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Metamorphic fluids in the deep crust: evidence from the Adirondacks

Nature volume 301, pages 226228 (20 January 1983) | Download Citation

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Abstract

Unusual mineral assemblages of tremolitic hornblende + diopside+enstatite+quartz have been examined to estimate the temperature (T) and water fugacity (fH2O) at the time of the granulite fades metamorphism in the Adirondacks. The pyroxenes are closely represented by the system CaO–MgO–SiO2 allowing application of experimental data1 with only small corrections for reduced values of activities. Tremolite equilibria2 buffered log fH2O= 3.0±0.1, equivalent to 0.9 kbar of water pressure (PH2O) at 7 kbar, 710 °C. Nearby marbles contain wollastonite+calcite+quartz and buffered PCO2 = 1.1 kbar, consistent with PH2O5.9 kbar. Inferred local gradients in fluid compositions suggest that deep crustal fluid conditions are complex and heterogeneous indicating that fluid movements, whether of mantle3 or deep crustal origin, are channelized rather than pervasive. Thus some granulite facies rocks were wholly or partly closed systems with respect to externally derived fluids permitting localized buffering of fluid compositions. Recognition of restricted fluid-mixing limits theories that call for massive amounts of pervasive fluid flow to facilitate melting, cause metasomatism, or to stabilize granulite facies mineralogy.

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Author information

Affiliations

  1. Department of Geology, Rice University, Houston, Texas 77251, USA

    • J. W. Valley
    •  & William Lamb
  2. Department of Geology, Colgate University, Hamilton, New York 13346, USA

    • James McLelland
  3. Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA

    • E. J. Essene

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https://doi.org/10.1038/301226a0

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