Threshold size for fluid inclusion decrepitation

Abstract

AbstractFluid inclusions in igneous and metamorphic rocks are equilibrated at pressures reaching a few tens of kilobars at mantle depths. These microsystems experience decompression as a consequence of uplift processes, such as subaerial volcanic eruptions. On decompression, inclusion fluid overpressure is known to bring about the mechanical failure of the crystal matrix through either stretch or decrepitation, depending on ductile or brittle failure mechanism of the matrix, respectively. On the one hand, laboratory experiments performed on synthetic inclusions at one atmosphere show that the decrepitation temperature is strongly size dependent, with the smaller cavities observed to decrepitate at higher temperatures. On the other hand, natural fluid inclusions, which undergo migration through a pressure gradient, are often found intact below a critical size. Here by modeling fluid inclusions as spherical cavities in a continuous elastic medium and by adopting a nonlocal stress approach to fracturing, we demonstrate that the decrepitation phenomenon is predicted to be characterized by a threshold size of the cavity, below which decrepitation would not be allowed. In order to validate our model, two independent experimental data sets relating internal pressure to cavity size are utilized to calculate pertinent model parameters and to evaluate their consistency.