Rapid Cenozoic ingrowth of isotopic signatures simulating “HIMU” in ancient lithospheric mantle: Distinguishing source from process

Chemical and isotopic heterogeneities in the lithospheric mantle are increasingly being recognised on all scales of examination, although the mechanisms responsible for generating this variability are still poorly understood. To investigate the relative behaviour of different isotopic systems in off-cratonic mantle, and specifically the origin of the regional southwest Pacific "HIMU" (high time integrated ²³⁸U/²⁰⁴Pb) Pb isotopic signature, we present the first U-Th-Pb, Rb-Sr, Sm-Nd and Re-Os isotopic dataset for spinel peridotite xenoliths sampling the subcontinental lithospheric mantle (SCLM) beneath Zealandia. Strongly metasomatised xenoliths converge to a restricted range of Sr and Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr = 0.7028-0.7033; εNd ≈ +3-+6) reflecting pervasive overprinting of their original melt depletion signatures by carbonatite-rich melts. In contrast, rare, weakly metasomatised samples possess radiogenic Nd isotopic compositions (εNd > +15) and unradiogenic Sr isotopic compositions (⁸⁷Sr/⁸⁶Sr < 0.7022). This is consistent with melt extraction at ca. 2.0 Ga and in accord with widespread Paleoproterozoic Re-Os model ages from both weakly metasomatised and the more numerous, strongly metasomatised xenoliths. The coupling of chalcophile (Os), and lithophile (Sr and Nd) melt depletion ages from peridotite xenoliths on a regional scale under Zealandia argues for preservation of a significant mantle keel (≥2 million km3) associated with a large-scale Paleoproterozoic melting event. Lead isotopic compositions are highly variable with ²⁰⁶Pb/²⁰⁴Pb = 17.3-21.3 (n = 34) and two further samples with more extreme compositions of 22.4 and 25.4, but are not correlated with other isotopic data or U/Pb and Th/Pb ratios in either strongly or weakly metasomatised xenoliths; this signature is thus a recent addition to the lithospheric mantle. Lead model ages suggest that this metasomatism occurred in the last 200 m.y., with errorchrons from individual localities providing ages younger than 116 Ma. When considered in the regional tectonic context the Pb isotopic signatures are best explained through interaction of the lithospheric mantle with a weak upwelling mantle plume that contained carbonatitic domains at ca. 110-115 Ma. Projection of the measured high U/Pb and Th/Pb signatures into the future predicts extreme Pb isotopic values distinct from any recognised terrestrial reservoir. We suggest that this type of young, carbonatite-related radiogenic Pb signature with extreme ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb, which is widely observed in the southwest Pacific, may reflect a secular change in mantle chemistry consistent with the increased prevalence of carbonatite sources during the Phanerozoic. This signature is referred to as "CarboHIMU", to differentiate it from the originally defined HIMU representing an ancient lower mantle component present in some ocean island basalts.AMW was supported by an Australian Postgraduate Award and J.C. Jaeger Fellowship at the ANU. We thank S. Zink for lab support and T. Ireland and V. Salters for comments on an earlier draft