High temperature structural and thermoelastic behaviour of mantle orthopyroxene: an in situ neutron powder diffraction study

The temperature induced structural evolution and thermoelastic behaviour of a natural (Pbca) orthopyroxene (Opx), with chemical formula M2(Mg0.856Ca0.025Fe2+ 0.119) M1(Mg0.957Fe2+ 0.011Fe3+ 0.016 Cr0.011Al0.005)Al0.032Si1.968O6, from a suite of high pressure ultramafic nodules of mantle origin, have been investigated by in-situ neutron powder diffraction at several temperatures starting from 1,200C down to 150C. Unit-cell parameter variations as a function of T show no phase transition within this temperature range. The volume thermal expansion coefficient, a = V–1(dV/dT)P0, varies linearly with T. The axial thermal expansion coefficients, aj = lj –1(dlj/dT)P0, increase non-linearly with T. The principal Lagrangian unit-strain coefficients (e//a, e//b, e//c), increase continuously with T. However, the orientation of the unit-strain ellipsoid appears to change with T. With decreasing T, the values of the unit-strain coefficients along the b and c axes tend to converge. The orientation at DeltaT = 1,080 C is maintained down to the lowest temperature (150 C). The two non-equivalent tetrahedral chains, TAnOA3n and TBnOB3n, are kinked differently. At room-T, the TBnOB3n chain is more strongly kinked by about 23° than the TAnOA3n chain. With increasing T, the difference decreases by 3° for the TBnOB3n chain. The intersite cation exchange reaction between M1 and M2 (Mg2+ and Fe2+) shows a slight residual order at 1,200 C followed by reordering with decreasing temperature although seemingly not with a definite progressive trend. At the lowest temperature reached (150 C), reordering has occurred with the same value of partitioning coefficient KD as that before heating. The absence of the expected phase transition is most likely due to the presence of minor amounts of Fe3+, Al, Ca and Cr which must play a crucial role on the thermoelastic behaviour and phase stability fields in natural Opx, with consequent important petrologic and geological implications.