Eruption risks from covert silicic magma bodies

Unintentional encounters with silicic magma at ~2–2.5 km depth have recently occurred during drilling at three volcanoes: Kilauea (Hawaii), Menengai (Kenya), and Krafla (Iceland). Geophysical surveys had failed to warn about shallow magma before each encounter, and subsequent surveys at Krafla have been unable to resolve the size or architecture of its silicic magma body. This presents a conundrum for volcano monitoring: Do such shallow "covert" magma bodies pose an eruption risk? Here, we show that Krafla's most recent explosive eruption, a mixed hydrothermal-magmatic event in 1724 C.E. that formed the Víti maar, involved rhyolite essentially indistinguishable in composition from magma encountered during drilling in 2009. Streaks of quenched basalt in some Víti pumices provide direct evidence for interaction between co-erupted rhyolitic and basaltic magmas, but crystals in these pumices show no evidence for late-stage heating or re-equilibration with more mafic melt, implying mixing time scales of at most several hours. Covert silicic magma thus presents an eruption risk at Krafla and may be mobilized with little warning. Difficulties in resolving magma bodies smaller than ~1 km3 with geophysical surveys mean that covert silicic magma may exist at many other volcanoes and should be considered in hazard and risk assessments

What lies beneath? Reconstructing the primitive magmas fueling voluminous silicic volcanism using olivine-hosted melt inclusions

Understanding the origins of the mantle melts that drive voluminous silicic volcanism is challenging because primitive magmas are generally trapped at depth. The central Taupō Volcanic Zone (TVZ; New Zealand) hosts an extraordinarily productive region of rhyolitic caldera volcanism. Accompanying and interspersed with the rhyolitic products, there are traces of basalt to andesite preserved as enclaves or pyroclasts in caldera eruption products and occurring as small monogenetic eruptive centers between calderas. These mafic materials contain MgO-rich olivines (Fo79–86) that host melt inclusions capturing the most primitive basaltic melts fueling the central TVZ. Olivine-hosted melt inclusion compositions associated with the caldera volcanoes (intracaldera samples) contrast with those from the nearby, mafic intercaldera monogenetic centers. Intracaldera melt inclusions from the modern caldera volcanoes of Taupō and Okataina have lower abundances of incompatible elements, reflecting distinct mantle melts. There is a direct link showing that caldera-related silicic volcanism is fueled by basaltic magmas that have resulted from higher degrees of partial melting of a more depleted mantle source, along with distinct subduction signatures. The locations and vigor of Taupō and Okataina are fundamentally related to the degree of melting and flux of basalt from the mantle, and intercaldera mafic eruptive products are thus not representative of the feeder magmas for the caldera volcanoes. Inherited olivines and their melt inclusions provide a unique “window” into the mantle dynamics that drive the active TVZ silicic magmatic systems and may present a useful approach at other volcanoes that show evidence for mafic recharge

Tracking Magma‐Crust‐Fluid Interactions at High Temporal Resolution: Oxygen Isotopes in Young Silicic Magmas of the Taupō Volcanic Zone

Abstract Oxygen isotopes are useful for tracing interactions between magmas, crustal rocks and surface‐derived waters. We use them here to consider links between voluminous silicic magmatism and large‐scale hydrothermal circulation in New Zealand's central Taupō Volcanic Zone (TVZ). We present >350 measurements of plagioclase, quartz, hornblende and groundmass glass δ18O values from 40 eruptions from three discrete magmatic systems: Ōkataina and Taupō calderas, and the smaller Northeast Dome system. For each mineral, mean δ18O values vary by ∼1‰ (δ18Oplag = +6.7–7.8‰, δ18Oqtz = +7.7–+8.7‰, δ18Ohbl = +5.4–+6.4‰, δ18Oglass = +7.1–+8.0‰), and inter‐mineral fractionations mostly reflect high‐temperature equilibria. Outliers (e.g., ∼+6‰ or >+10‰ plagioclase) represent contaminants incorporated on short‐enough timescales to preserve disequilibrium (∼102 yrs for plagioclase). Melt δ18O values calculated from phenocrysts are ∼+7.3–+8.0‰. Where multiple magmas were involved in the same eruption their δ18Omelt values are indistinguishable, implying that their parental mushes were isotopically well‐mixed. However, small (≤0.5‰) but consistent δ18Omelt value gradients occur over millennial timescales at Ōkataina and Taupō, with short‐term ∼0.4–0.5‰ decreases in δ18Omelt values over successive post‐caldera eruptions correlating with increases in 87Sr/86Sr. These changes reflect tens of percent assimilation of a mixture of hydrothermally altered silicic plutonic material and higher‐87Sr/86Sr greywacke. These examples represent the first evidence for assimilation of altered crust into TVZ magmas. The subtle and short‐lived isotopic signals of these interactions are only recognized through the high temporal resolution of the TVZ eruptive record and complementary radiogenic isotope data. Similar interactions may have been obscured in other nominally high‐ or normal‐δ18O magmatic systems

The permeability of loose magma mush

Models for the evolution of magma mush zones are of fundamental importance for understanding magma storage, differentiation in the crust, and melt extraction processes that prime eruptions. These models require calculations of the permeability of the evolving crystal frameworks in the mush, which influences the rate of melt movement relative to crystals. Existing approaches for estimating the crystal framework permeability do not account for crystal shape. Here, we represent magma mush crystal frameworks as packs of hard cuboids with a range of aspect ratios, all at their maximum random packing. We use numerical fluid flow simulation tools to determine the melt fraction, specific surface area, and permeability of our three-dimensional digital samples. We find that crystal shape exerts a first-order control both on the melt fraction at maximum packing and on the permeability. We use these new data to generalize a Kozeny-Carman model in order to propose a simple constitutive law for the scaling between permeability and melt fraction that accounts for crystal shape in upscaled mush dynamics simulations. Our results show that magma mush permeability calculated using a model that accounts for crystal shape is significantly different compared with models that make a spherical crystal approximation, with key implications for crustal melt segregation flux and reactive flow

Rifting and recharge as triggers of the mixed basalt-rhyolite Halarauður ignimbrite eruption (Krafla, Iceland)

International audienceWe present a petrologic study of the ca. 110 ka Halarauður eruption (7 ± 6 km3 magma), associated with collapse of Krafla caldera in northeast Iceland. Whole-rock compositions of juvenile Halarauður products span a continuous range between quartz tholeiite basalt (50.0 wt% SiO2, 5.0 wt% MgO; Mg# 42) and rhyolite (74.6 wt% SiO2). Linear correlations between all major elements are consistent with two-component mixing of sub-equal volumes of these end-member magmas, whereas correlations between trace elements are influenced by diffusive fractionation during chaotic mixing. Evolved compositions (andesite to rhyolite) and compositional heterogeneity are typical of early-erupted units, reflecting tapping of the upper, more silicic regions of a compositionally heterogeneous reservoir undergoing chaotic mixing. Later-erupted deposits are more compositionally homogeneous and grade smoothly upward from andesite to basalt, reflecting tapping of denser hybrid magma and uncontaminated basalt from lower in the chamber. All erupted products host 60-76) and augite (Mg# 68-75) macro-crysts crystallized from this basalt during shallow storage, while sparse glomerocrysts (plagioclase ± augite ± olivine ± orthopyroxene) in late-erupted basaltic material are derived from disaggregated cumulate mush and include more primitive compositions. Occasional narrow sodic rims on plagioclase crystals from the quartz tholeiite record short periods of re-equilibration with hybrid magmas during mixing, constrained by experimental growth rates as at most two months and possibly as short as tens of hours. A second population of calcic plagioclase (cores An83-91) with adhering primitive basaltic glass selvages (Mg# 53-59) occurs sparsely in deposits of the first eruptive phase and is scarce or absent in later-erupted units, providing evidence for eruption of a second, more primitive basalt that was of insufficient volume to skew whole-rock mixing trends. Nucleation delay models suggest that the absence of overgrowth rims or quench crystals in these glassy basaltic selvages reflect residence times of a few hours at rhyolitic temperatures before eruption. Short basalt-rhyolite mixing timescales reflect rapid destabilization of the magmatic system and triggering of the eruption by mafic recharge. The ascent of both primitive and evolved basaltic magmas from depth mirrors events in recent volcano-tectonic episodes in the north of Iceland, suggesting that mafic recharge was driven by a plate boundary rifting event

CALDERA: a scientific drilling concept to unravel Connections Among Life, geo-Dynamics and Eruptions in a Rifting Arc caldera, Okataina Volcanic Centre, Aotearoa New Zealand

Abstract. Silicic caldera volcanoes present major volcanic and seismic hazards but also host dynamic hydrothermal and groundwater systems and a rich but largely unexplored subsurface biosphere. Many of these volcanoes are hosted in rift settings. The intricate connections and feedbacks among magmatism, rifting, hydrothermal processes, and the biosphere in these complex systems remain poorly understood, necessitating subsurface joint observations that are only enabled by scientific drilling. The CALDERA (Connections Among Life, geo-Dynamics and Eruptions in a Rifting Arc caldera) project workshop funded by the International Continental Scientific Drilling Program (ICDP) gathered multi-disciplinary international experts in January 2023 to advance planning of a scientific drilling project within one of these dynamic, rift-hosted calderas, the Okataina Volcanic Centre (OVC), Aotearoa New Zealand. The OVC's high eruption rate, frequent unrest events and earthquake swarms, location in a densely faulted rapidly extending rift, abundant groundwater–geothermal fluid circulations, and diverse surface hot spring microbiota make it an ideal location for exploring a connected geo-hydro-biosphere via scientific drilling and developing a test bed for novel volcano monitoring approaches. Drilling configurations with at least two boreholes (∼ 200 and ∼ 1000–1500 m deep) were favoured to achieve the multi-disciplinary objectives of the CALDERA project. Decadal monitoring including biosphere activity and composition has the potential to evaluate the response of the hydro-bio system to volcano-tectonic activity. In addition to the OVC caldera-scale datasets already available, site surveys will be conducted to select the best drilling locations. The CALDERA project at the OVC would provide, for the first time, an understanding of volcanic–tectonic–hydrological–biological connections in a caldera–rift system and a baseline for global comparisons with other volcanoes, rifts, and hydrothermal systems. CALDERA would serve as an unprecedented model system to understand how and how quickly the subsurface biosphere responds to geologic activities. Discoveries will improve assessment of volcanic and seismic hazards, guide the sustainable management and/or conservation of groundwater and geothermal resources and microbial ecosystems, and provide a forum for interweaving mātauranga Māori and Western knowledge systems. </jats:p