Supplementary Online Text
1. Temperature determination
Measuring the temperature directly by thermocouple in the RDA is very challenging (Nishihara et al.,
2008). Therefore, all published work on the deformation of transition zone minerals using the RDA rely
on the temperature (and pressure) dependence on Ca-partitioning between enstatite and diopside
under a range of conditions. In the current study the thermal EOS of wadsleyite suggests pressures were
likely in the range of 19 to 21 GPa, and constrained by the presence of ringwoodite in some wadsleyite
samples.
The enstatite-diopside geothermo(baro)meter can be applied only under the conditions where
these two phases exist. Diopside is stable up to 18 GPa followed by the breakdown to majorite garnet
and perovskite-structured phases at higher pressures (Gasparik, 1996; Akaogi et al., 2004). Also,
following Gasparik (1996), the upper limit of pressure under which his geothermobarometer can be
used is ~16 GPa. However, we did not observe transformation products of the enstatite-diopside phases
using the raman spectrometer and X-ray diffractometer. This might be caused by the uncertainties in the
geothermobarometers. Also, non-static conditions in the RDA might have some effect as well. An
alternative explanation may be very sluggish phase transformation kinetics of the pyroxene mixture that
requires diffusion of Al and Si (e.g. van Mierlo et al. (2013)). We do not know exactly why both enstatite
and diopside were observed and majorite garnet was not observed, but as far as the enstatite-diopside
pair was present, it is possible to apply the published results of geothermobarometry (even if these
phases may exist metastably). The reason for this justification is as follows. The element partitioning
between two minerals is controlled by the size of that element relative to the size of the sites to which
that element goes in two phases and by the elastic properties of the host minerals. Both the size of the
site and elastic properties of a mineral do not change by stress or by the meta-stability in any significant
way. Finally, temperatures (and pressures) estimated this way are consistent with the conditions where
ringwoodite and wadsleyite co-exist and with the results of power-temperature calibrations.
2. References
Akaogi, M., Yano, M., Tejima, Y., Iijima, M., Kojitani, H., 2004. High-pressure transitions of diopside and
wollastonite: Phase equilibria and thermochemistry of camgsi2o6, casio3 and casi2o5–catisio5 system.
Physics of the Earth and Planetary Interiors 143–144, 145-156.
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Gasparik, T., 1996. Melting experiments on the enstatite-diopside join at 70-224 kbar, including the
melting of diopside. Contributions to Mineralogy and Petrology 124, 139-153.
Nishihara, Y., Tinker, D., Kawazoe, T., Xu, Y., Jing, Z., Matsukage, K.N., Karato, S.-i., 2008. Plastic
deformation of wadsleyite and olivine at high-pressure and high-temperature using a rotational
drickamer apparatus (rda). Physics of the Earth and Planetary Interiors 170, 156-169.
van Mierlo, W.L., Langenhorst, F., Frost, D.J., Rubie, D.C., 2013. Stagnation of subducting slabs in the
transition zone due to slow diffusion in majoritic garnet. Nature Geoscience 6, 400-403.
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