Uranium series constraints on rates of CO2 production by skarnification at Merapi volcano, Indonesia

Lavas erupting from arc volcanoes result from the transfer of water and other materials from subducted oceanic crust to their mantle sources. The release of this water and other gasses from magmas as they migrate towards the surface often results in explosive eruptions, which may create severe hazards for local populations. When magmas rise through limestones, they can chemically react with the limestones, which releases CO₂ gas and may add to the explosivity of eruptions.

Mount Merapi, the focus of this study, is a highly active arc volcano that lies 20 miles north of Yogyakarta, Indonesia, a major population center. It has had three major explosive eruptions in the past two decades (2006, 2010, and 2018-ongoing). Merapi overlies limestones, and its erupted lavas commonly contain enclaves composed of calcium-silicate minerals, which result from magma-carbonate interaction. Hence, it has been hypothesized that its explosivity is enhanced by this interaction.

This study presents the first 238U-²³⁰Th-²²⁶Ra-²¹⁰Pb data on these enclaves, with a goal of constraining rates of CO₂ production by reactions between magmas and limestones at Merapi. The relative abundances of these U-series nuclides can be used to determine timescales of magmatic processes because their half-lives are similar to these timescales. Our data indicate that Merapi’s magmas have assimilated limestones over a variety of timescales, and that some of this assimilation occurred within a few decades leading up to eruption.