Results And Discussion 41 Panarea island

In proximity of Panarea island (Aeolian islands, Southern Italy) a huge submarine volcanic-hydrothermal gas burst occurred during November, 2002. The high-pressure gas release created sinkholes with the collapse of the seafloor. During the first months of Panarea "activity", some vertical logs were performed in the area characterised by gas emissions (Fig. 2) which showed the presence of convective cells containing a mixture of water and gas from the vents. Furthermore, they revealed a modification of sea water pH from 8.0 to 5.0 and Eh from +80 mV to -200 mV, probably due to the presence of H2S in the emission points. The STS data logger permitted the temperatures of the gas emission points to be measured. The data acquisition interval was one measurement per hour in order to have a correlation with sea tide fluctuations. The gas emission temperatures range from 30 to 90°C, except for the "black point" whose mean value was around 120°C.

More than 100 samples (both water and gas) were collected from November 2002 to December 2004. Analyses were performed in order to determine major and minor elements, trace elements, dissolved gases in water and gases in free phase. Results suggest that the sea water was locally affected by the acidic, reducing and more saline fluids that circulated at high temperatures in the volcanic rocks.

Figure 2. Panarea island location: on the right a detailed map of the studied area where gas emission points have been monitored since November 2002.

Gas samples (Tab.1) are mainly composed of carbon dioxide (98%, v/v) and methane (around 10 ppm) except for the "black point", whose mean value is around 600 ppm.

Figure 3 shows the direct relationship between temperatures and CH4 content in the "black point", suggesting the thermogenic origin of the methane. Slightly elevated helium concentrations (mean value = 11 ppm) were found at every point. He isotopic analyses were performed on 32 samples in order to define its origin, with values from 4.1 to 4.5, suggesting a magmatic origin. Temporary variations of both H2 and H2S content have the same trend, inferring fractionation of gases by partial dissolution in water. The observed gaseous and chemical composition of the Panarea emission points is very different from that reported in the literature (Fig. 4), as data collected before November 2nd, 2002 have values typical of hydrothermal fields (represented by N2), whilst samples collected after the recent gaseous emission have concentrations typical of volcanic fields (high He concentrations). This data suggests an evolution of the system controlling the Panarea emissions.

Gas species

Mean value

CO2 (vol., %)

98

CH4 (ppm)

10

N2 (vol. %)

0.4

He (ppm)

11

H2 (ppm)

1100

H2S (vol. %)

2.2

130 140 130 120 110 100

—•— Tot parature —#_ Met tiara

/ / / /

/

/ / * /

\ /

J \v/ V

Epr-03 june-03 juhpC® sept-03 may-04 jufy-04

Epr-03 june-03 juhpC® sept-03 may-04 jufy-04

0.07

0,04

Figure 3. Comparison between methane (%, v/v) and temperature variations (°C) at the "black point" (Panarea island). The observed direct relationship infers a thermogenic origin of the methane gas.

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