A chlorine isotope study of DSDP/ODP serpentinized ultramafic rocks: Insights into the serpentinization process

Eight DSDP/ODP cores were analyzed for major ion concentrations and δ³⁷Cl values of water-soluble chloride (δ³⁷Cl_WSC) and structurally bound chloride (δ³⁷Cl_SBC) in serpentinized ultramafic rocks. This diverse set of cores spans a wide range in age, temperature of serpentinization, tectonic setting, and geographic location of drilled serpentinized oceanic crust. Three of the cores were sampled at closely spaced intervals to investigate downhole variation in Cl concentration and chlorine isotope composition.The average total Cl content of all 86 samples is 0.26 ± 0.16 wt.% (0.19 ± 0.10 wt.% as water-soluble Cl (X_WSC) and 0.09 ± 0.09 wt.% as structurally bound Cl (X_SBC)). Structurally bound Cl concentration nearly doubles with depth in all cores; there is no consistent trend in water-soluble Cl content among the cores. Chlorine isotope fractionation between the structurally bound Cl⁻ site and the water-soluble Cl⁻ site varies from − 1.08‰ to + 1.16‰, averaging to + 0.21‰. Samples with negative fractionations may be related to reequilibration of the water-soluble chloride with seawater post-serpentinite formation. Six of the cores have positive bulk δ³⁷Cl values (+ 0.05‰ to + 0.36‰); the other two cores (173-1068A (Leg-Hole) and 84-570) have negative bulk δ³⁷Cl values (− 1.26‰ and − 0.54‰). The cores with negative δ³⁷Cl values also have variable Cl⁻ / SO₄² ratios, in contrast to all other cores. The isotopically positive cores (153-920D and 147-895E) show no isotopic variation with depth; the isotopically negative core (173-1068A) decreases by ∼1‰ with depth for both the water-soluble and structurally bound Cl fractions.Non-zero bulk δ³⁷Cl values indicate Cl in serpentinites was incorporated during original hydration and is not an artifact of seawater infiltration during drilling. Cores with positive δ³⁷Cl values are most likely explained by open system fractionation during hydrothermal alteration, with preferential incorporation of ³⁷Cl from seawater into the serpentinite and loss of residual light Cl back to the ocean. Fluid / rock ratios were probably low as evidenced by the presence of water-soluble salts. The two isotopically negative cores are characterized by a thick overlying sedimentary package that was in place prior to serpentinization. We believe the low δ³⁷Cl values of these cores are a result of hydration of ultramafic rock by infiltrating aqueous pore fluids from the overlying sediments. The resulting serpentinites inherit the characteristic negative δ³⁷Cl values of the pore waters. Chlorine stable isotopes can be used to identify the source of the serpentinizing fluid and ultimately discern chemical and tectonic processes involved in serpentinization.     

Giant swarms of Precambrian mafic dikes and potential diamond resources of ancient platforms

By the examples of the Siberian Platform and Canadian Shield, it is shown that spatial juxtaposition of Phanerozoic diamond-bearing kimberlite fields with giant swarms of Precambrian mafic dikes is caused by both systematic and incidental events. The first of these include (1) origination of mantle plumes and associated lenses of high-temperature mantle melting in the subequatorial “hot belt” of the early Earth, (2) formation of magma chambers that generated mafic dikes in these asthenospheric lenses, (3) shear stress, and (4) ultrahigh-pressure metamorphism of igneous and country rocks. As a result, the association of diamond-bearing high-density mafic and ultramafic rocks was formed under favorable thermal and fluidal conditions. These processes occurred first in the embryonic (multiplate) Neoarchean tectonic setting at a depth of 40–60 km (present-day elevation marks) and then at a deeper (100–150 km) level during the transition to the Proterozoic true plate tectonics. These processes left behind giant swarms of Precambrian mafic dikes, as well as structurally and genetically related deep-seated morphological and density anomalies. The relatively high position of two lithospheric units of diamond-bearing rocks, each underlain by a thick layer of the cold mantle, prevented these units from thermal and mechanical erosion during subsequent plate-tectonic stages characterized by deeper location of asthenospheric layers. The occurrence of clusters of Phanerozoic diatremes in ancient giant swarms of mafic dikes, as well as the enrichment of pipes in xenogenic diamond-bearing material derived from different levels of the tectonically delaminated lithosphere, may be attributed to incidental events that controlled the fertility of a relatively small number of kimberlite pipes.     

Formation of iron-containing colloids by the weathering of phyllite

The formation of colloids during the weathering of phyllite was investigated by exposing ground phyllite to Milli-Q water. Secondary mineral colloids of 10¹–10² nm were detected in significant concentrations. At pH of about 8.5, the solution concentration of these colloids reached up to 10 mg/L (however, acidification to pH 4.0 prevented the formation of the colloids). The mineralogical composition of the secondary mineral colloids is assumed to be a mixture of ferrihydrite, manganese oxyhydroxides, aluminosilicates, amorphous Al(OH)₃ and gibbsite with possible additions of iron silicates and␣iron-alumino silicates. The colloids were stable over longer periods of time (at least several weeks), even in the presence of suspended ground rock. Direct formation of iron-containing secondary mineral colloids at the rock–water interface by the weathering of rock material is an alternative to the well-known mechanism of iron colloid formation in the bulk of water bodies by mixing of different waters or by aeration of anoxic waters. This direct mechanism is of relevance for colloid production during the weathering of freshly crushed rock in the unsaturated zone as for instance crushed rock in mine waste rock piles. Colloids produced by this mechanism, too, can influence the transport of contaminants such as actinides because these colloids have a large specific surface area and a high sorption affinity.     

Coexistence of garnet blueschist and eclogite in South Tianshan,NW China: dependence of P–T evolution and bulk‐rock composition

Coexisting garnet blueschist and eclogite from the Chinese South Tianshan high‐pressure (HP)–ultrahigh‐pressure (UHP) belt consist of similar mineral assemblages involving garnet, omphacite, glaucophane, epidote, phengite, rutile/sphene, quartz and hornblendic amphibole with or without paragonite. Eclogite assemblages generally contain omphacite >50 vol.% and a small amount of glaucophane (<5 vol.%), whereas blueschist assemblages have glaucophane over 30 vol.% with a small amount of omphacite which is even absent in the matrix. The coexisting blueschist and eclogite show dramatic differences in the bulk‐rock compositions with higher X(CaO) [=CaO/(CaO + MgO + FeO_total + MnO + Na₂O)] (0.33–0.48) and lower A/CNK [=Al₂O₃/(CaO + Na₂O + K₂O)] (0.35–0.56) in eclogite, but with lower X(CaO) (0.09–0.30) and higher A/CNK (0.65–1.28) in garnet blueschist. Garnet in both types of rocks has similar compositions and exhibits core–rim zoning with increasing grossular and pyrope contents. Petrographic observations and phase equilibria modelling with pseudosections calculated using thermocalc in the NCKMnFMASHO system for the coexisting garnet blueschist and eclogite samples suggest that the two rock types share similar P–T evolutional histories involving a decompression with heating from the P_max to the T_max stage and a post‐T_max decompression with slightly cooling stage, and similar P–T conditions at the T_max stage. The post‐T_max decompression is responsible for lawsonite decomposition, which results in epidote growth, glaucophane increase and omphacite decrease in the blueschist, or in an overprinting of the eclogitic assemblage by a blueschist assemblage. Calculated P–X(CaO), P–A/CNK and P–X(CO₂) pseudosections indicate that blueschist assemblages are favoured in rocks with lower X(CaO) (<0.28) and higher A/CNK (>0.75) or fluid composition with higher X(CO₂) (>0.15), but eclogite assemblages preferentially occur in rocks with higher X(CaO) and lower A/CNK or fluid composition with lower X(CO₂). Moreover, phase modelling suggests that the coexistence of blueschist and eclogite depends substantially on P–T conditions, which would commonly occur in medium temperatures of 500–590 °C under pressures of ~17–22 kbar. The modelling results are in good accordance with the measured bulk‐rock compositions and modelled temperature results of the coexisting garnet blueschist and eclogite from the South Tianshan HP–UHP belt.     

Feasibility,emission and fuel requirement analysis of hybrid car versus solar electric car: a comparative study

The problem regarding environment has been considered as contemporary issue, and to cater this, various technologies have been revolutionized in vehicle transport field. Efforts have been made to make vehicle engine efficient and introducing hybridized vehicles with the aim of reducing emissions and less fuel dependency. In essence of this, trends of solar electric cars in different countries have been reviewed. Feasibility analysis is done by doing fuel cost analysis of two cases, i.e., simple hybrid vehicle and hybrid vehicle equipped with solar module and increased battery energy storage capacity for a specific round trip distance between two cities, i.e., Rawalpindi and Islamabad in comparison with feasibility of third case, i.e., proposed solar electric car model. The solar module selection along with desired number of batteries with charging and discharging time and motor power required to carry five persons weight (70 kg each) is calculated for third case. Moreover, total carbon dioxide emission analysis has been carried out from car material production to its assembly, manufacturing solar module and nickel metal hydride battery for each case. The annual carbon dioxide emitted from fuel in first two cases relative to electric outlet in third case for specific distance has been analyzed. On large scale, emission analyses for hundred cars of each case have been done at 100 km distance. From calculations, it is revealed that overall emissions in third case on large scale and from its material production, assembly, solar module and batteries manufacturing perspective are comparatively less than other cases.     

The energetics of nanophase calcite

Calcium carbonate (CaCO₃) is an important component of the near-surface environment. Understanding the nature of its precipitation is important for a variety of environmental processes, as well as for the geologic sequestration of anthropogenic carbon dioxide. Calcite is the most thermodynamically stable bulk polymorph, but energy crossovers may exist that could favor the precipitation of vaterite or aragonite with decreasing particle size. The purpose of this study is to determine the surface energy of calcite, which is the first step towards understanding the effect of particle size on thermodynamic stability in the calcium carbonate system. The enthalpies of five well-characterized calcite samples (four nanophase and one bulk) were measured by acid solution isothermal and water adsorption calorimetric techniques. From the calorimetric data, the surface energies of calcite were determined to be 1.48 ± 0.21 and 1.87 ± 0.16 J/m² for hydrous and anhydrous surfaces. These values are similar to those measured for many oxides but larger than predicted from computational models for idealized calcite surfaces. The surfaces of synthetic CaCO₃ particles contain a range of planes and defect structures, which may give rise to the difference between the experimental and modeled values.     

Geochronology and genesis of subalkaline basaltic lava rivers at the Dzhavakheti highland,lesser Caucasus: K-Ar and Sr-Nd isotopic data

The paper reports newly obtained K-Ar isotopic-geochronological data on the age of three lava flows (Khrami, Mashavera, and Kura), which begin at the Dzhavakheti volcanic highland in southern Georgia. All of the dated rocks, including those from the Kura Flow, which was previously considered as the Pleistocene, are demonstrated to have a Pliocene age. The lavas of the longest Khrami Flow were erupted at 3.25–3.10 Ma, and those of the Kura and Mashavera Flows at 2.20–2.05 Ma, a fact testifying to two pulses of volcanic activity at the Dzhavakheti Highland. The petrogeochemical and isotopic characteristics of the rocks (⁸⁷Sr/⁸⁶Sr = 0.7039–0.7042; ∈_Nd = 3.4–5.1) indicate that they are subalkaline within-plate basalts formed by the fractional crystallization of a basic mantle melt with the usually discontinuous selective or rarely continuous contamination with material that was not in geochemical equilibrium with the melt. The volcanics of the Khrami Flow are characterized by the less radiogenic Sr isotopic composition and the highest ∈_Nd values, while the younger rocks of the Mashavera and Kura Flows have similar and more “crustal” isotopic signatures. The ⁸⁷Sr/⁸⁶Sr ratios of the Dzhavakheti subalkaline basalts are close to the initial Sr isotopic ratios of the Quaternary and Middle Pliocene dacite lavas from the same territory. Considered together with petrogeochemical and geological data, this suggests that all young rocks in Southern Georgia were produced in similar tectonic and geodynamic environments.     

Microscopic Fracture Processes in a Granite

Summary The deformation of a competent, brittle, granitic rock is thought to have two main components: elastic and brittle deformation, the latter caused by axial microcracking. Dynamic fatigue testing of Lac du Bonnet granite would, however, suggest the presence of a third mechanism, compaction. Compaction is not the same as elastic crack closure; compaction entails permanent damage along grain boundaries that are under high compression. During compaction, the axial stiffness (elastic modulus) of the rock increases and the permanent crack volume becomes negative (compression). Compaction is active at all stress levels, but it is most noticeable at low stress where its presence is not masked by dilation caused by axial microcracking.