SPINEL PERIDOTITES FROM THE HAWAIIAN HOTSPOT AS PROBES OF MANTLE HETEROGENEITY

Peridotites found as xenoliths in oceanic hot-spots provide a unique access to the oceanic mantle lithosphere and allow to test hypotheses related to plume-lithosphere interaction and chemical heterogeneity of the upper mantle. The xenoliths from Salt Lake Crater (SLC) and Pali vent in Oahu, Hawaii, are some of the best-studied oceanic xenoliths suites. Earlier Os-isotope work on SLC peridotites revealed a heterogeneous lithosphere with Re-depletion ages up to 2 Ga. The highly radiogenic Hf–isotopes (ε_Hf up to 115 in cpx and 130 in opx) at relatively low Lu/Hf ratios also support ancient depletions. As such, the SLC peridotites were interpreted as fragments of ancient recycled lithosphere brought to the surface by the plume (Bizimis et al. 2007; EPSL). The fewer Os and Hf isotope data of the nearby Pali peridotites instead suggest an origin from a MORB – type mantle source and the Pacific lithosphere. Here, we present compositional and Hf, Nd, and Os isotopes on spinel peridotite xenoliths from Kauai in order to test whether the ancient depletions in SLC peridotites are plume related or are inherited in the Pacific lithosphere and the upper (MORB-type) mantle.

The Kauai peridotites are, on average, more depleted and less metasomatized than the Oahu peridotites (Cr#_{spinel =} 0.1-0.6; cpx Ti= 2800 - 150 ppm, Ce/Yb_PM= 7 - < 0.002). Their Os isotopes (¹⁸⁷Os/¹⁸⁸Os ~ 0.1148 to 0.133) overlap the SLC peridotite range, but with only 2 of the 25 samples having ancient 1-2 Ga Re-depletions. In contrast to SLC, several Kauai cpx fall along ~ 100 Ma Sm/Nd and Lu/Hf isochrons similar to the age of the Pacific lithosphere under Hawaii, yet the cpx ε_Hf(initial) are often too radiogenic to be produced in 100 Ma from a MORB source. A cpx-opx pair from SLC also defines a ~100My isochron at radiogenic ε_Hf(initial) = 108. The Os–isotope distribution of all Hawaiian peridotites (n=53) is surprisingly similar to the global abyssal peridotite distribution, with prominent Re-depletion age peaks at 300-600My and at ~1.2 Ga in both groups. The new Hawaiian peridotite data leaves open the possibility that the observed heterogeneity in Os and Hf isotopes is inherited within the Pacific lithosphere. This has implications on the efficiency of mantle stirring to homogenize ancient depletion events which are best recognized in the peridotite xenoliths.