Static compression of iron to 300 GPa and Fe_0.8Ni_0.2 alloy to 260 GPa: Implications for composition of the core

Abstract

We report the results of X ray diffraction experiments with the diamond anvil cell to pressures above 300 GPa at room temperature on pure iron and an iron‐nickel alloy. These data extend throughout the pressure range of the bulk of the outer core of the Earth and provide for the first time direct pressure‐volume measurements on geophysically important materials at such conditions. Both iron and iron‐nickel are observed to remain in the hexagonal close‐packed structure to the maximum pressures. A combined fit to all recent compression data up to 300 GPa gives the following Birch‐Murnaghan equation‐of‐state (EOS) parameters for iron: V02 = 6.73(1) cm3 mol−1, K02 = 165(4) GPa, and K′02 = 5.33(9). (Value in parentheses refers to the uncertainty of the last digit; e.g., 6.73(1) refers to 6.73+0.01.). Similar parameters are obtained with a recent “universal” form of the EOS of solids. For an Fe0.8 Ni0.2 alloy, the equation‐of‐state parameters are nearly identical, within error: V02 = 6.737(5) cm3 mol−1, K02 = 172(2) GPa, and K′02 = 4.95(9). In terms of volume, the alloy equation‐of‐state is indistinguishable from that of pure iron and the densities differ (dominantly in proportion to their atomic weights) by ∼0.3 Mg m−3 at 330 GPa. Within the range of uncertainty in Earth model densities and trade‐offs with the percentage light component in the core, nickel could be present in the core in an amount at least equal to its estimated abundance in the Earth. A direct comparison with (solid) inner core densities is now possible and places direct constraints on the thermal models of the Earth's interior.

Iron sound velocities in shock wave experiments

“…At ambient pressure and up to 300 GPa, iron is known to be in the ε hcp phase. [7] Whether this ε hcp phase remains stable up to the melt line [6] or transforms to the β phase [4,8] can not be resolved from our data.…”

“…[2][3][4][5][6][7][8] Differing melting criteria and experimental techniques may have contributed to these apparent disparities.…”

“…Over the past twenty years or more, there have been many experimental and theoretical efforts to determine the melting line as well as the high pressure-high temperature phases of iron. [1][2][3][4][5][6][7][8][9][10][11][12][13] Such a phase diagram has long been sought after as a means to understanding the dynamics deep inside the Earth. These efforts have yet to produce an agreed-upon phase diagram for iron.…”

Thermal expansion of mantle and core materials at very high pressures

“…If the average temperature of the inner core is 6750 (+ 1000) K as inferred from shock and static Fe melting data [Williams et al, 1987], then oc and p of e-Fe at inner core conditions, using (2), (6), and the data of Mao et al [1990], are 6 x 10 -6 K -1 and 13.45 +0.21 g/cm 3, compared with the average inner core density from normal mode data of 12.85 + 0.15 g/cm 3 [Shearer and Masters, 1990]. This calculation ignores the small temperature-dependence of oc at high pressure and assumes that q = 1 is appropriate for e-Fe [Mao et al, 1990]. This result shows that the inner core is less dense than pure iron, in agreement with previous analyses [Jephcoat and Olson, 1987], and the density deficit of the inner core is 5 + 2%.…”

High Brilliance and High Pressure: A New Diamond Anvil Cell Facility at the Advanced Photon Source.

“…Although ED is limited by the intrinsically low energy resolution of the solid state detector (∆E/E ~1% FWHM), the peak positions are typically determined to an accuracy of ∆d/d<0.1%. Therefore, ED has been successfully used for determination of equations of state of known structures [4,5]. The peak broadness is a drawback for resolving overlapping peaks in polycrystalline diffraction.…”