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| Titel |
Peralkaline fluid composition in equilibrium with K-feldspar, muscovite and quartz at 10 kbar and 700°C: Al transport in crustal fluids |
| VerfasserIn |
A. Wohlers, C. E. Manning |
| 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 |
250070765
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| Zusammenfassung |
| Aluminum is commonly regarded as one of the least soluble elements during metamorphic
and metasomatic processes. However, abundant field evidence suggests that aluminum
transport can occur in natural hydrothermal processes. For example, late formed
aluminumsilicate-bearing and muscovite-bearing veins are widely observed in high-grade
metamorphic rocks, and provide a persuasive argument for considerable mobility of
aluminum in aqueous fluid. The present study explores the fluid composition coexisting with
K-feldspar (K-fsp), muscovite (ms), corundum (co) and quartz (qz) at deep crustal
metamorphic conditions, using a piston cylinder device at 10 kbar and 700°C. Starting
materials of natural microcline, quartz, synthetic corundum, reagent Al2O3 and KSi3O6.5
glass was used.
Ms and K-fsp dissolve incongruently to co + fluid and ms + fluid, respectively. Fluid
composition in equilibrium with co + ms and ms + K-fsp were located in experiments with
and without qz. In quartz-absent experiments fluid composition with co+ms (I1) is mAl =
0.11, mK = 0.15, mSi = 0.44, and with K-fsp + ms (I2) mAl = 0.18, mK = 0.28 and mSi =
0.81, where mi is molality of the subscripted element. Fluid compositions with qz are:
mAl = 0.08, mK = 0.11 and mSi = 1.18 (co + ms + qtz; II1) and mAl = 0.18, mK
= 0.29 and mSi = 1.58 for (K-fsp + ms +qtz; II2). Measured fluid compositions
are peralkaline (K/Al < 1.4). Bulk solubility of Al in pure H2O at this P and T is
reported to be ~0.3 wt% [1], and increase to ~1.9 wt % Al in the presence of SiO2
[2]. This study shows that Al solubility is further enhanced by the presence of K
and Si, increasing from ~4.07 wt% for (I1) to ~ 7.14 wt% at (I2). Presence of
quartz enhances the bulk solubility from ~ 7.63 wt% (II1) to ~ 12.05 wt % at
(II2).
Results indicate that substantial aluminum transfer may occur at deep-crust metamorphic
conditions in aqueous solutions equilibrated with common crustal bulk compositions such as
metapelites and granites. Such high Al mobility is promoted by K and Si. Large-scale Al
transfer may explain the formation of aluminumsilicate- and muscovite-bearing veins by
replacement of K-feldspar in metamorphic rocks, without appealing to either strongly acid or
basic solutions.
[1] Tropper and Manning (2007) Chem. Geol. 240, 54-60. [2] Manning (2007) Geofluids 7,
258-269. |
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