Transport of crystal cargo in high-Mg lava flows: a microstructural study of Reliance Formation komatiites, Belinqwe Belt, Zimbabwe

Komatiites are extrusive volcanic rocks with 18 wt % Mg, and are almost exclusively of Archean age (Arndt 1994). These high-Mg lavas have very low viscosity (0.1-10 Pa) and are expected to have flowed in a turbulent manner (Huppert and Sparks 1985). However, there is little field evidence for such flow behaviour (Cas et al. 2024). Additionally, as such hot lavas no longer erupt on Earth, we know little of the initial crystal cargo they may have transported. To address these questions, we investigated komatiite lava flows from the Reliance Formation, Belingwe Greenstone belt, Zimbabwe, using quantitative microstructural analyses together with electron backscatter diffraction (EBSD).

We investigated misorientation axes between neighbouring olivine grains in crystal clusters, to gain information about their crystallisation history and mode of aggregation. We found a systematic misorientation axis, [100], and three dominant misorientation angles (4˚, 40˚ and 60°). The low-angle misorientation axes are associated with olivine crystals with dendritic morphology, and are associated with regions of the crystals where branching occurs. In contrast, the 40˚ misorientation angles are found in crystal clusters formed by synneusis, whereas the 60° misorientation angles are associated with grains showing a twinning relationship (Wieser et al. 2019). We also investigated the crystallographic relationship between enclosed grains of Cr-rich spinels and their host olivine crystals. These show an epitaxial relationship; [100]_Ol [111]_Sp and [001]_Ol [110]_Sp.

Vance (1969) suggests that the likelihood of random collisions between olivine crystals is most likely in a turbulent flow, but Schwindinger & Anderson (1989) point out that such random collisions will not produce aggregates with systematic crystallographic alignment. Instead, they infer that synneusis with systematic alignments requires a “flowing fluid” (i.e. laminar flow). Therefore, we suggest that the relationships we observe in the Belingwe olivine clusters result from synneusis of crystals carried from their magmatic source in a laminar flow. Additionally, the observed epitaxial relationship between olivine and spinel suggests that spinel either exsolved from olivine, or nucleated heterogeneously on the olivine substrates with an epitaxial relationship. This will require further investigation.

 

Reference:

Arndt, N. T. In Developments in Precambrian Geology, vol. 11, pp. 11-44. Elsevier, 1994.

Huppert, H. E., and Sparks. R.S.J.  " Journal of Petrology 26, no. 3 (1985): 694-725.Cass et al 2024

Cas, R., Wright, J. V., & Giordano, G. (2024). Cham: Springer International Publishing.

Wieser, P. E., Vukmanovic, Z., Kilian, R., Ringe, E., Holness, M. B., Maclennan, J., & Edmonds, M. (2019). Geology, 47(10), 948-952.

Vance, J. A. (1969). On synneusis. Contributions to Mineralogy and Petrology, 24(1), 7-29

Schwindinger, K. R., & Anderson Jr, A. T. (1989). Contributions to Mineralogy and Petrology, 103(2), 187-198.