Controls on the stability of sulfide sols: Colloidal covellite as an example

The conditions under which dilute CuS sols might be stable in natural waters have been explored. In single electrolyte solutions at room temperature, coagulation data adhere approximately to the classic theory for electrostatic stabilization. From this theory, the Hamaker constant is estimated to be 4 × 10^?19J. In a simple, four-component, synthetic seawater, coagulation occurs at 0.14 g/kg salinity. The critical coagulation concentration for CaCl₂ drops by a factor of six between 25 and 175°C. These measurements, which appear to be the first over so large a temperature range, confirm previous suppositions about the effect of temperature. The results suggest that electrostatic stabilization will be ineffective in nature for substances like CuS with large Hamaker constants, because divalent cation concentrations in most aquatic environments are sufficient for coagulation. Three alternate mechanisms by which organic compounds can stabilize sulfide sols are reported: chelation of counterions. surface hydrophillization through adsorption, and steric stabilization by polymers. Humic acid, which contributes stability when present at less than 1 mg C/1, may stabilize by all three mechanisms. However, theory indicates that even in the absence of chemical stabilization, extremely dilute sols such as would be encountered in anoxic marine basins might require many years to coagulate, thus appearing stable even if not.     

A carbonaceous inclusion from the Krymka LL-chondrite: noble gases and trace elements

A black inclusion from the Krymka LL3 chondrite was analyzed for 20 trace elements and five noble gases, by radiochemical neutron activation and mass spectrometry. The trace element pattern somewhat resembles that of C1 or C2 chondrites, but with several unique features. Elements of nebular condensation T ? 1000 K (U, Re, Os, Ir, Ni, Pd, Au, Sb and Ge) are essentially undepleted, as in C1 chondrites, but $\text{Re}\text{Ir}$ is 1.49 × higher than the characteristic Cl value. Among elements condensing below 1000 K, Cs, Se, Te, and In are depleted to approximately C2 levels (~0.6 × C1), whereas Ag, Bi, Tl are enriched to ~ 1.6 × C1. Such enrichments are thought to be characteristic of late nebular condensates.The noble-gas pattern also is unique. Gas contents are higher than in C1s, by factors of 2.6 to 19 for Ne through Xe. The $\text{Ar}{}^{36}\text{Xe}{}^{132}$ ratio of 500 is higher than mean values for C1s or C2s (109 or 89) and exceeds even the highest value seen in C3Os, 420, whereas the $\text{He}{}^4\text{Ne}{}^{20}$ ratio of 62 is much lower than the values for C1s and C2s (200–370). The $\text{Xe}{}^{129}\text{Xe}{}^{132}$ and $\text{Xe}{}^{136}\text{Xe}{}^{\mathrm{l32}}$ ratios of 1.040 and 0.320 resemble those of C1 chondrites, and seem to imply typical proportions of radiogenic Xe¹²⁹ and ‘fissiogenic’ xenon.It appears that the inclusion represents a new primitive meteorite type, similar to C-chondrites, but probably a late condensate from a region of higher nebular pressure.     

Evolution of the Taupo-Hikurangi subduction system

The present day Taupo-Hikurangi subduction system is a southward extension of the Tonga-Kermadec Arc system into a sediment-rich continental margin environment. It consists of a shallow structural trench (the Hikurangi Trough), a 150 km wide, imbricate thrust controlled accretionary borderland (the continental slope, shelf, and coastal hills of eastern North Island), a frontal ridge (the main “greywacke” ranges of North Island), and a volcanic arc and marginal basin (the Taupo Volcanic Zone).Structural elements become progressively more elevated and subduction more oblique towards the south. The whole NNE-trending system is truncated at a largely strike-slip, transform boundary that extends along the southwestern part of the Hikurangi Trough and the Hope fault of South Island to the main Alpine Fault.The volcanic arc is 200–270 km from the structural trench and comprises a NNE trending chain of andesite-dacite volcanoes extending along the eastern side of the Taupo Volcanic Zone. Most of the andesites are olivine-bearing and have been erupted within the last 50,000 years.It is suggested the Taupo-Hikurangi margin has evolved by rotation of accretionary elements, from an original NW-trending subduction system north of New Zealand. The older elements of the prism were associated with subduction of a re-entrant of the Pacific Plate (and perhaps the South Fiji Basin) in Mid Tertiary times. They subsequently became separated from their NW-trending volcanic arc by dextral strike-slip movement along curved faults east of the main “greywacke” ranges. During the Plio-Pleistocene, oblique subduction and accretion intensified as the Taupo-Hikurangi margin rotated into line with the NNE-trending Kermadec system and a marginal basin was developed along a similar trend to form the Taupo Volcanic Zone. Within the last 50,000 years olivine-bearing andesite volcanism has commenced along the eastern side of the Taupo Volcanic Zone.     

Rare gases in separated whitlockite from the St. Severin chondrite: xenon and krypton from fission of extinct 244Pu

Helium, neon, argon, krypton and xenon data are presented from stepwise heating of samples of the mineral whitlockite from the chondritic meteorite St. Severin. The xenon is shown to be a uniform mixture derived from ²⁴⁴Pu fission and rare-earth element spallation. The krypton similarly contains spallation products and ⁸⁶Kr from ²⁴⁴Pu fission. Plutonium-244 fission yields of ⁸⁶Kr/¹²⁹Xe/¹³¹Xe/¹³²Xe/¹³⁴Xe/¹³⁶Xe = 1.9 ± 0.5/4.8 ± 5.5/24.6 ± 2.0/88.5 ±3.0/93.9 ± 0.8/  100 are obtained. The helium, neon and argon are dominated by spallation and radiogenic ⁴He and ⁴⁰Ar. A pile neutron irradiation experiment does not yield a unique Pu/U ratio for this mineral but yields ratios varying from 0.045 down to 0.017. Both the high concentration of ²⁴⁴Pu fission xenon and the high ratio of Pu/U previously reported by Wasserburget al. (J. Oeophys. Res. 74, 4221–4232, 1969) are thus confirmed.     

Mineralogy of the ejected plutonic blocks of the soufriere volcano St. Vincent: Olivine,pyroxene, amphibole and magnetite paragenesis

Ejected plutonic blocks from the Soufrière volcano, St. Vincent, consist of the mineral phases plagioclase (An₉₆?An₈₉, average An₉₃), olivine (Fo₇₉?Fo₆₇, most frequent interval Fo₇₄?₇₂), salite containing 5–6 percent Al₂O₃, hastingsitic amphibole and magnetite containing 6% Al₂O₃, 4% MgO and 7% TiO₂. Aluminous salitic pyroxenes are not confined to alkaline suites but also occur as high temperature phases in certain calc-alkaline suites. Similarly low silica amphiboles with comparatively high Fe⁺³+Ti contents are unusual in common sub-alkaline rocks. Magnetite in the St. Vincent blocks is an example of a single homogeneous iron oxide phase precipitated in equilibrium with the other four phases in the blocks. Conditions of crystallisation are just as important as silica activity in determining the compositions of phases separating from basaltic magmas at relatively shallow depths (P _tot< 8 kb). It is argued mainly on crystal chemical grounds that the mineral assemblage in the St. Vincent blocks can crystalline from a sub-alkaline magma under conditions of relatively high temperature, high water vapor pressure, and oxygen fugacity.     

Background concentrations of photochemically active trace constituents in the stratosphere and upper troposphere

Summary High resolution absorption spectroscopy can be used for obtaining very small background concentrations or very low upper limits of trace constituents, particularly from measurements inQ-branches of fundamental vibration-rotation bands. As examples, the results of a search for NH₃ and SO₂ in the Vermande and Dionne Mont-Louis solar spectra will be reported. The relative concentration of NH₃ is very much lower above the Pyrenees than the generally accepted minimum background concentrations near sea-level.     

Metal/silicate fractionation in the solar system

Fractionation between the metal and silicate components of objects in the inner solar system has long been recognized as a necessity in order to explain the observed density variations of the terrestrial planets and the H-group, L-group dichotomy of the ordinary chondrites. This paper discusses the densities of the terrestrial planets in light of current physical and chemical models of processes in the solar nebula. It is shown that the observed density trends in the inner solar system need not be the result of special fractionation processes, and that the densities of the planets may be direct results of simultaneous application of both physical and chemical restraints on the structure of the nebula, most notably the variation of temperature with heliocentric distance. The density of Mercury is easily attributed to accretion at temperatures so high that MgSiO₃ is only partially retained but Fe metal is condensed. The densities of the other terrestrial planets are shown to be due to different degrees of retention of S, O and H as FeS, FeO and hydrous silicates produced in chemical equilibrium between condensates and solar-composition gases. It is proposed that Mercury and Venus Have cores of Fe⁰, Earth has a core of Fe⁰ containing substantial amounts of FeS, and Mars has a quite small core of FeS with more FeO in its mantle than in Earth's. Geophysical and geochemical consequences of these conclusions are discussed.     

Volatile element chemistry in the solar nebula: Na,K, F,Cl, Br,and P

The results of the most extensive set to date of thermodynamic calculations of the equilibrium chemistry of several hundred compounds of the elements Na, K, F, Cl, Br, and P in a solar composition system are reported. The calculations are carried out over a wide range of temperatures and pressures and along an adiabat in the primitive solar nebula. Two extreme models of accretion are investigated. In one extreme complete chemical equilibrium between condensates and gases is maintained because the time scale for accretion is long compared to the time scale for cooling or dissipation of the nebula. Condensates formed in this homogeneous accretion model include several phases such as whitlockite, alkali feldspars, and apatite minerals which are found in chondrites. In the other extreme complete isolation of newly formed condensates from prior condensates and gases occurs due to a time scale for accretion that is short relative to the time required for nebular cooling or dissipation. The condensates produced in this heterogeneous accretion model include alkali sulfides, ammonium halides, and ammonium phosphates. None of these phases are found in chondrites. Available observations of the Na, K, F, Cl, Br, and P elemental abundances in the terrestrial planets are found to be compatible with the predictions of the homogeneous accretion model.