Decoupling of short-lived radiogenic and helium isotopes in the Marquesas hotspot

Abstract

The short-lived radiogenic 182Hf/182W (t½ = 9 Ma) and 146Sm/142Nd (t½ = 103 Ma) systems are useful tools to investigate differentiation processes in Earth's early history. Mantle reservoirs that have been preserved over long geological timescales could carry signatures formed early on in Earth's history and later could act as sources to modern mantle plumes. Negative μ182W anomalies in combination with high 3He/4He ratios have been detected globally as differently sloping negative trends in plume-derived ocean island basalts (OIB) from individual hotspots. Large low shear velocity provinces (LLSVP) and ultra-low velocity zones (ULVZ) are seismically anomalous regions in the lower mantle that have been suggested as sources of primordial mantle components near the core-mantle boundary (CMB) and accessed by deep-rooted mantle plumes.In this work, seven samples from the islands Hiva Oa and Fatu Hiva of the Marquesas Archipelago, showing a range in 3He/4He ratios, have been investigated motivated by its location above the Pacific LLSVP and a recently discovered “mega-ULVZ”. Despite elevated 3He/4He ratios of up to 14.4 R/RA, new high precision measurements using thermal ionization mass spectrometry revealed non-anomalous μ182W (−0.9 ± 4.9 ppm) and μ142Nd (1.3 ± 2.4 ppm, 95% confidence interval (c.i.)). These mark the Marquesas Archipelago as the first hotspot to show a decoupling of 3He/4He ratios and μ182W in OIB. A mixing model between an elevated 3He/4He-negative-μ182W and an EM2 or a HIMU endmember suggest that the addition of recycled material is not responsible for this decoupling. Instead, like many previously studied OIB, the Marquesas plume is most likely dominated by an LLSVP component proposed to contain high 3He/4He ratios and μ182W of 0. The unique observed absence of negative μ182W anomalies, on the other hand, implies a lack of involvement of the ULVZ source in the plume's melting region, despite the hotspot being located above a recently discovered mega-ULVZ. This may imply that the expression of ancient mantle domains in hotspot geochemistry is temporally variable, thus, delaying the arrival or involvement of ULVZ material in the origin of the Marquesas OIB.