Abstract
Olivine-hosted melt inclusions (MIs) are widely used as a tool to study the early stages of magmatic evolution. There are a series of processes that affect MI compositions after trapping, including post-entrapment crystallization (PEC) of the host mineral at the MI boundaries, exsolution of volatile phases into a “shrinkage bubble” and diffusive exchange between a MI and its host. Classical correction schemes applied to olivine-hosted MIs include PEC correction through addition of olivine back to the melt until it reaches equilibrium with the host composition and “Fe-loss” correction due to Fe-Mg diffusive exchange. These corrections rely on the assumption that the original host composition is preserved. However, for many volcanic samples the crystal cargo is thought to be antecrystic, and the olivine composition may thus have been completely re-equilibrated during long crystal storage times. Here, we develop a novel MI correction scheme that is applicable when the original host crystal composition has not been preserved and the initial MI composition variability can be represented by simple fractional crystallization (FC). The new scheme allows correction of MI compositions in antecrystic hosts with long and varied temperature histories. The correction fits a set of MI compositions to modelled liquid lines of descent generated by FC. A MATLAB®script (called MushPEC) iterates FC simulations using the rhyolite-MELTS algorithm. In addition to obtaining the corrected MI compositions, the application of this methodology provides estimations of magmatic conditions during MI entrapment. A set of MIs hosted in olivine crystals of homogeneous composition (Fo77–78) from a basaltic tephra of Akita-Komagatake volcano was used to test the methodology. The tephra sample shows evidence of re-equilibration of the MIs to a narrow Mg# range equivalent to the carrier melt composition. The correction shows that olivine hosts were stored in the upper crust (c.125 – 150 MPa) at undersaturated H2O contents ofc. 1 – 2 wt% H2O).
Highlights
Melt inclusions (MIs) have been widely used in the study of magmatic processes, as they can constitute snapshots of primary to pre-eruptive magmatic conditions (e.g., Lowenstern, 1995; Danyushevsky et al, 2002a; Danyushevsky et al, 2004; Kelley et al, 2006; Kent, 2008; Kelley et al, 2010; Cannatelli et al, 2016)
We develop a novel approach for post-entrapment crystallization (PEC) and Fe-Mg diffusion correction of olivine-hosted glassy MIs, which can be applied to a genetically related population of MIs that have been entrapped during co-crystallization of other mineral phases and that have suffered widespread Fe-Mg diffusive re-equilibration
The results show that MushPEC can accurately find the fractional crystallization (FC) models that provide the best fits to the corrected MI data
Summary
Melt inclusions (MIs) have been widely used in the study of magmatic processes, as they can constitute snapshots of primary to pre-eruptive magmatic conditions (e.g., Lowenstern, 1995; Danyushevsky et al, 2002a; Danyushevsky et al, 2004; Kelley et al, 2006; Kent, 2008; Kelley et al, 2010; Cannatelli et al, 2016). Olivinehosted MIs are especially valuable for the study of primary magma compositions This mineral phase is usually the first to crystallize from mafic melts, and the post-entrapment processes are well understood. These processes are easy to correct for because of the simplicity of the olivine structure and because relevant diffusion of major elements is reduced to Fe-Mg exchange (Danyushevsky et al, 2000). The use of these MIs for studies of primary magmas has typically been restricted to primitive olivine compositions (Fo > 80) where the original olivine composition is thought to be preserved and MI compositions are close to that of the primary magma
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