PRESSURE DEPENDENCE OF MAGNESITE AND DOLOMITE AGGREGATES

Intermediate depth (170-400 km) deep focus earthquakes are observed in subducting slabs, but unlike shallow (50-170 km) and deep (400-660 km) deep focus earthquakes, the mechanism(s) responsible for these earthquakes are not clear. Two common alteration products observed in peridotites, magnesite and dolomite, are stable along the pressure-temperature path of a subducting slab. Low pressure experiments indicate that these minerals are weaker than olivine, but there are no data to constrain the pressure dependence of magnesite or dolomite strengths.

In order to determine the pressure dependence of magnesite and dolomite deforming by low temperature plasticity mechanisms (dislocation glide and twinning), stacked magnesite and dolomite cores (1mm X 1mm) were deformed in series to 25-30% strain using the deformation-DIA (DDIA) at Argonne National Lab at 500^oC and three different pressures ranging from 2 - 6 GPa. Stacked samples allow direct observation of the materials’ relative strengths via differences in strain rates of the two materials.

At all conditions, dolomite deforms at a constant strain rate, which does not evolve with increasing strain. However, magnesite strain weakens in all experiments. Magnesite is initially isoviscous or stronger than dolomite, but with increasing strain, becomes weaker than dolomite. These results indicate that strain weakening of magnesite in subduction zones may cause strain localization and therefore be the source of intermediate depth deep focus earthquakes.