Strontium and oxygen isotope profiles across marble-silicate contacts,Lizzies Basin,East Humboldt Range,Nevada: constraints on metamorphic permeability contrasts and fluid flow |
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| Authors: | M J Bickle H J Chapman S M Wickham M T Peters |
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| Institution: | (1) Department of Earth Sciences, Downing Street, Cambridge CB2 3EQ UK, GB;(2) Department of Geophysical Sciences, University of Chicago, Chicago IL 60637, USA, US |
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| Abstract: | Sr isotope profiles across marble-silicate rock contacts are used in conjunction with previously published oxygen isotope
profiles to constrain fluid movement, porosity and permeability contrasts in migmatitic rocks from Lizzies Basin in the East
Humboldt Range, Nevada. The 18O/16O systematics in the high-grade sequence have been interpreted to reflect infiltration of ∼2×102 m3/m2 of a relatively low 18O hydrous fluid through the sequence, but with preservation of δ18O anomalies in thin marble bands due to a 30-fold lower porosity in the marble compared with silicate rocks (Wickham and Peters
1992). The Sr isotope profiles confirm that tracer exchange between marble and silicate rock was primarily by diffusion, and
in one case, indicate that porosities differed by less than a factor of four in the ∼10 cm boundary layer which exhibits diffusive
modification of 87Sr/86Sr ratios. This contrasts with modelling of the oxygen isotope profiles which imply porosity contrasts >10 for one marble
band and >50 for a second marble band. Either strontium and oxygen isotope diffusion reflect different events (possible if
fluid Sr contents varied with time) or porosity varied substantially with the silicate rocks. Oxygen isotope profiles in the
deeper part of the metamorphic section in which δ18O values of silicate rocks have been homogenised and lowered, indicate similar diffusion distances (and thus porosity-time
evolution) to oxygen isotopic profiles higher in the section. Comparison of strontium and oxygen isotope diffusion distances
constrains fluid Sr contents to between ∼50 and ∼500 ppm deep in the section, but less than ∼10 ppm higher in the section.
The difference is related to release of relatively saline, Sr-rich fluids, by the abundant leucogranites and associated skarns
deep in the section (cf. Peters and Wickham 1995).
Received: 9 December 1994/Accepted: 13 April 1995 |
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