Minor-Element Distribution in Olivine

Microprobe analyses of natural igneous olivines from many rocks confirm that Ni increases and Mn de-decreases with increasing Mg content. A strong crystal-chemical control is apparently not overridden by effects resulting from pressure-temperature variations during crystallization or from variations in bulk composition of the host magma. Calcium ranges from 0 to 1 wt percent and, although commonly increasing with Fe in zoned grains, does not correlate generally with major elements. However, most plutonic olivines have less than 0.10 percent and are separated almost completely from olivines from extrusive and hypabyssal rocks with over 0.10 percent Ca. Depth of crystallization appears to play a major role in determining Ca content, but the data provide no evidence for the correlation of Ca content with crystallization temperature anticipated from experimental work. Diffusion of minor elements in and out of olivine during slow cooling is demonstrated, but its effect on Ca content is uncertain. Some correlation with host-rock chemistry is shown by the olivines from highly undersaturated rocks which, for any given Mg content, are unusually high in Ca and Mn, yet low in Ni.