ALBERT

Description: Oxidation state is a sensitive indicator of geochemical processes within the upper mantle. Here we report results of a regional study of the oxidation state of spinel peridotite xenoliths from 45 volcanic centers distributed over ~20 000 km 2 in the Massif Central, France. The log f O 2 values relative to the fayalite–magnetite–quartz oxygen buffer (FMQ) were determined from the equilibrium between the Fe-bearing components in olivine, orthopyroxene and spinel, with the Fe 3+ content of spinel measured either by Mössbauer spectroscopy or by electron microprobe using secondary spinel standards. For the entire suite of samples, log f O 2 values range between FMQ – 0·47 and FMQ + 1·66. Our data confirm the presence of two distinct lithospheric mantle domains, previously reported in the literature, lying north and south of 45°30'N, respectively. The northern domain, with its refractory bulk composition, tends to record more oxidized conditions, having log f O 2 values mostly at or above FMQ + 1. The log f O 2 in the southern domain is more variable, including values below FMQ. Assuming that increasing equilibration temperatures among xenoliths reflect increasing depths of origin, samples from the northern domain suggest that the shallower part of the subcontinental lithospheric mantle (SCLM) is somewhat more oxidized than at deeper levels. On the other hand, such a general observation cannot be made for the southern domain. The high log f O 2 values of harzburgites suggest that they are more sensitive to resetting of their oxidation state by metasomatism than lherzolites. In terms of modally metasomatized xenoliths, the ‘melt’ leading to the addition of clinopyroxene apparently had a higher oxidation state (log f O 2 〉 FMQ + 1) than the agent responsible for crystallization of amphibole (log f O 2 ~ FMQ + 0·6). Furthermore, amphibole-bearing and amphibole-free peridotites exhibit the same range in f O 2 . Cryptic metasomatism can also reset oxidation state, sometimes very effectively. Metasomatic processes are probably the reason why the xenolith suite from the Massif Central records relatively high log f O 2 values compared with ‘normal’ non-cratonic SCLM. This study demonstrates the utility of using oxidation state to help characterize and delineate domains in the lithospheric mantle.