UC San Diego

Abstract: A suite of mantle xenoliths from Aitutaki, which forms part of the Cook-Austral hotspot chain, have been examined for highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances and Re-Os isotope systematics to investigate the compositional nature of Western Pacific mantle lithosphere. These data are complemented by olivine mineral analyses and whole-rock major and trace element abundances. The Aitutaki xenolith suite comprises low-Al dunite, harzburgite, and lherzolite, and high-Al olivine websterite, and pyroxenite lithologies. The xenolith suite is hosted within Aitutaki nephelinite lavas representing low degree (<1%) partial melts from an enriched mantle (EM1/EM2) source (187Os/188Os = ~0.136). High-Al xenoliths have experienced significant melt-rock interaction with melts similar in composition to the lavas that host the xenoliths, with pyroxenites having similar HSE abundances and Os isotope compositions to the lavas (Al2O3 = 5.7-8.3 wt.%; 187Os/187Os = 0.1263 to 0.1469). Conversely, Aitutaki lherzolites and harzburgites are similar in composition to abyssal peridotites with respect to their 187Os/188Os ratios (0.1264 ±43), HSE and major element abundances (MgO and Al2O3), forsterite content, and typical extents of melt depletion (~10 to 14 %). Aitutaki xenolith compositions would lead to the conclusion of a relatively fertile lherzolitic lithospheric mantle. The geochemical evidence for pervasive melt infiltration, however, suggests that Pacific oceanic plate mantle lithosphere is more depleted than previously thought. The results show that extensive and recent melt-rock reaction is superimposed on both recent melting at the ridge and prior melt depletion evident from Mesoproterozoic time of rhenium depletion ages (~1.5 Ga) in some Aitutaki xenoliths. Similar evidence for melt refertilization superimposed on Proterozoic melt depletion ages are exhibited in xenoliths from the Samoan islands of Savai’i and Tubuai, implying that a wide region of the Western Pacific oceanic mantle lithosphere preserves evidence for extensive melt-depletion in the Mesoproterozoic.