Fe-Mg zoning in olivine represents a disequilibrium state that reflects the physical and chemical environment the mineral has experienced. Olivine crystallization is affected by changes in the magmatic environment. The types of profiles we have observed include (1) homogeneous crystals, (2) homogeneous cores and zoned rims, (3) zoning extending uniformly across the crystal, (4) composition profiles that change in steps across the crystal, and (5) irregular zoning resulting from alteration and oxidation. Stepped profiles appear to have resulted from episodic changes in the magmatic environment. Calculations show that the steps could have formed owing to changing pressure as magma pulsed toward the surface.
Olivines from crater 160, a maar in the San Francisco volcanic field, Arizona, differ markedly in their zoning profiles. Comparison of separate flows indicates a common parent magma and suggests that a near-surface magma chamber did not exist. Probable correlations between a complex feeder dike and specific nearby flows were made.