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| Titel |
The formation and chemistry of low degree hydrous partial melt on top of the transition zone |
| VerfasserIn |
Daniel J. Frost, Mainak Mookherjee |
| 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 |
250041301
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| Zusammenfassung |
| There is some geophysical evidence for the presence of silicate melt on top of the 410 km
seismic discontinuity. It has also been argued that the difference in the water storage capacity
of upper mantle versus transition zone minerals may cause dehydration melting as material
up-wells across the 410. Studies have proposed that hydrous partial melts may be neutrally
buoyant in the mantle at these conditions. In order to assess these possibilities it is
important to determine the likely composition of small degree hydrous melts at these
conditions and to measure the H2O contents of mantle minerals coexisting with this melt
phase.
The composition of a hydrous melt in equilibrium with a mantle peridotite composition
has been determined at conditions of the 410 and 1450°C. Sandwich experiments were
performed where an “initial-guess” hydrous melt composition was equilibrated with 50%
anhydrous peridotite. The resulting melt composition was used to assemble a further
melt, which was then equilibrated in the same way. After several iterations it was
possible to derive a melt composition, which was in equilibrium with a mineral
assemblage identical to that observed for an anhydrous peridotite composition at the same
conditions. We assess whether this melt composition could be neutrally buoyant at
410km.
The 410 km discontinuity may also correspond to a transition in redox state in the mantle
from a reducing transition zone to a less reduced upper mantle. Volatiles may also
collect and induce melting at this horizon due to the oxidation of a rising mobile
reduced fluid phase containing CH4. Minerals in mantle upwelling out of a hydrous
melt layer would be expected to have H2O contents close to saturation; however,
this may not be the case if the melt layer also contains other volatile components
such as CO2 or CH4, which further lower the H2O activity in the melt. We assess
ranges of melt compositions that may be in equilibrium with minerals containing
relatively low H2O contents, and whether these melts could be neutrally buoyant at 410
km. |
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