Melt extraction and accumulation from partially molten rocks Paul D. Bons a, * , Jochen Arnold b , Marlina A. Elburg c , Jaan Kalda d , Alvar Soesoo e , Boudewijn P. van Milligen f a Institut fu ¨r Geowissenschaften, Eberhard Karls Universita ¨t Tu ¨bingen, Sigwartstrasse 10, D-72076 Tu ¨bingen, Germany b Institut fu ¨r Geowissenschaften, Johannes Gutenberg-Universita ¨t Mainz, D-55099 Mainz, Germany c Max Planck Institute for Chemistry, Department of Geochemistry, Postfach 3060, D-55020 Mainz, Germany d Institute of Cybernetics, Tallinn Technical University, Akadeemia tee 21, 12618 Tallinn, Estonia e Institute of Geology, Tallinn Technical University, Estonia Ave 7, Tallinn 10143, Estonia f Asociacio ´n Euratom-CIEMAT (Centro de Investigaciones Energe ´ticas, Medioambientales y Tecnolo ´gicas), Avenida Complutense 22, 28040 Madrid, Spain Received 6 March 2003; accepted 23 April 2004 Available online 28 August 2004 Abstract Current models for melt segregation and ascent are not adequate to accurately describe transport and accumulation in combination. We propose that transport is discontinuous and in batches, and that accumulation occurs by stepwise merging of batches. A simple numerical model of jostling spheres that merge when they touch was used to represent stepwise accumulation and transport of batches by propagation of hydrofractures. Results of the numerical model indicate that such a system may quickly develop into a self-organised critical (SOC) state. In this state, the distribution of melt batch volumes can be described by a power law, with an exponent m that lies between 2/3 and 1. Once a self-organised critical state is established, the system is capable of discharging any additional melt without further change to itself. Deformation aids melt extraction efficiency, as it increases the mobility of hydrofractures, enhances accumulation and hence lowers the exponent m. Full connectivity of melt needs never to be reached in the system and melt transport and extraction can occur at very low melt fractions. The chemical evolution of melt from source to emplacement level will be governed by the discontinuous mixing and mingling of batches, each with different histories, and possibly different sources. If no subsequent homogenisation occurs in a magma chamber or the final emplacement structure, the process can be identified by chemical heterogeneity of plutons and volcanic rocks. D 2004 Elsevier B.V. All rights reserved. Keywords: Migmatite; Melt segregation; Magma ascent; Hydrofractures; Self-organised criticality 1. Introduction Transfer of melt is the major mass transport and differentiation process in the Earth’s crust and upper- most mantle, and answering the question of how this 0024-4937/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2004.04.041 * Corresponding author. Tel.: +49 7071 2976469; fax: +49 7071 5059. E-mail address: paul.bons@uni-tuebingen.de (P.D. Bons). Lithos 78 (2004) 25 – 42 www.elsevier.com/locate/lithos