Major and minor element site occupancies in heated natural forsterite

Abstract

Using a new analytical transmission electron microscopic technique known as CHannelling Enhanced X‐ray Emission (CHEXE) spectroscopy, the M‐site occupancies of Fe, Ni, Mn, and Ca have been determined in a natural forsteritic olivine (Fo91) heat treated at different temperatures. The sample was taken as a single olivine grain from a spinel peridotite inclusion in an alkali basalt and contains 0.36 wt% NiO, 0.07 wt% MnO, and 0.09 wt% CaO. In the non‐heat‐treated sample, 49.6 at% of the Fe, 97 ± 5 at% of the Ni, and 1 ± 5 at% of the Mn in the sample occupy the M1 site. In the present study of samples quenched from different temperatures, the fraction of the Ni present at M1 is 87 ± 5% (6 days at 300°C), 83 ± 5% (48h at 600°C), 83 ± 5% (45 h at 900°C) and 80 ± 5% (24 h at 1050°C). We thus observe a lesser tendency for Ni to order than postulated by previous workers for Ni‐rich olivines. For Mn, typically 15% of the atoms occupy M1 in the heat treated samples. No significant deviation from complete ordering into M2 was observed for Ca. The Fe atoms are completely disordered with 50 ± 1% at each M‐site, except for a weak deviation at 300°C with 47.1 ± 1% at M1. The study indicates that exchange of cations between M‐sites may begin as low as 300°C. This implies that Ni and Mn distributions in natural olivines may be a useful indicator of cooling rate in rapidly cooled rocks.