Platinum–Group Minerals in Podiform Chromitite from the Kamuikotan Zone, Hokkaido, Northern Japan

Abstract: Ru–Os–Ir alloys have been found in two podiform chromitites located at the Chiroro and Bankei mines in the Sarugawa peridotite complex in the Kamuikotan zone, Hokkaido, Japan. This is the first report on the occurrence of PGM (= platinum-group minerals) from chromitites in Japan. The Ru–Os–Ir alloys most typically form polyhedra associated with other minerals (Ni–Fe alloys and heazlewoodite) in chromian spinel. The PGM are possibly pseudomorphs after some primary PGM such as laurite and are chemically highly inhomogeneous, indicating a low-temperature alteration origin. This is consistent with intense alteration (formation of serpentine, uvarovite and kämmererite) imposed on the Kamuikotan chromitites. High-temperature primary PGE (platinum–group elements)–bearing sulfides were possibly recrystallized at low temperatures into a new assemblage of PGM, Ni-Fe alloys and sulfides. Placer PGM around the peridotite complexes are chemically different from the PGM in dunite and chromitite possibly due to the, as yet, incomplete search for the rock-hosted PGM. The PGE content in chromitites is distinctly higher in those in the Kamuikotan zone than in those in the Sangun zone of Southwest Japan, consistent with the more refractory nature (Cr# of spinel, up to 0.8) of the former than the latter (Cr# of spinel, 0.5).     

Numerical modelling study of mechanisms of mid‐basin salt canopy evolution and their potential applications to the Northwestern Gulf of Mexico

Salt canopies are present in many of the worldwide large salt basins and are key players in the basins' structural evolution as well as in the development of associated hydrocarbon systems. This study employs 2D finite‐element models which incorporate the dynamical interaction of viscous salt and frictional‐plastic sediments in a gravity‐spreading system. We investigate the general emplacement of salt canopies that form in the centre of a large, autochthonous salt basin. This is motivated by the potential application to a mid‐basin canopy in the NW Gulf of Mexico (GoM) that developed in the late Eocene. Three different salt expulsion and canopy formation concepts that have been proposed in the salt‐tectonic literature for the GoM are tested. Two of these mechanisms require pre‐existing diapirs as precursory structures. We include their evolution in the models to assure a continuous, smooth evolution of the salt‐sediment system. The most efficient canopy formation takes place under the squeezed diapir mechanism. Here, shortening of a region containing pre‐existing diapirs is absorbed by the salt (the weakest part of the system), which is then expelled onto the seafloor. The expulsion rollover mechanism, which evacuates salt from beneath evolving rollover structures and expels it both laterally and to the surface, was not successfully captured by the numerical models. No rollover structures developed and only minor amounts of allochthonous salt emerged to the seafloor. The breached anticline mechanism requires substantial shortening of salt‐cored, pre‐weakened folds such that the salt breaches the anticlines and is expelled to the seafloor. The amount of shortening may be too large to occur in the central part of a salt basin, but may explain canopy evolution closer to the distal end of the allochthonous salt. When applying the different concepts to the northwestern GoM, none of the models adequately describes the entire system, yet the squeezed diapir mechanism captures most structural features of the Eocene paleocanopy. It is nevertheless possible that different mechanisms have acted in combination or sequentially in the northwestern GoM.     

Seasonal Forecasts of the Summer 2016 Yangtze River Basin Rainfall

The Yangtze River has been subject to heavy flooding throughout history,and in recent times severe floods such as those in 1998 have resulted in heavy loss of life and livelihoods.Dams along the river help to manage flood waters,and are important sources of electricity for the region.Being able to forecast high-impact events at long lead times therefore has enormous potential benefit.Recent improvements in seasonal forecasting mean that dynamical climate models can start to be used directly for operational services.The teleconnection from El Ni ?no to Yangtze River basin rainfall meant that the strong El Ni ?no in winter 2015/16 provided a valuable opportunity to test the application of a dynamical forecast system.This paper therefore presents a case study of a real-time seasonal forecast for the Yangtze River basin,building on previous work demonstrating the retrospective skill of such a forecast.A simple forecasting methodology is presented,in which the forecast probabilities are derived from the historical relationship between hindcast and observations.Its performance for2016 is discussed.The heavy rainfall in the May–June–July period was correctly forecast well in advance.August saw anomalously low rainfall,and the forecasts for the June–July–August period correctly showed closer to average levels.The forecasts contributed to the confidence of decision-makers across the Yangtze River basin.Trials of climate services such as this help to promote appropriate use of seasonal forecasts,and highlight areas for future improvements.     

Magma transfer processes at persistently active volcanoes: insights from gravity observations

Magma transfer processes at persistently active volcanoes are distinguished by the large magma flux required to sustain the prodigious quantities of heat and gas emitted at the surface. Although the resulting degassed magma has been conjectured to accumulate either deep within the volcanic edifice or in the upper levels of the sub-edifice system, no direct evidence for such active accumulation has been reported. Temporal gravity data are unique in being able to quantify mass changes and have been successfully used to model shallow magma movements on different temporal scales, but have not generally been applied to the investigation of postulated long-term accumulation of magma at greater spatial scales within volcanic systems. Here, we model the critical data acquisition parameters required to detect mass flux at volcanoes, we review existing data from a number of volcanoes that exemplify the measurement of shallow mass changes and present new data from Poas and Telica volcanoes. We show that if a substantial proportion of degassed magma lodges within the sub-edifice region, it would result in measurable annual to decadal gravity increases occurring over spatial scales of tens of kilometres and propose that existing microgravity data from Sakurajima and, possibly, Etna volcanoes could be interpreted in these terms. Furthermore, such repeat microgravity data could be used to determine whether the accumulation rate is in equilibrium with the rate of production of degassed magma as calculated from the surface gas flux and hence identify the build-up of gas-rich magma at depth that may be significant in terms of eruption potential. We also argue that large magma bodies, both molten and frozen, modelled beneath volcanoes from seismic and gravity data, could represent endogenous or cryptic intrusions of degassed magma based on order of magnitude calculations using present-day emission rates and typical volcano lifetimes.     

Salt tectonics driven by differential sediment loading: stability analysis and finite-element experiments

At many continental margins, differential sediment loading on an underlying salt layer drives salt deformation and has a significant impact on the structural evolution of the basin. We use 2‐D finite‐element modelling to investigate systems in which a linear viscous salt layer underlies a frictional‐plastic overburden of laterally varying thickness. In these systems, differential pressure induces the flow of viscous salt, and the overburden experiences updip deviatoric tension and downdip compression. A thin‐sheet analytical stability criterion for the system is derived and is used to predict conditions under which the sedimentary overburden will be unstable and fail, and to estimate the initial velocities of the system. The analytical predictions are in acceptable agreement with initial velocity patterns of the numerical models. In addition to initial stability analyses, the numerical model is used to investigate the subsequent finite deformation. As the systems evolve, overburden extension and salt diapirism occur in the landward section and contractional structures develop in the seaward section. The system evolution depends on the relative widths of the salt basin and the length scale of the overburden thickness variation. In narrow salt basins, overburden deformation is localised and characterised by high strain rates, which cause the system to reach a gravitational equilibrium and salt movement to cease earlier than for wide salt basins. Sedimentation enhances salt evacuation by maintaining a differential pressure in the salt. Continued sedimentary filling of landward extensional basins suppresses landward salt diapirism. Sediment progradation leads to seaward propagation of the landward extensional structures and depocentres. At slow sediment progradation rates, the viscous flow can be faster than the sediment progradation, leading to efficient salt evacuation and salt weld formation beneath the landward section. Fast sediment progradation suppresses the viscous flow, leaving salt pillows beneath the prograding wedge.     

Saturn's icy satellites

Hazel J McAndrews – and colleagues including the CAPS Team – presents initial results from the Cassini Plasma Spectrometer, based on the talk which won her a Rishbeth Prize at the UK MIST meeting in April.