Assessing the Origin of Olivines Based on Relative Age of Melt Inclusions

Abstract:

The reliability of melt inclusions (MI) for deciphering magmatic processes is the subject of debate. While the compositional variability of MI in single samples or phenocrysts can nominally be used to understand crystallization processes and magma dynamics of active volcanic systems, the observed variability can potentially result from modification after trapping or by boundary-layer effects. Key to assessing whether MI reliability is to study melt inclusion assemblages (MIA). MIA represent groups of MI trapped at the same time. Compositional differences between inclusions in an MIA can therefore guide interpretation of the modification processes that occurred after entrapment.

We analyzed melt inclusions in a euhedral olivine phenocryst from the Solchiaro eruption at Procida (Southern Italy) that is the most primitive eruption in the Phlegrean Volcanic District. The olivine is reversely zoned from Fo₈₄ at the core to Fo₈₇ at the rim. Selection of MIA near the rim and at the core was based on petrography and phosphorus X-Ray maps. We measured major elements plus S, Cl, P by electron microprobe in 12 MI belonging to the same MIA near the phenocryst rim. With the exception of Mg and Ca, element concentrations are consistent and independent of MI size. In contrast, Mg concentration increases and Ca concentration decreases with increasing MI diameter. After post-entrapment crystallization, a boundary layer formed at the olivine MI interface with concentration profiles developing toward the center of MI. Smaller MI will reequilibrate (homogenize) faster than larger MI. If the same melt was trapped in MI with different size and all the MI were measured at their centers, a correlation between concentration and size will result depending on the length of the profile.

Trapped melt compositions in a MIA from the core of the same olivine are significantly more evolved (Al/Ca 1.96±0.04 wt%) than those from the MIA at the rim (Al/Ca 1.30±0.06 wt%). In particular, K₂O is enriched in melt trapped in the core (3.70±0.05 wt%) relative to the melt trapped near the rim of the olivine (1.48±0.09 wt%). Thus, the composition of the melt inclusions is consistent with the reverse zoning of the crystal indicating that injection of more primitive basaltic melt mixed with a more evolved shallower melt prior to the eruption.