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 (δ 34 S = −2.5 to +3.8‰ and Δ 33 S = −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 δ 34 S, we show that primitive (MgO > 6 wt.%), least degassed glasses accurately record the δ 34 S signatures of their mantle sources. Compared to the depleted MORB source mantle (DMM; δ 34 S = −1.3±0.3‰), the Iceland mantle is shown to have a greater range of δ 34 S values between −2.5 and −0.1%. Similarly, Icelandic basalts are characterized by more variable and negatively shifted Δ 33 S values (−0.035 to +0.013‰) relative to DMM (0.004±006‰). Negative δ 34 S- Δ 33 S 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/R A , where R A 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- δ 34 S- Δ 33 S 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/R A) and negative μ 182 W anomalies define an ancient lower mantle reservoir with a near-chondritic Δ 33 S and δ 34 S 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. • Multiple sulfur isotope data presented for 62 Icelandic lavas. • Effects of degassing and sulfide immiscibility on δ 34 S evaluated. • Undegassed Icelandic basalts show greater S isotopic variability than MORBs. • Negative δ 34 S- Δ 33 S values tied to a subduction fluid-modified mantle component. • A near-chondritic S isotopic signature suggested for the primordial mantle. [ABSTRACT FROM AUTHOR]