A deuterium-hydrogen fractionation mechanism in magnesium oxide

Freund, Friedemann; Wengeler, H.; Martens, R.

doi:10.1016/0016-7037(82)90121-1

Cited by 23 publications

(4 citation statements)

References 22 publications

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“…Therefore, other chemical processes that lead to hydrogen fractionation during alteration of the oceanic lithosphere should be considered. For instance, a strong D/H fractionation associated with production of molecular hydrogen from OH" has been documented in an experimental study by Freund et al (1982). They have shown that the process of dissolution of water in oxides such as MgO and CaO not only produces OH" ions, but molecular hydrogen forms from a certain fraction of the dissolved hydroxyl ions without transition metal cations acting as reducing agents.…”

Section: A Depletion Inmentioning

confidence: 99%

Petrologic and Stable Isotope Constraints on Hydrothermal Alteration and Serpentinization of the EPR Shallow Mantle at Hess Deep (Site 895)

Früh-Green¹,

Plas²,

Lécuyer³

1996

Proceedings of the Ocean Drilling Program

106

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Leg 147 of the Ocean Drilling Program recovered sections of the East Pacific Rise lower crust and shallow mantle ( l Ma), tectonically exposed at the western end of the Cocos-Nazca propagator of the Hess Deep Rift Valley. These rocks record a polyphase history of hydrothermal alteration and provide new constraints on the depth and mechanisms of hydrothermal circulation at fast-spreading ridges. A complex sequence of harzburgite-dunite-troctolite-gabbro recovered at Site 895 is considered to be the result of processes of melt migration and wall-rock reaction close to the mantle/crust boundary. The peridotites are extensively serpentinized (50%-100%) and are cut by multiple generations of fracture-filling veins. In the gabbros, progressive alteration under greenschist to zeolite facies conditions is characterized by tremolite + chlorite + diopside + anorthjte ± prehnite assemblages in the least altered samples, and incipient rodingitization to prehnite + hydrogrossular + zeolite + clays as Cataclastic deformation and veining increases.Oxygen isotope ratios of mineral separates from the gabbros and peridotites from Site 895 show a depletion in I8O relative to mantle values and are consistent with high-temperature exchange with aqueous fluids. Dunite/harzburgite ratios of chlorite, serpentine, and tremolite, together with δ' 3 C values of CO 2 extracted from completely serpentinized dunites, suggest at least two, but possibly three, components of the hydrothermal fluids: hydrothermally altered seawater; magmatic volatiles; and H 2 released during serpentinization. These data combined with structural data imply that penetration of seawater and high-temperature hydrothermal alteration produced a low 18 O shallow mantle sequence at some distance off-axis of the East Pacific Rise, but at an early stage in the propagation of the Cocos-Nazca rift and formation of the Hess Deep Rift Valley. Mineral assemblages in the gabbroic rocks and the presence of antigorite at Hess Deep, combined with oxygen isotope ratios, suggest that faulting associated with the Cocos-Nazca propagator enhanced seawater penetration and hydrothermal alteration at temperatures above 350°C in this segment of the East Pacific Rise oceanic lithosphere. The results of this study suggest that seawaterperidotite interactions and high-temperature serpentinization processes may be an important contribution to the overall 18 Obudget in the oceanic lithosphere nd may represent a significant sink for mantle CO 2 and source of H 2 .

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Section: A Depletion Inmentioning

confidence: 99%

Petrologic and Stable Isotope Constraints on Hydrothermal Alteration and Serpentinization of the EPR Shallow Mantle at Hess Deep (Site 895)

Früh-Green¹,

Plas²,

Lécuyer³

1996

Proceedings of the Ocean Drilling Program

106

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“…guously identifiable as due to lattice-bound H 2 through the combination of the H-H stretching mode HH ν with a phonon mode[18]. This assignment has been confirmed through the HD ν band in MgO crystals grown from a melt containing dissolved D 2 O[19].…”

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confidence: 65%

From Where Did the Water Come That Filled the Earth’s Oceans? A Widely Overlooked Redox Reaction

Freund¹,

Freund²

2015

AJAC

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Though two-thirds of Earth's surface is covered by oceans, measurements of hydroxyl concentrations in upper mantle minerals, specifically in olivine, reportedly provide surprisingly low values. This has been interpreted to mean that there is little dissolved H2O in the Earth's mantle. By inference, when Earth formed, there might not have been able enough water to fill the oceans through volcanic degassing. It has therefore been proposed that the missing water was delivered to Earth from space, through comets and other impacting bodies. However, the reported low hydroxyl concentrations in olivine and similar mineralsis probably based on a profound misunderstanding of a solid state reaction that converts hydroxyls into something more difficult to detect. There is indeed a redox reaction that converts, during cooling, solute hydroxyls in the matrix of minerals into peroxy plus H2. This widely overlooked redox conversion takes place under thermodynamic non-equilibrium conditions. Its significance is that any mineral and any rock available for collection at the Earth surface has gone through a process that causes hydroxyls, the telltale sign of dissolved H2O, to change into peroxyplusH2. The H2 molecules are diffusively mobile and may leave even structurally dense mineral grains. The remaining peroxy thus become the memory of the "true" solute H2O content, besides a few residual hydroxyls. Though first described over 30 years ago, this redox conversion has been largely ignored. As a result it is unknown how much H2O is contained in the Earth's upper mantle but it is certainly much more than has been assumed until now on the basis of analysis of residual hydroxyls.

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“…It is not poisoned by H 2 , also unlike the silica center (149). It is postulated to be formed by high temperature dissociation of hydroperoxy (164) as discussed earlier (138). How this precursor becomes converted to a Vj center upon vacuum heating is unclear, but the spectroscopic assignment of Boudart, et al will be accepted for the purposes of this discussion.…”

Section: Catalysis Ofmentioning

confidence: 99%

“…The basis for the challenge to the catalytic mechanism stems from the apparent incongruity between the observed efficiency of the catalysis at 77 e Κ in the face of the high measured (752) and calculated (164) activation energy for dissociation of OH. Also the proposed high temperature formation mechanism for the center (138,164) as an equilibrium dissociation of two hydroxyls to produce a peroxy entity and molecular hydrogen might seem to predict poisoning of the center by H 2 , at least for the low temperature reaction. H 2 poisoning is not observed, even under "drastic conditions" (137).…”

Section: Catalysis Ofmentioning

confidence: 99%