Lang, S.; Mollo, S.; France, L.; Misiti, V.; Nazzari, M.
Chemical Geology, 2022, 601, 120870
Voir en ligne : https://doi.org/10.1016/j.chemgeo.2022.120870
Abstract :
The mechanism governing the kinetic growth of olivine in dynamic volcanic settings has been the subject of
considerable attention in recent years. Under variable cooling rate (CR) and undercooling (- ΔT) regimes, the
textual maturation of olivine proceeds from skeletal/dendritic crystals to polyhedral morphologies by infilling of
the crystal framework. Owing to the establishment of a diffusion-controlled growth regime, a sharp diffusive
boundary layer develops in the melt next to the advancing olivine surface. In this context, we have quantified the
apparent partitioning of Ti, Al, P, and Cr between olivine and a Hawaiian tholeiitic basaltic melt at P = 1 atm,
fO2 = QFM-2 buffer, and CR = 4, 20, and 60 ◦C/h over a constant -ΔT = 85 ◦C. Differences in charge and/or size
between the substituent minor cations and the major species in the olivine crystallographic site dominate the
energetics of homovalent and heterovalent cation substitutions. While the entry of Ti in the olivine lattice site
accounts for the simple exchange [TSi4+] ↔ [TTi4+], more complex charge-balancing coupled-substitution
mechanisms have been determined for the incorporation of Al, P, and Cr, i.e., [MMg2+, TSi4+] ↔ [MAl3+, TAl3+],
[2 TSi4+] ↔ [TP5+, TAl3+], and [MMg2+, TSi4+] ↔ [MCr3+, TAl3+], respectively. In order to maintain charge
balance, the disequilibrium uptake of minor cations in rapidly growing crystals is controlled by the same substitution
mechanisms observed under equilibrium crystallization. This finding is consistent with the achievement
of a local interface equilibrium at the olivine-melt interface independently of the diffusive boundary in the melt.
A statistical approach based on multivariate analysis of olivine/melt compositional parameters confirms that the
control of melt structure on the partitioning of Ti, Al, P, and Cr is almost entirely embodied in the olivine
structure and chemistry via charge compensation reactions. Therefore, the magnitude of minor element partition
coefficients is weakly dependent on diffusion kinetics in the melt but rather strongly governed by olivine zoning
patterns resulting from fast crystal growth rates.