The geochemical behaviour of Sn and In during mantle melting and crust formation
Zusammenfassung der Projektergebnisse
The main aim of the present project was to determine the behaviour of Sn and In during mantle melting and crust formation. In order to do this, a routine analytical procedure for the high-precision analyses of Sn and In via isotope dilution and MC-ICP-MS in geological silicate samples had to be developed. As such, tests were carried out involving different digestion techniques and –temperatures as well as optimization of separation and measurement protocols. A total of 16 different geological reference materials were measured and digested multiple times, while some of them were characterized for the first time for their Sn-In budget. During partial melting of MORB mantle sources, the Sn content correlates with the degree of partial melting of the mantle source with Sn being more compatible than its purported twin Sm and consequently a tentative compatibility sequence of In ~ Y > Sn > Sm can be established for mantle melting. Moreover, the behaviour of Sn and In is mainly controlled by partial melting-fractional crystallization during MORB mantle melting, leading to a clear decoupling between mantle residue and basaltic magmas. The results from various terrestrial silicate reservoirs (n = 65) imply a rather uniform Sn/Sm of ~0.27 for the mantle source(s) of MORB whereas depleted mantle samples can display Sn/Sm of up to 3.5. In subduction zones with a high flux of fluids and melts, particularly those derived from subducted sediments a fractionation of the Sn/Sm and In/Y ratios might be expected. As such, we investigated the Sn and In concentrations for 75 well-characterized samples from eight different subduction zones (Sunda arc, Aegean arc, Tonga- Kermadec, Kamchatka, Izu-Bonin-Marianas, Lesser Antilles, Solomon island arc, Bulgaria) that represent end-members for fluid- and/or sediment-melt mediated source enrichment. The In budget in all investigated arc lavas is quite invariant, and reflects a more compatible character compared to Sn and Sm and an only slightly more compatible behaviour than its “twin” Y. As such, the In budget is virtually entirely controlled by the mantle wedge composition and addition of slab or sediment melts/fluids having no impact. In subduction zones with a high flux of sediments the previously established invariant Sn/Sm pair is not confirmed and significant fractionations of Sn/Sm were found (up to values of 1). In contrast, in subduction environments where the trace element budget is controlled by fluids derived from sediments or the AOC Sn-Sm seem to be unfractionated. Our data suggests that Sn shows indeed considerable mobility in subduction zones, however, it is dependent on the previous degree of mantle depletion and type of subduction component. The Sn/In ratio is therefore suggested as the best proxy to assess Sn mobility in arc lavas, with potential applications to other terrestrial silicate reservoirs.
Projektbezogene Publikationen (Auswahl)
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(2015). Precise and accurate determination of In and Sn abundances in geological materials by isotope dilution MCICP-MS. Abstract Goldschmidt Prague
Kirchenbaur M., Wombacher F. Heuser A.
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(2017). The behaviour of Sn and In during mantle melting and crust formation. Abstract GeoBremen
Kirchenbaur M., Bragagni A., Sprung P., Harak M., Münker C.
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(2018). Determination of In and Sn mass fractions in sixteen geological reference materials by isotope dilution MC-ICP-MS. Geostandards and Geoanalytical Research
Kirchenbaur M., Heuser A., Bragagni A., Wombacher F.
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(2018). Revisiting the primitive mantle abundances of the moderately volatile elements Sn and In. Abstract GeoBonn
Kirchenbaur M., Bragagni A., Fonseca R.O.C., Münker C.
