The mineralogy and microtextures of samples of the Mercia Mudstone Group from Northgates, Leicester

This report presents the results of mineralogical, petrographical and surface area analyses carried out on a suite of five samples taken from the Mercia Mudstone Group. The samples were removed from boreholes drilled for the Northgates, Leicester CSO Improvements Scheme and were submitted for analysis by Mr. D. Ouston of Haswell Consulting Engineers Ltd. The client expressed a particular interest in characterizing the clay mineralogy of the samples and the identification and quantification of any 'swelling' clay minerals. The samples, four of which were provided in plastic cartons (two per sample) while the fifth was delivered in a plastic sack, all comprise dark red-brown slurries containing fragments of mudstone, siltstone and sandstone up to 5 cm in length. Sample details are shown in Table 1

A geochemical investigation of fragmentation and physical fractionation in pyroclastic flows from from the Soufriere Hills volcano, Montserrat

Abstract Geochemical analysis is used to investigate fragmentation and physical fractionation in pyroclastic flows. Bulk analyses of the matrices (<4 mm) and individual size fractions in pyroclastic flow deposits formed in the eruption of the Soufrière Hills volcano, Montserrat, West Indies are compared with analyses of associated ash fall deposits formed from lofting plumes above the flows, and with bulk lava analyses. Within the flow matrices intermediate grain size fractions (<4 mm to 125 μm) are depleted in the groundmass component of the lava (principally glass and micro-crystalline silica) and enriched in phenocryst components. Fine-grained size fractions (<125 μm) are enriched in groundmass components. Crushing of the lava in the laboratory with analysis of grain size fractions shows the same relationship, but enrichment and depletions are less pronounced. Comparison of the bulk compositions of matrices, ash fall deposits and lava show that the finest fractions, enriched in the groundmass component, have been selectively removed from the flows into the lofting ash plumes. Mass balance calculations indicate that typically about 10% of the mass of the pyroclastic flows are elutriated into lofting ash plumes to form ash fall deposits, which is consistent with data on relative volumes of the deposits. Three factors influence fragmentation and fractionation. First, the initial size distribution of crystals influences fragmentation with phenocrysts resisting break up, whereas fine groundmass minerals (mostly feldspar, glass and micro-crystalline silica) are preferentially fragmented to form the finest ash fractions. Second, the mineral phases and groundmass glass vary in strength so that vesicular glass fragments more rapidly than other silicate minerals. This interpretation is supported by crushing experiments on different rock types. Third, only the finest fractions are elutriated into the lofting ash plume from the pyroclastic flows. Description of the natural size distributions in terms of a power law and fractal dimensions indicates that fragmentation is dominated by a single stage fragmentation process with secondary crushing and abrasion only being of minor importance

Natural emissions of CO2 from the geosphere and their bearing on the geological storage of carbon dioxide

Carbon dioxide (CO2) capture and storage has the potential to reduce CO2 emissions from fossil fuel combustion. Although leakage from monitored CO2 injection sites has been minimal to non-existent, experience from the natural gas storage industry suggests that, if it becomes a widely deployed technology, leaks may be expected from some storage sites. Natural occurrences of CO2 in the geosphere, some of which have been exploited, provide insights into the types of emissions that might be expected from anthropogenic CO2 storage sites. CO2 emission sites are commonly found in clusters in CO2-prone geological provinces: the most common natural emissions sites in sedimentary basins consist of carbonated springs and mofettes. These represent at worst only a local hazard. In volcanic and hydrothermal provinces, more energetic emissions may occur due to active supply from degassing magma. These include rare, sudden emissions from fissures and craters that have caused fatalities. It is unlikely that such provinces would be considered for CO2 storage Major lake overturn events such as occurred at Lake Nyos in 1986 are considered highly unlikely to occur as a result of CO2 storage, not least because CO2 levels in lake waters can be monitored and remediated. Natural CO2 fields indicate that under favourable conditions CO2 can be retained in the subsurface for millions of years. The main risk from man-made CO2 storage sites that does not have any close analogy in nature is considered to be a well blowout. A blowout that took place at a natural CO2 field provides some indication of the likely hazard

Origin of CFB magmatism: Multi-tiered intracrustal picrite-rhyolite magmatic plumbing at Spitzkoppe, western Namibia, during early-Cretaceous Etendeka magmatism

Early Cretaceous tholeiitic picrite-to-rhyolite dykes around Spitzkoppe, western Namibia, are part of the extensive Henties Bay–Outjo swarm, penecontemporaneous with 132 Ma Etendeka lavas 100 km to the NW. Although only intermediate to rhyolitic dykes contain clinopyroxene phenocrysts, the behaviour of Ca, Al and Sc in the dyke suite shows that liquidus clinopyroxene—together with olivine—was a fractionating phase when MgO fell to 9 wt %. Both a plot of CIPW normative di–hy–ol–ne–Q and modelling using (p)MELTS show that a mid-crustal pressure of 0·6 GPa is consistent with this early clinopyroxene saturation. Sr, Nd, Hf and Pb isotope variations all show trends consistent with AFC contamination (assimilation linked to fractional crystallization), involving Pan-African Damara belt continental crust. The geochemical variation, including isenthalpic AFC modelling using (p)MELTS, suggests that the picrites (olivine-rich cumulate suspensions) were interacting with granulite-facies metamorphic lower crust, the intermediate compositions with amphibolite-facies middle crust, and the rhyolitic dykes (and a few of the basalts) with the Pan-African granites of the upper crust. The calculated densities of the magmas fall systematically from picrite to rhyolite and suggest a magmatic system resembling a stack of sills throughout the crust beneath Spitzkoppe, with the storage and fractionation depth of each magma fraction controlled by its density. Elemental and isotopic features of the 20 wt % MgO picrites (including Os isotopes) suggest that their parental melts probably originated by fusion of mid-ocean ridge basalt (MORB) source convecting mantle, followed by limited reaction with sub-continental lithospheric mantle metasomatized just prior to the formation of the parental magmas. Many of the distinctive features of large-volume picritic–basaltic magmas may not be derived from their ultimate mantle sources, but may instead be the results of complex polybaric fractional crystallization and multi-component crustal contamination