TWIST SO FINE: EBSD STUDY OF CLINOPYROXENE LATTICE ROTATION IN KILAUEA BASALT

We characterize the morphologies and crystallographic orientations of dozens of clinopyroxene (cpx) crystals in a sample of basaltic lava emplaced during the 2018 Kīlauea Lower East Rift Zone eruption in the context of ongoing analysis of feedbacks among lava solidification, rheology, and flow dynamics. Near the sample’s glassy margin, isolated cpx crystals ≤50 microns in length have a branching (dendritic) internal structure but are externally compact, bounded by a polyhedral external footprint. This domain contains isolated crystals and center-cut sections (indicated by morphological symmetry of central plateaus), thus affording estimation of tip propagation directions, or traces thereof, in a context of known thermal gradient orientation. Electron backscatter diffraction (EBSD) mapping was performed using a Zeiss Gemini 450 SEM with Oxford Symmetry detector and analyzed with AZtecCrystal software. Similar characteristics are noted in all cpx dendrites, including (a) crystal-lattice rotation about the b-axis, such that the a-axis progressively rotates toward the initial position of the c-axis, averaging 4° um^-1 in the section plane, (b) unidirectional sense of lattice rotation across the entire crystal, regardless of tip propagation direction, and (c) lattice rotation that is either incremental and continuous and or discrete and focused at first-order branch segment jumps. These results broadly accord with recent observations of feathery cpx in fulgerite by Griffiths et al. (2023). We concur with their inference that the rotation mechanism is intrinsic (i.e., controlled by the morphology and crystallography of the dendrites themselves rather than an external thermal or compositional field) and thus could be calibrated as an indicator of thermodynamic undercooling.

Griffiths, T. A. et al. The Origin of Lattice Rotation during Dendritic Crystallization of Clinopyroxene. J. Petrol. 64, 1–19 (2023).