Mantle source regions feeding hotspot volcanoes likely contain recycled subducted material. Anomalous sulphur (S) isotope signatures in hotspot lavas have tied ancient surface S to this deep geological cycle, but their potential modification by shallow magmatic processes has generally been overlooked. Here we present S isotope measurements in magmatic sulphides, silicate melt inclusions and matrix glasses from the recent eruption of a hotspot volcano at El Hierro, Canary Islands, which show that degassing induces strongly negative δ34S fractionation in both silicate and sulphide melts. Our results reflect the complex interplay among redox conditions, S speciation and degassing. The isotopic fractionation is mass dependent (Δ33S = 0‰), thus lacking evidence for the recycled Archaean crust signal recently identified at other hotspot volcanoes. However, the source has an enriched signature (δ34S ~ + 3‰), which supports the presence of younger 34S-rich recycled oceanic material in the Canary Island mantle plume.
Chemical, physical and biological processes within the mantle, crust, atmosphere and oceans fractionate sulphur isotopes, and material exchanges among these geological reservoirs lead to characteristic sulphur isotope signatures that have varied over time1. Sulphur isotope heterogeneity in the mantle, as sampled by sulphide inclusions2,3,4,5, melt inclusions6,7 or primitive volcanic rocks8,9, thus traces secular variation in the tectonic boundary conditions that influence mantle circulation, as well as the internal processes that have established the current mantle state. On the other hand, the full scope of mantle heterogeneity is best illustrated by variations in trace element and radiogenic isotope geochemistry of ocean island basalts (OIB), which define various mantle end-members thought to feed the sources of hotspots10,11. Therefore, it is of interest to investigate these mantle components from a sulphur isotope perspective to help set additional constraints on their origin and further our understanding of the deep sulphur cycle. This type of approach has been adopted in recent studies, which we summarise below.
Mass-independent fractionation (MIF) of sulphur isotopes is defined by non-zero Δ33S values, where Δ33S = δ33S – [(1 + δ34S)0.515–1] and δxSV-CDT = (xS/32S)sample/(xS/32S)V-CDT – 1, and is characteristic of sedimentary rocks of Archaean age (~2.5 Ga and older), reflecting the influence of photochemical processes in an atmosphere devoid of oxygen12. Sulphur in sedimentary rocks that post-date the Great Oxidation Event at ~2.4 Ga thus has Δ33S values around 0‰, and the associated S isotope fractionations are termed mass-dependent. However, negative, non-zero Δ33S values have now been reported twice for young volcanic rocks from hotspot settings: in olivine-hosted sulphides from Mangaia, Cook Islands4, and in sulphides from the Pitcairn hotspot5. These anomalous signatures are thought to reflect the cycling of Archaean sulphur from Earth’s surface to the mantle by subduction, and back to the surface via mantle plumes4,5,13. Since Mangaia is the representative end-member of the ancient ‘HIMU’ (high μ = 238U/204Pb) mantle component10,11, the potential positive covariation of Δ33S and δ34S values in Mangaia sulphides was first used to suggest a specific Archaean protolith to the HIMU source characterised by negative Δ33S and δ34S values4. The subsequent finding of S-MIF at Pitcairn, representative of the enriched mantle I end-member10,11 (EM-I; characterised by unradiogenic Pb isotope signatures), in association with negative δ34S values, lends support to this hypothesis, potentially resolving the positively-skewed imbalance of Δ33S values observed in Archaean surface reservoirs5,13. These studies thus imply that a missing Archaean sulphur pool is stored in the deep mantle and occasionally resurfaces at hotspots. Other plume-related lavas from Samoa, the type locality for the third common OIB mantle isotopic end-member, EM-II (characterised by the highest 87Sr/86Sr ratios), show coupled variations in S and Sr isotopes that indicate recycling of younger sulphur-rich sediments into a mantle source with a near-zero Δ33S value (mass-dependent) and a δ34S value of ~3‰9. The distinction in S isotope signatures between different mantle reservoirs suggests a long-lived and isotopically evolving surficial input into different hotspot source regions, highlighting the importance of understanding the causes of S isotope variability in the mantle.
Magmatic processes involving sulphur, such as degassing, sulphide segregation, mixing or assimilation, may also leave an imprint on the S isotope composition of volcanic rocks7,8,14,15, and separating these effects from the source signature can present challenges. However, melt inclusions, which represent droplets of silicate melt trapped in minerals during crystal growth, offer unique snapshots of an ascending magma at various depths16, and can help resolve this problem. Toward this end, we report the S isotope compositions of melt inclusions and matrix glasses spanning the degassing history of the volatile- and sulphur-rich magma erupted at El Hierro, Canary Islands, in 2011–201217. We also present δ34S and Δ33S signatures of magmatic sulphide inclusions — to our knowledge, this is the first time S isotopes are measured simultaneously in sulphides, melt inclusions and matrix glass for a single eruption. The dataset provides an unusually clear picture of the mechanisms by which sulphur isotopes fractionate during degassing and sulphide saturation in natural magmas, and shows that degassing can induce large δ34S fractionations of up to 10‰. In turn, this offers an exceptional opportunity to investigate in situ how the S isotope heterogeneity generated during magma evolution, ascent and eruption7,8,18,19 can be quantitatively discriminated from that inherited from the mantle sources for hotspot volcanism. In doing so, we find that the S isotope signals at El Hierro reflect a post-Archaean origin for recycled S in the Canary Island hotspot, contrasting with the recent findings of S-MIF at other OIB localities.
Geological setting and sample description
The Canary Island hotspot in the eastern Atlantic Ocean is characterised by an exotic geochemistry, producing OIB that is mostly alkaline in composition and displays isotopic affinities with the HIMU, EM and depleted MORB mantle (DMM) end-members20,21, suggesting that the mantle source constitutes a mix of different reservoirs21. This context thus offers the potential to confirm or contrast S isotope signals from the Canary Islands to those observed at hotspots in the South Pacific (i.e. Mangaia/HIMU; Pitcairn/EM-I; Samoa/EM-II). Additionally, ubiquitous fluid inclusions in mantle xenoliths and occurrence of carbonatite melt22,23 point to a mantle source enriched in volatiles. The recent submarine eruption off the south coast of El Hierro, the youngest and westernmost island of the archipelago, produced lava balloons containing olivine-hosted melt inclusions (Fig. 1a) with dissolved volatiles reaching concentrations in excess of 3000 ppm CO2, 3 wt.% H2O and 5000 ppm S (ref. 17). In addition, clinopyroxene and spinel (Fe–Ti oxide) phenocrysts in the same samples host abundant sulphide globules (Fig. 1b–f), revealing that the magma was saturated with an immiscible sulphide liquid for part of its history. However, sulphide inclusions are not present in olivine phenocrysts, nor do they occur as a free phase in the matrix glass.
We performed in situ sulphur isotope analyses by Secondary Ion Mass Spectrometry (SIMS) on a suite of 25 olivine- and spinel-hosted melt inclusions and 9 matrix glass chips, previously shown to have large ranges in volatile contents and sulphur speciation, with S contents linearly and positively correlated with H2O and S6+/ΣS (ref. 17). These measurements yielded δ34S values only, owing to the analytical difficulty of resolving low-abundance 33S at low levels of S in silicate melts. We also obtained the S isotope compositions (δ34S and Δ33S) of 49 clinopyroxene- and spinel-hosted sulphide droplets, and measured their chemical composition by electron probe micro-analysis (EPMA). Table 1 summarises the isotopic composition of the various samples. Details on our analytical techniques and associated uncertainties can be found in the Methods section. The Supplementary Information includes a discussion of potential matrix effects during isotopic analysis, which appear negligible. All quoted errors are 1σ propagated analytical uncertainties (see Methods for error treatment).
Sulphur isotopes in the silicate melt
The δ34S values of melt inclusions and matrix glasses (Supplementary Data 1) exhibit an exponential decline with decreasing S content (Fig. 2a), with S-rich inclusions having positive δ34S values and S-poor matrix glasses (<500 ppm S) having the most negative δ34S values. Spinel-hosted silicate melt inclusions have δ34S ranging from +3.1 ± 0.5‰ to +0.8 ± 0.5‰ while olivine-hosted melt inclusions span a greater range from +3.2 ± 0.1‰ to −5.9 ± 0.6‰. Matrix glasses have the lowest δ34S values, extending down to −8.2 ± 0.6‰ (Fig. 2a). Inclusion size (30–150 μm) show no relationship with isotopic composition.
Sulphur isotopes and petrography of the sulphides
All magmatic sulphides analysed have Δ33S values within error of the terrestrial mass-dependent fractionation line (i.e. 0‰), averaging 0.01 ± 0.06‰ (Fig. 3). However, they exhibit a large δ34S range from +1.0 ± 0.4‰ to −9.6 ± 0.4‰, and display a variety of morphologies and compositions (Figs 1, 3–5, Table 1).
The majority of sulphide inclusions analysed (n = 33) are homogeneous, smooth, circular to elliptical droplets (Fig. 1b) with compositions corresponding to monosulphide solid solution (mss) close to the pyrrhotite (Fe1–xS) end-member (Fig. 4a, Supplementary Data 2). These sulphides only occur completely enclosed in phenocrysts, and have δ34S values between +1.0 and −4.0‰ (Figs 3 and 5). We refer to them as group 1.
Another set of inclusions (n = 12), group 2, display heterogeneous mineralogy and more angular shapes (Fig. 1c–e). These consist of intergrowths of mss, intermediate solid solution (iss)24,25,26 and an interstitial oxide phase. The mss in these inclusions reaches higher Ni (0.2–6.4 wt.%) and Cu (0.8–5.8 wt.%) contents than the homogeneous sulphides (0.1–2.2 wt.% Ni and 0.4–1.9 wt.% Cu; Fig. 4a and Supplementary Data 2), and sometimes contains small exsolutions of a more Ni-rich phase, likely pentlandite (Fig. 1c, Supplementary Fig. 3b). The iss has a stoichiometry corresponding to cubanite (CuFe2S3) (Fig. 4a). The oxide phase was too fine-grained to analyse by EPMA, but elemental maps obtained with a scanning electron microscope (SEM) confirmed that its composition is essentially iron oxide (Supplementary Fig. 4). These inclusions are hosted primarily in spinel phenocrysts, commonly occur at the edge of crystals, and are sometimes in contact with the melt or even completely outside the crystal (Fig. 1e). They are also often associated with vesicles (cf. ref. 27). They have δ34S ranging from −1.8‰ to −7.1‰ (Figs 3 and 5).
Four additional globules have spherical shapes resembling the homogeneous inclusions of group 1, but have trellis-textured surfaces, which are often observed for quenched sulphide melt28,29, and also occur outside or at the edge of crystal hosts — we distinguish them as group 3 (Fig. 1f). They are characterised by the highest Fe and lowest S contents, as well as low Ni and Cu (Supplementary Data 2, Figs 4, 5). Their δ34S range is similar to that of the group 2 sulphides, from −2.3‰ down to −9.6‰. However, they do not contain a discrete oxide phase as seen in group 2, and have low EPMA element totals (~95 wt.%). This may be due to incorporation of O2− anions in the sulphide liquid30,31, given that oxygen was not included in the analytical routine. One of these inclusions also has an interstitial Cu-rich phase (Supplementary Fig. 3e).
Sulphides analysed from all three groups have similar size ranges from 25 μm up to 180 μm across, and there is no systematic variation in isotopic composition or geochemistry with size. Smaller droplets (<10 μm) were also present but not analysed due to the ~15 μm size of the ion beam (see Methods).
Decompression-induced degassing beginning at pressures of at least 300 MPa was the primary cause of CO2, H2O and S loss in the El Hierro magma17 (see Supplementary Fig. 5). Sulphur speciation data (S6+/ΣS) in the melt inclusions also show that the magma became more reduced during decompression and S loss to a fluid phase (Fig. 2b). However, the presence of sulphide globules in minerals also attests to S partitioning into an immiscible sulphide melt. The solubility of sulphur in magma before the separation of an immiscible sulphide liquid, also known as the sulphur concentration at sulphide saturation (SCSS)32,33, increases steeply with increasing oxygen fugacity (fO2) between the fayalite–magnetite–quartz (FMQ) buffer and two log units above it (FMQ + 2), corresponding to a switch from sulphide(S2-)- to sulphate(SO42-)-dominated conditions33. The most sulphur-rich melt inclusion has a S concentration (5080 ppm) well exceeding the S solubility of a relatively reduced magma (around FMQ)32,33, but at FMQ + 1.5 dissolved S can reach >1 wt.%33. The S6+/ΣS ratio of that inclusion suggests a minimum fO2 of 1.2 log units above FMQ (but likely higher; see ref. 17), hence we infer that the initially oxidised magma was not sulphide saturated. The presence of sulphide droplets therefore reflects the decrease in fO2 until intersection of the SCSS (Fig. 2b), at which point a sulphide liquid started to separate from the silicate melt. The absence of sulphide droplets in olivine phenocrysts supports a late saturation of sulphide. Alternatively, the lack of olivine-hosted sulphide inclusions could reflect preferential late-stage nucleation near rapidly-growing Fe–Ti oxide or clinopyroxene phenocrysts17.
The crystallisation of magnetite (Fe2+Fe3+2O2–4) has previously been argued to trigger sulphide saturation in hydrous, oxidised magmas from subduction zone settings34,35,36. This process could also reasonably be invoked for the El Hierro magma, supported by the ubiquitous association of sulphides with Fe–Ti oxides. However, the El Hierro oxides, with an average composition approaching Fe2+1.0Mg2+0.4Fe3+0.9Al3+0.3Ti4+0.4O2–4 (ref. 37), have a ferric iron content less than half that of the magnetite end-member, and so their crystallisation may not have modified the melt Fe3+/ΣFe ratio sufficiently to trigger sulphide saturation34. Moreover, there is no systematic variation between the FeO content of melt inclusions and their S6+/ΣS ratios17, which would be expected if spinel fractionation was the cause of magma reduction. Hence, we favour degassing-induced reduction17,38,39,40,41 as the cause of the intersection of the SCSS and separation of an immiscible sulphide liquid. Our observations are consistent with different degassing modelling trends obtained with the software D-Compress39, which all show reduction of the magma associated with S loss when the initial magma is oxidised (Fig. 2b). It is worth noting that while sulphide droplets are abundant in some phenocrysts, overall they represent a rather insignificant fraction of the S budget of the El Hierro magma (estimated at <1% of total S from thin section observations, see Methods), and therefore sulphide separation did not appreciably decrease the S content of the magma.
We now explore the intricate isotopic interplay among gas loss, sulphide saturation, and magmatic fO2 with quantitative calculations of S isotope fractionation during magma ascent. The positive exponential correlation of S content and δ34S in melt inclusions and matrix glasses (Fig. 2a) suggests that S loss via degassing is the primary process driving the strong mass-dependent isotopic fractionation, especially after a significant amount of S has escaped. There is no need to invoke mixing of isotopically distinct melts, which might be expected to produce linear arrays in Fig. 2a, and no evidence for late-stage assimilation of seawater-influenced components, which should drive δ34S to high positive values15. Isotopic fractionation of sulphur in magmatic systems depends strongly on the proportions of oxidised and reduced S species in the different phases present, since oxidised species are generally enriched in 34S relative to reduced species in the order SO42−(melt) > SO2(gas) > H2S(gas) ≈ S2−(melt) ≈ S2−(FeS) (ref. 15). Since the proportions of oxidised and reduced S species in the melt are linked to the oxidation state of the magma, as monitored by fO2 (ref. 33), processes involving the exchange of sulphur between different phases (melt, gas or sulphide liquid) have the potential to induce isotopic fractionation when these exchanges involve modifications in the valence state of S. For example, the isotopic fractionation factor between a sulphide liquid and S dissolved in the melt (αFeS–melt, where 1000lnαFeS–melt ≅ δ34SFeS – δ34Smelt) will vary strongly with the proportions of dissolved SO42− and S2− in the melt (i.e., the S6+/ΣS ratio). At oxidised conditions (e.g. ΔFMQ ≈+1.5), where significant SO42− is present, δ34SFeS is ~3‰ lower than δ34Smelt15,42,43, such that sulphide segregation would cause the melt to become enriched in 34S. At reduced conditions, S has the same valence (−2) in both sulphide and silicate melts, and the isotopic fractionation is small, with δ34SFeS about 0.4‰ higher than δ34Smelt at 1150 °C (ref. 15). This fractionation and the small contribution of sulphide to the total initial magmatic S content are not sufficient to produce the strong isotopic signal we observe (Fig. 2a). Instead, the positive exponential trend between S content and δ34S values in the melt implicates open-system degassing as the primary agent of isotopic change (Fig. 2a), even though gas loss is conventionally expected to favour light isotopes, a common misconception for redox-dependent isotope systems18.
The sulphur species incorporated in a fluid phase at magmatic conditions are mainly H2S and SO2 (refs 15,32), in which S has a valence of −2 and +4, respectively. The proportions of H2S and SO2 are therefore controlled by fO2, but they also depend on H2O fugacity (fH2O), which can be used as a proxy for pressure since H2O is the major constituent (in mol%) of the fluid phase (Supplementary Fig. 6). This can be described by the equilibrium reaction:8,14
Degassing-induced S isotope fractionation is therefore sensitive to both fO2 and fH2O (i.e. pressure)14 (Fig. 6). As a result, both of these intensive parameters must be known simultaneously to correctly model degassing-induced isotopic fractionation of sulphur during the evolution of the El Hierro magmatic system. We used the S6+/ΣS and CO2–H2O data of Longpré et al.17 combined with the parameterisations of Jugo et al.33 and Iacono-Marziano et al.44 to estimate fO2 and total pressure, respectively, in olivine-hosted melt inclusions (Fig. 2b and Supplementary Fig. 5). We considered two sets of end-member estimates42,45 for the fractionation factors among the relevant S species (SO42−(melt), S2−(melt), SO2(gas), H2S(gas))15,32. This allowed us to calculate a range of evolving gas–melt S isotope fractionation factors (αgas–melt) during gradual loss of sulphur from the El Hierro melt upon ascent from 300 to 1 MPa.
Under isothermal decompression at 1150° C17, calculations based on our estimated fO2 and pressure ranges produce lower- and upper-bound isotopic fractionation curves encompassing our entire dataset (Fig. 2a). The most recent experimentally determined fractionation factors45 better reproduce the strongly fractionated δ34S values we observe at low S abundances (Fig. 2a); details on these calculations are provided in the Methods section, and the differences between models are outlined in the Supplementary Discussion. Supplementary Data 3 contains the calculations used to model S isotope fractionation. Equivalent calculations, assuming S loss via sulphide segregation alone, demonstrate that this mechanism did not affect the S isotope fractionation we observe (Fig. 2a). As the δ34S range of sulphide inclusions mirrors that of the melt inclusions and matrix glasses, we suggest that trapped sulphides essentially track the isotopic composition of the silicate melt as sulphur is degassed15 (see below). Our calculations demonstrate that increased proportions of SO2 (S4+) in the gas phase, which is predicted at lower pressures, favour the degassing of heavy S isotopes since S in the El Hierro melt speciates toward S2− as oxygen fugacity decreases at lower pressures. While degassing of SO2 had previously been identified as a potential trigger for magma reduction and isotopic fractionation18,38,40,46, our data clearly demonstrate that it can exert the primary control on both in a positive feedback loop driven by decompression.
Compositional and textural trends in the magmatic sulphides can be explained as part of this framework. The smooth, homogeneous sulphides (group 1), which have the highest δ34S values (Table 1, Figs 3 and 5), represent the early sulphide liquid that was entrapped as pristine mss droplets in growing crystals of clinopyroxene and spinel. Sulphides occurring at the edge of crystals and in contact with the matrix glass (groups 2 and 3) reach significantly more negative δ34S values and exhibit more diverse features, such as angular shapes, heterogeneous mineralogy or quench textures. Moreover, these sulphides have a lower S content (Supplementary Data 2, Fig. 5a), suggesting that they formed or equilibrated in a S-poorer melt (i.e., at lower fS2)30,31, or were subject to S loss by degassing27,47. The Ni content of group 2 sulphides also increases in both mss and iss with decreasing δ34S (Fig. 5b). To first order, the isotopic composition of the sulphides should be controlled by the S isotopic composition of the dissolved sulphur species in the silicate melt from which they are segregating. The different groups thus likely represent different stages in the evolution of the sulphide melt, with the majority of droplets observed corresponding to the onset of sulphide saturation. A break in slope in the S vs FeO* trend of melt inclusions at a S concentration of ~1500 ppm may mark this onset17. This would have occurred after a significant amount of S was already degassed, but before the isotopic composition of the melt was substantially affected, consistent with the modest isotopic fractionation seen in the group 1 sulphides, with δ34S between +1.0 and −2.8‰, with one exception reaching −4.0‰ (Fig. 2a, b).
The separation of a sulphide melt should also deplete the silicate melt in metals such as Ni and Cu, which have ~1000 times more affinity for sulphide liquid25,48. Therefore, the higher concentration of Ni and Cu in the group 2 sulphides cannot be taken as evidence for later saturation, since later-formed sulphides would precipitate from a more Ni- and Cu-depleted magma. Nevertheless, their occurrence at the outer edges and outside crystals combined with their δ34S range of −1.8‰ to −7.1‰ suggests that they communicated with the silicate melt at a later stage than the group 1 sulphides, and may therefore have isotopically re-equilibrated with the 34S-depleted magma. The Ni and Cu partition coefficients between mss and sulphide liquid are <1 (ref. 28), hence fractional crystallisation of mss should enrich the residual sulphide liquid in these elements. This scenario is consistent with the expected evolution of the sulphide melt, which should first crystallise mss, followed by iss at a later stage49. The group 2 sulphides therefore appear to represent residual sulphide liquid that crystallised iss and higher-Ni mss at a late stage while the δ34S of the magma was plummeting, after removal of Ni-poorer mss (group 1).
This is counterintuitive, considering that these later-crystallised sulphides formed at lower fO2. However, incorporation of O in the sulphide liquid is also controlled by sulphur fugacity (fS2)30,31, where a lower fS2 represents a competing effect to lower fO2 and can result in a higher O content of the sulphide liquid. Model calculations39 indicate that fS2 decreased by several orders of magnitude more than fO2 during degassing of the El Hierro magma (Supplementary Fig. 7), and thus exerted the dominant control on this equilibrium. The presence of O in the sulphide phase may also explain some of the lower EPMA totals observed in the group 2 and 3 sulphides (Supplementary Data 2).
The group 3 sulphides, in contrast, do not contain a discrete oxide phase and their Ni and Cu contents are low, but they also show a large δ34S range towards very negative values. Their spherical shapes and fine trellis textures suggest quenching from sulphide liquid. We therefore interpret them as having formed at the latest stage from a Ni- and Cu-depleted silicate melt. Hence we propose that sulphide segregation occurred continuously once the magma reached sulphide saturation. The group 3 sulphides have the lowest S content, presumably because they did not crystallise oxide and thus retained O in the liquid. The lower S content of these sulphides should also have lowered their melting point51, allowing them to remain a liquid phase, which is consistent with the observed quench textures. The overlap between the isotopic compositions of group 2 and 3 sulphides with group 1 (Table 1, Fig. 3) supports the idea of a continuous sulphide segregation history, with groups 2 and 3 experiencing larger fractionations at a later stage, as expected from the exponential decrease in δ34S of the silicate melt upon S degassing (Fig. 2a).
The chemistry of mss sulphides has been shown to vary systematically between MORB and arc magmas, due to their different crystallisation histories and redox states26. The Ni content of sulphides is generally controlled by the amount of olivine fractionation that has occurred prior to sulphide saturation, since Ni is compatible in olivine. MORB sulphides have significantly higher Ni than sulphides from arc magmas (Fig. 4, Supplementary Fig. 8), which can be attributed to the lower fO2 of MORB parental magmas and, hence, earlier sulphide saturation prior to extensive removal of olivine26. In contrast, arc magmas are generally more oxidised and reach sulphide saturation at a later stage, when the magma is more depleted in Ni. The chemistry of the El Hierro sulphides is most similar to sulphides from arc and back-arc settings, with lower Ni and Ni/Cu ratios (Fig. 4, Supplementary Fig. 8), which is consistent with the relatively high oxidation state of the inferred parental magma, and thus later sulphide saturation. These results are supported by the S isotope data, and also indicate that S degassing can have an additional important effect on the composition of the sulphides and stabilisation of the sulphide melt47.
While the HIMU mantle reservoir is characterised by extreme 206Pb/204Pb ratios that constrain its age to >1 Ga to account for sufficient uranium decay10,11, we find no S-MIF in El Hierro sulphides (Fig. 3), which is consistent with a post-Archaean (<2.5 Ga) origin for any recycled material in the Canary Island plume. Sulphur isotope analyses on basalt glasses from St-Helena, another HIMU-type hotspot in the South Atlantic, also show no signs of MIF52, contrasting with the MIF-bearing Mangaia sulphides presumably derived from an older recycled source4. It appears that HIMU-type signatures may not originate from a single reservoir, but rather from a recurrent geological process. This is consistent with the slightly lower 206Pb/204Pb ratios of the HIMU component found in the Canary Islands and other OIB from the eastern Atlantic Ocean, such as the Cape Verde islands and Madeira53, which also suggests a shorter time-integrated addition of radiogenic Pb relative to Mangaia, and thus a more recent age of formation. By analogy, the Archaean MIF signal observed at Pitcairn5 may not require that all EM-I OIB have Archaean plume sources. Moreover, while degassing cannot explain the negative MIF Δ33S values observed at Mangaia4 and Pitcairn5, our results clearly show that it can impart negative δ34S signatures to volcanic rocks and their S-rich inclusions, potentially overprinting the isotopic composition of the source. Degassing thus offers an alternative mechanism to produce the highly negative δ34S values of the Mangaia and Pitcairn sulphides, which have been interpreted to reflect an intrinsic feature of the presumed Archaean protolith within these plumes4,5,13. Disentangling the effects of degassing from source heterogeneity is thus critical for understanding the geodynamic implications of S isotopes at volcanic hotspots.
By removing the effect of degassing, we find that the mantle source of the El Hierro magma was significantly enriched in 34S compared to chondrites and the bulk silicate Earth54, reaching a δ34S value of +3.2 ± 0.1‰ in S-rich melt inclusions. A strikingly similar value of +3.5‰ was previously reported for S-rich clinopyroxene-hosted melt inclusions from Gran Canaria where, like at El Hierro, the most S-rich inclusions also show the highest S6+/ΣS ratios6. The convergence of these results suggests a potential link between high oxidation state and enriched δ34S in the mantle sources of volcanic hotspots. This connects two emerging views: (1) in line with other isotopic proxies11,21,53, recycling of subducted sulphur best explains S isotope signatures observed at many OIB hotspots (Mangaia4, Pitcairn5, Samoa9, and now the Canary Islands), and (2) the mantle sources of OIB appear to be more oxidised than those of MORB33,41, but less so than those of arc magmas46,55. During subduction, altered and oxidised oceanic crust and sediments transfer redox potential to the mantle wedge underlying volcanic arcs, likely with sulphur as an oxidising agent46,55. However, a significant fraction of S might remain in the downgoing slab since estimates of S output at arcs are an order of magnitude lower than estimates of subducted S (refs 46,56.), which may act as a potent provider of S and oxidising power in hotspot sources. The source of subducted S can be generally constrained from the S isotope compositions inferred here. While sulphides produced by microbial sulphate reduction during alteration of the modern-day oceanic crust generally show negative δ34S values57,58, this may have been different at earlier stages in Earth’s history1. The high δ34S (+2.8‰) of recycled S at Samoa, for example, has been attributed to the positive δ34S that characterizes sulphides in Proterozoic sediments9. On the other hand, serpentinization of oceanic peridotites may be an important sink for the S in high-δ34S seawater sulphate59, as recent evidence points toward significant fluid uptake during bending and faulting of the oceanic lithosphere before subduction60,61. Whether the subducted slab remains more oxidising than the surrounding mantle at greater depths and over long timescales is, to our knowledge, poorly constrained, but redox heterogeneities introduced into the deep mantle via subduction46,55,62 conceivably play an important role in global geochemical dynamics, as documented by diamond-forming processes62,63. The survival of such heterogeneities until recycling at hotspots could be behind the documented S isotope differences between OIB and MORB, and may explain their apparent redox contrast as well.
Sample collection and preparation
Lava balloons floating on the surface of the ocean were collected during the eruptive activity at El Hierro on 27 November 2011, 6 December 2011 and 28 January 2012. These samples were crushed and sieved, and phenocrysts of clinopyroxene and spinel were picked and mounted in epoxy to expose sulphide inclusions. Silicate melt inclusions and matrix glasses were previously mounted in indium and analysed for volatile abundances and sulphur speciation17.
Secondary ion mass spectrometry
In situ sulphur isotope measurements on these same glasses (32S, 34S) and on the sulphide inclusions (32S, 33S, 34S) were performed using a CAMECA 1270 SIMS instrument at the University of California, Los Angeles (UCLA). A Cs+ primary beam was focused to a 10–15 µm spot size at a mass resolving power of 5500, with 32SH− fully resolved from 33S− for the triple isotope measurements.
The sulphides were measured in multi-collection mode with three Faraday cups, with beam current set at 5–6 nA. Charge buildup on the gold-coated sample surface was compensated using an electron flood gun. A cold finger surrounding the sample connected to a Dewar flask of liquid N2 was used to reduce hydrides in the sample chamber. A contrast aperture and field aperture were used to exclude aberrant ions. Beam centering in the Field Aperture was scanned before each analysis. Ions from an energy window of 30 V were collected. The magnetic field was set and subsequently controlled by Nuclear Magnetic Resonance. Isotope counts per second (CPS) were measured in multi-collection mode with three Faraday cups. Slits of a common size on each detector, together with entrance slits, contrast aperture and energy window, yielded a mass resolution power of ~ 5500 (mass/∆mass) with 32SH− fully resolved from 33S−. Faraday cup electrometers were calibrated immediately prior to analysis.
The analytical routine included a 60 s pre-sputter to clean the sample surface, centre the beam in the field aperture and achieve sputtering equilibrium before data collection. Measurements of each isotope in counts per second were collected in 6 cycles of 10 sec each to allow time for settling of the Faraday cup electronics between cycles. Standards used to calibrate the instrumental mass fractionation (Supplementary Table 1) included Balmat (pyrite), CAR123 (pyrite), Canyon Diablo Troilite and Anderson pyrrhotite64,65, which was also used as our in-run internal standard. The resulting calibration curve is shown in Supplementary Figure 1 and has a slope of 0.517 ± 0.002 (2σ). Triple sulphur isotope measurements (n = 55; Supplementary Table 2) (32S, 33S and 34S) were then obtained on exposed sulphide inclusions ranging in size from 30 to 100 μm. Regularly interspersed measurements (every 5–6 measurements) of our reference material, Anderson pyrrhotite (n = 15), gave a standard deviation (1σ) on δ34S of 0.4‰, compared with its known δ34S of 1.4 ± 0.3‰64.
Double isotope analyses (n = 44; Supplementary Data 1) were conducted on glasses using instrumental parameters and analytical technique identical as above except for the following differences: (1) 34S was measured on an electron multiplier; (2) the primary beam current was reduced to 3.15 nA in order to balance the errors associated with high count rates on the electron multiplier (deadtime error) and low count rates on the Faraday cup (background); (3) analyses consisted of 20 cycles of 10 s each. The standard used for calibration is P1326-2, described below.
Three repeat analyses of one large sulphide inclusion (EH_4-29), done at different times during the analytical session, show good reproducibility, yielding δ34S and Δ33S values of −0.64 ± 0.06‰ (1σ) and 0.00 ± 0.06‰ (1σ), respectively. Other sulphide inclusions analysed twice (n = 5) are reproducible within error, except for the most isotopically negative inclusion (δ34S = −8.9 ± 0.4‰ and −10.2 ± 0.4‰).
Matrix glasses show more heterogeneity; for example, three analyses of a single glass chip (# 9.25) give a δ34S range of 2.5‰, with an average of −0.9 ± 1.3 (1σ). Other glass chips analysed twice (n = 6) show as much as 3.8‰ difference between two measurements (see Methods for discussion of uncertainty).
Description of glass standard
Our glass standard used in SIMS analyses, P1326-2, is a MORB glass from the Juan de Fuca ridge, and has previously been described for major and trace elements by refs 66,67. It is highly homogeneous and contains very few microlites. Its volatile concentrations (CO2, H2O, F, S, Cl) have previously been measured by SIMS, on separate occasions by refs 17,67, which showed good reproducibility. Ref. 17 reports 340 ± 24 ppm CO2, 0.29 ± 0.09 wt.% H2O, 206 ± 6 ppm F, 1296 ± 26 ppm S and 187 ± 5 ppm Cl.
Isotope ratio mass spectrometry (IRMS)
The sulphur isotopic composition of P1326-2 was independently analysed by both SO2 continuous flow at the University of New Mexico (Thermo Delta Plus XL mass spectrometer) and SF6 dual inlet isotope-ratio mass spectrometry at McGill University (Thermo Scientific MAT 253). The SO2 measurements were converted to the SF6 scale using the regression of ref. 68. Sulphur extractions were achieved in two ways: (1) the Kiba reagent extraction method, which extracts all S species, and (2) combined chromium reduction and Thode reagent methods to separately extract S2− and S6+, respectively69,70,71. The extracted sulphur was re-precipitated as silver sulphide (Ag2S), before fluorination to SF6 and subsequent isotopic analysis. For the separate sulphide and sulphate extractions, we used the relative weights of Ag2S precipitates to estimate the S6+/ΣS ratio, followed by a mass balance to obtain a bulk δ34S value. Replicate extraction and analysis (n = 3) give δ34S = 0.8 ± 0.1‰ and S6+/ΣS = 0.20 ± 0.03. The full results of these analyses are summarised in Supplementary Data 4.
Electron probe micro-analysis (EPMA)
The major and minor element compositions of sulphides were determined at McGill University on a JEOL 8900 electron microprobe, using an acceleration voltage of 20 kV, a beam current of 30 nA and a beam size of 3 µm. The instrument was calibrated with the following standards: AsCoNi alloy (for As, Co and Ni), pyrrhotite and chalcopyrite (for Fe, S and Cu), galena (Pb) and sphalerite (Zn). A total of 55 analyses were obtained and are presented in Supplementary Data 2, including one analysis of Anderson pyrrhotite, which agrees very well with known Anderson pyrrhotite composition (Fe0.87S)64.
Scanning electron microscopy (SEM)
Elemental maps were acquired for six different sulphide inclusions (Supplementary Figs 3–4) using a Hitachi SEM at Queens College, City University of New York, equipped with a Bruker Quantax 400 energy-dispersive X-ray spectroscopy (EDS) detector.
Estimation of the S budget of magmatic sulphides
Thin section observations show that sulphides, while abundant in some clinopyroxene and spinel phenocrysts, actually represent a minor component of the El Hierro magma. A conservative estimate for the mass fraction of sulphide in the bulk rock is 0.01 wt.%. Assuming an FeS composition, the sulphides would then have ~36 wt.% S. This corresponds to an upper limit for the sulphide contribution to bulk S content of 36 ppm, hence <1% of the total initial S concentration (5080 ppm).
Calculation of uncertainty
The instrumental mass bias factor αi on the isotopic ratio 34S/32S was calculated by comparing the average of the raw measured ratios (Ravg) of an internal calibration standard with its known isotopic ratio:
For the sulphide inclusions the standard was Anderson pyrrhotite (Rknown = 34S/32Sknown = 0.0442244 or δ34SV-CDT = + 1.4 ± 0.3‰)64. The calculation of αi for 33S/32S was done similarly, assuming a δ33Sknown falling on the regression line between δ33S and δ34S shown in Supplementary Fig. 1, i.e. dependent on δ34Sknown. The uncertainty on αi is given by:
where σavg is the standard deviation on the average of the raw measured ratios and σknown the uncertainty on the known isotopic ratio of the standard.
The measured ratios xS/32S (Ri) were then divided by αi to obtain the corrected, ‘true’ isotopic ratios, and then converted to δxSV-CDT values:
where RV-CDT is the isotopic ratio of the international reference standard Vienna-Canyon Diablo Troilite64. The propagated analytical error on δxS values is then calculated with the following equation:
where σi is the analytical uncertainty on the raw isotopic ratios (Ri).
Once the corrected isotopic ratios are converted to δ34SV-CDT and δ33SV-CDT values, Δ33S is calculated with the following expression:
where k = 0.5166 represents the slope of the mass-dependent fractionation line in δ33SV-CDT - δ34SV-CDT space (Supplementary Fig. 1). The analytical uncertainty on Δ33S then becomes:
Duplicates were measured on 6 sulphide inclusions, and always agreed within uncertainty (1σ), except for the most 34S depleted inclusion, which had individual δ34S values of −8.9 ± 0.4‰ and −10.2 ± 0.4‰ (average of −9.6 ± 1.1‰).
For the sulphide inclusions, no instrumental drift was observed between the different standard measurements during the analytical session. For the melt inclusions and matrix glasses, analyses were conducted in a 2-day span, with 31 analyses on the first day and 13 on the second day. A linear correction was applied to the first day measurements to account for instrumental drift observed on measurements of our standard, P1326-2 (n = 24) (Supplementary Fig. 2). On the second day, no instrumental drift was observed.
The sulphur isotope data on glass measurements were corrected for instrumental mass bias (αi) using the average of measured 34S/32S ratios on P1326-2 (day 1: n = 24, αi = Ri/Rknown = 0.99223; day 2: n = 13, αi = 0.98871) compared to our independently determined value for P1326-2 (Rknown = 34S/32Sknown = 0.04420 or δ34SV-CDT = 0.9 ± 0.3‰).
Errors on δ34S values were computed as for the sulphide inclusions and range from 0.1 to 0.8‰.
Modelling of S isotope fractionation
The positive correlation between fO2 (in ΔFMQ log units, derived from S6+/ΣS ratios) of melt inclusions and their S content is used to estimate fO2 variation during degassing (Fig. 2b), but as mentioned in the main text the highest S6+/ΣS values obtained are minimum estimates, and their variability in the matrix glasses is high. To account for this uncertainty, our approach has been to place maximum and minimum bounds on ΔFMQ values based on the log fO2 (ΔFMQ) vs. S trend shown in Fig. 2b. This results in a sensitivity analysis both minimising and maximising the extent of S-isotope fractionation.
Vapour saturation pressures based on volatile concentrations in melt inclusions were calculated using the data of Longpré et al.17 and the model of Iacono-Marziano et al.44. The relationship between S content and the calculated pressures for olivine-hosted melt inclusions is shown in Supplementary Fig. 5. Spinel-hosted melt inclusions were excluded because their CO2 contents are thought to be affected by disequilibrium degassing and overestimate pressures significantly72. Matrix glasses were also excluded because they are supersaturated with respect to CO2 (ref. 17), giving calculated pressures significantly in excess of the hydrostatic pressure corresponding to eruption depth. A polynomial fit was applied to the data in order to obtain input pressures as a function of S content in the degassing isotopic fractionation model. Five data points from the January 2012 sample were excluded from the computation of the polynomial fit, as they exhibit a slightly different degassing behaviour, characterised by lower H2O and S content for comparatively high CO2.
These trends can be compared to degassing paths obtained with the software D-Compress39. This model requires the initial pressure, fO2, CO2 and H2O contents of the magma as inputs, which we estimate from the most volatile-rich melt inclusion. We start with an initial CO2 content of 3420 ppm and H2O content of 3.05 wt.%, pressures varying between 300 and 400 MPa and fO2 conditions between ΔFMQ = 1.2 and 1.7 (ref. 17). The programme outputs the initial S content and models each of these parameters (CO2–H2O–S–fO2) during decompression. While the output from D-Compress generally provides good fits to the melt inclusion data (Fig. 2b, Supplementary Fig. 5), we note two main discrepancies: (1) the output S content reaches minimum values of >1100 ppm at low pressure (~1MPa), more than twice higher than what we measure in the matrix glasses (~500 ppm); (2) computed melt S6+/ΣS ratios stay relatively constant as fO2 decreases. This decoupling of S6+/ΣS and fO2 in D-Compress may arise because the S6+/ΣS vs. fO2 relationship given by Jugo et al.33, used in our conversion of S6+/ΣS ratios to ΔFMQ values, could not be implemented in D-Compress (A. Burgisser, personal communication). Nevertheless, there is general agreement that fO2 decreases with S and H2O degassing. This is also supported by XANES data on Fe3+/ΣFe ratios in El Hierro melt inclusions (Y. Moussallam, personal communication).
All data generated during this study are included in the published article and Supplementary Data files.
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We thank the Spanish Instituto Geográfico Nacional for providing lava balloon samples from the submarine El Hierro eruption. This work was completed as part of P.B.’s Master’s thesis at CUNY Queens College, supported by awards from the Fonds de recherche Nature et technologies du Québec (FRQNT), the Geological Society of America and the Association of Environmental and Engineering Geologists to P.B., a Queens College Research Foundation grant, including partial support from the Paula and Jeffrey Gural Endowed Professorship in Geology, to M.-A.L. and NSERC Discovery and Accelerator grants to J.S. The Stable Isotope Laboratory at McGill University was supported by the FRQNT through the GEOTOP research centre. We thank Lang Shi for technical assistance with electron microprobe analyses of sulphide inclusions and M. de Moor and M. Campbell for preliminary S isotope analyses of the P1326-2 glass standard. This work benefited from fruitful discussions with D. Baker, P. Cartigny, T.L. Grove, S. Ono and W. Blanford, comments by J. Marsh and N.G. Hemming, and formal reviews by M. Keith.