 |
| Titel |
D/H diffusion in serpentine |
| VerfasserIn |
Hélène Pilorgé, Bruno Reynard, Laurent Remusat, Sylvie Le Floch, Gilles Montagnac, Hervé Cardon |
| Konferenz |
EGU General Assembly 2017
|
| Medientyp |
Artikel
|
| Sprache |
en
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250143677
|
| Publikation (Nr.) |
 EGU/EGU2017-7422.pdf |
|
|
|
|
|
| Zusammenfassung |
| Interactions between aqueous fluids and ultrabasic rocks are essential processes in a
broad range of contexts including hydrothermal alteration on the parent body of
carbonaceous chondrites, at mid-oceanic ridge, and in subduction zones. Tracking these
processes and understanding reaction kinetics require knowledge of the diffusion
of water in rocks, and of isotope fractionation in major minerals forming under
hydrous conditions, such as serpentines. We present a study of D/H inter-diffusion in
antigorite, a common variety of serpentine. High-temperature (HT) experiments
were performed in a belt apparatus at 540˚ C and 3.0 GPa on natural antigorite
powders saturated with interstitial D2O. A low-temperature (LT) experiment was
performed in diamond anvil cell at 350˚ C and 2.5 GPa on an antigorite single-crystal
loaded with pure D2O. D/(D+H) ratios were mapped using Raman spectroscopy
for the HT experiment and NanoSIMS for the LT experiment. As antigorite is a
phyllosilicate, diffusion coefficients were obtained for crystallographic directions
parallel and perpendicular to the antigorite layers (perpendicular and parallel to
the c∗-axis, respectively). The equations of D/H inter-diffusion coefficients were
determined to be DD∕H (m2/s) = 5.04 x 10−5 x exp(-170(±53) (kJ/mol) / RT) and DD∕H
(m2/s) = 1.52 x 10−7 x exp(-157(±32) (kJ/mol) / RT) perpendicular and along the
c∗-axis, respectively, and DD∕H (m2/s) = 7.29 x 10−6 x exp(-166(±14) (kJ/mol) / RT)
for the bulk diffusivity. These results are similar to those obtained on chlorite, in
agreement with the similar crystallographic structures and atomic bonds in the two
minerals. Assuming D/H inter-diffusion coefficients for antigorite are the same for
all serpentine species, closure temperature and diffusion durations are applied to
hydrothermal fields and in CI, CM and CR chondrites. Closure temperatures lie
below 300˚ C for terrestrial hydrothermal alteration and depend on serpentine
variety because they have different typical grain sizes. They lie below 130˚ C for
carbonaceous chondrites, indicating that D/H isotopic exchange may have persisted down to
very low temperatures on their parent bodies. D/H isotopic composition may be
associated with grain size heterogeneities in carbonaceous chondrites due to protracted
alteration of fine-grained material with the lowest closure temperatures (ca 50˚ C). |
|
|
|
|
|
|