Geofluids
Volume 2019 (2019), Article ID 6250393, 17 pages
https://doi.org/10.1155/2019/6250393

Research Article

Evidence of Tectonic Control on the Geochemical Features of the Volatiles Vented along the Nebrodi-Peloritani Mts (Southern Apennine Chain, Italy)

Francesco Italiano,¹ Pietro Bonfanti,² and Salvatore Roberto Maugeri²

¹Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Palermo, 90146, Italy
²Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania Osservatorio Etneo, 95125, Italy

Correspondence should be addressed to Pietro Bonfanti; pietro.bonfanti@ingv.it

Received 15 February 2019; Revised 30 April 2019; Accepted 30 May 2019; Published 4 July 2019

Academic Editor: Andrew H. Manning

Copyright © 2019 Francesco Italiano et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Investigations carried out over the southernmost portion of the Apennine chain (Nebrodi-Peloritani Mountains, Sicily, Italy) reveal a close connection between the tectonic setting and the regional degassing of CO₂-dominated volatiles. The geochemical features of the collected gases show that the pristine composition has been modified by gas-water interaction (GWI) and degassing processes. The ³He/⁴He isotopic ratio in the range of 0.7-2.8 Ra highlights variable contributions of mantle-derived helium, representing an unusual feature for the crustal regime of the study areas characterized by the widespread presence of ⁴He-producer metamorphic rocks. The degassing of mantle helium is coherent with the tectonics and related to the NW-SE extensional regime of the Calabro-Peloritan Arc (CPA). We propose that the degassing regime as well as the geochemical features of both the dissolved and bubbling gases is closely connected to the strain accumulation rate, inducing almost no temporal changes and insignificant deep-originated fluid contributions to the locked fault volumes. Investigations including discrete and continuous monitoring and degassing-rate estimations are useful tools to gain a better insight into the evolution of seismogenesis, considering the fault rupture as the final stage of a seismic cycle.

1. Introduction

Results of studies on fluid/fault relationships have widely shown the role of fluids both as triggering agents of seismic shocks and as fast carriers of information on processes occurring at deep levels, making them useful tools in gaining a better insight into the evolution of seismogenic processes. The Apennines are well known to be a still-developing chain, with tectonic movements driven by a large number of active faults often recognized as seismogenetic [1, 2]. The Southern portion of the Apennines, the Calabro-Peloritan Arc (CPA), develops over an area that has been struck by some of the most destructive seismic events ever seen in Europe (e.g., Messina 1908 and Calabria 1905, 1783). Indeed, its complicated tectonic setting has been satisfactorily constrained only in recent times and remains under investigation. The CPA is a major tectonic structure running across northeastern Sicily and Calabria (Figure 1). The Nebrodi and Peloritani mountains stretch E-W along the Northern Ionian and Tyrrhenian coasts for about 100 km.

Figure 1: Sketch map of the study area (Nebrodi-Peloritani Mountains, Southern Apennines, Sicily, Italy) with the location of the sampling sites and the main tectonic lines (red lines). The beach balls show the focal mechanisms associated with some main shocks. Black square: spring; blue triangle: well; green circle: bubbling gas.

The presence of thermal springs besides the degassing occurring over some areas (evidenced by bubbling gases in thermal waters and shallow sea waters) is a clue to the close connection of the fluids’ circulation pattern with the local tectonic structures. This paper accounts for the results of a fluid collection carried out over the Nebrodi and Peloritani Mts over the period 2004-2006. More samples collected in 2002, 2003, and 2007 were added to the time series of some of the most important sites. The survey aimed at defining the origin as well as the interactions of the fluids circulating over the area in order to evaluate possible relationships with local tectonic structures.

Improving the knowledge of the geochemical features of the fluids and their behaviour in the geodynamic context of the area can provide new information leading to a better understanding of local tectonic settings along with new tools for an insight into the development of the seismogenesis (stress accumulation, deformation, strain release, etc.).

The study area is characterized by the venting of geothermal fluids over both the Tyrrhenian and Ionian coasts, as well as by gas emissions over areas crossed by the Aeolian-Tindari-Letojanni fault (ATLF) system (Figure 1). The gases are mainly dissolved in groundwater and vented as a separate phase only in a few sites.

2. Geologic and Tectonic Setting

The current tectonic framework of the Calabro-Peloritan Arc (CPA) results from the N-S Africa-Eurasia convergence during the Neogene-Quaternary at a rate of 1–2 cm/yr during the last 5–6 My [3–7]. Despite this, a rapid E to SE motion affected the CPA at a rate of 5–6 cm/yr, with an uplift between 0.5 and 1.2 mm/year, in the last 1–0.7 My, mainly accommodated by normal faulting [8–10]. This NE corner of the chain exhibits an uplift with the highest rate in proximity to the Messina Strait (along the Ionian coast) and lower uplift rates along the Tyrrhenian coasts [11].

The motion is related to the roll-back of the subjacent Ionian transitional to oceanic slab and back-arc expansion in the Tyrrhenian Sea [12–14]. During the middle-late Pleistocene, roll-back and subduction slowed to less than 1 cm/yr [15].

The current structural framework of northern Sicily is the result of the Plio-Pleistocene activation of a complex network of fractures related to a W-E trending right-lateral regional shear zone extending from the Pantelleria Rift to the Aeolian Islands [16–20]. Some of these structures are still active and responsible for the shallow seismicity occurring both in inland Sicily and in the northern offshore in the Tyrrhenian Sea. Focal mechanisms are typically characterized by strike-slip and oblique kinematics consistent with low-dip NW-SE to NNW-SSE trending -axes [21–26], roughly consistent with the global convergence direction between the European and African plates [11, 27–31].

The seismological and geodetic data depict two main crustal domains marked by different stress regimes: a compressive domain in the northern Sicilian offshore and an extensional domain in northeastern Sicily and southern Calabria [24, 32–36]. The transition between the two domains occurs along the Aeolian-Tindari-Letojanni fault (ATLF) system which has been interpreted as a transfer crustal zone between the northern Sicily offshore thrust belt in the Tyrrhenian Sea and the accretionary wedge offshore the eastern Calabria in the Ionian Sea [29, 32, 37] or as a lithospheric tear fault bounding the western edge of the subducting Ionian slab [13, 38–41]. Extension, however, although at an immature tectonic stage, also occurs in a narrow band to the west of the ATLF in northern Sicily as documented by seismology and structural studies [25, 42]. In the field, the ATLF is formed by NW-SE-oriented en echelon segments characterized by prevailing right-transpressional movements in the Aeolian sector [43, 44] and by transtensional motion in the northern Sicily sector [45]. Seismological, geological, and geodetic data evidence that the ATLF is very active with more than 1500 earthquakes () occurring in the past 30 years in its northern portion. Fault plane solutions reveal prevailing normal faulting coupled with dextral solutions along the inland part of the ATLF [25]. Figure 1 summarizes the above-mentioned information alongside the distribution of the sampling sites.

Recent geodetic observations coupled to geological constraints have served to better elucidate the interplay of crustal blocks of the Nebrodi-Peloritani area [46]. The ATLF juxtaposes north-south contraction between Sicily and the Tyrrhenian block with northwest-southeast extension in northeastern Sicily and Calabria (Nebrodi-Peloritani chains). As a matter of fact, the Africa-Eurasia convergence in Sicily and southern Calabria is nowadays expressed by two different tectonic and geodynamic domains: a roughly N-S compression over the western region caused by continental collision and a NW-SE extension to the east (Calabro-Peloritan Arc) related to the S-E-directed expansion. The ATLF right-lateral shear zone accommodates the different deformation patterns of these two domains from the Ionian Sea (north of Mt. Etna) to the Aeolian Islands across the Peloritani chain, thus crossing our study area.

3. Methods

3.1. Field Investigations and Sample Collection

A suite of 158 samples taken at 67 different sites has been collected along the Nebrodi-Peloritani chain, the southernmost portion of the Apennines. They include natural springs, fountains, and boreholes as well as bubbling gases from thermal and cold ponds spread over an .

Table 1 lists the sample locations, the coordinates (in WGS84 notation), and the field data; Table 2, the analytical results for the dissolved gases (43 from the Nebrodi sector and 19 on the Peloritani sector); and Table 3 (8 different sites), the bubbling (free) gases. The samples are listed using ID numbers as well as the site names. The ID number identifies the site; thus, bubbling and dissolved gases taken at the same site display the same ID number but are listed in different tables.

Table 1: List of the sample sites with their location and field data.

Sample #
Site name
Type
Latitude
Longitude
Date
T (°C)
pH
EC (μS cm⁻¹)
Eh (mV)

1
Boffe
S
4218339
493362
31/05/2006
17.3
6.6
567
11.7

2
Usignolo
S
4223182
495604
31/05/2006
14
7.4
307
183

3
Belvedere
W
4213960
500401
31/05/2006
n.d.
6.2
373
n.d.

4
Margherita
S
4223774
494811
31/05/2006
17.2
6.8
n.d.
n.d.

5
Lettosanto
S
4203485
446098
26/10/2006
14.5
8.1
430
320

6
Sampieri
S
4201594
453979
26/10/2006
22
7.2
313
141

7
Leone
S
4215364
476353
26/10/2006
15
7.7
439
234

8
S. Mamma
S
4201769
463060
26/10/2006
17.9
7.8
743
251

9
Ramosa
S
4210970
459344
26/10/2006
18.9
6.6
795
-18.7

10
Ficuzza
S
4209011
455969
31/10/2006
20.1
6.6
807
218

11
Moglie
S
4201393
467921
31/10/2006
13.1
8.1
286
256

12
Piliegi
S
4200639
469645
31/10/2006
13.4
7.2
454
158

13
Cesarò
S
4188208
475206
31/10/2006
15.7
7.4
414
153

14
Torti
S
4194100
469379
31/10/2006
10.1
7.1
117.4
163

15
Muto
S
4199383
467806
31/10/2006
11.2
7.5
224
151

16
Sulipani
S
4210175
493398
16/11/2006
12.7
6.8
251
244

17
Giangalia
S
4206137
494519
16/11/2006
10.1
8.0
333
297

18
Pace
S
4204674
492680
16/11/2006
11.3
8.0
278
195

19
Sfaranda
S
4212196
485181
16/11/2006
14.9
7.1
349
212

20
Agrifoglio
S
4210452
471272
16/11/2006
15.9
7.9
983
195

21
Mezzalora
S
4188364
467379
22/11/2006
9.7
8.4
487
59

22
Margio Salice
S
4192999
484963
22/11/2006
11.9
7.9
511
124

23
Chiusitta
S
4197148
484907
22/11/2006
9
7.5
382
172

24
Tre Arie
S
4198001
486223
22/11/2006
9
8.0
291
58

25
Zarbata
S
4196816
490269
22/11/2006
9.7
7.3
330
196

26
S. Domenica
S
4209724
470900
22/11/2006
13.8
7.5
358
205.8

27
Piano Soprano
S
4207559
489767
22/11/2006
11.7
7.4
397
220

28
Casitti
S
4208940
487512
22/11/2006
11.4
8.0
287
193

29
Jardini
S
4205291
482034
23/11/2006
11.4
7.8
321
193.8

30
Fossaneve
S
4204120
482502
23/11/2006
8.6
7.6
126.5
19

31
Titi bassa
S
4207272
481640
23/11/2006
13.9
7.4
434
198

32
S. Anna
S
4209106
481785
23/11/2006
12
7.5
387
177

33
Balestra
S
4204036
482514
23/11/2006
7.2
7.1
109
132

34
S. Pietro
S
4205385
479831
23/11/2006
13.8
7.8
264
181

35
Filipelli
S
4208702
477706
23/11/2006
10.5
7.8
367
187

36
Iria
S
4209929
468000
29/11/2006
15.6
7.7
848
147

37
Abate
S
4208184
473990
29/11/2006
14.5
7.7
309
106

38
Feudo
S
4218642
480320
29/11/2006
15.4
6.3
300
280

39
Favarotta
S
4210817
476145
29/11/2006
13.7
7.5
388
261

40
Palamara
W
4204231
455414
20/12/2006
17
7.5
648
101

41
Neviera
S
4197698
443048
20/12/2006
10.6
6.9
161.9
35

42
Ramata
S
4193148
445498
20/12/2006
10.4
7.5
378
-28

43
Nocita
S
4193948
459898
20/12/2006
8.0
7.0
57
69

44
Marino SPA 1
W
4206888
537429
08/01/2004
28.1
6.12
6070
-178.5

45
Marino SPA 2
W
4206888
537429
07/04/2004
22.3
6.07
2120
112.9

45
Marino SPA 2
W
4206888
537429
06/05/2004
23.3
6.11
2270
158.0

45
Marino SPA 2
W
4206888
537429
18/10/2004
25
6.13
2630
-228.0

46
Parco Giochi
W
4206515
537444
04/02/2004
16.5
7.1
980
67.5

47
Berlinghieri
W
4206465
537262
04/02/2004
21.5
7.2
1002
124.5

48
La Magnolia 1
W
4206434
537276
01/06/2005
31.1
5.65
41000
-70.8

49
Granata
S
4206973
537759
23/03/2005
22.5
5.96
35600
-10

50
Granata 1
S
4206973
537759
24/03/2005
22.1
6.14
992
161

51
Calavà
B
4226175
493509
28/08/2002
n.d.
n.d.
n.d.
n.d.

52
Terme Vigliatore
S
4221034
513801
27/02/2006
30.4
6.8
4030
n.d.

53
Chiappe
S
4195686
512328
01/03/2007
16
n.d.
524
-274

54
Pintaudi
W
4211772
500245
26/01/2006
18.2
7.93
5100
-223

55
Biondo
W
4213979
498283
26/01/2006
16.5
6.67
557
n.d.

56
Fetente
S
4226066
494050
27/02/2006
14.9
7.40
6990
210

57
La Malfa
W
4226742
523732
12/01/2007
n.d.
n.d.
n.d.
n.d.

58
Acqua rugiada
S
4216468
519311
25/02/2004
n.d.
n.d.
n.d.
n.d.

59
Pezzino
W
4206270
452884
19/02/2004
17.3
7.20
1032
455

60
Canneto
W
4208324
446323
19/02/2004
15.8
7.43
1028
453

61
Maio
W
4222264
517189
11/04/2007
22.8
7.15
597
n.d.

62
Fontana WP43
S
4214022
536220
18/01/2007
n.d.
n.d.
n.d.
n.d.

63
Scuderi
S
4211597
536559
18/01/2007
n.d.
n.d.
n.d.
n.d.

64
La Magnolia 2
W
4206434
537276
28/04/2005
n.d.
n.d.
n.d.
n.d.

65
Alì
B
4206058
537395
28/08/2002
n.d.
n.d.
n.d.
n.d.

66
Gas in seawater
B
4206473
538547
23/03/2005
n.d.
n.d.
n.d.
n.d.

67
Rodì Milici
B
4216796
513441
n.a.
n.d.
n.d.
n.d.
n.d.

Geographical coordinates UTM WGS84; EC: electrical conductivity; S: spring; W: well; B: bubbling gas; n.d.: not analyzed; n.a.: not available. Data after [53]. See Figure 1 for sample locations.

Table 2: Chemical and isotopic composition of the bubbling gases from vents located along the Peloritani chain.

Site ID
Site
Data
He
Ne
O₂
N₂
CH₄
CO₂
δ¹³C
R/Ra
ε
He/Ne
CO₂/³He

48
La Magnolia 1
23/03/2005
<
<
<
3.26
3.4E-02
95.9
-2.15
<
<
<
<

48
La Magnolia 1
28/04/2005
4.7E-03
3.6E-05
0.03
2.0
2.1E-02
95.9
-2.15
0.67
0.0035
132.11
2.17E+10

48
La Magnolia 1
02/03/2006
4.7E-03
4.3E-05
24.0
6.0
2.2E-03
71.3
<
0.67
0.0049
108.66
1.63E+10

49
Granata
28/08/2002
1.5E-04
3.6E-06
<
0.27
4.6E-03
99.7
<
0.70
0.0161
24.23
6.77E+11

49
Granata
11/09/2002
1.8E-04
1.3E-05
<
0.40
1.2E-02
99.5
-1.03
0.70
0.0255
14.20
5.76E+11

49
Granata
23/03/2005
1.8E-04
1.6E-05
<
0.73
5.7E-03
98.3
-1.13
0.68
0.0076
11.36
5.72E+11

49
Granata
28/04/2005
1.6E-04
9.4E-06
<
0.81
7.2E-03
98.1
<
0.68
0.0104
17.48
6.33E+11

49
Granata
02/03/2006
2.4E-04
5.1E-05
0.0
1.1
1.5E-06
97.8
-1.30
0.90
0.0131
4.63
3.32E+11

49
Granata
06/04/2006
1.8E-04
2.6E-05
0.7
4.1
5.4E-03
93.7
<
0.71
0.0108
7.18
5.11E+11

51
Calavà
28/08/2002
2.0E-03
1.8E-05
0.41
1.62
5.4E-01
97.38
<
2.48
0.0243
115.35
1.39E+10

51
Calavà
11/09/2002
2.7E-03
3.2E-05
0.62
2.35
6.9E-01
96.40
<
2.46
0.0233
84.10
1.06E+10

51
Calavà
25/02/2004
2.5E-03
1.4E-05
0.73
2.75
8.1E-01
95.72
<
2.50
0.0206
171.03
1.12E+10

51
Calavà
01/04/2004
1.9E-03
1.3E-05
0.47
2.12
5.6E-01
95.68
<
2.55
0.0206
146.63
1.40E+10

51
Calavà
26/05/2004
1.5E-03
6.9E-06
0.35
1.61
5.1E-01
98.24
<
2.52
0.0239
216.59
1.87E+10

51
Calavà
28/06/2004
1.5E-03
1.9E-05
0.42
1.63
5.6E-01
97.17
<
2.54
0.0301
80.29
1.80E+10

51
Calavà
18/07/2004
2.1E-03
2.5E-05
0.28
1.62
5.2E-01
97.73
<
2.52
0.0321
84.43
1.30E+10

51
Calavà
15/10/2004
<
<
2.68
11.06
4.4E-01
84.46
<
<
<
<
<

51
Calavà
15/05/2005
2.2E-03
1.4E-05
0.48
2.22
6.7E-01
96.52
<
2.47
0.0129
150.42
1.30E+10

51
Calavà
12/08/2005
1.7E-03
1.1E-05
0.2
1.65
5.4E-01
97.38
<
2.49
0.0094
160.56
1.64E+10

52
Terme Vigliatore
06/05/2004
1.0E-05
8.7E-06

0.046
4.8E-03
99.76
<
1.51
0.2850
1.21
4.53E+12

52
Terme Vigliatore
—
<
<
<

6.0E-05
99.9
<
<
<
<

64
La Magnolia 2
28/04/2005
3.1E-03
5.1E-05
<
1.44
1.5E-02
97.0
<
0.67
0.0043
60.26
3.36E+10

65
Alì
28/08/2002
1.5E-04
6.2E-06
<
0.3
4.6E-03
99.7
<
0.70
0.0161
24.23
6.75E+11

65
Alì
11/09/2002
1.8E-04
1.3E-05
1.4
5.7
1.2E-02
93.5
<
0.70
0.0255
14.20
5.40E+11

65
Alì
11/11/2003
1.6E-04
1.7E-05
<
1.0
4.8E-03
99.5
<
0.67
0.0392
9.67
6.63E+11

66
Gas in sea water
23/03/2005
6.6E-03
<
0.3
4.2
6.0E-02
96.4
-1.71

67
Rodì Milici
—

<
1.36
96.1
1.3E-01
0.01
<
2.14
<
99.52
5.4E+04

Data in vol%; <: below detection limits or not analyzed; ε: error of the isotopic determination reported for all of the R/Ra data. Data after [53].

Table 3: Chemical composition of the dissolved gas phase.

(a)

Sample #
Site name
Type
Date
He
Ne
O₂
N₂
CH₄
CO₂
Air%

1
Boffe
S
31/05/2006
2.94E-05
1.02E-04
4.73
11.09
8.45E-05
39.62
24.38

2
Usignolo
S
31/05/2006
1.11E-04
1.64E-04
2.33
11.19
2.40E-04
1.37
44.82

3
Belvedere
W
31/05/2006
5.96E-05
1.90E-04
2.70
14.65
1.48E-04
69.10
8.93

4
Margherita
S
31/05/2006
6.76E-05
2.11E-04
4.71
10.30
7.20E-05
13.07
47.96

5
Lettosanto
S
26/10/2006
2.72E-05
7.05E-05
5.27
11.81
1.01E-04
1.27
82.09

6
Sampieri
S
26/10/2006
3.76E-05
7.43E-05
4.79
11.07
4.35E-05
8.27
56.71

7
Leone
S
26/10/2006
2.05E-05
6.65E-05
5.58
12.78
<
4.69
69.25

8
S. Mamma
S
26/10/2006
1.37E-05
4.60E-05
5.20
11.40
4.35E-05
3.66
73.46

9
Ramosa
S
26/10/2006
2.55E-05
7.88E-05
2.61
11.85
1.23E-03
47.40
12.04

10
Ficuzza
S
31/10/2006
3.04E-05
9.50E-05
2.61
12.21
5.07E-04
37.70
35.57

11
Moglie
S
31/10/2006
3,20E-05
8.94E-05
5.26
11.84
3.62E-04
1.27
81.86

12
Piliegi
S
31/10/2006
4.15E-05
1.25E-04
3.08
12.11
2.03E-03
21.24
24.17

13
Cesarò
S
31/10/2006
1.09E-05
2.82E-05
4.96
11.20
<
4.29
69.33

14
Torti
S
31/10/2006
2.83E-05
8.11E-05
6.82
16.89
2.04E-04
6.90
63.69

15
Muto
S
31/10/2006
2.37E-05
7.80E-05
5.28
11.31
2.39E-04
1.27
84.53

16
Sulipani
S
16/11/2006
2.23E-05
5.61E-05
1.88
11.78
1.53E-05
20.26
15.87

17
Giangalia
S
16/11/2006
2.29E-05
8.80E-05
4.92
10.31
3.71E-04
2.23
80.62

18
Pace
S
16/11/2006
3.53E-05
1.11E-04
4.76
11.39
2.82E-04
3.17
70.47

19
Sfaranda
S
16/11/2006
5.80E-05
1.69E-04
4.98
10.32
1.99E-04
8.22
60.59

20
Agrifoglio
S
16/11/2006
2.53E-05
8.65E-05
4.28
10.97
6.65E-05
8.54
51.48

21
Mezzalora
S
22/11/2006
2.52E-05
4.79E-05
1.96
7.28
9.89E-03
2.14
49.24

22
Margio Salice
S
22/11/2006
1.09E-05
3.25E-05
5.57
11.48
3.71E-04
3.95
75.87

23
Chiusitta
S
22/11/2006
1.32E-05
5.46E-05
3.38
10.67
2.83E-04
9.65
40.80

24
Tre Arie
S
22/11/2006
1.84E-05
6.91E-05
4.38
9.61
5.62E-05
2.87
74.32

25
Zarbata
S
22/11/2006
3.21E-05
6.77E-05
3.26
11.42
5.20E-05
12.39
34.47

26
S. Domenica
S
22/11/2006
2.24E-05
8.16E-05
2.58
10.03
2.55E-05
6.60
38.39

27
Piano Soprano
S
22/11/2006
2.52E-05
7.90E-05
3.08
9.76
<
12.39
34.87

28
Casitti
S
22/11/2006
1.80E-05
5.08E-05
5.23
11.51
<
1.34
82.71

29
Jardini
S
23/11/2006
2.64E-05
6.72E-05
5.43
13.72
3.05E-04
3.12
69.67

30
Fossaneve
S
23/11/2006
2.12E-05
6.31E-05
4.87
11.27
5.64E-03
.1.39
79.43

31
Titi bassa
S
23/11/2006
2.04E-05
6.36E-05
3.84
10.68
1.94E-05
6.75
51.62

32
S. Anna
S
23/11/2006
3.53E-05
1.19E-04
5.16
13.06
3.81E-05
4.74
64.28

33
Balestra
S
23/11/2006
1.47E-05
5.33E-05
3.35
11.45
<
9.53
39.39

34
S. Pietro
S
23/11/2006
1.57E-05
5.45E-05
4.39
9.79
4.04E-05
1.97
77.75

35
Filipelli
S
23/11/2006
1.80E-05
5.68E-05
5.04
11.94
2.46E-03
4.34
67.60

36
Iria
S
29/11/2006
2.85E-05
9.63E-05
3.31
12.07
2.03E-04
7.84
40.73

37
Abate
S
29/11/2006
2.84E-05
7.94E-05
5.22
11.22
3.11E-05
2.80
77.65

38
Feudo
S
29/11/2006
2.10E-05
6.81E-05
4.60
10.88
2.33E-05
30.64
28.51

39
Favarotta
S
29/11/2006
3.60E-05
8.44E-05
4.95
11.56
2.33E-05
6.13
62.50

40
Palamara
W
20/12/2006
4.74E-05
1.37E-04
3.70
14.21
1.61E-03
12.22
35.14

41
Neviera
S
20/12/2006
4.21E-05
1.20E-04
2.05
11.83
1.75E-03
13.42
21.47

42
Ramata
S
20/12/2006
3.16E-05
9.53E-05
4.46
11.13
1.07E-03
8.45
53.00

43
Nocita
S
20/12/2006
4.39E-05
1.12E-04
4.90
10.50
5.83E-04
4.20
71.52

ASW

4.06E-05

5
9.80
1.00E-06
0.23

(b)

Sample #
Site name
Type
Date
He
Ne
O₂
N₂
CH₄
CO₂
Air%

44
Marino SPA 1
W
08/01/2004
4.84E-04
8.38E-05
0.27
3.05
1.32E-02
509.71
0.15

44
Marino SPA 1
W
07/04/2004
2.57E-04
7.26E-05
0.23
2.61
9.24E-03
615.63
0.21

44
Marino SPA 1
W
06/05/2004
1.03E-04
3.51E-05
0.21
2.83
7.57E-03
499.69
0.12

44
Marino SPA 1
W
18/10/2004
1.61E-04
8.22E-05
0.10
3.54
6.40E-03
502.41
0.06

45
Marino SPA 2
W
08/01/2004
2.26E-04
6.10E-05
0.99
3.10
1.94E-03
664.14
0.43

45
Marino SPA 2
W
07/04/2004
1.47E-04
5.87E-05
0.84
2.62
7.45E-04
616.56
0.06

45
Marino SPA 2
W
06/05/2004
9.49E-05
2.71E-05
0.87
2.74
1.00E-03
606.51
0.43

45
Marino SPA 2
W
18/10/2004
6.94E-05
7.15E-05
0.10
3.54
6.40E-03
502.41
0.39

46
Parco Giochi
W
04/02/2004
2.64E-03
3.02E-04
2.05
17.02
1.22E-04
18.30
15.67

46
Parco Giochi
W
03/03/2004
2.49E-03
1.25E-04
3.48
<
<
13.00
32.58

46
Parco Giochi
W
07/04/2004
1.07E-03
2.36E-04
1.84
43.65
<
32.07
6.80

46
Parco Giochi
W
06/05/2004
1.12E-03
1.99E-04
3.76
16.36
<
9.83
35.90

46
Parco Giochi
W
12/10/2004
1.33E-03
1.30E-04
1.98
16.51
8.41E-04
55.01
7.69

46
Parco Giochi
W
18/10/2004
2.65E-03
1.22E-04
2.56
17.38
3.99E-04
48.95
10.63

46
Parco Giochi
W
28/12/2004
2.89E-04
1.50E-04
3.42
12.71
4.65E-04
18.13
28.55

46
Parco Giochi
W
10/02/2005
1.22E-03
2.51E-04
2.62
13.10
<
17.73
22.36

46
Parco Giochi
W
09/03/2005
6.10E-05
1.20E-04
3.42
14.60
6.49E-03
14.17
30.42

46
Parco Giochi
W
29/04/2005
1.06E-03
2.59E-04
1.72
14.02
2.28E-04
14.30
16.35

46
Parco Giochi
W
01/06/2005
2.12E-03
1.06E-04
2.93
16.14
1.01E-04
16.23
23.75

47
Berlinghieri
W
04/02/2004
1.34E-03
1.38E-04
1.66
10.14
6.98E-04
20.25
14.80

47
Berlinghieri
W
03/03/2004
1.86E-03
1.40E-04
2.11
11.45
1.91E-03
22.12
16.93

47
Berlinghieri
W
07/04/2004
1.36E-03
1.72E-04
2.27
11.12
2.35E-03
22.45
18.10

47
Berlinghieri
W
06/05/2004
1.37E-03
1.69E-04
3.10
15.31
6.38E-05
24.56
20.61

47
Berlinghieri
W
08/07/2004
6.68E-04
2.37E-04
2.86
11.79
<
16.71
26.11

47
Berlinghieri
W
12/10/2004
5.68E-04
1.47E-04
2.27
12.70
6.15E-04
20.44
18.31

47
Berlinghieri
W
18/10/2004
3.11E-04
8.79E-05
2.04
14.06
5.69E-03
20.21
16.05

47
Berlinghieri
W
03/03/2006
1.19E-03
8.36E-05
0.06
5.65
3.65E-04
0.49
2.95

47
Berlinghieri
W
06/04/2006
1.16E-03
8.84E-05
0.88
6.86
9.86E-05
0.51
30.49

48
La Magnolia
W
01/06/2005
<
<
0.13
4.29
8.84E-03
535.13
0.07

48
La Magnolia
W
29/06/2005
3.10E-04
7.83E-05
0.10
0.99
3.37E-03
483.37
0.06

48
La Magnolia
W
04/08/2005
7.27E-05
1.08E-05
0.06
2.26
4.62E-03
488.27
0.04

48
La Magnolia
W
14/09/2005
3.19E-05
3.01E-05
0.03
1.04
2.32E-03
52.67
0.16

48
La Magnolia
W
14/02/2006
6.77E-05
3.61E-05
0.08
3.68
9.07E-03
1065.52
0.12

48
La Magnolia
W
14/06/2006
<
<
0.12
2.31
4.74E-03
71.70
0.44

48
La Magnolia
W
18/07/2006
6.32E-05
3.52E-05
0.04
0.90
3.05E-03
795.63
0.01

49
Granata
S
23/03/2005
2.90E-05
1.01E-04
0.04
4.47
7.05E-04
373.87
0.03

49
Granata
S
13/10/2005
2.03E-05
1.56E-05
0.03
8.56
1.01E-03
359.08
0.02

49
Granata
S
02/03/2006
1.96E-05
3.62E-05
0.20
12.67
7.71E-04
39.24
1.11

49
Granata
S
06/04/2006
6.25E-05
3.73E-05
0.02
6.47
6.10E-04
29.76
0.12

50
Granata 1
S
24/03/2005
7.08E-05
1.14E-04
0.03
3.92
<
310.31
0.02

50
Granata 1
S
28/04/2005
4.12E-05
5.80E-05
0.05
4.15
3.93E-04
348.19
0.04

52
Terme Vigliatore
S
27/02/2006
2.12E-05
3.67E-05
0.08
2.11
4.20E-03
530.14
0.05

52
Terme Vigliatore
S
27/04/2006
1.68E-05
3.92E-05
0.05
1.74
4.85E-03
609.16
0.03

52
Terme Vigliatore
S
26/05/2006
7.27E-05
1.66E-04
0.08
7.79
7.45E-03
763.34
0.02

52
Terme Vigliatore
S
26/06/2006
4.48E-05
4.63E-05
0.05
2.88
4.37E-03
617.45
0.02

53
Chiappe
S
01/03/2007
4.85E-05
1.37E-04
0.03
7.76
3.79
22.21
0.27

53
S. Giorgio
S
11/10/2006
4.30E-03
1.60E-04
0.05
18.30
6.18E-01
432.18
0.03

53
S. Giorgio
S
18/02/2007
<
<
0.02
10.41
8.20E-03
43.02
0.12

54
Pintaudi
W
26/01/2006
2.45E-05
4.85E-05
3.54
10.81
7.23E-03
8.26
44.79

54
Pintaudi
W
27/02/2006
3.01E-05
7.49E-05
3.96
10.41
3.25E-03
6.75
53.63

55
Biondo
W
26/01/2006
1.86E-05
4.00E-05
0.09
11.51
1.26E-03
0.44
2.23

55
Biondo
W
27/02/2006
3.53E-05
6.32E-05
0.15
12.42
4.97E-03
1.79
3.07

55
Biondo
W
26/06/2006
3.38E-05
4.02E-05
0.01
10.5
4.81E-04
0.41
0.36

55
Biondo
W
10/10/2006
2.91E-05
4.39E-05
0.03
11.68
1.20E-02
0.55
0.73

56
Fetente
S
27/02/2006
2.91E-05
4.39E-05
4.73
10.34
2.95E-05
10.12
53.71

57
La Malfa
W
12/01/2007
3.88E-05
1.19E-04
2.45
10.32
1.15E-04
37.92
13.84

58
Acqua rugiada
S
25/02/2004
2.17E-04
7.45E-05
0.16
5.96
2.40E-02
482.20
0.10

59
Pezzino
W
19/02/2004
3.64E-05
1.12E-04
4.23
9.25
<
12.31
46.91

60
Canneto
W
19/02/2004
4.97E-05
1.64E-04
2.34
10.78
<
19.23
20.70

61
Maio
W
11/04/2007
<
<
0.26
16.97
1.82E-03
25.02
1.75

62
Fontana WP43
S
18/01/2007
2.58E-05
7.51E-05
5.51
11.38
2.92E-05
1.23
87.01

63
Scuderi
S
18/01/2007
5.43E-05
1.35E-04
5.92
12.11
<
1.18
88.17

ASW

4.06E-05

5
9.80
1.00E-06
0.23

Data in ccSTP/LH2O. (a) Samples from the Nebrodi Mts; (b) samples from the Peloritani Mts. The site number is the same as reported on the graphs. S: spring; W: well; <: below detection limits or not analyzed.

To carry out dissolved gas analyses, water samples were collected and stored in 240 ml Pyrex bottles sealed in the field using silicon/Teflon septa and purpose-built pliers, following the methodology and instrumentation described in Italiano et al. [47–49]. All of the samples were collected taking care to avoid even the tiniest bubbles in order to prevent atmospheric contamination.

To recover a pure gas sample, we collected gas bubbles using an inverted funnel placed on top the bubbles, driving them towards a Pyrex bottle of about 50 ml in volume with two vacuum-type valves at both ends. The sampling bottle was washed by the gas coming from the funnel and the sample taken by closing the two valves after a volume of at least one order of magnitude larger than that of the sampling bottle had been passed through.

All of the samples (dissolved and free gases) were analyzed for the chemical and isotopic composition of carbon (CO₂) and He.

3.2. Analytical Methods

Field measurements of temperature, pH, redox potential (Eh), and electrical conductivity (EC) were performed by a multiparameter device (Multi 350i, Weilheim) (Table 1).

In the laboratory, the chemical and isotopic composition (He and C) of the bubbling and dissolved gases were determined using the same analytical equipment. The dissolved gases were extracted after equilibrium was reached at constant temperature with a host-gas (high-purity argon) injected in the sample bottle through the rubber septum (for further details, see Italiano et al. [47, 49]). Chemical analyses were carried out by gas chromatography (PerkinElmer Clarus 500 equipped with a double TCD-FID detector) using argon as the carrier gas. Typical uncertainties are within ±5%.

Helium isotope analyses were performed on gas fractions extracted following the same procedure as for the gas chromatography and purified following methods described in the literature [50–52]. The purified helium fraction (either of dissolved or of bubbling gases) was analyzed by a static vacuum mass spectrometer (GVI5400TFT) that allows the simultaneous detection of ³He and ⁴He ion beams, thereby keeping the ³He/⁴He error of measurement to very low values. Typical uncertainties in the range of low-³He samples are within ±1%. During the same analytical procedure, the ⁴He/²⁰Ne ratio was measured by peak intensities on the mass spectrometer.

The isotopic composition of the total dissolved carbon (δ¹³CTDC) was measured in a sample of 2 ml of water introduced into containers injected with high-purity helium to remove atmospheric CO₂. The water samples were acidified with phosphorus pentoxide in an autosampler to ensure complete release of CO₂ from acidified waters. CO₂ was then directly admitted to a continuous flow mass spectrometer (AP2003). The results are reported in δ‰ units relative to the V-PDB (Vienna Pee Dee Belemnite) standard; standard deviation of the ¹³C/¹²C ratio was ±0.2‰.

4. Results

4.1. Chemical and Isotopic Composition

Table 2 shows the chemical and isotopic data of the bubbling gases as well as their He and C isotopic compositions and ⁴He/²⁰Ne and CO₂/³He ratios.

CO₂ is by far the main component of the bubbling gases with concentrations always above 90%, but sample Rodì Milici (after [53]) which has a composition dominated by N₂ (96.1%) with CO₂ content is as low as 0.01% with a large amount of O₂. The sample is largely air contaminated and also suffered from gas-water interactions as shown by the high helium and negligible CO₂ content (Table 2). The composition of the dissolved gas phase was calculated from the gas-chromatographic analyses, combining the solubility coefficients (Bunsen coefficient “β,” ccgas/mlwater STP) of each gas species, the volume of gas extracted (cm³), and the volume of the water sample and the equilibration temperature, as shown in the following equation:

where is the concentration of the selected gas specie, is its concentration measured by gas chromatography (vol%), and and represent the extracted and introduced gas volumes, respectively, while is the volume of the analyzed water sample (see also [47, 49] for further details). All volumes are carefully measured at the equilibration temperature. Data for dissolved gas compositions are expressed in cm³STP/LH2O and listed in Tables 3(a) and 3(b), as well as the amount of dissolved air (air% in Tables 3(a) and 3(b)). The air content estimation is based on the oxygen content and represents the minimum amount of dissolved air at the sampling site. Oxygen, in fact, can be consumed because of bacterial activity as well as oxidation reactions likely occurring during the water circulation; the percentage of atmospheric components during infiltration might be much higher. As we assume that the deep volatiles are oxygen-free, we recalculated the gas composition removing the atmospheric components (since atmospheric nitrogen is calculated from the oxygen content, for some sites, it might be underestimated). The gas analyses recalculated in vol%, allowing us a comparison with the analytical results of the bubbling gases, are listed in Tables 4(a) and 4(b) together with the helium and carbon isotopic compositions and the He/Ne ratios.

Table 4: (a, b) Recalculated chemical composition for the Nebrodi (a) and Peloritani Mts. (b) after air removal. Isotopic composition of He and total dissolved carbon are reported as well as the He/Ne and CO₂/³He ratios.

(a)

Sample #
Site name
Type
Date
He
Ne
N₂
CH₄
CO₂
He/Ne
R/Ra
ε
CO₂/³He
δ¹³CTDC

2
Usignolo
S
31/05/2006
1.4E-03
2.0E-03
45.98
2.9E-03
16.66
0.677
0.86
0.018
1.04E+10
-13.00

6
Sampieri
S
26/10/2006
3.6E-04
7.1E-04
9.01
4.2E-04
79.19
0.51
0.60
0.015
2.65E+11
-15.04

21
Mezzalora
S
22/11/2006
4.4E-04
8.3E-04
31.87
1.7E-01
37.05
0.527
0.83
0.046
7.34E+10
-8.68

25
Zarbata
S
22/11/2006
1.8E-04
3.8E-04
19.77
2.9E-04
69.83
0.474
0.86
0.031
3.24E+11
-12.42

39
Favarotta
S
29/11/2006
4.2E-04
9.9E-04
10.44
2.7E-04
72.17
0.427
0.90
0.031
1.36E+11
-12.82

(b)

Sample #
Site name
Type
Date
He
Ne
N₂
CH₄
CO₂
He/Ne
R/Ra
Error
CO₂/³He
δ¹³CCO2

44
Marino SPA 1
W
08/01/2004
9.4E-05
5.5E-04
0.49
2.6E-03
99.50
5.78
0.67
0.01
1.13E+12
<

44
Marino SPA 1
W
07/04/2004
<
<
0.43
2.1E-03
99.57
3.54
0.69
0.011
2.48E+12
<

44
Marino SPA 1
W
06/05/2004
<
<
0.47
1.7E-03
99.53
2.93
0.71
0.04
4.92E+12
<

44
Marino SPA 1
W
18/10/2004
4.2E-05
1.5E-04
0.35
1.5E-03
99.65
1.95
0.71
0.014
3.16E+12
<

45
Marino SPA 2
W
08/01/2004
2.0E-05
6.0E-05
0.49
1.5E-03
99.51
3.71
0.66
0.017
3.19E+12
<

45
Marino SPA 2
W
07/04/2004
3.2E-05
6.2E-05
0.66
1.3E-03
99.33
2.5
0.74
0.026
4.10E+12
<

45
Marino SPA 2
W
06/05/2004
3.4E-05
1.3E-04
0.19
2.9E-04
99.81
3.5
0.7
0.027
6.58E+12
<

45
Marino SPA 2
W
18/10/2004
<
0.0E+00
0.17
1.4E-04
99.83
0.97
0.78
0.024
6.68E+12
<

46
Parco Giochi
W
04/02/2004
<
0.0E+00
0.14
8.7E-05
99.86
8.73
0.7
0.007
7.09E+09
<

46
Parco Giochi
W
03/03/2004
2.4E-05
5.9E-05
0.17
1.2E-04
99.83
19.97
0.69
0.006
5.43E+09
<

46
Parco Giochi
W
07/04/2004
1.6E-05
5.5E-05
0.18
1.7E-04
99.81
4.54
0.69
0.009
3.13E+10
<

46
Parco Giochi
W
06/05/2004
1.4E-05
1.3E-05
0.66
1.3E-03
99.33
5.64
0.68
0.013
9.26E+09
<

46
Parco Giochi
W
12/10/2004
8.4E-03
7.3E-02
35.33
3.9E-04
58.03
10.27
0.63
0.006
4.69E+10
<

46
Parco Giochi
W
18/10/2004
3.8E-02
7.5E-01
24.68
<
63.10
21.76
0.63
0.007
2.10E+10
<

46
Parco Giochi
W
28/12/2004
1.5E-03
6.7E-03
51.85
<
44.36
1.93
0.65
0.006
6.95E+10
<

46
Parco Giochi
W
10/02/2005
5.8E-03
3.3E-02
31.26
<
51.19
4.87
0.64
0.006
1.64E+10
<

46
Parco Giochi
W
09/03/2005
3.7E+00
9.6E+01
0.00
<
<
0.51
1.19
0.037
1.40E+11
<

46
Parco Giochi
W
29/04/2005
2.0E-03
2.0E-02
17.46
1.2E-03
81.06
4.114
0.63
0.005
1.53E+10
<

46
Parco Giochi
W
01/06/2005
4.3E-03
9.4E-02
18.29
6.5E-04
79.44
19.916
0.63
0.004
8.81E+09
<

47
Berlinghieri
W
04/02/2004
1.2E-03
2.3E-03
18.53
1.9E-03
74.06
9.76
0.67
0.006
1.6E+10
<

47
Berlinghieri
W
03/03/2004
4.7E-03
2.3E-02
24.62
0.0E+00
68.27
13.26
0.67
0.006
1.27E+10
<

47
Berlinghieri
W
07/04/2004
2.7E-04
1.4E-04
25.57
2.9E-02
63.23
7.91
0.67
0.01
1.76E+10
<

47
Berlinghieri
W
06/05/2004
4.2E-03
1.7E-02
36.03
9.1E-04
56.90
8.08
0.68
0.013
1.9E+10
<

47
Berlinghieri
W
08/07/2004
7.9E-03
1.6E-01
30.21
3.7E-04
60.22
2.82
0.67
0.009
2.7E+10
<

47
Berlinghieri
W
12/10/2004
0.0E+00
0.0E+00
51.41
2.8E-04
48.37
3.87
0.69
0.01
3.75E+10
<

47
Berlinghieri
W
18/10/2004
4.9E-03
4.8E-02
22.01
2.6E-03
74.12
3.53
0.64
0.016
7.34E+10
<

47
Berlinghieri
W
03/03/2006
6.3E-03
8.3E-02
21.06
6.4E-03
74.56
14.2
0.67
0.006
4.38E+08
-8.61

47
Berlinghieri
W
06/04/2006
4.6E-03
3.7E-02
19.25
8.0E-03
76.45
13.12
0.62
0.009
5.07E+08
-8.25

48
La Magnolia
W
01/06/2005
4.0E-03
3.2E-02
22.22
1.9E-04
71.98
<
<
<
1.79E+12
<

48
La Magnolia
W
29/06/2005
2.9E-03
8.1E-03
20.61
0.0E+00
72.10
3.961
0.69
0.007
7.13E+12
<

48
La Magnolia
W
04/08/2005
2.0E-03
7.6E-03
23.96
2.1E-03
70.65
6.72
0.67
0.014
1.34E+13
<

48
La Magnolia
W
14/09/2005
1.0E-03
3.6E-03
28.28
1.9E-02
66.30
1.06
0.82
0.021
6.35E+11
<

48
La Magnolia
W
14/02/2006
1.4E-03
0.0E+00
15.40
6.6E-03
81.17
1.87
0.84
0.016
8.79E+12
0.54

48
La Magnolia
W
14/06/2006
3.1E-03
0.0E+00
21.78
3.1E-03
72.25
<
<
<
1.41E+12
<

48
La Magnolia
W
18/07/2006
6.3E-03
0.0E+00
23.63
1.2E-02
68.98
1.8
0.58
0.012
2.52E+13
<

49
Granata
S
23/03/2005
2.0E-02
2.8E-01
88.94
6.0E-03
8.05
0.29
0.72
0.08
1.29E+13
1.82

49
Granata
S
13/10/2005
2.0E-02
2.6E-01
63.16
1.7E-03
8.85
1.31
0.71
0.027
1.79E+13
<

49
Granata
S
02/03/2006
<
<
0.67
7.6E-04
99.33
0.54
0.73
0.012
1.97E+12
3.03

49
Granata
S
06/04/2006
<
<
0.54
9.3E-04
99.45
1.68
0.71
0.011
4.84E+11
<

50
Granata 1
S
24/03/2005
5.7E-05
2.3E-04
0.75
1.6E-03
99.25
0.62
0.62
0.008
5.09E+12
<

50
Granata 1
S
28/04/2005
1.5E-05
1.0E-04
0.17
7.0E-04
99.83
0.71
0.91
0.026
6.68E+12
<

52
Terme Vigliatore
S
27/02/2006
6.5E-06
6.9E-06
0.44
9.4E-04
99.56
0.58
0.98
0.116
1.84E+13
<

52
Terme Vigliatore
S
27/04/2006
2.3E-04
5.6E-04
1.83
4.3E-03
98.16
0.43
0.76
0.015
3.44E+13
<

52
Terme Vigliatore
S
26/05/2006
3.5E+01
6.5E+01
0.00
<
<
0.44
0.77
0.008
9.8E+12
<

52
Terme Vigliatore
S
26/06/2006
7.2E-06
2.6E-05
0.33
8.5E-04
99.67
0.97
0.48
0.009
2.08E+13
<

53
Chiappe
S
01/03/2007
<
<
0.79
3.0E-03
99.20
0.353
0.69
<
4.76E+11
-7.09

53
S. Giorgio
S
11/10/2006
8.6E-05
1.5E-04
2.82
6.4E-03
97.16
26.859
1.85
0.026
3.9E+10
-15.94

53
S. Giorgio
S
18/02/2007
4.1E-06
3.0E-06
0.10
3.8E-04
99.90
<
<
<
<
-16.33

54
Pintaudi
W
26/01/2006
<
<
2.50
4.7E-03
97.49
0.506
0.81
0.041
3.00E+11
<

54
Pintaudi
W
27/02/2006
<
<
1.38
4.3E-03
98.60
0.402
0.99
0.056
1.63E+11
<

55
Biondo
W
26/01/2006
<
<
0.21
3.6E-04
99.79
0.466
0.79
0.043
2.16E+10
<

55
Biondo
W
27/02/2006
<
<
0.00
<
<
0.56
0.7
0.036
5.19E+10
<

55
Biondo
W
26/06/2006
7.7E-06
2.2E-06
1.16
1.9E-04
98.84
0.84
0.7
0.026
1.67E+10
<

55
Biondo
W
10/10/2006
5.5E-06
7.2E-06
2.31
2.7E-04
97.68
<
<
<
<
<

56
Fetente
S
27/02/2006
3.8E-05
2.1E-05
23.59
1.5E-03
76.14
0.66
1.3
0.071
1.93E+11
<

57
La Malfa
W
12/01/2007
1.7E-04
2.9E-04
17.77
1.7E-03
82.21
0.326
0.79
<
8.93E+11
-13.82

58
Acqua rugiada
S
25/02/2004
2.3E-05
1.4E-05
1.23
<
98.77
2.91
1.13
0.022
1.41E+12
<

59
Pezzino
W
19/02/2004
1.2E-05
8.3E-06
1.15
1.1E-04
98.85
0.33
1.21
0.232
2.01E+11
<

60
Canneto
W
19/02/2004
1.1E-04
0.0E+00
3.73
3.1E-05
96.26
0.3
1.11
0.155
2.52E+11
<

61
Maio
W
11/04/2007
4.1E+01
5.9E+01
0.00
<
<
0.343
<
<
4.21E+10
-16.77

62
Fontana WP43
S
18/01/2007
4.0E-06
2.3E-06
0.37
7.9E-04
99.63
0.403
0.81
<
1.88E+10
<

63
Scuderi
S
18/01/2007
<
<
0.39
8.4E-04
99.61
0.506
0.84
<
1.93E+11
<

Data in vol%. See text for details. <: below detection limits or not analyzed; ε: error of the isotopic determination reported for all of the R/Ra data. Carbon isotopic ratios are expressed as δ‰ units vs. PDB. δ¹³CTDC and δ¹³CCO2r: isotopic composition of the total inorganic carbon and recalculated gaseous CO₂, respectively.

5. Discussion

5.1. Fluid Geochemistry

The chemical composition of the bubbling gases (Table 2) shows that CO₂ is the most abundant component with the concentration always above 95% by vol. The only exception is represented by the sample #67 (Rodì Milici, Table 2; after [53]), whose CO₂ content is the lowest of the entire dataset. Considering the high He concentration, this sample is a very fractionated gas, which lost almost all the CO₂ likely for intense GWI. It is noteworthy that it was possible to recover a free gas phase only over the Peloritani chain. The main difference in the gas chemistry is related to the amount of CH₄ and N₂ (Table 2) that besides CO₂ are the main components in crustal gases. All the CO₂-dominated volatiles are here classified as of deep origin, where the term “deep” may indicate an origin from either crustal or mantle/magmatic environments.

The composition of the dissolved gases (Tables 3(a) and 3(b)) shows the presence of air-derived gases (N₂ and O₂) along with the nonatmospheric gases CO₂ and CH₄. The estimated amount of dissolved air is significantly lower in the samples collected from the Peloritani chain than in those from the Nebrodi Mts. (Tables 3(a) and 3(b)): it ranges between 8.94 and 84.53% for the latter samples and from 0.01 to 53% for the former (Peloritani chain) with the exception of two samples (#62 and 63, Table 1(b)) showing air content as high as 87 and 88%, respectively. The relationships between atmospheric (O₂) and nonatmospheric gas species in the dissolved gases (Tables 3(a) and 3(b)) are shown in Figure 2.

Figure 2: CH₄-O₂-CO₂ composition of the gases dissolved in the studied waters. The dashed arrows show the trends produced by the addition and dissolution of CO₂ and addition of CH₄ to an atmospheric gas assemblage (see text for details). ASW: air saturated water. Open black marks: samples from Nebrodi Mts; blue filled marks: samples from Peloritani Mts. Labels refer to sample numbers as in Tables 1–4.

The arrows show the trends produced by the addition of CO₂ and CH₄ to an atmospheric gas assemblage as well as the effect of gas-water interactions (GWI) on the CO₂ dissolution. The graph clearly shows the presence of fluids from a crustal and/or a mantle source over the whole study area that interact with the groundwater changing their original atmospheric-derived gas assemblage. Gas-water interactions (GWI) allow dissolution of deep gas species as a consequence of the solubility coefficients of each of them, the temperature, and the flux intensity towards the surface.

The recalculated gas analyses show that even in the case of dissolved gases, the deep volatiles are always made by a CO₂-dominated gas assemblage despite a widely different extent of atmospheric contamination, namely, the air-derived/deep-originated volatile mixing proportion (Tables 3(a) and 3(b)).

It is worth noting that the investigated area is from 30 to more than 60 km away from the volcanic districts of Mt Etna and the Aeolian Islands and there is no evidence of melt intrusions in shallow crustal levels supporting the release of mantle-derived volatiles as already observed in other portions of the Southern Apennines [54].

5.2. Helium and Carbon Isotopes

The helium and carbon systematics may provide the necessary information to constrain the origin of the CO₂-dominated volatiles. It is well accepted that the isotopic ratio of helium is a very sensitive tracer of volatile mixing in volcanic and tectonic systems located near the Earth’s surface. During earth evolution and differentiation, the production of radiogenic ⁴He (alpha particles) as a function of U and Th concentrations modified the pristine ³He/⁴He ratio leading to a wide range of isotopic ratios both in the crust (from less than 0.01 to 0.05 Ra, where Ra is the atmospheric ³He/⁴He value of ) and in the mantle (e.g., ; ~8 Ra; [55–58]; 6.5 Ra in the Sub Continental European Mantle (SCEM) [59]).

The helium isotopic ratio observed in the gases dissolved in the groundwater of the Nebrodi and Peloritani chains, ranging from 0.5 to 1.85 Ra (Tables 4(a) and 4(b)) and increasing to 0.67-2.54 Ra (Table 2) in the bubbling gases of the Peloritani area, indicating significant ³He injections, thus pointing to a mantle origin for those volatiles.

Figure 3 plots the ³He/⁴He ratios (expressed as R/Ra values) versus the ⁴He/²⁰Ne ratio and includes results of both dissolved (Tables 4(a) and 4(b)) and bubbling (Table 2) gases. Assuming all neon to be of atmospheric origin, the ⁴He/²⁰Ne ratio provides an indication of the presence of an atmospheric-derived component in the gas assemblage.

Figure 3: Helium isotopes (normalized to the atmosphere) vs. ⁴He/²⁰Ne ratios. The samples represent a mixing between the atmosphere and a radiogenic-type end member. The mixing boundary lines are built with the following end members: atmosphere (as dissolved air) vs. ; mantle (Sub Continental European Mantle (SCEM)) vs (Dunai and Baur [59]); and continental crust vs. . Symbols and labels are as in Figure 2 (dissolved gas samples). Open blue marks refer to bubbling gases from Peloritani Mts.

The plot shows that the dissolved gases, although extracted from groundwater equilibrated with the atmosphere, display ⁴He/²⁰Ne ratios remarkably higher than the ASW. In particular, the ratios over the Peloritani Mts are higher than those from the Nebrodi Mts, most likely as a consequence of a larger contribution of CO₂-dominated, He-rich gases. Moreover, the isotopic composition of helium in both dissolved and bubbling gases of the Peloritani samples denotes a broadly higher contribution of mantle-type helium that might seem surprising, considering the geology of this portion of the chain, with its outcrops of high-grade metamorphic rocks (micaschists, gneiss) normally enriched in radioactive, ⁴He-producing elements.

The CO₂-³He-⁴He plot of Figure 4 shows the occurrence of GWI, inducing CO₂ loss, as bicarbonate ions in the pH range of the collected waters (Table 1).

Figure 4: ³He-⁴He-CO₂ diagram. The graph shows the occurrence of CO₂ dissolution phenomena occurring at variable extents for the sampled sites (grey lines). All the samples plot to the left of the grey line showing that their ³He content is always below the atmospheric. The helium isotopic ratio is mainly a binary mixing between the atmosphere and a ⁴He-rich end member, and the grey lines also indicate constant ³He/⁴He ratios. Symbols and labels are as in Figure 3. Open circles refer to bubbling gases from the Calavà and Alì areas.

The information is consistent with that from the geochemical features of the dissolved helium and its isotopic composition. The affinity of the heavy ¹³C for the liquid phase is responsible for the observed C fractionation depicting the increasing δ¹³C trend of the dissolved carbon species (total dissolved carbon (TDC)). The process is commonly observed in CO₂-rich waters (e.g., Eastern Alps [47] and Southern Apennines [60]) with the exception of waters with the presence of gas bubbling (gas oversaturated) where C isotopic fractionation is induced by CO₂ escaping (e.g., East Anatolian Fault Zone (EAFZ) [48]; Figure 5).

Figure 5: CO₂ content vs. δ¹³CTDC. The plot shows carbon isotopic fractionation due to the occurrence of GWI interactions and degassing. The chemical fractionation due to CO₂ dissolution during GWI induces isotopic fractionation of the carbon species. The evidence that the higher the CO₂ content in the dissolved gases, the lower the isotopic composition of the TDC, indicates that the origin of the dissolved carbon is mainly due to the preferential dissolution of the heavy carbon isotope as bicarbonate ion. Samples from the Southern Apennines (grey crosses) after Italiano et al. [60]; samples from the Eastern Alps (grey squared symbols) after Italiano et al. [47]. Other symbols are as in Figure 2.

To evaluate the extent that mantle volatiles contribute to the dissolved gas phase and to the soils, as well as elemental fractionations between He and CO₂, the correlations between CO₂/³He versus CO₂ (Figure 6(a)) and He (Figure 6(b)) were evaluated.

Figure 6: CO₂-³He relationships. CO₂/³He vs. CO₂ content (ml/l STP) (a) and (b) CO₂/³He vs. [He]. The sample distribution on (a) shows trends of CO₂ dissolution at variable extents (arrows) with almost constant ³He content. (b) Combines (a) with the information that helium isotopes do not denote simple crust/mantle mixes (see text); however, variable CO₂/³He ratios at constant ³He/⁴He ratios are possible results of subsequent elemental fractionation. All error bars are within the symbol size. CO₂/³He end members are in the range of (mantle-type volatiles) and crustal volatiles (vertical dashed lines). Symbols are as in Figure 2; labels not reported for a readable figure.

The CO₂/³He ratios along the Nebrodi-Peloritani chain spans over five orders of magnitude (from to ) covering the range proposed for mantle ( for MORBs [61]) and crustal continental fluids (10¹⁴ [62]). The samples from the two sections of the chain, however, span over different ranges: 10¹⁰-10¹² for the Nebrodi area and the whole range for the Peloritani chain.

Since He isotope ratios can only be modified by the admixture of He with a different isotopic signature, i.e., from a different reservoir, the coexistence of low ³He/⁴He ratios (namely, high crustal He component) and low, mantle-like CO₂/³He ratios suggests that the ratio is modified by a mixing of crustal and mantle-derived helium, or alternatively, the crustal component ratios span over a wide range due to chemical CO₂ fractionation.

The data are plotted on Figures 6(a) and 6(b) where the arrows in (a) display two concomitant trends here interpreted as (1) CO₂ addition to the ASW-type waters because of regional degassing and (2) increase in helium concentration because of CO₂ loss during GWI. Almost all the samples from the Nebrodi Mts and a group of samples from the Peloritani show CO₂/³He ratios in the range of 10¹⁰-10¹² broadly lower than the ratios detected for the dissolved gases from the Peloritani area.

It is of note that only samples from the Peloritani Mts show an antirelated CO₂/³He-He relationship as a consequence of GWI processes with CO₂ loss due to dissolution. In addition, the CO₂/³He ratio vs. [CO₂] content (Figure 6(b)) demonstrates a correlation trend indicating that whatever the GWI-induced CO₂ loss, samples from the Nebrodi Mts are characterized by broadly lower CO₂ content. The group of samples from the Peloritani area, marked by low He content with high ³He/⁴He ratios, denotes a different composition of the pristine gas phase.

6. Fluid/Fault Relationships

The active degassing of CO₂-dominated volatiles is considered responsible for the deep-originated dissolved gases we detected in the groundwater collected over the Nebrodi-Peloritani area. Although it was almost unexpected in the geologic context of the Southern Apennines, the evidence that a large amount of CO₂-dominated volatiles feeds the groundwater circulating over the study area indicates the close connection of the fluids’ geochemistry with the ATLF tectonic structure.

There is no evidence of melt intrusions in shallow crustal levels capable of releasing ³He-enriched fluids as already detected in other Apennine areas (e.g., [54]); thus, we propose that the presence of mantle fluids is related to lithospheric structures that, enhancing the vertical permeability, allow depressurization at the level of the upper mantle and the degassing of deep-originated fluids. A possible circulation model can be summarized as follows: CO₂-dominated fluids are produced by the partial melting of the upper mantle induced by depressurization due to lithospheric faults; the mantle-originated fluids are driven toward the surface through the 25-30 km thick crustal layers by the ATLF zone; the mantle volatiles permeate shallow crustal levels where crustal-type helium (⁴He) is originated by a wide range of metamorphic rocks; a further mixing occurs as the volatiles move across the groundwater circulating at relatively shallow depths where they suffer GWI (e.g., gas dissolution as a function of Bunsen coefficients and boundary pressure and temperature conditions) and mix with the atmospheric components (mainly represented by O₂ and N₂).

The upraising of large amounts of mantle-originated volatiles (as shown by the ³He content) at Alì Terme (Ionian Sea) and Capo Calavà (Tyrrhenian Sea) coincides with areas where the ATLF zone crosses the E-W-trending normal faults [63] on the Tyrrhenian Sea and the NE-SW normal structure of the Ionian Sea. The latter area that includes the eastern Peloritani area (Messina Strait and Ionian coasts, Figure 1) is shaped by a complex network of normal and transtensional faults [64] with extensional and right-lateral transtensional tectonics in the southern Messina Strait. This complex tectonic network looks to be able to generate high permeability zones where mantle volatiles can be driven to the surface, in full agreement with the obtained results.

On the other hand, the area is characterized by high strain rate (SR) [65], where the contemporary creeping of the upper and the lower crust may produce that high permeability zone where the mantle fluids move towards the surface without significant temporal changes, in full agreement with our findings. In terms of the escape of fluids, we found the highest CO₂ flow rates at the eastern side of the area (Peloritani chain). By contrast, the Nebrodi area, to the west of the ATLF, seems to accumulate strain at lower rates acting as a locked fault marked by lower CO₂ degassing rates and lower mantle volatile content. The ATLF zone cannot be simply described as a boundary line; however, it is a zone where many different fault segments separate locked fault volumes from areas where dilatancy goes on. Following the model proposed by Riguzzi et al. [65], the elastic energy accumulates in those areas where faults are locked and the strain rate (SR) is lower. High SR areas can be interpreted as regions where both the upper and the lower crust are creeping or alternatively where tectonic loading is more effective. Vice versa, lower strain rates suggest the presence of locked faults in a later stage in the seismic cycle [65]. Doglioni et al. [66] proposed that during an interseismic period, the brittle-ductile transition zone separates the brittle upper crustal layers, where the faults are almost locked, from the deeper shear zones where the ductile deep crust/shallow mantle layers are constantly creeping. In terms of the geochemical features of the fluids and their temporal behaviour, this model implies that deep, mantle-originated volatiles (marked, for example, by high helium isotopic ratios) are continuously released at the level of the ductile, upper mantle level. They cannot easily move toward the surface over areas marked by very low SR due to absence of creeping that is the lack of discontinuities where fluids can move across. Contrastingly significant contribution of mantle-type fluids and high degassing rates can be expected over areas undergoing dilatancy, namely, where high SR produce measurable crustal deformation due to creeping activity that allows a fast and, in terms of masses, significant contribution of mantle-derived volatiles.

The CPA is one of the highest seismic risk-prone territories in Italy and includes areas with both high and low strain rate accumulation [65], thus representing a suitable test site to better understand the behaviour of the fluids with respect to the tectonic setting.

7. Conclusions

The investigations carried out on the volatiles vented over the CPA show that a huge amount of CO₂ is daily dissolved in the groundwater. The dissolved gases carry helium with a typical, although variable, mantle signature here interpreted as induced by the lithospheric character of the ATLF.

During the evolution of the seismic cycle, deep lithospheric faults are able to drive mantle-derived fluids to the surface and to change the mixing proportions with the shallow fluids. Following the model proposed by Doglioni et al. [66], we may detect significant contribution of deep fluids over an area accumulating strain at a high rate in full agreement with the geochemical features of the fluids vented over the Peloritani and Nebrodi mountains. Due to high seismicity of the area, further investigations, including discrete and continuous monitoring activity as well as accurate estimations of the degassing rates, may provide a better insight in defining the role of the fluids during the final stage of a seismic cycle. Overall, an integrated approach coupling fluid geochemistry with accurate mapping of the actual SR analysis could represent an effective tool in order to better constrain those areas exposed to high seismic risk.

Data Availability

The data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that there is no conflict of interest regarding the publication of this paper.

Acknowledgments

We acknowledge the thorough and constructive reviews of two anonymous referees, which greatly improved the manuscript. The authors are also indebted to Prof. Alessandro Tripodo and Dott. Giuseppe Sabatino for their help during the field work. Mauro Martelli and Andrea Rizzo are kindly acknowledged for their support during the laboratory work. The research work was supported by the Istituto Nazionale di Geofisica e Vulcanologia (INGV)-DPC grants, S2 and V5 projects, Research Unit-Italiano.

References

R. Basili, G. Valensise, P. Vannoli et al., “The Database of Individual Seismogenic Sources (DISS), version 3: summarizing 20 years of research on Italy’s earthquake geology,” Tectonophysics, vol. 453, no. 1-4, pp. 20–43, 2008.

DISS Working Group, Database of Individual Seismogenic Sources (DISS), version 3.2.1: a compilation of potential sources for earthquakes larger than M 5.5 in Italy and surrounding areas, Istituto Nazionale di Geofisica e Vulcanologia, 2018, http://diss.rm.ingv.it/diss/.

F. Barberi, F. Innocenti, G. Ferrara, J. Keller, and L. Villari, “Evolution of Eolian arc volcanism (southern Tyrrhenian Sea),” Earth and Planetary Science Letters, vol. 21, no. 3, pp. 269–276, 1974.

A. Malinverno and W. B. F. Ryan, “Extension in the Tyrrhenian Sea and shortening in the Apennines as result of arc migration driven by sinking of the lithosphere,” Tectonics, vol. 5, no. 2, pp. 227–245, 1986.

J. F. Dewey, M. L. Helman, S. D. Knott, E. Turco, and D. H. W. Hutton, “Kinematics of the western Mediterranean,” Geological Society, London, Special Publications, vol. 45, no. 1, pp. 265–283, 1989.

M. Boccaletti, R. Nicolich, and L. Tortorici, “The Calabrian arc and the Ionian Sea in the dynamic evolution of the Central Mediterranean,” Marine Geology, vol. 55, no. 3-4, pp. 219–245, 1984.

E. Patacca, R. Sartori, and R. Scandone, Tyrrhenian basin and Apenninic arcs. Kinematic relations since late Tortonian times, vol. 45, Memorie della Società Geologica Italiana, 1990.

C. Monaco and L. Tortorici, “Active faulting in the Calabrian arc and eastern Sicily,” Journal of Geodynamics, vol. 29, no. 3-5, pp. 407–424, 2000.

S. Catalano and G. De Guidi, “Late Quaternary uplift of northeastern Sicily: relation with the active normal faulting deformation,” Journal of Geodynamics, vol. 36, no. 4, pp. 445–467, 2003.

S. Catalano, G. De Guidi, G. Romagnoli, S. Torrisi, G. Tortorici, and L. Tortorici, “The migration of plate boundaries in SE Sicily: influence on the large-scale kinematic model of the African promontory in southern Italy,” Tectonophysics, vol. 449, no. 1-4, pp. 41–62, 2008.

L. Ferranti, J. S. Oldow, B. D'Argenio et al., “Active deformation in southern Italy, Sicily and southern Sardinia from GPS velocities of the Peri-Tyrrhenian geodetic Array (PTGA),” Bollettino della Societa Geologica Italiana, vol. 127, pp. 299–316, 2008.

E. Gueguen, C. Doglioni, and M. Fernandez, “On the post-25 Ma geodynamic evolution of the western Mediterranean,” Tectonophysics, vol. 298, no. 1-3, pp. 259–269, 1998.

C. Faccenna, C. Piromallo, A. Crespo-Blanc, L. Jolivet, and F. Rossetti, “Lateral slab deformation and the origin of the western Mediterranean arcs,” Tectonics, vol. 23, no. 1, 2004.

G. Rosenbaum, M. Gasparon, F. P. Lucente, A. Peccerillo, and M. S. Miller, “Kinematics of slab tear faults during subduction segmentation and implications for Italian magmatism,” Tectonics, vol. 27, no. 2, 2008.

C. Faccenna, T. W. Becker, F. P. Lucente, L. Jolivet, and F. Rossetti, “History of subduction and back-arc extension in the Central Mediterranean,” Geophysical Journal International, vol. 145, no. 3, pp. 809–820, 2001.

M. Boccaletti, G. Cello, and L. Tortorici, “Strike-slip deformation as a fundamental process during the Neogene-Quaternary evolution of the Tunisian-Pelagian area,” Annales Tectonicae, vol. 4, p. 16, 1990.

F. Ghisetti and L. Vezzani, “Thin-skinned deformation of the Western Sicily thrust belt and relationships with crustal shortening mesostructural data on the M. Kumeta-Alcantara fault zone and related structures,” Bollettino della Societa Geologica Italiana, vol. 103, p. 28, 1984.

I. Finetti and A. Del Ben, “Geophysical study of the Tyrrhenian opening,” Bollettino di Geofisica Teorica ed Applicata, vol. 28, p. 81, 1986.

G. Giunta, D. Luzio, F. Agosta et al., “An integrated approach to investigate the seismotectonics of northern Sicily and southern Tyrrhenian,” Tectonophysics, vol. 476, no. 1-2, pp. 13–21, 2009.

G. Giunta, F. Nigro, A. Renda, and A. Giorgianni, The Sicilian-Maghrebides Tyrrhenian Margin: A Neotectonic Evolutionary Model, vol. 119, Memorie della Società Geologica Italiana, 2000.

A. Frepoli and A. Amato, “Fault plane solutions of crustal earthquakes in southern Italy (1988-1995): seismotectonic implications,” Annali di Geofisica, vol. 43, pp. 437–467, 2000.

S. Pondrelli, A. Morelli, G. Ekstrom, S. Mazza, E. Boschi, and A. M. Dziewonski, “European-Mediterranean regional centroid-moment tensors: 1997-2000,” Physics of the Earth and Planetary Interiors, vol. 130, no. 1-2, pp. 71–101, 2002.

S. Pondrelli, A. Morelli, and G. Ekstrom, “European-Mediterranean regional centroid-moment tensor catalog: solutions for years 2001 and 2002,” Physics of the Earth and Planetary Interiors, vol. 145, no. 1-4, pp. 127–147, 2004.

S. Pondrelli, S. Salimbeni, G. Ekstrom, A. Morelli, P. Gasperini, and G. Vannucci, “The Italian CMT dataset from 1977 to the present,” Physics of the Earth and Planetary Interiors, vol. 159, no. 3-4, pp. 286–303, 2006.

G. Neri, G. Barberi, G. Oliva, and B. Orecchio, “Spatial variations of seismogenic stress orientations in Sicily, South Italy,” Physics of the Earth and Planetary Interiors, vol. 148, no. 2-4, pp. 175–191, 2005.

G. Giunta, F. Nigro, A. Renda, F. Andaloro, A. Giorgianni, and V. Sparacino, “The structural and morphotectonic framework of the NW Sicilian Maghrebides, in a seismotectonic perspective,” Studi Geologici Camerti, vol. 5, 2002.

C. Hollenstein, H. G. Kahle, A. Geiger, S. Jenny, S. Goes, and D. Giardini, “New GPS constraints on the Africa-Eurasia plate boundary zone in southern Italy,” Geophysical Research Letters, vol. 30, no. 18, 2003.

F. Pepe, G. Bertotti, and S. Cloetingh, “Tectono-stratigraphic modelling of the North Sicily continental margin (southern Tyrrhenian Sea),” Tectonophysics, vol. 384, no. 1-4, pp. 257–273, 2004.

A. Billi, D. Presti, B. Orecchio, C. Faccenna, and G. Neri, “Incipient extension along the active convergent margin of Nubia in Sicily, Italy: Cefalù-Etna seismic zone,” Tectonics, vol. 29, no. 4, 2010.

G. Lavecchia, F. Ferrarini, R. de Nardis, F. Visini, and M. S. Barbano, “Active thrusting as a possible seismogenic source in Sicily (southern Italy): some insights from integrated structural-kinematic and seismological data,” Tectonophysics, vol. 445, no. 3-4, pp. 145–167, 2007.

M. Mattia, M. Palano, V. Bruno, F. Cannavo, A. Bonaccorso, and S. Gresta, “Tectonic features of the Lipari-Vulcano complex (Aeolian archipelago, Italy) from 10 years (1996-2006) of GPS data,” Terra Nova, vol. 20, no. 5, pp. 370–377, 2008.

G. Neri, G. Barberi, G. Oliva, and B. Orecchio, “Tectonic stress and seismogenic faulting in the area of the 1908 Messina earthquake, South Italy,” Geophysical Research Letters, vol. 31, no. 10, 2004.

L. Ferranti, C. Monaco, D. Morelli, F. Antonioli, and L. Maschio, “Holocene activity of the Scilla fault, southern Calabria: insights from coastal morphological and structural investigations,” Tectonophysics, vol. 453, no. 1-4, pp. 74–93, 2008.

M. Mattia, M. Palano, V. Bruno, and F. Cannavo, “Crustal motion along the Calabro-Peloritano Arc as imaged by twelve years of measurements on a dense GPS network,” Tectonophysics, vol. 476, no. 3-4, pp. 528–537, 2009.

E. Serpelloni, R. Bürgmann, M. Anzidei, P. Baldi, B. Mastrolembo Ventura, and E. Boschi, “Strain accumulation across the Messina Straits and kinematics of Sicily and Calabria from GPS data and dislocation modeling,” Earth and Planetary Science Letters, vol. 298, no. 3-4, pp. 347–360, 2010.

M. Cuffaro, F. Riguzzi, D. Scrocca, and C. Doglioni, “Coexisting tectonic settings: the example of the southern Tyrrhenian Sea,” International Journal of Earth Sciences, vol. 100, no. 8, pp. 1915–1924, 2011.

S. Goes, D. Giardini, S. Jenny, C. Hollenstein, H. G. Kahle, and A. Geiger, “A recent tectonic reorganization in the South-Central Mediterranean,” Earth and Planetary Science Letters, vol. 226, no. 3-4, pp. 335–345, 2004.

R. Nicolich, M. Laigle, A. Hirn, L. Cernobori, and J. Gallart, “Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution,” Tectonophysics, vol. 329, no. 1-4, pp. 121–139, 2000.

C. Doglioni, M. Ligi, D. Scrocca et al., “The tectonic puzzle of the Messina area (southern Italy): insights from new seismic reflection data,” Scientific Reports, vol. 2, no. 1, 2012.

G. Rosenbaum and G. S. Lister, “Neogene and quaternary rollback evolution of the Tyrrhenian Sea, the Apennines, and the Sicilian Maghrebides,” Tectonics, vol. 23, no. 1, 2004.

C. Chiarabba, P. De Gori, and F. Speranza, “The southern Tyrrhenian subduction zone: deep geometry, magmatism and Plio-Pleistocene evolution,” Earth and Planetary Science Letters, vol. 268, no. 3-4, pp. 408–423, 2008.

A. Billi, G. Barberi, C. Faccenna, G. Neri, F. Pepe, and A. Sulli, “Tectonics and seismicity of the Tindari Fault System, southern Italy: crustal deformations at the transition between ongoing contractional and extensional domains located above the edge of a subducting slab,” Tectonics, vol. 25, no. 2, 2006.

G. Ventura, “Tectonics, structural evolution and caldera formation on Vulcano Island (Aeolian archipelago, southern Tyrrhenian Sea),” Journal of Volcanology and Geothermal Research, vol. 60, no. 3-4, pp. 207–224, 1994.

R. Mazzuoli, L. Tortorici, and G. Ventura, “Oblique rifting in Salina, Lipari and Vulcano Islands (Aeolian Islands, southern Italy),” Terra Nova, vol. 7, no. 4, pp. 444–452, 1995.

F. Ghisetti, “Relazioni tra strutture e fasi trascorrenti e distensive lungo i sistemi Messina-Fiumefreddo, Tindari-Letojanni e Alia-Malvagna (Sicilia nord-orientale): uno studio microtettonico,” Geologica Romana, vol. 18, p. 35, 1979.

M. Palano, L. Ferranti, C. Monaco et al., “GPS velocity and strain fields in Sicily and southern Calabria, Italy: updated geodetic constraints on tectonic block interaction in the Central Mediterranean,” Journal of Geophysical Research: Solid Earth, vol. 117, no. B7, 2012.

F. Italiano, P. Bonfanti, M. Ditta, R. Petrini, and F. Slejko, “Helium and carbon isotopes in the dissolved gases of Friuli region (NE Italy): geochemical evidence of CO₂ production and degassing over a seismically active area,” Chemical Geology, vol. 266, no. 1-2, pp. 76–85, 2009.

F. Italiano, A. Sasmaz, G. Yuce, and O. O. Okan, “Thermal fluids along the East Anatolian Fault Zone (EAFZ): geochemical features and relationships with the tectonic setting,” Chemical Geology, vol. 339, pp. 103–114, 2013.

F. Italiano, G. Yuce, I. T. Uysal, M. Gasparon, and G. Morelli, “Insights into mantle-type volatiles contribution from dissolved gases in artesian waters of the Great Artesian Basin, Australia,” Chemical Geology, vol. 378-379, pp. 75–88, 2014.

D. R. Hilton, “The helium and carbon isotope systematics of a continental geothermal system: results from monitoring studies at Long Valley caldera (California, U.S.A.),” Chemical Geology, vol. 127, no. 4, pp. 269–295, 1996.

Y. Sano and H. Wakita, “Precise measurement of helium-isotopes in terrestrial gases,” Bulletin of the Chemical Society of Japan, vol. 61, no. 4, pp. 1153–1157, 1988.

F. Italiano, G. Martinelli, and P. M. Nuccio, “Anomalies of mantle-derived helium during the 1997-1998 seismic swarm of Umbria-Marche, Italy,” Geophysical Research Letters, vol. 28, no. 5, pp. 839–842, 2001.

S. Giammanco, M. Palano, A. Scaltrito, L. Scarfi, and F. Sortino, “Possible role of fluid overpressure in the generation of earthquake swarms in active tectonic areas: the case of the Peloritani Mts. (Sicily, Italy),” Journal of Volcanology and Geothermal Research, vol. 178, no. 4, pp. 795–806, 2008.

F. Italiano, M. Martelli, G. Martinelli, and P. M. Nuccio, “Geochemical evidence of melt intrusions along lithospheric faults of the southern Apennines, Italy: geodynamic and seismogenic implications,” Journal of Geophysical Research: Solid Earth, vol. 105, no. B6, pp. 13569–13578, 2000.

D. R. Hilton, K. Hammerschmidt, G. Loock, and H. Friedrichsen, “Helium and argon isotope systematics of the Central Lau Basin and Valu Fa Ridge - evidence of crust mantle interactions in a back-arc basin,” Geochimica et Cosmochimica Acta, vol. 57, no. 12, pp. 2819–2841, 1993.

D. R. Hilton, K. Hammerschmidt, S. Teufel, and H. Friedrichsen, “Helium isotope characteristics of Andean geothermal fluids and lavas,” Earth and Planetary Science Letters, vol. 120, no. 3-4, pp. 265–282, 1993.

K. A. Farley and E. Neroda, “Noble gases in the Earth’s mantle,” Annual Review of Earth and Planetary Sciences, vol. 26, no. 1, pp. 189–218, 1998.

M. Ozima and G. Igarashi, “The primordial noble gases in the Earth: a key constraint on Earth evolution models,” Earth and Planetary Science Letters, vol. 176, no. 2, pp. 219–232, 2000.

T. J. Dunai and H. Baur, “Helium, neon, and argon systematics of the European subcontinental mantle - implications for its geochemical evolution,” Geochimica et Cosmochimica Acta, vol. 59, no. 13, pp. 2767–2783, 1995.

F. Italiano, P. Bonfanti, L. Pizzino, and F. Quattrocchi, “Geochemistry of fluids discharged over the seismic area of the southern Apennines (Calabria region, southern Italy): implications for fluid-fault relationships,” Applied Geochemistry, vol. 25, no. 4, pp. 540–554, 2010.

B. Marty and A. Jambon, “C³He in volatile fluxes from the solid earth: implications for carbon geodynamics,” Earth and Planetary Science Letters, vol. 83, no. 1-4, pp. 16–26, 1987.

R. K. O'Nions and E. R. Oxburgh, “Heat and helium in the earth,” Nature, vol. 306, no. 5942, pp. 429–431, 1983.

G. Barreca, V. Bruno, F. Cultrera, M. Mattia, C. Monaco, and L. Scarfi, “New insights in the geodynamics of the Lipari-Vulcano area (Aeolian archipelago, southern Italy) from geological, geodetic and seismological data,” Journal of Geodynamics, vol. 82, pp. 150–167, 2014.

C. Doglioni, F. Innocenti, and G. Mariotti, “Why Mt Etna?” Terra Nova, vol. 13, no. 1, pp. 25–31, 2001.

F. Riguzzi, M. Crespi, R. Devoti, C. Doglioni, G. Pietrantonio, and A. R. Pisani, “Geodetic strain rate and earthquake size: new clues for seismic hazard studies,” Physics of the Earth and Planetary Interiors, vol. 206-207, pp. 67–75, 2012.

C. Doglioni, S. Barba, E. Carminati, and F. Riguzzi, “Fault on-off versus coseismic fluids reaction,” Geoscience Frontiers, vol. 5, no. 6, pp. 767–780, 2014.