Upper mantle mush zones beneath low melt flux ocean island volcanoes: insights from Isla Floreana, Galápagos
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The physicochemical characteristics of sub-volcanic magma storage regions have important implications for magma system dynamics and pre-eruptive behaviour. The architecture of magma storage regions located directly above high buoyancy flux mantle plumes (such as Kīlauea, Hawai’i and Fernandina, Galápagos) are relatively well understood. However, far fewer constraints exist on the nature of magma storage beneath ocean island volcanoes that are distal to the main zone of mantle upwelling or above low buoyancy flux plumes, despite these systems representing a substantial proportion of ocean island volcanism globally. To address this, we present a detailed petrological study of Isla Floreana in the Galápagos Archipelago, which lies at the periphery of the upwelling mantle plume and is thus characterised by an extremely low flux of magma into the lithosphere. Detailed in situ major and trace element analyses of crystal phases within exhumed cumulate xenoliths, lavas and scoria deposits, indicate that the erupted crystal cargo is dominated by disaggregated crystal-rich material (i.e., mush or wall rock). Trace element disequilibria between cumulus phases and erupted melts, as well as trace element zoning within the xenolithic clinopyroxenes, reveals that reactive porous flow (previously identified beneath mid-ocean ridges) is an important process of melt transport within crystal-rich magma storage regions. In addition, application of three petrological barometers reveal that the Floreana mush zones are located in the upper mantle, at a depth of 23.7±5.1 km. Our barometric results are compared to recent studies of high melt flux volcanoes in the western Galápagos, and other ocean island volcanoes worldwide, and demonstrate that the flux of magma from the underlying mantle source represents a first-order control on the depth and physical characteristics of magma storage.Keywords:
Scoria
Mantle plume
Magma chamber
Xenolith
Caldera
Trace element
Caldera
Magma chamber
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Annual surveys of trilateration and leveling networks in and around Long Valley caldera in the 1982–1985 interval indicate that the principal sources of deformation are inflation of a magma chamber beneath the resurgent dome and right‐lateral strike slip on a vertical fault in the south moat of the caldera. The rate of inflation of the magma chamber seems to have been roughly constant (0.02 km 3 /yr) in the 1982–1985 interval, but the slip rate on the south moat fault has decreased substantially. In addition, there is evidence for a shallow source of dilatation (possibly dike intrusion) beneath the south moat of the caldera in 1983 and less certain evidence for a deep source (possibly magma chamber inflation beneath Mammoth Mountain) in the western caldera in 1983–1985. Deformation in the 1985–1986 interval as inferred from trilateration alone seems to be similar to that observed in 1984–1985.
Caldera
Magma chamber
Dike
Dome (geology)
Trilateration
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縦長型マグマだまりの膨張ないし収縮によって形成されるカルデラを，アナログ実験で再現した．アナログ地殻は上新粉（米の粉末），アナログマグマだまりはゴム風船を使用した．実験結果は以下の通り．1．マグマだまりの膨張によってドーム隆起が生じ，ドーム頂部にグラーベン状に小さな漏斗型陥没が形成される．陥没の直径は，実際のスケールでは0.8～1.6 kmなので，カルデラというにはやや小さい．2．マグマだまりの収縮によって，平坦な底を持つ浅いカルデラと，その中心部の小径陥没が形成される．実際のスケールでは，カルデラの大きさは0.9～4.4 km，深さ200～400 mとなる．このモデルは，キラウエアカルデラと相似である．3．膨張後に収縮するマグマだまりによって形成されるカルデラは，先行するドーム隆起による地殻ダイラタンシーのため，マグマだまりが収縮するだけのモデルに比べて陥没量が小さくなる．
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Abstract We present the result of morphostratigraphic analysis and geological field observation to identify the distribution of scoria flow deposit related to the caldera-forming eruption of Lautan Pasir caldera as the youngest caldera in Bromo-Tengger caldera complex. Morphostratigraphic analysis derived from digital elevation model and volcanic stratigraphy shows that products of Lautan Pasir caldera-forming eruption(s) mainly distributed to the north filled up the Sapikerep valley, and further to the north formed Sukapura distal fan. While to the south, it filled the valley between Ijo and Old Semeru. Field observation in the deeply dissected part of Sapikerep valley found two massive pyroclastic flow deposits separated by lava flow. Above the lava flow is scoria rich pyroclastic flow deposit typical of Lautan Pasir caldera-forming eruptions. Below the lava flow is scoria poor pyroclastic deposit of older products, most likely, of Ngadisari caldera. The distribution of Ngadisari caldera-forming eruption products is very limited considering the following massive volcanism of Lautan Pasir caldera. This outcrop might provide the key information to further reconstruct the volcanism stage of Bromo-Tengger caldera complex
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Scoria
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Caldera
Magma chamber
Igneous differentiation
Fractional crystallization (geology)
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The caldera structure after collapse of the magma chamber was estimated by numerical simulation. In the simulation, the collapse of the magma chamber was approximated by the contraction of a small sphere in the elastic medium, and the distribution of plastic and/or rupturing area was calculated using the Coulomb failure criterion under the assumption of an elastic‐perfectly‐plastic material. Given an undefined or isotropic regional stress field, the plastic area (caldera) was found to develop as a circular depression on the surface, appearing funnelform in cross section. Under an anisotropic regional stress field, the caldera developed in the direction of the maximum compression (or minimum extension) axis. In all simulations, the collapse of the magma chamber resulted in the formation of an outward dipping reverse ring fault around the area above the chamber, and an inward dipping normal ring fault at the periphery of the caldera.
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Magma chamber
Stress field
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Explosive caldera-forming eruptions eject voluminous magma during the gravitational collapse of the roof of the magma chamber. Caldera collapse is known to occur by rapid decompression of a magma chamber at shallow depth, however, the thresholds for magma chamber decompression that promotes caldera collapse have not been tested using examples from actual caldera-forming eruptions. Here, we investigated the processes of magma chamber decompression leading to caldera collapse using two natural examples from Aira and Kikai calderas in southwestern Japan. The analysis of water content in phenocryst glass embayments revealed that Aira experienced a large magmatic underpressure before the onset of caldera collapse, whereas caldera collapse occurred with a relatively small underpressure at Kikai. Our friction models for caldera faults show that the underpressure required for a magma chamber to collapse is proportional to the square of the depth to the magma chamber for calderas of the same horizontal size. This model explains why the relatively deep magma system of Aira required a larger underpressure for collapse when compared with the shallower magma chamber of Kikai. The distinct magma chamber underpressure thresholds can explain variations in the evolution of caldera-forming eruptions and the eruption sequences for catastrophic ignimbrites during caldera collapse.
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Magma chamber
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The La Primavera caldera lies close to the triple junction of the Tepic-Zacoalco, Colima, and Chapala rifts in the western part of the Mexican Volcanic Belt. It is a promising geothermal field with 13 deep wells already drilled. We calculated solute geothermometric temperatures (Na–K, Na–Li, and SiO2) from the chemistry of geothermal water samples; determined values are generally between 99°C and 202°C for springs and between 131°C and 298°C for wells. Thermal modelling is an important geophysical tool as documented in the study of this and other Mexican geothermal areas. Using the computer program TCHEMSYS, we report new simulation results of three-dimensional (3-D) thermal modelling of the magma chamber underlying this caldera through its entire eruptive history. Equations (quadratic fit) describing the simulated temperatures as a function of the age, volume and depth of the magma chamber are first presented; these indicate that both the depth and the age of the magma chamber are more important parameters than its volume. A comparison of 3-D modelling of the La Primavera and Los Humeros calderas also shows that the depth of the magma chamber is more important than its volume. The best model for the La Primavera caldera has 0.15 million years as the emplacement age of the magma chamber, its top at a depth of 4 km, and its volume as 600 km3. Fresh magma recharge events within the middle part of the magma chamber were also considered at 0.095, 0.075, and 0.040 Ma. The simulation results were evaluated in the light of actually measured and solute geothermometric temperatures in five geothermal wells. Future work should involve a smaller mesh size of 0.050 or 0.10 km on each side (instead of 0.25 km currently used) and take into account the topography of the area and all petrogenetic processes of fractional crystallization, assimilation, and magma mixing as well as heat generation from natural radioactive elements.
Caldera
Magma chamber
Dome (geology)
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Caldera
Magma chamber
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Abstract 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 km 3 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.
Caldera
Magma chamber
Effusive eruption
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