Injection Of Magma Research Articles

First characterization of the volcanism in the southern Mozambique Channel: Geomorphological and structural analyses

The southern part of the Mozambique Channel is characterized by a cluster of isolated seamounts, including the Bassas da India atoll and Europa Island, ranging in latitude from 20°S to 22°S and from 38° to 40° in longitude, and located at the Rovuma-Lwandle plate boundary. Only Cenozoic carbonate platforms have previously been studied in this region, with very little work done on the volcanic history. We confirm here the volcanic nature of the basement of the Bassa da India/Europa complex by providing important constraints on the setting of this hitherto poorly understood volcanism.Recent bathymetric surveys and dredging operations allowed us to map and date these seamounts, comprising, from west to east, the Hall Bank, the Jaguar Bank, Bassas da India, Ptolemee, and Europa. In addition, we discovered, to the south of Bassas da India, two large new polygenetic volcanic edifices, Pamela Seamount 1 (PS1) and Pamela Seamount 2 (PS2), showing heights and diameters of up to 900 m and 13 km, respectively. Mapping and statistical analysis carried out revealed that the volcanic structures of the Bassas da India/Europa complex are organized along two main alignments with different stages of development: (i) a NE-SW volcanic alignment characterized by volcanic ridges up to 700 m in height, comprising small individual volcanic cones; and (ii) a NW-SE volcanic alignment in which many large and well-developed individual volcanic cones can be found. From this distribution of the volcanism, we suggest that the large volcanic edifices of the Bassas da India/Europa complex were fed by long-lived magma systems, repeatedly supplied from deep magma reservoirs through a significant network of dykes and faults, with lateral injections of magma guided by a dense network of faults allowing magma to reach the surface along rift-zones. 40Ar/39Ar dating confirms that the volcanism covers a period from the Oligo-Miocene to the Pleistocene, and probably extends to the present day. The two volcanic alignments are also consistent with the tectonic features already recognized for the region and are spatially superimposed by active seismicity. Magma ascent is strongly controlled by large pre-existing crustal structures.

Evolution of the magma plumbing system of Miyakejima volcano with periodic recharge of basaltic magmas

Defining the variations in petrological characteristics of erupted magmas within a high-resolution chronostratigraphy provides a necessary framework for monitoring the long-term activity and eruption potential of an active volcano. Here, we investigate the evolution of the magmatic system of Miyakejima volcano, Japan, between the last two caldera-forming eruptions, at ~ 2.3 ka and AD 2000, based on new stratigraphic constraints, radiocarbon ages, and whole-rock geochemical data. The activity of Miyakejima during this interval can be divided into three magmatic periods based on cyclic whole-rock compositional trends. Period 1 spans the interval between ~ 2.3 ka and the 7th century, from the Hatchodaira eruption with caldera collapse to immediately before the Suoana–Kazahaya eruption. Period 2 spans the time period between the seventh century and the fourteenth century, from the Suoana–Kazahaya to the Sonei–bokujyo eruptions. Period 3 covers the period from the two major flank eruptions that occurred in the sixteenth century to the end of the twentieth century until the last caldera-collapse event in AD 2000. The eruption rate decreased from 0.5 km3 per 1000 years in Period 1 to ~ 0.2 km3 per 1000 years in Period 2 and 3. Recharge of primitive basaltic magmas into shallower crustal systems triggered extensive basaltic fissure eruptions at the beginning of each period. Progressively increasing whole-rock SiO2 contents of the hybrid magmas in subsequent eruptions indicates continuous fractional crystallization in small shallow magma chambers which formed at the start of each magmatic period. Intermittent injections of basaltic magma into shallow magma chambers induced magma mixing that caused eruption of hybrid basaltic andesite in each period. We suggest that some basaltic magmas formed isolated magma reservoirs at shallow depth, in which rapid fractionation was able to occur. Rupturing of these isolated magma storage regions filled with gas-rich evolved magma can lead to violent ejection of andesitic magmas, such as for the Suoana–Kazahaya eruption in the seventh century. Our results suggest two main scenarios of eruption for the basaltic magma system at Miyakejima and similar mafic volcanoes in the northern Izu–Bonin arc; (1) eruption of voluminous basaltic lavas after the recharge of primitive basaltic magmas into the shallow magmatic system, and (2) explosive fissure eruption by rupturing of isolated magma bodies filled with gas-rich evolved magmas.Graphical

Spatio-temporal evolution of the magma plumbing system at Masaya Caldera, Nicaragua

Volcanic unrest in calderas can be exhibited through a variety of different mechanisms, such as changes in seismicity and ground deformation, as well as variations in thermal and/or gas emissions. However, not all caldera unrest results in explosive caldera-forming volcanic activity. Alternative activity may include periods of quiescence, passive degassing, effusive activity (e.g., lava flows lava lakes and dome formation), and/or magma injection into the shallow magma system. In this study, we perform a long-term study (spanning 2011–2019) of ground deformation at Masaya using six Interferometric Synthetic Aperture Radar (InSAR) datasets. Masaya exhibited bi-modal eruptive behavior between 2011 and 2019, dominated by open-vent lava lake activity and punctuated by short-lived summit explosions. The Multidimensional Small BAseline Subset time-series analysis approach was used to take advantage of the temporally dense SAR datasets. Between 2012 and early 2015, we observed degassing-induced pressurization of the Masaya Central Reservoir (MCR) at an estimated volume change rate of ~ 0.28 $$\times$$ 106 m3/year. In May 2015, magma was supplied into the MCR at a rate of ~ 5.6 $$\times$$ 106 m3/year, leading to the appearance of a summit lava lake in December 2015. Over the next 6 months, rapid magma supply continued to drive lava lake activity and was followed by a cessation of magma supply into the MCR for another 11 months. From mid-2017 to end-2019, we observed depressurization (~ − 0.67 $$\times$$ 106 m3/year) of the MCR due to a lack of magma supply and continued high rates of degassing in-conjunction with declining lava lake activity.

Petrography Characteristic of Porphyritic Dacite Rock in Olele Village, Kabila Bone, and Bone Bolango

Porphyritic Dacite volcanic rocks are abundant in Olele Village, Kabila Bone, Bone Bolango. This research aims to determine the petrography characteristics of porphyritic dacite rock in Olele. The methods used in this research were field observation and petrography analysis to determine rock texture, mineral composition, and micro-texture plagioclase. Olele Porphyritic Dacite has a porphyritic texture composed phenocryst of plagioclase, quartz, hornblende, sanidine, opaque mineral, and secondary minerals such as sericite and clay minerals. Rock textures include glomeroporphyritic, seriate, and embayment. The micro-textures in plagioclase are fine oscillatory zoning, synneusis, glomerocryst, fine sieve, coarse sieve, resorption surface, and broken crystal. Magmatic processes that occur in the magma chamber include fractional crystallization, magma injection, magma mixing, convection, adiabatic decompression, syn-eruption decompression, and undercooling. The secondary minerals indicated alteration hydrothermal process.

Injection of enriched lithospheric mantle magmas explains the formation of microgranular enclaves in the Rio Jacaré Batholith, Borborema Province, Brazil

The Rio Jacaré Batholith (RJB; 617 ± 4 Ma) is inserted in the Poço Redondo Domain, Sergipano Orogenic System. This batholith is formed by monzodiorite, quartz monzodiorite, monzonite, and quartz monzonite, with abundant microgranular enclaves (MEs). The MEs vary from black to light gray and exhibit globular to slightly elongated shapes with clear-cut, crenulated, and cuspate, or, more rarely, diffuse contacts. They correspond to diorites, monzodiorites, quartz monzodiorites, and monzonites, and textural features indicate mixing of magmas, such as compositional zoning in plagioclase, inclusion zones in plagioclase phenocrysts, poikilitic alkali feldspar, acicular apatite, and ocellar quartz. Calculations of linear correlations of major elements showed that the smallest fraction of mafic magma involved in the mixing was 0.43. MEs represent the breakdown and cooling of a mafic magma that was injected into a cooler felsic magmatic chamber. Emplacement of this mafic magma occurred at different stages of crystallization of the RJB magmatic chamber. The MEs are magnesian and metaluminous, with affinity to the shoshonitic series. Ratios for Ba/Nb (> 23), Ba/La (> 15), and Nb/La (0.22–0.69) are characteristic of magmas generated from partial melting of an enriched lithospheric mantle source. Batch melting modeling suggests that source melting rates of less than 3% are necessary to generate magmas similar to those of the RJB MEs.

FIELD AND PETROGRAPHIC EVIDENCES OF GRANITOIDS AND MAFIC MAGMAS INTERACTION IN THE NEOPROTEROZOIC CENTRAL AFRICAN FOLD BELT IN CAMEROON (MAKENENE AREA)

Field and petrographic studies of the Makenene area in the Central African Fold Belt in Cameroon reveals several features testifying the mafic and felsic magmas interactions and their coeval nature. They are: (1) the Mafic Magmatic Enclaves (MMEs) scattered throughout the Makenene granitoid pluton and displaying sub-rounded shape and back veining, (2) flow structures consisting of schlierens at the tails of MMEs, folded MMEs along with felsic host granitoids with hinge indicating the flow direction, (3) irregular or cuspate boundary between MMEs and host granitoids, (4) quenching of apatite and biotites minerals, (5) MMEs enclosing other MMEs or felsic host granitoids. The mafic magma injection operated during at least four stages (from early to late crystallization state of the host magma) leading respectively to the formation of homogenized granitoid; sub-spherical MMEs scattered in the pluton; dismembered dyke and undisturbed synplutonic mafic dyke. The Makenene area registered four deformation phases (D1 to D4). The first two deformation phases occurred before the magmatism and migmatization events. The third phase is coeval to the magmatism and the migmatization period (at the Eburnean orogeny (2.08-2.07 Ga)). The fourth phase is related to the Panafrican orogeny.

Relationship Between Dike Injection and b‐Value for Volcanic Earthquake Swarms

AbstractDike swarms are the fossil remains of regions of the crust that have undergone repeated magma injections. Volcanic earthquake swarms and geodetic measurements are, at least in part, a record of active injection of fluids (water, gas, or magma) into fractures. Here, we link these two ways of observing magmatic systems by noting that dike thicknesses and earthquake magnitudes share similar scaling parameters. In the Jurassic Independence dike swarm of eastern California median dike thickness is ∼1 m, similar to other swarms worldwide, but glacially polished exposures reveal that a typical dike comprises a number of dikelets that are lognormally distributed in thickness with a mean of ∼0.1 m. Assuming that dikes fill penny‐shaped cracks of a given aspect ratio, the geodetic moment and earthquake magnitude of a diking event can be estimated. A Monte Carlo simulation of dike‐induced earthquakes based on observed dike thickness variations yields a frequency‐magnitude distribution remarkably like observed volcanic earthquake swarms, with a b‐value of ∼1.7. We suggest that swarms of dikes composed of dikelets, as well as plutons built incrementally by sheet intrusions, are physical complements to volcanic seismic swarms, and that at least some earthquake swarms are a palpable expression of incremental magma emplacement.

Linking surface deformation to thermal and mechanical magma chamber processes

Surface deformation at active volcanoes reflects a wide variety of magmatic and crustal processes, and the typical kinematic models employed to interpret these geodetic data are highly nonunique. This work presents a new model for surface deformation linked to magma chamber evolution in response to magma injection, eruptions, cooling and crystallization, volatile exsolution, and viscoelastic deformation of the crust. The model is applied to investigate surface displacements during 1) magma chamber crystallization and second boiling without magma injection; 2) a single episode of magma injection to the chamber; and 3) steady magma injection over multiple eruption cycles. Results indicate that the magnitude of surface velocities associated with crystallization and second boiling should not exceed ∼1 mm/yr unless the magma cooling rate is substantially elevated above normal conductive cooling rates. The surface response to an episode of magma injection depends on magma compressibility and how the timescale of injection compares to the timescale for viscous relaxation. When the injection time is short compared to the relaxation time, and when the magma is equally or less compressible compared to the wall rocks, post-injection deflation can occur without any volume contraction of the chamber. For more compressible magmas, or longer-duration injections, inflation continues over a protracted time following injection. Over multiple eruption cycles, uplift accumulates when more magma is added to the chamber than lost by eruptions; because both the eruption frequency and uplift rate are sensitive to the time-averaged magma supply and chamber volume, geologic data from the eruptive record can be integrated with geodetic data to place tighter constraints on the size and state of subvolcanic magma systems.

Shallow magmatic processes revealed by cryptic microantecrysts: a case study from the Taupo Volcanic Zone

Arc magmas typically contain phenocrysts with complex zoning and diverse growth histories. Microlites highlight the same level of intracrystalline variations but require nanoscale resolution which is globally less available. The southern Taupo Volcanic Zone (TVZ), New Zealand, has produced a wide range of explosive eruptions yielding glassy microlite-bearing tephras. Major oxide analyses and textural information reveal that microlite rims are commonly out of equilibrium with the surrounding glass. We mapped microlites and microcrysts at submicron resolution for major and trace element distributions and observed three plagioclase textural patterns: (1) resorption and overgrowth, (2) oscillatory zoning, and (3) normal (sharp) zoning. Pyroxene textures are diverse: (1) resorption and overgrowth, (2) calcium-rich bands, (3) hollow textures, (4) oscillatory zoning, (5) sector zoning, (6) normal zoning and (7) reverse zoning. Microlite chemistry and textures inform processes operating during pre-eruptive magma ascent. They indicate a plumbing system periodically intruded by short-lived sub-aphyric dykes that entrain microantecrysts grown under diverse physico-chemical conditions and stored in rapidly cooled, previously intruded dykes. Changes in temperature gradients between the intrusion and the host rock throughout ascent and repeated magma injections lead to fluctuations in cooling rates and generate local heterogeneities illustrated by the microlite textures and rim compositions. Late-stage degassing occurs at water saturation, forming thin calcic microcryst rims through local partitioning effects. This detailed investigation of textures cryptic to conventional imaging shows that a significant proportion of the micrometre-sized crystal cargo of the TVZ is of antecrystic origin and may not be attributed to late-stage nucleation and growth at the onset of volcanic eruptions, as typically presumed.

A Crystal Mush Perspective Explains Magma Variability at La Fossa Volcano (Vulcano, Italy)

The eruptive products of the last 1000 years at La Fossa volcano on the island of Vulcano (Italy) are characterized by abrupt changes of chemical composition that span from latite to rhyolite. The wide variety of textural features of these products has given rise to several petrological models dealing with the mingling/mixing processes involving mafic-intermediate and rhyolitic magmas. In this paper, we use published whole-rock data for the erupted products of La Fossa and combine them in geochemical and thermodynamic modelling in order to provide new constrains for the interpretations of the dynamics of the active magmatic system. The obtained results allow us to picture a polybaric plumbing system characterized by multiple magma reservoirs and related crystal mushes, formed from time to time during the differentiation of shoshonitic magmas, to produce latites, trachytes and rhyolites. The residing crystal mushes are periodically perturbated by new, fresh magma injections that, on one hand, induce the partial melting of the mush and, on the other hand, favor the extraction of highly differentiated interstitial melts. The subsequent mixing and mingling of mush-derived melts ultimately determine the formation of magmas erupted at La Fossa, whose textural and chemical features are otherwise not explained by simple assimilation and fractional crystallization models. In such a system, the compositional variability of the erupted products reflects the complexity of the physical and chemical interactions among recharging magmas and the crystal mushes.

The Historical Seismicity of the Puebla-Tlaxcala Region (Trans-Mexican Volcanic Belt) during Early Novohispanic Times (A.D. 1542–1740) and the Structure of the Tlaxcala-Huamantla Half-Graben

AbstractThis article documents earthquake intensity observations during early novohispanic times (A.D. 1542–1740) in the Puebla-Tlaxcala region (central Mexico), mostly based on Nahuatl-language historical annals and chronicles, and includes (as a supplement) a descriptive earthquake catalog for this period. Although it is difficult to assign intensities from fragmentary accounts, the largest shocks (A.D. 1582 and 1711) caused damage through the entire region. Earthquake ground shaking in the study area, located within the active volcanic arc crossing central Mexico, can result from intraslab normal faulting in the subducted Cocos plate, subduction interface thrust earthquakes, motion on the faults of the intra-arc Tlaxcala-Huamantla half-graben, as well as extension accommodated through magmatic injection. Most of the historical sources describe earthquakes felt only locally and sometimes report prolonged sequences of shocks, which is indicative of magmatic dike-induced earthquakes or else resulted from motion on the faults of the Tlaxcala-Huamantla half-graben. This structure, ∼70 km long and 15–20 km wide, is the only major tectonic depression in the eastern part of the trans-Mexican volcanic belt. It is filled by a volcano-sedimentary sequence, up to 400 m thick, of unknown but likely Pliocene age. The half-graben is bound in the north by an east–west-striking and south-dipping normal fault system, which is morphologically most pronounced near the town of Tlaxcala where the footwall uplift reaches a maximum of 350 m. The fault array is likely to be active; it vertically displaces Quaternary Totolqueme volcano.

High pressure experimental investigation of clinopyroxene dissolution in a K-basaltic melt

Dissolution of clinopyroxene (cpx) in a K-basaltic melt from the Campi Flegrei Volcanic District (Italy) has been investigated through dissolution and dissolution-crystallization experiments at pressure of 0.8 GPa, superliquidus temperature of 1350 °C, and dwell times between 0.5 and 1 h. The obtained dissolution rates range from 7.9·10−6 cm s−1 to 6.1·10−6 cm s−1 as a function of dwell time. In the dissolution-crystallization experiment (1300 °C; 0.8 GPa; 2 h), the formation of overgrowth rims accompanied by new cpx crystals suggests that the injection of recharge magmas in basaltic reservoir may lead to inverse or oscillatory zonation. The interaction between cpx and K-basaltic melt at ~1570 °C was studied by in situ radiography using synchrotron radiation combined with the Paris-Edinburgh press. This resulted in cpx resorption to occur depending on the temperature conditions with respect to the liquidus temperature of the cpx (TcpxL). The calculated cpx dissolution rates are ~5·10−3 cm s−1 at T ≤ TcpxL and ~ 3·10−2 cm s−1 at T ≥ TcpxL. The role of crystal dissolution in the estimation of magma residence times has been also tested for a natural magmatic system by interpolating the dissolution rates (~10−5–10−6 cm s−1) with the textural data of cpx phenocrysts from the Agnano-Monte Spina pyroclastic deposit at Campi Flegrei caldera (Campanian region, Italy). Results from calculations indicate that the time required for partial or complete resorption of phenocrysts varies from ~0.5 to ~40 h, and that the effect of crystal dissolution may be relevant to estimate magma residence times whether significant dissolution occurs during magma mixing processes.

Subsidence, not erosion: Revisiting the emplacement environment of the Giant's Causeway, Northern Ireland

For almost a century it has been recognised that the anomalous thickness of columnar-jointed tholeiite basalts at the Giant's Causeway, Northern Ireland, was due to ponding of lava in a topographic depression. For more than 80 years this feature has been ascribed to fluvial incision, yet new field observations have found no evidence for erosion of the underlying Lower Basalt Formation anywhere in the studied area. Basalt weathering and erosion rates also suggest that there was insufficient time for fluvial incision to occur on the scale required. Local dips on the Lower Basalt Formation broadly converge about a low point centred on the Giant's Causeway itself and are discordant with the regional dip of the overlying Causeway Tholeiite Member, observations consistent with a subsidence-based model. The absence of sediment and limited development of hyaloclastite beneath the Causeway Tholeiite Member here indicates that the interval between subsidence of the land surface and emplacement of tholeiite lava was insufficient for sediment, or even a significant depth of water, to accumulate. It implies that subsidence may have been directly related to draining of a relatively shallow magma reservoir from which the tholeiite lavas were erupted. The style of faulting in the Lower Basalt Formation outcrop suggests that a brief period of inflation, perhaps associated with magma injection, may have preceded subsidence caused by magma withdrawal. This study demonstrates that even at sites as well-trodden as the Giant's Causeway, new discoveries can be made from simple observations.

Dynamics of Episodic Magma Injection and Migration at Yellowstone Caldera: Revisiting the 2004–2009 Episode of Caldera Uplift With InSAR and GPS Data

AbstractThe 2004–2009 caldera uplift is the largest instrumentally recorded episode of unrest at Yellowstone caldera. We use GPS and Interferometric Synthetic Aperture Radar (InSAR) time series spanning 2004–2015, with a focus in the aforementioned event to understand the mechanisms of unrest. InSAR data recorded ∼25 and ∼20 cm of uplift at the Sour Creek (SCD) and Mallard Lake (MLD) resurgent domes during 2004–2009, and ∼8 cm of subsidence at the Norris Geyser Basin (NGB) during 2004–2008. The SCD/MLD uplift was followed by subsidence across the caldera floor with a maximum at MLD of ∼1.5–2.5 cm/yr, and no deformation at NGB. The best‐fit source models for the 2004–2009 period are two horizontal sills at depths of ∼8.7 and 10.6 km for the caldera source and NGB, respectively, with volume changes of 0.354 and −0.121 km3, and an overpressure of ∼0.1 MPa. The InSAR and GPS time series record exponentially increasing followed by exponentially decreasing uplift between 2004 and 2009, which is indicative of magma injection into the caldera reservoir, with no need for other mechanisms of unrest. However, magma extraction from NGB to the caldera is unable to explain the subsidence coeval with the caldera uplift. Models of magma injection can also explain other episodes of caldera uplift like that in 2014–2015. Distributed sill opening models show that magma is stored across the caldera source with no clear boundary between MLD and SCD. Since the magma overpressure is orders of magnitude below the tensile strength of the encasing rock, historical episodes of unrest like these are very unlikely to trigger an eruption.

Igneous rocks related to porphyry Cu‐Au mineralization at the Dizon mine, Philippines

AbstractThe Dizon Au‐rich porphyry Cu deposit, 0.67 Mt Cu and 174 t Au, is hosted by diorite and andesite porphyry intrusions, dated at ~2.5 Ma. Amphibole and Fe‐Ti oxides in relatively unaltered rocks were used to evaluate the magma conditions of intrusions. Parental magma for diorite porphyry was ~950°C at a depth of ~15 km, whereas the parental magmas for the andesite porphyry had lower temperatures, 760–820°C at a depth of ~5 km. The deposit formed at the locus of multiple intrusions, with evidence for injections of hot mafic magmas, including destabilization texture of plagioclase phenocrysts. Parental magmas at Dizon were oxidized, above FMQ + 2.0, and water‐rich, >5 wt%, comparable to conditions of many large porphyry deposits elsewhere in the world. The occurrence of thick opacitic bands of amphibole in the diorite porphyry at Dizon reflects the release of aqueous fluids from the magma; such magmatic fluids were likely responsible for magmatic hydrothermal mineralization of the Dizon porphyry deposit. Subduction of the Scarborough Seamount caused a compressive regime in the overlying plate, which likely contributed to favourable tectonic conditions for mineralization.

Stress fields around magma chambers influenced by elastic thermo-mechanical deformation: implications for forecasting chamber failure

Defining the conditions that lead to the rupture of a magma chamber is essential to forecast eruptions. So far, models simulating magma chamber dynamics have neglected the effects of elastic thermal expansion in the host rocks surrounding a new injection of magma, focusing instead primarily on elastic-plastic deformation and more recently, on visco-elastic deformation. Here we fill this gap by building a suite of elastic thermo-mechanical models to determine the stress field around a variably heated crustal magma chamber. We first consider linear elastic mechanical models with only the effect of magma pressure. We then present purely thermal models simulating heat distribution around a heated chamber. Finally, we present coupled linear elastic thermo-mechanical models that highlight the influence of temperature on the distribution of crustal stresses. Results show that thermal expansion–induced stresses generate two competing consequences: (1) they increase the level of shear stress around the magma chamber and (2) they partially suppress the level of tensile stress generated by the magmatic pressure. These competing effects influence the short-timescale conditions required for the failure of immature magmatic systems and hence the nucleation of dikes which may ultimately feed eruptions during unrest. Therefore, soon after a new magmatic recharge event, the contribution of temperature increase in the host rocks, following the new influx of magma into a crustal magma chamber, should be considered.

Timescales of crystal mush mobilization in the Bárðarbunga-Veiðivötn volcanic system based on olivine diffusion chronometry

AbstractThe timescales of magmatic processes within a volcanic system may be variable over a volcano’s geological history. Crystals reflect environmental perturbations under which they grew, and compositional gradients quenched inside crystals on eruption can be exploited to extract timescales of magmatic processes. Here, we use multi-element diffusion chronometry in olivine macrocrysts to recover their residence time in a melt that ultimately erupted at the surface. The macrocrysts were mobilized by the carrier melt from mushy layers in the magma reservoir, and diffusion timescales likely reflect the time interval between mush disaggregation, ascent, and eruption. To unravel the evolution of mush disaggregation timescales with time, we target early-Holocene, middle-Holocene, and historical magmatic units erupted in the Bárðarbunga-Veiðivötn volcanic system in Iceland’s Eastern Volcanic Zone. Macrocryst contents vary between samples; early-Holocene samples are highly phyric (10–45 vol% macrocrysts) and contain gabbroic nodules, whereas middle-Holocene (5–15 vol%) and historical units (5–10 vol%) tend to be generally less phyric. Early-Holocene olivine macrocrysts have core compositions in the range Fo84–87, while middle-Holocene and historical samples record a wider range in core compositions from Fo80 to Fo86.5. Olivine rims are in chemical equilibrium with their carrier liquid and are slightly more evolved in early-Holocene units (Fo76–81) compared to middle-Holocene (Fo78–80) and historical (Fo81–83) units. Diffusion chronometry reveals that the timescale between mush dis-aggregation and eruption has changed over time, with timescales getting shorter approaching recent times. Early-Holocene olivine macrocrysts dominantly record Fe-Mg diffusion timescales between 200–400 days, while middle-Holocene and historical units typically record timescales of about 70 and 60 days, respectively. Barometric studies suggest that melts and crystals are likely stored and gradually transferred throughout an interconnected multi-tiered system that ultimately culminates in a mid-crustal reservoir(s) at about 6.8–7.5 ± 2.5 km depth, where final disaggregation by the carrier liquid took place. We argue that, as a result of extensional processes enhanced by rifting events, well-mixed melts got drawn into mid-crustal reservoir(s), causing crystal mush loosening and mobilization. In addition, we propose that more energy in the form of heat and/or melt supply was required in the early-Holocene to break up the dense mush fabric and convert it into an eruptible magma. Conversely, as evidenced by the diverse macrocryst content of the historical units and by the lack of gabbroic nodules, the system has become characterized by a less compact mush fabric since at least the middle-Holocene, such that fresh injection of melt would easily loosen and mobilize the mush, resulting in an eruption within a couple of months. This study provides evidence that along axial rift settings, rifting-related processes can help to “pull the mush apart” with no requirement for primitive magma injection as an eruption trigger. Furthermore, we provide evidence that in the Bárðarbunga-Veiðivötn volcanic system specifically, the time between mush disaggregation and eruption has decreased considerably with time, indicating shorter warning times before imminent eruptions.

Vent distribution and structural inheritance in an embryonic rift: The example of the Chyulu Hills off-rift magmatic range (South Kenya)

In the East African Rift System (EARS) tectonic inheritance rules the location, the shape and the development of rift segments. Such lithospheric/crustal-scale inherited structures may be exploited by magma favouring its ascent to shallow crustal level and, possibly, to the surface producing volcanism. In the eastern branch of the EARS in southeast Kenya, the Pleistocene Chyulu Hills volcanic field (CHVF) is a volcanic range formed east of the main rift along a NW-SE trending Proterozoic crustal-scale shear zone (the Emali-Voi shear zone). The volcanic range consists of hundreds (at least 514) of scoria cones of basaltic composition, often coalescent and well-aligned along the trace of the crustal shear zone. Morphostructural and spatial analyses of vents and fractures in the CHVF show that vent density and clustering change moving south-eastward along the ridge mimicking the age pattern of volcanism with the youngest activity in the southeast of the CHVF. Notably, the parameters defining the vent self-similar clustering indicate a decreasing thickness of the upper brittle crust along the CHVF from about 12 km in the north-west to about 7 km in the southeast, in agreement with the crustal structure of the area as imaged by seismic and geophysical data sets. Here we propose an evolutionary model of the CHVF occurring at spatial and time scales quite different from that of magma-assisted rifting processes, where CHVF is an example of an embryonic rift stage with a focussed magma injection into the continental lithosphere localized along a weakness discontinuity represented by an inherited crustal shear zone.

Zircon geochronology suggests a long-living and active magmatic system beneath the Ciomadul volcanic dome field (eastern-central Europe)

Ciomadul is the youngest volcano in eastern-central Europe. Although its last eruption occurred at ca. 30 ka, there are independent indications for a high-crystallinity magma reservoir persisting beneath the volcano until present. In order to further test the hypothesis of long-lived melt presence and to better constrain the nature and timescales associated with the subvolcanic magma storage system, over 500 zircon U-Th and U-Pb spot ages (crystal interiors and outer surfaces) were interpreted from dacitic rocks of the most productive eruptive period (the Young Ciomadul Eruptive Period; YCEP, 160-30 ka). Zircon surface ages from lava dome and pumice samples range from ca. 600 ka up to the youngest eruption event at 30 ka. They form a continuous age distribution and some single crystals reveal significant age zonation (>150 kyr difference from core to rim). The oldest zircon ages of YCEP overlap with the last eruption events of the Old Ciomadul Eruptive Period (1000–330 ka). The zircon age spectra, combined with textural data, point to a prolonged (several 100's kyr) residence in a highly crystalline mush state. The range in zircon crystallization temperature (from ∼750°C to the solidus at ∼680°C) is consistent with the results of thermometry on amphibole and plagioclase from felsic crystal clots, which represent crystal mush fragments. To maintain magma reservoir for such a long time above solidus, continuous magma input by deeper recharge is required. Zircon crystallization model calculations constrained by thermal modelling imply an average rate of magma input of about 1.3 × 10−4 km3/yr over 2 Myr. Such estimate allows us to calculate an extrusive/intrusive ratio of 1:25–1:30. The model calculations suggest that a crystal mush zone of about 35 km3 is still present within the subvolcanic magma reservoir. Importantly, the Ciomadul plumbing system thus remains thermally primed and renewed magma injection could lead to rapid reawakening and eruption of the apparently inactive volcano.

Generation of Cretaceous high-silica granite by complementary crystal accumulation and silicic melt extraction in the coastal region of southeastern China

Abstract It is generally hypothesized that high-silica (SiO2 > 75 wt%) granite (HSG) originates from crystal fractionation in the shallow crust. Yet, identifying the complementary cumulate residue of HSG within plutons remains difficult. In this work, we examine the genetic links between the porphyritic monzogranite and HSG (including porphyritic granite, monzogranite, and alkali feldspar granite) from the coastal area of southeastern China using detailed zircon U-Pb ages, trace elements, Hf-O isotopes, and whole-rock geochemistry and Nd-Hf isotopic compositions. Zircon U-Pb ages indicate that the porphyritic monzogranite and HSG are coeval (ca. 96–99 Ma). The HSG and porphyritic monzogranite have similar formation ages within analytic error, identical mineral assemblages, similar Nd-Hf isotopic compositions, and consistent variations in their zircon compositions (i.e., Eu/Eu*, Zr/Hf, and Sm/Yb), which suggests that their parental magma came from a common silicic magma reservoir and that the lithological differences are the result of melt extraction processes. The porphyritic monzogranite has relatively high SiO2 (70.0–73.4 wt%), Ba (718–1070 ppm), and Sr (493–657 ppm) contents, low K2O and Rb concentrations and low Rb/Sr ratios (0.1–0.2), and it displays weak Eu anomalies (Eu/Eu* = 0.57–0.90). Together with the petrographic features of the porphyritic monzogranite, these geochemical variations indicate that the porphyritic monzongranite is the residual silicic cumulate of the crystal mush column. The HSG (SiO2 = 75.0–78.4) has variable Rb/Sr ratios (2–490) and very low Sr (1–109 ppm) and Ba (9–323 ppm) contents. Zircon from the HSG and porphyritic monzogranite overlap in Eu/Eu*, Zr/Hf, and Sm/Yb ratios and Hf contents; however, some zircon from the HSG show very low Eu/Eu* (<0.1) and Zr/Hf ratios. These features suggest that the HSG represents the high-silica melt that was extracted from a crystal-rich mush. The injection of mantle-derived hotter mafic magma into the mush column and the exsolution of F/Cl−-enriched volatiles (or fluids) from the interstitial melt rejuvenated the pre-existing highly crystalline mush. Subsequent extraction and upward migration of silicic melt resulting from compaction of the mush column formed the HSG at shallow crustal levels, which left the complementary crystal residue solidified as porphyritic monzogranite at the bottom.