C. B. Till

Months between rejuvenation and volcanic eruption at Yellowstone caldera, Wyoming

Rejuvenation of previously intruded silicic magma is an important process leading to effusive rhyolite, which is the most common product of volcanism at calderas with protracted histories of eruption and unrest such as Yellowstone caldera (Wyoming), Long Valley caldera (California), and Valles caldera (New Mexico) in the United States. Although orders of magnitude smaller in volume than rare caldera-forming supereruptions, these relatively frequent effusions of rhyolite are comparable to the largest eruptions of the 20th century, and pose a considerable volcanic hazard. However, the physical pathway from rejuvenation to eruption of silicic magma is unclear, particularly because the time between reheating of a subvolcanic intrusion and eruption is poorly quantified. This study uses nanometer-scale trace element diffusion in sanidine crystals to reveal that rejuvenation of a near-solidus or subsolidus silicic intrusion occurred in ∼10 mo or less following a protracted period (220 k.y.) of volcanic repose, and resulted in effusion of ∼3 km3 of high-silica rhyolite lava at the onset of Yellowstone’s last volcanic interval. The future renewal of effusive silicic volcanism at Yellowstone will likely require a comparable energetic intrusion of magma that remelts the shallow subvolcanic reservoir and generates eruptible rhyolite on month to annual time scales.

Rapid cooling and cold storage in a silicic magma reservoir recorded in individual crystals

Taupo Volcanic Zone magma spent more than 90% of its life deep and crystalline before rapid shallow accumulation and eruption. Quick eruption after a long bake Minerals such as zircon can record the storage conditions of magma before volcanic eruption. Rubin et al. combined traditional 238U-230Th dating with lithium concentration profiles in seven zircons from the Taupo supervolcanic complex in New Zealand to determine magma storage conditions. The zircons spent more than 90% of their lifetime in an uneruptible, mostly crystalline, and deep magmatic reservoir. The zircons were eventually transported to hotter, shallower, and eruptible magma bodies, where they spent only decades to hundreds of years before eruption. The result suggests a two-stage model for magmatic systems with large thermal variations. Science, this issue p. 1154 Silicic volcanic eruptions pose considerable hazards, yet the processes leading to these eruptions remain poorly known. A missing link is knowledge of the thermal history of magma feeding such eruptions, which largely controls crystallinity and therefore eruptability. We have determined the thermal history of individual zircon crystals from an eruption of the Taupo Volcanic Zone, New Zealand. Results show that although zircons resided in the magmatic system for 103 to 105 years, they experienced temperatures >650° to 750°C for only years to centuries. This implies near-solidus long-term crystal storage, punctuated by rapid heating and cooling. Reconciling these data with existing models of magma storage requires considering multiple small intrusions and multiple spatial scales, and our approach can help to quantify heat input to and output from magma reservoirs.

A review and update of mantle thermobarometry for primitive arc magmas

Abstract Erupted lavas and tephras remain among the best tools we have to ascertain the mantle processes that give rise to the compositional diversity and spatial distribution of near-primary magmas at volcanic arcs. A compilation of mantle-melt thermobarometry for natural, primitive arc magmas to date reveals published estimates vary between ∼1000−1600 °C at ∼6–50 kbar. In addition to the variability of mantle melting processes within and between different arcs, this range of conditions is the result of different methodology, such as the nature of reverse fractional crystallization calculations, the choice of thermobarometer, how magmatic H2O was quantified and its calculated effect on pressure and temperature, and choices about mantle lithology and oxygen fugacity. New and internally consistent reverse fractionation calculations and thermobarometry for a representative subset of the primitive arc samples with adequate published petrography, measured mineral and melt compositions, and constraints on pre-eruptive H2O content suggest a smaller range of global mantle-melt equilibration conditions (∼1075−1450 °C at ∼8−19 kbar) than the literature compilation. The new pressure and temperature estimates and major element modeling are consistent with a model whereby several types of primitive arc magmas, specifically hydrous calc-alkaline basalt, primitive andesite and hydrous high-MgO liquid such as boninite, first form at the location of the water-saturated mantle solidus at pressures of ∼20−35 kbar and rise into the hot core of the mantle wedge reacting with the mantle en route. Due to their re-equilibration during ascent, these hydrous magmas ultimately record the conditions in the hot, shallow nose of the mantle wedge at the end of their mantle ascent path rather than the conditions at their point of origin as often interpreted. When the mantle residue for this process is lherzolite, calcalkaline basalt is generated. When the mantle residue is harzburgite to dunite, either high-Mg primitive andesite or high-MgO liquid is generated, depending on the H2O content. A different type of primitive arc magma, specifically nominally anhydrous arc tholeiite, is generated by near-fractional decompression melting at or near the anhydrous lherzolite solidus in the upwelling back limb of corner flow at ∼25−10 kbar and is focused into the same region of the shallow mantle wedge as the hydrous melts. The similarity in the terminus of the mantle ascent paths for both wet and dry primitive arc magmas likely explains their eruption in close spatial and temporal proximity at many arcs. The conditions of last mantle equilibration for primitive arc tholeiites generated by decompression melting also imply that the convecting mantle extends to 10 kbar (∼30 km) or less below most arcs. The range of mantle-melt equilibration conditions calculated here agrees well with the range of temperatures predicted for the shallow mantle wedge beneath arcs by geodynamic models, although it suggests some subduction zones may have higher maximum temperatures at shallower depths in the wedge than originally predicted. Primitive hydrous arc magmas also constrain natural variation on the order of 200−250 °C in the maximum temperature in the hot shallow nose of the mantle wedge between arcs. Thus the new primitive magma thermobarometry presented here is useful for understanding melt migration processes and the temperature structure in the uppermost part of the mantle wedge, as well as the origin of different primitive magma types at arcs.

Super-volcanic investigations

Multi-disciplinary analyses of Earth’s most destructive volcanic systems show that continuous monitoring and an understanding of each volcano’s quirks, rather than a single unified model, are key to generating accurate hazard assessments.

Response to Comment on “Rapid cooling and cold storage in a silicic magma reservoir recorded in individual crystals”

In a recent paper, we used Li concentration profiles and U-Th ages to constrain the thermal conditions of magma storage. Wilson and co-authors argue that the data instead reflect control of Li behavior by charge balance during partitioning and not by experimentally determined diffusion rates. Their arguments are based on (i) a coupled diffusion mechanism for Li, which has been postulated but has not been documented to occur, and (ii) poorly constrained zircon growth rates combined with the assumption of continuous zircon crystallization.

Melting the Earth's Upper Mantle

Abstract The magmas that erupt from volcanoes on Earth originate primarily through partial melting processes initiated in the Earths mantle. This chapter will discuss the structure and physical properties of the Earths upper mantle that influence magma generation, physical mechanisms leading to melting, and the chemical compositions of mantle melts.

Big Data and Quantifying Variability Top Scientific Trends List

How will big data change the way research is conducted? What kind of impact will an increasing focus on interdisciplinary and transdicisplinary science have on funding and management models? How is the need to quantify variability changing Earth and space sciences? These questions are indicative of just a few of the trends highlighted in a new report by AGU.