Preservation of chemical residue-melt equilibria in natural anatexite: the effects of deformation and rapid cooling

A combined petrologic, textural and chemical approach is used to interpret the relationship between chemical exchange and deformation during partial melting of a granodioritic gneiss in the contact aureole of the Bergell Pluton (Central Alps). In contrast to most regional metamorphic anatexites, chemical equilibrium between residue and segregated melt was attained and preserved by several elements, as shown by their present distribution. Equilibrium involved both the major and the trace elements hosted in the rock-forming minerals and the light rare earth elements (LREE), hosted primarily in allanite. Equilibrium with respect to LREEs was achieved because allanite had a reactive behaviour during water-present melting, in contrast to the refractive behaviour of xenotime, which mostly controls the distribution of the HREE's. The attainment and preservation of equilibrium between leucosome and residue requires peculiar boundary conditions. We propose that the achievement of equilibrium between melt and residue was promoted by deformation, operating via a mechanism of melt present granular flow (i.e. dissolution-accommodated grain-boundary sliding active during partial melting). Microstructural observations indicate that melt-present granular flow was the dominating deformation mechanism in these anatexites. The preservation of residue-melt equilibrium is inferred to result from rapid cooling under contact metamorphic conditions and from the lack of deformation below the solidus.     

Geological Consequences of the Accretion of Benham Rise to Northern Luzon, Philippines

Benham Rise is a large igneous province that has been accreted to the eastern seaboard of northern Lu-zon since Early Miocene. It started forming during the Eocene at a hotspot/mantle plume in the vicinity of the Central basin Spreading Center in the West Philippine Basin (CBSC) of the Philippine Sea Plate. Seafloor spreading from the CBSC and Parece Vela pushed Benham Rise towards Luzon. Eventually Benham Rise jammed against Luzon at the end of the Oligocene, with consequences that impacted on the geology of the Philippines which have been similarly noted in colli-sions of large igneous provinces in other areas. These are manifested as follows:     

Mixed deposits of complex magmatic and phreatomagmatic volcanism: an example from Crater Hill,Auckland, New Zealand

 A series of alternating phreatomagmatic ("wet") and magmatic ("dry") basaltic pyroclastic deposits forming the Crater Hill tuff ring in New Zealand contains one unit (M1) which can only be interpreted as the products of mixing of ejecta from simultaneous wet and dry explosions at different portions of a multiple vent system. The principal characteristics of M1 are (a) rapid lateral changes in the thicknesses of, and proportions in juvenile components in individual beds, and (b) wide ranges of juvenile clast densities in every sample. M1 appears to have been associated with an elongate source of highly variable and fluctuating magma : water ratios and magma discharge rates. This contrasts with the only other documented mixed (wet and dry) basaltic pyroclastic deposits where mixing from two point sources of quite different but stable character has been inferred. Received: July 11, 1995 / Accepted: February 13, 1996     

Arm regeneration frequency in eight species of ophiuroidea (Echinodermata) from European sea areas

Eight ophiuroid species, six from the northern Kattegat-eastern Skagerrak and three from the northern Adriatic Sea, were examined for regeneration of arms. The species were separated into groups based upon mode of feeding and habitat. Comparison between groups collected in the northern Kattegat-eastern Skagerrak showed that infaunal suspension- and deposit-feeding species (Amphiura filiformis and A. chiajei) had significantly more scars per arm (mean number 0.78) than epibenthic suspension feeders (Ophiothrix fragilis and Ophiocomina nigra, 0.29) or epibenthic carnivores and deposit feeders (Ophiura ophiura and O. albida, 0.13). Spatial variation in arm regeneration incidence was found between sampling sites in the northern Kattegat-eastern Skagerrak for Amphiura filiformis and in the northern Adriatic Sea for Ophiothrix quinquemaculata. The ash-free dry weight (AFDW) and nitrogen (N) contents were measured in arms of six species of brittle-stars from the northern Kattegat-eastern Skagerrak. Differences between species were found, with highest concentrations of AFDW and N in Amphiura filiformis, intermediate in A. chiajei, Ophiocomina nigra and Ophiothrix fragilis, and lowest in Ophiura ophiura and O. albida. As the infaunal suspension- and deposit-feeding brittle-stars (Amphiura spp.) had the highest proportions of damaged arms and highest AFDW and N contents in their arms in this comparison, it is suggested that selective cropping of arms by demersal fish is the main cause of arm damage on Amphiura spp. in this area.     

Lower hybrid instability driven by charged dust beam

This brief note investigates a lower hybrid instability driven by a beam of negatively charged dust in a collisionless magnetized dusty plasma. Application to the generation of fluctuations by Space Shuttle exhaust in the ionosphere is discussed.     

Intermittency of riometer auroral absorption observed at South Pole

Auroral radio-wave absorption values measured at South Pole for 3 years using a riometer are analyzed in order to test whether they show evidence of intermittency. The properties of the parameters of the probability density functions determined for several magnetic local time sectors are found to be significantly different. The probability density functions for the pre-midnight sector show the typical shape associated to intermittency. No results are given for the afternoon sector because few auroral absorption events meet the selection criteria to give statistically significant results. It is suggested that if the precipitating particle population responsible for the riometer auroral absorption shared the intermittency features of the absorption then the present results would allow the study of the properties of the induction component of magnetospheric turbulence.     

Efficiency of phosphorus accumulation by plankton,periphyton developed on submerged artificial substrata and metaphyton: in-situ observation in two shallow ponds

Phosphorus overenrichment of shallow ponds prevailing in wetlands leads to their eutrophication causing the collapse of those vulnerable habitats. The potential...