Delavault, Hélène, Chauvel, Catherine, Thomassot, Emilie, Devey, Colin W. and Dazas, Baptiste (2016) Sulfur and lead isotopic evidence of relic Archean sediments in the Pitcairn mantle plume. PNAS Proceedings of the National Academy of Sciences of the United States of America, 113 (46). pp. 12952-12956. DOI 10.1073/pnas.1523805113.

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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.

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