Long Valley Caldera: Monitoring studies of gas composition and helium, argon, and carbon isotopes

Technical Report · Tue Sep 25 04:00:00 EDT 1990

OSTI ID:5551337

Craig, H

We have now completed our 3-year intensive monitoring programme of fumarole and hot spring gas chemistry at Long Valley caldera in the Sierra Nevadas. The study was initiated in response to earlier findings at the Isotope Laboratory linking variations in helium isotope ratios ({sup 3}He/{sup 4}He) to the presence of magma-based hydrothermal activity in the western United States. In an attempt to establish what relationship, if any, exists between variations in ({sup 3}He/{sup 4}He) and the timing, magnitude and location of seismic events in the area we have focused on 6 sites within the caldera and immediate vicinity, and sampled them a total of 12 times during the period 1986--88. Again, our analytical effort has centered on helium isotope ratios ({sup 3}He/{sup 4}He), because of their extreme sensitivity for monitoring the balance between magmatic and crustal additions to the volatile inventory; however, in an attempt to evaluate other geochemical parameters, we have also analysed for major gas chemistry (CO{sub 2}, O{sub 2}, N{sub 2}, Ar), other trace gases (CH{sub 4}), and the isotopic composition of carbon (CO{sub 2} and CH{sub 4}). This report represents on overview of our results for the 1986--1988 sampling period. It is divided into two sections, and deals with temporal helium isotope variations and preliminary remarks on the helium-carbon relations. 6 figs., 3 tabs.

• Research Organization: California Univ., La Jolla, CA (United States)
• Sponsoring Organization: DOE; USDOE, Washington, DC (United States)
• DOE Contract Number: FG03-84ER13267
• OSTI ID: 5551337
• Report Number(s): DOE/ER/13267-T1; ON: DE91016953
• Country of Publication: United States
• Language: English

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58 GEOSCIENCES
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