The sources of granitic melt in Deep Hot Zones

A Deep Hot Zone develops when numerous mafic sills are repeatedly injected at Moho depth or are scattered in the lower crust. The melt generation is numerically modelled for mafic sill emplacement geometries by overaccretion, underaccretion or random emplacement, and for intrusion rates of 2, 5 and 10 mm/yr. After an incubation period, melts are generated by incomplete crystallisation of the mafic magma and by partial melting of the crust. The first melts generated are residual from the mafic magmas that have low solidi due to concentration of H2O in the residual liquids. Once the solidus of the crust is reached, the ratio of crustal partial melt to residual melt increases to a maximum. If wet mafic magma, typical of arc environments, is injected in an amphibolitic crust, the residual melt is dominant over the partial melt, which implies that the generation of I-type granites is dominated by the crystallisation of mafic magma originated from the mantle and not by the partial melting of earlier underplated material. High ratios of crustal partial melt over residual melt are reached when sills are scattered in a metasedimentary crust, allowing the generation of S-type granites. The partial melting of a refractory granulitic crust intruded by dry, high-T mafic magma is limited and subordinate to the production of larger amount of residual melt in the mafic sills. Thus the generation of A-type granites by partial melting of a refractory crust would require a mechanism of selective extraction of the A-type mel

Report on potential sampling biases in the LaMEVE database of global volcanism

Abstract We investigate whether the disproportionate contribution of individual volcanoes in the Large Magnitude Explosive Volcanic Eruption database (LaMEVE) potentially compromises the treatment of LaMEVE as a globally representative database of volcanic activity. We find that 41% of volcanoes which contribute at least one eruption to LaMEVE only contribute one eruption (10% of all eruptions), and the six most prolific volcanoes contribute 11% of eruptions. However, there is no systematic bias with respect to the eruption magnitude or date for volcanoes contributing one eruption. Also, no bias can be discerned for when the smallest or largest eruption at a volcano occurs in its eruptive record. Half of the volcanoes contributing one or more eruptions to the LaMEVE database had their first eruption prior to 36.4 ka. We find LaMEVE is representative – while there are well-known issues of eruption under-reporting, LaMEVE is not overly biased by the activity of a few volcanoes

Report on potential sampling biases in the LaMEVE database of global volcanism

Abstract We investigate whether the disproportionate contribution of individual volcanoes in the Large Magnitude Explosive Volcanic Eruption database (LaMEVE) potentially compromises the treatment of LaMEVE as a globally representative database of volcanic activity. We find that 41% of volcanoes which contribute at least one eruption to LaMEVE only contribute one eruption (10% of all eruptions), and the six most prolific volcanoes contribute 11% of eruptions. However, there is no systematic bias with respect to the eruption magnitude or date for volcanoes contributing one eruption. Also, no bias can be discerned for when the smallest or largest eruption at a volcano occurs in its eruptive record. Half of the volcanoes contributing one or more eruptions to the LaMEVE database had their first eruption prior to 36.4 ka. We find LaMEVE is representative – while there are well-known issues of eruption under-reporting, LaMEVE is not overly biased by the activity of a few volcanoes

Transtensional deformation of Montserrat revealed by shear wave splitting

Here we investigate seismic anisotropy of the upper crust in the vicinity of Soufrière Hills volcano using shear wave splitting (SWS) analysis from volcano-tectonic (VT) events. Soufrière Hills, which is located on the island of Montserrat in the Lesser Antilles, became active in 1995 and has been erupting ever since with five major phases of extrusive activity. We use data recorded on a network of seismometers between 1996 and 2007 partially spanning three extrusive phases. Shear-wave splitting in the crust is often assumed to be controlled either by structural features, or by stress aligned cracks. In such a case the polarization of the fast shear wave (ϕ) would align parallel to the strike of the structure, or to the maximum compressive stress direction. Previous studies analyzing SWS in the region using regional earthquakes observed temporal variations in ϕ which were interpreted as being caused by stress perturbations associated with pressurization of a dyke. Our analysis, which uses much shallower sources and thus only samples the anisotropy of the upper few kilometres of the crust, shows no clear temporal variation. However, temporal effects cannot be ruled out, as large fluctuations in the rate of VT events over the course of the study period as well as changes in the seismic network configuration make it difficult to assess. Average delay times of approximately 0.2 s, similar in magnitude to those reported for much deeper slab events, suggest that the bulk of the anisotropy is in the shallow crust. We observe clear spatial variations in anisotropy which we believe are consistent with structurally controlled anisotropy resulting from a left-lateral transtensional array of faults which crosses the volcanic complex

Tapered Fluidized Beds and the Role of Fluidization in Mineral Emplacement

One of the most prominent features of fluidized beds is their ability to mix and segregate. This is of great importance for many industrial processes, but takes on a particular significance for mineral extraction where a small amount of valuable matter is mixed with a large amount of waste. In this study we consider the occurrence of diamonds in the volcanic rock called “kimberlite”. These are often emplaced (erupted and deposited) in large volcanic pipes commonly referred to as “diatremes” (length scale of the order of a kilometre) with a vent at the bottom through which the minerals were introduced along with other fragmental particulate matter and a gas flow. The purpose of this study is to gain an understanding of the processes that led to the dispersal of minerals before their emplacement to allow efficient extraction. The paper describes experimental observations of a tapered fluidized bed. The objective was to identify the physical behaviour of gas and particles; so, of particular interest are the extent to which fluidization takes place within the bed, and the arrangements of particles seen. Gas flow-rate, particle size, and degree of taper were all varied. These observations can be used to identify the structures and processes that can take place; it is then possible to understand field data in terms of the physics that led to the emplacement of material. This will be shown using new data taken from southern Africa. Scale-up of evidence is of obvious difficulty in this system and this is discussed in terms of the possible behaviour of the bubbles that have generated mixing of material before emplacement