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

Abstract

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Variable sources for Cretaceous to recent HIMU and HIMU-like intraplate magmatism in New Zealand

“…206 Pb and 208 Pb can change due to rapid ingrowth caused by relatively recent addition of U and Th to the melt source whereas 207 Pb will not change significantly. Recent studies have demonstrated that the metasomatic signature and trace element (including Th and U) enrichment in the lithospheric xenoliths is consistent with addition of a metasomatic component related to carbonatite/CO 2 -rich melts (Scott et al, 2014a(Scott et al, , 2014bMcCoy-West et al, 2016) which would create a lithospheric reservoir where rapid ingrowth of radiogenic Pb can occur. We stress that for the purpose of this study, the type of metasomatic enrichment (carbonatite or silicate melt) is not crucial, as long as enriched domains with high U+Th/Pb are created.…”

“…This implies that the intraplate magmas are unlikely to be derived from melting of ancient pyroxenite/eclogite residing in the asthenospheric mantle. Under the proposed asthenosphericsource model (Hoernle et al, 2006;Timm et al, 2010;McGee et al, 2013), the observed enrichment of lithospheric mantle documented by Scott et al (2014aScott et al ( , 2014b, McCoy-West et al (2016), Scott et al (2016) and Dalton et al (2017) would have occurred when enriched basaltic melts derived from melting of carbonated pyroxenite in the asthenosphere ascended through the lithosphere. However, this is inconsistent with (1) the lack of radiogenic 207 Pb/ 204 Pb ratios associated with ancient enriched domains; (2) the presence of cooling diffusion profiles of pyroxenes in the Cenozoic-erupted peridotite xenoliths which require that lithospheric mantle enrichment occurred before xenolith entrainment (Scott et al, 2014a(Scott et al, , 2014bMcCoy-West et al, 2016;Dalton et al, 2017); and (3) the unradiogenic Pb isotopic compositions for the local asthenosphere in the Cretaceous, as approximately represented by Tasman MORB (Mortimer et al, 2012).…”

“…La/Yb) and carbonatitic signatures e.g. low Ti/Eu (Scott et al, 2014a(Scott et al, , 2014bMcCoy-West et al, 2016;Dalton et al, 2017). Unmetasomatised portions of lithosphere under the former fore-arc area have distinctly more radiogenic Nd (>+15.5) and extend to less radiogenic Sr (∼0.702) compositions (Scott et al, 2014b;McCoy-West et al, 2016;Dalton et al, 2017).…”

“…Tappenden, 2003;Sprung et al, 2007;McCoy-West et al, 2010) compared to Cretaceous seamounts on the Hikurangi Plateau , Cenozoic intraplate volcanism (Timm et al, 2010) and Tasman MORB (Mortimer et al, 2012). Second column shows comparison with mantle reservoirs; the Anita Peridotite (Czertowicz et al, 2016), peridotites from Cenozoic intraplate centres (Scott et al, 2014a(Scott et al, , 2014b and Cretaceous exhumed peridotites from Westland and the Chatham Islands (McCoy-West et al, 2016). For explanation of the four different symbols for Westland Alkaline Lamprophyres see Fig.…”

“…A suite of Sr-Nd-Pb-Hf isotope data from the most geographically extensive -but least studied -intraplate occurrence in New Zealand (the Cretaceous Westland Dike Swarm, Waight et al, 1998a;van der Meer et al, 2013van der Meer et al, , 2016, coupled with published data from elsewhere in Zealandia, is used here to examine the mantle sources for these intraplate alkaline magmas. Our dataset comprises 49 new samples and >150 published samples and is the first to be compared against the Zealandia peridotite mantle SrNd-Pb-Hf isotope reference frames recently established by Scott et al (2014aScott et al ( , 2014bScott et al ( , 2016, Czertowicz et al (2016), McCoy-West et al (2016) and Dalton et al (2017). This dataset permits analysis of the temporal and spatial isotopic variability of intraplate magma sources on a continental-scale.…”