Tracing and quantifying magmatic carbon discharge in cold groundwaters: lessons learned from Mammoth Mountain, USA

Abstract A major campaign to quantify the magmatic carbon discharge in cold groundwaters around Mammoth Mountain volcano in eastern California was carried out from 1996 to 1999. The total water flow from all sampled cold springs was ≥1.8×10 7 m 3 /yr draining an area that receives an estimated 2.5×10 7 m 3 /yr of recharge, suggesting that sample coverage of the groundwater system was essentially complete. Some of the waters contain magmatic helium with 3 He/ 4 He ratios as high as 4.5 times the atmospheric ratio, and a magmatic component in the dissolved inorganic carbon (DIC) can be identified in virtually every feature sampled. Many waters have a 14 C of 0–5 pmC, a δ 13 C near −5‰, and contain high concentrations (20–50 mmol/l) of CO 2(aq) ; but are otherwise dilute (specific conductance=100–300 μS/cm) with low pH values between 5 and 6. Such waters have previously escaped notice at Mammoth Mountain, and possibly at many other volcanoes, because CO 2 is rapidly lost to the air as the water flows away from the springs, leaving neutral pH waters containing only 1–3 mmol/l HCO 3 − . The total discharge of magmatic carbon in the cold groundwater system at Mammoth Mountain is ∼20 000 t/yr (as CO 2 ), ranging seasonally from about 30 to 90 t/day. Several types of evidence show that this high discharge of magmatic DIC arose in part because of shallow dike intrusion in 1989, but also demonstrate that a long-term discharge possibly half this magnitude (∼10 000 t/yr) predated that intrusion. To sustain a 10 000 t/yr DIC discharge would require a magma intrusion rate of 0.057 km 3 per century, assuming complete degassing of magma with 0.65 wt% CO 2 and a density of 2.7 t/m 3 . The geochemical data also identify a small (