Recycled Components in Mantle Plumes Deduced From Variations in Halogens (Cl, Br, and I), Trace Elements, and 3He/4He Along the Hawaiian-Emperor Seamount Chain.

Michael W Broadley,David W Graham,Ray Burgess,Hirochika Sumino,Chris J Ballentine

doi:10.1029/2018gc007959

Abstract

Halogens are primarily located within surface reservoirs of the Earth; as such they have proven to be effective tracers for the identification of subducted volatiles within the mantle. Subducting lithologies exhibit a wide variety of halogen compositions, yet the mantle maintains a fairly uniform signature, suggesting halogens may be homogenized during subduction to the mantle or during eruption. Here we present halogen (Cl, Br, and I), K, noble gas, and major and trace element data on olivines from three seamounts along the Hawaiian‐Emperor seamount chain to determine if the deep mantle source has retained evidence of halogen heterogeneities introduced through subduction. High Ni contents indicate that the Hawaiian‐Emperor mantle source contains a recycled oceanic crust component in the form of pyroxenite, which increases from the 46% in the oldest (Detroit) to 70% in the younger seamount (Koko). Detroit seamount retains mid‐ocean ridge basalts (MORB)‐like Br/Cl and I/Cl, while the Br/Cl and I/Cl of Suiko and Koko seamounts are higher than MORB and similar to altered oceanic crust and dehydrated serpentinite. Helium isotopes show a similar evolution, from MORB‐like values at Detroit seamount toward higher values at Suiko and Koko seamounts. The correlation between pyroxenite contributions, Br/Cl, I/Cl, and 3He/4He indicates that subducted material has been incorporated into the primordial undegassed Hawaiian mantle plume source. The identification of recycled oceanic crustal signatures in both the trace elements and halogens indicates that subduction and dehydration of altered oceanic crust may exert control on the cycling of volatile elements to the deep mantle.

Highlights

The wide range of differing geochemical signatures within submarine basalts is considered indicative of the heterogeneous nature of the Earth's mantle (Hofmann, 2003; Kurz et al, 1982; Mukhopadhyay, 2012; White, 2010)
We present halogen (Cl, Br, and I), K, noble gas, and major and trace element data on olivines from three seamounts along the Hawaiian‐Emperor seamount chain to determine if the deep mantle source has retained evidence of halogen heterogeneities introduced through subduction
We present a combined halogen and noble gas study of olivine‐hosted fluid inclusions, as well as bulk olivine separates, together with trace element analysis within several of the Emperor seamounts, to gain further understanding of the nature of subducted components within the primitive Hawaiian mantle source

Summary

Introduction

The wide range of differing geochemical signatures within submarine basalts is considered indicative of the heterogeneous nature of the Earth's mantle (Hofmann, 2003; Kurz et al, 1982; Mukhopadhyay, 2012; White, 2010). OIB are often characterized by their high 3He/4He ratios, there is significant geochemical variation between different ocean islands and even within the same island chain, indicating that the mantle source is heterogeneous over relatively small length scales (Huang et al, 2011; Jackson et al, 2014; Weis et al, 2011). Ocean islands that are believed to sample the remnants of subducted oceanic crust (e.g., Canary Islands, St Helena, Azores etc.) are characterized by lower 3He/4He and 143Nd/144Nd, and higher radiogenic 87Sr/86Sr and 206Pb/204Pb signatures, than the primordial‐type plumes

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Discussion

Conclusion