Chemistry of micas from kimberlites and xenoliths—I. Micaceous kimberlites

Micaceous kimberlites from South Africa and Canada contain two types of groundmass mica less than 1 mm across. Very rare Type I micas are relatively iron-rich with $\text{mg [ = Mg/(Mg + Fe)] 0.45–0.65}$, TiO₂ 3–6 wt%, Al₂O₃ 14–16wt%, no Fe³⁺ required in tetrahedral sites, low NiO (~0.02 wt%), and relatively high na [Na₂O/(Na₂O + K₂O)] 0.02–0.03. The much more abundant Type II micas are variable in composition, but relative to Type I micas are more magnesium (mg 0.80-0.93), lower in TiO₂ (0.7–4.0 wt%) and Al₂O₃ (6.8–14.2 wt%), have substantial Fe³⁺ in tetrahedral sites, and have relatively low na. Both types may have rims with compositions indicative of mica-‘serpentine’ mixtures resulting from reaction with a highly aqueous fluid. The petrographically-determined ‘serpentine’ is chemically of two types: Fe-rich serpentine and Fe-rich talc. Associated phases in the ground-mass vary from one kimberlite to another: calcite, dolomite, diopside, chromite, Mg-ilmenite, perovskite, barite, pyrite, pentlandite, millerite?, heazlewoodite?, quartz.Inter-grain variations in composition of Type II micas may result from establishment of local reservoirs on a mm scale, consequent upon mechanical mixing and competition of other phases for minor elements (e.g. chromite for Cr, serpentine for Ni).Type I micas may result from an intrusive precursor (carbonatitic?) to kimberlite, perhaps genetically related, which was incorporated into a later pulse of kimberlite from which the Type II micas crystallized.     

Comparisons of 210Pb and pollen methods for determining rates of estuarine sediment accumulation

Comparisons of sedimentation rates obtained by ²¹⁰Pb and pollen analyses of 1-m cores collected throughout the Potomac Estuary show good agreement in the majority of cores that can be analyzed by both methods. Most of the discrepancy between the methods can be explained by the analytical precision of the ²¹⁰Pb method and by the exactness with which time horizons can be identified and dated for the pollen method. X-radiographs of the cores and the distinctness of the pollen horizons preclude significant displacement by reworking and/or mixing of sediments. Differences between the methods are greatest where uncertainties exist in assigning a rate by one or both methods (i.e., ²¹⁰Pb trends and/or “possible” horizon assignments). Both methods show the same relative rates, with greater sediment accumulation more common in the upper and middle estuary and less toward the mouth. The results indicate that geochronologic studies of estuarine sediments should be preceded by careful observation of sedimentary structures, preferably by X-radiography, to evaluate the extent of mixing of the sediments. Time horizons, whether paleontologic or isotopic, are generally blurred where mixing has occurred, precluding precise identification. Whenever possible, two methods should be used for dating sediments because a rate, albeit erroneous, can be obtained isotopically in sediments that are mixed; accurate sedimentation rates are also difficult to determine where the time boundary is a zone rather than a horizon, where the historical record does not provide a precise date for the pollen horizon, or where scouring has removed some of the sediment above a dated pollen horizon.     

Hydrocarbons in age-dated sediment cores from two basins in the Southern California Bight

The distributions of hydrocarbons in sediment cores dated by ²¹⁰Pb (1845–1977) from San Pedro and San Nicolas Basins in the Southern California Bight have been determined by gas Chromatographic and combined gas chromatographic-mass spectrometric analysis. The chromatograms of the hydrocarbons contain peaks of resolved alkanes and cycloalkanes, as well as an unresolved complex mixture which decrease in content with increasing depth in both of the cores. The concentrations of o,p' andp,p'-DDE are highest in the top 50 mm of the San Pedro core section. The 45–50 mm segment of this core was deposited in the 1945–1950 period when DDT (the precursor of DDE compounds) came into common use. The relatively high content of hydrocarbons and DDE in this core is attributable to the proximity of the site to the San Pedro Harbor which receives petroleum residues from shipping, sewage outfalls and industrial effluents. The presence of only traces of DDE throughout the San Nicolas core, and the low hydrocarbon content are explainable by the greater distance of this basin from anthropogenic inputs and probably a greater rate of degradation of deposited organic matter during bioturbation. δ¹³C, δ¹⁵N and electron-spin resonance analyses of kerogens and humic substances in these cores, indicate that most of this organic matter in these sediments is of a marine origin.     

Stability properties of the Newmark,Houbolt and Wilson θ methods

This paper analysis the stability of several methods for obtaining numerical solutions of second-order ordinary differential equations. The methods are popular in structural and geotechnical engineering applications and are direct, that is they do not require the transformation of the second-order equation into a first-order system. They include Newmark's method in both implicit and explicit forms, Wilson's θ-method, Houbolt's method and some variants on this latter method. We shall examine the stability of the methods when applied to the second-order scalar test equation where a and c are real.     

The role of phytoplankton in the diets of adult and larval shipworms,Lyrodus pedicellatus (Bivalvia: Teredinidae)

The shipwormLyrodus pedicellatus does not feed exclusively on the wood into which it burrows. This paper demonstrates the ability of adults to ingest and incorporate radioactivity from¹⁴C-labelled phytoplankton. The free-swimming larvae of this species, however, do not feed while in the plankton. This is related to their limited ability to delay metamorphosis in the absence of a suitable wood substrate.     

Paleomagnetism and tectonics of the Cretaceous Mt. Stuart Batholith of Washington: translation or tilt?

The Mt. Stuart Batholith is a composite pluton of Late Cretaceous age that intrudes the crystalline North Cascades terrane of northwestern Washington. Its paleomagnetic direction (D=10.0°, I=45.5°; α₉₅=4.9°) is markedly different from the direction expected for Cretaceous rocks from northwestern Washington (D=330.5°, I=73.0°), which means that the Mt. Stuart Batholith either has moved relative to the North American interior, or has been tilted through a substantial angle, or both, since it acquired its magnetism. Either tilt or translation are possible, geometrically, but translation is more likely, because: (1) local geology apparently does not support tilt in the required direction; (2) it probably is not possible for a panel of rock the size of the Mt. Stuart Batholith to tilt through the necessary angle (ca. 35°) after its deeper parts have cooled sufficiently to retain remanent magnetization; (3) the sense of paleomagnetic discordance found in the Mt. Stuart Batholith (clockwise-rotation of declination, flattened inclination) is identical to that found in nearly every other allochthonous terrane in the western North American Cordillera, including every other Cretaceous batholith studied except the Sierra Nevada.     

Results from an aeromagnetic investigation of the nares strait region

An analysis of published and recently collected aeromagnetics in the Naires Strait region shows that short wavelength, moderate amplitude magnitic anomalies are coincident with previously mapped Precambrian Shield rocks under southeast Ellesmere Island. Similar magnetic anomalies are also observed in northwestern Greenland across Nares Strait. On Greenland the characteristic magnetic anomalies persist about 105 km beyond and northeast of the exposed Precambrian outcrops. This suggests that the Greenland Pre-Cambrian Shield structures previously shown as only occuring as far north as Inglefield Land actually extend at least 100 km further north. Correlation of the magnetic anomalies with mapped geological units suggest a 105+−10 km left lateral displacement in the boundary between the Lower Paleozoic and Precambrain terrains across Nares Strait. This, we interpret, is due to the relative motion between Greenland and Ellesmere Island during the evolution of the Labrador Sea and Baffin Bay in the Early to Mid Tertiary.     

Nares Strait—A suture zone: Geophysical and geological implications

The motion of Greenland relative to Ellesmere Island along Nares Strait is determined from poles of rotation which provide control for the motion independent of local geology and geophysics. The plate kinematics around the North Atlantic Ocean, the Norwegian and Greenland Seas and the Eurasian Basin of the Arctic Ocean constrain motion along Nares Strait. These motions are checked by examining the stability characteristics of the triple junctions. These junctions are found to be stable. The motion along Nares Strait between anomalies 34 and 13 is a combination of strike-slip and compression. The regional geology is found to support the plate reconstructions. The local geology of the Nares Strait area is reviewed and found not to refute the predicted motions. The geophysical and geological data are interpreted in terms of the Wilson cycle, the opening and closing of an ocean. The Nares Strait area has the characteristics of a cryptic suture, a join between regions of collided continental crust.