Sulfur and lead isotopic evidence of relic Archean sediments in the Pitcairn mantle plume

Abstract

The isotopic diversity of oceanic island basalts (OIB) is usually attributed to the influence, in their sources, of ancient material recycled into the mantle, although the nature, age, and quantities of this material remain controversial. The unradiogenic Pb isotope signature of the enriched mantle I (EM I) source of basalts from, for example, Pitcairn or Walvis Ridge has been variously attributed to recycled pelagic sediments, lower continental crust, or recycled subcontinental lithosphere. Our study helps resolve this debate by showing that Pitcairn lavas contain sulfides whose sulfur isotopic compositions are affected by mass-independent fractionation (S-MIF down to Δ33S = −0.8), something which is thought to have occurred on Earth only before 2.45 Ga, constraining the youngest possible age of the EM I source component. With this independent age constraint and a Monte Carlo refinement modeling of lead isotopes, we place the likely Pitcairn source age at 2.5 Ga to 2.6 Ga. The Pb, Sr, Nd, and Hf isotopic mixing arrays show that the Archean EM I material was poor in trace elements, resembling Archean sediment. After subduction, this Archean sediment apparently remained stored in the deep Earth for billions of years before returning to the surface as Pitcairn´s characteristic EM I signature. The presence of negative S-MIF in the deep mantle may also help resolve the problem of an apparent deficit of negative Δ33S anomalies so far found in surface reservoirs.

Light Mg Isotopic Composition in the Mantle Beyond the Big Mantle Wedge Beneath eastern Asia

“…Mixing curves between peridotitic mantle and recycled carbonate minerals for δ 26 Mg and 87 Sr/ 86 Sr. Mixing curves are marked in 5% increments. The isotopic compositions and element concentrations of endmembers are given in Table S7 (with data from Aizawa et al, ; Delavault et al, ; Hauff et al, ; Ho et al, ; Huang & Xiao, ; Huang, Shen, et al, ; Huang et al, ; Johnson & Plank, ; Kessel et al, ; Li et al, ; Liu, Zhao, et al, ; Salters & Stracke, ; Teng, ; Xue et al, ; Zhao & Fan, ). Symbols are equivalent to those in Figure .…”

“…If this is correct, then the question arises how the similar sources have generated melts with significantly different Mg isotopic composition (average δ 26 Mg = −0.41 ± 0.03‰ for Huihe nephelinites, average δ 26 Mg = −0.28 ± 0.03‰ for Chaihe basalts in this study, and average δ 26 Mg = −0.29 ± 0.04‰ for Chaihe Sr. Mixing curves are marked in 5% increments. The isotopic compositions and element concentrations of endmembers are given in Table S7 (with data from Aizawa et al, 1999;Delavault et al, 2016;Hauff et al, 2003;Ho et al, 2013;Huang & Xiao, 2016;Huang, Shen, et al, 2015;Huang et al, 2018;Johnson & Plank, 1999;Kessel et al, 2005;Li et al, 2017;Salters & Stracke, 2004;Teng, 2017;Xue et al, 2019;Zhao & Fan, 2012). Symbols are equivalent to those in Figure 6.…”

Missing Archean sulfur returned from the mantle

“…Delavault et al (6) use a three-stage Monte Carlo model to evaluate scenarios for radiogenic lead-isotopic evolution in the Pitcairn EM1 end-member. The model is a refined version of one used previously (10) to constrain the formation age of the EM1 mantle domain sampled by Pitcairn lavas, and the modified model yields ages as great as 2.75 Ga, with a significant number of solutions converging on 2.5-2.6 Ga.…”

“…S for two mantle endmembers sampled by plume-fed hot spots-the Mangaia extreme HIMU flavor (12) (attributed to recycled ancient oceanic crust) (3,12) and the Pitcairn EM1-type lavas (with suggested links to recycled sediment) (10, 13)-led Delavault et al (6) to speculate that recycling of surface-derived sulfur to the mantle also may resolve a long-standing geochemical question related to closing the isotope balance of Earth's early sulfur cycle. A bias toward MIF-S with positive Δ (16).…”

New Constraints on the Origin of the EM‐1 Component Revealed by the Measurement of the La‐Ce Isotope Systematics in Gough Island Lavas

“…For example, hafnium isotope compositions measured in lavas from several Hawaiian volcanoes have revealed the participation of old pelagic sediments in their deep source (Blichert‐Toft et al, ). Similarly, mass independent Δ 33 S measured in olivine‐hosted sulfides from Mangaia and Pitcairn lavas are explained by the recycling of surface material that existed before the Great Oxygenation Event (GOE) in a reduced atmosphere (Cabral et al, ; Delavault et al, ). In a more general way, extreme Pb‐Sr‐Nd isotope signatures identified in Ocean Island Basalts (OIB) and defining the mantle end‐members (HIMU, EM‐1, EM‐2: Zindler & Hart, ) have been interpreted as proof of deep recycling of surface components (Hofmann & White, ; White & Hofmann, ).…”