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The fate of volatiles in magma on Mercury

Periodic Reporting for period 1 - VOLATILES_MERCURY (The fate of volatiles in magma on Mercury)

Reporting period: 2019-08-01 to 2021-07-31

Mercury has been extensively characterized by the NASA MESSENGER spacecraft that was orbiting the planet from 2011 to 2015. The surface of Mercury is covered by lavas and magmatic processes have structured the planet into an extremely large core, a thin mantle and a relatively thick crust, which is highly reduced, iron-depleted, and rich in volatiles. Some volatiles (sulfur and carbon) have been measured at the surface in relatively high abundances compared to other terrestrial planets but the speciation, role and their fate under highly reducing conditions are still unclear. In this research program, the understanding of the evolution of volatiles in magma on Mercury has been partially advanced by combining MESSENGER data and laboratory experiments. Relevant data has been obtained from high-temperature and low- to high-pressure experiments using furnaces, presses and multi-anvil apparatus. We experimentally investigated compositions corresponding to the possible primitive mantle composition, and to surface compositions provided by X-Ray Spectroscopy data from MESSENGER. These recent data provide a unique opportunity to perform innovative experiments under the extremely reducing conditions characteristic for Mercury on sulfides liquid immiscibility, mantle partial melting, and speciation and solubility of volatiles in magmas. Our objectives provided new and firm constraints on (1) the P-T position of the solidus of the mantle of Mercury; (2) the speciation, role and fate of carbon and sulfur volatiles in magmas on Mercury and (3) the partitioning of trace elements between sulphide melts (FeS and (Ca,Mg,Fe)S) and silicate melt in the Mercurian mantle.
Task 1: Solidus of the primitive magma ocean (CMAS±S±Na) on the Mercury
24 successful high pressure-temperature experiments have been performed by gas-mixing furnace for 1 atm and piston-cylinder press for 0.5 – 4 GPa. The starting compositions are CaO-MgO-Al2O3-SiO2 + Na2O and CaO-MgO-Al2O3-SiO2 + S, and all experimental samples have been analysed by electron-microprobe (EMPA). Our current experimental results show that the Na2O or S can significantly decrease the solidus of Mercurian mantle over 80 C. This preliminary result significantly changed the dynamic numerical model of the evolution of Mercurian interior.

Task 2: Speciation, role and fate of volatiles (Sulfur and Carbon) on Mercury under highly reducing conditions
27 successful high pressure-temperature experiments focusing on S have been done by piston-cylinder press at the pressure of 1 – 4 GPa and the temperature of 1200 – 1600 C. The major and trace elements in our experimental samples have been analyzed by electron-microprobe (EMPA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), respectively. We did find the S-bearing phases (e.g. (Ca,Mg)S) at the extremely reduced condition, and the experimental temperature significantly affects the solubility of S in a mercurian conditions. Unfortunately, we have not done the Raman measurements for testing the speciation of S in our experiments or the role of C under highly reducing conditions, due to the influence of Covid pandemic.

Task 3: Distribution of trace elements (U, Th and K) in the mantle of Mercury
19 successful high pressure-temperature experiments have been performed by the piston-cylinder press at the pressure of 0.5 – 4 GPa and the temperature of 1200 – 1600 C. Some of experiments have been analysed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The other experiments have not been measured, due to the pandemic and the travel ban to Germany for LA-ICP-MS measurement. Although the current experimental data show a result consistent with what we expected, more experiments and data are needed to accomplish this task because the effects of S concentration and oxygen fugacity on partitioning of those elements are still unclear.
In this project, experiments on planetary materials were performed on compositions and under conditions that had never been investigated before. The new data provided by the MESSENGER spacecraft provide unique opportunities for significant breakthrough on Mercury. A major originality is to combine experimental petrology, geochemical analyses and spacecraft measurements. Specifically, this project is the first to attempt experiments aimed at understanding the evolution of volatiles in Mercury’s mantle by determining the solidus of the entire mantle, investigating the partitioning of radioactive elements between sulfides and silicate melt under extremely reduced conditions, and exploring whether a diamond layer might exist in the deep mantle. Experiments were performed at pressure and temperature conditions relevant to the entire Mercurian mantle. This is highly challenging and requires the development of new approaches and methods that are not commonly used in experimental petrology. The results of this project provided the following new insights into primitive silicate liquids depleted in FeO: 1) the solidus of the entire Mercurian mantle, 2) the speciation and solubility of sulfur and carbon, 3) the distributions of volatiles and radioactive elements in the mantle, and 4) the structure of the Mercurian deep mantle.

This project was rooted in igneous petrology and in the combination of experimental petrology, geochemical analyses, numerical and thermodynamic modelling, and spacecraft measurements of the surface of Mercury acquired by MESSENGER. The experimental strategy and the interpretations of data also strongly rely on geophysical measurements for the internal structure of Mercury. Our experiments can also be used for the calibration of the MERTIS instrument (Mercury Radiometer and Thermal Imaging Spectrometer) installed on the Mercury Planetary Orbiter of the BepiColombo spacecraft (to be launched to Mercury in October 2018). We discussed with the Principal Investigator Harald Hiesinger (University of Münster, Germany) and co-Principal Investigator Jörn Helbert (DLR Institut für Planetenforschung, Berlin, Germany) and shared our experiments' data that have relevant compositions and mineralogy for the surface of the planet. MERTIS is an infrared imaging spectrometer and these wavelengths have a high potential for mineral identification because it is in this region where the major rock-forming minerals have their fundamental vibration bands.
Plausible evolutionary pathways of Mercury, from the accretion to its present-day state