Shock compression of Fe‐Ni‐Si system to 280 GPa: Implications for the composition of the Earth's outer core

Abstract

The shock Hugoniot of an Fe-9 wt %Ni-10 wt %Si system as a model of the Earth's core has been measured up to~280 GPa using a two-stage light-gas gun. The samples had an average density of 6.853 (±0.036) g/cm 3 . The relationship between shock velocity (U s ) and particle velocity (u p ) can be described by U s (km/s) = 3.95 (±0.15) + 1.53 (±0.05) u p (km/s). The calculated Hugoniot temperatures and the melting curve indicate that the model composition melts above a shock pressure of~168 GPa, which is significantly lower than the shock-melting pressure of iron (~225 GPa). A comparison of the pressure-density (P-ρ) profiles between the model composition and the preliminary reference Earth model gives a silicon content close to 10 wt %, necessary to compensate the density deficit in the Earth's outer core from seismological observations, if silicon is present as a major light element in the Fe-Ni core system.

Equation of State for Shocked Fe‐8.6 wt% Si up to 240 GPa and 4,670 K

“…Open circles represent Fe‐9Si (this work). Green down triangles and open squares represent Fe‐9Ni‐10Si (Zhang et al, ) and Fe‐8Ni‐10Si (Zhang et al, ). Solid circles, open triangles, and open diamonds represent Fe‐4Si (Balchan & Cowan, ), Fe‐3.8Si (Marsh, ), and pure Fe (Brown et al, ).…”

“…The fitted result is slightly different from that of the D S -u relationship, but it is considered more reliable because the initial density used in equation (9) is normalized. In Figure 2b, we also plotted the P H -ρ data for iron (Brown et al, 2000), Fe-3.8Si (by weight, Marsh, 1980), Fe-4.0Si (by weight, Balchan & Cowan, 1966), Fe-9Ni-10Si (by weight, Zhang et al, 2014), and Fe-8Ni-10Si (by weight, Zhang et al, 2018) for comparison. The data of Fe-9Si are close to the data of Fe-8Ni-10Si (Zhang et al, 2018) but differ from their earlier data on Fe-9Ni-10Si (Zhang et al, 2014).…”

Limitations on silicon in the outer core: Ultrasonic measurements at high temperatures and high dK/dP values of Fe‐Ni‐Si liquids at high pressures

“…Notably, the adiabatic temperature increase across the outer core is expected to shallow the isothermal equation of state of silicon‐bearing liquids that we show in Figure , which will enhance the discrepancy between the pressure‐density slope of the Si‐bearing liquid and PREM. It is, however, critical to note that the thermal properties of iron‐silicon liquids at these conditions are not well constrained; assumed values of, for example, the logarithmic derivative of the Grüneisen parameter and electronic contributions to thermodynamic parameters, coupled with reliance on the behavior of solid materials, are, by necessity, deployed [ Zhang et al ., ; Fischer et al ., ]. Indeed, for shock measurements on the Fe‐Si system in particular [ Zhang et al ., ; Balchan and Cowan , ], the starting material is typically a two‐phase aggregate, and whether the high‐pressure shock data actually record the properties of a homogeneous Fe‐Si alloy, rather than simply a shocked, inhomogeneous multiphase material, is unclear; the timescale (and degree) of compositional homogenization relative to the timeframe of the shock experiment is poorly constrained.…”

“…It is, however, critical to note that the thermal properties of iron-silicon liquids at these conditions are not well constrained; assumed values of, for example, the logarithmic derivative of the Grüneisen parameter and electronic contributions to thermodynamic parameters, coupled with reliance on the behavior of solid materials, are, by necessity, deployed [Zhang et al, 2014;Fischer et al, 2014]. Indeed, for shock measurements on the Fe-Si system in particular [Zhang et al, 2014;Balchan and Cowan, 1966], the starting material is typically a two-phase aggregate, and whether the Figure 7. Equations of state of iron, iron-silicon, and iron-nickel-silicon alloys relative to PREM across the pressure range of the outer core.…”

“…These are isothermal curves, and hence, adiabatic curves would be anticipated to be shallower. Shock data from Zhang et al [2014] on Fe-9 wt %Ni-10 wt %Si are shown by the solid squares (the three highest pressure points are inferred to be in the liquid state); PREM [Dziewonski and Anderson, 1981] is the central solid line, closed circles are 300 K compression results on Fe-8.7 wt %Si [Hirao et al, 2004], and triangles are high-temperature data spanning betweeñ 2200 and 4000 K on Fe-9 wt %Si [Fischer et al, 2014]. The iron 300 K equation of state and Hugoniot are from Mao et al [1990] and Brown and McQueen [1986], respectively.…”