A petrological assessment of diamond as a recorder of the mantle nitrogen cycle

During part of this study, SM was employed at Bristol on NERC grant NE/J008583/1 to Michael J Walter. DH is funded by a DAAD PRIME fellowship. SM is grateful to the Deep Carbon Observatory for funding the fortnightly ‘Deep Carbon’ meeting at the Carnegie Institution of Washington 358 (USA), where the idea for this study first originated.Nitrogen is fundamental to the evolution of Earth and the life it supports, but for reasons poorly understood, it is cosmochemically the most depleted of the volatile elements. The largest reservoir in the bulk silicate Earth is the mantle, and knowledge of its nitrogen geochemistry is biased, because ≥90% of the mantle nitrogen database comes from diamonds. However, it is not clear to what extent diamonds record the nitrogen characteristics of the fluids/melts from which they precipitate. There is ongoing debate regarding the fundamental concept of nitrogen compatibility in diamond, and empirical global data sets reveal trends indicative of nitrogen being both compatible (fibrous diamonds) and incompatible (non-fibrous monocrystalline diamonds). A more significant and widely overlooked aspect of this assessment is that nitrogen is initially incorporated into the diamond lattice as single nitrogen atoms. However, this form of nitrogen is highly unstable in the mantle, where nitrogen occurs as molecular forms like N2 or NH4+, both of which are incompatible in the diamond lattice. A review of the available data shows that in classic terms, nitrogen is the most common substitutional impurity found in natural diamonds because it is of very similar atomic size and charge to carbon. However, the speciation of nitrogen, and how these different species disassociate during diamond formation to create transient monatomic nitrogen, are the factors governing nitrogen abundance in diamonds. This suggests the counter-intuitive notion that a nitrogen-free (Type II) diamond could grow from a N-rich media that is simply not undergoing reactions that liberate monatomic N. In contrast, a nitrogen-bearing (Type I) diamond could grow from a fluid with a lower N abundance, in which reactions are occurring to generate (unstable) N atoms during diamond formation. This implies that diamond’s relevance to nitrogen abundance in the mantle is far more complicated than currently understood. Therefore, further petrological investigations are required to enable accurate interpretations of what nitrogen data from mantle diamonds can tell us about the deep nitrogen budget and cycle.PostprintPeer reviewe

Metasomatism is a source of methane on Mars

MR and SM acknowledge support from NERC standard grant (NE/PO12167/1) and UK Space Agency Aurora grant (ST/T001763/1). DAS acknowledges support by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Geosciences program under Award Number DE-SC0019830 as well as NSF Petrology and Geochemistry Grant Number 2032039.The abundance of inactive Martian volcanic centres suggests that early Mars was more volcanically active than today. On Earth, volcanic degassing releases climate-forcing gases such as H2O, SO2, and CO2 into the atmosphere. On Mars, the volcanic carbon is likely to be more methane-rich than on Earth because the interior is, and was, more reducing than the present-day Terrestrial upper mantle. The reports of reduced carbon associated with high-temperature minerals in Martian igneous meteorites back up this assertion. Here, we undertake irreversible reaction path models of the fluid-rock interaction to predict carbon speciation in magmatic fluids at the Martian crust-mantle boundary. We find methane is a major carbon species between 300 and 800 °C where logfO2 is set at the Fayalite = Magnetite + Quartz redox buffer reaction (FMQ). When logfO2 is below FMQ, methane is dominant across all temperatures investigated (300–800 °C). Moreover, ultramafic rocks produce more methane than mafic lithologies. The cooling of magmatic bodies leads to the release of a fluid phase, which serves as a medium within which methane is formed at high temperatures and transported. Metasomatic methane is, therefore, a source of reduced carbonaceous gases to the early Martian atmosphere and, fundamentally, for all telluric planets, moons, and exoplanets with Mars-like low logfO2 interiors.Peer reviewe

Low surface gravitational acceleration of Mars results in a thick and weak lithosphere : implications for topography, volcanism, and hydrology

The first author acknowledges funding from an Initiative d’Excellence (IDEX) “Attractivité” grant (VOLPERM), funded by the University of Strasbourg. M.H. also acknowledges support from the CNRS (INSU 2016-TelluS-ALEAS).Surface gravitational acceleration (surface gravity) on Mars, the second-smallest planet in the Solar System, is much lower than that on Earth. A direct consequence of this low surface gravity is that lithostatic pressure is lower on Mars than on Earth at any given depth. Collated published data from deformation experiments on basalts suggest that, throughout its geological history (and thus thermal evolution), the Martian brittle lithosphere was much thicker but weaker than that of present-day Earth as a function solely of surface gravity. We also demonstrate, again as a consequence of its lower surface gravity, that the Martian lithosphere is more porous, that fractures on Mars remain open to greater depths and are wider at a given depth, and that the maximum penetration depth for opening-mode fractures (i.e., joints) is much deeper on Mars than on Earth. The result of a weak Martian lithosphere is that dykes—the primary mechanism for magma transport on both planets—can propagate more easily and can be much wider on Mars than on Earth. We suggest that this increased the efficiency of magma delivery to and towards the Martian surface during its volcanically active past, and therefore assisted the exogeneous and endogenous growth of the planet's enormous volcanoes (the heights of which are supported by the thick Martian lithosphere) as well as extensive flood-mode volcanism. The porous and pervasively fractured (and permeable) nature of the Martian lithosphere will have also greatly assisted the subsurface storage of and transport of fluids through the lithosphere throughout its geologically history. And so it is that surface gravity, influenced by the mass of a planetary body, can greatly modify the mechanical and hydraulic behaviour of its lithosphere with manifest differences in surface topography and geomorphology, volcanic character, and hydrology.PostprintPeer reviewe

On the origin(s) and evolution of Earth’s carbon

SM acknowledges support from the National Environmental Research Council (grant no. NE/PO12167/1) and the Carnegie Trust for the Universities of Scotland (grant no. RIG007794). EF acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 715028).The isotopic “flavor” of Earth’s major volatiles, including carbon, can be compared to the known reservoirs of volatiles in the solar system and so determine the source of Earth’s carbon. This requires knowing Earth’s bulk carbon isotope value, which is not straightforward to determine. During Earth’s differentiation, carbon was partitioned into the core, mantle, crust, and atmosphere. Therefore, although carbon is omnipresent within the Earth system, scientists have yet to determine its distribution and relative abundances. This article addresses what we know of the processes involved in the formation of Earth’s carbon reservoirs, and, by deduction, what we know about the possible origins of Earth’s carbon.Publisher PD

The effects of planetary and stellar parameters on brittle lithospheric thickness

P.K.B. acknowledges support from North Carolina State University. Funding for S.M. was provided by NERC standard grant NE/PO12167/1 and UK Space Agency Aurora grant ST/T001763/1. M.J.H. thanks the Institut Universitaire de France (IUF) for support.The thickness of the brittle lithosphere—the outer portion of a planetary body that fails via fracturing—plays a key role in the geological processes of that body. The properties of both a planet and its host star can influence that thickness, and the potential range of those properties exceeds what we see in the Solar System. To understand how planetary and stellar parameters influence brittle lithospheric thickness generally, we modeled a comprehensive suite of combinations of planetary mass, surface and mantle temperature, heat flux, and strain rate. Surface temperature is the dominant factor governing the thickness of the brittle layer: smaller and older planets generally have thick brittle lithospheres, akin to those of Mercury and Mars, whereas larger, younger planets have thinner brittle lithospheres that may be comparable to the Venus lowlands. But certain combinations of these parameters yield worlds with exceedingly thin brittle layers. We predict that such bodies have little elevated topography and limited volatile cycling and weathering, which can be tested by future telescopic observations of known extrasolar planets.Publisher PDFPeer reviewe

Hot climate inhibits volcanism on Venus : constraints from rock deformation experiments and argon isotope geochemistry

M.J. Heap acknowledges funding from an Initiative d’Excellence (IDEX) “Attractivité” grant (VOLPERM), funded by the University of Strasbourg.The disparate evolution of sibling planets Earth and Venus has left them markedly different. Venus’ hot (460 °C) surface is dry and has a hypsometry with a very low standard deviation, whereas Earth’s average temperature is 4 °C and the surface is wet and has a pronounced bimodal hypsometry. Counterintuitively, despite the hot Venusian climate, the rate of intraplate volcano formation is an order of magnitude lower than that of Earth. Here we compile and analyse rock deformation and atmospheric argon isotope data to offer an explanation for the relative contrast in volcanic flux between Earth and Venus. By collating high-temperature, high-pressure rock deformation data for basalt, we provide a failure mechanism map to assess the depth of the brittle–ductile transition (BDT). These data suggest that the Venusian BDT likely exists between 2–12 km depth (for a range of thermal gradients), in stark contrast to the BDT for Earth, which we find to be at a depth of ~25-27 km using the same method. The implications for planetary evolution are twofold. First, downflexing and sagging will result in the sinking of high-elevation structures, due to the low flexural rigidity of the predominantly ductile Venusian crust, offering an explanation for the curious coronae features on the Venusian surface. Second, magma delivery to the surface—the most efficient mechanism for which is flow along fractures (dykes; i.e., brittle deformation)—will be inhibited on Venus. Instead, we infer that magmas must stall and pond in the ductile Venusian crust. If true, a greater proportion of magmatism on Venus should result in intrusion rather than extrusion, relative to Earth. This predicted lower volcanic flux on Venus, relative to Earth, is supported by atmospheric argon isotope data: we argue here that the anomalously unradiogenic present-day atmospheric 40Ar/36Ar ratio for Venus (compared with Earth) must reflect major differences in 40Ar degassing, primarily driven by volcanism. Indeed, these argon data suggest that the volcanic flux on Venus has been three times lower than that on Earth over its 4.56 billion year history. We conclude that Venus’ hot climate inhibits volcanism.PostprintPeer reviewe

Metamorphic evolution of carbonate-hosted microbial biosignatures

This work was funded by a Royal Society of Edinburgh Research Fellowship. FF and FW acknowledge funding from the CNRS and CNES.Microbial biosignature assemblages captured within mineral substrates experience extreme pressures (P) and temperatures (T) during rock burial and metamorphism. We subjected natural microbial biofilms hosted within thermal spring carbonate to six high pressure, high temperature (HPHT) conditions spanning 500 and 800 MPa and 200 to 550 °C, to investigate the initial petrographic transformation of organic and inorganic phases. We find biogenic and amorphous silica mineralises increasingly mature organic matter (OM) as temperature and pressure increase, with OM expelled from recrystallised calcite at the highest HPHT, captured within a quartz phase. Sulfur globules associated with microbial filaments persist across all HPHT conditions in association with microbially-derived kerogen. These data demonstrate how microbial material captured within chemically-precipitated sediments petrographically evolves in high grade rocks during their first stages of transformation.Publisher PDFPeer reviewe

Hydrothermal recycling of sedimentary ammonium into oceanic crust and the Archean ocean at 3.24 Ga

Funding was provided by a Natural Environment Research Council studentship (grant NE/R012253/1) to T.J. Boocock, and a National Science Foundation grant (grant EARPF 1725784) and an American Philosophical Society Lewis and Clark Grant, both to B.W. Johnson.The Archean ocean supported a diverse microbial ecosystem, yet studies suggest that seawater was largely depleted in many essential nutrients, including fixed nitrogen. This depletion was in part a consequence of inefficient nutrient recycling under anoxic conditions. Here, we show how hydrothermal fluids acted as a recycling mechanism for ammonium (NH4+) in the Archean ocean. We present elemental and stable isotope data for carbon, nitrogen, and sulfur from shales and hydrothermally altered volcanic rocks from the 3.24 Ga Panorama district in Western Australia. This suite documents the transfer of NH4+ from organic-rich sedimentary rocks into underlying sericitized dacite, similar to what is seen in hydrothermal systems today. On modern Earth, hydrothermal fluids that circulate through sediment packages are enriched in NH4+ to millimolar concentrations because they efficiently recycle organic-bound N. Our data show that a similar hydrothermal recycling process dates back to at least 3.24 Ga, and it may have resulted in localized centers of enhanced biological productivity around hydrothermal vents. Last, our data provide evidence that altered oceanic crust at 3.24 Ga was enriched in nitrogen, and, when subducted, it satisfies the elemental and isotopic source requirements for a low-N, but 15N-enriched, deep mantle nitrogen reservoir as sampled by mantle plumes.PostprintPeer reviewe

Low-Molecular Weight Protamine Overcomes Chondroitin Sulfate Inhibition of Neural Regeneration

Protamine is an arginine-rich peptide that replaces histones in the DNA-protein complex during spermatogenesis. Protamine is clinically used in cardiopulmonary bypass surgery to neutralize the effects of heparin that is required during the treatment. Here we demonstrate that protamine and its 14-22 amino acid long fragments overcome the neurite outgrowth inhibition by chondroitin sulfate proteoglycans (CSPGs) that are generally regarded as major inhibitors of regenerative neurite growth after injuries of the adult central nervous system (CNS). Since the full-length protamine was found to have toxic effects on neuronal cells we used the in vitro neurite outgrowth assay to select a protamine fragment that retains the activity to overcome the neurite outgrowth inhibition on CSPG substrate and ended up in the 14 amino acid fragment, low-molecular weight protamine (LMWP). In contrast to the full-length protamine, LMWP displays very low or no toxicity in our assays in vitro and in vivo. We therefore started studies on LMWP as a possible drug lead in treatment of CNS injuries, such as the spinal cord injury (SCI). LMWP mimicks HB-GAM (heparin-binding growth-associated molecule; pleiotrophin) in that it overcomes the CSPG inhibition on neurite outgrowth in primary CNS neurons in vitro and inhibits binding of protein tyrosine phosphatase (PTP) sigma, an inhibitory receptor in neurite outgrowth, to its CSPG ligand. Furthermore, the chondroitin sulfate (CS) chains of the cell matrix even enhance the LMWP-induced neurite outgrowth on CSPG substrate. In vivo studies using the hemisection and hemicontusion SCI models in mice at the cervical level C5 revealed that LMWP enhances recovery when administered through intracerebroventricular or systemic route. We suggest that LMWP is a promising drug lead to develop therapies for CNS injuries.Peer reviewe