The Serpentinite Multisystem Revisited: Chrysotile Is Metastable

Evans, Bernard W.

doi:10.2747/0020-6814.46.6.479

Cited by 473 publications

(378 citation statements)

References 97 publications

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“…Serpentinized oceanic peridotites are dominantly composed of low-temperature (<300°C) polymorphs lizardite and chrysotile, and to a lesser extent antigorite, which forms at temperatures >250°C (17). In modern oceanic serpentinites, antigorites have δD between −46 and −30‰ (Figs.…”

Section: Serpentine Isotope Geochemistrymentioning

confidence: 99%

“…Serpentine-group minerals are hydrous magnesian silicates commonly formed by hydrothermal infiltration of seawater into ultramafic oceanic crust, hydration of the mantle wedge by slabderived fluids, or fluid-rock interaction in continentally emplaced ultramafic lithologies (16)(17)(18). Serpentinized oceanic peridotites are dominantly composed of low-temperature (<300°C) polymorphs lizardite and chrysotile, and to a lesser extent antigorite, which forms at temperatures >250°C (17).…”

Section: Serpentine Isotope Geochemistrymentioning

confidence: 99%

See 1 more Smart Citation

Isotope composition and volume of Earth’s early oceans

Pope

Bird

Rosing

2012

Proc. Natl. Acad. Sci. U.S.A.

128

145

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Oxygen and hydrogen isotope compositions of Earth's seawater are controlled by volatile fluxes among mantle, lithospheric (oceanic and continental crust), and atmospheric reservoirs. Throughout geologic time the oxygen mass budget was likely conserved within these Earth system reservoirs, but hydrogen's was not, as it can escape to space. Isotopic properties of serpentine from the approximately 3.8 Ga Isua Supracrustal Belt in West Greenland are used to characterize hydrogen and oxygen isotope compositions of ancient seawater. Archaean oceans were depleted in deuterium [expressed as δD relative to Vienna standard mean ocean water (VSMOW)] by at most 25 AE 5‰, but oxygen isotope ratios were comparable to modern oceans. Mass balance of the global hydrogen budget constrains the contribution of continental growth and planetary hydrogen loss to the secular evolution of hydrogen isotope ratios in Earth's oceans. Our calculations predict that the oceans of early Earth were up to 26% more voluminous, and atmospheric CH 4 and CO 2 concentrations determined from limits on hydrogen escape to space are consistent with clement conditions on Archaean Earth. Both interpretations have been questioned based on the susceptibility of chemical sediments to diagenetic alteration over geologic time (2, 4), and for the latter, a lack of geologic evidence for the extreme greenhouse gas concentrations required to sustain such high temperatures under a less luminous young Sun (9).Minerals formed by metasomatic reactions between seawater and oceanic lithosphere provide an alternative proxy for early ocean chemistry (10-14). Here we report hydrogen and oxygen isotope compositions of 114 silicate mineral separates from the ca. 3.8 Ga Isua Supracrustal Belt (ISB) of West Greenland, including metamorphosed basalts, gabbros, and ultramafic rocks that represent fragments of Eoarchaean oceanic crust (see Fig. S1, Table S2). Minerals formed during heterogeneous late-stage CO 2 -or meteoric-dominated metamorphic overprinting are distinguished by characteristic trends in their coupled δD and δ 18 O values (Fig. S2). In contrast, serpentine minerals formed by reaction of seawater with ultramafic rocks (peridotites) preserved in a low-strain enclave of the ISB (15) define a different isotopic trend. Here we use these serpentines to constrain D∕H and 18 O∕ 16 O ratios of the Eoarchaean ocean. Serpentine Isotope GeochemistrySerpentine-group minerals are hydrous magnesian silicates commonly formed by hydrothermal infiltration of seawater into ultramafic oceanic crust, hydration of the mantle wedge by slabderived fluids, or fluid-rock interaction in continentally emplaced ultramafic lithologies (16)(17)(18). Serpentinized oceanic peridotites are dominantly composed of low-temperature (<300°C) polymorphs lizardite and chrysotile, and to a lesser extent antigorite, which forms at temperatures >250°C (17). In modern oceanic serpentinites, antigorites have δD between −46 and −30‰ (Figs. 1 and 2A, filled squares), values that indicate equilibrium with modern-...

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Section: Serpentine Isotope Geochemistrymentioning

confidence: 99%

Section: Serpentine Isotope Geochemistrymentioning

confidence: 99%

Isotope composition and volume of Earth’s early oceans

Pope

Bird

Rosing

2012

Proc. Natl. Acad. Sci. U.S.A.

128

145

View full text Add to dashboard Cite

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“…At first, the dehydroxylation starts with a condensation of adjacent hydroxyl groups to form an H 2 O molecule or the liberation of OH − and/or H + to diffuse along the interlayer (one-dimensional diffusion). This process starts along the outer sheets as proposed by the chrysotile structure (Evans 2004). An inward-migrating dehydroxylation front causes a predominantly amorphous outer layer (disordered chrysotile), which inhibits a bulk diffusion/radial diffusion.…”

Section: Chrysotilementioning

confidence: 99%

Kinetics of the chrysotile and brucite dehydroxylation reaction: a combined non-isothermal/isothermal thermogravimetric analysis and high-temperature X-ray powder diffraction study

2013

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a much higher apparent E a characterised by an initial stage of around 290 kJ/mol. Afterwards, the apparent E a comes down to around 250 kJ/mol at α ~ 65 % before rising up to around 400 kJ/mol. The delivered kinetic data have been investigated by the z(α) master plot and generalised time master plot methods in order to discriminate the reaction mechanism. Resulting data verify the multi-step reaction scenarios (reactions governed by more than one rate-determining step) already visible in E a versus α plots.Keywords Serpentine dehydroxylation · Generalised master plot · Chrysotile · Brucite · Thermogravimetry · In situ high-temperature X-ray powder diffraction

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“…The most problematic choice was for the equilibrium constant of reaction (1) (Table 2), (log K1 value). Several log K constants for the dissolution of serpentine are found in the literature (Table 3), and most are controversial because of the different crystal structures of serpentines and the close proximity of these structures in the same sample (Evans, 2004). The various equilibrium constants were calculated from the respective free energies of formation (Table 4).…”

Section: Theorymentioning

confidence: 99%

Ni Enrichment and Stability of Al-Free Garnierite Solid-Solutions: A Thermodynamic Approach

Galí

Soler

Proenza

et al. 2012

Clays and clay miner.

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Abstract-Garnierites represent significant Ni ore minerals in the many Ni-laterite deposits worldwide. The occurrence of a variety of garnierite minerals with variable Ni content poses questions about the conditions of their formation. From an aqueous-solution equilibrium thermodynamic point of view, the present study examines the conditions that favor the precipitation of a particular garnierite phase and the mechanism of Ni-enrichment, and gives an explanation to the temporal and spatial succession of different garnierite minerals in Ni-laterite deposits. The chemical and structural characterization of garnierite minerals from many nickel laterite deposits around the world show that this group of minerals is formed essentially by an intimate intermixing of three Mg-Ni phyllosilicate solid solutions: serpentine-népouite, kerolite-pimelite, and sepiolite-falcondoite, without or with very small amounts of Al in their composition. The present study deals with garnierites which are essentially Al-free. The published experimental dissolution constants for Mg end-members of the above solid solutions and the calculated constants for pure Ni end-members were used to calculate Lippmann diagrams for the three solid solutions, on the assumption that they are ideal. With the help of these diagrams, congruent dissolution of Ni-poor primary minerals, followed by equilibrium precipitation of Ni-rich secondary phyllosilicates, is proposed as an efficient mechanism for Ni supergene enrichment in the laterite profile. The stability fields of the solid solutions were constructed using [log a SiO 2 (aq) , log ((a Mg 2+ + a Ni 2+)/(a H +) 2 )] (predominance) diagrams. These, combined with Lippmann diagrams, give an almost complete chemical characterization of the solution and the precipitating phase(s) in equilibrium. The temporal and spatial succession of hydrous MgNi phyllosilicates encountered in Ni-laterite deposits is explained by the small mobility of silica and the increase in its activity.

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The Serpentinite Multisystem Revisited: Chrysotile Is Metastable

Cited by 473 publications

References 97 publications

Isotope composition and volume of Earth’s early oceans

Isotope composition and volume of Earth’s early oceans

Kinetics of the chrysotile and brucite dehydroxylation reaction: a combined non-isothermal/isothermal thermogravimetric analysis and high-temperature X-ray powder diffraction study

Ni Enrichment and Stability of Al-Free Garnierite Solid-Solutions: A Thermodynamic Approach

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