Palaeomagnetism of the Tudor gabbro, Ontario: evidence for divergence between Grenvillia and interior Laurentia

Summary. After thermal and alternating field (AF) cleaning, the characteristic high blocking temperature A component of natural remanent magnetization (NRM) of the Tudor gabbro of southern Ontario has a mean direction D = 326°, I =–46° ( k = 132, α₉₅= 4.8°, N = 8 sites). The corresponding palaeopole, 133°E, 12°N ( dp = 4°, dm = 6°), confirms the palaeopole 137°E, 17°N (α₉₅= 8.4°) reported earlier by Palmer & Carmichael, based on AF cleaning only. The A NRM has unblocking temperatures > 515–525°C which exceed the estimated 500°C peak temperature reached locally during ∼ 1050 Ma Grenvillian regional metamorphism. The A NRM therefore predates metamorphism and is probably a primary thermoremanence (TRM). The age of the Tudor NRM has previously been taken to be about 675 Ma, but recent ⁴⁰Ar/³⁹Ar dating by Baksi has shown that this is the time of post-metamorphic cooling to 200–250°C. Hornblendes record initial cooling of the intrusion to 590±20°C at 1110 Ma and this is the best estimate of the age of the A remanence. Successful Thellier-type palaeointensity determinations on 11 Tudor samples confirm that the A NRM is a TRM and indicate a palaeofield at this time of 18–27 μT, about 50–70 per cent of the present field intensity at 27° magnetic latitude. The anomalous Tudor A palaeopole, which lies well to the west of both 1000–800 Ma Grenvillian palaeopoles and 1100–1050 Ma poles from Interior Laurentia, is interpreted as recording divergence between Grenvillia and Interior Laurentia just before the Grenvillian orogeny, rather than a post-metamorphic extension of the apparent polar wander path as previously assumed.     

The elastic properties of single-crystal fayalite as determined by dynamical measurement techniques

We present new elasticity measurements on single-crystal fayalite and combine our results with other data from resonance, pulse superposition interferometry, and Brillouin scattering to provide a set of recommended values for the adiabatic elastic moduliC _ij and their temperature variations. We use a resonance method (RPR) with specimens that were previously investigated by pulse superposition experiments. The nineC _ij of fayalite are determined from three new sets of measurements. One set of our newC _ij data is over the range 300–500 K. We believe that the relatively large discrep ncies found in someC _ij are due in large part to specimen inhomogeneities (chemical and microstructural) coupled with differences in the way various techniques sample, rather than only systematic errors associated with experimental procedures or in the preparations of the specimens.Our recommendeaC _ij's (GPa) and (C _ij/T) _p (GPa/K) are: The resulting values for the isotropic bulk and shear moduli,K _s and , and their temperature derivatives are:K _s=134(4) GPa; =50.7(0.3) GPa; (K _s/T) _p =–0.024(0.005) GPa/K; and (/T) _p =–0.013(0.001) GPa/K. An important conclusion is thatK _s increases as the Fe/(Fe+Mg) ratio in olivine is increased.     

The spectra and light curves of two gamma-ray bursts

During the last half of 1977 the UCSD/MIT Hard X-Ray and Low Energy Gamma-Ray Experiment of HEAO-1 observed two of the three gamma-ray bursts detected by at least three satellites. The first of these bursts (20 October, 1977) had a fluence of (3.1±0.5)×10^–5 erg cm^–2 integrated over the energy range 0.135–2.05 MeV and over its duration of 38.7 s, placing it among the largest bursts observed. The second (10 November, 1977) had a fluence of (2.1±0.8)×10^–5 erg cm^–2 integrated over the energy range 0.125–3 MeV and over its duration of 2.8 s. The light curves of both bursts exhibit time fluctuations down to the limiting time resolution of the detectors (0.1 s). The spectrum of the 20 October, 1977 burst can be fitted with a power law (index –1.93±0.16), which is harder than other reported gamma-ray burst spectral fits. This burst was detected up to 2.05 MeV, and approximately half of its energy was emitted at photon energies above 0.5 MeV. The spectrum of the 10 November, 1977 burst is softer (index –2.4±0.7) and is similar to the spectrum of the 27 April, 1972 burst.Paper presented at the Symposium on Cosmic Gamma-Ray Bursts held at Toulouse, France, 26–29 November, 1979.     

Thermal expansion and crystal structure of FeSi between 4 and 1173 K determined by time-of-flight neutron powder diffraction

The thermal expansion and crystal structure of FeSi has been determined by neutron powder diffraction between 4 and 1173?K. No evidence was seen of any structural or magnetic transitions at low temperatures. The average volumetric thermal expansion coefficient above room temperature was found to be 4.85(5)?×?10^?5?K^?1. The cell volume was fitted over the complete temperature range using Grüneisen approximations to the zero pressure equation of state, with the internal energy calculated via a Debye model; a Grüneisen second-order approximation gave the following parameters: θ_D=445(11)?K, V ₀=89.596(8)?ų, K ₀′=4.4(4) and γ′=2.33(3), where θ_D is the Debye temperature, V ₀ is V at T=0?K, K ₀′ is the first derivative with respect to pressure of the incompressibility and γ′ is a Grüneisen parameter. The thermodynamic Grüneisen parameter, γ_th, has been calculated from experimental data in the range 4–400?K. The crystal structure was found to be almost invariant with temperature. The thermal vibrations of the Fe atoms are almost isotropic at all temperatures; those of the Si atoms become more anisotropic as the temperature increases.     

Drought in Bulgaria and atmospheric synoptic conditions over Europe

Drought in Bulgaria is analyzed from the multiple viewpoints of statistical occurrence, spatial patterns, and synoptic conditions. A new index of drought, the SD (spatial-dryness) index, characterizes drought by both intensity and spatial extent. The occurrence of the SD index is analyzed using global gridded data sets. Examination of transitional probabilities of multiple months and years with drought occurrence suggests persistence is sufficiently frequent to be important for climate-related environmental planning. Finally, it is shown that specific seasonal synoptic patterns are associated with wet and dry conditions in Bulgaria. This revised version was published online in July 2006 with corrections to the Cover Date.     

Recent ventifact development on the central Oregon coast,western USA

The unusual location of ventifacts, on a boulder‐built jetty at the mouth of the Siuslaw River, Oregon coast, western USA, allows ventifact age and wind abrasion rates to be estimated with some precision. The jetty was built mainly between 1892–1901 and extended throughout the twentieth century. Consideration of historical shoreline position and the history of jetty construction and repair suggests the ventifacts have formed since about 1930. Morphologically the ventifacts are aligned south‐to‐north reflecting winter winds and sediment transport from the adjacent beach. Wind‐parallel grooves and ridges with sharp, sinuous crests are developed on inclined boulder surfaces on top of the jetty and reflect suspended sand transport in wind vortices. Deeply pitted surfaces on steep boulder surfaces nearest the beach reflect impact by saltating sand grains. Based on present wind regimes (1992–2000) from three regional weather stations, southerly winds above the sand transport threshold occur for 21·9–29·6 per cent of the time. Based on estimated depth of loss from boulder surfaces, wind abrasion rates are calculated to be on the order of 0·24–1·63 mm a^?1. This is the first well‐constrained field estimate of ventifact age and ventifaction rate from a modern coastal environment. Copyright © 2003 John Wiley & Sons, Ltd.     

The Coolgardie Goldfied,Western Australia: district-scale controls on an Archaean gold camp in an amphibolite facies terrane

Archaean lode gold deposits in the Coolgardie Goldfield, Western Australia, occur in mafic and ultramafic rocks which have been metamorphosed to the amphibolite facies. Mineralisation was broadly synchronous with peak metamorphism, the main phase of granitoid emplacement, and regional deformation. Several different structural styles are represented by the deposits of the Coolgardie Goldfield. Mineralisation occurs along sheared felsic porphyry-ultramafic rock contacts, in gabbro-hosted quartz-vein sets, in fault-bounded quartzvein sets, and in laminated quartz reefs sited in brittleductile shear zones. The structures hosting mineralisation formed in response to a progressive deformation event, related to granitoid emplacement in an east-west compressional far-field stress régime, but with local heterogeneous stress orientations. This occurred after an earlier period of thrust-stacking, with probable north-south directed tectonic transport. Two contrasting styles of goldrelated wallrock alteration are associated with the auriferous lodes of the Coolgardie Goldfield. A high-temperature assemblage (formation temperature >500°C), characterised by the proximal alteration assemblage garnet+ hornblende + plagioclase + pyrrhotite, contrasts with a medium-temperature assemblage (formation temperature <500°C), consisting of calcic amphibole + biotite + plagioclase + calcite + arsenopyrite + pyrrhotite. The distribution of the two styles of gold-related alteration is controlled by distance from voluminous syntectonic granitoids located to the west of the Coolgardie Goldfield, with the high-temperature style of alteration more proximal to the granitoid-greenstone contact than the medium-temperature style. The occurrence of gold deposits that formed under amphibolite facies conditions throughout the Coolgardie Goldfield supports a crustal continuum model for Archaean lode-gold deposits, which proposes that gold is deposited in metamorphic environments that range from the sub-greenschist to granulite facies. In addition, the data from Coolgardie suggest that syntectonic, synmineralisation granitic plutons may play a significant role in controlling the style of gold associated wallrock-alteration at deep crustal levels.