Assessment of the Zr/Hf fractionation in oceanic basalts and continental materials during petrogenetic processes

In order to evaluate the widely accepted assumption that Zr/Hf ratios are u niform and chondritic (i.e. equal to 36.6) in terrestrial rocks [Jochum et al., Geochim. Cosmochim. Acta 50 (1986) 1173-1183], precise Zr/Hf measureme nts on oceanic basalts, continental materials and chondrites have been obta ined by isotope dilution technique using thermal ionisation mass spectromet ry and magnetic sector-multiple collector ICP-MS. The results indicate that Zr/Hf ratio may substantially fractionate during petrogenetic processes. A well-defined negative correlation observed between Sc concentrations and Z r/Hf ratios indicates that during fractional crystallisation, the latter ar e controlled by the precipitation of clinopyroxene. Although clinopyroxene is the major phenocryst phase, minor mineral phases such as sphene and amph ibole must be taken into account to explain the fractionation of highly evo lved alkaline suites. On the other hand, the comparison between mid-ocean r idge basalts (MORB) and oceanic island basalts (OIB) suggests that D-Zr < D Sm < D-Hf < D-Eu during partial melting. Finally, after filtering the data for such fractionation effects, we observe that MORE and continental materi al (with the exception of granites) display relatively uniform chondritic Z r/Hf ratios, ranging from 35.41 to 38.37 and from 36.28 to 38.71, respectiv ely. This result implies that the extraction of the continental crust from the initially primitive mantle did not result in large Zr/Hf variations. By contrast, OIB are characterised by distinctively higher Zr/Hf ratios, rang ing from 36.86 to 43.93, and this may strongly constrain the addition of pl ume-related material to the chemical budget of the continental crust. Moreo ver, residual garnet influence will result in the generation of OIB with lo wer Zr/Hf ratios and recycling of continental and oceanic crustal materials in the sources of OIB will not strongly modify their Zr/Hf ratios. These o bservations thus give additional support to our explanation that the variat ion of Zr/Hf ratios in OIB mainly reflect the melting process. (C) 2000 Els evier Science B.V. All rights reserved.