The relaxation of residual inclusion pressure and implications to Raman-thermobarometry

Abstract

Abstract. Residual pressure can be preserved in mineral inclusions, e.g. quartz-in-garnet, after exhumation due to differential expansion between inclusion and host crystals. Raman spectroscopy has been applied to infer the residual pressure and provides information on the entrapment temperature and pressure conditions. However, the amount of residual pressure relaxation cannot be directly measured. An underestimation of pressure relaxation may lead to significant errors between calculated and actual entrapment pressure. This study focuses on three mechanisms responsible for the residual-pressure relaxation: 1) viscous creep; 2) plastic yield; 3) proximity of inclusion to thin-section surface. Criteria are provided to quantify how much of the expected residual pressure is relaxed due to these three mechanisms. An analytical solution is introduced to demonstrate the effect of inclusion depth on the residual pressure field when the inclusion is close to thin-section surface. It is shown that for quartz-in-garnet system, the distance between thin-section surface and inclusion centre needs to be at least two times the inclusion radius to avoid pressure relaxation. In terms of viscous creep, representative case studies on quartz-in-garnet system show that viscous relaxation may occur from temperatures as low as 600∼700 °C depending on the particular P-T path and various garnet compositions. For quartz entrapped along the prograde P-T path and subject to viscous resetting at peak T above 600∼700 °C, its residual pressure after exhumation may be higher than predicted from its true entrapment conditions. Moreover, such a viscous resetting effect may introduce apparent overstepping of garnet nucleation that is not related to reaction affinity.