Thermal diffusivity of MORB-composition rocks to 15 GPa: implications for triggering of deep seismicity

Abstract

We have measured the thermal diffusivity of eclogite and majorite with a model MORB composition at pressures of 3 and 15 GPa, respectively. Both phase assemblages show inverse dependences of their thermal diffusivities on temperature: D _eclogite=9(10)×10⁻¹⁰+7(1)×10⁻⁴/T(K) m ²/s and D _majorite=6.2(5)×10⁻⁷+3.0(5)×10⁻⁴/T(K) m ²/s. The values for majorite are in good agreement with previous measurements for other garnets and are considerably lower than thermal diffusivities of wadsleyite and ringwoodite, which are the main components of the mantle transition zone. We discuss the implications of the low thermal conductivity of subducted oceanic crust in the transition zone for the triggering of deep seismicity.

Keywords:

• thermal diffusion
• MORB
• majorite
• deep earthquakes
• high pressure

Acknowledgements

We acknowledge financial support from the following sources: Marie Curie Research Training Network ‘c2c’ Contract No. MRTN-CT-2006-035957, The Royal Society (DPD), NERC grant NE/E012922/2 (AMW); NSF grant EAR0809397 (LL). Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. We thank W. Crichton and two anonymous reviewers for helpful comments.