Ultra-Refractory Peridotites of Phanerozoic Mantle Origin: the Papua New Guinea Ophiolite Mantle Tectonites

Abstract

Harzburgites and dunites forming the base of the Late Cretaceous–Paleocene Papuan Ultramafic Belt (PUB) and Marum ophiolites of Papua New Guinea (PNG) are amongst the most refractory mantle peridotites on Earth. We present a new integrated dataset of major element, bulk plus mineral trace element and Re–Os isotopic analyses aimed at better understanding the genesis of these peridotites. The PUB harzburgites contain olivine (Fo92–93), low-Al enstatite (less than or equal to 0.5 wt. % Al2O3 and CaO), and Cr-rich spinel (Cr# = 0.90–0.95). The Marum harzburgites are less refractory with olivine (Fo91.9–92.7), enstatite (~0.5–1.0 wt. % Al2O3 and CaO), minor clinopyroxene (diopside), and spinel (Cr# = 0.71–0.77). These major element characteristics reflect equivalent or greater levels of melt depletion than that experienced by Archean cratonic peridotites. Whereas bulk-rock heavy rare earth element (HREE) abundances mirror the refractory character indicated by the mineral chemistry and major elements, large-ion lithophile elements (LILEs) indicate a more complex melting and metasomatic history. In-situ olivine and orthopyroxene REE measurements show that harzburgites and dunites have experienced distinct melt-rock interaction processes, with dunite channels/lenses, specifically, showing higher abundances of HREE in olivine. Distinctive severe inter-element fraction of platinum group elements and Re result in complex patterns that we refer to as “M-shaped”. These fractionated highly siderophile element (HSE) patterns likely reflect the dissolution of HSE-rich phases in highly depleted peridotites by interaction with subduction-related melts/fluids, possibly high-temperature boninites. Osmium isotope compositions of the PNG peridotites are variable (187Os/188Os = 0.1204 to 0.1611), but fall within the range of peridotites derived from Phanerozoic oceanic mantle, providing no support for ancient melt depletion, despite their refractory character. This provides further evidence that highly depleted peridotites can be produced in the modern Earth, in subduction zone environments. The complex geochemistry indicates a multi-stage process for the formation of the PNG mantle peridotites in a modern geodynamic environment. The first stage involves partial melting at low-pressure (<2 GPa) and high-temperature (~1250–1350 0C) to form low-K, low-Ti tholeiitic magmas that formed the overlying cumulate peridotite–gabbro and basalt (PUB only) sequences of the ophiolites. This is inferred to have occurred in a fore-arc setting at the initiation of subduction. Later stages involved fluxing of the residual harzburgites with hydrous fluids and melts to form replacive dunites and enstatite dykes, and interaction of the residual peridotites in the overlying mantle wedge with high-temperature hydrous melts from the subducting slab to generate the extremely refractory harzburgites. This latter stage can be linked to the eruption of low-Ca boninites at Cape Vogel, and other arc-related volcanics, in a nascent oceanic island arc. Both ophiolites were emplaced shortly after when the embryonic oceanic island arc collided with the Australian continent.

Formation of Ultra‐Depleted Mantle Peridotites and Their Relationship With Boninitic Melts: An Example From the Kamuikotan Unit, Hokkaido, Japan

“…The ultra‐depleted to fertile peridotites bodies are distributed in the Kamuikotan unit more than 100 km scale. Other circum‐Pacific ultra‐depleted peridotites bodies also occur with depleted fertile peridotites bodies at a similar scale (Figure 1; Barrett et al., 2022; Choi, Mukasa, & Shervais, 2008; Choi, Shervais, & Mukasa, 2008; Secchiari et al., 2020b; Xu et al., 2022). This commonality suggests that the distribution of ultra‐depleted to fertile peridotite bodies in the mantle was probably >100 km scale.…”

“…Other circum-Pacific ultra-depleted peridotites bodies also occur with depleted fertile peridotites bodies at a similar scale (Figure 1; Barrett et al, 2022;Secchiari et al, 2020b;Xu et al, 2022). This commonality suggests that the distribution of ultra-depleted to fertile peridotite bodies in the mantle was probably >100 km scale.…”

“…Dunite layers in the mantle sections containing slightly higher Ti in olivine compared to host harzburgites were also observed in ultra‐depleted and depleted peridotites bodies and their origins were interpreted as a product of melt‐rock interaction (Barrett et al., 2022; Lawley et al., 2020). Olivines from the dunite layers in the mantle sections have much lower Ti, Ca, and Y abundances than those in olivine from replacive dunites at the crust‐mantle transition zone (Figure 6a).…”

“… Distribution of the clinopyroxene‐free ultra‐depleted peridotite bodies in the circum‐Pacific belt (Barrett et al., 2022; Choi, Shervais, & Mukasa, 2008; Czertowicz, Scott, et al., 2016; Czertowicz, Toy, & Scott, et al., 2016; Ishiwatari et al., 2004; Jean et al., 2010; Jean & Shervais, 2017; Marchesi et al., 2009; Pirard et al., 2013; Secchiari et al., 2020a; Tamura et al., 1999; Ulrich et al., 2010; Xu et al., 2021). Compilation of mineral modes, major element and trace element compositions from these ultra‐depleted peridotites are available at Zenodo (additional Table S1 in Supporting Information S1) (https://doi.org/10.5281/zenodo.7263366).…”

“…However, studies of ultra‐depleted peridotites bodies with clinopyroxene‐free (<1 vol %) assemblages are very limited from the circum‐Pacific belt (Figure 1). These include examples such as Papua New Guinea (e.g., Barrett et al., 2022), New Caledonia (e.g., Secchiari et al., 2020a; Xu et al., 2021), Coast Range (e.g., Choi, Shervais, & Mukasa, 2008; Jean & Shervais, 2017), Anita peridotite massif (e.g., Czertowicz, Scott, et al., 2016; Scott et al., 2019), Shelting and Mainits zone, eastern Russia (Ishiwatari et al., 2004), and the Kamuikotan unit, Japan (this study; Igarashi et al., 1985; Tamura et al., 1999).…”