Missing Archean sulfur returned from the mantle

Abstract

Lavas erupted at intraplate volcanic hot spots are thought to be melts of buoyant plumes that upwell from the deep mantle (1, 2). A long-standing hypothesis argues that hot-spot lavas are melts of oceanic crust or sediments injected into the mantle at ancient subduction zones and recycled back to the near surface in mantle plumes (3, 4). Ocean island basalts (OIBs) erupted at hot spots possess heterogeneous radiogenic isotopic compositions that reveal three broad compositional end-members: high µ (HIMU) (where µ = 238U/204Pb), Enriched Mantle I (EM1), and Enriched Mantle II (EM2). These end-members preserve information about this recycled material (5). Understanding the processes by which these end-member reservoirs formed, and the timing of their formation, is of considerable interest to mantle geochemists because of the implications they hold for long-term mass exchange between Earth’s surface and its deep mantle reservoirs. New data from sulfur isotopes presented in PNAS by Delavault et al. (6) provide important clues that speak to the age and origin of the EM1 OIB end-member. Delavault et al. (6) report unusual sulfur isotopic compositions for EM1-type OIB from Pitcairn Island in the South Pacific. They identify two submarine lavas with sulfides preserving 33S depletions (negative Δ33S) relative to isotopic compositions for typical mantle sulfur. The negative Δ33S observed in the Pitcairn lavas are uniquely associated with surface reservoirs preserved in the Archean rock record. Such negative (and positive) Δ33S formed in the Archean atmosphere via photochemical-induced mass-independent reactions. Mass-independent reactions obey fractionation laws that deviate from those of classical mass-dependent isotope partitioning (7). After inheriting distinct mass-independently fractionated sulfur (MIF-S) isotopic signatures in the Earth’s atmosphere, sulfur was then transferred to seawater where it was incorporated into sediments … [↵][1]1To whom correspondence should be addressed. Email: jfarquha{at}umd.edu. [1]: #xref-corresp-1-1