Mg/Mn partitioning as a test for equilibrium between coexisting Fe-Ti oxides

Partitioning of Mg and Mn between titanomagnetite and ferrian ilmenite of volcanic rocks provides a test for equilibrium between coexisting phases. A plot of log(Mg/Mn)-, vs. log(Mg/Mn),, for 213 homogeneous oxide pairs from volcanic rocks yields a straight line over more than two orders of magnitude variation in Mg/Mn. Analyses that plot within reasonable limits of analytical precision of this line are consistent with preservation of equilibrium compositions. The test appears to be valid for fresh volcanic rocks but may not be applicable to oxides in metamorphic, plutonic, and altered volcanic rocks in which oxide minerals typicatly have oxidized or exsolved at subsolidus temperatures. It can be used to discriminate between multiple populations of an oxide phase or to evaluate equilibration of inclusions in silicate phenocrysts for Fe-Ti oxide geothermometry and oxygen barometry.