GEOLOGICA BALCANICA, 27. 3-4, , Decemb. 1997, p. 113-118

Helium-3 evidence for a spot of thinned lithosphere in the eastern part of the Balkan Peninsula

N. B. Piperov

Institute of General and Inorganic Chemistry Bulgarian .Academy of Sciences. 1113 Sofia, (Submitted 12.03.1997; accepted for publication: 18.11.1997)

H. llunepOB -/(aHH'ble o coaepxaHuu 1He KaK doKa3a­ .Abstract. The presence of a mantle component in the meAbcmao o npucymcmauu y&~acmKtl ymoHbUleHHOU AU­ crust-to-atmosphere volatile flux may be considered as mocfjjepbl 60 BOCmOIIHOU &~acmu 5aAKaHCKOZO noAyocm­ evidence for increased crustal permeability. The air-cor­ pOBa. llpHcyTCTBHe Ma.HTHiiHOro J:OMDOBeHTa B DOTOJ:e rected 3HerHe = R ratio, usually normalized to R . , is JieryqHX H3 ICOp B aTMoccllepy MO:KeT paCCMaTpHBaTLC.II well known as a starting point for such an evaluation."tbe JCU .nonJaTenLCTBO o6 yse.IIH'IeHBoi r;opoaol BpOHHQa­ relative abundance of 3He, however, may be underesti­ eMOCTH. KoppHrHpOBa.HJWe OTHOWCHH.II 3HerHe o6J.111- mated especially in the case of helium-rich fluids. The HO HOpMaJIRJOBaHHI>Ie r; R .. - xopomo HJBeCTHu om­ lHefONe ratio is proposed as an alternative geochemical paBHU TO'IJCa ,nJIJI nr;oA oQeHJ:R. HecMOTp.ll aa 3TO, OT­ tracer for mantle exhalation, normalizing the 3He abun­ HOCRTeJILHoe HJo6HJIHe 3He Mo:a:e-r 6hlTL ae,nooQe&eao, dance to a small and comparatively constant atmospher­ oco6o B CJIY'f&JIX npRCYTCTBH.II 6oraTYX reJIHeM ctunoa­ ic component: neon . .nos. OTHomeHRe 3Hef0Ne npe.nnarae-rc.11 B J:a'leCTBe The 3Hef0Ne ratio for 21 mineral water sites from the aJILTepHaTHBHOrO reOXHMH'IeCJ:OrO Tpelcepa ,nJI.II Ma.H­ eastern part of the Balkan peninsula are calculated on the THIHYX 3JCCraJIJI.IIQHI, BOpMaJIB3YJI BJ06HJIBe 3He B CO­ bases of previous measurements. .The regional distribu­ ,nep:Ka.HRH cpaBHHTeJILHO DOCTO.IIHBOrO B aTMOCcllePe H tion of the values found is compared with the data avail­ npHCYTCTBYJOmero B Manm J:O.IIH'Iecnax J:OMnoaeHTa, a able on the depth of Moho discontinuity and lithosphere/ HMeHHO, HeOHa. asthenosphere boundary. An evident correlation between OTHoweaae BLI'IHCJieHo .nn• 21 o6"Lerra MHHe­ 3Hef0Ne maximum values and an area with thin lithos­ panLHYX BO,n BO BOCTO'IHOI 'laCTH liaJIJ:a.HCJ:OrO DOJIYOC­ phere is observed. TpOBa Ha OCHOBe 6onee paBHHX R3MepeHHI. Perao­ HaJILHOe pacnpe,neJieHHe CTOHMOCTel 3Hef0Ne cpaBRB­ BaeTCJI co cyilleCTBYJOmHMH ,na.HBliiMH o rny6aae Moxo Ho rpaHHQe JIHTqepLI c aCTe&occPepoi. Ha6JIJO.nae-rc.ll O'leBH,nHU r;oppeJIJIQHJI MaJ:CHMaJILHLlX 3Ha'leHBI 3He/ 2l1Ne c Y1faCTs:oM YTOHLmeHHol naToccllePLI.

Piperov, N. 1997. Helium-3 evidence for a spot of thinned lithosphere in the eastern part of the Balkan Peninsula- Geologica Bale., 27, 3-4; 113-118. Key words: Helium-3; thinned lithosphere

Introduction stikhin, 1984) and revealing the gas flux from the mantle to the atmosphere. The presence of The significant difference in 3HerHe = R ratios mantle derived helium in mineral water springs in the three main terrestrial reservoirs, i .e. at­ may be considered as an indication of en­ mosphere (R = 1.4xl0-6), crust (R ~ lxlo-•) and hanced crustal permeability as is mostly ob­ mantle (R ~ lxlo-s) provides the unique possi­ served in zones of active tectonics (MaTBeeBa et bility of establishing the source of helium, and, al., 1978; Hooker et al., 1985 a and b; Polyak, by correlation also allows the identification of Tolstikhin, 1985; Oxburgh, O'Nions, 1987; Sano other mantle derived volatiles (Mamyrin, Tol- et al., 1987; O'Nions, Oxburgh, 1988; Sano et

15 113 al., 1989; Wakita et al., 1987; Griesshaber et al., 1992; Hoke et al., 1994), along rifts, deep fault and lineaments (liamopHH, 1980; Huixin, Zu­ huang, 1986; Sugisaki, 1987; Sianisyan, 1992) and the volcanic manifestations appertaining (Torgersen et al., 1982; Takaoka, 1985; Torgers­ en et al., 1987; Sano et al., 1988). As mentioned above, the ratio 3HerHe can be used in evaluating mantle, crustal, and atmo­ spheric components in a natural fluid. Conven­ tionally the measured 3HerHe ratio is normal­ ized by the (lHerHe) = 1.39xl0-6 (Mamyrin, Tolstikhin, 1984). ~s. RfR = (3He/ •He) :(lHerHe) . is oftenly used as an·indica­ tor oT participation of mantle derivatives, tak­ ing into account that the main source of 3He is the Earth's mantle. Hence, R/R > 1 is indica­ tive for relatively high (lHerHe)• ratio (higher 6 ... than 1.4xt0· ) and shows the presence of man- tle helium (e.g. Torgersen et al., 1982; Sano et al., 1989). Typical crustal R/R values are 0.01, whereas MORB values reach '8 (O'Nions, Ox­ Fig. I. 3He/Ar, vs. lHef'Ne linear correlation graph. ASW burgh, 1988) and the mantle plumes show as =air saturatea water (at T = 100 C) high R/R values as 12.7 (Basu et al., 1995) and even 25-jo (Kurz et al., 1982). In any case, the represented by a high total helium content in presence of atmospheric helium (He ) may in­ the gas. Thus it may be assumed that air-cor­ terfere with mantle-to-crustal helium•ratios de­ rected 3HerHe ratios do not reflect adequately creasing the high HeJHe ratios when R/R > 1 the mantle helium flux at high total helium and increasing these values at R/R < 1. Th•e air concentration: comparatively low (lHerHe) correction (ac) may be made either by conve­ (i.e. (RfR ) < I) ratio does not show a wealC nient method for estimating (R/R ) (e.g. 3He flux iitany case; this flux could be diluted Griesshaber et al., 1992) or by evahiation of by a strong addition of crustal 4He. The region 4He (Tolstikhin, 1986). Both the methods give under consideration is known as an area of clos·e results using neon data (lONe respective­ mineral waters with high helium concentration ly), which is considered entirely atmospheric. in the gases spontaneously evolved (lleHqes et Thus the air-corrected ratios (R/R ) represent al., 1964; Kones et al., 1984; Piperov et al., in a more clear form the relationSbTp between 1994). ·This consideration needs an alternative mantle and crustal components. solution of the problem. The author believes that mantle 3He may be re­ vealed at R~ < 1 and high total helium content Hypothesis, data and discussion (i.e. high Hej'Ne ratio) using as a reference not the crustal 4He, as it is in the R -ratios, but an atmo­ As it was mentioned above, the (R/R ) ratio spheric component, e.g. lONe. Thus a sensitive may help in evaluation of the mantle-io~c:atmo­ geochemical parameter should be the ratio lHe/ sphere gas flux. For such a calculation also the lONe (Lupton, 1979; Takaoka, 1985; TonCTHXHH, ) knowledge of the total helium flux from a geo­ 1986). The atmospheric gases (Ne, Ar , N2 are graphic area (i.e. the total gas flux and the con­ permanent components of the undergroUnd aqui­ centration of helium in it) is needed (Martel et fer systems, being dissolved in the water at its pen­ al., 1989). For many regions, including the in­ etration into the lithosphere. For that reason the vestigated one, this set of data is not available, concentration (the ratios) of these gases are com­ however, and only a tentative estimation, based paratively constant, as their ratios are determined on the R_jR data seems possible. by dissolving-degassing equihbrium. The role of When R: jk ratio is smaller than 1, some dif­ the mass fractionation is small and may be ne­ ficulties m·ay arise in the discussion of the re­ glected in this discussion. Hence, the 3Hej20Ne (or sults. The (lHerHe)ac: ratio is small either in the lHe/ArJ ratio seems to be a sensitive geochemical case of very weak mantle flux (low concentra- criterion for mantle exhalation, preferable proba­ tion of 3He) or in the case of large addition bly. in the cases of R.JR. < 1 and high HejNe of crustal helium, i.e. high 4He concentration rattos.

114 Table 1 Some ratios between the mantle (m) and atmospheric (a) components as well as the air-co"ected values RjR,for 21 gases released from mineral waters in the eastern part of the Balkan peninsula. The estimations are based on the analytical data published by Piperov et al. ( 1994).

4 Site Site R.JR. He,/He. 3HeFNe • 3He/Ar code• code x104 x106 • Merichleri l.l.Mr MER 0.31 850 27 1.51 1.2.Kh HAR 0.12 150 6.0 0.90 2.1.SB SAB 0.16 12 0.46 0.018 2.2.Bg BLG 0.18 33 1.2 0.065 2.3.Sm SIM 0.28 62 2.2 0.10 Kozhukh 2.4.Kz KZH 0.64 18 0.59 0.021 Marikostinovo 2.5.Mk MRK. 0.48 7.7 0.28 0.028 Gouliina Banya 3.l .GB GLB 0.14 5.0 0.19 O.Ql1 Ognyanovo 3.2.0g OGN 0.11 2.3 0.09 0.0068 3.3.Vg VLG 0.11 3.3 0.16 0.011 Mihalkovo 3.4.Mi MIH 0.26 97 3.5 0.40 Bedenski Bani 3.5.BB BEB 0.41 77 2.9 0.12 Narechen 3.6.Nr NAR 0.14 47 1.9 0.14 Banite 3.7.Bn BAN 0.29 107 4.4 0.24 Erma Reka ERR 0.41 8.8 0.73 0.14 4.1.PB PAB 0.14 16 0.64 0.023 Slivenski Bani 4.2.SI SLV 0.78 920 21 0.96 Medovo 5.l.Md MED (0.14) 0.03 0.006 0.0007 Troyanovo 6.l.Tr TRO 0.60 76 2.5 0.20 Stefan-Karadzhovo 6.2.SK SKA 0.76 11 0.39 0.029 Voneshta Voda 7.l.VV VNV 0.50 56 2.0 0.14 Air 0.0044 00.00078 ASW# 0.0034 0.00042

Notes: • Piperov et al. (1994) # air saturated water at T = 10° C.

The present investigation gives a new reading He+ Ne fraction was collected in a glass am­ of the 3He-data available for the eastern part of poule and sealed for transport to a special mass the Balkan peninsula, already published (Piperov spectrometer (MI 1201, At>atity, Russia) for et al., 1994). Two considerations determine the He-isotope analysis. choice of the sample sites: 1) a good number of A detailed description of the sampling and active tectonic zones to be presented and 2) a pos­ analytical procedures was given by Piperov et sibility for gas sampling with minimum of air al. (1994). Some elemental and isotopic data, "contamination". So, the following zones are tak­ including the 3Hef'Ne ratios, were also avail­ en into account (cf. Table 1): fault zone able in the study referred. This data set is used (MER, HAR); Struma fault zone (SAB, BLG, to calculate the mantle- to atmospheric com­ SIM, KZH, MR.K.); Rhodope massif (GLB, OGN, ponent ratios eHeufHe and 3He/Ar) (Table VLG, MIH, BEB, NAR. BAN, ERR); Trans-Bal­ 1). The pretty goOd correlation (r) ·between kan fault zone (PAB, SLV); synclinorium them (r = 0.90 for 4He_fHe vs. 3Hef'Ne and r (MED); Strandzha- Sakar region (fRO, SKA); = 0.94 for 3He/Ar vs. 31lef~e) shows that they Fore-Balkan region (VNV). are, on the one ~and, equivalent, and, on the The analytical procedures need three aliquot other, highly informative. The correlation be­ (ml, STP) of the gas samples: 1) About 0.5 ml tween 3Hef'Ne and 3He/Ar is plotted in Fig. 1. gas were let into a mass spectrometer for gen­ The regional distribution· of the 3Hef'Ne ra­ eral analysis; 2) Another volume (5-50 ml) was tio is shown on Fig. 2; black circles being pro­ introduced into a vacuum line for analysis of portional to the estimated values. It should be the light noble gases. Heated metallic calcium noted that the sites with maximum mantle val­ (600°C) and cooled activated charcoal (-195°C) ues (SLY, MER, HAR. TRO) are located in a were used for removing the chemically active relative narrow area, which indicate a more sig­ components and a combination of volumetry nificant participation of mantle volatiles (lHe) and mass spectrometry was applied for deter­ and, hence, reveal an enhanced permeability of mination of He {= 4He), Ne, and Ar. 3) A gas the crust (Oxburgh, O'Nions, 1987). The center fraction for 3HetHe and precise Ne determina­ of this spot on the Earth's surface is approxi­ tion was isolated from a 50-200 ml aliquot by mately 42°30' N and 26° E. adsorption of all other gas specia, except He It is interesting that many authors assume and Ne, on activated charcoal at -195°C. This that "there is something" in the depth beneath 115 •SKA

3HeJ20Ne (x1 o-4) a • • o - 2 0.7 <0.2 7 20

0 50 100 km

28° E

Fig. 2. Topographic distn'bution of the mineral water sites under consideration and lHefDNe values (Ta­ ble 1) presented by the area of the black circles. The bold dashed lines denote the submeridional axes and the approximate width of the presumable tensional tectonic cross (.lioaqea, 1971); the hatched band shows the situation of part of the deep submeridional fault according to illaaoa, Paarenoa (1986). The isobathes of Moho discontinuity (solid) (Boykova, 1996) and lithosphere/asthenosphere boundary (dashed) (Shanov, Kostadinov, 1992) are also presented. The numbers give the depth (km) beneath the Earth's surface the surface of the region mentioned. Thus More recently IDaHos, PaareJIOB (1986), us­ Bonchev (1958) has noticed a belt of strike-slip ing new geophysical data - a map of the mag­ faults situated "on a special lineament with nitudes of the earthquakes with hypocenters Rhine orientation". Later the same author deeper than 70 km - have also supported the (lioaqes, 1965) has introduced the concept of idea about existence of a deep NfS trending Tvurditsa lineament uniting a band of tectonic fault in the eastern part of the Balkan peninsu­ dislocations crossing the predominating fault la from the Carpathian Arc (Vrancea Moun­ systems (of NW- to SE-orientation) and form­ tain, Romania) to the island of Crete (Fig.2). ing an arc from Tvurditsa (Central Bulgaria) to The more intensive leakage of mantle volatil­ Kavala (North Greece). Using geophysical es is connected as a rule with the places of most data, mainly the depth of the Conrad disconti­ permeable crust; the extension and thinning of nuity, again lioaqes (1971) has launched the the crust respectively, favour this process idea about the existence of a tensional tectonic (O'Nions, Oxburgh, 1988). A clear correlation cross on the territory of Bulgaria, whose ap­ is observed between an extended crust and ele­ proximate submeridional axis is the ­ vated asthenospheric materials, oftenly melted, Kurdzhali line (probable width 30-50 km, Fig. 2). on the one hand, and mantle helium flux, on Gergelchev et al. (1975; repreJiqes et al., the other (Martel et al., 1989; Marty et al., 1992; 1977) assume that the tectonic dislocations of Hoke et al., 1994). submeridional direction from the Lower Some years ago Shanov, Kostadinov (1992) Danube to the central part of the Aegean Sea published data for the deep geophysical discon­ are elements of a newly discovered structure: tinuities beneath the territory of Bulgaria, in­ Aegean-Danubian rift. cluding Moho and the lithospherefasthenos-

116 phere boundary. The isobathes of this boundary kan peninsula. - Compt. Rend. Acad. bulg. Sci., 28; are shown on Fig. 2 in combination with the 787-790. more modern data of Moho isobathes (Boyko­ Gergelchev, V.N., Petkov,I.N., Panov, G .B., Yosifov, D.S. 1977. A general geological-geophysical and metallo­ va, 1996). It is evident a good coincidence of genic features of the rift zones of the Balkan peninsula the elevated Moho discontinuity (up to 30-35 (Bulgaria). The Role of the Riftogenesis in the Geologi­ km beneath the Earth surface) and lithosphere/ cal History. Nauka, Novosibirsk, 173-180 (in Russian). asthenosphere boundary (up to 95-100 km be­ Griesshaber, E., O'Nions, R.K.., Oxburgh, E.R. 1992. He­ neath the Earth surface) with the comparative­ lium and carbon isotope systematics in crustal fluids 3 3 0 from the Eifel, the Rhine Graben and Black Forest, ly intensive He flux, revealed by high Hef Ne F.R.G. - Chem. Geol., 99; 213-235. ratios (MER, SLV, HAR, TRO). This finding is Hoke, L., Hilton, D.R., Lamb, S.H., Hammerschmidt, K.., in a good agreement with the suggestion of Friedrichsen, H. 1994. 3He evidence for a wide zone of many authors (O'Nions, Oxburgh, 1988; Martel active mantle melting beneath the Central Andes. - et al., 1989; Griesshaber et al., 1992; Hoke et al., Earth Planet. Sci. Lett., 128; 341 -355. Hooker, P.J., O'Nions, R.K.., Oxburgh, E.R. 1985a. Heli­ 1994) on the existence of a detectable flux of um isotopes in North Sea gas fields and the Rhine rift. mantle derivatives through the zones with -Nature (London), 318; 273-275. thinned lithosphere and elevated mantle mate­ Hooker, P.J., Bertrami, R., Lombardi, S., O'Nions, R.K.., rials. · Oxburgh, E.R. 1985a. Helium-3 anomalies and crust­ mantle interaction in Italy. - Geochim. Cosmochim. It also may be noted that the sites SAB, GLB, Acta, 49; 2505-2513. OGN, KZH, MRK., and VLG, showing com­ Huixin, Shi, Zubuang, Cai 1986. Geochemical character­ paratively low 3Hef0Ne ratios, are situated on a istics of underground fluids in some active fault zones thicker crust: Moho depth 45-50 km and lower in China.- J. Geophys. Res., 91, (B12); 12282-12290. lithosphere boundary at approximately 130 km. Kurz, M.D., Jenkins, W.J., Hart, S.R. (1982) Helium iso­ topic systematics of oceanic islands and mantle heter­ It should be remarked additionally that the ogeneity.- Nature (London), 297; 43-47. R jR ratios, usually used in such a discussion, Lupton, J.E. 1979. Helium-3 in the Guamas basin: evi­ vary rn a relatively narrow range (0.11-0. 78) in dence for injection of mantle volatiles in the Gulf of the region studied (Table 1), showing only a California. - J. Geophys. Res., 84, (B13); 7446-7452. vague trend in the same direction as 3Hef0Ne Mamyrin, BA., Tolstikhin, LN. 1984. Helium Isotopes in ratios. Nature (Developments in Geochemistry, 3). Elsevier, Amsterdam, 274 pp. Martel, D.J., Deak, J., Di:ivenyi, P., Horvath, F., O'Nions, R.K.., Oxburgh, E.R., Stegena, L., Stute, M. 1989. Leak­ Conclusions age of helium from the Pannonian basin. - Nature (London), 342; 908-912. . (1) 3Hef0Ne ratios may be successfully used as Marty, B., O'Nions, R.K.., Oxburgh, E.R., Martel, D., Lombardi, S. 1992. Helium isotopes in Alpine regions. a geochemical criterion for revealing mantle ex­ - Tectonophysics, 206; 71-78. halation in the helium-rich mineral waters. Matveeva, E.S., Tolstikhin, I.N., Yalcutseni, V.P. 1978. (2) On the basis of 3Hef0Ne survey a new ev­ Helium isotopes criterion for the origin of the gases idence for a spot of thinned lithosphere in the and for neotectogenesis zones revelation. Caucasus as eastern part of Balkan peninsula is provided. example.- Geolchimiya, 307-317 (in Russian). O'Nions, R.K.., Oxburgh, E.R. 1988. Helium volatile flux­ es and the development of continental crust. - Earth Planet. Sci. 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117 geophysical discontinuities beneath the territory of Fe011. UHCm., 6; 3-14. Bulgaria. - Geologica Balcanica, 22; 71-79. lioH'Iea, E. 1971. Ilpo611eMu Ha 6&AzapcKama zeomeK­ Sugisaki, Ryuichi 1987. Behavior a.nd origin of helium, moHuKa. TeXHBia, Co$u; 204 c-rp. neon, argon, and nitrogen from active faults. -J. Geo­ Konea, E., KyJMaaoa, Jl., Mapaaoa, li., Uaaoa, Un. phys. Res., 92, (B12); 12523-12530. jlaal{IB, H. 1984. B1.pxy xenaeaoTo CD.Ill>PzaHHC H Takaoka, Nobuo 1985. Study of volcanic gas by isotopic R30TODCH CDCTaB &a HaiOB DO,Il3CMHB BO,IlH B liDnra­ analysis of noble gas for eruption prediction by pu. - Cnyc. Suz. ze011. c>-so, 45; 318-328. geochemical technique.- Kazan, 30; 185-195 (in Jap­ lleH'ICB, H., Hop.llaHOB, H. Koqeaa, Jl. 1964. 0 co,llepxa­ anese, with English abstract). HRJIX renaa H aproaa B CDOBTIUIIILIX fa38X BCIOTOpLIX Torgersen, T., Lupton, J.E., Sheppard, D.S., Giggenbach, 6onrapCIHX MHBepan&BWX BCTO'IHBIOB. - ~OK/I . W.F. 1982. Helium isotope variations in the thermal ar­ SA.H, 17; 133-136. eas of New Zealand. - J. Volcano/. Geotherm. Res.,12; llOJUli, li., llpaconoB, E., liy&'IH,IllC, r., KOBOHOB, B., 283-298. MllMWJ)BH, li., Cypoal{eaa, Jl., Xa6apaa, H., IO.IlCBH'I, Torgersen, T., Clarke, W.B., Habermehl, MA. 1987. He­ B. 1979. He a Ar HlOTonBYA coCTaa clunoH.IlOB An&no­ lium isotopic evidence for recent subcrustal volcanism AnneBHBcioro peraoaa aero CBJDH c ayniaHHJMOM. in eastern Australia.- Geophys. Res. LetL, 14; 1215- - ~oiC.II. A.H CCCP, cep. zeo11., 247; 1220-1225. 1218. C.IIBHCD, C.C. 1992. Bo.llopaCTBopellllhdt rend a l{CBT­ Wakita, Hiroshi, Sano, Yuji, Mizoue, Megumi 1987. High pan&aol 'laCTH Tepe.:-Kacnalc.:oA ,llenpeccaa. - 3He erqanation and seismic swarms observed in a non­ BecmH. MocK. y-mema, cep. 4 - Fe011.; 37-44. volcanic, forearc region. -J. Geophys. Res., 92, (812); TonCTHXHB, H. 1986. H3omonHa.R zeoxUMUR Ze/IIIUR, 12539-12546. apzOHa, u pec)~eux za30B. Hayn, JleBBHrpa,ll; 200 CTp. liamopaa, B. H. 1980. 1980. renaihlu CDCMJ:a J:ai MCTO.Il Waaoa, C., Paarenoa, li. 1986.0 cymeCTBOBaHBH rny6o­ Rl)"'CHRJI ,llOnfOIRB)"'HX pa3nOMHLIX ,llHcnOIQHI. - .:oro cy6MepH,IlHOB&nbBOfO pa3nOMa 88 JianiaHCIOM ~oiC.II . A.H CCCP, cep. zeo11., 255; 932-935. nonyoc-rpoae. - In: Communications of the Bulgarian Jioaqea, E. 1958. B1>pxy C.llBH noac aa OTCC.IlBO pa3nOMa­ Geologists, 27th IGC (Moscow, 1984). Tehnika, Sofia, BaHe, pa3nonollle& .:oco aa lianuHHJUITe. - H3s. 77-86.

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