Crustal CO2 contribution to subduction zone degassing recorded through calc-silicate xenoliths in arc lavas

Sean Whitley,Valentin R Troll,Ralf Halama,Katie Preece,Frances M Deegan,Ralf Gertisser

doi:10.1038/s41598-019-44929-2

Sean Whitley, Valentin R Troll + Show 4 more

Open Access

https://doi.org/10.1038/s41598-019-44929-2

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Abstract

Interaction between magma and crustal carbonate at active arc volcanoes has recently been proposed as a source of atmospheric CO2, in addition to CO2 released from the mantle and subducted oceanic crust. However, quantitative constraints on efficiency and timing of these processes are poorly established. Here, we present the first in situ carbon and oxygen isotope data of texturally distinct calcite in calc-silicate xenoliths from arc volcanics in a case study from Merapi volcano (Indonesia). Textures and C-O isotopic data provide unique evidence for decarbonation, magma-fluid interaction, and the generation of carbonate melts. We report extremely light δ13CPDB values down to −29.3‰ which are among the lowest reported in magmatic systems so far. Combined with the general paucity of relict calcite, these extremely low values demonstrate highly efficient remobilisation of crustal CO2 over geologically short timescales of thousands of years or less. This rapid release of large volumes of crustal CO2 may impact global carbon cycling.

Highlights

Crustal magma-carbonate interaction has been suggested as a process that may dominate the CO2 output in several volcanic arcs[1,2] and a possible source of magmatic carbonate melts[3,4,5]
The upper crust underlying Merapi consists of a 8–11 km thick unit of Cretaceous to Cenozoic limestones, marls and volcaniclastic deposits24, 1School of Geography, Geology and the Environment, Keele University, Keele, ST5 5BG, UK. 2Department of Geography, College of Science, Swansea University, Swansea, SA2 8PP, UK. 3Section for Mineralogy, Petrology and Tectonics (MPT), Department of Earth Sciences, Uppsala University, 752 36, Uppsala, Sweden. 4Faculty of Geological Engineering, Universitas Padjajaran (UNPAD), Bandung, Indonesia
Magmatic skarn xenoliths contain abundant glass that is CaO-enriched (1 to 12 wt%) relative to host lava dacite groundmass glasses, and dominantly comprise wollastonite which contains numerous CaO-enriched melt inclusions compositionally similar to the contaminated groundmass glass. These wollastonite-hosted melt inclusions and additional Fe-rich growth rims on wollastonite crystals testify to crystallisation from a melt that has assimilated a significant quantity of CaO (c.f.29)

Summary

Introduction

Crustal magma-carbonate interaction has been suggested as a process that may dominate the CO2 output in several volcanic arcs[1,2] and a possible source of magmatic carbonate melts[3,4,5] Direct evidence for this process often remains elusive, but the occurrence of calc-silicate (skarn) xenoliths in the eruptive products of some active volcanoes[6,7,8,9] provide a unique opportunity to study high temperature magma-carbonate interaction, and the subsequent effects on the host magmatic system. This combination of processes documents fast and highly efficient, rapid liberation of crustal CO2 into the atmosphere

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