Trace element partitioning between pyrochlore, microlite, fersmite and silicate melts
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eochemical Transactions Open Access
RESEARCH ARTICLE
Trace element partitioning between pyrochlore, microlite, fersmite and silicate melts Stephan Klemme* and Jasper Berndt
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 DZr and DHf below or equal to unity, and DNb and D Ta significantly above unity. We find that DTa pyrochlore-group mineral/silicate melt is always higher than DNb, 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, DTh 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: Pyrochlore, Microlite, Nb, Ta, Ore deposit, Fersmite, Trace element, Partition coefficients, Alkaline rocks, Experimental petrology, LA-ICP-MS, Electron microprobe Introduction To understand the behavior of trace elements in igneous rocks, trace element partition coefficients between minerals and melts are needed. Over the last decades, numerous experimental studies focused on the trace element partitioning between major rock forming minerals and melts [1], but few experimental partition coefficients are available for accessory phases such as rutile, ilmenite, spinel or apatite in basaltic compositions [2–12] and even less data are available for accessory mineral phases such as perovskite or pyrochlore in alkaline rock compositions [13]. As the aforementioned accessory mineral phases commonly occur in alkaline igneous rocks, they may exert a strong control on the trace element evolution of alkaline magmas. *Correspondence: stephan.klemme@uni‑muenster.de Institut für Mineralogie, Universität Münster, Corrensstraße 24, 48149 Münster, Germany
In this study we focus on the Nb- and Ta-rich accessory phases pyrochlore, microlite, and fersmite, which are accessory phases in alkaline silicate rocks [14–20] and, perhaps more commonly, carbonatites [18, 21–23]. Note that these mineral phases are the most important hosts for Nb and Ta in ore deposits (e.g., [18, 24–27]. The pyrochlore-group of minerals in silicate rocks and carbonatites encompasses a very large und very complex group of minerals [28]. The general formula of minerals of the pyrochlore supergroup is A2B2O7, where the A-site is often occupied by monovalent or divalent cations, and the B-site is mainly occupied by pentavalent cations such as Nb or
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