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Oxygen isotopes composition of sapphires from the French Massif Central : implications for the origin of gem corundum in basaltic fields

Gaston Giuliani , Anthony Fallick, Daniel Ohnenstetter and Guy Pegere

Gaston Giuliani1, 2 , Anthony Fallick3, Daniel Ohnenstetter2 and Guy Pegere4

(1) Institut de Recherche pour le Développement (UR154), LMTG, 14 avenue Edouard Belin, 31400 Toulouse, France (2) Present address : CRPG, Nancy-Université, CNRS, BP 20, 54501 Vandœuvre-lès-Nancy, France (3) Isotope Geosciences Unit, Scottish Universities Environmental Research Centre, East Kilbride, Rankine Avenue, Glasgow, G75 0QF, Scotland, UK (4) Avenue d’Auvergne, 43100 Brioude, France

Received : 26 February 2008 Accepted : 29 September 2008 Published online : 29 October 2008 Editorial handling : R. Moritz Abstract Alluvial and colluvial gem sapphires are common in the basaltic fields of the French Massif Central (FMC) but sapphire-bearing xenoliths are very rare, found only in the Menet trachytic cone in Cantal. The O-isotope composition of the sapphires ranges between 4.4 and 13.9‰. Two distinct groups have been defined : the first with a restricted isotopic range between 4.4 and 6.8‰ (n = 22 ; mean δ18O = 5.6 ± 0.7‰), falls within the worldwide range defined for blue-green-yellow sapphires related to basaltic gem fields (3.0 < δ18O < 8.2‰, n = 150), and overlaps the ranges defined for magmatic sapphires in syenite (4.4 < δ18O < 8.3‰, n = 29). A second group, with an isotopic range between 7.6 and 13.9‰ (n = 9), suggests a metamorphic sapphire source such as biotite schist in gneisses or skarns. The δ18O values of 4.4–4.5‰ for the blue sapphire-bearing anorthoclasite xenolith from Menet is lower than the δ18O values obtained for anorthoclase (7.7–7.9‰), but suggest that these sapphires were derived from an igneous reservoir in the subcontinental spinel lherzolitic mantle of the FMC. The presence of inclusions of columbite-group minerals, pyrochlore, Nb-bearing rutile, and thorite in these sapphires provides an additional argument for a magmatic origin. In the FMC lithospheric mantle, felsic melts crystallized to form anorthoclasites, the most evolved peraluminous variant of the alkaline basaltic melt. The O-isotopic compositions of the first group suggests that these sapphires crystallized from felsic magmas under upper mantle conditions. The second group of isotopic values, typified for example by the Le Bras sapphire with a δ18O of 13.9‰, indicates that metamorphic sapphires from granulites were transported to the surface by basaltic magma.

Keywords France - French Massif Central - Basalt - Oxygen isotopes - Sapphires

Voir en ligne : http://dx.doi.org/doi:10.1007/s0012...




publié vendredi 3 avril 2009