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      Mineral disequilibrium in lavas explained by convective self-mixing in open magma chambers.

      1 , ,
      Nature
      Springer Science and Business Media LLC

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          Abstract

          Characteristic features of many porphyritic andesite and dacite lavas are that they are rich in crystals and display a range of disequilibrium features, including reversely zoned crystals, resorption surfaces, wide ranges of mineral compositions and minerals which are not in equilibrium with the surrounding rock matrix. These features are often interpreted as evidence of the mixing of magmas of contrasting composition, temperature and origin. Here, however, we propose that such features can also be caused by convection within a magma body with a single composition, that is heated from below and cooled from above. We describe petrological observations of andesite lava erupted at the Soufrière Hills volcano, Montserrat, which indicate a heating event and the intermingling of crystals that have very different thermal histories. We present experimental data on a representative groundmass composition of this lava, which indicate that it is difficult to explain the calcic compositions of plagioclase overgrowth rims and microphenocrysts unless parts of the magma were at temperatures much higher than the inferred average temperature. The concept of convective self-mixing allows us to explain the occurrence of compositions of minerals that apparently cannot coexist under equilibrium conditions.

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          Author and article information

          Journal
          Nature
          Nature
          Springer Science and Business Media LLC
          0028-0836
          0028-0836
          Jun 28 2001
          : 411
          : 6841
          Affiliations
          [1 ] Department of Earth Sciences, Bristol University, Bristol BS8 1RJ, UK. Susan.Couch@bristol.ac.uk
          Article
          35082540
          10.1038/35082540
          11429601
          d26eb51f-c6da-495e-8188-70b9734cfe23
          History

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