Lithos 182–183 (2013) 114–124

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Lithos

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Abiogenic hydrocarbon isotopic signatures in granitic rocks: Identifyingpathways of formation

Joanna Potter a,⁎, Stefano Salvi b, Fred J. Longstaffe a

a Department of Earth Sciences, The University of Western Ontario, London, Ontario N6A 5B7, Canadab Géosciences Environnement Toulouse (GET), Observatoire Midi Pyrénées, Université de Toulouse, CNRS, IRD, 14 avenue E. Belin, F-31400 Toulouse, France

⁎ Corresponding author.E-mail addresses: jpotter6@uwo.ca (J. Potter), stefano.s

flongsta@uwo.ca (F.J. Longstaffe).

0024-4937/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.lithos.2013.10.001

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 May 2013Accepted 1 October 2013Available online 10 October 2013

Keywords:Abiogenic hydrocarbonsFischer–Tropsch-reactionsMantle hydrocarbonsStable isotopesPeralkaline rocks

The stability and isotopic composition of hydrocarbons formed in themantle are controversial subjects. Knowingthe range in isotopic compositions of abiogenically-derived hydrocarbons is important for recognising biologicalsignatures in ancient and extra-terrestrial materials. In an effort to enhance this database, stable isotope resultsare reported here for hydrocarbon-bearing fluid inclusions hosted in two peralkaline igneous complexes fromLovozero, Russia and Strange Lake, Canada. Based on the distribution of isotopic compositions, we proposethree pathways for abiogenically-generated hydrocarbons. Type-1 (δ13CCH4 N δ13CCO2 and δ13CC2+) representsmantle-derived hydrocarbons generated in equilibrium with hyperagpaitic magmas in the upper mantle,and suggests that mantle CH4 and C2H6 δ13C and δ2H could have values of ~−5.3 and ~−112‰, and ~−10.8and −162‰, respectively. Type-2 (δ13CCO2 N δ13CCH4 N δ13CC2+) represents Fischer–Tropsch-type hydrocarbongeneration in quartz-bearing peralkaline rockswith CO2 as the initialmagmatic gas phase. For Type-2, δ13CCO2 valuesare ~−2‰, whereas δ13CCH4 and δ2HCH4 values range from−32 to−20‰ and−170 and−160‰, respectively. Theδ13CC2H6 values are slightly lower than associated δ13CCH4 values. Type-3 (δ13CCO2N δ13CCH4b δ13CC2+) represents anoverprint of Type-2, formed during low temperature alteration of quartz-bearing peralkaline rocks. Here, δ13CCO2values are variable,−14.0 to−9.5‰, δ13CCH4 values range from−30.8 to−20.8‰while δ13CC2H6 values are higherthan associated δ13CCH4 values. Both Type-2 and Type-3 isotopic compositions mimic patterns normally consideredto be thermogenic in origin, thus demonstrating that isotopic data alone cannot be used reliably to distinguishbetween hydrocarbons of abiogenic versus biogenic origin.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

The compound-specific stable isotopic compositions of carboniccompounds formed in magmatic-related environments are becomingincreasingly important for distinguishing materials of abiotic originfrom those of microbial or thermogenic origins. However, the carbon-and hydrogen-isotope compositions of abiogenic hydrocarbons canvary substantially depending on their pathway of formation andoverlapwith fields known for thermogenically-derived hydrocarbons (McCollomand Seewald, 2007; Sherwood-Lollar et al., 2002, 2006). This overlapmakes unequivocal identification of ancient organic materials difficultin ancient rocks and meteorites and potentially limits future recognitionof biological signatures in extraterrestrial investigations (Lancet andAnders, 1970; Onstott et al., 2006; Ueno et al., 2004). Therefore, anevaluation of the possible ranges of δ13C and δ2H results from igneoussystems for carbonic gases is essential before assigning them a microbialor thermogenic origin.

alvi@get.obs-mip.fr (S. Salvi),

ghts reserved.

Much attention has been paid to the discovery of high hydrocarbonconcentrations in ultramafic-hosted hydrothermal vent fluids. Isotopicresults for these fluids have been interpreted to indicate a largecomponent of abiogenically-derived hydrocarbons (Table 1; Charlouet al., 2002; Proskurowski et al., 2008; Whelan and Craig, 1983).The presence of abiogenic hydrocarbons generated in deep crustalenvironments has also been suggested based on gas isotopiccompositions (Sherwood-Lollar et al., 2002, 2006). Other isotopicevidence for abiogenic hydrocarbon generation comes from moreexotic rock types such as meteorites (Lancet and Anders, 1970) andperalkaline complexes (Beeskow et al., 2006; Graser et al., 2008;Konnerup-Madsen and Rose-Hansen, 1982; Nivin et al., 1995; Potterand Longstaffe, 2007; Potter et al., 2004a; Voytov, 1992).

Here we report new isotopic data for carbonic fluid inclusiongases from the Lovozero peralkaline complex, Russia and the StrangeLake peralkaline complex, Canada. The origin of hydrocarbon gasesin peralkaline complexes including Khibina and Lovozero, Russia,Strange Lake, Canada and Ilímaussaq, Greenland remains controversial,with possibilities ranging from (1) a direct mantle source (Graser et al.,2008; Krumrei et al., 2007; Markl et al., 2010; Potter and Longstaffe,2007), (2) late-magmatic re-speciation of a C–O–H fluid (Konnerup-Madsen and Rose-Hansen, 1982), (3) late Fischer–Tropsch-type (F–T-t)

Table1

Exam

ples

ofprev

ious

publishe

dda

taforab

ioge

nically

-derived

gasesfrom

variou

sen

vironm

ents.

Locality

Complex

Rock

type

δ13C C

O2

δ13C C

H4

δ2HCH4

δ13C C

2H6

δ2HC2H6

δ13C C

3H8

Referenc

e

EPR

21°EP

RHyd

rothermal

–−

17.6

to−

15.0

−12

6to

−10

2–

––

Whe

lanan

dCraig(198

3)MAR

Rainbo

wHyd

rothermal

−4.0to

−3.0

−18

.2to

−13

.0–

––

–Ch

arlouet

al.(20

02)

MAR

Lost

City

Hyd

rothermal

–−

13.6

to−

9.4

−14

7to

−11

9−

15.2

to−

13.3

−16

6to

−12

7−

16.0

to−

13.4

Prosku

rowski

etal.(20

08)

Cana

dian

Shield

Kid

Cree

kMine

Crystalline

basemen

t–

−40

.7to

−32

.7−

419to

−39

0−

41.1

to−

36.3

−32

1to

−29

9−

40.4

to−

34.9

Sherwoo

d-Lo

llar

etal.(20

02)

SouthAfrica

Witsw

atersran

dVolcano

sedimen

ts–

−55

.5to

−28

.7−

452to

−21

8−

50.5

to−

24.6

−34

8to

−19

8−

47.2

to−

25.2

Sherwoo

d-Lo

llar

etal.(20

06)

KolaPe

nins

ula

Khibina

Peralkaline

−18

.6to

−14

.6−

13.3

to−

7.6

−11

8to

−52

−23

.3to

−16

.1−

175to

−12

0−

21.3

to−

14.1

Potter

and

Long

staffe

(200

7)KolaPe

nins

ula

Khibina

Peralkaline

–−

22.4

to−

5.4

-−

19.2

to−

14.3

–−

19.6

to−

13.0

Beesko

wet

al.(20

06)

KolaPe

nins

ula

Khibina

Peralkaline

–−

12.8

to−

3.2

–−

16.4

––

Voy

tov(199

2)KolaPe

nins

ula

Khibina

Peralkaline

–−

167to

−13

2–

Nivin

etal.(19

95)

Green

land

Ilímau

ssaq

Si-sat

Peralkaline

−21

.0to

−4.6

−43

.4to

−22

.6−

176to

−12

1−

43.3

to−

28.2

−21

6−

41.4

to−

27.3

Graseret

al.(20

08)

Green

land

Ilímau

ssaq

Si-und

ersat

Peralkaline

−17

.3to

−4.6

−5.9to

−3.1

−12

1−

14.7

to−

10.1

−13

3−

17.3

Graseret

al.(20

08)

Note:

EPR—

East

PacificRise,M

AR—

Mid

Atlan

ticRidg

e.

115J. Potter et al. / Lithos 182–183 (2013) 114–124

reactions during low temperature (~350 °C) alteration (Potter et al.,2004a; Salvi and Williams-Jones, 1997), (4) a mixture of magmaticand thermogenic components (Beeskow et al., 2006), and (5) athermogenic source (Laier and Nytoft, 2012). The suite of samplesselected here have been characterised extensively by Salvi andWilliams-Jones (1990, 1992, 1997, 2006), Potter (2000) and Potteret al. (2004a) and provide a rare opportunity to obtain isotopicanalyses of hydrocarbon fluid inclusion assemblages previouslyinterpreted to be abiogenic in origin. We use this suite of isotopicdata for carbonic fluid species to determine the origin of thesegases and to understand the isotopic variability associated with thereactions by which these hydrocarbons may have formed.

2. Summary of the geology of the Lovozero and StrangeLake complexes

The 370–360Ma Lovozero complex is a laccolithic, layered complex,located in the Kola Alkaline Province, NWRussia. It is spatially related tothe Kontozero–Khibina graben and intrudes Archean basement rocks(Dudkin and Mitrofanov, 1994; Kramm et al., 1993). The complexconsists of nepheline syenites, urtites, foyaites and lujavrites. Theserocks are mineralogically agpaitic in nature, ranging from low agpaiticurtites composed of nepheline, arfvedsonite, titanite and sodalite tohyperagpaitic lujavrites composed of nepheline, arfvedsonite, aegirine,eudialyte and rare Na–T–Zr-silicates (Potter et al., 2004a).

The 1240± 2MaMa Strange Lake complex is a small intrusion ofperalkaline granite, located in the Rae tectonic province of the CanadianShield intruding Archaean basement and Elsonian quartz monzonite(Currie, 1985; Miller, 1986; Miller et al., 1997; Pillet, 1985). Thecomplex consists mostly of hypersolvus and subsolvus granites.Pegmatites are found mainly in the subsolvus granite but also arepresent in the hypersolvus granites. The hypersolvus granite iscomposed of perthite, quartz and arfvedsonite, whereas the subsolvusgranite is composed of quartz, arfvedsonite, albite and microcline. Thepegmatites are composed of quartz, microcline, albite, arfvedsonite,aegirine and rare-metal minerals and interpreted to have formedpenecontemporaneously with fluid phase separation duringcrystallisation of the subsolvus granite (Salvi and Williams-Jones,1990). Most units have been affected by high temperature (~350 °C)alteration; it consists of replacement of arfvedsonite by aegirine,magnetite and quartz, interpreted to be caused by orthomagmatic fluidsreleased during crystallisation of the subsolvus granite (Salvi andWilliams-Jones, 1990, 2006). A low temperature (150–200 °C)alteration overprint, related to hematisation, Ca-metasomatism andHFSE mineralisation, can be observed in the apical region of thesubsolvus granite; this overprint is associated with local infiltration ofCa-rich fluids that mixed with magmatic fluids producing HFSE-enrichment in the ore zone (Salvi and Williams-Jones, 1996, 2006).

3. Sample selection and isotopic analysis protocol

The Lovozero samples analysed here are part of the collection ofPotter (2000). Mineralogical, petrographical, fluid-inclusionmicrothermometric and compositional data (GC and Laser-Raman)have been reported for these samples by Potter et al. (2004a). TheStrange Lake samples were obtained from the collections of Salvi andWilliams-Jones (1990, 1992, 1997, 2006) and were described in detailin those reports.

The Lovozero samples comprise the range of rock types (urtite,foyaite, lujavrite, eudialyte lujavrite and loparite juvite) present inthe complex. The Strange Lake samples consist of hypersolvus andsubsolvus peralkaline granites that display high-temperature and/orlow-temperature alteration features. Pegmatites hosted in the StrangeLake hypersolvus granites were also analysed.

Isotopic data were obtained at the University of Western Ontariousing an online-crushing GC–irMS technique described by Potter and

116 J. Potter et al. / Lithos 182–183 (2013) 114–124

Longstaffe (2007; cf. also Salvi and Williams-Jones, 2003). Mineralseparates were hand-picked based on previous identification of fluidinclusion assemblage abundance in specific minerals: nepheline fromLovozero samples and quartz from Strange Lake samples. Mineralsize-fraction typically ranged between 0.5 and 2 mm. Between 1 and2 g of material was picked, cleaned with distilled water and oven-dried before being placed in the crusher sample chambers, whichwere then attached on-line to a gas chromatograph–isotope ratiomass spectrometer system. Samples were baked overnight at ~120 °Cin a constant He flow (~1 ml/min) to remove surface contaminants.When each sample was ready for crushing, the He flow rate waschanged to ~20 ml/min. A hydraulic ram was pumped to ~7000 psiand then released in one quick stroke onto the piston of the crusher.Volatiles released from the sample chamber passed through heatedstainless steel tubing to a slurry trap (−70 °C) to condense H2O,and then to a 13 nm molecular sieve trap immersed in liquid nitrogen(−196 °C) to capture all gas phases present. He-flow through themolecular sieve trap was then changed to 1–2 ml/min and the sieverapidly heated, releasing the carbonic gases to a 25 m × 0.32 mmPoraplot Q GC capillary column at 30 °C. Eluting gases were passedthrough a CuNiPt combustion reactor (940 °C) to convert anyhydrocarbon species to CO2 for carbon-isotope analysis or a hollowceramic pyrolysis reactor (1420 °C) to crack hydrocarbons generatingH2 for hydrogen-isotope analysis. A ThermoFinnigan DeltaPlusXL

continuous-flow mass-spectrometer was used to measure the isotopicratios. All samples were re-crushed to obtain additional aliquots ofgas, in order to assess reproducibility of the process.

Carbon-isotope internal standards for methane, carbon dioxide,ethane and propane, which have been evaluated against NBS 19-calibrated NIST natural gas standards RM8559 and 8560,were analysedthroughout the study to determine the accuracy, precision and linearityof the system. The carbon-isotope results are reported in δ-notationrelative to VPDB at 1σ error. The δ13C values measured for CH4, CO2,C2H6 and C3H8 internal standards during this study were −44.1 ±0.2‰, −44.2± 0.2‰, −30.9± 0.2‰ and −24.9± 0.2‰, respectively,which compares well with accepted values of −44.0, −44.3, −30.8

50 μm

a

aeg

50 μm

c primary aqueous-carbonic inclusions with fluorescenthydrocarbons

b

d

Fig. 1. Photomicrographs of (a) primary monophase hydrocarbon inclusions attached to aeghydrocarbon inclusion cross-cutting larger primary inclusions of the same composition fropegmatites from Strange Lake. Image taken under partial UV illumination; (d) pseudomorph cryby primary fluid inclusions.

and −25.0‰, respectively. Hydrogen-isotope compositions wereevaluated against two internal house CH4 and C2H6 standards (EIL 7and 8) and NIST natural gas standard RM8560, as evaluated relative toVSMOW and VSLAP. The hydrogen-isotope results are reported in δ-notation relative to VSMOW at 1σ error. The δ2H values measured forEIL 8 CH4, EIL 7 C2H6 and RM8560 CH4 standards during this studywere −64 ± 2‰, −324 ± 2‰ and −174 ± 2‰, respectively, whichcompares well with accepted values of −65, −325 and −175‰,respectively. To provide an extra geochemical discrimination tool, ratiosof CO2/CH4 and C1/C2 + C3 were calculated from peak area responsesrecorded by the isotope-ratio mass spectrometer, as calibrated usingknown natural gas mixtures (RM8559 and 8560). Compositional gaschromatographic analysis was not available in this study.

4. Results

4.1. Summary of fluid inclusion petrography

The Lovozero samples analysed here contain rare, primarymonophase gas inclusions commonly associated with aegirine needles(Fig. 1a). The most abundant fluid-inclusion assemblages occur assecondary trails of monophase gas inclusions in healed fractures withinnepheline, intersecting larger, primary fluid inclusions (Fig. 1b). Thereare no discernable compositional differences between primary andsecondary inclusions in these samples, both of which consist mostly ofa low-density gas composed of CH4 (52–90%) and H2 (4–26%), withhigher hydrocarbons making up the rest of the gas phase (up to C6;Potter et al., 2004a). Aqueous inclusions are extremely rare in allsamples. Trace CO2 was detected using GC–irMS analysis but not byother methods.

The Strange Lake samples contain primary monophase carbonic andmulti-phase (aqueous and aqueous-carbonic)fluid inclusions, hosted inquartz. Pegmatites in the hypersolvus and subsolvus granites containabundant, primary monophase aqueous-carbonic inclusions thatare Na-rich and fluoresce under UV light (Fig. 1c). Similar inclusionsoccur in granite samples that were affected by the high-temperature

50 μm

early primaryinclusion

secondary trails

0.5mm

primary inclusions in pseudomorph crystal

irine microlites in nepheline from Lovozero samples; (b) secondary trails of monophasem Lovozero; (c) primary aqueous-carbonic and monophase hydrocarbon inclusions instals in the ore zone at Strange Lake. The outer edges of the precursor crystals are outlined

Table 2Data for fluid inclusion gases from Lovozero and Strange Lake.

Sample Rock type Type Al %H2 δ13CCO2 1σ δ13CCH4 1σ δ2HCH4 1σ δ13CC2H6 1σ δ2HC2H6 1σ δI3CC3H8 1σ CO2/CH4 C1/(C2+ C3)

ΔCO2–CH4 T

LovozeroL-16-19 Lop Juvite 1 1.32 25.90 −28.5 3.3 −5.4 0.8 −103 – −9.6 0.3 −175 – −7.8 0.5 0.0 7.4 −23.1 395a

L-16-11 Eud Lujavrite 1 1.24 5.13 −27.8 3.4 −5.5 0.1 −112 – −11.9 0.4 −147 – −10.7 – 0.0 18.6 −22.3 415a

L 167 Eud Lujavrite 1 1.36* – −29.9 1.7 −5.2 0.8 −117 – −11.2 0.4 −169 – −12.5 – 0.0 8.2 −24.7 365a

146-6 Lujavrite 1 1.28 35.24 −25.3 3.0 −5.5 – −138 – −9.8 0.8 – – −8.5 – 0.1 5.8 −19.8 475a

L 135 Lujavrite 1 1.34* – −26.4 4.3 −7.2 0.3 −93 4 −13.2 0.2 −164 – −11.8 – 0.0 40.7 −19.2 495a

L 228 Lujavrite 1 1.34* – −27.3 2.4 −4.7 0.1 −99 2 −11.6 0.2 −157 2 −10.0 0.6 0.0 18.6 −22.6 410a

727-9 Foyaite 1 1.23* 0.04 −26.4 1.7 −7.3 0.1 −138 4 −15.3 0.5 −175 – −15.9 – 0.0 42.5 −19.1 495a

L 76 Foyaite 1 1.23* – −27.7 3.7 −11.6 0.7 −126 – −17.0 0.7 −173 – −15.1 – 0.0 34.7 −16.1 610a

UG 54 Urtite 1 1.04 27.76 −16.3 2.5 −5.9 0.2 −143 – −9.9 0.2 −211 – −9.6 – 0.0 5.1 −10.4 N900a

Strange LakeSL-2-4 Pegmatite

(high T alt in HSG)1 – – −18.0 0.5 −10.3 0.3 −176 3 −15.4 1.2 −216 – −15.4 – 0.0 10.3 −7.8 N900a

SL-58-C-2 Pegmatite(high T alt in HSG)

1 – – −16.0 – −11.8 0.2 −240 4 −18.3 0.3 −303 – −16.6 0.1 0.3 6.1 −4.2 N900a

SL-48-A-7 Pegmatite(high T alt in HSG)

1 – – −21.5 1.6 −12.0 – – – −16.4 0.3 – – – – 0.3 11.5 −9.5 N900a

SL-4-1 Pegmatite(high T alt in SSG)

2 – – −1.8 – −19.7 0.2 −171 – −23.3 – – – −22.3 – 0.7 10.8 17.9 430b

SL-46-A-3 Subsolvus granite(high T alt)

2 – – −2.0 1.3 −32.0 0.6 −160 – −33.0 0.6 – – – – 1.0 12.0 30.0 240b

SL-38-A-1 Hypersolvus granite(low T alt)

3 – – −9.5 – −28.8 0.6 – – – – – – – – 6.2 – 19.3 400b

TTR-16 Subsolvus granite(high+ low T alt)

3 – – −12.9 – −26.0 – – – −24.9 – – – – – 1.1 15.9 13.1 570b

SL-37-D-1 Hypersolvus granite(low T alt)

3 – – −14.3 – −30.8 – – – – – – – – – 2.2 – 16.5 465b

SL-1-16 Pegmatite(high+ low T alt in SSG)

3 – – −9.8 0.9 −30.2 1.3 – – −24.9 – – – – – 4.2 14.2 20.4 380b

SL-1-8+ 9

Pegmatite(high+ low T alt in SSG)

3 – – −13.0 – −30.1 – – – −22.1 – – – – 6.2 12.5 17.1 450b

Type number refers to carbon-isotope distribution as discussed in text; AI — agpaicity index (Na+K/Al) and %H2 data and estimates* from Potter (2000); CO2/CH4 ratios calculated frommass spectrometer peak area response; C1/(C2+C3) ratioscalculated from mass spectrometer peak area response calibrated against NIST natural gas standards, RM 8559 and 8560; temperatures calculated from CH4–CO2 kinetic fractionation factora (Kiyosu and Krouse, 1989) and CO2–CH4 equilibriumfractionation factorb (Horita, 2001). Temperature errors are ~± 30 °C and±10 °C, respectively. HSG — host is hypersolvus granite, SSG — host is subsolvus granite.

117J.Potter

etal./Lithos182

–183(2013)

114–124

-35.0

-30.0

-25.0

-20.0

-15.0

-10.0

-5.0

0.0

CO2 CH4 C2H6 C3H8 CO2 CH4 C2H6 C3H8

a) Lovozero:Type-1 distribution

-35.0

-30.0

-25.0

-20.0

-15.0

-10.0

-5.0

0.0

b) Strange Lake:Type-1 distribution

-35.0

-30.0

-25.0

-20.0

-15.0

-10.0

-5.0

0.0

c) Strange Lake:Type-2 distribution

-35.0

-30.0

-25.0

-20.0

-15.0

-10.0

-5.0

0.0

d) Strange Lake:Type-3 distribution

13C

‰ (

VP

DB

)

13C

‰ (

VP

DB

)13

C ‰

(V

PD

B)

CO2 CH4 C2H6 C3H8 CO2 CH4 C2H6 C3H8

Fig. 2. The carbon-isotope distribution of CO2 and hydrocarbons in: (a) Type-1 in Lovozero samples, and (b) Type-1, (c) Type-2, and (d) Type-3 in Strange Lake samples (data in Table 1).

118 J. Potter et al. / Lithos 182–183 (2013) 114–124

alteration event. The fluid contained in these inclusions is recognised asorthomagmatic (Salvi and Williams-Jones, 1997, 2006). Granitesamples overprinted by low-temperature alteration contain primarycarbonic-aqueous inclusions and Ca-rich, aqueous inclusions of variablesalinity entrapped in pseudomorphs after primary rare-metal phases(Fig. 1d). These inclusions are interpreted to represent mixing betweenthe orthomagmatic fluid and an external, low-salinity, Ca-rich oxidisingfluid (cf. Salvi andWilliams-Jones, 2006 for a review). Gases entrappedin rocks affected by high-temperature alteration consist predominantlyof CH4 (54–80%) and H2 (1–35%); other gases include C2–C6hydrocarbons, CO2 and N2. Gases in rocks affected by low temperaturealteration consist of CH4 (8–40%), CO2 (25–62%), H2 (15–27%) andminor C2–C6 hydrocarbons. Salvi and Williams-Jones (1997) attributedthe elevated content of CO2 in the latter inclusions to oxidation oforthomagmatic hydrocarbons by meteoric fluids.

4.2. Isotopic results for the fluid-inclusion gases

Three distributions of isotopic compositions have been identifiedbased on the δ13C values of these gases (Table 2; Fig. 2), and are henceforward labelled: Type-1 — δ13CCH4 N δ13CCO2 and δ13CC2+; Type-2 —

δ13CCO2N δ13CCH4N δ13CC2+, and Type-3 — δ13CCO2N δ13CCH4b δ13CC2+.Fluid-inclusion gases in the Lovozero agpaitic samples all have a

Type-1 distribution (Fig. 2a), with δ13CCH4 values ranging from −11.6to −4.7‰. Methane from hyperagpaitic lujavrite–juvite samples ismost enriched in 13C (~−5.6‰) and its δ13CCH4 values are consistentlyhigher than associated δ13CCO2 values, which are as low as −30‰.The δ13CC2+ values (−15.9 to −7.8‰) are lower than associatedδ13CCH4 values. The δ2HCH4 values range from −143 to −93‰, whichis consistently higher by 35 to 70‰ than associated δ2HC2H6 results

(−211 to −147‰). The CO2/CH4 ratios are extremely low (b0.1) andC1/(C2+C3) ratios range from 5.1 to 42.5 (Fig. 3a,b).

Fluid-inclusion gases from three agpaitic pegmatite samplesfrom Strange Lake, hosted in the hypersolvus granite, also show aType-1 distribution (Fig. 2b). The δ13CCH4 values range from −12.0 to−10.3‰, higher than associated δ13CCO2 (−21.5 to −16.0‰) andδ13C2+ values (−18.3 to −15.4‰). The δ2HCH4 values are −240 and−176‰, higher by 40 to 60‰ than associated δ2HC2H6 values (−303to −216‰). The CO2/CH4 ratios are also low (b0.4) and C1/(C2 + C3)ratios range from 6.1 to 11.5 (Fig. 3a,b).

Fluid-inclusion gases from one pegmatite hosted in subsolvusgranite and one subsolvus peralkaline granite from Strange Lake showType-2 distribution (Fig. 2c). These samples have been affected byhigh-temperature alteration and gas concentrations are exponentiallylower than samples displaying a Type-1 distribution. The δ13CCO2 valuesrange from −2.0 to −1.8‰ and δ13CCH4 values are lower thanassociated δ13CCO2 values (−32.0 to −19.7‰), which in turn areslightly higher than δ13CC2+ values (−33.0 to −22.3‰). The δ2HCH4

values lie between −171 and −160‰. Ethane concentrations weretoo low for hydrogen-isotope measurements. The CO2/CH4 ratios (0.7to 1.0) are higher than in Type-1 (Fig. 3a). The C1/(C2 + C3) ratios forthese Type-2 samples range from 10.8 to 12.0 (Fig. 3b).

Fluid-inclusion gases from the remainder of the Strange Lakesamples, all of which have been affected by low-temperature alteration,display Type-3 distribution (Fig. 2d). The δ13CCO2 values range from−14.3 to −9.5‰. The δ13CCH4 values are always lower thanassociated δ13CCO2 values and fall within a narrower range (−30.8 to−26.0‰). The δ13CC2H6 values in these samples (−24.9 to −22.1‰)are higher than associated δ13CCH4 values. Concentrations of CH4 andC2H6 were too low for hydrogen-isotopemeasurements. These samples

20.0

30.0

40.0

50.0

60.0 Lovozero

Strange Lake

b)

Type 1

3.0

4.0

5.0

6.0

7.0C

O2/

CH

4Lovozero

Strange Lake

a)Type 3

0.0

10.0

C1/

(C2+

C3)

Type 2 & 3

0.0

1.0

2.0

-40.0 -30.0 -20.0 -10.0 0.0

-40.0 -30.0 -20.0 -10.0 0.0

Type 1Type 2

13CCH4 ‰ (VPDB)

13CCH4 ‰ (VPDB)

Fig. 3. Plots for the carbon-isotope distribution of Types 1–3: (a) CO2/CH4 versus δ13CCH4,and (b) C1/(C2+ C3) versus δ13CCH4.

-50-150-250-350-4500

-20

-40

-60

-80

-100

-120

Abiogenic/MantleType-1

MicrobialMethyl-typeFermentation

mix

transition

early matureThermogenic

mature

Atmospheric

Geothermal, Hydrothermal, Crystalline

Microbial CO2

ReductionBacterial

exchange with H O2

F-T-t CH4

Type-2

δ2HCH4 ‰ (VSMOW)

δ13C

CH

4 ‰

(V

PD

B)

Fig. 4. A δ13C versus δ2H diagram showing established fields for CH4 (based on data fromSchoell, 1983, 1988; Whiticar, 1999; Whiticar et al., 1986 and reviews of McCollom andSeewald, 2007; Potter and Konnerup-Madsen, 2003). Open triangles (Lovozero) andopen circles (Strange Lake) show Type-1 abiogenic/mantle CH4 values. Filled circles(Strange Lake) show Type-2 F–T-t generated CH4 values. Arrows indicate the shift of CH4

isotopic compositions resulting from exchange with H2O or because of Fischer–Tropsch-type reactions (F–T-t CH4).

119J. Potter et al. / Lithos 182–183 (2013) 114–124

have high CO2/CH4 ratios (1.1 to 6.2) and C1/(C2+C3) ratios between12.5 and 15.9 (Fig. 3a,b).

5. Discussion

5.1. Typical pathways of hydrocarbon generation

5.1.1. Microbially- and thermogenically-derived hydrocarbonsMethane and higher hydrocarbons can be formed by various

mechanisms, which result in a variety of isotopic distributions amongspecies. The most common distribution in nature is that observed forthermogenic hydrocarbons derived from the cracking of complexorganic molecules in response to heating during diagenesis. During thisprocess, 12C\1H and 12C\12C bonds are preferentially broken to formsimpler hydrocarbon species that are enriched in the lighter isotope.This produces an isotopic distribution in which δ13C and δ2H valuesdecrease with decreasing carbon number and results in δ13CCH4 andδ2HCH4 values that range from −60 to −20‰ and −350 to −150‰,respectively, depending on the thermal maturity of the gases (Fig. 4;Schoell, 1983, 1988; Whiticar, 1999). Associated CO2 is generallyenriched in 13C relative to CH4. The C1/(C2+ C3) ratios in thermogenicgases are b100 (Bernard et al., 1978; Horita and Berndt, 1999).

Microbially-derived methane can usually be identified bydepletion of 13C and 2H relative to thermogenic methane (Fig. 4).Depending on the pathway, such methane can have δ13C and δ2Hvalues as low as −90 and −450‰, respectively, and the system iscommonly characterised by increased C1/(C2 + C3) ratios (N1000;Whiticar, 1999; Whiticar et al., 1986).

5.1.2. Abiogenically-derived hydrocarbons via Fischer–Tropsch-type (F–T-t)reactions

The most commonly invoked reaction for abiotic generation ofhydrocarbons is the modified Fischer–Tropsch-type reaction:

nCO2 þ 3nþ 1ð ÞH2→CnH2nþ2 þ 2nH2O: (R1)

Fischer–Tropsch-type reactionswere initially recognised in industrialprocesses for generating hydrocarbons by reacting CO andH2 over nativemetal catalysts (Anderson, 1984). These step reactions have beenextrapolated to natural systems, invoking CO2 as the initial reactant asshown in reaction 1 (R1). In this case,magmatic CO2 is the inferred initialreactant, along with H2 produced from hydrothermal redox reactions(e.g., Salvi and Williams-Jones, 1997; Sherwood-Lollar et al., 2002). Theredox reaction usually connected to abiogenic hydrocarbon generationvia reaction 1 (R1) is related to serpentinisation of ultramafic–maficcrust (Konn et al., 2009; Sherwood-Lollar et al., 1993) and can besummarised as:

A similar redox reaction has been proposed that explains theformation of hydrocarbons during high-temperature hydrothermalalteration of peralkaline rocks, where arfvedsonite is commonly alteredto aegirine at ≥350 °C (Salvi and Williams-Jones, 1997):

During Fischer–Tropsch-type reactions, it is predicted that 12C\16Obonds are broken preferentially and 12C incorporated by CH4, whichleads to depletion of 13C in CH4 relative to initial CO2. Furtherintermediate reactions preferentially break 12C\1H bonds and form12C\12C bonds, thus producing systematically lower δ13C values in the

0.10.50.99

CH + O4 2

CO + H O2 2

HMhyperagpaitic

miaskiticmixed volatiles?

300 400 500 600 700 800 900-35

-30

-25

-20

-15

-10

lgo

fO2

T (°C)

limit of solidus

Si-sat peralkaline

FMQ

Fig. 5. The trajectory of the estimated paths of crystallisation in fO2-T space with respect to CO2- and CH4-fluid stability for hyperagpaitic, miaskitic and Si-saturated peralkaline magmas.Based on calculations by Ryabchikov and Kogarko (2006), Schöneberger andMarkl (2008) andMarkl et al. (2010). Finely dashed lines and associated numbers represent varying CH4/CO2

ratios at the fugacity that these magmas formed.

120 J. Potter et al. / Lithos 182–183 (2013) 114–124

higher hydrocarbons relative to CH4. The formation of successive higherhydrocarbons during these reactions requires two hydrogen atoms tobe ejected at each step. The H “lost” is expected to be 1H, which leadsto 2H-enrichment in the new higher hydrocarbons relative to CH4

(Sherwood-Lollar et al., 2002). This pattern has been observed duringlow-temperature serpentinisation processes in nature (Sherwood-Lollar et al., 2002, 2008). However, experimental studies of abiogenichydrocarbon gas generation and isotopic fractionation of thesehydrocarbons during hydrothermal alteration of mafic minerals atvarious temperatures and pressures have produced mixed results. Thedata range from the predicted 13C-depletion with increasing carbonnumber (Horita and Berndt, 1999; Lancet and Anders, 1970; Taranet al., 2007) to minimal isotopic variation between hydrocarbon species(Fu et al., 2007) to 13C-enrichment with increasing carbon number(McCollom and Seewald, 2006; McCollom et al., 2010; Taran et al.,2007, 2010). Hydrogen-isotope results are rare in these studies giventhe limited amount of hydrocarbons generated, but some reporteddata exhibit the pattern of 2H-enrichment between CH4 and C2H6

proposed by Sherwood-Lollar et al. (2008) for Fischer–Tropsch-typereactions (McCollom et al., 2010; Taran et al., 2010).

5.2. Discussion of Type-1 distribution

A large proportion of the isotopic data presented here shows Type-1distribution (all the Lovozero samples and three Strange Lake pegmatitesamples). In these samples, CO2 is depleted of 13C relative to CH4. Thisbehaviour indicates that CO2 was not a precursor to CH4 and precludesformation of CO2 and CH4 by thermogenic or Fischer–Tropsch-typereactions; CO2-reduction by chemical reactive pathways should produce13C-enrichment in the residual CO2 in relation to CH4 (e.g., Giggenbach,1997; Horita, 2001). This inference is supported by the fact that theconcentration of CO2 in these samples is negligible, i.e., below thedetection level of Laser-Raman microscopy (CO2/CH4 ratios b0.1;Fig. 3a). Instead, the CO2 was most likely generated by kinetic (non-equilibrium) CH4-oxidation at trace levels. Kinetic CH4-oxidation cangenerate CO2 depleted of 13C relative to CH4 (Kiyosu and Krouse,1989), in contrast to equilibrium isotopic exchange between CO2 andCH4 (Horita, 2001). The CH4–CO2 carbon-isotope kinetic fractionationfactor derived by Kiyosu and Krouse (1989) from experimental CH4-oxidation over Fe- and Cu-oxide compounds yields calculatedtemperatures of 365 to 610 °C for the majority of Lovozerohyperagpaitic samples. These results may indicate that limited CH4-oxidation occurred during early cooling of the Lovozero intrusion,with associated remobilisation and re-entrapment of the hydrocarbonfluids as secondary inclusions. One Lovozero sample and all Strange

Lake samples showing Type-1 distribution yield anomalously hightemperatures for these rock types, well above predicted solidustemperatures, using the Kiyosu and Krouse (1989) fractionation factor.This outcome could point to kinetic CH4-oxidation but presently there islittle understanding of non-equilibrium fractionation at such tracelevels.

The 13C-depletion of these samples with increasing carbon numbermimics hydrocarbon formation by proposed Fischer–Tropsch-typereactions in natural systems. However, the 2H-enrichment expected inhigher hydrocarbons relative to CH4 during these reactions is absent.Instead, 2H-depletion is observed. Production of higher hydrocarbonsby radical recombination of CH4 at high temperatures (N500 °C) mayinstead explain these results. In this scenario, small amounts of CH4 aretransformed to CH3 radicals during interactions with a catalytic surface(i.e., MeAl2O3) and recombined to produce C2H6 (Sinev, 2003). Suchrecombination preferentially incorporates 12C and 1H atoms, producingthe pattern of 13C- and 2H-depletion observed for the Type-1 fluid-inclusion gases.

The majority of fluid-inclusion gases showing a Type-1 distributionare hosted in the agpaitic–hyperagpaitic rocks of Lovozero. The threepegmatite samples showing a Type-1 distribution at Strange Lake arehosted in the early hypersolvus granite and are also agpaitic incomposition. Similar results have been reported for miaskitic–agpaiticrocks from the Khibina complex, Russia (Beeskow et al., 2006; Potterand Longstaffe, 2007) and for hyperagpaitic ussingite veins from theIlímaussaq complex, Greenland (Graser et al., 2008). The pattern of 13C-and 2H-depletion suggests that these hydrocarbon gases formed undermagmatic conditions in the absence of CO2 (N500 °C) in equilibriumwith the agpaitic magmas.

The fO2 calculations for these magmas range from FMQ 0 to FMQ−5 (Markl et al., 2010; Ryabchikov and Kogarko, 2006; Salvi andWilliams-Jones, 1997). The persodic nature of these magmaspromotes crystallisation of sodic pyroxene and amphibole, which alsorequires incorporation of Fe3+. Oxidation of Fe2+ to Fe3+ in themagma reduces CO2 and H2O in the fluid phase, producing a fluidcomposed of CH4 ± H2 in equilibrium with the melt (Fig. 5; Marklet al., 2010). This can be demonstrated by the redox equation:

This is apparent in the Lovozero samples where H2O fluid inclusionsare exceptionally rare. However, Khibina samples are dominated by a

121J. Potter et al. / Lithos 182–183 (2013) 114–124

miaskitic mineral assemblage of augite, titanite and zircon with rareaegirine and arfvedsonite (Potter, 2000). Such an assemblage shouldbe in equilibrium with a CO2-H2O fluid (Ryabchikov and Kogarko,2006). The Khibina rocks sit near the CO2–CH4 stability boundary,close to the transition betweenmiaskitic and agpaitic rocks; the agpaiticindex generally ranges between 0.99 and 1.04 (Potter, 2000), similar totheMotzfeld complex, Greenland, where a change from CO2–H2O fluidsto CH4–H2O fluids is observed with increasing agpaicity (Schönebergerand Markl, 2008). Mixed aqueous-CH4 inclusions are observed inKhibina samples and H2 is rare, indicating these rocks are not as “dry”as the hyperagpaitic rocks of Lovozero and may sit close to the CO2–

H2O stability field (Fig. 5; Beeskow et al., 2006; Potter et al., 1998).Such may also be the case for the Strange Lake pegmatites that show

Type-1 distribution. Si-saturated peralkaline rocks are more oxidisedthan Si-undersaturated magmas and are usually in equilibrium with aCO2–H2O fluid (Fig. 5; Markl et al., 2010). The presence of arfvedsoniteand aegirine in these samples, however, requires low FMQ conditions(bFMQ −2; 400–550 °C; Ernst, 1962), where CH4 ±H2O±H2 may bethe stable magmatic fluid phase. The presence of CH4, CO2, H2O and H2

suggests that these samples formed close to the stability boundary forall these phases. The presence of H2O may also explain the lower δ2HCH4

values at Strange Lake relative to Lovozero (Fig. 4; Table 2). Hydrogen-isotope exchange between CH4 and H2O in the Strange Lake samplescould have altered the original δ2HCH4 values. The hydrogen-isotopefractionation between H2O(l) and CH4 is approximately 110 to 120‰(Horibe and Craig, 1995), and between H2O(v) and CH4, approximately80‰ (Bottinga, 1969) at 350–500°C, placing the associated δ2HH2O valuesin the two Strange Lake samples between−130 and−55‰or−150 and−85‰ respectively, which overlaps δ2H values known for magmaticfluids (Kyser and O'Neil, 1984; Sheppard, 1986).

The CH4 and higher hydrocarbons in these rocksmay, therefore, haveformed under upper mantle conditions. These alkaline complexes weregenerated in the upper mantle before emplacement at N3 km depths(Konnerup-Madsen and Rose-Hansen, 1982; Kramm and Kogarko,1994; Krumrei et al., 2007). The majority of δ13CCH4 values fall withinthe range of typically defined upper mantle carbon (−8 to −5‰;Deines, 2002; Fig. 4). Lower δ13CCH4 values (to −12‰) may be relatedto subtle changes inmagmatic fO2 conditions; lower values are generallyobtained for hydrocarbons hosted in the less agpaitic foyaites andpegmatites or may indicate some isotopic heterogeneity in the uppermantle carbon due to localised mixing with metasomatic fluids. Theδ2HCH4 values generally lie between those suggested for mantle waterand minerals (−80 to −40‰; Kyser and O'Neil, 1984; Sheppard,1986) and hydrothermal abiogenically derived CH4 (−170 to −90‰;e.g. Proskurowski et al., 2008). The majority of δ2HCH4 values for theLovozero lujavrite samples fall between −120 and −90‰, and likelyare the most representative of potential upper mantle δ2HCH4 values.If we extend this inference to the higher hydrocarbons, δ13C and δ2Hvalues for hydrocarbons formed at mantle/magmatic conditionsfall mainly between−15 to−10‰ and−175 to−150‰, respectively.The C1/(C2 + C3) ratios in these Type-1 samples are variable, rangingfrom 5.1 to 42.5 (Fig. 3b), within the thermogenic gas field but similarto other abiogenically derived hydrocarbons reported by McCollom andSeewald (2007) and Tassi et al. (2012).

The lower C1/(C2 + C3) ratios (5.1 to 8.2) obtained for four of theLovozero samples coincide with exceptionally high H2 concentrations(25–35mol%), in comparison to sample 727–9, which has the highestC1/(C2 + C3) ratio (42.5) and negligible H2 (Table 2). The significantamounts of H2may be the preserved by-product of higher hydrocarbonsduring methane radical recombination.

In summary, the isotopic compositions of the Type-1 samples arehighly distinctive. They are characterised by 13C-enriched CH4 values(N−12‰), 13C-depleted CO2 values relative to CH4, δ2HCH4 valuesconsistently higher than δ2HC2H6 values and very low CO2/CH4 ratios(b0.1), indicating CH4 was the dominant stable carbonic gas speciesduring magmatic crystallisation.

5.3. Discussion of Type-2 distribution

In contrast to the Type-1 distribution, two samples of subsolvusgranite and associated pegmatite (SL-4-1 and SL-46-A-3) from StrangeLake have a Type-2 distribution, that is, δ13CCO2 values that are higherthan associated δ13CCH4 values. The CO2/CH4 ratios of these samples arealso higher than observed for Type-1 (Fig. 3a). This isotopic distributionindicates that CO2 was the initial magmatic gas phase, with CH4

subsequently formed by CO2-reduction. The CO2 has higher δ13C values(−2.0 to −1.8‰) than typical magmatic CO2 (−8 to −5‰; Deines,2002). Reduction of CO2 to CH4 preferentially causes the residual CO2

to become enriched in 13C. Such an outcome is anticipated in a closedsystem but can also occur in more open environments where carbonconcentration is low.

Two possible pathways could produce the distribution of δ13C valuesobserved for these two Strange Lake samples: (1) late magmatic re-speciation of a CO2-dominated fluid to a CH4-dominated fluid atb500 °C involving chemical and isotopic exchange of CO2 and CH4, asproposed by Konnerup-Madsen and Rose-Hansen (1982), or (2) post-magmatic Fischer-Tropsch-type reactions at b450 °C involving CO2-reduction via chemical and kinetic reactions, as proposed by Salvi andWilliams-Jones (1997). Calculated CO2–CH4 equilibrium temperaturesare 240 and 440 °C, indicating isotopic exchange or CO2-reductiondown to 240 °C. These temperatures are below the solidus of thesemelts. In addition, late-magmatic equilibrium re-speciation of these C–O–H fluids would not generate the higher hydrocarbon and H2

concentrations measured for these gases (Salvi and Williams-Jones,1997). Therefore, a Fischer–Tropsch-type process is likely the mainhydrocarbon-generating reaction that occurred in these samples. Unlikethe pegmatites showing a Type-1 isotopic distribution, these Type-2samples must have been at redox conditions above the CH4-stabilityfield during crystallisation (i.e. CO2 was the initial magmatic gasphase), as has been predicted for most Si-saturated peralkaline rocks(Fig. 5; Markl et al., 2010). Post-magmatic Fischer–Tropsch-typereactions reduced the CO2 magmatic phase to CH4 and higherhydrocarbons, fractionating δ13C values of CO2 and CH4 in a fashionthat mimics CO2-reduction, depleting the resultant CH4 in 13C andenriching the residual CO2 in 13C. These samples, therefore, provide anexample of abiotically generated hydrocarbons in an orthomagmaticsystem where the hydrocarbon δ13C and δ2H values shift towards theisotopic field for thermogenic gas (Fig. 4). The δ2HCH4 values are lowerthan those measured for the Lovozero samples, most likely because ofisotopic exchange with magmatic H2O, as observed for the Type-1pegmatites. The C1/(C2 + C3) ratios (10.8 to 12.0; Fig. 3b) lie withinthe typical range of thermogenic gases. However, as previouslydiscussed, evidence is growing that abiogenically-derived gases haveC1/(C2 + C3) ratios that are similar to, or slightly higher thanthermogenic gases (Tassi et al., 2012). Thus, there is a need for cautionin interpreting genesis solely from such ratios.

These peralkaline rock samples show the high-temperature alteration(arfvedsonite to aegirine) that provides H2 to drive Fischer–Tropsch-typereactions, as demonstrated in (R3). The 13C-depletion of C2H6 relative toCH4 is the only distinguishable feature separating these abiogenic gasesfrom thermogenically-derived hydrocarbons.

5.4. Discussion of Type-3 distribution

The five Strange Lake samples showing Type-3 distribution of δ13Cvalues are affected by late, low-temperature, Ca-metasomatic alteration.Their CO2 concentrations are significantly higher, and their C2H6

concentrations much lower, than other samples (Fig. 3a). The δ13CCO2values are higher than associated δ13CCH4 values, similar to Type-2samples, which may indicate CH4 formation by CO2-reduction viaFischer–Tropsch-type reactions. However, the δ13CCO2 values show alarge variability and are shifted to compositions (−14.3 to −9.5‰)lower than the typical magmatic range. The δ13CCH4 values fall within a

122 J. Potter et al. / Lithos 182–183 (2013) 114–124

narrow range (−30.8 to −26.0‰) and the δ13CC2H6 values, wheremeasurable, are higher than associated δ13CCH4 values, as might be thecase for a thermogenic hydrocarbon source. While the Ca-metasomaticalteration was produced by infiltration of Ca-rich meteoric fluids, noevidence exists for infiltration of thermogenically derived hydrocarbonfluid into these rocks (as discussed in Salvi and Williams-Jones, 1997).The compositions of the fluid-inclusion gases in these samples areremarkably similar in H2O, CH4, C2+, H2 and N2 concentrations to theorthomagmatic fluids related to the high-temperature alteration (Salviand Williams-Jones, 1997). The main difference is the elevated CO2

concentrations in the Type-3 samples. Therefore, these fluid-inclusiongases are interpreted to represent orthomagmatic fluids that wereoxidised by infiltrating Ca-rich fluids in the apical region of the intrusion.Oxidation is consistent with observable hematisation of the rocks duringthis event (Salvi andWilliams-Jones, 1996, 1997). This process modifiedthe δ13C values of both CO2 and C2H6 and significantly increased CO2

concentrations (Fig. 3a; Salvi and Williams-Jones, 1997, 2006). Thevariability in δ13CCO2 values and high δ13CC2H6 values are most likelythe product of oxidation of C2H6 (and to a certain extent CH4). Thisprocess could have enriched residual C2H6 in 13C relative to CH4 (CH4

being the dominant carbonic species present) and produced thevariability and shift to lower δ13C values observed for the resultant CO2.The C1/(C2 + C3) ratios are also slightly higher (≥12.5) than those ofType-2 samples (Fig. 3b), again suggestive of C2H6 consumption. CH4-oxidation may also have occurred but a shift in δ13CCH4 values is not asapparent as for C2H6, possibly because of initial high CH4 concentrationsrelative to C2H6. Calculated CO2–CH4 equilibration temperatures rangefrom 380 to 570 °C. The higher ‘temperatures’ (N450 °C) are most likelyincorrect, given the lowering of δ13CCO2 values arising from hydrocarbonoxidation and the resulting anomalously low value for Δ13CCO2–CH4. Theremainder of these ‘temperatures’, however, are consistent with theestimates of Salvi and Williams-Jones (1992, 1997), and hence maystill provide a reasonable estimate for initial CH4 formation duringpost-magmatic Fischer–Tropsch-type reactions (380–450 °C).

5.5. The thermogenic-gas origin controversy

Coming full circle, as mentioned in the introduction, a purelythermogenic origin has recently been postulated for hydrocarbons inthese peralkaline complexes (Laier and Nytoft, 2012). Detailed analysisof bitumens extracted from hand samples from the Ilímaussaq complexrevealed biomarker signatures that point to a mix of marine andterrestrial organic matter not older than Cretaceous in age. From thisevidence, the authors inferred that the ubiquitous hydrocarbons foundthroughout these peralkaline complexes are the result of the migrationof external thermogenically-derived hydrocarbon fluids into thesecomplexes. They argue that the 13C-enriched values observed for CH4

are the product of extensive modification of the residual CH4 gasresulting from diffusive loss of 13C-depleted CH4 before entrapment assecondary fluid inclusions. We would contend this hypothesis basedon the following observations:

(1) The identification of primary hydrocarbon inclusions in nephelineand sodalite crystals in Ilímaussaq and Khibina samples (Beeskowet al., 2006; Krumrei et al., 2007).

(2) The presence of high-density liquid hydrocarbon inclusions atdepth from drill core samples in Ilímaussaq and Lovozero(Krumrei et al., 2007; Potter et al., 2004a).

(3) The presence of up to 35 mol% H2 retained in primary andsecondary hydrocarbon inclusions in Lovozero and StrangeLake (Potter et al., 2004a; Salvi and Williams-Jones, 1997). Itspresence brings into question the hypothesis that the 13C-enriched CH4 values obtained from these samples is due tolarge-scale diffusive/migratory loss of “light” CH4 prior toentrapment; H2 would be the first gas phase to disappear ifthat were the case.

(4) The extent of 13C-enrichment required (+20 to+25‰) requiredby such diffusive processes. Such enrichments have yet tobe clearly confirmed in any natural samples. One study notedby Laier and Nytoft (2012) referred to the progressive 13C-enrichment of CH4 observed in primary inclusions up-sequencein an undisturbed evaporite sequence (−50 to +21‰; Potteret al., 2004b). This 13C-enrichment was attributed to Rayleighfractionation by evaporative processes during evaporiteprecipitation. Yet again, the uppermost sequence (13C-enriched)also contained high concentrations of H2, indicatingminimal gasdiffusion.

(5) The ubiquitous presence of hydrocarbons in these rocks contrastswith their absence in the adjacent host rocks, implying littleevidence for migration of substantial hydrocarbons from anexternal source.

The observation of Cretaceous or younger biomarkers in surfacehand samples is certainly interesting. Beeskow et al. (2006) noted thattowards the exterior contact of the Khibina complex, the δ13CCH4 valueswere 13C-depleted (−24‰) in relation to δ13CCH4 values at the core ofthe complex (~−5‰), indicating that these complexes have potentiallyexchanged with local externally-derived hydrocarbon fluids where incontact with the host rocks or unroofed specimens but an extensivesurvey of bitumens from deep drill cores will be needed before a purelythermogenic origin for these hydrocarbons can be fully put to the test.

6. Summary and implications

These unusual peralkaline igneous complexes provide a rarewindowinto the isotopic compositions of abiogenically-derived hydrocarbonfluids formed by different reaction pathways. Hydrocarbons hosted inhyperagpatitic samples, the majority of which are from Lovozero, havea carbon-isotope distribution, labelled Type-1, that is indicative ofhydrocarbon formation in equilibrium with peralkaline magmas. Wepropose that methane hosted in the hyperagpaitic samples has carbon-and hydrogen-isotope compositions close to those expected for theupper mantle (~−5.3 and −112‰). Higher hydrocarbons have lowerδ13C and δ2H values than coexisting CH4 (~−10.8 and −162‰). Theassociated CO2 is consistently depleted of 13C relative to CH4, and is theproduct of late-postmagmatic kinetic CH4-oxidation at trace levels.

Hydrocarbons hosted in agpaitic silica-saturated peralkalinerocks of Strange Lake have a carbon-isotope distribution, labelledType-2, that is indicative of formation of hydrocarbons by lateFischer–Tropsch-type reactions at ~350 °C where CO2 was the initialmagmatic fluid phase. The δ13CCO2 values are ~−2‰, indicating 13C-enrichment of the residual CO2 phase resulting from CO2-reductionvia Fischer–Tropsch-type reactions in a restricted system that formedhydrocarbons with carbon- and hydrogen-isotope compositionsbetween −30 and −20‰ and −170 and −160‰, respectively. Theresulting CH4 can have δ13C and δ2H values that plot within the fieldtraditionally assigned to thermogenic gas. However, C2H6 shows a slight13C-depletion relative to CH4, as predicted for abiogenic formation ofhydrocarbons.

A Type-3 distribution of carbon-isotope compositions has beenidentified in the Strange Lake samples affected by low temperature(150–200 °C) Ca-metasomatism and associated oxidation. Here,δ13CC2H6 values (−25 to −22‰) are higher than δ13CCH4 values(−30.8 to −26.0‰) and could easily be misinterpreted asthermogenic. However, all evidence points to an initial exsolvedorthomagmatic fluid with generation of hydrocarbons via Fischer–Tropsch-type reactions (as in Type-2). These fluids have beensubsequently modified by infiltration of oxidising Ca-rich meteoricfluids, which increased CO2 concentrations and decreased C2H6

concentrations, with associated 13C-enrichment of residual C2H6 relativeto the larger reservoir of CH4 in these fluids.

123J. Potter et al. / Lithos 182–183 (2013) 114–124

As such, carbon andhydrogen isotopic data alonemay be insufficientfor conclusive identification of thermogenic versus biogenic versusabiogenic methane in certain environments. In this study, it is shownthat a combination of analytical tools (petrography, fluid inclusionmicrothermometric and laser-Raman analyses and GC compositionalanalyses) are required in order to fully interpret the origin of thesefluids with regard to their isotopic compositions. Obtaining bothcarbon- and hydrogen-isotope compositions for not only CH4 butalso CO2 and higher hydrocarbons is essential as a first step inunderstanding source and subsequent reaction pathways in combinationwith other geochemical tools. Without careful scrutiny of results, manyabiogenically-derived gases could be interpreted as having a large, if nottotal, thermogenic gas component. As analytical techniques improve,other trace gas components can provide more robust parameters fordetermining abiogenic versus biogenic sources of hydrocarbons in variousenvironments. For example, noble gas isotopes are becoming increasinglyimportant for identifying source contributions in most hydrocarbon gascomponents (Schlegel et al., 2011).

Acknowledgements

We thank the Natural Sciences and Engineering Research Council ofCanada, the Canada Foundation for Innovation, the Ontario ResearchFund and the Canada Research Chairs Program for the support of thisresearch. We would also like to thank the reviewers Katie Smart andYuri Taran for providing comments on a previous version of thismanuscript that enabled us to improve this contribution. This isLaboratory for Stable Isotope Science Contribution #283.

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