The Pleistocene cinder cones surrounding Volcán Colima, Mexico re-visited: eruption ages and volumes, oxidation states, and sulfur content

Located at the volcanic front in the western Mexican arc, in the Colima Rift, is the active Volcán Colima, which lies on the southern end of the massive (∼450 km 3 ) Colima-Nevado volcanic complex. Along the margins of this andesitic volcanic complex, is a group of 11 scoria cones and associated lavas, which have been dated by the 40 Ar/ 39 Ar method. Nine scoria cones erupted ∼1.3 km 3 of alkaline magma (basanite, leucite-basanite, minette) between 450 and 60 ka, with >99% between 240 and 60 ka. Two additional cones (both the oldest and calc-alkaline) erupted <0.003 km 3 of basalt (0.5 Ma) and <0.003 km 3 of basaltic andesite (1.2 Ma), respectively. Cone and lava volumes were estimated with the aid of digital elevation models (DEMs). The eruption rate for these scoria cones and their associated lavas over the last 1.2 Myr is ∼1.2 km 3 /Myr, which is more than 400 times smaller than that from the andesitic Colima-Nevado edifice. In addition to these alkaline Colima cones, two other potassic basalts erupted at the volcanic front, but ∼200 km to the ESE (near the historically active Volcán Jorullo), and were dated at 1.06 and 0.10 Ma. These potassic suites reflect the tendency in the west-central Mexican arc for magmas close to the volcanic front to be enriched in K 2 O relative to those farther from the trench.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47809/1/445_2005_Article_15.pd

Production of andesite in the western Mexican arc: Constraints from argon chronology, volume analysis, and geochemistry.

40Ar/39Ar geochronology is coupled with quantitative volume determinations (utilizing field mapping, digital elevation models, ortho-airphotos and Geographic Information System software) to constrain magma eruption rates at the Ceboruco-San Pedro and the Volcan Tepetiltic volcanic fields in the western Trans-Mexican arc. Each volcanic field is dominated by an intermediate composition stratovolcano (&sim;51 and &sim;42 km 3, respectively) that was built in tens of kyrs. The smaller volume peripheral lavas (3) are more geochemically diverse than the large volume flows associated with the main edifices. The peripheral flows tend to be crystal-poor (<5% phenocrysts) and show little evidence for magma mingling or disequilibria. Based on silica content, there is no systematic progression to the composition of magma type (basalt, andesite, dacite, rhyolite) erupted with time, at either the individual stratocones or across the entire volcanic field. On the basis of eruptive sequence, proportions of lava types, phenocryst assemblages, and geochemistry, the lavas in the Ceboruco-San Pedro volcanic field and the lavas at Volcan Tepetiltic do not each reflect the differentiation of a single parental liquid in a long-lived magma chamber. The distinct geochemical signatures were present prior to magma emplacement in the upper crust, whereupon subsequent degassing and crystallization led to variable phenocryst abundances and assemblages. The crystal-poor lavas peripheral to the central stratocones provide insight into the generation of magmas at volcanic arcs. The composition of plagioclase is strongly dependent on the water concentration of the melt from which it crystallized. This relationship is used to develop a new plagioclase hygrometer to estimate water content in the melt. The plagioclase in the crystal-poor lavas span a broad range in composition and reflect crystallization from magmas with different amounts of dissolved water. Textures in the plagioclase (abundant melt inclusions) reflect rapid crystallization. These two separate lines of evidence suggest that crystallization is degassing induced. The broader tectonic evolution of western Mexico is explored with a detailed geochronology study of the silicic volcanism over the last &sim;80 Myr. A previously unrecognized volcanic flare-up in the Pliocene suggests that western Mexico may have been part of an aborted rifting event.Ph.D.Earth SciencesGeologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/125081/2/3186631.pd

Petrology of the 2020-21 effusive to explosive eruption of La Soufrière volcano, St Vincent:Insights into plumbing system architecture and magma assembly mechanism

The 2020-21 eruption of La Soufrière, St Vincent began with extrusion of a viscous lava dome, which was destroyed upon transition to a major explosive phase. Here we present petrological data to reconstruct the processes leading up to these events. Bulk-rock SiO2 contents range from 52.8 to 55.4 wt. %, classifying the lava and the subsequent scoria as basaltic andesite, the latter being slightly more mafic. Macrocrystal chemistry and modes (plag-cpx-opx-tmt-ol) and crystallinity (45–50 vol. %) are largely identical for both phases of the eruption. Pyroxenes are homogenous and precipitated mostly from andesitic melts. Conversely, plagioclase shows strong normal zonation resulting from magma ascent and stalling at multiple crustal levels. Clinopyroxene thermobarometry reveals that crystallisation predominantly took place between 8 and 13 km depth at temperatures of 997+18−35°C. A lack of evidence for mafic recharge and changes in volatile content and the omnipresence of xenoliths, suggests pre-eruptive destabilisation of an andesitic-dacitic melt pocket that disrupted and entrained antecedent mush. Olivine diffusion profiles show that this interaction preceded the onset of eruption. Low dissolved sulfur contents (≤270 ppm S) place constraints on the total SO2 gas release. Melt-mush disruption appears to be a dominant driver of eruptions at La Soufrière. Supplementary material at https://doi.org/10.6084/m9.figshare.c.648487

Risk of COVID-19 after natural infection or vaccinationResearch in context

Summary: Background: While vaccines have established utility against COVID-19, phase 3 efficacy studies have generally not comprehensively evaluated protection provided by previous infection or hybrid immunity (previous infection plus vaccination). Individual patient data from US government-supported harmonized vaccine trials provide an unprecedented sample population to address this issue. We characterized the protective efficacy of previous SARS-CoV-2 infection and hybrid immunity against COVID-19 early in the pandemic over three-to six-month follow-up and compared with vaccine-associated protection. Methods: In this post-hoc cross-protocol analysis of the Moderna, AstraZeneca, Janssen, and Novavax COVID-19 vaccine clinical trials, we allocated participants into four groups based on previous-infection status at enrolment and treatment: no previous infection/placebo; previous infection/placebo; no previous infection/vaccine; and previous infection/vaccine. The main outcome was RT-PCR-confirmed COVID-19 >7–15 days (per original protocols) after final study injection. We calculated crude and adjusted efficacy measures. Findings: Previous infection/placebo participants had a 92% decreased risk of future COVID-19 compared to no previous infection/placebo participants (overall hazard ratio [HR] ratio: 0.08; 95% CI: 0.05–0.13). Among single-dose Janssen participants, hybrid immunity conferred greater protection than vaccine alone (HR: 0.03; 95% CI: 0.01–0.10). Too few infections were observed to draw statistical inferences comparing hybrid immunity to vaccine alone for other trials. Vaccination, previous infection, and hybrid immunity all provided near-complete protection against severe disease. Interpretation: Previous infection, any hybrid immunity, and two-dose vaccination all provided substantial protection against symptomatic and severe COVID-19 through the early Delta period. Thus, as a surrogate for natural infection, vaccination remains the safest approach to protection. Funding: National Institutes of Health

Risk of COVID-19 after natural infection or vaccinationResearch in context

Background: While vaccines have established utility against COVID-19, phase 3 efficacy studies have generally not comprehensively evaluated protection provided by previous infection or hybrid immunity (previous infection plus vaccination). Individual patient data from US government-supported harmonized vaccine trials provide an unprecedented sample population to address this issue. We characterized the protective efficacy of previous SARS-CoV-2 infection and hybrid immunity against COVID-19 early in the pandemic over three-to six-month follow-up and compared with vaccine-associated protection. Methods: In this post-hoc cross-protocol analysis of the Moderna, AstraZeneca, Janssen, and Novavax COVID-19 vaccine clinical trials, we allocated participants into four groups based on previous-infection status at enrolment and treatment: no previous infection/placebo; previous infection/placebo; no previous infection/vaccine; and previous infection/vaccine. The main outcome was RT-PCR-confirmed COVID-19 >7–15 days (per original protocols) after final study injection. We calculated crude and adjusted efficacy measures. Findings: Previous infection/placebo participants had a 92% decreased risk of future COVID-19 compared to no previous infection/placebo participants (overall hazard ratio [HR] ratio: 0.08; 95% CI: 0.05–0.13). Among single-dose Janssen participants, hybrid immunity conferred greater protection than vaccine alone (HR: 0.03; 95% CI: 0.01–0.10). Too few infections were observed to draw statistical inferences comparing hybrid immunity to vaccine alone for other trials. Vaccination, previous infection, and hybrid immunity all provided near-complete protection against severe disease. Interpretation: Previous infection, any hybrid immunity, and two-dose vaccination all provided substantial protection against symptomatic and severe COVID-19 through the early Delta period. Thus, as a surrogate for natural infection, vaccination remains the safest approach to protection. Funding: National Institutes of Health