We report the first high-precision δ
18
O analyses of glass, δ
18
O of minerals, and trace element concentrations in glass and minerals for the 260–79 ka Central Plateau Member (CPM) rhyolites of ...Yellowstone, a >350 km
3
cumulative volume of lavas erupted inside of 630 ka Lava Creek Tuff (LCT) caldera. The glass analyses of these crystal-poor rhyolites provide direct characterization of the melt and its evolution through time. The δ
18
O
glass
values are low and mostly homogeneous (4.5 ± 0.14 ‰) within and in between lavas that erupted in four different temporal episodes during 200 ka of CPM volcanism with a slight shift to lower δ
18
O in the youngest episode (Pitchstone Plateau). These values are lower than Yellowstone basalts (5.7–6 ‰), LCT (5.5 ‰), pre-, and extracaldera rhyolites (~7–8 ‰), but higher than the earliest 550–450 ka post-LCT rhyolites (1–2 ‰). The glass δ
18
O value is coupled with new clinopyroxene analyses and previously reported zircon analyses to calculate oxygen isotope equilibration temperatures. Clinopyroxene records >900 °C near-liquidus temperatures, while zircon records temperatures <850 °C similar to zircon saturation temperature estimates. Trace element concentrations in the same glass analyzed for oxygen isotopes show evidence for temporal decreases in Ti, Sr, Ba, and Eu—related to Fe–Ti oxide and sanidine (±quartz) crystallization control, while other trace elements remain similar or are enriched through time. The slight temporal increase in glass Zr concentrations may reflect similar or higher temperature magmas (via zircon saturation) through time, while previosuly reported temperature decreases (e.g., Ti-in-quartz) might reflect changing Ti concentrations with progressive melt evolution. Multiple analyses of glass across single samples and in profiles across lava flow surfaces document trace element heterogeneity with compatible behavior of all analyzed elements except Rb, Nb, and U. These new data provide evidence for a three-stage geochemical evolution of these most recent Yellowstone rhyolites: (1) repeated batch melting events at the base of a homogenized low-δ
18
O intracaldera fill resulting in liquidus rhyolite melt and a refractory residue that sequesters feldspar-compatible elements over time. This melting may be triggered by conductive "hot plate" heating by basaltic magma intruding beneath the Yellowstone caldera resulting in contact rhyolitic melt that crystallizes early clinopyroxene and/or sanidine at high temperature. (2) Heterogeneity within individual samples and across flows reflects crystallization of these melts during preeruptive storage of magma at at lower, zircon-saturated temperatures. Compatible behavior and variations of most trace elements within individual lava flows are the result of sanidine, quartz, Fe–Ti oxide, zircon, and chevkinite crystallization at this stage. (3) Internal mixing immediately prior to and/or during eruption disrupts, these compositional gradients in each parental magma body that are preserved as melt domains distributed throughout the lava flows. These results based on the most recent and best-preserved volcanic products from the Yellowstone volcanic system provide new insight into the multiple stages required to generate highly fractionated hot spot and rift-related rhyolites. Our proposed model differs from previous interpretations that extreme Sr and Ba depletion result from long-term crystallization of a single magma body—instead we suggest that punctuated batch melting events generated a sanidine-rich refractory residue and a melt source region progressively depleted in Sr and Ba.
The 2016–2017 eruption of Bogoslof primarily produced crystal-rich amphibole basalts. The dominant juvenile tephra were highly microlitic with diktytaxitic vesicles, and amphiboles had large reaction ...rims. Both observations support a magma history of slow ascent and/or shallow stalling prior to eruption. Plagioclase-amphibole-clinopyroxene mineralogy are also suggestive of shallow magma crystallization. Lavas were emplaced as shallow submarine lava domes and cryptodomes that produced 70 relatively short-lived and water-rich explosions over the course of the 9-month-long eruption. The explosions ejected older trachyandesite lavas that were likely uplifted by cryptodome emplacement that began in December 2016 and continued for many months. Trachyte pumice, similar in composition to a 1796 lava dome, was entrained in basalts by the end of the eruption. The pumice appears to be a largely crystalline magma that was rejuvenated, entrained in the basalt, and heated to ~ 1000 °C. The composition of trachytes require differentiation through stronger amphibole control than the apparent shallow crustal evolution implied for the basalt. This suggests that they are magmas derived from a mid-crustal zone of amphibole crystallization. Nearby arc-front volcanoes that notably lack amphibole have strikingly similar compositional trends. Trace element signatures of the Bogoslof basalts, however, suggest derivation from a mantle source with residual garnet and lower-degree melting than basalts from nearby arc-front volcanoes. The diversity of magmas erupted at Bogoslof thus provides an opportunity not only to probe rare backarc compositions from the Aleutian arc but also to examine the apparent role of amphibole in generating evolved compositions more broadly in arc environments.
The long-term evolution of continental magmatic arcs is episodic, where a few transient events of high magmatic flux or flare-ups punctuate the low-flux magmatism or “steady state” that makes up most ...of the arc history. How this duality manifests in terms of differences in crustal architecture, magma dynamics and chemistry, and the time scale over which transitions occur is poorly known. Herein we use multiscale geochemical and isotopic characteristics coupled with geothermobarometry at the Purico–Chascon Volcanic Complex (PCVC) in the Central Andes to identify a transition from flare-up to steady state arc magmatism over ∼800 kyr during which significant changes in upper crustal magmatic dynamics are recorded.
The PCVC is one of the youngest volcanic centers related to a 10–1 Ma ignimbrite flare-up in the Altiplano–Puna Volcanic Complex of the Central Andes. Activity at the PCVC initiated 0.98±0.03 Ma with the eruption of a large 80–100 km3 crystal-rich dacite ignimbrite. High, restricted 87Sr/86Sr isotope ratios between 0.7085 and 0.7090 in the bulk rock and plagioclase crystals from the Purico ignimbrite, combined with mineral chemistry and phase relationships indicate the dacite magma accumulated and evolved at relatively low temperatures around 800–850 °C in the upper crust at 4–8 km depth. Minor andesite pumice erupted late in the ignimbrite sequence records a second higher temperature (965 °C), higher pressure environment (17–20 km), but with similar restricted radiogenic bulk rock 87Sr/86Sr = 0.7089–0.7091 to the dacites. The compositional and isotopic characteristics of the Purico ignimbrite implicate an extensive zone of upper crustal mixing, assimilation, storage and homogenization (MASH) between ∼30 and 4 km beneath the PCVC ∼1 Ma.
The final eruptions at the PCVC <0.18±0.02 Ma suggest a change in the magmatic architecture beneath the PCVC. These eruptions produced three small <6 km3 crystal-rich dacite lava domes with radiogenic bulk rock 87Sr/86Sr ratios ranging from 0.7075 to 0.7081, that contain abundant basaltic-andesite inclusions with relatively low bulk rock 87Sr/86Sr ratios of 0.7057–0.7061. Plagioclase and amphibole in the host lava of Cerro Chascon, the largest of the domes, record two distinct magmatic environments; an upper crustal environment identical to that recorded in the Purico ignimbrite, and a second deeper, ∼15–20 km depth, higher temperature (∼922–1001 °C) environment. This deeper environment is recorded in textures and compositions of distinct mineral phases, and in intracrystalline isotope ratios. Plagioclase cores in the host dacite lava and mafic inclusions have in situ87Sr/86Sr isotopic compositions of 0.7083 to 0.7095, broadly similar to plagioclase from the Purico ignimbrite. In contrast, plagioclase rims and microphenocrysts in the mafic inclusions are isotopically distinct with lower 87Sr/86Sr isotope ratios (0.7057 to 0.7065 and 0.7062 to 0.7064, respectively) that overlap with the regional isotopic “baseline” compositions that are parental to the modern arc lavas.
The textural and compositional characteristics of the PCVC attest to two distinct stages in its history. At ∼1 Ma the system was broadly homogeneous and dominantly dacitic recording extensive upper crustal magmatism. By ∼0.2 Ma the PCVC had transitioned to a more compositionally heterogeneous, smaller volume, mixed dacite to basaltic-andesite system, coinciding with the appearance of less-enriched “baseline” compositions. The evolution of PCVC is a microcosm of the Central Andean arc in this region where, from 10 to 1 Ma, upper crustal MASH processes resulted in the production and eruption of large volumes of homogeneous crystal-rich dacite during a regional ignimbrite flare-up. Since ∼1 Ma, decreasing explosivity, smaller eruptive volumes, increasing heterogeneity, and the emergence of less isotopically enriched basaltic-andesite to dacite composite volcanoes signal a return to steady-state arc volcanism.
We posit that the transition from flare-up to steady state captured at the PCVC tracks the waning of the arc scale “thermal engine”. High magmatic fluxes during the flare-up would lead to elevated geothermal gradients and efficient crustal processing leading to a dominantly “crustal” magmatism feeding the large volume Purico ignimbrite. This upper crustal MASH zone would act as an efficient filter to any parental compositions precluding them from the eruption record. As magmatic flux and thermal energy wanes, crustal isotherms would relax leading to greater thermal contrast between parental magmas, upper crust, and remnant felsic magmas stored in the upper crust. These changes are manifested in the preservation of textural and compositional heterogeneity and the survival of less isotopically enriched magmas in the upper crust. The chemical imprint of these arc-scale changes in magma dynamics is recorded at all scales from bulk rock to intra-crystalline. The distinct magma dynamics and chemical signatures of the two modes of arc magmatism detailed here should provide a model for investigations of mature continental arc evolution through time and space.
•The PCVC records the transition from flare-up to steady-state arc magmatism.•Transition recorded in the volumes, textures, and compositions of erupted products.•∼1 Ma the PCVC was dominated by upper crustal magmatism.•∼0.2 Ma magmatism at the PCVC was predominantly from the upper mantle/lower crust.•Changes in magma generation and storage record decreasing regional geotherms.
Magma‐water interaction can dramatically influence the explosivity of volcanic eruptions. However, syn‐ and post‐eruptive diffusion of external (non‐magmatic) water into volcanic glass remains poorly ...constrained and may bias interpretation of water in juvenile products. Hydrogen isotopes in ash from the 2009 eruption of Redoubt Volcano, Alaska, record syn‐eruptive hydration by vaporized glacial meltwater. Both ash aggregation and hydration occurred in the wettest regions of the plume, which resulted in the removal and deposition of the most hydrated ash in proximal areas <50 km from the vent. Diffusion models show that the high temperatures of pyroclast‐water interactions (>400°C) are more important than the cooling rate in facilitating hydration. These observations suggest that syn‐eruptive glass hydration occurred where meltwater was entrained at high temperature, in the plume margins near the vent. Ash in the drier plume interior remained insulated from entrained meltwater until it cooled sufficiently to avoid significant hydration.
Plain Language Summary
Explosive volcanic eruptions produce plumes of volcanic ash and gas that commonly mix with water from overlying seawater, glaciers, or hydrothermal systems. Within these plumes, volcanic glass (rapidly cooled magma) can lose its dissolved magmatic water or gain additional water from the surrounding environment. This study uses water concentrations in volcanic glass, and the hydrogen isotopes of that water, to identify if water was lost or gained in ash during the 2009 eruption of Redoubt Volcano, Alaska, USA. Results show that most of the magmatic water was lost, and some external water was gained in samples that fell closest to the volcano. Numerical models show that external water is most easily gained in glass at high temperatures even at the fastest cooling rates. These findings suggest external water was incorporated into the margins of the eruption plumes during the eruption. Ash hydration and aggregation occurred in these wet plume margins near the vent and preferentially deposited it closer to the vent. Ash in the hotter plume core that encounters water at cooler temperatures is erupted to higher altitudes and disperses the drier ash to further distances.
Key Points
Water contents and hydrogen isotopes in volcanic ash record syn‐eruptive hydration during the wet 2009 eruption of Redoubt Volcano, Alaska
The temperature of pyroclast‐water interaction, more than the pyroclast cooling rate, dictates the extent of syn‐eruptive glass hydration
More extensive hydration of proximal ashfall suggests wetter plume margins and drier, higher transport of the plume interiors
Variable eruptive style and explosivity is common in basaltic to basaltic andesite volcanoes but can have uncertain origins. Veniaminof volcano in the Alaska-Aleutian arc is a frequently active ...open-vent center, regularly producing Strombolian eruptions and small lava flows from an intracaldera cone within an intracaldera ice cap. The September–December 2018 eruption of Veniaminof evolved in explosivity over time. The eruption was documented with frequent satellite observations, syn- and post-eruption structure-from-motion photo surveys, and post-eruption sampling of lava flows and tephra preserved in the syn-eruption snowpack. Lava flows with a total volume of ~ 6 × 10
6
m
3
flowed down the cone flanks into the ice cap, overthickening at ice marginal flow fronts. Smaller tephra deposits were estimated at ~ 1–2 × 10
6
m
3
dense rock equivalent, with almost half of this volume deposited directly on the eruptive cone itself. Erupted products were basaltic andesite, and composition (54 ± 0.7 wt% bulk rock, 58 ± 0.7 wt% glass SiO
2
), sideromelane microlite crystallinity (20–30%), and microlite number density (plagioclase 6.4 ± 2.6 × 10
5
n/mm
3
) did not change significantly over the eruption suggesting a similar magma source and ascent rate. We defined tephra componentry with groundmass microcrystalline textures using backscatter electron images. The componentry of tephra groundmass showed significant increases in tachylite grains, defined here by the presence of dendritic interstitial nanolites, corresponding to increasing seismic tremor and periods of increased ash emissions. We suggest that these componentry changes reflect increasing undercooled zones on the conduit margins that increased brittle shearing, fragmentation, and ultimately ash emissions.
Large-volume effusive rhyolite lava flows are a common but poorly understood occurrence from silicic volcanic centers. We integrate characterization of lava flow topographic morphology and ...petrographic textures and zoning of crystals with physical models of viscous fluid flow in order to interpret the eruption durations and discharge rates for the most recent effusive volcanic eruptions from Yellowstone. These large-volume (10–70 km3) crystal-poor rhyolite lavas erupted within the Yellowstone caldera as 100–200 m thick flows and have a cumulative erupted volume of 650 km3 that is similar to less frequent caldera-forming events, but occur as individual eruptions spread over ∼100 ka. Most of this work is focused on the axisymmetric 124 ka, ∼50 km3 Summit Lake flow. We examined crystallinity, major and trace element concentrations, oxygen and hydrogen isotopic values, and quartz morphology and zoning in samples from the center to margin of this flow. Water contents down to 0.1 wt.% and δD values of −110‰ are low and require closed-system degassing until near-surface lithostatic pressure, while major elements are consistent with water-undersaturated pre-eruptive storage and crystallization at ∼4–8 km depth. We found some evidence for subtle km-scale zoning within the lavas but describe significant microscopic scale compositional diversity including sharp boundaries between high-Ti cores and ∼200 μm thick rims on quartz phenocrysts. Embayed quartz external morphology and rim growth may be the result of undercooling during coalescence of magma bodies during shallow transport between dikes and sills. Modeling the emplacement of the lava flow as a simple viscous fluid suggests that emplacement of rhyolite lava at ∼800 °C occurred over ∼2 to 5 yrs with high discharge rates >100 m3/s. Such high magma discharge rates are accommodated through ∼6 km-long fissures that allow for slower magma ascent velocities of <1 cm/s required for eruptions to remain dominantly effusive. Lower temperatures will result in >10 yr flow durations and significant cooling of the flow front that should result in a more complex compound flow morphology than observed. Higher temperatures require unrealistically wide (>50 m) dike widths to accommodate large discharge rates. Petrographic and isotopic evidence from crystals suggests recharge and merger of individual magma batches occurs on a similar timescale to the eruption duration and may directly cause overpressure and emplacement of these rhyolite lava flows from a shallow, ephemeral magma chamber. Large-volume rhyolitic lavas are able to erupt effusively through elongate fissures that utilize preexisting zones of crustal weaknesses such as ring fractures. Less-common explosive eruptions at Yellowstone may result when ascending magmas are forced through narrower conduits or when recharge rates are especially high. The results of this study provide a unique “top-down” constraint on effusive eruption rates, make new interpretations of common petrographic textures, and presents a comprehensive model for eruption control.
•Yellowstone large-volume (∼50 km3) rhyolitic lavas erupt at ∼800 °C in 2–5 yrs.•Timescales of crystal residence estimated by Ti-diffusion in quartz are similar.•Eruption occurs at slow ascent but high discharge rates via 6 km-long fissures.•Explosive/effusive style is controlled by conduit dimensions and recharge rate.•Pre- or syn-eruptive amalgamation of magmas sustains overpressure during eruption.
The eruption and storage temperatures of rhyolitic magmas are critical factors for understanding the mechanisms of their eruption and petrogenesis. Temperatures are particularly important when ...comparing the magmatic histories of hot-dry rhyolites from the Yellowstone-Snake River Plain (YSRP) and Iceland to cold-wet rhyolites such as the Bishop Tuff. Here we employ mineral pair oxygen isotope fractionations for estimating rhyolite temperatures independent of pressure and other compositional factors. We report high precision oxygen isotope analyses of quartz, pyroxene, magnetite, and zircon that we use to estimate crystallization and storage temperatures. Temperatures for YSRP and Icelandic rhyolites are highest for quartz-magnetite and quartz-clinopyroxene (∼950 °C), with lower quartz-zircon (850 °C) temperatures that are similar to estimates of zircon saturation. The magnitude and pattern of these temperatures is consistent with crystallization from near-liquidus rhyolites. In contrast, oxygen isotope temperatures calculated for the Bishop and other "cold-wet" type tuffs define low ∼760 °C temperatures for all three mineral pairs consistent with prolonged mineral residence at near-solidus conditions. Preservation of a down-temperature crystallization sequence of hot magnetite and clinopyroxene with colder zircon in hot-dry YSRP and Icelandic rhyolites suggest <1000 yr magma residence, where magnetite does not have sufficient time to diffusively equilibrate oxygen in a lower temperature melt. This is consistent with recently determined high precision U-Pb crystallization ages zircons from the same units indicating magma generation shortly before eruption.
Concentrations of H2O and CO2 in olivine-hosted melt inclusions can be used to estimate crystallization depths for the olivine host. However, the original dissolved CO2 concentration of melt ...inclusions at the time of trapping can be difficult to measure directly because in many cases substantial CO2 is transferred to shrinkage bubbles that form during post-entrapment cooling and crystallization. To investigate this problem, we heated olivine from the 1959 Kīlauea Iki and 1960 Kapoho (Hawai‘i) eruptions in a 1-atm furnace to temperatures above the melt inclusion trapping temperature to redissolve the CO2 in shrinkage bubbles. The measured CO2 concentrations of the experimentally rehomogenized inclusions (⩽590ppm for Kīlauea Iki n=10; ⩽880ppm for Kapoho, with one inclusion at 1863ppm n=38) overlap with values for naturally quenched inclusions from the same samples, but experimentally rehomogenized inclusions have higher within-sample median CO2 values than naturally quenched inclusions, indicating at least partial dissolution of CO2 from the vapor bubble during heating. Comparison of our data with predictions from modeling of vapor bubble formation and published Raman data on the density of CO2 in the vapor bubbles suggests that 55–85% of the dissolved CO2 in the melt inclusions at the time of trapping was lost to post-entrapment shrinkage bubbles. Our results combined with the Raman data demonstrate that olivine from the early part of the Kīlauea Iki eruption crystallized at <6kmdepth, with the majority of olivine in the 1–3kmdepthrange. These depths are consistent with the interpretation that the Kīlauea Iki magma was supplied from Kīlauea’s summit magma reservoir (∼2–5kmdepth). In contrast, olivine from Kapoho, which was the rift zone extension of the Kīlauea Iki eruption, crystallized over a much wider range of depths (∼1–16km). The wider depth range requires magma transport during the Kapoho eruption from deep beneath the summit region and/or from deep beneath Kīlauea’s east rift zone. The deeply derived olivine crystals and their host magma mixed with stored, more evolved magma in the rift zone, and the mixture was later erupted at Kapoho.
The hydrogen isotope value (δD) of water indigenous to the mantle is masked by the early degassing and recycling of surface water through Earth's history. High 3He/4He ratios in some ocean island ...basalts, however, provide a clear geochemical signature of deep, primordial mantle that has been isolated within the Earth's interior from melting, degassing, and convective mixing with the upper mantle. Hydrogen isotopes were measured in high 3He/4He submarine basalt glasses from the Southeast Indian Ridge (SEIR) at the Amsterdam–St. Paul (ASP) Plateau (δD = −51 to −90‰, 3He/4He = 7.6 to 14.1 RA) and in submarine glasses from Loihi seamount south of the island of Hawaii (δD = −70 to −90‰, 3He/4He = 22.5 to 27.8 RA). These results highlight two contrasting patterns of δD for high 3He/4He lavas: one trend toward high δD of approximately −50‰, and another converging at δD = −75‰. These same patterns are evident in a global compilation of previously reported δD and 3He/4He results. We suggest that the high δD values result from water recycled during subduction that is carried into the source region of mantle plumes at the core–mantle boundary where it is mixed with primordial mantle, resulting in high δD and moderately high 3He/4He. Conversely, lower δD values of −75‰, in basalts from Loihi seamount and also trace element depleted mid-ocean ridge basalts, imply a primordial Earth hydrogen isotopic value of −75‰ or lower. δD values down to −100‰ also occur in the most trace element-depleted mid-ocean ridge basalts, typically in association with 87Sr/86Sr ratios near 0.703. These lower δD values may be a result of multi-stage melting history of the upper mantle where minor D/H fractionation could be associated with hydrogen retention in nominally anhydrous residual minerals. Collectively, the predominance of δD around −75‰ in the majority of mid-ocean ridge basalts and in high 3He/4He Loihi basalts is consistent with an origin of water on Earth that was dominated by accretion of chondritic material.
•δD determined on submarine glasses from Loihi and Amsterdam–St. Paul hotspots.•Primordial δD = −75‰ characterized by Loihi glasses with 3He/4He = 20–30 RA.•Amsterdam–St. Paul δD trends towards −50‰ from mixing with recycled water.•Low δD values globally of −90 to −100‰ often found in ultra-depleted fracture zones.•Global δD variations explained as mixing between these endmember components.
Mt. Jefferson is a large composite volcano located in the central Oregon Cascades that has erupted a diverse compositional suite of lavas from basalt to rhyodacite (50–72 wt. % SiO2). Individual ...eruptive units contain multiple populations of plagioclase, and a variety of mafic textural/mineralogical components often preserved as large centimeter to millimeter‐sized enclaves. Understanding the processes active in any volcanic center requires that we document the products of those processes. In this contribution, we documented the small‐scale compositional diversity within a single eruptive unit at Mt. Jefferson, the Whitewater Creek andesite, in order to answer three questions: (1) what are the characteristics and scale of diversity in a single eruptive unit, (2) what is the provenance of the observed components, and (3) how does that observed small‐scale diversity relate to the larger‐scale diversity observed between other flows at Mt. Jefferson. Our analyses are based on major, trace element concentrations for phenocrysts, and melt inclusions from a single eruptive unit, the Whitewater Creek andesite which is one of the most heterogeneous units erupted at Mt. Jefferson. We have identified at least four distinct components present at the centimeter scale. These components, identified on the basis of their mineralogy and composition, include three mafic (two pyroxene + plagioclase; plagioclase‐hornblende, and olivine‐orthopyroxene) and one silicic component (dacitic groundmass). To understand the relationship between the observed textural components and the magma types erupted at Mt Jefferson, equilibrium liquid compositions were calculated from the phenocryst compositions. The range exhibited by those calculated liquids covers much of the entire range represented by the lavas at Mt. Jefferson. However, it is difficult to directly connect them to specific magma types observed at Mt. Jefferson. We attribute our inability to directly link the textural components to the known magma types to post mixing diffusive reequilibration during storage and transport. These results suggest that great care should be taken in interpretation of whole rock data.
Key Points:
We compared compositional diversity at the micrometer to meter scale in a single unit
Early erupted components undergone storage degassing at P < 2.5 kbar
Significant post mixing reequilibration happened during storage and transport
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