How does inherited zircon survive in partially molten mantle: Insights on modes of magma transport in the mantle from nanoscale melt-crystal interaction experiments

Abstract

Inherited zircons found in mantle-derived rocks are increasingly relevant for characterizing the history and petrogenetic processes of these rocks. However, the occurrence of inherited zircons in mafic igneous rocks poses an enigma because, as shown by numerous experiments combined with modelling studies, zircon, e.g., 150 μm size, is expected to dissolve extremely swiftly, e.g., c. 1 day, in the presence of mafic melts. Slow dissolution kinetics may explain zircon survival in fast ascending magmas that scavenged zircons near their final emplacement, but it cannot explain the persistence of inherited zircons that dwelled in the magma-bearing mantle for more extended periods. To understand how these zircons can survive over long times in contact with mafic melts, we performed a series of experiments, including some novel nano-scale experiments, to explore the behaviour of zircons in contact with different volumes of melt. Our results demonstrate that if melt occurs in narrow, spatially restricted domains around zircon grains, they get rapidly saturated with Zr. From this point onwards, the melt acts as a protective layer preventing further dissolution. These results carry important implications for the mode of melt transfer in mantle segments from which inherited zircons have been found: zircon crystals in partially molten mantle systems can only survive long in mush-like zones in which the melt migrates through porous flow, the wetting behaviour of basaltic melt on zircon would appear to further facilitate such survival. Zircon crystals would be rapidly consumed in high melt fraction regions of channelised (e.g., dike) flow. Such regions are therefore constrained to be short-lived (weeks to months) and pulsed in nature. Thus, the presence of zircons in mantle-derived rocks helps to place constraints on modes and timescales of melt transfer in the respective mantle segments.