Pc. Burnley et al., DIAMOND-ANVIL CELL STUDY OF THE TRANSFORMATION MECHANISM FROM THE OLIVINE TO SPINEL PHASE IN CO2SIO4, NI2SIO4, AND MG2GEO4, J GEO R-SOL, 100(B9), 1995, pp. 17715-17723
Transformation textures and kinetics of the olivine to spinel phase tr
ansformation were observed in situ in ungasketed samples using a diamo
nd anvil cell (DAC). The low-temperature kinetic limit for reconstruct
ive transformation from olivine (alpha) to spinel phase (gamma) observ
ed in DAC experiments is approximately 150 degrees C lower than when o
bserved under more hydrostatic conditions. The spinel phase, which in
many of the samples is distributed in an annular pattern, exhibits rec
onstructive textures including grain boundary nucleation, and lack of
topotaxy; in some cases it forms lenses similar to those associated wi
th transformation-induced mechanical failure. Although spinel phase la
mellae, formed by a martensiticlike mechanism, are observed in the spe
cimens, the lamellae remain extremely thin (similar to 10 nm) and do n
ot produce enough spinel to be optically visible. The observation of r
econstructive textures within the annular transformed regions leads us
to conclude that high shear stress and plastic strain enables reconst
ructive transformation at temperatures where transformation rates woul
d otherwise be virtually zero. High transient differential stresses an
d rapid deformation accompany deep earthquakes. Therefore knowledge of
the kinetics of this transformation under these conditions is importa
nt for understanding the connection between phase transformation and d
eep earthquakes in subducting lithospheric slabs.