Re-equilibration of melt inclusions trapped by magnesian olivine phenocrysts from subduction-related magmas: petrological implications

Abstract

We describe and model a potential re-equili- bration process that can affect compositions of melt inclusions in magnesian olivine phenocrysts. This pro- cess, referred to as ``Fe-loss'', can operate during natural pre-eruptive cooling of host magma and results in lower FeOt and higher MgO contents within the initially trapped volume of inclusion. The extent of Fe-loss is enhanced by large temperature intervals of magma cooling before eruption. The compositions of homoge- nised melt inclusions in olivine phenocrysts from several subduction-related suites demonstrate that (1) Fe-loss is a common process, (2) the maximum observed degree of re-equilibration varies between suites, and (3) within a single sample, variable degrees of re-equilibration can be recorded by melt inclusions trapped in olivine pheno- crysts of identical composition. Our modelling also demonstrates that the re-equilibration process is fast going to completion, in the largest inclusions in the most magnesian phenocrysts it is completed within 2 years. The results we obtained indicate that the possibility of Fe-loss must be considered when estimating composi- tions of parental subduction-related magmas from nat- urally quenched glassy melt inclusions in magnesian olivine phenocrysts. Compositions calculated from glassy inclusions affected by Fe-loss will inherit not only erroneously low FeOt contents, but also low MgO due to the inherited higher Mg# of the residual melt in re- equilibrated inclusions. We also demonstrate that due to the higher MgO contents of homogenised melt inclu- sions affected by Fe-loss, homogenisation temperatures achieved in heating experiments will be higher than original trapping temperatures. The extent of overheat- ing will increase depending on the degree of re-equili- bration, and can reach up to 50 degrees C in cases where complete re-equilibration occurs over a cooling interval of 200 degrees C.