Oxidation State of the Lithospheric Mantle below the Massif Central, France

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 similar to 20 000 km(2) in the Massif Central, France. The Delta log fO(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 Fe3+ content of spinel measured either by Mossbauer spectroscopy or by electron microprobe using secondary spinel standards. For the entire suite of samples, Delta log fO(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 degrees 30'N, respectively. The northern domain, with its refractory bulk composition, tends to record more oxidized conditions, having Delta log fO(2) values mostly at or above FMQ+1. The Delta log fO(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 Delta log fO(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 (Delta log fO(2)>FMQ+1) than the agent responsible for crystallization of amphibole (Delta log fO(2) similar to FMQ+0.6). Furthermore, amphibole-bearing and amphibole-free peridotites exhibit the same range in fO(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 Delta log fO(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.