Extreme 176Hf/ 177Hf in the sub-oceanic mantle

Abstract

The results of Hf isotopic analysis of peridotites are presented. Development of a hot-SIMS technique for Hf isotope analysis has made this possible at concentration levels that were previously prohibitively low. 176Hf/ 177Hf ratios in abyssal peridotites and in two xenoliths from the subcontinental lithosphere fall within the range observed for present-day oceanic basalts. 176Hf/ 177Hf ratios in anhydrous spinel lherzolites from Salt Lake Crater, Hawaii, however, range to extremely radiogenic values, although 143Nd/ 144Nd is within the range of present-day MORBs. These high 176Hf/ 177Hf ratios (ε Hf up to 76) are well outside the range of oceanic volcanics and indicate that peridotites such as these cannot have contributed significantly to Hawaiian volcanism. The high 176Hf/ 177Hf, combined with the enriched trace element characteristics of the Salt Lake Crater peridotites may be explained by either metasomatism of a depleted peridotite by a melt which underwent extensive re-equilibration with material similar to the suboceanic lithosphere beneath Hawaii, or by ancient melt depletion (> 1 Ga) and recent enrichment of material stored in the sublithospheric mantle. The good correlation between Hf and Nd isotopes in OIBs indicates that the OIB source is not influenced by the high 176Hf/ 177Hf type mantle identified in the Salt Lake Crater xenoliths. In contrast, the lack of correlation between Hf and Nd isotopes in MORB can be explained by pollution of the MORB reservoir with small amounts of this high-field-strength-element-depleted, high 176Hf/ 177Hf, lherzolitic material.