Abstract
The phase relations of iron-rich olivine and its high-pressure polymorphs are important for planetary science and meteoritics because these minerals are the main constituents of terrestrial mantles and meteorites. The olivine–ahrensite binary loop was previously determined by thermochemical calculations in combination with high-pressure experiments; however, the transition pressures contained significant uncertainties. Here we determined the binary loop of the olivine–ahrensite transition in the (Mg,Fe)2SiO4 system at 1740 K in the pressure range of 7.5–11.2 GPa using a multi-anvil apparatus with the pressure determined using in situ X-ray diffraction, compositional analysis of quenched run products, and thermochemical calculation. Based on the determined binary loop, a user-friendly software was developed to calculate pressure from the coexisting olivine and ahrensite compositions. The software is used to estimate the shock conditions of several L6-type chondrites. The obtained olivine–ahrensite phase relations can also be applied for precise in-house multi-anvil pressure calibration at high temperatures.
Highlights
Fe-rich olivine could be an important constituent of the mantles of oxidized terrestrial planets
The composition of more distant terrestrial planets can be estimated from the composition of CI chondrites (McDonough and Sun 1995), and assuming that all Fe is present in their mantles as F e2+, their upper mantles would be composed of olivine with Mg# = 0.55
We determined the binary loop of the olivine–ahrensite transition in the M g2SiO4–Fe2SiO4 system at a temperature of 1740 K using the multi-anvil technique with the pressure determined using in situ X-ray diffraction (XRD), compositional analysis of quenched run products by electron microprobe, and thermochemical calculation
Summary
Fe-rich olivine could be an important constituent of the mantles of oxidized terrestrial planets. The phase relations of olivine and its high-pressure polymorphs in intermediate to Fe-rich compositions are, significant for understanding the structure and dynamics of oxidized terrestrial planets. Olivine (α-(Mg,Fe)2SiO4) is well known to transform to a polymorph with the spinel structure (γ-(Mg,Fe)2SiO4) at high pressure (Ringwood 1958). This polymorph with Mg2SiO4 composition has been named ringwoodite (Binns et al 1969). Hereafter we will call the α- and γ-(Mg,Fe)2SiO4 with more or equal to 50 mol% Mg2SiO4 as forsterite and ringwoodite, respectively, whereas α- and γ-(Mg,Fe)2SiO4 with less than 50 mol% M g2SiO4 as fayalite and ahrensite, respectively Both ringwoodite and ahrensite in meteorites formed from the host olivine via a solid-state transformation
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