Fluid-Magma decoupling in a hot-spot volcano
H. Bureau, C. Gondé, B. Ménez, M. Semet logo_tutelle logo_tutelle 

Melt inclusion (200 µm diameter) trapped in olivine from the Piton Kapor (Piton de la Fournaise 1998 eruption).

Melt and fluid inclusions in olivines from the last 197 days long eruption of Piton de la Fournaise (Réunion Island) that began March 9, 1998, have recorded a range of volatile partial pressures. The highest pressures (400-540 MPa) so far for this volcano are found in unusually Fo-rich olivines from a vent remote from the main eruptive activity. Such pressures indicate that fractionation of olivine (and other crystal phases) occurs below the crust-mantle boundary in a CO2-rich volatile saturated environment. Together with previously published data, the model that emerges for Piton de la Fournaise is one where olivines which have crystallized from multiple past magma injection events, may be picked up by newly intruding magma, anywhere from upper-mantle depths to the surface, providing a record of the composition of a rather permanent and independent gas phase. The proposed model (H. Bureau, N. Métrich, M. Semet, T. Staudacher (1999) GRL, 26, 3501-3504) is likely to apply to other shield volcanoes. Magma production and transfers to the surface is accompanied by continuous open-system degassing through the permeable volcanic pile. The calculated H2O content of primary basalts (MgO ≈ 12-14 wt.%) from Piton de la Fournaise may reach 0.7-1 wt.% implying a rather H2O-rich hot spot mantle source. We are developing analytical protocols for Particle Induced X-ray Emission (PIXE, Laboratoire Pierre Süe, CEA Saclay) and Synchrotron X-Ray Fluorescence (SXRF, ESRF Grenoble) for the determination of major and trace element concentrations in these melt inclusions that had already been studied for major and volatile elements. Calibration of each of these two methods are done using International standards (NIST glasses) and analytically well documented glasses. Consistent results are obtained for elements ranging for K to U. The doubly polished basaltic melt inclusions (50-150 µm diameter) are investigated by the two methods, allowing comparison for the following elements: Ni, Cu, Zn, Rb, Sr, Zr, Pb etc... Compared to other techniques usually used for melt inclusions study, such as SIMS, SXRF and PIXE yield promising complementary results.  

Maj : 25/03/2005 (364)

 

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