Magma Migration Research Articles

Post-rift magmatism on the northern South China Sea margin

Abstract Intense magmatism in the form of widespread volcanoes and lava flows is identified in high-resolution 3-D seismic data over a post-rift sequence of the northern South China Sea (SCS). Such a magmatism post-dates the end of seafloor spreading in the SCS by at least 6.8 m.y. A detachment (boundary) fault propagating into a deep-seated magma chamber provided the main vertical pathway for magma migration. In turn, normal faults and dykes constituted a shallow plumbing system through which the magma migrated from the boundary fault and was extruded onto the paleo-seafloor. Volcanism occurred in the study area from ca. 8.2 Ma to ca. 1.1 Ma in the form of two distinct events, dated ca. 5.2 Ma and ca. 2.8 Ma, which are correlated with the Dongsha Event. Extrusive magma formed volcano edifices and extensive lava flows; the latter of which were confined to the troughs of sediment waves or, instead, flowed along submarine canyons. As a corollary, this study shows that in the SCS: (1) young magmatism is widespread on the northern continental margin, (2) seafloor morphology greatly influences the architecture of deep-water volcanoes, and (3) syn-rift faults (especially detachment faults) reactivated by regional tectonics closely control the magma plumbing systems.

Polybaric/polythermal magma transport and trace element partitioning recorded in single crystals: A case study of a zoned clinopyroxene from Mt. Etna

We present elemental maps and intra-crystal compositional profiles conducted on a representative clinopyroxene phenocryst from the 1974 eccentric lava flows at Mt. Etna volcano (Sicily, Italy). The eruption was fed by deep-seated and primitive magmas ascending through pathways bypassing the central volcanic conduits. These magmas show MgO and Cr contents higher (and REE + Y lower) than those characterizing younger and more evolved eruptions, albeit the bulk rock compositions of both primitive and more evolved products are invariably classified as trachybasalts in the TAS (total alkali vs. silica) diagram. Mafic recharge episodes are recorded by the complex textural features of the clinopyroxene, with a subrounded core enclosed within a concentrically zoned mantle. The core is enriched in Mg + Fe2++Na and depleted in Fe3++Ca relative to the mantle. The jadeite (Jd) component decreases from core to mantle and is counterbalanced by higher Ca-Tschermak (CaTs) contents, as the number of TAl cations in tetrahedral coordination increases. The Jd-rich core incorporates high proportions of rare earth elements and Y (REE + Y) and low concentrations of high field strength elements (HFSE) and transition elements (TE, such as Ni, Cr and Sc), whereas the opposite occurs for the CaTs-rich mantle. The decoupling of REE + Y and HFSE argues against simple changes in melt composition and indicates an additional mechanism driving trace element zonations. Thermobarometric calculations indicate that the early-formed Jd-rich core equilibrated with the host magma at mantle depths (750–950 MPa and 1190–1210 °C), whereas the later CaTs-rich mantle formed at shallower crustal levels (400–700 MPa and 1150–1180 °C) after magma recharge. Quantitative modeling of apparent cation partitioning between clinopyroxene and melt (Di) indicates that DHFSE increase from the Jd-rich core to the CaTs-rich mantle. In contrast, DREE+Y increase up to one order of magnitude at the Jd-rich core due to the enhanced stability of an Na0.5REE + Y0.5MgSiO6 end-member. We infer that compositional changes in clinopyroxene due to the different P-T conditions of the plumbing system may control the concentrations of REE + Y in residual melts derived after partial crystallization and differentiation of primitive magmas, such as those feeding the 1974 eccentric eruption. On this basis, we use DREE+Y measured across the core-mantle interface to constrain the geochemical evolution of recent 2000–2013 magmas at Mt. Etna volcano by Rayleigh fractional crystallization. Results indicate that magma dynamics proceed via a stepwise polybaric-polythermal process accounting for 1) crystallization of Jd-rich clinopyroxenes at high-P, high-T conditions, 2) upward migration of crystal-bearing magmas due to replenishment phenomena with input of fresh magmas and 3) crystallization of CaTs-rich clinopyroxene in low-P, low-T reservoirs. The resulting total amount (~40 vol%) of clinopyroxene fractionated agrees with geophysical data suggesting the presence of highly crystalline magmatic bodies at shallow to intermediate crustal levels below Mt. Etna.

The 2018 unrest phase at La Soufrière of Guadeloupe (French West Indies) andesitic volcano: Scrutiny of a failed but prodromal phreatic eruption

After 25 years of gradual increase, volcanic unrest at La Soufrière of Guadeloupe reached its highest seismic energy level on 27 April 2018, with the largest felt volcano-tectonic (VT) earthquake (ML 4.1 or MW 3.7) recorded since the 1976–1977 phreatic eruptive crisis. This event marked the onset of a seismic swarm (180 events, 2 felt) occurring after three previous swarms on 3–6 January (70 events), 1 st February (30 events, 1 felt) and 16–17 April (140 events, 1 felt). Many events were hybrid VTs with long-period codas, located 2–4 km below the volcano summit and clustered within 2 km along a regional NW-SE fault cross-cutting La Soufrière. Elastic energy release increased with each swarm whereas inter-event time shortened. At the same time, summit fractures continued to open and thermal anomalies to extend. Summit fumarolic activity increased significantly until 20 April, with a maximum temperature of 111.4 °C and gas exit velocity of 80 m/s, before declining to ~95 °C and ~33 m/s on 25 April. Gas compositions revealed increasing C/S and CO2/CH4 ratios and indicate hydrothermal P-T conditions that reached the critical point of pure water. Repeated MultiGAS analysis of fumarolic plumes showed increased CO2/H2S ratios and SO2 contents associated with the reactivation of degassing fractures (T = 93 °C, H2S/SO2 ≈ 1). While no direct evidence of upward magma migration was detected, we attribute the above phenomena to an increased supply of deep magmatic fluids that heated and pressurized the La Soufrière hydrothermal system, triggering seismogenic hydro-fracturing, and probable changes in deep hydraulic properties (permeability) and drainage pathways, which ultimately allowed the fumarolic fluxes to lower. Although this magmatic fluid injection was modulated by the hydrothermal system, the unprecedented seismic energy release and the critical point conditions of hydrothermal fluids suggest that the 2018 sequence of events can be regarded as a failed phreatic eruption. Should a similar sequence repeat, we warn that phreatic explosive activity could result from disruption of the shallow hydrothermal system that is currently responsible for 3–9 mm/y of nearly radial horizontal displacements within 1 km from the dome. Another potential hazard is partial collapse of the dome's SW flank, already affected by basal spreading above a detachment surface inherited from past collapses. Finally, the increased magmatic fluid supply evidenced by geochemical indicators in 2018 is compatible with magma replenishment of the 6–7 km deep crustal reservoir feeding La Soufrière and, therefore, with a potential evolution of the volcano's activity towards magmatic conditions.

A Reappraisal of Seismicity Recorded During the 1996 Gjálp Eruption, Iceland, in Light of the 2014–2015 Bárðarbunga–Holuhraun Lateral Dike Intrusion

This study performs a reanalysis of the seismicity recorded during the 1996 Gjalp eruption that occurred at NW Vatnajokull, Iceland. The seismicity was recorded by the temporary HOTSPOT network consisting of 30 three-component broadband stations. In total 301 events were identified between 29 September and 12 October and their phases were manually picked. A velocity model was estimated from P-phase travel times by using VELEST. Events were first located using the algorithm NONLINLOC in order to obtain absolute locations. Precise relative locations were obtained with HYPODD by utilizing catalog and cross-correlation differential travel times. Results show that events clustered first along the SW rim of the Barðarbunga caldera and later along the Gjalp fissure, with most hypocentral depths located between 3 and 8 km. Waveforms of the 10 largest events that followed the Barðarbunga earthquake were inverted in order to obtain moment tensors. For all events we found that the deviatoric moment tensor fits the data better than pure double-couple or full moment tensor solutions. Events along the Barðarbunga caldera exhibited reverse focal mechanisms, while those at the Gjalp fissure exhibited mostly strike-slip faulting. Seismic velocity variations calculated using ambient noise interferometry, point to the possibility that a small subglacial eruption occurred at Barðarbunga before the main earthquake. This removed melt from the magma chamber causing its roof to collapse, and also resulted in the lateral migration of magma towards the Gjalp fissure. The 2014–2015 Barðarbunga–Holuhraun eruption shares common characteristics with the 1996 Gjalp eruption, although the size of the latter was much smaller.

Copper Isotope Variations During Magmatic Migration in the Mantle: Insights From Mantle Pyroxenites in Balmuccia Peridotite Massif

AbstractThe crust shows large variations in Cu isotopic composition (δ65Cu relative to NIST 976) resulting from redox reactions and other supergene processes. The significant range of δ65Cu in mantle peridotites thus could be ascribed to recycled crustal materials and/or oxidative mantle metasomatism. However, the influence of normal magmatic fractionation processes on δ65Cu variations remains poorly understood, and it is unclear whether the magmatic processes also lead to large δ65Cu variations in mantle rocks. To address the issue, we present bulk‐rock δ65Cu of fresh mantle pyroxenites from the Balmuccia peridotite massif, Northern Italy. These pyroxenites formed by melt‐peridotite reaction and mineral accumulation from MORB‐ to OIB‐like, sulfide‐saturated basic magmas. Mass balance calculations show that sulfide phases host >98 wt % of the Cu budget of bulk rocks (87−484 μg/g). The pyroxenites show significant variations of δ65Cu from −0.66‰ to 0.66‰, which cover the range known for peridotites, including metasomatized ones. Three subsamples from the same pyroxenite layer display >0.3‰ decrease in δ65Cu with progressive differentiation. Nevertheless, δ65Cu does not display correlations with indicators of magmatic differentiation (e.g., Mg# and Cu/Pd) for all pyroxenites in this study. This might be attributed to variable extents of magmatic sulfide segregation for different samples and/or varying δ65Cu of the pyroxenite parent magmas, which were also affected by reactive migration through peridotites. The combined processes can change δ65Cu of evolving magmas and reacted peridotites and lead to Cu isotopic heterogeneity in the mantle, without involvement of oxidative metasomatism or recycled crustal materials.

Pore-scale permeability prediction for Newtonian and non-Newtonian fluids

Abstract. The flow of fluids through porous media such as groundwater flow or magma migration is a key process in geological sciences. Flow is controlled by the permeability of the rock; thus, an accurate determination and prediction of its value is of crucial importance. For this reason, permeability has been measured across different scales. As laboratory measurements exhibit a range of limitations, the numerical prediction of permeability at conditions where laboratory experiments struggle has become an important method to complement laboratory approaches. At high resolutions, this prediction becomes computationally very expensive, which makes it crucial to develop methods that maximize accuracy. In recent years, the flow of non-Newtonian fluids through porous media has gained additional importance due to, e.g., the use of nanofluids for enhanced oil recovery. Numerical methods to predict fluid flow in these cases are therefore required. Here, we employ the open-source finite difference solver LaMEM (Lithosphere and Mantle Evolution Model) to numerically predict the permeability of porous media at low Reynolds numbers for both Newtonian and non-Newtonian fluids. We employ a stencil rescaling method to better describe the solid–fluid interface. The accuracy of the code is verified by comparing numerical solutions to analytical ones for a set of simplified model setups. Results show that stencil rescaling significantly increases the accuracy at no additional computational cost. Finally, we use our modeling framework to predict the permeability of a Fontainebleau sandstone and demonstrate numerical convergence. Results show very good agreement with experimental estimates as well as with previous studies. We also demonstrate the ability of the code to simulate the flow of power-law fluids through porous media. As in the Newtonian case, results show good agreement with analytical solutions.

Gravimetric study of the shallow basaltic plumbing system of Tenerife, Canary Islands

Recent studies on intraplate volcanic islands reveal that magma ascends sub-vertically through these volcanic edifices, following well-defined zones in the lithospheric mantle and oceanic crust, and occasionally suffering lateral propagation along rift zones when it reaches very shallow depths. Visualizing the internal structure of the shallowest part of such magma plumbing systems is crucial to understanding magma transport and the location of basaltic volcanism in these volcanic environments. Tenerife (Canary Islands) is a very good example of an intraplate volcanic island, in which the distribution of basaltic volcanism includes well-defined rift systems, and wider monogenetic fields where cinder cones form clusters along different tectonic alignments. In order to characterize the structure of the uppermost part of the basaltic plumbing system of Tenerife, we conducted a detailed gravimetric study of the north-western, central, and southern sectors of the island. We developed a 3D density contrast model of the subsoil based on the application of a genetic algorithm for the inversion of gravity data. The results of our inverse modelling suggest that the observed gravimetric anomalies below the northwestern rift zone are shallow and aligned parallel to this structure. This is consistent with a shallow character for the rift system and a lateral migration of magma along it that comes from a deeper zone located towards the east of this rift zone. Conversely, the anomalies located in the south are also shallow but are distributed along more than one alignment and are connected to much deeper sub-vertical similar density anomalies. These results support the existence of several main channels at the north and south of the island that allow the ascent of deep basaltic magma to shallower zones of Tenerife. There, magma may be either caught by the rifts systems, being distributed laterally and occasionally erupting, or may continue its ascent to the surface using a secondary system of shallow fractures that dominates the southern part of Tenerife. The results of our inverse model also offer new insights on others aspects of the internal structure of Tenerife, such as the distribution of the basaltic shield or that of Las Cañadas caldera.

Dike Channelization and Solidification: Time Scale Controls on the Geometry and Placement of Magma Migration Pathways

AbstractWe investigate the conditions under which magma prefers to migrate through the crust via a dike or a conduit geometry. We performed a series of analogue experiments, repeatedly injecting warm, liquid gelatin, into a cold, solid gelatin medium and allowing the structure to evolve with time. We varied the liquid flux and the time interval between discrete injections of gelatin. The time interval controls the geometry of the migration, in that long intervals allow the intrusions to solidify, favoring the propagation of new dikes. Short time intervals allow the magma to channelize into a conduit. These times are characterized by the Fourier number (Fo), a ratio of time and thermal diffusion to dike thickness, so that long times scales have Fo > 102 and short time scales have Fo < 100. Between these time scales, a transitional behavior exists, in which new dikes nest inside of previous dikes. The flux controls the distance a dike can propagate before solidifying, in that high fluxes favor continual propagation, whereas low fluxes favor dike arrest due to solidification. For vertically propagating dikes, this indicates whether or not a dike can erupt. A transitional behavior exist, in which dikes may erupt at the surface in an unstable, on‐and‐off fashion. We supplemented the experimental findings with a 2‐D numerical model of thermal conduction to characterize the temperature gradient in the crust as a function of intrusion recurrence frequency. For very infrequent intrusions (Fo > 104 to 105) all thermal energy is lost, while more frequent intrusions allow heat to build up nearby.

Real-time assessment of potential seismic migration within a monitoring network using Red-flag SARA

Magma opening new fluid pathways through the crust can generate migrating seismic sources following the trail of the magma. By using Seismic Amplitude Ratio Analysis (SARA), it is possible to detect this seismic migration simply from the amplitudes of continuous data recorded at different stations in a network, without having to do any picking of seismic phases. In this study, we present a modified method – Red-flag SARA, which adapts SARA for real-time monitoring. Red-flag SARA provides a quantitative tool to analyse amplitude ratios between stations in a network and detect temporal changes in these ratios. Since such changes imply seismic source location variations, Red-flag SARA is a handy tool during seismic crises to quickly answer the question of whether seismic activity, and therefore magma, is migrating or not. We tested Red-flag SARA on synthetic data and validated it using real data from two volcanoes – Piton de la Fournaise, Reunion Island, and Gede, Indonesia, for three scenarios: 1) magma migration ending as intrusion, 2) migration leading to eruption and 3) a burst of seismicity with no magma migration.

Is Himalayan leucogranite a product by in situ partial melting of the Greater Himalayan Crystalline? A comparative study of leucosome and leucogranite from Nyalam, southern Tibet

Widespread leucogranites in the Himalayan orogenic belt are thought to have originated by in situ partial melting of the Greater Himalayan Crystalline (GHC) when it underwent high-grade metamorphism during Cenozoic orogenesis. Therefore, the leucogranites and associated migmatites can be used to constrain the exhumation history of the GHC. However, the petrogenetic relationship between the GHC, leucogranites, and migmatites is not well-constrained. As such, we carried out a detailed petrographic, mineralogical, geochronological, and geochemical study of leucosomes and leucogranites from the Nyalam region in southern Tibet. Monazite U–(Th)–Pb dating indicates that anatexis of the GHC occurred during the late Eocene and Miocene (40–14 Ma), whereas leucogranite emplacement occurred from 27 to 14 Ma. There are marked differences between the leucosomes in migmatites and leucogranites in terms of field geology, mineralogy, and geochemistry, suggesting different origins. The leucosomes occur mainly as pockets or are interlayered with melanosomes in stromatic metatexites. The leucosomes contain oligoclase and Fe-rich biotite, and have whole-rock compositions with high K2O contents (4.8–7.4 wt%) and K2O/Na2O ratios (1.35–2.97), positive Eu anomalies (Eu/Eu* = 0.93–2.61), and low rare-metal (Li, Be, Cs, Sn, and Ta) contents. These features are consistent with an origin by muscovite dehydration melting of the GHC. However, the leucogranites intrude the GHC and occur as small plutons along the South Tibetan Detachment System. In contrast to the leucosomes, the plagioclase and biotite in the leucogranites are albite and siderophyllite, respectively. The leucogranites have relatively low K2O (4.3–4.7 wt%) contents and K2O/Na2O ratios (1.04–1.24), high rare-metal contents, and marked negative Eu anomalies (Eu/Eu* = 0.47–0.70), indicating an origin by extensive fractional crystallization. We propose that the leucogranites were magmas produced in the deeper GHC during peak metamorphism, and subsequent extensive fractional crystallization occurred during long-distance, upward migration of magma along the South Tibetan Detachment System (STDS) during exhumation of the GHC. During these processes, the high-grade metamorphic rocks of the GHC were partially melted, resulting in the formation of leucosomes within migmatites. Therefore, the leucosomes and leucogranites have different origins and should be considered separately in studies of Himalayan orogenesis.

Space- and Ground-Based Geophysical Data Tracking of Magma Migration in Shallow Feeding System of Mount Etna Volcano

After a month-long increase in activity at the summit craters, on 24 December 2018, the Etna volcano experienced a short-lived lateral effusive event followed by a rapid resumption of low-level explosive and degassing activity at the summit vents. By combining space (Moderate Resolution Imaging Spectroradiometer; MODIS and SENTINEL-2 images) and ground-based geophysical data, we track, in near real-time, the thermal, seismic and infrasonic changes associated with Etna’s activity during the September–December 2018 period. Satellite thermal data reveal that the fissural eruption was preceded by a persistent increase of summit activity, as reflected by overflow episodes in New SouthEast Crater (NSE) sector. This behavior is supported by infrasonic data, which recorded a constant increase both in the occurrence and in the energy of the strombolian activity at the same crater sectors mapped by satellite. The explosive activity trend is poorly constrained by the seismic tremor, which shows instead a sudden increase only since the 08:24 GMT on the 24 December 2018, almost concurrently with the end of the infrasonic detections occurred at 06:00 GMT. The arrays detected the resumption of infrasonic activity at 11:13 GMT of 24 December, when tremors almost reached the maximum amplitude. Infrasound indicates that the explosive activity was shifting from the summit crater along the flank of the Etna volcano, reflecting, with the seismic tremor, the intrusion of a gas-rich magma batch along a ~2.0 km long dyke, which reached the surface generating an intense explosive phase. The dyke propagation lasted for almost 3 h, during which magma migrated from the central conduit system to the lateral vent, at a mean speed of 0.15–0.20 m s−1. Based on MODIS and SENTINEL 2 images, we estimated that the summit outflows erupted a volume of lava of 1.4 Mm3 (±0.5 Mm3), and that the lateral effusive episode erupted a minimum volume of 0.85 Mm3 (±0.3 Mm3). The results presented here outline the support of satellite data on tracking the evolution of volcanic activity and the importance to integrate satellite with ground-based geophysical data in improving assessments of volcanic hazard during eruptive crises.

Surface displacements of Aso volcano after the 2016 Kumamoto earthquake based on SAR interferometry: implications for dynamic triggering of earthquake–volcano interactions

SUMMARYThe 2016 Kumamoto earthquake involved a series of events culminating in an Mw 7.0 main shock on 2016 April 16; the main-shock fault terminated in the caldera of Aso volcano. In this study, we estimated surface displacements after the 2016 Kumamoto earthquake using synthetic aperture radar interferometry analysis of 16 Phased Array Type L-band Synthetic Aperture Radar-2 images acquired from 2016 April 18 to 2017 June 12 and compared them with four images acquired before the earthquake. Ground subsidence of about 8 cm was observed within about a 3 km radius in the northwestern part of Aso caldera. Because this displacement was not seen in data acquired before the 2016 Kumamoto earthquake, we attribute this displacement to the 2016 Kumamoto earthquake. Furthermore, to estimate the source depth of the surface displacement, we applied the Markov chain Monte Carlo method to a spherical source model and obtained a source depth of about 4.8 km. This depth and position are nearly in agreement with the top of a low-resistivity area previously inferred from magnetotelluric data; this area is thought to represent a deep hydrothermal reservoir. We concluded that this displacement is due to the migration of magma or aqueous fluids.

Noneruptive Unrest at the Caldera of Alcedo Volcano (Galápagos Islands) Revealed by InSAR Data and Geodetic Modeling

AbstractUnderstanding volcanic unrest is crucial to forecasting eruptions. At active mafic calderas unrest culminates in eruption more frequently than at felsic calderas. However, the mafic caldera of Alcedo Volcano (Ecuador) has experienced repeated episodes of unrest without erupting, since at least 1992, when geodetic monitoring began. Here we investigate the unrest that occurred between 2007 and 2011 using interferometric synthetic aperture radar (InSAR) data and geodetic modeling. We observe an initial asymmetric uplift of the southern caldera floor (~30 cm of vertical motion) from 2007 to 2009, followed by subsidence of the uplifted area and contemporary uplift of the northwestern caldera rim between January and June 2010. Finally, from June 2010 through March 2011, caldera uplift resumed. The first uplift episode is best explained by inflation of a sill and the activation of an inner ring fault. Successive caldera subsidence and rim uplift are compatible with the withdrawal of magma from the previously inflated sill and its northwestern migration. The resumption of uplift is consistent with the repressurization of the sill. This evolution suggests episodic magma emplacement in a shallow reservoir beneath the caldera, with aborted lateral magma migration, probably due to the discontinuous supply from depth. This short‐term deformation pattern matches well geological observations showing a longer‐term (hundreds of years at least) asymmetric uplift of the caldera floor, culminating in a weak resurgence of ~30 m. We propose that the monitored episodes of uplift represent short‐term stages of the rarely observed incremental growth of a resurgent basaltic caldera.

Theoretical models and experimental determination methods for equations of state of silicate melts: A review

Silicate melts are very active in the interior of the Earth and other terrestrial planets, and are important carriers for the transport of material and energy. The determination of the equation of state (EOS) for silicate melts and the acquisition of a precise quantitative relationship between molar volume (or density) and temperature, pressure, and composition is essential for simulating the generation, migration, and eruption processes of magmas and the evolution of the magma ocean stage during the early formation of the Earth and other terrestrial planets, for calculating and modeling the phase equilibria involving silicate melts, and for revealing the variation of the microstructure of silicate melts with pressure. However, it is experimentally challenging to determine the volumetric properties of silicate melts and the accumulated density data at high pressure are still very limited due to a series of problems such as: the high liquidus temperature of silicate rocks; proneness for silicate melts to react with sample capsules to change the melt composition; and proneness for melts to flow and leak during the high pressure and high temperature experiments. In recent years, there is rapid progress in the high pressure and high temperature experimental techniques, in terms of not only the extension of temperature and pressure ranges but also the improvement on the accuracy of measurements, and the emergence of new methods for in-situ measurements. Here, we review the widely-used theoretical models of ambient-pressure and high-pressure EOS for silicate melts, and illustrate some problems that need to be solved urgently: (1) the room pressure EOS for iron- and titanium-bearing silicate melts needs to be improved; (2) the partial molar properties of the H2O and CO2 components in silicate melts containing volatile components may vary markedly with the melt composition, which need to be addressed in high-pressure EOS; (3) how the formulation and applicable range of EOS correspond to changes in melt structure and compression mechanism requires further study. We highlight the basic principle and applicable range of various methods for determining the EOS for silicate melts, and compare the advantages and disadvantages of double-bob Archimedes method, fusion curve analysis, shock compression experiments, sink-float method, X-ray absorption, X-ray diffraction and ultrasonic interferometry. Future trends in this field are to develop experimental techniques for in situ measurements on melt density or sound velocity at high temperature and high pressure and to accumulate more experimental data, and on the other hand, to improve the theoretical models of the EOS for silicate melts by a combination of research on the microstructure and compression mechanisms of silicate melts.

Rift structures and its related unconformities on and adjacent the Dongsha Rise: insights into the nature of the high-velocity layer in the northern South China Sea

It is curious that the typical feature of high-velocity layer (HVL) underneath the crust for the magma-rich margin was found in the magma-poor margin of the South China Sea. In order to better understand the nature and age of the HVL in the South China Sea, here we provide seismic imaging on the Cenozoic structure and sedimentation using a regional seismic reflection profile combined with existing drilling data and seismic refraction lines in the northeastern South China Sea. An important finding is that the extent of erosion since Neogene is only on the Dongsha Rise and is much smaller than that of the HVL observed across the northern margin of the South China Sea, which suggests the occurrence of the HVL has no influence on the erosion on Dongsha Rise through Neogene. On the regional seismic profiles, the different structures between the deepwater rifted margins and the continental shelf were also observed. The faults in the Zhuyi Sub-basins on the continental shelf are imaged to be high-angle and bound a series of grabens and horsts. In contrast, the deepwater region is characterized by tilted fault blocks and an array of listric normal faults, which extremely thinned the crust to be ~ 5–10 km. After middle Miocene, active magmatism occurred in the deepwater region at the time of post-spreading of oceanic crust. These listric faults on the hyper-extended crust is interpreted to be conduits for magma migration to the shallower levels. This typical structure in the deepwater region changed the geophysical nature of the HVL underneath the extreme thinned crust, which should be formed before the event of hyper-extension and is likely to be associated with the subduction of the paleo-Pacific toward the Asia in the late Cretaceous. Our study will answer the question why the South China Sea with the HVL underneath the crust is a magma-poor passive margin.

Spatial and temporal variations in the geochemistry of Cretaceous high-Sr/Y rocks in Central Tibet

Recent studies based on low-temperature chronology and sedimentology have proposed the existence of a proto-Tibetan Plateau (p-TP); however, the timing and mechanisms of its formation and evolution remain ambiguous. High-Sr/Y rocks are an important petrological indicator of thickening. Here, we compile geochemical data of Cretaceous rocks to interpret their petrogenesis and to constrain deep geodynamic processes. Geochemical characteristics, in combination with zircon Hf isotopic compositions, indicate that the high-Sr/Y rocks were derived from the partial melting of thickened juvenile lower crust, with or without contamination by mantle peridotite. Comparing geochronological and geochemical data, we observe a correlation between magma migration and the composition of high-Sr/Y rocks. Based on these observations, we propose a revised tectonomagmatic evolution model for central Tibet, involving crustal thickening, retreating delamination, and breakoff. Our research suggests that the rapid uplift of the p-TP was a consequence of the removal of isostatic load during the Mesozoic.

Constraining Spatiotemporal Characteristics of Magma Migration at Piton De La Fournaise Volcano From Pre-eruptive Seismicity.

Volcano‐tectonic seismicity has been recorded for decades on various volcanoes and is linked with the magma transport and reservoir pressurization. Yet earthquakes often appear broadly distributed such that magma movement is difficult to infer from its analysis. We explore the seismicity that occurred before eruptions at Piton de la Fournaise in the last 5 years. Using template matching and relocation techniques, we produce a refined image of the summit seismicity, demonstrating that most earthquakes are located on a ring structure. However, only a portion of this structure is activated before each eruption, which provides an indication as to the direction of the future eruptive site. Furthermore, we show that the delay between the pre‐eruptive swarm and the eruption onset is proportional to the distance of the eruptive fissures relative to the summit cone. This reveals that the beginning of the intrusion already bears information regarding the future eruption location.

Post-spreading deformation and associated magmatism along the Iberia-Morocco Atlantic margins: Insight from submarine volcanoes of the Tore-Madeira Rise

A new high-resolution bathymetric map combined with a regional Digital Elevation Model (DEM) analysis reveal the modalities of occurrence and emplacement of post-spreading magmatism along the NNE-SSW oriented, 1000 km long Tore-Madeira Rise (TMR) as well as its relationship with the activity of major fault systems including the Estremadura Fault System (ESF) and the Azores-Gibraltar Fracture Zone (AGFZ). Morphological and structural analysis of the bathymetric map were performed to map volcanic features such as eruptive cones, vents and fissures together with faults along the TMR. The new bathymetric map shows that the main NNW-SSE seamount alignment is formed by three structurally distinct volcanic massifs, the Tore, the Josephine and the Southern Volcanic Groups. The majority of the volcanoes of each group emplaced within or along specific portion of pre-existing faults (ESF and AGFZ) including splay fault, releasing bend, fault tips and interaction zones between different segments. Magmas were channelled into sub-vertical pre-existing lithospheric faults that acted as preferential pathways for the vertical magma ascent. Migration and final eruption of magma are controlled by the local stress variation induced by complex fault geometries, change in plate kinematics as well as strong shear zone anisotropy as suggested by the emplacement within localised areas of transtension. We conclude that post-spreading magma emplacement in the southern part of the Iberia margin was related to the development of a transtensional plate boundary between the Iberian and African Plate during the Late Cretaceous. More generally, our findings emphasize that the distribution of volcanism as the expression of the interaction between shallow plate tectonic and mantle processes should be included in plate kinematic reconstruction. This study also demonstrates that the accurate mapping of oceanic seafloor is pivotal to better understand tectono-magmatic evolution of volcanic seamount chains and geological processes in oceanic domains.

Crystal and melt inclusion timescales reveal the evolution of magma migration before eruption

Volatile element concentrations measured in melt inclusions are a key tool used to understand magma migration and degassing, although their original values may be affected by different re-equilibration processes. Additionally, the inclusion-bearing crystals can have a wide range of origins and ages, further complicating the interpretation of magmatic processes. To clarify some of these issues, here we combined olivine diffusion chronometry and melt inclusion data from the 2008 eruption of Llaima volcano (Chile). We found that magma intrusion occurred about 4 years before the eruption at a minimum depth of approximately 8 km. Magma migration and reaction became shallower with time, and about 6 months before the eruption magma reached 3–4 km depth. This can be linked to reported seismicity and ash emissions. Although some ambiguities of interpretation still remain, crystal zoning and melt inclusion studies allow a more complete understanding of magma ascent, degassing, and volcano monitoring data.

Magma Plumbing Systems: A Geophysical Perspective

Over the last few decades, significant advances in using geophysical techniques to image the structure of magma plumbing systems have enabled the identification of zones of melt accumulation, crystal mush development, and magma migration. Combining advanced geophysical observations with petrological and geochemical data has arguably revolutionised our understanding of, and afforded exciting new insights into, the development of entire magma plumbing systems. However, divisions between the scales and physical settings over which these geophysical, petrological, and geochemical methods are applied still remain. To characterise some of these differences and promote the benefits of further integration between these methodologies, we provide a review of geophysical techniques and discuss how they can be utilised to provide a structural context for and place physical limits on the chemical evolution of magma plumbing systems. For example, we examine how Interferometric Synthetic Aperture Radar (InSAR), coupled with Global Positioning System (GPS) and Global Navigation Satellite System (GNSS) data, and seismicity may be used to track magma migration in near real-time. We also discuss how seismic imaging, gravimetry and electromagnetic data can identify contemporary melt zones, magma reservoirs and/or crystal mushes. These techniques complement seismic reflection data and rock magnetic analyses that delimit the structure and emplacement of ancient magma plumbing systems. For each of these techniques, with the addition of full-waveform inversion (FWI), the use of Unmanned Aerial Vehicles (UAVs) and the integration of geophysics with numerical modelling, we discuss potential future directions. We show that approaching problems concerning magma plumbing systems from an integrated petrological, geochemical, and geophysical perspective will undoubtedly yield important scientific advances, providing exciting future opportunities for the volcanological community.