EPISODIC CHANGES IN HYDROTHERMAL DISCHARGE: SOME EXAMPLES FROM THE WESTERN UNITED STATES

The USGS has monitored hydrothermal discharge in the western U.S. intermittently for over 30 years and has recently begun to develop continuous (e.g. hourly) hydrothermal records suitable for comparison with other continuous monitoring data (seismic, geodetic). Much of the episodic hydrothermal behavior documented to date is periodic, reflecting the influence of near-surface conditions rather than changes at the magma-hydrothermal interface. For instance, seasonal variability in the hydrothermal solute flux from magmatically influenced springs of the Cascade Range ranges up to a factor of 5-10. Changes associated with episodes of volcanic unrest are more difficult to capture but are of particular interest, both from the standpoint of volcano-hazards monitoring and economic geology. Indeed, much of our knowledge about hydrothermal fluids between the magma-hydrothermal interface and the land surface owes to the study of hydrothermal veins in ore deposits. Given the apparent brevity and intensity of some veining episodes, it seems reasonable to associate vein formation with volcanic unrest. Episodes of rapid vein formation would presumably entail seismicity, ground deformation, and increases in hydrothermal venting. High rates of vapor and metal release have been documented at restless volcanoes, and the 1991 Pinatubo eruption is believed to have destroyed part of an incipient porphyry deposit. The continuous-monitoring effort in the western U.S. has yet to capture such dramatic changes. However, a few recent episodes of volcanic unrest permit linkage between hydrothermal and geophysical observations. For instance, variations in the surficial flux of magmatic CO₂ and ³He/⁴He ratios at Mammoth Mountain (CA) have been convincingly related to releases of magmatic fluids from basaltic intrusions in 1989 and 2006-2009 that are suggested by seismic and geodetic records. And a thermal pulse at Mount Baker (WA) in 1975 (from 11+6 MW to 80+44 MW; Friedman and Frank, 1980), with concomitant increases in sulfur and CO₂ output, may have deposited sufficient pyrite to explain a +1800 μGal local gravity increase (Crider and others, 2008). These and similar episodes worldwide have prompted the USGS to refocus hydrothermal monitoring on relatively active volcanoes, in an effort to capture full cycles of volcanic unrest.