Experimental evidence of fast transport of trace elements in planetary basaltic crusts by high temperature metamorphism
Résumé
Incompatible elements (IEs) such as K, P, Ti, and rare earth elements (REEs) provide important constraints on the geochemistry and chronology of basaltic meteorites, most of which experienced complicated post-crystallization histories. These elements are immobile under subsolidus conditions because of their slow diffusion rates in planetary basalt minerals such as pyroxene and plagioclase. Thus, IEs are considered to preserve in most cases, reliable records of formation processes, even in ancient rocks that have undergone moderate thermal processing. However, observations of natural planetary samples suggest that melting of IE accessory carrier phases enhances the mobilization of such elements. Here we show that IEs are rapidly transported by near-solidus partial melting of highly IE-enriched minor phases including Ca-phosphate and Ti-rich phases. These partial melts occur as interconnected veins along cracks and fractures, and as thin films on surfaces of pore spaces, indicating that the melt mobilization is driven by surface tension. The melt transport provides the necessary condition for melt migration consistent with the presence of the depleted basaltic eucrites. Also, reaction between major minerals pyroxene and plagioclase, and partial melts may cause disturbance and resetting of some isotopic systems. These results have important implications for a range of geochemical investigations. In particular, the elemental fractionation resulting from such partial melting may result in improper age determinations.