Volcanic Magma Research Articles

The inclined conductive column effect: a new simple model for magnetotelluric anomalous phases

SUMMARY Magnetotelluric data are sometimes accompanied by ‘anomalous’ impedance phases ($\phi $xy and $\phi $yx) in the off-diagonal components deviating from the first (0° < $\phi $xy < 90°) or third (−180° < $\phi $yx < −90°) quadrant, especially in long-period bands. This phenomenon is called the phases out-of-quadrant (POQ). The POQ poses a challenge in magnetotelluric modelling because simple 1-D or 2-D models cannot explain it. Previous studies have reported that strong inhomogeneity, anisotropy, or particular 3-D structures, such as the L-shaped or cross-shaped conductors, could explain the POQ. Aside from these models, we have discovered that a slanted columnar conductor also generates the POQ. Our systematic investigation through the synthetic forward modelling of an inclined conductive column with a varying geometry showed that the inclination angle and the column length may affect the POQ appearance. We investigated herein the behaviour of the electric currents around the inclined conductive column embedded in a resistive half-space. We found that the induced electric field in the region with the POQ tends to point in the opposite direction to the surrounding vectors. This result can reasonably explain the inverted phase in long-period bands. Furthermore, we confirmed that current is sucked into one end of the column, but discharged from the other end, suggesting that the column works as a current channel. The localized reverse vectors are associated with the current channelling along the inclined conductor, which generates the POQ. A volcanic conduit within a resistive host rock is one of the typical field examples of such an inclined channel. Our study suggests that the POQ is a helpful clue in imaging the geometry of a volcanic magma plumbing system through magnetotelluric surveys.

The Heating of Oceans by Volcanic Magma - Quantifying Global Warming by Submarine Volcanoes

The Heating of Oceans by Volcanic Magma - Quantifying Global Warming by Submarine Volcanoes

Pn Velocity and Anisotropy Tomography With Nonuniform Grid Beneath the Ryukyu–Taiwan–Philippines Region

AbstractTo enhance our comprehension of the dynamic processes associated with complex plate subduction, volcanic magmatism, and lithosphere deformation beneath the Ryukyu–Taiwan–Philippines region, we utilized nonuniform inversion grids for Pn velocity and anisotropy tomography, and obtained the uppermost mantle structure of this area. The results demonstrate remarkable characteristics: cold oceanic subducting plates display high Pn velocities, whereas volcanic arcs, the extinct mid‐ocean ridge in the South China Sea, and the Palawan region exhibit low Pn velocities. The extinct mid‐ocean ridge also displays a low‐velocity anomaly, indicating that residual heat persists even after approximately 15 million years since seafloor spreading. The occurrence of a discontinuous low‐velocity beneath the Ryukyu arc supports the presence of a slab window at approximately 123°E. The volcanic arcs of all subduction zones within the study area displayed trench‐parallel Pn anisotropy. The observed Pn fast directions beneath the Taiwan orogenic belt are consistent with crustal anisotropy, providing evidence for the crust–mantle coupled deformation. Moreover, our results shed light on the deep structural characteristics of the complex subduction zones beneath the Philippines region. Plate subduction causes partial melting due to dehydration; then, the melt ascends and accumulates at the uppermost mantle, showing low Pn velocities. However, the low‐velocity anomaly is not widely distributed but corresponds to a narrow band. In particular, the east–west bidirectional subduction of the Philippines and Negros formed separate low‐velocity anomalies. Low Pn velocities and trench‐parallel anisotropy indicate the location of different subduction zones and finely characterize their impact on the uppermost mantle structure.

Why are S-type plutonic rocks more chemically variable than S-type volcanic rocks and S-types more variable than I-types?

S-type granitic magmas form essentially through partial melting of crustal metasedimentary rocks, so this eliminates many possibilities for the origins of chemical variations in their parental magmas. Global, whole-rock, major-oxide data show that S-type volcanic series have far less chemical variability (analytical scatter) than S-type plutonic rocks. On the other hand, published data show far smaller contrasts between metaluminous ‘I-type’ silicic plutonic and volcanic rocks. There are several possible explanations for these features, but sampling bias and analytical errors can be discounted. Likewise, the differences cannot be explained by plutonic S-types being crystal cumulates or products of wall-rock assimilation, nor can they be due to volcanic suites being products of magma mixing or having compositions controlled by a different differentiation mechanism to that which might control S-type plutonic compositions. Variations in major-oxide concentrations suggest that the most probable explanation is that S-type plutonic magmas are derived from somewhat more aluminous metasedimentary source rocks, with higher ratios of metapelites to metagreywackes. If so, there are complementary implications that plutonic S-type magmas may be formed at lower temperatures, that they may be more hydrous than the S-type volcanic magmas, and that the differences in emplacement mechanism reflect this. The weaker contrasts between ‘I-type’ volcanic and plutonic rocks may be due to their crustal magma sources being more homogeneous and the melting reactions stoichiometrically similar, regardless of source-rock type.

Mineralization age of the Xiangshan uranium ore field, South China redefined by hydrothermal apatite U-Pb geochronology

The Xiangshan volcanic-related uranium ore field, located in the Ganzhou-Hangzhou Uranium-Polymetallic Metallogenic Belt (GHMB) in South China, has been the country’s largest historic uranium producer. Although this ore field has been intensely studied for decades, the absolute age of uranium ore formation was still debated. Conventional U-Pb dates on U-minerals (i.e., pitchblende, brannerite) and Ar-Ar dates on associated alteration minerals (i.e., illite) from different uranium deposits yield a wide range of ages (122 to 85 Ma). The uncertainty regarding the exact timing of uranium mineralization at Xiangshan has limited the development of accurate ore genesis models and precluded a better understanding of the links between metallogenesis and regional-scale geodynamics during the Cretaceous-Tertiary period in South China.In this work, we focus on applying in-situ LA-ICP-MS U-Pb geochronology to magmatic and hydrothermal apatite from two typical uranium ore deposits at Xiangshan, intending to indirectly constrain the exact timing of uranium mineralization and reconstruct the magmatic-hydrothermal history. U-Pb age determinations on magmatic zircon and apatite from the three types of ore-hosting volcanic rocks yielded precise crystallization ages of 133 to 136 Ma and 135 to 140 Ma, respectively. These results are identical within uncertainty to previously reported U-Pb dates obtained in the study area, and thereby tightly constrain the emplacement age of the Xiangshan volcanic complex that hosts uranium deposits. Moreover, the potassium feldspar-biotite Ar-Ar dating for the major volcanic rocks demonstrates that these host rocks were cooled and completely solidified through the biotite/potassium feldspar closure temperature (below 300 °C) at 133–131 Ma. Subhedral or anhedral aggregates of hydrothermal apatite ubiquitously formed in the high-grade uranium ores at the Shannan and Zoujiashan deposits and are commonly intergrown with fluorite, illite and uranium minerals. Relative to magmatic apatite from the host rocks, the hydrothermal apatite is typically characterized by much higher U and Th contents, depletion of LREEs, enrichment in HREEs and a much more pronounced negative Eu anomaly. As such, the hydrothermal apatite is unequivocal of hydrothermal origin and formed synchronously with uranium mineralization. LA-ICP-MS U-Pb dating of these hydrothermal apatite crystals yielded an absolute 206Pb/238U age of 126.7 ± 0.6 Ma for the Shannan deposit and 131.3 ± 7.2 Ma for the Zoujiashan deposit. The new ages are much older (>10 Ma) than those previously reported ages on ore or gangue minerals such as pitchblende and illite. Thus, one unidentified mineralization event likely occurred at ca. 131–127 Ma, soon after solidification and subsequent brittle deformation or faulting within the volcanic caldera. We, therefore, proposed that volcanic magmatism was highly likely to be the ‘heat engine’ driving hydrothermal fluid convection and possibly provide the essential conditions (i.e., magmatic fluids, U sources) for the formation of the U mineralization. Another implication of this study is that dating hydrothermal apatite with high U concentrations associated with uranium mineralization could be considered a complementary geochronological tool in the study of analogous hydrothermal uranium deposits worldwide.

Devonian ignimbrites of central Victoria: explosive magmas from multiple sources, and deep crustal structure in the Selwyn Block

The magmas that formed each Devonian, S-type, volcanic complex in central Victoria were generated through partial melting of a distinct assemblage of rock types in the crustal source region. The more silicic rhyolites were formed independently of the rhyodacitic to dacitic magmas, rather than as differentiates. Within every complex, each ignimbritic unit is also heterogeneous in its chemical and isotopic characteristics. Thus, in this part of Paleozoic Australia, magma heterogeneity occurred on at least three scales—the individual rock unit, the complex and across the region. This means that the Selwyn Block, the logical source of the ignimbrite magmas, is highly heterogeneous, probably in three dimensions. The statistically significant differences in the ages of the various complexes suggest that the heat sources for partial melting of the deep crust were themselves rather local in extent, and probably reflected numerous asynchronous intrusions of mantle-derived mafic magmas, most likely in relatively small batches. Thus, there should be no thick underplated or intraplated layer of gabbroic rock in the deep crust here. Thinner bodies of hot mafic magmas were probably emplaced in different places and at different levels, inducing local high-grade metamorphic conditions. Published radiometric dates for various volcanic units suggest that the magmatic heating of the crust occurred over a period of at least 17 Myr, although maximum intensity occurred between ca 375 and ca 371 Ma. Thus, a model of the deep crust here should involve numerous, perhaps thin metasedimentary units with highly variable ages and original clay contents, as well as sills of Late Devonian mafic rocks scattered both vertically and horizontally and intruded episodically. The dominantly meta-igneous sources of crustally derived I-type magmas generally lie at higher structural levels, where they did not experience metamorphic temperatures sufficient to cause the higher-temperature melting reactions needed to form I-type volcanic magmas. KEY POINTS Each Late Devonian, S-type, volcanic magma in central Victoria formed through melting of a distinct assemblage of crustal source rocks, most likely within the Selwyn Block. The rhyolites formed independently of the rhyodacitic to dacitic magmas, not as differentiates. The mafic magma heat sources for partial melting of the deep crust were emplaced as numerous, relatively small batches, in sheets distributed horizontally and vertically. The dominantly meta-igneous and shallower crustal sources of I-type magmas did not experience the higher temperatures required to form I-type volcanic magmas.

Geochemistry, geochronology and Sr–Nd–Hf isotopes of Paleozoic granitoids in the Chinese Altai, NW China: constraints on the conversion from subduction–accretion to syn-/post-collision

The Altai region of China features abundant granitoids, which can be used to reconstruct the tectonic evolution of the orogen. We conducted petrological, geochronological, geochemical and isotopic analyses on several representative granitoids from the Altai region. U‒Pb dating revealed ages of 444 ± 3 Ma for gneissic granites, 406 ± 8 Ma for syenogranites, 280 ± 6 Ma for diorites, 278 ± 3 Ma for two-mica monzonitic granites and 269 ± 3 Ma for muscovite granites. The Ordovician gneissic granites have I-type arc-related element characteristics and are derived from volcanic magmas extracted from the mantle wedge and then metasomatized by subducted sediment in a continental arc. The Devonian syenogranites show dual I-type arc and within-plate granite geochemical signatures, suggesting derivation from volcanic magmas extracted from the lithospheric mantle metasomatized by subducted fluid/melt in a back-arc basin. The Permian diorites are Mg-diorites derived from a mixed-magma source involving residual subducted basaltic oceanic crust and lithospheric mantle. The two-mica monzonitic granites are S-type granites originating from the crustal recycling of sedimentary rocks, whereas the muscovite granites are leucogranites resulting from the anatexis of ancient metasedimentary rocks. Their tectonic setting is syn-/post-collisional. By combining these results with previously published data, we propose that the Chinese Altai region experienced mid- to late Ordovician continental arc magmatism, early to mid-Devonian back-arc extension and early to mid-Permian arc–arc syn-/post-collision. Supplementary material: Supplementary Tables S1–S3 are available at https://doi.org/10.6084/m9.figshare.c.6420236

Geochronology and geochemistry of igneous rocks of the Dassa region, Central-Benin: evidence of an Ediacaran emplacement of alkali-calcic and alkaline plutonic and volcanic magmas

Geochronology and geochemistry of igneous rocks of the Dassa region, Central-Benin: evidence of an Ediacaran emplacement of alkali-calcic and alkaline plutonic and volcanic magmas

LA–ICP-MS Zircon U–Pb and Alunite 39Ar/40Ar Dating of the Saozhouhe Alunite Deposit: Implications for the Metallogenic Significance of the Xiaotian Basin

As an important part of the northern Huaiyang tectonic belt, the Mesozoic Xiaotian basin hosts a series of gold and alunite deposits. However, the ages of these deposits remain unclear, constraining the further understanding of the genesis of these deposits. In this work, zircon LA–ICP-MS and alunite 39Ar/40Ar isotopic dating studies were carried out on the andesitic porphyrite and alunite of the Saozhouhe alunite deposit, respectively. LA–ICP-MS zircon U–Pb dating for the Saozhouhe andesitic porphyrite yields a weighted average age of 122.8 ± 0.9 Ma (MSWD = 1.8), indicating that these volcanic rocks were formed in the Early Cretaceous period. The alunite 39Ar/40Ar dating yields a plateau age of 121.0 ± 1.1 Ma (MSWD = 4.4) and an isochron age of 121.2 ±1.9 Ma (MSWD = 8.5), indicating that the alunite was also formed in the Early Cretaceous period. Our finding confirms that the formation of the Saozhouhe alunite deposit is genetically related to the Cretaceous volcanic magmatism, which has great significance in the metallogenetic regularity of and further ore prospecting in the Xiaotian basin.

The long-lived transcrustal magmatic systems of Southeast China in response to paleo-Pacific plate subduction, recorded by the Cretaceous volcanic sequences in southeastern Zhejiang Province

Processes taking place in subduction zones are highly debated. The Cretaceous volcanic rocks are voluminously distributed along the coastal area of southeastern (SE) China. To elucidate their petrogenesis and relationship with subduction, we use new zircon U-Pb ages and Hf-O isotopes for the representative Cretaceous volcanic sequences in Zhejiang Province, China. According to stratigraphic investigations, zircon U-Pb ages, and Hf-O isotopes, these volcanic rocks can be divided into different groups corresponding to three stages of volcanic activity: the early (Stage 1, 136−129 Ma), middle (Stage 2, 125−115 Ma), and late (Stage 3, 110−94 Ma) stages. Diverse zircon populations (including antecrysts, autocrysts, and xenocrysts) are recognized. Hf-O isotopes of autocrysts suggest different protoliths for the identified three stages of volcanic activity. Xenocrysts show obvious different compositions from autocrysts. Antecrysts share similar compositions with autocrysts, which favor similarities in the magmas from which they were generated. Our observations (very small age intervals between antecrysts and autocrysts, sources for volcanic rocks throughout the entire crust, and volcanic magmatism with long-term and discontinuous characteristics), were inconsistent with the traditional melt-dominated magma chamber model. Consequently, we propose that those volcanic rocks were derived from long-lived transcrustal magmatic systems (TCMS), dominated by crystal mush, instead of melt-dominated magma chambers, maintained and recharged by a discontinuous contribution of contemporaneous underplated mantle-derived magmas, triggered by paleo-Pacific plate subduction. We suggest the different stages of volcanic activity and corresponding long-lived TCMS were produced by the change of Pacific plate motion beneath SE China during the Cretaceous period.

The Oligocene Avaj volcanic – plutonic complex of Central Iran: A record of magma evolution and mineral equilibria

• High-An plagioclase feldspar cores formed during early stage of evolution of the primary magma . • Mineral-melt equilibrium conditions are determined in clinopyroxenes and plagioclase cores. • Zoning in plagioclase and smooth CSD patterns invoke an unsteady-state magma system. • Parental mafic magmas were hydrous and contained <4 wt% H 2 O. • The residence time of plutonic and volcanic magmas was 117 and 13 years, respectively. The Avaj Oligocene volcanic – plutonic complex is part of extensive Cenozoic magmatic activity within the Urumieh-Dokhtar magmatic arc of Iran. We use whole rock geochemistry, mineral compositions and crystal size distributions (CSD) in a suite of co-genetic basalt, basaltic andesite and gabbro to determine their petrogenesis. Ca-rich cores in plagioclase (An 79-86 ) overlap empirically modelled compositions, indicating equilibrium crystallization from melts represented by the whole-rock compositions. Clinopyroxene compositions (Mg# 74–80) are compatible with mildly fractionated mantle-derived magmas in an arc setting. Mineral-melt equilibrium is inferred from high Al contents and close correspondence between the measured DiHd and predicted Kd Fe–Mg (0.23–0.32) in clinopyroxenes, and Kd (Plg/melt) (An–Ab) values of plagioclase cores (0.11–0.15). Clinopyroxene-melt thermometers indicate crystallization at 1119–1173 °C for volcanic and 1099–1134 °C for plutonic rocks. Plagioclase crystal core saturation temperatures range from 1088–1162 °C (volcanic) and 1121–1163 °C (plutonic); these values overlap calculated mineral-melt equilibrium temperatures. Plagioclase CSDs are nearly straight for both volcanic and plutonic samples, with higher nucleation density and steeper slopes for the plutonic samples. Major element variations suggest the Avaj rocks represent co-genetic magmas related by fractional crystallization of the observed mineral phases. We suggest minor crustal assimilation occurred during ascent from a deeper reservoir to a shallower one; CSD data indicate longer magma residence time for plagioclase in the plutonic samples (∼117 years) compared to the residence time of basaltic samples (∼13 years).

Do Oceanic Convection and Clathrate Dissociation Drive Europa’s Geysers?

Abstract Water vapor geysers on Europa have been inferred from observations made by the Galileo spacecraft, the Hubble Space Telescope, and the Keck Observatory. Unlike the water-rich geysers observed on Enceladus, Europa’s geysers appear to be an intermittent phenomenon, and the dynamical mechanism permitting water to sporadically erupt through a kilometers-thick ice sheet is not well understood. Here we outline and explore the hypothesis that the Europan geysers are driven by CO2 gas released by dissociation and depressurization of CO2 clathrate hydrates initially sourced from the subsurface ocean. We show that CO2 hydrates can become buoyant to the upper ice–water interface under plausible oceanic conditions, namely, if the temperature or salinity conditions of a density-stratified two-layer water column evolve to permit the onset of convection that generates a single mixed layer. To quantitatively describe the eruptions once the CO2 has been released from the hydrate state, we extend a one-dimensional hydrodynamical model that draws from the literature on volcanic magma explosions on Earth. Our results indicate that for a sufficiently high concentration of exsolved CO2, these eruptions develop vertical velocities of ∼700 m s−1. These high velocities permit the ejecta to reach heights of ∼200 km above the Europan surface, thereby explaining the intermittent presence of water vapor at these high altitudes. Molecules ejected via this process will persist in the Europan atmosphere for a duration of about 10 minutes, limiting the timescale over which geyser activity above the Europan surface may be observable. Our proposed mechanism requires Europa’s ice shell thickness to be d ≲ 10 km.

Accumulation and Distribution of Natural Gas Reservoir in Volcanic Active Area: A Case Study of the Cretaceous Yingcheng Formation in the Dehui Fault Depression, Songliao Basin, NE China

Tight gas sandstone and volcanic gas reservoirs have received global attention in the energy arena for further exploration and exploitation attempts. Considering the Yingcheng Formation of Dehui fault depression in the Songliao Basin as an example, this study focused on the accumulation and distribution of natural gas reservoirs in volcanic area in a fault depression basin. Volcanic activities occurred in the Yingcheng Formation, which is distributed centrally in the northwest of the study area. During the sedimentation of the Yingcheng Formation, fan-delta, lacustrine, and nearshore subaqueous fan facies were deposited. The source rocks of the Yingcheng Formation have high abundance of organic matter mainly in type III at high-overmature stages, indicating favorable conditions for gas production. The porosity of volcanic reservoir is 3.0%-14.8%, the permeability is 0.0004 mD-2.52 mD, and the pore types are mainly secondary dissolved pores and fractures. Besides, the porosity of the tight sandstone reservoir is 0.5%-11.2%, and the permeability is 0.0008 mD-3.17 mD. The pore types are mainly interparticle pores, with a small proportion of intraparticle pores and microfractures. The intrusion of late volcanic magma provided sufficient heat for the thermal maturity progression of organic matter in Yingcheng Formation and promoted the generation of natural gas in large quantities. Volcanic rocks formed at the early and middle stages of volcanic activities occupied the sedimentary space and hindered the development of sedimentary sand bodies to a certain extent. However, volcanic rocks can become the seal to promote the formation of tight sandstone gas traps. Comparing tight sandstone reservoirs with volcanic ones, the latter are less affected by compaction; thus, their petrophysical properties do not vary much with depth, showing more homogeneous characteristics. The pyroclastic rocks influenced by volcanic activity and the secondary pores formed by dissolution in the later stages also provide reservoir space for gas accumulation. Ultimately, the tight sandstone and volcanic rocks in the study area form a complex gas reservoir system, which can become a reference for exploration and exploitation of natural gas in other petroliferous fault depressions that are affected by volcanisms.

Age and Geochemistry of Late Jurassic Mafic Volcanic Rocks in the Northwestern Erguna Block, Northeast China

The northwestern Erguna Block, where a wide range of volcanic rocks are present, provides one of the foremost locations to investigate Mesozoic Paleo-Pacific and Mongol-Okhotsk subduction. The identification and study of Late Jurassic mafic volcanic rocks in the Badaguan area of northwestern Erguna is of particular significance for the investigation of volcanic magma sources and their compositional evolution. Detailed petrological, geochemical, and zircon U-Pb dating suggests that the Late Jurassic mafic volcanic rocks formed at 157–161 Ma. Furthermore, the geochemical signatures of these mafic volcanic rocks indicate that they are calc-alkaline or transitional series with weak peraluminous characteristics. The rocks have a strong MgO, Al2O3, and total alkali content, and a SiO2 content of 53.55–63.68 wt %; they are enriched in Rb, Th, U, K, and light rare-earth elements (LREE), and depleted in high-field-strength elements (HFSE), similar to igneous rocks in subduction zones. These characteristics indicate that the Late Jurassic mafic volcanic rocks in the Badaguan area may be derived from the partial melting of the lithospheric mantle as it was metasomatized by subduction-related fluid and the possible incorporation of some subducting sediments. Subsequently, the fractional crystallization of Fe and Ti oxides occurred during magmatic evolution. Combined with the regional geological data, it is inferred that the studied mafic volcanic rocks were formed by lithospheric extension after the closure of the Mongol-Okhotsk Ocean.

Geodetic monitoring of the Changbaishan volcano activity and its relationship with earthquakes, 1999–2017

The major earthquakes often trigger unrest of surrounding volcanic magma chambers. In recent years, three major earthquakes occurred around the Changbaishan volcano, but it was unclear whether these earthquakes triggered the unrest of the magma chamber. Based on geodetic data, we analyzed the volcanic activity according to the Global Position System (GPS) and leveling sites time-series and re-estimated the location and volume change of magma chamber from 2002 to 2005. Meanwhile, we calculated the dilatational strain variations of the deep magma chamber resulting from coseismic deformations caused by the 1999 Mw7.0 and 2002 Mw7.3 Wangqing deep earthquakes and the 2011 Mw 9.0 Tohoku-oki earthquake. Our results show: (1) Changbaishan has experienced four stages of unrest since 1999, and the biggest unrest of the shallow magma chamber occurred from 2002 to 2005; (2) the parameters of the shallow magma chamber simulated by the Mogi and Point Compound Dislocation Model (pDCM) show that the magma chamber is located in the northern part of the crater, with a depth of approximately 7 km. The volume of the magma chamber increased by 25–28 × 106 m from 2002 to 2005; (3) the strain variation beneath the Changbaishan volcano corresponding to the 1999 Wangqing earthquake was small. The 2002 Wangqing earthquake produced an expansion strain of about 4.4 nano-strain on the magma chamber at a depth of 550 km, and probably promoted the unrest of the Changbaishan volcano. The 2011 Tohoku earthquake induced the expansion of the shallow magma chamber and compression of the deep magma chamber. Although this event promoted shallow magma unrest, it inhibited deep magma unrest. This may explain why the Changbaishan did not show obvious unrest after the 2011 Tohoku earthquake. Therefore, more attention should be paid to earthquakes that can promote deep magma unrest in the Changbaishan volcano.

Thematic vent opening probability maps and hazard assessment of small-scale pyroclastic density currents in the San Salvador volcanic complex (El Salvador) and Nejapa-Chiltepe volcanic complex (Nicaragua)

Abstract. The San Salvador volcanic complex (El Salvador) and Nejapa-Chiltepe volcanic complex (Nicaragua) have been characterized by a significant variability in eruption style and vent location. Densely inhabited cities are built on them and their surroundings, including the metropolitan areas of San Salvador (∼2.4 million people) and Managua (∼1.4 million people), respectively. In this study we present novel vent opening probability maps for these volcanic complexes, which are based on a multi-model approach that relies on kernel density estimators. In particular, we present thematic vent opening maps, i.e., we consider different hazardous phenomena separately, including lava emission, small-scale pyroclastic density currents, ejection of ballistic projectiles, and low-intensity pyroclastic fallout. Our volcanological dataset includes: (1) the location of past vents, (2) the mapping of the main fault structures, and (3) the eruption styles of past events, obtained from critical analysis of the literature and/or inferred from volcanic deposits and morphological features observed remotely and in the field. To illustrate the effects of considering the expected eruption style in the construction of vent opening maps, we focus on the analysis of small-scale pyroclastic density currents derived from phreatomagmatic activity or from low-intensity magmatic volcanism. For the numerical simulation of these phenomena we adopted the recently developed branching energy cone model by using the program ECMapProb. Our results show that the implementation of thematic vent opening maps can produce significantly different hazard levels from those estimated with traditional, non-thematic maps.

Calcium isotopic fractionation during magma differentiation: Constraints from volcanic glasses from the eastern Manus Basin

To better constrain Ca isotopic fractionation during volcanic magma differentiation, we report Ca isotope data for a series of volcanic glasses from the eastern Manus Basin that represent a continuum of compositions comprising basaltic andesite, andesite, dacite, and rhyolite. Our results show that these volcanic glasses exhibit ~0.25‰ variation in Ca isotopic compositions, with δ44/40Ca ranging from 0.81 ± 0.04 to 1.05 ± 0.04‰. The similar trace element patterns and Sr-Nd isotopic compositions exclude source heterogeneity as the cause of the varied Ca isotopic compositions. Due to their low K/Ca ratios and young age, the Ca isotopic variations cannot be attributed to 40K decay. Instead, the observed Ca isotopic variations are primarily controlled by their lithology. Specifically, δ44/40Ca of basaltic andesite (0.81 ± 0.04) and andesite (0.82 ± 0.01) are similar to the averages of reported mid-ocean ridge basalts (MORBs; 0.84 ± 0.10; 2SD, N = 26) and back-arc basin basalts (BABBs; 0.80 ± 0.08; 2SD, N = 21) in literature, but do not correlate with SiO2 or MgO contents, indicating the Ca isotopic fractionation is insignificant during intermediate-mafic magma evolution. As magma differentiation continues, δ44/40Ca of dacite and rhyolite evolves from 0.85 ± 0.01 to 1.05 ± 0.04‰. Moreover, δ44/40Ca of felsic glasses are negatively correlated with Eu/Eu*, which suggests that the increasing δ44/40Ca of felsic glasses is most likely related to plagioclase (Pl) fractional crystallization. Therefore, we conclude that Ca isotopes can behave differently at different stages of volcanic magma differentiation. Modeling of fractional crystallization using rhyolite-MELTS shows that the relative Ca fractions in crystallized minerals (e.g., Pl vs. Cpx) and the Ca isotopic fractionation factor between the residual melt and crystallized Pl (αmelt-Pl, which is likely related to the An number of Pl, magma temperature and kinetic effects during crystallization), provide a plausible explanation for the behavior of Ca isotopes during volcanic magma differentiation.

A black hole particle swarm optimization method for the source parameters inversion: application to the 2015 Calbuco eruption, Chile

The traditional genetic algorithm and simulated annealing methods have been widely used in geophysical modeling. However, these nonlinear inversion methods require a lot of calculations, many control parameters and are unstable. In this paper, a particle swarm optimization algorithm combined with black hole strategy (BH-PSO) is proposed to solve these problems. The comprehensive experiments show that the BH-PSO method consumes less time than the simulated annealing (SA) method and has a higher accuracy than the genetic algorithm (GA). It is more applicable to the inversion of parameters of volcanic magma chamber, and easier to be generalized to other kinematic source parameters inversion. Based on BH-PSO method, Sentinel-1 data, composite dislocation model (CDM), Yang model and Mogi model, the magma chamber parameters of Calbuco eruption in 2015 were retrieved. The results show that the RMSE of CDM model is 1.1 cm, which can better fit the surface deformation than the Mogi model and Yang model. The final results show that the magma chamber is located about 0.8 km northeast of the crater, about 9 km below the surface, and the total volume of the erupted volcanic material obtained with the CDM Model is of 0.209 km3, without considering dense rock equivalent.

Reactivation of Magma Pathways: Insights From Field Observations, Geochronology, Geomechanical Tests, and Numerical Models

AbstractField observations and unmanned aerial vehicle surveys from Caldera Taburiente (La Palma, Canary Islands, Spain) show that pre‐existing dykes can capture and re‐direct younger ones to form multiple dyke composites. Chill margins suggest that the older dykes were solidified and cooled when this occurred. In one multiple dyke example, an 40Ar/39Ar age difference of 200 kyr was determined between co‐located dykes. Petrography and geomechanical measurements (ultrasonic pulse and Brazilian disc tests) show that a microscopic preferred alignment of plagioclase laths and sheet‐like structures formed by non‐randomly distributed vesicles give the solidified dykes anisotropic elastic moduli and fracture toughness. We hypothesize that this anisotropy led to the development of margin‐parallel joints within the dykes, during subsequent volcanic loading. Finite element models also suggest that the elastic contrast between solidified dykes and their host rock elevated and re‐oriented the stresses that governed subsequent dyke propagation. Thus, the margin‐parallel joints, combined with local concentration and rotation of stresses, favored the deflection of subsequent magma‐filled fractures by up to 60° to form the multiple dykes. At the edifice scale, the capture and deflection of active intrusions by older ones could change the organization of volcanic magma plumbing systems and cause unexpected propagation paths relative to the regional stress. We suggest that reactivation of older dykes by this mechanism gives the volcanic edifice a structural memory of past stress states, potentially encouraging the re‐use of older vents and deflecting intrusions along volcanic rift zones or toward shallow magma reservoirs.

Altimeter-derived marine gravity variations reveal the magma mass motions within the subaqueous Nishinoshima volcano, Izu–Bonin Arc, Japan

Variations of marine gravity reflect mass changes in Earth’s interior, including magma motions during volcanic eruption, which manifest in submarine mass transports. Absolute gravimeters have been used to detect and study evolutions of magma mass motions, however, only for land volcanoes. Here for the first time, we used radar altimeter-derived time-varying marine gravity fields, corresponding to before, during and after the Nishinoshima volcanic eruption, Izu–Bonin arcs near Japan, to quantify the evolution of undersea volcanic magma mass motions. The magma depths were observed to become shallower and < 2 km, which are almost in exact accordance with seismic results. We find that the magma volume decreased beneath the Nishinoshima volcano, while increased or being fed by deeper reservoirs to the east and northeast of Nishinoshima. We conclude that the Nishinoshima volcano may continue to be active in the future. This study highlights the use of satellite radar altimetry as an innovative and viable tool to study subaqueous volcanism.