Hydrogen isotope exchange reactions between hydrous minerals and molecular hydrogen. 1. A new approach for the determination of hydrogen isotope fractionation at moderate temperatures

Détails

ID Serval
serval:BIB_486987B1B1D1
Type
Article: article d'un périodique ou d'un magazine.
Collection
Publications
Titre
Hydrogen isotope exchange reactions between hydrous minerals and molecular hydrogen. 1. A new approach for the determination of hydrogen isotope fractionation at moderate temperatures
Périodique
Geochimica et Cosmochimica Acta
Auteur(s)
Vennemann T.W., O'Neil J.R.
ISSN-L
0016-7037
Statut éditorial
Publié
Date de publication
1996
Peer-reviewed
Oui
Volume
60
Pages
2437-2451
Langue
anglais
Résumé
Molecular hydrogen was used as an exchange medium to make indirect
determinations of hydrogen isotope fractionation factors between hydrous
minerals (epidote, kaolinite, muscovite, biotite, and hornblende) and
water at temperatures between 150 and 400 degrees C. Hydrogen isotope
exchange between hydrous minerals and Hz is found to be unusually rapid
at these temperatures and in the absence of reducible iron, may
represent true diffusive exchange. Two methods were used for the
exchange: (1) the molecular hydrogen provided an `'infinite'' reservoir
of H-2 and (2) the mineral provided an `'infinite'' reservoir of H-2.
Results of these methods are in good agreement for epidote and
kaolinite, suggesting that `'surface effects'' are not important for
these experiments. Fractionation factors in the epidote-H-2 system,
expressed as 1000/n alpha, increase linearly with increasing 1/T-2 (or
decreasing temperature) between temperatures of 150 and 400 degrees C.
This behavior is also true for epidote-water fractionation factors
calculated from published H2O(vapor) - H-2 and H2O(vapor) - H2O(liquid)
fractionation factors. Values of 1000/n alpha for kaolinite-H-2 and
muscovite-H-2 fractionation factors are similar and also decrease
Linearly with increasing 1/T-2 between 200 and 275 degrees C, and 200
and 400 degrees C, respectively. There is a small departure from such
linearity at temperatures of less than 200 degrees C for the
kaolinite-H-2 system. The trends of the calculated kaolinite-H2O
fractionation factors with temperature are similar to those previously
published, with a maximum at about 200 degrees C. Muscovite-H2O
fractionation factors also decrease with decreasing temperatures and
become increasingly positive at temperatures lower than about 225
degrees C. Problems of reduction of iron and a concomitant increase in
the amount of water were encountered in exchange experiments involving
biotite, hornblende, and, to a lesser extent, muscovite. The
consequences of these reactions were the production of apparent
relatively small mineral-H-2 and large mineral-H2O equilibrium
fractionation factors and support the hypothesis that hydroxyl groups
are added to sites linked to iron and that there is indeed a
compositional effect on hydrogen isotope fractionations in mineral-H2O
systems. The new equilibrium fractionation factors determined in this
study do not unambiguously resolve serious conflicts in data published
by previous workers, but do provide some important constraints on the
direction and magnitude of the D/H fractionation factors between water
and the common rock-forming minerals.
Création de la notice
29/09/2012 16:23
Dernière modification de la notice
20/08/2019 13:55
Données d'usage