Roof Of Magma Chamber Research Articles

What Prevents the Crystallization of Magma from the Roof Downwards in Large Basaltic Magma Chambers?

Abstract A common feature of layered intrusions is the absence of the Upper Border Series (UBS)—a rock sequence that is expected to grow from the roof downwards in large basaltic magma chambers. This is surprising because magma cooling occurs predominantly through the roof of magma chambers. The lack of the UBS is thus indicative of some fundamental reason that prevents basaltic magmas from crystallizing at the chamber roof. Here, we propose that this could be attributed to a layer of anatectic granitic melt that develops on the top of evolving basaltic chambers. The layer may be formed by partial melting of silica-rich crustal rocks in response to the upward transfer of heat from the crystallizing magma. The heat transfer takes place through the initial UBS, resulting in this unit becoming suspended between two buoyant liquids: granitic melt above and basaltic melt below. This unstable position leads to the disruption of the UBS and its collapse onto the temporary chamber floor. A two-layer magma system thus forms, with a light granitic melt floating atop a denser basaltic magma. The granitic melt is expected to be superheated with respect to its liquidus temperature via heat transfer from the underlying basaltic magma. In the absence of a solid surface at the top of the basaltic magma layer, the growth of the UBS cannot resume due to the lack of a solid place for new crystals to attach. Further crystallization of basaltic magma may therefore only continue from the base upwards, producing a layered intrusion that consists of only floor rock sequences.

The 2008–2010 Subsidence of Dallol Volcano on the Spreading Erta Ale Ridge: InSAR Observations and Source Models

In this work, we study the subsidence of Dallol, an explosive crater and hydrothermal area along the spreading Erta Ale ridge of Afar (Ethiopia). No volcanic products exist at the surface. However, a diking episode in 2004, accompanied by dike-induced faulting, indicates that Dallol is an active volcanic area. The 2004 diking episode was followed by quiescence until subsidence started in 2008. We use InSAR to measure the deformation, and inverse, thermoelastic and poroelastic modelling to understand the possible causes of the subsidence. Analysis of InSAR data from 2004–2010 shows that subsidence, centered at Dallol, initiated in October 2008, and continued at least until February 2010 at an approximately regular rate of up to 10 cm/year. The inversion of InSAR average velocities finds that the source causing the subsidence is shallow (depth between 0.5 and 1.5 km), located under Dallol and with a volume decrease between −0.63 and −0.26 × 106 km3/year. The most likely explanation for the subsidence of Dallol volcano is a combination of outgassing (depressurization), cooling and contraction of the roof of a shallow crustal magma chamber or of the hydrothermal system.

Formation of the Weiya magmatic Fe–Ti oxide deposit and its ore-hosting layered gabbro intrusion, Eastern Tianshan (Xinjiang, NW China)

The Weiya is the largest magmatic Fe–Ti oxide deposit in the Chinese Eastern Tianshan orogen, and is divided into an eastern, central and western section by younger granitoid and diabase intrusions. The Weiya eastern section contain Fe-Ti oxide ores and ore-bearing apatite-poor layered gabbro, which display significant difference in mineral assemblage from the layered gabbro in the central section. We presented new data on the geology, mineralogy, petrology, geochemistry, geochronology, and Sr-Nd isotopes of the gabbroic rocks and the Fe-Ti oxide ores from the eastern section. Igneous zircons from the Weiya anorthosite yielded a SIMS U–Pb concordia age of 237.9 ± 0.71 Ma, coeval with the apatite-rich gabbro in the central section. The gabbroic rocks and Fe-Ti oxide ores in the eastern section and the central section have parallel chondrite-normalized REE patterns and N-MORB-normalized multi-element patterns, together with highly-similar Sr-Nd isotope features, suggesting that the two are comagmatic. Olivine Fo value (62.0–66.7), clinopyroxene Mg# number (68.1–76.2) and plagioclase An contents (39.9–55.4) suggest that the Weiya parental magma was highly evolved after extensive fractional crystallization in the deeper magma chamber, and the residual magma became increasingly oxidized and metals- and volatiles-rich. During the magma emplacement, plagioclase accumulated extensively in the magma chamber roof and further enriched Fe–Ti in the residual melts, leading to Fe–Ti oxide mineralization.

Tectonic and magmatic controls on the evolution of post-collapse volcanism. Insights from the Acoculco Caldera Complex, Puebla, México

The Acoculco Caldera Complex (ACC; 2.7–0.06 Ma) is located in the eastern part of the Trans Mexican Volcanic Belt, México. The ACC produced three major explosive eruptions: the Acoculco andesitic ignimbrite (AI; 2.7 Ma; Syn-caldera), Piedras Encimadas rhyolitic ignimbrite (1.3 Ma; Late post-caldera), and the Tecoloquillo rhyolitic ignimbrite (TI; 0.8 Ma; Late post-caldera). Faulting has been active since the Pliocene to the present; the regional stress regime developed a NW-SE normal fault system (graben-horst structures) where the ACC was built on the Rosario-Acoculco horst.We investigated the depth of the Acoculco and Tecoloquillo magmatic chambers with H2O-CO2 content in glass inclusions, magmatic processes (magma mixing), and origin of magmas with major oxides-trace-element geochemistry and mineral chemistry. Magma chambers for TI and AI magmas are isolated from each other and located approximately 6–10 km depth. We related the local stress field to the magmatic processes that modified the AI (syn-caldera) and TI (post-caldera) magmas of the ACC. The extensional regime promoted the ascent of calc-alkaline magma and stopped by a local compressive stress regime (transfer zones; Acoculco fault), generating an entrapment zone (1.5–2.5 kbar). The new ascent of calc-alkaline magmas from a deep reservoir provoked magma mixing that triggered the AI eruption and the collapse of the magma chamber roof. The collapse event destroyed the feeding conduits and created new ascent paths that favored the formation of a new plumbing system. The ascent of melt used the pre-existing structures, probably deep graben-horst NE-SW faults, allowing peralkaline magma ascent that stopped in the entrapment zone. The new alkaline and calc-alkaline magma injection caused magma mixing that catalyzed the TI explosive eruption through NE-SW trending structures forming the TI plumbing system.

High-silica rhyolites in the terminal stage of massive Cretaceous volcanism, SE China: Modified crustal sources and low-pressure magma chamber

Late Cretaceous felsic volcanism erupted as calderas in the terminal stage of the massive Cretaceous volcanism in SE China. This paper reports comprehensive geochemical and isotopic data for high-silica rhyolites from the Shiniushan and Xiaoxiong calderas in eastern Fujian–Zhejiang provinces, SE China. Zircon U-Pb dating constrains these rhyolites eruptions to 94–92 Ma and 93–89 Ma in the sites of the two calderas. All the samples possess high-silica contents with peraluminous characteristics and are enriched in light rare earth elements (LREEs), Rb, and Th-U and depleted in Ba, Nb-Ta, Sr and Eu. These samples have high whole-rock F (114–596 ppm) but low Cl (3.3–54.8 ppm) contents. Apatite minerals separated from the high-silica rhyolites are classified as fluorapatite and show F-rich (F > 3.56 wt%), Cl-poor (Cl < 0.02 wt%), low (La/Sm)N and high (Gd/Yb)N ratios, with a negative Eu anomaly. The high-silica rhyolites yield initial (87Sr/86Sr)i ratios of 0.70405 to 0.70789, negative whole-rock εNd(t) (−3.3 to −6.3), negative apatite εNd(t) (−5.2 to −6.6) and negative zircon εHf(t) values (−0.1 to −9.6). Our data suggest that their primary magma was derived from modified crustal sources that could contain a mixture of ancient and juvenile components, and it then evolved, was injected into a low-pressure magma chamber, and finally erupted with the release of volatiles at the magma chamber roof. We infer that SE China was mainly under a setting of lithospheric extension from the Early to early Late Cretaceous because of slab roll-back and steepening of the subducted Paleo-Pacific Plate.

The basaltic plinian eruption of the ~6 ka San Antonio Tephra and formation of the Masaya caldera, Nicaragua

Masaya is unusual for a basaltic caldera because it formed by piston-subsidence in response to large-volume magma withdrawal by highly explosive eruptions, i.e. in a fashion typical of silicic calderas. The first and most voluminous of the three explosive eruptions formed the 6 ka old basaltic San Antonio Tephra (SAT). This eruption is also unusual in that most of the 9 km3 DRE basaltic magma was discharged by a plinian eruption. The subsequent eruptions of the basaltic Masaya Triple Layer (MTL, 2.1 ka) and the Masaya Tuff/Ticuantepe Lapilli (MT-TIL, 1.9 ka) each discharged 2 km3 DRE magma and enlarged the Masaya caldera.The SAT consists of a lower sequence of alternating scoria lapilli and ash layers, interpreted as an alternation between more or less phreatomagmatically influenced fallout events. These are followed by two prominent well-sorted lapilli layers: the first one formed by a climactic plinian eruption whose column height reached 21–29 km and discharged most of the total erupted mass including about 35 Mt SO2. The second, lithic-rich lapilli layer probably formed by a phreatoplinian event when partial collapse of the magma chamber roof initiated increasing magma-water interaction which ultimately formed the upper sequence of phreatomagmatic cross-bedded surge deposits, accretionary lapilli-rich tuffs and a final fallout of dense lapilli. Phreatomagmatic activity may have been related to disruption of a hydrothermal system reflected in hydrothermally altered lithics, and/or by the caldera floor subsiding closer to the groundwater table.The bulk-rock chemical composition of the SAT is basaltic but the bimodal glass compositions demonstrate mixing of a basaltic with an andesitic melt probably in the conduit during eruption. The SAT basalt differentiated in a reservoir near the MOHO at 20 km depth by fractional crystallization of olivine, plagioclase, and minor clinopyroxene forming a tholeiitic fractionation trend. Minor intermediate-An plagioclase crystallized from the basaltic melt at H2O concentrations of about 2 wt% as measured by FTIR in melt inclusions. However, a key observation is that the melt inclusions are not in equilibrium with the high-An plagioclases hosting them. Re-equilibration of the inclusions requires initially higher water contents (about 5–6 wt%) which also fits the high Ba/La ~ 80 indicating input from the strongly hydrated subducting slab. Therefore, while the SAT magma evolved under hydrous conditions at depth, it was then stored at shallow level long enough to adjust to the low saturation pressure and to precipitate some intermediate-An plagioclase but still preserving its high temperature (around 1100 °C) and phenocryst-poor composition. Large overpressure due to connection to the deep-seated reservoir and water degassing during ascent limited the storage time at shallow level and drove the unusually intense and voluminous plinian-style eruption that facilitated piston-type collapse of the chamber roof.

Mechanisms and patterns of magmatic fluid transport in cooling hydrous intrusions

Volatile outgassing from hydrous magma intrusions emplaced and cooling in the Earth's upper crust is key to a number of geologic processes including volcanic eruptions and ore deposition, yet the physical interactions between production, storage and transport of a magmatic volatile phase within magmatic intrusions and their large-scale thermal evolution have remained elusive. We performed numerical simulations of aqueous volatile transport in generic magma chambers as they crystallize in response to conductive and convective cooling and thereby transition from a crystal - volatile - melt suspension to a mush and eventually to a rigid rock. We used simplified but realistic material properties based on published phase equilibrium experiments, published grain-scale modeling results and general geological constraints.We found that the intrusion depth and water content exert decisive control on the rates and mechanisms of intrusion outgassing. Above a critical volatile content, highly permeable capillary tubes develop in crystal mush regions with intermediate crystal volume fractions (∼ 0.5 to 0.7), and such grain-scale tubes provide rapid intrusion-scale degassing paths. These tubes are located in a ring-like mush region inside the intrusion, allowing strongly lateral flow and focusing towards the top of the intrusion. If the water content of the magma is close to saturation at the time of emplacement (5 to 6 wt% H2O) and the depth of the magma chamber roof is at least 4 km deep, initially distributed breakthrough points may coalesce to a single area of fluid release. Large-scale intrusion geometry determines this location of focused fluid release, and cooling rate and size of the intrusion determine overall fluid flux. Feedbacks between local hydrofracturing and heat advection break up the focused fluid expulsion into many short-lived pulses. These conditions for large-scale fluid focusing favor the formation of porphyry copper deposits, but might also trigger caldera-forming ignimbrite eruptions.

Variation in b-value of caldera earthquakes during recent activity of the Bárðarbunga Volcano in Iceland

The magnitude distribution of caldera earthquakes in the subglacial Bárðarbunga volcano in Central Iceland, characterized by the b-value, shows a systematic variation that is consistent with stress changes anticipated in the roof of an inflating magma chamber beneath the caldera. The b-value was 0.83 prior to the rupture of the chamber in August 2014 when a dike propagated laterally from the volcano to feed the eruption in Holuhraun. The b-value was relatively high following the collapse of the caldera, reflecting low stress in the magma chamber roof. Half a year later a decrease was observed in the b-value, concurrent with an increase in the seismicity, consistent with indications of recharging of the volcano magma chamber. The magnitude distribution was anomalous during the slow collapse of the caldera in association with the eruption. During this period the earthquake sequence appeared to consist of two populations, only one of which followed the conventional Gutenberg–Richter distribution. For a subglacial volcano, where geodetic methods are difficult to implement, the b-value of caldera earthquakes provides an important additional parameter for the monitoring of magma pressure variations.

Eruptive chronology of the Acoculco caldera complex – A resurgent caldera in the eastern Trans-Mexican Volcanic Belt (México)

The Acoculco caldera complex (ACC) is located in the eastern part of the Trans-Mexican Volcanic Belt in the northern part of the State of Puebla. The complex sits at the intersection of two regional fault systems with NE-SW and NW-SE orientations. The ACC was built atop Cretaceous limestones, the Zacatán basaltic plateau of unknown age, early Miocene domes (~12.7–10.98 Ma), and Pliocene lava domes (~3.9–3 Ma). Detailed field mapping and stratigraphy studies complemented by 40Ar/39Ar and 14C dating allowed the division of the ACC volcanic succession into 30 volcanic units. Based on the new results and previous studies, the ACC eruptive chronology was grouped in four eruptive phases: syn-caldera, early post-caldera, late post-caldera, and extra-caldera. Inception of the ACC volcanism began around 2.7 Ma with the dispersion of an andesitic ignimbrite followed by the collapse of the magma chamber roof as attested by the presence of a lithic breccia in isolated parts of the caldera rim. The collapse produced a 18 × 16 km caldera depression which was partly filled by the ignimbrite (total volume of ~127 km3) followed by the establishment of an intracaldera lake of unknown total extension. Early post-caldera collapse activity (2.6–2.1 Ma) was restricted within the caldera producing 27 km3 of lava flows and domes dominantly of basaltic trachyandesite to basaltic composition. Late post-caldera collapse activity (2.0-<0.016 Ma) migrated dominantly to the caldera rim and periphery emplacing 90 km3 of magma as rhyolitic domes, lava flows, scoria cones, and two younger ignimbrites. The 1.2 Ma Encimadas ignimbrite (26 km3) was vented through the eastern margin of the caldera and dispersed to the northeast, and the 0.6–0.8 Ma Tecoloquillo ignimbrite and dome (11 km3) erupted from the southwestern margin of the caldera. The most recent eruption of this phase was vented close to the southeastern caldera rim producing the Cuatzitzingo (<16,710 ± 50 years BP) scoria cone. Extra-caldera activity (2.4–0.19 Ma) of the Apan–Tezontepec volcanic field produced scoria cones and lava flows of basaltic trachyandesite to basaltic andesite composition that are interbedded with the products of the caldera complex.Aeromagnetic data further constrains the edge of the caldera rim and is consistent with the presence of at least four intrusive bodies at depths of >1 km hosted in the Cretaceous limestones. These bodies might represent a series of horizontal mafic intrusions located at different depths that provide the energy that maintains the Acoculco geothermal system active.

Volatile segregation and generation of highly vesiculated explosive magmas by volatile-melt fining processes: The case of the Campanian Ignimbrite eruption

The late Pleistocene trachytic Campanian Ignimbrite (>300 km3 DRE, ca. 39 ka) covers the Campanian Plain (Italy) around Naples. It is found behind ridges >1000 m high at 80 km from the source in the Campi Flegrei caldera. This points towards very dilute currents, that together with the huge amount of discharged magmatic material, suggest a magma reservoir highly enriched in volatiles. The huge volume of magma that extruded during the Campanian Ignimbrite eruption differentiated and mixed at shallow depth (6–3 km), as shown by modelling of the petrologic and geochemical features of the erupted products together with melt inclusion-based studies of gas-melt saturation. With respect to compositionally similar but lower magnitude Phlegraean eruptions (e.g., Agnano-Monte Spina, >1 km3 DRE, ca. 4.6 ka), the large amount of volatiles discharged by Campanian Ignimbrite was likely due to fractional crystallization of the lowermost portion of its magmatic reservoir. Because of the long residence time of the Campanian Ignimbrite magmatic system within the crust, an efficient decoupling took place during the upward migration of volatile elements in response to a chromatographic fractionation between H2O and CO2. This caused early pulses of nearly pure CO2 (CO2-fluxing), followed by a long-lasting H2O enrichment which yielded dissolved H2O contents up to 6–7 wt%. An overpressurized CO2-dominated gas cap was consequently produced, uniformly distributed at the top of the magma chamber. The onset of the eruption tapped this cap and generated Plinian columns, causing depressurization and fast volume decrease that facilitated, or even drove, the caldera collapse. H2O-rich magma was discharged during the following phase, characterised by pyroclastic density currents. The uniform distribution of the high values of void fraction (>70%) and the high degree of vesicle connectivity throughout the magma body testify to the huge abundance of volatiles. These percolate from the crystallizing basal layers and determine the volatile-melt fining process yielding an efficient separation between H2O and CO2. The gas saturation-based estimates of the tapped foamy magma are compatible with the extent of magma chamber roof collapse, the strong expansion revealed by textural data, and the transport and deposition mechanisms, reflecting depressurization and inflation of the volatile-rich magma within the collapsed and laterally confined caldera.

The Petrogenesis of Plagioclase-ultraphyric Basalts from La Réunion Island

Plagioclase-bearing ultraphyric basalts, which can have up to 35% accumulative, millimetre-sized plagioclase crystals, were episodically erupted during some stages of building of La Re union Island volcanoes. Selected rock samples were analysed from the two volcanoes of the island (four samples from Piton des Neiges and two from Piton de la Fournaise). We summarize the results of petrographic and geochemical studies of the whole-rocks and silicate melt inclusions trapped within plagioclase (An 84 2–71 7 ) macrocrysts, which contain aliquots of the parental melts of their hosts. Melt inclusion compositions are used to discuss the origin of La Re union plagioclase- bearing ultraphyric basalts, with special emphasis on the magma storage system that led to their production. Experimentally re-homogenized melt inclusions were analysed by electron microprobe and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for their major and trace elements. Together with the textural observations, these results indicate that the host crystals are inherited, although their parental magma compositions are close to that of the host lava. Petrographic lines of evidence suggest that the plagioclase macrocrysts originate from batches of plagioclase-rich mush or fragments of anorthositic gabbro ripped from the magma chamber walls. The parental melts of the plagioclases evolve predominantly by clinopyroxene þ plagioclase crys- tallization. The density contrasts between minerals and liquid allow their efficient segregation. We propose that plagioclases are collected below the magma chamber roof as a flotation cumulate, thereby forming plagioclase-rich mush and anorthositic gabbro, whereas clinopyroxenes sink. The plagioclase-ultraphyric basalts are derived from the remobilization of this mush, or fragments of the anorthositic cumulate, upon the input of a new batch of magma that triggers their eruption. We postulate that the periodic occurrence of these striking basalts during specific periods of the volcano’s growth corresponds to decreases in the magma supply, which promoted plagioclase crystallization and its segregation by flotation. Finally, we suggest that the plagioclase-ultraphyric basalts from the Rivie ` re des Remparts and Rivie ` re de l’Est valleys, previously regarded as compo- nents of the Piton de la Fournaise volcano, are instead products of the declining activity of an older volcano, possibly Les Alize s volcano

Trace elements in anatectic products at the roof of mid-ocean ridge magma chambers: An experimental study

At fast-spreading mid-ocean ridges (MORs), the horizon between the axial melt lens (AML) and the overlying sheeted dikes is characterized by extensive anatectic processes. The heat flux of the AML in combination with hydrothermal fluids from above causes high-grade contact metamorphism, which may result in anatexis of the roof rocks above the AML. The products of this process are silica-rich anatectic melts that have the potential to contaminate MOR basalts and residual hornfels. Here, we simulate the complex igneous and metamorphic processes occurring at the AML roof by hydrous partial melting experiments and provide corresponding trace element partition coefficients between melt and residues, which are useful to quantify those processes. We present trace element patterns from experimental anatectic felsic melts and the related residue produced by hydrous partial melting of various types of AML roof rocks. The starting materials used are sheeted dikes and hornfelses from Hole 1256D drilled by the Integrated Ocean Drilling Program. Results are compared with directly-related natural lithologies (i.e., felsic veins and granoblastic hornfels) from the same site. The trace element contents generally overlap with natural examples and experimental melts produced at low water activity (aH2O<0.5) can be highly enriched in trace elements despite relatively low SiO2 contents (58.9 to 65.7wt%). A low aH2O is required to reproduce the low Al2O3 contents observed in natural silica-rich rocks. However, low aH2O implies that the presence of residual amphibole is not required for anatectic processes Even though residual amphibole is often used as an important phase for explaining trace element characteristics in relevant felsic rocks formed at MORs when modeling anatexis. Because amphibole is lacking in any experimental residue, which is in agreement with natural hornfelses from the dike/gabbro transition at Site 1256, we assume that partial melting within the AML roof rocks proceeds without the participation of amphibole as residual phase. We present a comprehensive set of trace element compositions as well as bulk and mineral/melt trace element partition coefficients obtained from our amphibole-free experimental results for different potential protoliths over a large range of temperature and at different aH2Os.

Surface displacements resulting from magma-chamber roof subsidence, with application to the 2014–2015 Bardarbunga–Holuhraun volcanotectonic episode in Iceland

The conditions which lead to caldera collapse are still poorly constrained. As there have only been four, possibly five, well-documented caldera forming events in the past century, the geodetic signals produced during chamber roof subsidence, or chamber volume reduction (shrinkage) in general, are not well documented or understood. In particular, when two or more geodetic sources are operating and providing signals at the same time, it is important to be able to estimate the likely contribution of each. Simultaneous activities of different geodetic sources are common and include pressure changes in magma chambers/reservoirs occurring at the same time as dyke emplacement. Here we present results from numerical models designed to simulate the subsidence of a magma-chamber roof, either directly (chamber shrinkage) or through ring-fault displacement, and the induced surface deformation and crustal stresses. We consider chamber depths at 3km, 5km, and 7km below the crustal surface, using both non-layered (isotropic) and layered (anisotropic) crustal models. We also model the effects of a caldera lake and of a thick ice cover (ice sheet) on top of the caldera. The results suggest that magma-chamber roof subsidences between 20m and 100m generate large (tens of centimetres) vertical and, in particular, horizontal displacements at the surfaces of the ice and the crust out to distances of up to tens of kilometres from the caldera/chamber centre. Crustal layering tends to reduce, but increasing chamber depth to enlarge, the horizontal and vertical surface displacements. Applying the results to the ice subsidence in the Bardarbunga Caldera during the 2014–2015 Bardarbunga–Holuhraun volcanotectonic episode indicates that the modelled ice displacements are less than those geodetically measured. Also, the geodetically measured crustal displacements are less than expected for a 60m chamber-roof subsidence. The modelling results thus suggest that only part of the ice subsidence is due to chamber-roof subsidence, the other part being related to flow in and down-bending of the ice. We show that such a flow is likely within the caldera as a result of the stress induced by the 45-km-long regional dyke emplaced (primarily in vertical magma flow) during the episode. This conclusion is further supported by the model results suggesting that the ring-fault (piston-like) displacements must have been much less than the total 60m ice subsidence, or else faults with tens-of-metres displacements would have cut through the ice (these are not observed). We suggest that the ring-fault subsidence was triggered by small doming of the volcanic field and system hosting the Bardarbunga Caldera and that this doming occurred as a result of magma inflow and pressure increase in a deep-seated reservoir. The doming is confirmed by GPS measurements and supported by the seismicity results. The magmatic pressure increase in the reservoir was, in terms of the present model, responsible for the regional dyke emplacement, the Holuhraun eruption, and part of the stress concentration around, and displacement of, the Bardarbunga Caldera.

Three‐dimensional geometry of axial magma chamber roof and faults at Lucky Strike volcano on the Mid‐Atlantic Ridge

AbstractWe present results from three‐dimensional (3‐D) processing of seismic reflection data, acquired in June 2005 over the Lucky Strike volcano on the Mid‐Atlantic Ridge as a part of the Seismic Study for Monitoring of the Mid‐Atlantic Ridge survey. We use a 3‐D tomographic velocity model derived from a coincident ocean bottom seismometer experiment to depth convert the poststack time‐migrated seismic volume and provide 3‐D geometry of the axial magma chamber roof, fault reflectors, and layer 2A gradient marker. We also generate a high‐resolution bathymetric map using the seismic reflection data. The magma chamber roof is imaged at 3.4 ± 0.4 km depth beneath the volcano, and major faults are imaged with dips ranging between 33° and 50°. The magma chamber roof geometry is consistent with a focused melt supply at the segment center and steep across‐axis thermal gradients as indicated by the proximity between the magma chamber and nearby faults. Fault scarps on the seafloor and fault dip at depth indicate that tectonic extension accounts for at least 10% of the total plate separation. Shallow dipping reflectors imaged in the upper crust beneath the volcano flanks are interpreted as buried lava flow surfaces.

Anatexis at the roof of an oceanic magma chamber at IODP Site 1256 (equatorial Pacific): an experimental study

Replenished axial melt lenses at fast-spreading mid-oceanic ridges may move upward and intrude into the overlying hydrothermally altered sheeted dikes, resulting in high-grade contact metamorphism with the potential to trigger anatexis in the roof rocks. Assumed products of this process are anatectic melts of felsic composition and granoblastic, two-pyroxene hornfels, representing the residue after partial melting. Integrated Ocean Drilling Program Expeditions 309, 312, and 335 at Site 1256 (eastern equatorial Pacific) sampled such a fossilized oceanic magma chamber. In this study, we simulated magma chamber roof rock anatectic processes by performing partial melting experiments using six different protoliths from the Site 1256 sheeted dike complex, spanning a lithological range from poorly to strongly altered basalts to partially or fully recrystallized granoblastic hornfels. Results show that extensively altered starting material lacking primary magmatic minerals cannot reproduce the chemistry of natural felsic rocks recovered in ridge environments, especially elements sensitive to hydrothermal alteration (e.g., K, Cl). Natural geochemical trends are reproduced through partial melting of moderately altered basalts from the lower sheeted dikes. Two-pyroxene hornfels, the assumed residue, were reproduced only at low melting degrees (<20 vol%). The overall amphibole absence in the experiments confirms the natural observation that amphibole is not produced during peak metamorphism. Comparing experimental products with the natural equivalents reveals that water activity (aH2O) was significantly reduced during anatectic processes, mainly based on lower melt aluminum oxide and lower plagioclase anorthite content at lower aH2O. High silica melt at the expected temperature (1000–1050 °C; peak thermal overprint of two-pyroxene hornfels) could only be reproduced in the experimental series performed at aH2O = 0.1.

Appearance and disappearance of pigeonite due to ultrametamorphism: Partial melting and assimilation at the roof of magma chamber beneath fast-spreading ocean ridges

Appearance and disappearance of pigeonite due to ultrametamorphism: Partial melting and assimilation at the roof of magma chamber beneath fast-spreading ocean ridges

Contamination of MORB by anatexis of magma chamber roof rocks: Constraints from a geochemical study of experimental melts and associated residues

Mid-ocean ridge basalts (MORBs) are the most abundant magmas produced on Earth. They are widely studied to infer mantle compositions and melting processes. However, MORB liquids are also the complex end-product of a variety of intra-crustal processes such as partial or fractional crystallization, melt–rock interaction, and contamination. Deciphering the relative contribution of these different processes is of first-order importance. Contamination at ocean crustal levels is likely, and may occur at magma chamber margins where fresh magmas can interact with previously hydrothermally altered rocks. Characterizing the composition of this crustal contaminant component is critical if we are to understand the relative importance of each component in the resulting MORB liquid. Here we present the results of experiments designed to reproduce the processes occurring at oceanic magma chamber roofs, where crustal contamination should be most extensive, by melting a representative sample of the sheeted dike complex. Anatectic melts thus produced are likely to represent the principal crustal contaminant in MORB. These melts were characterized for major and trace elements, showing B, Zr, Hf, and U enrichment, and Sr, Ti, and V depletion relative to original MORB liquids. In comparison to the starting material, relative element fractionations are observed in the anatectic melts, with enrichments of: U relative to Ba, Nb, and Th; LREE and MREE relative to Sr; and Zr–Hf relative to LREE. Bulk partition coefficients for element partitioning during magma chamber roof anatexis are derived and proposed as valuable tools for tracking MORB contamination. Comparison with natural samples from the East Pacific Rise and the Oman ophiolite shows that anatectic melts can crystallize in situ to form oceanic plagiogranite intrusions, and that residual assemblages associated with the hydrous partial melting stage are represented by hornfelsic dikes and enclaves (also named granoblastic basalts). We now recognize these as commonplace at the root of the sheeted dike complex both at present-day and fossil oceanic spreading centers.

Microgranitoid enclaves in the felsic Looanga monzogranite, New England Batholith, Australia: Pressure quench cumulates

Sparse microgranitoid enclaves (MGE) in the leucocratic I-type Looanga monzogranite near Bendemeer, N.S.W. Australia, range from microdiorite to micromonzogranite and all have fine to medium grainsize igneous microstructures. The enclaves that vary from SiO2 53 to 69wt.% are all less silicic than the host monzogranite (71–76wt.%). Although compositionally diverse, the enclaves and host monzogranite pluton share a common mineralogy of quartz, oligoclase, ferro-edenitic hornblende, iron-rich (mg ~35) biotite, fluor-apatite and ±K-feldspar. Except for the core of one double enclave, the enclaves have the same 87Sr/86Sr initial ratio as the host pluton. A characteristic of the enclaves is high MnO/(MnO+MgO+FeO) ratios with MnO abundances of the more mafic enclaves up to 0.8wt.%, higher than any common magma. The enclaves have a wide range of Na2O/K2O ratios (0.5 to 2.8) and, in common with the host pluton, contain hornblendes with Na2O/K2O ratios varying from 1.5 to 2.3. The hornblendes in two enclaves have lower Na2O/K2O ratios than their host enclave, making it unlikely that the hornblende could have crystallised from a melt of the same composition as these enclaves.Chemically and mineralogically the more mafic enclaves have characteristics expected of cumulates formed from a magma of similar composition as the host pluton, in that they contain the same minerals but are enriched in the near-liquidus phases (hornblende, plagioclase and biotite) and depleted in the near-solidus phases (quartz and K-feldspar). Except for some minor replacement of pyroxene by hornblende the minerals do not show microstructural evidence of being made over from other minerals. It is argued that the mineral chemistry of these enclaves is also a primary feature rather than the result of mineralogical equilibration with the host monzogranite magma. The two most felsic enclaves are medium-grained monzogranites (SiO2 68 and 70wt.%) and are considered to be compositionally little different from the magmas from which they crystallised. These two exhibit hydrothermal alteration and are considered to be fragments of an earlier roof phase of the intrusion.The quench microstructure and cumulate chemistry of the more mafic enclaves are argued to result from PH2O reduction events within the upper parts of the magma chamber due to roof fracture brought on by the pressure increase imparted to the magma by the fluid release from a water saturated magma. The sudden reduction in fluid pressure results in an increase in the liquidus and solidus temperatures of the water saturated magma and this rather than a drop in temperature produces quench conditions. The reduction in PH2O shifts the cotectic compositions in the Q–Ab–Or system closer to K-feldspar and quartz and this and the heat of crystallisation restrict the amount of these two near solidus minerals that crystallise. The enclaves form as crystal cluster cumulates around minerals already in the magma and/or any other solid substrate available including the magma chamber roof. This pressure–quench–cumulate mechanism explains why the Looanga enclaves are mineralogically similar to the host granite and we suggest that this process may be more widely applicable to enclaves in granitic rocks.

Supervolcano eruptions driven by melt buoyancy in large silicic magma chambers

Supervolcano eruptions dwarf all historical eruptions, but their trigger mechanisms are unclear. Experimental measurements of magma density at high pressures and temperatures show that the buoyancy of magma alone can impose sufficient pressure at the roof of a supervolcano magma chamber to induce an eruption.

液体中における密に詰まった粉粒体の重力不安定

To better understand the processes which occur in magma chambers and in liquefied sediments, we conduct laboratory experiments of gravitational instability of a close packed granular medium in a liquid. First we consider a case in which a granular medium overlies a liquid layer. Here the lowermost thin layer of a granular medium dilates and becomes mobile to form a rheological boundary layer (RBL), from which narrow granular plumes grow downwards. We measure the wavelength and the growth rate of the instability and constrain the thickness of the RBL to be only about twice the particle size, and its packing fraction as Φ = 0.42, indicating dilation. When the granular medium is thicker, we find that the granular plumes organize themselves into convection cells. Importantly, the ascent velocity of the boundary between the granular medium and the liquid layer is approximately constant regardless of the thickness ratio of the two layers. A scaling in which the packing fraction of the RBL governs the ascent velocity is found to explain the results well. Next we extend the first experiment and model magma chamber roof melting and subsequent particle settling. An experimental cell consists of a lower layer of a thermally convecting molten wax, and an upper layer comprising of a mixture of particles and solid wax. As the interstitial solid wax of the upper layer melts, the particles become mobile and settle downwards. From experiments with different particle sizes, we find that when the particle is smaller than 0.1 mm, cycles in which melting starts and stops, occur spontaneously. Melting stops due to the formation of a stable stratification in the melt layer which suppresses the vertical heat transfer. Melting resumes by the convective overturning because the cell is heated from below. A dimensionless Buoyancy number is found to explain the critical particle size well. As a third experiment, we consider the instability caused by liquefaction. Here an experimental cell consists of a water-immersed granular medium which is size-graded into 2 layers such that the upper layer has a smaller permeability. When the cell is shaken vertically, liquefaction occurs and the water temporarily accumulates at the 2 layer interface resulting in a gravitational instability. From experiments in which we vary the acceleration and frequency of shaking, we find that there is a critical acceleration for instability to occur, and that this acceleration is minimum at a frequency of about 100 Hz. We show that the frequency dependence can be interpreted from a combined condition of energy and jerk of shaking exceeding their respective critical values.