Abstract:From a geological perspective, hydrogen has been neglected. It is not as common as biogenic or thermogenic methane, which are ubiquitous in hydrocarbon basins, or carbon dioxide, which is common in geologically-active areas of the world. Nevertheless, small flows of hydrogen naturally reach the Earth's surface, occur in some metal mines and emerge beneath the oceans in a number of places worldwide. These occurrences of hydrogen are associated with abiogenic and biogenic methane, Further research should aim to … Show more
“…Serpentinization, that is hydration of olivine and/or pyroxene, produces H 2 which then may react with C-gases (CO 2 or CO) forming CH 4 . The FTT synthesis, however, may be independent of serpentinization, whereby H 2 in many rocks can derive from other processes, such as radiolysis, cataclasis of silicates in fault zones, or magmatic degassing [Smith et al, 2005;Onstott et al, 2006].…”
[1] Over the last 30 years, geochemical research has demonstrated that abiotic methane (CH 4 ), 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) CH 4 . 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 CH 4 in natural gases, which are frequently mixtures of multiple sources. Although it has been traditionally assumed that abiotic CH 4 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 CH 4 and the primary approaches for differentiating abiotic from biotic CH 4, including novel potential tools such as clumped isotope geochemistry. A diagnostic approach for differentiation is proposed.
“…Serpentinization, that is hydration of olivine and/or pyroxene, produces H 2 which then may react with C-gases (CO 2 or CO) forming CH 4 . The FTT synthesis, however, may be independent of serpentinization, whereby H 2 in many rocks can derive from other processes, such as radiolysis, cataclasis of silicates in fault zones, or magmatic degassing [Smith et al, 2005;Onstott et al, 2006].…”
[1] Over the last 30 years, geochemical research has demonstrated that abiotic methane (CH 4 ), 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) CH 4 . 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 CH 4 in natural gases, which are frequently mixtures of multiple sources. Although it has been traditionally assumed that abiotic CH 4 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 CH 4 and the primary approaches for differentiating abiotic from biotic CH 4, including novel potential tools such as clumped isotope geochemistry. A diagnostic approach for differentiation is proposed.
“…The H 2 necessary for FTT-Sabatier reaction can derive from different sources: serpentinization, radiolysis, cataclasis of silicates in fault zones, or magmatic degassing (Smith et al, 2005 ). Serpentinization, in particular, is widely invoked as a source of CH 4 on Mars ( e.g., Oze and Sharma, 2005 ; Atreya et al, 2007 ).…”
Methane on Mars is a topic of special interest because of its potential association with microbial life. The variable detections of methane by the Curiosity rover, orbiters, and terrestrial telescopes, coupled with methane's short lifetime in the martian atmosphere, may imply an active gas source in the planet's subsurface, with migration and surface emission processes similar to those known on Earth as “gas seepage.” Here, we review the variety of subsurface processes that could result in methane seepage on Mars. Such methane could originate from abiotic chemical reactions, thermogenic alteration of abiotic or biotic organic matter, and ancient or extant microbial metabolism. These processes can occur over a wide range of temperatures, in both sedimentary and igneous rocks, and together they enhance the possibility that significant amounts of methane could have formed on early Mars. Methane seepage to the surface would occur preferentially along faults and fractures, through focused macro-seeps and/or diffuse microseepage exhalations. Our work highlights the types of features on Mars that could be associated with methane release, including mud-volcano-like mounds in Acidalia or Utopia; proposed ancient springs in Gusev Crater, Arabia Terra, and Valles Marineris; and rims of large impact craters. These could have been locations of past macro-seeps and may still emit methane today. Microseepage could occur through faults along the dichotomy or fractures such as those at Nili Fossae, Cerberus Fossae, the Argyre impact, and those produced in serpentinized rocks. Martian microseepage would be extremely difficult to detect remotely yet could constitute a significant gas source. We emphasize that the most definitive detection of methane seepage from different release candidates would be best provided by measurements performed in the ground or at the ground-atmosphere interface by landers or rovers and that the technology for such detection is currently available. Key Words: Mars—Methane—Seepage—Clathrate—Fischer-Tropsch—Serpentinization. Astrobiology 17, 1233–1264.
“…An increasing number of discoveries in both continental and oceanic lithosphere suggest that free H 2 is not as rare as once thought [Apps and van de Kamp, 1993;Smith et al, 2013]. A quantitative understanding of H 2 production by serpentinization, as well as other known abiotic and biotic sources, represents a challenging but important research frontier that bears on many open questions across the geosciences [e.g., Apps and van de Kamp, 1993;Hoehler et al, 2001;Bach and Edwards, 2003;Sherwood Lollar et al, 2014;Telling et al, 2015].…”
Section: 1002/2016gl069066mentioning
confidence: 99%
“…Of the various processes that can generate free hydrogen gas (H 2 ) in the lithosphere, field evidence suggests that one of the principal mechanisms is the hydration of ferrous iron minerals or “serpentinization” reactions [ Apps and van de Kamp , ]. The H 2 produced by serpentinization in turn figures prominently in theories regarding (1) the origin and early evolution of life on Earth [e.g., Canfield et al , ; Sleep and Bird , ], (2) the basal fuel‐source sustaining the subsurface biosphere [e.g., Charlou et al , ; Kelley et al , ; Menez et al , ], (3) the formation of abiogenic hydrocarbons [ McCollom and Seewald , ; Sherwood Lollar et al , ; Proskurowski et al , ] as well as (4) native metal alloys [ Dekov , ; McCollom and Bach , ], (5) the biogeochemical cycling of elements such as sulfur and carbon [ Alt et al , ], and (6) the potential use of H 2 as a substitute for fossil fuels [ Smith et al , ].…”
It has recently been estimated that serpentinization within continental lithosphere produces H2 at rates comparable to oceanic lithosphere (both are ~1011 mol H2/yr). Here we present a simple model that suggests that H2 production rates along the mid‐oceanic ridge alone (i.e., excluding other marine settings) may exceed continental production by an order of magnitude (~1012 mol H2/yr). In our model, H2 production rates increase with spreading rate and the net thickness of serpentinizing peridotite (S‐P) in a column of lithosphere. Lithosphere with a faster spreading rate therefore requires a relatively smaller net thickness of S‐P to produce H2 at the same rate as lithosphere with a slower rate and greater thickness of S‐P. We apply our model globally, incorporating an inverse relationship between spreading rate and net thickness of S‐P to be consistent with observations that serpentinization is more common within lithosphere spreading at slower rates.
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