-rhyolite. Here, we explore the possibility of using Zr and P systematics to evaluate the importance of some of these processes. Direct melting of the mantle generates magmas with low Zr ( 100 ppm) and P2O5 (>0.2 wt.%) in island- and continental-arc magmas initially increase to levels higher than what can be achieved if andesites form by direct mantle melting. As Si increases, both Zr and P decrease with Zr decreasing at higher Si, and hence lagging the decrease in P. These systematics, particularly the decoupled decrease in Zr and P, cannot be explained by mixing, and instead, are more easily explained if andesites are dominantly formed by crystal-liquid segregation from moderately hydrous basalt, wherein P and Zr are controlled, respectively, by early and later saturation in apatite and zircon. Although there is clear isotopic and outcrop (enclaves) evidence for mixing in magmatic systems, crystal-liquid segregation appears to be the dominant process in generating intermediate magmas, with mixing playing a secondary role. Finally, recent studies have suggested that the abundance of certain magma compositions in a given v olcanic setting may be dictated by the optimal crystallinity window for efficient crystal-liquid separation (50-70 vol%). We show that the SiO2 content of the residual liquid in this crystallinity window increases with increasing water content. We thus speculate that high water contents (> 2w t.% H 2O) may favor extraction of andesitic and dacitic liquids while lower water contents favor extraction of more basaltic magmas. If continental arc magmas tend to be more andesitic, as often believed, it follows that they may begin more water-rich than island arc magmas, which are basaltic. In any case, if intermediate arc magmas are formed dominantly by crystal-liquid fractionation, large volumes of complementary mafic cumulates must be generated during the formation of andesitic magmas, as is seen in well-exposed crustal sections.

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of eight zircon reference materials and synthetic zircon-hafnon end-members indicate that corrections for abundance sensitivity and molecular zirconium sesquioxide ions (Zr 2 O 3 + ) are critical for reliable determination of 230 Th abundances in zircon. Other polyatomic interferences in the mass range 223–233 amu are insignificant. When corrected for abundance sensitivity and interferences, activity ratios of ( 230 Th)/( 238 U) for the zircon reference materials we used average 1.001 ± 0.010 (1σ error; mean square of weighted deviates MSWD = 1.45; n = 8). This includes the 91500 and Plesovice zircons, which were deemed unsuitable for calibration of ( 230 Th)/( 238 U) by Ito (2014). Uranium series zircon ages generated by LA-ICP-MS without mitigating (e.g., by high mass resolution) or correcting for abundance sensitivity and molecular interferences on 230 Th such as those presented by Ito (2014) are potentially unreliable.

International audience; The eruption style of silicic magmas is affected by the loss of gas (outgassing) during ascent. We investigate outgassing using a numerical model for one-dimensional, two-phase, steady flow in a volcanic conduit. By implementing Forchheimer's equation rather than Darcy's equation for outgassing we are able to investigate the relative influence of Darcian and inertial permeability on the transition between effusive and explosive eruptions. These permeabilities are defined by constitutive equations obtained from textural analysis of pyroclasts and determined by bubble number density, throat-bubble size ratio, tortuosity, and roughness. The efficiency of outgassing as a function of these parameters can be quantified by two dimensionless quantities: the Stokes number, the ratio of the response time of the magma and the characteristic time of gas flow, and the Forchheimer number, the ratio of the viscous and inertial forces inside the bubble network. A small Stokes number indicates strong coupling between gas and magma and thus promotes explosive eruption. A large Forchheimer number signifies that gas escape from the bubble network is dominated by inertial effects, which leads to explosive behaviour. To provide context we compare model predictions to the May 18, 1980 Mount St. Helens and the August-September 1997 Soufrière Hills eruptions. We show that inertial effects dominate outgassing during both effusive and explosive eruptions, and that in this case the eruptive regime is determined by a new dimensionless quantity defined by the ratio of Stokes and Forchheimer number. Of the considered textural parameters, the bubble number density has the strongest influence on this quantity. This result has implications for permeability studies and conduit modelling.

International audience; Scoriae produced by four mafic historic eruptions of Volcán Llaima are used to elucidate magma differentiation, recharge, mixing, and eruption triggering. Whole-rock, mineral, and olivine-hosted melt inclusion chemistry suggest that basaltic andesites (~ 52-53.5 wt.% SiO2) stall at immediately sub-edifice depths (≤ 4 km beneath the base of the volcano), where they crystallize to large extents and form mush bodies. Melt inclusion trends overlap with the whole-rock data trends defined by the entire volcano up to 53-55% SiO2, but more evolved compositions define divergent trends for some elements, with up to 2.2 wt% TiO2 (increasing Zr) and Al2O3 as low as 12.5 wt% (decreasing Sr) at 57-58 wt% SiO2. These more evolved melt compositions are inferred to be the result of shallow evolution of interstitial melt during the formation of crystal mush bodies, as a consequence of degassing and crystallization with a reduced participation of Fe-Ti-oxides. The inferred suppression of Fe-Ti-oxide stability and the modal dominance of plagioclase crystallization are consistent with low fluid-saturation pressures of ~ 40-70 MPa inferred from average H2O contents for mafic arc magmas (dominantly 1-4 wt%) but low CO2 contents (dominantly 0-300 ppm) in melt inclusions. The broad range in olivine core compositions (Fo69-83) and the absence of correlated degassing and magma evolution trends in historic Llaima magmas suggest that they are stored as multiple dike-like bodies created by a high frequency of magma replenishment relative to the frequency of large eruptions. This temporal-spatial disconnection leads to isolated evolution and degassing of discrete magma batches, followed by remobilization and assembly just prior to eruption. Eruptions are probably triggered by recharge of relatively hot, mafic, and much less degassed magma, in accord with dominantly reversely-zoned olivine crystals, and higher olivine-melt temperatures recorded by the relatively primitive matrix glasses and coexisting olivine rims.

Super-volcanic eruption deposits, such as the Ammonia Tanks Tuff (AT; ~ 900 km 3 ), typically record the explosive evacuation of a chemically and thermally zoned shallow magma reservoir. Such zoning patterns require either 1) mixing between two magmas or 2) in-situ differentiation driven by crystal–liquid separation. Here we show, using a geochemical model firmly grounded on natural observations, that these processes are not mutually exclusive. Using an initial trachydacitic magma, the interstitial liquid evolves to a rhyolitic composition as the magma reaches high crystallinity (mush state; ≥~50% crystals). The low-density rhyolitic melt then migrates upwards and leaves behind a trachyandesite cumulate with distinctive trace element concentrations (e.g., high Ba, Sr, Zr) that can be modeled by crystal fractionation. During eruption, both liquid cap and underlying cumulate are evacuated from a shallow reservoir, leading to the observed chemical gradient. However, to account for the thermal gradient and low crystal fraction (~ 25 vol.%) in the cumulate trachyandesite of the AT tuff, a mixing stage with hot mafic recharge must have occurred prior to eruption. The late recharge induced heating, partially melting a significant portion of the crystal cargo and ultimately triggering the eruption.

Thermal reactivation of locked crystal mushes in the upper crust is a fundamental step towards volcanic eruptions of crystal-rich magmas. Models of such reactivation events indicate that partial melting of the crystalline framework is energetically costly and lead to average crystallinities that are lower than those observed in many erupted crystal mushes. Here, we show that internal overpressurization of the mush induced by small amounts of melting (10–20 % ) breaks the crystalline framework by microfracturing and allows for efficient unlocking of the mush. Hence, this melting-induced overpressurization, enhanced by addition of gas in wet magmatic systems, plays an important role in generating volcanic deposits with crystal contents close to the rheological lock-up (∼ 50 vol % crystals) by accelerating the incorporation of highly crystalline parts of the magma chamber (self-assimilation). It can also participate in disintegrating pieces of country rock that are commonly scavenged in magmas, leading to bulk assimilation of crustal lithologies in shallow reservoirs.

If fractional crystallization coupled with some crustal assimilation (AFC) is the dominant process for generating evolved magmas in the Earth's crust, large quantities of igneous cumulates must be present in the rock record. However, despite some reported occurrences, intermediate to silicic rocks characterized by an unambiguous crystal accumulation signature have remained elusive. The signature is expected to be subtle in viscous silicic magmas, and sensitive criteria need to be developed to test whether these cumulates are truly rare or simply difficult to discern. We propose models that use specific trace element concentrations and ratios (Zr, Ba, Eu/Eu*, and Zr/Hf) within a well-constrained framework for the physical extraction of liquid from crystals (mostly occurring between 50 and 70 vol.% crystals) to identify silicic cumulates. The results of these models indicate that crystal accumulation (or melt extraction) has occurred in multiple natural examples around the world (both in plutonic and volcanic realms). However, these examples were selected because they represent conditions that are most favorable (high amount of melt extraction, well-constrained initial liquid compositions) for unambiguously identifying cumulate residue; in many cases, we show that the trace element signature for crystal accumulation will remain largely concealed. In addition, the different liquid lines of descent related to the various tectonic settings on Earth (dry-reduced in hot spots-divergent margins and wetter-more oxidizing in arc environments) will lead to different trace element evolution and each criterion must be applied on a case-by-case basis. Despite these limitations, the undeniable but subtle presence of silicic cumulates in the mid- to upper-crust lends support to the idea that igneous differentiation is dominantly driven by melt extraction from partially crystalline reservoirs.

High crystallinity silicic ignimbrites (such as the Monotonous Intermediates) typically erupt magma with an average crystallinity ranging from 40 to 50%. This average crystallinity is believed to be just under the threshold at which magma behaves as a solid (50–60% crystals), i.e. the locking point crystallinity, where convection is suppressed and large eruptions are unlikely. These magmas often display textural features which suggest that their average crystallinity was once higher and decreased before the eruption as a result of reheating through the injection of new magma. In this study, we use a theoretical 1D heat conduction model with phase change to test the ability of different melting scenarios of crystal mushes to meet the 40 to 50% crystallinity constraint observed in the field. Our heat conduction and melting models allow us to derive analytical solutions for the average crystallinity in the magma body (initially a crystal mush). We focus on the propagation of the melting front coinciding with the locking point crystallinity for different crystallinity–temperature relationships and various choices of temperature boundary conditions. We develop another analytical model based on stagnant-lid convection scaling to assess the role of convection on the expected average crystallinity of the magma subjected to wholesale steady-state convection. We find that, for all realistic melting scenarios, the average crystallinity of a silicic magma body that passed through the rheological transition is always substantially lower than what is observed in the field. We further show with a simple energy balance that the thermal energy needed to unlock/remobilize these magma bodies requires the intrusion of about an order of magnitude of more magma than the mush. Based on these results we argue that, although melting is a key process in the thermal reactivation of high crystallinity magma bodies, another coupled process is required in order to reactivate large volumes of crystal mushes. Moreover this additional process has to (1) be more efficient energetically and (2) lead to a smaller overall crystallinity reduction than melting alone.

Heterogeneities in crustal magma chambers are generated by incomplete mixing of different batches of magmas, precipitation or remelting of solids ~ 10–20% denser than surrounding liquids at cooling/heating boundaries, and by liquid extraction from more dense crystalline residues. Volcanic eruptions commonly sample that heterogeneity as many deposits from large explosive events show gradients in composition, crystallinity and temperature. However, some of the most viscous of all magmas – crystal-rich rhyolites–dacites and large silicic plutons – are, in contrast, strikingly homogeneous. This observation is in stark contradiction to the common assumption that the main homogenizing process in magma chambers is mechanical mixing by convection. The timescale for convective stirring is governed by flow velocities. The faster the convective currents, the more rapidly mixing occurs. However, it is the total amount of strain that determines whether homogeneity can be attained. Here, we show that convectiving magma body needs 5–10 overturns following the introduction of heterogeneities to be homogenized to lengthscales at which diffusion is effective, irrespective of the vigor of the convection. For heterogeneities that are discrete in time and space and mechanically passive (such as patches of magmas with slightly different characteristics), the threshold of 5–10 overturns can potentially be reached if enough time is allowed for the system to convect. For heterogeneities that are continuously re-established or introduced by the convective process itself (such as density instabilities generated by crystallization at the cooling boundaries of magma chambers), convective stirring is unable to produce complete homogeneity. Therefore, to explain the low variability in major element whole-rock composition in crystal-rich dacitic/rhyolitic ignimbrites and silicic plutons, we propose that another mechanism of homogenization acts to decrease thermal, and related crystallinity variability at high crystal fraction (> 50 vol.%). It is induced by latent heat buffering of silicic magmas close to the haplogranitic eutectic, and leads to the rapid equilibration of temperature (and crystal fraction) throughout the magma reservoir. As this process drives the magma body towards a uniformly high crystallinity (“crystal mush”), we refer to it as “mushification”. Nevertheless, the homogeneity of both large crystal-rich silicic ignimbrites and granites also require (1) incremental growth dominated by addition of magma batches that are compositionally similar to reach the observed large sizes without inducing unmixable chemical heterogeneities and (2) some late convective stirring at high crystallinity to explain the dispersal of externally derived crystalline material, e.g. xenocrysts and antecrysts. Late mechanical mixing mostly occurs at high crystallinity (~ 30–50 vol.% crystals) following the intrusion of hotter magmas (and gas) from below, triggering whole-reservoir convective overturn without introducing new significant gradients.

We develop a lattice Boltzmann method to couple thermal convection and pure-substance melting. The transition from conduction-dominated heat transfer to fully-developed convection is analyzed and scaling laws and previous numerical results are reproduced by our numerical method. We also investigate the limit in which thermal inertia (high Stefan number) cannot be neglected. We use our results to extend the scaling relations obtained at low Stefan number and establish the correlation between the melting front propagation and the Stefan number for fully-developed convection. We conclude by showing that the model presented here is particularly well-suited to study convection melting in geometrically complex media with many applications in geosciences.

The Earth's magmatism produces both volcanic and plutonic rocks. These two rock types share many similarities, but also display significant differences that have led to a tendency to view (and study) them as separate realms. This review tries to bridge the gap to provide more incentive to integrate data from all magmatic rocks in a hope to better understand the processes that led to the differentiation of the Earth and generation of a silicic continental crust. We strongly reinforce recent statements made in the literature suggesting that most disparities between volcanic and plutonic rocks can be resolved if volcanic rocks are seen as erupted melt-rich regions (magma “chambers”) expelled from crystal-rich (mushy) reservoirs, which later crystallize to form plutons. © 2007 Elsevier B.V. All rights reserved.

The ∼ 5000 km3 Fish Canyon Tuff (FCT) is an important unit for the geochronological community because its sanidine, zircon and apatite are widely used as standards for the 40Ar/39Ar and fission track dating techniques. The recognition, more than 10 years ago [Oberli, F., Fischer, H. and Meier, M., 1990. High-resolution 238U–206Pb zircon dating of Tertiary bentonites and Fish Canyon Tuff; a test for age “concordance” by single-crystal analysis. Seventh International Conference on Geochronology, Cosmochronology and Isotope Geology. Geological Society of Australia Special Publication Canberra, 27:74], of a ≥ 0.4 Ma age difference between the U–Pb zircon ages and 40Ar/39Ar sanidine ages has, therefore, motivated efforts to resolve the origin of this discrepancy. To address this controversial issue, we initially performed 37 U–Pb analyses on mainly air-abraded zircons at ETH Zurich and nearly 200 40Ar/39Ar measurements on hornblende, biotite, plagioclase and sanidine obtained at the University of Geneva, using samples keyed to a refined eruptive stratigraphy of the FCT magmatic system. Disequilibrium-corrected 206Pb/238U ages obtained for 29 single-crystal and three multi-grain analyses span an interval of ∼ 28.67–28.03 Ma and yield a weighted mean age of 28.37 ± 0.05 Ma (95% confidence level), with MSWD = 8.4. The individual dates resolve a range of ages in excess of analytical precision, covering ∼ 600 ka. In order to independently confirm the observed spread in zircon ages, 12 additional analyses were carried out at the Berkeley Geochronology Center (BGC) on individual zircons from a single lithological unit, part of them pre-treated by the “chemical abrasion” (CA) technique [Mattinson, J.M., 2005. Zircon U–Pb chemical abrasion (“CA-TIMS”) method: Combined annealing and multi-step partial dissolution analysis for improved precision and accuracy of zircon ages. Chemical Geology, 220(1–2): 47–66]. Whereas the bulk of the BGC results displays a spread overlapping that obtained at ETH, the group of CA treated zircons yield a considerably narrower range with a mean age of 28.61 ± 0.08 Ma (MSWD = 1.0). Both mean zircon ages determined at ETH and BGC are older than the ∼ 28.0 Ma 40Ar/39Ar eruption age of FCT – even when considering the possibility that the latter may be low by as much as ∼ 1% due to a miscalibration of the 40K decay constants – and is thus indicative of a substantial time gap between magma crystallization and extrusion. The CA technique further reveals that younger FCT zircon ages are likely to be associated with chemically unstable U-enriched domains, which may be linked to crystallization during extended magma residence or may have been affected by pre-eruptive and/or post-eruptive secondary loss of radiogenic lead. Due to their complex crystallization history and/or age bias due to Pb loss, the FCT zircon ages are deemed unsuitable for an accurate age calibration of FCT sandine as a fluence monitor for the 40Ar/39Ar method. Even though data statistics preclude unambiguous conclusions, 40Ar/39Ar dating of sanidine, plagioclase, biotite, and hornblende from the same sample of vitrophyric Fish Canyon Tuff supports the idea of a protracted crystallization history. Sanidine, thought to be the mineral with the lowest closure temperature, yielded the youngest age (28.04 ± 0.18 Ma at 95% c.l., using Taylor Creek Rhyolite [Renne, P.R. et al., 1998. Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating. Chemical Geology, 145: 117–152.] as the fluence monitor), whereas more retentive biotite, hornblende and plagioclase gave slightly older nominal ages (by 0.2–0.3 Ma). In addition, a laser step-heating experiment on a 2-cm diameter feldspar megacryst produced a “staircase” argon release spectrum (older ages at higher laser power), suggestive of traces of inherited argon in the system. Thermal and water budgets for the Fish Canyon magma indicate that the body remained above its solidus (∼ 700 °C) for an extended period of time (> 105 years). At these temperatures, argon volume diffusion is thought to be fast enough to prevent accumulation of radiogenic Ar. If this statement were true, an existing isotopic record should have been completely reset within a few hundred years, regardless of the phase and initial age of the phenocryst. As these minerals are unlikely to be xenocrysts that were incorporated within such a short time span prior to eruption, we suggest that a fraction of radiogenic Ar can be retained > 105 years, even at T ∼ 700 °C.

Several crystal-rich, intermediate to silicic magmas erupted at arc volcanoes record a reheating event shortly prior to eruption: they provide evidence for remobilization of crystal mushes by mafic magmas. As hybridization between the mush and the mafic magma is often limited, bulk mixing could not be the dominant process in transferring heat. Conductive heating from a basaltic underplate plays a role, but a few characteristics of these rejuvenated mushes suggest that reheating occurs faster than predicted by conduction. In the upper crust, a process that can transport heat faster than conduction, and still remain chemically nearly imperceptible, is the upward migration of a hot volatile phase (bgas spargingQ) that originates in underplated mafic magmas. Using numerical simulations, we quantified the thermal effects of two-phase flow (a silicic melt phase and a H2O–CO2 fluid phase) in the pore space of shallow silicic mushes that have reached their rheological lock-up point (i.e., rigid porous medium, crystallinity z50 vol.%). Results show that the reheating rates are significantly faster than conduction for volatile fluxes N0.1 m 3 /m 2 yr. Considering that volatiles can be rapidly exsolved from the underplated mafic magma, these high fluxes can be promptly

In order to understand outgassing during volcanic eruptions, we performed permeability measurements on trachy-phonolitic pyroclastic products from the Campanian Ignimbrite and Monte Nuovo, two explosive eruptions from the active Campi Flegrei caldera, Southern Italy. Viscous (Darcian) permeability spans a wide range between 1.22 x 10(-14) and 9.31 x 10(-11) m(2). Inertial (non-Darcian) permeability follows the same trend as viscous permeability: it increases as viscous permeability increases, highlighting the strong direct correlation between these two parameters. We observe that vesicularity does not exert a first order control on permeability: the Monte Nuovo scoria clasts are the most permeable samples but not the most vesicular; pumice clasts from the Campanian Ignimbrite proximal facies, whose vesicularity is comparable with that of Monte Nuovo scoriae, are instead the least permeable. In addition, we find that sample geometry exhibits permeability anisotropy as samples oriented parallel to vesicle elongation are more permeable than those oriented perpendicular. We compare our results with permeability values of volcanic products from effusive and explosive activity, and discuss the role of melt viscosity and crystallinity on magma permeability.