Rare earth element, 87Sr/86Sr,and 143Nd/144Nd compositions of Cenozoic orogenic dacites from Baja California,northwestern Mexico,and adjacent west Texas: evidence for the predominance of a subcrustal component

Dacitic lavas and ignimbrites were examined from seven localities that span the entire 700 km width of the mid- to late Cenozoic magmatic arc of northwestern Mexico and adjacent west Texas. These rocks have remarkably similar REE patterns that are parallel in the heavy REE and have modest negative Eu anomalies. Samples from three localities including Baja California, the Sierra Madre Occidental, and the Chihuahuan Basin and Range have initial ⁸⁷Sr/⁸⁶Sr between 0.7044 and 0.7050 and _Nd near 0.0±1.0. These dacites are isotopically similar to associated basalts, and they show no systematic isotopic variation that is correlated with age or composition of the basement. There is no evidence that magmas parental to these dacites interacted significantly with continental crust. Samples form three other localites in the Basin and Range vary in initial ⁸⁷Sr/⁸⁶Sr from 0.7051 to 0.7070 and _Nd from about -1 to –2. The composition of these rocks reflects contamination of the parental magmas by relatively small amounts of Precambrian crust. Collectively, the dacites of this study show much less isotopic variation than do Mesozoic granitoids (Farmer and DePaolo 1983) and late Cenozoic olivine tholeiites (Hart 1985) from similar transects of the western United States. The distinctive source region for the magmas parental to the Mexican dacites was relatively uniform isotopically, but it was enriched in LIL and HFS elements beneath the eastern Basin and Range.     

Variable stars in the field of open cluster NGC 6819

We report on the discovery of 25 variable stars plus 13 suspected variables found in the field of the open cluster NGC 6819. The stars were identified from time-series photometric data obtained on the Isaac Newton Telescope, La Palma, during two observing runs covering the 19 nights between 1999 June  22–30  and 1999 July  22–31  . The variables found include 12 eclipsing binaries with an additional three suspected, nine BY Draconis systems, plus four variables of other types, including one star believed to be a Cepheid. Three of the 15 eclipsing binaries are believed to be cluster members. Details of a further 10 suspected variable stars are also included.     

Petrophysics and mineral exploration: a workflow for data analysis and a new interpretation framework

As mineral exploration seeks deeper targets, there will be a greater reliance on geophysical data and a better understanding of the geological meaning of the responses will be required, and this must be achieved with less geological control from drilling. Also, exploring based on the mineral system concept requires particular understanding of geophysical responses associated with altered rocks. Where petrophysical datasets of adequate sample size and measurement quality are available, physical properties show complex variations, reflecting the combined effects of various geological processes. Large datasets, analysed as populations, are required to understand the variations. We recommend the display of petrophysical data as frequency histograms because the nature of the data distribution is easily seen with this form of display. A petrophysical dataset commonly contains a combination of overlapping sub-populations, influenced by different geological factors. To understand the geological controls on physical properties in hard rock environments, it is necessary to analyse the petrophysical data not only in terms of the properties of different rock types. It is also necessary to consider the effects of processes such as alteration, weathering, metamorphism and strain, and variables such as porosity and stratigraphy. To address this complexity requires that much more supporting geological information be acquired than in current practice. The widespread availability of field portable instruments means quantitative geochemical and mineralogical data can now be readily acquired, making it unnecessary to rely primarily on categorical rock classification schemes. The petrophysical data can be combined with geochemical, petrological and mineralogical data to derive explanations for observed physical property variations based not only on rigorous rock classification methods, but also in combination with quantitative estimates of alteration and weathering. To understand how geological processes will affect different physical properties, it is useful to define three end-member forms of behaviour. Bulk behaviour depends on the physical properties of the dominant mineral components. Density and, to a lesser extent, seismic velocity show such behaviour. Grain and texture behaviour occur when minor components of the rock are the dominate controls on its physical properties. Grain size and shape control grain properties, and for texture properties the relative positions of these grains are also important. Magnetic and electrical properties behave in this fashion. Thinking in terms of how geological processes change the key characteristics of the major and minor mineralogical components allows the resulting changes in physical properties to be understood and anticipated.     

WASP-1b and WASP-2b: two new transiting exoplanets detected with SuperWASP and SOPHIE

We have detected low-amplitude radial-velocity variations in two stars, USNO-B1.0 1219–0005465 (GSC  02265–00107 = WASP–1  ) and USNO-B1.0 0964–0543604 (GSC  00522–01199 = WASP–2  ). Both stars were identified as being likely host stars of transiting exoplanets in the 2004 SuperWASP wide-field transit survey. Using the newly commissioned radial-velocity spectrograph SOPHIE at the Observatoire de Haute-Provence, we found that both objects exhibit reflex orbital radial-velocity variations with amplitudes characteristic of planetary-mass companions and in-phase with the photometric orbits. Line-bisector studies rule out faint blended binaries as the cause of either the radial-velocity variations or the transits. We perform preliminary spectral analyses of the host stars, which together with their radial-velocity variations and fits to the transit light curves yield estimates of the planetary masses and radii. WASP-1b and WASP-2b have orbital periods of 2.52 and 2.15 d, respectively. Given mass estimates for their F7V and K1V primaries, we derive planet masses 0.80–0.98 and 0.81–0.95 times that of Jupiter, respectively. WASP-1b appears to have an inflated radius of at least 1.33 R _Jup, whereas WASP-2b has a radius in the range 0.65–1.26 R _Jup.     

Formation of the prelunar accretion disk

According to the single-impact hypothesis for forming the Moon, the angular momentum needed for the present Earth-Moon system can be imparted to the proto-Earth by a collision with a body having one-tenth of the mass or more. The collision must vaporize a large amount of rock which must stay in the form of vapor after expanding in density by a factor of several, so that pressure gradients can accelerate significant amounts of the matter into orbital motion about the proto-Earth. A successful theory must put considerably more than a lunar mass into orbit, having considerably more angular momentum than is needed to assemble a lunar mass in orbit at 3 Earth radii. Such a collision has been simulated by a particular form of a particle-in-cell representation of hydrodynamics and 78 cases have been run representing variations in a variety of parameters. A significant fraction of the cases were successful in creating a satisfactory prelunar accretion disk. A fairly common characteristic of these cases was the presence of an excess velocity in the collision (above that of a parabolic orbit), implying that the projectile involved in the collision existed in an Earth-crossing orbit of significant ellipticity. A majority of the mass of the prelunar accretion disk is contributed by the projectile.     

Differential signal of change among multiple components of West African rainfall

Theoretical and Applied Climatology - Rainfall components likely differ in the magnitude and direction of their long-term changes for any given location, and some rainfall components may carry a...     

Pre-main sequence stellar evolution with mass loss

A semi-empirical formulation is given of the rate of stellar mass loss by stellar winds. Evolutionary studies of stars in the pre-main sequence (T Tauri) stage are presented for a variety of rates of mass loss. It has been found that different mass loss rates produce only small changes in the positions of equal evolutionary time lines in HR diagrams. Thus it is concluded that the spread of points in HR diagrams of young clusters results from a spread in their times of formation. This is consistent with the initiation of star formation by violent hydrodynamic compression of a typical interstellar cloud.     

Hydrogeochemical methods for base metal exploration in the northern Canadian shield

The use of lake waters for base metal exploration has been studied in the northern part of the Slave Geological Province of the Canadian Shield. The area is north of the treeline, within the zone of continuous permafrost, and, like most other regions of the Shield, has a high density of small lakes.A regional sampling of 1218 lakes established that less than 2 ppb (μg/l) Zn or Cu is typical of waters from unmineralized terrane. These samples had a median pH of 6.8 and a median specific conductivity of 19.5 μmhos. Lake waters were also taken from the areas surrounding five massive sulphide occurrences: High Lake, Canoe Lake, Takiyuak Lake, Hackett River and Agricola Lake. In all cases there are unambiguous anomalies for Zn. Anomalies are also present for Cu, but are less intense and extensive. This difference between the two elements is related to the superior mobility of Zn in surface waters and its more consistent presence as a major constituent of massive sulphides.A water sampling apparatus has been developed and tested on a light turbine helicopter. Using this, thirty sites may be sampled each hour when sampling at a density of 1 site per 2.8 km². Measurement of pH, conductivity and water temperature are recorded in the helicopter during sampling.A number of factors have been investigated that may influence the utility of lake water sampling for base metal exploration:

1. (1) Seasonal variability, while present to moderate degree, is unlikely to hinder application of the method.

2. (2) For the size of lakes sampled (2 km² or less), elements are homogeneously distributed across the lake surface during the ice-free season. During the initial period of break-up there are marked variations in element content around the ice-free lake margin. Sampling during this period may help define the source of metals for anomalous lakes.

3. (3) Study of sample preservation suggests that mobile elements, such as Zn, that are stable in solution within lakes, are also relatively stable when untreated water is stored in plastic bottles.

4. (4) Care must be taken to avoid contamination of the samples, particularly from the bottle.

The areal extent of lake water base metal anomalies appears to be less than equivalent lake sediment anomalies. Thus for wide-interval, regional geochemical reconnaissance, lake sediment sampling is the method of choice. Lake waters are an appropriate medium for detailed exploration of areas of interest, such as volcanic belts. For this application, the principal attractions are rapid sampling rates, and hence low costs, high contrast anomalies, and a uniform sampling medium.     

The starburst in the Wolf-Rayet nucleus of the Liner NGC 6764

We present high resolution millimeter, near-infrared, and optical data on the Wolf-Rayet nucleus of the Liner NGC 6764. The millimeter¹²CO(1-0) maps were obtained using the Nobeyama Millimeter Interferometer. Near-infrared images in the K-band continuum and the 2.12µm H₂, 2.06µm He I, 2.17µm Br, and 1.64µm [Fe II] lines were taken with the MPE imaging spectrometer FAST at the William Herschel Telescope on La Palma, Spain. The optical data were obtained at the 3.5m telescope on Calar Alto, Spain. The measurements indicate a strong concentration of molecular gas and a massive starburst at the nucleus of NGC 6764. The interferometric position velocity map of the nucleus shows the presence of distinct molecular cloud complexes with an apparently asymmetric velocity field shifted towards the blue with respect to the systemic velocity of 2420 km s^–1. The distribution of the 2.12µm H₂ line flux exhibits extensions approximately perpendicular to the bar which are in agreement with structural features in VLA radio maps and IRAM 30m maps of the¹²CO(2–1) line emission. This may represent evidence for combination of a nuclear outflow and a central oval distortion of gas predicted by gas dynamical calculations as a response to a bar like potential. A detailed investigation of the Wolf-Rayet-feature at 466 nm indicates that it is spatially extended on a scale of a few arcseconds.