and CH4-enriched hydrothermal fluids of Socorro Island, Revillagigedo Archipelago, Mexico ...

13 Pages·2010·0.51 MB·English

Geofluids(2010) doi:10.1111/j.1468-8123.2010.00314.x
1
2
Geochemistry of H - and CH -enriched hydrothermal fluids
3 2 4
4 of Socorro Island, Revillagigedo Archipelago, Mexico.
5
Evidence for serpentinization and abiogenic methane
6
7
8 Y. A. TARAN1, N. R. VARLEY2, S. INGUAGGIATO3 AND E. CIENFUEGOS4
9
1Institutode Geofı´sica, Universidad NacionalAuto´noma deMe´xico, Coyoacan, Mexico D.F.,Mexico;2Universidad De
10
Colima, Colima, Mexico; 3Istituto Nazionale diGeofisica eVulcanologia, Palermo,Italy; 4Institutode Geologia, Universidad
11
NacionalAutonoma deMexico, Coyoacan, Mexico D.F.,Mexico
12
13
14
ABSTRACT
15
16
Socorro Island is the exposed part of an approx. 4000-m-high volcanic edifice rising from the oceanic floor to
17
approx.1000maslatthenorthernpartoftheMathematicianRidge,WesternPacific.Thevolcanoisactive,with
18
the most recent basaltic eruption in 1993. Moderate fumarolic activity and diffuse degassing with a total CO
2
19 fluxofapprox.20totalday)1areconcentratedinthesummitregionofthevolcanocomposedofagroupofrhy-
20
olite domes. Low-temperature, boiling point, fumaroles discharge gas with high H (up to 20 mol% in dry gas)
2
21
andCH (upto4mol%). Bothcarbonand Heisotopic ratios andabundances correspond tothosein MORB flu-
4
22 ids (d13C (cid:2) )5&; 3He⁄4He = 7.6 R , CO ⁄3He = (2–3) · 109, where R is the atmospheric ratio 3He⁄4He of
CO2 a 2 a
23 1.4 · 10)6. Light hydrocarbons (CH , C H , C H , and C H ) are characterized by a high C ⁄C ratio of
4 2 6 3 8 4 10 1 2+
24 approx.1000.Methaneis enrichedin 13C(d13C from)15to)20&)and2H(d2Hfrom)80to)120&),and
CH4
25 hydrocarbons show an inverse isotopic trend in both d13C and d2H (ethane is isotopically lighter than methane).
26
These isotopic and concentration features of light hydrocarbons are similar to those recently discovered in fluids
27
fromultramafic-hostedspreadingridgeventsandmayberelatedtotheserpentinizationprocesses:H generation
2
28
and reduction of CO to CH within high-temperature zone of volcano-seawater hydrothermal system hosted in
2 4
29
basaltic and ultramafic rocks beneath a volcano edifice. The thermodynamic analysis of this unusual composition
30
of the Socorro fluids and the assessment of endmember compositions are complicated by the near-surface cool-
31
ing,condensationandmixingwithmeteoricwater.
32
33 Keywords:abiogenichydrocarbons,isotopegeochemistry,spreadingcenters,steam-dominatedhydrothermal
34 systems,volcanicgas
35
Received28February2010;accepted7September2010
36
37 Correspondingauthor:YuriTaran,InstituteofGeophysics,UNAM,MexicoD.F.04510,Mexico.
38 1 Email:[email protected]:[phone].Fax:XXXXX.
39
Geofluids(2010)
40
41
42
43 associated with deep-seated waters of Precambrian shields
INTRODUCTION
44 (Sherwood Lollar etal. 2002, 2006, 2008), and in
45 Methane-rich fluids discharging in environments where occluded gases from igneous alkaline rocks of Kola Penin-
46 sedimentary matter is limited or almost absent may have sula(Galimov&Petersilie1967;Potteretal.2004).These
47 methane and light hydrocarbons of abiotic origin, synthe- gases are characterized by high H and CH contents and
2 4
48 sized under reduced conditions in the Earth’s crust from sometimes by a rare ‘inverse’ trend in the carbon isotope
49 the oxidized carbon (CO , carbonates) and hydrogen. composition of the light hydrocarbons, with ethane isoto-
2
50 Such fluids have been found as cold seeps in ophiolites pically lighter than methane, in contrast to ‘thermogenic’
51 2 (Abrajano etal. 1990; Lyon & Giggenbach 1990; Sano trend with the enrichment in 13C of longer chains, com-
52 etal. 1993), in moderate- and high-temperature ultra- mon for most natural hydrocarbons. Boiling-point fuma-
53 mafic-hosted spreading ridge vents (Charlou etal. 2002; roles of Socorro Island, a giant shield basaltic volcano built
54 3 Proskurowski etal. 2008; Konn etal., 2008), in gases at the former spreading center on the ocean floor in the
(cid:2)2010BlackwellPublishingLtd
G F L 3 1 4 B Dispatch:29.9.10 Journal: GFL CE:BalajiPrasad
Journal Name ManuscriptNo. AuthorReceived: No.ofpages:13 PE:Sangeetha
2 Y. A. TARAN et al.
1 Eastern Pacific, produce gases with very high concentra- cones. According to Bohrson & Reid (1995), silicic peral-
2 tions of H and CH , which have been rarely observed in kaline rocks comprise up to 80% of the surface of the
2 4
3 other hydrothermal orvolcanic gases (Taranetal.2002). island, rendering Socorro unique in the Pacific. Bohrson &
4 In this study, new data are presented on the Socorro Reid (1995) divide the eruptive history of the island into
5 gases with an emphasis on the hydrocarbon isotopic geo- pre-, syn-, and postcaldera phases. They suggest that the
6 chemistry. The carbon and hydrogen isotopic composition whole volcanic edifice of approx. 2400 km3 is composed
7 of C –C hydrocarbons in Socorro gases are characterized mainly of basalts, but the subaerially exposed deposits are
1 4
8 by the inverse isotopic trend in both C and H isotopes. silicic peralkaline rocks including ignimbrites that erupted
9 These trends are discussed in terms of the mixing of gas between approx. 540 and 370 ka. Postcaldera silicic rocks
10 produced from the high-temperature hydrothermal ser- have been erupted between approx. 180 and 15 ka and
11 pentinization of mafic and⁄or ultramafic rocks and a typical cover the northern, western, and southern quadrants of
12 hydrothermal fluid composed of magmatic gases and air- the island. Postcaldera alkaline basalts (approx. 20 vents
13 saturated meteoric water located above a magma chamber with lava flows, Lomas Coloradas) are largely restricted to
14 inside the volcanoedifice. thesoutheastern part ofthe island (Fig.1B).
15 The presence of the caldera may indicate that the associ-
16 ated silicic magma chamber is shallow and probably resides
GEOLOGICAL SETTING
17 within the upper oceanic crust or the edifice (Bohrson
18 Socorro Island (18(cid:3)47¢N⁄110(cid:3)57¢W) is the largest island etal. 1996). Fumarolic activity at the Cerro Everman
19 of the Revillagigedo Archipelago and is located 700 km dome indicates that a hydrothermal system is developed
20 west of Manzanillo on the Pacific coast of Mexico abovethis shallow magmachamber.
21 (Fig.1A). Its area is approx. 120 km2 and it lies at the
22 northern end of the Mathematician Ridge near the inter-
Hydrothermal activity
23 section with the Clarion Fracture Zone. The northern
24 Mathematician Ridge marks the location of a mid-ocean Morethan100 individual steam ventsand several bubbling
25 ridge spreading center that was abandoned at approx. 3.5 and boiling water and mud pools can be seen around the
26 Ma when activity shifted to the East Pacific Rise (Mamm- hydrothermally active dome and on its slopes almost up to
27 erickx etal. 1988). Volcanic activity in this region has con- the summit (Fig.1C). The most intensive, ‘noisy’ fuma-
28 tinued to the present as demonstrated by the 1952–1953 roles are situated on the SE slope of the dome in a short
29 eruption of San Benedicto Island, 50 km north of Socorro, and shallow canyon. All fumaroles are characterized by the
30 and a submarine basaltic eruption approx. 3 km west of boiling point temperature near 97(cid:3)C at this elevation
31 Socorro Island in 1993 (Siebe etal. 1995). Socorro Island (950–1050 m asl). The water contained in a series of hot
32 represents the emergent peak of a large basaltic volcano pools is chloride-free, steam condensate. The island is
33 that rises from a sea floor depth of approximately 3000– located in the tropical zone with marked dry and wet sea-
34 1050 m above sea level. The highest point of the island is sons and with annual precipitations of about 700 mm
35 the summit of a pantelleritic dome, Cerro Everman, where (Atlas de Agua). There are no cold springs on the island,
36 significant fumarolic activity occurs. The surface landscape and the only freshwater sources are small rainwater-filled
37 is dominated by lava flows, domes, scoria, and cinder pools or temporary lakes within the cinder cones. Taran
38
39 (A)
G
40 FI
41 R
O
42
L
43 (B) (C) O
C
44
N
45 O
46 TI
U
47 L
O
48 S
E
49 Fig.1. (A)LocationofSocorroIslandrelativeto 31R
50 the Mexican Pacific coast. (B) Location of ther- W
51 mal springs and the summit fumarolic field; cal- O
dera rim is shown by a dashed curve. (C) L
52
AlterationandfumarolicfieldoftheCerroEver-
53
mandome.StarsaresampledfumarolesF5,F10,
54 andF40.
(cid:2)2010BlackwellPublishingLtd
Socorro hydrocarbons 3
1 etal. (2002) suggested the presence of a steam-dominated 3He-rich gases down to values lower than 0.1%. 3He⁄4He
2 aquifer beneath the caldera based on the chemical and iso- ratios were corrected for atmospheric contamination on
3 topic analyses of fluids collected from two fumaroles and a the basis of the difference between the 4He⁄20Ne of the
4 steam-heated pool. The surface of the dome and surround- sampleand inthe air (Sano& Wakita1985).
5 ing slopes are intensively altered within an area of approx. Carbon isotope analysis of CO and hydrocarbons was
2
6 1.5 km2 (Fig.1C). Besides, there are several hydrothermal conductedonaGC-isotoperatiomassspectrometersystem
7 manifestations on the island. Playa Armada and Playa composed of an Aglient 6890A capillary gas chromato-
8 Blanca warm springs discharge diluted seawater from frac- graph connected to a Thermo Finnigan MAT 253 mass
9 turesinthelavaflowsonthetidelinesofthesouth,south- spectrometer. A PoraBond Q column (50 m · 0.32 mm)
10 west, and west shoreline. Palma Sola springs discharges was used. The temperature program started at 40(cid:3)C for 10
11 diluted water (approx. 300 ppm of Cl) and thus is the best min before increasing to 220(cid:3)C at 5(cid:3)C min)1 with a final
12 representative for the isotopic composition of meteoric hold 8 min. An Oztech tank with CO (d13C = )10.99&
2
13 water. In a lava tunnel named La Gruta, 1.5 km from the V-PDB) was used as a working standard. Matheson Tri
14 southern coast, within the Lomas Coloradas area, there is a Gasmicro MAT14 mixture with1000 ppmV oflinear alk-
15 smallareaof steamingground. anes C –C in He was used for the calibration of C –C
1 6 2 4
16 concentrations and determination of the precision of the
17 isotopic analyses of hydrocarbons. Concentrations of eth-
SAMPLING AND ANALYSIS
18 ane in the headspace of Giggenbach’s bottles usually were
19 Gas samples from fumaroles and pools were taken into approx. 200 ppmV. At these concentrations and the ali-
20 Giggenbach’s bottles filled with 40–50 ml of 4–5 N NaOH quot volumeof 2 ml, theprecision wasapprox. 1permil as
21 or KOH solution (Giggenbach 1975) using a Ti-tube or a determined on 10 analyses of the standard mixture. For
22 plastic funnel and a short Tygon silicon hose. Because of CO andCH , theprecision was±0.3permil.
2 4
23 the high fraction of noncondensable gases (H > CH > ForthedDanalysisofhydrocarbons,thesamePoraBond
2 4
24 N ), the flasks were filled rapidly, and the headspace pres- Q column was used together with a pyrolysis oven at
2
25 sure was never lower than 0.5 atm. Several gas ampoules 1450(cid:3)C. A Molecular Sieves 5A column was used for the
26 were filled with dry gas simultaneously at each sampling separation of H and CH . An Oztech tank with H (dD
2 4 2
27 location by pumping condensate through two consecutive = )124.15& v-SMOW) was used as a working standard.
28 bubblers cooled with ice. These samples were used for the Precision of measurements for the standard hydrocarbon
29 carbon and hydrogen isotope analyses in CO , H , CH , mixtureusuallywas better than±5&.
2 2 4
30 as well as hydrogen and oxygen isotopes in the vapor con-
31 densate. Splits of the headspace gases were used for the
RESULTS
32 chemical analysis of hydrocarbons, carbon and hydrogen
33 isotope analysis in hydrocarbons and Heisotopes.
Chemicalcompositionof gases
34 Gas analysis was performed by using a Perkin-Elmer
35 8500 and a Gow-Mac 350 gas chromatographs with Ar Chemical compositions (without hydrocarbons) of 20 gas
36 and He as carrier gases, packed columns with 5A Molecu- samples collected in 2001–2008 are shown in Table1.
37 lar Sieves and TCD detector. Argon was separated from Samples collected from different vents are generally similar:
38 O using a composite CT-III Altech column with He as they have about 95 mol% of water vapor (mean gas con-
2
39 the carrier gas at room temperature. Carbon monoxide tent X (cid:2) 50 mmol mol)1), and the predominant compo-
g
40 was determined using FID detector, a Carbosieve S-II col- nent of dry gas is carbon dioxide. The most intriguing
41 umn, and a methanizer only in the 2008 dry gas samples. feature is the high H and CH contents: up to 20% and
2 4
42 Dry gases were analyzed for the CO to avoid effects of the 4% in dry gas, respectively. Giggenbach (1987) proposed
43 formate formation in the alkaline solution during the stor- the use of R ¼logðX =X Þ as a redox parameter for
H H2 H2O
44 age of samples (Giggenbach & Matsuo 1991). The detec- hydrothermal systems instead of f (X are concentrations
O2
45 tion limits were about 5 ppmV for He, 3 ppmV for H , inmolefractionsoranyotherunits).TheSocorrogeother-
2
46 and 0.5 ppmV for CO. Concentrations of hydrocarbons mal steam is characterized by a mean R value of )2.0,
H
47 C2–C4 were analyzed simultaneously with carbon isotopes almost one order of magnitude higher than analyzed fluids
48 in Instituto deGeologia,UNAM. fromother steam-dominated hydrothermal fields.
49 Helium and nitrogen isotopes were analyzed in the
50 National Institute of Geophysics and Volcanology (INGV),
He, 3He⁄4He, andCO
51 Palermo. The 3He⁄4He ratios were measured by a static 2
52 vacuum mass spectrometer (VG-5400TFT, VG Isotopes) Data on 3He⁄4He together with gas concentrations are
53 modified to detect 3He⁄4He ion beams simultaneously, shown in Table1. All measured samples are characterized
54 reducing the error of the 3He⁄4He measurements in by high 3He⁄4He (7.2–7.6 R) along with high He⁄Ne
a
(cid:2)2010BlackwellPublishingLtd
4 Y. A. TARAN et al.
1 Table1 Chemicalcomposition(mol%indrygas)andheliumisotoperatios(R⁄Ra)offumarolicandbubblinggasesoftheSocorroIsland.Xg–gascontent
2 inmmolmol)1(H2O=1000)Xg).
3
FumNo Date T(cid:3)C Xg CO2 H2S H2 N2 Ar He CH4 CO R⁄Ra 4He⁄20Ne CO2⁄3He(·109)
4
5 F5 December2001 98 53 77.5 1.12 15 3.09 0.048 0.0027 3.15 nd 7.55 185 2.8
F10 December2001 98 50 76.6 1.35 17.3 2.1 0.025 0.0035 2.6 nd 7.56 432 2.1
6
F10 March2002 98 36 83.5 1.5 13 0.86 0.012 0.0042 2.62 nd 7.41 161 2.1
7
P1 March2002 48 77.2 0.61 18.9 0.54 0.01 0.0021 2.6 nd
8 F5 November2003 98 48 80.8 1.1 11.1 2.8 0.046 0.0031 4.12 nd 7.61 62 2.5
9 F5 November2003 98 56 80.6 1.32 13.1 1.45 0.034 0.0038 3.52 nd 6.24 8 2.5
10 F10 February2004 99 46 81.9 1.08 10.2 2.3 0.031 0.0034 3.52 nd 7.54 15 2.3
F40 February2004 99 32 82.1 0.95 13.2 0.5 0.026 0.0023 3.03 nd 7.59 311 3.4
11
P1 February2004 56 78.7 0.71 16.2 0.91 0.013 0.0026 3.46 nd 7.53 557 2.9
12
F5 March2005 98 45 77.7 1.86 16.9 0.23 0.004 0.0036 2.2 nd 7.48 465 2.1
13 F5 March2005 98 67 77.4 1.09 17.1 1.02 0.0095 0.0041 2.63 nd 7.66 220 1.8
14 F40 March2005 98 54 74.5 1.85 20.7 0.34 0.0028 0.0029 3.11 nd 7.21 530 2.5
15 F40 March2005 98 45 80.6 1.83 14.9 0.41 0.0059 0.0026 2.26 nd 7.54 230 2.9
F5 March2008 98 34 77.5 1.35 17.8 0.20 0.005 0.0072 3.15 0.0008
16
F5 March2008 98 48 77.4 1.61 16.9 0.92 0.022 0.0034 3.19 0.0007
17
F10 March2008 98 54 80.4 1.16 13.7 0.27 0.004 0.0067 4.47 0.0012
18 F40 March2008 98 49 76.1 0.94 19.1 1.16 0.029 0.0040 2.70 0.0005
19 F40 March2008 98 51 77.4 1.13 17.7 0.92 0.026 0.0064 2.84 0.0013
20 P1 March2008 52 75.1 0.58 18.9 1.44 0.033 0.0031 3.97 0.0006
P1 March2008 52 74.8 0.93 19.6 0.75 0.021 0.0078 3.90 0.0012
21
22
23
ratios indicating little direct air contamination of the sam-
24 G
25 ples. He concentrations are high, and CO2⁄3He ratios are FI
within the range of MORB glass values of (1–6) · 109 N
26 O
(Marty& Jambon1987).
27 TI
U
28 L
O
29 Waterisotopes
S
30 E
Table2 shows data on the isotopic composition of water R
31
samples from Socorro island. Analyses of meteoric water W
32 O
are limited to two samples of rainwater (Table2, Fig.2).
33 L
Permanent cold springs are absent on the island. There are
34
three warm coastal springs: Playa Blanca (31(cid:3)C), Palma
35
Sola (35–37(cid:3)C), and Playa Armada (31.5(cid:3)C) shown in
36
37
38 Table2 Isotopic composition (permil, V-SMOW) of fumarolic steam,
39 thermalandmeteoricwatersofSocorroIsland.
40
Site⁄year dD d18O Site⁄year dD d18O
41
42 Condensates Thermalpools Fig.2. dD versus d18O plot for waters of Socorro Island. Inserted is a 32
F10⁄2004 )53.2 )11.4 P1⁄2004 )12.3 )2.0 modelfordDofvaporafterasingle-stepsteamseparationatdifferentboil-
43
F5⁄2003 )49.7 )10.4 P2⁄2004 )14.9 )1.6 ing temperatures from water with dD=)40& and d18O=)6.25&. PS,
44 F5⁄2004 )56.6 )11.6 P3⁄2004 )11.0 0.1 PalmaSolaspring.Alsoshownareasforarcmagmaticwaters(Taranetal.
45 F40⁄2008 )46.2 )9.9 P7⁄2004 )16.8 )1.5 1989)andwaterfromundegassedMORBglasses(e.g.Poreda1985).See
46 F5⁄2008 )42.2 )7.3 P1⁄2008 )4.5 2.7 textfordetailsanddiscussion.
47 F10⁄2008 )50.1 )8.7 P4⁄2008 )12.6 )0.9
SG2⁄1998 )47.0 )9.2 M1⁄1998 )7.4 0.5
48
Rains M2⁄1998 )5.5 1.6
49 02.2004 )35.5 )6.2 M4⁄1998 )15.0 )2.0 Fig.1. Water from Playa Blanca is significantly (40–70%)
50 01.2008 )37.6 )5.3 M1⁄2000 )8.7 2.0 mixed with seawater. The Palma Sola warm spring dis-
51 PacificOcean Thermalsprings(2006) charges thermal water of a low salinity (Cl < 300 ppm)
52 2004 )2.5 )0.4 PalmaSola )35.2 )7.2 with isotopic composition close to rain waters and thus
53 PlayaBlanca )7.5 )0.8 represents the local meteoric water composition with dD (cid:2)
54 PlayaArmada )25.0 )4.1 )38 ±5&(Fig.2).
(cid:2)2010BlackwellPublishingLtd
Socorro hydrocarbons 5
12 Tligahbtleh3ydCroOc2a,rbHo2n,sC(Hm4m(omlomlo%l)1inCdHr4y)ginas)f,umanad- Site Date CO2 H2 CH4 C2H6 C3H8 n-C4 iC4 C1⁄C2
rolesandbubblinggas.
3 F5 November2003 80.8 11.1 4.12 0.91 0.57 0.06 0.17 1098
4 F10 February2004 81.9 10.2 3.52 1.23 0.76 0.12 0.25 813
F40 February2004 82.1 13.2 2.03 1.16 0.29 0.09 0.06 862
5
P1 February2004 78.7 16.2 3.46 2.68 0.89 0.16 0.06 373
6
F5 March2008 77.5 17.8 3.15 1.1 0.6 0.2 0.1 909
7 F10 March2008 80.4 13.7 4.47 1.52 0.52 0.12 0.25 657
8 F40 March2008 76.1 19.1 2.70 1.09 0.51 0.11 0.06 917
9 F40 March2008 77.4 17.7 2.84 1.2 0.45 0.06 0.06 833
P1 March2008 75.1 18.9 3.97 1.68 0.53 0.21 0.09 595
10
11
12
13 bToabnlsea4ndCarhbyodnroigseontopiseostoinpeCsOin2 ahnyddrohcyadrrboocanrs-, d13C&V-PDB dD&V-SMOW
14
15 molecularH2,andwatervapor. Site Date CO2 CH4 C2H6 C3H8 C4H10 H2 CH4 C2H6 C3H8 H2O
16 F5 November2003 )5.3 )17.9 )21.5 nd nd )568 )110 nd nd )49.7
17 F10 February2004 )5.4 )17.2 )24 )24.7 )24.2 )580 )74 nd nd )53.2
18 F40 February2004 )5.6 )19.3 )23.4 )23.1 bd )558 )92 nd nd nd
19 P1 February2004 )3.8 )16.7 )19.8 )17.3 )19.8 )565 )117 nd nd )12.3
F5 March2008 )5.1 )19.4 )21.4 )26.5 )23.5 )565 )73.7 )107 )96 nd
20
F10 March2008 )4.9 )20.2 )22.7 )26.3 bd )562 )70 )120 bd )50.1
21 F40 March2008 )5.3 )18.9 )21.6 )25.3 )22.7 )566 )102 )107 )116 )46.2
22 F40 March2008 )5.4 )19.3 )22 bd bd )556 )99 )121 bd )46.2
23 P1 March2008 )5.4 )21.1 nd nd nd nd nd nd nd nd
24
bd,belowdetectionlimit;nd,notdetermined.
25
26
27
C –C hydrocarbons
28 1 4 G
29 Chemical abundances of C–H–O gases including H ,CO , FI
2 2
N
30 CH4, ethane, propane, and butanes in Socorro gases are O
31 given in Table3. Methane concentrations vary from 2.0 to TI
32 4.5 mol% in dry gas. It can be seen also that CH ⁄C H U
4 2 6 L
33 ratios are high (approx. 400–1100), similar to those O
S
34 reported for gases associated with serpentinization (Lyon E
R
35 4 and Giggenbach, 1996; Proskurowski etal. 2008) and
W
36 hydrothermal gases from Nysiros island (Fiebig etal. O
37 2009). The distribution of C hydrocarbons in Socorro L
2+
38 gases is monotonic: on average C ⁄C (cid:2) C ⁄C (cid:2) 3
2 3 3 4
39 (Table3).
40
41
Carbon andhydrogenisotopes inCO ,H , and
42 2 2
hydrocarbons
43
44 Carbon isotopes in CO , CH , and C –C alkanes and
2 4 2 4
45 hydrogen isotopes in H , H O, and hydrocarbons are pre-
2 2
46 sented in Table4. Isotopically heavy methane with d13C Fig.3. Plot of dD versus d13C of methane for the samples from Socorro 33
47 from )15 to )20& (V-PDB) is a common characteristic islandandforfluidsfromothersitesanddifferentorigin(Schoelldiagram;
Schoell,1988).Seetextformoredetails.
48 for fluids from spreading ridge submarine vents (Welhan &
49 Craig 1983; Scott 1997; Charlou etal. 2002; Proskurow-
50 5 ski etal. 2008). The Schoell diagram (Schoell, 1988) in mud volcanoes in Turkey, and Kola rocks. Additionally,
51 Fig.3 shows dD and d13C in methane from Socorro and the hydrocarbons from Socorro demonstrate an ‘inverse’
52 various other sources. Methane from Socorro gases isotopic trend, where ethane is isotopically lighter than
53 enriched with both 13C and deuterium is plotted close to methane, which is not common for natural gases.
54 other ‘heavy’ methane from Lost City submarine vents, Figure4(A,B) shows that in Socorro fluids, such a trend is
(cid:2)2010BlackwellPublishingLtd
6 Y. A. TARAN et al.
1 Taran (1988), Chiodini & Marini (1998), and Lowern-
G (A)
2FI stern & Janik (2003) have shown that there were no
3N hydrothermal systems found with RH > )2.5, and the only
4O samples with R > )2.8 (a few samples) were reported for
TI H
5U the Geysers vapor-dominated field (Lowernstern & Janik
L
6O 2003). Usually, very high H2 concentrations in dry gas
7S correspond to a high water vapor content with the result-
E
8R ingR << )2.8.
H
9W The Socorro Island R value of 2.0 may indicate that
H
O
10 redox conditions in the Socorro aquifer are controlled by a
L
11 more reduced mineral assemblage than that of continental
12 or arc-related hydrothermal system where the aquifers are
13 composed of altered basaltic to rhyolitic or sedimentary
14 rocks.
15 Analyses of deep-sea hydrothermal fluids that have inter-
(B)
16 acted with hot basalts typically produce H concentrations
2
17 in the 1–2 mmol kg)1 range (Scott 1997 and references
18 therein). H concentrations up to 20 mmol kg)1 are
2
19 reported forsubmarine hydrothermal fluids that have inter-
20 acted with ultramafic rocks (Charlou etal. 2002; Prosku-
21 rowski etal., 2005). These concentrations correspond to 6
22 R in the )4.7 to )3.5 range, which is close to the ‘rock
H
23 buffer’ control at350(cid:3)C in liquid phase.
24 To demonstrate a possible redox control for the Socorro
25 hydrothermal gases, a set of diagrams is presented in
26 Figs5–7. The concentrations of following components
27 depend upon the redox conditions: H , CO , CO, CH ,
2 2 4
28 34 Fig.4. A:CarbonisotopiccompositionofhydrocarbonsinSocorrofluidsas and H S. Three reactions are chosen with the correspond-
2
afunctionofcarbonnumber.B:hydrogenisotopiccompositionasafunc-
29 ing equilibrium constants computed using the HSC-6
tionofcarbonnumber.
30 thermochemical code(Roine 2006):
31
32 observed for both carbon and hydrogen isotopes in C1–C4 Water(cid:3)gasshiftreaction: CO2þH2,COþH2 ðR1Þ
33 hydrocarbons. Sabatier0sreaction: CO þ4H ,CH þ2H O ðR2Þ
2 2 4 2
34
35 and
DISCUSSION
36
Reductionofpyrite: FeS þH ,FeSþH S ðR3Þ
2 2 2
37
Chemicalcompositionof gases:redox stateandchemical
38 For the first two reactions, simple expressions can be
equilibriaof macro-species
39 derived for the ratios RCO¼logðXCO=XCO2Þ,
40 High concentrations of H2 and CH4 are rare but not RC ¼logðXCH4=XCO2Þ,RS¼logðXH2S=XH2OÞ andRH:
41 unique among geothermal gases. Gases from The Geysers
R ¼R þ2:024(cid:3)2082=T ð1Þ
42 steam-dominated field in California are characterized by CO H
43 even higher H and CH on the water-free basis, up to R ¼4R (cid:3)10:29þ9067=T ð2Þ
2 4 C H
44 30% and 10%, respectively (Lowernstern & Janik 2003). In
R ¼R þ4:732(cid:3)2746=T ð3Þ
S H
45 gases from the Namafiall geothermal field in Iceland, up to
46 25% H was reported (Arnorsson & Gunnlaugsson 1985). where X are concentrations in mole fractions in the total
2 i
47 Theuniquefeature of theSocorro gasesis theanomalously gasdischargeand Tis temperature in K.
48 high H ⁄H O ratio. Giggenbach (1987) showed that the Each of three panels in Figs5–7 show analytical points
2 2
49 upper threshold R value (R ¼logðX =X Þ) for together with lines representing phase equilibria (vapor–
H H H2 H2O
50 hydrothermal systems is around )2.8, which corresponds liquid) for three different redox buffers: FeO–FeO (faya-
1.5
51 to an R value in the vapor phase in equilibrium with lite-hematite-quartz) or the ‘rock buffer’ of Giggenbach
H
52 Fe(II)- and Fe(III)-bearing wall rock (primary or altered). (1987), fayalite-magnetite-quartz (FMQ) buffer, and pyr-
53 The proxy thermodynamic system for this ‘rock buffer’ is a ite-pyrrhotite-magnetite (PPM) buffer under saturated
54 mineral assemblage fayalite+hematite+quartz and water. water vapor pressure. Equilibria in the liquid phase were
(cid:2)2010BlackwellPublishingLtd
Socorro hydrocarbons 7
1
G G
2FI FI
3N N
O O
4
TI TI
5U U
L L
6O O
7S S
E E
8R R
9W W
O O
10
L L
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36 Fig.6. PlotoflogðXCH4=XCO2ÞversusRHforSocorrogases.The‘theoreti- 36
cal’ grids, as in Fig.2, correspond to the ‘rock buffer’, pyrite-pyrrhotite-
37
magnetite (PPM), andfayalite-quartz-magnetite(FMQ) redox buffers (see
38
Fig.5).
39 35 Fig.5. PlotoflogðXCO=XCO2ÞversusRHforSocorrogases.Three‘theoreti-
cal’gridsareshownwiththeredoxcontrolbythe‘rockbuffer’ofGiggen-
40
bach, pyrite-pyrrhotite-magnetite (PPM), and fayalite-quartz-magnetite
41 plotted far from any buffered equilibrium and the Sabatier
(FMQ)buffers.
42 equilibrium temperatures (reaction R2) are very high, in
43 therange of400–600(cid:3)C.
44 7 computed using the Giggenbach (1992) approximation, Fast equilibrating gas species (H , CO, and H S) most
2 2
45 where liquid water is considered to be in equilibrium with probably are indicators of equilibrium conditions in the
46 watervapor,whichis,inturn, inchemicalequilibriumwith vapor-dominated aquifer beneath the Everman dome,
47 the corresponding mineral assemblage. Additionally, an Socorro Island. High R values may indicate that the
H
48 equilibrium line at 300(cid:3)C and saturation pressure in the redox conditions in this aquifer are controlled by a
49 gasphaseisdrawn oneachpanelinFig.6toshowequilib- highly reduced mineral assemblage, a hydrothermal ana-
50 rium relationships without redox buffering. It can be seen log of the FMQ buffer. These are uncommon hydrother-
51 that the CO⁄CO pair (Fig. 5) and the H S mole fraction mal redox conditions confirmed also by a high
2 2
52 (Fig.7) are close to equilibrium at 200–250(cid:3)C, under the concentration of CH and the apparent absence of the
4
53 FMQ buffering within the two-phase region, closer to the sedimentary organic matter within this geologically
54 vapor line. In contrast, the CH ⁄CO points (Fig.6) are young volcanic environment.
4 2
(cid:2)2010BlackwellPublishingLtd
8 Y. A. TARAN et al.
1 denote gaseous and dissolved gases, respectively. Experi-
G
2FI mental results (Seyfried etal. 2007) show that the concen- 8
3N tration of H2 produced as a result of the interaction of
4O seawater with peridotite at 200(cid:3)C, 500 bar and a water⁄-
5UTI rock(w⁄r)ratiocloseto1mayreach70mmolkg)1,which
L
6O is about 20% of the theoretically predicted equilibrium H2
7S concentration (Palandri & Reed 2004; Sleep etal. 2004;
E
8R McCollom & Bach 2009). According to thermodynamic
9W modeling by Palandri & Reed (2004) and McCollom &
O
10 Bach (2009), the hydrogen generation in serpentinization
L
11 reaches about 350 mmol kg)1 at 300(cid:3)C, under pressure
12 when no free gas phase can be formed. The equilibrium
13 concentration of H strongly increases with decreasing
2
14 water⁄rock ratio.
15 Thestoichiometricweightwater⁄rockratioforthegener-
16 alized reaction of serpentinization of olivine (Mg# = 0.86)
17 isabout0.15asitfollowsfromthereactionequation:
18
19
3Fe SiO þ15Mg SiO þ23H O
20 2 4 2 4 2
fayalite forsterite
21
)9Mg Si O ðOHÞ þ3MgðOHÞ þ 2Fe O þ 2H
22 3 2 5 4 2 3 4 2 ðR6Þ
serpentine brucite magnetite
23
24
25 This equilibrium can be considered as a relevant redox
26 buffer of ultramafic rocks under hydrothermal conditions
27 (Sleep etal. 2004). However, the equilibrium partial
28 pressure of H computed using this reaction in the 100–
2
29 350(cid:3)C rangeis sohighthat, evenat500 barsoftotal pres-
30 sure, hydrogen separates into a free gas phase with a low
31 fraction of water vapor. Hydrogen production decreases
32 when the modeling takes into account the substitution of
33 Mg for Fe(II) in chrysotile (serpentine) and brucite and
34 thus makes Fe(II) partially unavailable for H production
35 37 Fig.7. PlotoflogPH2S versusRHforSocorrogases.The‘theoretical’grids (Sleep etal. 2004; McCollom & Bach 20092). Therefore,
correspond to the ‘rock buffer’, pyrite-pyrrhotite-magnetite (PPM), and
36 fayalite-quartz-magnetite(FMQ)redoxbuffers(seeFig.5). the modeling predicts that the observed concentrations of
37 H in the ultramafic-hosted submarine vents should corre-
2
38 Serpentinization asapossible mechanism producinghigh spond to a water⁄rock ratio >>1 and a significant substitu-
39 H2andCH4 inSocorro fluids tionofMgbyFeinserpentinites.
40 The methane concentration in the computer simulations
Serpentinization is a hydration reaction of water with mafic
41 of Palandri & Reed (2004) was pre-determined by the
minerals such as olivine and pyroxene. Serpentinization is
42 concentration of HCO(cid:3) in seawater or meteoric water,
driven by the instability of these minerals in water at tem- 3
43 peratures <300(cid:3)C (e.g. Palandri & Reed 2004; Sleep etal. andtherefore, 2–4orders ofmagnitude lowerthanequilib-
44 2004). The oxidation of Fe(II) of fayalite and⁄or ferrosilite rium concentrations of H2. In Socorro gases, the H2⁄CH4
45 reduces water, producing H , and reduces CO , HCO(cid:3) or ratio is within the range of 3–7, close to that of the unltra-
2 2 3
46 mafic-hosted submarine vents of Lost City (Proskurowski
carbonates producing CH :
4
47 etal.2008)and Rainbow (Charlou etal.2002).
48 3FeOðol;pxÞþH2O)Fe3O4ðmtÞþH2ðg;aqÞ ðR4Þ Correlations of H2–CH4 for these vent gases and gases
49 of Socorro are shown in Fig.8. Despite significant scatter-
12FeOðol;pxÞþH CO þH O
2 3 2
50 ing, all three thermal fields show a similar H –CH rela-
)4Fe O ðmtÞþCH ðg;aqÞ ðR5Þ 2 4
51 3 4 4 tionship. The main source for CH in the Lost City and
4
52 Rainbow vent fluids is thought to be magmatic CO . The
2
53 Symbols in parenthesis show thatFeO is apartof olivine same should be suggested for the Socorro gases. There-
54 or pyroxenes, ‘mt’ denotes magnetite, and ‘g’ and ‘aq’ fore, serpentinization is one of the most probable reasons
(cid:2)2010BlackwellPublishingLtd
Socorro hydrocarbons 9
1 lying between the air value of 83.6 and the air-saturated
2 water (ASW) ratio of approx. 40 (Fig.9). This indicates
3 thatlittle H O,N , orAroriginated fromadeepfluid.
2 2
4
5
He,3He⁄4He, andCO asin MORBfluids
6 2
7 In the absence of data on 40Ar⁄36Ar, it can only be specu-
8 latedastotheSocorro4He⁄40Ar*ratios(40Ar*is40Arfrom
9 the mantle). Taran etal. (2002) reported the maximum
10 40Ar⁄36Arof311inasamplewith0.021%ofArindrygas.
11 This may indicate that only 5% (approx. 0.001% instead of
12 0.021%) of the discharging Ar may have a mantle origin. If
13 this is the case for the collected gases (Table1), the
14 4He⁄40Ar* ratio should be between approx. 2 and approx.
15 8, which is close to values for undegassed (approx. 2) and
16 Fig.8. RelationshipbetweenH2andCH4inthermalfluidsassociatedwith degassed (approx. 8) MORB glasses (Marty & Zimmer-
17 ultramafic-hostedsubmarinehydrothermalsystems(mmolkg)1scale,Char- mann 1999). Taking into account that the mantle N ⁄Ar
2
18 lou etal. 2002; Proskurowski etal. 2008) and gases from Socorro fuma- ratioiscloseto100(Marty&Zimmermann1999),itcould
roles(mol%indrygasscale).
19 besuggestedthatthefractionofthemantlenitrogeninthe
20 Socorro gases is also about 5% of the total discharging N ,
2
21 for high H and CH concentrations in Socorro gases, tak- and thus, the mantle component of the Socorro gases can
2 4
22 ing into account the tectonic setting of Socorro Island and bepresented(inmole%)asCO =99.93%,N =0.06%,Ar
2 2
23 unusually high R values. = 0.001%, and He = 0.004%. Because we assume that all
H
24 CO has mantle origin, this estimate leads to very high
2
25 CO ⁄N ratio (approx. 1700), significantly higher than the
Waterisotopes, N ,Ar,andmeteoric contribution 2 2
26 2 expected100–200(Marty&Zimmermann1999).
27 Water from Playa Armada spring water is a mixture of
28 about 20% of seawater with groundwater and the resulting
C –C hydrocarbons: chemicalcomposition
29 dD = )27&and d18O = )4.1&(Table 2,Fig.2). Plotting 1 4
30 the isotopic compositions of water from drainless steam- ThemonotonicdistributionofC hydrocarbons(Table 3)
2+
31 heated pools within the fumarolic field produces a slope istypicalforCH -richnaturalgases(e.g.Jendenetal.1993;
4
32 close to 3, which is a typical ‘evaporation trend’ for hot Hulstonetal.2001),andthistypeofdistributionisusually
33 pools (Giggenbach & Stewart 1982). This trend intersects called Anderson-Flory-Schulz distribution—a monotonic
34 the meteoric water line somewhere between dD = )35 and
35 )25& (Fig.2). Points representing the results for conden- G
36 sates of fumarolic steam form another trend, in the oppo- FI
N
37 site direction, compatible with water from the Palma Sola O
38 thermal spring (Fig.2). A model of the single-step steam TI
39 separation from water with dD = )40& and d18O = LU
40 )6.25& is inserted into Fig.2. It can be seen from Fig.2 O
S
41 that the predominant fraction of the fumarolic steam may E
R
42 be formed at 150–200(cid:3)C as a result of boiling of ground
W
43 water of meteoric origin. This groundwater, most proba- O
44 bly, is the condensed steam ‘envelope’, recorded by the L
45 transient electromagnetic survey by Varley etal. (2004) at
46 shallow depths of 50–100 m under the surface. It might
47 be suggested that the initial fraction of water vapor prior
48 to partial shallow condensation would be much lower than
49 that measured approx. 95 mol% in the Socorro fumaroles
50 (Table1) and thus with much higher H ⁄H O ratio than
2 2
51 themeasured (also very high)values approx. 0.01. Fig.9. The N2–Ar correlation for fumarolic gases of Socorro island. The 38
upper line shows the direct air contamination with the corresponding
52 Owing to the high proportion of a meteoric component
N2⁄Ar=83.6. The lower line represents N2 and Ar concentrations (mole
53 within the volcanic vapor, the ‘atmogenic’ components N
2 fractions) in steam separated from the air-saturated water (ASW) with
54 and Ar show a good correlation, with the N2⁄Ar ratio numberscorrespondingtothesteamfraction(weight%).
(cid:2)2010BlackwellPublishingLtd
10 Y. A. TARAN et al.
1 decrease in C concentrations as a function of carbon num- source of CH , undistinguishable by its isotopic composi-
i 4
2 ber following an exponential law (e.g., Anderson 1984; tion from the deeper inorganic source cannot be excluded
399;;1100 Giggenbach 1997a,b). Fiebig etal. (2009) suggested a cri- fortheLostCity fluids.
4 terion formethane producedabiogenically: if the CH ⁄C He-isotope ratios in Socorro gases (approx. 7.6 R) are
4 2+ a
5 ratio inahydrothermalgasishigherthanthatcommonfor lower than those for the Mid Atlantic Ridge systems (7.5
6 thermogenic hydrocarbons (<100), the excess methane to 8.8 R) but coincide with 3He⁄4He of the northern
a
7 wouldhavebeensynthesizedabiogenicallybythereduction East Pacific Rise vents (7.5–7.8 R, Proskurowski etal.
a
8 of CO or carbonates under appropriate redox conditions; 2008). Therefore, it can be suggested that Socorro hydro-
2
9 butC hydrocarbonsinsuchamixturemaybeofthermo- thermal fumaroles discharge vapors of a mixed origin: (i)
2+
10 genic origin. Ifso, the reduction of CO , orany other oxi- CO andHefromtheuppermantle(orMORB);(ii)water
2 2
11 dized form of carbon, should be very specific allowing the vapor, N , and Ar from the atmosphere (and⁄or ASW);
2
12 productionofCH butnothigheralkanes.Inindustry,this (iii) H and CH are secondary products of reaction of
4 2 4
13 is possible only using so-called Sabatier process, which is a water with hot ultramafic rocks.
14 catalytic reduction of CO by H on Ni or other transition The main difference between Socorro gases and gases
2 2
15 metals(butnotFe)withanexcessofH (e.g.Lunde&Kest- fromsubmarineventswithahighCH ⁄C ratioistheC
2 4 2+ 2+
16 er1973).Innature,theonlyoptionforthistypeoftheCH concentration as a function of carbon number: hydrocar-
4
17 production is microbial CO reduction. Examples for such bons C –C from Lost City, according to Proskurowski
2 2 4
18 methanearegashydrates.‘Regular’Fischer-Tropschsynthe- etal. (2008), and is characterized by concentration ratios
19 sis on Fe and Co catalysts produces hydrocarbons with low C ⁄C approx. C ⁄C approx. 10. Approximately the same
2 3 3 4
20 CH ⁄C ratios(e.g.Anderson1984;Taranetal.2007). ratios have been recorded from C hydrocarbons in cold
4 2+ 2+
21 seepsfromophiolitesofthePoisonBay,SouthIsland,New
22 Zealand (Lyon & Giggenbach 1990). In Socorro gases,
Carbon andhydrogenisotopes inCO ,H , and
23 2 2 C ⁄C (cid:2)C ⁄C (cid:2)3.Thelatteraretypicalratiosfortherm-
hydrocarbons 2 3 3 4
24 ogenic hydrocarbons in CH -rich natural gases (Jenden
4
25 The inverse carbon isotopic trend found in Socorro island etal. 1993; Hulston etal. 2001) and hydrocarbons from
26 (Fig.3) has been reported for ultramafic-hosted submarine hydrothermal gases: for example, fumarolic gases (160(cid:3)C)
27 vents (Proskurowski etal. 2008; Konn etal. 2009), in fromLaSolfatara,Italy,haveCH ⁄C (cid:2)100,C ⁄C (cid:2)7,
4 2+ 2 3
28 gases occluded in the alkaline igneous rocks of Kola penin- and C ⁄C (cid:2) 4 (Capaccioni & Mangani 2001). Therefore,
3 4
29 sula (Galimov & Petersilie 1967; Potter etal. 2004; Potter iftheabiogenicoriginformethaneintheultramafic-hosted
30 11 and Longstaffe, 2007), in free gases of Precambrian rocks vents, cold seeps from ophiolites, and in Socorro gases is
31 of the Canadian Shield (Sherwood Lollar etal. 2002, almost unambiguous, especially, in the hot (350(cid:3)C) fluids
32 2008), in gasesfrom somemud volcanoes in Turkey(Hos- of Rainbow and Logachev vents, the origin of heavier
33 gormez etal. 2008), and in some gases from a gas field in hydrocarbonsisstillunclear.FollowingFiebigetal.(2009),
34 12 China (Tarim basin, Liu etal. 2008). The inverse isotopic it can be assumed that in gases with CH ⁄C >> 100,
4 2+
35 trend for hydrogen isotopes was observed in gases from methane is a mixture between abiogenic, ‘synthetic’ CH ,
4
36 rocks from the Kola peninsula (only between CH and andthermogenichydrocarbonswithCH ⁄C £100.Ifthis
4 4 2+
37 C H , Potter and Longstaffe, 2007), Lost City gases isthecase,ithastobeexplainedwhyd13CanddDofmeth-
2 6
38 (Proskurowski etal. 2008) and in some gas samples from aneandC intheLostCityfluidsareallwithintheranges
2+
39 theTarim basin(Liu etal.2008).Among thesegases, only )15& to )10& and )160& to )120&, respectively
40 those from the Lost City ultramafic-hosted hydrothermal (Proskurowski etal. 2008), i.e., ‘thermogenic’ hydrocar-
41 submarine field are similar in terms of the chemical and bonshavealmostthesameisotopiccompositionastheabio-
42 isotopic composition of the hydrocarbon components to genicmethane.Socorrohydrocarbonsarealsocharacterized
43 Socorrogases (Proskurowski etal.2008). Othergaseshave bysimilarvaluesofd13CanddD(Table4),whichcouldbe
44 low CH ⁄C ratios (Canadian Shield, Kola Peninsula, oil- evidence of mainly syngenetic, not mixed, origin of the
4 2+
45 gas fields) or have isotopically light methane and hydrogen hydrocarbons. Assuming that the methane is abiogenic, or
46 (Canadian Shield, oil-gas fields), or have a clear positive ‘synthetic’,thentheotherhydrocarbonsshouldalsobepro-
47 trend in the hydrogen isotopic composition of hydrocar- duced from the same carbon source by the same abiotic
48 bons (Canadian Shield). A serious discrepancy in the case process. One spectacular example of the mixing of hydro-
49 of the Lost City gases is that the total organic carbon carbons of different origins was reported by Hosgormez
50 (TOC) in the carbonate chimneys (up to 0.6 wt%) has the etal.(2008).Inthiscase,methane-richgasseepagesonthe
51 same range of isotopic compositions ()15 ± 5&) as meth- Mediterranean coast of Antalya, Turkey, are characterized
52 13 ane and its homologues in gases (Bradley etal. 2008). by d13C–CH within the range )12& to )8& (abiogenic
4
53 Taking into account the moderate temperatures of the source), but d13C of the C hydrocarbons varied from
2+
54 Lost City venting (40–90(cid:3)C), an additional microbial )26&to)23&,withinthe‘thermogenic’range.
(cid:2)2010BlackwellPublishingLtd