Fig. 3.

Summary of high-pressure ⁵⁷Fe Mössbauer spectroscopy results, at 6–300 K, on (Mg,Fe)O magnesiowüstite, which along with silicate perovskite is thought to make up the bulk of the Earth's lower mantle (>60% of the planetary interior on a molar basis; refs. 7 and 8). At high pressures and low temperatures, magnesiowüstite is antiferromagnetic (blue lines), with a site-magnetization that increases under pressure. A new signal indicative of a nonmagnetic (diamagnetic, black lines) site appears abruptly at pressures above 30–90 GPa, depending on composition, and is interpreted as the onset of the high- to low-spin transition (gray surface). For mantle compositions, with bulk Mg/(Mg+Fe) ratio as high as x = 0.9, the spin transition occurs at pressures as low as 30–40 GPa, corresponding to the shallowest part of the lower mantle. This figure illustrates only a subset of pressure–temperature paths explored for various compositions (Table 1), with spectra also being collected as a function of temperature at constant (high) pressure (see also ref. 4). We have not determined the Néel (light blue surface) and spin transitions for compositions with higher Mg content than x = 0.8; the former presumably exhibits a critical line beyond which antiferromagnetic ordering is impossible because of the low concentration of Fe, whereas the spin transition would be expected to take place even for trace-impurity levels of ferrous iron in MgO.