Crystallization kinetics of clinopyroxene and titanomagnetite growing from a trachybasaltic melt: New insights from isothermal time-series experiments

Abstract

In order to investigate the role of crystallization kinetics in mafic alkaline systems, textural measurements, mineral compositional changes and diffusion modelling calculations have been carried out on isothermal time-series experiments. The data were obtained at 400 MPa and 1100 °C under anhydrous (nominally 0 wt% H2O) and hydrous (2 wt% H2O added) conditions. A synthetic trachybasaltic melt was first heated up to the superliquidus temperature of 1300 °C and then rapidly cooled at 80 °C/min down to 1100 °C. The final target temperature was kept constant over variable dwell times in the range of 0.5–24 h. Results from textural analysis indicate the attainment of fast crystal growth kinetics at the shortest experimental run duration, with early achievement of stable crystal sizes for clinopyroxene and titanomagnetite. The surface area to volume ratio weakly decreases with increasing dwell time, according to the development of euhedral crystal morphologies. Crystal growth rates are also observed to progressively decrease from 0.5 to 24 h. Due to the effect of fast growth kinetics, the morphological maturation of clinopyroxene progresses by attachment of dendrite branches, infilling and overgrowth phenomena, leading to the formation of well-faced and euhedral crystals. The kinetically-controlled cation exchange (Si + Mg) → (TAl + Fe3+) controls the clinopyroxene compositional variation, expanding the stability of Tschermak component at the expense of diopside. Conversely, titanomagnetite is characterized by an almost constant composition that, however, is enriched in incompatible Al and Mg cations, as typically observed under rapid crystal growth conditions. Titanomagnetite crystals show always euhedral morphology that develops by heterogeneous nucleation on early-formed clinopyroxene dendrites. Overall, the effect of undercooling causes strong supersaturation phenomena in the trachybasaltic melt, resulting in enhanced nucleation kinetics and fast attainment of a high crystallinity. As the dwell time increases, the bulk system tends to minimize the interfacial free energy between crystals and surrounding melt. This results in the progressive replacement of the early dendritic shapes developed in a diffusion-limited growth regime, by the formation of euhedral morphologies typical of interface-limited regimes that still retain the chemical evidences of the dendritic stage as complex zoning patterns in clinopyroxene.