Abstract
The weathering of mantle peridotite tectoni-cally exposed to the atmosphere leads commonly to natural carbonation processes. Extensive cryptocrystalline mag-nesite veins and stock-work are widespread in the ser-pentinite sole of the New Caledonia ophiolite. Silica is systematically associated with magnesite. It is commonly admitted that Mg and Si are released during the laterization of overlying peridotites. Thus, the occurrence of these veins is generally attributed to a per descensum mechanism that involves the infiltration of meteoric waters enriched in dissolved atmospheric CO 2. In this study, we investigate serpentinite carbonation processes, and related silicifica-tion, based on a detailed petrographic and crystal chemical study of serpentinites. The relationships between serpen-tine and alteration products are described using an original method for the analysis of micro-X-ray fluorescence images performed at the centimeter scale. Our investigations highlight a carbonation mechanism, together with precipitation of amorphous silica and sepiolite, based on a dis-solution–precipitation process. In contrast with the per descensum Mg/Si-enrichment model that is mainly concentrated in rock fractures, dissolution–precipitation process is much more pervasive. Thus, although the texture of rocks remains relatively preserved, this process extends more widely into the rock and may represent a major part of total carbonation of the ophiolite.