Rheology and Multiphase Flow in Congested Ammonia-Water-Ice Slurries

Cryomagmas are natural examples of complex multiphase fluids. Some preliminary statements are made about the rheology of densely packed suspensions that have important bearing on their flow rates and eruptive style. Abstract no.104

Addams Crater, Venus, Venus: outflow analogous with a submarine debris flow?

The extraordinary outflow length and morphology of Addams crater deposits are compared with the Saharan submarine debris flow off Northwest Africa. Vapor cloud modelling and comparison of sonar data with radar images suggest a possible similar origin for the Addams crater outflow deposits

Goalposts on the Move

The impact profile of recent UK geoscience research is set to change, say Nick Petford & Jonathan Adam

Bucking the trend: part-time Master’s students at the University of Northampton

Nick Petford, Vice Chancellor of the University of Northampton, and Nick Allen, Executive Officer at the University of Northampton, explain how their institution has bucked the decline in part-time study for postgraduates and push the benefits of working with local industry partners on bespoke programmes

Deformation-Induced Mechanical Instabilities at the Core-Mantle Boundary

Post-Perovskite: The Last Mantle Phase Transition Our understanding of the core-mantle boundary (CMB) region has improved significantly over the past several years due, in part, to the discovery of the post-perovskite phase. Sesimic data suggest that the CMB region is highly heterogeneous, possibly reflecting chemical and physical interaction between outer core material and the lowermost mantle. In this contribution we present the results of a new mechanism of mass transfer across the CMB and comment on possible repercussions that include the initiation of deep, siderophile-enriched mantle plumes. We view the nature of core-mantle interaction, and the geodynamic and geochemical ramifications, as multiscale processes, both spatially and temporally. Three lengthscales are defined. On the microscale (1-50 km), we describe the effect of loading and subsequent shearing of the CMB region and show how this may drive local flow of outer core fluid upwards into D". We propose that larger scale processes operating on a mesoscale (50-300 km) and macroscale regimes (> 300 km) are linked to the microscale, and suggest ways in which these processes may impact on global mantle dynamics

Changing Sources of Magma Generation Beneath Intra-Oceanic Islands Arcs: An Insight From the Juvenile Kohistan Island Arc, Pakistan Himalaya

The Kohistan arc, situated in the Pakistan Himalaya, is a Cretaceous intraoceanic island arc which was initiated during the northward movement of the Indian Plate. The arc was sutured to Asia at ca. 100 Ma. It was subsequently tilted northward when underplated by Indian continental crust during the early stages of India–Asia collision. Deep erosion of this tilted section provides a spectacular section through the whole arc sequence and offers a profound insight into the mechanisms of early stages of arc formation. Geochemical analysis and rare earth element modelling of basaltic sequences which date from the intraoceanic stages of arc development allow identification of three main magma source types in the mantle beneath the juvenile arc. The ‘E-type’ Kamila Amphibolites, with a MORB-type chemistry, form the intraoceanic basement to the arc. The ‘D-type’ Kamila Amphibolites are the earliest of the arc volcanic rocks. These were extracted from a primitive spinel-bearing mantle source, above a north-dipping subduction zone. The stratigraphically younger basalts of the Jaglot Group and Ghizar Formation of the Chalt Volcanic Group were derived from partial melting of a garnet-bearing source at greater depth. The Hunza Formation of the Chalt Volcanic Group contains the youngest mafic volcanic rocks of the intraoceanic arc. Although coeval with the Ghizar Formation of the Chalt Volcanic Group, they were generated by melting of a depleted, spinel-bearing mantle source rock and were erupted into a spatially and temporally restricted back-arc basin developed behind the volcanic front. The Chalt Volcanic Group was therefore formed from two different, adjacent, mantle source regions active at the same time. Results of REE modelling are consistent with models for intraoceanic arc formation in which the earliest volcanic rocks are derived from shallow level spinel-bearing peridotite, and later ones from a deeper garnet-bearing source. This is consistent with the melt region becoming deeper with time as subduction continues. A two-stage model is proposed for the back-arc basalts of the Hunza Formation in which a mantle source, depleted from a previous melting event, is underplated beneath the arc and later remelted during decompression as a consequence of extension and rifting of the arc

Thermally activated reversal in exchange-coupled structures

In this paper, we study the thermally activated reversal of IrMn/CoFe exchange-coupled structures using Lorentz microscopy and magnetometry. An asymmetry and a training effect were found on the hysteresis loops both with and without holding the film at negative saturation of the ferromagnetic layer. Holding the film at negative saturation results in the hysteresis loop shifting toward zero field. We believe that, in this system, two energy barrier distributions with different time constants coexist. The large-time-constant thermally activated reversal of the antiferromagnetic layer contributes to a increasing shift of the entire hysteresis loop toward zero field with increased period of time spent at negative saturation of the ferromagnetic layer. The small-time-constant thermal activation contributes to asymmetry in the magnetization reversal and training effects

Which effective viscosity?

Magmas undergoing shear are prime examples of flows that involve the transport of solids and gases by a separate (silicate melt) carrier phase. Such flows are called multiphase, and have attracted much attention due to their important range of engineering applications. Where the volume fraction of the dispersed phase (crystals) is large, the influence of particles on the fluid motion becomes significant and must be taken into account in any explanation of the bulk behaviour of the mixture. For congested magma deforming well in excess of the dilute limit (particle concentrations >40% by volume), sudden changes in the effective or relative viscosity can be expected. The picture is complicated further by the fact that the melt phase is temperature- and shear-rate-dependent. In the absence of a constitutive law for the flow of congested magma under an applied force, it is far from clear which of the many hundreds of empirical formulae devised to predict the rheology of suspensions as the particle fraction increases with time are best suited. Some of the more commonly used expressions in geology and engineering are reviewed with an aim to home in on those variables key to an improved understanding of magma rheology. These include a temperature, compositional and shear-rate dependency of viscosity of the melt phase with the shear-rate dependency of the crystal (particle) packing arrangement. Building on previous formulations, a new expression for the effective (relative) viscosity of magma is proposed that gives users the option to define a packing fraction range as a function of shear stress. Comparison is drawn between processes (segregation, clustering, jamming), common in industrial slurries, and structures seen preserved in igneous rocks. An equivalence is made such that congested magma, viewed in purely mechanical terms as a high-temperature slurry, is an inherently non-equilibrium material where flow at large Péclet numbers may result in shear thinning and spontaneous development of layering