An experimental study of grain scale melt segregation mechanisms in two common crustal rock types

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doi: 10.1046/j.1525-1314.2002.00381.x
Authors:Holyoke, C. W.; Rushmer, T.
Author Affiliations:Primary:
University of Vermont, Department of Geology, Burlington, VT, United States
Volume Title:Journal of Metamorphic Geology
Source:Journal of Metamorphic Geology, 20(5), p.493-512. Publisher: Blackwell, Oxford, United Kingdom. ISSN: 0263-4929
Publication Date:2002
Note:In English. 56 refs.; illus., incl. 1 table
Summary:Creation of pathways for melt to migrate from its source is the necessary first step for transport of magma to the upper crust. Two common crustal rock types were experimentally deformed to test the role of different dehydration-melting reactions in the development of permeability during partial melting and deformation in the crust. A muscovite-biotite metapelite and a biotite gneiss were deformed at conditions below, at and above their fluid-absent solidus. The muscovite-dehydration melting reaction was investigated using the metapelite, with T ranging from 650 to 800°C at Pc 700 MPa. Biotite dehydration-melting under lower crustal conditions was investigated using the biotite gneiss, at T ranging from 850 to 950°C at Pc 1000 MPa. Deformation for both sets of experiments was performed at the same strain rate of 1.37 × 10-5/s. In the presence of deformation, the positive ΔV and associated high dilational strain of the muscovite dehydration-melting reaction produces an increase in melt pore P with partial melting of the metapelite. In contrast, the biotite dehydration-melting reaction is not associated with a large dilational strain and during deformation and partial melting of the biotite gneiss, melt pore P builds more gradually. Due to the different rates in pore P increase, melt-enhanced deformation microstructures reflect the different dehydration melting reactions themselves. Permeability development in the two rocks differs because grain boundaries control melt distribution to a greater extent in the gneiss. Muscovite-dehydration melting may develop melt pathways at low melt fractions due to a larger volume of melt, in comparison with biotite-dehydration melting, generated at the solidus. This may be a viable physical mechanism in which rapid melt segregation from a metapelitic source rock can occur. Alternatively, the results from the gneiss experiments suggest continual draining of biotite-derived magma from the lower crust with melt migration paths controlled by structural anisotropies in the protolith. [P.J.T.]
Sections:Experimental mineralogy
Subjects:Biotite; Crust; Deformation; Dehydration; Dilation; Experimental studies; Magma transport; Magmas; Melts; Metamorphic rocks; Metapelite; Metasedimentary rocks; Mica group; Muscovite; Segregation; Sheet silicates; Silicates; Strain; Upper crust
Abstract Numbers:03M/1510
Record ID:2002054629
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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