Heterogeneous nucleation of crystals in synthetic trachybasalts

Description

In systems that are weakly to moderately supersaturated with respect to a mineral, heterogeneous crystal nucleation on a pre-existing surface is energetically more favourable than homogeneous nucleation from a melt (Lofgren 1983). For this reason, heterogeneous nucleation is considered one of the most important clustering mechanisms occurring in both natural magmas and experimental analogues (Hammer et al., 2010). However, in volcanic samples with a complex pressure-temperature crystallization history, robust criteria for proving heterogeneous nucleation are lacking.
In this study, we present microstructural observations supporting heterogeneous nucleation of clinopyroxene (cpx) and oxides (ox) in time-series dynamic crystallization experiments performed at 4 kbar using a piston cylinder apparatus on dry (nominally 0 wt.% H2O) and wet (2 wt.% H2O added) trachybasaltic melts. After 30 minutes of superheating at 1300°C, the samples were cooled at a rate of 80°C/min to the final resting temperatures of 1050, 1100 and 1150 °C, corresponding to degrees of undercooling (ΔT expressed as T liquidus – T experiment) ranging from 30° to 160°. The duration of the experiments at the resting temperature varied from 5 minutes to 22 hours in the different experiments.
High resolution synchrotron X-ray microtomography (SR-μCT), electron backscatter diffraction (EBSD) maps, and microprobe (EMPA) chemical analysis have been used to extract the 3D distribution and the crystallographic orientation relationships (CORs) between touching crystals, and the compositional zoning of crystals and melt. All the experimental samples contain cpx (5 to 55 vol.%), and ox (1 to 5 vol.%). Cpx and ox morphology varies respectively from skeletal to dendritic, and from skeletal to anhedral with increasing ΔT.
Microstructural observations supporting the hypothesis of heterogeneous nucleation of ox on top of cpx crystals and vice versa are the following: a) a high fraction of ox touching cpx crystals (>0.95); b) radial arrangements of cpx crystals around single ox crystals (rosette structures); c) ox crystals decorate the tips or the edges of cpx crystals, with flat to curved cpx-ox interfaces; d) adjacent oxides on the same dendritic branch are regularly spaced, and e) the fraction of total cpx-ox boundary length following a COR exceeds 70%. Combining multiple methods, including 3D and crystallographic information, is essential to identify heterogeneous nucleation with confidence.
Funded by the Austrian Science Fund (FWF): P 33227-N
Hammer J.E., Sharp T.G. & Wessel P. 2010. Heterogeneous nucleation and epitaxial crystal growth of magmatic minerals. Geology 38:367-370.
Lofgren G.E. 1983. Effect of Heterogeneous Nucleation on Basaltic Textures: A Dynamic Crystallization Study. J Petrol 24:229–255..

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	National