Sr–Pb–Nd–Hf isotopes and 40Ar/39Ar ages reveal a Hawaii–Emperor-style bend in the Rurutu hotspot

Abstract

Current Pacific absolute plate motion (APM) models include two major, long-lived hotspot tracks: the ∼80 Ma Hawaii–Emperor and ∼79 Ma Louisville seamount tracks. Prior to ∼50 Ma, these two tracks show significant inter-hotspot drift, most likely due to a strong southern motion of the Hawaiian plume. With the addition of a third long-lived Pacific hotspot track, featuring a well-constrained hotspot location and a Hawaii–Emperor-style bend, we can achieve a more robust evaluation of the relationship between APM models and inter-hotspot drift. Such a candidate exists in the Rurutu hotspot track. The youngest volcanism (∼10 to 0.2 Ma) associated with the Rurutu hotspot is located in the Cook-Austral Islands and is known as the “Atiu” or “Young Rurutu” trend. Here we show with 40Ar/39Ar ages and Sr–Pb–Nd–Hf “isotopic fingerprints” that islands and seamounts ranging between ∼70 to 40 Ma fit the compositional trend and APM model-predicted locations of the Rurutu hotspot.The Rurutu hotspot has a distinctive HIMU- to FOZO-like composition, characterized by high 206Pb/204Pb (≥20.0) and low 87Sr/86Sr (≤0.704) ratios. HIMU is a rare composition, found in only two other hotspots worldwide. Uniquely, elevated Rurutu 208Pb/204Pb ratios relative to 206Pb/204Pb define a steeper regression that more closely reproduces the Northern Hemisphere Reference Line than the shallower regression defined by traditional HIMU. Rurutu also is the only HIMU-type hotspot known to continue into the West Pacific, allowing its unique composition to effectively be used as a fingerprint. The Rurutu signature can be traced through non-Samoan “interloper seamounts” in the Samoan region, to a hotspot bend in the southern Tuvalu Islands, continuing through the ∼74 to 63 Ma Gilbert Ridge and the ∼120 to 78 Ma Wake seamounts. However, precisely locating the Hawaii–Emperor style bend in Rurutu is complicated by the younger Samoan hotspot partially overlapping the Rurutu hotspot track, and Cretaceous volcanic structures predating regional Rurutu volcanism. Therefore, we present a computational method that tests a range of preexisting APM models, all generating Hawaii–Emperor style bend locations for Rurutu within error of each other. Our results confirm the presence of a third major hotspot track in the Pacific as originally proposed by W.J. Morgan (1972), with a ∼50.5–49.0 Ma bend at 178.48°E and 8.56°S.Paleomagnetic data for Hawaii indicate that changes in southern hotspot motion may explain its bend, while negligible latitudinal motion in Louisville and an unchanging distance to Rurutu (Tarduno et al., 2003; Koppers et al., 2012; Konrad et al., 2018) are consistent with subduction-driven plume motion explaining the Hawaii–Emperor bend. The presence of a defined ∼135° bend in the Rurutu hotspot track, coupled with minimal inferred latitudinal motion at that time (Konrad et al., 2018), leaves room for a small change in Pacific plate motion at ∼50 Ma.