Homogeneous and heterogeneous nucleation in synthetic trachybasalts: microstructural evidence from Titanomagnetite crystals of different populations

Abstract

Homogeneous and heterogeneous nucleation are two important mechanisms of mineral formation. Heterogeneous nucleation is theoretically more favourable because it requires overcoming a lower energetic barrier, and is thus supposed to be the prevalent nucleation mechanism during magma crystallisation and crystal cluster formation. However, it remains challenging to identify whether a given crystal population is formed by homogeneous or heterogeneous nucleation.Here we present results that allow us to reconstruct the nucleation mechanism of titanomagnetite (Tmt) crystals formed alongside dendritic clinopyroxene (Cpx) from a synthetic trachybasaltic melt (with 2 wt.% added H2O) in crystallisation experiments carried out in a piston-cylinder apparatus at a constant pressure of 4 kbar. 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 1150°C and 1100°C. These temperatures correspond to a respective undercooling (∆T expressed as T liquidus – T experiment) of 30° and 80°. The dwell times at these temperatures were 30 minutes and 8 hours, respectively.High-resolution synchrotron X-ray computed microtomography (µCT) and subsequent 3D image processing and analysis allow to discriminate three main Tmt populations: a) Tmt grains > 100 µm in size, skeletal in shape, and mostly isolated in the melt (population 1); b) Tmt grains between 2 and 100 µm in size, anhedral in shape, and always decorating Cpx grain surfaces (population 2); c) Acicular Tmt grains almost completely enclosed within Cpx grains (population 3).The 3D spatial distribution of the centroids of the Tmt grains was employed to understand if the grains of the different populations are randomly distributed, ordered, or clustered, using the pair correlation function g(x). Tmt grains of population 1 have g(x) near 1, a sign of an ordered point pattern. We attribute this to a homogeneous nucleation origin. Populations 2 and 3 have peak g(x) values up to 1.5 at interpoint distances between 10 and 30 µm, denoting strong clustering at these distances. We attribute this to heterogeneous nucleation of Tmt on Cpx, which is corroborated by 3D microstructure and the relationship between Tmt grains and compositional zoning of Cpx.Electron backscatter diffraction analysis enables us to clarify the crystallographic relationships between Cpx and nearby Tmt crystals. More than 85% of the Cpx-Tmt boundary length in Tmt populations 2 and 3 follow a crystallographic orientation relationship (COR). This strongly points to formation by heterogeneous nucleation of Tmt on top of Cpx grains for these populations. Single grains in Tmt population 1 which touch Cpx crystals show a COR with Cpx less frequently (cpx-tmt boundaries sharing a COR are <60%, confirming that at least some proportion are the result of homogeneous nucleation. Population 1 Tmt with CORs may represent large heterogeneously nucleated grains or potentially reflect Tmt-Cpx interaction after nucleation.In conclusion, multiple, potentially simultaneous Tmt nucleation events led to observable differences in microstructure, clustering, and CORs that enable the crystallisation process to be reconstructed.  Funded by the Austrian Science Fund (FWF): P 33227-N