Stable isotope geochemistry and formation mechanisms of quartz veins; Extreme paleoaltitudes of the Central Alps in the Neogene


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Stable isotope geochemistry and formation mechanisms of quartz veins; Extreme paleoaltitudes of the Central Alps in the Neogene
American Journal of Science
Sharp Z.D., Masson H., Lucchini R.
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Quartz veins ranging in size from less than 50 cm length and 5 cm width
to greater than 10 m in length and 5 m in width are found throughout the
Central Swiss Alps. In some cases, the veins are completely filled with
milky quartz, while in others, sometimes spectacular void-filling quartz
crystals are found. The style of vein filling and size is controlled by
host rock composition and deformation history. Temperatures of vein
formation, estimated using stable isotope thermometry and mineral
equilibria, cover a range of 450 degrees C down to 150 degrees C. Vein
formation started at 18 to 20 Ma and continued for over 10 My. The
oxygen isotope values of quartz veins range from 10 to 20 permil, and in
almost all cases are equal to those of the hosting lithology. The
strongly rock-buffered veins imply a low fluid/rock ratio and minimal
fluid flow. In order to explain massive, nearly morromineralic quartz
formation without exceptionally large fluid fluxes, a mechanism of
differential pressure and silica diffusion, combined with pressure
solution, is proposed for early vein formation. Fluid inclusions and
hydrous minerals in late-formed veins have extremely low delta D values,
consistent with meteoric water infiltration.
The change from rock-buffered, static fluid to infiltration from above
can be explained in terms of changes in the large-scale deformation
style occurring between 20 and 15 Ma. The rapid cooling of the Central
Alps identified in previous studies may be explained in part, by
infiltration of cold meteoric waters along fracture systems down to
depths of 10 km or more. An average water flux of 0.15 cm 3 cm(-2)yr(-1)
entering the rock and reemerging heated by 40 degrees C is sufficient to
cool rock at 10 km depth by 100 degrees C in 5 million years.
The very negative delta D values of < -130 permil for the late stage
fluids are well below the annual average values measured in meteoric
water in the region today. The low fossil delta D values indicate that
the Central Alps were at a higher elevation in the Neogene. Such a
conclusion is supported by an earlier work, where a paleoaltitude of
5000 meters was proposed on the basis of large erratic boulders found at
low elevations far from their origin.
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