Reversals of the orthoenstatite‐clinoenstatite transition at high pressures and high temperatures

Abstract

The MgSiO3 orthoenstatite‐clinoenstatite (OEn‐CEn) phase boundary has been reversed between pressures of 70 and 110 kbar and temperatures of 900°C and 1700°C with a split‐sphere anvil apparatus (USSA‐2000). Starting materials contained PbO‐PbF2 (1∶1) flux to promote equilibration of the charges and eliminate a potential effect of deviatoric stresses on the phase boundary. The phase boundary separating orthoenstatite from clinoenstatite at high pressures and temperatures can be described by the equation P (kbar) = 0.031 T (°C)+50. Our results are consistent with previous unreversed determinations of the boundary at these high‐pressure, high‐temperature conditions. However, the dP/dT slope determined in the present study is much smaller than that implied by earlier experimental studies of the orthoenstatite/low clinoenstatite boundary at low pressures and temperatures. We propose that the clinoenstatite observed at high pressures and temperatures is a new high‐pressure clinoenstatite phase which is possibly an analogue of MgGeO3 clinopyroxene.

Core Sizes and Internal Structure of Earth's and Jupiter's Satellites

“…The EOS fitted parameters, H o f and S o for orthoenstatite and low-pressure clinoenstatite are from Fabrichnaya and Kuskov (1994), Skinner (1966), Pacalo and Gasparik (1990), Angel and Hugh-Jones (1994), Yang and Ghose (1994), Hugh-Jones (1997), Jackson et al (1999), and Shinmei et al (1999). Using K S = 107.7 GPa and K = 5 for orthoenstatite, the difference between experimental (Angel and Hugh-Jones 1994) and calculated volumes for orthoenstatite at pressures up to 8.5 GPa is no more than 0.1-0.3%; see also Jackson et al (1999).…”

The elastic constants of an aluminous orthopyroxene to 12.5 GPa

“…In addition, the chemistry of coexisting phases is dependent on pressure and temperature. Pacalo and Gasparik [1990] and Kanzaki [1991] have shown that pure enstatite transforms from the orthorhombic structure to the high clinoenstatite structure near 8 GPa and 1000øC. At still higher pressure, the components of pyroxene dissolve into the majorite garnet phase.…”

Determining the Origin of Ultrahigh-Pressure Lherzolites

“…The bulk TiO 2 contents of individual olivine grains from both localities were analyzed with a comprehensive grid of 5-, 10-, and 20-m diameter electron beams at 15 kV accelerating voltage, 15 to 100 nA sample current, and counting times of 200 to 2000 seconds (Table 1). All analyzed grains yielded bulk TiO 2 contents of hundreds of parts per million (15), including three inclusion-rich grains, "ol-1," "ol-2," and "ol-3" from sample 3-73-AA, loaned to us by L. Dobrzhinetskaya. To pursue this factor of 10 or more disparity between our measured TiO 2 contents and their calculated TiO 2 contents, we isolated an area of 225 ϫ 575 m from grain "ol-1" to analyze in greater detail (Fig.…”

“…To our knowledge, this is the only occurrence of clinoenstatite in diopside recorded. A possible explanation would be exsolution of high-pressure clinoenstatite from diopside at very high pressures [P Ͼ 8 GPa for a normal mantle geotherm (15)] and inhibition of transformation to orthoenstatite during subsequent pressure reduction because of sluggish nucleation kinetics or coherency stresses between the lamellae and diopside host. The high-pressure polymorph of clinoenstatite cannot be preserved because at lower pressures it undergoes a nonquenchable second-order transformation to the low pressure form (16).…”