Geochemistry of the Adamello massif (northern Italy)

The Tertiary Adamello massif, outcropping over an area of more than 550 km² in the southern Alps (northern Italy) is composed mainly of granitoid rocks (granodiorite, tonalite, quartz diorite) with minor amounts of diorite and gabbro. The major and trace element composition of these rocks is comparable to calc-alkaline volcanic rocks of continental margins. The granitoid rocks display spatial and temporal variations in their composition, particularly in Na, P, Sr, La, Nb and Y contents and ⁸⁷Sr/⁸⁶Sr ratios. The variations were probably produced by concurrent contamination/wall-rock assimilation and fractional crystallization of high-alumina basaltic magma.     

Sulphur-containing compounds in sulphur-rich crude oils from hypersaline lake sediments and their geochemical implications

Three sulphur-rich commercial crude oils have been studied, which contain sulphur as high as up to 4–12%. These samples were collected from Tertiary hypersaline lake sediments of the Jianghan Basin, Hubei Province at different depths, but above the oil generation threshold (2200m). FPD-GC and GC-MS data show that aromatic fractions of the crude oils are composed of different homologues of sulphur-containing compounds, including long-chain normal alkyl-thiophenes and-thiolanes, long-chain isoprenoid-thiophenes and -thiolanes, and benzothiophenes. It is worth noting that the distribution patterns of long-chain alkyl-thiophenes and -thiolanes from two shallow-seated crude oils are quite similar to those of normal alkanes showing marked even-odd predominance. It seems that the even-odd predominance of sulphur-containing compounds decreases with increasing burial depth of the crude oils. The major component of aliphatic fraction is phytane, and similarly the major peaks of aromatic fractions also represent C₂₀ isoprenoid thiophenes. Some preliminary conclusions have been drawn from the above discussion: (1) Abundant sulphur-containing compounds may be used as an indicator of low mature or immature crude oils produced from hypersaline lake sediments; (2) Sulphur-containing compounds are considered to be early diagenetic products of reactions between elemental sulphur or sulfides and alkanes or their precursors (phytols, fatty acids, alcohols, etc.), or of bacterial activities, but not direct inputs of organisms.     

Nannofossil age constraints for the northern KwaZulu-Natal shelf-edge wedge: Implications for continental margin dynamics, South Africa, SW Indian Ocean

Samples collected from the shelf-edge wedge using surface grab samples and the Jago submersible constrain the KwaZulu-Natal shelf-edge wedge to a late Pliocene age on the basis of the absence of Gephyrocapsa oceanica s.l. and Discoaster brouweri, and the presence of Calcidiscus macintyrei. This correlates with proposed Tertiary sea-level curves for southern Africa and indicates relative sea-level fall during the late Pliocene coupled with hinterland uplift. Exposed failure scarps in the upper portions of submarine canyons yield sediment samples of early Pleistocene ages, indicating the uppermost age of deposition of clinoform topsets exposed in the scarp walls. Partially consolidated, interbedded silty and sandy deposits of similar age outcrop in the thalweg of Leven canyon at a depth of 150 m. These sediments provide an upper age limit of the shelf-edge wedge of early Pleistocene, giving a sedimentation rate of this wedge of 162–309 m/Ma. The distribution of widespread basal-most Pleistocene sediments on the upper slope indicates that these sediments escaped major reworking during sea-level falls associated with Pleistocene glaciations and remain as relict upper slope veneers. The absence of more recent sediments suggests that this area has been a zone of sediment bypass or starvation since the early Pleistocene. Areas where younger sediments mantle deposits of early Pleistocene ages represent areas of offshore bedload parting, re-distributing younger Holocene sediment offshore and downslope.     

THE SASKATCHEWAN FIREBALL OF 1922 AND A POSSIBLE LINK WITH THE WYNYARD CHONDRITE

A brilliant smoking meteor appeared in a clear sky in bright sunlight at 11 a.m., July 24, 1922 near Wynyard, Saskatchewan, Canada. The sight and thunderous sounds were witnessed by many hundreds of people in the rural district but no craters or meteorites were found at that time. Investigation and interviews with surviving witnesses in 1981 indicated a defined area near Big Quill Lake in which a meteorite may have fallen. Field investigation led to the Wynyard meteorite which had been found by a farmer sometime in the late 1960's at 104° 11'W 51°33'N. The Wynyard meteorite is a chondrite weighing 3.5 kg. It is moderately weathered and it may or may not have been part of the 1922 fall.     

Origin of eclogite and garnet pyroxenite from the Moldanubian Zone of the Bohemian Massif, Czech Republic and its implication to other mafic layers embedded in orogenic peridotites

Summary The granulite terrane of the Czech part of the Gf?hl unit includes numerous small bodies of mantle derived peridotite, some of which contain layers or lenses of eclogite and garnet pyroxenite. These eclogitic rocks have generally been considered to be high-pressure crystal cumulates formed in the upper mantle. We present new analyses of whole-rock major and trace element contents for three kynanite-quartz eclogite samples taken from the Nové Dvory garnet peridotite body. Integrating these data with previously published analyses from the literature on eclogitic rocks from this terrane, we demonstrate that a magnesian group of eclogites, including these three new samples, were originally formed as cumulus gabbros, which were later transformed to eclogites in the mantle. A gabbroic origin for some mafic layers (Type II) has been advocated for other orogenic peridotites, such as Beni Bousera (Morocco), Ronda (Spain), and Horoman (Japan). By comparing these sets of data with those from the Bohemian Massif, we propose a simple method of identifying groups of metagabbros by utilizing MgO-normalization in oxide ratio plots for whole-rock major element analyses.     

The timing of ultrahigh-temperature metamorphism in Southern India: U–Th–Pb electron microprobe ages from zircon and monazite in sapphirine-bearing granulites

We report here U–Pb electron microprobe ages from zircon and monazite associated with corundum- and sapphirine-bearing granulite facies rocks of Lachmanapatti, Sengal, Sakkarakkottai and Mettanganam in the Palghat–Cauvery shear zone system and Ganguvarpatti in the northern Madurai Block of southern India. Mineral assemblages and petrologic characteristics of granulite facies assemblages in all these localities indicate extreme crustal metamorphism under ultrahigh-temperature (UHT) conditions. Zircon cores from Lachmanapatti range from 3200 to 2300 Ma with a peak at 2420 Ma, while those from Mettanganam show 2300 Ma peak. Younger zircons with peak ages of 2100 and 830 Ma are displayed by the UHT granulites of Sengal and Ganguvarpatti, although detrital grains with 2000 Ma ages are also present. The Late Archaean-aged cores are mantled by variable rims of Palaeo- to Mesoproterozoic ages in most cases. Zircon cores from Ganguvarpatti range from 2279 to 749 Ma and are interpreted to reflect multiple age sources. The oldest cores are surrounded by Palaeoproterozoic and Mesoproterozoic rims, and finally mantled by Neoproterozoic overgrowths. In contrast, monazites from these localities define peak ages of between 550 and 520 Ma, with an exception of a peak at 590 Ma for the Lachmanapatti rocks. The outermost rims of monazite grains show spot ages in the range of 510–450 Ma.While the zircon populations in these rocks suggest multiple sources of Archaean and Palaeoproterozoic age, the monazite data are interpreted to date the timing of ultrahigh-temperature metamorphism in southern India as latest Neoproterozoic to Cambrian in both the Palghat–Cauvery shear zone system and the northern Madurai Block. The data illustrate the extent of Neoproterozoic/Cambrian metamorphism as India joined the Gondwana amalgam at the dawn of the Cambrian.     

Sulphur saturation in the lower and critical zones of the Eastern Bushveld Complex

A suite of ultramafic and mafic rocks from the lower, critical and lower portion of the main zones of the Bushveld Complex has been analysed for Th, Cs, Zr, Ni, Cr and Au by INAA and XRF spectrometry. The incompatible elements Th, Cs, and Zr correlate positively, and show a gradual upward increase in abundance. Assuming constant average proportion of intercumulus material, this upward increase implies that the zones of the Complex studied represent crystallization of a single magma type some 3600 m thick. Pyroxenites dominate the lower portion of the section studied and their Ni content shows an initial rapid decrease from 850 ppm in the lowermost rocks, to around 500 ppm, with considerable scatter. This distribution is most likely to have resulted from bottom crystallization with superimposed convective overturn near the transient floor of the chamber. Gold abundances are generally higher in chromitites, and correlate positively with Ni, indicating the presence of significant amounts of cumulus immiscible sulphide. In the silicate rocks, Au does not correlate with any of the analysed elements, and it is concluded that Au was trapped in small quantities of immiscible sulphide which precipitated continuously during crystallization. There is an upward increase in the amount of cumulus immiscible sulphide, indicating a progressive increase in sulphur solubility in the magma.     

Analysis of Short-Period Rayleigh Waves Recorded in the Bohemian Massif Area During Celebration 2000 Experiment

The aim of this paper is to show the application of short-period surface waves recorded during deep seismic sounding experiment for constraining shallow velocity structure of the crust. Phase velocity of fundamental mode Rayleigh waves, observed along the CELEBRATION 2000 experiment profile CEL09, were obtained by a p-ω method and has been subsequently inverted for one-dimensional shear velocity models for the top 2 km. Multiple filter technique applied to one shot gather was used to carry out a joint inversion of phase and group velocity data and to provide γ_R data to be used for Q_β inversion. Validity of obtained V_S and Q_β models was confirmed by the reflectivity method. Noticeably, no clear dispersive wawes were observed in the Tepla-Barrandian Unit. Quasi-2D model based on the individual 1D V_S models is well correlated with the surface geology. Lower V_S are observed in the Saxothuringian Zone in comparison to the Moldanubian Zone. In the vicinity of the Central Bohemian and Moldanubian Plutons, the near-surface V_S values are relatively low, but below 1 km depth, they are higher than in surrounding areas. We interpret it as the result of the weathering and cracks within the granitoid rocks.     

Dissolved nutrient balance and net ecosystem metabolism in a Mediterranean-climate coastal lagoon: San Diego Bay

The temporal and spatial variability of dissolved inorganic phosphate (DIP), nitrogen (DIN), carbon (DIC) and dissolved organic carbon (DOC) were studied in order to determine the net ecosystem metabolism (NEM) of San Diego Bay (SDB), a Mediterranean-climate lagoon. A series of four sampling campaigns were carried out during the rainy (January 2000) and the dry (August 2000 and May and September 2001) seasons. During the dry season, temperature, salinity and DIP, DIC and DOC concentrations increased from oceanic values in the outer bay to higher values at the innermost end of the bay. DIP, DIC and DOC concentrations showed a clear offset from conservative mixing implying production of these dissolved materials inside the bay. During the rainy season, DIP and DOC increased to the head, whereas salinity decreased toward the mouth due to land runoff and river discharges. The distributions of DIP and DOC also showed a deviation from conservative mixing in this season, implying a net addition of these dissolved materials during estuarine mixing within the bay. Mass balance calculations showed that SDB consistently exported DIP (2.8–9.8 × 10³ mol P d⁻¹), DIC (263–352 × 10³ mol C d⁻¹) and DOC (198–1233 × 10³ mol C d⁻¹), whereas DIN (5.5–18.2 × 10³ mol N d⁻¹) was exported in all samplings except in May 2001 when it was imported (8.6 × 10³ mol N d⁻¹). The DIP, DIC and DOC export rates along with the strong relationship between DIP, DIC or DOC and salinity suggest that intense tidal mixing plays an important role in controlling their distributions and that SDB is a source of nutrients and DOC to the Southern California Bight. Furthermore, NEM ranged from −8.1 ± 1.8 mmol C m⁻² d⁻¹ in September to −13.5 ± 5.8 mmol C m⁻² d⁻¹ in January, highlighting the heterotrophic character of SDB. In order to explain the net heterotrophy of this system, we postulate that phytoplankton-derived particulate organic matter, stimulated by upwelling processes in the adjacent coastal waters, is transported into the bay, retained and then remineralized within the system. Our results were compared with those reported for the heterotrophic hypersaline coastal lagoons located in the semi-arid coast of California–Baja California, and with those autotrophic hypersaline systems found in the semi-arid areas of Australia. We point out that the balance between autotrophy and heterotrophy in inverse estuaries is dependent on net external inputs of either inorganic nutrients or organic matter as it has been indicated for positive estuaries.     

The concentration and distribution of dimethyl sulfide in the marine atmospheric boundary layer near the equator

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