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Clinopyroxene dissolution records rapid magma ascent
Get PDFMagma ascent rates control volcanic eruption styles. However, the rates at which basaltic magmas ascend through the crust remain highly uncertain. Although recent studies have successfully exploited records of decompression driven degassing to estimate the rates at which H2O-rich basalts ascend, such approaches cannot readily be applied to primitive and H2O-poor basalts that erupt in ocean island and mid-ocean ridge settings. Here we present magma ascent rates obtained by modelling the dissolution of clinopyroxene crystals in a wehrlitic nodule from the primitive Borgarhraun lava flow in North Iceland. High-Al2O3 clinopyroxene core compositions are consistent with crystallisation near the Moho (_800 MPa), whereas low-Al2O3 clinopyroxene rims and inclusion compositions are consistent with crystallisation at or near the surface. We interpret low-Al2O3 rims and inclusions as the crystallised remnants of boundary layers formed by the dissolution of high-Al2O3 clinopyroxene during magma ascent. By combining characteristic rim dissolution lengths of 50–100 m with published experimental calibrations of clinopyroxene dissolution behaviour, we estimate that the Borgarhraun magma most likely decompressed and ascended at rates of 3.0–15 kPa.s-1 and 0.11–0.53 m.s-1 respectively. These rates are slightly faster than published estimates obtained by modelling the diffusive re-equilibration of olivine crystals, suggesting that the Borgarhraun magma either accelerated upwards or that it stalled briefly at depth prior to final ascent. Comparisons with other basaltic eruptions indicate that the H2O-poor magma that fed the dominantly effusive Borgarhraun eruption ascended at a similar rate to some H2O-rich magmas that have fed explosive eruptions in arc settings. Thus, magma ascent rates do not appear to correlate simply with magma H2O contents. Overall, our findings confirm that primitive and H2O-poor basalts can traverse the crust within days, and may erupt with little precursory warning of magma ascent
Geophysical imaging of ophiolite structure in the United Arab Emirates
Get PDFThe Oman-United Arab Emirates ophiolite has been used extensively to document the geological processes that form oceanic crust. The geometry of the ophiolite, its extension into the Gulf of Oman, and the nature of the crust that underlies it are, however, unknown. Here, we show the ophiolite forms a high velocity, high density, >15 km thick east-dipping body that during emplacement flexed down a previously rifted continental margin thereby contributing to subsidence of flanking sedimentary basins. The western limit of the ophiolite is defined onshore by the Semail thrust while the eastern limit extends several km offshore, where it is defined seismically by a ~40–45°, east-dipping, normal fault. The fault is interpreted as the southwestern margin of an incipient suture zone that separates the Arabian plate from in situ Gulf of Oman oceanic crust and mantle presently subducting northwards beneath the Eurasian plate along the Makran trench
The effect of temperature on sulfur and oxygen isotope fractionation by sulfate reducing bacteria (Desulfococcus multivorans)
No full textTemperature influences microbiological growth and catabolic rates. Between 15 and 35 °C the growth rate and cell specific sulfate reduction rate of the sulfate reducing bacterium Desulfococcus multivorans increased with temperature. Sulfur isotope fractionation during sulfate reduction decreased with increasing temperature from 27.2 ‰ at 15 °C to 18.8 ‰ at 35 °C which is consistent with a decreasing reversibility of the metabolic pathway as the catabolic rate increases. Oxygen isotope fractionation, in contrast, decreased between 15 and 25 °C and then increased again between 25 and 35 °C, suggesting increasing reversibility in the first steps of the sulfate reducing pathway at higher temperatures. This points to a decoupling in the reversibility of sulfate reduction between the steps from the uptake of sulfate into the cell to the formation of sulfite, relative to the whole pathway from sulfate to sulfide. This observation is consistent with observations of increasing sulfur isotope fractionation when sulfate reducing bacteria are living near their upper temperature limit. The oxygen isotope decoupling may be a first signal of changing physiology as the bacteria cope with higher temperatures
Tightly shut: flexible valve margins and microstructural asymmetry in pterioid bivalves
Get PDFAn organic-rich columnar prismatic outer shell layer, which extends far beyond the underlying nacre, has allowed pterioid bivalves (the pearl oysters and their allies) to develop flexible valve margins, allowing a tight hermetic seal when shut. In some taxa, the microstructural arrangement is known to be asymmetrically developed between the two valves. The asymmetry was surveyed across 29 taxa of pterioids (including representatives of known genera) confirming that it is typically the right valve which has a greater expanse of prism-only shell (and less nacre) and showing that this portion of the right valve has more organic content (more than twice the value in some instances) than the equivalent in the left. A more detailed investigation of prismatic material in Pteria penguin comparing the right and left valves revealed that the right valve flange has a higher density of smaller prisms, each with its organic envelope, and not a greater thickness of the organic envelopes themselves. The flange is also thinner on the right valve and shown here to be very flexible when wet. This allows it to bend against the rigid left valve when the shell is closed. Comparison of this structural asymmetry in the pterioids with five outgroup taxa in the Ostreidae and Pinnidae suggests that clades with the asymmetry have been freed from the constraints of a flattened valve morphology and to develop inequivalved forms
Shapes, Plasmonic Properties, and Reactivity of Magnesium Nanoparticles
Get PDFLocalized surface plasmon resonances have attracted much attention due to their ability to enhance light–matter interactions and manipulate light at the subwavelength level. Recently, alternatives to the rare and expensive noble metals Ag and Au have been sought for more sustainable and large-scale plasmonic utilization. Mg supports plasmon resonances, is one of the most abundant elements in earth’s crust, and is fully biocompatible, making it an attractive framework for plasmonics. This feature article first reports the hexagonal, folded, and kite-like shapes expected theoretically from a modified Wulff construction for single crystal and twinned Mg structures and describes their excellent match with experimental results. Then, the optical response of Mg nanoparticles is overviewed, highlighting Mg’s ability to sustain localized surface plasmon resonances across the ultraviolet, visible, and near-infrared electromagnetic ranges. The various resonant modes of hexagons, leading to the highly localized electric field characteristic of plasmonic behavior, are presented numerically and experimentally. The evolution of these modes and the associated field from hexagons to the lower symmetry folded structures is then probed, again by matching simulations, optical, and electron spectroscopy data. Lastly, results demonstrating the opportunities and challenges related to the high chemical reactivity of Mg are discussed, including surface oxide formation and galvanic replacement as a synthetic tool for bimetallics. This Feature Article concludes with a summary of the next steps, open questions, and future directions in the field of Mg nanoplasmonics
Fault weakening across the frictional-viscous transition zone, Karakoram Fault Zone, NW Himalaya
No full textThe mechanical behaviour of antigorite strongly influences the strength and deformation of the subduction interface. Although there is microstructural evidence elucidating the nature of brittle deformation at low pressures, there is often conflicting evidence regarding the potential for plastic deformation in the ductile regime at higher pressures. Here, we present a series of spherical nanoindentation experiments on aggregates of natural antigorite. These experiments effectively investigate the single-crystal mechanical behaviour because the volume of deformed material is significantly smaller than the grain size. Individual indents reveal elastic loading followed by yield and strain hardening. The magnitude of the yield stress is a function of crystal orientation, with lower values associated with indents parallel to the basal plane. Unloading paths reveal more strain recovery than expected for purely elastic unloading. The magnitude of inelastic strain recovery is highest for indents parallel to the basal plane. We also imposed indents with cyclical loading paths, and observed strain energy dissipation during unloading–loading cycles conducted up to a fixed maximum indentation load and depth. The magnitude of this dissipated strain energy was highest for indents parallel to the basal plane. Subsequent scanning electron microscopy revealed surface impressions accommodated by shear cracks and a general lack of dislocation-induced lattice misorientation. Based on these observations, we suggest that antigorite deformation at high pressures is dominated by sliding on shear cracks. We develop a microphysical model that is able to quantitatively explain Young’s modulus and dissipated strain energy data during cyclic loading experiments, based on either frictional or cohesive sliding of an array of cracks contained in the basal plane
Mechanisms of fault mirror formation and fault healing in carbonate rocks
No full textThe development of smooth, mirror-like surfaces provides insight into the mechanical behaviour of crustal faults during the seismic cycle. To determine the thermo-chemical mechanisms of fault mirror formation, we investigated carbonate fault systems in seismically active areas of central Greece. Using multi-scale electron microscopy combined with Raman and electron energy loss spectroscopy, we show that fault mirror surfaces do not always develop from nanogranular volumes. The microstructural observations indicate that decarbonation is the transformation process that leads to the formation of smooth surface coatings in the faults studied here. Piercement structures on top of the fault surfaces indicate calcite decarbonation, producing CO2 and lime (CaO). Lime subsequently reacts to portlandite (Ca(OH)2) under hydrous conditions. Nanoscale imaging and electron diffraction reveal a thin coating of a non-crystalline material sporadically mixed with nano-clay, forming a complex-composite material that smooths the slip surface. Spectroscopic analyses reveal that the thin coating is non-crystalline carbon. We suggest that ordering (hybridisation) of amorphous carbon led to the formation of partly-hybridised amorphous carbon but did not reach full graphitisation. Calcite nanograins, &lt; 50 nm in diameter, are spatially associated with the carbon and indicate that the decomposition products acted as a crystallisation medium. Within this medium, portlandite back-reacted with CO2 to form nanocrystalline calcite. Consequently, two types of calcite nanograins are present: nanograins formed by grain-size reduction (primary nanograins, &gt; 100 nm) and new nanograins formed by back-reaction (secondary nanograins, &lt; 50 nm). Hence, we suggest that the new, secondary nanograins are not the result of comminution during slip but originate from pseudomorphic replacement of calcite after portlandite. The continuous coverage of partly-hybridised amorphous carbon on all samples suggests that calcite decarbonation products may develop across the entire fault surface, controlling the formation of carbonate fault mirrors, and may facilitate slip on a decarbonation-product glide film.</p
Last glacial atmospheric CO2 decline due to widespread Pacific deep-water expansion
Get PDFOcean circulation critically affects the global climate and atmospheric carbon dioxide through redistribution of heat and carbon in the Earth system. Despite intensive research, the nature of past ocean circulation changes remains elusive. Here we present deep-water carbonate ion concentration reconstructions for widely distributed locations in the Atlantic Ocean, where low carbonate ion concentrations indicate carbon-rich waters. These data show a low-carbonate-ion water mass that extended northward up to about 20° S in the South Atlantic at 3–4 km depth during the Last Glacial Maximum. In combination with radiocarbon ages, neodymium isotopes and carbon isotopes, we conclude that this low-carbonate-ion signal reflects a widespread expansion of carbon-rich Pacific deep waters into the South Atlantic, revealing a glacial deep Atlantic circulation scheme different than commonly considered. Comparison of high-resolution carbonate ion records from different water depths in the South Atlantic indicates that this Pacific deep-water expansion developed from approximately 38,000 to 28,000 years ago. We infer that its associated carbon sequestration may have contributed critically to the contemporaneous decline in atmospheric carbon dioxide, thereby helping to initiate the glacial maximum
A spectral survey of an ultra-hot Jupiter
Get PDFContext. KELT-9 b exemplifies a newly emerging class of short-period gaseous exoplanets that tend to orbit hot, early type stars – termed ultra-hot Jupiters. The severe stellar irradiation heats their atmospheres to temperatures of ~4000 K, similar to temperatures of photospheres of dwarf stars. Due to the absence of aerosols and complex molecular chemistry at such temperatures, these planets offer the potential of detailed chemical characterization through transit and day-side spectroscopy. Detailed studies of their chemical inventories may provide crucial constraints on their formation process(es) and evolution history.
Aims. We aim to search the optical transmission spectrum of KELT-9 b for absorption lines by metals using the cross-correlation technique.
Methods. We analysed two transit observations obtained with the HARPS-N spectrograph. We used an isothermal equilibrium chemistry model to predict the transmission spectrum for each of the neutral and singly ionized atoms with atomic numbers between three and 78. Of these, we identified the elements that are expected to have spectral lines in the visible wavelength range and used those as cross-correlation templates.
Results. We detect (>5σ) absorption by Na I, Cr II, Sc II and Y II, and confirm previous detections of Mg I, Fe I, Fe II, and Ti II. In addition, we find evidence of Ca I, Cr I, Co I, and Sr II that will require further observations to verify. The detected absorption lines are significantly deeper than predicted by our model, suggesting that the material is transported to higher altitudes where the density is enhanced compared to a hydrostatic profile, and that the material is part of an extended or outflowing envelope. There appears to be no significant blue-shift of the absorption spectrum due to a net day-to-night side wind. In particular, the strong Fe II feature is shifted by 0.18 ± 0.27 km s−1, consistent with zero. Using the orbital velocity of the planet we derive revised masses and radii of the star and the planet: M* = 1.978 ± 0.023 M⊙, R* = 2.178 ± 0.011 R⊙, mp = 2.44 ± 0.70 MJ and Rp = 1.783 ± 0.009 RJ