Date of Graduation

Spring 2017

Document Type

Honors Research Project

Degree Name

Bachelor of Science

Major

Geology

Research Sponsor

Dr. Caleb Holyoke

First Reader

Dr. John Peck

Second Reader

Dr. Meagan Ankney

Abstract

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 them 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 about the pressure dependence of the strength of magnesite or dolomite. Magnesite and dolomite cylinders (1mm by 1mm) were deformed in stacked series to 25-30% strain using the deformation-DIA (DDIA) at Argonne National Lab at 500°C and three different pressures of 3.1 GPa, 5.8 GPa, and 6.2 GPa. In order to determine the pressure dependence of magnesite and dolomite deformed by low temperature plasticity, stacked samples allow direct observation of the materials’ relative strengths via differences in strain rates. At all conditions, dolomite deformed at a constant strain rate, which does not evolve with increasing strain. However, magnesite strain weakened during all experiments. Magnesite is initially isoviscous or stronger than dolomite, but with increasing strain, became weaker than dolomite. These results indicate that presence of magnesite in subduction zones can cause strain localization and therefore be the likely source of intermediate depth deep focus earthquakes.

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