Early mantle heterogeneities in the Réunion hotspot source inferred from highly siderophile elements in cumulate xenoliths

Abstract

Ultramafic cumulate rocks form during intrusive crystallization of high-MgO magmas, incorporating relatively high abundances of compatible elements, including Cr and Ni, and high abundances of the highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re). Here, we utilize a suite of cumulate xenoliths from Piton de la Fournaise, La Réunion (Indian Ocean), to examine the mantle source composition of the Réunion hotspot using HSE abundances and Os isotopes. Dunite and wherlite xenoliths and associated lavas from the Piton de la Fournaise volcanic complex span a range of MgO contents (46 to 7 wt. %), yet exhibit remarkably homogenous 187 Os/ 188 Os (0.1324 ± 0.0014, 2σ), representing the Os-isotopic composition of Réunion hotspot primary melts. A significant fraction of the xenoliths also have primitive upper-mantle (PUM) normalized HSE patterns with elevated Ru and Pd (PUM-normalized Ru/Ir and Pd/Ir of 0.8-6.3 and 0.2-7.2, respectively). These patterns are not artifacts of alteration, fractional crystallization, or partial melting processes, but rather require a primary magma with similar relative enrichments. Some highly olivine-phyric (>40 modal percent olivine) Piton de la Fournaise lavas also preserve these relative Ru and Pd enrichments, while others preserve a pattern that is likely related to sulfur saturation in evolved melts. The estimate of HSE abundances in PUM indicates high Ru/Ir and Pd/Pt values relative to carbonaceous, ordinary and enstatite chondrite meteorite groups. Thus, the existence of cumulate rocks with even more fractionated HSE patterns relative to PUM suggests that the Réunion hotspot samples a yet unrecognized mantle source. The origin of fractionated HSE patterns in Reunion melts may arise from sampling of a mantle source that experienced limited late accretion (<0.2 percent by mass) compared with PUM (0.5-0.8 percent), possibly involving impactors that were distinct from present-day chondrites, or limited core-mantle interactions. Given the remarkably homogenous Os, Pb, and noble-gas isotopic signatures of Réunion, which plot near the convergence point of isotopic data for many hotspots, such a conclusion provides evidence for an early differentiated and subsequently isolated mantle domain that may be partially sampled by some ocean island basalts.

186Os–187Os and highly siderophile element abundance systematics of the mantle revealed by abyssal peridotites and Os-rich alloys

“…In the most recent iterations, Becker et al (2006) and Fischer-Gödde et al (2011) proposed a PM composition with Pd/Ir and Ru/Ir ratios significantly higher than in bulk chondrite meteorites. Similar 'non-chondritic' HSE patterns have also been observed in mantle peridotites from different settings (Pattou et al, 1996;Snow & Schmidt, 1998;Rehkamper et al, 1999), as well as in the mantle sources of some lavas (Peters et al, 2016). However, the reliability of using massif peridotites compositions has been called into question, due to refertilization processes that these samples may experience (e.g., Marchesi et al, 2014;Lorand & Luguet, 2016).…”

“…An important aspect of the estimates of HSE abundances in the PM from our study and that of Becker et al (2006) is that both have high Ru/Ir, relative to carbonaceous, ordinary and enstatite chondrite meteorites. Whether this enrichment reflects a combination of metal-sulfide-silicate partitioning and late accretion (e.g., Peters et al, 2016), late accretionary impactors with high Ru/Ir (±Pd/Ir) (e.g., Morgan et al, 2001), or a partial melting and refertilization effect in the mantle, remains unresolved (e.g., Lorand & Luguet, 2016). One possible cause is fractionation by sulfide liquids at high temperatures and pressures, which can cause Ru/Ir fractionation (Laurenz et al, 2015).…”

Combined Lithophile‐Siderophile Isotopic Constraints on Hadean Processes Preserved in Ocean Island Basalt Sources

“…ε 176 Hf = +8.8. The Deccan parental magma is assumed to have had trace element abundances identical to the Réunion parental magma, which are calculated according to the MgO-trace element correlations present in our samples (Peters et al, 2016; MgO = 13 wt.%, Nd = 18 ppm, Hf = 3.0 ppm, W = 0.18 ppm). This assumption is made based on the shared heritage of Deccan and Réunion lavas inferred from Sr-Nd-Os isotopic systematics .…”

“…Consequently, it is an ideal location to search for a relationship between Hadean 142 Nd/ 144 Nd and 182 W/ 184 W isotopic signatures and explore the relationship of any observed heterogeneity in these systems compared to the long-lived 147 Sm- 143 Nd and strongly olivine-and/or clinopyroxene-phyric basaltic lavas (e.g., sample RU0714, MgO = 35 wt.%). Many of these samples have been previously characterized for their 3 He/ 4 He (Füri et al, 2011), 142,143 Nd/ 144 Nd (Peters et al, 2018) and 187 Os/ 188 Os isotopic compositions (Peters et al, 2016. One cumulate dunite xenolith from the Piton Chisny volcanic complex of Piton de la Fournaise was also analyzed for its 182 W/ 184 W ratio in order to determine whether the isotopic compositions of pre-eruptive and posteruptive igneous rocks are consistent.…”

Extensive CO2 degassing in the upper mantle beneath oceanic basaltic volcanoes: First insights from Piton de la Fournaise volcano (La Réunion Island)

“…Basalts are the only products of PdF activity and they are inferred to originate from partial melting of the upper mantle (4 GPa; Fretzdorff and Haase, 2002). The mantle source was described as homogeneous with constant noble gases and Sr-Nd-Hf-Os isotope signature (Graham et al, 1990;Staudacher et al, 1990;Albarède et al, 1997;Hanyu et al, 2001;Schiano et al, 2012;Peters et al, 2016). Only minor isotope variations are documented in the eruptive products and have been attributed to either (1) a distinct component intrinsic to the plume itself or to the incorporation of the ambient Indian asthenosphere into the upwelling plume (Pietruszka et al, 2009;Di Muro et al, 2014;Vlastelic and Pietruzska, 2016), and/or (2) small degrees of late-stage crustal contamination (Luais, 2004;Pietruszka et al, 2009).…”