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Polycrystalline samples of San Carlos olivine were deformed at high‐pressure (2.8–7.8 GPa), high‐temperature (1153 to 1670 K), and strain rates between 7.10−6 and 3.10−5 s−1, using the D‐DIA apparatus. Stress and strain were measured in situ using monochromatic X‐rays diffraction and imaging, respectively. Based on the evolution of lattice strains with total bulk strain and texture development, we identified three deformation regimes, one at confining pressures below 3–4 GPa, one above 4 GPa, both below 1600 K, and one involving growth of diffracting domains associated with mechanical softening above ∼1600 K. The softening is interpreted as enhanced grain boundary migration and recovery. Below 1600 K, elasto‐plastic self‐consistent analysis suggests that below 3–4 GPa, deformation in olivine occurs with large contribution from the so‐called “a‐slip” system [100](010). Above ∼4 GPa, the contribution of the a‐slip decreases relative to that of the “c‐slip” [001](010). This conclusion is further supported by texture refinements. Thus for polycrystalline olivine, the evolution in slip systems found by previous studies may be progressive, starting from as low as 3–4 GPa and up to 8 GPa. During such a gradual change, activation volumes measured on polycrystalline olivine cannot be linked to a particular slip system straightforwardly. The quest for “the” activation volume of olivine at high pressure should cease at the expense of detailed work on the flow mechanisms implied. Such evolution in slip systems should also affect the interpretation of seismic anisotropy data in terms of upper mantle flow between 120 and 300 km depth.

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[1] Multianvil experiments with long experimental durations have been made with the anorthite composition CaAl 2 Si 2 O 8 at pressure-temperature (P-T) conditions of 14-25 GPa and 1400-2400 C. At subsolidus conditions, these experiments demonstrated three phase assemblages, grossular (Gr) + kyanite (Ky) + stishovite (St) at $14 GPa, Gr + calcium-alumino-silicate phase (CAS) + St at $18 GPa, and CAS + CaSiO 3 -perovskite (CaPv) + St at above $20 GPa, which are related by the reactions Gr + Ky = CAS + St and Gr + St = CAS + CaPv. Following the method of Schreinemakers, we combined our data with the literature data to deduce a P-T phase diagram for a portion of the CaO-Al 2 O 3 -SiO 2 system at subsolidus conditions, which subsequently helped to solve some long-lasting discrepancies in the high-P behavior of the compositions of anorthite and grossular. The crystal chemistry of the CAS and CaPv solid solutions was examined, and new substitution mechanisms were firmly established. Along the solidus, the melting reaction at $14 GPa is peritectic while that at $22 GPa is eutectic. For both pressures, St is the first phase to melt out and the melt is generally andesitic. For the An composition, its density starts to be significantly higher than the density of pyrolite at $2.5 GPa, a much lower pressure than that for the Or, Ab or Qtz composition ($7.5-10 GPa), so that the An-enriched continental crust material should readily plunge into the upper mantle.Citation: Liu, X., H. Ohfuji, N. Nishiyama, Q. He, T. Sanehira, and T. Irifune (2012), High-P behavior of anorthite composition and some phase relations of the CaO-Al 2 O 3 -SiO 2 system to the lower mantle of the Earth, and their geophysical implications,

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