Geochemical Journal, Vol. 28, pp. 173 to 184, 1994 Chemical characters of crater lakes in the Azores and Italy: the anomaly of Lake Albano MARINO MARTINI,' LUCIANO GIANNINI,' FRANCO PRATI,' FRANCO TASSI,' BRUNO CAPACCIONI2 and PAOLO IOZZELLI3 'Department of Earth Sciences , University of Florence, 50121 Florence, Italy 2Institute of Volcanology and Geochemistry , University of Urbino, 61029 Urbino, Italy 3Department of Pharmaceutical Sciences , University of Florence, 50121 Florence, Italy (Received April 23, 1993; Accepted January 10, 1994) Investigations have been carried out on crater lakes in areas of recent volcanism in the Azores and in Italy, with the aim of detecting possible evidence of residual anomalies associated with past volcanic activities; data from crater lakes of Cameroon have been considered for comparison. Among the physical chemical characters taken into account, the increases of temperature, ammonium and dissolved carbon dioxide with depth are interpreted as providing information about the contribution of endogene fluids to the lake water budgets. The greater extent of such evidence at Lakes Monoun and Nyos (Cameroon) appears associated with the disasters that occurred there during the last decade; some similarities observed at Lake Albano (Italy) suggest a potential instability also for this crater lake. INTRODUCTION parison. With reference to the data collected so far and considering the possibility that the actual Crater lakes in active volcanic systems have chemical characters of crater lakes are influenced been investigated with reference to changes oc by residual thermal anomalies in the hosting vol curring in water chemistry in response to different canic systems, an effort has been made to verify stages of activity, and interesting information is whether and to what extent these anomalies can available about Ruapehu (Giggenbach, 1974), be revealed by simple observations. Kusatsu-Shirane (Ossaka et al., 1980), Chichon Be this procedure sufficiently correct, moni (Casadevall et al., 1984), Pods (Casertano et al., toring a large number of crater lakes would be 1985), and Maly Semyachik (Fazllullin, pers. come a viable program even where high-level comm.). Much of the attention of scientists and technology is not available and a wealth of infor public officials, however, has been recently cap mation could be easily collected. According to the tured by crater lakes not influenced by active intensity of thermal and chemical anomalies, pos volcanism; in fact, the events in Cameroon, at sibly related to past volcanic activities, different Lakes Monoun (Sigurdsson et al., 1987) and Nyos levels of probability could be estimated for po (Sigvaldason, 1989), represent the consequences tentially hazardous events. of a rather uncommon type of geologic hazard. Crater lakes appear thus to deserve serious atten VOLCANOLOGICAL SETTINGS tion as potential sources of disasters significantly affecting the populations in the surrounding areas. Given the general aim of this investigation, Systematic investigations of water chemistry main attention was paid to the location of lakes have been carried out at crater lakes in areas of within volcanic systems, and the difference be recent volcanism in Italy and in the Azores; other tween craters representing the whole structure or results from Cameroon have been used for com pertaining to larger calderas is not supposed to 173 174 M. Martini et al. OCorvo Ay ORES 0 Fl ores Graciosa San rge QTerceira Faial 0<, Pica San Miguel S.ta Maria 4 Sete Cidades Fogo Furnas S. MIGUEL 0 10 I i Km. Fig. 1. Location of the crater lakes in the Azores. le CS • BOLSENA q. 0 CO ACCJkNO r •MARTIONANO 1 Roma 0ALBANO•NEMI TYRRHENIAN 10 SEA Napol i Fig. 2. Location of the crater lakes in Italy. Crater lakes in Azores and Italy 175 affect to a significant extent the chemical charac comm.). Martignano lake pertains to the same ters under consideration. structure. The activity of Alban Hills volcanic Significant volcanic activity is reported for the complex occurred during the period 0.70-0.036 Azores, crossed in a north-south direction by the m.y. b.p. with hydromagmatic explosions from axis of the Mid-Atlantic ridge (Fig. 1); besides eccentric craters in its final phase; Nemi and the last surtseyan eruption of Capelinhos at Faial Albano lakes are filling two of these craters (De in 1957-58 (Machado et al., 1962), strombolian Rita et al., 1991). Phlegrean Fields are located activity with lava flows occurred in 1761 at inside a large caldera possibly produced during a Terceira, and four explosive eruptions are recorded giant ignimbrite eruption about 0.035 m.y. ago for S. Miguel in 1563, 1638, 1640, and 1811 (Rosi et al., 1983). The crater hosting Lake Averno (Booth et al., 1983). Fogo lake fills the summit is considered as a monogenetic tuff ring formed depression of Agua de Pau volcano, about 3 km during the recent subaerial period; it is dated at wide; its present morphology results from about 3.800 y.b.p. The last eruptive event in the cratering, volcano-tectonic collapse, and landslides. area occurred in 1538 with the build up of a 140 Sete Cidades lake is located inside a polygenetic m high tuff cone. caldera, 5 km in diameter, produced during a long series of plinian events, the last probably occurred SAMPLING AND ANALYTICAL PROCEDURES in 1444. Furnas lake occupies the summit of the easternmost active stratovolcano, truncated by a Water sampling has been carried out along a large polygenetic collapse caldera, about 6 km vertical profile above the deepest point of lake wide. A well documented historical eruption is bottom; the position of the boat was constantly reported in 1640; intense thermal activity is at checked by means of a portable GPS (Sony IPS present occurring inside the caldera. 360). Samples from depth have been collected in With reference to Italian lakes (Fig. 2) Bolsena closed containers. Temperature was measured in lake is a large volcano-tectonic depression, about situ, pH at the surface, at the same time of the 13 km in diameter, probably produced by a series collection of samples for dissolved CO2 determi of huge ignimbrite forming eruptions during the nation, in 100 ml glass bottles containing 10 ml period 0.60-0.35 m.y. b.p. (Varekamp, 1980; of a 1 M Na2CO3 solution. The C032 excess Gillot et al., 1991). The volcanic activity contin subsequent to the reaction: H20 + CO2 + C032 ued in the western Vulsini area probably up to => 2HCO3 allows then the calculation of dis 0.125 m.y. ago, where the well shaped polygenetic solved C02 Latera caldera developed (Nappi et al., 1991; The analytical methods used are as follows. Turbeville, 1993). Anions and dissolved CO2 ' Automatic titration The activity of Vico volcano occurred from with a Titroprocessor Metrohm 636 equipped with 0.40 to 0.09 m.y. b.p.; its summit appears as ion selective electrodes. Cations and Boron => Ion truncated by a polygenetic collapse caldera, about chromatography with a Dionex DX 100. NH4 = 8 km wide (Bertagnini and Sbrana, 1986), and half Potentiometric determination. Li Atomic ab of the caldera floor is occupied by the lake, sorption spectrophotometry. probably established after a giant ignimbritic eruption dated at about 0.15 m.y. (Locardi, 1965). OBSERVATION DATA AND DISCUSSION Explosive paroxysmal eruption with subordinate effusive episodes characterized the Sabatini area The analytical results of major components for during a period of 0.60 m.y., and the development the samples collected from crater lakes at the of the volcano-tectonic depression of Bracciano surface and at approximately the maximum depth lake followed the exhaustion of the eruptive ac are given in Table 1. tivity about 40,000 years ago (De Rita, pers. Our attention has been mainly focussed on an 176 M. Martini et al. 10 O 00 ~--~ ~--i M min v) N N C1 O ~O 00 ' '0 00 0 ..ti N 01 M v) N N ~+ M en N en en 10 ~--~ N 1 Ion N .--~0 I'0 N N N N M 10 U 00 00 N N N-+ MO MO N 00 NTM 'ChM MN MN N.-~ N~ - N N l001 O O O O O N N 0 to 00 0 en O in O M M O a1 01 ~O O O O .-r 0 0 0 0 0 0 O N M M N N 01 NO --~ .--~ 00 N a, O N N M en N \O 00 et d N M -+ M N 0 U M t` In o0 O O -O O\ 00 -t 0 Vn N N kn d 'It IC 00 N t 0 -~ V) N N N t0 N W) tn N M O N t U x N N N 00N "oW) v) N"D Nc 00 00 M 10o0 ~,O 10 00 10 00 00 M -+ n O N O .--i kn tn M N N N k) t t` N M IC N N 01 d en ~O N cd IIO O d 10 N 0\ N tlM U O M M N N N d M M ct N N N N O - N M - N 10 ---~ 01 N N N N M M N N N ct N N ~--i M M .-+ N N 10 tl v) N M N O N - 1,0 1,0 M N 1C to 'Tt W) O a1 .--. .--. N 00 N M N V l - N 1.0 M N It N N "t m - d' d' N N ~,o N cd z - M d o-oN N N M N N N IT W) -+ M t- N N h 110 10 10 0 110 M 10 N 01 00 N N M N W) kn .-+ N N M 00 01 t~ N 10 00 10 n O C': In M oo l0 O .