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

摘要

The deformation behaviour of partially molten rocks was investigated using in situ analogue experiments with norcamphor + ethanol, as well as partially molten KNO_3 + LiNO_3. Three general deformation regimes could be distinguished during bulk pure shear deformation. In regime Ⅰ, above ca. 8-10 vol.% liquid (melt) fraction (φ_(bulk)) > deformation is by compaction, distributed granular flow, and grain boundary sliding (GBS). At φ_(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 φ_(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 (φ_(LT)) depends mainly on the grain geometry and is therefore comparable in both analogue systems. The transition liquid fraction between regimes Ⅱ and Ⅲ (φ_(GBS-L)) varies between 4-7 vol.% for norcamphor-ethanol and ca. 1 vol.% for KNO_3+LiNO_3 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.