Fig. 6: Sulfur isotopic composition of CF MIs.
From: Escalation of caldera unrest indicated by increasing emission of isotopically light sulfur
a, MI sulfur isotopic composition plotted against sulfur content (measured by secondary ion mass spectrometry; data from Supplementary Table 2). The red band shows the δ34S range of mid-ocean-ridge basalts40. Grey diamonds and crosses are published MI and glass data from arc volcanics in Central America42 and the Cascades41, respectively. The yellow-brown line shows the results of a total least-squares regression fitted to the data, assuming equilibrium closed-system degassing, while the shaded area is 1 standard error of the regression. The large pentagon symbol shows the predicted δ34S of the undegassed melt. Yellow, green and red lines are forward degassing models calculated using Sulfur_X (ref. 27) and combined with sulfur isotope fractionation factors for sulfide-sulfate, H2S-sulfide and H2S-SO2 (Methods; all calculations reported in Supplementary Data 1). Solid, dashed and dotted lines show model runs with variable CO2 contents (2.0, 0.8 and 0.4 wt%). All error bars are 2 s.d. of the mean. b, Same as a but reporting the model evolution of the equilibrium gas sulfur isotope compositions (for closed-system degassing).
