A postglacial coleopterous assemblage from Lockport Gulf,New York

The Lockport Gulf site near Lockport, New York, is a 1.9-m sequence of organic-rich marls having a basal date of approximately 10,920 yr B.P. Six bulk samples with a combined weight of 48 kg produced over 780 individual fossil insects representing five orders, as well as molluscs and abundant plant macrofossils. Coleoptera were represented by 24 families. Several major beetle groups (Carabidae, ground beetles; Hydrophilidae, water scavenger beetles; Elmidae, riffle beetles; Staphylinidae, rove beetles; and Scolytidae, bark beetles) indicate a riffle-and-pool stream, surrounded by marsh, with open riparian habitats and nearby trees. Two zones were recognized based on the Coleoptera assemblages. The Zone 1 fauna (ca. 10,920–9800 yr B.P.) was dominated by boreal forest taxa with abundant bark beetles indicating the presence of spruce trees. In Zone 2 (ca. 9700-9100 yr B.P.) the combination of species with a restricted modern distribution in the Great Lakes-St. Lawrence Forest region and pine and deciduous tree inhabitants suggests a change in vegetation by 9700 yr B.P. Thermal estimates from a faunal analysis indicate that the climatic conditions were stable across the spruce-pine transition, with the mean July temperature in the range of 16° to 18°C.     

Ries impact crater,southern Germany: search for meteoritic material

Twenty-three samples from the Ries crater, representing a wide range of shock metamorphism, were analyzed for seven siderophile elements (Au, Ge, Ir, Ni, Os, Pd, Re) and five volatile elements (Ag, Cd, Sb, Se, Zn). Taking Ir as an example, we found siderophile enrichments over the indigenous level of 0.015 ppb Ir occur in only eight samples. The excess is very modest; even the most enriched samples (a weakly shocked biotite gneiss and a metal-impregnated amphibolite) have Ir, Os corresponding to ~4 × 10^?4 C1 chondrite abundances. Of five flädle glasses analyzed only one shows excess Ir. Suevite matrix and vesicular glass have slight enrichment, but homogenous glass from the same rock does not. In flädle glasses, Ni and Se are strongly correlated and apparently reside in Ir, Os-poor Sulfides [pyrrhotite, chalcopyrite, pentlandite(?)]of terrestrial, probably sedimentary, origin. The Ir, Os and Ni enrichments of the metal-bearing amphibolite are compatible with chondritic ratios, but these are ill-defined because of uncertainty in Ni. In the other samples enriched in siderophiles Ir(Os), Ni and Se are mutually correlated; $\text{Ni}\text{Ir}$ and $\text{Ni}\text{Os}$ ~ 11 × C1 and are much higher than any chondritic ratios; $\text{Se}\text{Ni}\text{~ 2 × C1}$ and suggests a sulfide phase, rather than metal may be the host of the correlated elements. Lacking a plausible local source, this material is apparently meteoritic in origin. The unusual elemental ratios, coupled with the very low enrichments, tend to exclude chondrites and most irons as likely projectile material. Of the achondrites, aubrites seem slightly preferable. Ratios of excess siderophiles in Ries materiel match tolerably those of an aubrite (possibly atypical) occurring as an inclusion in the Bencubbin meteorite, Australia. The Hungaria group of Mars-crossing asteroids may be a source of aubritic projectiles.     

Pb isotopic zoning of K-feldspar megacrysts determined by Laser Ablation Multi-Collector ICP-MS: Insights into granite petrogenesis

This study investigates Pb isotopic zoning in magmatic K-feldspar megacrysts from the Monte Capanne pluton (Elba, Italy) using Laser Ablation Multi-Collector-ICPMS. The studied crystals provide an ideal opportunity to use in situ techniques to assess the extent of open-system processes and better characterize the components involved in the genesis of complex magma systems. Earlier investigations of the pluton identified the importance of magma mixing between mantle and crustal-derived magmas.The investigated K-feldspar megacrysts exhibit strong zoning in ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb, correlated with lead elemental variations. We interpret these variations as reflecting growth zoning, as opposed to secondary diffusive exchange. Despite a great variety of zoning patterns, we were able to correlate different events of megacryst growth, reflecting crystallization in a dynamic magma system. Our two-step model includes (1) growth of a granitic magma chamber by addition of low ²⁰⁸Pb/²⁰⁶Pb magma to a high ²⁰⁸Pb/²⁰⁶Pb magma contaminated with crustal material (i.e., the megacryst cores) and (2) recharge by mantle-derived magma (i.e., the megacryst rims). We interpret the thorogenic nature of the megacryst rims to reflect the mantle-derived component involved in the mixing process. Taking account of other data from the Tuscan Magmatic Province, the mantle source is inferred to have been metasomatized by continental material during subduction. TIMS Sr isotopic data from microdrilled cores in one megacryst provides general support for the model but show that the two isotopic systems are decoupled.     

Re-Os ages for Archean molybdenite and pyrite, Kuittila-Kivisuo, Finland and Proterozoic molybdenite, Kabeliai, Lithuania: testing the chronometer in a metamorphic and metasomatic setting

Seven ¹⁸⁷Re-¹⁸⁷Os ages were determined for molybdenite and pyrite samples from two well-dated Precambrian intrusions in Fennoscandia to examine the sustainability of the Re-Os chronometer in a metamorphic and metasomatic setting. Using a new ¹⁸⁷Re decay constant (1.666 × 10⁻¹¹y⁻¹) with a much improved uncertainty (±0.31%), we determined replicate Re-Os ages for molybdenite and pyrite from the Kuittila and Kivisuo prospects in easternmost Finland and for molybdenite from the Kabeliai prospect in southernmost Lithuania. These two localities contain some of the oldest and youngest plutonic activity in Fennoscandia and are associated with newly discovered economic Au mineralization (Ilomantsi, Finland) and a Cu-Mo prospect (Kabeliai, Lithuania). Two Re-Os ages for vein-hosted Kabeliai molybdenite average 1486 ± 5 Ma, in excellent agreement with a 1505 ± 11 Ma U-Pb zircon age for the hosting Kabeliai granite pluton. The slightly younger age suggests the introduction of Cu-Mo mineralization by a later phase of the Kabeliai magmatic system. Mean Re-Os ages of 2778 ± 8 Ma and 2781 ± 8 Ma for Kuittila and Kivisuo molybdenites, respectively, are in reasonable agreement with a 2753 ± 5 Ma weighted mean U-Pb zircon age for hosting Kuittila tonalite. These Re-Os ages agree well with less precise ages of 2789 ± 290 Ma for a Rb-Sr whole-rock isochron and 2771 ± 75 Ma for the average of six Sm-Nd T_DM model ages for Kuittila tonalite. Three Re-Os analyses of a single pyrite mineral separate, from the same sample of Kuittila pluton that yielded a molybdenite separate, provide individual model ages of 2710 ± 27, 2777 ± 28, and 2830 ± 28 Ma (Re = 17.4, 12.1, and 8.4 ppb, respectively), with a mean value of 2770 ± 120 Ma in agreement with the Kuittila molybdenite age. The Re and ¹⁸⁷Os abundances in these three pyrite splits are highly correlated (r = 0.9994), and provide a ¹⁸⁷Re-¹⁸⁷Os isochron age of 2607 ± 47 Ma with an intercept of 21 ppt ¹⁸⁷Os (MSWD = 1.1). It appears that the Re-Os isotopic system in pyrite has been reset on the millimeter scale and that the 21 ppt ¹⁸⁷Os intercept reflects the in situ decay of ¹⁸⁷Re during the ∼160 to 170 m.y. interval from ∼2778 Ma (time of molybdenite ± pyrite deposition) to ∼2607 Ma (time of pyrite resetting). When the Re-Os data for molybdenites from the nearby Kivisuo prospect are plotted together with the Kuittila molybdenite and pyrite data, a well-constrained five-point isochron with an age of 2780 ± 8 Ma and a ¹⁸⁷Os intercept (−2.4 ± 3.8 ppt) of essentially zero results (MSWD = 1.5). We suggest that the pyrite isochron age records a regional metamorphic and/or hydrothermal event, possibly the time of Au mineralization. A proposed Re-Os age of ∼2607 Ma for Au mineralization is in good agreement with radiometric ages by other methods that address the timing of Archean Au mineralization in deposits worldwide (so-called “late Au model”). Molybdenite, in contrast, provides a robust Re-Os chronometer, retaining its original formation age of ∼2780 Ma, despite subsequent metamorphic disturbances in Archean and Proterozoic time. Received: 25 September 1996 / Accepted: 27 August 1997     

FLAIR observations of planetary nebulae in the Large Magellanic Cloud

This paper presents new observations of 97 planetary nebulae in the Large Magellanic Cloud (LMC) obtained using the FLAIR system on the UK 1.2-m Schmidt Telescope. These nebulae are mostly at the fainter end of the known population, and about 75 per cent have not been observed before in spectroscopic mode. Radial velocities have been measured using cross-correlation techniques, and represent an increase of 66 per cent in the sample of LMC planetary nebulae with known radial velocities. The major line ratios are given, and are analysed in conjunction with published data. One-quarter of the faint nebulae are Type I objects with very strong [N II ] and [S II ] lines; most of the other faint nebulae have low density, low excitation and relatively strong [S II ] lines.     

RXTE spectroscopy of GRS 1915+105

The galactic superluminal motion source GRS 1915+105 has been extensively observed with the RXTE satellite over the last two years. More than 250 RXTE pointings have been performed until mid-May 1998 with more than 1.5 Msec exposure time on roughly a weekly basis. Here we report on first results of our spectral analysis of a major part of these pointed RXTE observations. We establish the existence of at least 5 spectral components and present the changes of these components over the last two years.