By William C. Evans1, Robert H. Mariner1, Steven E. Ingebritsen1, B. Mack Kennedy2, Matthias C. van Soest2, and Mark A. Huebner1
1U.S. Geological Survey, Menlo Park, CA
2Lawrence Berkeley National Laboratory, Berkeley, CA
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An ongoing episode of crustal uplift centered in the Separation Creek drainage
of the Three Sisters area, central Oregon Cascades, may result from a magmatic
intrusion that began in 1998. An investigation of springs in this drainage in
summer 2001 revealed slightly elevated water temperatures and chloride (Cl-)
concentrations of up to about 5°C and 20 milligrams per liter (mg/L), respectively,
above background. The total discharge of anomalous Cl- in Separation
Creek was 9.2 grams per second, which in combination with the temperature-Cl-
relation in the springs results in a total advective heat discharge of 16 MW
(megawatts). Comparison with similar findings obtained a decade earlier suggests
that total Cl- and heat discharges in the groundwater drainage are
unaffected by the current uplift. However, the isotopic composition of the dissolved
inorganic carbon (DIC) in the spring waters (delta carbon-13 (13C)
= -9.03 to -11.6; carbon-14 (14C) <25 pmC) combined with helium-3/helium-4
(3He/4He) ratios near 8 RA and C/3He ratios <1010 in two of the springs are
indicative of a magmatic source. The high 3He/4He ratios indicate that the magmatic
gas is derived from a relatively recent, if not ongoing, intrusion. The concentration
of magmatic carbon is low, a few millimoles per liter (mmol/L) at most, with
an average value of 1.53 mmol/L for all the springs sampled in the drainage.
Combining this average with the late-summer water flow in Separation Creek suggests
a discharge of 21 tonnes/day of magmatic carbon dioxide (CO2). The presence
of magmatic carbon in the shallow groundwater system, and the fact that DIC
is uncorrelated with Cl-, suggests that some magmatic gas could escape diffusely
through the soils.
Abstract
Introduction
Background
Procedures
Results
Comparisons with Past Data
Chloride/Bromide Ratios
Alkalinity and Dissolved Inorganic Carbon
Carbon: Isotopes and Sources
Gas Results
Discussion
Effects of Intrusion?
Heat and Carbon Discharge
New Data and the Conceptual Mode
The Need for Cold Spring Studies
Conclusions
Acknowledgements
References
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