Synchrotron radiation microtomographic analysis of a synthetic trachybasalts: insight into titanomagnatite nucleation mechanisms

Description

Titanomagnetite (Tmt) and Clinopyroxene (Cpx) commonly form clusters in mafic magmas. Studying the mechanisms of active crystal clustering and the formation conditions of clusters offers important insights into the nature and evolution of volcanic plumbing systems and crystal mushes. 2D images are not sufficient to capture the geometry and true degree of clustering between two phases. To overcome this, we present a phase-contrast synchrotron X-ray microtomographic analysis of Tmt-Cpx clusters in a synthetic trachybasalt sample with 2 wt% H2O added. The sample is one of a series of isothermal time series experiments, produced in a Quick Press piston-cylinder apparatus at 4 kbar. After 30 minutes of superheating at 1300°C the sample was cooled to 1150°C and held for 30 minutes before quenching. 3D X-ray imaging reveals a vertical gradient in crystallinity and in the size of the mineral phases caused by an intrinsic temperature (and thus undercooling) gradient in the experimental apparatus (temperature decreases of approximately 30 °C from bottom to top of the capsule). At the cold end of the capsule, the crystallinity is maximum, the crystal phases are small dendritic Cpx grains (between 100 µm to 300 µm in size) and small anhedral Tmt grains (1 µm to 25 µm in diameter). In the central portion of the sample, the crystallinity is lower but the overall size of Cpx (> 500 µm) and Tmt (skeletal, 80 µm to 150 µm in diameter) increases. The shape of the crystals is less dendritic/anhedral, respectively. In the hot end of the capsule (where the temperature is controlled by the thermocouple), the sample is dominated by glass, and only isolated skeletal Tmt grains crystallized. Cpx and Tmt grains are clustered together whenever both crystal phases are present. Although large skeletal Tmt grains are more abundant in the central portion while small anhedral Tmt grains dominate the cold end of the sample, the occurrence of the two Tmt morphologies overlaps, and both can be found in contact with the same Cpx grain. The coexistence of two different Tmt grain morphologies and sizes implies two different nucleation mechanisms. Large skeletal Tmt in the bottom (hotter) and central portion of the sample likely crystallized by homogeneous nucleation and probably predated the crystallization of Cpx grains. Smaller, anhedral Tmt crystals, which decorate dendrites in the central to upper (cooler) portion of the sample, probably crystallized by heterogeneous nucleation on already crystallized Cpx grains. In future, EBSD analysis on the sample will assess if crystallographic orientation relationships between Tmt and Cpx in clusters differ depending on Tmt morphology and inferred nucleation mechanism.

Period	21 Sept 2021
Event title	MinPet2021: Biennial Meeting of the "Österreichische Mineralogische Gesellschaft"
Event type	Conference
Location	Wien, AustriaShow on map
Degree of Recognition	National