Element signatures of subduction-zone fluids. An experimental study of the element partitioning (Dfluid/rock) of natural partly altered igneous rocks from the ODP drilling site 1,256

The trace element signatures of fluids were investigated by leaching experiments on natural samples of partly altered mafic igneous rocks recovered from the drilling site 1,256 of ODP Leg 206 on the Cocos plate (Central America). Experiments with ultrapure water were performed at 400 °C/0.4 GPa and 500 °C/0.7 GPa. Both fluids and residual solids were examined to obtain the partition coefficients (D^fluid/rock) of various trace elements. Element partition coefficients (D^fluid/rock) obtained at 500 °C/0.7 GPa are significantly lower compared to results obtained at 400 °C/0.4 GPa, which is in contrast to observations at higher pressures (2.2–6 GPa) and temperatures between 700 and 1,400 °C (Kessel et al. in Earth Planet Sci Lett 237: 873–892, 2005a; Spandler et al. in Chem Geol 239: 228–249, 2007). This finding may indicate a considerable pressure effect on the leaching processes and strongly divergent fluid–rock interactions in the upper part of a subduction zone at 0.4–0.7 GPa compared to deeper subduction areas with higher pressures. Furthermore, this may be interpreted as one of the earliest fractionation processes during the subduction of crustal material.     

Settling and compaction of chromite cumulates employing a centrifuging piston cylinder and application to layered mafic intrusions

The time scales and mechanics of gravitationally driven crystal settling and compaction is investigated through high temperature (1,280–1,500 °C) centrifuge-assisted experiments on a chromite-basalt melt system at 100–1,500g (0.5 GPa). Subsequently, the feasibility of this process for the formation of dense chromite cumulate layers in large layered mafic intrusions (LMIs) is assessed. Centrifugation leads to a single cumulate layer formed at the gravitational bottom of the capsule. The experimentally observed mechanical settling velocity of a suspension of ~24 vol% chromite is calculated to be about half (~0.53) of the Stokes settling velocity, with a sedimentation exponent n of 2.35 (3). Gravitational settling leads to an orthocumulate layer with a porosity of 0.52 (all porosities as fraction). Formation times for such a layer from a magma with initial chromite contents of 0.1–1 vol% are 140–3.5 days, equal to a growth rate of 0.007–0.3 m/day for grain sizes of 1–2 mm. More compacted chromite layers form with increasing centrifugation time and acceleration through chemical compaction: An increase of grain contact areas and grain sizes together with a decrease in porosity is best explained by pressure dissolution at grain contacts, reprecipitation and grain growth into the intergranular space and a concomitant expulsion of intergranular melt. The relation between the porosity in the cumulate pile and effective pressure integrated over time (Δρ · h · a · t) is best fit with a logarithmic function, in fact confirming that a (pressure) dissolution–reprecipitation process is the dominant mechanism of compaction. The experimentally derived equation allows calculating compaction times: 70–80 % chromite at the bottom of a 1-m-thick chromite layer are reached after 9–250 years, whereas equivalent compaction times are 0.2–0.9 years for olivine (both for 2 mm grain size). The experiments allow to determine the bulk viscosities of chromite and olivine cumulates to be of magnitude 10⁹ Pa s, much lower than previously reported. As long as melt escape from the compacting cumulate remains homogeneous, fluidization does not play any role; however, channelized melt flow may lead to suspension and upward movement of cumulate crystals. In LMIs, chromitite layers are typically part of a sequence with layers of mafic minerals, compaction occurs under the additional weight of the overlying layers and can be achieved in a few years to decades.     

Global climate policy and deep decarbonization of energy-intensive industries

If we are to limit global warming to 2 °C, all sectors in all countries must reduce their emissions of GHGs to zero not later than 2060–2080. Zero-emission options have been less explored and are less developed in the energy-intensive basic materials industries than in other sectors. Current climate policies have not yet motivated major efforts to decarbonize this sector, and it has been largely protected from climate policy due to the perceived risks of carbon leakage and a focus on short-term reduction targets to 2020. We argue that the future global climate policy regime must develop along three interlinked and strategic lines to facilitate a deep decarbonization of energy-intensive industries. First, the principle of common but differentiated responsibility must be reinterpreted to allow for a dialogue on fairness and the right to development in relation to industry. Second, a greater focus on the development, deployment and transfer of technology in this sector is called for. Third, the potential conflicts between current free trade regimes and motivated industrial policies for deep decarbonization must be resolved. One way forward is to revisit the idea of sectoral approaches with a broader scope, including not only emission reductions, but recognizing the full complexity of low-carbon transitions in energy-intensive industries. A new approach could engage industrial stakeholders, support technology research, development and demonstration and facilitate deployment through reducing the risk for investors. The Paris Agreement allows the idea of sectoral approaches to be revisited in the interests of reaching our common climate goals.

Policy relevance

Deep decarbonization of energy-intensive industries will be necessary to meet the 2 °C target. This requires major innovation efforts over a long period. Energy-intensive industries face unique challenges from both innovation and technical perspectives due to the large scale of facilities, the character of their global markets and the potentially high mitigation costs. This article addresses these challenges and discusses ways in which the global climate policy framework should be developed after the Paris Agreement to better support transformative change in the energy-intensive industries.     

Speckle imaging of Titan at 2 microns: surface albedo, haze optical depth, and tropospheric clouds 1996-1998

We present results from 14 nights of observations of Titan in 1996-1998 using near-infrared (centered at 2.1 microns) speckle imaging at the 10-meter W.M. Keck Telescope. The observations have a spatial resolution of 0.06 arcseconds. We detect bright clouds on three days in October 1998, with a brightness about 0.5% of the brightness of Titan. Using a 16-stream radiative transfer model (DISORT) to model the central equatorial longitude of each image, we construct a suite of surface albedo models parameterized by the optical depth of Titan's hydrocarbon haze layer. From this we conclude that Titan's equatorial surface albedo has plausible values in the range of 0-0.20. Titan's minimum haze optical depth cannot be constrained from this modeling, but an upper limit of 0.3 at this wavelength range is found. More accurate determination of Titan's surface albedo and haze optical depth, especially at higher latitudes, will require a model that fully considers the 3-dimensional nature of Titan's atmosphere.     

Assessment of the interstellar processes leading to deuterium enrichment in meteoritic organics

Abstract— The presence of isotopic anomalies is the most unequivocal demonstration that meteoritic material contains circumstellar or interstellar components. In the case of organic compounds in meteorites and interplanetary dust particles (IDPs), the most useful isotopic tracer has been deuterium (D). We discuss four processes that are expected to lead to D enrichment in interstellar materials and describe how their unique characteristics can be used to assess their relative importance for the organics in meteorites. These enrichment processes are low‐temperature gas phase ion‐molecule reactions, low‐temperature gas‐grain reactions, gas phase unimolecular photodissociation, and ultraviolet photolysis in D‐enriched ice mantles. Each of these processes is expected to be associated with distinct regiochemical signatures (D placement on the product molecules, correlation with specific chemical functionalities, etc.), especially in the molecular population of polycyclic aromatic hydrocarbons (PAHs). We describe these differences and discuss how they may be used to delineate the various interstellar processes that may have contributed to meteoritic D enrichments. We also briefly discuss how these processes may affect the isotopic distributions in C, O, and N in the same compounds.     

Near-infrared laboratory spectra of solid H2O/CO2 and CH3OH/CO2 ice mixtures

We present near-IR spectra of solid CO₂ in H₂O and CH₃OH, and find they are significantly different from that of pure solid CO₂. Peaks not present in either pure H₂O or pure CO₂ spectra become evident when the two are mixed. First, the putative theoretically forbidden CO₂ (2ν₃) overtone near 2.134 μm (4685 cm⁻¹), that is absent from our spectrum of pure solid CO₂, is prominent in the spectra of H₂O/CO₂=5 and 25 mixtures. Second, a 2.74-μm (3650 cm⁻¹) dangling OH feature of H₂O (and a potentially related peak at 1.89 μm) appear in the spectra of CO₂-H₂O ice mixtures, but are probably not diagnostic of the presence of CO₂. Other CO₂ peaks display shifts in position and increased width because of intermolecular interactions with H₂O. Warming causes some peak positions and profiles in the spectrum of a H₂O/CO₂=5 mixture to take on the appearance of pure CO₂. Absolute strengths for absorptions of CO₂ in solid H₂O are estimated. Similar results are observed for CO₂ in solid CH₃OH. Since the CO₂ (2ν₃) overtone near 2.134 μm (4685 cm⁻¹) is not present in pure CO₂ but prominent in mixtures, it may be a good observational (spectral) indicator of whether solid CO₂ is a pure material or intimately mixed with other molecules. These observations may be applicable to Mars polar caps as well as outer Solar System bodies.