Radiogenic helium degassing and rock fracturing - Wiley Online Library

PUBLICATIONS Journal of Geophysical Research: Solid Earth RESEARCH ARTICLE 10.1002/2014JB011462 Key Points: • Extraordinary high radiogenic helium flux in continental region 4 • Release of crustal He due to rock fracturing • Relationship between rock involved in earthquake and radiogenic He flux

Radiogenic helium degassing and rock fracturing: A case study of the southern Apennines active tectonic region Antonio Caracausi1 and Michele Paternoster2 1

Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Palermo, Italy, 2Dipartimento di Scienze, Università della Basilicata, Potenza, Italy

Abstract

Correspondence to: A. Caracausi, [email protected]

Citation: Caracausi, A., and M. Paternoster (2015), Radiogenic helium degassing and rock fracturing: A case study of the southern Apennines active tectonic region, J. Geophys. Res. Solid Earth, 120, 2200–2211, doi:10.1002/2014JB011462. Received 15 JUL 2014 Accepted 22 FEB 2015 Accepted article online 25 FEB 2015 Published online 15 APR 2015

Gas from mud volcanoes, dry mofettes, springs, and wells were sampled in a region of active tectonics and high seismicity in the southern Apennines (Italy), where there is a long history of disastrous earthquakes, with the latest (Ms = 6.9) occurring in 1980. The fluids consist of a mixture of mantle-derived and crust-derived volatiles, with a low atmosphere-derived contribution, as identified by the He isotope signature and He/Ne ratio measurements. One year of monthly monitoring of the He concentrations and He isotopes revealed no seasonal modifications or variations induced by low seismicity. There are extraordinary high outputs of 4He produced in the crust in the area (up to 2.5 × 1028 atoms yr1). These outputs cannot be solely due to the whole-rock production rate and a long-lasting diffusion degassing through the crust of the produced 4He. This study explored the relation between the volume of fractured rock and the related release of He. The results support that crustal degassing can be controlled by tectonic events resulting in earthquakes. The high seismicity in this sector of the Apennines provides the conditions necessary for a massive release of He that has accumulated in the rock over a long time period. We identified that the assessed high crustal 4He output can be attributed to an intense fracturing of a calculable volume of rock, which gives new constraints on the volume of rock involved in high-magnitude earthquakes in the region.

1. Introduction Helium is recognized as a powerful tracer in various fields, including groundwater hydrology, hydrocarbon exploration, mantle processes, and magma degassing [Burnard, 2013]. Melting and the release of volatiles from magma are effective processes for degassing He from mantle. Degassing of He produced in the crust occurs under different conditions, and it mainly consists of two stages that act on different scales: (1) the release of volatiles from the mineral/rocks and (2) their transport toward the surface [Ballentine and Burnard, 2002]. There is large variability in the degassing 4He flux from the continental crust [Torgersen, 2010]. Furthermore, the amount of He accumulated within the crustal rocks themselves and the rapidity with which He can be purged have not yet been fully explored [i.e., Torgersen, 2010; Lowenstern et al., 2014]. While He is highly mobile, it cannot extensively escape without a network of pathways allowing advective fluid flow in the crust, and it remains trapped in very low permeability rocks. These features mean that 4He can reside undisturbed in tectonically stable regions, allowing it to accumulate over a very long time scale and potentially be used as a groundwater dating tool [e.g., Torgersen, 1980; Stute et al., 1992; Kulongoski et al., 2005; Zhou and Ballentine, 2006]. Furthermore, there are also considerable evidences that large-scale vertical transport of fluids in the continental crust is probably both advective and episodic [e.g., Etheridge et al., 1983, 1984; Torgersen and Clarke, 1985; Hu et al., 2009]. Experimental studies and theoretical calculations have highlighted that noble gases (e.g., He and Ar) produced in the crust can be effectively released under compression and the related dilatancy [Scholz et al., 1973; Honda et al., 1982]. Furthermore, the flux of noble gases produced in the crust is also controlled by the fracturing of rock [Torgersen and O’Donnell, 1991]. Scholz et al. [1973] highlighted that the release of volatiles from rock can be explained using a dilatancy model, which represented the first indication of the role of dilatancy in triggering earthquakes. Honda et al. [1982] subsequently highlighted that the amount of rare gases degassed from compressed rock is proportional to the degree of dilatancy, probably due to the creation of newly exposed surfaces by microcracking, which allows atoms of rare gases residing in the vicinity of the fresh surface to escape. Dilatant fractures in normal fault zones are widely recognized as

CARACAUSI AND PATERNOSTER

©2015. American Geophysical Union. All Rights Reserved.

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pathways of fluid flow in the crust, but the structure of these fracture networks, their connectivity, and their temporal evolution are poorly understood [Holland et al., 2011]. The transfer of He through the crust, from the production site to the near-surface system, needs a driving force. This can take the form of concentration and pressure gradients that result in diffusion and advective fluid flow, respectively. Helium remains in trace amounts in the pore fluids, and its movement is strongly dependent on the behavior of the fluids occupying the pore spaces [Ballentine and Burnard, 2002]. Hence, the transfer of fluids within the pore space can carry the crustal He away from the site of production. The southern Apennines is characterized by the active degassing of mantle-derived volatiles (e.g., He and CO2) away from the volcanic centers [e.g., Italiano et al., 2000, 2001; Minissale, 2004; Caracausi et al., 2013; Nuccio et al., 2014]. Furthermore, in the central southern Apennines, there is a relatively high potential to mobilize accumulated crustal volatiles [e.g., 4He produced by U and Th decay in the crust (henceforth referred to as 4Hecrust)]. This is due to active tectonics affecting the investigated region since the Apennine orogeny [e.g., Patacca and Scandone, 2001] and the disastrous earthquakes that have occurred since historical times. The last damaging earthquake (Ms = 6.9 [Deschamps and King, 1983]) occurred in 1980 in the Campania-Lucania region, which killed more than 3000 people. The background seismicity in this area is mainly controlled by a high pore fluid pressure [Amoroso et al., 2014]. Geological evidences coupled to lithological properties as well as the measured distribution of microseismicity in the southern Apennine highlight that the increase in the pore fluid pressure in fluid-filled cracks around major faults leads to earthquake nucleation [Amoroso et al., 2014]. Hence, seismic pumping along major faults carries fluids through the conduit system represented by the intensely fractured zone. Here we discuss the crustal output of radiogenic He in the natural emissions of CO2 and CH4 in the southern Apennines. The present study investigated the possible role of rock fracturing in the 4Hecrust output in a tectonically active region characterized by high seismicity.

2. Sampling and Analytical Method The samples consisted of gas taken from dry mofettes at Mefite D’Ansanto, San Sisto, and Maschito sites (MDA, SSi, and Mas, respectively); mud volcanoes at Pineto and Malvizia (Pin and Mal, respectively); and bubbling gases taken from water at Telese, Ailano, Ciorlano, San Cataldo, and Tramutola (Tel, Ail, Cio, SCa, and Tra, respectively). The locations of the sampling sites are shown in Figure 1. Gas samples were collected in Pyrex bottles with vacuum valves at both ends, taking care to prevent air contamination. The gases emitted from the dry mofettes together with those emitted at Tel, Ail, and Cio are dominated by CO2 [Italiano et al., 2000; Caracausi et al., 2013; Istituto Nazionale Geofisica e Vulcanologia (INGV)-Palermo data], whereas Tra, Pin, and Mal are rich in CH4 [Etiope et al., 2007; Caracausi et al., 2013], and SCa gases are dominated by N2 [Caracausi et al., 2013]. The 3He/4He ratios were measured in a split-flight-tube mass spectrometer (GVI Helix SFT). Ion beams of 3He and 4He were simultaneously detected by a double collector system that measured the isotope ratios to a precision of within 0.05. The analytical error for He concentrations was ≤15%. Purified atmospheric He was used as a running standard. 4He/20Ne ratios were measured with a quadrupole mass spectrometer. All of the samples were analyzed within 1 week of their collection, and all analyses were carried out in the laboratories of the Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo.

3. Results and Discussion 3.1. He Isotopes The He isotope ratios along with the He concentrations and the 4He/20Ne ratios of the sampled fluids are reported in Table 1. All of the 4He/20Ne ratios are higher than the value for the atmosphere (0.318 [Ozima and Podosek, 2002]), indicating that air contamination was negligible in all of the investigated fluids (Table 1 and Figure 2). According to previous investigations [Italiano et al., 2000; Caracausi et al., 2013], the He isotope signature of the fluids emitted in this sector of the Apennines exhibits a wide range of variability (Table 1 and Figure 2), and contributions of He from the mantle was present in most of the samples with high 4He/20Ne values. R/Rac values of >0.1 (R/Rac is the helium signature corrected for air contributions [Giggenbach et al., 1993]) are indicative of a contribution of He from the mantle [Ballentine et al., 2002], and



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Figure 1. Map of southern Italy. Geological map of the southern Appenines (from Bentivenga et al. [2004], modified) with the location of the sampling sites (black circles: CO2-rich fluids, black rectangles: CH4-rich fluids, and black star: N2-rich fluid).

the Rc/Ra ratios in fluids emitted from all of the mofettes are higher than 1.10 Ra. In addition to previous investigations [Caracausi et al., 2013, and references therein], the high R/Rac values at Ail and Cio (>0.6) highlight that the degassing of mantle-derived He also extends to the central Apennines. In contrast, the He in gas emitted from the mud volcanoes (Pin and Mal), which are dominated by CH4, is exclusively of radiogenic origin (0.03 < R/Rac < 0.07). The Tra well volatiles are rich in CH4 and have an R/Rac ratio of 1.16 ± 0.05, while the R/Rac ratio in the N2-dominated gas of the SCa spring is 0.11 ± 0.05. We sampled the gases emitted at MDA, SSi, Tra, and SCa monthly from June 2006 to October 2007 with the purpose of identifying possible seasonal variations and/or modifications due to the low seismicity occurring in the region (Ml < 3; http://iside.rm.ingv.it/iside/standard/index.jsp). The gases emitted at the selected sites have different chemical compositions, with both SSi and MDA emissions being dominated by CO2 and Tra and SCa being dominated by CH4 and N2, respectively. No variations in either the He concentrations or He isotope ratios were recorded over a 1 year period (Figure 3) or relative to previously obtained data [Italiano et al., 2000, 2001; Caracausi et al., 2013]. However, Italiano et al. [2001] highlighted earthquake-related variations in the chemistry of the fluids emitted at Tra, corresponding to the 1996 Irpinia earthquake (Ml = 4.9). The wide ranges of the measured He isotope compositions and He/Ne ratios in the collected gases can be explained in terms of mixing between three sources of He: atmosphere, mantle, and crust (Figure 2). According to the geodynamic scenario of this sector of the Apennine, the He signature in the mantle below western side of central southern Apennine suffers of a stronger enrichment by metasomatic fluids derived from subduction than that of the eastern side and the He isotope signature of the mantle below the western Apennine is lower than the signature below the eastern Apennine [Martelli et al., 2004, 2008; Caracausi et al., 2013]. Figure 2 shows illustrative curves for the mixing with different geological fluid end-members, where fluid 1 is a mantle end-member with 3He/4He = 6.0 Ra and fluids 2 and 3 are mantle end-members with a He isotope end-member of 2.8 and 3.6 Ra, respectively. Fluid 1 is representative of the mantle source below the eastern side of the Apennine [Caracausi et al., 2013]. In contrast, fluids 2 and 3 are representative of the mantle source on the western side of the Apennine, and it can be assumed that the He isotopes of the source range from 2.8 to 3.6 R/Ra; these values are representative of those measured in the fluid inclusions and gas emitted from Mount Vesuvius and the Phlegrean Field areas [e.g., Sano et al., 1989; Tedesco et al., 1990; Marty et al., 1994; Martelli et al., 2004, and references therein]. Considering the position of the investigated



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Table 1. Geochemical Data Sample Mal Mal Mal Mas Mas Mas Mas Ail Cio Tel Pin SCa SCa SCa SCa SCa SCa SCa SCa SCa SCa SCa SCa SCa SCa SSi SSi SSi SSi SSi SSi SSi SSi SSi SSi SSi SSi SSi SSi SSi SSi

Date

He/Ne

He

R/Ra

R/Rac

Sample

Date

He/Ne

He

R/Ra

R/Rac

6 Oct 6 Nov 6 Oct 5 Jul 6 Mar 10 May 11 May 7 Mar 7 Mar 7 Feb 7 Mar 5 Jul 6 Jun 6 Jul 6 Aug 6 Oct 6 Nov 6 Dec 7 Jan 7 Feb 7 Apr 7 Jun 7 Jul 7 Aug 7 Oct 6 Mar 6 Jun 6 Jul 6 Sep 6 Nov 6 Dec 7 Jan 7 Feb 7 Mar 7 Apr 7 Jun 7 Jul 7 Sep 7 Oct 7 Nov 13 Jun

126.3 192.4 126.3 943.0 115.8 1391.7 415.9 21.4 648.2 56.3 30.0 25.0 14.9 17.8 16.2 16.0 172.1 18.3 18.1 19.5 18.8 14.5 7.0 19.5 12.2 n.a. 262.8 n.a. 18.1 219.0 199.6 1446.6 9.9 1186.8 1395.0 1250.0 418.0 494.6 710.1 942.9 341.2

433.8 489.8 433.8 49.9 66.2 58.8 50.4 0.60 19.8 56.3 98.0 240.3 261.7 235.9 268.8 180.0 193.3 186.6 243.7 237.6 n.a n.a. n.a. 208.4 268.3 17.0 21.5 20.3 16.0 18.3 21.5 19.3 17.4 18.1 17.8 15.8 20.5 19.2 21.8 22.2 20.1

0.05 0.07 0.05 4.61 4.72 4.64 4.68 0.80 0.66 1.89 0.03 0.11 0.11 0.10 0.13 0.11 0.11 0.11 0.11 0.11 n.a n.a n.a. 0.10 0.11 n.a. 1.27 n.a. 1.25 1.31 1.33 1.28 1.30 1.29 1,33 1.30 1.27 1.33 1.28 1.32 1.31

0.05 0.07 0.05 4.62 4.73 4.64 4.68 0.80 0.66 1.89 0.03 0.10 0.10 0.09 0.12 0.10 0.10 0.10 0.10 0.10 n.a n.a. n.a. 0.09 0.09 n.a. 1.27 n.a. 1.26 1.31 1.33 1.28 1.32 1.29 1.33 1.30 1.27 1.34 1.28 1.32 1.31

MdA MdA MdA MdA MdA MdA MdA MdA MdA MdA MdA MdA MdA MdA MdA MdA MdA MdA MdA MdA MdA MdA MdA Tra Tra Tra Tra Tra Tra Tra Tra Tra Tra Tra Tra Tra Tra Tra Tra Tra Tra Tra

6 Mar 6 Jun 6 Jul 6 Nov 6 Mar 6 Jun 6 Jul 6 Nov 7 Dec 7 Jan 7 Feb 7 Mar 7 Apr 7 May 7 Jun 7 Jul 1 Nov 2007 20 Nov 2007 29 Nov 2007 9 Mar 9 Apr 9 Nov 13 Jun 5 Jul 6 Mar 6 Jun 6 Jul 6 Aug 6 Sep 6 Oct 6 Nov 6 Dec 7 Jan 7 Feb 7 Mar 7 Apr 7 May 7 Jun 7 Jul 7 Aug 7 Nov 13 Jun

53.8 303.4 96.6 22.0 53.8 303.4 96.6 543.6 133.8 1055.3 1033.8 581.3 770.8 971.7 2075.4 115.7 590.0 138.4 1236.7 40.5 30.9 90.1 86.3 943.6 278.6 595.0 494.3 19.1 396.1 685.2 606.2 743.1 764.1 849.8 1008.8 1515.4 963.3 1338.6 731.4 583.1 538.8 705.5

14.9 14.6 14.4 15.1 14.9 14.6 14.4 12.0 13.7 13.5 14.4 14.5 13.7 14.2 13.0 14.9 15.6 15.3 16.4 12.8 13.9 13.0 14.4 297.8 318.4 288.9 329.8 301.0 264.2 314.5 279.6 317.6 358.6 354.1 334.8 304.3 324.9 330.1 369.4 353.9 274.0 365.3

2.71 2.75 2.68 2.65 2.71 2.75 2.68 2.75 2,67 2.71 n.a 2.72 2.70 2.69 2.72 2.68 2.74 2.78 2.77 2.69 2.68 n.a 2.69 1.17 1.16 1.15 1.15 1.12 1.15 1.14 1.16 1.16 1.13 1.14 n.a. 1.16 1.11 1.16 1.15 1.17 1.16 1.15

2.72 2.75 2.68 2.68 2.72 2.75 2.68 2.75 2.67 2.71 n.a. 2.72 2.70 2.69 2.72 2.69 2.74 2.78 2.77 2.70 2.70 n.a 2.69 1.17 1.16 1.15 1.15 1.13 1.15 1.14 1.16 1.16 1.13 1.14 n.a. 1.16 1.11 1.16 1.15 1.17 1.16 1.15

a

3

4

He concentrations are expressed in ppmv. He isotope ratios are expressed in R/Ra units, where R is the He/ He ratio 3 4 6 [Ozima and Podosek, 2002]). R/Rac is the R/Ra value of the sample and Ra is the He/ He ratio in air (1.39 × 10 corrected for air contributions [Giggenbach et al., 1993].

manifestations in the geological context of the area, in order to calculate the percentage of He coming from the mantle in the sampled fluids, we assumed fluid 1 to be the mantle source for Mas and fluids 2 and 3 to be those for the other mofettes and Tra. In fact, Mas is 25 km from Mount Vulture and is located along a regional tectonic discontinuity connected to that volcano [Caracausi et al., 2013; Nuccio et al., 2014]. In contrast, the other mofettes and Tra are located in the central western sector of the Apennines. Furthermore, the He isotope signature of the fluids emitted at MDA, which is characterized by the highest natural emissions of CO2-rich gases from a nonvolcanic environment ever measured worldwide, is in the range of the values of the fluids emitted from the fumaroles at Mount Vesuvius [Chiodini et al., 2010]. The sampled fluids contain mantle-derived He from 2% (SCa) to 80% (MDA) and from 3% (SCa) to 67% (SSi) for isotopic He with a mantle end-member characterized by an isotope signature of 3.6 and 2.8 Ra, respectively. Assuming an He mantle end-member with 2.8 Ra, the He at MDA is almost exclusively derived from the mantle. Moreover, assuming an isotope signature of 6.0 Ra for the end-members in the region of



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Figure 2. R/Ra ratio versus He/ Ne. He isotope ratios and He/ Ne ratios for fluids emitted in the southern Apennines. The wide range of values can be explained by different mixing percentages between three possible He sources: a mantle 4 end-member, the atmosphere (R/Ra = 1), and the Hecrust (R/Ra = 0.01). In accordance with the geodynamical framework of the area, we considered three possible mantle end-members: the black, green, and red lines are for mixing with mantle end-members with 6.0 Ra, 3.6 Ra, and 2.8 Ra, respectively (see discussion in the text).

Mount Vulture (fluid 1), He outgassed at Mas, which is 20 km from the volcano, contains 76 ± 2% mantle-derived He. The He isotope ratios of fluids emitted at the mud volcanoes (Pin and Mal) ranges from 0.03 to 0.07 Rac. These values are within the range of a typical radiogenic end-member [Ozima and Podosek, 2002], and their position in the graph of R/Ra versus He/Ne (Figure 2) is independent of the value of the mantle He end-member (Figure 2). The above described results confirm that the main natural gas emissions in this region of the Apennines predominantly comprise mantle-derived He, with the exception of SCa and the mud volcanoes, where He is exclusively due to a crustal radiogenic production. Furthermore, they highlight that the degassing of mantle-derived He in the southern Apennines is not mainly localized to the western sector, since it also occurs up to 50 km from the Tyrrhenian coast (Figure 1). 3.2. Radiogenic He Flux Mantle-derived He emitted in central southern Apennines, away from the volcanic complex, is due to the outgassing of melts that have intruded into the crust [Italiano et al., 2000; Caracausi et al., 2013; Nuccio et al., 2014]. The outgassing of mantle-derived volatiles in this sector of the Apennine is coupled to high heat flows, which are in the range of those in Italian volcanic areas [Italiano et al., 2001, and references therein]. The geochemical evidence of the occurrence of melt intrusions into the crust is strongly supported by geophysical data [e.g., Doglioni et al., 1996; Mongelli et al., 1996; Mele et al., 1997; Florio et al., 2009; Ökeler et al., 2009; Cella and Fedi, 2012]. The presence of intrusive magmatic bodies located in correspondence of the Mount Vulture line, which cuts from east to west the Apennine [D’Orazio et al., 2007; Caracausi et al., 2013], has been recognized on the basis of inversion of potential field data [Cella and Fedi, 2012]. Waveform modeling of regional surface waves showed that partial melting due to asthenospheric upwelling occurs beneath the Apennine [Ökeler et al., 2009]. An interpretation of aeromagnetic data indicates the presence of intrusions, probably linked to the activity of the Mount Vulture [Florio et al., 2009]. An attenuation of shear wave observed by Mele et al. [1997] indicated that the lithospheric mantle below the internal units of the Apennine and the western Italy is contaminated by advection of a relatively hot asthenospheric mantle at shallow depth, which produces a drastic change of its thermal structure.



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Figure 3. Temporal variations. (a, c, e, and g) He concentrations (in ppmv) over time at SCa (San Cataldo), SSi (San Sisto), 3 4 MDA (Mefite D’Ansanto), and Tra (Tramutola), respectively. (b, d, f, and h) He/ He ratios (as R/Rac values) over time at SCa, SSi, MDA, and Tra, respectively.

Italiano et al. [2000] calculated the He output from the main CO2-rich natural emissions in the Apennines based on the measured CO2 output and CO2/He ratio. The computed output of mantle-derived He is within the range of outputs from the main active volcanic areas in Italy, such as Vulcano (Aeolian Island) and the Phlegrean Fields. Furthermore, the absence of temporal variation of the computed flux indicates that the gas emissions do not predominantly come from a shallow local crustal reservoir [Italiano et al., 2000]. Degassing of mantle-derived He is typical of continental areas subject to recent volcanism and/or distensive tectonics [e.g., O’Nions and Oxburgh, 1988] or to the ascent of metasomatic fluids related to subduction [Umeda et al., 2007]. In contrast, outgassing of 4Hecrust dominates the stable continental crust, which is characterized by compressive tectonics [O’Nions and Oxburgh, 1988]. Lowenstern et al. [2014] recently highlighted an extraordinary degassing of a billion of years of accumulated crustal radiogenic He at



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a

Hecrust Output

% rad (End-Member 1)

% rad (End-Member 2)

3 54 34 60 76

20 62 45 65

4

Hecrust Output (End-Member 1) 9.4 E + 26 3.2 E + 27 1.2 E + 27 2.3 E + 28 6.8 E + 26 6.4 E + 22

4

Hecrust Output (End-Member 2) 6.2 E + 27 3.7 E + 27 1.6 E + 27 2.5 E + 28

4

The % rad (end-member 1) is the percentage of crustal He by assuming R/Ra = 2.8 for the mantle end-member. The % rad (end-member 2) is referenced to an R/Ra value of 3.6 for the mantle end-member. For the Mas sample, the assumed R/Ra end-member value is 6.0. These percentages were computed based on the approach proposed by Giggenbach et al. [1993]. This table lists the average value of all the samples for each site of the computed “% rad.” 4 The Hecrust output is expressed in atoms per year. b Pin represents the pure crust-derived He term.

Yellowstone. This area became stabilized 2.8 Gyr ago, since when it has undergone relatively little tectonism, which has allowed He to accumulate over a very long geological time, only to be liberated over the past 2 Ma by intense crustal metamorphism [Lowenstern et al., 2014]. The Apennine is an active tectonic region characterized by high seismicity, where elevated He flux is a specific and permanent characteristic on the regional scale and it is not related to quick discharge of gas accumulated in shallower reservoirs [Italiano et al., 2000]. Previous geochemical investigations [e.g., Chiodini et al., 2010; Caracausi et al., 2013] have highlighted that the C/He ratio in the emitted fluids is representative of their source. Therefore, in accordance with Italiano et al. [2000], in the present study, we calculated the output of 4He produced in the crust (4Hecout) at MDA, SSI, Tel, Tra, and Mas by using the output of the main gaseous species (i.e., CO2 and CH4), the C/4Hecrust ratio (4Hecrust is the crustal-derived 4 He; Table 2). These values of 4Hecout range from 9.4 × 1026 to 2.5 × 1028 atoms yr1 (at MDA and Tra, respectively). Furthermore, Etiope et al. [2007] investigated the CH4 emissions from seeps and mud volcanoes in the central southern Apennines and highlighted that these are typical crustal-derived fluids. These fluids are CH4 rich (CH4 < 95%), and the origin of CH4 is biogenic or thermogenic [Etiope et al., 2007]. Helium is typically radiogenic [Minissale, 2004; INGV Palermo data set]. The total CH4 output from the Pin mud volcano is 2.7 t yr1 [Etiope et al., 2007]. Based on the CH4 /He ratio and assuming that the emitted 4 He is completely produced in the crust (R/Ra = 0.03), we computed that the 4Hecout is 6.4 × 1022 atoms yr1. In order to investigate if the assessed 4Hecout in the southern Apennine can be due to long-lasting diffusion degassing of the 4He produced over the entire crust, we calculated the rate of 4Hecrust production beneath the central Apennines, which is equivalent to QCHe ¼

My α NA

(1)

where QCHe is in moles per year, My is the mass of the crust beneath the investigated area in grams, NA is the Avogadro’s constant, and α is the crustal production of 4He in atoms per gram per year. In turn, α ¼ 3:115106 ½U235 þ 1:272 105 ½U238 þ 7:710 105 ½Th 235

(2)

238

where [U] and [Th] are, respectively, the concentrations of U (both U and U, and hence, two terms are included) and Th in the crust in parts per million by weight [Lowenstern et al., 2014]. Assuming a typical crustal composition (U = 1.4 ppm and Th = 5.6 ppm [Rudnick and Fountain, 1995]), a crust thickness from 25 to 35 km in the studied area [Piana Agostinetti and Amato, 2009], and a surface area of 0.01 km2 (the same as at Pin [Etiope et al., 2007]), the rate of production of 4He (QCHe ) ranges from 6.6 × 1021 to 9.2 × 1021 atoms yr1. Assuming the same crust thickness (25–35 km) and surface area (0.01 km2), for a U-rich upper crust (12 ppm Th and 4 ppm U [Rudnick and Fountain, 1995]), the QCHe ranges from 1.6 × 1022 to 2.3 × 1022 atoms yr1. This implies that the whole-crust QCHe below Pin is slightly lower than the computed 4Hecout (6.4 × 1022 atoms yr1) in the same area and a long-lasting diffusion degassing of the 4He produced over the entire crust cannot exactly explain the computed 4Hecout at Pin site. However, a larger productive area and/or an accumulation of He in a crustal reservoir would be more consistent with the computed radiogenic He output. We also



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computed the whole-crust radiogenic production QCHe in correspondence of each investigated manifestation on the basis of the same approach (see equations (1) and (2)), and a steady state degassing of 4He produced in the crust cannot still explain the computed 4Hecout from the these manifestations (MDA, Mas, SSi, Tel, and Tra; Table 2), which range from 9.4 × 1026 and 2.5 × 1028 atoms yr1 (at MDA and Tra, respectively). In fact, the largest degassing area in the southern Apennine is at MDA (about 0.4 km2), and the whole-crust QCHe in this area is 3.7 × 1023 atoms yr1. At the same manifestation (MDA), the 4Hecout values are from 9.4 × 1026 to 6.2 × 1027 atoms yr1, for 2.8 and 3.6 Ra, respectively, as mantle end-members. If similar amounts of He were discharged over the 2000 years of activity at MDA [Sinno, 1969], then 32 Myr of accumulated radiogenic He must have been emitted during that time. To explain the computed 4Hecout at MDA, which is about 1.6 × 104 times the whole-crust QCHe , we need to invoke either a productive area of about 100 km2 or the degassing of He that has accumulated in the crust over a long time. 3.3. Fracturing of Rock and He Release Experimental investigations have identified that noble gases (He and Ar) are degassed from compressed rocks [Honda et al., 1982]. According to these studies, this process depends primarily on the generation of new surface areas by dilatancy, which can be attributed to microcracking processes occurring in the rock prior to the occurrence of macroscopic fracturing. Furthermore, He degasses continuously from rock under compression until it fractures, in contrast to the degassing of other noble gases (e.g., Ar) depending on parameters unrelated to dilatancy, such as the rock type and compression conditions. This means that earthquakes tend to induce the release of He from rocks. Torgersen and O’Donnell [1991] developed a one-dimensional model for the release of He and Ar due to rock fracturing, with the results of their calculations highlighting that Hec can be released over a short time scale (