Partitioning behaviour of Hf and Zr between amphibole, clinopyroxene, garnet, and silicate melts at high pressure.

T.H. Green and A. Fujinawa

(GEMOC, School of Earth Sciences, Macquarie University, Sydney, N.S.W. 2109, Australia)

Partition coefficients (D) have been determined for Hf (mainly) and Zr for amphibole, clinopyroxene and garnet crystallizing from Zr and Hf enriched basanite and andesite compositions at 0.5-3.0 GPa and 1000 - 11000C (basanite) or 900 - 9500C (andesite). DHf and DZr for amphibole and clinopyroxene decrease with increasing P, and DHf > DZr, consistently. For garnet however, DZr > DHf. Also, for amphibole and clinopyroxene DZr and DHf correlate strongly with DTi ,and the pressure effect on Dís is most noteworthy between 0.5 and 1.5 GPa (about a 3-fold decrease in DHf for amphibole). This needs to be considered in modelling Zr, Hf behaviour in magmas, especially at crustal depths, although major change in Zr/Hf is unlikely to be produced through amphibole, clinopyroxene/silicate melt fractionation. When these Zr, Hf partitioning results are linked with established partitioning patterns for REE in amphibole, clinopyroxene and garnet crystallizing from basanite, it is evident that (1) behaviour of Sm/Hf does not discriminate between these 3 minerals (DSm > DHf for each); (2) behaviour of Sm/Nd relative to Lu/Hf, linked with the Nd and Hf isotope systems, does not require such an important role for garnet in generation of oceanic basalts, and for clinopyroxene, these ratios remain unchanged with pressure from 0.5-2.5 GPa.; (3) on mantle normalized diagrams for clinopyroxene/basanite partitioning at 2.0-2.5 GPa, 1050oC there is a negative DZr anomaly, but for garnet/basanite partitioning there is no evident positive DZr anomaly relative to DSm. Thus constancy of Sm/Zr in mantle melts cannot be attributed to a balance of clinopyroxene and garnet involvement.