GeochemicalJournal,Vol.28,pp. 173to 184,1994 C h em ic al c h a r acters of cr ater la k es in th e A z o res a n d Italy: th e a n o m aly o f L a k e A lb a n o M ARlNO M ARTlNl,1 L UCIANO GIANNINl,1 FRANCO PRATI,1 FRANCO TASSI,l B RUNO CAPACCIONl2 and PAOLO IOZZELL13 IDepartm ent ofEarth Sciences,U niversity ofFlorence, 50121 Florence,Italy 2lnstitute ofV olcanology and G eochemistry, University ofUrbino, 61029 Urbino,Italy 3Departm ent ofPharm aceutical Sciences, University ofFlorence,50121 Florence,Italy (Received April23, 1993,･Accepted January 10, 1994) Investigations have been carried out on craterlakesin areas ofrecent volcanism in the A zores and in Italy, with the aim of detecting possible evidence of residual anom alies associated with past volcanic activities;data from craterlakes ofCam eroon have been considered for com parison. A m ong the physical- chem ical ch aracters taken into account, the increases of tem perature, am m onium and dissolved carbon dioxide with depth are interp reted as providing inform ation aboutthe contribution of endogene fluidsto the lake w ater budgets. The greater extent of such evidence at Lakes M onoun and N yos (Cam eroon) appears associated withthe disastersthatoccurred there duringthe lastdecade;som e sim ilarities observed atLake Albano (Italy)suggesta potentialinstability also forthis craterlake. parison. W ith reference to the data collected so INTRODUCT ION far and considering the possibility that the actual Crater lakes in active volcanic system s have chem ical characters ofcrater lakes are influenced been investigated with reference to change s oc- by residualtherm al anom alies in the hosting vol- curing in w ater chem istry in response to different canic system s, an effort has been m ade to verify stages of activity, and interesting inform ation is w hether and to w hat extent these anom alies can available about R u apehu (Giggenbach, 1974), be revealed by sim ple observations. K usatsu-Shirane (O ssaka et al., 1980), C hich6n B e this pr ocedure sufficiently correct, m oni- (C asadevall et al., 1984), Po~s (Casert ano et al., toring a larg e num ber of crater lakes w ould be- 1985), and M aly Sem yachik (Fazllu llin, pers. com e a via ble program even w here high-level com m .). M uch of the atte ntion of scientists and technology is not available and a w ealth o finfor- public officials, how ever, has bee n recently cap- m ation co uld be e asily collected. A ccording to the tured by crater lakes not influenced by active intensity oftherm al and chem ical anom alies, pos- volcanism ; in fact, the events in C am eroon, at sibly related to past volcanic activities, diff erent Lakes M onoun (Sigurdsson etal.,1987) and N yos levels of probability could be estim ated for po- (Sigvaldason, 1989), represent the consequences tentially hazardous events. of a rather uncom m on type of geologic hazard. Crater lakes appear thus to deserv e serious atten- V OLCANOLOGICAL SETTINGS tion as potential sources of disasters significantly affecting the populations in the sur ounding areas. Given the general aim of this investigation, System atic investigations of w ater chemistry m ain attention w as paid to the location oflakes have been car ied out at crater lakes in areas of within volcanic system s, and the difference be- recent volcanism in Italy and in the A zores; other tw een craters representing the w hole structure or results from Cam eroon have been used for com- pert aining to larger calderas is not supposed to 173 174 M. M artinietal. OCor¥vo A g O R E S O FIores Graciosa ¥~ e) ~'~ sanJ~~rg:e o Tercetrcl FoialO ~ ;pico _ san Miguel ~ ~~ ) S.t~a M aria ~ SeteCidades F'ogo Furnas S. M I G U E L L_ _ JloKm Fig. 1. Location of the craterlakesin the Azores. / N ¥?) O ･ s Y~ e eo LsENA <) evlco ~~ Ac~ANO 'OMARTIGNANO R ~ALBANO eN EMI T Y R R H E N I A N S E A RNO Napoli Fig.2. Location of the craterlakesin Italy. Craterlakesin A zores and Italy 175 affectto a significant extentthe chem ical charac - com m .). M artignano lake pertains to the sam e ters un der consideration. structure. The activity of A lban Hills volcanic Signi ficant volcani c activity isreported fort he com plex occur ed during the period 0.70-0.036 A zores, crossed in a north-sout h directi on by the m.y. b.p. with hydrom agm atic explosions from axis o f the M id- Atlantic ri dge (Fig. 1); besi des eccentric craters in its final phase; N em i and t he last surtseyan eru pti on of Capelinhos at Faial Albano lakes are filling tw o oft hese craters (D e in 1957-58 (M achado et al., 1962), strom bolian Rit a et a l., 1991). Phlegrean Fields are located activity w ith lava n ow s occurred in 1761 at inside a large caldera possibly produced during a Terceira, and four explos ive eru ptions arerecorded giant ignim brite eru ption about 0.035 m.y. ago for S. M iguel in 1563, 1638, 1640, and 1811 (R osietal., 1983). The crater hosting Lake A vern o (B ooth et al., 1983). Fogo lake fills the sum m it is considered as a m onogenetic tuff ring form ed depression of A gua de Pau volcano, about 3 km during the recent subaerial period; it is dated at about 3.800 y.b.p. The last eru ptive event in the w ide; its present m orphology results from area occur ed in 1538 with the build up of a 140 cratering, volcano-tectonic collapse, and landslides. Sete Cidades lake is located inside a polygenetic m high tuff cone. caldera, 5 km in diam eter, produced during a long series ofplinian events,the lastprobably occur ed SAM PLlNG AND A NALYTICAL PROCEDURES in 1444. Furn as lake occupies the sum mit ofthe eastern m ost active stratovolcano, tru ncated by a W ater sam pling has been carried out along a large polygenetic collapse caldera, about 6 km vertical profile above the deepest point of lake wide. A w ell doctim ented historical eru ption is bottom ; the position of the boat w as constantly reported in 1640; intense therm al activity is at checked by m eans of a portable G P S (Sony IPS- present occuring inside the caldera. 360). Sam ples from depth have been collected in W ith referenceto Italian lakes (Fig. 2) B olsena closed containers. Tem perature w as m easured in lake is a large volcano-tectonic depression, about situ, pH at the surface, at the sam e tim e of the 13 km in diam ete r,probably produced by a series collection of sam ples for dissolved C 02 determ i- of huge ignim brite forming eru ptions during the nation,in 100 m l glass bottles containing 10 m l period 0.60-0.35 m .y. b.p. (V arekam p, 1980; of a I M N a2C 03 solution. The C 032- excess Gillot et al., 1991). The volcanic activity contin- subsequent to the reaction: H20 + C0 2 + C0 32 ued in the we stern V ulsini area probably up to => 2H C 03- allow s then the calculation of dis- solved C 0 2. O.125 m.y.ago,w here the w ellshaped polygenetic L atera caldera develop ed (N appi et al., 1991; The analytical m ethods used are as follow s. Turbeville, 1 993). A nions and dissolv ed C 0 2 ~> A utom atic titration The activity of V i co volcano occu rred f rom with a Titroprocessor M etrohm 636 equipped with ion selective electrodes.C ations and B oron ~> Ion- 0.40 to 0.09 m .y. b.p.; its sum m i t appears as tru ncated by a polygenetic collapse caldera, about chrom atography with a Dionex D X IOO. N H4 ~> 8 kn wide(Bertagnini and Sbrana,1986),and half Potentiom etric determination. Li ~> A tom ic ab- of the caldera fl oor is occupied by the lake, sorp tion spectrophotom etry . probably established after a giant ignim britic eru ption dated atabout O.15 m.y.(Locardi, 1965). O BSERVATION D ATA AND DISCUSSION Explosive paroxysm al eru ption with subordinate effusive episodes characterized the Sabatini area The analytical results of m ajor com ponents for during a period of0.60 m .y., andthe developm ent the sam ples collected from crater lakes at the of the volcano-tectonic depression of Bracciano surface and at approxim ately the m axim um depth lake follow ed the exhaustion of the eru ptive ac- are given in Table 1. tivity about 40,000 years ago (D e Rita, pers. O ur att entio n has been m ainly focussed on an- 176 M. M artinietal, ~:) ~ O,r} ee ･~,-4cYl olr) -Lr~ (~ c~] C~ 0 ~0 00 a$ cs o o oe o (,o~ - c~ e~cf)Lr) C~](~1 - ~ ~rcf)cf) c~] cf) cf)~O tr)(~ -c~] c~] ~r~o- c~] (~ ~r~ ~ c~o~cr)~ ren~~o ~5 Oe C,O ~(~~¥ O Ot¥ Oecf)en ~l~･-1 =~¥ OLr)(~ c; c; c; c~ c; ~i -1,-4(~].~ rrl c,~ -.-1 ~t ~tcf)cf)(~ C~].-,C~(~~~¥O C:~cf] ~: ~:cf] o c~ c~ ~t¥o Lr O)eOe c:-)cfO) -Q V-) Cl)O~ (~ Qflcfl,N (~i O~ o - - oe (~ c~ o c~i(~j(Y~ cr; ~ ~o C;(:;(:i(:~ c; C; - (~ ~r ~r c~]cl~ - (~-c~f) (Lc+) ~ cf) - ~ ~rl/~ oe O O ~O Cr~ Cr~ ~r Otr)(~q¥O ~,r] ~r ~t ¥O O�(~ ~ O (~ -Lr) C~1(~ C~¥O ~,!)Lf) o ¥OLr~Lr)¥O~O eO OOcf) oo (~ Cn O Q~II!)~O ~ (~1C~ - -(~ C~l - -(~](~ - - cf) ~O = ¥C hC :~ ¥O O�e~ ~r cfl -Lr) o c~] ~ (:;,-1.-4 -~ ~~'-r)1.Lr~),c-n'.c-~1~,c-~~.c-~.-~,L-r1)'-1~,t-~¥.,-~1,(~-1~,c-f4],~-1o c¥i(~i a~ ~c)r ~:(e~n ~o 0 ~r ~o c~ cF~(~ t¥ cf! ~ c;cf;ef;(~ (~i[c~l c~~lt(e¥]n(cf~) c(~1r cn~(c~]1c(¥~]c(~l ~ro,~~-(~1 c-nc~~r '-1 c~¥c,c･f)~ ~t cr~ c~1.(~.~O r:th.