40Ar39Ar and KAr dating of altered glassy volcanic rocks: the Dabi Volcanics,P.N.G.

KAr and ${}^{40}\text{Ar}{}^{39}\text{Ar}$ ages have been determined for altered submarine tholeiitic and boninite (high-Mg andesite) lavas from the Dabi Volcanics, Cape Vogel Peninsula, Papua New Guinea. ${}^{40}\text{Ar}{}^{39}\text{Ar}$ whole rock total fusion and plateau ages identify a Late Paleocene age for the tholeiitic lavas (58.9 ± 1.1 Ma) and also for the boninitic lavas (58.8 ± 0.8 Ma). Apparent KAr ages for the same samples range from 27.2 ± 0.7 to 63.9 ± 4.5 Ma, and young KAr ages for glassy boninites are probably due to variable radiogenic ⁴⁰Ar (⁴⁰Ar¹) loss. These new ages effectively reconcile previously ambiguous age data for the Dabi Volcanics and indicate contemporaneous tholeiitic and boninitic volcanism occurring in southeast PNG during the Late Paleocene.Smectites, developed as alteration products after glass in oceanic lavas commonly do not retain ³⁹Ar during or subsequent to irradiation, but in some cases may contain ⁴⁰Ar1. In the absence of other factors modifying K and Ar contents, samples which have not lost ⁴⁰Ar¹ from smectite and suffer ³⁹Ar loss only, are interpreted to have been altered immediately subsequent to the crystallization of the lava; whereas samples which have lost ⁴⁰Ar¹ as well as ³⁹Ar may be the result of either recent alteration, or of continuous ⁴⁰Ar¹ loss since the time of crystallization.     

Mass transfer and volume change during alkali metasomatism at Kisingiri,Western Kenya

Mass transfer estimates for metasomatic rocks are dependent on volume-change determinations and a knowledge of the original composition. In particular, such estimates are highly sensitive to uncertainties in the original rock composition. A volume-change model is derived for Kisingiri fenites by considering textural evidence, the known transfer of SiO₂ to the crystallizing ijolite magma, the immobility of Al in low grade fenites, the effect of the stress field around an intrusion, and by considering major element ‘ion content’ concentration gradients across the fenites for various degrees of volume change. The model suggests volume decreases of up to 20% in a zone adjacent to the intrusions and approximately constant volume beyond this zone. Such volume decreases are explained by the removal of quartz by dissolution and diffusion into the crystallizing magma. Aluminium was almost immobile according to this model.     

Chemical and isotopic investigation of warm springs associated with normal faults in Utah

Thermal springs associated with normal faults in Utah have been analyzed for major cations and anions, and oxygen and hydrogen isotopes. Springs with measured temperatures averaging greater than 40°C are characterized by Na + K- and SO₄ + Cl-rich waters containing 10³ to 10⁴ mg/l of dissolved solids. Lower temperature springs, averaging less than 40°C, are more enriched in Ca + Mg relative to Na + K. Chemical variations monitored through time in selected thermal springs are probably produced by mixing with non-thermal waters. During the summer months at times of maximum flow, selected hot springs exhibit their highest temperatures and maximum enrichments in most chemical constituents.Cation ratios and silica concentrations remain relatively constant through time for selected Utah thermal springs assuring the applicability of the geothermometer calculations regardless of the time of year. Geothermometer calculations utilizing either the quartz (no steam loss), chalcedony or Mg-corrected Na/K/Ca methods indicate that most thermal springs in Utah associated with normal faults have subsurface temperatures in the range of 25 to less than 120°C. This temperature range suggests fluid circulation is restricted to depths less than about three kilometers assuming an average thermal gradient of about 40°C/km.Thermodynamic calculations suggest that most thermal springs are oversaturated with respect to calcite, quartz, pyrophyllite, (Fe, Mg)-montmorillonite, microcline and hematite, and undersaturated with respect to anhydrite, gypsum, fluorite and anorthite. Chalcedony and cristobalite appear to be the only phases consistently at or near saturation in most waters. Theoretical evaluation of mixing on mineral saturation trends indicates that anhydrite and calcite become increasingly more undersaturated as cold, dilute groundwater mixes with a hot (150°C), NaCl-rich fluid. The evolution of these thermal waters issuing from faults appears to be one involving the dissolution of silicates such as feldspars and micas by CO₂-enriched groundwaters that become more reactive with increasing temperature and/or time. Solution compositions plotted on mineral equilibrium diagrams trend from product phases such as kaolinite or montmorillonite toward reactant phases dominated by alkali feldspars.Isotopic compositions indicate that these springs are of local surface origin, either meteoric (low TDS, < 5000 mg/l) or connate ground water (high TDS, > 5000 mg/l). Deviations from the meteoric water line are the result of rock-water isotopic exchange, mixing or evaporation. Fluid source regions and residence times of selected thermal spring systems (Red Hill, Thermo) have been evaluated through the use of a σ D-contour map of central and western Utah. Ages for waters in these areas range from about 13 years to over 500 years. These estimates are comparable to those made for low-temperature hydrothermal systems in Iceland.     

Alternating field characteristics of pseudo-single-domain (2–14 μm) and multidomain magnetite

We report normalized AF demagnetization curves of anhysteretic remanences (ARM's) produced by 1-, 10- and 40-Oe steady fields and of saturation isothermal remanence (IRM_s) in a suite of dispersed, unannealed magnetite powders with median sizes of 2, 4, 6, 10 and 14 μm (pseudo-single-domain or PSD size range) and 100 μm (multidomain or MD size). Interpreted in the light of the domain structure test first proposed by Lowrie and Fuller [12], the relative stability trend of curves for the 2 μm sample is of single-domain (SD) type, the 1-Oe ARM being most resistant to demagnetization followed by the 10-Oe and 40-Oe ARM's and IRM_s. For the 100-μm sample, the trend is exactly reversed and is of MD-type. In the 4–14 μm samples, hitherto undescribed transitional trends between SD-type and MD-type occur. At 6 μm, 1-Oe, 10-Oe and 40-Oe ARM's preserve an SD-type trend but for all AF's > 75 Oe, IRM_s is more resistant than any of these remanences. At 10 μm, this trend is unmistakable, and only at 14 μm do the 1-Oe, 10-Oe and 40-Oe ARM curves merge. We conclude (1) that the Lowrie-Fuller test distinguishes between small MD grains enhanced by PSD remanence and large MD grains lacking PSD remanence, rather than between SD and MD structures per se, and (2) that in the PSD transition region from 6 to 14 μm in magnetite, IRM_s changes over to MD-type relative stability around 6 μm, whereas 10-Oe and 40-Oe ARM's achieve an MD-type trend around 14 μm, in accord with the predicted field dependence of the PSD threshold size.Our theoretical interpretation assumes that the intrinsic (internal field) coercive force spectra of weak-field and strong-field remanences are identical but that the observed (external field) spectrum is shifted to lower fields as a result of the internal demagnetizing field — NJ_r of the remanence J_r. The effect is slight for weak-field J_r's but substantial for IRM_s. Since all coercivities, high as well as low, are shifted, the result of the Lowrie-Fuller test is determined simply by the shape of the intrinsic coercivity spectrum or the corresponding AF demagnetization curve. Depending on the model of self-demagnetization used, either subexponential or sublinear AF decay curves of weak-field remanence will automatically lead to an MD-type trend, whereas by either model the decay curves that characterize SD and PSD remanences (decaying slowly initially and then more rapidly) will always produce and SD-type trend.     

Nontronite from a low-temperature hydrothermal system on the Juan de Fuca Ridge

A deposit of Fe-rich, Al-poor, hydrothermal nontronite was recovered from the Juan de Fuca Ridge. Analyses show the deposit to be mineralogically and chemically similar to nontronite described at other oceanic localities. The deposit is located near the tip of a propagating segment of the Juan de Fuca Ridge. Rare earth elements and Sr isotopes indicate that the nontronite precipitated from seawater. A formation temperature of 57°C is suggested by oxygen isotopic composition. The low-temperature nontronite deposits apparently form from newly established hydrothermal systems associated with the propagating rift segment. More mature hydrothermal systems that deposit sulfide on the seafloor may develop from these low-temperature systems.     

DDT,PCB and benzo(a)pyrene levels in white croaker (Genyonemus lineatus) from southern California

In 1978 the California Department of Fish and Game carried out a major, independent sportfishing survey in southern California. The results of that survey indicated that approximately one million angler-trip hours per year were spent fishing and that one in three fish caught was a white croaker (Genyonemus lineatus). There was no attempt by the Department of Fish and Game to determine the disposition of caught fish. We conducted a subsequent survey in 1980 which included 12 popular sportfishing sites in the Los Angeles area. Our results also indicated that one in three fish caught was a white croaker. In addition our survey showed that 82% of the sportfishermen ate the white croaker they caught and the median consumption was 14.8 g day^?1 person^?1. White croaker were then sampled by hook-and-line from 11 of the 12 survey sites plus two control sites and analysed for PCB, DDT and BaP. Median PCB levels ranged from 0.014 to 0.41 mg wet kg^?1; median DDT from 0.059 to 7.5 mg wet kg^?1 and BaP was below detectable limits of 1 ng for all sites.     

Aerial flux of particulate hydrocarbons to the Chesapeake Bay Estuary

Bulk hydrocarbon deposition rates have been measured over a 15 month period at four stations in south-eastern Virginia surrounding the lower Chesapeake Bay. A nearly linear trend of atmospheric particulate deposition was recorded. Deposition rates at the urban station (195 μg m^?2 day^?1) were aproximately three times greater than those recorded for nonurban and coastal locations (mean value 69 μg m^?2 day^?1). The increased levels at the urban location were attributed to localized source inputs. Anthropogenic hydrocarbons accounted for approximately 50% of the total deposition at all stations. Significant biogenic inputs were indicated by the odd/even n-alkane distribution. A minimum flux to the water surface, based on mean nonurban deposition rates (24 mg yr^?1), indicated an annual particulate hydrocarbon flux of +275 metric tons. Little information is available for the comparison of additional source inputs; however, the data reported here indicate that the aerial deposition of hydrocarbons is of the same order of magnitude as the input from municipal wastewater facilities and accidental discharge and is a potentially significant source of hydrocarbon pollution to the Chesapeake Bay Estuary.     

The thermal boundary-layer interpretation of D″ and its role as a plume source

The “anomalous” layer in the lowermost mantle, identified as D″ in the notation of K.E. Bullen, appears in the PREM Earth model as a 150 km-thick zone in which the gradient of incompressibility with pressure, $\text{d}\text{K}\text{d}\text{P}$, is almost 1.6, instead of 3.2 as in the overlying mantle. Since PREM shows no accompanying change in density or density gradient, we identify D″ as a thermal boundary layer and not as a chemically distinct zone. The anomaly in $\text{d}\text{K}\text{d}\text{P}$ is related to the temperature gradient by the temperature dependence of K_s, for which we present a thermodynamic identity in terms of accessible quantities. This gives the numerical result (?K_s/?T)_P=?1.6×10⁷ Pa K^?1 for D″ material. The corresponding temperature increment over the D″ range is 840 K. Such a layer cannot be a static feature, but must be maintained by a downward motion of the lower mantle toward the core-mantle boundary with a strong horizontal flow near the base of D″. Assuming a core heat flux of 1.6 × 10¹² W, the downward speed is 0.07 mm y^?1 and the temperature profile in D″, scaled to match PREM data, is approximately exponential with a scale height of 73 km. The inferred thermal conductivity is 1.2 W m^?1 K^?1. Using these values we develop a new analytical model of D″ which is dynamically and thermally consistent. In this model, the lower-mantle material is heated and softened as it moves down into D″ where the strong temperature dependence of viscosity concentrates the horizontal flow in a layer ～ 12 km thick and similarly ensures its removal via narrow plumes.