Multiphase petrography of volcanic rocks using element maps: a method applied to Mount St. Helens, 1980–2005

Quantitative textural analyses including crystal size distributions (CSDs) provide insights into crystallisation kinetics of magmatic systems. Investigations of volcanic crystal textures often rely on greyscale variations on backscattered electron images to identify crystal phases, which must then be thresholded and/or traced manually, a laborious task, and investigations are typically restricted to a single crystal phase. A method is presented that uses energy-dispersive X-ray element maps to generate textural data. Each pixel is identified as a crystal phase, glass or vesicle according to relative chemical composition enabling concurrent acquisition of multiphase CSD, crystallinity and mineral mode data. Data processing is less time intensive for the operator but considerable instrument time is required to generate element maps. The method is applied to 17 dacite samples from the 1980–1986 and 3 from the 2004–2005 eruptive periods of Mount St. Helens volcano (USA) to provide quantitative insights into multiphase textural evolution. All of the CSDs are curved and concave-up in the standard CSD plot with curvature increasing with plagioclase content. To facilitate comparisons with previous studies, CSDs for microlites (<50?μm length crystals) are approximated as straight lines. The line intercepts and slopes provide information on n ⁰ (nucleation density) and characteristic length or Gτ (the product of growth rate (G) and residence time (τ)), respectively. These parameters, as well as the total groundmass crystallinity, show distinct differences between explosive deposits from summer 1980 and post-summer 1980 domes. Post-summer 1980 microlite n ⁰ values are mostly at the lower end of the range of those measured for summer 1980 samples. Total groundmass crystallinities during summer 1980 are between 10 and 30?vol.%, whereas post-summer 1980 crystallinity increases to between 39 and 51?vol.%. The range of n ⁰ values is similar to those previously published for Mount St. Helens but Gτ is consistently higher. Gτ of a May 1985 sample analysed in this study is approximately 2?μm higher compared with previously published data for the same sample when processed using similar methodologies. Groundmass crystallinity data show similar trends to those previously published for the 1980 to 1986 eruption, increasing sharply after summer 1980 then increasing more gradually during the dome-building phase of the eruption. The effects of varying L, the apparent crystal size, and crystal aspect ratio on resultant CSDs are also investigated. Whilst relative temporal variations in CSDs for a given dataset can be investigated, absolute values from different studies cannot be compared unless methods of data acquisition and processing are exactly the same.     

Analysis of coupled axial and lateral deformation of roots in soil

Plant roots can help to stabilise slopes. Existing analytical models to predict their mechanical contribution are however limited: they typically focus on the ultimate limit state, employ various empirical factors, and ignore much of the underlying root-soil interaction. A new model was developed based on large deflection Euler-Bernoulli elastic beam theory that can be used to study the mobilisation of root strength under various loading conditions (direct shear and pull-out). Both lateral and axial loading of the root by the soil were incorporated, based on existing methodologies for foundation piles (p-y and t-z curves). The model is able to take the key parameters into account (root biomechanical properties, root architectural properties, and soil properties) while remaining quick to solve using a numerical boundary value problem solver. The model was compared with experimental direct shear test data using various root analogues (rubber, plastic, and wood) in dry sand with various densities and effective stress levels and was able to accurately predict the measured shear force-displacement behaviour. Comparison with experimentally measured pull-out force-displacement curves using rubber and wooden root analogues with various architectures in dry and partially saturated sands was also satisfactory. In the future, this model can aid with addressing long-standing problems in the root-reinforcement community: quantifying the effect of (sequential) mobilisation of root strength in direct shear, the effect of the angle at which the root crosses a shear plane, the effect of root topology on root-reinforcement or the effect of root bending, and root shear shear forces on root-reinforcement.     

Chemical composition of short sediment cores from Thermaikos Gulf (Eastern Mediterranean): Sediment accumulation rates, trawling and winnowing effects

Four cores recovered within the framework of the INTERPOL Project have been analysed for their grain size and geochemistry; sediment accumulation rates (SARs) were also determined from ²¹⁰Pb and ¹³⁷Cs profiles. Two cores are representative of the Axios and Aliakmon Rivers depositional environment, whilst the third core represents the Pinios River province; the fourth core represents an environment of outer shelf relict sands. Apparent SARs ranged between 0.667 g cm⁻² yr⁻¹ (Axios and Aliakmon Rivers) and 0.414 g cm⁻² yr⁻¹ (Pinios River). Trawling activities and biomixing are critical processes that may be responsible for the mixing of the surface sediments, as observed from the excess ²¹⁰Pb profiles. The thickness of the surface mixed layer was 4.5 cm in the vicinity of Axios and Aliakmon Rivers and in the area of Pinios River, 3.75 cm on the outer shelf and 1 cm in the area where no trawling was observed. Sediment accumulation appeared to be regulated by variations in the riverine discharge, shelf transport pathways and winnowing processes. Major element variations, such as Si, Al, Ti, V and Ni, were dominated by terrigenous supply as aluminosilicate minerals and quartz, whereas most Ca and Sr were biogenic. Si/Al and Ca/Al ratios have been used to express changes in sediment accumulation and winnowing. Redox processes were depicted by Mn, which showed an increase in the depth of its redoxcline, from 1 cm in inshore stations to 2 cm on the outer shelf. Si/Al ratios follow the Ca/Al ratios and can be used to assess percentage winnowing in the sediment. Increases in these ratios indicate a decrease in sediment input rates and are seen in the upper parts of most of the cores. Anthropogenic or ‘excess’ metal contents have been calculated from Zn/V and Pb/V ratios. Their distributions in the cores showed that by far the highest contamination is associated with the Axios River output, whilst sediments influenced by the Pinios River were relatively uncontaminated.     

The microfossils from the proterozoic Urquhart Shale,Mount Isa,Queensland, and their significance in relation to the depositional environment,diagenesis, and mineralisation

The Urquhart Shale from Mount Isa, Queensland, hosts major lead-zinc and copper orebodies. Re-examination of organic matter from the lead-zinc ore bodies has shown that abundant microfossils characteristic of lacustrine environments are present, and substantiates earlier results of Love and Zimmerman (1961). The framboid residues described by these authors are not microfossils but nevertheless the occurrence of framboidal pyrite has important implications for the timing of lead-zinc mineralisation. A diagenetic model is proposed for the lead-zinc ore bodies, and a later transgressive event accounts for the mode of occurrence of the copper ore bodies.