ALBERT

Description: The Hideaway Park tuff is the only preserved extrusive volcanic unit related to the Red Mountain intrusive complex, which produced the world-class Henderson porphyry Mo deposit. Located within the Colorado Mineral Belt, USA, Henderson is the second largest Climax-type Mo deposit in the world, and is therefore an excellent location to investigate magmatic processes leading to Climax-type Mo mineralization. We combine an extensive dataset of major element, volatile, and trace element abundances in quartz-hosted melt inclusions and pumice matrix glass with major element geochemistry from phenocrysts to reconstruct the pre-eruptive conditions and the source and evolution of metals within the magma. Melt inclusions are slightly peraluminous topaz rhyolitic in composition and are volatile-charged (≤6 wt % H 2 O, ≤600 ppm CO 2 , ~0·3–1·0 wt % F, ~2300–3500 ppm Cl) and metal-rich (~7–24 ppm Mo, ~4–14 ppm W, ~21–52 ppm Pb, ~28–2700 ppm Zn, 〈0·1–29 ppm Cu, ~0·3–1·8 ppm Bi, ~40–760 ppb Ag, ~690–1400 ppm Mn). Melt inclusion and pumice matrix glass chemistry reveal that the Hideaway Park magma evolved by large degrees of fractional crystallization (≤60–70%) during quartz crystallization and melt inclusion entrapment at pressures of ≤300 MPa (≤8 km depth), with little to no crystallization upon shallow ascent and eruption. Filter pressing, crystal settling, magma recharge and mixing of less evolved rhyolite melt, and volatile exsolution were important processes during magma evolution; the low estimated viscosities (~10 5 –10 10 Pa s) of these H 2 O- and F-rich melts probably enhanced these processes. A noteworthy discrepancy between the metal contents in the pumice matrix glass and in the melt inclusions suggests that after quartz crystallization ceased upon shallow magma ascent and eruption, the Hideaway Park magma exsolved an aqueous fluid into which Mo, Bi, Ag, Zn, Mn, Cs, and Y strongly partitioned. Given that the Henderson deposit contains anomalous abundances of not only Mo, but also W, Pb, Zn, Cu, Bi, Ag, and Mn, we suggest that these metals were sourced from similar fluids exsolved from unerupted portions of the same magmatic system. Trace element ratios imply that Mo was sourced deep, from either the lower crust or metasomatized mantle. The origin of sulfur remains unresolved; however, given the extremely low S solubility of rhyolite melts in the shallow crust we favor the possibility that another source of S might supplement or account for that present in the ore deposit, probably the comagmatic, mantle-derived lamprophyres that occur in minor quantities with the voluminous topaz rhyolites in the area. To account for the 437 Mt of MoS 2 (~1·0 x 10 6 t Mo) present in the Henderson ore deposit, a volume of ~45 km 3 of Hideaway Park rhyolite magma would have been necessary to supply the Mo (a cylindrical pluton measuring 3·1 km x 6·0 km) along with sparging of ~6·8 x 10 5 t of S from ~0·05 km 3 of lamprophyre magma. Based on a weighted mean 40 Ar/ 39 Ar age of 27·58 ± 0·24 Ma, similar melt geochemistry, and characteristically F-rich biotite phenocrysts, we conclude that the Hideaway Park tuff was cogenetic with the intrusions at Red Mountain that formed the Henderson deposit.

Description: Arc magmas erupted at the Earth’s surface are commonly more oxidized than those produced at mid-ocean ridges. Possible explanations for this high oxidation state are that the transfer of fluids during the subduction process results in direct oxidation of the sub-arc mantle wedge, or that oxidation is caused by the effect of later crustal processes, including protracted fractionation and degassing of volatile-rich magmas. This study sets out to investigate the effect of disequilibrium crustal processes that may involve coupled changes in H 2 O content and Fe oxidation state, by examining the degassing and hydration of sulphur-free rhyolites. We show that experimentally hydrated melts record strong increases in Fe 3+ /Fe with increasing H 2 O concentration as a result of changes in water activity. This is relevant for the passage of H 2 O-undersaturated melts from the deep crust towards shallow crustal storage regions, and raises the possibility that vertical variations in f O 2 might develop within arc crust. Conversely, degassing experiments produce an increase in Fe 3+ /Fe with decreasing H 2 O concentration. In this case the oxidation is explained by loss of H 2 as well as H 2 O into bubbles during decompression, consistent with thermodynamic modelling, and is relevant for magmas undergoing shallow degassing en route to the surface. We discuss these results in the context of the possible controls on f O 2 during the generation, storage and ascent of magmas in arc settings, in particular considering the timescales of equilibration relative to observation as this affects the quality of the petrological record of magmatic f O 2 .

Description: Tephra and lava pairs from two summit eruptions ( ad 2008 and 1957) and a flank fissure eruption (~ ad 1850) are compared in terms of textures, phenocryst contents, and mineral zoning patterns to shed light on processes responsible for the shifts in eruption style during typical eruptive episodes at Volcán Llaima (Andean Southern Volcanic Zone, Chile). The mineralogy and whole-rock compositions of tephra and lavas are similar within eruptive episodes, suggesting a common magma reservoir for Strombolian paroxysms and lava effusion. The zoning profiles and textures of plagioclase record successive and discrete intrusions of volatile-rich mafic magma accompanied by mixing of these recharge magmas with the resident basaltic-andesitic crystal mushes that are commonly present at shallow levels in the Llaima system. Each recharge event destabilizes the plagioclase in equilibrium with the resident crystal mush melt and stabilizes relatively An-rich plagioclase, as is recorded by the numerous resorption zones. Lavas typically have ~15–20 vol. % more phenocrysts than the tephra. Differences in plagioclase and olivine textures and zoning, combined with different phenocryst contents, indicate that a greater volume fraction of recharge magma is present in the explosively erupted magma than in subsequent effusively erupted magma. We propose that Strombolian paroxysms at Volcán Llaima are triggered by interactions with large volume fractions of recharge magma, which decrease the bulk viscosity and increase the volatile contents of the erupted magmas, leading to the conditions required for the fragmentation of basaltic-andesite. Lava effusion ensues from reduced interactions with the recharge magma, after it has partially degassed and crystallized, thereby impeding rapid ascent. This process could be operating at other steady-state basaltic volcanoes, wherein shallow reservoirs are periodically refilled by fresh, volatile-rich magmas.

Description: The question of how the mineral layering in layered intrusions forms has been extensively debated for many decades. There are many types of layering and it is of course possible that a number of mechanisms are involved. Of particular interest is how chromite layers form, because these may contain valuable metals (such as platinum-group elements) in addition to Cr. One model for the formation of these layers is that they formed through slumping of semi-consolidated cumulates from the margins of the intrusion into the magma chamber. During this slumping, the grains are sorted by density and/or size differences. This study examines the viability of this process using analogue modelling. Starting materials (beads and glycerine) were scaled to match the density and size of the minerals (chromite, orthopyroxene and plagioclase) present in layered mafic–ultramafic layered intrusions and to match the density and viscosity of the silicate magma. A Perspex flume tank divided by a removable partition at one end was fully filled with glycerine. A homogenized mixture of the beads was placed in the smaller partition of the tank (representing the margins of the magma chamber). The tank was then inclined between 16° and 45°. The partition was removed and the beads flowed into the main part of the box. The experiments were recorded by video camera, allowing us to follow the dynamics of the flow during each run. Segregation of the beads was observed in the final deposits: the larger, less dense beads (representing plagioclase) concentrated at the top of the flow, with the intermediate-sized and medium density beads (representing orthopyroxene) in the middle and the smaller, denser beads (representing chromite) at the bottom, thus mimicking natural examples. In experiments where the angle of inclination was low, long, thin layers formed, such as those found in the Bushveld Complex. In experiments where the angle of inclination was high, thick but short layers formed. A dimensionless analysis allows better understanding of the dynamics of the flow. At the macroscopic scale, the flow regime is strongly influenced by the viscosity of the fluid and is considered macro-viscous, where the role of the interstitial liquid is non-negligible.

Description: Magma degassing, characterized by changes in permeability and porosity distribution, has a crucial control on the style of eruption. During ascent, magma might develop large porosities and crystallize while it is subjected to shear. Shear, in turn, enhances complex fabrics that result from the reorganization of the different phases (crystals, gas, melt). Such fabrics have not yet been evaluated experimentally on a three-phase system. We performed torsion experiments on a synthetic crystal-rich hydrous magma at subsolidus conditions with 11 vol% porosity to establish a link between strain partitioning and porosity redistribution. Crystals induce non-Newtonian deformation, resulting in localization of the shear strain. Three-dimensional microtomography and two-dimensional scanning electron microscope imaging show gas accumulation in local microstructures caused by shear-induced crystal fabric. Our data show that strain localization is a mechanism that could enable magma degassing at very low vesicularity.

Description: Magma dominantly exists in a slowly cooling crystal-rich or mushy state. Yet, observations of complexly zoned crystals, some formed in just one to ten years, as well as time-transgressive crystal fabrics imply that magmas mix and transition rapidly from a locked crystal mush to a mobile and eruptable fluid. Here we use a discrete-element numerical model that resolves crystal-scale granular interactions and fluid flow, to simulate the open-system dynamics of a magma mush. We find that when new magma is injected into a reservoir from below, the existing magma responds as a viscoplastic material: fault-like surfaces form around the edges of the new injection creating a central mixing bowl of magma that can be unlocked and become fluidized, allowing for complex mixing. We identify three distinct dynamic regimes that depend on the rate of magma injection. If the magma injection rate is slow, the intruded magma penetrates and spreads by porous media flow through the crystal mush. With increasing velocity, the intruded magma creates a stable cavity of fluidized magma that is isolated from the rest of the reservoir. At higher velocities still, the entire mixing bowl becomes fluidized. Circulation within the mixing bowl entrains crystals from the walls, bringing together crystals from different parts of the reservoir that may have experienced different physiochemical environments and leaving little melt unmixed. We conclude that both granular and fluid dynamics, when considered simultaneously, can explain observations of complex crystal fabrics and zoning observed in many magmatic systems. © 2015 Macmillan Publishers Limited. All rights reserved.