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

Abstract

We describe and model a potential re-equilibration process that can aect compositions of melt inclusions in magnesian olivine phenocrysts. This process, referred to as``Fe-loss'', can operate during natural pre-eruptive cooling of host magma and results in lower FeO t 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 homogenised 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 phenocrysts 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 compositions of parental subduction-related magmas from naturally quenched glassy melt inclusions in magnesian olivine phenocrysts. Compositions calculated from glassy inclusions aected by Fe-loss will inherit not only erroneously low FeO t contents, but also low MgO due to the inherited higher Mg# of the residual melt in reequilibrated inclusions. We also demonstrate that due to the higher MgO contents of homogenised melt inclusions aected by Fe-loss, homogenisation temperatures achieved in heating experiments will be higher than original trapping temperatures. The extent of overheating will increase depending on the degree of re-equilibration, and can reach up to 50°C in cases where complete re-equilibration occurs over a cooling interval of 200°C.

High-K ankaramitic melt inclusions and lavas in the Upper Cretaceous Eastern Srednogorie continental arc, Bulgaria: Implications for the genesis of arc shoshonites

“…These processes have been described in details by Danyushevsky et al (2000Danyushevsky et al ( , 2002, who demonstrated that the degree of modification of the melt inclusions strongly depend on the cooling rate and chemical composition of the melt and host. The submarine emplacement and glassy nature of the groundmass of the cumulate SG12 suggest that the entire chemical modification of the melt inclusions most probably occurred before the eruption of the lava.…”

Constraints from melt inclusions and their host olivines on the petrogenesis of Oligocene-Early Miocene Xindian basalts, Chifeng area, North China Craton

“…There is a negative correlation between FeO contents of melt inclusions and Fo values of host olivines, the so-called Fe-loss (Fig. 2a), which is caused by post-entrapment re-equilibration between melt inclusions and their host olivines (e.g., Danyushevsky et al 2000). The measured major element compositions of these inclusions therefore need to be recalculated.…”

“…Thus, we assume a constant FeO content during the Fe-loss correction. We used the method of Danyushevsky et al (2000) and the model of Ford et al (1983) for ''Fe-loss'' correction and post-crystallization, by assuming the total FeO contents of Xindian lavas to be 10.9 wt%, and oxygen fugacity to be QFM. On a total alkalis-silica (TAS) diagram ( Fig.…”

Relationships between Campi Flegrei and Mt. Somma volcanism: evidence from melt inclusions in clinopyroxene phenocrysts from volcanic breccia xenoliths

“…Clinopyroxene phenocrysts were separated and mounted into epoxy, polished and examined for the presence of melt inclusions. Grains containing melt inclusions suitable for reheating experiments [see Danyushevsky et al (2000) for detailed description of the reheating technique] were polished on both sides to form doublypolished wafers $ 300 microns thick. Experiments were performed at the University of Tasmania using a low-inertia heating stage designed at the Vernadsky Institute, Moscow (Sobolev and Slutskii, 1984).…”

“…Experiments were performed at the University of Tasmania using a low-inertia heating stage designed at the Vernadsky Institute, Moscow (Sobolev and Slutskii, 1984). Reheating experiments were continued until the inclusions were deemed to be close to homogenisation (Danyushevsky et al, 2000), at which point they were quenched by switching the power off. A detailed description of the experimental procedures will be presented elsewhere (Lima et al, in preparation).…”