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

Abstract

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+ 0.016Cr0.011Al0.005)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 1,200°C down to 150°C. Unit-cell parameter variations as a function of T show no phase transition within this temperature range. The volume thermal expansion coefficient, α = V −1(∂V/∂T) P0, varies linearly with T. The axial thermal expansion coefficients, αj = l j −1 (∂l j/∂T)P0, increase non-linearly with T. The principal Lagrangian unit-strain coefficients (ɛ//a, ɛ//b, ɛ//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 ΔT = 1,080°C is maintained down to the lowest temperature (150°C). The two non-equivalent tetrahedral chains, TA n OA3n and TB n OB3n , are kinked differently. At room-T, the TB n OB3n chain is more strongly kinked by about 23° than the TA n OA3n chain. With increasing T, the difference decreases by 3° for the TB n OB3n 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 K D 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.