Future Benefits of Time-Varying Gravity Missions to Ocean Circulation Studies

A summary is offered of the potential benefits of future measurements of temporal variations in gravity for the understanding of ocean dynamics. Two types of process, and corresponding amplitudes are discussed: ocean basin scale pressure changes, with a corresponding amplitude of order 1 cm of water, or 1 mm of geoid height, and changes in along-slope pressure gradient, at cross-slope length scales corresponding to topographic slopes, with a corresponding amplitude of order 1 mm of water, or a maximum of about 0.01 mm of geoid. The former is feasible with current technology and would provide unprecedented information about abyssal ocean dynamics associated with heat transport and climate. The latter would be a considerable challenge to any foreseeable technology, but would provide an exceptionally clear, quantitative window on the dynamics of abyssal ocean currents, and strong constraints on ocean models. Both options would be limited by the aliassing effect of rapid mass movements in the earth system, and it is recommended that any future mission take this error source explicitly into account at the design stage. For basin-scale oceanography this might involve a higher orbit than GRACE or GOCE, and the advantages of exact-repeat orbits and multiple missions should be considered.     

Another look at theR-modes

Possible oscillation modes for a rotating star are listed. The only assumption made is that oscillations are adiabatic and that rotation is uniform. It is found that two modes not present for non-rotating stars are possible. Oscillation frequencies of these modes are rather different from those given in the literature for so-calledr-modes.     

The Nyquist frequency for irregularly spaced time-series: a calculation formula

Eyer & Bartholdi showed that the Nyquist frequency of irregularly sampled time-series can be very high. In this paper, a calculation formula for the Nyquist frequency is presented. In practice there is an upper limit of 0.5/Δ on this frequency, where Δ is the best accuracy with which time is recorded.     

New insights into volcanic processes from deep mining of the southern diatreme within the Argyle lamproite pipe,Western Australia

Underground mining and deep drilling of the richly diamondiferous ~1.2 Ga Argyle lamproite in Western Australia has prompted a re-evaluation of the geology of the pipe. Argyle is considered to be a composite pipe that formed by the coalescence of several diatremes and has been offset and elongated by post-emplacement faulting. Recent geological studies have recognised at least five distinct volcaniclastic lamproite lithofacies with differing diamond grades. The new data suggest that the centre of the southern (main) diatreme is occupied by well-bedded, olivine lamproite lapilli tuff with very high diamond grades (>10 ct/t). Characteristic features include a clast-supported fabric and high modal abundance of densely packed lamproite lapilli and coarse-grained, likely mantle-derived olivine now replaced by serpentine and/or talc. The persistence of small-scale graded and cross-bedding in this lithofacies to depths of ~1.5 km below the original surface prior to erosion suggests phreatomagmatic volcanism forming the diatreme was syn-eruptively accompanied by subsidence of the tephra, maintaining a steep-walled diatreme in the water-saturated country rock sediments.

     

Th/U ratios in metamorphic zircon

The Th/U ratios of zircon crystals are routinely used to help understand their growth mechanism. Despite the wide application of Th/U ratios in understanding the geological significance of zircon U–Pb ages, the main controls on the Th/U ratio in metamorphic zircon are poorly understood. Here, phase equilibria modelling coupled with solubility expressions for accessory minerals are used to investigate the controls on the Th/U ratios of suprasolidus metamorphic zircon in an average amphibolite facies metapelite composition. We also present a new database of metamorphic Th/U ratios in zircon from Western Australia. Several factors affecting the Th/U ratio are investigated, including the bulk rock concentrations of Th and U, the amount of monazite in the system, and open v. closed system behaviour. Our modelling predicts that the main controls on the Th/U ratio of suprasolidus metamorphic zircon are the concentrations of Th and U in the system, and the breakdown and growth of monazite in equilibrium with zircon. Furthermore, the relative timing of zircon and monazite growth during cooling and melt crystallization has an important role in the Th/U ratio of zircon. Early grown zircon near the peak of metamorphism is expected to have elevated Th/U ratios whereas zircon that grew near the solidus is predicted to have relatively low Th/U ratios, which reflects the coeval growth of monazite during cooling and melt crystallization. Our modelling approach aims to provide an improved understanding of the main controls of Th/U in metamorphic zircon in migmatites and hence better apply this geochemical ratio as a tool to assist in interpretation of the genesis of metamorphic zircon.     

A conceptual model of avalanche hazard

This conceptual model of avalanche hazard identifies the key components of avalanche hazard and structures them into a systematic, consistent workflow for hazard and risk assessments. The method is applicable to all types of avalanche forecasting operations, and the underlying principles can be applied at any scale in space or time. The concept of an avalanche problem is introduced, describing how different types of avalanche problems directly influence the assessment and management of the risk. Four sequential questions are shown to structure the assessment of avalanche hazard, namely: (1) What type of avalanche problem(s) exists? (2) Where are these problems located in the terrain? (3) How likely is it that an avalanche will occur? and (4) How big will the avalanche be? Our objective was to develop an underpinning for qualitative hazard and risk assessments and address this knowledge gap in the avalanche forecasting literature. We used judgmental decomposition to elicit the avalanche forecasting process from forecasters and then described it within a risk-based framework that is consistent with other natural hazards disciplines.