High precision Hf isotope measurements of MORB and OIB by thermal ionisation mass spectrometry: insights into the depleted mantle

Abstract

The existing Hf isotope database for mid-ocean ridge basalts (MORB) is limited in both quantity and precision. Nevertheless, in Hf–Nd isotope space, MORBs show a wide variation in 176 Hf / 177 Hf over a relatively restricted range in 143 Nd / 144 Nd . The highest 176 Hf / 177 Hf ratios (≥0.283355) within the MORB range are restricted to just four samples (<6.5% of total). Of these high 176 Hf / 177 Hf MORBs, three are from ridge segments adjacent to known active plumes and one is from a ridge segment for which a plume influence has been suggested. By comparison, MORBs from `normal' ridge segments show a more limited range in 176 Hf / 177 Hf ratios (0.283040 to 0.283311). We suggest that NMORB and the depleted MORB mantle reservoir (DMM) are characterised by a similarly limited range in 176 Hf / 177 Hf ratios. Furthermore, we suggest that the high 176 Hf / 177 Hf MORB-like basalts may ultimately be related to mantle plumes and represent melts of a depleted component entrained by the plumes before they traverse the shallow MORB mantle. We illustrate our preferred model with new hafnium isotope data on 11 MORB samples from the Atlantic and Pacific Oceans, two oceanic gabbros from the Indian Ocean (all collected away from known plume localities) and basalts associated with the Iceland and Azores plumes obtained using a new high precision thermal ionisation mass spectrometry (TIMS) technique. The new TIMS technique routinely yields 176 Hf beam intensities of 150–700 mV (total Hf beam of 2.8–13.5 V), allowing a routine internal precision on the measured 176 Hf / 177 Hf ratio of 0.002–0.006%, to be achieved using just 1–3 μg of Hf separate. This represents a considerable improvement over the 0.008–0.056% internal precision quoted as typical for conventional single or triple filament TIMS analysis of Hf. The external reproducibility for the international Hf standard JMC 475 has also been significantly improved over conventional TIMS and is currently ∼0.002%. This is comparable with the 0.003% external reproducibility currently obtained on the new Fisons Instruments Plasma 54 at the Ecole Normale Superieure de Lyon.