Alteration of olivine in a basalt from central France

The Belbex basalt is a Miocene alkaline basalt of the kind typical of the Massif Central of France and of continental rift systems generally. It outcrops as part of the Cantal stratovolcano in the vicinity of Aurillac. The current Cambisol overlies a relatively simple weathering profile that shows the progressive enrichment of SiO2, Al2O3, Fe2O3 and TiO2 characteristic of alteration under humid climatic conditions. On the scale of individual grains, olivine follows a weathering trend that conserves Si and Al, and contrasts in this respect from an earlier path of deuteric alteration. Paradoxically, SiO2, which acts as a mobile component in the weathering macrosystem, is essentially immobile within the microsystem. A likely reason is that the activity of SiO2 in the microsystem is buffered by the breakdown of glass, the first phase to weather in the parent basalt. The composition of the weathered olivine is consistent with an evolution towards a smectite–goethite assemblage, two of the secondary minerals found in the macrosystem.