The effect of strain on crystal alignment and clustering during crystallisation of basalt in experiment

Description

During magma transport and effusive eruption, crystallisation often occurs. Investigating the effect of crystallisation in flowing melt on microstructure, crystallographic preferred orientation (CPO), and cluster formation can shed light on the interaction between crystallisation and rheological behaviour of magma, and provides new tools to detect syn-eruptive crystallisation in nature.
At the University of Perugia Petro-Volcanology Research Group lab, basaltic glass synthesised from crushed Mt. Etna lava was placed in a cylindrical crucible in a furnace (at 1 atmosphere in air). After melting and super-liquidus homogenisation, a central spindle was inserted, rotating at a fixed strain rate of 1s-1 at its rim. The sample was cooled to 1170 °C (~ 30 °C undercooling), then subjected to ten sub-liquidus temperature oscillations of 40°C to create chemical zoning. After ~ 18 hours, spindle rotation was stopped, and the sample was annealed for 48 hours at 1150°C before quenching in air.
Electron backscatter diffraction scans were acquired at different distances from the spindle to quantify microstructure, CPO, and crystallographic orientation relationships (CORs). The sample contains glass (42 – 51%), subhedral plagioclase (Plg, 25 – 27%) and clinopyroxene (Cpx, 18 – 25%), and skeletal magnetite (Mt, 5 – 6%). In general, phase proportions are constant, whereas average grain size increases and average number density decreases with increasing distance from the spindle for all phases. Only Plg shows a strong shape preferred orientation (SPO), weakening far from the spindle. Near to the spindle, Plg exhibits a weak CPO (J-index 1.3 – 1.4), while Cpx exhibits a very weak CPO (J-index ≤ 1.2). Far from the spindle, both CPOs weaken further but do not disappear. In detail, variations occur at the same radial distance. Very close to the spindle, localised very fine grained domains with reduced melt fraction occur. Far from the spindle, phase proportions remain constant, but some domains have weaker Plg SPO and larger Cpx grains. Heterogeneous radial distribution of microstructures underlines the complex relationship between microstructure and rheology in crystallisation under strain.
Near to the spindle, low-index planes in crystals are preferentially aligned subparallel to low-indexed planes in their touching neighbours. {110}Plg aligns preferentially with (010)Cpx, and there is also an increased fraction of low angle Cpx-Cpx grain boundaries. Far from the spindle, these alignments become more dispersed, but do not disappear. Both the statistical nature of CORs and their variation with radial distance suggest they arise due to physical interaction of grains in the melt. The fact that simultaneous deformation and crystallisation led to weak or very weak CPOs, but CORs resulting from physical crystal alignment were always clearly detectable, suggests CORs may be a key indicator of crystallisation in flowing magma.
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