ALBERT

Notes: The Re–Os (rhenium–osmium) chronometer applied to molybdenite (MoS2) is now demonstrated to be remarkably robust, surviving intense deformation and high-grade thermal metamorphism. Successful dating of molybdenite is dependent on proper preparation of the mineral separate and analysis of a critical quantity of molybdenite, unique to each sample, such that recognized spatial decoupling of 187Re parent and 187Os daughter within individual molybdenite crystals is overcome. Highly precise, accurate and reproducible age results are derived through isotope dilution and negative thermal ion mass spectrometry (ID-NTIMS). Spatial decoupling of parent–daughter precludes use of the laser ablation ICP-MS microanalytical technique for Re–Os dating of molybdenite. The use of a reference or control sample is necessary to establish laboratory credibility and for interlaboratory comparisons. The Rb–Sr, K–Ar and 40Ar/39Ar chronometers are susceptible to chemical and thermal disturbance, particularly in terranes that have experienced subsequent episodes of hydrothermal/magmatic activity, and therefore should not be used as a basis for establishing accuracy in Re–Os dating of molybdenite, as has been done in the past. Re–Os ages for molybdenite are almost always in agreement with observed geological relationships and, when available, with zircon and titanite U–Pb ages. For terranes experiencing multiple episodes of metamorphism and deformation, molybdenite is not complicated by overgrowths as is common for some minerals used in U–Pb dating (e.g. zircon, monazite, xenotime), nor are Re and Os mobilized beyond the margins of individual crystals during solid-state recrystallization. Moreover, inheritance of older molybdenite cores, incorporation of common Os, and radiogenic Os loss are exceedingly rare, whereas inheritance, common Pb and Pb loss are common complications in U–Pb dating techniques. Therefore, molybdenite ages may serve as point-in-time markers for age comparisons.

Notes: Abstract Remnant stands of the non-bradysporous Banksia integrifolia on the Mornington Peninsula, Victoria appear to be in decline due to old age and other unknown factors. Many stands have been unburnt for 〉55-100 years and there is little evidence of successful regeneration by seed. The cause(s) for this lack of recruitment are unclear. In the present study we quantified (i) seed production and germinability in four remnant forests in the region; (ii) the microsite conditions necessary for seedling emergence; and (iii) the effect of herbivore browsing on early seedling survival in order to understand this apparent recruitment failure. Despite large intra-annual variation, no evidence was found that old forest trees do not produce viable seed. Emergence and survival of seedlings were not inhibited by the presence of groundlayer vegetation. Rather, vegetation facilitated early seedling survival by providing protection from browsing. Seedling transplants exhibited extremely high mortality (≥75%) due to browsing and summer soil desiccation and these factors appear to be major contributors to the lack of recruitment at the study site. Stand-replacing recruitment is therefore considered to remain unlikely under the present conditions. Developing fire and/or grazing management regimes will be necessary to conserve the structural integrity of these coastal ecosystems.

Notes: Abstract 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 TDM 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.

Notes: [Auszug] The solar wind flux of Re is given1 as 10?3 atoms cm?2 s?1, which deposits approximately 10?8 (g Re) cmr2 in 109 yr. To estimate the likelihood of detection the probable Re abundance of the lunar surface must first be considered. The preliminary examination of Apollo 11 samples3 shows only that Re ...

Notes: [Auszug] Fig. 1. The three largest craters in the Henbury meteorite crater group in Central Australia (photo by W. I). E., taken from the east at an altitude of 2,000 feet). We wish to point out that at least one terrestrial example of multiple impact craters with dividing walls essentially retained does ...

Notes: [Auszug] The Os isotopic composition of modern mantle reservoirs is heterogeneous. Rhenium is a mildly incompatible element during mantle melting, so melt depletion reduces the Re/Os of mantle residues4. In contrast, the injection of Re-rich basaltic melt or crust into a mantle domain increases the Re/Os of ...

Notes: [Auszug] Determinations of uranium and thorium in carbonaceous chondrites by neutron activation show that both uranium and thorium are inhomogeneously distributed within Type I and are generally homogeneously distributed within Type II carbonaceous chondrites. If Type I carbonaceous chondrites represent ...

Notes: [Auszug] As part of a neutron activation study of rhenium and osmium abundances in tektites and possible tektite parent materials3, a shale from the State Circle formation7, Etheridge Creek, Australian Capital Territory, Australia, was investigated. The actual specimen analysed was described as sample 13 by ...