Petrological Constraints on the Thermal History of Magma Storage in the Crust

The processes involved in the formation and storage of magma within the Earth's upper crust are of fundamental importance to volcanology. Many volcanic eruptions, including some of the largest, result from the eruption of components stored for tens to hundreds of thousands of years before eruption. Although the physical conditions of magma storage and remobilization are of paramount importance for understanding volcanic processes, they remain relatively poorly known. Eruptions of crystal-rich magma are often suggested to require the mobilization of magma stored at near-solidus conditions; however, accumulation of significant eruptible magma volumes has also been argued to require extended storage of magma at higher temperatures. What has been lacking in this debate is clear observational evidence linking the thermal (and therefore physical) conditions within a magma reservoir to timescales of storage-that is, thermal histories. Here we present a method of constraining such thermal histories by combining timescales derived from uranium-series disequilibria, crystal sizes and trace-element zoning in crystals. At Mount Hood (Oregon, USA), only a small fraction of the total magma storage duration (at most 12 per cent and probably much less than 1 per cent) has been spent at temperatures above the critical crystallinity (40-50 per cent) at which magma is easily mobilized. Partial data sets for other volcanoes also suggest that similar conditions of magma storage are widespread and therefore that rapid mobilization of magmas stored at near-solidus temperatures is common. Magma storage at low temperatures indicates that, although thermobarometry calculations based on mineral compositions may record the conditions of crystallization, they are unlikely to reflect the conditions of most of the time that the magma is stored. Our results also suggest that largely liquid magma bodies that can be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption.

The storage conditions of super-eruptions, particularly regarding the timescales and thermal status of their magma reservoirs, are under debate. We present cathodoluminescence images and Ti contents of quartz crystals from two super-eruptions, the ~0.78 Ma Oldest Toba Tuff (OTT) on Sunda arc and the ~0.63 Ma Lava Creek Tuff (LCT) at Yellowstone hotspot, to compare their pre-eruptive thermal evolution histories and shed light on the links between magma storage conditions and tectonic settings. The OTT quartz crystals are commonly unzoned and crystallize at temperatures <750°C with a residence time of ~100–200 kyr constrained by Ti-in-quartz diffusion chronometry, indicating a prolonged near-solidus and steady-state magma storage. In addition, the wide range of the formation time (~1000–20 000 years) of the Ti-rich, CL-bright rims of the zoned quartz crystals reveal progressive thermal maturation of the OTT magma reservoir fed by continuous magma fluxes. In contrast, the LCT quartz crystals mostly exhibit complex oscillatory zoned patterns and crystallize above the lock-up temperature with a residence time of ~1–10 kyr, indicating rapid generation of voluminous eruptible magmas through frequent magma recharge events within the LCT silicic magma reservoir. Both the OTT and the LCT magma reservoirs show heterogeneous storage features, and the differences in thermal histories between them were also observed in other super-eruptions from the Toba volcanic system and the Yellowstone Plateau Volcanic Field, which could be associated with the distinct magma supply rates feeding large magma reservoirs in continental arc and hotspot settings.

Determinations of the temperatures and pressures of volcanic products during their formation are helpful for understanding magmatic system behaviour and igneous processes. Whereas estimates of the former give insights into the thermal evolution of magmas, the latter provides estimation of the storage depth of magmatic reservoirs. Together they provide practical constraints for understanding the magma storage conditions beneath active volcanic areas and their implications in other research fields, such as geothermal exploration. The Asal-Ghoubbet rift is one of the emergent segments of the Aden Gulf oceanic ridge, which spreads westward on land into the triple junction zone of the Afar depression. The rift-in-rift area has witnessed repeated magmatic and tectonic activity over its ~1 Ma evolution. Most recently, in November 1978, a one-week-long basaltic fissure eruption led to the birth of the Ardoukôba volcano. Due to its active and unique location, the area has been subject to geothermal exploration since the 1970s. However, although intensive geological, geochemical, and geophysical studies have been conducted in the area, detailed knowledge is still lacking regarding the evolution of storage conditions in the magmatic system beneath the Asal-Ghoubbet rift. For this study, eleven samples, representing the time span from ~300 ka to today, were collected in the rift, and geochemical and geothermobarometric analyses were conducted. Two samples representing the oldest basalts in Djibouti (BD and BS) were also added for comparison. The rock samples are tholeiitic to transitional basalts with MgO contents between 3.3 and 10.2 wt%. The negative correlations of TiO2 and FeO contents, and positive of CaO, with MgO indicate that fractional crystallization is an important process in the magmatic system. The crystal cargo of the erupted lavas consists of plagioclase, clinopyroxene and olivine, in order of decreasing amount. Anorthite contents of plagioclase vary between 45and 89, with a main population at ~An85. Most crystals are in disequilibrium with their carrier melts. Clinopyroxenes have Mg# ranging from 47 to 87,with crystals of Dalha basalts (BD) being the most differentiated. Olivine cores and mantles are mostly primitive with a main composition of Fo84. The majority of clinopyroxenes and olivines are in equilibrium with carrier melts, except for BD. Based on geothermobarometric calculations, we propose the existence of two magma storage depths beneath the Asal-Ghoubbet rift. A mid-deep crustal reservoir is located at ~13 km with a mean temperature of 1200°C. It represents the main magma body with higher temperatures and where most of the minerals crystallized. The second one is found at shallow crustal level (5 to 7 km) with lower temperatures (~ 1120°C). Few crystals formed at this depth, which could indicate a smaller size reservoir. Textural and chemical variations of minerals suggest a connection between the two magma bodies, forming the magma plumbing system. Magma storage conditions appear to have been maintained over time, which implies the continuous renewal of the geothermal heat source in the Asal-Ghoubbet rift.

Geophysical, geochemical, and petrological studies indicate that magma storage and chemical differentiation can occur at various depths in the continental crust. Examination of exposed ancient magmatic systems reveal a continuum from a refractory lower crust to a silica-rich upper crust, prompting a debate on the mechanisms governing magma storage and differentiation in vertically extensive magmatic systems.Conceptual models for magma storage and chemical differentiation within the crust range from (i) a low crystallinity magma body, where differentiation occurs primarily by fractional crystallisation and partial melting of the crust, to (ii) a trans-crustal high crystallinity ‘mush’ reservoir consisting of a porous crystal matrix with melt in the pore space. Chemical differentiation in these mush reservoirs is driven by the compaction of the crystal matrix, buoyant upward reactive flow of melt, and partial melting of the surrounding crust.We use a numerical model to investigate magma storage and chemical differentiation in the continental crust. The model assumes a magmatic system sustained by the intrusion of mantle-derived basalt into the lower crust.  We find that intrusion of basalt creates a high crystallinity reservoir in the lower crust. Reactive flow and compaction cause melt to accumulate at the top of the reservoir, creating a layer of low crystallinity, chemically differentiated magma.  Buoyancy overpressure causes this magma to  evacuate via dikes and we assume the magma intrudes the mid-crust to form a second high crystallinity mush reservoir. Reactive flow and compaction once again lead to the accumulation of low-crystallinity, evolved magma at the top of the reservoir that can evacuate and intrude the upper crust, driving volcanic eruptions or forming shallow plutons.The compositional evolution of magma as it ascends through the system is influenced by the flux of the parental basalt and the fertility of the crust. A higher basalt flux leads to warmer reservoirs that evacuate less evolved magma. Reservoirs in infertile crust that cannot melt are formed only of hot, parental magma, and also evacuate less evolved magma. Partial melting of fertile crust allows melt to percolate upwards and accumulate in cooler crust, leading to the evacuation of evolved (silicic) magma.In most of our simulation cases, the reservoirs remain as discrete bodies in the lower- and mid-crust, with magma transfer occurring via dikes; a trans-crustal reservoir does not form. The lower-crust reservoir evacuates mafic to intermediate magma, whilst the cooler mid-crust reservoir primarily evacuates evolved, silicic magma, consistent with vertical compositional trends in exposed, ancient magmatic systems. Both reservoirs comprise primarily low-melt fraction mush, consistent with geophysical imaging of contemporary systems.  Layers of accumulated low-crystallinity magma are transient and likely too thin to resolve in geophysical data. The predicted volume, composition, and frequency of episodic magma intrusions into the upper crust are consistent with observed data from large volcanic eruptions. Our results suggest that reactive flow in multiple mush reservoirs controls magma storage and differentiation.

Constraining magma storage conditions at a restless volcano in the Main Ethiopian Rift using phase equilibria models

The last three eruptions at the Cordon Caulle volcanic complex, Chile, have been strikingly similar in that they have started with relatively short pre-eruptive warning and produced chemically homogeneous rhyolite to rhyodacite magma with glassy to aphyric texture. These characteristics collectively call for an understanding of the storage conditions leading to the rise and extraction of crystal-poor silicic magma from volcanoes. We have analyzed and experimentally reproduced the mineral assemblage and glass chemistry in rhyolite magma produced in the most recent eruption of Cordon Caulle, and we use these to infer magma storage and ascent conditions. Fe–Ti oxide mineral geothermometry suggests that the rhyolite was stored at ∼870–920 °C. At these temperatures, the phenocryst assemblage (plag∼An37 > cpx + opx > mag + ilm) can be reproduced under H2O-saturated conditions of between 100 and 50 MPa, corresponding to crustal depths between about 2.5 and 5.0 km. The shallow and relatively hot magma storage conditions have implications for the rapid onset, degassing efficiency, and progression from explosive to mixed pyroclastic-effusive eruption style at Cordon Caulle.

Establishing the conditions and dynamics of pre-eruptive magma storage and transfer within transient transcrustal storage networks is a major focus of quantitative volcanic petrology. In Iceland, the behaviour, conditions and timescales of magmatic processes within on-rift plumbing systems are increasingly well constrained. However, relatively little is known about magma storage and transfer in off-rift zones, despite off-rift volcanoes being able to generate hazardous explosive eruptions after centuries or millennia of dormancy (e.g. 2010 AD Eyjafjallajökull; 1362 AD Öræfajökull; 3.0 ka, 4.2 ka and 1104 AD Hekla). We present a combined geochemical and geothermobarometric study of magma storage and transfer recorded in the products of the postglacial Búðahraun (∼5.0–8.0 ka) and Berserkjahraun (∼4.0 ka) eruptions within the Snæfellsnes volcanic zone. The eruption products contain diverse and compositionally heterogeneous macrocryst cargoes recording complex petrogenetic histories of crystal evolution and inheritance from different parts of the sub-volcanic plumbing systems. Geothermobarometry indicates two compositionally and thermally heterogeneous magma storage regions located in the lower (20 ± 4 km) and upper-mid (11 ± 3 km) crust. Crystallization pressure and depth estimates coincide with comparable data from Vatnafell, a small sub-glacial table mountain (tuya) in the centre of the Snæfellsnes volcanic zone, indicating that the nature and conditions of magma storage have remained unchanged since the Upper Pleistocene. Trace element zoning of clinopyroxene macrocrysts indicates that mafic recharge into the upper-mid-crustal storage zone triggered the eruptions of Búðahraun and Berserkjahraun. Evidence for eruption-triggering mafic recharge and basaltic cannibalism involving the transfer and amalgamation of crystals with different evolutionary histories sets the Búðahraun and Berserkjahraun eruptions apart from other studied eruptions in Iceland. We propose that the compositional and textural diversity preserved within the crystal cargoes are a direct consequence of the reduced heat flow beneath the Snæfellsnes volcanic zone, which favours the formation of isolated melt pockets in which compositionally diverse macrocryst populations formed. Periodic flushes of primitive basaltic magma from depth promote widespread mixing with evolved melts, resulting in the assembly of crystals with diverse ancestries from different parts of the sub-volcanic systems. Insights gained from the diverse macrocryst cargoes of Búðahraun and Berserkjahraun and comparisons with recent off-rift volcanism in Iceland are essential for the development of future monitoring efforts and hazard evaluation. Although volcanism within the Snæfellsnes volcanic zone differs fundamentally from that in rift zones where eruptions are controlled by extensional spreading, magma ascent from depth still appears to follow pre-existing tectonic escape routes. This could result in extremely short advance warning times on the order of a few days.

Calderas and magma reservoirs

A variety of methods exist to constrain sub-volcanic storage conditions of magmas. Petrological, seismological and satellite geodetic methods are integrated to determine storage conditions of peralkaline magmas beneath Dabbahu Volcano, Afar, Ethiopia. Secondary ion mass spectrometry (SIMS) analysis of volatile contents in melt inclusions trapped within phenocrysts of alkali feldspar, clinopyroxene and olivine from pantellerite obsidians representing the youngest eruptive phase (<8 ka) show H2O contents ≤5.8 wt.% and CO2 contents generally below 500 ppm, although rarely as high as 1,500 ppm. Volatile saturation pressures (at 679–835°C) are in the range 43–207 MPa, consistent with published experimental data for similar pantellerites, which show that the phenocryst assemblage of alkali feldspar + cpx + aenigmatite ± ilmenite is stable at 100 to 150 MPa. Inferred magma storage depths for these historic eruptions are ~1–5 km below sea-level, consistent with the depths of earthquakes, associated with magma chamber deflation following a dyke intrusion in the period Oct 2005–Apr 2006. Interferometric synthetic aperture radar (InSAR) data for the same period reveal a broad ~20 km diameter area of uplift. Modelling of different geometries reveals that a series of stacked sills over a 1–5 km depth range best matches the InSAR data. The consistency of depth estimates based on petrological study of ancient eruptions and the seismicity, inflation and deflation of Dabbahu observed in relation to the dyking event of 2005, suggest a small but vertically extensive and potentially long-lived magma storage region.

Bezymianny volcano (Kamchatka, Russia) is an andesitic island arc stratovolcano that started to erupt in 1955 after ~ 1000 years of dormancy. On March 30, 1956, the climactic phase of the eruption was preceded by a 4-month-long emplacement of a shallow cryptodome, which triggered a flank collapse violently decompressing the magma into a laterally directed blast followed by an explosive phase emplacing extensive pumice concentrated pyroclastic density currents (pumice C-PDC). Aiming at constraining the plumbing system below Bezymianny volcano prior to the 1956 eruption, we performed a multiphase textural and petrological study using dense to vesiculated clasts of the blast and pumice samples from the post-blast C-PDC deposits. We inferred the pressure and temperature conditions of magma storage using sample vesicularity, amphibole destabilization rims, volatile contents in melt inclusions, microlite textures, and phase compositions (phenocrysts, microlites, and glasses). We propose a three-level magma storage characterized by a deep reservoir (≥ 200–350 MPa, ≥ 840 °C, ~ 4.0–8.0 wt% H2O and CO2 up to 1500 ppm, where amphibole is stable), a shallow reservoir (50–100 MPa, 850–900 °C, 1.5–4.0 wt% H2O and CO2 < 250 ppm, where amphibole is unstable and quartz crystallizes) in which the pre-cryptodome magma resided and from which the post-blast pumiceous magma originated, and a subsurface cryptodome (< 25 MPa, ~ 900 °C, cristobalite crystallized) from which the blast was initiated. This plumbing system provides the framework for constraining the timescales of the 1956 eruptive dynamics (companion paper). The three-stage architecture proposed for the 1956 andesitic reservoir compares to the present-day plumbing system emitting mafic lavas, thus suggesting that the timescales of the eruptive dynamics (e.g., magma residence time and ascent rate) may be the key to determining evolved or mafic magmas.

The eruptions of Campi Flegrei (Southern Italy), one of the most studied and dangerous active volcanic areas of the world, are fed by mildly potassic alkaline magmas, from shoshonite to trachyte and phonotrachyte. Petrological investigations carried out in past decades on Campi Flegrei rocks provide crucial information for understanding differentiation processes in its magmatic system. However, the compositional features of rocks are a palimpsest of many processes acting over timescales of 100–104 years, including crystal entrapment from multiple reservoirs with different magmatic histories. In this work, olivine, clinopyroxene and feldspar crystals from volcanic rocks related to the entire period of Campi Flegrei’s volcanic activity are checked for equilibrium with combined and possibly more rigorous tests than those commonly used in previous works (e.g., Fe–Mg exchange between either olivine or clinopyroxene and melt), with the aim of obtaining more robust geothermobarometric estimations for the magmas these products represent. We applied several combinations of equilibrium tests and geothermometric and geobarometric methods to a suite of rocks and related minerals spanning the period from ~59 ka to 1538 A.D. and compared the obtained results with the inferred magma storage conditions estimated in previous works through different methods. This mineral-chemistry investigation suggests that two prevalent sets of T–P (temperature–pressure) conditions, here referred to as “magmatic environments”, characterized the magma storage over the entire period of Campi Flegrei activity investigated here. These magmatic environments are ascribable to either mafic or differentiated magmas, stationing in deep and shallow reservoirs, respectively, which interacted frequently, mostly during the last 12 ka of activity. In fact, open-system magmatic processes (mixing/mingling, crustal contamination, CO2 flushing) hypothesized to have occurred before several Campi Flegrei eruptions could have removed earlier-grown crystals from their equilibrium melts. Moreover, our new results indicate that, in the case of complex systems such as Campi Flegrei’s, in which different pre-eruptive processes can modify the equilibrium composition of the crystals, one single geothermobarometric method offers little chance to constrain the magma storage conditions. Conversely, combined methods yield more robust results in agreement with estimates obtained in previous independent studies based on both petrological and geophysical methods.

In the islands of Faial and Pico (the Azores), fluid inclusions are hosted in megacrysts of olivine (Mg#80–88) and clinopyroxene (Mg#79–90) in highly porphyritic lavas and in mineral assemblages of ultramafic xenoliths. Rare inclusions are contained in olivine phenocrysts (Mg# &lt; 80) and plagioclases in poorly porphyritic lavas. Trails of late‐stage inclusions are predominant over isolated early‐stage inclusions. Almost all inclusions are re‐equilibrated and the trapped fluid consists of pure CO2 (Tm from −56.5 to −57.2). Rare early‐stage inclusions may contain dypingite or Mg‐calcite, which indicates that in earlier times some water was present along with CO2. Barometric data indicate that CO2 inclusions in xenoliths from the two islands equilibrated at maximum pressures of 570–586 MPa (19.7–21.2 km), while in poorly porphyritic lavas from all the fissure zones at 465–508 MPa (16.4–18.1 km). Maximum pressure values of 463 MPa (16.8 km) and 492 MPa (17 km) were recorded for the central volcanoes of Pico and Faial, respectively. Further trapping/re‐equilibration was recorded at 156 MPa in Faial (5.6 km), in plagioclase phenocrysts in mugearites. All these pressures correspond to magma ponding sites and to its crystallization and can be useful for tracing the progressive thickening of a dense transition zone, below the geophysical Moho. The ability to extract rapidly the stored magmas from these volcanic systems strictly depends on the different tectonic styles, acting in this transition zone. Magmatic evolution in small and short‐lived intracrustal reservoirs, not necessarily coaxial with main conduit system, was enhanced at the intersection of differently oriented lineaments.

The Virunga Volcanic Province (VVP), located in the western branch of the East African Rift System, hosts a variety of alkaline lavas erupted from closely spaced volcanic centers. However, the magmatic system of this region, particularly in its eastern sector, remains insufficiently constrained. In this study, we present a petrological and geochemical investigation of basaltic to trachytic lavas from the eastern VVP. Thermobarometric analysis of mineral phases indicates that basalts originated from magma storage zones between 4 and 30 km deep, with crystallization temperatures of ~1200 °C and melt H2O contents lower than 1 wt%. In contrast, more evolved magmas crystallized at similar depths, but at lower temperatures (~1050 °C) and higher H2O contents, ranging from 2 to 4 wt%. Thermodynamic modelling suggests that extensive (up to 70%) fractional crystallization of an assemblage dominated by olivine, clinopyroxene, and plagioclase can produce the more evolved trachytic derivatives from basaltic parental melts. When integrated with previous studies from other VVP volcanoes, our findings deepen the understanding of the architecture of the magmatic system beneath the region, suggesting it resembles a well-developed multi-level plumbing system.

Phase petrology reveals shallow magma storage prior to large explosive silicic eruptions at Hekla volcano, Iceland

Melt generation and magma storage conditions of primitive arc lavas in the Macolod Corridor, southwestern Luzon arc, Philippines