[1] Over the last 30 years, geochemical research has demonstrated that abiotic methane (CH4), formed by chemical reactions which do not directly involve organic matter, occurs on Earth in several specific geologic environments. It can be produced by either high-temperature magmatic processes in volcanic and geothermal areas, or via low-temperature (<100°C) gas-water-rock reactions in continental settings, even at shallow depths. The isotopic composition of C and H is a first step in distinguishing abiotic from biotic (including either microbial or thermogenic) CH4. Herein we demonstrate that integrated geochemical diagnostic techniques, based on molecular composition of associated gases, noble gas isotopes, mixing models, and a detailed knowledge of the geologic and hydrogeologic context are necessary to confirm the occurrence of abiotic CH4 in natural gases, which are frequently mixtures of multiple sources. Although it has been traditionally assumed that abiotic CH4 is mainly related to mantle-derived or magmatic processes, a new generation of data is showing that low-temperature synthesis related to gas-water-rock reactions is more common than previously thought. This paper reviews the major sources of abiotic CH4 and the primary approaches for differentiating abiotic from biotic CH4, including novel potential tools such as clumped isotope geochemistry. A diagnostic approach for differentiation is proposed.

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Abiotic methane on earth

Summary The Gardar province (c. 1320–1120 Ma old) relates to repetitive rifting within the Mid-Proterozoic continent and the attendant large-scale generation of alkali and transitional olivine basaltic and hawaiitic magmas. Lavas from the earliest episodes are preserved in down-faulted units (the Eriksfjord formation), whereas younger basic magmatism is recorded only as intrusions, mainly dykes. The province shows a compositional continuum from basaltic to salic alkaline compositions. The latter are represented among the early lavas, as dykes and, most prominently, as central-type complexes commonly involving layered (cumulate) syenitic intrusions. Most of the central complexes display either a silica-oversaturated trend (augite syenite-quartz syenite-(A-type) alkali granite) or a silica-undersaturated trend (augite syenite-pulaskite-foyaite). Strongly peralkaline products are present in each association with agpaitic rocks occurring as differentiates at Ilímaussaq and Motzfeldt. The oversaturated and undersaturated complexes, irrespective of age, define southerly and northerly zones respectively in the province. ‘Giant-dyke complexes’, produced in late-Gardar rifting events (contemporaneous with Keweenawan activity in N America) are important in providing a link between simple dilatational dykes and the central-type complexes. An ultramafic lamprophyre-carbonatite association plays a subordinate role in the province, and carbonatites are also associated with some of the salic complexes. The carbonatites at Grønnedal-Ika are thought to be residual from CO2-rich phonolitic magmas, whereas carbonate (siderite)-fluoride rocks at Ivigtut crystallized from volatile-rich residues associated with an alkali granite. The salic alkaline magmas were produced from alkali or transitional olivine basaltic parent magmas, with crystal fractionation being the dominant mechanism. Generally low 87Sr/86Sr(i) ratios in the alkaline complexes suggest little assimilation of old radiogenic crustal materials. The basaltic magmas were characterized by high Al2O3/CaO and FeO/MgO ratios, and it is inferred that they themselves were residues from more magnesian parent magmas that had undergone extended olivine and clinopyroxene fractionation at sub-crustal levels. The primary magmas may have been derived from garnet lherzolite source rocks with high garnet to clinopyroxene ratios. Late-Gardar basic magmas of the Tugtutôq-Ilímaussaq-Nunataq zone are geochemically distinct from all other Gardar basic magmas in being enriched in P, Ba, Nb and light rare earth elements. This feature may be due to metasomatic enrichment of source rocks along a lineament sub-parallel to the Archaean craton margin. Gardar alkaline magmas were rich in F, Cl and C compounds (oxides and/or hydrocarbons) while being relatively anhydrous. Many crystallized under low fO2 conditions. The fundamental cause of the repetitive alkaline magma generation may have been the ascent of F-, Cl- and C-rich fluids from the deep mantle, leading to partial melting of the lithospheric mantle close to the boundary between Archaean and Proterozoic crustal units.

Mid-Proterozoic alkaline magmatism in southern Greenland: the Gardar province

The occurrence and geoscience of natural hydrogen: A comprehensive review

The 1.13 Ga Ilimaussaq intrusive complex, South Greenland, is composed of various types of alkali granite and silica-undersaturated alkaline to agpaitic nepheline syenites related to three subsequently intruded magma batches. Mineral chemistry indicates continuous fractionation trends within each rock type, but with distinct differences among them. The last, peralkaline magma batch is the most fractionated in terms of XFemafic mineral, feldspar composition and mineral assemblage. This indicates that an evolving magma chamber at depth discontinuously released more highly fractionated alkaline melts. Fluid inclusions in some sodalites record a pressure drop from 3.5 to 1 kbar indicating that crystallization started during magma ascent and continued in the high-level magma chamber. On the basis of phase equilibria and preliminary fluid inclusion data, crystallization temperature drops from 1000 °C (augite syenite liquidus) to 500 °C (lujavrite solidus) and silica activity decreases from ~0.8 to <0.3. An almost pure methane fluid phase at high temperatures and an almost pure aqueous fluid phase in the last crystallization stages of the agpaitic rocks indicate a strong increase in water activity. NaCl activity drops from 0.4 during magmatic sodalite crystallization to <0.01 (3 wt % NaCl equiv) in the late magmatic aqueous fluids. Relative oxygen fugacity [ [{Delta}] FMQ, where FMQ is fayalite-magnetitequartz)] depends on silica and water activity via two solid-solid buffer reactions. It decreases during fractionation in the augite syenite from about FMQ-1 to below FMQ-4, but increases in the peralkaline stage. The extreme peralkaline fractionation trend appears to be governed by low water activity and low SiO2 activity in the parental melt. Only then is methane a stable fluid phase during most of the crystallization history, which prevents early unmixing of an aqueous NaCl-bearing fluid phase. J. Petrol. (2001), 42, 2231-2258

Phase Equilibrium Constraints on Intensive Crystallization Parameters of the Ilímaussaq Complex, South Greenland

Alteration, HFSE mineralisation and hydrocarbon formation in peralkaline igneous systems: Insights from the Strange Lake Pluton, Canada

Hydrocarbon gases and bituminous substances in rocks from the Ilímaussaq alkaline intrusion, South Greenland: (Contribution to the Mineralogy of Ilímaussaq No. 18)

I.A. Petersilie

Papers

Hydrocarbon gases and bituminous substances in rocks from the Ilímaussaq alkaline intrusion, South Greenland: (Contribution to the Mineralogy of Ilímaussaq No. 18)