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 (ϕbulk), deformation is by compaction, distributed granular flow, and grain boundary sliding (GBS). At ϕbulk<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 ϕbulk (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 (ϕLT) depends mainly on the grain geometry and is therefore comparable in both analogue systems. The transition liquid fraction between regimes II and III (ϕGBS-L) varies between 4–7 vol.% for norcamphor–ethanol and ca. 1 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.