Photon Factory Activity Report 2019 #37 (2020)

NE1A/2019G003

High-pressure behavior of Ar
1

Shigeaki ONO1,*Research Institute for Marine Geodynamics, Japan Agency for Marine-Earth Science andTechnology (JAMSTEC), Yokosuka 237-0061, Japan

We have found that an abundance of solid argon (Ar) may exist in the Earth’s deep interior. This conclusion is based on
the results of numerical models, as well as high-temperature and high-pressure experiments, conducted on asupercomputer to reproduce the conditions of Ar in extreme environments. Ar, which is the third most abundant elementin the atmosphere after nitrogen and oxygen, is also found dissolved in seawater, from which it may be incorporated into
oceanic plates and descend into the Earth’s interior via subduction along ocean trenches.
1 Introduction
Noble gases are key tracers to understand the evolutionof the Earth because of their inert nature and isotope
variations. However, the mechanism of the recycling of
noble gases in the deep mantle is still an open question.
Recently, an experimental study demonstrated highsolubility of noble gases in amphibole. Amphibole iscommonly observed in the altered oceanic crust, with asignificant amount of noble gases measured in naturalrock samples. Amphibole has a mineralogical A-site,which is an energetically favourable position for noblegases. Lattice structures of some hydrous minerals, suchas serpentine and chlorite, are similar to the A-site inamphibole. This indicates that these hydrous minerals are
candidates for the host phases of noble gases. As thebreakdown of serpentine finishes at the upper part of theupper mantle (~ 200 km depth), it is difficult to transport
to great depths in the deep mantle.
2 Experiment
The high-pressure X-ray diffraction experiments
were performed using a diamond anvil cell high-pressure
apparatus. After reaching the required pressure, we perfor
med the laser-heated annealing, which reduced the diffent
ial stress on the compression. The high puarity Ar (purity= 99.999%) was used as a starting material, which was cr
yogenically loaded into the sample chamber. The sample
was probed using angle-dispersive X-ray diffraction, emp
loying the AR-NE1A synchrotron beamline at KEK, Japa
n. The angle-dispersive X-ray diffraction patterns were obtained on an image plate system (Rigaku R-AXIS, Japan).
In this study, we determined the pressure-volume relationof Ar at room temperature using diamond anvil cell experi
ments. We also performed the first-principles calculations
to investigate high-teperature bihaviours of Ar. The combi
nation of high-pressure experiments and first-principles calculations allows us to determine reliable physical properties, such as the EOS and melting temperature, over a wid
e range of pressures and temperatures.
3 Results and Discussion
Calculated results revealed that the melting temperatureof argon increases sharply as its pressure increases, indicating that argon descends into the Earth’s interior in a stable, solid condition (Fig. 1). As experimental results indicar
ed thtat solid argon is lighter in density than the minerals t
hat comprise the mantle, it is possible that solid argon acc
elerates mantle upwelling flow. Furthermore, solid argon
may stably exist in extreme environments, with temperatu
res of 2000ºC or higher and pressures of 100 GPa or more, and it is thus believed to occur throughout the entire lower mantle (~55% of the Earth’s total volume) [1]. Thesefindings provide insight into the mechanism of global arg
on cycling, which has previously remained enigmatic. Th
ese findings may lead to understand other noble gases, su
ch as helium, neon, and xenon, as well as volatiles, such as water and carbon dioxide, with the aim of understanding
the evolution and roles of volatile compounds throughout
Earth’s history.

Fig. 1: Fig. 1. Crystal structure of solid argon, which canremain stable under extreme conditions comparable tothose in Earth’s core [1].
AcknowledgementWe thank the PF staffs for their helps in carrying out
the experiments.
References[1] S. Ono (2020) Fate of subducted argon in the deep
mantle, Sci. Rep. 10, 1393 (2020).
* sono@jamstec.go.jp