Xenon and iodine behaviour in magmas

Abstract

Iodine (I) and xenon (Xe) are two key elements that trace Earth's differentiation (e.g. atmosphere formation) and dynamics (e.g. volcanism and recycling at subduction zones). Iodine and Xe abundances are linked through the decay of the extinct 129 I that produced 129 Xe, which is today depleted in the Earth's atmosphere compared to the composition of the solar system (i.e. chondrites). Iodine and Xe cycles and storage in the deep Earth are almost unknown, which is in large part due to the fact that their behaviour in magmas and fluids, key agents of mass transfer through planetary enveloppes, are poorly known. Here, the solubility of Xe and I in melts is measured under high pressure (P) and temperature (T) conditions using large volume presses, and Xe and I behaviour in melts and fluids is monitored in situ under high P-T conditions using resistive heating diamond anvil cells combined with synchrotron x-ray fluorescence (XRF) and Raman spectroscopy. Xenon, I and H (H2O) contents were measured in quenched glasses by particle x-ray Emission (PIXE) and Elastic Recoil Detection Analysis (ERDA). Solubility, speciation and degassing processes are investigated for two different compositions: haplogranitic melt (HPG analogue for crustal melts) and basaltic melts (MORB and IAB). Experimentally measured solubilities for both elements are much higher than their natural abundances in terrestrial magmas. Xenon solubility at 3.5 GPa reaches 4.00 wt.% in HPG and 0.40 wt.% in basalts. Iodine solubility is 0.46 wt.% at 0.4 GPa on average in HPG, and reaches 1.42 wt.% in basalts at 2 GPa. The in situ Raman spectroscopic study shows that I forms I-I bonds in hydrous high P fluids/melts unlike Xe that was previously shown to oxidize in high P melts. The XRF monitoring of I and Xe partitioning between aqueous fluids and silicate melts during decompression (i.e. water degassing) shows that Xe degassing is strongly P-T dependent and can be retained in the melt at deep crust conditions,, while I is totally washed out from the silicate melt by the aqueous phase. Xenon and I degassing processes are based on different mechanisms, which implies that the atmospheric isotopic signature of Xe cannot be inherited from a process involving volcanic water degassing. Instead, 129 Xe depletion may originate from a separation of both elements at depth, by deep fluids, a proposition that agrees with a deep storage of Xe in minerals.

Iodine local environment in high pressure borosilicate glasses: An X-ray photoelectron spectroscopy and X-ray absorption spectroscopy investigation

“…Cicconi et al [14] dissolved ~4 mol.% I in B 2 O 3 -rich glass compositions held at pressure of 150 MPa; however, their investigated compositions (~60 wt.% B 2 O 3 ) were less relevant for the I immobilization in nuclear waste matrices. A comparable behaviour was also observed as a function of pressure for I in boron-free silicate glasses compositions with applications to Earth Sciences [16,17]. Leroy et al [17] showed that I solubility can reach up to 1.5 mol.% in Si-poor H 2 Orich silicate glasses synthesised at 3.5 GPa.…”

“…A comparable behaviour was also observed as a function of pressure for I in boron-free silicate glasses compositions with applications to Earth Sciences [16,17]. Leroy et al [17] showed that I solubility can reach up to 1.5 mol.% in Si-poor H 2 Orich silicate glasses synthesised at 3.5 GPa.…”

“…If it is currently observed for ISG glass samples, it is the opposite of NH glass samples: BO 4 increases with increasing I content and BO 3 decreases. In any case, further experimental work is currently required using 17 O NMR to determine the nature of the oxygen involved in the dissolution mechanisms of iodine as Ispecies.…”

Predicting iodine solubility at high pressure in borosilicate nuclear waste glasses using optical basicity: an experimental study

“…More recently, Morizet et al [33] showed that the iodine solubility could be further increased by using highly oxidizing conditions promoting the formation of iodate species (I 5+ ) within the glass structure. Regardless, the strong increase in the iodine solubility is ascribed to the increase in thermodynamic activity of the fluidic iodine in equilibrium with the melt phase upon pressure [30,34,35]. Although, this represents a major breakthrough in the race for immobilizing efficiently iodine radioisotopes, we still lack a clear fundamental understanding on how iodine behave as a function of glass composition.…”

The influence of iodide on glass transition temperature of high‐pressure nuclear waste glasses

“…There has been a growing interest in the incorporation of Iodine (I) in glasses in the past few years, especially in the field of Earth sciences 1,2 and in the nuclear waste glasses community. [3][4][5][6][7][8] I has an active radioisotope ( 129 I) that is produced from the nuclear fission of actinides.…”