Giuliani Gaston, Fallick A., Ohnenstetter D., Pegere G. (2009). Oxygen isotopes composition of sapphires from the French Massif Central : implications for the origin of gem corundum in basaltic fields. Mineralium Deposita, 44 (2), p. 221-231. ISSN 0026-4598.
Titre du document
Oxygen isotopes composition of sapphires from the French Massif Central : implications for the origin of gem corundum in basaltic fields
Giuliani Gaston, Fallick A., Ohnenstetter D., Pegere G.
Mineralium Deposita, 2009,
44 (2), p. 221-231 ISSN 0026-4598
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 13.9aEuro degrees. Two distinct groups have been defined: the first with a restricted isotopic range between 4.4 and 6.8aEuro degrees (n = 22; mean delta O-18 = 5.6 +/- 0.7aEuro degrees), falls within the worldwide range defined for blue-green-yellow sapphires related to basaltic gem fields (3.0 < delta O-18 < 8.2aEuro degrees, n = 150), and overlaps the ranges defined for magmatic sapphires in syenite (4.4 < delta O-18 < 8.3aEuro degrees, n = 29). A second group, with an isotopic range between 7.6 and 13.9aEuro degrees (n = 9), suggests a metamorphic sapphire source such as biotite schist in gneisses or skarns. The delta O-18 values of 4.4-4.5aEuro degrees for the blue sapphire-bearing anorthoclasite xenolith from Menet is lower than the delta O-18 values obtained for anorthoclase (7.7-7.9aEuro degrees), 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 delta O-18 of 13.9aEuro degrees, indicates that metamorphic sapphires from granulites were transported to the surface by basaltic magma.