Quantifying lithological variability in the mantle

We present a method that can be used to estimate the amount of recycled material present in the source region of mid-ocean ridge basalts by combining three key constraints: (1) the melting behaviour of the lithologies identified to be present in a mantle source, (2) the overall volume of melt production, and (3) the proportion of melt production attributable to melting of each lithology. These constraints are unified in a three-lithology melting model containing lherzolite, pyroxenite and harzburgite, representative products of mantle differentiation, to quantify their abundance in igneous source regions. As a case study we apply this method to Iceland, a location with sufficient geochemical and geophysical data to meet the required observational constraints. We find that to generate the 20 km of igneous crustal thickness at Iceland's coasts, with 30±10%30±10% of the crust produced from melting a pyroxenitic lithology, requires an excess mantle potential temperature (ΔTp) of ⩾130 °C (View the MathML sourceTp⩾1460°C) and a source consisting of at least 5% recycled basalt. Therefore, the mantle beneath Iceland requires a significant excess temperature to match geophysical and geochemical observations: lithological variation alone cannot account for the high crustal thickness. Determining a unique source solution is only possible if mantle potential temperature is known precisely and independently, otherwise a family of possible lithology mixtures is obtained across the range of viable ΔTp. For Iceland this uncertainty in ΔTp means that the mantle could be >20% harzburgitic if View the MathML sourceΔTp>150°C (View the MathML sourceTp>1480°C). The consequences of lithological heterogeneity for plume dynamics in various geological contexts are also explored through thermodynamic modelling of the densities of lherzolite, basalt, and harzburgite mixtures in the mantle. All lithology solutions for Iceland are buoyant in the shallow mantle at the ΔTp for which they are valid, however only lithology mixtures incorporating a significant harzburgite component are able to reproduce recent estimates of the Iceland plume's volume flux. Using the literature estimates of the amount of recycled basalt in the sources of Hawaiian and Siberian volcanism, we found that they are negatively buoyant in the upper mantle, even at the extremes of their expected ΔTp. One solution to this problem is that low density refractory harzburgite is a more ubiquitous component in mantle plumes than previously acknowledged

Scum of the Earth: A Hypothesis for Prebiotic Multi-Compartmentalised Environments.

Compartmentalisation by bioenergetic membranes is a universal feature of life. The eventual compartmentalisation of prebiotic systems is therefore often argued to comprise a key step during the origin of life. Compartments may have been active participants in prebiotic chemistry, concentrating and spatially organising key reactants. However, most prebiotically plausible compartments are leaky or unstable, limiting their utility. Here, we develop a new hypothesis for an origin of life environment that capitalises upon, and mitigates the limitations of, prebiotic compartments: multi-compartmentalised layers in the near surface environment-a 'scum'. Scum-type environments benefit from many of the same ensemble-based advantages as microbial biofilms. In particular, scum layers mediate diffusion with the wider environments, favouring preservation and sharing of early informational molecules, along with the selective concentration of compatible prebiotic compounds. Biofilms are among the earliest traces imprinted by life in the rock record: we contend that prebiotic equivalents of these environments deserve future experimental investigation

Post-main sequence thermal evolution of planetesimals

White dwarfs that have accreted planetary materials provide a powerful tool to probe the interiors and formation of exoplanets. In particular, the high Fe/Si ratio of some white dwarf pollutants suggests that they are fragments of bodies that were heated enough to undergo large-scale melting and iron core formation. In the solar system, this phenomenon is associated with bodies that formed early and so had short-lived radionuclides to power their melting, and/or grew large. However, if the planetary bodies accreted by white dwarfs formed during the (pre)-main sequence lifetime of the host star, they will have potentially been exposed to a second era of heating during the star's giant branches. This work aims to quantify the effect of stellar irradiation during the giant branches on planetary bodies by coupling stellar evolution to thermal and orbital evolution of planetesimals. We find that large-scale melting, sufficient to form an iron core, can be induced by stellar irradiation, but only in close-in small bodies: planetesimals with radii $\lesssim$ 30 km originally within $\sim$ 2 AU orbiting a 1$-$3$\,M_{\odot}$ host star with solar metallicity. Most of the observed white dwarf pollutants are too massive to be explained by the accretion of these small planetesimals that are melted during the giant branches. Therefore, we conclude that those white dwarfs that have accreted large masses of materials with enhanced or reduced Fe/Si remain an indicator of planetesimal's differentiation shortly after formation, potentially linked to radiogenic heating.Comment: 19 pages, 18 figure

Oxidised micrometeorites as evidence for low atmospheric pressure on the early Earth

Reconstructing a record of the partial pressure of molecular oxygen in Earth’s atmosphere is key for understanding macroevolutionary and environmental change over geological history. Recently, the oxidation state of iron in micrometeorites has been taken to imply the presence of modern Earth concentrations of oxygen in the upper atmosphere at 2.7 Ga, and therefore a highly chemically stratified atmosphere (Tomkins et al., 2016). We here explore the possibility that the mixing ratio of oxygen in Earth’s upper atmosphere, that probed by micrometeorites, may instead be sensitive to the surface atmospheric pressure. We find that the concentrations of oxygen in the upper atmosphere required for micrometeorite oxidation are achieved for a 0.3 bar atmosphere. In this case, significant water vapour reaches high up in the atmosphere and is photodissociated, leading to the formation of molecular oxygen. The presence of oxidised iron in micrometeorites at 2.7 Ga may therefore be further evidence that the atmospheric pressure at the surface of the early Earth was substantially lower than it is today

Prebiosignature Molecules Can Be Detected in Temperate Exoplanet Atmospheres with JWST

The search for biosignatures on exoplanets connects the fields of biology and biochemistry to astronomical observation, with the hope that we might detect evidence of active biological processes on worlds outside the solar system. Here we focus on a complementary aspect of exoplanet characterisation connecting astronomy to prebiotic chemistry: the search for molecules associated with the origin of life, prebiosignatures. Prebiosignature surveys in planetary atmospheres offer the potential to both constrain the ubiquity of life in the galaxy and provide important tests of current prebiotic syntheses outside of the laboratory setting. Here, we quantify the minimum abundance of identified prebiosignature molecules that would be required for detection by transmission spectroscopy using JWST. We consider prebiosignatures on five classes of terrestrial planet: an ocean planet, a volcanic planet, a post-impact planet, a super-Earth, and an early Earth analogue. Using a novel modelling and detection test pipeline, with simulated JWST noise, we find the detection thresholds of hydrogen cyanide (HCN), hydrogen sulfide (H2S), cyanoacetylene (HC3N), ammonia (NH3), methane (CH4), acetylene (C2H2), sulfur dioxide (SO2), nitric oxide (NO), formaldehyde (CH2O), and carbon monoxide (CO) in a variety of low mean molecular weight (<5) atmospheres. We test the dependence of these detection thresholds on M dwarf target star and the number of observed transits, finding that a modest number of transits (1-10) are required to detect prebiosignatures in numerous candidate planets, including TRAPPIST-1e with a high mean molecular weight atmosphere. We find that the NIRSpec G395M/H instrument is best suited for detecting most prebiosignatures.Comment: 28 pages, 12 figures, accepted for publication in A

Origin of Life's Building Blocks in Carbon- and Nitrogen-Rich Surface Hydrothermal Vents.

There are two dominant and contrasting classes of origin of life scenarios: those predicting that life emerged in submarine hydrothermal systems, where chemical disequilibrium can provide an energy source for nascent life; and those predicting that life emerged within subaerial environments, where UV catalysis of reactions may occur to form the building blocks of life. Here, we describe a prebiotically plausible environment that draws on the strengths of both scenarios: surface hydrothermal vents. We show how key feedstock molecules for prebiotic chemistry can be produced in abundance in shallow and surficial hydrothermal systems. We calculate the chemistry of volcanic gases feeding these vents over a range of pressures and basalt C/N/O contents. If ultra-reducing carbon-rich nitrogen-rich gases interact with subsurface water at a volcanic vent they result in 10 - 3 ⁻ 1 M concentrations of diacetylene (C₄H₂), acetylene (C₂H₂), cyanoacetylene (HC₃N), hydrogen cyanide (HCN), bisulfite (likely in the form of salts containing HSO₃-), hydrogen sulfide (HS-) and soluble iron in vent water. One key feedstock molecule, cyanamide (CH₂N₂), is not formed in significant quantities within this scenario, suggesting that it may need to be delivered exogenously, or formed from hydrogen cyanide either via organometallic compounds, or by some as yet-unknown chemical synthesis. Given the likely ubiquity of surface hydrothermal vents on young, hot, terrestrial planets, these results identify a prebiotically plausible local geochemical environment, which is also amenable to future lab-based simulation

Growth and evolution of secondary volcanic atmospheres: I. Identifying the geological character of hot rocky planets

The geology of Earth and super-Earth sized planets will, in many cases, only be observable via their atmospheres. Here, we investigate secondary volcanic atmospheres as a key base case of how atmospheres may reflect planetary geochemistry. We couple volcanic outgassing with atmospheric chemistry models to simulate the growth of C-O-H-S-N atmospheres in thermochemical equilibrium, focusing on what information about a planet's mantle fO$_2$ and bulk silicate H/C ratio could be determined by atmospheric observation. 800K volcanic atmospheres develop distinct compositional groups as the mantle fO$_2$ is varied, which can be identified using sets of (often minor) indicator species: Class O, representing an oxidised mantle and containing SO$_2$ and sulfur allotropes; Class I, formed by intermediate mantle fO$_2$'s and containing CO$_2$, CH$_4$, CO and COS; and Class R, produced by reduced mantles, containing H$_2$, NH$_3$ and CH$_4$. These atmospheric classes are robust to a wide range of bulk silicate H/C ratios. However, the H/C ratio does affect the dominant atmospheric constituent, which can vary between H$_2$, H$_2$O and CO$_2$ once the chemical composition has stabilised to a point where it no longer changes substantially with time. This final atmospheric state is dependent on the mantle fO$_2$, the H/C ratio, and time since the onset of volcanism. The atmospheric classes we present are appropriate for the closed-system growth of hot exoplanets, and may be used as a simple base for future research exploring the effects of other open-system processes on secondary volcanic atmospheres.Comment: Accepted for publication in JGR:Planet

A Statistical description of concurrent mixing and crystallisation during MORB differentiation: Implications for trace element enrichment

The pattern of trace element enrichment and variability found in differentiated suites of basalts is a sim- ple observable, which nonetheless records a wealth of information on processes occurring from the mantle to crustal magma chambers. The incompatible element contents of some mid-ocean ridge basalt (MORB) sample suites show progressive enrichment beyond the predictions of simple models of fractional crystalli- sation of a single primary melt. Explanations for this over-enrichment have focused on the differentiation processes in crustal magma chambers. In this paper we consider an additional mechanism, and focus instead on the deviation from simple fractionation trends that is possible by mixing of diverse mantle-derived melts supplied to magma chambers. A primary observation motivating this strategy is that there is significant chemical diversity in primitive high MgO basalts, which single liquid parent models cannot match. Models were developed to simulate the compositional effects of concurrent mixing and crystallisation (CMC): diverse parental melts were allowed to mix, with a likelihood that is proportional to the extent of fractional crys- tallisation. Using a simple statistical model to explore the effects of concurrent mixing and crystallisation on apparent liquid lines of descent, we show how significant departure from Rayleigh fractionation is possible as a function of the diversity of trace elements in the incoming melts, their primary MgO, and the relative proportion of enriched to depleted melts. The model was used to make predictions of gradients of trace element enrichment in log[trace element]– MgO space. These predictions were compared with observations from a compilation of global MORB and provide a test of the applicability of CMC to natural systems. We find that by considering the trace element variability of primitive MORB, its MgO content and degree of enrichment, CMC accurately predicts the pattern of trace element over-enrichment seen in global MORB. Importantly, this model shows that the relationship between over-enrichment and incompatibility can derive from mantle processes: the fact that during mantle melting maximum variability is generated in those elements with the smallest bulk K d . Magma chamber processes are therefore filtering the signal of mantle-derived chemical diversity to produce trace element over-enrichment during differentiation. Finally, we interrogate the global MORB dataset for evidence that trace element over-enrichment varies as a function of melt supply. There is no correlation between over-enrichment and melt supply in the global dataset. Trace element over-enrichment occurs at slow-spreading ridges where extensive steady-state axial magma chambers, the most likely environment for repeated episodes of replenishment, tapping and crystallisation, are very rarely detected. This supports a model whereby trace element over-enrichment is an inevitable consequence of chemically heterogeneous melts delivered from the mantle, a process that may operate across all rates of melt supply

Distinguishing Oceans of Water from Magma on Mini-Neptune K2-18b

Mildly irradiated mini-Neptunes have densities potentially consistent with them hosting substantial liquid-water oceans ("Hycean" planets). The presence of CO2 and simultaneous absence of ammonia (NH3) in their atmospheres has been proposed as a fingerprint of such worlds. JWST observations of K2-18b, the archetypal Hycean, have found the presence of CO2 and the depletion of NH3 to 4 μm region, where CO2 and CO features dominate: magma ocean models suggest a systematically lower CO2/CO ratio than estimated from free-chemistry retrieval, indicating that deeper observations of this spectral region may be able to distinguish between oceans of liquid water and magma on mini-Neptunes