On the appearance of PCBs in the surficial sediments of Casco Bay,Maine     

Peter F. Larsen  Donald F. Gadbois  Anne C. Johnson  Richard F. Maney 《Marine pollution bulletin》1984,15(12):452-453

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Sediment PCB distribution in the Penobscot Bay region of the Gulf of Maine     

Peter F. Larsen  Donald F. Gadbois  Anne C. Johnson 《Marine pollution bulletin》1984,15(1):34-35

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Triple junction magmatism: a geochemical study of Neogene volcanic rocks in western California   总被引：1，自引：0，他引：1  

Clark M. Johnson  James R. O&#x;Neil 《Earth and Planetary Science Letters》1984,71(2):241-262

Inception of volcanism at late Oligocene to Recent centers in the eastern Coast Ranges of California (ECR suite) regularly decreases in age northward and is correlated with the northward migration of the transform-transform-trench Mendocino triple junction (MTJ). Miocene volcanism in the southern California basin (SCB suite) is spatially and temporally associated with the transform-ridge-trench Rivera triple junction (RTJ). The tholeiitic to calc-alkaline rocks in both suites were erupted through older trench melange while arc magmatism was occurring several hundred kilometers to the east. Therefore they are not related to subduction zone magmatism, but instead to interactions of the MTJ and RTJ with the continental margin.The ECR rocks, dominantly intermediate to silicic in composition, have relatively high δ¹⁸O values up to 11.3, ⁸⁷Sr/⁸⁶Sr ratios up to 0.7055, as well as relatively high Th contents, suggesting that crustal anatexis played a dominant role in their generation. Coupled crystal fractionation and crustal assimilation by an initially basaltic magma cannot explain the high δ¹⁸O values and ⁸⁷Sr/⁸⁶Sr ratios because greater than 95% of the basalt would need to crystallize. In contrast, the SCB rocks, dominantly mafic to intermediate in composition, have relatively low δ¹⁸O values down to 5.2 and ⁸⁷Sr/⁸⁶Sr ratios down to 0.7025 suggesting that these rocks were derived dominantly from a mantle source.Whether crustal anatexis occurs is determined largely by the type of stress a triple junction imposes upon the continental margin. Both the MTJ and RTJ are associated with high heat flow and magma fluxes from the mantle. The transform-transform-trench MTJ is associated with locally variable mild extension to compression and therefore allows pooling of basaltic magma in the crust to initiate crustal melting. The high rates of continental extension associated with the transform-ridge-trench RTJ prevents such pooling of magma.The space created by decoupling of the subducted slab at a transform-transform-trench triple junction might promote passive upwelling of mantle material to fill it and induce melting to generate basalts. Mafic volcanic rocks of this origin may provide a unique view of the subcontinental mantle at the continental margin. ⁸⁷Sr/⁸⁶Sr ratios as low as 0.70255 for mafic volcanic rocks in the Sonoma-Tolay center associated with the MTJ contrast with high La/Sm ratios of 1.1 to 1.3 and low Zr/Nb,Hf/Ta,La/Th, and La/Ta ratios of 5.0 to 6.7, 2.6 to 3.5, 4.7 to 8.8, and 10.2 to 12.5, respectively. These data suggest that the mantle beneath parts of western California may have originally been depleted but has been enriched relatively recently. Such enrichment might have occurred by metasomatic processes associated with crustal accretion and/or juxtaposition of differing lithospheric mantle in the Mesozoic and Cenozoic.  相似文献   

Dredged basalts from the western Nazca plate and the evolution of the East Pacific Rise     

John Campsie  G. Leonard Johnson  Mogens H. Rasmussen  Jens Laursen 《Earth and Planetary Science Letters》1984,68(2):271-285

Ocean-floor basalts and glasses were recovered from three stations along the western Nazca plate, from a sublinear topographic feature believed to represent the proto-East Pacific Rise (EPR), and include abyssal tholeiites, FeTi-basalts and glasses, as well as transitional and little fractionated compositions. When compared with their coexisting fresh glasses, the FeTi-basalts have higher total alkalies, TiO₂ and MgO, and lower FeO^*, suggesting that they have also been affected by non-oxidative post-magmatic alteration processes. The FeTi-glasses form a remarkably uniform compositional group through space and time. A little fractionated composition having anMg-number= 73, similar to those reported from the Mathematician Ridge, has higher Na₂O and TiO₂, and slightly lower CaO than similar compositions from the slowly accreting Mid-Atlantic Ridge. The basalts and glasses reported here exhibit the compositional diversity expected for propagating rifts and probably represent more than one volcanic episode.Both geochemical and geophysical interpretations support the inference that the EPR grew from Miocene times by the progressive growth and propagation of mantle perturbations, leaving a remnant sublinear zone of rough topography characteristic of slower accretion as the trace of the proto-EPR. Continuing translations and rotations of axial segments are occurring along the EPR, probably in response to self-reorganizations of mantle flow patterns arising from rapid melting and depletion of the source regions. The data allow the inference that the youthful rift systems of the eastern Pacific are far from thermodynamic equilibrium as might be expected if such systems were to drive fundamental life processes.  相似文献   

Experimental determination of the partitioning of gallium between solid iron metal and synthetic basaltic melt: Electron and ion microprobe study     

Michael J. Drake  Horton E. Newsom  Stephen J.B. Reed  M.Clare Enright 《Geochimica et cosmochimica acta》1984,48(8):1609-1615

The distribution of Ga between solid Fe metal and synthetic basaltic melt has been investigated experimentally at two temperatures over a limited range of oxygen fugacities. Reversal experiments were conducted, indicating a close approach to equilibrium. Analysis of run products was performed using an electron and an ion microprobe. At 1 bar total pressure, the solid metal/silicate melt partition coefficient. D(Ga), is given by: 1190°C: logD(Ga) = ?0.92 logfO₂ ? 11.91330°C: logD(Ga) = ?0.77 logfO₂ ? 8.8. For the common assumption of a valence state for Ga of 3 in silicates and assuming ideal solution of Ga in both phases, a slope of ?0.75 is predicted. The slope obtained at 1300°C (?0.77) is indistinguishable from this value, while the slope at 1190°C (?0.92) is somewhat higher. Henry's law is obeyed over the concentration range 0.007 wt.% to 0.15 wt.% Ga in metal, a factor of 20 in concentration. These partition coefficients may be utilized to evaluate metal/silicate fractionation processes in the Earth, Moon and Eucrite Parent Body. The lunar mantle appears to be depleted in Ga by a factor of 20–40 relative to CI abundances. This depletion is consistent with extraction of Ga into a geophysically plausible lunar core if Ga was initially present at a subchondritic concentration. A similar explanation probably accounts for the depletion of Ga in the Eucrite Parent Body. The upper mantle of the Earth appears to be depleted in Ga by a factor of 4– 7 relative to CI abundances. This depletion is far smaller than would be expected as a result of core formation, but is consistent with two quite different hypotheses: (i) a small amount of solid metal and sulfur-bearing metallic liquid was retained in the mantle after core formation; or (ii) addition of a late-stage chondritic component involving 5% to 10% of the upper mantle following core formation.  相似文献   

Summary of Arctic Geophysics     

H.R Jackson  G.L Johnson 《Journal of Geodynamics》1986,6(1-4)

The Cenozoic history of the Eurasian Basin is well understood because it involves the Eurasian (EU) and North American (NA) plates and is therefore constrained by data from more southerly regions and contains a readilly decipherable magnetic pattern. Reconstruction of the older portion of the Arctic Ocean is more difficult; however, information collected on ice station CESAR in 1983, interpreted in conjunction with regional geologic and geophysical data, provides insight into its oceanic affinities and age. A dredged outcrop of the Alpha ridge, consisted of weathered fragmental alkaline volcanic rocks. Refraction data reveal a thick crust nearly 40 km and a high velocity lower crust on this ridge. The basal layer velocity is typical of all plateaus known to be oceanic crust. It is hypothesized that Alpha Ridge therefore represents a late Cretaceous oceanic plateau. A more recent analogue is the Iceland-Faeroe Ridge. The magnetic information are shown to be consistent with this interpretation of oceanic crust. The Amerasia Basin is closed by rotating the Arctic-Alaska plate against NA during the Cretaceous. This reconstruction, its timing and its position are consistent with the geology of the Canadian Arctic Islands and Alaska.  相似文献   

A theory of concentric, kink, and sinusoidal folding and of monoclinal flexuring of compressible, elastic multilayers: I. Introduction     

Arvid M. Johnson  Stephenson D. Ellen 《Tectonophysics》1974,21(4):301-339

Most folds in stratified rock are similar in form to ideal kink, concentric or chevron folds, in which there are discontinuities in slope or curvature of bedding planes. In this respect most folds appear to be closely related to faults, traces of which can be considered to be lines across which there are discontinuities of displacement of layers. Further, the close association of reverse faults and folds or monoclinal flexures seems to indicate that theories of faulting and folding should be closely related.The theory of characteristics is a mathematical tool with which we can obtain insights into processes involving discontinuities. Theoretical characteristic lines are directions across which certain variables might be discontinuous and they are directions along which discontinuities propagate. The theory has been widely applied in plasticity theory and in fluid mechanics and theoretical studies of faulting have suggested that faults are analogous to the lines of discontinuity predicted by plasticity theory. Elasticity and viscosity theories, on which theories of folding have been founded, exclude the existence of characteristic lines in the materials unless the equilibrium equations, rheological properties or strains are nonlinear. However, all folding theories are nonlinear to some extent and the theories can be modified so that they predict lines of discontinuity for some conditions of loading and deformation.Theories of folding will be developed in subsequent papers of this series in order to predict conditions under which characteristic lines can exist in multilayered materials and in order to determine the conditions that must be satisfied across and along the characteristic lines. The theory should help us to recognize lines of apparent discontinuity in natural and experimental folds and study of these lines should provide further understanding of mechanisms of folding.Experimental studies of folding of a wide variety of materials, including alternating layers of rubber and gelatin, modeling clay and grease or graphite, and potter's clay and rubber or cardboard, suggest that the patterns of folding in these materials begin with sinusoidal forms, transform into concentric or kink forms and then into chevron forms as the multilayers are shortened axially. A suitable theory of folding of multilayers should account for these observations.  相似文献   

Strontium isotope ratios of quaternary volcanic rocks from Papua New Guinea     

R.W Page  R.W Johnson 《Lithos》1974,7(2):91-100

Strontium isotope data for Quaternary volcanic rocks from six separate areas in Papua New Guinea suggest a subdivision into two broad groups. One group consists of island volcanoes that have lower Sr³⁷/Sr³⁸ ratios (0.7034–0.7043) and show isotopic homogeneity within each area sampled. Except for the St. Andrew Strait samples, it is considered that these rocks were produced from relatively homogeneous source regions in the upper mantle. Volcanoes of the second group (on the Papua New Guinea mainland) have generally a higher and wider range of Sr³⁷/Sr³⁶ ratios, compared to those of the first group. It is thought that the magmas of these mainland volcanoes were affected by different degrees of sialic crustal contamination, or were derived from heterogeneous sources in the upper mantle.  相似文献   

A longitudinal seismic reflection profile of the Reykjanes Ridge: Part II — Implications for the mantle hot spot hypothesis     

P.R. Vogt  G.L. Johnson 《Earth and Planetary Science Letters》1973,18(1):49-58

A longitudinal seismic reflection profile of the Reykjanes Ridge, together with earthquake seismicity patterns, is interpreted in terms of the mantle plume hypothesis. Between 52°N and 57°N Reykjanes Ridge is cut by about 12 fractures whose trend, inferred from other data, is approximately east-west. North of 57° there is little or no indication of east-west fracturing.The 57°N transition from fractured to unfractured basement occurs about 900 km southwest of the postulated Iceland mantle plume. The fractured province exhibits higher seismicity and rougher basement, on transverse profiles, than does the unfractured province. A similar transition to rougher, more seismic ridge crest also occurs 900 km northeast of Iceland. We propose that flowage of hot, basalt-rich asthenosphere away from the Iceland hot spot keeps the axial lithosphere hot, thin, sparsely fractured, and relatively aseismic out to 900 km from the plume. Similar effects are evident in the vicinity of some other plumes located near spreading axes. Some plumes also exhibit a greater number of earthquakes at some distance from the spreading axis — possibly a reflection of non-axial igneous activity or fracturing due to local, plume-generated stresses.The regional basement slope along the longitudinal profile is about 8 × 10^?4. If this slope represents a balance between viscous and gravity forces in the flow, a viscosity of the order 10¹⁹ poises can be estimated from the Poiseuille equation.A peculiarly flat, opaque reflector was discovered near the Reykjanes axis, about 300 km southwest of Iceland. Several hypotheses are advanced to account for such reflectors by the exceptional volcanic activity associated with high plume discharge.  相似文献   

The minor structures of deformed rocks — a photographic atlas: L.E. Weiss. Springer-Verlag, Berlin, 1972, 431pp., DM 94.00     

M.R.W. Johnson 《Tectonophysics》1973,18(1-2)

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