The evolution and storage of primitive melts in the Eastern Volcanic Zone of Iceland: the 10 ka Grímsvötn tephra series (i.e. the Saksunarvatn ash)

Major, trace and volatile elements were measured in a suite of primitive macrocrysts and melt inclusions from the thickest layer of the 10 ka Grímsvötn tephra series (i.e. Saksunarvatn ash) at Lake Hvítárvatn in central Iceland. In the absence of primitive tholeiitic eruptions (MgO > 7 wt.%) within the Eastern Volcanic Zone (EVZ) of Iceland, these crystal and inclusion compositions provide an important insight into magmatic processes in this volcanically productive region. Matrix glass compositions show strong similarities with glass compositions from the AD 1783–84 Laki eruption, confirming the affinity of the tephra series with the Grímsvötn volcanic system. Macrocrysts can be divided into a primitive assemblage of zoned macrocryst cores (An_78–An_92, Mg#_cpx = 82–87, Fo_79.5–Fo_87) and an evolved assemblage consisting of unzoned macrocrysts and the rims of zoned macrocrysts (An_60–An_68, Mg#_cpx = 71–78, Fo_70–Fo_76). Although the evolved assemblage is close to being in equilibrium with the matrix glass, trace element disequilibrium between primitive and evolved assemblages indicates that they were derived from different distributions of mantle melt compositions. Juxtaposition of disequilibrium assemblages probably occurred during disaggregation of incompatible trace element-depleted mushes (mean La/Yb_melt = 2.1) into aphyric and incompatible trace element-enriched liquids (La/Yb_melt = 3.6) shortly before the growth of the evolved macrocryst assemblage. Post-entrapment modification of plagioclase-hosted melt inclusions has been minimal and high-Mg# inclusions record differentiation and mixing of compositionally variable mantle melts that are amongst the most primitive liquids known from the EVZ. Coupled high field strength element (HFSE) depletion and incompatible trace element enrichment in a subset of primitive plagioclase-hosted melt inclusions can be accounted for by inclusion formation following plagioclase dissolution driven by interaction with plagioclase-undersaturated melts. Thermobarometric calculations indicate that final crystal-melt equilibration within the evolved assemblage occurred at ~1140°C and 0.0–1.5 kbar. Considering the large volume of the erupted tephra and textural evidence for rapid crystallisation of the evolved assemblage, 0.0–1.5 kbar is considered unlikely to represent a pressure of long-term magma accumulation and storage. Multiple thermometers indicate that the primitive assemblage crystallised at high temperatures of 1240–1300°C. Different barometers, however, return markedly different crystallisation depth estimates. Raw clinopyroxene-melt pressures of 5.5–7.5 kbar conflict with apparent melt inclusion entrapment pressures of 1.4 kbar. After applying a correction derived from published experimental data, clinopyroxene-melt equilibria return mid-crustal pressures of 4±1.5 kbar, which are consistent with pressures estimated from the major element content of primitive melt inclusions. Long-term storage of primitive magmas in the mid-crust implies that low CO_2 concentrations measured in primitive plagioclase-hosted inclusions (262–800 ppm) result from post-entrapment CO_2 loss during transport through the shallow crust. In order to reconstruct basaltic plumbing system geometries from petrological data with greater confidence, mineral-melt equilibrium models require refinement at pressures of magma storage in Iceland. Further basalt phase equilibria experiments are thus needed within the crucial 1–7 kbar range.D.A.N. was supported by a Natural Environment Research Council studentship (NE/1528277/1) at the start of this project. SIMS analyses were supported by Natural Environment Research Council Ion Microprobe Facility award (IMF508/1013).This is the final version of the article. It first appeared from Springer via http://dx.doi.org/10.1007/s00410-015-1170-

South African HIV-1 subtype C transmitted variants with a specific V2 motif show higher dependence on α4β7 for replication.

CAPRISA, 2015.Abstract available in pdf

Identification of potentially dangerous glacial lakes in the northern Tien Shan

Like in many other parts of the world, the glaciers in northern Tien Shan are receding, and the permafrost is thawing. Concomitantly, glacial lakes are developing. Historically, outbursts of these glacial lakes have resulted in severe hazards for infrastructures and livelihood. Multi-temporal space imageries are an ideal means to study and monitor glaciers and glacial lakes over large areas. Geomorphometric analysis and modelling allows to estimate the potential danger for glacial lake outburst floods (GLOFs). This paper presents a comprehensive approach by coupling of remote sensing, geomorphometric analyses aided with GIS modelling for the identification of potentially dangerous glacial lakes. We suggest a classification scheme based on an additive ratio scale in order to prioritise sites for detailed investigations. The identification and monitoring of glacial lakes was carried out semi-automatically using band ratioing and the normalised difference water index (NDWI) based on multi-temporal space imagery from the years 1971 to 2008 using Corona, ASTER and Landsat data. The results were manually edited when required. The probability of the growth of a glacial lake was estimated by analysing glacier changes, glacier motion and slope analysis. A permafrost model was developed based on geomorphometric parameters, solar radiation and regionalised temperature conditions which permitted to assess the influence of potential permafrost thawing. Finally, a GIS-based model was applied to simulate the possibly affected area of lake outbursts. The findings of this study indicate an increasing number and area of glacial lakes in the northern Tien Shan region. We identified several lakes with a medium to high potential for an outburst after a classification according to their outburst probability and their downstream impact. These lakes should be investigated more in detail