The impact environment of the Hadean Earth

Impact bombardment in the first billion years of solar system history determined in large part the initial physical and chemical states of the inner planets and their potential to host biospheres. The range of physical states and thermal consequences of the impact epoch, however, are not well quantified. Here, we assess these effects on the young Earth's crust as well as the likelihood that a record of such effects could be preserved in the oldest terrestrial minerals and rocks. We place special emphasis on modeling the thermal effects of the late heavy bombardment (LHB) – a putative spike in the number of impacts at about 3.9 Gyr ago – using several different numerical modeling and analytical techniques. A comprehensive array of impact-produced heat sources was evaluated which includes shock heating, impact melt generation, uplift, and ejecta heating. Results indicate that ∼1.5–2.5 vol.% of the upper 20 km of Earth's crust was melted in the LHB, with only ∼0.3–1.5 vol.% in a molten state at any given time. The model predicts that approximately 5–10% of the planet's surface area was covered by >1 km deep impact melt sheets. A global average of ∼600–800 m of ejecta and ∼800–1000 m of condensed rock vapor is predicted to have been deposited in the LHB, with most of the condensed rock vapor produced by the largest (>100-km) projectiles. To explore for a record of such catastrophic events, we created two- and three-dimensional models of post-impact cooling of ejecta and craters, coupled to diffusion models of radiogenic Pb*-loss in zircons. We used this to estimate what the cumulative effects of putative LHB-induced age resetting would be of Hadean zircons on a global scale. Zircons entrained in ejecta are projected to have the following average global distribution after the end of the LHB: ∼59% with no impact-induced Pb*-loss, ∼26% with partial Pb*-loss and ∼15% with complete Pb*-loss or destruction of the grain. In addition to the relatively high erodibility of ejecta, our results show that if discordant ca. 3.9 Gyr old zones in the Jack Hills zircons are a signature of the LHB, they were most likely sourced from impact ejecta.     

Estimating very high resolution urban surface temperature using a spectral unmixing and thermal mixing approach

Land surface temperature (LST) plays a critical role in characterizing energy exchanges of the Earth's surface and atmosphere. Recent advances in thermal infrared (TIR) remote sensing technology enable the emergence of airborne very-high-resolution (VHR) TIR sensors to identify detailed LST distribution for environmental, geological and urban applications. However, the usage of airborne VHR TIR data may be limited by its high cost, long acquisition period, extensive data processing, etc. A cost-effective alternative could be VHR LST estimation. We proposed a physically based method, referred to as the VHR spectral unmixing and thermal mixing (VHR-SUTM) approach, to estimate LST at the meter level. Particularly, considering both spectral and thermal properties, spectral unmixing was employed to estimate fractional urban compositions for a comprehensive representation of heterogeneous urban surfaces. Further, VHR LST was modeled as a summation of the thermal features of representative urban compositions weighted by their respective abundances. Results suggest a high agreement between the resampled VHR LST estimates and the retrieved LSTs. With relatively high estimation accuracy (RMSE of 2.02 K and MAE of 1.51 K), the VHR-SUTM technique could serve as a promising and practical method for various applications in urban and environment studies.     

Urban warming and global warming: Combined effect on thermal discomfort in the desert city of Beer Sheva,Israel

The effect of climatic changes on human comfort levels was investigated through examination and statistical analysis of long-term trends in human discomfort during summer months in the desert city of Beer Sheva, Israel and in the adjacent rural area of Wadi Hatzerim. In an era of global warming, the urban warming effect is likely to be amplified and as a result increase human discomfort, especially during summer.Climate data for the city of Beer Sheva over the last 40 years shows an increase of temperature and air humidity in comparison to the surrounding rural area. Wind velocity data for Wadi Hatzerim show that changes are inconstant and not significant, while in Beer Sheva, wind velocity is significantly reduced. Two indices – the Discomfort Index (DI) and Physiological Equivalent Air temperature (PET) – were used to evaluate the effect of these climatic changes on human discomfort. Although the bio-meteorological indices showed the same tendency of increasing heat stress values and duration, in Beer Sheva they were more pronounced and more significant than in the desert environment. The study concludes that these combined climatic effects negatively impact human comfort and are more noticeable in desert cities at peak daytime hours during summer.     

Analytical modeling of a diver dry suit enhanced with micro-encapsulated phase change materials

Over the years, various forms of phase change materials (PCMs) have been utilized as thermal storage mediums for cooling and heating applications. Recently, the U.S. Navy has investigated using phase change materials inside divers' dry suits to enhance thermal protection in extremely cold water applications. The thermal performance of optional types of insulative Comfortemp¹ foam mediums, medium thicknesses, and foam-to-Thermosorb² ratios are evaluated analytically in this paper to compare the relative thermal performances of PCM-enhanced dry suits with existing commercial dry suits. An analytical model of dry suit systems, both conventional and those containing Comfortemp materials, is developed to predict their thermal performances in simulated ocean environments. This analytical model is used to compare the relative thermal performances of PCM-enhanced dry suits with existing commercial dry suits, and to assess the relative thermal performances of candidate Comfortemp foams, including those interspersed with different weight percentages of micro-encapsulated PCMs, in an effort to maximize thermal storage capabilities. The observations from this modeling effort provides guidelines and recommendations for materials selection and thickness requirements for candidate Comfortemp foams to be used in prototype dry suits for divers.     

First experimental studies of large samples of gas-laden amorphous “cometary” ices

In a unique machine, the first of its kind, large (200 cm² × 10 cm) samples of gas-laden amorphous ice were prepared at 80 K and 10⁻⁵ Torr. The sample consisted of a fluffy agglomerate of 200-μm ice grains, similar to what is presumed to be the structure of comet nuclei. The sample was heated from above by IR radiation. The properties studied were gas content in the ice and its emanation from the ice upon warming and bearing on the gas/water vapor ratio observed in cometary comae vs this ratio in cometary nuclei and the effect of internal trapped gas on the thermal conductivity of the ice and the density and mechanical properties of pure ice vs gas-laden ice. These findings might have significance for the interpretation of comet observations, the forthcoming ESA’s Rosetta space mission to Comet 46P/Wirtanen in 2012, and to other comet missions.     

Thermal volume change of poorly draining soils II: Model development and experimental validation

This study presents the development of an alternative constitutive model for predicting the thermal volume change of saturated and unsaturated soils using the concept of thermally-accelerated secondary compression. The model requires observation of the secondary compression response prior to drained heating to estimate the coefficient of secondary compression. The model was found to successfully capture the trends in thermal volume change arising from accelerated secondary compression resulting from prior mechanical loading for unsaturated, compacted Bonny silt. The model was also used successfully to represent data from the literature for normally consolidated and heavily overconsolidated soils prepared with different loading paths.     

Petrology,geochemistry and thermobarometry of the northern area of the Flamenco pluton,Coastal Range batholith,northern Chile. A thermal approach to the emplacement processes in the Jurassic andean batholiths

The Flamenco pluton is part of a N–S alignment of Late Triassic to Early Jurassic intrusive belt comprising the westernmost part of the Coastal Range batholith in northern Chile. The Jurassic-Cretaceous voluminous magmatism related to subduction in the western active continental margin of Gondwana is emplaced in the predominantly metasedimentary Paleozoic host-rocks of the Las Tórtolas formation, which in the northern area of the Flamenco pluton present an intense deformation, including the Chañaral mélange.Geochemically, the Flamenco pluton shows a wide compositional variability (SiO₂ between 48wt % and 67wt %). Gabbros, Qtz-diorites and tonalites, mesocratic and leucocratic granodiorites are classified as calc-alkaline, calcic, magnesian and metaluminous magmatism. Flamenco granitoids define cotectic linear evolution trends, typical of magmatic fractionation processes. Geochemical trends are consistent with magmas evolved from undersaturated and low-pressure melts, even though the absence of transitional contacts between intrusive units precludes in-situ fractionation. Although some granodioritic samples show crossed geochemical trends that point to the compositional field of metasediments, and large euhedral prismatic pinnite-biotite crystals, typical Crd pseudomorph, are observed in contact magmatic facies, geochemical assimilation processes are short range, and the occurrence of host-rocks xenoliths is limited to a few meters from the pluton contact.A thermal approach to the emplacement process has been constrained through the thermobarometric results and a 2D thermo-numerical model of the contact aureole. Some Qtz-diorites and granodiorites located in the north area of the pluton exhibit granulitic textures as Hbl-Pl-Qtz triple junctions, poikiloblastic Kfs and Qtz recrystallization. The Hbl–Pl pairs have been used for the thermobarometric study of this metamorphic process, resulting granoblastic equilibrium temperatures between 770 and 790 °C, whereas Hbl–Pl pairs in domains that preserve the original igneous textures yield temperatures above 820 °C. This is characteristic of self-granulitization processes during the sequential emplacement of composite batholiths.In addition, the thermal modeling was used in order to compare the expected and observed thermal contact aureole of the intrusive body. Model P-T conditions have been established between 3 and 4 kbars (extracted from the thermobarometric results), and temperatures between 1159 °C (liquidus temperature for a tonalitic composition) and 992 °C (fixed at the rheological threshold of a 50% crystal fraction). The thermal modeling estimates a homogeneous contact aureole, where the established temperatures for the melting reactions in the host-rocks are located at distances between 200 and 650 m from the magma chamber boundary, whereas the temperatures for Crd stabilization extend 1500 m far from the contact in the case of the emplacement at liquidus temperatures and 4 kbars. According to field observations, the contact aureole presents a scarce development in the northern area of the Flamenco pluton, with few migmatite outcrops and less than 1 Km in thickness for Crd-schists. However, in the southern contact, partially melted rocks are described at distances up to 2 km from the Flamenco pluton boundary.The processes of self-granulitization and the differences between the observed and calculated (by the thermal modeling of one single pulse) contact aureole suggests a process of incremental emplacement for the Flamenco pluton, by accretion of magmatic pulses from north to south (in its current position), where the thermal maturity reached through the repeated magmatic intrusion generates a more extensive area of high-grade metamorphism.     

Organic matter geochemistry and petrography of Late Cretaceous (Cenomanian-Turonian) organic-rich shales from the Belle Fourche and Second White Specks formations,west-central Alberta,Canada

Organic-rich mudstones with up to 10 wt% TOC from the upper portion of the Belle Fourche Formation and the lower part of the Second White Specks Formation in the Western Canada Sedimentary Basin were evaluated as source rocks. Both geochemistry and organic petrography indicate an open marine paleoenvironment with deposition of Type II kerogen based on the predominance of marine alginite and amorphous organic matter (OM), limited amounts of terrigenous vitrinite and inertinite macerals, the presence of marine fossils, and the low ratio of TOC to total sulfur (∼1.26). The prevalence of short-chain n-alkanes (n-C₁₃ to n-C₁₉), a predominance of C₂₈ αββ(H)-20S steranes, and small concentrations of oleanane confirm the dominantly algal and planktonic origin of OM. Alternating oxic to anoxic paleoenvironmental sedimentary conditions are proposed based on common bioturbation, abundant inoceramid prisms, and good organic richness. Biomarker distributions are consistent with intermittent anoxia, without unequivocal evidence for water column stratification or hypersalinity. The thermal maturity measured in seven sediment cores by different methods consistently indicates a westward increase in maturity according to vitrinite reflectance, T_max, and hopane and sterane biomarkers. Two cores are thermally immature (∼0.42 %Ro), one is early mature (∼0.65 %Ro), and four cores are within the oil window (∼0.78 to 0.89 %Ro). All thermally mature cores retain good to very good hydrocarbon potential (248 mg HC/g rock) and are dominantly oil-prone and minor gas-prone based on their maceral compositions. The upper Belle Fourche and lower Second White Specks Formations represent potential targets for unconventional light shale oil production.     

http://www.sciencedirect.com/science/article/pii/S1674987114000656

Apatite fission track (AFT) analysis on samples collected from a Paleozoic series is used to constrain the cooling history of the Bogda Mountain, northwest China. AFT ages range from 136.2 to 85.6 Ma a...     

Behavior of 10-Å phase at low temperatures

The thermal evolution of 10-Å phase Mg₃Si₄O₁₀(OH)₂·H₂O, a phyllosilicate which may have an important role in the storage/release of water in subducting slabs, was studied by X-ray single-crystal diffraction in the temperature range 116–293 K. The lattice parameters were measured at several intervals both on cooling and heating. The structural model was refined with intensity data collected at 116 K and compared to the model refined at room temperature. As expected for a layer silicate on cooling in this temperature range, the a and b lattice parameters undergo a small linear decrease, α _a  = 1.7(4) 10^?6 K^?1 and α _b  = 1.9(4) 10^?6 K^?1, where α is the linear thermal expansion coefficient. The greater variation is along the c axis and can be modeled with the second order polynomial c _T  = c ₂₉₃(1 + 6.7(4)10^?5 K^?1ΔT + 9.5(2.5)10^?8 K^?2(ΔT)²) where ΔT = T ? 293 K; the monoclinic angle β slightly increased. The cell volume thermal expansion can be modeled with the polynomial V _T  = V ₂₉₃ (1 + 8.0 10^?5 K^?1 ΔT + 1.4 10^?7 K^?2 (ΔT)²) where ΔT = T ? 293 is in K and V in ų. These variations were similar to those expected for a pressure increase, indicating that T and P effects are approximately inverse. The least-squares refinement with intensity data measured at 116 K shows that the volume of the SiO₄ tetrahedra does not change significantly, whereas the volume of the Mg octahedra slightly decreases. To adjust for the increased misfit between the tetrahedral and octahedral sheets, the tetrahedral rotation angle α changes from 0.58° to 1.38°, increasing the ditrigonalization of the silicate sheet. This deformation has implications on the H-bonds between the water molecule and the basal oxygen atoms. Furthermore, the highly anisotropic thermal ellipsoid of the H₂O oxygen indicates positional disorder, similar to the disorder observed at room temperature. The low-temperature results support the hypothesis that the disorder is static. It can be modeled with a splitting of the interlayer oxygen site with a statistical distribution of the H₂O molecules into two positions, 0.6 Å apart. The resulting shortest O_bas–O_W distances are 2.97 Å, with a significant shortening with respect to the value at room temperature. The low-temperature behavior of the H-bond system is consistent with that hypothesized at high pressure on the basis of the Raman spectra evolution with P.