Proton microprobe determined partitioning of Rb, Sr, Ba, Y, Zr, Nb and Ta between experimentally produced amphiboles and silicate melts with variable F content

Abstract

A proton microprobe was used to measure partition coefficients for Rb, Sr, Ba, Y, Zr, Nb and Ta between experimentally produced amphiboles and hydrous basaltic melts. A limited amount of data was also obtained for the distribution of trace elements in clinopyroxene and mica. Partition coefficients for trace elements in amphibole and basanite melts are (at 1σ): Rb 0.34 ± 0.14; Sr 0.33 ± 0.07; Ba 0.46 ± 0.16; Y 0.6 ± 0.2; Ti 0.95 ± 0.19; Zr 0.25 ± 0.06; Nb 0.08 ± 0.01; and Ta 0.09 ± 0.03. Only small, generally non-systematic differences in these values are observed with variation in pressure (10–20 kbar) and temperature (1000–1050°C), but large differences accompany changes in melt composition and F content. For a (F-free) basaltic andesite melt, at 20 kbar and 950°C, amphibole/melt distribution coefficients are: Rb 0.07 ± 0.01; Sr 0.35 ± 0.03; Y 1.3 ± 0.1; Ti 1.75 ± 0.12; Zr 0.35 ± 0.06; Nb 0.21 ± 0.01; and Ta 0.19 ± 0.02. The data support proposals that residual amphibole in mantle source regions for some nephelinites explains their relatively high HFSE/LILE ratios. In contrast, the data do not favour amphibole as the cause of characteristically low HFSE/LILE observed in mantle-derived island arc basalts. The increases in partition coefficients (excepting for Rb) for the more SiO 2-rich melt are consistent with similar trends observed in phenocryst-matrix pairs from volcanic rocks. The compositional dependence of HFSE partition coefficients increases with increasing field strength. This trend can be related to steric effects within polymerised aluminosilicate units of the melt phase. Amphiboles grown from F-enriched melts are relatively depleted in TiO 2, Al 2O 3, CaO and incompatible trace elements. These effects are only large, however, at high F concentrations (> 2 wt%). The effects of F on incompatible elements will be least in melts containing high concentrations of Al 2O 3, FeO, MgO and CaO. For these reasons, it is unlikely that concentrations of HFSE and other incompatible elements in natural magmas (with the possible exception of some rare F-and SiO 2-rich magmas) are significantly affected by F.