What Controls the CO2/SO2 Ratio in Arc Volcanic Gas?

Lead PI: Terry Plank

Unit Affiliation: Geochemistry, Lamont-Doherty Earth Observatory (LDEO)

September 2020 - August 2022
Project Type: Research

DESCRIPTION: The degassing of carbon dioxide and sulfur from arc volcanoes is fundamentally important to eruption forecasting and cycling of volatiles through subduction zones. Measurements of carbon dioxide/sulfur dioxide in high-temperature volcanic gases have become the main approach to quantifying the arc volcanic carbon dioxide and sulfur outflux to the atmosphere. New high-rate, long-time series measurements of gas emissions from several volcanoes have now shown precursory signals in the rise of carbon dioxide/sulfur dioxide weeks before the eruption, which is typically considered to reflect deep, high pressure gas moving into the system. On the other hand, recent work has shown that volcanoes with intrinsically high carbon dioxide/sulfur dioxide gas ratios seem to correlate globally with regions of carbonate subduction. Such correlations have led to the suggestions that carbon dioxide/sulfur dioxide reflects the source of the parental magma. The source of variation in carbon dioxide/sulfur dioxide of magmatic gas is thus debated. This project aims at resolving the pressure vs. source effects on carbon dioxide/sulfur dioxide by providing novel data on the melt phase for volcanoes with published gas records. This project bears directly on the interpretation of carbon dioxide/sulfur gas precursors to volcanic eruptions, of relevancy to developing timely forecasts and warnings to local populations. This project will form the core of the postdoctoral project of an early-career female scientist at Lamont-Doherty Earth Observatory. It will also enable an undergraduate student to participate in the proposed research via an NSF-funded research experience for undergraduates program at Lamont-Doherty.

Microanalysis of water, carbon dioxide, and sulfur in mineral-hosted melt inclusions can be a powerful tool for the study of carbon dioxide and sulfur degassing. However, vapor ?shrinkage? bubbles present in melt inclusions can cause a significant underestimate of carbon dioxide contents in the bulk melt inclusion and lead to false estimates of degassing pressures and carbon dioxide outflux. The published melt inclusion database is rife with this issue. This project will constrain the carbon dioxide and sulfur contents in parental and degassing arc magmas by experimentally rehomogenizing olivine-hosted melt inclusions in the eruptive products from five volcanoes (San Cristóbal, Telica, Masaya, Poás and Turrialba) along the Costa Rica-Nicaragua volcanic arc. The rehomogenized melt inclusions will be used 1) to infer the carbon dioxide and sulfur contents of parental magmas (from basaltic end-members) and entrapment pressures (from water-carbon dioxide barometry), and 2) to calculate the coexisting fluid/vapor composition to compare with the volcanic gas data and to inform existing thermodynamic models. In this way, our data will test the extent to which the pressure of degassing versus the parental magma drive carbon dioxide/sulfur dioxide variations in arc volcanic gas, and quantify the origin of eruptive gas precursors.