Geochemical Characterization of Naturally occurring ...

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occurring fluids on the Island of Pantelleria (ltaly) ... guite tra il 1987 ed il 1990, su campioni di fluidi prelevati sull'isola di ... The oldest outcropping rocks are.

Miner. Petrogr. Acta

VoI. XXXVII,

pp.

91-102

(1994)

Geochemical Characterization of Naturally occurring fluids on the Island of Pantelleria (ltaly) WALTER D'ALESSANDRO", GAETANO DONGARRÀ'd', SERGIO GURRIERP, FRANCO PARELLO""* & MARIANO VALENZA"-'''''' * Istituto di Geochimica dei Fluidi, C.N.R., via Torino 27d, 90133 Palermo, Italy ** Istituto di Scienze della Terra, Università di Messina, Salita Sperone 31, 98166 S. Agata di Messina, Italy *',,' Istituto di Mineralogia, Petrografia e Geochimica, Università di Palermo, via Archirafi 36, 90123 Palermo, Italy

RIASSUNTO - Vengono riportati i risultati di alcune indagini geochimiche, ese­ guite tra il 1987 ed il 1990, su campioni di fluidi prelevati sull'isola di Pantelleria. Sono stati analizzati 70 campioni di acque ipotermali e termali e 73 campioni di gas liberi e disciolti nelle acque. Oltre ad una sostanziale costanza di composizione nel tempo delle sorgenti termali, vengono conferma­ ti i principali processi che determinano le caratteristiche chimico-fisiche delle acque: l) mescolamento di acqua meteorica locale con acqua di mare; 2) inte­ razione acqua-roccia con arricchimento nella fase fluida di ioni sodio e potas­ sio e rimozione di magnesio; 3) apporto di CO2 profonda. Tutte le acque, eccet­ to due campioni (Nikà ed Arrnani), possono essere definite «acque immature» e quindi non adatte per considerazioni geotermiche. La composizione chimica della sorgente Nikà e del pozzo Arrnani suggeriscono un'equilibrio con le rocce ad una temperatura di circa 150-180°C. Le principali emissioni fumaroliche sono caratterizzate sia da bassi flussi che da basse temperature « 100°C). Calcoli geotermometrici indicano la probabile presenza in profondità di fluidi geotermici con temperature comprese tra 180 e 230°C. I rapporti isotopici 3He/4He e 13C/12C della CO2 suggeriscono, per questi due gas, un contributo di origine profonda. Termini chiave: Geochimica dei fluidi, acque termali Pantelleria,

geotermometria.

ASSTRACT - Between 1987 and 1990, samplings of both thermal and cold waters (total 70 samples) and free and dissolved gases (73 samples) were car­ ried out on the island of Pantelleria, located between Sicily and Tunisia. The analytical data point to the substantial geochemical constancy in tirne of the main thermal springs, which are characterized by the following geochemical processes: 1) mixing between local meteoric water and seawater; 2) water-rock interaction at high temperature with enrichment of Na- and K+ and depletion of Mg+' in the Iluids: 3) addition of CO2 of deep origino AlI but two water sam­ ples may be defined as «immature waters» unsuitable far the application or ionic solute geothermometers. Only the Nikà spring and Armani well seem to approach water-rock equilibrium, their equilibration temperature being 150180°C. The principal fumarole emanations are characterized by low flow and low emission temperature (dOO°C). Geothermometric calculations indicate a fluid source with a temperature in the range 180-230°C. The carbon isotope ratios (13C/12C) or CO2 and isotope ratios or helium (3He/4He) indicate the deep origin of these two gases. Key Words: Gas geochemistry,

Introduction

Pantelleria lies on a submerged conti-

water geochcrnistry,

thermal waters Pantelleria.

nental rift in the Strait of Sìcily, between Sicily and Tunisia, It is the emerging sum­ mit of a very large submarine volcano rising from the seafloor for about 2000 m,

92

w D'Alessandro,

G. Dongarrà, S. Gurrieri, F. Parello, M. Valenza

of which only 836 m are above sea level. From the geodynamic viewpoint, Pantelleria may be considered an in tra­ plate volcano, being inside the African plate and lying on a thin crust of conti­ nental type. Crustal thinning is due to a distensive tectonic regime which started at the beginning of the Pliocene, is still active, and led to the formation of the Strait of Sicily rift (Colombi et al. 1973, Cassinis et al. 1979, Finetti 1984). The island is characterized by a bimodal association of basaltic and trachytic-rhyoli­ tic products, the latter prevailing (approx. 94% outcropping) (Rittmann 1968, Villari 1974). The oldest outcropping rocks are about 300,000 years old (Civetta et al. 1984). The most recent paroxysmic activity was the submarine eruption of 1891, 7 km north-west of the island. More recent subaerial activity produced the scoriae and basalt flows of Mursia (Civetta et al. 1984). The products of explosive activity are quite widespread, the most recent being about 5000 years old (Mahood & Hildreth 1986). On the basis of geochemical considera­ tions on the evolution of the Pantelleria magmas, some authors (e.g. Civetta et al. 1988) believe that explosive activity will probably recommence in the next 2000 years. The present activity is mainly cha­ racterized by the presence of some thermal waters and some fumarolic manifestations. Pantelleria has no surface waters at all, due both to low rainfall (about 350 mm/year) and to the high permeability of its rocks. The only surface water is repre­ sented by a lake, an endoreic basin inside a caldera, fed by rainfall and thermal springs (Azzaro et al. 1983). The «fresh water. aquifer of the island is quite thin and becomes significantly larger only near the town of Pantelleria. The springs, all of

small discharge, are located near the coast. This paper reports the chemical and isotopic composition of the natural occur­ ring fluids on the island and discusses their source and geochemical evolution. Sampling and Analytical Methods Fig. 1 shows the sampling sites of wells and springs and of free and dissolved gases. Water temperature, pH and con­ ductivity were measured in the field at the time of collection. Laboratory determina­ tions were carri ed out by means of atomic spectrophotometry and ion chrornato­ graphy. Alkalinity was determined by titration and silica by colorimetry with ammonium molybdate on diluted sarn­ ples. Samples were prepared for 1)'80 analysis according to Epstein & Mayeda (1953) and measured with a mass spectro­ meter Varian Mat 250. The standard reproducibility for 1)'80 values was ± 0.2 %0 (la): the oxygen isotope values are reported versus the V-SMOW standard.

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