Relative strength of the pyrope–majorite solid solution and the flow-law of majorite containing garnets

Even though the garnet phase is the second most abundant phase in the upper-mantle and transition-zone, no previous studies have directly measured the effect of majorite content on the strength of garnet under mantle conditions. Here we report the results of constant strain-rate and stress-relaxation experiments on garnets in the pyrope–majorite solid solution which constrain the strength of majoritic containing garnets relative to pyrope as a function of majorite content and temperature. We find that at temperatures below 650 °°C both pure pyrope and majoritic garnets have the same strength. Conversely, above 650 °°C we find that majoritic garnets are initially stronger than pure pyrope but weaken with increasing temperature and majorite content and with significant majorite contents are weaker than pyrope above approximately 800 °°C. We develop a flow law for the entire pyrope–majorite solid solution as a function of temperature and majorite content.