A lead isotope study of mineralization in the Saudi Arabian Shield

New lead isotope data are presented for some late Precambrian and early Paleozoic vein and massive sulfide deposits in the Arabian Shield. Using the Stacey Kramers (1975) model for lead isotope evolution, isochron model ages range between 720 m.y. and 420 m.y. Most of the massive sulfide deposits in the region formed before 680 m.y. ago, during evolution of the shield. Vein type mineralization of higher lead content occurred during the Pan African event about 550 m.y. ago and continued through the Najd period of extensive faulting in the shield that ended about 530 m.y. ago. Late post-tectonic metamorphism may have been responsible for vein deposits that have model ages less than 500 m.y. Alternatively some of these younger model ages may be too low due to the mineralizing fluids acquiring radiogenic lead from appreciably older local crustal rocks at the time of ore formation.The low²⁰⁷Pb/²⁰⁴Pb ratios found for the deposits in the main part of the shield and for those in north-eastern Egypt, indicate that the Arabian craton was formed in an oceanic crustal environment during the late Precambrian. Involvement of older, upper-crustal material in the formation of the ore deposits in this part of the shield is precluded by their low²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb characteristics.In the eastern part of the shield, east of longitude 44°20E towards the Al Amar-Idsas fault region, lead data are quite different. They exhibit a linear²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb relationship together with distinctly higher²⁰⁸Pb/²⁰⁴Pb characteristics. These data imply the existence of lower crustal rocks of early Proterozoic age that apparently have underthrust the shield rocks from the east. If most of the samples we have analyzed from this easterly region were mineralized 530 m.y. ago, then the age of the older continental rocks is 2,100±300 m.y. (2).The presence of upper crustal rocks, possibly also of early Proterozoic age, is indicated by galena data from Hailan in South Yemen and also from near Muscat in Oman. These data are the first to indicate such old continental material in these regions.     

Depositional and Diagenetic History and Petroleum Geology of the Jurassic Norphlet Formation of the Alabama Coastal Waters Area and Adjacent Federal Waters Area

The discovery of deep (20,000 ft) gas reservoirs in eolian sandstone of the Upper Jurassic Norphlet Formation in Mobile Bay and offshore Alabama in the late 1970s represents one of the most significant hydrocarbon discoveries in the nation during the past several decades. Estimated original proved gas from Norphlet reservoirs in the Alabama coastal waters and adjacent federal waters is 7.462 trillion ft3 (Tcf) (75% recovery factor). Fifteen fields have been established in the offshore Alabama area. Norphlet sediment was deposited in an arid environment in alluvial fans, alluvial plains, and wadis in updip areas. In downdip areas, the Norphlet was deposited in a broad desert plain, with erg development in some areas. Marine transgression, near the end of Norphlet deposition, resulted in reworking of the upper part of the Norphlet Formation. Norphlet reservoir sandstone is arkose and subarkose, consisting of a simple assemblage of three minerals, quartz, albite, and K-feldspar. The present framework grain assemblage of the Norphlet is dominantly diagenetic, owing to albitization and dissolution of feldspar. Despite the simple framework composition, the diagenetic character of the Norphlet is complex. Important authigenic minerals include carbon ate phases (calcite, dolomite, Fe-dolomite, and breunnerite), feldspar (albite and K-feldspar), evaporite minerals (anhydrite and halite), clay minerals (illite and chlorite), quartz, and pyrobitumen. The abundance and distribution of these miner als varies significantly between onshore and offshore regions of Norphlet production. The lack of sufficient internal sources of components for authigenic minerals, combined with unusual chemical compositions of chlorite (Mg-rich), breunnerite, and some minor authigenic minerals, suggests that Louann-derived fluids influenced Norphlet diagenesis. In offshore Alabama reservoirs, porosity is dominantly modified primary poros ity. Preservation of porosity in deep Norphlet reservoirs is due to a combination of factors, including a lack of sources of cement components and lack of pervasive early cement, so that fluid-flow pathways remained open during burial. Below the dominantly quartz-cemented tight zone near the top of the Norphlet, pyrobitumen is a major contributor to reduction in reservoir quality in offshore Alabama. The highest reservoir quality occurs in those wells where the present gas water contact is below the paleohydrocarbon water contact. This zone of highest reservoir quality is between the lowermost occurrence of pyrobitumen and the present gas water contact.     

Chlamydomonas sajao nov. sp. (Chlorophyta,Volvocales)

A new species ofChlamydomonas, namely,C. sajao nov. sp. of the Volvocales, Chlorophyta was isolated from a duckweed growing near a ricefield in the vicinity of Guangzhou, China. This interesting unicellular green alga, similar toC. mexicana from Mexico, secretes quantities of extracellular mucilaginous polysaccharides, and may be employed in improving soil quality. The new species resemblesC. waldenburgensis Moewus in most characteristics but differs in three important features.     

Formation of freshwater Fe-Mn coatings on gravel and the behavior of 60Co, 90Sr,and 137Cs in a small watershed

Iron and manganese oxide coatings are actively forming on stream substrates in the White Oak Creek watershed in East Tennessee. Although oxidizing conditions are required for Fe-Mn oxide precipitation, coatings accumulate only if dissolved iron and manganese exceed 50 μg/L. Below this, coatings are lost by abrasion as fast as or faster than they form. Annual rates of formation of 3 mg/g on substrate (gravel) were observed. Manganese is dissolved from coatings between Eh values of 100 to 300 mV and below 1 mg/L dissolved O₂ at pH 6.5 to 7.5. Iron oxides can be precipitated under these conditions.Uncontaminated gravels with oxide coatings (composed of illite, quartz, and feldspar) adsorbed radionuclides rapidly from waters below MPC (Maximum Permissible Concentration) levels. Contaminated gravels placed in uncontaminated waters lost ⁶⁰Co by abrasion in oxidizing conditions and by dissolution of manganese coatings in reducing conditions. Exchangeable ⁹⁰Sr was completely lost after one month whereas nonexchangeable ⁹⁰Sr was lost more slowly; ¹³⁷Cs was totally retained by the gravels. Gravels such as these can be used to monitor the radionuclide content of waters in the environment.     

Origin and structure of the Galactic disc(s)

We examine the chemical and dynamical structure in the solar neighbourhood of a model Galaxy that is the endpoint of a simulation of the chemical evolution of the Milky Way in the presence of radial mixing of stars and gas. Although the simulation's star formation rate declines monotonically from its unique peak and no merger or tidal event ever takes place, the model replicates all known properties of a thick disc, as well as matching special features of the local stellar population such as a metal-poor extension of the thin disc that has high rotational velocity. We divide the disc by chemistry and relate this dissection to observationally more convenient kinematic selection criteria. We conclude that the observed chemistry of the Galactic disc does not provide convincing evidence for a violent origin of the thick disc, as has been widely claimed.     

Expansion of the northern hemisphere subtropical high pressure belt: trends and linkages to precipitation and drought

A critical issue in research concerning long-term climate change is the relationship between circulation features and global temperature variations. We establish that the annual areal size of the northern hemisphere subtropical high pressure belt (SHPB), as defined by seven 500-hPa height isohypses, shares over 70% of the variability with global annual near-surface air temperature since 1948. The area enclosed by the 5850-m isohypse of the 500-hPa surface in the northern hemisphere has more than doubled since the 1950s, with greatest increases over northern Africa, the Middle East and India. A long-term historical run of a coupled global climate model shows rapidly increasing SHPB annual sizes, since the mid-1970s. Since the SHPB’s descending air produces increased aridity, SHPB expansion may transition humid regions to more arid lands. To examine this aspect, first, variations in recorded precipitation using a gridded database for the region experiencing expansion of the SHPB show a decrease in precipitation (though significant only at the 88% confidence level) over the last 60?years. Second, variations in a 0.5° spatial resolution monthly drought index, the Standardized Precipitation Evapotranspiration Index, are highly correlated (+0.78) with annual variations in the area enclosed by the 500-hPa height isohypses. These results support those of previous investigations that suggest further northward expansion of the northern hemisphere subtropical dry zones with continued global climate change.     

TIME-DISTANCE HELIOSEISMOLOGY WITH THE MDI INSTRUMENT: INITIAL RESULTS

In time-distance helioseismology, the travel time of acoustic waves is measured between various points on the solar surface. To some approximation, the waves can be considered to follow ray paths that depend only on a mean solar model, with the curvature of the ray paths being caused by the increasing sound speed with depth below the surface. The travel time is affected by various inhomogeneities along the ray path, including flows, temperature inhomogeneities, and magnetic fields. By measuring a large number of times between different locations and using an inversion method, it is possible to construct 3-dimensional maps of the subsurface inhomogeneities. The SOI/MDI experiment on SOHO has several unique capabilities for time-distance helioseismology. The great stability of the images observed without benefit of an intervening atmosphere is quite striking. It has made it possible for us to detect the travel time for separations of points as small as 2.4 Mm in the high-resolution mode of MDI (0.6 arc sec pixel^-1). This has enabled the detection of the supergranulation flow. Coupled with the inversion technique, we can now study the 3-dimensional evolution of the flows near the solar surface.