Trace element partitioning between pyrochlore, microlite, fersmite and silicate melts

Stephan Klemme; Jasper Berndt

doi:10.1186/s12932-020-00072-w

Trace element partitioning between pyrochlore, microlite, fersmite and silicate melts

Geochem Trans. 2020 Aug 24;21(1):9. doi: 10.1186/s12932-020-00072-w.

Authors

Stephan Klemme¹, Jasper Berndt²

Affiliations

¹ Institut für Mineralogie, Universität Münster, Corrensstraße 24, 48149, Münster, Germany. stephan.klemme@uni-muenster.de.
² Institut für Mineralogie, Universität Münster, Corrensstraße 24, 48149, Münster, Germany.

Abstract

We present experimentally determined trace element partition coefficients (D) between pyrochlore-group minerals (Ca₂(Nb,Ta)₂O₆(O,F)), Ca fersmite (CaNb₂O₆), and silicate melts. Our data indicate that pyrochlores and fersmite are able to strongly fractionate trace elements during the evolution of SiO₂-undersaturated magmas. Pyrochlore efficiently fractionates Zr and Hf from Nb and Ta, with D_Zr and D_Hf below or equal to unity, and D_Nb and D_Ta significantly above unity. We find that D_Ta pyrochlore-group mineral/silicate melt is always higher than D_Nb, which agrees with the HFSE partitioning of all other Ti-rich minerals such as perovskite, rutile, ilmenite or Fe-Ti spinel. Our experimental partition coefficients also show that, under oxidizing conditions, D_Th is higher than corresponding D_U and this implies that pyrochlore-group minerals may fractionate U and Th in silicate magmas. The rare earth element (REE) partition coefficients are around unity, only the light REE are compatible in pyrochlore-group minerals, which explains the high rare earth element concentrations in naturally occurring magmatic pyrochlores.

Keywords: Alkaline rocks; Electron microprobe; Experimental petrology; Fersmite; LA-ICP-MS; Microlite; Nb; Ore deposit; Partition coefficients; Pyrochlore; Ta; Trace element.