Melt segregation rates in migmatites: review and critique of common approaches
Leo M. Kriegsman, Annika I. Nyström, 2003. "Melt segregation rates in migmatites: review and critique of common approaches", Geochronology: Linking the Isotopic Record with Petrology and Textures, D. Vance, W. Müller, I. M. Villa
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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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Isotope geochemistry has produced many technical developments in the past decade or so that have revolutionized the potential information available on the tectonics of metamorphic belts from geochronology. These include the ability to date minerals and rocks on small spatial scales, scales that at last approach those from which other types of information — structural and petrological — are obtained. However, interpreting the new data, and their integration with the other datasets available, is not straightforward and requires careful chemical and textural observations that go hand-inhand with the geochronology. The increasing realization of the importance of this approach has led to a number of symposia at international conferences devoted to this topic in recent years. The set of papers in this book emanates from one such symposium and describes recent progress in integrating this new information with other datasets from metamorphic petrology on a mineral and sub-mineral scale.