Melting behavior of the lower-mantle ferropericlase across the spin crossover: Implication for the ultra-low velocity zones at the lowermost mantle

• Published in: Earth and planetary science letters Vol. 503; no. C; pp. 1 - 9
• Main Authors: Fu, Suyu, Yang, Jing, Zhang, Youjun, Liu, Jiachao, Greenberg, Eran, Prakapenka, Vitali B., Okuchi, Takuo, Lin, Jung-Fu
• Format: Journal Article
• Language: English
• Published: United States Elsevier B.V 01-12-2018; Elsevier
• Subjects: ferropericlase; lower mantle; melting behavior; spin crossover; lower mantle; spin crossover; ferropericlase; melting behavior
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2018.09.014

Preferential iron partitioning into melt during melting and crystallization of lower mantle minerals – bridgmanite and ferropericlase – can play a critical role in our understanding of the origin of the early Earth and its evolution to form chemically and seismically distinct regions in the present lowermost mantle. Of particular interest is the consequence of iron spin crossover in ferropericlase on the physical and chemical properties of the molten materials under relevant pressure–temperature (P–T) conditions of the lowermost mantle. However, the spin crossover in liquid (Mg, Fe)O and its effects on melting curves, iron partitioning, melt density – and thus the evolution of an early basal magma ocean – remain poorly studied. Here we conducted high P–T melting experiments on ferropericlase with a starting composition of (Mg0.86Fe0.14)O using synchrotron X-ray diffraction up to ∼120 GPa and ∼5400 K in laser-heated diamond anvil cells, together with chemical analyses on quenched samples using focused ion beam and energy dispersive spectroscopy technique. An ideal solid solution model could be satisfactorily used to fit the experimental data of the liquidus and solidus of (Mg, Fe)O for pure high-spin (HS, below ∼83 GPa), and low-spin (LS, above ∼120 GPa) states, respectively. The experimental solidus and liquidus at 99 GPa and ∼4000–5200 K strongly deviate from ideal solid solution behavior for pure HS and LS states alone, but can be qualitatively explained using a thermodynamics model for a mixture of HS and LS states across the spin crossover. We found that LS (Mg, Fe)O exhibits ∼6–8% lower solidus and liquidus temperature than its HS counterpart. Furthermore, our results show that iron preferentially partitions into melt within the spin crossover to generate iron-rich LS melt. Such iron-rich LS (Mg, Fe)O is ∼27(±5)% denser than materials expected for lowermost mantle and could potentially persist as residual melt in the lowermost mantle at the late stage of magma ocean crystallization. Modeled results indicate that the existence of the dense, iron-rich LS (Mg, Fe)O melt in the lowermost mantle could provide plausible explanations for characteristic seismological signatures of ultra-low velocity zones (ULVZs).
•We constructed temperature–composition phase diagram of (Mg, Fe)O up to ∼120 GPa.•We observed and modeled non-ideal melting of ferropericlase across the spin crossover.•The spin crossover in liquid (Mg, Fe)O was experimentally constrained the first time.•Significant iron partitions into the melt phase during the melting of ferropericlase.•The dense LS iron-rich (Mg, Fe)O melt in the lowermost mantle could explain ULVZs.