Deformation of melt-bearing systems - insight from in situ grain-scale analogue experiments

The deformation. behaviour of partially molten rocks was investigated using in situ analogue experiments with norcamphor + ethanol, as well as partially molten KNO3 + LiNO3. Three general deformation regimes could be distinguished during bulk pure shear deformation. In regime I, above ca. 8-10 vol.% liquid (melt) fraction (phi bulk), deformation is by compaction, distributed granular flow, and grain boundary sliding (GBS). At phi bulk <8-10 vol.% (regime II), GBS localizes in conjugate shear zones. Liquid segregation is inefficient or even reversed as the dilatant shear zones draw in melt to locally exceed the 8-10 vol.% threshold. At even lower phi(bulk) (regime III), grains form a coherent framework that deforms by grain boundary migration accommodated dislocation creep, associated with efficient segregation of remaining liquid. The transition liquid fraction between regimes I and II (phi(LT)) depends mainly on the grain geometry and is therefore comparable in both analogue systems. The transition liquid fraction between regimes II and III (OGBS-L) varies between 4-7 vol.% for norcamphor-ethanol and ca. I vol.% for KNO3 + LiNO3 and depends on system specific parameters. Regime II behaviour in our experiments can explain the frequently observed small melt-bearing shear zones in partially molten rocks and in HT experiments. (C) 2005 Elsevier Ltd. All rights reserved.