Volatile emissions to the atmosphere associated with the Siberian Traps eruptions at the Permian-Triassic boundary were sourced from the outgassing of primary magmas and the sedimentary host rocks ...into which they were intruded. Halogens in volcanic gases may have played an important role in environmental degradation and in stratospheric ozone destruction. Here we investigate how halogens behave during the interaction between salts and basalt magma emplaced as sills and erupted as lava. We present whole-rock, trace, and halogen concentrations for a suite of samples from three locations in the Siberian Traps Large Igneous Province, including basalt lavas erupted, and dolerites intruded into both organic-bearing shales and evaporites. Dolerites are enriched in Cl, Br, and I; their enrichment in Cl is similar to MORB and OIB that have been inferred to have assimilated seawater. The dolerites exhibit halogen compositional systematics, which extend towards both evaporites and crustal brines. Furthermore, all analyzed samples show enrichment in Rb/Nb; with the dolerites also showing enrichment in Cl/K similar to MORB and OIB that have been inferred to have assimilated seawater. We infer that samples from all three locations have assimilated fluids derived from evaporites, which are components of crustal sedimentary rocks. We show that up to 89% of the chlorine in the dolerites may have been assimilated as a consequence of the contact metamorphism of evaporites. We show, by thermal modeling, that halogen transfer may occur via assimilation of a brine phase derived from heating evaporites. Halogen assimilation from subcropping evaporites may be pervasive in the Siberian Traps Large Igneous Province and is expected to have enhanced emissions of Cl and Br into the atmosphere from both intrusive and extrusive magmatism.
Dome-forming eruption is a frequent eruptive style and a major hazard on numerous volcanoes worldwide. Lava domes are built by slow extrusion of degassed, viscous magma and may be destroyed by ...gravitational collapse or explosion. The triggering of lava dome explosions is poorly understood: here we propose a new model of superficial lava-dome explosivity based upon a textural and geochemical study (vesicularity, microcrystallinity, cristobalite distribution, residual water contents, crystal transit times) of clasts produced by key eruptions. Superficial explosion of a growing lava dome may be promoted through porosity reduction caused by both vesicle flattening due to gas escape and syn-eruptive cristobalite precipitation. Both processes generate an impermeable and rigid carapace allowing overpressurisation of the inner parts of the lava dome by the rapid input of vesiculated magma batches. The relative thickness of the cristobalite-rich carapace is an inverse function of the external lava dome surface area. Explosive activity is thus more likely to occur at the onset of lava dome extrusion, in agreement with observations, as the likelihood of superficial lava dome explosions depends inversely on lava dome volume. This new result is of interest for the whole volcanological community and for risk management.
Before the 1991–1992 activity, a large andesite lava dome belonging to the penultimate Pinatubo eruptive period (Buag ∼
500 BP) formed the volcano summit. Buag porphyritic andesite contains abundant ...amphibole-bearing microgranular enclaves of basaltic–andesite composition. Buag enclaves have lower K
2O and incompatible trace element (LREE, U, Th) contents than mafic pulses injected in the Pinatubo reservoir during the 1991–1992 eruptive cycle. This study shows that Buag andesite formed by mingling of a hot, water-poor and reduced mafic magma with cold, hydrous and oxidized dacite. Depending on their size, enclaves experienced variable re-equilibration during mixing/mingling. Re-equilibration resulted in hydration, oxidation and transfer of mobile elements (LILE, Cu) from the dacite to the mafic melts and prompted massive amphibole crystallization. In Buag enclaves, S-bearing phases (sulfides, apatite) and melt inclusions in amphibole and plagioclase record the evolution of sulfur partition among melt, crystal and fluid phases during magma cooling and oxidation. At high temperature, sulfur is partitioned between andesitic melt and sulfides (Ni-pyrrhotite). Magma cooling, oxidation and hydration resulted in exsolution of a S–Cl–H
2O vapor phase at the S-solubility minimum near the sulfide–sulfate redox boundary. Primary magmatic sulfide (pyrrhotite) and xenocrystic sulfide grains (pyrite), recycled together with olivines and pyroxenes from old mafic intrusives, were replaced by Cu-rich phases (chalcopyrite, cubanite) and, partially, by Ba–Sr sulfate. Sulfides degassed and transformed into residual spongy magnetite in response to fS
2 drop during final magma ascent and decompression. Our research suggests that a complete evaluation of the sulfur budget at Pinatubo must take into account the en route S assimilation from the country rocks. Moreover, this study shows that the efficiency of sulfur transfer between mafic recharges and injected magmas is controlled by the extent and rate of mingling, hydrous flushing and melt oxidation. Vigorous mixing/mingling and transformation of the magmatic recharge into a spray of small enclaves is required in order to efficiently strip their primary S-content that otherwise remains locked in the sulfides. Hydrous flushing increases the magma oxidation state of the recharges and modifies their
primary volatile concentrations that cannot be recovered by the study of late-formed mineral phases and melt inclusions. Conversely, S stored in both late-formed Cu-rich sulfides and interstitial rhyolitic melt represents the
pre-eruptive sulfur budget immediately available for release from mafic enclaves during their decompression.
Using temperature gradients measured in 10 holes at 6 sites, we generate the first high fidelity heat flow measurements from Integrated Ocean Drilling Program drill holes across the northern and ...central Lesser Antilles arc and back arc Grenada basin. The implied heat flow, after correcting for bathymetry and sedimentation effects, ranges from about 0.1 W/m2 on the crest of the arc, midway between the volcanic islands of Montserrat and Guadeloupe, to <0.07 W/m2 at distances >15 km from the crest in the back arc direction. Combined with previous measurements, we find that the magnitude and spatial pattern of heat flow are similar to those at continental arcs. The heat flow in the Grenada basin to the west of the active arc is 0.06 W/m2, a factor of 2 lower than that found in the previous and most recent study. There is no thermal evidence for significant shallow fluid advection at any of these sites. Present‐day volcanism is confined to the region with the highest heat flow.
Key Points
Heat flow in the Lesser Antilles is similar to other volcanic arcs
No evidence for subsurface fluid flow
Volcanism is confined to the region with high heat flow
In the 15
years following the last crisis of phreatic type in 1975–77, the hydrothermal and seismic activity of La Soufrière de Guadeloupe has displayed an episodic behavior superimposed on a ...long-term decay, as inferred from seismicity and degassing, monitored through fumarolic emissions and Cl content in thermal springs. In particular, the evolution of the Cl content can be explained by a succession of magmatic gas injections into the hydrothermal system. This episodic supply of gas may be associated to the thermodynamic evolution of a batch of magma intruded at shallow depth around 1976. Progressive magma cooling, crystallization, gas exsolution and pressure buildup lead to cyclic failure of host rock around the intrusion, and episodic release of gas, with a decreasing frequency as cooling proceeds. This physical model allows to reproduce the pattern of degassing activity of La Soufrière, including the decreasing frequency of Cl pulses with time. Our preferred forward model of Cl pulses involves a magma intrusion volume in the range 0.01–0.52
km
3 and a tensile strength of the surrounding rock in the range 0.1–0.6
MPa. These results reinforce the hypothesis of a shallow magma intrusion triggering the 1975–77 crisis and provide an explanation for the degassing and seismic activities observed until 1992.
► A magma intrusion may have triggered the last phreatic crisis of La Soufrière. ► After-crisis episodicity in degassing and seismicity may result from this intrusion. ► A physical model for episodic degassing of a magma intrusion is applied. ► Magma intrusion volume and tensile strength of the surrounding rocks are determined.
The products of eruptive and mass-wasting processes that built island arc volcanoes are better preserved in marine deposits than on land. Holes U1397A and U1399A drilled during IODP Expedition 340 ...provide a 1.5 Ma record of the volcanic history of Martinique.
14
C dating and δ
18
O patterns are used to reconstitute the chronostratigraphy of tephra, volcaniclastic turbidites, and mass-wasting events (traced by debris avalanches, debrites, and duplication and deformation of pre-existing sediments), leading to a new volcanic history of Montagne Pelée and Pitons du Carbet volcanoes. The top 50 m of core U1397A provides a continuous high-resolution sedimentation record over the last ∼130 ka. The sedimentation record deeper than 50 m in core U1397A and in the whole core U1399A is discontinuous because of the numerous sliding and deformation events triggered by debris avalanches related to flank collapses. Three successive activity periods are identified since ∼190 ka: the “Old Pelée” until 50 ka, the “Grand Rivière” (50–20 ka), and the “Recent Pelée” (20 ka—present day). The first two periods have the highest volcanic deposition rates offshore but very little outcrop on land. The whole magmatic activity of Mt Pelée comprises silicic andesites, but mafic andesites were also emitted during the whole “Grand Rivière.” At ∼115 ka, a major flank collapse (“Le Prêcheur”) produced a debris avalanche and submarine landslide that affected sea floor sediments by erosion and deformation up to ∼70 km from the shore. The Pitons du Carbet volcano was active from 1.2 Ma to 260 ka with numerous large flank collapses at a mean rate of 1 event every 100 ka. The average deposition rate of tephra fall offshore is much less than that at Mt Pelée. Our data show that correlations between the timing of large landslides or emission of mafic magmas and rapid sea level rise or lowstands suggested by previous studies are not systematic. The reconstituted chronostratigraphy of cores U1397A and U1399A provides the framework necessary for further studies of the magma petrology and production rates and timing of the mechanisms triggering flank collapses and related submarine landslides of Mt Pelée and Pitons du Carbet
.
Flank-collapse events are now recognized as common processes of destruction of volcanoes. They may occur several times on a volcanic edifice pulling out varying volumes of material from km3 to ...thousands of km3. In the Lesser Antilles Arc, a large number of flank-collapse events were identified. Here, we show that some of the largest events are correlated to significant variations in erupted magma compositions and eruptive styles.
On Montagne Pelée (Martinique), magma production rate has been sustained during several thousand years following a 32ka old flank-collapse event. Basic and dense magmas were emitted through open-vent eruptions that generated abundant scoria flows while significantly more acidic magmas were produced before the flank collapse. The rapid building of a new cone increased the load on magma bodies at depth and the density threshold. Magma production rate decreased and composition of the erupted products changed to more acidic compared to the preceding period of activity. These low density magma generated plinian and dome-forming eruptions up to the Present. In contrast at Soufrière Volcanic Centre of St. Lucia and at Pitons du Carbet in Martinique, the flank-collapses have an opposite effect: in both cases, the acidic magmas erupted immediately after the flank-collapses. These magmas are highly porphyritic (up to 60% phenocrysts) and much more viscous than the magmas erupted before the flank-collapses. They have been generally emplaced as voluminous and uptight lava domes (called “the Pitons”). Such magmas could not ascent without a significant decrease of the threshold effect produced by the volcanic edifice loading before the flank-collapse.
•Flank-collapse effect on magma plumbing system in the Lesser Antilles arc•Threshold effect of volcanic edifice on magma emission•By-pass of the threshold due to the unload by flank-collapse•By-pass of the threshold by magma differentiation•Combining on-land and marine tephrochronology data on Montagne Pelée
The use of volcanic glass as recorder of paleoenvironmental conditions has existed for 30years. In this paper we investigate the methodological aspects of the determination of water content, isotopic ...composition, and water speciation in volcanic glass using the High Temperature Conversion/Elemental Analyzer (TCEA) mass spectrometer system on milligram quantities of glass concentrates. It is shown here that the precision and the reproducibility of this method is comparable to off-line conventional methods that require 100 times greater amount of material (δD±3‰; H2Otot±10relative% if <1wt% and ±5 relative% if >1wt%) but is quicker and permits easy replication. This method extracts 100% of the water as verified by FTIR measurements. Finally, this study confirms the interest of DRIFT spectroscopy in the NIR range for the study of porous samples such as volcanic pumices and tephra, to determine the water speciation (H2O/OH). It may complement conventional FTIR transmission measurements in the MIR or NIR range that usually require homogeneous transparent sections or high degree of sample dilution in a non-absorbing matrix.
Using these methods, we attempt to discriminate residual magmatic from secondary meteoric water in volcanic glass. Using mafic to differentiated samples from different geological settings and different climatic conditions, we show that the H-isotope composition and water content of volcanic glass alone are not always sufficient to provide clear distinction between magmatic and meteoric origin. However if the magma is known to have a δD between −90‰ and −40‰ (−60‰ for MORB mantle source), it is quite easy to resolve the δD evolution during magmatic degassing from post-depositional rehydration by meteoric water with δD<−50‰ or δD>−20‰. Water speciation measurements may provide additional information. In most cases, isotopic and total water measurements should be complemented by characterization of water speciation. During magmatic degassing (from 6wt% to ~0.1wt% water) the H2O/OH is expected to decrease from 2 to close to 0. However, our dataset suggests that during secondary glass hydration (from 0.1wt% to 6wt% water) the H2O/OH ratio decreases from ~5 to 2, which is the complete opposite.
Overall our results support the use of H-isotopes of volcanic glass to discuss the composition of meteoric waters and paleo-climate within a specific region. To this purpose, the volcanic glass has to be almost fully rehydrated in order to fingerprint the isotopic composition of the ambient environmental water. As rehydration is exponentially faster with increasing temperature, efficient rehydration taking months to years, may occur in a cooling volcanic deposits that are meters-thick and thus can remain at a few hundred °C for a years to hundreds of years after the eruption. Such deposits could then provide a snap-shot view of climatic conditions at the time of the studied eruption.
•TC/EA method is a convenient tool to measure δD and H2Otot in volcanic glass.•The DRIFT spectroscopy in the NIR is useful to determine OH/H2O in volcanic samples.•Discrimination between magmatic and rehydration water in volcanic glass
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