Dissolution, growth and survival of zircons during crustal fusion: kinetic principals, geological models and implications for isotopic inheritance 论文

摘要

ABSTRACT: Finite difference numerical simulations were used to characterise the rates of diffusion-controlled dissolution and growth of zircon in melts of granitic composition under geologically realistic conditions. The simulations incorporated known solubility and Zr diffusivity relationships for melts containing 3 wt% dissolved H 2 O and were carried out in both one and thre dimensions under conditions of constant temperature, linearly time-dependent temperature and for a variety of host system thermal histories. The rate of zircon dissolution at constant temperature depends systematically on time (t½−12;), temperature (exp T −1 ) and degree of undersaturation of the melt with respect to zircon (in ppm Zr). Linear dissolution and growth rates fall in the range 10 −19 10 −15 cm s −1 at temperatures of 650-850°C. Radial rates are strongly dependent on crystal size (varying in inverse proportion to the radius, r ): for r>30 μm, dissolution and growth rates fall between 10 −17 and 10 −13 cm s −1 . During crustal magmatism, the chances of survival for relict cores of protolith zircons depend on several factors, the most important of which are: the initial radius of the zircon; the intensity and duration of the magmatic event; and the volume of the local melt reservoir with which the zircon interacts. In general, only the largest protolith zircons (>120 μm radius) are likely to survive magmatic events exceeding 850°C. Conversely, only the smallest zircons (<50 μm radius) are likely to be completely consumed during low-temperature anatexis (i.e. not exceeding ≍700°C). The effects of stirring the zircon-melt system are unimportant to dissolution and growth behaviour; except under circumstances of extreme shearing (e.g. filter pressing?), zircon dissolution is controlled by diffusion of Zr in the melt.