Abstract:
In order to evaluate the widely accepted assumption that Zr/Hf ratios are uniform and chondritic (i.e. equal to 36.6) in terrestrial rocks [Jochum et al., Geochim. Cosmochim. Acta 50 (1986) 1173-1183], precise Zr/Hf measurements on oceanic basalts, continental materials and chondrites have been obtained by isotope dilution technique using thermal ionisation mass spectrometry 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 Zr/Hf ratios indicates that during fractional crystallisation, the latter are controlled by the precipitation of clinopyroxene. Although clinopyroxene is the major phenocryst phase, minor mineral phases such as sphene and amphibole must be taken into account to explain the fractionation of highly evolved alkaline suites. On the other hand, the comparison between mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB) suggests that DZr<DSm<DHf<DEu during partial melting. Finally, after filtering the data for such fractionation effects, we observe that MORB and continental material (with the exception of granites) display relatively uniform chondritic Zr/Hf ratios, ranging from 35.41 to 38.37 and from 36.28 to 38.71, respectively. 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, ranging from 36.86 to 43.93, and this may strongly constrain the addition of plume-related material to the chemical budget of the continental crust. Moreover, residual garnet influence will result in the generation of OIB with lower 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 observations thus give additional support to our explanation that the variation of Zr/Hf ratios in OIB mainly reflect the melting process.