Triple oxygen isotope variations in magnetite from iron-oxide deposits, central Iran, record magmatic fluid interaction with evaporite and carbonate host rocks
Date
2020
Authors
Peters, Stefan T. M.
Alibabaie, Narges
Pack, Andreas
McKibbin, Seann J
Raeisi, Davood
Nayebi, Niloofar
Torab, Farhad
Ireland, Trevor
Lehmann, Bernd
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Geological Society of America Inc
Abstract
Oxygen isotope ratios in magnetite can be used to study the origin of iron-oxide ore deposits.
In previous studies, only 18O/16O ratios of magnetite were determined. Here, we report triple
O isotope data (17O/16O and 18O/16O ratios) of magnetite from the iron-oxide-apatite (IOA)
deposits of the Yazd and Sirjan areas in central Iran. In contrast to previous interpretations of
magnetite from similar deposits, the triple O isotope data show that only a few of the magnetite
samples potentially record isotopic equilibrium with magma or with pristine magmatic water
(H2O). Instead, the data can be explained if magnetite had exchanged O isotopes with fluids
that had a mass-independently fractionated O isotope composition (i.e., MIF-O), and with
fluids that had exchanged O isotopes with marine sedimentary carbonate rocks. The MIF-O
signature of the fluids was likely obtained by isotope exchange with evaporite rocks of early
Cambrian age that are associated with the IOA deposits in central Iran. In order to explain
the triple O isotope composition of the magnetite samples in conjunction with available iron
isotope data for magnetite from the deposits, we propose that magnetite formed from magmatic fluids that had interacted with evaporite and carbonate rocks at high temperatures and
at variable water/rock ratios; e.g., magmatic fluids that had been released into the country
rocks of a magma reservoir. Additionally, the magnetite could have formed from magmatic
fluids that had exchanged O isotopes with SO2 and CO2 that, in turn, had been derived by the
magmatic assimilation and/or metamorphic breakdown of evaporite and carbonate rocks.
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Geology
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Journal article
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