Abstract
The crystallization of the magma ocean resulted in the present layered structure of the Earth’s mantle. An open question is the electronic spin state of iron in bridgmanite (the most abundant mineral on Earth) crystallized from a deep magma ocean, which has been neglected in the crystallization history of the entire magma ocean. Here, we performed energy-domain synchrotron Mössbauer spectroscopy measurements on two bridgmanite samples synthesized at different pressures using the same starting material (Mg0.78Fe0.13Al0.11Si0.94O3). The obtained Mössbauer spectra showed no evidence of low-spin ferric iron (Fe3+) from the bridgmanite sample synthesized at relatively low pressure of 25 gigapascals, while that directly synthesized at a higher pressure of 80 gigapascals contained a relatively large amount. This difference ought to derive from the large kinetic barrier of Fe3+ rearranging from pseudo-dodecahedral to octahedral sites with the high-spin to low-spin transition in experiments. Our results indicate a certain amount of low-spin Fe3+ in the lower mantle bridgmanite crystallized from an ancient magma ocean. We therefore conclude that primordial bridgmanite with low-spin Fe3+ dominated the deeper part of an ancient lower mantle, which would contribute to lower mantle heterogeneity preservation and call for modification of the terrestrial mantle thermal evolution scenarios.
Highlights
The crystallization of the magma ocean resulted in the present layered structure of the Earth’s mantle
The Mössbauer spectrum of MAOS3265 bridgmanite at 14 GPa was well fitted by two Lorentz-type doublets, corresponding to F e2+ having higher center shift (CS) and quadrupole splitting (QS) values ( Fe2+ #1) than F e3+ (Fe3+ #1) (Fig. 2b)
Spectra at the higher pressures 50 and 80 GPa required another component of F e3+ #2 with a low CS (~ 0.05 mm/s) for satisfactory fitting, which is likely to be attributed to low spin (LS) F e3+ in the B site, as found in previous s tudies[21,38,39] (Figs. 2c,d and 3)
Summary
The crystallization of the magma ocean resulted in the present layered structure of the Earth’s mantle. The obtained Mössbauer spectra showed no evidence of low-spin ferric iron (Fe3+) from the bridgmanite sample synthesized at relatively low pressure of 25 gigapascals, while that directly synthesized at a higher pressure of 80 gigapascals contained a relatively large amount This difference ought to derive from the large kinetic barrier of Fe3+ rearranging from pseudo-dodecahedral to octahedral sites with the high-spin to low-spin transition in experiments. Whether iron and aluminium (Fe,Al)-bearing bridgmanite undergoes a spin transition in the lower mantle has remained elusive for decades Those synthesized at relatively low pressure in a multi-anvil apparatus (MA) show no evidence of the spin transition[13,14,15,16,17,18], implying a high-spin Fe throughout the lower mantle, which is consistent with theoretical studies[31,32,33,34]. Described below, which is suggested to be a plausible process for bridgmanite containing low spin (LS) F e3+ to be generated from initial high spin (HS)[20]
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