MANTLE-TO-SURFACE NEOTECTONIC CONNECTIONS IN THE SAN JUAN MOUNTAINS DOCUMENTED BY 3HE/4HE, CO2 FLUX MEASUREMENTS, AND HYDROCHEMICAL ANALYSES OF THE GEOTHERMAL SYSTEM

This project investigates the controls on geothermal-fluid conduit systems which may account for high mantle signatures of geothermal fluids in areas of high crustal thickness. The field laboratory is the western San Juan Mountains of southwestern Colorado where the structural setting and hydrochemistry of carbonic springs suggest potential connections between surface hot springs, fault networks, CO₂ degassing, significant geothermal potential, young volcanic and plutonic rocks (< 7 Ma), and low-velocity upper mantle. The Rico Hot Springs have the highest mantle volatile component of any spring in Colorado (73% mantle helium component from an Rc/Ra value of 5.88). This near-MORB mantle helium value at Rico indicates that volatiles degassing from the mantle are rapidly transmitted into the groundwater system along deep-seated faults. Rico itself has many associated geologic features that are important for our investigation of volatile transport and spring chemistry controls, including a complex fault network involving the Precambrian-cored Rico Dome, ~4 Ma intrusive rocks at Calico Peak and Priest Gulch , and a low-velocity shallow upper mantle. Therefore, Rico and the surrounding region is a natural laboratory for studying geothermal fluid and mantle volatile pathways. Additional noble gas analyses and hydrochemistry data were gathered from regional springs and modeled via chemical geothermometers. New noble gas measurements from this study, paired with literature values, reveal highest mantle volatiles at Rico (4.09-5.88 Rc/Ra), Dunton (3.11-4.54 Rc/Ra), Geyser Warm Spring (3.39 Rc/Ra), and Paradise Warm Spring (2.72 Rc/Ra). Water volume is dominated by meteoric fluids as shown by stable isotope data but hydrochemistry indicates high TDS, high CO2, high He geothermal fluid endmembers that can explain chemistry variations between spatially proximal springs. CO₂ flux measurements (up to 36.2 g/m²/hr) vary across structural features and demonstrate that the faults act as pathways for CO₂ flux suggesting ongoing neotectonic activity. Overall, we find that local high mantle helium signature is controlled by proximity to a low-velocity upper mantle domain while variation in spring hydrochemistry reflects complex mixing in the groundwater system.