MANTLE-TO-SURFACE NEOTECTONIC CONNECTIONS IN THE SAN JUAN MOUNTAINS DOCUMENTED BY 3HE/4HE, CO2 FLUX MEAUREMENTS, AND HYDROCHEMICAL ANALYSIS OF THE GEOTHERMAL SYSTEM NEAR RICO, COLORADO

This project investigates the controls on geothermal fluids and their conduit systems which may account for high mantle helium components of geothermal fluids in intracratonic continental regions. The field laboratory is the western San Juan Mountains of southwestern Colorado where the structural setting and hydrochemistry of carbonic springs suggest potential connections among surface hot springs, fault networks, CO2 degassing, significant geothermal potential, young volcanic and plutonic rocks (\u3c 7 Ma), and low-velocity upper mantle. The Rico Hot Springs have the highest mantle volatile component of any spring in Colorado with air-corrected values of 3He/4He = 5.88 RA indicating 73% mantle helium component. This near-MORB mantle helium value at Rico indicates that volatiles degassing from the mantle must be rapidly transmitted into the groundwater system along deep-seated faults such that accumulation of 4He from radiogenic crust has not swamped mantle-derived primordial 3He. Geologic features that are important for volatile transport and spring chemistry controls include a complex conduit system and fault network involving the Precambrian-cored Rico Dome, ~4 Ma intrusive rocks at Calico Peak and Priest Gulch, and a low-velocity 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 air corrected 3He/4He at Rico (4.09-5.88 RA), Dunton (3.11-4.54 RA), Geyser Warm Spring (3.39 RA), and Paradise Warm Spring (2.72 RA). Water volume is dominated by meteoric fluids as shown by stable isotope data but hydrochemistry indicates high TDS, high CO2, and high He come from a geothermal fluid endmember. Variable mixing and water-rock interactions are processes 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 degassing. Overall, we find that local high mantle helium signature is localized directly above regions of lowest upper mantle velocity in the San Juan mantle anomaly and is derived from neotectonic mantle melts in those regions