James M. Watkins

Bubble geobarometry: A record of pressure changes, degassing, and regassing at Mono Craters, California

Water concentration profiles around bubbles offer a new kind of geobarometer. We measure H 2 O and CO 2 concentrations in glass adjacent to bubbles in pyroclastic obsidian from Mono Craters, California (United States). H 2 O and CO 2 concentration gradients are preserved during the eruption and record nonequilibrium degassing. A key result is that H 2 O is enriched in the glass surrounding the bubbles, indicating that bubbles were resorbing into the melt just prior to the eruption. The required pressure increase for the observed water enrichment is inferred to be the last in a series of pressure cycles with amplitude 5–30 MPa that are caused by repeated fragmentation and annealing. CO 2 concentrations vary substantially in individual obsidian clasts, suggesting that slow diffusion of CO 2 and nonequilibrium degassing contributes to high CO 2 /H 2 O ratios in pyroclastic obsidian from Mono Craters. These data are direct evidence for vapor-melt disequilibrium and demonstrate that degassing paths from a single parental melt need not be unidirectional. Hence volatile concentration gradients offer a tool for evaluating degassing models and inferring time scales of magmatic processes.

Spherulites as in-situ recorders of thermal history in lava flows

Spherulites in rhyolitic obsidian provide a record of the thermal history of their host lava during the interval of spherulite growth. We use trace element concentration profiles across spherulites and into the obsidian host from Yellowstone National Park (USA) to reconstruct the conditions that existed during spherulite formation. The measured transects reveal three behaviors: expulsion of the most diffusively mobile elements from spherulites with no concentration gradients in the surrounding glass (type 1); enrichment of slower-diffusing elements around spherulites, with concentration gradients extending outward into the glass (type 2); and complete entrapment of the slowest-diffusing elements by the spherulite (type 3). We compare the concentration profiles, measured by laser ablation–inductively coupled plasma–mass spectrometry and Fourier transform infrared spectroscopy, to the output of a spherulite growth model that incorporates known diffusion parameters, the temperature interval of spherulite growth, the cooling rate of the lava, and data on the temporal evolution of spherulite radius. Our results constrain spherulite nucleation to the temperature interval 700–550 °C and spherulite growth to 700–400 °C in a portion of lava that cooled at 10 –5.2 ± 0.3 °C s –1 , which matches an independent experimental estimate of 10 –5.3 °C s –1 measured using differential scanning calorimetry. Maximum spherulite growth rates at nucleation are on the order of 1 μm hr –1 and are inferred to decrease exponentially with time. Hence, spherulites may serve as valuable in-situ recorders of the thermal history of lava flows.

Nonequilibrium degassing, regassing, and vapor fluxing in magmatic feeder systems

Magma degassing models typically invoke volatile depletion of a single parental melt, with permeable loss of exsolved gas having served for many years as the paradigm for the transition from volatile-rich, explosive eruptions to volatile-depleted lava flows. These degassing models are guided by measurements of H2O, CO2, and hydrogen isotope variations retained in melt that quenched to glass, but the existing models are not uniquely constrained by the data. There also remains uncertainty surrounding the origin and significance of volcanic glass fragments. We show that individual obsidian pyroclasts from Mono Craters, California (USA), are heterogeneous in dissolved H2O and CO2, suggesting that clasts are assembled from juvenile melt and rewelded ash during magma ascent. This is in contrast to the conventional view that clasts are chemically homogeneous and sample the chilled, glassy margins of conduit walls. The new measurements of dissolved H2O and CO2 help reconcile existing open-system degassing models used to explain elevated CO2/H2O ratios, provide time scales based on diffusion modeling for pyroclast formation, and show that magma does not necessarily lose volatiles monotonically during ascent-driven decompression.

A refined method for calculating paleotemperatures from linear correlations in bamboo coral carbon and oxygen isotopes

Bamboo corals represent an emerging paleoclimate archive with the potential to record variability at intermediate depths throughout much of the global ocean. Realizing this potential has been complicated by biologically mediated vital effects, which are evident in linear correlations of skeletal carbon (δ13C) and oxygen (δ18O) isotope composition. Previous efforts to develop a bamboo coral δ18O paleothermometer by accounting for such vital effects have not been completely successful as they still rely on empirical calibrations that are offset from the temperature dependence of abiogenic experiments. Here we describe an approach that better corrects for bamboo coral vital effects and allows paleotemperatures to be calculated directly from the abiogenic temperature dependence. The success of the method lies in calculating apparent equilibrium carbon and oxygen isotope fractionation at the temperature, pH, and growth rate of each coral, as well as in the use of model II regressions. Rigorous propagation of uncertainty suggests typical errors of ±2–3°C, but in select cases errors as low as ±0.98°C can be achieved for densely sampled and strongly correlated data sets. This lower limit approaches the value attributed to uncertainty in pH and growth rate estimates alone, as predicted by a series of pseudoproxy experiments. The incorporation of isotopically light metabolic CO2 appears to be negligible in most Pacific corals, but may be significant in Atlantic specimens, potentially requiring an additional correction. The success of the method therefore hinges on how well complex environmental systems and biomineralization strategies are constrained, with the most reliable temperatures occurring when calcifying fluid pH, growth rate, and incorporation of metabolic carbon into skeletal calcite are constrained using multiple geochemical proxies.

Ascent rates of rhyolitic magma at the onset of three caldera-forming eruptions

Abstract Important clues to the initiation and early behavior of large (super-) eruptions lie in the records of degassing during magma ascent. Here we investigate the timescales of magma ascent for three rhyolitic supereruptions that show field evidence for contrasting behavior at eruption onset: (1) 650 km3, 0.767 Ma Bishop Tuff, Long Valley; (2) 530 km3, 25.4 ka Oruanui eruption, Taupo; and (3) 2500 km3, 2.08 Ma Huckleberry Ridge Tuff, Yellowstone. During magma ascent, decompression causes volatile exsolution from the host melt into bubbles, leading to H2O and CO2 gradients in quartz-hosted re-entrants (REs; unsealed inclusions). These gradients are modeled to estimate ascent rates. We present best-fit modeled ascent rates for H2O and CO2 profiles for REs in early-erupted fall deposits from Bishop (n = 13), Oruanui (n = 9), and Huckleberry Ridge (n = 9). Using a Matlab script that includes an error minimization function, Bishop REs yield ascent rates of 0.6–13 m/s, overlapping with and extending beyond those of the Huckleberry Ridge (0.3–4.0 m/s). Re-entrants in Oruanui quartz crystals from the first two eruptive phases (of 10) yield the slowest ascent rates determined in this study (0.06–0.48 m/s), whereas those from phase three, which has clear field evidence for a marked increase in eruption intensity, are uniformly higher (1.4–2.6 m/s). For all three eruptions, the interiors of most REs appear to have re-equilibrated to lower H2O and CO2 concentrations when compared to co-erupted, enclosed melt inclusions in quartz. Such reequilibration implies the presence of an initial period of slower ascent, likely resulting from movement of magma from storage into a developing conduit system, prior to the faster (<1–2.5 h) final ascent of magma to the surface. This slower initial movement represents hours to several days of reequilibration, invalidating any assumption of constant decompression conditions from the storage region. However, the number of REs with deeper starting depths increases with stratigraphic height in all three deposits (particularly the Bishop Tuff), suggesting progressive elimination of the deep, sluggish ascent stage over time, which we interpret to be the result of maturing of the conduit system(s). Our results agree well with ascent rates estimated using theoretical approximations and numerical modeling for plinian rhyolitic eruptions (0.7–30 m/s), but overlap more with the slower estimates.

Nucleation rates of spherulites in natural rhyolitic lava

Abstract The rates of nucleation and crystal growth from silicate melt are difficult to measure because the temperature-time path of magma is often unknown. We use geochemical gradients around spherulites in obsidian glass to estimate the temperature-time interval of spherulite crystallization. This information is used in conjunction with new high-resolution X-ray computed tomography (HRXCT) data on the size distributions of spherulites in six samples of rhyolite obsidian lava to infer spherulite nucleation rates. A large data set of geochemical profiles indicate that the lavas cooled at rates of 10-2.2 to 10-1.2°C/h, and that the spherulites grew at rates that decreased exponentially with time, with values of 10-0.70 to 100.30µm/h at 600°C. Spherulites are estimated to have begun nucleating when undercooling [ΔT, = liquidus T (≈800°C) minus nucleation T] reached 100-277°C, and stopped when ΔT = 203-365°C, with exact values dependent on assumed cooling and growth rates. Regardless of rates, we find that spherulites nucleated within a ~88-113°C temperature interval and, hence, began when ΔT ≈ 0.65-0.88 x TL, peaking when ΔT ≈ 0.59-0.80 x TL. A peak rate of nucleation of 0.072 ± 0.049 cm-3h-1 occurred at 533 ± 14 °C, using cooling and growth rates that best fit the data set of geochemical profiles. While our inferred values for ΔT overlap those from experimental studies, our nucleation rates are much lower. That difference likely results from experimental studies using hydrous melts; the natural spherulites grew in nearly anhydrous glass.