HYDROGEN FRACTIONATION DURING EARTH’S CORE FORMATION: PROGRESS ON HIGH-PRESSURE TEM EXPERIMENTS

Our recent model of planetary formation suggests that a small but significant fraction of Earth’s water likely originated from nebular hydrogen in the early solar system (Wu et al., JGR-Planets, 2018). A combination of ingassing and iron hydrogenation is hypothesized to have resulted in relatively low D/H values in early Earth’s magma ocean and core. Therefore, the average D/H value widely assumed for Earth, which is based on surface and mantle sampling, may be significantly inaccurate.

To test this hypothesis, we need an accurate knowledge of i) the solubility (x) of hydrogen in iron and ii) the D/H fractionation factor (a) for hydrogen dissolution in iron under magma-ocean conditions. Although both elements are abundant naturally and important industrially, neither reliable x nor a has been determined experimentally because iron hydrides, the product of hydrogen-iron reaction, are unstable at room conditions. To circumvent this problem, we designed experiments to make the measurements: 1) iron is encapsulated within multi-shelled, fullerene-like carbon nano-onions (Fe@CNOs); 2) these Fe@CNOs are irradiated by energetic electrons or ions to induce self-compression of the CNO walls, thus producing gigapascal internal pressures; 3) the self-compressed Fe@CNOs are then hydrogenated at high pressure and high temperature in a hydrogen-loaded, diamond anvil cell (DAC) coupled with laser heating; once the Fe@CNOs are recovered at room conditions from the DAC, the iron hydrides are expected to remain stabilized at the internal pressure of the enclosing CNOs; 4) heating within either the vacuum line of a high-sensitivity isotope-ratio mass spectrometer (IRMS) or a nano secondary-ion mass spectrometer (nano-SIMS) will be used to break the CNO walls and thus release the internal pressure, causing hydride decomposition and consequent hydrogen release into the spectrometer, where both the elemental and isotopic hydrogen compositions are to be measured. We will describe the status of the several experimental steps and their implications for the origin of water on Earth and other planets.