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Absence of molybdenum isotope fractionation during magmatic differentiation at Hekla volcano, Iceland

Yang, Jie; Siebert, Christopher; Barling, Jane; Savage, Paul; Liang, Yu-Hsuan; Halliday, Alex N.

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Authors

Jie Yang

Christopher Siebert

Jane Barling

Paul Savage

Yu-Hsuan Liang

Alex N. Halliday



Abstract

This study investigates the behaviour of molybdenum (Mo) isotopes during magmatic differentiation. Molybdenum isotope compositions, as well as concentrations of rare earth elements and a selection of trace elements, have been determined for a well characterised sequence of lavas from Hekla volcano, Iceland, covering a compositional range from basalt to rhyolite (46–72 wt.% SiO2), and thought to have developed by differentiation and mixing of melts derived from a cogenetic source. All samples have identical Mo isotopic compositions with an average δ98Mo of −0.15 ± 0.05‰ (2 s.d.; n = 23). There is therefore no resolvable Mo isotope fractionation during magmatic differentiation at Hekla. This finding is supported by the fact that Mo remains highly incompatible in Hekla lavas, increasing from 1.3 to 4.6 μg/g from basalt to rhyolite, indicating that the crystallising phases are extracting only limited amounts of Mo from the magma and therefore that significant fractionation of Mo isotopes is unlikely. It has previously been proposed that cerium (Ce) and Mo have similar bulk distribution coefficients and are equally incompatible during mantle melting. While both Ce and Mo remain incompatible in Hekla lavas, the Ce/Mo ratio decreases from 50 to 36 during magmatic differentiation indicating that Mo is more incompatible than Ce. Comparison of Mo with other incompatible trace elements indicates that Mo is as incompatible as La and slightly less incompatible than K. Sulphur (S) decreases strongly from ∼200 to as low as ∼2 μg/g from basalt to andesite and more evolved compositions, yet this has no effect on the Mo isotopes. Therefore, Mo does not exhibit significant chalcophile behaviour in Hekla magmas. The Mo isotopic signature therefore may be used as an indicator of parent magma composition and a potential discriminant of assimilation processes.

Citation

Yang, J., Siebert, C., Barling, J., Savage, P., Liang, Y., & Halliday, A. N. (2015). Absence of molybdenum isotope fractionation during magmatic differentiation at Hekla volcano, Iceland. Geochimica et Cosmochimica Acta, 162, 126-136. https://doi.org/10.1016/j.gca.2015.04.011

Journal Article Type Article
Acceptance Date Apr 4, 2015
Publication Date Aug 1, 2015
Deposit Date Apr 23, 2015
Publicly Available Date Mar 28, 2024
Journal Geochimica et Cosmochimica Acta
Print ISSN 0016-7037
Publisher Meteoritical Society
Peer Reviewed Peer Reviewed
Volume 162
Pages 126-136
DOI https://doi.org/10.1016/j.gca.2015.04.011

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