Understanding noble gas incorporation in mantle minerals: An atomistic study

Ab initio calculations in forsterite (Mg2SiO4) are used to gain insight into the formation of point defects and incorporation ofnoble gases. We calculate the enthalpies of incorporation both at pre-existing vacancies in symmetrically non-equivalent sites,and at interstitial positions. At high pressure, most structural changes affect the (MgO6) units and the enthalpies of point defectsincrease, with those involving Mg and Si vacancies increasing more than those involving O sites. At 15 GPa Si vacanciesand Mg interstitials have become the predominant intrinsic defects. We use these calculated enthalpies to estimate the totaluptake of noble gases into the bulk crystal as a function of temperature and pressure both in the presence and absence ofother heterovalent trace elements. For He and Ne our calculated solubilities point to atoms occupying mainly interstitial sitesin agreement with previous experimental work. In contrast, Ar most likely substitutes for Mg due to its larger size and thedeformation it causes within the crystal. Incorporation energies, as well as atomic distances suggest that the incorporationmainly depend on the size mismatch between host and guest atoms. Polarization effects arising from the polarizability of thenoble gas atom or the presence of charged defects are minimal and do not contribute significantly to the uptake. Finally, thediscrepancies between our results and recent experiments suggest that there are other incorporation mechanisms such asadsorption at internal and external interfaces, voids and grain boundaries which must play a major role in noble gas storageand solubility.