LONG-TERM GEOCHEMICAL SURVEILLANCE OF FUMAROLES AT SHOWA-SHINZAN DOME, USU VOLCANO, JAPAN

This study investigates 31 years of fumarole gas and condensate (trace elements) data from Showa-Shinzan, a dacitic dome-cryptodome complex that formed during the 1943-1945 eruption of Usu volcano. Forty-two ga s samples were collected from the highest-temperature fumarole, named A-1, from 1954 (800 degrees C) to 1985 (336 degrees C), and from lower -temperature vents. Condensates were collected contemporaneously with the gas samples, and we reanalyzed ten of these samples, mostly from t he A-1 vent, for 32 cations and three anions. Modeling using the therm ochemical equilibrium program, SOLVGAS, shows that the gas samples are mild disequilibrium mixtures because they: (a) contain unequilibrated sedimentary CH4 and NH3; (b) have unequilibrated meteoric water; or ( c) lost CO, either by air oxidation or by absorption by the sodium hyd roxide sampling solution. SOLVGAS also enabled us to restore the sampl es by removing these disequilibrium effects, and to estimate their equ ilibrium oxygen fugacities and amounts of S-2 and CH4. The restored co mpositions contain > 98% H2O with minor to trace amounts of CO2, H-2, HCl, SO2, HF, H2S, CO, S-2 and CH4. We used the restored gas and conde nsate data to test the hypotheses that these time-series compositional data from the dome's fumaroles provide: (1) sufficient major-gas data to analyze long-term degassing trends of the dome's magma-hydrotherma l system without the influence of sampling or contamination effects; ( 2) independent oxygen fugacity-versus-temperature estimates of the Sho wa-Shinzan dacite; (3) the order of release of trace elements, especia lly metals, from magma; and (4) useful information for assessing volca nic hazards. The 1954-1985 restored A-1 gas compositions confirm the f irst hypothesis because they are sufficient to reveal three long-term degassing trends: (1) they became increasingly H2O-rich with time due to the progressive influx of meteoric water into the dome; (2) their C /S and S/Cl ratios decreased dramatically while their Cl/F ratios stay ed roughly constant, indicating the progressive outgassing of less sol uble components (F approximate to Cl > S > C) from the magma reservoir ; and (3) their H2O/H-2, CO2/CO and H2S/SO2 ratios increased significa ntly in concert with equilibrium changes expected for the similar to 5 00 degrees C temperature drop. When plotted against reciprocal tempera ture, the restored-gas log oxygen fugacities follow a tight linear tre nd from < NNO + 0.5 at > 800 degrees C to NNO + 2.5 at similar to 400 degrees C. This trend largely disproves the second hypothesis because the oxygen fugacities for the < 800 degrees C restored gases can only be explained by mixing of hot magmatic gases with similar to 350 degre es C steam from superheated meteoric water. But above 800 degrees C th is trend intersects the opposing linear trend for other Usu eruptive p roducts, implying a log oxygen fugacity of -11.45 at 902 degrees C for the Showa-Skinzan magma. The time-series trace-element data also disp rove the third hypothesis because rock- and incrustation-particle cont aminants in the condensates account for most of the trace-element vari ation. Nonetheless, highly volatile elements like B and As are relativ ely unaffected by this particle contamination, and they show similar t ime-series trends as Cl and F. Finally, except for infrequent sampling around the 1977 Usu eruption, the results generally confirm the fourt h hypothesis, since the time-series trends for the major gases and sel ected trace elements indicate that, with time, the system cooled, dega ssed and was infiltrated by meteoric water, all of which are positive signs that volcanic activity declined over the 31-year history. This s tudy also suggests that second boiling of shallow magma within and pos sibly beneath the cryptodome sustained magmatic degassing for at least 20 years after emplacement.