Size effects resolve discrepancies in 40 years of work on low-temperature plasticity in olivine — Science Advances 2017

 

One-sentence summary:

When deforming by low-temperature plasticity, the strength of the mantle mineral olivine is controlled by its grain size.

At a glance:

The deformation of olivine (the main mineral of the upper mantle) at low temperatures controls the way that tectonic plates bend and break. There have been several studies that have used experiments to try to understand the strength of olivine under low-temperature conditions, but they all disagree with each other. This study points out that there is a relationship between the experimentally-predicted strength of olivine and the grain size of the experiments: tests on samples with smaller grain sizes exhibit a higher strength than tests on samples with larger grain sizes!

Abstract:

The strength of olivine at low temperatures and high stresses in Earth's lithospheric mantle exerts a critical control on many geodynamic processes, including lithospheric flexure and the formation of plate boundaries. Unfortunately, laboratory-derived values of the strength of olivine at lithospheric conditions are highly variable and greatly disagree with those inferred from geophysical observations. We demonstrate via nanoindentation that the strength of olivine depends on the length-scale of deformation, with experiments on smaller volumes of material exhibiting larger yield stresses. This "size effect" resolves discrepancies among previous measurements of olivine strength using other techniques. It also corroborates the most recent flow law for olivine, which proposes a much weaker lithospheric mantle than previously estimated, this bringing experimental measurements into closer alignment with geophysical constraints. Further implications include an increased difficulty of activating plasticity in cold, fine-grained shear zones and an impact on the evolution of fault surface roughness due to the size-dependent deformation of nanometer- to micrometer-sized asperities.

 

Links:
Article - Science Advances 
Press summary - University of Oxford
Press summary - University of Delaware  
EARTH Magazine article

 

Kumamoto, K.M., C.A. Thom, D. Wallis, L.N. Hansen, D.E.J. Armstrong, J.M. Warren, D.L. Goldsby, and A.J. Wilkinson, 2017, Size effects resolve discrepancies in 40 years of work on low-temperature plasticity in olivine, Science Advances, vol. 3, e1701338.

 
Katie Kumamoto