Observed major element metasomatism in 5 amphibolite facies ductile fault zones can be explained as the inevitable consequence of aqueous fluid flow along normal temperature gradients under conditions of local chemical equilibrium. The metasomatism does not require the infiltration of chemically exotic fluids. Calculations suggest that metasomatized ductile fault zones are typically infiltrated by ∼105 moles H2O/cm2, fluid flow is in the direction of decreasing temperature, and fluids contain about 1.0 molal total chloride. Where available, stable isotopic alteration data confirm both flow direction and fluid fluxes calculated from major element metasomatism. The fluid fluxes inferred from metasomatism do not require large-scale fluid recirculation or mantle sources if significant lateral fluid flow occurs in the deep crust. Time-integrated fluid fluxes are combined with estimates of flow duration to constrain average flow rates and average permeabilities. Rocks in ductile fault zones are probably much more permeable during metasomatism (average permeabilities of 10-17 to 10-15 m2) than rocks normally are during regional metamorphism (10-21 to 10-18 m2). Estimated average fluid flow rates (3.5×10-3 to 0.35 m/yr) are insufficient, however, to significantly elevate ambient temperatures within ductile faults. Fluid flow in the direction of decreasing temperature may increase the ductility of silicate rocks by adding K to the rocks and thereby driving mica-forming reactions.

Metasomatism and fluid flow in ductile fault zones

Prograde dehydration reactions within metabasalts at the blueschist- eclogite facies transition produce kyanite veins, usually with quartz, sometimes with omphacite or chloritoid, of up to several meters length and 20 cm width. Bulk rock analyses show clear depletion haloes for Al2O3 and SiO2 near the veins. Mass balance calculations support the suggestion that the vein material must have been derived from the immediate surroundings. These veins are interpreted as local segregation phenom- ena and not as fluid dehydration channels of the eo-Alpine subduction zone. Calcula- tions have been performed for a stagnant vein formation model that considers the rate of the dehydration reactions (k) responsible for providing the H2O for Al and Si transport and the rate of diffusion (D) of these from wall rock to growing vein. The model is satisfied for quite a wide range of values for k/D and upper limits on the duration of diffusional vein formation of about 1 Ma are indicated.

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Local origin of high pressure vein material in eclogite facies rocks of the zermatt-saas zone, switzerland

The Imjingang belt in the central Korean Peninsula is a strong candidate to represent the extension of the Dabie-Sulu ultrahigh-pressure (UHP) belt in China. This fold-thrust belt consists primarily of: (1) the northern, Jingok unit, characterized by Barrovian-type metapelites; and (2) the southern, Samgot unit, comprising calc-silicate rocks and amphibolites with or without garnet. The metamorphic grade of the Jingok unit increases southward from garnet through staurolite to kyanite zones. Peak metamorphic conditions estimated from the kyanite zone range up to ~11 kbar and 700°C. In contrast, no significant change in metamorphic assemblages is apparent in the Samgot unit. Characteristic minerals of the calc-silicate rocks include garnet, diopside, hornblende, scapolite, and plagioclase, and are compatible with upper amphibolite facies estimated from garnet amphibolite. P-T estimates from the Samgot amphibolites are generally in the range of 8.5-11.5 kbar and 660-780°C. These P-T estimates, together with ...

Metamorphic Evolution of the Imjingang Belt, Korea: Implications for Permo-Triassic Collisional Orogeny

http://www2.ess.ucla.edu/~manning/pdfs/m10.pdf

Premelting polymerization of crustal and mantle fluids, as indicated by the solubility of albite + paragonite + quartz in H2O at 1 GPa and 350–620 °C

Partitioning of trace elements between olivine and aqueous fluids at highP-T conditions: implications for the effect of fluid composition on trace-element transport

In this article we present the results of some studies of the composition of an aqueous phase in equilibrium with various three-phase assemblages in the system K2O-Al2O3-SiO2 (figure 1) such as quartz-feldspar-muscovite, muscovite-leucite-corundum, and several others. We interpret the results as indicating that the solutes are dominantly uncharged species, specifically alkali-alumina and alkalialumina- silica species.
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Figure 1
The system K2O-Al2O3-SiO2 and the solid phases used in this study.

M. L. Pascal

Papers

Aluminum Speciation in Metamorphic Fluids