A neodymium,strontium, and oxygen isotopic study of the Cretaceous Samail ophiolite and implications for the petrogenesis and seawater-hydrothermal alteration of oceanic crust

In the Samail ophiolite,¹⁴⁷Sm-¹⁴³Nd,⁸⁷Rb-⁸⁷Sr, and¹⁸O/¹⁶O isotopic systems have been used to distinguish between sea-floor hydrothermal alteration and primary magmatic isotopic variations. The Rb-Sr and¹⁸O/¹⁶O isotopic systems clearly exhibit sensitivity to hydrothermal interactions with seawater while the Sm-Nd system appears essentially undisturbed. Internal isochrons have been determined by the¹⁴⁷Sm-¹⁴³Nd method using coexisting plagioclase and pyroxene and give crystallization ages of 130 ± 12m.y. from Ibra and 100 ± 20 m.y. from Wadi Fizh. These ages are interpreted as the time of formation of the Samail oceanic crust and are older than the inferred emplacement age of 65–85 m.y. The initial¹⁴³Nd/¹⁴⁴Nd ratios for a tectonized harzburgite, cumulate gabbros, plagiogranite, sheeted dikes and a basalt have a limited range in ε_Nd of from 7.5 to 8.6 for all lithologies, demonstrating a clear oceanic affinity and supporting earlier interpretations based on geologic observations and geochemistry. The⁸⁷Sr/⁸⁶Sr initial ratios on the same rocks have an extremely large range of from 0.70296 to 0.70650 (ε_Sr = ?19.7 to +30.5) and the δ¹⁸O values vary from 2.6 to 12.7. These large variations are clearly consistent with hydrothermal interaction of seawater with the oceanic crust. A model is presented for the closed system exchange of Sr and O, that in principle illustrates how the Sr isotopic data may be utilized to estimate the water/rock ratio and subsequently used to evaluate the temperature of equilibration between the water and silicates from the¹⁸O/¹⁶O water-rock fractionation.     

Fallout radionuclides in the Pacific Ocean: Vertical and horizontal distributions,largely from GEOSECS stations

From GEOSECS stations, largely, the 1974 distributions of Pu and of¹³⁷Cs are described in the Pacific Ocean north of about 20°S latitude. Changes in some of these distributions are described from 1978 cruises by the authors.The Pacific exhibited, everywhere, a shallow subsurface layer of Pu-rich water with its concentration maximum at about 465 m in 1974; over a large portion of the central North Pacific a second layer of Pu-labelled water, less concentrated than the shallow layer, lay just above the bottom. Similar features were not observed in the case of¹³⁷Cs.The inventories of both Pu and¹³⁷Cs in the water column at most 1974 stations are substantially greater than those to be expected from world-wide fallout alone; these inventory excesses appear to be attributable to close-in fallout, but only if the ratio Pu/¹³⁷Cs in this source was much higher than in world-wide fallout. The North Pacific mean ratio of the inventories is 2.2 times that observed in world-wide fallout.Resolubilization of Pu both from sinking particles and from sediments explains peculiarities of its depth distributions.There is little evidence for tracer movement by sliding downward along density surfaces;¹³⁷Cs appears to have moved to depth by downmixing at the edge of the Kuroshio, and then moved horizontally and upward alongσ_t contours. The shallow Pu-rich layer shows no coordination with density, salinity or O₂ isopleths. The deep Pu-rich layer is restricted to a narrow range of O₂ concentrations that confirm its origin in the Aleutian Trench and rapid spread southward and laterally. Near-bottom circulation processes have been much more active than here-to-fore described.     

A mathematical model to predict the inelastic response of a steel frame: Formulation of the model

The purpose of this research is to use data from experiments to formulate a mathematical model that will predict the non-linear response of a single-storey steel frame to an earthquake input. The process used in this formulation is system identification. The form of the model is a second-order non-linear differential equation with linear viscous damping and Ramberg—Osgood type hysteresis. The damping coefficient and the three parameters in the hysteretic model are to be established. An integral weighted mean squared error function is used to evaluate the [goodness of fit] between the model's response and the structure's response when both are subjected to the same excitation. The function includes errors in displacement and acceleration and is integrated from zero to a time T, which may be the full duration of the recorded response or only a portion of it. The parameters are adjusted using a modified Gauss-Newton method until the error function is minimized. The computer program incorporating these steps in the system identification process is verified with simulated data. Results given in the paper show that in every case the program converges in few iterations to the assigned set of parameters.     

A mathematical model to predict the inelastic response of a steel frame: Establishment of parameters from shaking table experiments

The purpose of this research is to use data from experiments to formulate a mathematical model that will predict the non-linear response of a single-storey steel frame to an earthquake input. The process used in this formulation is system identification. In experiments performed on a shaking table, the frame was subjected to two earthquake motions at several intensities. In each case the frame underwent severe inelastic deformation. A computer program which incorporates the concepts of system identification makes use of the recorded data to establish four parameters in a non-linear mathematical model. When different amounts of data are used in the program, parameter sets are established which give the best model response for that amount of test data. The resulting sets of parameters reflect the way in which the properties of the structure change during the excitation. However, when the full durations of the different excitations are used, the sets of parameters are almost identical. For each of these sets of parameters, the correlation of the computed accelerations with the measured is excellent, and the shape of the computed displacement response compares very well with the measured response, although the permanent offset of the displacements is not computed exactly. Suggestions are given on how to overcome this deficiency in the mathematical model.     

Chromite in the mantle section of the Oman ophiolite: A new genetic model

Abstract   This paper reviews the compositional data (major elements, platinum group element [PGE] concentrations, Os- and O-isotopes) for chromites from the mantle section of the Oman ophiolite. Chromites in chromitite from the Oman ophiolite lie on a compositional spectrum between high-Cr♯, boninite-like and low-Cr♯, mid-oceanic ridge basalt-like end-members. The high-Cr♯ end-member is low in Ti, has a fractionated PGE pattern and is enriched in iridium group-platinum group elements (IPGE). The low-Cr♯ end-member has higher Ti and an unfractionated PGE pattern. The compositional variation in the chromitites reflects their crystallization from a range of different melt compositions. It is proposed that this wide variation in melt compositions was produced by the process of a melt–rock reaction, whereby a basaltic melt has reacted with harzburgitic mantle to yield successively more Cr-rich melts. In contrast to previous models, this approach does not require a change in the tectonic environment to explain the different chromite types.     

O/O mapping and hydrogeology of a short-lived (≈10 years) fumarolic (>500°C) meteoric–hydrothermal event in the upper part of the 0.76 Ma Bishop Tuff outflow sheet, California

¹⁸O/¹⁶O data from the 200-m-thick, 0.76 Ma Bishop Tuff outflow sheet provide evidence for a vigorous, short-lived (≈10 years), high-temperature, fumarolic meteoric–hydrothermal event. This is proved by: (1) the juxtaposition in the upper, partially welded Bishop Tuff of low-¹⁸O groundmass/glass (δ¹⁸O=−5 to +3) with coexisting quartz and feldspar phenocrysts having magmatic δ¹⁸O values (+8.7±0.3; +7.5±0.3); and (2) the fact that these kinds of ¹⁸O/¹⁶O signatures correlate very well with morphological features and mapped zones of fumarolic activity. Profiles of δ¹⁸O with depth in the Bishop Tuff within the fumarole area define a 40- to 50-m-thick, low-¹⁸O, stratigraphic zone that is sandwiched between the essentially unwelded near-surface portion of the tuff and an underlying, densely welded black tuff that displays magmatic ¹⁸O/¹⁶O values. Shallow-dipping columnar joints and other fumarolic features (i.e., subhorizontal tubular conduits and steep fissures) correlate very well with these pervasively devitrified, low-¹⁸O zones. The base of the low-¹⁸O zone is extremely sharp (3‰ per meter) and is located directly above the transition from partially welded tuff to densely welded black tuff. The observed average whole-rock ¹⁸O-depletions within this low-¹⁸O zone are about 6–7‰, requiring meteoric water/rock ratios in excess of 0.24 in mass units. Rainfall on the surface of the tuff would not have been high enough to supply this much H₂O in the short lifetime of fumarolic activity, suggesting that some recharge must have been from groundwater flow through the upper part of the tuff, above the sloping (1°–5°) top of the impermeable lower zone. This is compatible with the observation that the fumarolic areas roughly correlate with the preeruptive regional drainage pattern. Some of this recharge may in part have been from the lake that filled Long Valley caldera, which was dammed by the Bishop Tuff up to the level of this boundary between the partially and densely welded zones (≈7000 ft, the elevation of the highest Long Valley Lake shorelines). Gazis et al. had previously shown that the 2.8-Ma intracaldera Chegem Tuff from the Caucasus Mountains exhibits exactly the same kind of ¹⁸O-signature that we have correlated with fossil fumaroles in the Bishop Tuff outflow sheet. Although not recognized as such by McConnell et al.; ¹⁸O/¹⁶O data from drill-hole samples from the intracaldera Bishop Tuff in Long Valley also display this characteristic ¹⁸O signature (i.e., analogous δ¹⁸O-depth profiles, as well as low-¹⁸O groundmass coexisting with high-¹⁸O feldspar phenocrysts). This fumarolic ¹⁸O/¹⁶O signature is observed to much greater depths (≈650–750 m) in the intracaldera tuffs (≈1500 m thick) than it is in the ≈200-m-thick Bishop Tuff outflow sheet (≈80 m depth).     

Zircon xenocrysts from the Kambalda volcanics: age constraints and direct evidence for older continental crust below the Kambalda-Norseman greenstones

The Hangingwall Basalt at Kambalda, Western Australia, contains zircons that have been shown by ion microprobe analyses to have very high U and Th contents and a wide variety of crystallization ages. Nearly all of these zircons certainly are xenocrysts; a few might relate to intrusive veinlets. The age of the youngest xenocrysts, 2693 ± 50Ma(2 σ), shows that the eruptive age of the basalt cannot exceed 2743 Ma. This confirms that the apparent SmNd isochron giving 3200 Ma [1,2] for Kambalda mafic and ultramafic rocks is a mixing-line [2] between unrelated components enriched and depleted in light rare earth elements. Mixing probably occurred at depth by erosion of 3200–3500 Ma old felsic crust from the walls of the HWB conduits. The zircon xenocryst ages are the first direct evidence for the presence of very old felsic crust in the eastern Yilgarn Block. The latter implies that the Kalgoorlie-Norseman greenstone sequences were formed in a continental rather than an oceanic environment.