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.