Gem corundum deposits of Madagascar: A review

Madagascar is one of the most important gem-producing countries in the world, including ruby and sapphires. Gem corundum deposits formed at different stages in the geological evolution of the island and in contrasting environments. Four main settings are identified: (1) Gem corundum formed in the Precambrian basement within the Neoproterozoic terranes of southern Madagascar, and in the volcano-sedimentary series of Beforona, north of Antananarivo. In the south, high-temperature (700 to 800 °C) and low-pressure (4 to 5 kbar) granulites contain deposits formed during the Pan-African orogenesis between 565 and 490 Ma. They accompany mafic and ultramafic complexes (ruby deposits of the Vohibory group), skarns at the contact between Anosyan granites and the Proterozoic Tranomaro group (sapphire deposits of the Tranomaro–Andranondambo district), and shear-zone corridors cross-cutting feldspathic gneisses, cordieritites and clinopyroxenites in the Tranomaro, Vohimena and Androyan metamorphic series (biotite schist deposits of Sahambano and Zazafotsy, cordieritites of Iankaroka and Ambatomena). The circulation of fluids, especially along discontinuities, allowed in-situ alkaline metasomatism, forming corundum host rocks related to desilicified granites, biotitites, “sakenites” and “corundumites”. (2) Gem corundum also occurs in the Triassic detrital formations of the Isalo group, as giant palaeoplacers in the Ilakaka–Sakaraha area. Here, sapphires and rubies may come from the metamorphic granulitic terranes of southern Madagascar. (3) Gem corundum deposits occur within the Neogene-Quaternary alkali basalts from Ankaratra (Antsirabe–Antanifotsy area) and in the Ambohitra Province (Nosy Be, Ambato and Ambondromifehy districts). Primary deposits are rare, except at Soamiakatra where ruby in gabbroic and clinopyroxenite xenoliths within alkali-basalts probably derive from mantle garnet peridotites. The blue-green-yellow sapphires typical of basaltic fields are always recovered in palaeoplacer (in karst formed upon Jurassic limestones from the Montagne d'Ambre, Antsiranana Province) and alluvial and soil placers (Ankaratra volcanic massif). (4) Deposits occur within Quaternary eluvial, colluvial and alluvial concentrations, such as high-quality rubies from the Andilamena and Vatomandry deposits.     

MuSET: A multiparameter and high precision sensor for downhole spontaneous electrical potential measurements

On the basis of an existing multiparameter borehole fluid sensor (p, T, Cw, pH, Eh), a new downhole tool designed for 200 bars and 75 °C was developed to measure the spontaneous electrical potential in situ with great precision (better than a μV). To this end, the new sensor is based on the use of unpolarizable Pb/PbCl₂ electrodes either at the surface or downhole. In situ testing has demonstrated a capacity to identify several subsurface sources of natural electrical potential such as diffusion ones (membrane potential in the presence of clays, Fickian processes due to pore fluid salinity gradients), or else the electrokinetic mechanisms with gradients in pore fluid pressure. As a consequence, Multi-Sensors Electrical Tool (MuSET) might be used as an “electrical flowmeter” sensitive to both horizontal and vertical fluid flow in a vertical borehole.     

Genetic responses to metal contamination in two clams: Ruditapes decussatus and Ruditapes philippinarum

Coastal ecosystems are subjected to a wide variety of disturbances, including those due to xenobiotics of agricultural and industrial origin. These pollutants as heavy metals can modify the genetic diversity of populations by favouring or counter-selecting certain alleles or genotypes by differential mortality. In the present study, two genetic markers (phosphoglucomutase and glucosephosphate isomerase) and a protein marker (metallothionein) were monitored in order to determine the impact of heavy metals in different clam populations. Analysis of the genetic structure of the clam populations examined reveals that those inhabiting environments contaminated by heavy metals exhibit a higher allelic diversity and possess alleles at PGM loci that could be selected by the presence of heavy metals. The evaluation of metallothionein levels using a specific polyclonal antibody developed in the Pacific oyster (Crassostrea gigas) demonstrated the existence of a relationship between metallothionein concentrations and the level of metal pollution for clam populations sampled from different sites. An inter-specific difference was also detected between Ruditapes decussatus and Ruditapes philippinarum living in sympatry at the same site, suggesting a differential response of these two species upon exposure to an identical heavy metal concentration.     

Activity of Mount Etna preceding the February 1999 fissure eruption: inferred mechanism from seismological and geochemical data

The February 1999 eruption of Mt. Etna took place through a fissure on the SSE flank of the cone of the summit SE Crater. This event was preceded by continuous activity since 1995, sometimes accompanied by violent outbursts from one or more of the three other summit craters (NE Crater, Voragine or Chasm, and Bocca Nuova), and finally by a series of 20 short-lived eruptions from the SE Crater between September 1998 and January 1999. These phenomena could be accounted for by invoking gradual invasion of a shallow small reservoir by more primitive, basic and gas-rich magma coming from depth. The shallow “chamber” is more likely to be a plexus of dikes, which had developed during the previous years (1995–1997), following variations of the local stress field owing to enhanced magma generation and accumulation at the top of the mantle. Magma injection and mixing is evidenced through geochemistry, whereas the state of stress of the volcanic pile and underlying crust is determined using earthquake distributions and focal mechanisms. The behaviour of the seismic tremor amplitude appears to be a good indicator of the state of unrest of the volcano, although not always directly linked to the relative energy of degassing phenomena.     

The city,the (Member) state,and the European Union

This contribution to a Debate on “The Nation-State and the city” foregrounds the recently adopted Urban Agenda for the European Union. It discusses the new role of cities in the European policy making process and its relevance for relations between State and City in the light of EU membership.     

North Atlantic extratropical and subpolar gyre variability during the last 120 years: a gridded dataset of surface temperature,salinity, and density. Part 1: dataset validation and RMS variability

We present a binned annual product (BINS) of sea surface temperature (SST), sea surface salinity (SSS), and sea surface density (SSD) observations for 1896–2015 of the subpolar North Atlantic between 40° N and 70° N, mostly excluding the shelf areas. The product of bin averages over spatial scales on the order of 200 to 500 km, reproducing most of the interannual variability in different time series covering at least the last three decades or of the along-track ship monitoring. Comparisons with other SSS and SST gridded products available since 1950 suggest that BINS captures the large decadal to multidecadal variability. Comparison with the HadSST3 SST product since 1896 also indicates that the decadal and multidecadal variability is usually well-reproduced, with small differences in long-term trends or in areas with marginal data coverage in either of the two products. Outside of the Labrador Sea and Greenland margins, interannual variability is rather similar in different seasons. Variability at periods longer than 15 years is a large part of the total interannual variability, both for SST and SSS, except possibly in the south-western part of the domain. Variability in SST and SSS increases towards the west, with the contribution of salinity variability to density dominating that of temperature in the western Atlantic, except close to the Gulf Stream and North Atlantic Current in the southwest area. Weaker variability and larger relative temperature contributions to density changes are found in the eastern part of the gyre and south of Iceland.

     

A virtual capital for the European Union?

This article explores the websites of the key institutions of the European Union to consider whether these applications of the new information and communication technologies may become a functional equivalent of the national capital city in the emerging supranational political arena. Three functions of capital cities are evaluated: that of command centre, showcase and cultural arena. The websites under consideration do provide a representation of the EU command centre, and they also offer detailed information useful for those monitoring and partaking in the EU decision-making process. They are underused as supranational showcase, with the noticeable exception of their institutionalisation of multilingualism. And they do offer a rudimentary EU-wide public space. The findings suggest that there is a real potential that could be exploited by the EU institutions to foster the expression of the supranational community.     

Modelling indicators of water security,water pollution and aquatic biodiversity in Europe

Abstract

The GWAVA (Global Water AVailability Assessment) model for indicating human water security has been extended with a newly developed module for calculating pollutant concentrations. This module is first described and then illustrated by being used to model nitrogen, phosphorus and organic matter concentrations. The module uses solely input variables that are likely to be available for future scenarios, making it possible to apply the module to such scenarios. The module first calculates pollutant loading from land to rivers, lakes and wetlands by considering drivers such as agriculture, industry and sewage treatment. Calculated loadings are subsequently converted to concentrations by considering aquatic processes, such as dilution, downstream transport, evaporation, human water abstraction and biophysical loss processes. Aquatic biodiversity is indicated to be at risk if modelled pollutant concentrations exceed certain water quality standards. This is indicated to be the case in about 35% of the European area, especially where lakes and wetlands are abundant. Human water security is indicated to be at risk where human water demands cannot be fulfilled during drought events. This is found to be the case in about 10% of the European area, especially in Mediterranean, arid and densely-populated areas. Modelled spatial variation in concentrations matches well with existing knowledge, and the temporal variability of concentrations is modelled reasonably well in some river basins. Therefore, we conclude that the updated GWAVA model can be used for indicating changes in human water security and aquatic biodiversity across Europe.

Editor Z.W. Kundzewicz

Citation Dumont, E., Williams, R., Keller, V., Voss, A., and Tattari, S., 2012. Modelling indicators of water security, water pollution and aquatic biodiversity in Europe. Hydrological Sciences Journal, 57 (7), 1378–1403.