V020-0022
MushPEC: A novel approach for post-entrapment corrections of melt inclusions hosted in olivine antecrysts

Thursday, 10 December 2020
Poster
Raimundo Brahm1, Georg F Zellmer2, Takeshi Kuritani3, Daniel Coulthard Jr1, Mitsuhiro Nakagawa4, Naoya Sakamoto5, Hisayoshi Yurimoto6 and Eiichi Sato7, (1)Massey University, Palmeston North, New Zealand, (2)Massey University, Volcanic Risk Solutions, Palmeston North, New Zealand, (3)Osaka City University, Sumiyoshi-Ku, Osaka, Japan, (4)Hokkaido Univ, Sapporo, Japan, (5)Hokkaido University, Isotope Imaging Laboratory, Sapporo, Japan, (6)Hokkaido University, Division of Earth and Planetary Sciences, Sapporo, Japan, (7)Osaka Prefecture University, Higher Education Development, Osaka, Japan
Abstract:
The use of olivine-hosted melt inclusions (MIs) for the estimation of primary magma compositions is usually reserved for very primitive and rapidly erupted olivine phenocrysts. The classical post-entrapment correction methodology for olivine-hosted MIs defined by Danyushevsky et al. (2000) assumes that the olivine composition at the moment of MI entrapment is preserved and that the MI is genetically related to the magma that carries its host crystal. In many cases, some the crystal cargo of magmas is antecrystic, i.e. picked-up during ascent from crystal-rich mushes. Olivine antecrysts have a long storage history, which completely re-equilibrates the Fe/Mg ratios of the olivine host and the MI to the interstitial melt composition, erasing the constraints that would allow to backtrack the initial MI composition.

We developed a novel MI correction scheme, coded in a MATLAB® script named MushPEC, applicable to antecrystic olivine-hosted MIs, where the host olivine (and MI) composition has been completely re-equilibrated during storage. This new method relies on fitting a genetically related MI set to liquid lines of decent (LLD) generated from fractional crystallization (FC) simulations. The script iterates FC simulations using the MELTS algorithm implemented in the alphaMELTS front end. The iterations are generated from a 5D grid of initial conditions (P, fO2, initial H2O content, olivine addition to the initial composition and initial MgO/FeOt ratios), starting from the most primitive MI composition of the set.

The fitting process consist on the minimization of the compositional distance between the LLD of the FC model and the trend of olivine addition to each MI in the set. We use the definition of the Aitchison distance for a compositional space geometry. We also use the combination of the minimum distances of all fitted MIs to each LLD as a measure of “goodness of fit” and a statistical assessment is done to find the best fit results. MushPEC returns corrected MI compositions and P, H2O contents, fO2, and the mineral proportions within the mush.

Danyushevsky, L.V., Della-Pasqua, F.N. and Sokolov, S., 2000. Re-equilibration of melt inclusions trapped by magnesian olivine phenocrysts from subduction-related magmas: Petrological implications. Contributions to Mineralogy and Petrology, 138(1): 68-83.

<< Previous Abstract | Next Abstract >>