Melting at the base of a terrestrial magma ocean controlled by oxygen fugacity

During accretion, conversion of kinetic energy from impacts to heat is thought to have led to extensive melting of early Earth’s silicate mantle, resulting in deep magma oceans covering (parts of) the surface. To assess the effect of oxygen fugacity on conditions at the bottom of such oceans, a research team led by Dr. Yanhao Lin from HPSTAR used lab techniques to mimic the extreme conditions of deep Earth at high oxygen fugacity and experimentally determined the solidus of mantle pyrolite at pressures of 16–26 gigapascals (GPa) at high oxygen fugacities. Across this pressure range, the solidus in experiments at oxidising conditions is found to be at least 230–450 °C lower than at more reducing conditions, which suggests that for a given magma ocean temperature, the ocean floor deepens by ~60 km for each log unit increase in mantle oxygen fugacity. This finding published in Nature Geoscience has major consequences for core formation and thermal evolution models of the early Earth, which invoke multiple log unit variations in mantle oxygen fugacity during Earth evolution due to temporal variations in the redox state of impactors and progressive lower mantle self-oxidation. This work provides a new perspective for our understanding of the evolution of early Earth’s magma ocean.  

The concept of deep magma oceans covering parts of the Earth during its earliest history is well established. One major problem with magma ocean formation models is that experimental data on the solidus of deep primitive mantle materials have not converged to accepted values to date. It is now generally accepted that the oxygen fugacity in Earth’s mantle varied significantly during accretion and core formation as the Earth was forming, and subsequently during mantle evolution. Here, Lin and his colleagues quantify the effect of fO2 on the solidus of a primitive mantle composition, pyrolite at mantle transition zone pressures to constrain the conditions at the floor of a deep terrestrial magma ocean by melting pyrolite at pressures equivalent to mantle depths between ~470 km and ~720 km at oxidizing conditions.  

Their results indicate that the oxygen fugacity induced shift of the pyrolite solidus is significant from the mantle transition zone to the top of the lower mantle, and that the increase of the solidus over a decrease of 3.2 log fO2 units fO2 is at least 340 ± 110 °C on average over this depth range. This is equivalent to moving the floor of a magma ocean at a given temperature down by ~200 km over this fO2 range.

Caption: The solidus temperature profile of primitive materials in the Earth’s mid-mantle pressure range at different oxygen fugacity.

The solidus of the early Earth’s mantle at a pressure of 40 GPa and an fO2 of IW-4 is calculated to be ~3210 °C, >500 °C higher than the average temperature at the bottom of a magma ocean at this depth of ~2700 °C used by Wade and Wood in their reduced-start core formation model. Given the very strong effect of oxygen fugacity on high-pressure mantle melting, models of core formation and the thermal evolution of the early Earth need to be re-evaluated. At a minimum, geochemical models in which oxygen fugacity variations during core formation are assessed should take into account the large variations in mantle melting temperatures that accompany such variations. Meanwhile, their results could also provide an explanation for the apparent discrepancy between the modeled low oxygen fugacities predicted for the Earth’s deep mantle after completion of core formation (~IW-2) and the high oxygen fugacities observed in Archaean (>3.0 Ga) magmatic rocks formed by deep mantle melting.

Caption: Comparison between temporal evolutions of Earth’s mantle temperature and the mantle solidus at pressures of (a) 16 GPa (470 km depth), (b) 21 GPa (580 km depth), (c) 26 GPa (710 km depth).

“We note that if the solidus shifts identified here persists to lower pressures, even minor variations in oxygen fugacity could lead to significant variations in magma production in the Earth as well as in the mantles of other rocky planets,” said Dr. Lin “Magma generation could occur by raising oxygen fugacity without increasing the water and/or CO2 content of mantle sources, and without raising the mantle temperature, and magma production in oxidized mantles (e.g. in Mars) could be higher than in more reduced mantles of similar composition.”

近日，北京高压科学研究中心的林彦蒿研究员和毛河光院士及其国际合作者在地球深部’富氧易熔’这一原创研究领域取得了重要新进展。地球早期的深层岩浆海洋被普遍认为是由增生撞击产生的热量引发的早期地球硅酸盐地幔的大规模融化所致。随着地球岩浆洋的演化，不断添加的氧化撞击体和下地幔自氧化作用促使了地幔的氧逸度提高，但氧逸度对深部地幔岩石的固相线的影响方面的研究仍是空白区。本研究通过高温高压实验限定了在470–710千米深以及相对高的氧逸度条件下地幔岩的固相线，并推算出了早期地球内部氧逸度与地球岩浆洋底部温度的关系。这一研究成果成功地将‘富氧易熔’原创理论应用到了地球深部，并为探讨早期地球演化条件及状态等前沿的地学科学问题提供了重要的实验约束。此研究以"Melting at the base of a terrestrial magma ocean controlled by oxygen fugacity "为题，发表于《Nature Geoscience》

论文链接https://doi.org/10.1038/s41561-024-01495-1。