Phase equilibrium and stable isotope constraints on the formation of metasomatic garnet-vesuvianite veins (SW Adamello,N Italy) |
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| Authors: | R Abart |
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| Institution: | Institut für Mineralogie-Kristalographie und Petrologie, Universit?t Graz, Universit?tsplatz 2, A-8010 Graz, Austria, AT
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| Abstract: | Metasomatic garnet-vesuvianite veins occur within the contact metamorphic marble sequence of the Lower Triassic Prezzo formation
in a narrow, 1–5 m wide zone along an intrusive marble-granodiorite contact at the southwestern border of the Tertiary Adamello
batholith. The metasomatic mineral assemblage is comprised of garnet, vesuvianite, clinopyroxene, wollastonite, and pyrrhotite,
which were precipitated from the vein-forming fluid in a preexisting calcite matrix at conditions of about 2800 bars and 630° C.
The veins are enriched in silicon, aluminum, iron, magnesium, titanium and depleted in calcium with respect to the unaltered
contact metamorphic marble. Graphite, which is present in the unaltered Prezzo Marble is absent in the veins. Irregularly
shaped mineralogically distinct zones with different degrees of silicification can be distinguished within the veins. The
isotopic compositions of calcite (cc) in the unaltered marble are about δ18O (SMOW; Standard mean Ocean Water)=21.0‰ and δ13C(PDB; Peedee belemnite)=0.0‰. They are reset to significantly lower values within the veins, where δ18Occ is 15.0 to 16.0‰ and δ13Ccc is −4.5 to −3.5‰. The isotopic front coincides with an abrupt change in the microscopic texture of matrix carbonate which
occurs at the sharp boundary between graphite-bearing and graphite-free material. Within the veins the oxygen isotope fractionation
between calcite and garnet (gar) varies systematically with distance from highly silicified zones. The variations in Δ18Occ-gar are as large as 2‰, on a millimeter scale, indicating garnet-calcite isotopic disequilibrium. Vein formation was due to the
infiltration of a water rich fluid of magmatic provenance into the carbonate country rock along fractures. Removal of graphite
from the wall rock by dissolution through the metasomatic fluid induced recrystallization of matrix calcite. Permeability
was enhanced during calcite recrystallization facilitating material transport into the wall rock and metasomatic alteration.
Vein garnet was precipitated in isotopic equilibrium with the metasomatic fluid. The isotopic composition of preexisting calcite
was initially out of equilibrium with the vein-forming fluid and it was shifted towards equilibrium by surface-reaction controlled
calcite-fluid isotopic exchange during calcite recrystallization. Due to the short lifetime of the metasomatic system, calcite-fluid
isotopic equilibrium was generally not attained. Within the veins, oxygen and carbon transport was fast relative to mineral-fluid
exchange of their isotopes and the geometry of the isotopic pattern is largely controlled by the kinetics of mineral-fluid
exchange.
Received: 16 June 1994/Accepted: 20 May 1995 |
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