Methane flux and origin in the Othrys ophiolite hyperalkaline springs, Greece

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    c, D. Christodoulou e, G. Papatheodorou e

    ezione Rob Faculty of Environmental Science and Engineering, Babec Section of Earth Materials, Department of Geology, Univd University of Brunei Darussalam, Department of Geosciee Laboratory of Marine Geology and Physical Oceanograph

    a r t i c l e i n f o

    Article history:Received 22 December 2012Received in revised form 28 March 2013Accepted 1 April 2013Available online 8 April 2013

    ial, notwithstanding it being a typical product of serpentinization; this could be due to complete H2 consumption

    l., 1989; Holm et al.,ars and other planetst al., 2012) and could

    Chemical Geology 347 (2013) 161174

    Contents lists available at SciVerse ScienceDirect

    Chemical

    l seultramac rocks are a theme of growing interest for its implicationin planetary geology, astrobiology and energy resource exploration.Serpentinization (i.e., hydration of peridotite including olivine and

    contribute, although in a subsidiary way, to feed hydrocarbon poolsin igneous reservoir rocks (Szatmari, 1989; Sherwood Lollar et al.,2002; Szatmari et al., 2011).1. Introduction

    Origin and exhalation of abiotic methane (CH4) in serpentinized

    energy for the origin of life (e.g., Russell et a2006; Russell et al., 2010), they may occur on M(Tobie et al., 2006; Atreya et al., 2007; Etiope eby CO2 reduction in a limited or decreased H2 production system due, for example, to a late stage of increasedsilica activity, as suggested by preliminary petrographic observations. The low geothermal gradient of the areaand the present-day serpentinization imply that, whatever the CH4 production mechanism, it took place at tem-peratures below those traditionally considered for the origin of abiotic methane in hydrothermal systems.

    2013 Elsevier B.V. All rights reserved.pyroxene) and the generated gases (CH4 athe pioneering work of Thayer (1966), are

    Corresponding author at: INGV, via V. Murata, 605,0651860394; fax: +39 0651860338.

    E-mail address: etiope@ingv.it (G. Etiope).

    0009-2541/$ see front matter 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.chemgeo.2013.04.003shields, which were considered dominantly abiotic and probably derived from FischerTropsch Type reactions.The paucity of CO2, which is the norm in hyperalkaline waters, and of other hydrocarbons prevents from evalu-ating possiblemixing of gas of different sources, includingmicrobial methanogenesis. Also the H2 content is triv-Editor: David R. Hilton

    Keywords:MethaneHydrogenSerpentinizationHyperalkaline springsOphiolitema 2, Italys-Bolyai University Cluj-Napoca, Romaniaersity of Patras, Greecences, Jalan Tungku Link, Gadong, BE1410 Bandar Seri Begawan, Brunei Darussalamy, Department of Geology, University of Patras, Greece

    a b s t r a c t

    The occurrence and origin of methane (CH4) generated by serpentinization of ultramac rocks is of currenttimely interest in planetary geology, astrobiology and energy resource exploration, as it may contribute, inparticular, to decipher the source of methane on Mars, the origin of life and the potential of abiotic hydrocar-bon synthesis. Methane of dominant abiotic origin in serpentinized peridotites on continents (ophiolites origneous intrusions) has been documented so far, with complete carbon and hydrogen isotope composition,in six countries, in the Philippines, Turkey, Oman, New Zealand, Japan and Italy. We report the discovery oftwo new sites in Greece, at Archani and Ekkara, located in the Othrys ophiolite massif. Portable sensorsbased on Fourier Transform InfraRed spectrometry (FTIR) and Tunable Diode Laser Absorption Spectroscopy(TDLAS) allowed to realize that out of 21 ophiolitic springs, methane is released only by four hyperalkaline(pH from 10.7 to 11.3) and calcium hydroxide (CaOH) type waters; all other 17 springs with pH b 8.7and magnesium-rich waters in the Pindos, Vourinos and Veria ophiolites, do not show methane. This corre-lation between gas occurrence and water type seems to occur worldwide; accordingly, CH4 production appearsto be intimately related to the depth and residence time of the circulating meteoric waters. Methane is emittedinto the atmosphere also from the soil surrounding the hyperalkaline springs, with uxes of the same order ofmagnitude (~102103 mg m2 day1) of seepage typically observed over conventional petroleum systems.Othrys CH4 has an isotopic composition (13C from 27 to 37.3 VPDB, 2H from 250 to 311VSMOW) similar to that reported in ultramac rocks in New Zealand and Japan, and in Precambrian crystallinea Istituto Nazionale di Geosica e Vulcanologia, SG. Etiope a,b,, B. Tsikouras c,d, S. Kordella e, E. IfandiMethane ux and origin in the Othrys opsprings, Greece

    j ourna l homepage: www.end H2), observed sinceconsidered a source of

    00143 Rome, Italy. Tel.: +39

    rights reserved.olite hyperalkaline

    Geology

    vie r .com/ locate /chemgeoMany studies are focused around the discovery of submarineperidotite-hosted CH4 + H2 uids in hydrothermal mid-ocean ridgesystems, where serpentinization (driven by seawater) and gas synthe-sis occurred at temperature generally above 100150 C (Logatchev,Rainbow, and Lost City; Charlou et al., 2002; Proskurowski et al., 2006,2008; Lang et al., 2010). Abiotic CH4 synthesis during present-day

  • concentration values, was reported for the Leka ophiolite in Norway(Okland et al., 2012). Gas can be released to the atmosphere viasprings (like in Oman, Canada, Italy, Japan) or through dry gas seepsalong faults, not associatedwith water discharge (Chimaera in Turkey,Zambales in the Philippines and Point Bay in New Zealand).

    Here we report the discovery of two new sites with methane inhyperalkaline springs located in the Othrys ophiolite in Greece. Weinvestigated 21 springs issuing from serpentinized peridotite out-crops spread in four different ophiolitic complexes, Othrys, Pindos,Vourinos and Veria (Fig. 1). Methane and its ux to the atmospherewere detected and measured in the eld by portable sensors basedon Fourier Transform InfraRed spectrometry (FTIR) and TunableDiode Laser Absorption Spectroscopy (TDLAS). The gas emissionrate characterizes the potential of the gas source and its measurementcontributes to develop the global data-set of geologic emissions ofmethane to the atmosphere, nowadays considered a major source ofgreenhouse gases (Etiope and Klusman, 2010; US EPA, 2010; Etiope,2012). Stable C and H isotopic composition of CH4 and molecularcomposition of gas dissolved in the water were analyzed in differentseasons (October 2011 and June 2012) and laboratories, and werecompared with those of abiotic gas reported in other ophiolites andcrystalline rocks. Preliminary petrographic observations were madeto evaluate the main features of the serpentinization of the Othrys pe-ridotite at the spring sites.

    2. Geologic setting

    Ophiolites in Greece comprise part of the DinarideTauride zonein the eastern Mediterranean region and crop out along two distinctbelts trending in a NNWSSE direction; both have obducted on either

    162 G. Etiope et al. / Chemical Geology 347 (2013) 161174serpentinization (driven by meteoric water) of ultramac rocks on thecontinents, i.e., in ophiolites, was less investigated but it provides fur-ther important implications because of the lower temperatures (gener-ally b100 C; Coleman and Keith, 1971; Barnes et al., 1978; Etiope et al.,2011b; Boschetti et al., 2013) and direct contact with the atmosphere.The lower temperatures imply that high enthalpy hydrothermal sys-tems would not be necessary for abiotic methane synthesis, whichcould then occur in a wider range of environments, either on Earth orother planets. The direct contact with atmosphere makes gas-bearingophiolites a further natural source of methane for the atmosphericgreenhouse-gas budget (Etiope and Klusman, 2002; Etiope, 2012),while the gas released at deep seaoors (>500 m), like at the hydro-thermal vents in mid-ocean ridges, never reaches the atmosphere(e.g., McGinnis et al., 2006). Serpentinized ultramac rocks, then, repre-sent a terrestrial analog of olivine-rich rocks occurring on Mars; under-standing methane generation mechanisms and uxes in the ophiolitesmay contribute to explain the origin of methane apparently observedin the Martian atmosphere (Oze and Sharma, 2005; Etiope et al.,2011b, 2012; Szponar et al., 2012).

    It is generally assumed that the mechanism of abiotic methanegeneration in serpentinized ultramac rocks is related to FischerTropsch Type (FTT) reactions, like the Sabatier reaction,

    CO2 4H2 CH4 2H2O 1

    whereby H2 is produced by peridotite hydration and CO2 may havevarious origins (mantle, limestones, soil, atmosphere). Laboratory ex-periments were performed simulating the hydrothermal conditionsof mid-ocean serpentinization, and showed CH4 production by FTT re-actions supported by metal catalysts, such as magnetite (Fe3O4),chromite (FeCr2O4) and awaruite (Ni3Fe), at temperatures above200 C (e.g., Berndt et al., 1996; Foustoukos and Seyfried, 2004;McCollom and Seewald, 2006; Taran et al., 2007; McCollom andBach, 2009). CH4 may however also form through uncatalyzed aque-ous reactions with a sequential reduction of formic acid, formalde-hyde and methanol as intermediaries (Seewald et al., 2006).

    The Sabatier reaction is however thermodynamically favored bylow temperatures; below 100 C the reaction is expected to evolvevery slowly but over geologic time scales gas production can be con-siderable, as suggested by the huge amount of abiotic CH4 releasedfrom the Chimaera ophiolitic seep in Turkey, where gas synthesisoccurred at temperatures of around 50 C (Etiope et al., 2011b).Neubeck et al. (2011) reported methane and hydrogen productionafter olivine dissolution at low temperatures (3070 C), catalyzedby magnetite and chromite, but they did not evaluate possib