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 Earth's 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 MgSi_1.5Fe_0.15Al_0.32H_2.6O₆, 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 _KT0 = 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 _KT0 = 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.