GEMOC ARC National Key Centre

T.H. Green, GEMOC, Macquarie
 

New partition coefficients (Ds) determined at high pressure for Nb, Ta, Zr and Hf between rutile and silicate and carbonatitic liquids and aqueous fluid are combined with literature data to assess the impact of rutile on HFSE behaviour. Rutile decouples Nb and Ta from Zr and Hf to varying extent, depending on the melt or fluid composition, from DNb,Ta/DZr,Hf of 3-4 for basanite to 17 for rhyodacite to 3-12 for carbonatite to 4-12 for fluid. All rutile/melt pairs except for one (a highly silicic composition) have DNb/DTa <1, indicating that such melts will have higher Nb/Ta than their source, if rutile is residual in the melting process. In contrast, highly silicic melts will have low Nb/Ta. It is significant that rutile from eclogites from kimberlites has median Nb/Ta of 30, and is possibly residual from extraction of a low Nb/Ta silicic melt as the oceanic crust was subducted. The rutile/fluid Nb and Ta results show DNb/DTa <1. Thus fluid does not appear to be crucial in causing the low Nb/Ta for continental crust, and available D data suggest that an effective fractionation mechanism capable of producing this low Nb/Ta is rutile residual to highly silicic melts. The results for Zr and Hf indicate that rutile will have much less effect on Zr/Hf than on Nb/Ta, so that these two element pairs are also decoupled in terms of their ratios. In contrast to this proposal for rutile controlling Nb/Ta in continental crust, low Nb/Ta ratios (<< chondritic value of 17) in some island arc volcanics may result from a 2-stage melting process and not necessarily involve rutile. This may occur because major residual silicate minerals controlling fractionation in the first melting stage (clinopyroxene, orthopyroxene and garnet) all have DNb/DTa <1, leaving a source for later melting with Nb/Ta <<17.