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| Titel |
Composition of COH fluids at 1 GPa: an experimental study on speciation and solubility |
| VerfasserIn |
Carla Tiraboschi, Simone Tumiati, Sandro Recchia, Peter Ulmer, Thomas Pettke, Patrizia Fumagalli, Stefano Poli |
| 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 |
250097891
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| Publikation (Nr.) |
 EGU/EGU2014-13515.pdf |
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| Zusammenfassung |
| COH fluids play a fundamental role in many geological processes, controlling the location of
melting in subduction zones and promoting mass transfer from the subducting litosphere to
the overlying mantle wedge. The properties of COH fluids are strictly dependent on the
composition of the fluid in subduction systems, i.e., the speciation of the volatile components
of the fluid itself and the presence of solutes deriving from the dissolution of rock-forming
minerals. In the scientific literature, the speciation of COH fluids has been generally
determined through thermodynamic calculations using equations of state of simple
H2O-non-polar gas systems (e.g., H2O-CO2-CH4), equations that do not consider the
complexity related to dissolution processes, which are substantially unexplored in COH fluids
and limited so far to aqueous fluids (Newton & Manning, 2002). The aim of this
work is to investigate experimentally the speciation and the dissolution of mantle
minerals in carbon-saturated COH fluids at buffered fO2 conditions. Our experimental
approach relies on two different techniques: 1) analysis by means of quadrupole mass
spectrometer (QMS) of the fluids from pierced run capsules to retrieve speciation
of volatile components and 2) analysis of frozen COH fluid with laser-ablation
inductively coupled plasma-mass spectrometry (LA-ICP-MS) to measure the amount of
solutes.
Experiments were conducted at pressure of 1 GPa and temperatures from 800 to 900°C
using a rocking piston cylinder apparatus. Mantle minerals in equilibrium with COH fluid are
represented by synthetic forsterite. fO2 conditions were controlled using the double
capsule technique and NNO buffer (ΔFMQ=-0.61 at 800°C; ΔFMQ =-0.98 at 900Ë
C).
For the speciation experiments, oxalic acid dihydrate and graphite have been used to
generate carbon-saturated COH fluid. The speciation was determined by analyzing the
quenched COH fluid, retrieved by piercing the capsule in a gas-tight vessel at T =80°C and
convoying evolved gases to a QMS through a heated line to avoid the condensation of
water. This type of analyzer ensures superior performances interms of selectivity
of molecules to be detected,high acquisition rates and extended linearresponse
range. The influence of dissolved solutes on fluid speciation has been evaluated
by comparing experiments in the pure COH system and in the COH+forsterite
system.
To determine the solubility of forsterite in COH fluids we performed a second set of
experiments at the same P , T and fO2 conditions above. Fluids trapped in a diamond layer
were analysed by the cryogenic LA-ICP-MS technique described by Aerts et al.
(2010). With this method the aqueous part of the COH fluid is frozen prior the
opening and maintained frozen during the analysis to avoid any precipitation of
solutes.
The results will highlight the importance of fluids for the mass transport in subduction
zones. Comparison between experimental data and thermodynamic calculation will also be
shown.
References:
Aerts, M., Hack, A.C., Reusser, E., Ulmer, P. (2010) Am. Mineral. 95, 1523-1526.
Newton, R.C., Manning, C.E. (2002) Geochim. Cosmochim. Ac. 66, 4165-4176. |
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