Formation of H 2 , CH 4 and N-species during low-temperature experimental alteration of ultramafic rocks

Altered mafic and ultramafic rocks were studied in correspondence with hyperalkaline, CH4-bearing and very low-hydrogen spring waters in the Othrys ophiolite, whose chemical features are typical of present day serpentinisation. The H2 paucity is interpreted as the result of the incorporation of high-silica, aqueous fluids, probably derived from mafic rocks. The vein assemblage of serpentine + magnetite is related to circulation of low-silica fluids whereas serpentine + talc, tremolite after garnet and Fe-rich serpentine in the interior of serpentine veins reflect a late circulation of low-temperature (likely below 120 °C), high silica activity fluids. The highsilica conditions might have limited or interrupted the production of H2, which was subsequently consumed by CO2 hydrogenation to produce CH4. The lack of H2 could also be due to peridotite alteration by CO2-rich fluids. This would imply that the Othrys peridotites, among similar methane-bearing peridotites, may be considered as terrestrial analogues of Martian ultramafic rocks, which are thought to contribute to methane emission in the atmosphere of Mars. Understanding the mechanism of methane abiotic production will likely shed light to the details of some crucial aspects as the greenhouse-gas budget, the production of hydrocarbons and the origin of life on Earth.

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Formation of Clay Minerals During Low Temperature Experimental Alteration of Obsidian

Clays and Clay Minerals ◽

10.1346/ccmn.1993.0410404 ◽

1993 ◽

Vol 41 (4) ◽

pp. 431-441 ◽

Cited By ~ 16

Author(s):

Motoharu Kawano

Keyword(s):

Low Temperature ◽

Clay Minerals ◽

Experimental Alteration

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Temperatures of Neoproterozoic Regional Carbonate Alteration in the Eastern Desert of Egypt

10.5194/egusphere-egu2020-18336 ◽

2020 ◽

Author(s):

Arman Boskabadi ◽

Tobias Kluge ◽

Iain Pitcairn ◽

Rabea Ali ◽

Mokhles Azer ◽

...

Keyword(s):

Low Temperature ◽

Homogenization Temperature ◽

Eastern Desert ◽

Ultramafic Rocks ◽

Wide Range ◽

Two Samples ◽

Ophiolitic Rocks ◽

Clumped Isotope ◽

Host Rocks ◽

Different Sources

Neoproterozoic ophiolites in the Eastern Desert (ED) of Egypt are pervasively carbonated and listvenitized. Two types of carbonation are recognized: 1) intergrown magnesite (and to lesser extent dolomite) with serpentine and talc that in cases form pure carbonate veins, and 2) cryptocrystalline magnesite veins filling the fractures crosscutting other ophiolitic host rocks. Few studies address the conditions of carbonate alteration of ultramafic rocks, especially the temperature of altering fluids. We employ clumped isotope thermometry on natural dolomite and magnesite from 17 variably carbonated ophiolitic rocks and veins in the ED. Five samples of antigorite-bearing serpentinite, talc-carbonate, and associated carbonate veins yield wide range temperatures of magnesite and dolomite between 213 to 426&#176;C (285&#177;73&#176;C). These temperatures are comparable with previous fluid inclusion thermometry carried out on some of the vein samples (homogenization temperature between 225 to 383&#176;C; Boskabadi et al. 2017). Ten samples of fully quartz-carbonate altered peridotites (i.e. listvenites) record even a wider range of clumped isotope carbonation temperatures between 90 and 452&#176;C (227&#177;112&#176;C). In contrast, two samples of late-stage veins of cryptocrystalline magnesite record lower temperatures of 19 and 28&#176;C. While the constraints on the pressure of carbonation are lacking, the wide range of temperatures for the carbonates in antigorite-bearing serpentinite, talc-carbonate, and listvenite lithologies suggest that carbonation probably occurred at variable depths, whereas the low temperature of cryptocrystalline magnesite veins points to conditions nearer the surface most likely associated with post-obduction processes. Therefore, different sources of carbon and CO2-bearing fluids should have been responsible for the formation of high- and low-temperature carbonates in the region.&#160;&#160; Boskabadi et al. 2017. International Geology Review 59, 391&#8211;419.

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Fluid-Assisted Hydration Processes on Chondrite Parent-Bodies: Low-Temperature Experimental Alteration

10.46427/gold2020.592 ◽

2020 ◽

Author(s):

Elena Dobrica ◽

Joseph Nuth ◽

Adrian Brearley

Keyword(s):

Low Temperature ◽

Experimental Alteration

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Mafite-ultramafites (Peridotitovy stream, Polar Urals): mineralogy, estimation of P–T-parameters of formation

Vestnik of Geosciences ◽

10.19110/geov.2021.4.2 ◽

2021 ◽

Vol 4 ◽

pp. 10-19

Author(s):

N. S. Ulyasheva ◽

◽

O. V. Udoratina ◽

A. S. Shuisky

Keyword(s):

Low Temperature ◽

Ultramafic Rocks ◽

Thrust Faults ◽

Temperature Changes ◽

Polar Urals ◽

Metamorphic Facies ◽

Primary Minerals ◽

The Polar Urals

The petrographic features and conditions of the formation of metamorphosed mafic-ultramafic rocks of the Ampelshor complex, which are part of a small massif in the southern part of the Marunke u block of the Polar Urals (Peridotitovyi stream) and are controlled by tectonic thrust faults, have been studied. Muscovite-albite-clinozoisite-amphibole rocks (metabasites) and pyroxene-amphibole-chlorite (metautramafic) rocks are described. Structural, textural, and mineralogical features of metamagmatites indicate the hypabyssal nature of metabasite and plutonic nature of metaultramaphite. In metabasites, primary minerals are represented by relics of amphiboles (pargasite, edenite), and in metaultramaphites — by olivine and, possibly, clinopyroxene (augite, diopside). Magnesiohastingsite and chermakite in metautramafite were formed either at the late magmatic or metamorphic stages of rock transformation. The studied rocks underwent low-temperature changes (t — 468–380 °C, P — 2–3 kbar), corresponding to the greenschist metamorphic facies.

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Decreasing extents of Archean serpentinization contributed to the rise of an oxidized atmosphere

Nature Communications ◽

10.1038/s41467-021-27589-7 ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

James Andrew M. Leong ◽

Tucker Ely ◽

Everett L. Shock

Keyword(s):

Low Temperature ◽

Molecular Hydrogen ◽

Ultramafic Rocks ◽

Early Earth ◽

Free Atmosphere ◽

Great Oxidation Event ◽

The Past ◽

Thermodynamic Simulations

AbstractAt present, molecular hydrogen (H2) produced through Fe(II) oxidation during serpentinization of ultramafic rocks represents a small fraction of the global sink for O2 due to limited exposures of ultramafic rocks. In contrast, ultramafic rocks such as komatiites were much more common in the Early Earth and H2 production via serpentinization was a likely factor in maintaining an O2-free atmosphere throughout most of the Archean. Using thermodynamic simulations, this work quantifies the global O2 consumption attributed to serpentinization during the past 3.5 billion years. Results show that H2 generation is strongly dependent on rock compositions where serpentinization of more magnesian lithologies generated substantially higher amounts of H2. Consumption of >2 Tmole O2 yr−1 via low-temperature serpentinization of Archean continents and seafloor is possible. This O2 sink diminished greatly towards the end of the Archean as ultramafic rocks became less common and helped set the stage for the Great Oxidation Event.

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Gold and silver accessory minerals in ultramafites of the Kyzyr-Burlyuksky ultramafic massif (Western Sayan)

Ores and metals ◽

10.47765/0869-5997-2021-10031 ◽

2022 ◽

pp. 109-120

Author(s):

Alexey Yurichev

Keyword(s):

Low Temperature ◽

Ultramafic Rocks ◽

Natural Setting ◽

Accessory Minerals ◽

Ultramafic Massif ◽

Ore Minerals ◽

Native Silver ◽

Temperature Transformation ◽

Western Sayan ◽

Epigenetic Processes

The study focuses on gold and silver accessory minerals (native silver, cuprous gold, luanheite (Ag3Hg), unspecified mineral phase (Cu,Ag,Hg), first diagnosed in dunites and apodunite serpentinites of the Kyzyr-Burlyuksky ultramafic massif, which is part of the Kurtushibin ophiolite belt of Western Sayan. The revealed ore minerals are mainly observed in the form of single hypidiomorphic, irregular microscopic precipitates (0.5– 3.0 μm) mainly inside magnetite, much less often in grains of avaruite. Typomorphic and chemical features of ore minerals, their natural setting in rock-forming silicate matrix are characterized. Formation and concentration of these accessory minerals is associated with superimposed low-temperature transformation (hydration) processes affecting original ultramafic rocks. At the same time, the presence of luanheite and an unnamed phase (Cu,Ag,Hg), along with the previously identified potarite (PdHg), is probably evidence of low-temperature conditions of mineral formation during the manifestation of epigenetic processes of serpentinization (lowgrade metamorphism) due to solutions enriched in mercury. The source of such solutions could be gabbro intrusions that penetrated later into the main ultramafic body.

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Olivine fabrics in the Bay of Islands Ophiolite: implications for oceanic mantle structure and anisotropy

Canadian Journal of Earth Sciences ◽

10.1139/e85-186 ◽

1985 ◽

Vol 22 (12) ◽

pp. 1757-1766 ◽

Cited By ~ 5

Author(s):

Matthew H. Salisbury ◽

Nikolas I. Christensen

Keyword(s):

High Temperature ◽

Low Temperature ◽

Plastic Flow ◽

Upper Mantle ◽

Mafic Rock ◽

Ultramafic Rocks ◽

Slip Systems ◽

Mantle Structure ◽

Ophiolite Complex ◽

Crust And Upper Mantle

Petrofabric analysis of oriented ultramafic and mafic rock samples from six traverses representing all four massifs of the Bay of Islands ophiolite complex, Newfoundland, indicate that the ultramafic rocks are tectonites displaying fabrics consistent with high-temperature plastic flow on the olivine (010) [100] and (0kl) [100] slip systems. The fabric orientation is uniform in three of the four massifs but varies between massifs, suggesting differential rotation before or during emplacement. Within North Arm Mountain, the olivine a axes are aligned approximately perpendicular to the sheeted dikes in both the ultramafic tectonites and the overlying gabbroic tectonites. In Blow Me Down Mountain, the olivine a axes in the gabbros are perpendicular to the dikes, but they are parallel to them in the ultramafic rocks. It is concluded that the ultramafic rocks on Blow Me Down Mountain were rotated 90° during emplacement or that local decoupling and rotation occurred between the crust and upper mantle prior to emplacement. Within the Lewis Hills, the olivine fabrics rotate and weaken near the shear zone in the center of the massif. A second deformation, perhaps associated with low-temperature plastic flow, appears to have obliterated the fabric patterns still observed in the ultramafic rocks to the east.

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