Ancient and recycled sulfur sampled by the Iceland mantle plume

Abstract

Stable sulfur isotope ratios of mid-ocean ridge and ocean island basalts (MORBs and OIBs) preserve unique information about early Earth processes and the long-term volatile cycles between Earth's mantle and the surface. Icelandic basalts present ideal material to examine the oldest known terrestrial mantle reservoir, accessed through a deep-rooted mantle plume, but their multiple sulfur isotope systematics have not been explored previously. Here, we present new sulfur concentration (30–1570 ppm) and isotope data (δ34S = −2.5 to +3.8‰ and Δ33S = −0.045 to +0.016‰; vs. Canyon Diablo Troilite) from a sample suite (n = 62) focused on subglacially erupted basaltic glasses obtained from Iceland's neovolcanic zones. Using these data along with trace element systematics to account for the effects of magmatic processes (degassing and immiscible sulfide melt formation) on δ34S, we show that primitive (MgO > 6 wt.%), least degassed glasses accurately record the δ34S signatures of their mantle sources. Compared to the depleted MORB source mantle (DMM; δ34S = −1.3±0.3‰), the Iceland mantle is shown to have a greater range of δ34S values between −2.5 and −0.1%. Similarly, Icelandic basalts are characterized by more variable and negatively shifted Δ33S values (−0.035 to +0.013‰) relative to DMM (0.004±006‰). Negative δ34S-Δ33S signatures are most prominent in basalts from the Snæfellsnes Volcanic Zone and the Kverkfjöll volcanic system, which also have the lowest, most MORB-like 3He/4He (8–9 R/RA, where RA is the 3He/4He of air) and the highest Ba/La (up to 12) in Iceland. We propose that subduction fluid-enriched, mantle wedge type material, possibly present in the North Atlantic upper mantle, constitutes a low-δ34S-Δ33S component in the Icelandic mantle. This suggests that volatile heterogeneity in Iceland, and potentially at other OIBs, may originate not only from diverse plume-associated mantle components, but also from a heterogeneous ambient upper mantle. By contrast, a set of samples with high 3He/4He (up to 25.9 R/RA) and negative μ182W anomalies define an ancient lower mantle reservoir with a near-chondritic Δ33S and δ34S signature of ∼0‰. The difference between DMM and the high high-3He/4He mantle may reflect separate conditions during core-mantle differentiation, or a previously unidentified flux of sulfur from the core to the high-3He/4He reservoir.