Resetting of zircon inclusions in garnet: Implications for elastic thermobarometry

Elastic thermobarometry of host-inclusion systems for back-calculating pressure (P) and temperature (T) conditions of inclusion entrapment relies on the assumption that the hostinclusion rheology is purely elastic. In this study, we have explored both the elastic and nonelastic behavior of zircon-in-garnet (ZiG) systems by in situ Raman spectroscopy at high T and ambient P. We show that upon heating, plastic relaxation takes place immediately after the zircon inclusions experience tensile stress conditions with respect to a free crystal at the same T. On subsequent cooling, the inclusions develop a new stress state, and thus the inclusion pressures have been reset from those corresponding to their original entrapment. Resetting of inclusion pressures therefore strongly depends on the exhumation P-T path. This explains why elastic thermobarometry using ZiG systems is reliable when applied to low-P high-T rocks where the cooling path after inclusion entrapment passes quickly into the compression domain of the inclusion. On the other hand, high-P rocks exhumed along quasi-isothermal paths take zircon inclusions into the tensile domain where they are reset until significant cooling commences at low P. ZiG systems in ultrahigh-P rocks therefore commonly indicate pressures on clockwise exhumation paths instead of the conditions of original entrapment.