EXTREME HF-176 HF-177 IN THE SUB-OCEANIC MANTLE

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. Hf-176/Hf-177 ratios in abyssal peridotites and in two xenoliths from the subcontinental lithosphere fall within the range observed for present-day oceanic basalts. Hf-176/Hf-177 ratios in anhydrous spinel lherzolites from Salt Lake Crater, Hawaii, however, range to extremely radiogenic values, although Nd-143/Nd-144 is within the range of present-day MORBs. These high Hf-176/Hf-177 ratios (epsilon(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 Hf-176/Hf-177, 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 Hf-176/Hf-177 type mantle identified in the Salt Lake Crater xenoliths. In contrast, the lack of correlation between Hf and Nd isotopes in MORE can be explained by pollution of the MORE reservoir with small amounts of this high-field-strength-element-depleted, high Hf-176/Hf-177, lherzolitic material.