Thermodynamic Consideration of Post-Entrapment Crystallization in Igneous Phenocrysts

The proliferation and refinement of micro-beam analytical techniques has revolutionized the use of melt inclusion chemistry as a window into pre-eruptive conditions in igneous environments. Several well recognized processes can obscure or destroy information originally contained in the inclusion. Among these processes is post-entrapment crystallization (PEC). One successful approach to the problem of looking through the effects of PEC has been to incrementally add estimated equilibrium host phase to the inclusion until the melt is in equilibrium with the bulk host composition. PEC is not an isobaric process. This is due both to the higher thermal expansivity of melts relative to solids and to the fact that silicates have negative volumes of crystallization. To the extent to which the thermal expansion of solid can be neglected, PEC can be considered an isochoric process. PEC is not purely isochoric, however, as the thermal expansivity of the host phase is not zero. The effect of crystallization on pressure is about three times that of host thermal expansion, though this depends on the composition considered. Phase relations based on isothermal-isobaric equilibrium are of limited applicability in estimating equilibrium during PEC, and thus in backing out pre-entrapment conditions. Simple heating experiments on melt inclusions in-situ can be used to restore pre-entrapment conditions, however, it is not possible to measure pressure or composition during the experiment lessening the usefulness of this approach. The only viable way to explore PEC in detail is through thermodynamic calculation. Using thermodynamic models from the MELTS package we explore relative contributions and effects of each aspect of PEC. Cooling of an alkali olivine basalt inclusion 100° below the liquidus at 2 kbar results in crystallization of 8 wt% olivine and a pressure drop of 1.2 kbar. Cooling the same inclusion isobaricly over the same temperature interval results in crystallization of 8 wt% olivine and 10 wt% cpx. The pressure drop accompanying cooling suppresses cpx crystallization in the semi-isochoric case. Relative oxygen fugacity changes accompanying PEC is variable, ranging from near zero in the alkali olivine basalt case to nearly a log unit in an olivine tholeiite. The approach described above can be reversed to calculate pre-entrapment conditions from input melt composition and bulk host-phase composition. This method is used to refine conditions of mixed magmas from recent Popocatépetl eruptions.