Titanomagnetite-clinopyroxene clustering in synthetic trachybasalts: Insight into nucleation mechanisms from new experimental samples

Crystal clusters of titanomagnetite (Tmt) and clinopyroxene (Cpx) are ubiquitous features in undercooled trachybasaltic magmas and can affect the rheology and chemical differentiation of volcanic plumbing systems. Furthermore, the circumstances of cluster formation may provide petrological information about processes occurring at depth.

We carried out isothermal time-series experiments on synthetic trachybasaltic melts in a non-end loaded piston cylinder apparatus at 4 kbar, to investigate the formation and evolution of Tmt-Cpx clusters. Samples were held for 30 minutes above the liquidus, before cooling at 80°C/minute to the experimental temperature. We varied the following parameters: i) the degree of undercooling ∆T (expressed as the difference T _liquidus - T _experiment), ranging from 30 to 80 °C; ii) melt H₂O content (either anhydrous or 2 wt.% H₂O added); and iii) dwell time, ranging from 5 minutes to 8 hours.

Phase-contrast synchrotron X-ray microtomographic analysis (SR-µCT) was used to obtain a high-resolution (0.9 µm voxel size) 3D reconstruction of the experimental samples, allowing us to visualize the 3D geometry of the contacts between Cpx and Tmt crystals.

The overall crystallinity of the samples increases with increasing degree of undercooling. Crystal phases are mainly Cpx and Tmt. Their shapes vary from mostly skeletal in samples with lower undercooling and longer dwell time, to mostly dendritic at higher undercooling and/or shorter dwell time. 3D X-ray imaging reveals three Tmt populations: i) large skeletal to euhedral Tmt (up to 150 µm) isolated in the glass or partially embedded in Cpx crystals (Population 1); ii) small skeletal to anhedral Tmt grains (from 1 µm to 50 µm) touching Cpx crystals at a straight interface (Population 2); and iii) arrow/cigar-shaped Tmt grains (major axis ranging from 10 µm to 60 µm, minor <10 µm) almost completely embedded in larger Cpx crystals, with only a small interface (<5 µm) exposed to the melt (Population 3). All population 3 Tmt within the same Cpx grain share a common shape preferred orientation (SPO).

The coexistence of different morphologies of clustered Tmt suggests the coexistence of at least two Tmt nucleation mechanisms within the samples. Population 1 Tmt are inferred to nucleate homogeneously, as they are also found isolated in the glass. Population 2 and 3 Tmt are both inferred to have nucleated heterogeneously in contact with pre-existing Cpx grains.

The reason for the different morphologies of the two heterogeneously nucleated Tmt populations is unknown. It may be due to different timings of heterogeneous nucleation of population 2 and 3, or simultaneous growth of population 3 Tmt together with Cpx. In the future, studies of compositional zoning and crystallographic orientation will enable us to identify the reasons for the difference.

In conclusion, multiple populations of Tmt crystals with different sizes, morphologies and clustering behaviour can arise even from a single cooling event and subsequent annealing at constant temperature. This has important implications for the interpretations of microstructures and crystal size distributions in natural magmas.

Funded by the Austrian Science Fund (FWF): P 33227-N