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| Titel |
Incipient fluid migration through the deep mantle by dissolution-precipitation: crystal growth constraints |
| VerfasserIn |
Anton Shatskiy, Konstantin Litasov, Yury Borzdov, Tomoo Katsura, Eiji Ohtani |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250038206
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| Zusammenfassung |
| The mechanism and driving forces for transport of incipient C-O-H-bearing fluid/melt species
through the Earth’s mantle is a key issue in geosciences. Several mechanisms of the volatile
transport, mainly solid state diffusion and fluid/melt porous flow have been considered. The
grain boundary diffusion of hydrogen and carbon is in the range of 10-10-10-11 m2/s. These
values suggest failure of the solid state diffusion hypothesis to explain melt migration. Since
the extensive partial melting of the Earth’s mantle seems improbable, the porous flow
model of fluid or melt migration, accepted for the shallow upper mantle, can not be
applied for the volatile transport through the deep mantle. At the mantle conditions
water and carbonates are the excellent silicate solvents. Hence the migration of
insulated portions of fluid through the solid matrix should proceed by means of the
dissolution-precipitation mechanism. The major driving force for this process would be
pressure or temperature gradient, differences in stable and metastable phase solubilities, and
stress.
In order to estimate the reliability of proposed mechanism we measured migration rate of
carbonate, water-carbonate, or water-rich liquid layer through the solid silicate matrix at the
upper and lower mantle PT conditions. The thermal gradient was employed as a driving
force. The kinetic constant of the migration rates were estimated to be 8Ã10-8
m/s/K for H2O, 5Ã10-9 m/s/K for K2Mg(CO3)2+2H2O, and 3Ã10-10 m/s/K
for K2Mg(CO3)2 solvents. In order to extrapolate obtained data to the Earth we
assumed that (a) mass transfer of silicate components through the melt layer is
limited by diffusion and (b) the thickness of the melt layer is not enough to establish
convection. The large lateral thermal gradient, 1-4 oC/km, proposed for mantle
plumes reveals lateral fluid migration rate relevant to the plume lifetime (25-50 Ma).
Nevertheless, the radial (vertical) migration rate is quite slow, about 1 km in 12.5 Ga.
Obtained experimental data clearly demonstrate that dissolution-precipitation could be
the predominant mechanism of volatile migration in the deep mantle. However,
pressure or temperature gradients cannot be considered as major driving forces for this
process.
The lower pressure silicate polymorphs may remains metastably in a subducted slab due
to slow kinetics at low temperatures. At the same time the warming of subducted material
should cause the decomposition of hydrous silicates and appearance of water-bearing
supercritical fluid. The difference in solubility of stable and metastable phase is in order of
tens wt.%, which is one order of magnitude higher than that realized in the experiments by
using thermal gradient. This driving force can be most powerful and should cause the
migration of fluids towards lower pressure and temperature conditions, until corresponding
phase equilibrium and/or back incorporation of the fluid into hydrous minerals occur. Since
the ratio of the strain rate to the diffusive flux in the upwelling mantle is significantly
larger than unity, the re-equilibration of the texture by diffusion processes does not
destroy that formed by straining the rock. The stress relaxation can be achieved by
recrystallization through the liquid phase rather than by solid state diffusion or
dislocation creep. In this case insulated fluid pockets will migrate towards maximum
stress. The driving force for this process is the solubility difference of deformed and
undeformed crystals in the fluid, which is caused by surface energy differences. This
mechanism provides the migration rate in order of several μm per day, which is
reasonably fast to explain fluid segregation in the upwelling mantle. Since melt
formation and rock deformation in the mantle operate simultaneously in time and space,
the fluid segregation by dissolution-precipitation can play important role in the
melting in the Earth’s mantle and can occur primarily at the subduction slab and
mantle plume conditions, which would be the regions of most intense deformations. |
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