Josef Dufek

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.

Snake River Plain – Yellowstone silicic volcanism: implications for magma genesis and magma fluxes

Abstract The origin of large-volume, high-temperature silicic volcanism associated with onset of the Snake River Plain – Yellowstone (SRPY) hotspot track is addressed based on evolution of the well-characterized Miocene Bruneau–Jarbidge (BJ) eruptive centre. Although O–Sr–Pb isotopic and bulk compositions of BJ rhyolites exhibit strong crustal affinity, including strong 18O-depletion, Nd isotopic data preclude wholesale melting of ancient basement rocks and implicate involvement of a juvenile component – possibly derived from contemporaneous basaltic magmas. Several lines of evidence, including limits on 18O-depletion of the rhyolite source rocks due to influx of meteoric/hydrothermal fluids, constrain rhyolite generation to depths shallower than mid-upper crust (<20 km depth). For crustal melting driven by basaltic intrusions, sustenance of temperatures exceeding 900 °C at such depths over the life of the BJ eruptive centre requires incremental intrusion of approximately 16 km of basalt into the crust. This minimum basaltic flux (c. 4 mm year−1) is about one-tenth that at Kilauea. Nevertheless, emplacement of such volumes of magma in the crust creates a serious room problem, requiring that the crust must undergo significant extensional deformation – seemingly exceeding present estimates of extensional strain for the SRPY province.

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.

Dynamics and deposits generated by the Kos Plateau Tuff eruption: Controls of basal particle loss on pyroclastic flow transport

The explosive eruption of voluminous silicic magmas often produces widespread and massive deposits formed from pyroclastic density currents. While these punctuated events dramatically alter the landscape and have potential climate-altering impact, our understanding of the internal structure and transport dynamics of these eruptions is hampered by a lack of direct observations. We utilize the natural boundary conditions encountered by the eruption of the Kos Plateau Tuff to probe its internal structure as well as constrain the neotectonic activity in the region and eruption duration of this moderate to large (>60 km3) event. At the time of the eruption, 161 ka, the lower sea level in the Mediterranean may have resulted in flows that traversed mostly land to the north of the eruptive vent, while flows to the south may have encountered an expanse of water. Steep topography and overwater transport can impede the transport of the dense basal portions of the flow where particles make multiple or sustained contact with the bed. We use an Eulerian-Eulerian-Lagrangian computational approach coupled with overwater and overland boundary conditions, including topography, to determine the role of bed load versus suspended load in the transport of these flows. We find that a ring vent structure and eruptive fluxes greater than ∼2 × 106 m3/s are required for the spatial distribution of the KPT. The maximum grain size and deposit locations of the first voluminous ignimbrite unit (D) can be explained by suspended flow to the south, consistent with overwater transport, and bed load and suspended load transport to the north, consistent with overland transport. However, the maximum lithic size for the largest and last ignimbrite unit (E) requires some bed load transport in all directions. We propose that the boundary conditions were significantly altered during the course of the eruption, through either the in-filling of a shallow sea to the south or the development of a thick pumice raft to aid saltation. On the basis of the inferred eruptive flux, we estimate that the duration of the eruption climax, in which most of the material was erupted, likely only lasted from a few hours to a day.

226Ra/230Th excess generated in the lower crust: Implications for magma transport and storage time scales

Excesses of 226 Ra in arc magmas have been interpreted as resulting from flux melting of the mantle above subducting slabs, followed by fast ascent rates of magma from slab to surface (up to 1000 m/yr). However, we demonstrate that incongruent melting of the lower crust could either maintain or augment mantle-derived 226 Ra excesses, and so reduce inferred vertical transport rates. We developed an incongruent, continuous melting model, and both the incongruent melting reaction and ingrowth effects contribute to the 226 Ra excess. In particular, we found that dehydration melting of amphibolite can produce modeled 226 Ra excesses greater than 300%. Mixtures of such amphibolite dehydration melts with mantle melts will likely retain a 238 U excess (subducted slab) signature. This amphibolite dehydration melting process will also produce elevated ratios of light rare earth elements to heavy rare earth elements, similar to those observed in several arc settings, which may distinguish these magmas from those with 226 Ra excesses produced by slab dewatering alone.

Ina pit crater on the Moon: Extrusion of waning-stage lava lake magmatic foam results in extremely young crater retention ages

The enigmatic Ina feature on the Moon was recently interpreted to represent extrusive basaltic volcanic activity within the past 100 m.y. of lunar history, an extremely young age for volcanism on the Moon. Ina is a 2 × 3 km D-shaped depression that consists of a host of unusual bleb-like mounds surrounded by a relatively optically fresh hummocky and blocky floor. Documentation of magmatic-volcanic processes from shield volcano summit pit craters in Hawai’i and new insights into shield-building and dike evolution processes on the Moon provide important perspectives on the origin of Ina. We show that the size, location, morphology, topography, and optical maturity of Ina are consistent with an origin as a subsided summit pit crater lava lake on top of a broad ~22-km-diameter, ~3.5-b.y.-old shield volcano. New theoretical treatments of lunar shield-building magmatic dike events predict that waning-stage summit activity was characterized by the production of magmatic foam in the dike and lake; the final stages of dike stress relaxation and closure cause the magmatic foam to extrude to the surface through cracks in the lava lake crust to produce the mounds. The high porosity of the extruded foams (>75%) altered the nature of subsequent impact craters (the aerogel effect), causing them to be significantly smaller in diameter, which could bias the crater-derived model ages. Accounting for this effect allows for significantly older model ages, to ~3.5 b.y., contemporaneous with the underlying shield volcano. Thus extremely young volcanic eruptions are not required to explain the unusual nature of Ina.

A new bubble dynamics model to study bubble growth, deformation, and coalescence

We propose a new bubble dynamics model to study the evolution of a suspension of bubbles over a wide range of vesicularity, and that accounts for hydrodynamical interactions between bubbles while they grow, deform under shear flow conditions, and exchange mass by diffusion coarsening. The model is based on a lattice Boltzmann method for free surface flows. As such, it assumes an infinite viscosity contrast between the exsolved volatiles and the melt. Our model allows for coalescence when two bubbles approach each other because of growth or deformation. The parameter (disjoining pressure) that controls the coalescence efficiency, i.e., drainage time for the fluid film between the bubbles, can be set arbitrarily in our calculations. We calibrated this parameter by matching the measured time for the drainage of the melt film across a range of Bond numbers (ratio of buoyancy to surface tension stresses) with laboratory experiments of a bubble rising to a free surface. The model is then used successfully to model Ostwald ripening and bubble deformation under simple shear flow conditions. The results we obtain for the deformation of a single bubble are in excellent agreement with previous experimental and theoretical studies. For a suspension, we observe that the collective effect of bubbles is different depending on the relative magnitude of viscous and interfacial stresses (capillary number). At low capillary number, we find that bubbles deform more readily in a suspension than for the case of a single bubble, whereas the opposite is observed at high capillary number.

The Syrtis Major volcano, Mars: A multidisciplinary approach to interpreting its magmatic evolution and structural development

Very weak crustal magnetic fields over the Syrtis Major volcanic complex imply almost total thermal demagnetization via magmatic intrusions over a large area less than ~4 Ga. We fit a model of these intrusions and the resulting thermal demagnetization to maps of crustal magnetic field strength at 185 km altitude. The best fits are most consistent with a “dog bone”-shaped region of intrusive material, elongated approximately north-south, with an area of ~350,000 km2 and an inferred volume of ~4–19 × 106 km3. Such a large volume is best explained by a long-lived mantle plume beneath the Syrtis edifice. A free-air gravity anomaly high over the Syrtis Major caldera is consistent with dense mafic residue remaining at depth following crystal fractionation that produced the silicic magmas seen at the surface. The elongation of this region is consistent with ascent and north-south emplacement of magma enabled by structures parallel to and associated with the preexisting Isidis impact basin.

The effects of dynamics on the triboelectrification of volcanic ash

Lightning is often observed during explosive volcanic eruptions, and the charging processes associated with these displays have been attributed to several mechanisms. In this work we delineate a set of experiments designed to quantify silicate-based triboelectric charging in the volcanic context. Using natural samples from three different volcanoes, we show that the rate of triboelectrification in a fluidized bed depends on the energy input into the granular system. Experiments are conducted employing nonintrusive electrostatic sensors, ensuring that all charge exchange arises solely from particle-particle collisions. At higher fluidization energies, particles undergo more frequent and energetic collisions, facilitating the transfer of charge. This finding implies that triboelectric charging could help promote charging in regions of the eruptive system that contain numerous particle-particle collisions such as the conduit and gas thrust regions. Our experiments also suggest that surface charge density is capped, at least in part, by atmospheric conditions, specifically the breakdown characteristics of the gas.

Pyroclastic Density Currents: Processes and Models

Pyroclastic density currents (PDCs) are gravity currents produced by the collapse and lateral spreading of particle-laden mixtures produced during volcanic eruptions. PDCs encompass a vast array of physical processes operating at many scales of motion from the grain scale to the scale of the whole current, and these currents are widely regarded as some of the most hazardous volcanic phenomena for populations near volcanic edifices due to their rapid propagation along the ground and high temperatures. In this chapter we review the physical processes operating inside PDCs and how these processes govern the current dynamics and eventual deposits.