Energize New Mexico



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This paper investigates the significance of faults and young magmatism as geothermal fluid conduit systems which may account for high mantle signature of geothermal fluids in areas with 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, significant geothermal potential, young volcanic and plutonic rocks (lt 7 Ma), and low-velocity upper-mantle. Rico Hot Springs has an Rc/Ra of 5.88 (73% mantle helium component) and is above a low-velocity upper-mantle domain. Rico is furthermore located along a complex fault network that involves the Precambrian-cored Rico Dome, and near young igneous rocks are such as the Calico Peak (4.7 Ma) and Priest Gulch (4.0 Ma) stocks. The near-MORB mantle helium value at Rico indicates that volatiles degassing from the mantle are rapidly transmitted into the groundwater system along as yet unexplained conduit systems. In contrast, Orvis hot spring is also located over a relatively low-velocity upper-mantle domain and yet exhibits little mantle helium (Rc/Ra=0.059). The goal is to investigate geothermal fluid and mantle volatile pathways to explain this dramatic local variation in mantle helium signature. The young igneous activity may have facilitated mantle-to-surface volatile conduits. If so, high mantle helium signature may be measured at Lemon Hot Spring because this spring is near the youngest volcanic rocks in the San Juan Mountains, the 614 ka Specie Mesa basalt. By directly connecting volatiles at depth to the surface, this extrusive rock at Lemon may provide a more rapid geothermal pathway than the plutons associated with other regional springs. This study applies hydrochemistry, noble gas chemistry, carbon dioxide flux and structural analysis to investigate the role of faults and young volcanism in helping explain local variation of mantle helium signature within the western San Juan Mountains. Overall we find that proximity to low velocity mantle domains and presence of bicarbonate type waters are more significant controls on anomalous mantle signatures than association of springs with faults of young magmatism.


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