Successive episodes of reactive liquid flow through a layered intrusion (Unit 9, Rum Eastern Layered Intrusion, Scotland)

We present a detailed microstructural and geochemical study of reactive liquid flow in Unit 9 of the Rum Eastern Layered Intrusion, Scotland. Unit 9 comprises an underlying lens-like body of peridotite overlain by a sequence of troctolite and gabbro (termed allivalite), with some local and minor anorthosite. The troctolite is separated from the overlying gabbro by a distinct, sub-horizontal, undulose horizon (the ‘major wavy horizon’). Higher in the stratigraphy is another, similar, horizon (the ‘minor wavy horizon’) that separates relatively clinopyroxene-poor gabbro from an overlying gabbro. To the north of the peridotite lens, both troctolite and gabbro grade into poikilitic gabbro. Clinopyroxene habit in the allivalite varies from thin rims around olivine in troctolite to equigranular crystals in gabbro and to oikocrysts in poikilitic gabbro. The poikilitic gabbros contain multiple generations of clinopyroxene, with Cr-rich (~1.1 wt% Cr2O3) anhedral cores with moderate REE concentrations (core1) overgrown by an anhedral REE-depleted second generation with moderate Cr (~0.7 wt% Cr2O3) (core2). These composite cores are rimmed by Cr-poor (~0.2 wt% Cr2O3) and REE-poor to -moderate clinopyroxene. We interpret these microstructures as a consequence of two separate episodes of partial melting triggered by the intrusion of hot olivine-phyric picrite to form the discontinuous lenses that comprise the Unit 9 peridotite. Loss of clinopyroxene-saturated partial melt from the lower part of the allivalite immediately following the early stages of sill intrusion resulted in the formation of clinopyroxene-poor gabbro. The spatial extent of clinopyroxene loss is marked by the minor wavy horizon. A second partial melting event stripped out almost all clinopyroxene from the lowest allivalite to form a troctolite, with the major wavy horizon marking the extent of melting during this episode. The poikilitic gabbro formed from clinopyroxene-saturated melt moving upwards and laterally through the remobilized cumulate pile and precipitating clinopyroxene en route. This process, called reactive liquid flow, is potentially important in open magma chambers

Reconstructing grain-shape statistics from electron back-scatter diffraction microscopy

Reconstructing the three-dimensional (3D) size and shape distribution of randomly oriented grains using only images of cross sections remains an important challenge. Even for ellipsoids, a solution is only possible when they are solids of revolution, and may still be numerically unstable. Here we show that crystallographic orientation data, for example from electron back-scatter diffraction (EBSD), provides enough additional information to obtain moments of the 3D grain distribution, provided grain shapes can be assumed to align with crystal axes. We show that this moment method can give an average 3D grain size and shape (with error estimate) which is rigorous for ellipsoids and a good approximation for cuboidal grains, indicating that it may be a useful technique for polycrystalline materials in general. High throughput image analysis and EBSD now make the necessary sample sizes practical. We illustrate by applying the method to a basaltic rock specimen

The Skaergaard trough layering: sedimentation in a convecting magma chamber

Reconstructing the three-dimensional (3D) size and shape distribution of randomly oriented grains using only images of cross sections remains an important challenge. Even for ellipsoids, a solution is only possible when they are solids of revolution, and may still be numerically unstable. Here we show that crystallographic orientation data, for example from electron back-scatter diffraction (EBSD), provides enough additional information to obtain moments of the 3D grain distribution, provided grain shapes can be assumed to align with crystal axes. We show that this moment method can give an average 3D grain size and shape (with error estimate) which is rigorous for ellipsoids and a good approximation for cuboidal grains, indicating that it may be a useful technique for polycrystalline materials in general. High throughput image analysis and EBSD now make the necessary sample sizes practical. We illustrate by applying the method to a basaltic rock specimen

Orientation of tabular mafic intrusions controls convective vigour and crystallization style

The microstructure in basaltic dykes is significantly different from that in sills and lava lakes of the same bulk composition. For a given width of intrusion (or depth of lava lake), vertical tabular bodies are coarser grained than horizontal bodies, with an invariant plagioclase shape across the intrusion. When comparing samples from sills and dykes for which the average grain size is the same, the dyke samples contain fewer small grains and fewer large grains than the sill samples. In contrast, the variation of median clinopyroxene-plagioclase-plagioclase dihedral angles in dykes correlates precisely with that observed in sills and is a function of the rate of diffusive heat loss. These patterns can be accounted for if the early stages of crystallization in dykes primarily involve the growth of isolated grains suspended in a well-mixed convecting magma, with the final stage (during which dihedral angles form) occurring in a crystal-rich static magma in which heat loss is primarily diffusive. In contrast, crystallization in sills occurs predominantly in marginal solidification fronts, suggesting that any convective motions are insufficient to entrain crystals from the marginal mushy layers and to keep them suspended while they grow. An exception to this general pattern is provided by members of the Mull SolitDykes, which propagated 100-1000 km SE from the Mull Palaeogene Igneous Centre, Scotland, through the shallow crust. These dykes, where sampled >100 km from Mull, have a microstructure indistinguishable from that of a sill of comparable thickness. We suggest that sufficient nucleation and crystallization occurred in these dykes to increase the viscosity sufficiently to damp convection once unidirectional flow had ceased

POPULATION, BREEDING, DIET AND CONSERVATION OF THE CROZET SHAG PHALACROCORAX [ATRICEPS] MELANOGENIS AT MARION ISLAND, 1994/95–2002/03

The number of Crozet shags or cormorants Phalacrocorax [atriceps] melanogenis breeding at subantarctic Marion Island decreased by 68&#37 from 841 pairs in 1994/95 to 272 pairs in 2002/03. The mean number of pairs at colonies also decreased and was significantly related to the overall number of birds breeding in any given season. The decreases coincided with a period of warming and reduced precipitation at Marion Island and with a decrease in the number of gentoo penguins Pygoscelis papua breeding there. Both these seabird species forage inshore and there is considerable overlap in their diets. Nototheniid fish and the decapod Nauticaris marionis continued to be important in the diet of Crozet shags, but a change in dominance among nototheniid prey suggests availability of prey to shags changed after the mid 1980s. Crozet shags breed for the first time when aged three years. It is probable that about 25&#37 of the mature population did not breed in 1997/98, coincident with a strong El Niño Southern Oscillation event. In four seasons, breeding pairs on average fledged 0.30 chicks from first clutches, an amount thought inadequate to sustain the population. Crozet shags at the Prince Edward Islands should now be regarded as Endangered. Placing breeding colonies in the most highly protected zone on Marion Island, considering the establishment of an ex situ population and undertaking a genetic study of the specific status of the Crozet shag are recommended conservation measures.Afr. J. mar. Sci. 25: 537–54

Compositional boundary layers trigger liquid unmixing in a basaltic crystal mush

Abstract: The separation of immiscible liquids has significant implications for magma evolution and the formation of magmatic ore deposits. We combine high-resolution imaging and electron probe microanalysis with the first use of atom probe tomography on tholeiitic basaltic glass from Hawaii, the Snake River Plain, and Iceland, to investigate the onset of unmixing of basaltic liquids into Fe-rich and Si-rich conjugates. We examine the relationships between unmixing and crystal growth, and the evolution of a nanoemulsion in a crystal mush. We identify the previously unrecognised role played by compositional boundary layers in promoting unmixing around growing crystals at melt-crystal interfaces. Our findings have important implications for the formation of immiscible liquid in a crystal mush, the interpretations of compositional zoning in crystals, and the role of liquid immiscibility in controlling magma physical properties

POPULATION AND BREEDING OF THE GENTOO PENGUIN PYGOSCELIS PAPUA AT MARION ISLAND, 1994/95–2002/03

The numbers of gentoo penguins Pygoscelis papua breeding at subantarctic Marion Island fell by 40&#37 from 1994/95 to 2002/03, from 1 352 pairs to 806 pairs. Apart from a slight increase in 1998/99, there was a steady decrease in numbers breeding between 1995/96 and 2000/01, when the population stabilized. There is indication that in some years not all breeders nested and that some birds relocated to another colony after disturbance. From first clutches, pairs on average fledged between 0.01 chicks in 1997/98 and 0.58 chicks in 2002/03 (mean 0.38 &#177 0.21). In 1994/95, replacement clutches increased the overall production of fledged chicks by 11&#37. Based on demographic parameters measured at other localities, the production of chicks at Marion Island was inadequate to maintain the population during the period 1995/96–2000/01. Consistency in trends in breeding success at five colonies suggests that factors operating at a mesoscale, rather than those specific to particular colonies, often influenced breeding success. Laying was later than normal in 1997/98, when there was almost total breeding failure with large losses of eggs and small chicks to returning Subantarctic skuas Catharacta antarctica. Future research on this Near Threatened species at Marion Island must take full account of its susceptibility to human disturbance.Afr. J. mar. Sci. 25: 463–47