It is generally accepted that mid-ocean ridge basalts (MORB's) exhibit a wide variation in hafnium (Hf) isotopic composition over a relatively restricted range in neodymium isotopic composition. However, the high 176Hf/177Hf end of the so-called 'MORB' field is currently defined by only three samples, all of which were collected from ridge segments adjacent to the known plume localities of Iceland, Azores and Ascension (Figure 1). Rather than appealing to a decoupling of Lu/Hf and Sm/Nd in the MORB source to explain the MORB field (Salters and Hart, 1991; Beard and Johnson, 1993) we argue that the high 176Hf/177Hf samples are not representative of 'normal' MORB and that the depleted MORB mantle (DMM) component is characterised by a more restricted range in 176Hf/177Hf ratios than previously suggested (0.283058 to 0.283306 as opposed to 0.283058 to 0.283476). Our new Hf isotope data for MORB, collected from ridge segments away from plume sites, doubles the size of the MORB data base and clearly supports our preferred model for the Hf isotopic composition of DMM (Fig. 1). It appears that the high 176Hf/177Hf component is ultimately related to mantle plumes and represents depleted mantle entrained by plumes before they traverse the shallow MORB mantle.
The relationship between the high 176Hf/177Hf component and mantle plumes is clearly demonstrated by our new, high-precision, Hf isotope data for samples from Iceland and ODP Leg 152. Traditionally, the isotopic variability of Icelandic volcanics has been attributed to mixing of an enriched plume and entrained DMM components, whereas the the variations along the Reykjanes Ridge reflect the diminshing influence of the plume away from Iceland. This interpretation is inconsistent with our Hf isotope data shown in Figure 1. Samples from Iceland define a field, oblique to the main ocean island basalt (OIB) array, which extends toward a component with higher 176Hf/177Hf than DMM. Samples from Leg 152 show a very similar trend to Iceland, though two samples do appear to have more MORB-like Hf isotopic compositions. These new data for DMM, Iceland and Leg 152 suggest that DMM itself does not represent the depleted component within the Iceland Plume. The high 176Hf/177Hf depleted component is a long-lived and intrinsic feature of the Iceland plume.
Beard, B.L. & Johnson, C.M., Earth. Planet. Sci. Lett. 119, 495-509 (1993).
Salters, V.J.M. & Hart, S.R., Earth. Planet. Sci. Lett. 104, 364-380 (1991).
Fig. 1: Variation of 176Hf/177Hf with 143Nd/144Nd in oceanic basalts. Except for Azores, Iceland and Ascension, published data for Atlantic and Pacific MORB and OIB are shown as fields whereas symbols are used for our new Hf isotope data . Where errors are not shown for Hf they are less than, or equal, to the size of the symbol.