PROTON MICROPROBE DETERMINED PARTITIONING OF RB, SR, BA, Y, ZR, NB AND TA BETWEEN EXPERIMENTALLY PRODUCED AMPHIBOLES AND SILICATE MELTS WITH VARIABLE F CONTENT

A proton microprobe was used to measure partition coefficients for Rb, Sr, Ba, Y, Zr, Nb and Ta between experimentally produced amphiboles a nd hydrous basaltic melts. A limited amount of data was also obtained for the distribution of trace elements in clinopyroxene and mica. Part ition coefficients for trace elements in amphibole and basanite melts are (at 1sigma): 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-degrees-C), but large differences accompany changes in melt composition and F content. For a (F-free) basaltic an desite melt, at 20 kbar and 950-degrees-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 sou rce regions for some nephelinites explains their relatively high HFSE/ LILE ratios. In contrast, the data do not favour amphibole as the caus e of characteristically low HFSE/LILE observed in mantle-derived islan d arc basalts. The increases in partition coefficients (excepting for Rb) for the more SiO2-rich melt are consistent with similar trends obs erved in phenocryst-matrix pairs from volcanic rocks. The compositiona l dependence of HFSE partition coefficients increases with increasing field strength. This trend can be related to steric effects within pol ymerised aluminosilicate units of the melt phase. Amphiboles grown fro m F-enriched melts are relatively depleted in TiO2, Al2O3, CaO and inc ompatible trace elements. These effects are only large, however, at hi gh F concentrations (> 2 wt%). The effects of F on incompatible elemen ts will be least in melts containing high concentrations of Al2O3, FeO , MgO and CaO. For these reasons, it is unlikely that concentrations o f HFSE and other incompatible elements in natural magmas (with the pos sible exception of some rare F- and SiO2-rich magmas) are significantl y affected by F.