Hidden Water in Magma Ocean Exoplanets

We demonstrate that the deep volatile storage capacity of magma oceans has significant implications for the bulk composition, interior, and climate state inferred from exoplanet mass and radius data. Experimental petrology provides the fundamental properties of the ability of water and melt to mix. So far, these data have been largely neglected for exoplanet mass–radius modeling. Here we present an advanced interior model for water-rich rocky exoplanets. The new model allows us to test the effects of rock melting and the redistribution of water between magma ocean and atmosphere on calculated planet radii. Models with and without rock melting and water partitioning lead to deviations in planet radius of up to 16% for a fixed bulk composition and planet mass. This is within the current accuracy limits for individual systems and statistically testable on a population level. Unrecognized mantle melting and volatile redistribution in retrievals may thus underestimate the inferred planetary bulk water content by up to 1 order of magnitude.

Hydrogen diffusion in olivine: challenges and opportunities

<p>Understanding rates and mechanisms of diffusion in geologically relevant materials is important when considering, for example, electrical conductivity, rheology and, of course, diffusion chronometry. Olivine has received much attention in this regard – not only is it important in upper mantle and many volcanic settings, but its wide range of stability in pressure-temperature-chemical activity space makes it extremely amenable to experimental petrology. Furthermore, olivine is simple enough to study systematically, but contains different crystallographic sites, diffusion pathways and is anisotropic, thus has sufficient complexity to remain interesting. Like many common rock-forming minerals, olivine is nominally anhydrous, but normally contains trace amounts of hydrogen. This is generally bonded to structural oxygen, forming hydroxyl groups. These can be easily imaged by infrared spectroscopy, which simultaneously elucidates both their concentration and associated point defect chemistry.</p><p>The combination of a mineral that is quite straightforward to study experimentally, and the ability to distinguish between different H substitution mechanisms, a major strength of infrared spectroscopy, has proved to be hugely useful. However, the more we know, the more complex the system seems to become. For example, firstly, small changes in the major element composition of olivine were shown to have considerable effects on H diffusion. Secondly, close inspection of infrared spectra from experiments and natural samples revealed the presence of point defects that, according to the generally invoked theory, should not be there. Thirdly, small variations in experimental design between different studies apparently led to major discrepancies in results, even if the experiments were designed to measure ostensibly the same process. Fourthly, apparent diffusivities extracted from well-constrained natural samples showed results in complete disagreement with experiments in the same system.</p><p>On the one hand, these complexities have the potential to severely limit the accuracy of diffusion chronometry using H diffusion. On the other hand, complexity is opportunity. Given the wealth of published studies, both experimental and natural, and given that H-bearing point defects in olivine can be easily distinguished, we are presented with a unique possibility to truly unravel the diffusive behaviour of H in olivine. Recently developed theories suggest that treating H mobility as diffusion alone is insufficient (even if multiple diffusion mechanisms are invoked), and instead it is necessary to consider the way in which different H-bearing point defects interact within the crystal. A model describing this process in both pure and trace element-doped forsterite will be presented, which reconciles, to some extent, these previous discrepancies. The model suggests that the true mobility of H is one to two orders of magnitude higher than that which has been directly measured when assuming simple diffusion. Work is in progress to expand the model towards crystals with chemistries relevant for nature. If a similar model can be invoked for natural olivine, then this will require that models of processes invoking H diffusion (e.g. rheology, diffusion chronometry, electrical conductivity) will need to be reevaluated.</p>

Conductive Channels in the Deep Oceanic Lithosphere Could Consist of Garnet Pyroxenites at the Fossilized Lithosphere–Asthenosphere Boundary

Magnetotelluric (MT) surveys have identified anisotropic conductive anomalies in the mantle of the Cocos and Nazca oceanic plates, respectively, offshore Nicaragua and in the eastern neighborhood of the East Pacific Rise (EPR). Both the origin and nature of these anomalies are controversial as well as their role in plate tectonics. The high electrical conductivity has been hypothesized to originate from partial melting and melt pooling at the lithosphere–asthenosphere boundary (LAB). The anisotropic nature of the anomaly likely highlights high-conductivity channels in the spreading direction, which could be further interpreted as the persistence of a stable liquid silicate throughout the whole oceanic cycle, on which the lithospheric plates would slide by shearing. However, considering minor hydration, some mantle minerals can be as conductive as silicate melts. Here I show that the observed electrical anomaly offshore Nicaragua does not correlate with the LAB but instead with the top of the garnet stability field and that garnet networks suffice to explain the reported conductivity values. I further propose that this anomaly actually corresponds to the fossilized trace of the early-stage LAB that formed near the EPR about 23 million years ago. Melt-bearing channels and/or pyroxenite underplating at the bottom of the young Cocos plate would transform into garnet-rich pyroxenites with decreasing temperature, forming solid-state high-conductivity channels between 40 and 65 km depth (1.25–1.9 GPa, 1000–1100 °C), consistently with experimental petrology.

Carbonatite-Related REE Deposits: An Overview

The rare earth elements (REEs) have unique and diverse properties that make them function as an “industrial vitamin” and thus, many countries consider them as strategically important resources. China, responsible for more than 60% of the world’s REE production, is one of the REE-rich countries in the world. Most REE (especially light rare earth elements (LREE)) deposits are closely related to carbonatite in China. Such a type of deposit may also contain appreciable amounts of industrially critical metals, such as Nb, Th and Sc. According to the genesis, the carbonatite-related REE deposits can be divided into three types: primary magmatic type, hydrothermal type and carbonatite weathering-crust type. This paper provides an overview of the carbonatite-related endogenetic REE deposits, i.e., primary magmatic type and hydrothermal type. The carbonatite-related endogenetic REE deposits are mainly distributed in continental margin depression or rift belts, e.g., Bayan Obo REE-Nb-Fe deposit, and orogenic belts on the margin of craton such as the Miaoya Nb-REE deposit. The genesis of carbonatite-related endogenetic REE deposits is still debated. It is generally believed that the carbonatite magma is originated from the low-degree partial melting of the mantle. During the evolution process, the carbonatite rocks or dykes rich in REE were formed through the immiscibility of carbonate-silicate magma and fractional crystallization of carbonate minerals from carbonatite magma. The ore-forming elements are mainly sourced from primitive mantle, with possible contribution of crustal materials that carry a large amount of REE. In the magmatic-hydrothermal system, REEs migrate in the form of complexes, and precipitate corresponding to changes of temperature, pressure, pH and composition of the fluids. A simple magmatic evolution process cannot ensure massive enrichment of REE to economic values. Fractional crystallization of carbonate minerals and immiscibility of melts and hydrothermal fluids in the hydrothermal evolution stage play an important role in upgrading the REE mineralization. Future work of experimental petrology will be fundamental to understand the partitioning behaviors of REE in magmatic-hydrothermal system through simulation of the metallogenic geological environment. Applying “comparative metallogeny” methods to investigate both REE fertile and barren carbonatites will enhance the understanding of factors controlling the fertility.

The mass flux of volatiles from volcanic eruptions on Mercury

<p>Mercury has been extensively resurfaced by large, effusive lava plains [1–2]. Similar lava plains on the Moon, the maria, are known to contain volatiles [3–4] and are estimated to have outgassed ~10<sup>16</sup> kg of CO and S and ~10<sup>14</sup> kg of H<sub>2</sub>O, with the bulk of volatiles being released during peak mare emplacement ~3.5 Ga ago [5]. If volcanic activity released substantial volatiles on the Moon [6–7], then it is possible that substantial volatiles were also volcanically released on Mercury, albeit with different chemical species [6–9]. Here we seek to understand the potential contribution of outgassing to volatile deposits, specifically for Mercury’s volatile species (S, CH<sub>4</sub>, Cl, and N-H).</p><p>We analyze the production function of volcanic plains deposits on Mercury and find that the volume of outgassed basalts on Mercury is 2 to 3 orders of magnitude larger than that predicted for the Moon [8]. We use a variety of experimental petrology studies [10–12] to predict the dominant species and their abundances associated with these eruptions on Mercury, providing estimates for both low-gas and high-gas scenarios for different oxygen fugacities (IW-3 and IW-7). The most prevalent volatile species predicted for Mercury (S, CH<sub>4</sub>, and Cl) are 1 to 4 orders of magnitude more abundant than what is predicted for the most abundant volatiles outgassed on the Moon (CO, S, and H<sub>2</sub>O) [5].</p><p>On the Moon, it has been predicted that volatiles outgassed from the formation of the maria may have been present in sufficient volumes to produce a transient atmosphere capable of aiding in the transport of H<sub>2</sub>O to cold-trapping regions [5]. At mantle pressures and Mercury’s extremely reducing conditions, H<sub>2</sub>O is not predicted to be present in the magma [e.g., 6–12]. Therefore, Mercury’s outgassed volatiles are of a different composition from the H<sub>2</sub>O ice observed at Mercury’s poles today [e.g., 13], and the polar H<sub>2</sub>O-ice deposits are better explained by some external delivery mechanism (likely cometary impacts). But the fate of large volumes of volatiles other than H<sub>2</sub>O is an important unanswered question for Mercury.</p><p>The large volumes of outgassed volatiles calculated here suggest that volcanism on Mercury may have resulted in the transient production of anomalously high atmospheric pressures of short lifetime due to solar proximity. If Mercury’s atmospheric loss rate was insufficient to lose all of the erupted gases, then it is possible that ancient, outgassed volatiles remain trapped in the planet’s subsurface today. The fate of Mercury’s outgassed volatiles is an important open question that we discuss in this work.</p><p>References: [1] Head et al. (2011). [2] Denevi et al. (2013). [3] Boyce et al. (2010). [4] McCubbin et al. (2010). [5] Needham and Kring (2017). [6] Nittler et al. (2011). [7] Zolotov et al. (2013). [8] Peplowski et al. (2016). [9] Greenwood et al. (2018). [10] Anzures et al. (2017). [11] Armstrong et al. (2015). [12] Libourel et al. (2003). [13] Lawrence et al. (2013).</p>