Investigating crystallization history in synthetic trachybasalts using electron backscatter diffraction

Description

Small differences in magma composition can strongly affect the crystallization history and microstructure of igneous rocks. Here we present results of crystallisation experiments carried out in a piston cylinder apparatus at constant pressure of 4 kbar.
Three synthetic trachybasaltic starting glasses were used, with different minor
elements composition as follows: Glass 1 - different transition metals and REE elements added (total of 1 wt.% dopants); Glass 2 - doped only with Cr2O3 (0.5 wt.%); Glass 3 - undoped. The major element compositions of Glasses 2 and 3 are almost identical, and similar to Glass 1. Experiments using doped and undoped compositions were carried out under anhydrous and hydrous conditions (nominally 0 and 2 wt.% H2O, respectively).
After 30 minutes of superheating at 1300°C the samples were cooled at a rate of 80°C/min to the final resting temperature ranging between 1150°C and 1050°C. These temperatures correspond to nominal undercooling (ΔT = Tliq - Texp) between 60° and 160° for the anhydrous glasses and between 30° and 130° for the hydrous ones. Dwell time at the final resting T was between 5 minutes and 8 hours for different experiments.
Dendritic clinopyroxene (Cpx) is the main crystal phase, comprising up to 50 vol% of the samples. Spinel crystals (Spl) comprise 3 to 5 vol%. Spl crystals from Glass 1 are mainly Titanomagnetite (Tmt), homogenously distributed through the sample, often decorating Cpx crystals and rarely found isolated in the glass.
Occasionally, they surround a Cr-oxide at the centre of a Cpx rosette (i.e., radially arranged dendritic Cpx). Spl crystals from Glass 2 show Cr-rich cores and Ti-rich rims. They can be found embedded in larger Cpx crystals, but also isolated in the glass, or forming clusters of Spl crystals comprising > 100 grains. Spl crystals from Glass 3 are Tmt, and always decorate Cpx edges or tips. Cpx
crystals with size of 1 - 5μm, formed during quenching, nucleate around Spl crystals isolated in the melt in the hydrous runs. Electron Backscatter Diffraction maps have been acquired to investigate the Crystallographic Orientation Relationships (CORs) between Spl and Cpx crystals and shed light on their crystallisation order and clustering mechanism. More than 70% of Tmt crystals decorating Cpx from glasses 1 and 3 show CORs with Cpx, suggesting a
process of heterogeneous nucleation on top of pre-existing Cpx. Less than 10% of Spl crystals from Glass 2 show CORs with large Cpx crystals, while 20 to 50% of the small Cpx quenched crystals around Spl grains show CORs with Spl. Spl crystals from Glass 2 runs are likely formed by homogenous nucleation before Cpx crystallization. The Cr content of trachybasaltic melts strongly affects the crystallisation order of Spl and Cpx at large ΔT, impacting the size, shape,
and distribution of Spl as well as the geometry of the Cpx crystal framework
developed.
Funded by the Austrian Science Fund (FWF): P 33227-N.

Period	12 Jun 2023 → 15 Jun 2023
Event title	18th International Symposium on Experimental Mineralogy, Petrology and Geochemistry
Event type	Conference
Location	Milano, ItalyShow on map
Degree of Recognition	International