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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - NEWDIA4Planet (Development of the new internally-heated diamond-anvil cell for planetary mineral physics: Application to high-pressure melting of H2O ice)

Teaser

Summary

The front-line discoveries on high pressure mineralogy of planetary materials coincide with development of high pressure (P) and temperature (T) technology. Diamond anvil cell (DAC) experiment is a major approach to generate static P-T conditions up to that of Earthâ€™s inner core. In particular, laser-heated DAC technique has been used to make the important mineralogical discoveries of planetary interiors including the Earthâ€™s. However, there is a critical drawback of the laser heated DAC, involving a large temperature uncertainty due to laser fluctuation and large temperature gradients in the sample (ca.Â±10%). Precise experimental temperature determination is crucial to implications for planetary interior models; e.g. the location of the phase transition is a key to understand the origin of the seismic discontinuity; high-pressure melting temperatures of planetary materials place important constraints on the temperature of deep planetary interiors. Therefore, the Fellow aimed to provide a new heating system for the DAC, which offers stable and homogeneous heating to a variety of samples under wide pressure and temperature conditions.
The basic design of the new heating system was adopted from a so-called internally heated DAC (IHDAC). The IHDAC is the most advanced high P-T generating system, in which a metallic foil inside a sample chamber is heated through supplied electricity. The internal heating benefits from steady and uniform heating due to resistive heating, resulting in smaller temperature uncertainty than laser heating (ca.Â±5%). The Fellow improved the IHDAC by developing a micron-sized heater made of chemically inert metals that heat up an adjacent sample. This was significant improvement to make the new IHDAC highly versatile i.e. applicable to diverse samples, because the conventional IHDAC can heat only metals. This was due to the fact that the sample also served as a heater in the conventional system to allow passage of enough electricity necessary for resistive heating.
Another important aspect of the new IHDAC is its applicability to H2O. H2O is a major constituent of planetary bodies in the outer solar system, such as Uranus and Neptune. However, the high-pressure melting temperatures of H2O differs significantly between previous studies (Â±300 degC at P = 40 GPa), hence there is no clear-cut answer to a basic question whether H2O is in liquid or solid state in these planets. Therefore, as the second part of this project, the Fellow re-examined the high-pressure melting curve of H2O using the newly developed IHDAC. H2O, or water, is particularly challenging material for the DAC experiments because of its liquid state and high chemical activity at the ambient condition. In addition to the inert metal heater, the Fellow employed a unique sample-loading method for water using liquid nitrogen. This further delivered variations to the sample heatable in the IHDAC. By using the newly developed IHDAC system, the high-pressure melting temperature of H2O was measured up to P = 44 GPa in this project.

Work performed

The new IHDAC developed in this project was proven to be suitable for studying high-pressure mineral physics of non-metallic planetary materials such as H2O, Al2O3, and Mg2SiO4. High-P-T experiments using the new IHDAC demonstrated that it can generate high-P-T conditions at least to P = 44 GPa and T = 1833 K, which corresponds to the P-T condition in Earthâ€™s mid-lower mantle. Using the new IHDAC, the melting temperatures of H2O at P = 42-44 GPa were determined to be T = 1493-1500 K, which was consistent with one of the highest estimates in the previously reported melting curves.

The summary of the exploitable results are as follow.
â€¢ Experimental methods developed for the new IHDAC
â€¢ High-pressure melting temperatures of H2O determined using the new IHDAC

The followings are the summary of exploitation and dissemination activities carried out during the project.
â€¢ Presenting the projectâ€™s overview and results at a seminar
â€¢ Giving a poster presentation at an international conference
â€¢ Managing project website
â€¢ Demonstrating solidification and melting of H2O at high pressure during outreaching events and eplaining implication of high-pressure ice to planetary sciences
In addition to above activities, two research papers on the experimental methods of the new IHDAC and on the results of H2O melting experiments with its planetary implications are in preparation, intended for submission to international peer-reviewed journals.

Final results

Using the newly developed IHDAC, high-pressure melting temperatures of H2O were measured. Two independent melting experiments were performed using rhenium (Re) and platinum-iridium (Pt-Ir) alloy heaters at P = 42 and 44 GPa, respectively. As the sample in a DAC is extremely small (< 100 microns), a micron-sized heater was created from thin foils with initial thickness of 10-25 microns by compressing them until their thickness become less than 5 microns. The starting material was in liquid state (water), thus it required cryogenic loading method in order to suppress water flowing out of a sample chamber. By cooling the DAC below the freezing point of H2O, the sample was maintained inside the sample chamber even at high pressure. The pressure was then increased to corresponding pressures. The temperature was increased by supplying electric current to the heater from a DC power supply. The temperature was measured by using the spectroradiometric optics. The melting of H2O ice was identified based on Raman spectra taken before and after heating using high-resolution Raman microscope. Formation of ReO2 and solid O2 were observe in Raman spectra at similar P-T conditions (P =42 GPa, T = 1500 K & P = 44 GPa, T = 1493 K) in Re and Pt-Ir alloy heater experiments, respectively, suggesting the melting of H2O ice occurred at these conditions.
Uranus and Neptune, or so called â€œIce Giantsâ€, have non-dipolar, non-axisymmetric magnetic fields unlike that of the Earth which has an axial dipole. It has been predicted that a high-pressure phase(s) of H2O inside these planets contributes to the formation of such complex magnetic fields. Unravelling the dynamics of these outer planets will not only help us to understand the history of our solar system, but also place valuable constrain on the evolution of our planet Earth. As such, high-pressure phase diagram of H2O has been extensively studied by high-pressure research community since the discovery of high-pressure ice polymorphs in 1930â€™s. Melting temperature of H2O is one of the most prioritised properties being studied in the interest of seeking the answer to a fundamental question whether the interiors of Ice Giants are solid, melt, or partially molten. The question, however, was left unresolved due to a large temperature gap between the high-pressure melting curves of H2O in each of earlier studies. The melting temperatures measured in the present project are in good agreement with the highest estimates among the earlier studies. This supports the existence of solid H2O (ice) in Ice Giants, which could lead us to unlock the mystery of their complex magnetic fields.