Crystal alignment and clustering during crystallisation of experimental basalt under strain

Description

Crystallisation in volcanic systems rarely occurs in static conditions. The effects of melt flow on crystal shape, size, orientation, and clustering behaviour may have important consequences for rheology and melt-crystal separation. Experiments carried out in the Petro-Volcanology Research Group lab of the University of Perugia allow observation of the effect of strain on shape preferred orientation (SPO), crystallographic preferred orientation (CPO), and clustering behaviour during igneous crystallisation. A basaltic starting glass synthesised from lava from the 2002/3 eruption of Mt. Etna was placed in a 37 mm diameter crucible in a high temperature furnace. After heating above the liquidus (at room pressure in air), a central spindle was rotated to constantly stir the melt. The strain rate was fixed at 1s-1 at the spindle rim. The sample was cooled to 1170 °C (~30 °C undercooling), then subjected to sub-liquidus temperature oscillations to create chemical zoning, before air-quenching after 8 hours.
The sample contains mostly glass, subhedral plagioclase (Pl) and clinopyroxene (Cpx), with some skeletal magnetite (Mt). Electron backscatter diffraction (EBSD) mapping was carried out for two domains, one near to and one far from the spindle. Near the spindle, Pl exhibits a strong SPO subparallel to the shear direction and a weak CPO, with Pl (010) normals oriented subperpendicular to the shear plane. Cpx shows only a very weak SPO subparallel to the shear direction, and a very weak CPO characterised by Cpx [001] axes lying preferentially in the shear plane. Far from the spindle, the Pl SPO and CPO weaken considerably, the Cpx SPO disappears, the median Cpx grain length is 3x higher, and the area fraction of glass is 7% higher.
Near the spindle, touching Pl-Cpx and Cpx-Cpx grain pairs show preferential crystallographic alignments. Plotting crystal orientations with respect to their neighbours reveals a {110}Pl||{010}Cpx rotational statistical crystallographic orientation relationship (COR) for Pl-Cpx grain pairs, while Cpx-Cpx grain pairs follow a dispersional statistical COR ({110}Cpx~||{110}Cpx and {001}Cpx~||{001}Cpx). Far from the spindle, very few boundary segments follow these statistical CORs. The CORs are thus inferred to indicate strain-induced alignment and clustering of crystals in the melt (synneusis). Heterogeneous nucleation is unlikely as individual grain pairs deviate from fixed (specific) CORs, and heterogeneous nucleation of Cpx on Cpx is energetically unfavourable vs. growth.
In conclusion, even where a strong SPO is present, CPO is weak. Therefore, detecting crystallisation under strain using anisotropy-based geophysical methods may be difficult. Statistical CORs resulting from crystal alignment and attachment are observed even where both SPO and CPO are weak, suggesting CORs may provide a more reliable indicator of crystallisation in flowing magma than SPO or CPO alone.
Funded by the Austrian Science Fund (FWF): P 33227-N

Period	21 Sept 2022
Event title	Geosciences for a Sustainable Future: SGI SIMP Torino 2022
Event type	Conference
Location	Turin, ItalyShow on map
Degree of Recognition	International