Ben S. Ellis

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.

Quantifying crystallization and devitrification of rhyolites by means of X-ray diffraction and electron microprobe analysis

Abstract Devitrification of silicic volcanic rocks is a relatively common process, resulting in the production of microcrystalline silica and feldspar components. Here we investigate how the products of pervasive devitrification may be characterized using the combined techniques of X-ray powder diffraction, electron microprobe analysis, and X-ray fluorescence analysis to provide a new calibrated approach to calculating the crystallinity and mineral modes in both glassy vitrophyre and devitrified volcanics. Using the integrated areas of the X-ray diffraction peaks associated with both the crystalline and amorphous components, the relative proportions of groundmass crystallites and amorphous material from both glassy and devitrified material can be calculated. A detailed calibration indicates a linear relationship among the ratio of the integrated counts and bulk crystallinity. Mineral proportions are also calculated from X-ray fluorescence measurements of whole-rock and groundmass separates and are well correlated to crystallinities calculated from both X-ray diffraction and electron microprobe image analysis for vitrophyre samples. Devitrification products in a pervasively devitrified sample are tridymite, quartz, sanidine, and a Ca-rich aluminosilicate component. Mineral analysis and X-ray mapping by electron microprobe analysis indicates that the Ca-rich aluminosilicate component appears to be the dominant metastable or amorphous phase in the devitrified sample with proportions calculated from X-ray mapping (~32%) in reasonable agreement with the calculated proportion of amorphous material determined by means of X-ray diffraction (~38%). These results demonstrate the robustness of this combined X-ray diffraction and electron microprobe imagery technique for quantifying and characterizing crystallization in complex samples.

Rhyolite Generation prior to a Yellowstone Supereruption: Insights from the Island Park–Mount Jackson Rhyolite Series

The Yellowstone volcanic field is one of the largest and best-studied centres of rhyolitic volcanism on Earth, yet it still contains little-studied periods of activity. Such an example is the Island Park–Mount Jackson series, which erupted between the Mesa Falls and Lava Creek caldera-forming events as a series of rhyolitic domes and lavas. Here we present the first detailed characterisation of these lavas and use our findings to provide a framework for rhyolite generation in Yellowstone between 1·3 and 0·6 Ma, as well as to assess whether magmatic evolution hints at a forthcoming super-eruption. These porphyritic (15–40% crystals) lavas contain mostly sanidine and quartz with lesser amounts of plagioclase (consistent with equilibrium magmatic modelling via rhyolite-MELTS) and a complex assemblage of mafic minerals. Mineral compositions vary significantly between crystals in each unit, with larger ranges than expected from a single homogeneous population in equilibrium with its host melt. Oxygen isotopes in quartz and sanidine indicate slight depletions (δ18Omagma of 5·0–6·1‰), suggesting some contribution by localised remelting of hydrothermally altered material in the area of the previous Mesa Falls Tuff-related caldera collapse. The preservation of variable O isotopic compositions in quartz requires crystal entrainment less than a few thousand years prior to eruption. Late entrainment of rhyolitic material is supported by the occurrence of subtly older sanidines dated by single-grain 40Ar/39Ar geochronology. The eruption ages of the lavas show discrete clusters illustrating that extended quiescence (>100 kyr) in magmatic activity may be a recurring feature in Yellowstone volcanism. Ubiquitous crystal aggregates, dominated by plagioclase, pyroxene and Fe–Ti oxides, are interpreted as cumulates co-erupted with their extracted liquid. Identical crystal aggregates are found in both normal-δ18O and low-δ18O rocks from Yellowstone, indicating that common petrogenetic processes characterise both volcanic suites, including the late-stage extraction of melt from an incrementally built upper crustal mush zone.

Strontium isotopes and magma dynamics: Insights from high-temperature rhyolites

Volcanic rocks often exhibit internal heterogeneity in radiogenic isotopes. Isotopic disequilibrium between coexisting phenocrysts and isotopic zoning within single crystals has been demonstrated in basalts, andesites, dacites, and rhyolites. High-temperature Snake River–type rhyolites appear to be an exception. Despite the occurrence of Snake River Plain rhyolites in a region of isotopically highly variable crust and mantle, and significant differences from rhyolite unit to rhyolite unit, little to no Sr isotopic zoning is found within their feldspar phenocrysts, and feldspars within a single unit define tightly grouped unimodal populations. High-temperature rhyolitic magmas possess a unique combination of temperature and melt viscosity. Although typically 200 °C hotter than common rhyolites, the temperature effect on viscosity is offset by lower water contents ( 5 –10 6 Pa s). However, magmatic temperatures are in the same range as basaltic andesites and andesites; consequently, cation diffusion rates are orders of magnitude greater than common rhyolites. We hypothesize that this combination of characteristics promotes crystal isotopic homogeneity: viscosities are too high to permit crystal transfer between liquids of contrasting 87 Sr/ 86 Sr on time scales shorter than those required for diffusive homogenization of Sr within phenocrysts (500–10,000 yr). This is not true for most magma types, in which crystal transfer is rapid ( 5 –10 6 yr).

Post-caldera Volcanism at the Heise Volcanic Field: Implications for Petrogenetic Models

The Heise volcanic field is the second youngest caldera complex of the Yellowstone–Snake River Plain province (USA) and represents a polycyclic caldera system with rhyolitic volcanism extending over more than 2 Myr. The products of the Heise volcanic field include four regionally extensive ignimbrites, including the Blacktail Creek and Kilgore tuffs, which both have volumes estimated at >1000 km, separated by sequences of smaller volume tuffs, lavas and sedimentary deposits. Rhyolites from the Heise volcanic field are both normal-dO and low-dO, making it a key locality for investigating rhyolite petrogenesis. However, the occurrence of abundant young basaltic lava has limited our ability to fully characterise this volcanic centre, particularly in terms of post-caldera volcanism. Here we describe rhyolitic samples from both a >700 m thick section of drillcore within the Snake River Plain and the exposed outflow stratigraphy on the margins of the plain. Based on a combination of bulk-rock and mineral geochemical, isotopic, and geochronological evidence, we conclude that the rhyolites from the drillcore are not exposed at the surface, nor are the surficial rhyolites found in the drillcore. High-precision isotope dilution thermal ionisation mass spectrometry U-Pb geochronology dates the rhyolite at the base of the drillcore to 4 0248 6 0 0011 Ma, 0 4 Myr younger than the youngest caldera-forming ignimbrite at Heise, the 4 48 Ma Kilgore Tuff, whereas U-Pb secondary ionisation mass spectrometry dates the uppermost portion of rhyolite in the drillcore to 3 86 6 0 19 Ma. The combined geochemistry and stratigraphic relations suggest that the drillcore penetrates the intracaldera stratigraphy. The intracaldera rhyolites are compositionally and mineralogically similar to the outflow stratigraphy with high-temperature magmas (>800 C) persisting for the full >3 Myr history of the Heise centre. The dO values of pyroxene, sanidine, and quartz from the unaltered drillcore samples are consistent with high-temperature equilibrium and return magma dO values that are low (4 1–6 0‰ based on DO melt–sanidine of 0 6‰) but somewhat higher than the value for the preceding Kilgore Tuff magma of 3 3‰. Buried deep within the drillcore are also hydrothermally altered rhyolites with bulk dO ranging from –3 5‰ to þ1 0‰ (SMOW) with complex X-ray diffraction spectra revealing the presence of epidote, quartz and chlorite. These altered samples are, however, not markedly different in bulk major or trace elemental geochemistry from the unaltered Heise rhyolites. Rhyolite-MELTS models using these hydrothermally altered samples as potential assimilants can reproduce the compositions, mineralogy, and crystallinity of the low-dO Kilgore Tuff with 40–50% assimilation while also satisfying the mass balance constrained on the basis of dO. These results support a cannibalisation model for Heise volcanism while highlighting that the lowest dO rhyolites may require large amounts of extremely O-depleted hydrothermally altered material available for assimilation.

Lithostratigraphic analysis and geochemistry of a vitric spatter-bearing ignimbrite: the Quaternary Adeje Formation, Cañadas volcano, Tenerife

The 1.5-Ma Adeje Formation in SW Tenerife contains an ignimbrite sheet with remarkable textural and chemical complexity. A basal Plinian pumice-fall layer is overlain by a partly welded compound ignimbrite in which phonolitic pumice lapilli and dense obsidian spatter rags with irregular, fluidal-shaped margins are supported in a poorly sorted tuff matrix. The lower ignimbrite flow-unit contains accretionary lapilli in its upper part, overlain by an ash-pellet-bearing fallout layer from a co-ignimbrite plume. The upper ignimbrite flow-unit comprises a locally welded massive lapilli-tuff that grades up into lithic breccia containing juvenile obsidian blocks and both cognate and vent-derived lithic blocks. Geochemically, the Adeje Formation shows two distinct juvenile populations that relate to crystal-poor and crystal-rich magma types. Crystal-rich juvenile clasts contain multiple compositions of ilmenite and magnetite, and crystal aggregates of bytownite (An79–86). The varied assemblage of juvenile clasts reflects an eruptive style that may have involved rapid changes in magma chamber pressure associated with caldera collapse, and possibly the disruption of a lava lake. The Adeje eruption started with a Plinian explosive phase that rained ash and pumice lapilli across SW Tenerife; followed by pyroclastic fountaining feeding density currents with explosive ejecta of juvenile glassy material producing the coarse, spatter-bearing ignimbrite facies. A short pause between pyroclastic density currents is recorded by the co-ignimbrite ash and pellet-fall bed. The climactic phase of the eruption probably involved caldera subsidence as recorded by a widespread massive heterolithic breccia.

The use of biotite trace element compositions for fingerprinting magma batches at Las Cañadas volcano, Tenerife

Accurate identification of individual volcanic events in the field is crucial for constraining eruption volumes and calculating recurrence intervals between eruptive episodes. Due to complexities of pyroclastic transport and deposition and intra-unit textural variability, such identification can be challenging. We present a novel method for fingerprinting ignimbrites via trace element chemistry (V, Co, Nb) in biotite by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Using samples from the alkaline magmatic series of Tenerife, we are able to demonstrate (1) clustering of previously characterized units into distinct, homogeneous groups based on V, Co, and Nb concentrations in biotite, despite the presence of extreme variation and zonation in other trace elements (Ba, Sr, Rb) that indicate complex petrogenetic processes, and (2) biotite compositions are similar throughout a deposit and relatively independent of stratigraphic height or juvenile clast texture (crystal-rich vs crystal-poor). Our results show that trace elements in biotite can be used to fingerprint eruptions and correlate geographically separated volcanic deposits, including those preserved in offshore turbidite records.

Post-eruptive mobility of lithium in volcanic rocks

To reflect magmatic conditions, volcanic rocks must retain their compositions through eruption and post-eruptive cooling. Mostly, this is the case. However, welded ignimbrites from the Yellowstone–Snake River Plain magmatic province reveal systematic modification of the lithium (Li) inventory by post-eruptive processes. Here we show that phenocrysts from slowly cooled microcrystalline ignimbrite interiors consistently have significantly more Li than their rapidly quenched, glassy, counterparts. The strong association with host lithology and the invariance of other trace elements indicate that Li remains mobile long after eruption and readily passes into phenocrysts via diffusion as groundmass crystallisation increases the Li contents of the last remaining melts. Li isotopic measurements reveal that this diffusion during cooling combined with efficient degassing on the surface may significantly affect the Li inventory and isotopic compositions of volcanic rocks. Utilisation of Li for petrogenetic studies is therefore crucially dependent on the ability to ‘see through’ such post-eruptive processes.Lithium, an increasingly economically important element, is also used to trace the cycling of materials through the Earth system. Here the authors show that post-eruptive processes such as degassing and groundmass crystallisation control the inventory of lithium in volcanic deposits.