NAL phase in K-rich portions of the lower mantle

The stability of the K-rich new aluminous (NAL) phase was examined on the join Na_1.00Mg_2.00Al_4.80Si_1.15O ₁₂-K_1.00Mg_2.00Al_4.80Si _1.15O₁₂ (Na100-K100) up to 144 GPa by X-ray diffraction in a laser-heated diamond anvil cell. Single-phase K100 and Na50K50 NAL were formed up to the lower mantle conditions, and the NAL phase coexisted with the calcium ferrite-type (CF) phase at 120 GPa and 2300 K for the Na75K25 bulk composition. This is a striking contrast to the K-free (Na100) NAL that becomes unstable above 27 GPa at 1850 K, which suggests that potassium stabilizes NAL at significantly higher pressures. K-rich NAL may host potassium in the lower mantle that contains K₂O more than 0.09 wt %. In addition, the NAL phase likely formed owing to partial melting in the ultralow-velocity zone or because of a basal magma ocean. Future seismological observations may clarify whether NAL is a radiogenic heat source above the core-mantle boundary. Key Points Potassium expands the stability of the NAL phase dramatically K-rich NAL is stable o the lowermost mantle conditions NAL forms in the whole lower mantle where the K2O content is higher than 900 ppm