Dynamic deformation of volcanic ejecta from the Toba caldera: Possible relevance to Cretaceous/Tertiary boundary phenomena

Plagioclase and biotite phenocrysts in ignimbrites erupted from the Toba caldera, Sumatra, show microstructures and textures indicative of shock stress levels higher than 10 GPa. Strong dynamic deformation has resulted in intense kinking in biotite and, with increasing shock intensity, the development in plagioclase of planar features, shock mosaicism, incipient recrystallization, and possible partial melting. Microstructures in quartz indicative of strong shock deformation are rare, however, and many shock lamellae, if formed, may have healed during post-shock residence in the hot ignimbrite; they might be preserved in ash falls. Peak shock stresses from explosive silicic volcanism and other endogenous processes may be high and if so would obviate the need for extraterrestrial impacts to produce all dynamically deformed structures, possibly including shock features observed near the Cretaceous/Tertiary boundary

Prognostic model to predict postoperative acute kidney injury in patients undergoing major gastrointestinal surgery based on a national prospective observational cohort study.

Background: Acute illness, existing co-morbidities and surgical stress response can all contribute to postoperative acute kidney injury (AKI) in patients undergoing major gastrointestinal surgery. The aim of this study was prospectively to develop a pragmatic prognostic model to stratify patients according to risk of developing AKI after major gastrointestinal surgery. Methods: This prospective multicentre cohort study included consecutive adults undergoing elective or emergency gastrointestinal resection, liver resection or stoma reversal in 2-week blocks over a continuous 3-month period. The primary outcome was the rate of AKI within 7 days of surgery. Bootstrap stability was used to select clinically plausible risk factors into the model. Internal model validation was carried out by bootstrap validation. Results: A total of 4544 patients were included across 173 centres in the UK and Ireland. The overall rate of AKI was 14·2 per cent (646 of 4544) and the 30-day mortality rate was 1·8 per cent (84 of 4544). Stage 1 AKI was significantly associated with 30-day mortality (unadjusted odds ratio 7·61, 95 per cent c.i. 4·49 to 12·90; P < 0·001), with increasing odds of death with each AKI stage. Six variables were selected for inclusion in the prognostic model: age, sex, ASA grade, preoperative estimated glomerular filtration rate, planned open surgery and preoperative use of either an angiotensin-converting enzyme inhibitor or an angiotensin receptor blocker. Internal validation demonstrated good model discrimination (c-statistic 0·65). Discussion: Following major gastrointestinal surgery, AKI occurred in one in seven patients. This preoperative prognostic model identified patients at high risk of postoperative AKI. Validation in an independent data set is required to ensure generalizability

Geochemistry and evolution of the fuego volcanic complex, Guatemala

Four closely spaced vents along a fissure make up the Fuego and Acatenango volcanic centers in western Guatemala. The Fuego complex is composed of the Fuego and Meseta vents, but historic activity has consisted exclusively of high-Al2O3 basalts from the Fuego vent. The Meseta vent is inactive and deeply exposed. Prehistoric lavas from Fuego and Meseta are generally more silicic than historic Fuego lavas, but all the rocks form a single coherent geochemical variation pattern. Major element chemistry of these rocks is consistent with plagioclase, olivine, augite, and magnetite (POAM) fractionating from high-Al2O3 basalt. Separate batches of magma can be recognized from trace-element data throughout the history of the Fuego complex. This suggests that closed-system, POAM fractionation of distinct magma bodies occurs at Fuego. Trace-element data requires that deep fractionation of olivine, clinopyroxene, and perhaps magnetite from primary olivine tholeiite occurs before arrival of new magma into the shallow (8–16 km) magma chamber at Fuego. Migration of activity from Meseta to Fuego along the fissure is correlated with the change towards more mafic compositions at Fuego. The shift of the vents may have resulted in shorter repose periods and less time for fractionation before eruption. A minimum age of 17,000 years was required to build the Fuego complex. The andesitic rocks from the adjacent, larger composite volcanoes of Acatenango and Agua have higher incompatible element concentrations, different incompatible element ratios, and lower CaO, Na2O, and Al2O3contents than Fuego\u27s lavas. We believe the magmatic evolution of Acatenango and Agua is much more complex than Fuego

Worldwide dispersal of ash and gases from earth\u27s largest known eruption: Toba, Sumatra, 75 ka

The eruption of the Youngest Toba Tuff (YTT) occurred 75 ± 2 ka in North Sumatra (2°45′N, 98°45′E). The eruption produced at least 2800 km3 of dense rock equivalent (DRE) rhyolite magma (7 × 1018 g). Much of the volume of magma was preserved as a non-welded outflow sheet covering 20,000–30,000 km2 and a thick, welded intracaldera tuff. At least 800 km3 (2 × 1018 g) of Toba ash was deposited in an extensive ash blanket over the Indian Ocean and Southern Asia. Detailed studies of the chemistry of minerals and glasses in pumices from the YTT enable estimation of the minimum masses of gaseous components released to earth\u27s atmosphere during the eruption: H2O:. The high eruption rate, with the entire event lasting perhaps 9–14 days make stratospheric venting of a significant fraction of these gases and associated silicate ash and aerosol particles likely. The masses of ash and gases released is nearly two orders of magnitude higher than any known historic eruption

Worldwide dispersal of ash and gases from earth\u27s largest known eruption: Toba, Sumatra, 75 ka

The eruption of the Youngest Toba Tuff (YTT) occurred 75 ± 2 ka in North Sumatra (2°45′N, 98°45′E). The eruption produced at least 2800 km3 of dense rock equivalent (DRE) rhyolite magma (7 × 1018 g). Much of the volume of magma was preserved as a non-welded outflow sheet covering 20,000-30,000 km2 and a thick, welded intracaldera tuff. At least 800 km3 (2 × 1018 g) of Toba ash was deposited in an extensive ash blanket over the Indian Ocean and Southern Asia. Detailed studies of the chemistry of minerals and glasses in pumices from the YTT enable estimation of the minimum masses of gaseous components released to earth\u27s atmosphere during the eruption: H2O:. The high eruption rate, with the entire event lasting perhaps 9-14 days make stratospheric venting of a significant fraction of these gases and associated silicate ash and aerosol particles likely. The masses of ash and gases released is nearly two orders of magnitude higher than any known historic eruption. © 1990

Stratigraphy of the Toba Tuffs and the evolution of the Toba Caldera complex, Sumatra, Indonesia

During the past 1.2 m.y., a magma chamber of batholithic proportions has developed under the 100 by 30 km Toba Caldera Complex. Four separate eruptions have occurred from vents within the present collapse structure, which formed from eruption of the 2800 km3 Youngest Toba Tuff (YTT) at 74 ka. Eruption of the three older Toba Tuffs alternated from calderas situated in northern and southern portions of the present caldera. The northern caldera apparently developed upon a large andesitic stratovolcano. The calderas associated with the three older tuffs are obscured by caldera collapse and resurgence resulting from eruption of the YTT. Samosir Island and the Uluan Block are two sides of a single resurgent dome that has resurged since eruption of the YTT. Samosir Island is composed of thick YTT caldera fill, whereas the Uluan Block consists mainly of the Oldest Toba Tuff (OTT). In the past 74000 years lava domes have been extruded on Samosir Island and along the caldera\u27s western ring fracture. This part of the ring fracture is the site of the only current activity at Toba: updoming and fumarolic activity. The Toba eruptions document the growth of the laterally continuous magma body which eventually erupted the YTT. Repose periods between the four Toba Tuffs range between 0.34 and 0.43 m.y. and give insights into pluton emplacement and magmatic evolution at Toba

Quartz crystals in Toba rhyolites show textures symptomatic of rapid crystallization

Textural and chemical heterogeneities in igneous quartz crystals preserve unique records of silicic magma evolution, yet their origins and applications are controversial. To improve our understanding of quartz textures and their formation, we examine those in crystal-laden rhyolites produced by the 74 ka Toba supereruption (\u3e2800 km3) and its post-caldera extrusions. Quartz crystals in these deposits can reach unusually large sizes (10–20 mm) and are rife with imperfections and disequilibrium features, including embayments, melt inclusions, titanomagnetite and apatite inclusions, spongy morphologies, hollow faces, subgrain boundaries, multiple growth centers, and Ti-enriched arborescent zoning. Using a combination of qualitative and quantitative analyses (petrography, CL, EBSD, X-ray CT, LA-ICPMS), we determine that those textures commonly thought to signify crystal resorption, crystal deformation, synneusis, or fluctuating P–T conditions are here a consequence of rapid disequilibrium crystal growth. Most importantly, we discover that an overarching process of disequilibrium crystallization is manifested among these crystal features. We propose a model whereby early skeletal to dendritic quartz growth creates a causal sequence of textures derived from lattice mistakes that then proliferate during subsequent stages of slower polyhedral growth. In a reversed sequence, the same structural instabilities and defects form when slow polyhedral growth transitions late to fast skeletal-dendritic growth. Such morphological transitions result in texture interdependencies that become recorded in the textural-chemical stratigraphy of quartz, which may be unique to each crystal. Similar findings in petrologic experimental studies allow us to trace the textural network back to strong degrees of undercooling and supersaturation in the host melt, conditions likely introduced by dynamic magmatic processes acting on short geologic timescales. Because the textural network can manifest in single crystals, the overall morphology and chemistry of erupted quartz can reflect not only its last but its earliest growth behavior in the melt. Thus, our findings imply that thermodynamic disequilibrium crystallization can account for primary textural and chemical heterogeneities preserved in igneous quartz and may impact the application of quartz as a petrologic tool

The enigmatic origin and emplacement of the Samosir Island lava domes, Toba Caldera, Sumatra, Indonesia

The 60 × 20-km Samosir resurgent dome within the 74-ka Toba Caldera features several clusters of rhyolitic lava domes. Because the previously known Tuk-Tuk and Samosir Fault lava domes occur near the base of a major resurgent dome fault, they appear associated with resurgent uplift, and dating them could provide constraints on the timing of resurgence. Towards that goal, we mapped and sampled the Tuk-Tuk and Samosir Fault lava domes, discovered two additional lava dome clusters in the uplifted interior of Samosir Island, then determined modal mineralogy, major and trace element geochemistry, and 40Ar/39Ar sanidine ages on several samples from each cluster. These quartz-bearing rhyolite lava domes have SiO2 contents ranging from ~ 70-76 wt%, crystallinities of 28-54%, and 40Ar/39Ar ages (~ 74-76 ka) that overlap with those of the Youngest Toba Tuff (YTT), ~ 74-75 ka. Our field observations and the remarkable similarity in mineralogy, geochemistry, and ages among the dome clusters and YTT support the interpretation that these domes represent remnant YTT magma that erupted shortly after the climactic YTT eruption. Later, during resurgence, these pre-resurgent lava domes were uplifted to their present locations, some near current lake level, others up to 440 m above lake level

Application of color-TV microscopy to self-taught optical mineralogy

Eleven 40–60 minute color videocassettes have been produced to supply all the practical and theoretical background for an eleven week optical mineralogy training course. The tapes use extensive down-the-tube microscopy which was produced with a direct color-TV linkage. The TV camera is also used for live laboratory discussions. The tapes greatly relieve the problem of recognition of optical properties and figures and achieve, with rapid transition from diagram to microscopic views, a strong link between simple theory and practice. Interactive computer graphics exercises in three-dimensional crystallography and optical crystallography are being assembled to aid in visualization. The goal is self-taught, self-spaced optical mineralogy