Negatively charged ions in the deep Earth : quantifying the chemical speciation of F and N in silicate melts and phases

Nitrogen and fluorine are essential volatile elements to study in the bulk silicate Earth (BSE) due to their respective influence over the onset of habitability on Earth or over physical and chemical properties of the phases which contain them. Nitrogen is abnormally depleted in the BSE relative to CI chondrite, while fluorine is abnormally enriched, hence questions arise about their chemical speciation and storage mechanisms within planetary reservoirs. The speciation of nitrogen in high pressure silicate melts was studied using solid-state nuclear magnetic resonance (NMR) and Raman spectroscopy and was found to be heavily influenced by oxygen fugacity. The former technique provided superior results in quantifying the abundance of individual species and higher sensitivity, however it requires 15N enrichment, low-Fe samples and performs only bulk analysis, while the latter was found to be an efficient in situ technique regardless of sample composition, but it struggled at detecting low-abundance N species. The speciation of fluorine in silicate melts was studied via NMR spectroscopy at BSE-like concentrations, thanks to this techniques high F sensitivity. F was found to be binding solely with Mg at atmospheric pressure, and this speciation remains predominant until c.a. 8 GPa, where relevant quantities of F start binding with Ca. This is likely due to the changing coordination number of the major components of the melt, but a change in the coordination of fluorine itself might also be occurring. The ordering of fluoride and hydroxide in the framework of humite group minerals was studied via NMR spectroscopy and computational modelling. The incorporation of one F and one OH anion in neighbouring sites was found to be favoured relative to the incorporation of two identical ions, thanks to the formation of a hydrogen bond. This likely explains the extended stability field of clinohumite when it is F rich

Petrology and geochronology of the Loro Intrusive Complex (Ivrea-Verbano Zone)

The Loro Intrusive Complex is located in Valle d’Ossola (southern Alps) and is part of the Ivrea-Verbano Zone. Literature data on this igneous complex are relatively scarce (Boriani, 1966) and a detailed geochemical and geochronological study is missing. Therefore, for its location between the northern and the southern Ivrea domains, thispoorly studied igneous complex is important to correctly understand the structure and igneous evolution of the whole Ivrea-Verbano Zone.The Loro Intrusive Complex crops out along the Canavese Line and is in contact with the “Scisti di Fobello e Rimella” to the west and with mafic and felsic granulites of the Kinzigite formation to the east. Within the whole complex slices of basement rocks including milonitic granulites, marbles and serpentinites are locally found. Dioritesand hornblendites are the main lithologies of the Loro Intrusive Complex. Diorites are relatively fine-grained and consist of pargasitic amphibole (60 vol%) and plagioclase (40 vol%) with average An contents of 47 mol%. A strongly altered clinopyroxene (Mg# = 0.7) is locally found. Hornblendites consists of brown amphibole and accessoryplagioclase. The chondrite normalized Rare Earth Element (REE) pattern of clinopyroxene is characterized by a marked light–(L)REE enrichment (22 times CI chondrite) relative to heavy (H)-REE, which are at about 5 times CI chondrite (GdN/YbN up to 3.8). The REE pattern of amphibole is enriched in LREE (40 times CI chondrite) and HREE are about 15 times CI chondrite (GdN/YbN = 1.5). Occasionally a positive Eu anomaly characterizes the REE pattern of amphibole.U-Pb geochronology was carried out with laser ablation ICP-MS on zircons from diorites. Zircon grains have round shape, thus suggesting a xenocrystic origin. Under cathodoluminescence they are generally homogeneous and rarely show ghost zoning. Most of zircons gave discordant U-Pb dates, only few grains are concordant and allowed to calculate a concordant date at 278±3 Ma (2s). This date is interpreted as the age of reset of the U-Pb system induced by the intrusion of the dioritic rocks.Similarities between the Loro and the Finero intrusive complexes were already suggested by Boriani (1966). These new results demonstrate a close geochemical affinity between the two complexes; in particular, peculiar similarities interms of trace element composition of minerals are observed (e.g., the positive Eu anomaly in amphibole). The inferred age of intrusion suggests that the Loro intrusive complex belongs to the same mantle-derived magmatism that gave origin to the Mafic Complex of the southern section of the Ivrea-Verbano Zone and perfectly overlap the Permian hightemperature event recognized in the Kinzigite formation in the Finero section of the Ivrea-Verbano Zone

The speciation of Fluorine in silicate melts

Understanding the behaviour of the halogens (F, Cl, Br, I) is required to explain the dynamic interplay between the silicate interior and volatile-rich hydrospheres and atmospheres of planets, moons, and exoworlds (Clay et al., 2017; 2019). We identify fluorine as a starting point because it is the most abundant and compatible of the halogens in the Earth system, and can directly impact the nature of Earth’s interior. For example, fluorine affects the thermal stability of mineral phases and the viscosity of silicate melts (Webster et al., 2018). Fluorine is a highly electronegative, monoisotopic, negatively charged atom with a low boiling point and ionic radius similar to both oxygen and hydroxyl ions (OH-) (Crepisson et al., 2014; Dalou et al., 2012). Therefore, understanding the pathways followed by fluorine might illuminate the pathways followed by water within silicate systems (McCubbin et al., 2015). Thus, the mechanics of fluorine dissolution in silicate melts (speciation) require attention.We have conducted experiments at high temperatures (1250°C) across a range of pressures (0-3 GPa) in a simple Ca-Mg-Al-Si-O system (CMAS) which reproduces the viscosities of intermediate to mafic silicate melts. The speciation of fluorine is studied using solid-state nuclear magnetic resonance (NMR) spectroscopy. Our results show most of the fluorine is bound with aluminium in the melt structure, but minor abundances of Mg-F and Ca-F are also observed in the 19F NMR spectrum. There is no observable effect of pressure and/or temperature. These observations challenge the assumption that fluorine and hydroxyl ions are kin, because OH- form stable compounds with Mg2+ (i.e. as Mg(OH)2, Mookherjee et al. 2008). These data predict decoupling of F- and OH- irrespective of their similar ionic radius and charge. Therefore, assuming the relative behaviours of F- and OH- follow an equilibrium exchange relationship might be (counterintuitively) flawed

The speciation of nitrogen in silicate phases

One of the ultimate goals in science is to illuminate the processes responsible for the development of habitable conditions on Earth, and this project will provide fundamental constraints on the storage, cycling, and release of nitrogen to Earth’s surface environment. Achieving this ambitious goal requires an understanding of the physical-chemical controls on the development of habitability on the planet’s surface, sub-surface, and marine environments. In this regard, the chemical nature of nitrogen is paramount. The addition of nitrogen to a planet’s surface is the result of both surficial and subsurface processes, including weathering and meteoritic influx, but are primarily added through though volcanism. Therefore, how nitrogen is stored within rock-forming minerals is information essential to understanding how their chemical activity varies though time, and their provenance in Earth’s formative years, as life got started.We have conducted experiments at 2 GPa and 1450 oC using the CMAS system to examine the speciation of nitrogen in silicate melts as a function of oxygen fugacity, which is currently believed to be the main factor that drives it. The speciation of nitrogen in melts is thought to be crucial for its incorporation in minerals. According to literature, the other main factors that control nitrogen speciation in melts are temperature, pressure and melt composition, which might also have an important influence on nitrogen solubility in silicate melts. The speciation of nitrogen is being and will be studied by the use of solid-state nuclear magnetic resonance (NMR) spectroscopy. This technique has been chosen for its unparalleled precision and accuracy in determining an element’s bonding environment.We will present the effects of oxygen fugacity and other factors on nitrogen speciation and discuss the possibility of creating a predictive model for nitrogen solubility and speciation in silicate melts. We will also address the application of these results to the study of nitrogen’s incorporation mechanism in common rock-forming minerals