An assessment of the utility of staurolite in U-Pb dating of metamorphism

Pb isotope data, major and trace element compositions, fission track and synchrotron X-ray fluorescence analyses are presented for staurolites from nine pelitic schists in the continental United States to evaluate their potential use in U-Pb geochronology. Seven U-Pb analyses from Lanzirotti and Hanson (1995) are reexamined with respect to this additional data which was not available at the time. These data are then compared to 21 new U-Pb analyses of staurolite of varying composition from a variety of localities. The primary goals of this study are to: (1) evaluate the variability in U and Pb abundance and U/Pb ratio in staurolites of varying composition; (2) constrain how much of the measured U and Pb is derived from radiogenic solid inclusions such as monazite and zircon; (3) constrain how much of the measured U and Pb is derived from staurolite itself and evaluate any possible correlation of U and Pb abundance and U/Pb ratio to major element composition; (4) place preliminary constraints on closure temperature to Pb diffusion in staurolite; (5) evaluate how meaningful U-Pb ages can better be calculated for the low U/Pb ratio minerals. In the staurolite fractions analyzed U abundances range from 0.2 to 24.9 ppm, Pb from 0.13 to 2.41 ppm, the ²³⁸U/²⁰⁴Pb ratios vary from 135 to 9447, and the ²⁰⁶Pb/²⁰⁴Pb ratios from 23 to 623. For many of the fractions analyzed precise U-Pb ages can be calculated (±10 Ma or better) that appear to be consistent with available age constraints on the time of peak metamorphism. Mass balance calculations, fission track analysis, and synchrotron X-ray fluorescence trace element mapping show that although radiogenic inclusions are almost always present in large staurolite porphyroblasts, it is difficult for inclusions to account for the measured Pb isotopic compositions. It is also demonstrated that the U-Pb ages calculated for staurolites from Connecticut are at least 20 Ma older than nearby Rb-Sr muscovite and ⁴⁰Ar-³⁹Ar hornblende ages. This is consistent with staurolite having a closure temperature to U and Pb diffusion significantly higher than 500 °C. Received: 14 July 1995 / Accepted: 16 May 1997     

Stresses in the Moon

It is argued that a reliable theory of the stress history of the Moon should take into account several factors; and that direct observations of the Moon's surface can throw much light on this subject.     

The influence of rock structures on coast-line and cliff development around Tintagel,North Cornwall

The paper deals with the morphology of the stretch of coast between Boscastle and the Treligga Cliffs in North Cornwall. The prevalent cliff type is slope-over-wall, either hog's-back or bevelled as defined by Miss Arber (1949). Flat-topped cliffs are less common. It is considered that geological structures—bedding, jointing, faulting and, to a lesser extent, rock-type—have not only had considerable influence on the shape of the cliffs, but have also guided the marine attack on the coast. The evolution of most of the coast features, when seen in plan or profile, has been controlled by the local geological structure. The southern stretch of the coast is sub-parallel to seaward-dipping normal faults which have been stripped of their hanging-walls. Such a cliff line is dominantly a faultline scarp which has suffered slight retrogression.     

Submarine slope processes on a fan delta,Howe Sound,British Columbia

Modern side-scan sonar technology was used to resurvey the site of the Howe Sound submarine slide described by Terzaghi. Chutes, hummocky topography, and subparallel scarps indicative of submarine mass movement are widespread. Submarine slope processes appear to be far more important to the development of this coarsegrained fan delta than suspected.     

Semiconductors exposed to oils in pressure-tolerant electronic systems

The present study indicates that most semiconductor components are fabricated such that they are not affected by operation in a dielectric oil environment. A quantitative estimate of the conditions under which oil-soluble ions will invert semiconductor surfaces and interfere with proper device operation is developed. The results are discussed in terms of pressure-tolerant electronic systems—solid-state components operated immersed in chemically inert dielectric oils when these components are subject to, and experience, ambient hydrostatic pressures of the ocean depths. The feasibility of pressure-tolerant electronic systems is demonstrated.