Trace element diffusion in jadeite and diopside melts at high pressures and its geochemical implication

Diffusivities of geochemically important trace elements (eleven rare earth elements (REE), Rb, Sr, Ba, and Y) in jadeite and diopside melts and those of Zr, Nb, Th, and U in jadeite melt have been measured at pressures betwee n 7.5 and 20 kbar and at temperatures 50-200 degrees above the liquidus, us ing diffusion couples and ion microprobe analysis. The concentrations of th ese elements in the experimental charges are close to those in natural igne ous rocks. In the jadeite melt which is nearly fully polymerized (NBO/T sim ilar to 0), (1) diffusivities of REE increase with increasing ionic radii, (2) diffusivities of tri- and tetravalent elements increase with increasing pressure at constant temperature, whereas those of mono- and divalent elem ents do not change significantly with pressure. and (3) diffusivities of th ese elements decrease with increasing their ionic charge at constant pressu re and temperature. In the diopside melt which is considerably less polymer ized (NBO/T similar to 2), (1) diffusivities of these trace elements depend mainly on their ionic radii rather than ionic charges; the diffusivities o f mono- and divalent ions decrease with increasing ionic radii at constant pressure and temperature, and (2) diffusivities of REE are nearly the same as that of Ca and decrease with increasing pressure at constant temperature . The behavior of these trace elements is correlated with that of major ele ments; in the jadeite melt, tri- and tetravalent elements behave similarly to network-forming cations Al and Si, whereas mono- and divalent elements b ehave as network-modifying cations similarly to Na. In the diopside melt, t he diffusion behavior of all these trace elements is similar to that of net work-modifying cations Ca and Mg. The results of the present experiments suggest that the abundance of some t race elements in igneous rocks may have been affected by diffusion process at the magmatic stage. In the case of REE, for example. if two different ma gmas with high and low REE concentrations become in contact one another by multiple intrusion, and a zoned magma chamber is formed, diffusion begins t o take place between them, and near the interface, the REE-enriched magma w ill become more light REE-depleted, whereas REE-depleted magma will become more light REE-enriched, and in addition, if magmas are reduced, the former will show a negative Eu anomaly, whereas the latter will show a positive E u anomaly. Copyright (C) 1998 Elsevier Science Ltd.