The Influence of Magma Bodies on CO2 Degassing Throughout the Central Volcanic Zone of the Andean Convergent Margin

The Central Volcanic Zone (CVZ) within the Andean Convergent Margin formed from the subduction of the Nazca Plate under the South American plate. It is one of the most active volcanic regions on Earth and features a distinctive form of subduction, known as flat-slab subduction, in which the subducting plate shoals in some sections to subduct at an angle less than 30. There is evidence of flat-slab subduction occurring from 21S and migrating southward beginning at least 35 million years ago. This region also features the largest known magma body, the Altiplano-Puna Magma Body (APMB), which is 200 km in diameter and 11 km thick from 21S-23.5S, and may consist of up to 25% partial melt. Investigating the impacts of this tectonic history and magma body's effects on volatile release dynamics have not previously been done, although various studies have detected variations in net volcanic outputs above the APMB.

We present new He and C isotope results from (n=37) samples retrieved during a 2022 field campaign in the CVZ. Data were collected in both water and gas samples, greatly increasing the resolution of sampling and characterization of the region, especially above the APMB. Helium isotopes, corrected for air contamination, in gas samples range from 0.98 to 5.80 Rc/RA and in water samples range from 0.07 to 3.87 Rc/RA, agreeing well with published data and indicating varying degrees of crustal contamination. Carbon isotopes ranged from -10.6 to -3.6 ‰ vs. Vienna Pee Dee Belemnite (VPDB), and also fall within expected ranges for the region.

Novel gas-phase data, together with compiled data, reveal the highest CO2/3He occur above the APMB, with samples in this region showing an average CO2/3He of 5.08x1011 in comparison with 1.81x1011, which is the average ratio for all other samples in the CVZ. In addition, the samples above the APMB which show the highest CO2/3He correlate strongly with significantly lower δ13C, suggesting greater subducted sedimentary input which may be a residual signal from the period of flat-slab subduction in addition to overall crustal carbon assimilation. Further exploration is thus needed of this region to better constrain impacts of previous flat-slab subduction and to better understand the role of magma bodies in volatile release dynamics.