Magma Flux Research Articles

Phreatomagmatic eruptions through unconsolidated coastal plain sequences, Maungataketake, Auckland Volcanic Field (New Zealand)

Maungataketake is a monogenetic basaltic volcano formed at ~85–89ka in the southern part of the Auckland Volcanic Field (AVF), New Zealand. It comprises a basal 1100-m diameter tuff ring, with a central scoria/spatter cone and lava flows. The tuff ring was formed under hydrogeological and geographic conditions very similar to the present. The tuff records numerous density stratified, wet base surges that radiated outward up to 1km, decelerating rapidly and becoming less turbulent with distance. The pyroclastic units dominantly comprise fine-grained expelled grains from various sedimentary deposits beneath the volcano mixed with a minor component of juvenile pyroclasts (~35vol.%). Subtle lateral changes relate to deceleration with distance and vertical transformations are minor, pointing to stable explosion depths and conditions, with gradual transitions between units and no evidence for eruptive pauses. This volcano formed within and on ~60m-thick Plio/Pleistocene, poorly consolidated, highly permeable shelly sands and silts (Kaawa Formation) capped by near-impermeable, water-saturated muds (Tauranga Group). These sediments rest on moderately consolidated Miocene-aged permeable turbiditic sandstones and siltstones (Waitemata Group). Magma–water fuelled thermohydraulic explosions remained in the shallow sedimentary layers, excavating fine-grained sediments without brittle fragmentation required. On the whole, the resulting cool, wet pyroclastic density currents were of low energy. The unconsolidated shallow sediments deformed to accommodate rapidly rising magma, leading to development of complex sill-like bodies and a range of magma–water contact conditions at any time. The weak saturated sediments were also readily liquefied to provide an enduring supply of water and fine sediment to the explosion loci. Changes in magma flux and/or subsequent stabilisation of the conduit area by a lava ring-barrier led to ensuing Strombolian and fire-fountaining eruption phases. Future eruptions in littoral environments around Auckland are likely to be of this type, producing base surges that rapidly decrease in energy over short runout distances (~1km).

Pre-eruptive storage conditions and eruption dynamics of a small rhyolite dome: Douglas Knob, Yellowstone volcanic field, USA

The properties and processes that control the size, duration, and style of eruption of rhyolite magma are poorly constrained because of a paucity of direct observations. Here, we investigate the small-volume, nonexplosive end-member. In particular, we determine the pre-eruptive storage conditions and eruption dynamics of Douglas Knob, a 0.011-km3 obsidian dome that erupted from a 500-m-long fissure in the Yellowstone volcanic system. To determine pre-eruptive storage conditions, we analyzed compositions of phenocrysts, matrix glass, and quartz-hosted glass inclusions by electron microprobe and Fourier-transform infrared analyses. The pre-eruptive melt is a high-silica rhyolite (∼75 wt.% SiO2) and was stored at 760 ± 30 °C and 50 ± 25 MPa prior to eruption, assuming vapor saturation at depth. To investigate emplacement dynamics and kinematics, we measured number densities and orientations of microlites at various locations across the lava dome. Microlites in samples closest to the inferred fissure vent are the most aligned. Alignment does not increase with distance traveled away from the vent, suggesting microlites record conduit processes. Strains of <5 accumulated in the conduit during ascent after microlite formation, imparted by a combination of pure and simple shear. Average microlite number density in samples varies from 104.9 to 105.7 mm−3. Using the magma ascent model of Toramaru et al. (J Volcanol Geotherm Res 175:156–157, 2008), microlite number densities imply decompression rates ranging from 0.03 to 0.11 MPa h−1 (∼0.4–1.3 mm s−1 ascent rates). Such slow ascent would allow time for passive degassing at depth in the conduit, thus resulting in an effusive eruption. Using calculated melt viscosity, we infer that the dike that fed the eruption was 4–8 m in width. Magma flux through this dike, assuming fissure dimensions at the surface represent its geometry at depth, implies an eruption duration of 17–210 days. That duration is also consistent with the shape of the dome if produced by gravitational spreading, as well as the ascent time of magma from its storage depth.

Magmatic lulls in the Sierra Nevada captured in zircon from rhyolite of the Mineral King pendant, California

The Mineral King pendant in the Sierra Nevada batholith (California, USA) contains at least four rhyolite units that record high-silica volcanism during magmatic lulls in the Sierran magmatic arc. U-Th-Pb, trace element (single crystal spot analyses via sensitive high-resolution ion microprobe–reverse geometry, SHRIMP-RG), and bulk oxygen isotope analyses of zircon from these units provide a record of the age and compositional properties of the magmas that is not available from whole-rock analysis because of intense hydrothermal alteration of the pendant. U-Pb spot ages reveal that the Mineral King rhyolites are from two periods, the Early Jurassic (197 Ma) and the Early Cretaceous (134–136 Ma). These two rhyolite packages have zircons with distinct compositional trends for trace elements and δ 18 O; the Early Jurassic rhyolite shows less evidence of crustal influences on the rhyolites and the Early Cretaceous rhyolite shows evidence of increasing crustal influences and crystal recycling. These rhyolites capture evidence of magmatism during two periods of low magmatic flux in the Sierran Arc; however, they still show that magmas were derived from interactions of maturing continental crust, increasing from the Early to Late Jurassic. This finding likely reflects the transition of the North America margin from one of docking island arcs in the Early Jurassic to one of a more mature continental arc in the Early Cretaceous. This also shows the utility in examining zircon spot ages combined with trace element and bulk isotopic composition to unlock the petrogenetic history of altered volcanic rocks.

Solubility of C[sbnd]O[sbnd]H volatiles in graphite-saturated martian basalts

To determine the speciation and concentrations of dissolved COH volatiles in graphite-saturated martian primitive magmas, we conducted piston-cylinder experiments on graphite-encapsulated synthetic melt of Adirondack-class Humphrey basaltic composition. Experiments were performed over three orders of magnitude in oxygen fugacity (IW+2.3 to IW−0.8), and at pressures (1–3.2GPa) and temperatures (1340–1617°C) similar to those of possible martian source regions. Oxygen fugacities were determined from compositions of coexisting silicate melt+FePt alloy, olivine+pyroxene+FePt alloy, or melt+FeC liquid. Infrared spectra of quenched glasses all show carbonate absorptions at 1430 and 1520cm−1, with CO2 concentrations diminishing under more reduced conditions, from 0.50wt% down to 26ppm. Carbon contents of silicate glasses and FeC liquids were measured using secondary ion mass spectrometry (SIMS) yielding 36–716ppm and 6.71–7.03wt%, respectively. Fourier transform infrared (FTIR) and SIMS analysis produced similar H2O contents of 0.26–0.85 and 0.29–0.40wt%, respectively. Raman spectra of glasses reveal evidence for OH− ions, but no indication of methane-related species. FTIR-measured concentrations of dissolved carbonate diminish linearly with oxygen fugacity, but more reduced conditions yield greater dissolved carbonate concentrations than would be expected based on oxidized conditions in previous work. C contents of silicate glasses determined by SIMS are consistently higher than C as carbonate determined by FTIR. Their difference, termed non-carbonate C, correlates well with additional IR absorptions found in reduced glasses (fO2<IW+0.4) at 1615, 2205, and 3370cm−1. These absorption bands are not seen in more oxidized glasses, except B441 (IW+1.7), presumably because they represent reduced C-bearing complexes. The 2205cm−1 peak is attributed to a CO complex, possibly an Fe-carbonyl ion. The 1615cm−1 peak does not correlate with that at 2205cm−1, but does correlate with non-carbonate C and is in a region commonly associated with CO bonding. The origin of the peak at 3370cm−1 is poorly understood and could potentially be owing to a variety of COH species or to NH bonding. The intensities of the 1615 and 3370cm−1 peaks correlate with each other leading us to provisionally attribute both to an unspecified complex with both CO and NH bonds. These results suggest that dissolved species such as carbonyl or other CO-bearing species could be a significant source of C fluxes to the martian atmosphere, with minor additions of CO2 and negligible methane contributions. By assuming that degassed, reduced C ultimately becomes atmospheric CO2, reduced C outgassing may be incorporated in models of martian atmospheric evolution. At Humphrey source region conditions (1350±50°C, 1.2±0.1GPa) the total C contents are equivalent to 1200ppm CO2 at IW+1 and 475ppm CO2 at IW, which are 2 and 4 times higher than the CO2 derived from CO32− alone. For reasonable magmatic fluxes over the last 4.5Ga of martian history, such graphite-saturated magmas would produce 0.25 and 0.60bars from sources at IW and IW+1, significantly more than expected solely from consideration of dissolved CO2. The carbon contents of FeC liquids in this study are consistent with graphite-saturated carbide liquids becoming more C-rich with increasing temperature. Experiments with melt and FeC liquid have values of DCall/sil between 1.3×103 and 2.2×103, potentially allowing planetary mantles to retain significant C following core formation.

Quantification of the Intrusive Magma Fluxes during Magma Chamber Growth at Soufriere Hills Volcano (Montserrat, Lesser Antilles)

Magma fluxes in the crust control the thermal viability and mechanical stability of magma chambers. We estimated the magma fluxes required to generate the negative seismic velocity anomaly observed below Soufriere Hills volcano, Montserrat. Growth of a magma body by accretion of andesitic sills was simulated numerically and the resulting temperatures and melt fractions were used to calculate a synthetic anomaly of seismic wave velocity, which was filtered to be comparable with the velocity anomaly obtained from a tomographic experiment. Petrology indicates that before it was reheated, remobilized and erupted, the temperature of the magma residing in the chamber was about 850°C. We ran simulations where convection is assumed to be low and heat transfer is mostly by conduction and simulations where convection is assumed to be vigorous enough to rapidly cool the magma chamber to 850°C. In both cases, magma chamber growth over the last 350 years results in tomography anomalies that are too strong, unless the magma was emplaced at an unlikely low melt fraction (<0·5). Good fits between the modelled and the observed velocity anomaly were obtained with sills 2–5 km in radius emplaced over 6000–150 000 years, depending on the temperature and melt fraction of the emplaced magma. Because of a trade-off between intrusion dimensions and emplacement durations, the volumetric magma fluxes are restricted to 7 × 10?4 and 5 × 10?3 km3 a?1. The velocity anomaly can be reproduced with a chamber containing high melt-fraction magma or with a mush of crystals and melt. The range of magma ages in the modelled magma chamber is much wider than the crystal residence time of the erupted andesite. This suggests that the eruption taps small pockets of recently assembled magma and that the velocity anomaly is mostly due to a non-eruptible mush.

Petrogenesis of Mount Rainier andesite: Magma flux and geologic controls on the contrasting differentiation styles at stratovolcanoes of the southern Washington Cascades

Quaternary Mount Rainier (Washington, USA) of the Cascades magmatic arc consists of porphyritic calc-alkaline andesites and subordinate dacites, with common evidence for mingling and mixing with less evolved magmas encompassing andesites, basaltic andesites, and rarely, basalts. Basaltic andesites and amphibole andesites (spessartites) that erupted from vents at the north foot of the volcano represent some of Mount Rainier’s immediate parents and overlap in composition with regional basalts and basaltic andesites. Geochemical (major and trace elements) and isotopic (Sr, Nd, Pb, O) compositions of Mount Rainier andesites and dacites are consistent with modest assimilation (typically ≤20 wt%) of evolved sediment or sediment partial melt. Sandstones and shales of the Eocene Puget Group, derived from the continental interior, are exposed in regional anticlines flanking the volcano, and probably underlie it in the middle to lower crust, accounting for their assimilation. Mesozoic and Cenozoic igneous basement rocks are unsuitable as assimilants due to their high 143 Nd/ 144 Nd, diverse 206 Pb/ 204 Pb, and generally high δ 18 O. The dominant cause of magmatic evolution at Mount Rainier, however, is inferred to be a version of in situ crystallization-differentiation and mixing (Langmuir, 1989) wherein small magma batches stall as crustal intrusions and solidify extensively, yielding silicic residual liquids with trace element concentrations influenced by accessory mineral saturation. Subsequent magmas ascending through the intrusive plexus entrain and mix with the residual liquids and low-degree re-melts of those antecedent intrusions, producing hybrid andesites and dacites. Mount St. Helens volcanic rocks have geochemical similarities to those at Mount Rainier, and may also result from in situ differentiation and mixing due to low and intermittent long-term magma supply, accompanied by modest crustal assimilation. Andesites and dacites of Mount Adams isotopically overlap the least contaminated Mount Rainier magmas and derive from similar parental magma types, but have trace element variations more consistent with progressive crystallization-differentiation, probably due to higher magma fluxes leading to slower crystallization of large magma batches, allowing time for progressive separation of minerals from melt. Mount Adams also sits atop the southern projection of a regional anticlinorium, so Eocene sediments are absent, or are at shallow crustal levels, and so are cold and difficult to assimilate. Differences between southwest Washington stratovolcanoes highlight some ways that crustal geology and magma flux are primary factors in andesite generation.

Thermomechanics of shallow magma chamber pressurization: Implications for the assessment of ground deformation data at active volcanoes

In this study, we utilize thermomechanical models to investigate how magma chambers overpressurize as the result of either magmatic recharge or volatile exsolution. By implementing an adaptive reservoir boundary condition we are able to track how overpressure dissipates as the magma chamber expands to accommodate internal volume changes. We find that the size of the reservoir greatly impacts the resultant magma chamber overpressure. In particular, overpressure estimates for small to moderate-sized reservoirs (1–10 km3) are up to 70% lower than previous analytical predictions. We apply our models to Santorini volcano in Greece where recent seismic activity and ground deformation observations suggested the potential for eruption. The incorporation of an adaptive boundary condition reproduces Mogi flux estimates and suggests that the magma reservoir present at Santorini may be quite large. Furthermore, model results suggest that if the magma chamber is >100 km3, overpressures generated due to the high magma flux may not exceed the strength of the host rock, thus requiring an additional triggering mechanism for eruption. Although the adaptive boundary condition approach does not calculate the internal evolution of the magma reservoir, it represents a fundamental step forward from elastic Mogi models and fixed boundary solutions on which future investigations of the evolution of the magma can be built.

Modeling volcano growth on the Island of Hawaii: Deep-water perspectives

Recent ocean-bottom geophysical surveys, dredging, and dives, which complement surface data and scientific drilling at the Island of Hawaii, document that evolutionary stages during volcano growth are more diverse than previously described. Based on combining available composition, isotopic age, and geologically constrained volume data for each of the component volcanoes, this overview provides the first integrated models for overall growth of any Hawaiian island. In contrast to prior morphologic models for volcano evolution (preshield, shield, postshield), growth increasingly can be tracked by age and volume (magma supply), defining waxing alkalic, sustained tholeiitic, and waning alkalic stages. Data and estimates for individual volcanoes are used to model changing magma supply during successive compositional stages, to place limits on volcano life spans, and to interpret composite assembly of the island. Volcano volumes vary by an order of magnitude; peak magma supply also varies sizably among edifices but is challenging to quantify because of uncertainty about volcano life spans. Three alternative models are compared: (1) near-constant volcano propagation, (2) near-equal volcano durations, (3) high peak-tholeiite magma supply. These models define inconsistencies with prior geodynamic models, indicate that composite growth at Hawaii peaked ca. 800–400 ka, and demonstrate a lower current rate. Recent age determinations for Kilauea and Kohala define a volcano propagation rate of 8.6 cm/yr that yields plausible inception ages for other volcanoes of the Kea trend. In contrast, a similar propagation rate for the less-constrained Loa trend would require inception of Loihi Seamount in the future and ages that become implausibly large for the older volcanoes. An alternative rate of 10.6 cm/yr for Loa-trend volcanoes is reasonably consistent with ages and volcano spacing, but younger Loa volcanoes are offset from the Kea trend in age-distance plots. Variable magma flux at the Island of Hawaii, and longer-term growth of the Hawaiian chain as discrete islands rather than a continuous ridge, may record pulsed magma flow in the hotspot/plume source.

A model for calculating eruptive volumes for monogenetic volcanoes — Implication for the Quaternary Auckland Volcanic Field, New Zealand

Monogenetic basaltic volcanism is characterised by a complex array of behaviours in the spatial distribution of magma output and also temporal variability in magma flux and eruptive frequency. Investigating this in detail is hindered by the difficulty in evaluating ages of volcanic events as well as volumes erupted in each volcano. Eruptive volumes are an important input parameter for volcanic hazard assessment and may control eruptive scenarios, especially transitions between explosive and effusive behaviour and the length of eruptions. Erosion, superposition and lack of exposure limit the accuracy of volume determination, even for very young volcanoes. In this study, a systematic volume estimation model is developed and applied to the Auckland Volcanic Field in New Zealand. In this model, a basaltic monogenetic volcano is categorised in six parts. Subsurface portions of volcanoes, such as diatremes beneath phreatomagmatic volcanoes, or crater infills, are approximated by geometrical considerations, based on exposed analogue volcanoes. Positive volcanic landforms, such as scoria/spatter cones, tephras rings and lava flow, were defined by using a Light Detection and Ranging (LiDAR) survey-based Digital Surface Model (DSM). Finally, the distal tephra associated with explosive eruptions was approximated using published relationships that relate original crater size to ejecta volumes. Considering only those parts with high reliability, the overall magma output (converted to Dense Rock Equivalent) for the post-250 ka active Auckland Volcanic Field in New Zealand is a minimum of 1.704km3. This is made up of 1.329km3 in lava flows, 0.067km3 in phreatomagmatic crater lava infills, 0.090km3 within tephra/tuff rings, 0.112km3 inside crater lava infills, and 0.104km3 within scoria cones. Using the minimum eruptive volumes, the spatial and temporal magma fluxes are estimated at 0.005km3/km2 and 0.007km3/ka. The temporal–volumetric evolution of Auckland is characterised by an increasing magma flux in the last 40ky, which is inferred to be triggered by plate tectonics processes (e.g. increased asthenospheric shearing and backarc spreading of underneath the Auckland region).

Preferential eruption of andesitic magmas: Implications for volcanic magma fluxes at convergent margins

Abstract Andesitic magmas (herein defined as having c. 55–65 wt% SiO 2 ) are abundant in convergent margin volcanoes. Although andesites and other intermediate magmas can plausibly be produced by a number of mechanisms, a lack of evidence for andesitic liquids relative to the abundance of andesite volcanic rocks, together with widespread evidence for hybridization and recharge, suggests that many erupted andesites form by mixing between relatively mafic magmas that ultimately derive from the mantle wedge, and more felsic magmas or mush zones derived from differentiation and crustal melting. A model of recharge filtering accounts for the high abundance of andesitic volcanic rocks and reflects the simple idea that the processes that create hybridized andesitic magmas in the shallow crust can also initiate volcanic activity, resulting in preferential eruption of andesitic magmas relative to the parental magmas that mix to produce them. This occurs via mafic recharge – intrusion of a mafic magma into a silicic magma or mush within the shallow crust. The overall abundance and variability from volcano to volcano of andesitic volcanism at convergent margins also suggests important roles for hydrous mafic magma in promoting mixing and eruption during mafic recharge, and the crust for modulating the compositions of volcanic outputs.

The Late Jurassic to present evolution of the Andean margin: Drivers and the geological record

AbstractUncommonly long‐lived subduction and variable plate geometry along the South American Andean Plate margin resulted in diverse relationships between magmatic flux and extensional and contractional deformation, as recorded by the overriding continental plate. Convergence velocities, absolute overriding plate velocities, and subducting slab ages were resolved along the trench from 170 Ma to the present using a recently developed kinematic global plate model to identify any relationship between subduction conditions, deformation style, and magmatic features in the overriding plate. Key correlations reflect the dependence of macroscopic crustal strain style on subduction mechanism and relative plate vectors. Extensional back‐arc basins involving mafic/oceanic crust developed only when the overriding plate velocity of South America was directed away from the trench and the modeled age of the subducting slab was older than 50 Myr. The development of fold and thrust belts, and uplift of major plateaus, was accompanied by trench normal convergence velocities in excess of 4 cm/yr. Parameters investigated in this study revealed no correlation with the timing of major magmatic events, nor was any correlation observed with the structural style of fold and thrust belts.

Time and space variations in magmatic and phreatomagmatic eruptive processes at Easy Chair (Lunar Crater Volcanic Field, Nevada, USA)

The products of monogenetic volcanoes often record complex sequences of eruptive processes. Easy Chair volcano (Lunar Crater Volcanic Field, Nevada, USA) was formed by a monogenetic eruption along a ∼2.5-km-long series of en echelon fissure vents. Hawaiian to Strombolian fountains along the fissures dominated initial activity, producing a series of agglomerate ramparts. Focusing of eruptive activity to two central vents and the formation of two overlapping scoria cones followed the early phase. Fountain-fed lavas from those cones merged to form a channel that fed lava onto a flow field at the foot of the cones. Focusing of subsurface magma flow toward the central conduits may have reduced magma flux in the remaining fissures, and the southern segment(s) entered a phase of phreatomagmatic explosions that destroyed the early agglomerate rampart and formed a maar and tephra ring composed of lapilli tuff rich in clasts derived from pre-Easy Chair lavas and early agglomerates. The eruption closed with a minor phase of magmatic activity that deposited scoria lapilli and bombs on top of the phreatomagmatic deposits. The eruptive sequence indicates that relatively low hazard Strombolian to Hawaiian activity can be replaced by more hazardous phreatomagmatic explosions well into a monogenetic eruption.

Asthenospheric Control of Melting Processes in a Monogenetic Basaltic System: a Case Study of the Auckland Volcanic Field, New Zealand

High-resolution sampling in monogenetic fields has the potential to reveal fine-scale heterogeneity of the mantle, a feature that may be overwhelmed by larger fluxes of magma, or missed by under-sampling.The Quaternary Auckland Volcanic Field (AVF) in northern New Zealand is a basaltic field of 51 small-volume volcanic centres, and is one of the best-sampled examples of a monogenetic volcanic field. We present data for 12 centres in the volcanic field. These show the large compositional variations between volcanoes as well as through single eruptive sequences. Whole-rock compositions range from subalkaline basalt in the larger centres, through alkali basalt to nephelinite in the smallest centres. Fractional crystallization has had a limited effect in many of the centres, but high-pressure clinopyroxene crystallization may have occurred in others. Three endmembers are observed in Pb isotope space, indicating that distinct mantle source components are involved in the petrogenesis of the magmas. Whole-rock multi-element patterns show that the larger centres have prominent positive Sr anomalies and lack K anomalies, whereas the smaller centres have prominent negative K anomalies and lack Sr anomalies. The melting parameters and compositions of the sources involved are modelled using trace element ratios and multi-element patterns, and three components are characterized: (1) fertile peridotite with a Pb-isotope composition similar to Pacific mid-ocean ridge basalt; (2) eclogite domains with a HIMU-like isotope composition dispersed within the fertile peridotite; (3) slightly depleted subduction-metasomatized peridotitic lithospheric mantle (containing c. 3% subduction fluids). Modelling shows that melting in the AVF begins in garnet-bearing fertile asthenosphere (with preferential melting of eclogite domains) and that melts are variably diluted by melts of the lithospheric source. The U^Th isotope compositions of the end-members in the AVF show 230 Th excess [( 230 Th/ 232 Th) ratios of 1·11^1·38], with the samples of lower ( 230 Th/ 232 Th) exhibiting higher ( 238 U/ 232 Th), which we attribute to the dilution effect of the melts from the lithospheric mantle source. Modelling reveals a correlation between melting in the asthenosphere, the degree of melting and incorporation of the metasomatized lithospheric mantle source, and the resultant size of the volcanic centre. This suggests that the scale of the eruption may essentially be controlled by asthenospheric mantle dynamics.

Unbedded diatreme deposits reveal maar-diatreme-forming eruptive processes: Standing Rocks West, Hopi Buttes, Navajo Nation, USA

Diatremes provide partial records of how dyke-delivered magma, periodically interacting with water, produces the largest known cylindrical conduit structures of any volcano type. We address how pre-eruptive country rock is disrupted and redistributed to form the diatreme structure during an eruption by establishing the internal architecture of the unbedded diatreme at Standing Rocks West, and the volumes and sources of wall-rock within the diatreme and in a complementary tephra ring at Teshim, both in the Hopi Buttes volcanic field. The unbedded diatreme is dominantly exposed as a massif comprising multiple subvertical columns, interpreted as ephemeral-conduit deposits of well-mixed, poorly sorted, composite and juvenile pyroclast-rich deposits that truncate marginal layered deposits, plus a peripheral heterolithic country rock breccia. Wall-rock clasts are scarce in the massif and were mainly sourced from ~75 to 245 m depth below the paleosurface. By contrast, the tephra ring has abundant wall-rock fragments, dominantly from the upper 75 m of the pre-eruptive sedimentary sequence. The diatreme structure is interpreted to have been formed by many small-volume explosions fed by a low flux of basaltic magma. Late stage explosive activity was rooted mostly within pyroclastic debris at shallow to intermediate depths within the diatreme structure. This resulted in damped and shifting intra-diatreme explosions and jets that facilitated gradual mixing, recycling and remobilization of debris in the diatreme, with incremental addition of juvenile material and possibly a local rise in the crater floor.

Pre-eruptive Magmatic Conditions at Augustine Volcano, Alaska, 2006: Evidence from Amphibole Geochemistry and Textures

Variations in the geochemistry and texture of amphibole phenocrysts erupted from Augustine Volcano in 2006 provide new insights into preand syn-eruptive magma storage and mixing. Amphiboles are rare but present in all magma compositions (lowto high-silica andesites) from the 3 month long eruption. Unzoned magnesiohornblende in the highand low-silica andesites exhibit limited compositional variability, relatively high SiO2 (up to 49·7 wt %), and relatively lowAl2O3 (511·1wt %). Intermediate-silica andesites and quenched mafic enclaves contain amphiboles that vary in composition (e.g. SiO2 40·8^48·9 wt %, Al2O3 6·52^15·2 wt %) and classification (magnesiohornblende^magnesiohastingsite^ tschermakite). Compositional variation in amphibole is primarily controlled by temperature-dependent substitutions. Both highand low-silica andesites represent remnant magmas that were stored in the shallow crust at 4^8 km depth, remaining distinct owing to a complex subsurface plumbing system. Intermediate-silica andesites and quenched mafic inclusions represent pre-eruptive hybrids of resident highand low-silica andesite magmas and an intruding basalt. Amphiboles in explosive phase high-silica andesites are largely euhedral and unreacted, consistent with the high magma flux rates from depth during this phase (up to 13 800 m s). Phenocrysts from the other lithologies have reaction rims that range from 1 to41000 m in thickness. Reaction rim microlite sizes correlate with reaction rim thicknesses. Reaction rims550 m thick contain microlites 1^10 m in length whereas reaction rims480 m thick contain microlites 10^100 m in length. Differentiating between heatingand decompression-induced amphibole reaction rim formation is problematic because of a lack of experimental constraints.We attempt a new approach to assessing reaction rim formation, based on a kinetic theory of crystal nucleation and growth, in which the differences in reaction rim textures represent different degrees of amphibole disequilibrium. Large crystals and low number densities suggest relatively lower levels of disequilibrium resulting in growth-dominated crystallization. Smaller crystals and larger number densities are indicative of higher nucleation rates and a high driving force.

On the longevity of large upper crustal silicic magma reservoirs

Research Article| July 01, 2013 On the longevity of large upper crustal silicic magma reservoirs Sarah E. Gelman; Sarah E. Gelman 1Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA Search for other works by this author on: GSW Google Scholar Francisco J. Gutiérrez; Francisco J. Gutiérrez 1Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA2Advanced Mining and Technology Center, and Departamento de Geología, Universidad de Chile, Santiago, Chile Search for other works by this author on: GSW Google Scholar Olivier Bachmann Olivier Bachmann 1Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA3Department of Earth Sciences, Institute of Geochemistry and Petrology, ETH Zurich, Switzerland Search for other works by this author on: GSW Google Scholar Geology (2013) 41 (7): 759–762. https://doi.org/10.1130/G34241.1 Article history received: 15 Nov 2012 rev-recd: 08 Feb 2013 accepted: 12 Feb 2013 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Sarah E. Gelman, Francisco J. Gutiérrez, Olivier Bachmann; On the longevity of large upper crustal silicic magma reservoirs. Geology 2013;; 41 (7): 759–762. doi: https://doi.org/10.1130/G34241.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Understanding the processes involved in the formation and maturation of upper crustal magma reservoirs, ultimately sourcing the largest volcanic eruptions on Earth, is one of the most fundamental aspects of volcanology. While such reservoirs are known to assemble incrementally over extended periods of time, debate persists regarding the time scales of melt preservation in the cold upper crust. If rapid cooling individually freezes incoming replenishing intrusions, accumulations of eruptible magma are impossible, precluding the construction of voluminous volcanic reservoirs for all but the highest magma emplacement rates. Recent numerical thermal models have been used to assess the viability of upper crustal silicic magma survival, and have suggested that supervolcanic reservoirs must form on geologically short time scales with anomalously high injection rates, and subsist only ephemerally, making their long-term evolution less predictable. Motivated by geological observations suggesting the contrary, we have improved upon these models by incorporating two fundamental features of natural systems not previously considered: (1) a nonlinear crystallization-temperature relationship adapted for upper crustal silicic magmas and (2) a temperature-dependent thermal conductivity. We demonstrate that the incorporation of both of these properties can allow an upper crustal reservoir to remain above its solidus for hundreds of thousands of years when fed by magma fluxes typical of large magmatic provinces. While the crystallization-temperature path plays the most significant role in maintaining a large pool of eruptible magma, the incorporation of temperature-dependent thermal properties significantly extends the lifetime of such reservoirs. Furthermore, while deeper emplacement levels (e.g., 10 km depth) can both extend magma survival and increase melt availability, we show that a 5 km depth can also provide an adequate magma storage environment. These results provide strong support for long-lived upper crustal mushes as a staging ground for accumulation of highly eruptible, crystal-poor silicic magmas, and further assert the evolutionary link between volcanic and plutonic systems. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Magma storage, transport and degassing during the 2008–10 summit eruption at Kīlauea Volcano, Hawai‘i

The 2008–current summit eruption at Kīlauea Volcano, Hawai‘i offers a unique opportunity to test models of degassing and magma plumbing and to improve our understanding of the volatile budget. The aim of this work was to test the hypothesis that gases emitted from a summit lava lake will be rich in carbon dioxide (CO2) and similar to those measured during the persistent lava lake activity in the early 20th century at Kīlauea Volcano (Gerlach and Graeber, 1985). We measured the sulfur dioxide (SO2) and CO2 concentrations in the gas plume from Halema‘uma‘u using electrochemical and non-dispersive infrared sensors during April 2009. We also analysed olivine-hosted melt inclusions from tephra erupted in 2008 and 2010 for major, trace and volatile elements. The gas and melt data are both consistent with the equilibration of a relatively evolved magma batch at depths of 1.2–2.0km beneath Halema‘uma‘u prior to the current degassing activity. The differences in the volatile concentrations between the melt inclusions and matrix glasses are consistent with the observed gas composition. The degassing of sulfur and halogen gases from the melt requires low pressures and hence we invoke convection to bring the magma close to the surface to degas, before sinking back into the conduit. The fluxes of gases (900 and 80t/d SO2 and CO2 respectively) are used to estimate magma fluxes (1.2–3.4m3/s) to the surface for April 2009. The observation of minimal loss of hydrogen from the melt inclusions implies a rapid rise rate (less than a few hours), which constrains the conduit radius to 1–2m. The inferred conduit radius is much narrower than the lava lake at the surface, implying a flared geometry. The melt inclusion data suggest that there is a progressive decrease in melt volatile concentrations with time during 2008–2010, consistent with convection, degassing and mixing in a closed, or semi-closed magma system. The degassing regime of the current summit lava lake activity is not similar to that observed in the early 20th century; instead the gases are extensively depleted in CO2.

Revised tectonic evolution of the Eastern Indian Ocean

Published plate tectonic models for the Australian‐Antarctic plate pair imply geologically improbable scenarios at either or both ends of the Cretaceous rift and spreading system. Controversy also exists around the location of and motion at the plate boundary extending west of Australia‐Antarctica, through the Kerguelen Plateau region. We present a plate tectonic model of relative motions among India, Australia, and Antarctica from the onset of continental rifting to the establishment of rapid seafloor spreading, at ~43 Ma. The model conforms to a wide range of geological/geophysical evidence and reconstructs the formation of both the western Kerguelen region and the eastern Tasman region. The incorporation of spatiotemporally continuous plate boundaries reveals the presence of a plate boundary beneath the contiguous Central Kerguelen Plateau and Broken Ridge for ~65 Ma. To investigate the relationship between the plate boundary system and the Kerguelen plume, we test three alternative absolute reference frames. Using a fixed hot spot reference frame, the Indian Ocean mid‐ocean ridge system remains within 500 km of the Kerguelen plume, while the proximity of the plate boundaries and the plume is more variable with a moving hot spot reference frame. Proximity between the plume, plate boundaries, and the Central Kerguelen Plateau/Broken Ridge for ~65 Myr suggests that these specific features were not formed by a single, short‐lived (5–10 Myr) pulse of magmatic activity, but rather by a ~25 Myr period of relatively high magma flux followed by ~40 Myr period of lower volume magmatic activity, an interpretation not excluded by the relatively sparse dredge and drill ages.

The composition of the foundered complement to the continental crust and a re-evaluation of fluxes in arcs

Most primitive arc melts are basaltic in composition, yet the bulk continental crust, thought to be generated in arcs, is andesitic. In order to produce an andesitic crust from primitive arc basalts, rocks complementary to the andesitic crust have to be fractionated and subsequently removed, most likely through density sorting in the lower arc crust. The Kohistan Arc in northern Pakistan offers a unique opportunity to constrain the composition and volume of material fluxes involved in this process. In a lower crustal section >10km cumulates (dunites, wehrlites, websterites, clinopyroxene-bearing garnetites and hornblendites, and garnet-gabbros) are exposed that are 0.1–0.3g/cm3 denser than the underlying mantle. The cumulates combine with the andesitic bulk Kohistan Arc crust to reproduce the major and trace element composition of primitive basaltic arc melts. Our petrochemical analysis suggests that fractionation and subsequent foundering of wehrlites+ultramafic hornblende–garnet–clinopyroxene cumulates+garnet-gabbros is a viable mechanism for producing andesitic crust from a calc-alkaline/tholeiitic primitive high-Mg basalt. The mass of the foundered material is approximately twice that of the arc crust generated. For an overall andesitic arc composition, we estimate a magma flux into the arc (11–15km3/yr) about three times the rate of arc crust production itself. Foundering fluxes of cumulates (6.4–8.1km3/yr) are one third to half those of the globally subducted oceanic crust (~19km3/yr). Hence, the delaminate forms a volumetrically significant, albeit refractory and depleted geochemical reservoir in the mantle. Owing to its low U/Pb and high Lu/Hf the foundered material evolves with time to a reservoir characterized by unradiogenic Pb and highly radiogenic Hf isotopes, unlike any of the common mantle endmembers defined by OIB chemistry. The unradiogenic Pb of the foundered arc cumulates could counterbalance the radiogenic Pb composition of the depleted mantle. The predicted highly radiogenic Hf (at rather unradiogenic Nd) of the foundered material can explain the εHf–εNd systematics observed in some abyssal peridotites and mantle xenoliths.

Fluid ascent and magma storage beneath Gunung Merapi revealed by multi-scale seismic imaging

Magma is fed to a volcano through a complex “plumbing” system that involves not only shallow structures beneath the volcano edifice, but also deep structures and processes within the underlying crust and upper mantle. This paper summarizes seismic experiments carried out over many years at Gunung Merapi in Central Java. These have resolved the 3D seismic velocity structure of the Merapi edifice, and provided a 3D structural image of the lithosphere and subduction zone beneath Central Java. Earthquake locations reveal that with distance from the trench, the dip of the subducting slab steepens from nearly horizontal (0–150km), through 45° (150–250km), to 70° (>250km). The slab appears as a 30km thick double layer of seismicity in a depth range of 80km to 150km, and it can be identified seismically to a depth of more than 600km. The active volcanoes of Merapi, Sumbing, and Lawu are located at the edge of a large low velocity body that extends from the upper crust to the upper mantle beneath Central Java. Shear wave signals recorded above this anomaly are strongly attenuated compared to neighboring areas. The anomalous body has a detected volume of >50,000km3 and a decrease in P and S velocities relative to adjacent regions of up to 30%. The resulting Vp/Vs ratio of up to 1.9 is unusually high for lower crust. Additionally, the anomaly extends along a 45 degree-slope downward from beneath the volcanic arc and meets the slab at 100km depth. We interpret this sloping anomaly as a pathway for fluids and partial melts. Increased seismicity is observed at depths of ~100km, possibly as a result of dehydration of the subducting slab with related fluid releases causing partial melting of overlying mantle material. The large velocity reduction and high Vp/Vs ratio in the region are consistent with an increase in temperature, a reduction of shear strength, and the presence of fluids or melts of 13 to 25vol.%. The detected strong anomaly beneath Central Java is unique in size and amplitude compared to other subduction zones. The geophysical evidence suggests that this segment of the arc has a high magma flux and is thus capable of developing even larger shallow crustal reservoirs and more voluminous explosive eruptions in the future.