ARE SPINEL LHERZOLITE XENOLITHS REPRESENTATIVE OF THE ABUNDANCE OF SULFUR IN THE UPPER MANTLE

In order to address systematic discrepancies of sulfur contents between orogenic Iherzolites and ultramafic xenoliths in alkali lavas, a study combining whole-rock sulfur geochemistry and microscopic investigation in reflected light has been performed on 40 type-I spinel Iherzolite xenoliths from the Neogene alkali basalts in the French Massif Central. These measurements yield sulfur contents ranging between 0 and 90 ppm S, with most values being below 60 ppm S, irrespective of rock texture and/or chemistry. Spinel Iherzolite xenoliths are strongly depleted in S relative to similar rocks found in orogenic spinel Iherzolite bodies or to the average sulfur content of 300 ppm calculated for the MORB mantle source region. Microscopic studies indicate S loss during secondary oxihydration of mantle-derived solid sulfides. This alteration seems to be independent of the effect of the host basalts and probably is related to latestage circulation of meteoric water. It is concluded that, except for a few samples armoured in massive basaltic lava flows, almost all the spinel Iherzolite xenoliths in continental basalts analyzed so far do not provide representative analyses of the sulfur content in the upper mantle. © 1990.