Grande Comore Island Research Articles

Genetic Characterization of the Population of Grande Comore Island (Njazidja) According to Major Blood Groups

The Comorian population is historically considered a blend of influences from African Bantus, Arabs, and possibly Austronesians. In this study we present the first genetic data on the current Comorian population. Serologic analysis of the six major blood group systems (ABO, RH, KEL, FY, JK, and MNS) was performed on 164 individuals from Grande Comore Island (Njazidja). In addition, Duffy genotypes were determined by polymerase chain reaction using allele-specific primers. Our findings establish a high frequency of the Fy(a- b-) phenotype (86%), presenting the same genetic background as in sub-Saharan Africa. Analysis of genetic frequencies, distances, and admixture with other populations indicates that African Bantus made the main contribution to the gene pool (73.2%+/-15.5%). The Arab contribution from the Arabian peninsula was smaller (24.2%+/-7%) and the Indonesian contribution was minor (2.6%+/-9%). The major Bantu contribution was commensurate with the Bantu cultural influence. The contribution from the Arabian peninsula seemed in relation to its permeating religious and linguistic influence. As with the language, the Indonesian contribution to the Comorian gene pool was small. These results are in agreement with historical, sociological, and linguistic data.

Hydrothermal system mapped by CSAMT on Karthala volcano, Grande Comore Island, Indian Ocean

Controlled source audio-magnetotelluric (CSAMT) has been used to investigate the resistivity structure of the summit region of Karthala volcano. The major purpose of this CSAMT survey is to locate the active hydrothermal system. The presence of the hydrothermal system had already been inferred from surface evidence of hydrothermal activity and from self-potential (SP) mapping of the Karthala summit zone. The results of the 1D CSAMT inversion indicate the following: (1) a highly resistive (500–5000 Ω m) 200- to 400-m thick surface layer, that is characteristic of dry basaltic rocks, and made up of lava flows and/or tuff breccias; (2) a 300- to 1200-m thick layer of intermediate resistivity (20–400 Ω m), thought to be representative of the groundwater body; and (3) a deep conductor with a resistivity of less than 2 Ω m, which might be related to the active hydrothermal system. This hydrothermal system appears to be bounded by caldera edges and shows the same north–south trend. Its depth ranges from more than 1 km to less than 0.7 km in the northern part of the caldera and its resistivity ranges from 2 to 0.5 Ω m in the northern part of the caldera. Less resistive zones and the shallowest depths of this conductive layer are well correlated with the largest SP positive anomalies and are assumed to be generated by hot fluid circulation. The most active hydrothermal zone is situated in the northern part of the caldera.

GLASSES IN MANTLE XENOLITHS: A TOOL FOR UNDERSTANDING MANTLE PROCESSES

A new perspective in the evolution of mantle material was developed along with the concept of metasomatism, that is peridotitic mantle could not only be depleted through melting process, but it could also get enriched by migration and reaction with fluids coming from deeper mantle portions. Anydrous mantle parageneses alone are not able to generate the wide magma spectrum which characterise, for example, alkaline intraplate magmatism and the presence of hydrous minerals, such as amphibole and phlogopite, has to be invoked. However, not every magmatic event occurring in the astenospheric or even lithospheric mantle reach the surface and, hence, may be detected. Most very-low degree partial melts, such as lamproites, lamprophyres or carbonatites, rarely, if ever, reach the surface, but, nevertheless, they may deeply modify the overlying mantle portions. In this context mantle glasses represent a powerful tool for investigating these basic petrogenetic problems. Their study may contribute to highlight a) which are the relationships between glasses, metasomatic processes and the formation of amphibole, phlogopite and other metasomatic phases, and b) which might be the relationships between glasses, geochemical components and basaltic magma production. In the following petrographical and geochemical (major and trace elements) features of three well-studied glass populations from oceanic and continental lithospheric mantle are reported. They were found within mantle xenoliths, in intra-plate magmatic settings, from Grande Comore island (Indian Ocean), Sal island (Cape Verde, Atlantic Ocean), and Veneto Volcanic Province (Southern Alps, Italy). Afterward, their characteristics will be compared with those of mantle glasses reported world wide. All studied xenoliths bear spinel as the only aluminiferous phase, then investigation is confined in the spinel stability field. The three groups of mantle xenoliths present quite distinctive geochemical features (both for major and trace element contents), such as Na2O/K2O ratio, and SiO2, CaO, TiO2, Zr, Sr, Rb and Ba contents, which allow to put some constrain on the nature of the original metasomatic agents. The final goals will be to identify i) what do glasses represent and which is the process controlling their formation, ii) what are the metasomatic fluids which might be responsible for their genesis and iii) what are geochemical markers which may be suitable for discriminating between Na- or Krich alkali-silicate vs carbonatite metasomatism.

Carbonatite Metasomatism of the Oceanic Upper Mantle: Evidence from Clinopyroxenes and Glasses in Ultramafic Xenoliths of Grande Comore, Indian Ocean

Petrological and geochemical data are presented for a suite of ultramafic xenoliths (lherzolites and wehrlites) from La Grille volcano, Grande Comore island. These xenoliths have peculiar textural features characterized by reaction rims or glassy patches, where clinopyroxene grows at the expense of orthopyroxene. The glasses vary from very strongly silica-oversaturated to strongly undersaturated varieties. They are generally homogeneous within a single xenolith, are extremely alkali rich, with Na2O contents (up to 14 wt %) higher than any previously encountered, and are highly evolved (mg-number 34–37). The major element (especially Na) and trace element abundances, together with some remarkable Ti and Zr negative anomalies in chondrite-normalized diagrams, suggest that the metasomatizing agent was an ephemeral alkali-rich carbonatite. These metasomatic reactions appear to have involved an increase in oxygen fugacity from lherzolites (−0.56 to 1.29Δlog units) to wehrlites (up to 2.68Δlog units), with a concomitant decrease in temperature (lherzolites: 908–993°C; wehrlites: 816–893°C). Geobarometric indicators point to equilibration in the range of 7–12 kbar, above the carbonate stability field. An experimentally determined carbonatite composition reacting with a harzburgitic–lherzolitic protolith was successfully balanced with the observed secondary paragenesis allowing calculation of the trace element content of the metasomatizing carbonatitic fluid/s. These modelling procedures were subsequently used to estimate a set of Kd cpx/carb values for mantle depths, which compare well with (scarce) data from both natural occurrences and experimental runs. The origin of the CO2 and H2O in the metasomatizing melts or fluids is problematic. The melts or fluids may have originated from recycled material in or, most probably, below the asthenospheric mantle, brought down in a subducting slab at a former convergent plate margin.

Carbonatite Metasomatism of the Oceanic Upper Mantle: Evidence from Clinopyroxenes and Glasses in Ultramafic Xenoliths of Grande Comore, Indian Ocean

material in or, most probably, below the asthenospheric mantle, Petrological and geochemical data are presented for a suite of brought down in a subducting slab at a former convergent plate ultramafic xenoliths (lherzolites and wehrlites) from La Grille margin. volcano, Grande Comore island. These xenoliths have peculiar textural features characterized by reaction rims or glassy patches, where clinopyroxene grows at the expense of orthopyroxene. The glasses vary from very strongly silica-oversaturated to strongly undersaturated varieties. They are generally homogeneous within a single xenolith, are extremely alkali rich, with Na2O contents (up

Ra–Th–Sr isotope systematics in Grande Comore Island: a case study of plume–lithosphere interaction

Here, we present new mass spectrometry measurements of U–Th–Ra disequilibria and Sr isotopes for historical volcanics from the Karthala and La Grille volcanoes in Grande Comore Island, Comores Archipelago. Alkali basalts from the Karthala are characterised by large 230Th (33–47%) and 226Ra excesses (21–53%), and radiogenic Sr compositions (0.7034–0.7041). In contrast, La Grille basanites have less radiogenic 87Sr/ 86Sr (∼0.7032) with less 226Ra excesses (21%) but similar 230Th– 238U disequilibria (45%). The Karthala samples display much more scatter in the Ra–Th isochron diagram than in the Th–U isochron diagram, suggesting a decoupling of the two parent–daughter systems. Correlations between the Sr isotopes and U–Th–Ra disequilibria suggest a mixing relationship between La Grille and Karthala sources. La Grille basanites are shown to result from partial melting of the old metasomatised oceanic lithosphere beneath the archipelago whereas Karthala lavas are derived from the Comore plume. The presence of amphibole in the lithosphere is responsible for Ra–Ba, Ba–Th and Ra–Th fractionation during melting such that D Ba > D Th > D Ra. The partitioning of these elements clearly differs from partial melting of garnet–lherzolite where D Th > D Ra > D Ba. Part of the variations in the 226Ra excesses is attributed to radioactive decay between the source and the surface. Short residence times (<1000 year) preclude the presence of a large magma chamber and may be an argument for rapid segregation of the melts through cracks or veins during melt transport.

Low-degree partial melting trends recorded in upper mantle minerals

The study of glass inclusions inside mantle minerals provides direct information about the chemistry of naturally occurring mantle-derived melts and the fine-scale complexity of the melting process responsible for their genesis. Minerals in a spinel lherzolite nodule from Grande Comore island contain glass inclusions which, after homogenization by heating, exhibit a continuous suite of chemical compositions clearly distinct from that of the host basanitic lava. The compositions range from silicic, with nepheline–olivine normative, 64 wt% SiO 2 and 11 wt% alkali oxides, to almost basaltic, with quartz normative, 50 wt% SiO 2 and 1–2 wt% alkali oxides. Within a single mineral phase, olivine, the inferred primary melt composition varies from 54 to 64 wt% SiO 2 for MgO content ranging from 8 to 0.8 wt%. An experimental study of the glass and fluid inclusions indicates that trapped melts represent liquids that are in equilibrium with their host phases at moderate temperature and pressure ( T≈1230°C and P≈1.0 Gpa for melts trapped in olivine). Quantitative modelling of the compositional trends defined in the suite shows that all of the glasses are part of a cogenetic set of melts formed by fractional melting of spinel lherzolite, with F varying between 0.2 and 5%. The initial highly silicic, alkali-rich melts preserved in Mg-rich olivine become richer in FeO, MgO, CaO and Cr 2O 3 and poorer in SiO 2, K 2O, Na 2O, Al 2O 3 and Cl with increasing melt fractions, evolving toward the basaltic melts found in clinopyroxene. These results confirm the connection between glass inclusions inside mantle minerals and partial mantle melts, and indicate that primary melts with SiO 2 >60 wt%, alkali oxides >11%, FeO <1 wt% and MgO <1 wt% are generated during incipient melting of spinel peridotite. The composition of the primary melts is inferred to be dependent on pressure, and to reflect both the speciation of dissolved CO 2 and the effect of alkali oxides on the silica activity coefficient in the melt. At pressures around 1 GPa, low-degree melts are characterized by alkali and silica-rich compositions, with a limited effect of dissolved CO 2 and a decreased silica activity coefficient caused by the presence of alkali oxides, whereas at higher pressures alkali oxides form complexes with carbonates and, consequently, alkali-rich silica-poor melts will be generated.

The Process of Plume-Lithosphere Interactions in the Ocean Basins--the Case of Grande Comore

Grande Comore Island is formed by two contemporaneously active INTRODUCTION volcanoes, Karthala and La Grille. Previous work has shown that Most individual oceanic islands display significant isotopic La Grille lavas are alkalic low-degree melts of a metasomatized variations in erupted basalts, and the cause of these lithospheric mantle source containing amphibole. This study presents variations is still a matter of great debate. There is major, trace element and Sr–Nd–Pb isotope analyses of a detailed still no consensus as to whether they reflect intrinsic sample suite from Karthala. The lavas are alkali basalts displaying heterogeneity of the deep mantle source, mixtures of the trace element and isotopic variations that indicate mixing between plume source with entrained mantle, or contamination the deep mantle plume source and the lithospheric mantle source of of plume-derived melts by shallow lithospheric mantle La Grille. This study delineates how the process of plume–lithosphere or crust. Recently there has been increased recognition interaction occurs in Grande Comore, characterizes the composition of the significance of lithospheric contributions to the of the metasomatized lithospheric mantle, and identifies its origin. isotopic diversity found in oceanic islands, especially those Lithosphere-derived La Grille lavas show similar trace element located near continents and on oceanic plateaux (Gerlach patterns to plume-related Karthala lavas, but lower trace element et al., 1988; Hoernle et al., 1991; Barling et al., 1994; contents for comparable degrees of partial melting. Addition of a Class et al., 1996; Milner & le Roex, 1996). The possibility few percent of plume melt to anhydrous depleted lithospheric mantle of lithospheric contributions to Hawaiian tholeiitic lavas generates a mantle composition that can serve as the source of La has been widely discussed [e.g. Tatsumoto (1978), StauGrille lavas. This plume-related metasomatism occurs pervasively digel et al. (1984), Chen & Frey (1985), Hegner et al. at the base of the lithospheric mantle, lowers its solidus temperature, (1986), Stille et al. (1986), Kurz & Kammer (1991), Eggins and causes it to be a major contaminant of plume melts. Karthala (1992), Eiler et al. (1996), Hauri et al. (1996), and references lavas are shown to be mixtures of plume melts with high-degree therein], although these studies disagree on the identity melts of the lithospheric mantle. To the extent that this process is of the lithospheric component. Nevertheless, models of valid in other locations, it implies that isotopic variations of other lithospheric contributions to plume lavas have met reoceanic islands may reflect contamination of nearly uniform plume sistance on theoretical, geophysical, and geochemical melts with lithospheric melts. grounds. Significant melting of anhydrous lithosphere as

Lava caves of Grande Comore, Indian Ocean: An initial reconnaissance, September 1997

What are believed to have been the first speleological investigations in the Comoros Islands were carried out on Grande Comore island between 7 and 13 September 1997. A number of caves were located with the help of local informants and the more significant ones surveyed. Exploration of some caves was not able to be completed. The potential for further significant discoveries is believed to be high.

Impact de l’utilisation du poisson larvivorePoecilia reticulatasur la transmission du paludisme en RFI des Comores

Field tests were conducted in the Grande Comore Island, Federal Islamic Republic of Comoros, in order to evaluate the potential of the larvivorous fish Poecilia reticulata for the control of the malaria vector Anopheles gambiae s.s. Due to the high permeability of soil, Anopheles breeding sites in all island occurs only in the man-made water reservoirs. The study was carried out from November 1987 to November 1988 within a framework of a malaria and filariasis control programme, supported by WHO and UNDP in collaboration with the Government of the FIR of Comoros. All larval breeding places of An. gambiae existing in the village of Hantsambou were recorded (59 ablutions basins and 61 cisterns) and provided initially with 3-5 specimens of P. reticulata/m3 in November 1987, after the importation of the larvivorous species from Mayotte Island. The percentage of breeding places positive for An. gambiae decreased from 41% to 6% after one year. Pyrethrum spray catch showed a reduction of indoor resting density from 5.5 to 0.3, while the ma value, number of Anopheles bites/man/night, obtained by night-biting catches, decreased from 6.3 to 1.2. At the same time of the reduction of entomological indices parasite index for P. falciparum and spleen rate drop steadily in 5-9 years school children. The tested vector control method, well accepted by the community, could be implemented in malaria control through primary health care, being the ecological conditions in the entire island very peculiar.