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


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Danyushevsky, LV, Della-Pasqua, FN and Sokolov, S 2000 , 'Re-equilibration of melt inclusions trapped by magnesian olivine phenocrysts from subduction-related magmas: petrological implications' , Contributions to Mineralogy and Petrology, vol. 138, no. 1 , pp. 68-83 , doi: 10.1007/PL00007664.

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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.

Item Type: Article
Authors/Creators:Danyushevsky, LV and Della-Pasqua, FN and Sokolov, S
Keywords: melt inclusions, olivine, petrology, geochemistry, subduction, magmatism, re-equilibration
Journal or Publication Title: Contributions to Mineralogy and Petrology
DOI / ID Number: 10.1007/PL00007664
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