Spin transition of Fe3+ in Al-bearing phase D: An alternative explanation for small-scale seismic scatterers in the mid-lower mantle

Among dense-hydrous magnesium silicates potentially transporting H2O into Earthʼs deep interior, phase D (MgSi2H2O6) exhibits the highest P–T stability range, extending into the lower mantle along cold slab geotherms. We have studied the compressibility and spin state of Fe in Al-bearing phase D up to 90 GPa using synchrotron X-ray diffraction and X-ray emission spectroscopy. Fe–Al-bearing phase D was synthesized at 25 GPa and 1400 °C with approximate composition MgSi1.5Fe0.15Al0.32H2.6O6, where nearly all of the Fe is ferric (Fe3+). Analysis of Fe-Kβ emission spectra reveals a gradual, pressure-induced high-spin (HS) to low-spin (LS) transition of Fe3+ extending from 40 to 65 GPa. The fitted equation of state for high-spin Fe–Al-bearing phase D results in a bulk modulus K T 0 = 147 ( 2 ) GPa with pressure derivative K ′ = 6.3 ( 3 ) . An equation of state over the entire pressure range was calculated using the observed variation in low-spin fraction with pressure and a low-spin bulk modulus of K T 0 = 253 ( 30 ) GPa , derived from the data above 65 GPa. Pronounced softening in the bulk modulus occurs during the spin transition, reaching a minimum at 50 GPa (∼1500 km) where the bulk modulus of Fe–Al phase D is about 35% lower than Fe–Al-bearing silicate perovskite. Recovery of the bulk modulus at 50–65 GPa results in a structure that has a similar incompressibility as silicate perovskite above 65 GPa. Similarly, the bulk sound velocity of Fe–Al phase D reaches a minimum at ∼50 GPa, being about 10% slower than silicate perovskite. The potential association of Fe–Al phase D with subducted slabs entering the lower mantle, along with its elastic properties through the Fe3+ spin transition predicted at 1200–1800 km, suggests that phase D may provide an alternative explanation for small-scale mid-lower mantle seismic scatterers and supports the presence of deeply recycled sediments in the lower mantle.