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 (46–72wt.% 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 4.6μg/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 ∼2μg/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.
Highlights
As a redox sensitive element with seven stable isotopes, molybdenum (Mo) holds great potential for deciphering the geological past
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
The Mo concentrations of GSP-2 analysed in this study show exellent consistency for 15 digestions and the average concentration of 2.1 ± 0.4 lg/g is in agreement with the USGS recommended value, 2.1 ± 0.6 lg/g
Summary
As a redox sensitive element with seven stable isotopes, molybdenum (Mo) holds great potential for deciphering the geological past. Molybdenum isotope data coupled with Mo concentrations have been used to reconstruct the. Siebert et al, 2003; Anbar, 2004; Pearce et al, 2008; Gordon et al, 2009). It has become clear that Mo isotope fractionation can occur at low-temperatures during redox reactions Tossell, 2005; Nagler et al, 2011), Mn–Fe oxides absorption (e.g. Barling and Anbar, 2004; Wasylenki et al, 2008), and biological utilisation
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
