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The 1982 eruption of El Chichón volcano ejected more than 1 km 3 of anhydrite-bearing trachyandesite pyroclastic material to form a new 1-km-wide and 300-m-deep crater and uncovered the upper 500 m of an active volcano-hydrothermal system. Instead of the weak boiling-point temperature fumaroles of the former lava dome, a vigorously boiling crater spring now discharges 1 20 kg/s of Cl-rich (F15 000 mg/kg) and sulphur-poor ( 1 200 mg/kg of SO 4 ), almost neutral (pH up to 6.7) water with an isotopic composition close to that of subduction-type magmatic water (dDp-15‰, d 18 Opc6.5‰). This spring, as well as numerous Clfree boiling springs discharging a mixture of meteoric water with fumarolic condensates, feed the crater lake, which, compared with values in 1983, is now much more diluted (F3000 mg/kg of Cl vs 24 030 mg/kg), less acidic (pHp2.6 vs 0.56) and contains much lower amounts of S ( 1 200 mg/kg of SO 4 , vs 3550 mg/kg) with d 34 Sp0.5-4.2‰ (c17‰ in 1983). Agua Caliente thermal waters, on the southeast slope of the volcano, have an outflow rate of approximately 100 kg/s of 71 7C Na-Ca-Cl water and are five times more concentrated than before the eruption (B. R. Molina, unpublished data). Relative N 2 , Ar and He gas concentrations suggest extensional tectonics for the El Chichón volcanic centre. The 3 He/ 4 He and 4 He/ 20 Ne ratios in gases from the crater fumaroles (7.3R a , 2560) and Agua Caliente hot springs (5.3R a , 44) indicate a strong magmatic contribution. However, relative concentrations of reactive species are typical of equilibrium in a two-phase boiling aquifer. Sulphur and C isotopic data indicate highly reducing conditions within the system, probably associated with the presence of buried vegetation resulting from the 1982 eruption. All Cl-rich waters at El Chichón have a common source. This water has the appearence of a "partially matured" magmatic fluid: condensed magmatic vapour neutralized by interaction with fresh volcaniclastic deposits and depleted in S due to anhydrite precipitation. Shallow ground waters emerging around the volcano from the thick cover of fresh pumice deposits (Red waters) are Ca-SO 4 -rich and have a negative oxygen isotopic shift, probably due to ongoing formation of clay at low temperatures.

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