Deformation of Partially Molten Rocks at Lithospheric Temperatures and Pressures

Partial melting accompanies deformation in many tectonic settings. The deformation-induced alignment of anisotropic minerals and/or stress-driven alignment of melt pockets in these settings produces a seismic signature detectable by remote sensing. This experimental study aims to provide first-order constraints on the seismic and mechanical properties of those partially molten, deforming olivine aggregates.

We performed a series of general shear experiments using a Griggs solid medium deformation apparatus. San Carlos olivine with ≈3wt% of basalt added was hot pressed at 1250˚C under a confining pressure of 300 MPa. This material was cut into thin discs and held inside a nickel ellipse, then placed between alumina forcing blocks pre-cut at 45˚ and weld-sealed in platinum jackets. The samples were subsequently deformed at Pc = 1.0 - 1.5 GPa, T = 900 - 1000˚C, at shear strain rates of 10^-3 - 10^-4 s^-1, resulting in final shear strains between 0.5 - 5.

At 900˚C, the samples show a positive pressure dependence (300 MPa peak shear stress at Pc = 1.0 GPa vs. 600 MPa at Pc = 1.5 GPa) and abruptly weaken after reaching peak strength. Samples deformed at T=1000˚C and Pc = 1.5 GPa reached steady-state shear stresses of ≈ 200 MPa, indicating a large drop in strength as the basalt undergoes glass transition.

Microstructural observation using SEM and EBSD show that pervasive fracturing and brittle deformation occurs at 900˚C despite the fact that stresses fall below Goetze's criterion. At 1000˚C much of the shear strain is accommodated in new bands of small (~< 3 µm diam.) grains within the existing fabric.

Samples deformed to higher strain generally show a large number of these fine-grained bands; samples deformed to lower strain do not. Furthermore, larger grains show internal misorientations, indicating that dynamic recrystallization leads to the observed grain size reduction.