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Book Chapter

Melt segregation rates in migmatites: review and critique of common approaches

By
Leo M. Kriegsman
Leo M. Kriegsman
National Museum of Natural History/NaturalisPO Box 9517, NL-2300 RA Leiden, The Netherlandskriegsman@naturalis.nnm.nl
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Annika I. Nyström
Annika I. Nyström
Department of Geology, University of TurkuFIN-20014 Turun Yliopisto, Finlandannika.nystrom@utu.fi
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Published:
January 01, 2003

Abstract

The rate of melt segregation in regional migmatite terrains can be estimated by various lines of research. Firstly, the segregation rate of a melt batch, with a volume below the melt percolation threshold, cannot exceed the melt production rate and is therefore limited by the heating rate derived from geothermometry and geochronology (method A). Other estimates come from physical models for melt percolation (method B) and from the degree of (dis)equilibrium reached between melt and source rocks (method C). The first method is restricted by the current time resolution of isotopic techniques. Results from the second and third approaches depend heavily on assumed values of melt viscosity and other parameters (B); on the correct recognition of (dis)equilibrium trace element distributions (C); and on the migmatization model used (B and C).

The validity of method C is undermined by the mathematical equivalence of trace element models for five different scenarios: (1) disequilibrium melting (with or without melt escape) followed by in situ crystallization of non-segregated melt; (2) equilibrium melting, followed by equilibrium crystallization and major melt escape; (3) disequilibrium melting, followed by equilibrium crystallization and minor melt escape; (4) pervasive retrograde re-equilibration; and (5) subsolidus differentiation. Hence, trace element data in support of model 1 (implying fast melt segregation rates) are equally consistent with models 2 to 4 (implying slow melt segregation rates), and even with melt-absent model 5. The level of trace element saturation reached during accessory phase dissolution into melt may not provide better answers, as it is largely controlled by textural constraints, e.g. shielding of accessories by porphyroblasts.

We conclude that the way forward is to directly couple microtextural and microgeochemical information with time constraints. This requires high-resolution (space and time) geochronology, possibly with more advanced methods than presently available.

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Contents

Geological Society, London, Special Publications

Geochronology: Linking the Isotopic Record with Petrology and Textures

D. Vance
D. Vance
Royal HollowayUniversity of London, UK
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W. Müller
W. Müller
The Australian National UniversityAustralia
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I. M. Villa
I. M. Villa
University of BernSwitzerland
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Geological Society of London
Volume
220
ISBN electronic:
9781862394681
Publication date:
January 01, 2003

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