Mafic explosive volcanism at Llaima Volcano: 3D x-ray microtomography reconstruction of pyroclasts to constrain shallow conduit processes

Mafic volcanic activity is dominated by effusive to mildly explosive eruptions. Plinian and ignimbrite-forming mafic eruptions, while rare, are also possible; however, the conditions that promote such explosivity are still being explored. Eruption style is determined by the ability of gas to escape as magma ascends, which tends to be easier in low-viscosity, mafic magmas. If magma permeability is sufficiently high to reduce bubble overpressure during ascent, volatiles may escape from the magma, inhibiting violent explosive activity. In contrast, if the permeability is sufficiently low to retain the gas phase within the magma during ascent, bubble overpressure may drive magma fragmentation. Rapid ascent may induce disequilibrium crystallization, increasing viscosity and affecting the bubble network with consequences for permeability, and hence, explosivity. To explore the conditions that promote strongly explosive mafic volcanism, we combine microlite textural analyses with synchrotron x-ray computed microtomography of 10 pyroclasts from the 12.6 ka mafic Curacautín Ignimbrite (Llaima Volcano, Chile). We quantify microlite crystal size distributions (CSD), microlite number densities, porosity, bubble interconnectivity, bubble number density, and geometrical properties of the porous media to investigate the role of magma degassing processes at mafic explosive eruptions. We use an analytical technique to estimate permeability and tortuosity by combing the Kozeny-Carman relationship, tortuosity factor, and pyroclast vesicle textures. The groundmass of our samples is composed of up to 44% plagioclase microlites, > 85% of which are < 10 µm in length. In addition, we identify two populations of vesicles in our samples: (1) a convoluted interconnected vesicle network produced by extensive coalescence of smaller vesicles (> 99% of pore volume), and (2) a population of very small and completely isolated vesicles (< 1% of porosity). Computed permeability ranges from 3.0 × 10−13 to 6.3 × 10−12 m2, which are lower than the similarly explosive mafic eruptions of Tarawera (1886; New Zealand) and Etna (112 BC; Italy). The combination of our CSDs, microlite number densities, and 3D vesicle textures evidence rapid ascent that induced high disequilibrium conditions, promoting rapid syn-eruptive crystallization of microlites within the shallow conduit. We interpret that microlite crystallization increased viscosity while simultaneously forcing bubbles to deform as they grew together, resulting in the permeable by highly tortuous network of vesicles. Using the bubble number densities for the isolated vesicles (0.1-3−3 × 104 bubbles per mm3), we obtain a minimum average decompression rate of 1.4 MPa/s. Despite the textural evidence that the Curacautín magma reached the percolation threshold, we propose that rapid ascent suppressed outgassing and increased bubble overpressures, leading to explosive fragmentation. Further, using the porosity and permeability of our samples, we estimated that a bubble overpressure > 5 MPa could have been sufficient to fragment the Curacautín magma. Other mafic explosive eruptions report similar disequilibrium conditions induced by rapid ascent rate, implying that syn-eruptive disequilibrium conditions may control the explosivity of mafic eruptions more generally.

Severity of Central Sleep Apnea Does Not Affect Sleeping Oxygen Saturation During Ascent to High Altitude

Central sleep apnea (CSA) is characterized by periodic breathing (PB) during sleep, defined as intermittent periods of apnea/hypopnea and hyperventilation, with associated acute fluctuations in oxyhemoglobin saturation (SO2). CSA has an incidence of ~50% in heart failure patients but is universal at high-altitude (HA; ≥2,500 m), increasing in severity with further ascent and/or time at altitude. However, whether PB is adaptive, maladaptive, or neutral with respect to sleeping SO2 at altitude is unclear. We hypothesized that PB severity would improve mean sleeping SO2 during acclimatization to HA due to relative, intermittent hyperventilation subsequent to each apnea. We utilized portable sleep monitors to assess the incidence and severity of CSA via apnea-hypopnea index (AHI) and oxygen desaturation index (ODI), and peripheral oxygen saturation (SpO2) during sleep during two ascent profiles to HA in native lowlanders: (I) rapid ascent to and residence at 3,800 m for 9 days/nights (n=21) and (II) incremental ascent to 5,160 m over 10 days/nights (n=21). In both ascent models, severity of AHI and ODI increased and mean sleeping SpO2 decreased, as expected. However, during sleep on the last night/highest altitude of both ascent profiles, neither AHI nor ODI were correlated with mean sleeping SpO2. In addition, mean sleeping SpO2 was not significantly different between high and low CSA. These data suggest that CSA is neither adaptive nor maladaptive with regard to mean oxygen saturation during sleep, owing to the relative hyperventilation between apneas, likely correcting transient apnea-mediated oxygen desaturation and maintaining mean oxygenation.

Risk and protective factors for acute mountain sickness: a meta-analysis

BackgroundCurrently, many people reach 3000-meter summits or greater. Some of these climbers suffer from acute mountain sickness (AMS), while others remain symptom-free. Some risk factors for AMS are well defined, such as lack of pre-acclimatization, rapid ascent, maximum altitude, and resilience at low altitude. However, there are other poorly described associated factors, such as sex, age, smoking, body mass index, medication use, and awareness or history of AMS. The objective of this meta-analysis was to establish the risk and protective factors associated with AMS. MethodsPubMed, UNIKA, and Scopus databases were searched in July 2020 for articles to include in the analysis. AMS was separately evaluated by the Lake Louise Score (LLS) and Hackett’s Score (HS). After screening and application of eligibility criteria, 14 articles were included in the meta-analysis (LLS = 12 and HS = 2). ResultsA total of 18,938 participants were included in the study with 17,450 in the LLS group and 1,488 in the HS group. In the LLS group, smoking (odds ratio [OR] 0.76 [0.63-0.92]; χ2 = 10.3; I2 = 61.2%), history of AMS (OR 1.16 [1.03-1.32]; χ2 = 12.2; I2 = 67%), altitude preexposure (OR 0.68 [0.5-0.91]; χ2 = 22.21; I2 = 82%; τ2 = 0.06), and maximum altitude (OR 2.01 (1.07-3.77) (χ2 = 19.15. I2 = 89.6%. τ2 = 0.26) were statistically significant. In the HS group, age >35 years (OR 0.72 [0.52-0.99]; χ2 = 2.14; I2 = 6.7%) and a history of AMS (OR 8.2 [3.28-20.54]; χ2 = 3.7; I2 = 73%; τ2 = 0.32) were found to be statistically significant. ConclusionsThis study suggests that younger age, non-smoking, history of AMS, nonacclimatization, and hiking to a high maximum altitude (>2500m) increases the possibility of suffering from AMS.

Mafic Explosive Volcanism at Llaima Volcano

Mafic eruptions, which are typically effusive to mildly explosive, can produce much stronger explosive eruptions. Eruption style is determined by the ability of gas to escape through the permeable network. If the permeability is sufficiently high to reduce vesicle overpressure during ascent, the volatiles may escape from the magma, inhibiting violent explosive activity. In contrast, if the permeability is sufficiently low to retain the gas phase within the magma during ascent, bubble overpressure may drive magma fragmentation. Rapid ascent rates may induce disequilibrium crystallization, increasing viscosity and explosivity, and have consequences for the geometry of the vesicle network. Quantitative vesicle texture analyses are commonly measured in 2D. However, 2D vesicle analyses do not provide sufficient information about the internal vesicle structures for permeability analysis. Here we use synchrotron X-ray computed microtomography of 10 pyroclasts from the 12.6 ka mafic Curacautín Ignimbrite (Llaima Volcano, Chile) to reconstruct and quantify pyroclast textures in three dimensions. Our goal is to obtain 3D measurements of porosity, bubble interconnectivity, bubble number density, and geometrical properties of the porous media to investigate the role of magma degassing processes at mafic explosive eruptions. We use an analytical technique to estimate permeability and tortuosity by combing empirical relationships and pyroclasts vesicle textures. We identified two populations of vesicles: (1) a convoluted connected vesicle network produced by extensive coalescence of smaller vesicles (> 99% of pore space), and (2) a population of very small and completely isolated vesicles (< 1% of porosity network). Bubble numbe2r density measurements are 1-29×103 bubbles per mm3, implying an average decompression rate of 1.4 MPa/s under heterogeneous nucleation. We computed tortuosities factor between 1.89 and 4.4, with higher values in the less vesicular samples. Permeability ranges are between 3×10-13 and 6.27×10-12 m2. 3D vesicle textures evidence rapid ascent rates that induced high disequilibrium, promoting rapid syn-eruptive crystallization of microlites and late vesiculation. We propose that the increase in viscosity due to crystallization and vesiculation, combined with rapid ascent, inhibited outgassing and increased bubble overpressures, leading to explosive fragmentation. We estimated that a bubble overpressure greater than 5.2 MPa could have been sufficient to fragment the Curacautín magma. Other mafic explosive eruptions report similar disequilibrium conditions induced by rapid ascent rate, implying that syn-eruptive disequilibrium may control the explosivity of mafic eruptions more generally.

Subduction initiation-induced rapid emplacement of garnet-bearing peridotites at a nascent forearc: Petrological and Os-Li isotopic evidence from the Purang ophiolite, Tibet

Garnet-bearing peridotites commonly occur in the deeper parts of mature or thickened oceanic lithosphere, and are rarely exhumed and emplaced onto the seafloor. The Purang ophiolitic peridotites in south Tibet contain rare symplectite pseudomorphs after garnet, offering a unique window into the still poorly understood evolution of the deep oceanic lithosphere. Here, integrated petrologic and Os-Li isotopic data are used to constrain the evolution and dynamics of emplacement for these garnet peridotite protoliths. The Purang peridotites show wide variations of chemical compositions (spinel Cr#: 0.2−0.8) and Os model ages (up to 2.0 Ga), thus representing a piece of heterogeneous oceanic mantle lithosphere. Dunite channels show two distinctive groups of Cr# of spinels and Os-isotope compositions, with the low- to medium-Cr# (0.2−0.6) and high-Cr# (0.7−0.8) dunites reflecting the reaction of host lherzolites/harzburgites with percolating mid-ocean ridge basalt−like and boninitic melts, respectively. This confirms recent subduction initiation-related melt percolation in the Purang peridotites. Coexisting olivines and pyroxenes in the peridotites show systematic Li elemental and isotopic disequilibrium, suggesting fast cooling of the peridotites to Li closure temperature shortly after the melt percolations, likely during exhumation of the peridotites onto the seafloor. This supports a close link between subduction initiation and tectonic emplacement of the Purang peridotites. Combined with other geological evidence, we suggest the Purang peridotites may originate from the deep part of old, thick oceanic lithosphere of the Neo-Tethys. This thick oceanic lithosphere was progressively weakened and thinned likely during widespread plume-lithosphere interaction, triggering the transformation of garnet peridotite protoliths to spinel peridotites. Subsequently, initiation of a new subduction zone along the lithospheric weakness caused rapid ascent and emplacement of the Purang peridotites at a nascent forearc.

Time Course and Magnitude of Ventilatory and Renal Acid-Base Acclimatization Following Rapid Ascent to and Residence at 3,800 m over Nine Days

Rapid ascent to high altitude imposes an acute hypoxic and acid-base challenge, with ventilatory and renal acclimatization countering these perturbations. Specifically, ventilatory acclimatization improves oxygenation, but with concomitant hypocapnia and respiratory alkalosis. A compensatory, renally-mediated relative metabolic acidosis follows via bicarbonate elimination, normalizing arterial pH(a). The time-course and magnitude of these integrated acclimatization processes are highly variable between individuals. Using a previously-developed metric of renal reactivity (RR), indexing the change in arterial bicarbonate concentration (∆[HCO3-]a; renal response) over the change in arterial pressure of CO2 (∆PaCO2; renal stimulus), we aimed to characterize changes in RR magnitude following rapid ascent and residence at altitude. Resident lowlanders (n=16) were tested at 1,045 m (Day [D]0) prior to ascent, on D2 within 24-hours of arrival, and D9 during residence at 3,800 m. Radial artery blood draws were obtained to measure acid-base variables: PaCO2, [HCO3-]a and pHa. Compared to D0, PaCO2 and [HCO3-]a were lower on D2 (P<0.01) and D9 (P<0.01), whereas significant changes in pHa (P>0.058) and RR (P=0.056) were not detected. As pHa appeared fully compensated on D2 and RR did not increase significantly from D2 to D9, these data demonstrate renal acid-base compensation within 24-hours at moderate steady-state altitude. Moreover, RR was strongly and inversely correlated with ∆pHa on D2 and D9 (r≤-0.95; P<0.0001), suggesting that a high-gain renal response better protects pHa. Our study highlights the differential time-course, magnitude, and variability of integrated ventilatory and renal acid-base acclimatization following rapid ascent and residence at high altitude.

Hyperbaric oxygen for decompression sickness: 2021 update

Hyperbaric oxygen for decompression sickness: 2021 update Decompression sickness (DCS, “bends”) is caused by the formation of bubbles in tissues and/or blood when the sum of dissolved gas pressures exceeds ambient pressure (supersaturation). This may occur when ambient pressure is reduced during: ascent from a dive; rapid ascent to altitude in an unpressurized aircraft or hypobaric chamber; loss of cabin pressure in an aircraft [2]; and during space walks. In diving, compressed-gas breathing is usually necessary, although occasionally DCS has occurred after either repetitive or very deep breath-hold dives