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. 2019 Apr 1;9:5442. doi: 10.1038/s41598-019-41901-y

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© The Author(s) 2019

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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The proxy approach for estimating the CO₂/S_T ratio for “unmeasured” volcanoes (i.e., those for which no gas data exist), based on the averaged trace-element composition of the corresponding volcanic rocks. The procedure is illustrated for Pacaya volcano in Guatemala. Firstly, the association between CO₂/S_T ratios in volcanic gases (corresponding to CO₂/SO₂ gas ratios in the high-temperature systems studied here) and whole-rock Ba/La ratios is established at the scale of the Central American Volcanic Arc (CAVA) segments, using data for volcanoes for which both gas and trace element data are available (see Table S1a). Secondly, the gas vs. trace-element trend is fitted via either a linear or logarithm best-fit regression function. Tests made excluding (panel A) or including (panel B) the compositional point of the Depleted Mid-ocean ridge Mantle (DMM; refs^123,124) in the data-fitting found that the second option systematically led to the best-data fits (see Table 2). Finally, the preferred regression model function (RM3 in the Pacaya example; see panel B and Table 2) is used to calculate a “predicted” gas CO₂/SO₂ from available Ba/La data for Pacaya whole-rocks (uncertainty is estimated from confidence interval at one standard deviation on the regression). Our inferred gas CO₂/S_T ratio (1.4 ± 0.75; Table 2) is well within the magmatic gas range (CO₂/SO₂ ratio of 1.1 ± 1.0.) measured during recent plume observations⁴⁶. A similar CO₂/S_T ratio (see Table 2) is predicted using the CAVA gas vs. Sr/Nd ratio association (panel C). In this plot, the yellow and green dashed lines are the linear best-fit regression lines for Group 1 and 2 sub-populations, respectively.