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

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 1000-1050°C (basanite) or 900-950°C (andesite). D_Hf and D_Zr for amphibole and clinopyroxene decrease with increasing P, and D_Hf is consistently > D_Zr; for garnet, however, D_zr > D_Hf. The pressure effect on D's is most noteworthy between 0.5 and 1.5 GPa (about a 3-fold decrease from 0.61 to 0.20 in D_Hf for amphibole). This needs to be considered when modelling Zr, Hf behaviour in magmas, especially at crustal depths, although major change in Zr/Hf is unlikely to be produced through fractionation of amphibole and clinopyroxene from silicate melts. The Hf partitioning results are linked with established partitioning patterns for REE in clinopyroxene and garnet crystallizing from a similar basanite, and confirm that the depth of initiation of melting for oceanic basalts may be shallower than originally constrained in the consideration of the "Hf paradox" (Salters & Hart, 1989). Also the results show that the near-chondritic constancy of Sm/Hf in mantle-derived melts cannot be attributed to a combined role of residual garnet and clinopyroxene.