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| Titel |
ALUMINUM MOBILITY IN CRUSTAL FLUIDS: THE ROLE OF Al-Si COMPLEXING |
| VerfasserIn |
C. E. Manning, R. Thomas, P. Tropper |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250061433
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| Zusammenfassung |
| The low solubility of Al in pure H2O at crustal metamorphic conditions has led to the
common assumption that this element is immobile during fluid flow; however, Al-rich
minerals in metamorphic veins and segregations suggest otherwise. High fluid fluxes are
typically not supported by other data, and alternatives such as H+ metasomatism or
complexing with alkalis or halides require special conditions if they are to provide a general
explanation for this apparent inconsistency. A more plausible explanation is Al complexing
with SiO2 because of its high concentrations in metamorphic pore fluids present in a
wide range of crustal lithologies. We investigated this hypothesis via rapid-quench,
hydrothermal piston-cylinder experiments on corundum solubility in SiO2-bearing
H2O at 700-950Ë C and 0.5-1.5 GPa. Three sets of runs were conducted at fixed P
and T: 1 GPa & 700Ë C, 1 GPa & 800Ë C, and 1.5 GPa & 800Ë C. Corundum
solubility increases with SiO2 concentration in each case, signaling Al-Si complexing.
Quartz-saturated experiments at 1.5 GPa, 800-950Ë C, and at 800Ë C, 0.5-1.5 GPa, show that
(1) both Al and Si solubility are enhanced in the presence of corundum+quartz
relative to that expected for saturation in a single oxide mineral, and (2) Al and Si
solubility enhancements increase with P and T, indicating progressively higher
concentrations of Al-Si complexes. The nature of the Al-Si complex(es) can be
determined from the solubility patterns. At 800Ë C, 1 GPa, the predominant Al
and Si aqueous species are the neutral Al monomer (AlO1.5(m)) and Si monomer
(SiO2(m)) and dimer (Si2O4(d)). Adopting a standard state of unit activity of one mole
of the species and assuming ideal mixing, mass balance relations can be coupled
with thermodynamic properties of equilibrium between SiO2(m) and Si2O4(d) to
obtain the stoichiometry and thermodynamic properties of the homogeneous reaction
AlO1.5(m) + nSiO2(m)= AlSinO2n+1.5. We obtain n = 2.01 and logK = 5.1±0.4
(1Ïă). Average deviation from the experimental measurements is 17%. That the
best fit value for n corresponds almost exactly to an integer reaction coefficient
of 2 strongly suggests that the mechanism for Al interaction with Si is formation
of a simple trimer complex at this P and T. In addition, we find that Si>Al at all
conditions measured. The results show that, by forming polymeric clusters with
silica in solution, Al is readily mobilized in metamorphic fluids simply by H2O
equilibration with the major minerals of the crust: quartz, feldspars and micas. Al
mobility should thus be expected during fluid-rock interaction in deep crustal settings. |
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