Abstract
Earth’s outer core is liquid and dominantly composed of iron and nickel (~5–10 wt%). Its density, however, is ~8% lower than that of liquid iron, and requires the presence of a significant amount of light element(s). A good way to specify the light element(s) is a direct comparison of density and sound velocity measurements between seismological data and those of possible candidate compositions at the core conditions. We report the sound velocity measurements of a model core composition in the Fe-Ni-Si system at the outer core conditions by shock-wave experiments. Combining with the previous studies, we found that the best estimate for the outer core’s light elements is ~6 wt% Si, ~2 wt% S, and possible ~1–2.5 wt% O. This composition satisfies the requirements imposed by seismology, geochemistry, and some models of the early core formation. This finding may help us to further constrain the thermal structure of the Earth and the models of Earth’s core formation.
Highlights
Earth’s outer core is liquid and dominantly composed of iron and nickel (~5–10 wt%)
Hypervelocity impact experiments were conducted to a pressure of ~280 GPa so as to obtain the sound velocity and melting behavior of Fe-9Ni-10Si system (81Fe9Ni10Si in weight per cent), which was proposed as a model composition of the outer core
Abrupt changes of the sound velocity for Fe-9Ni-10Si along the Hugoniot were found at shock pressures of 147 ± 5 GPa and 183 ± 5 GPa, that were interpreted as the onset and complete melting (Fig. 1), respectively
Summary
Earth’s outer core is liquid and dominantly composed of iron and nickel (~5–10 wt%). Its density, is ~8% lower than that of liquid iron, and requires the presence of a significant amount of light element(s). Combining with the previous studies, we found that the best estimate for the outer core’s light elements is ~6 wt% Si, ~2 wt% S, and possible ~1–2.5 wt% O This composition satisfies the requirements imposed by seismology, geochemistry, and some models of the early core formation. O may be the likely element that would be expelled from the recrystallized inner core and accumulated in the outer core due to its high incompatibility with solid iron inferred by ab initio calculations[12] This explanation is consistent with the observed density jump across the inner-core boundary. In order to identify the light element(s) of the outer core more tightly, the most conventional approach is a direct comparison of density and sound velocity for possible candidates at the core conditions with seismic data such as the preliminary reference Earth model (PREM)[16]. Density measurements for liquid Fe-Ni-S and Fe-Ni-Si17 have been performed up to 94 GPa in laser-heated diamond anvil cells, at higher pressures, only solid samples[18,19]
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