High-Pressure Melting Curve of FeH: Implications for Eutectic Melting
between Fe and Non-Magnetic FeH
Abstract
While hydrogen could be an important light alloying element in planetary
iron cores, phase relations in the Fe-FeH system remain largely unknown
at high pressures and temperatures (P-T). A speculative
Fe-H2 phase diagram has been proposed assuming
continuous solid solution between Fe and FeH and eutectic melting
between FeH and H2. Recent studies revealed that
stoichiometric FeH becomes non-magnetic above ~40 GPa,
which might affect its melting behavior. Here we examined the melting
curve of non-magnetic FeH between 43 and 152 GPa by a combination of
laser-heated diamond-anvil cell (DAC) techniques and synchrotron X-ray
diffraction (XRD) analyses. The melting temperature was determined by
employing the appearance of additional hazy XRD signals upon quenching
temperature as a melting criterion. We also performed thermodynamic
modeling, which well reproduces the change in the curvature of FeH
melting curve upon the loss of magnetism and extrapolates the
experimental constraints to inner core pressures. The XRD data showed
that non-magnetic FeH melts congruently at temperatures higher than the
known eutectic melting curve for FeHx (x
> 1). Combined with the fact that the endmembers exhibit
different crystal structures, these results indicate that Fe and
non-magnetic FeH form a eutectic system. The dT/dP slope
of the FeH melting curve is comparable to that for Fe, suggesting that
the eutectic liquid composition of FeH0.42 (Fe + 0.75
wt% H) previously estimated at ~40 GPa changes little
with increasing pressure.