Shallow Magma Reservoir Research Articles

Anorthosites in Nishiyama volcanic products from the Hachijo–jima island, Izu–Bonin arc: The direct evidence for ‘plagioclase control’ in shallow magma reservoir

Anorthosites in Nishiyama volcanic products from the Hachijo–jima island, Izu–Bonin arc: The direct evidence for ‘plagioclase control’ in shallow magma reservoir

Pre- and syn-eruptive conditions of a basaltic Plinian eruption at Masaya Volcano, Nicaragua: The Masaya Triple Layer (2.1 ka)

The Masaya Triple Layer tephra was deposited ~2100 years ago during a basaltic Plinian eruption of Masaya caldera, Nicaragua, and is one of few known examples of this extreme endmember of basaltic explosive volcanism. Masaya caldera is located approximately 25 km from Managua, the capital of Nicaragua, and a Plinian eruption presents a high potential risk to its population of 1 million people. Here we use geochemical and petrological tools to constrain the pre- and syn-eruptive physico-chemical magmatic conditions, in order to understand how low-viscosity basaltic magmas can erupt with Plinian style.By combining thermometric models with Rhyolite-MELTS simulations, we find that the Masaya Triple Layer magma was last stored between 1080 and 1100 °C and 21–42 MPa, with a pre-eruptive volatile content of ~2 wt% H2O. A small phenocryst volume fraction of 0.1 crystallised under water-saturated conditions within a shallow magma reservoir located at 0.8–1.6 km depth. During ascent, rapid microlite crystallisation occurred and lateral velocity gradients across the conduit produced heterogeneous inter-mingling regions of varying crystallinity, between 20 and 50 vol%. Crystal size distribution analysis shows substantial microlite crystallisation over a timescale of 1–5 min, which induced significant changes in viscosity and magma rheology during ascent, increasing the effective magma viscosity from ~10 Pa s to 106 Pa s and approaching the brittle fragmentation threshold. Comparable pre- and syn-eruptive conditions have been deduced for other examples of basaltic Plinian activity, indicating a common eruption mechanism. We find that the common requirements for a basaltic Plinian eruption are a low magmatic H2O concentration of ~2 wt%, storage at moderate temperatures of 1080–1100 °C, and rapid crystallisation during magma ascent. Ultimately, these conditions contributed to the production of this highly hazardous Plinian eruption of Masaya.

Progressive Changes in Magma Transport at the Active Serreta Ridge, Azores

AbstractVolcanism in the Eastern Azores Plateau occurs at large central volcanoes and along subaerial and submarine fissure zones, resulting from a mantle melting anomaly combined with transtensional stresses. Volcanic structures are aligned WNW‐ESE and NW‐SE, reflecting two tectonic stress fields that control the direction of lateral melt transport. Terceira Island is influenced by both stress fields, dividing the island into an eastern and western part. Several submarine volcanic ridges with variable orientations are located west of Santa Bárbara, the youngest central volcano on Terceira. Major, trace element and Sr‐Nd‐Pb‐Hf isotope compositions from submarine lavas and glasses, in part associated with the 1998–2001 Serreta Ridge eruption, vary between different lava suites, suggesting a formation from different mantle sources. Submarine lavas are more primitive than those from Santa Bárbara volcano, indicating that they are not laterally connected with the shallow magma reservoir located in 2‐ to 5‐km depth beneath the central volcano. Mineral thermobarometric data suggest that the older Serreta magmas were laterally transported at depths >5 km from Santa Bárbara predominantly in WNW direction. We propose that lithospheric extension controls magma transport from the central volcano to Serreta Ridge. The youngest Serreta lavas differ from Santa Bárbara and other submarine ridges in having less radiogenic Pb and higher Hf isotope ratios representing a new magma pulse ascending from the mantle. We conclude that lateral magma transport and the morphology of volcanic ridges are controlled by tectonic stresses in the lithosphere, whereas vertical melt transport is initiated by processes in the mantle.

A Recurrent Magmatic Pattern on Observable Timescales Prior to Plinian Eruptions From Nevado de Toluca (Mexico)

AbstractNevado de Toluca is a stratovolcano in the densely populated Mexican Highland and the source of three late Pleistocene Plinian eruptions. While the characterization of the magmatic processes and timescales, leading to these events, is crucial to evaluate potential signals of the reawakening of this volcano, they are not well constrained. Here we present insights gathered from the analysis of mineral zoning. Abrupt changes in plagioclase compositions (anorthite [An] >10 mol. %) record repeated thermochemical disturbance of a shallow magma reservoir prior to the three eruptions investigated. In particular, sudden chemical changes in the outermost crystal rims indicate that the eruptions were triggered by recharge of less evolved magma. Recalculated melt compositions reveal that magma input is chemically heterogeneous. Orthopyroxene compositions span a large range (En53‐En92, Cr2O3 of up to 0.9 wt. %) resulting from magma hybridization, which is consistent with a late‐stage temperature increase recorded by amphibole chemistry. Using diffusion chronometry, we show that this recharge typically occurs decades to centuries before Plinian eruptions and hence on timescales relevant for volcano monitoring. Additionally, modeling of Mg profiles in plagioclase constraints the duration of differentiation cycles to be millennial in order of magnitude. Our study shows that similar processes preceded explosive eruptions from Nevado de Toluca, suggesting that analogous paths of unrest should be considered when evaluating potential future activity. We emphasize that signs of deep crustal activity, such as relatively broad deformation pattern surrounding the volcano, could herald the reactivation of Nevado de Toluca.

Noise-based passive ballistic wave seismic monitoring on an active volcano

SUMMARY Monitoring temporal changes of volcanic interiors is important to understand magma, fluid pressurization and transport leading to eruptions. Noise-based passive seismic monitoring using coda wave interferometry is a powerful tool to detect and monitor very slight changes in the mechanical properties of volcanic edifices. However, the complexity of coda waves limits our ability to properly image localized changes in seismic properties within volcanic edifices. In this work, we apply a novel passive ballistic wave seismic monitoring approach to examine the active Piton de la Fournaise volcano (La Réunion island). Using noise correlations between two distant dense seismic arrays, we find a 2.4 per cent velocity increase and −0.6 per cent velocity decrease of Rayleigh waves at frequency bands of 0.5–1 and 1–3 Hz, respectively. We also observe a −2.2 per cent velocity decrease of refracted P waves at 550 m depth at the 6–12 Hz band. We interpret the polarity differences of seismic velocity changes at different frequency bands and for different wave types as being due to strain change complexity at depth associated with subtle pressurization of the shallow magma reservoir. Our results show that velocity changes measured using ballistic waves provide complementary information to interpret temporal changes of the seismic properties within volcanic edifices.

Remotely Sensed Deformation and Thermal Anomalies at Mount Pagan, Mariana Islands

Mt Pagan is an active volcano in the Mariana Island Arc, having produced a large eruption (>0.1 km3) in 1981, and recent smaller eruptions (>0.001 km3) in2011 and 2012. In this study we examine available Interferometric Synthetic Aperture Radar(InSAR) from Envisat and ALOS between October 2004 and June 2010 for precursory deformationleading up to the most recent eruptions. Although we do not observe evidence for shallowmagma reservoir inflation, we do observe a zone of subsidence of up to -3 cm/yr located onthe northern flank likely caused by continuing scoria compaction of 1981 eruptive deposits.CosmoSkyMed InSAR data spanning the October 2012 eruption also shows deposit subsidence,but no detectable co-eruptive deflation. We also present remotely-sensed thermal time series fromthe MODIS and ASTER instruments spanning over a decade. There are two distinct locationsof anomalous surface temperatures on the island, which correlate with previously documented,fumarolically active vents at South Pagan and Mt. Pagan. While elevated temperatures duringthe eruptions are visible from space in both ASTER and MODIS data, we find no clear thermalprecursors before eruption. The available data may have had insufficient spatial and temporalresolution to detect either deformation or thermal precursors if the magmatic system had rapidtransitions between quiescence and eruption. Looking ahead, satellite sensors with improvedspatial and temporal resolution will help characterize transient dynamics at small and remotevolcanic islands such as Pagan.

Cyclic activity of the Fuego de Colima volcano (Mexico): insights from satellite thermal data and nonlinear models

Abstract. The Fuego de Colima volcano (Mexico) shows a complex eruptive behavior, with periods of rapid and slow lava dome growth punctuated by explosive activity. We reconstructed the weekly discharge rate average between 1998 and 2018 by means of satellite thermal data integrated with published discharge rate data. By using spectral and wavelet analysis, we found a multiyear long-term, multi-month intermediate-term, and multi-week short-term cyclic behavior during the period of the investigated eruptive activity like that of many other dome-forming volcanoes. We use numerical modeling in order to investigate the nonlinear cyclic eruptive behavior considering a magma feeding system composed of a dual or a single magma chamber connected to the surface through an elastic dyke developing into a cylinder conduit in the shallowest part. We investigated cases in which the periodicity is controlled by (i) the coupled deep–shallow magma reservoirs, (ii) the single shallow chamber, and (iii) the elastic shallow dyke when it is fed by a fixed influx rate or constant pressure. Due to the limitations of the current modeling approach, there is no single configuration that can reproduce all the periodicities on the three different timescales. The model outputs indicate that the observed multiyear periodicity (1.5–2.5 years) can be described by the fluctuations controlled by a shallow magma chamber with a volume of 20–50 km3 coupled with a deep reservoir of ca. 500 km3, connected through a deep elastic dyke. The multi-month periodicity (ca. 5–10 months) appears to be controlled by the shallow magma chamber for the same range of volumes. The short-term multi-week periodicity (ca. 2.5–5 weeks) can be reproduced considering a fixed influx rate or constant pressure at the base of the shallower dyke. This work provides new insights on the nonlinear cyclic behavior of Fuego de Colima and a general framework for comprehension of the eruptive behavior of andesitic volcanoes.

Mechanisms of Unrest and Eruption at Persistently Restless Volcanoes: Insights From the 2015 Eruption of Telica Volcano, Nicaragua

AbstractMany of Earth's volcanoes experience well‐defined states of “quiescence” and “unrest,” with unrest occasionally culminating in eruption. Some volcanoes, however, experience an unusually protracted (i.e., decades‐long) period of noneruptive unrest and are thus categorized as “persistently restless volcanoes” (PRVs). The processes that drive persistently restless volcanism are poorly understood, as our knowledge of PRVs is currently based on a small number of case studies. Here we examine multidisciplinary observations of the 2015 eruptive episode at Telica Volcano, Nicaragua, in the context of its long‐term behavior. We suggest that the latter phases of the 2015 eruption were ultimately driven by destabilization of its shallow magma reservoir. Based on previous geodetic‐seismic studies of Telica (Geirsson et al., 2014, https://doi.org/10.1016/j.jvolgeores.2013.11.009; Rodgers et al., 2013, https://doi.org/10.1016/j.jvolgeores.2013.08.010 and 2015, https://doi.org/10.1016/j.jvolgeores.2014.11.012) and on multiparameter observations at Telica over a 7‐year period, we propose that three distinct states of unrest occur at Telica over decadal timescales: a stable open state involving steady conduit convection and two distinct “unstable” states that may lead to eruptions. In the “weak sealing” state, phreatic explosions result from steady conduit convection underlying a weak seal. In the “destabilized” state, destabilization of the top of the convecting magma in the conduit leads to rapid accumulation of high pressures leading to strong/impulsive phreatomagmatic explosions. Our observations and interpretations suggest that continuous seismic, ground‐based deformation, gas emission, and thermal monitoring and interpretation of these data within a paradigm of sustained conduit convection modulated by episodes of sealing and destabilization of shallow magma reservoirs may allow robust forecasting of eruption potential, energy, and duration at Telica and similar PRVs worldwide.

Fault‐Associated Magma Conduits Beneath Volcán de Colima Revealed by Seismic Velocity and Attenuation Tomography Studies

AbstractWe present the results of tomographic studies using seismic velocity and attenuation in the area of the Colima Volcanic complex (CVC). Our dataset comprises body waves from local earthquakes recorded by the temporary seismic stations of the CODEX network in the Colima area and a few stations of the regional Mapping the Rivera Subduction Zone (MARS) networks, both deployed in 2006–2008. We obtain three‐dimensional distributions of seismic velocities and attenuation in the crust beneath the CVC area. At shallow depths, we observe a large negative anomaly to the south of CVC, coinciding with the location of the Central Colima Graben. This anomaly may represent debris avalanche deposits, as well as shallow magma reservoirs feeding the eruptions of the presently active Volcán de Colima. In contrast, the volcano edifice of Nevado de Colima, which is built of rigid igneous rocks, is associated with high‐velocity and low‐attenuation anomalies at shallow depths. In the deeper section, a major anomaly with high Vp/Vs, low Vs, and high S wave attenuation corresponds to the location of the regional Tamazula fault. As this represents a mechanically weakened zone of the crust, it may form the pathway that feeds CVC. Both velocity and attenuation models show that the fault‐associated conduit brought magma from the mantle through the lower crust to a depth of 15 km. Then, a light fraction of magma may continue to ascend, forming shallow reservoirs beneath the southern flank of CVC.

Parental Melts and Magma Storage of a Large-volume Dacite Eruption at Vetrovoy Isthmus (Iturup Island, Southern Kuril Islands): Insights into the Genesis of Subduction-zone Dacites

Abstract Detailed mineralogical and melt and fluid inclusion constraints on magma storage, and the evolution of melts, are presented for the large-volume caldera eruption on the Vetrovoy Isthmus on Itutrup Island (Kuril Islands, Russia). The shallow magma reservoir beneath the Vetrovoy Isthmus is composed of a mush of plagio-rhyolitic melt, phenocrysts and the products of peritectic reaction(s). The melt appears to have formed as a result of partial melting of previously erupted rocks, which probably had andesitic to basaltic compositions and were metamorphosed into amphibole-bearing assemblages. The breakdown of amphibole in the partially melted precursor rocks led to the formation of early Mg-rich clino- and orthopyroxene, along with plagioclase and Fe–Ti oxides, and the release of aqueous fluids. Variations in fluid pressure are recorded by a strong increase of An contents in plagioclase. Crystallization took place at around 850°C with pressure ranging from 0·9 to 3 kbar. This study demonstrates that dacitic magmas erupted during the course of a 20 kyr voluminous eruption were the result of mixing between plagio-rhyolitic partial melts and the breakdown reaction minerals (i.e. pyroxenes, plagioclase and Fe–Ti oxides). Plagioclase and quartz were the last minerals to crystallize from these melts prior to eruption.

Cyclic magma recharge pulses detected by high-precision strainmeter data: the case of 2017 inter-eruptive activity at Etna volcano

Unprecedented ultra-small strain changes (~10−8–10−9), preceding and accompanying the 2017 explosive-effusive activity, were revealed by a high precision borehole strainmeter at Etna. No pre- or co-eruptive deformation was detected by the GPS measurements, which often fail to detect ground deformation engendered by short-term small volcanic events due to their limited accuracy (millimetres to few centimetres). Through the analysis and detection of ultra-small strain changes (few tens of nanostrain), revealed by filtering the raw data, a significant time correspondence with the eruptive activity is observed. For the first time, cyclic fast exponential strain changes, preceding the onset of eruptive events, with a timescale of about 2–7 days, were detected. These variations are attributable to the expansion of the shallow magma reservoir, which is replenished with new magma from depth during the inter-eruptive periods. Interpreting the strain changes in terms of pressurization/depressurization of the chamber due to the cyclic influx and withdrawal of magma, allows placing some constraints on the magma recharge volume rate. A Finite Element model has been developed to simulate the temporal evolution of the strain changes generated by the re-pressurization of a spheroidal magma source using a dynamical approach. An average total mass budget of about 1–2 × 109 kg, which is in the range of the erupted mass, is estimated to be accumulated within a shallow vertically elongated magma chamber during the inter-eruptive periods. Such evidence demonstrates that the near-real time analysis of strainmeter records is remarkable for its ability to record small transients and highlight recharging phases preceding eruptive activity, which would go undetected with other current methodologies. Under these conditions, the ability to simulate inter-eruptive periods offers an opportunity to estimate the magma recharge rate with important implications for volcano hazard assessment.

Magma Reservoir Below Laguna del Maule Volcanic Field, Chile, Imaged With Surface‐Wave Tomography

AbstractThe Laguna del Maule (LdM) volcanic field comprises the greatest concentration of postglacial rhyolite in the Andes and includes the products of ~40 km3 of explosive and effusive eruptions. Recent observations at LdM by interferometric synthetic aperture radar and global navigation satellite system geodesy have revealed inflation at rates exceeding 20 cm/year since 2007, capturing an ongoing period of growth of a potentially large upper crustal magma reservoir. Moreover, magnetotelluric and gravity studies indicate the presence of fluids and/or partial melt in the upper crust near the center of inflation. Petrologic observations imply repeated, rapid extraction of rhyolitic melt from crystal mush stored at depths of 4–6 km during at least the past 26 ka. We utilize multiple types of surface‐wave observations to constrain the location and geometry of low‐velocity domains beneath LdM. We present a three‐dimensional shear‐wave velocity model that delineates a ~450‐km3 shallow magma reservoir ~2 to 8 km below surface with an average melt fraction of ~5%. Interpretation of the seismic tomography in light of existing gravity, magnetotelluric, and geodetic observations supports this model and reveals variations in melt content and a deeper magma system feeding the shallow reservoir in greater detail than any of the geophysical methods alone. Geophysical imaging of the LdM magma system today is consistent with the petrologic inferences of the reservoir structure and growth during the past 20–60 kyr. Taken together with the ongoing unrest, a future rhyolite eruption of at least the scale of those common during the Holocene is a reasonable possibility.

Repeated Rhyolite Eruption From Heterogeneous Hot Zones Embedded Within a Cool, Shallow Magma Reservoir

AbstractDespite the hazard posed by explosive silicic eruptions, the magma storage conditions and dynamics that precede these events remain controversial. The Laguna del Maule volcanic field, central Chile, is an exceptional example of postglacial (younger than ca. 20,000 years) rhyolite volcanism and sustained unrest driven by a large, shallow, active silicic magma system. New zircon petrochronologic data reveal that compositionally distinct domains developed concurrently within the Laguna del Maule magma reservoir, which produced two episodes of concentrated rhyolitic eruptions at 23–19 and 8–2 ka. Zircon crystallization ages record 160 kyr of magma emplacement resulting in a several hundreds of cubic kilometers reservoir that has been imaged geophysically. The average magma emplacement rate inferred from the zircon geochronology and tomographically defined magma volume is consistent with those required by thermal models to maintain a shallow silicic system. Ti‐in‐zircon temperatures of crystal cores and rims and hiatuses in crystal growth indicates most of this volume persisted in a near‐solidus state. However, consistent patterns of trace element zoning in crystal interiors and crystallization rates derived from a model of diffusion‐limited zircon growth suggest the erupted rhyolite magma batches originated from long‐lived hot zones of extractable mush embedded within the larger, cool reservoir of rigid mush. These contrasting, coeval magma storage conditions obviate a simple hot versus cold storage dichotomy for large silicic magma systems.

Impact of Crustal Rheology on Temperature‐Dependent Viscoelastic Models of Volcano Deformation: Application to Taal Volcano, Philippines

AbstractWe use ALOS‐1 interferometric synthetic aperture radar data spanning the period of 2007–2011 to obtain time‐dependent ground deformation data over Taal caldera related to the 2010–2011 volcanic unrest, which did not lead to an eruption. We employ finite element models that account for subsurface thermomechanical properties to test for viscoelastic deformation. We also examine the variability of crustal rheological parameters of the temperature‐dependent Arrhenius formulation for viscosity, to investigate the effects on surface deformation. Deformation at Taal is observed to be time dependent and explained by a linear rate of pressure increase in a shallow magma reservoir at 5‐km depth within a hot and viscoelastic upper crust. The best‐fitting rheological properties of the local setting are consistent with either a felsic or hydrothermally altered uppermost crust. We show the significantly different effects that the variation of rheological parameters has on surface deformation, highlighting the importance of incorporation into future studies of deformation at different volcanic settings.

Models of the Formation of the 29 February 2016 New Spatter Cone Inside Mount Nyiragongo

AbstractA new and unusual eruptive event occurred on 29 February 2016 within the summit crater of the Mount Nyiragongo volcano. Based on field campaigns performed between July 2015 and September 2017, and building on a previously published buoyancy‐driven bidirectional magma flow model explaining the progression of Mount Nyiragongo lava lake level, we provide the first quantitative estimations of volumes of erupted lava outpouring from the new spatter cone. Besides matching field data of the lava lake level covering the period December 2002 to September 2017, numerical solutions of the model reveal that the most probable dike path is one originating from the shallow magma reservoir, and not from the lava lake or branching from its feeding conduit. According to these simulations, the reservoir and erupted lava volumes are respectively estimated to the order of 10 km3 and 20 M m3. Magma overpressure at the level of the shallow reservoir is estimated in the range 12 to 16 MPa, high enough to potentially initiate new erupting events.

Geological evidence for recurrent collapse-driven phreatomagmatic pyroclastic density currents in the Holocene activity of Stromboli volcano, Italy

We present a multidisciplinary geological study of lithified to unconsolidated, ash-rich lapilli-tuff deposits bearing accretionary lapilli that crop out in scattered sites along the lower slopes of the Stromboli volcano (southern Italy). These deposits are related to three pyroclastic successions (called Semaforo Nuovo, Secche di Lazzaro and Semaforo Labronzo) with distinctive lithologic and volcanologic features, and different stratigraphic position within the Neostromboli volcano-stratigraphy (Holocene). Their juvenile clasts have slightly different major and trace element compositions (whole-rock and glass), isotopic ratios and mineralogical characteristics, although they are all within the most evolved high-K shoshonites typical of Neostromboli. The three successions are independently recognized in distinct sectors of the volcano, displaying an asymmetric areal distribution that is interpreted as the result of directed pyroclastic density currents (and minor fallout) generated from summit eruptive vents. Lithological characteristics and morphoscopic features of ash particles indicate phreatomagmatic fragmentation (combined with mechanisms of volatile exsolution or decompression), which is atypical with respect to the dominant Strombolian/effusive eruptive behaviour of Stromboli during the Holocene. We suggest that the three phreatomagmatic eruptions are linked to different major lateral collapses (associated to flank fissure eruptions) that occurred along the NW-dipping Sciara del Fuoco collapsed flank. It is argued that the collapses have unloaded the magmatic and hydrothermal system enabling magma-water interaction and explosive decompression of a shallow magma reservoir. The recurrent collapse-eruption link with generation of directed PDCs able to reach the lower slopes of the volcano highlights a previously underestimated source of volcanic hazard at Stromboli.

Evidence of a shallow magma reservoir below Los Humeros volcanic complex: Insights from the geochemistry of silicate melt inclusions

Los Humeros volcanic complex (Eastern Trans Mexican Volcanic belt) is one of the three most important geothermal fields in Mexico and is currently producing electricity (ca. 90 MW). It is therefore pertinent to constrain the magma chamber characteristics, such as its depth, and the magmatic evolution of the complex in order to better define the superficial geothermal activity. In this paper we present the first geochemical data (major and trace elements) for 15 silicate melt inclusions and their five host rocks from Los Humeros volcanic complex.We constrained the evolution of the volcanic complex by coupling X-ray spectrometry, ICP-MS, microprobe, X-ray microtomography and NanoSIMS analyses together with thermodynamical modeling on both whole rocks and silicate melt inclusions.A ponding reservoir located within an interval betwwen 5 and 13 km at depth is evidenced by silicate melt inclusions entrapment pressures. This ponding depth's interval is in agreement with previous studies by thermal modeling, resistivity and geobaromatry done in the same area. The geothermal activity of Los Humeros volcanic area is expected to be the result of the cooling of the magma in the shallow chamber. A large geochemical variability, in less than 1 Ma, is demonstrated by trace and volatile element compositions evolution through time. Subduction contamination is expected into the mantle source with the highest contamination event at 69 Ka, during the second caldera event of Los Humeros volcanic complex.

Crustal Magmatism and Deformation Fabrics in Northeast Japan Revealed by Ambient Noise Tomography

AbstractMapping both isotropic and anisotropic velocity structures of the crust provides insight into the dominant mechanisms that produce the deformation. We performed ambient noise tomography for shear velocity Vs in the Tohoku region, Japan, where plate kinematics remains simple as the Pacific plate is subducting under the Okhotsk plate. Cross‐correlation functions were retrieved from Hi‐net recordings and phase velocities of 3–16 s were measured, followed by a one‐step, wavelet‐based multiscale inversion for both Vs and its azimuthal anisotropy. The isotropic model shows that high Vs anomalies underlie the Cretaceous aged Kitakami and Abukuma Mountains from shallow crust to lower crust. Low Vs is found to track the active volcanoes with the strongest anomalies concentrated at roughly 10 km depth, which we interpret as the shallow magma reservoirs of the Quaternary magmatic system. The resolved azimuthal anisotropy exhibits a switch of anisotropy in the upper crust from island‐parallel to a random or suppressed state in 5–15 km depth interval and back to island‐parallel. The island‐parallel fast direction may map the structural fabrics of the crustal deformation, and the spatial correlation between velocity and anisotropy patterns suggests that continuous volcanic activities and magma ponding disrupt the otherwise consistent fabrics in the upper crust. Below 20 km depth, the anisotropy turns to convergence‐parallel, which may result from shearing either imposed by the return flow in the mantle wedge or frozen‐in from the last stage of extension of the continental margin that opened the Sea of Japan.

The dynamics of the southern Okinawa Trough magmatic system: New insights from the microanalysis of the An contents, trace element concentrations and Sr isotopic compositions of plagioclase hosted in basalts and silicic rocks

The Okinawa Trough (OT) is a back-arc, initial marginal sea basin located behind the Ryukyu arc–trench system. Analysis of the plagioclase phenocrysts from two basalts and two silicic rocks dredged from different locations in the southern Okinawa Trough (SOT) provides a detailed record of the nature and composition of and changes in the magma within the magmatic system. Plagioclase phenocrysts from the basalt record 87Sr/86Sr values of 0.704015–0.704360 and 0.704686–0.704863, respectively, which overlap with previously reported whole-rock values and suggest that the parent magma of the plagioclase represents upwelling mantle materials influenced by a subducted component. Plagioclase-melt thermobarometry records the crystallization of plagioclase phenocrysts in the basalt at the bottom of the lower crust near the Moho (depths of ~20 ± 7.5 km and ~25 ± 7.5 km). The temporal record of the Sr and Ba contents in the melt recorded by these crystals, which were calculated using partition coefficients and the Sr isotopic compositions of the plagioclases from the basalts, demonstrates the existence of two distinct mafic magmas within the SOT due to the presence of different subducted components (i.e., the subduction of the Gagua Ridge).Significant textural variations and wide ranges in the An35-89 and trace element concentrations and in situ Sr isotopic compositions (0.705748–0.707520) are observed in plagioclase crystals from silicic rocks in the SOT. The barometry results for the plagioclases from silicic rocks yield a wide range of crystallization depths ranging from 6.6 to 17.8 km. The Sr isotopic compositions and trace element concentrations of high-An80 plagioclases in basalt and silicic rocks indicate that there is no direct relationship between these two magma chambers. The negative correlation between the An contents and the Sr isotopic compositions in the plagioclases from silicic rocks indicates that more upper continental crust materials joined the magma source as its evolution proceeded. The presence of core-sieved plagioclase in the silicic rock indicates that decompression and/or magma mixing occurred. Thus, we propose a multilayer magma chamber system to explain the complex plagioclase crystals in silicic rocks. Two potential mush zones were recognized; their generation may have been controlled by the adiabatic decompression melting of lower crustal materials resulting from crustal extension or the post-collisional extension of the Northern Taiwan Mountain Belt. The two potential mush zones at greater depths are relatively basic, whereas the shallow magma reservoirs, which are the host magmas of the plagioclase, are relatively silicic.

Lithium diffusion in olivine records magmatic priming of explosive basaltic eruptions

The short duration (e.g., hours to days) of magmatic processes leading up to volcanic eruptions are challenging to characterize using conventional petrologic and geophysical methods. This is especially true for eruptions that occurred prior to the implementation of seismic and geodetic monitoring (mid-20th century). We present a new application of diffusion chronometry that utilizes lithium zoning in olivine phenocrysts to investigate the timing of late-stage, pre-eruptive magma mixing at Kīlauea Volcano (Hawai‘i). The diffusive re-equilibration of sub-ppm changes in lithium concentration are modeled in rapidly quenched, well-oriented olivine from six eruptions during a dominantly explosive period (1500–1820 CE). Lithium timescales reveal repeated intrusion of primitive composition magma into Kīlauea's shallow (∼2–6 km) reservoir system typically only a few days prior to each eruption. These timescales are shorter than the weeks to years retrieved from modeling Fe–Mg and Ni zoning in the same olivine crystals, which record earlier magma mixing events. The short Li timescales indicate that an intrusion primed the shallow magma reservoir potentially only hours to a few days before an eruption. Late-stage intrusions and mixing events should thus be investigated at other basaltic volcanoes to better understand pre-eruptive processes, so that eruption scenarios can be better constrained.