A direct determination of the oxygen isotope fractionation between quartz and magnetite at 600 and 800°C and 5 kbar

The partitioning of oxygen isotopes between quartz and magnetite was measured at 600 and 800 C and 5 kbar, using an internally heated pressure vessel equipped with a Shaw-membrane. Through adjustment of the hydrogen fugacity the starting material, fayalite, was decomposed to form quartz and magnetite. The isotopic composition difference between the separated quartz and magnetite fraction was measured directly. As the mineral separates were not completely pure, a correction was applied to the measured fractionations. The independence of the measured isotopic composition differences of run length, the initial isotopic composition of the starting materials as well as the isotopic composition difference between them suggests that the fractionations, 1000 In α_Q-M = 7.8 ± 0.4% at 600°C and 1000 In α_Q-M = 6.1 ± 0.2% at 800°C, represent equilibrium values.     

Gas hydrates (clathrates) causing pore-water freshening and oxygen isotope fractionation in deep-water sedimentary sections of terrigenous continental margins

The occurrence of gas hydrates in deep-water sections of the continental margins predicted from anomalous acoustic reflectors on seismic profiles has been confirmed by recent deep-sea drilling results. On the Pacific continental slope off Guatemala gas hydrates were brought up for the first time from two holes (497, 498A) drilled during Leg 67 of the DSDP in water depths of 2360 and 5500 m, respectively. The hydrates occur in organic matter-rich Pleistocene to Miocene terrigenous sediments. In the hydrate-bearing zone a marked decrease in interstitial water chlorinities was observed starting at about 10–20 m subbottom depth. Pore waters at the bottom of the holes (near 400 m subbottom) have as little as half the chlorinity of seawater (i.e. 9‰). Similar, but less pronounced, trends were observed during previous legs of the DSDP in other hydrate-prone segments of the continental margins where recharge of fresh water from the continent can be excluded (e.g. Leg 11). The crystallization of hydrates, like ice, excludes salt ions from the crystal structure. During burial the dissolved salts are separated from the solids. Subsidence results in a downward motion of the solids (including hydrates) relative to the pore fluids. Thawing of hydrates during recovery releases fresh water which is remixed with the pore fluid not involved in hydrate formation. The volume of the latter decreases downhole thus causing downward decreasing salinity (chlorinity). Hydrate formation is responsible for oxygen isotope fractionation with¹⁸O-enrichment in the hydrate explaining increasingly more positive δ¹⁸O values in the pore fluids recovered (after hydrate dissociation) with depth.     

Stable carbon isotopes of HCO3− in oil-field waters—implications for the origin of CO2

The δ¹³C values of dissolved HCO₃^? in 75 water samples from 15 oil and gas fields (San Joaquin Valley, Calif., and the Houston-Galveston and Corpus Christi areas of Texas) were determined to study the sources of CO₂ of the dissolved species and carbonate cements that modify the porosity and permeability of many petroleum reservoir rocks. The reservoir rocks are sandstones which range in age from Eocene through Miocene. The δ¹³C values of total HCO₃^? indicate that the carbon in the dissolved carbonate species and carbonate cements is mainly of organic origin.The range of δ¹³C values for the HCO₃^? of these waters is ?20–28 per mil relative to PDB. This wide range of δ¹³C values is explained by three mechanisms. Microbiological degradation of organic matter appears to be the dominant process controlling the extremely low and high δ¹³C values of HCO₃^? in the shallow production zones where the subsurface temperatures are less than 80°C. The extremely low δ¹³C values (< ?10 per mil) are obtained in waters where concentrations of SO₄^2? are more than 25 mg/l and probably result from the degradation of organic acid anions by sulfate-reducing bacteria (SO₄^2? + CH₃COO^? → 2HCO₃^? + HS^?). The high δ¹³C values probably result from the degradation of these anions by methanogenic bacteria (CH₃COO^? + H₂OaiHCO₃^? + CH₄).Thermal decarboxylation of short-chain aliphatic acid anions (principally acetate) to produce CO₂ and CH₄ is probably the major source of CO₂ for production zones with subsurface temperatures greater than 80°C. The δ¹³C values of HCO₃^? for waters from zones with temperatures greater than 100°C result from isotopic equilibration between CO₂ and CH₄. At these high temperatures, δ¹³C values of HCO₃^? decrease with increasing temperatures and decreasing concentrations of these acid anions.     

Transition metal ions in silicate melts—I. Manganese in sodium silicate melts

Optical absorption spectra obtained on glasses quenched from sodium silicate melts show Mn³⁺ to be the dominant species for melts heated in air and Mn²⁺ to be the dominant species for melts heated at Po₂ = 10^?17 bar. The absorption spectrum of Mn³⁺ consists of an intense band at 20,000cm^?1 with a 15,000cm^?1 satellite possibly arising from the Jahn-Teller effect. The independence of the spectrum from melt composition and the high band intensity is offered as evidence for a distinct Mn³⁺ complex in the melt. The spectrum of Mn²⁺ is weak and many expected bands are not observed. A two-band luminescence spectrum from Mn²⁺ has been tentatively interpreted as due to Mn²⁺ in interstitial sites in the network and Mn²⁺ coordinated by non-bridging oxygens.     

Cyclic geomorphic patterns of washover on a barrier Island in southeastern North Carolina

A detailed analysis of historic aerial photographs provided the data for determining the magnitude and importance of oceanic overwash on Masonboro Island, southeastern North Carolina. Overwash, which is both temporally and spatially distributed, produces a suite of physiographic features on the subaerial portion of the island. Four physiographic types are recognized including: A) small coalescing loosely vegetated dunes, B) intact, well-vegetated dunes and terraces, C) individual, isolated washover fans, and D) washover terraces. Vegetation patterns, including shrub thickets and black needle rush marshes are related to old fan sites. Sites dominated by saltmeadow cordgrass and goldenrod are associated with recent overwashes. A process-response model, which synthesizes the physiographic types and vegetation patterns, provides input for a management program for the island. Five sections on Masonboro Island are delineated on the basis of washover history and potential for future washovers. The response of Masonboro Island to overwash is similar to that observed on Core Banks, North Carolina; however there are several differences, probably because of the rapidity with which dune ridges redevelop after washovers.     

Effects of annealing on deformation textures in galena

Experimental annealing of galena samples with known deformation histories shows that this mineral has the necessary properties to be a valuable source of information about low-grade deformational environments. Annealed galena displays recovery and/or recrystallization features dependent upon the type of texture inherited from the tectonic event, which in turn is closely linked to deformation temperatures.In samples deformed at temperatures less than 200 ° C in the laboratory, later annealing produced subgrains, mosaics of new grains, or rapid grain boundary migration as the annealing temperatures were varied from 200 ° C to 700 ° C. Kink bands maintained characteristic straight simple boundaries inherited from the deformation event. Samples deformed above 300 ° C developed syntectonically recrystallized textures. Kink bands had been converted to elongate grains with complex sutured grain boundaries during deformation, and mosaics of new grains were found in highly deformed regions. These textures were extremely stable through later annealing. Despite our changing annealing temperatures through 500 ° C, we did not produce similar textures from both low and high temperature deformation runs.Examination of polished and etched galena from low-grade tectonic settings may well be worth the effort since galena textures appear to display features indicative of deformational evironments, even after being subjected to considerable post-tectonic thermal perturbations.     

Chemical fractionations in meteorites—VIII. Iron meteorites and the cosmochemical history of the metal phase

The composition of the metal phase is traced through an idealized, traditional history from condensation, oxidation and accretion in the nebula to melting, segregation and freezing in a parent body. Fifteen elements are considered: Au, Co, Cu, Fe, Ga, Ge, Ir, Mo, Ni, Os, Pd, Pt, Re, Rh and Ru. All are strongly siderophile but differ in volatility and melting-freezing behavior. This simplifies the problem yet provides a means to resolve chemical trends which evolve at different stages in the metal's history.The parent bodies of 5 (IC, IIAB, IIC, IID and IIIAB) of the 12 iron meteorite groups resolved by Scott and Wasson (1975) seem to have had a traditional history. That is condensation, oxidation to various levels, accretion, melting, segregation and fractional crystallization during freezing, presumably in cores. The others seem to have had more unusual histories. The composition of the metal in group IVB matches that predicted for the metal condensate at 1270°K (at$\text{P}{}_{\mathrm{T}}\text{= 10}{}^{\mathrm{?5}}\text{atm}$). This implies accretion at high temperatures; no other combination of the processes can produce this composition. It does not rule out secondary processing, however. The metal in group IVA, whose members have different cooling rates (7–200°/myr), has a composition indicative of aggregates in a body undergoing progressive stages of partial melting. This is consistent with a model in which molten metal collects into pods or raisins at various depths. The composition of the metal in group IAB is indicative of a partial melt which refroze during the initial stages of segregation, before it had managed to aggregate into a single mass. The physical setting implied is consistent with observed inhomogeneities in the metal and abundant inclusions.Three of the 12 groups are deficient in volatiles (IIIF, IVA and IVB) implying a high accretion temperature. In all three cases, cooling rates are comparatively rapid, indicating small bodies or low radioactive element contents. Conceivably all three were deficient in K.     

The late Pleistocene geomagnetic field as recorded by sediments from Fargher Lake,Washington, U.S.A.

Measurement of the remanent magnetization of a 6.88-m oriented core of soft sediments and tephras from Fargher Lake near Mount St. Helens in southwestern Washington State shows that no significant geomagnetic reversals were recorded in the sediments of the lake. Radiocarbon and palynological dating of the tephra layers from the lake bed indicates deposition during the interval 17, 000–34, 000 years B.P. although geochemical correlation of a prominent tephra layer in the core with tephra set C of Mount St. Helens could mean that the maximum age of the sediments may be at least 36, 000 years B.P. The core was divided into specimens 0.02 m long, each representing approximately 55 years of deposition assuming a constant rate of sedimentation. Pilot alternating field demagnetization studies of every tenth specimen indicated a strong, stable remanence with median destructive field of 15 mT, and the remaining specimens were subsequently demagnetized in fields of this strength. The mean inclination for all specimens exclusive of the unstably magnetized muck and peat from near the surface is 56.1° which is 8° shallower than the present axial dipole field at this site, perhaps because of inclination error in the detrital remanent magnetization of the sediments, although because of the variability in the data, this departure from the axial dipole field may not be significant. The ranges of inclination and declination are comparable to those of normal secular variation at northern latitudes. Although three isolated specimens have remanence with negative inclination, these anomalous directions are due to sampling and depositional effects. Measurement of a second core of 6.86 m length also revealed only normal magnetic polarity, but this result is of little stratigraphic value as this core failed to penetrate the distinctive tephra found near the base of the former core.Studies of a concentrate of the magnetic minerals in the sediments by optical microscopy and X-ray diffraction indicate that the primary magnetic constituent is an essentially pure magnetite of detrital origin. The magnetite occurs in a wide range of grain sizes with much of it of sub-multidomain size (< 15 μm).As a whole, this study provides substantial evidence against the existence of large-scale worldwide geomagnetic reversals during the time interval of Fargher Lake sedimentation, a segment of geological time for which many excursions and reversals have been reported elsewhere.