Isotopic Evidence for Multiple Recycled Sulfur Reservoirs in the Mangaia Mantle Plume

Abstract

Mangaia, an ocean island in the Cook-Austral volcanic chain, is the type locality for the HIMU mantle reservoir and has also been shown to exhibit evidence for recycled sulfur with anomalous δ 34 S and Δ 33 S that has been attributed an Archean origin. Here we report bulk S-isotope data from sulfide inclusions in olivine and pyroxene phenocrysts from one of the previously analyzed and four additional Mangaia basalts to further test for the prevalence of anomalous S in the HIMU mantle source feeding Mangaia. We document compositions that range from −5.13‰ to +0.21‰ (±0.3 2σ), +0.006‰ to +0.049‰ (±0.016 2σ), −0.81‰ to +0.69‰ (±0.3 2σ) for δ 34 S, Δ 33 S, and Δ 36 S, respectively. These data extend the range of measured compositions and suggest S-isotope heterogeneity in the HIMU mantle source at Mangaia. We show that S-isotope compositions of bulk sulfide in olivine is not in isotopic equilibrium with bulk sulfide in pyroxene from the same samples and that samples from a confined area (M4, M10, M12, and M13) in the northern central part of the island show a distinct covariation for δ 34 S and Δ 33 S. This isotopic variation (forming an array) suggests mixing of sulfur from two sources that were captured at different stages of crystallization by phenocrysts in the Mangaia HIMU sulfur endmember. Plain Language Summary The sulfur isotope composition of the mantle is heterogenous from crust formation and recycling continental and oceanic crust. Basalts, from an intraplate volcanic hotspot volcano in the Pacific Ocean, Mangaia, show isotopic evidence for hosting Archean-related sulfur that is sourced from a recycled oceanic crustal component. The prevalence of this Archean-related sulfur in the plume source that feeds Mangaia is not well constrained. In this work, we present sulfur isotope data on sulfur from minerals that crystallized deep in the magma chamber and identify a new, potentially younger, recycled sulfur source that demonstrates the diversity of materials mixing in the magma chamber prior to eruption.

Sulfur Isotope Evidence for a Geochemical Zonation of the Samoan Mantle Plume

“…(2020b) show that the data from Vailulu‘u plot on a slope of ∼−7, which is best attributed to a contribution of post‐Archean sulfur at Samoa. Similarly, at Mangaia, bulk S measured in olivine‐ and clinopyroxene‐hosted melt inclusions from pooled magmatic phenocrysts demonstrate a contribution from Proterozoic sulfur (Dottin et al., 2020a). Such commonality in the age of recycled sulfur suggests that at HIMU localities with resolvable Δ 33 S, the recycled S is usually of post‐Archean nature and the crustal protolith with Archean S that is argued to exist on the basis of large negative Δ 33 S down to −0.35 (Cabral et al., 2013) measured in individual sulfides from Mangaia basalts may not be widely distributed among mantle plumes.…”

Sulfur isotope evidence of geochemical zonation of the Samoan mantle plume

“…Dottin et al (2020b) show that the data from Vailulu'u plot on a slope of ~-7, which is best attributed to a contribution of post-Archean sulfur at Samoa. Similarly, at Mangaia, bulk S measured in olivineand clinopyroxene-hosted melt inclusions from pooled magmatic phenocrysts demonstrate a contribution from Proterozoic sulfur (Dottin et al, 2020a). Such commonality in the age of recycled sulfur suggests that at HIMU localities with resolvable Δ 33 S, the recycled S is usually of post-Archean nature and the crustal protolith with Archean S that is argued to exist on the basis of large negative Δ 33 S down to -0.35 (Cabral et al, 2013) measured in individual sulfides from Mangaia basalts may not be widely distributed among mantle plumes.…”

“…Instead, we provide additional insights into the nature and age of the HIMU component within the context of recently published work on HIMU-Mangaia (Dottin et al, 2020a).…”

“…However, we acknowledge uncertainty in these compositions as the composition of modern oceanic crust may not be a perfect analogue for oceanic crust in deep time. Such diversity in the S-isotope composition of HIMU related materials is reflected by HIMU-influenced MORBs and the Canary Islands showing no resolvable Δ 33 S variations (Beaudry et al 2018, Labidi et al, 2014 with rather positive δ 34 S signatures, whereas HIMU at Mangaia whole rock lavas show both negative and positive Δ 33 S anomalies with δ 34 S signatures far lower than the upper mantle (Dottin et al 2020a).…”

“…At Mangaia, the type locality for the HIMU mantle reservoir, work by Cabral et al (2013) identified anomalous S isotope compositions in the form of negative δ 34 S and negative Δ 33 S that are argued to reflect the recycling of Archean S stored in oceanic crust. Dottin et al (2020a) suggested that Mangaia basalts also preserve slightly positive Δ 33 S that is linked to recycled post-Archean sulfur. At Pitcairn, an EM I locality, Delavault et al (2016) observed negative δ 34 S and negative Δ 33 S in individual sulfide grains and argued that the compositions originate from recycled Archean S.…”