Olivier Bachmann

Quantum magmatism: Magmatic compositional gaps generated by melt-crystal dynamics

Compositional gaps are common in volcanic series worldwide. The pervasive generation of compositional gaps influences the mechanical and thermal properties of the crust, and holds clues on how our planet differentiates. We have explored potential mechanisms to generate these gaps using numerical simulations coupling crystallization kinetics and multiphase fluid dynamics of magma reservoirs. We show that gaps are inherent to crystal fractionation for all compositions, as crystal-liquid separation takes place most efficiently within a crystallinity window of ∼50–70 vol% crystals. The probability of melt extraction from a crystal residue in a cooling magma chamber is highest in this crystallinity window due to (1) enhanced melt segregation in the absence of chamber-wide convection, (2) buffering by latent heat of crystallization, and (3) diminished chamber-wall thermal gradients. This mechanical control of igneous distillation is likely to have played a dominant role in the formation of the compositionally layered Earth9s crust by allowing multiple and overlapping intrusive episodes of relatively discrete or quantized composition that become more silicic upward.

The Magma Reservoirs That Feed Supereruptions

The vigor and size of volcanic eruptions depend on what happens in magma reservoirs in the Earth’s crust. When magmatic activity occurs within continental areas, large reservoirs of viscous, gas-rich magma can be generated and cataclysmically discharged into the atmosphere during explosive supereruptions. As currently understood, large pools of explosive magma are produced by extracting interstitial liquid from long-lived “crystal mushes” (magmatic sponges containing >50 vol% of crystals) and collecting it in unstable liquid-dominated lenses.

Rejuvenation of the Fish Canyon magma body: A window into the evolution of large-volume silicic magma systems

Voluminous, unzoned, phenocryst-rich pyroclastic deposits, considered as erupted batholiths, provide a unique opportunity to investigate magmatic processes in silicic magmas. The Fish Canyon Tuff, a well-documented example of these monotonous ignimbrites, displays evidence for simultaneous dissolution of feldspars 1 quartz and crystallization of hydrous phases during gradual near-isobaric reheating from ;720 to 760 8C. These observations, along with a high crystallinity (45%) and near-solidus mineral assemblage, suggest that the Fish Canyon magma cooled to a rigid crystal mush before being partly remelted prior to eruption. Rejuvenation was triggered by intrusion of water-rich mafic magmas at the base of the Fish Canyon mush, but the mechanisms of heat transfer remain poorly understood. The growth of amphibole during reheating requires addition of mafic components, but the absence of any measurable gradients and the paucity of mafic enclaves in the Fish Canyon magma rule out a reheating event dominated by convective mixing with a mafic magma. Closed-system processes, such as heat conduction and convective self-mixing, could not account for the transport of externally derived mafic components. We performed numerical simulations of upward percolation of a hot, low-density H2O-CO2 fluid phase (gas sparging) through a crystalline framework saturated with rhyolitic melt to assess the efficiency of such a process in rejuvenating silicic mushes in open systems. Sparging by ;20‐40 km 3 of gas extracted from ;3000 km 3 of mafic magma is capable of reheating 7500 km 3 of silicic crystal mush by .40 8C in 150‐200 k.y. Moreover, the vertical thermal gradient after 150 k.y. in most of the mush is small (;25 8C in the upper 65%). Gas sparging also produces an increase in the internal pressure of silicic crystal mushes and may lead to the formation of crystal-poor rhyolites by expelling interstitial melt. However, our simulations predict that filter pressing driven by sparging of externally derived gas could not solely account for the generation of the most voluminous rhyolites.

On the longevity of large upper crustal silicic magma reservoirs

Understanding the processes involved in the formation and maturation of upper crustal magma reservoirs, ultimately sourcing the largest volcanic eruptions on Earth, is one of the most fundamental aspects of volcanology. While such reservoirs are known to assemble incrementally over extended periods of time, debate persists regarding the time scales of melt preservation in the cold upper crust. If rapid cooling individually freezes incoming replenishing intrusions, accumulations of eruptible magma are impossible, precluding the construction of voluminous volcanic reservoirs for all but the highest magma emplacement rates. Recent numerical thermal models have been used to assess the viability of upper crustal silicic magma survival, and have suggested that supervolcanic reservoirs must form on geologically short time scales with anomalously high injection rates, and subsist only ephemerally, making their long-term evolution less predictable. Motivated by geological observations suggesting the contrary, we have improved upon these models by incorporating two fundamental features of natural systems not previously considered: (1) a nonlinear crystallization-temperature relationship adapted for upper crustal silicic magmas and (2) a temperature-dependent thermal conductivity. We demonstrate that the incorporation of both of these properties can allow an upper crustal reservoir to remain above its solidus for hundreds of thousands of years when fed by magma fluxes typical of large magmatic provinces. While the crystallization-temperature path plays the most significant role in maintaining a large pool of eruptible magma, the incorporation of temperature-dependent thermal properties significantly extends the lifetime of such reservoirs. Furthermore, while deeper emplacement levels (e.g., 10 km depth) can both extend magma survival and increase melt availability, we show that a 5 km depth can also provide an adequate magma storage environment. These results provide strong support for long-lived upper crustal mushes as a staging ground for accumulation of highly eruptible, crystal-poor silicic magmas, and further assert the evolutionary link between volcanic and plutonic systems.

Ignimbrites to batholiths: Integrating perspectives from geological, geophysical, and geochronological data

Multistage histories of incremental accumulation, fractionation, and solidification during construction of large subvolcanic magma bodies that remained sufficiently liquid to erupt are recorded by Tertiary ignimbrites, source calderas, and granitoid intrusions associated with large gravity lows at the Southern Rocky Mountain volcanic field (SRMVF). Geophysical data combined with geological constraints and comparisons with tilted plutons and magmatic-arc sections elsewhere are consistent with the presence of vertically extensive (>20 km) intermediate to silicic batholiths (with intrusive:extrusive ratios of 10:1 or greater) beneath the major SRMVF volcanic loci (Sawatch, San Juan, Questa-Latir). Isotopic data require involvement of voluminous mantle-derived mafic magmas on a scale equal to or greater than that of the intermediate to silicic volcanic and plutonic rocks. Early waxing-stage intrusions (35–30 Ma) that fed intermediate-composition central volcanoes of the San Juan locus are more widespread than the geophysically defined batholith; these likely heated and processed the crust, preparatory for ignimbrite volcanism (32–27 Ma) and large-scale upper-crustal batholith growth. Age and compositional similarities indicate that SRMVF ignimbrites and granitic intrusions are closely related, but the extent to which the plutons record remnants of former magma reservoirs that lost melt to volcanic eruptions has been controversial. Published Ar/Ar-feldspar and U-Pb-zircon ages for plutons spatially associated with ignimbrite calderas document final crystallization of granitoid intrusions at times indistinguishable from the tuff to ages several million years younger. These ages also show that SRMVF caldera-related intrusions cooled and solidified soon after zircon crystallization, as magma supply waned. Some researchers interpret these results as recording pluton assembly in small increments that crystallized rapidly, leading to temporal disconnects between ignimbrite eruption and intrusion growth. Alternatively, crystallization ages of the granitic rocks are here inferred to record late solidification, after protracted open-system evolution involving voluminous mantle input, lengthy residence (105–106 yr) as near-solidus crystal mush, and intermittent separation of liquid to supply volcanic eruptions. The compositions of the least-evolved ignimbrite magmas tend to merge with those of caldera-related plutons, suggesting that the plutons record nonerupted parts of long-lived cogenetic magmatic systems, variably modified prior to final solidification. Precambrian-source zircons are scarce in caldera plutons, in contrast to their abundance in some peripheral waning-stage intrusions of the SRMVF, implying dissolution of inherited crustal zircon during lengthy magma assembly for the ignimbrite eruptions and construction of a subvolcanic batholith. Broad age spans of zircons (to several million years) from individual samples of some ignimbrites and intrusions, commonly averaged and interpreted as “intrusion-emplacement age,” alternatively provide an incomplete record of intermittent crystallization during protracted incremental magma-body assembly, with final solidification only when the system began to wane. Analyses of whole zircons cannot resolve late stages of crystal growth, and early growth in a long-lived magmatic system may be poorly recorded due to periods of zircon dissolution. Overall, construction of a batholith can take longer than recorded by zircon-crystallization ages, while the time interval for separation and shallow assembly of eruptible magma may be much shorter. Magma-supply estimates (from ages and volcano-plutonic volumes) yield focused intrusion-assembly rates sufficient to generate ignimbrite-scale volumes of eruptible magma, based on published thermal models. Mid-Tertiary processes of batholith assembly associated with the SRMVF caused drastic chemical and physical reconstruction of the entire lithosphere, probably accompanied by asthenospheric input.

Crystal-poor versus crystal-rich ignimbrites: A competition between stirring and reactivation

Ignimbrites, providing unique windows into magma reservoirs prior to explosive volcanic eruptions, are of two main types: (1) crystal-rich dacites, and (2) dominantly crystal-poor rhyolites. Crystal-rich dacites are typically homogeneous, while crystal-poor ignimbrites can display strong gradients in composition and crystallinity. This presents a conundrum, as the more viscous, crystal-rich units should be less prone to stirring and mixing. As ignimbrites typically erupt following a reheating event induced by recharge from below, this dichotomy reflects the competition between two time scales: (1) a thermal reactivation time scale that measures the time necessary to make a locked crystal mush rheologically eruptible (<50% crystals), and (2) a homogenization time scale associated with convective stirring. Using a well-constrained thermo-mechanical model of a magma reservoir, we show that the reactivation time scale of locked mushes is much greater than the time necessary to homogenize reservoirs by convective stirring. Hence, crystal-rich units, which require a reactivation stage, are inevitably well stirred. In contrast, crystal-poor magmas are rheologically ready to be mobilized without reactivation and need not be thoroughly mixed prior to eruption. This model provides an integrated picture of upper crustal reservoirs and has major implications for the link between shallow plutonic and volcanic rocks.

Synchrotron X-ray microtomography and lattice Boltzmann simulations of gas flow through volcanic pumices

To illustrate the advances made in permeability calculations combining X-ray microtomography and lattice Boltzmann method simulations, a sample suite of different types of pumices was investigated. Large three-dimensional images at high spatial resolution were collected at three different synchrotron facilities (Elettra, SLS, and ESRF). Single phase gas flow simulations were done on computer clusters with a highly parallelized lattice Boltzmann code, named Palabos. Permeability measurements obtained by gas flow simulation were compared to lab measurements of pumices produced by the Kos Plateau Tuff eruption to validate the method. New permeability data for pumices from other silicic volcanic deposits is presented, and an empirical model for permeability is tested using geometrical and topological data, i.e., tortuosity, specific surface area, and total and connected porosity.

Cumulate fragments in silicic ignimbrites: The case of the Snake River Plain

Large silicic ignimbrites commonly erupt from compositionally variable reservoirs. Although ignimbrite compositional architecture is often consistent with evacuation of a single zoned magma body, other examples are better interpreted as the products of amalgamation of multiple discrete subvolcanic melt-rich lenses. For example, multiple populations of pyroxene crystals and glass fragments within single ignimbrites from the central Snake River Plain (Idaho, USA) support the multibatch model. This presents a conundrum in terms of magma generation and storage; if the crystal-poor silicic magma batches are not generated nearly in situ in the upper crust, they must traverse, and reside within, a thermally hostile environment with large temperature gradients, resulting in low survivability in their shallow magmatic hearths. Ubiquitous crystal aggregates in central Snake River Plain rhyolites hint at another model. These aggregates contain the same plagioclase, pyroxene, and oxide mineral compositions as single phenocrysts of the same minerals in their host rocks, but they have significantly less silicic bulk compositions and lack quartz and sanidine, which occur as single phenocrysts in the deposits. These observations imply significant crystallization followed by melt extraction from mushy margins of the magma reservoirs. The extracted melt then pools and continues to evolve (crystallizing sanidine and quartz) while the melt-depleted walls and/or floors provide an increasingly rigid and refractory network segregating the crystal-poor batches of magma. Such hot refractory margins insulate the crystal-poor lenses, allowing (1) extended residence in the upper crust, and (2) preservation of chemical heterogeneities among batches. In contrast, systems that produce cumulates richer in low-temperature phases (quartz, K-feldspars, and/or biotite) can melt extensively upon recharge, leading to less segregation of eruptible melt pockets and the formation of gradationally zoned ignimbrites.

Pre-eruptive reheating during magma mixing at Quizapu volcano and the implications for the explosiveness of silicic arc volcanoes

Effusive-explosive transitions are observed in volcanoes erupting water-rich magmas that lack mineralogical evidence of degassing, suggesting that gas loss occurred rapidly and ascent was fast despite the high viscosity of rhyodacitic to rhyolitic melts. Here, we show that pre-eruptive heating of water-rich, silicic magma by mixing with hot recharge in subvolcanic chambers controls the effusive-explosive transition. Volcan Quizapu (Chile) emitted a 5 km3 mingled dacite-andesite lava flow in A.D. 1846–1847 that was ∼130 °C hotter than the 5 km3 of the same dacite that lead to a Plinian eruption in A.D. 1932. We explain the suppression of explosive fragmentation in the 1846–1847 lavas by enhanced syneruptive magma degassing as a consequence of late reheating following mixing with andesite recharge. Higher magma temperatures led to the transitioning into effusive eruptive behavior by significantly accelerating volatile diffusion and lowering melt viscosity, which facilitated bubble nucleation, growth, and coalescence, while also inhibiting brittle fragmentation. Thus, recharge by mafic magmas in subvolcanic magma chambers may reduce the risk of explosive eruptions in cases where heat can be efficiently transferred to cold, volatile-rich silicic magmas.

Bubble accumulation and its role in the evolution of magma reservoirs in the upper crust

Volcanic eruptions transfer huge amounts of gas to the atmosphere. In particular, the sulfur released during large silicic explosive eruptions can induce global cooling. A fundamental goal in volcanology, therefore, is to assess the potential for eruption of the large volumes of crystal-poor, silicic magma that are stored at shallow depths in the crust, and to obtain theoretical bounds for the amount of volatiles that can be released during these eruptions. It is puzzling that highly evolved, crystal-poor silicic magmas are more likely to generate volcanic rocks than plutonic rocks. This observation suggests that such magmas are more prone to erupting than are their crystal-rich counterparts. Moreover, well studied examples of largely crystal-poor eruptions (for example, Katmai, Taupo and Minoan) often exhibit a release of sulfur that is 10 to 20 times higher than the amount of sulfur estimated to be stored in the melt. Here we argue that these two observations rest on how the magmatic volatile phase (MVP) behaves as it rises buoyantly in zoned magma reservoirs. By investigating the fluid dynamics that controls the transport of the MVP in crystal-rich and crystal-poor magmas, we show how the interplay between capillary stresses and the viscosity contrast between the MVP and the host melt results in a counterintuitive dynamics, whereby the MVP tends to migrate efficiently in crystal-rich parts of a magma reservoir and accumulate in crystal-poor regions. The accumulation of low-density bubbles of MVP in crystal-poor magmas has implications for the eruptive potential of such magmas, and is the likely source of the excess sulfur released during explosive eruptions.