Volcano-tectonic controls on magma residence time in arc crusts: Insights from noble gas geochemistry in the Andean Southern Volcanic Zone

The Southern Volcanic Zone of the Andes (SVZ) is a unique natural laboratory to explore the interaction between tectonic and volcanic processes. This study used the helium isotopic composition (R = 3He/4He of the sample) of volcanic rocks and geothermal gases to determine magma storage timescales across different volcano-tectonic settings. Precisely, we assess how transtensional and transpressional crustal settings linked to the interaction of the intra-arc Liquiñe-Ofqui Fault System (LOFS) and the margin-oblique Andean Transverse Faults (ATF), influence magma storage conditions and residence times. We focused on two volcanic systems: Sollipulli and Cordón Caulle. Measured helium isotope ratios normalized to the air ratio (or R/Ra) in geothermal gases range from 5.88 - 6.98 Ra for the Sollipulli and 2.37 - 5.42 Ra for Cordón Caulle, while the expected mantle endmember in this region is 7.9 Ra. Cordón Caulle's rocks showed the larger range of helium isotope ratios, with the highest of 5.71 Ra. We solved magma aging models, incorporating Monte Carlo simulations, to assess the progressive dilution of the magmatic 3He/4He endmember due to the addition of locally produced radiogenic 4He from decay of U and Th contained in magma chambers. Our results show shorter residence times at Sollipulli (31.8 ± 16.4 Ky), associated with transtensional LOFS-controlled setting, and longer residence times at Cordón Caulle (86.8 ± 27.8 Ky), linked to a transpressional ATF regime. These findings underscore the influence of both fault systems and deformation style on magma residence time, offering new insights into the processes that drive volcanic activity in the SVZ.