Garnet petrochronology reveals the lifetime and dynamics of phonolitic magma chambers at Somma-Vesuvius

Magma chambers feeding hazardous Plinian eruptions of Somma-Vesuvius have been present throughout most of the volcano’s lifetime.

Large silicic magma bodies and very large magnitude explosive eruptions

Over the last 20 years, new concepts have emerged into understanding the processes that lead to build up to large silicic explosive eruptions based on integration of geophysical, geochemical, petrological, geochronological and dynamical modelling. Silicic melts are generated within magma systems extending throughout the crust by segregation from mushy zones. Segregated melt layers become unstable and can assemble into ephemeral upper crustal magma chambers rapidly prior to eruption. In the next 10 years, we can expect major advances in dynamical models as well as in analytical and geophysical methods, which need to be underpinned in field research.

Seismic Diffraction Analysis of a Fluid Escape Pipe beneath the Submarine Gas Bubble Plume in the Haima Cold Seep Area

Pipe structures are considered as fluid conduits beneath cold seeps. These structures have been observed in many geological settings and are widely accepted as the most critical pathway for fluid migration. One of such pipe structures in the Haima cold seep region is investigated herein. The pipe structure extends from below the BSR and reaches the seafloor. It is characterized by a string of events with short and strong seismic amplitudes, similar to the string of bead reflections (SBRs) associated with small-scale caves in carbonate reservoirs. This leads to the hypothesis that multiple small-scale bodies exist within the pipe structure. We test this hypothesis by analysis of diffraction waves and numerical seismic modeling. Travel time pattern analysis indicates that the diffractors within the pipe structure caused the rich diffraction waves on the shot records, and the reversed polarity indicates that the diffractors have a lower impedance than the surrounding sediments. These low-impedance bodies are interpreted as gas pockets within the pipe structures. Based on these interpretations, a conceptual model is proposed to describe the fluid migration process within the pipe. Briefly, we propose that gas pockets within the pipe structure could be analogue to the magma chambers located beneath volcanoes and this may provide a new insight into how gases migrate through the pipe structure and reach the seafloor.

A 5-km-thick reservoir with >380,000 km3 of magma within the ancient Earth's crust

Several recent studies have argued that large, long-lived and molten magma chambers1–10 may not occur in the shallow Earth’s crust11–23. Here we present, however, field-based observations from the Bushveld Complex24 that provide evidence to the contrary. In the eastern part of the complex, the magmatic layering was found to continuously drape across a ~4-km-high sloping step in the chamber floor. Such deposition of magmatic layering implies that the resident melt column was thicker than the stepped relief of the chamber floor. Prolonged internal differentiation within such a thick magma column is further supported by evolutionary trends in crystallization sequence and mineral compositions through the sequence. The resident melt column in the Bushveld chamber during this period is estimated to be >5-km-high in thickness and >380,000 km3 in volume. This amount of magma is three orders of magnitude larger than any known super-eruptions in the Earth’s history25 and is only comparable to the extrusive volumes of some of Earth’s large igneous provinces26. This suggests that super-large, entirely molten and long-lived magma chambers, at least occasionally, occur in the geological history of our planet. Therefore, the classical view of magma chambers as ‘big magma tanks’1–10 remains a viable research concept for some of Earth’s magmatic provinces.

Catching the Main Ethiopian Rift evolving towards plate divergence

Magmatism accompanies rifting along divergent plate boundaries, although its role before continental breakup remains poorly understood. For example, the magma-assisted Northern Main Ethiopian Rift (NMER) lacks current volcanism and clear tectono-magmatic relationships with its contiguous rift portions. Here we define its magmatic behaviour, identifying the most recent eruptive fissures (EF) whose aphyric basalts have a higher Ti content than those of older monogenetic scoria cones (MSC), which are porphyritic and plagioclase-dominated. Despite these differences, calculations highlight a similar parental melt for EF and MSC products, suggesting only a different evolutionary history after melt generation. While MSC magmas underwent a further step of storage at intermediate crustal levels, EF magmas rose directly from the base of the crust without contamination, even below older polygenetic volcanoes, suggesting rapid propagation of transcrustal dikes across solidified magma chambers. Whether this recent condition in the NMER is stable or transient, it indicates a transition from central polygenetic to linear fissure volcanism, indicative of increased tensile conditions and volcanism directly fed from the base of the crust, suggesting transition towards mature rifting.

Geochronology, Correlation and Magnetic Studies of Quaternary Ignimbrites in the Taupo Volcanic Zone, New Zealand

<p>Voluminous, rhyolitic ignimbrites erupted from calderas in the Taupo Volcanic Zone (TVZ) of North Island, New Zealand during the last ca. 1.6 Ma, are characterised by geochemical, paleomagnetic, magnetic fabric and isotopic age techniques to determine their stratigraphy and source vent areas. Most of the welded ignimbrites record distinctive thermoremanent magnetism (TRM) directions that can be defined with a precision of less than 5 degrees. On this basis, individual ignimbrites may be identified and correlated. These data indicate that the voluminous Whakamaru group ignimbrites, mapped by various names in different parts of the TVZ, were probably erupted over a period of as little as 100 years. The Kaingaroa and Matahina ignimbrites display very similar TRM directions and may have been emplaced contemporaneously. Ahuroa and Mamaku ignimbrites display TRM directions widely different to that expected from a dipole field, and were emplaced during polarity transitions in Earth's magnetic field. Geochemically, glasses and FeTi-oxides from the TVZ ignimbrites are homogeneous and typical of high-SiO v2 (>75 wt percent) rhyolites. They indicate little evidence of derivation from physically or compositionally zoned magma chambers, and allow individual eruptives to be fingerprinted. Variable compositions of whole pumice clasts from welded units, previously interpreted as evidence for chemical zonation can be explained by glass alteration and variable mineral components. Geochemical and chronological data suggest the Rocky Hill Ignimbrite and/or Unit E ignimbrite (ca. 1 Ma) may be correlatives of the Potaka tephra, found in sedimentary basins outside the TVZ. Rock magnetic fabric studies using anisotropy of magnetic susceptibility of ignimbrites allow paleoflow patterns to be determined. These patterns are generally consistent with source areas inferred from other data. The source for Mamaku Ignimbrite is consistent with an area on the western margin of Lake Rotorua. The Whakamaru group ignimbrites appear to have originated north of Lake Taupo, and in particular from an area near the Western Dome Belt. Glass shards from nonwelded bases of ignimbrites are well suited to dating by the isothermal plateau fission track (ITPFT) method. Any partial fading of the spontaneous tracks has been corrected by a single-step heat treatment of 150 degrees C for 30 days. The resulting ages and their uncertainties are comparable is caret 40Ar/caret 39Ar plagioclase determinations. The following new eruption ages were determined: Whakamaru group ignimbrites (0.34 Plus-minus 0.03 Ma), Matahina Ignimbrite (0.34 Plus-minus 0.02 Ma), Kaingaroa ignimbrite (0.33 Plus-minus 0.02 Ma), informally named unit Downer 8 (0.33 Plus-minus 0.02 Ma), and Mamaku Ignimbrite (0.23 Plus-minus 0.01 Ma). These data suggest a major phase of activity, with several different caldera forming events in the interval ca. 0.35-0.32 Ma. The age of Mamaku Ignimbrite constrains the paleomagnetic excursion recorded in the unit to ca. 0.23 Ma, similar to the age of the Pringle Falls geomagnetic episode recorded in the western USA.</p>

Geochronology, Correlation and Magnetic Studies of Quaternary Ignimbrites in the Taupo Volcanic Zone, New Zealand

<p>Voluminous, rhyolitic ignimbrites erupted from calderas in the Taupo Volcanic Zone (TVZ) of North Island, New Zealand during the last ca. 1.6 Ma, are characterised by geochemical, paleomagnetic, magnetic fabric and isotopic age techniques to determine their stratigraphy and source vent areas. Most of the welded ignimbrites record distinctive thermoremanent magnetism (TRM) directions that can be defined with a precision of less than 5 degrees. On this basis, individual ignimbrites may be identified and correlated. These data indicate that the voluminous Whakamaru group ignimbrites, mapped by various names in different parts of the TVZ, were probably erupted over a period of as little as 100 years. The Kaingaroa and Matahina ignimbrites display very similar TRM directions and may have been emplaced contemporaneously. Ahuroa and Mamaku ignimbrites display TRM directions widely different to that expected from a dipole field, and were emplaced during polarity transitions in Earth's magnetic field. Geochemically, glasses and FeTi-oxides from the TVZ ignimbrites are homogeneous and typical of high-SiO v2 (>75 wt percent) rhyolites. They indicate little evidence of derivation from physically or compositionally zoned magma chambers, and allow individual eruptives to be fingerprinted. Variable compositions of whole pumice clasts from welded units, previously interpreted as evidence for chemical zonation can be explained by glass alteration and variable mineral components. Geochemical and chronological data suggest the Rocky Hill Ignimbrite and/or Unit E ignimbrite (ca. 1 Ma) may be correlatives of the Potaka tephra, found in sedimentary basins outside the TVZ. Rock magnetic fabric studies using anisotropy of magnetic susceptibility of ignimbrites allow paleoflow patterns to be determined. These patterns are generally consistent with source areas inferred from other data. The source for Mamaku Ignimbrite is consistent with an area on the western margin of Lake Rotorua. The Whakamaru group ignimbrites appear to have originated north of Lake Taupo, and in particular from an area near the Western Dome Belt. Glass shards from nonwelded bases of ignimbrites are well suited to dating by the isothermal plateau fission track (ITPFT) method. Any partial fading of the spontaneous tracks has been corrected by a single-step heat treatment of 150 degrees C for 30 days. The resulting ages and their uncertainties are comparable is caret 40Ar/caret 39Ar plagioclase determinations. The following new eruption ages were determined: Whakamaru group ignimbrites (0.34 Plus-minus 0.03 Ma), Matahina Ignimbrite (0.34 Plus-minus 0.02 Ma), Kaingaroa ignimbrite (0.33 Plus-minus 0.02 Ma), informally named unit Downer 8 (0.33 Plus-minus 0.02 Ma), and Mamaku Ignimbrite (0.23 Plus-minus 0.01 Ma). These data suggest a major phase of activity, with several different caldera forming events in the interval ca. 0.35-0.32 Ma. The age of Mamaku Ignimbrite constrains the paleomagnetic excursion recorded in the unit to ca. 0.23 Ma, similar to the age of the Pringle Falls geomagnetic episode recorded in the western USA.</p>

The fate of particles in a volumetrically heated convective fluid at high Prandtl number

The dynamics of suspensions plays a crucial role in the evolution of geophysical systems such as lava lakes, magma chambers and magma oceans. During their cooling and solidification, these magmatic bodies involve convective viscous fluids and dispersed solid crystals that can form either a cumulate or a floating lid by sedimentation. We study such systems based on internal heating convection experiments in high Prandtl fluids bearing plastic beads. We aim to determine the conditions required to produce a floating lid or a sedimented deposit. We show that, although the sign of particles buoyancy is the key parameter, it is not sufficient to predict the particles fate. To complement the model we introduce the Shields formalism and couple it with scaling laws describing convection. We propose a generalized Shields number that enables a self-consistent description of the fate of particles in the system, especially the possibility to segregate from the convective bulk. We provide a quantification of the partition of the mass of particles in the different potential reservoirs (bulk suspension, floating lid, settled cumulate) through reconciling the suspension stability framework with the Shields formalism. We illustrate the geophysical implications of the model by revisiting the problem of the stability of flotation crusts on solidifying rocky bodies.

Magma chamber decompression during explosive caldera-forming eruption of Aira caldera

Decompression of a magma chamber is a fundamental condition of caldera collapse. Although theoretical models have predicted the decompression of magma chambers before caldera collapse, few previous studies have demonstrated the amount of magma chamber decompression. Here, we determine water content in quartz glass embayments and inclusions from pyroclastic deposits of a caldera-forming eruption at Aira volcano approximately 30,000 years ago and apply this data to calculate decompression inside the magma chamber. We identify a pressure drop from 140–260 MPa to 20–90 MPa during the extraction of around 50 km3 of magma prior to the caldera collapse. The magma extraction may have caused down-sag subsidence at the caldera center before the onset of catastrophic caldera collapse. We propose that this deformation resulted in the fracturing and collapse of the roof rock into the magma chamber, leading to the eruption of massive ignimbrite.

Misinterpretation of zircon ages in layered intrusions

A recent re-interpretation of the Bushveld Complex and other layered intrusions as stacks of randomly emplaced, amalgamated sills is mostly fuelled by finding of zircon ages that are not getting progressively younger from the base upwards, as expected from a classical model for the formation of layered intrusions. Rather, they display several reversals from older to younger ages and vice-versa with moving up-section through the layered intrusions. Here, we show that the reported zircon ages are at odds with the relative ages of rocks as defined by cross-cutting relations in potholes of the Bushveld Complex. This indicates that interpretation of the zircon isotopic data as the emplacement age of the studied rocks/units is incorrect, making a new emplacement model for layered intrusions baseless. This conclusion is further buttressed by the phase equilibria analysis showing that regular cumulate sequences of layered intrusions are not reconcilable with a model of randomly emplaced sills. In this model, the late sills are free to intrude at any stratigraphic position of the pre-existing rocks, producing magmatic bodies with chaotic crystallization sequences and mineral compositional trends that are never observed in layered intrusions. There are thus no valid justifications for the re-evaluation of the current petrological model of the Bushveld Complex and other layered intrusions as large, long-lived and largely molten magma chambers. A fundamental implication of this analysis is that the current high-precision U-Pb TIMS ages from layered intrusions are inherently unreliable on the scale of several million years and cannot therefore be used for rigorous estimations of the timing of crystallization, duration of magmatism, and cooling of these intrusions.