Fig. 2

Degassing-induced sulphur isotope fractionation. a Sulphur content and isotopic composition measured in olivine-hosted (MI-ol; navy blue circles) and spinel-hosted (MI-sp; blue diamonds) melt inclusions and matrix glasses (light blue triangles). Shaded regions represent the possible ranges of isotopic fractionation due to degassing, with changing pressure and oxygen fugacity, computed with the model of Marini et al.¹⁴ with the empirical fractionation factors of Miyoshi et al.⁴² (grey fill) and Fiege et al.⁴⁵ (hatched field). Minimum and maximum starting δ³⁴S values are chosen as the δ³⁴S value of the most S-rich inclusion and the maximum δ³⁴S value, respectively. The lower and upper bounds for each field correspond to S–fO₂ covariation as shown by the grey and black solid lines in b, with varying pressure as modelled in Supplementary Fig. 5. The dashed line shows the effect of FeS segregation on melt δ³⁴S values. Error bars represent 1σ propagated analytical uncertainties (see Methods). b Melt oxygen fugacity (ΔFMQ: log fO₂ units relative to the fayalite–magnetite–quartz buffer) as a function of S content (F(S)), derived from S⁶⁺/ΣS ratios of melt inclusions and matrix glass with the equation of Jugo et al.³³. Solid lines are linear relations drawn to encompass the data, representing minimum fO₂ (grey line) and maximum fO₂ (black line) for a given S content, and are used for modelling the lower and upper-bound fractionation estimates for each model shown in a. Dashed arrows represent degassing paths calculated with D-Compress (ref. ³⁹) in the C–S–O–H–Fe system, starting at a pressure of 300 MPa, initial volatile contents as measured in the most volatile-rich inclusion, and minimum (grey arrow) and maximum (black arrow) estimated initial fO₂. The inset shows the expected trajectory of the magma, with initially high S⁶⁺/ΣS ratio (black dot) decreasing upon S degassing (black dashed arrow) until sulphide saturation is reached (yellow dot). At that point the magma follows the SCSS upon further degassing (grey dotted line). Error bars for ΔFMQ represent the analytical uncertainty on S⁶⁺/ΣS values, converted to ΔFMQ values^17,33