ISSN:
1432-0967
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract Impure limestones with interstratified metachert layers were contact metamorphosed and metasomatized by the Bufa del Diente alkali syenite. Massive marbles exhibit mineralogical and stable isotope evidence for limited fluid infiltration, confined to a 17 m wide zone at the contact. Influx of magmatic brines along most metacherts produced up to 4 cm thick wollastonite rims, according to calcite (Cc)+quartz (Qz)= wollastonite (Wo)+CO2, and were observed at distances of up to 400 m from the contact. The produced CO2 exsolved as an immiscible low density CO2-rich fluid. Chert protolith isotope compositions were δ18O (Qz)= 27–30‰ and δ18O (Cc)=24–27‰. Many wollastonites in infiltrated metacherts have low δ18O ranging from 11–17‰ and confirm that decarbonation occurred in presence of a magmatic-signatured fluid. Large gradients in δ18O (Wo) across the rims may reach 6‰. The δ18O of remaining quartz is often lowered to 15–20‰ whereas calcites largely retained their original compositions. The isotopic reversals of up to 10‰ between quartz and calcite along with reaction textures demonstrate non-equilibrium between infiltrating fluid in the aquifer and the assemblage calcite+quartz+wollastonite. This is compatible with the assumption of a down-temperature flow of magmatic fluids that occurred exclusively in the remaining quarzite layer. The δ13C (Cc) and δ18O (Cc) of marble calcites measured perpendicular to two metachert bands reveal significant isotopic alterations along distances of 4.5 cm and 7.5 cm from the wollastonite-marble boundary only into the hanging wall marble, suggesting an advection process caused by a fluid phase which moved upwards. Covariation trends of δ13C (Cc) and δ18O (Cc) across the alteration front indicate that this fluid was CO2-rich. Mass balance calculations show that all CO2-rich fluid produced by the decarbonation reaction was lost into overlying marble. The metachert aquifers did not leak with respect to water-rich fluids.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00286935
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