Environmental radioactivity at Stromboli (Aeolian Islands)

Applied Radiation and Isotopes 57 (2002) 99–107

Environmental radioactivity at Stromboli (Aeolian Islands) M. Braia,*, S. Basilea, S. Belliab, S. Hauserb, P. Puccioc, S. Rizzoc, A. Bartolottad, A. Licciardellob a

Dipartimento di Fisica e Tecnologie Relative and Unita" INFM, Universita" di Palermo, Via G. Parlavecchio 3, 90127 Palermo, Italy b Dipartimento di Chimica e Fisica della Terra, Universita" di Palermo, Via Archirafi 36, 90123 Palermo, Italy c Dipartimento di Ingegneria Nucleare, Universita" di Palermo, Viale delle Scienze, 90128 Palermo, Italy d Dipartimento Farmacochimico, Tossicologico e Biologico, Universita" di Palermo, Via Archirafi 32, 90123 Palermo, Italy Received 16 July 2001; received in revised form 20 September 2001; accepted 10 October 2001

Abstract HPGe gamma spectrometry, thermoluminescence dosimetry, X-ray diffractometry and fluorescence techniques have been used to analyze the natural radionuclides content of soil and rock samples, air kerma and geochemical features on the island of Stromboli, belonging to the Aeolian Islands, in the Mediterranean Sea. The 214Bi, 238Ac, and 40K contents obtained are in agreement with the magmatic evolution of the rock formation, as shown by the correlations between radionuclide and chemical elements abundacies, depending on the various magmatic differentiation mechanisms. Correlations between radiometric, lithological and geochemical data have been assessed in order to obtain some hints on the geochronology of the magmatic products. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Environmental radioactivity; Thermoluminescence; Magmatic rocks; Geochronology

1. Introduction Among primordial radionuclides, 238U, 232Th, and K mainly contribute to the total dose from natural background. Acid igneous rocks generally exhibit high concentrations of these radionuclides with respect to the basic ones as pointed out by Larsen and Gottfried (1960). Several papers (Civetta et al., 1970; Civetta and Gasparini, 1973; Brai et al., 1995; Bellia et al., 1997) have confirmed that some of the Southern Italy volcanoes are generally characterized by high contents of 238U, 232Th, and 40K. Most of those studies have dealt with the understanding of magmatological processes affecting the enrichment of primordial elements in the rocks (Bellia et al., 1996). Moreover, the study of parent–daughter radioactive systems in rock samples provides two type of information: the timing of geological processes giving rise to their formation (Condomines and Allegre, 1980; 40

*Corresponding author. Fax: +39-091-6552949. E-mail address: [email protected] (M. Brai).

Allegre and Condomines, 1976; Gillot and Keller, 1993) and indications on the chemical changes occurring during such processes (Hornig-Kjarsgaard et al., 1993). The main scope of the present work was to determine the activity of some radioactive daughters of the U–Th system in recent and in older ones volcanic rocks of Stromboli. The radionuclide activities were correlated with the age and magmatic evolution of this volcanic island. Comparisons were made with other data of another island of the Aeolian archipelago, namely, Vulcano.

2. Geological settings The Aeolian Islands are a volcanic arc of seven islands and three seamounts, located on the south-eastern continental slope of the Tyrrhenian Sea abyssal plate. Of its seven islands, only Vulcano and Stromboli have erupted recently. Stromboli, particularly, is well known for its continuous explosive activity.

0969-8043/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 0 2 ) 0 0 0 7 4 - X

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M. Brai et al. / Applied Radiation and Isotopes 57 (2002) 99–107

The island of Stromboli is the northernmost island of the Aeolian arc, in the Mediterranean Sea, north-east of Sicily. The island extension is about 12.2 km2, with an almost circular shape and a maximum height of 924 m above the sea level. The base of the volcano cone is located at about 1500 m under the sea level and is regularly shaped up to the top. The island can be divided into two large areas, an older one on the east that comprises two volcanic units belonging to different eruptive cycles: Paleostromboli (PS) and Scari-Vancori (SV). The younger part, classified according to its main features, is Neostromboli-Recent Stromboli (NS-RS). The geochemical evolution is characterized by strictly calcalkaline compositions in the oldest lavas and most of them are either K-rich andesites or Al-rich basalts and some rare shoshonites (Hornig-Kjarsgaard et al., 1993). The lavas from the younger area are shoshonitic with relatively low content of SiO2 and high K2O levels and a K2O/ Na2O ratio close to unity. However, detailed mapping and chemical characterization (Francalanci et al., 1993; Hornig-Kjarsgaard et al., 1993) have shown a progressive enrichment in K. A simplified map showing the main geochronological units and the sampling sites is shown in Fig. 1. The

squares indicate calcalkaline lavas (CA); the circles represent calcalkaline high in K series (HK-CA); the triangles are for the shoshonitic rocks (SHO). Filled symbols have been used to indicate soil samples (sites 24, 31 and 35). The numbers refer to the samples as in Table 1.

3. Materials and methods Twenty-nine rocks and three soil samples have been collected, representative of the different units and geochemical series (see Fig. 1). The specific activities were evaluated by g-ray spectrometry on samples crushed or milled into a powder with a particle size of o5 mm. Samples were dried and sealed into a ‘‘Marinelli’’ beaker. Measurements were performed after 20 days to be sure that secular equilibrium between 226Ra, 222Rn and its daughters had been reached. Spectrometry measurements of g-activity were carried out using an HPGe detector, with a relative efficiency of 32% and a resolution of 1.8 keV. It was calibrated against a standard soil (NBS SRM 4353) containing all the radionuclides of interest. The activities of 226Ra and 232Th were computed from the 214Bi

Fig. 1. Simplified sketch map of Stromboli with distribution of the main volcanic periods. The squares are for the calcalkaline rocks (CA); circles are for high in K calcalkaline rocks (HK-CA); triangles are for shoshonitic rocks (SHO). Filled symbols indicate soil samples. Horizontal shaded areas belong to the Paleostromboli unit; vertical ones belong to the Scari-Vancori period; the unshaded areas belong to the Neostromboli-Recent Stromboli period.

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Table 1 Major-element data of the Stromboli volcano. Data are grouped by geochronology and geochemical series of the investigated samples. The values (in wt%) refer to anhydrous samples Site STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR

01 21 22 23 02 03 04 05 20 25 26 27 28 13 14 29 30 16 17 18 19 06 07 08 09 10 11 12 15

Unit

Geochemical series

SiO2

TiO2

Al2O3

Fe2O3

MnO

MgO

CaO

Na2O

K2O

P2O5

Recent Stromboli Recent Stromboli Recent Stromboli Recent Stromboli Neostromboli Neostromboli Neostromboli Neostromboli Neostromboli Neostromboli Neostromboli Neostromboli Neostromboli Vancori Vancori Vancori Vancori Scari Scari Scari Scari Paleostromboli Paleostromboli Paleostromboli Paleostromboli Paleostromboli Paleostromboli Paleostromboli Paleostromboli

HK-CA HK-CA HK-CA HK-CA SHO SHO SHO SHO SHO SHO SHO SHO SHO SHO SHO SHO SHO HK-CA HK-CA HK-CA SHO HK-CA CA CA CA HK-CA HK-CA CA HK-CA

51.80 51.97 47.31 48.34 54.34 53.72 55.01 52.27 52.43 52.16 52.42 52.30 51.86 52.84 60.74 52.81 57.85 53.43 52.96 51.43 54.51 55.79 57.41 53.68 55.70 60.34 59.85 51.03 57.72

0.78 0.78 0.73 0.81 0.91 1.09 0.83 0.97 0.94 0.95 0.90 0.93 0.97 1.07 0.70 0.77 0.81 1.14 0.98 0.96 0.80 0.81 0.93 0.74 0.68 0.72 0.77 0.97 0.87

18.52 18.33 11.07 12.95 19.15 20.23 20.61 18.28 18.21 17.73 17.26 17.92 17.15 19.37 17.61 18.27 18.26 17.58 18.63 18.34 17.56 17.88 18.72 16.89 16.25 17.62 17.95 18.67 18.14

8.45 8.36 13.59 11.67 7.49 8.95 6.68 8.68 8.24 8.68 8.42 8.56 8.89 8.89 5.87 6.79 7.02 9.80 8.36 9.33 9.02 7.72 7.56 8.90 8.12 6.08 6.47 10.32 7.18

0.17 0.20 0.33 0.29 0.18 0.22 0.15 0.21 0.18 0.20 0.20 0.20 0.20 0.21 0.20 0.18 0.17 0.23 0.19 0.23 0.16 0.20 0.17 0.25 0.21 0.19 0.19 0.24 0.21

5.83 6.12 13.62 11.21 2.65 2.74 2.10 3.77 4.11 4.55 4.78 4.37 4.92 3.06 1.68 2.03 1.93 4.30 4.36 4.87 3.02 3.96 3.44 6.02 5.69 2.29 2.39 5.57 2.88

10.32 10.14 10.75 11.57 7.52 7.63 7.22 8.90 9.03 9.22 9.43 9.21 9.49 8.64 4.86 6.20 6.39 8.82 9.02 9.92 6.45 8.54 6.73 9.67 9.33 6.03 6.14 9.60 7.13

2.38 2.36 1.33 1.60 2.86 2.07 2.93 3.22 3.05 2.39 2.35 2.28 2.30 3.05 4.13 3.96 3.57 2.29 2.91 2.75 4.31 2.87 2.84 2.37 2.47 3.69 3.37 2.07 3.25

1.58 1.55 1.05 1.28 4.46 2.98 4.13 3.16 3.31 3.59 3.69 3.69 3.73 2.45 3.93 3.57 3.65 2.14 2.31 1.90 3.75 2.02 1.96 1.33 1.42 2.81 2.64 1.29 2.36

0.18 0.20 0.22 0.28 0.45 0.38 0.33 0.55 0.50 0.51 0.53 0.53 0.48 0.41 0.31 0.34 0.36 0.29 0.26 0.26 0.42 0.21 0.24 0.14 0.13 0.22 0.22 0.23 0.24

609 keV and 228Ac 911 keV lines, respectively. If secular equilibrium in the materials examined does exist, the measured values of 226Ra should correspond to those of 238 U. The minimum detectable activity, for a 1000 min counting time, ranged between 3.2 and 8.9 Bq kg 1, 0.26 and 0.61 Bq kg 1, 0.59 and 1.8 Bq kg 1, for 40K, 228Ac, and 214Bi, respectively. Spectrometry data have been corrected to take into account different sample density (which ranged between 0.6 and 1.6 g cm 3), according to a method developed by some of us (Rizzo and Puccio, 1992). Mineralogical and chemical features of the samples were determined by X-ray diffractomety (XRD) and fluorescence (XRF) analyses. The air kerma due to the natural g terrestrial radiation were also measured by thermoluminescent detectors. The chips used were LiF:Cu,Mg,P (GR-200A), of disk shape with diameter of 4.5 mm and thickness of 0.8 mm. Each dosimeter contained three thermoluminescent chips packed with 40 mg cm 2 opaque cardboard to

avoid environmental influence (light, humidity, dust, etc.). Measurements were peformed using a thermoluminescence dosimetry (TLD) reader (Model 7300C, Teledyne Isotopes, 50 Van Buren Ave, Westwood, NJ 07675, USA) without N2 flow. The lowest detectable dose was 0.19 mGy. Error levels were evaluated to vary within 8.3 15.5%. The calculation of TLD error in a single measurement includes the standard error (78%) of the calibration curve. For calibration, a 137Cs source of 1 GBq (27 mCi) activity, calibrated against an ionization chamber was used. The dosimeters, contained inside a 160 mg cm 2 thick plastic tube, were placed in 15 sites, which in order to get comparable data, were chosen to be the same where rock and soil samples were collected. Dosimeters were left in the field for 1 year at about 1 m above the ground. All the TLD readings can be considered to underestimate the real dose by a factor of about 13%, due to fading during exposure time.

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4. Results and discussion Table 1 shows the major-element data, obtained by XRF technique. The values, in wt%, have allowed us to classify the samples in five main units. The K2O–SiO2 diagram, as generally recommended for orogenic magmatic rocks (Peccerillo and Taylor, 1976), has also been used, as shown in Fig. 2. The reason to use the K2O–SiO2 diagram is linked to the extreme variability of K content in products of Stromboli volcanism. Data in the diagram shown in Fig. 2 are grouped for the three time cycles, Paleostromboli, ScariVancori cycles, Neostromboli and Recent Stromboli. It is evident from the figure that the geochemical evolution in time is far away from linear and it is marked by sudden changes in magma composition (Pasquare" et al., 1993). Table 2 shows the radionuclide specific activities for rock and soil samples grouped according to the three geochronologic units and the three major geochemical series. Samples STR 24, 31 and 35 are soils. Specific activities have been found to range between 31 and 112 Bq kg 1 for 214Bi, between 30 and 106 Bq kg 1 for 228Ac, and between 340 and 1427 Bq kg 1 for 40K. These values are well above (about 50 times larger) than the lowest detection limits.

In the same table the comparison between the computed and measured air kerma values is shown in the last two columns. Air kerma values, measured by thermoluminescence technique, and after subtraction of cosmic contribution, ranged between 0.8 and 1.3 mGy yr 1, and the calculated values between 0.5 and 1.4 mGy yr 1. The conversion factors used to compute the air kerma per unit of specific activity of natural g-ray sources, uniformly distributed in the ground, were: 41.7 pGy h 1 for 40K, 463 pGy h 1 for 238U and 604 pGy h 1 for 232Th (Saito and Jacob, 1995). The discrepancies between measured and calculated air kerma values can be due to the treatment of the sample before the measurements. For example, the different densities of powdered and dried samples with respect to ‘‘in situ’’ ones, lead to an overestimation of fluence and air kerma rate. Moreover, it must be pointed out that the prevalent contribution to the exposure is due to the superficial part of the outcropping rocks. The presence of soil layers hence could influence considerably the experimental results. Figs. 3–5 show the radionuclide content vs. SiO2. The straight line is a linear fitting of SV data. For all three radionuclide specific activity values, it is evident that the straight line acts as a separation line between older and younger magmas. It is evident the absolute lack of

5 Shoshonite Latite

4

Basalt Shoshonite

K2O (%)

3

HK-CA Andesite HK-CA

2 NS-RS

HK Basalt

Andesite

1 Basalt

Basalt Andesite

SV PS

0 50

55

60

65

SiO2 (%) Fig. 2. K2O vs. SiO2 diagram. PS, SV and NS-RS are for Paleostromboli, Scari-Vancori and Neostromboli-Recent Stromboli periods.

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Table 2 Specific activities of 214Bi, 228Ac, 40K in rock and soil samples. Data are grouped according to Table 1. The calculated and measured annual air kerma (in mGy yr 1) values are reported in the last two columns Site STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR STR

01 21 22 23 24 02 03 04 05 20 25 26 27 28 31 35 13 14 29 30 16 17 18 19 06 07 08 09 10 11 12 15

Unit

Geochemical series

214

Bi (Bq kg 1)

228

Ac (Bq kg 1)

40

K (Bq kg 1)

Measured kerma (mGy yr 1)

Calculated kerma (mGy yr 1)

Recent Stromboli Recent Stromboli Recent Stromboli Recent Stromboli Recent Stromboli Neostromboli Neostromboli Neostromboli Neostromboli Neostromboli Neostromboli Neostromboli Neostromboli Neostromboli Neostromboli Neostromboli Vancori Vancori Vancori Vancori Scari Scari Scari Scari Paleostromboli Paleostromboli Paleostromboli Paleostromboli Paleostromboli Paleostromboli Paleostromboli Paleostromboli

HK-CA HK-CA HK-CA HK-CA Soil on HK-CA SHO SHO SHO SHO SHO SHO SHO SHO SHO Soil on SHO Soil on SHO SHO SHO SHO SHO HK-CA HK-CA HK-CA SHO HK-CA CA CA CA HK-CA HK-CA CA HK-CA

52 52 67 96 112 106 78 101 79 79 89 86 88 88 91 98 66 97 88 76 47 60 45 94 48 49 45 31 58 60 44 54

45 44 30 41 67 99 73 94 80 77 75 73 76 74 68 69 69 106 93 81 54 65 52 93 50 54 48 32 66 68 48 60

545 533 340 457 764 1427 910 1386 1048 1095 1173 1190 1163 1157 752 747 831 1215 1081 989 652 757 595 1309 659 624 690 500 894 876 455 777

884 n.a. 823 986 1109 n.a. n.a. 997 989 1157 n.a. n.a. n.a. n.a. 1211 1085 1278 n.a. n.a. n.a. 1008 n.a. 1029 n.a. n.a. n.a. 1002 n.a. n.a. n.a. 781 1076

649 n.a 555 775 1089 n.a n.a 1413 1127 1127 n.a n.a n.a n.a 1006 1037 935 n.a n.a n.a 714 n.a 678 n.a n.a n.a 685 n.a n.a n.a 600 820

correlation for data from rocks of NS-RS unit, pointing out that there is a poor differentiation in more recent lavas. Fig. 6 shows the correlation between the specific activities of 228Ac and 214Bi. There is not a linear correlation due to two reasons, the lack of secular equilibrium for U and Th family and the inhomogeneous magmatic evolution and hence differentiation of lavas. Nevertheless, a strong correlation is evident for older lavas (PS). The fitting parameters are: 0.77 for the slope, 0.99 for the correlation coefficient and 1.9 for the w2 value. The radiometric evolution in time (Fig. 7), as represented by 228Ac/214Bi ratio vs. age, is far away from linear and it is marked by sudden changes going from recent magmas to ancient ones. The scarcity of highly evoluted products is well pointed out by comparing the 214Bi vs. 228Ac diagram

of the Stromboli survey with the data of radiometric measurements on rock samples from Vulcano island (Fig. 8). The only degree of differentiation observed in the oldest lavas of Stromboli, generally remains in the range basalt–basaltic andesite, while the evolution of Vulcano lavas reaches also the higher acidic levels.

5. Conclusions Differences in the volcanism of arcs with physical similarities have been already reported in literature, for instance between the Cascades arc in Northern America and the western Mexican subduction zone (Righter, 2000). Using radiometric and X-ray techniques we have shown that, although Stromboli and Vulcano islands belong to the same volcanic arc and so of the same

M. Brai et al. / Applied Radiation and Isotopes 57 (2002) 99–107

104

150 NS-RS SV PS SOIL

214

Bi (Bq m-3)

100

50

0 45

50

55

60

65

SiO2 (%) Fig. 3. 214Bi content vs. SiO2 weight percent for the rock samples. PS, SV and NS-RS as in Fig. 2. A linear fitting of SV data is also shown.

150

228

Ac (Bq kg-1)

NS-RS SV PS SOIL 100

50

0 45

50

55

60

65

SiO2 (%) Fig. 4. As in Fig. 3 for

geodynamic environment, they each generate eruptive products that are characterized by different levels of primordial radionuclides.

228

Ac.

On the other hand, the non-linear correlation of shoshonitic products of Stromboli suggests the existence of complex evolutive processes.

M. Brai et al. / Applied Radiation and Isotopes 57 (2002) 99–107

105

2000 NS-RS SV PS

SOIL

1000

40

K (Bq kg-1)

1500

500

0 46

48

50

52

54

56

58

60

62

SiO2 (%) Fig. 5. As in Fig. 3 for

40

K.

214

Bi (Bq kg-1)

150

100

50 NS-RS SV PS 0 0

50 228

Fig. 6.

214

Bi content vs.

100

150

-1 Ac (Bq kg )

228

Ac content for rock and soil samples (filled symbols).

These processes have modified the primary basaltic magma generated by a homogeneous or inhomogeneous mantle source.

According to some authors (Francalanci et al., 1993; Hornig-Kjarsgaard et al., 1993), the following mechanism has been suggested: the heterogeneous

M. Brai et al. / Applied Radiation and Isotopes 57 (2002) 99–107

106

228

Ac /

214

Bi

1

0.8

SHO

HK-CA

0.6

CA

10

3

10

4

10

5

age (year)

Fig. 7.

228

214

Ac/

Bi ratio vs. estimated age.

250

214

Bi (Bq kg-1)

200

150

100

50

0 0

50

100 228

150

200

250

-1

Ac (Bq kg )

Fig. 8. A comparison of

214

Bi content vs.

228

Ac content for Stromboli (symbols as in Fig. 6) and Vulcano samples (black points).

mantle source generates different primary magmas, the magmas mix, and afterwards the contamination and fractional crystallization under different pressure, temperature and O2 conditions take place.

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