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| Titel |
Water incorporation in NAMs after antigorite and chlorite dehydration reactions |
| VerfasserIn |
José Alberto Padrón-Navarta, Jörg Hermann |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250100170
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| Publikation (Nr.) |
 EGU/EGU2014-16062.pdf |
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| Zusammenfassung |
| Subduction zones play a fundamental role in the deep water cycle making the Earth unique
among other terrestrial planets. Water is incorporated into hydrous minerals during seafloor
alteration of the oceanic lithosphere. During subduction of the oceanic lithosphere,
dehydration of these hydrous minerals produces a fluid phase. A part of this fluid phase will
be recycled back to the Earth’s surface through hydrothermal aqueous fluids or through
hydrous arc magmas, whereas another part of the water will be transported to the deep
mantle by Nominally Anhydrous Minerals (NAMs) such as olivine, pyroxene and
garnet. The partitioning of water between these two processes is crucial for our
understanding of the mantle-scale water recycling in the Earth. This can be investigated
experimentally under water-saturated conditions because this situation is met during
dehydration reactions. However relatively low temperature conditions for such reactions
make challenging these experiments. An alternative can be found in the natural
record.
The Alpine Betic-Rif orogen together with Central and Western Alps offer an invaluable
diversity of ultramafic lenses that record a significant range of pressure-temperature and
cooling rates. Hence these samples portray an excellent data set of 24 samples to survey the
transfer of fluids from hydrous phases (brucite, antigorite and chlorite) to NAMs
(olivine, orthopyroxene, clinopyroxene and garnet). Well-studied samples from these
localities have been selected for water measurement using FTIR spectroscopy. The
selected suite comprises the following high-pressure peridotite outcrops: Malenco
serpentinite, Cerro del Almirez (1.6–1.9 GPa and 680–710ºC), Alpe Arami (3.2 GPa and
840ºC), Cima di Gagnone (3.0 GPa and 750-800ºC) and Alpe Albion (0.6 GPa and
730ºC).
The infrared signature of olivine in all localities contains water (hydroxyl groups)
associated to intrinsic defects (mostly point defects related to Ti4+) and extrinsic
submicroscopic hydrous lamellae (titanoclinohumite). In the following only water contents
related to intrinsic defects are reported. At low temperature (400-450 ºC) the spectra of
olivine coexisting with antigorite are dominated by OH associated to silica-vacancies and
contains 12-20 wt. ppm H2O (using site-specific infrared OH absorption coefficients
from [1]). Olivine in equilibrium with orthopyroxene and chlorite formed after the
antigorite breakdown (650-700 ºC) at high pressure (1.6-1.9 GPa) from Cerro del
Almirez contains 14-17 ppm and is associated to Ti4+ and abundant extrinsic defects.
Surprisingly the associated orthopyroxene is nearly dry (1-3 wt. ppm) resulting in
D[opx/ol] 1. After chlorite breakdown (750-800ºC) olivine contains 21-68 wt. ppm
H2O.
In summary there is a systematic correlation between PT conditions and water content in
olivine and orthopyroxene. The dependence is however different for both resulting in
significant changes in the water partition coefficient. This dataset represents a first step in the
quantification of the water budget in the slab and in the mantle wedge of NAMs coexisting
with hydrous phases and after their breakdown.
[1] Kovacs, I., O’Neill, H.S.C., Hermann, J., Hauri, E.H., 2010. Site-specific infrared O-H
absorption coefficients for water substitution into olivine. Am. Miner. 95, 292-299. |
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