P-V-T-X Evolution of Olivine-hosted Melt Inclusions and Implications for Interpretation of Homogenization Experiments

Abstract:

Olivine-hosted melt inclusions (MIs) provide unique insights into physico-chemical conditions of magma at depth. MIs are often rehomogenized in laboratory to bring them back (or close) to the original state. At homogenization temperature (T_h), the last bubble disappears and MI becomes a homogeneous liquid phase. However, during 1-atm experiments in heating stages, homogenization is usually achieved at T_h higher than expected entrapment T and systematic increase of T_h with time is observed. This reveals occurrence of physico-chemical processes that irreversibly lower internal pressure (P_int) (relative to entrapment P) during magma ascent and laboratory treatments.

We combined theoretical modeling with experimental observations on H₂O-poor and H₂O–rich basaltic MIs in order to: a) identify the reversible (olivine elastic deformation and olivine dissolution-crystallization on MI walls) and irreversible (water loss from MIs and olivine plastic deformation) processes responsible for evolution of MI volume, P_int and composition upon heating, and b) examine how these processes affect the results of typical homogenization experiments. Due to P_int drop caused by olivine elastic deformation, performing experiments at 1 atm prevents achievement of homogenization at T_h equal to entrapment T. Predicted increase of T_h ranges from some to tens of degrees depending on entrapment conditions, melt composition and volatile contents. Presence of a gas bubble must be considered for the correct prediction of P-volume evolution. In H₂O-rich MIs, faster increase of T_h with time shown by small MIs is consistent with increase of T_h mainly driven by water loss. In H₂O-poor MIs, occurrence of olivine elastoplastic deformation is mainly responsible for the increase of T_h with time. Distance from MI wall to olivine rim is a critical parameter to take into account, as short distances enhance depressurization related with both elastoplastic deformation and water loss and contribute to increase T_h.