THE MEANING OF CHEMICAL ZONING IN OLIVINE: PERSPECTIVES FROM CRYSTALLIZATION EXPERIMENTS
Experimental studies have so far generally focused on olivine crystallization in melts with few components (e.g. CMAS), and chemical diffusion in olivine without surrounding melt. Series of 1-atm crystallization experiments involving high-MgO (~11.4 wt.%) Kilauea basalt were performed to investigate the development of chemical zoning during initial growth, and subsequent modifications by diffusion under isothermal conditions. Low to moderate degrees of undercooling (ΔT=10-60°C) were applied by cooling the runs rapidly from near-liquidus conditions to their final temperature. Results show that little to no zoning occurs in most major and minor elements (Fe-Mg, Ni, Ca, Cr, Mn) during rapid growth. In contrast, nominally incompatible trace elements like Al or P show striking variations in composition. Phosphorus appears to substitute for Si, and yields similar ‘apparent’ partition coefficients (~0.8). Diffusion in most major and minor elements is apparently much faster than predicted by relationships known from melt-free experiments, raising questions about the timescales that we can infer from diffusion modeling in natural crystals.