No abstract available

No abstract available

[en] Fission track ages have been determined on sphene and apatite from the granitic rocks of King Island in Bass Strait, southeastern Australia. In all cases sphene and apatite ages are markedly discordant. Sphene ages compare very closely to earlier K-Ar measurements and indicate an emplacment age of about 350 m.y. for the east coast group of granites and their important scheelite mineralization. Apatite ages are all younger by about 80-200 m.y. suggesting that fission tracks were not fully retained in this mineral until the Cretaceous. During the Cretaceous King Island was at the edge of the developing Otway Rift Valley which resulted in the breakup of Australia and Antarctica. Uplift of the basement rocks along the rift margin with consequent rapid erosion allowed the apatites to cool below about 110⁰C and begin accumulating fission tracks for the first time. Differing degrees of uplift, at least partly fault controlled, have produced a regular pattern of apatite ages across the island. A relationship between apatite fission track ages and continental breakup may be a widespread phenomenon which could give valuable insight into the thermal and tectonic development of rifted continental margins. (Auth.)

[en] Induced-fission track densities have been compared on mica external track detectors (2π geometry) and internal mineral surfaces (4π geometry) for the minerals apatite, zircon and sphene. The track-density ratios indicate that the geometry factor relating these two track-registration geometries is indistinguishable from the ideal value of 0.5 (i.e. 2π/4π) for these minerals. External track detectors are not geometrically equivalent to external etching surfaces of a uranium-bearing material. Values significantly higher than 0.5 are caused by a low etching efficiency on the internal mineral surface. The etching efficiency varies with crystallographic orientation in zircon, and may be lowered after thermal annealing in sphene. Etching a mineral surface for spontaneous tracks has no observable effect on the uranium concentration of that surface. These observations have important implications for the use of the external-detector method in fission-track dating. (author)

No abstract available

[en] Many Type B Ca-Al-rich inclusions (CAI's) in the Allende carbonaceous chondrite contain two types of spinel structures, 'framboids' and 'palisades'. Framboids are clumps of spinel grains generally < 100 μm across. Experimental studies show that they probably formed in situ by solid state growth processes. Palisades are texturally different and consist of ovoid shells of spinel grains that appear in thin sections as long arcs or rings with diameters ranging from approx. 50 μm up to 2 cm. No in situ formation process seems able to explain the variety of sizes and morphologies of palisades nor the different compositions and textures of the enclosed and enclosing materials. We therefore suggest that palisades are the spinel rims of smaller, earlier-formed Type B CAI's that were incorporated into other CAI material in various ways - by capture into liquid drops, by solid condensate overgrowths and by the partial melting or welding of agglomerates containing the bodies. As some Type B bodies have been found inside Type A host material it appears that in at least some regions, and probably generally, Type A CAI's formed after Type B. Proposals are made concerning the origin of the CAI material. (author)

[en] A compilation of the lengths of confined fission tracks in a wide variety of apatites from different geological environments has shown that the distribution of confined track lengths can provide unique thermal history information in the temperature range below about 150⁰C over times of the order of 10⁶ to 10⁹ years. The continuous production of tracks through time, coupled with the fact that the length of each track shrinks to a value characteristic of the maximum temperature it has experienced, gives a final length distribution which directly reflects the nature of the variation of temperature with time. Most distinctive of the myriad possible forms of the final distribution are the bimodal distributions, which give clear evidence of a two-stage history, including high and low temperature phases. The study of confined length distributions therefore offers invaluable evidence on the meaning of any fission-track age, and bears the potential of providing rigorous constraints on thermal history in the temperature regime below about 150⁰C. The results of this study strongly suggest that any apatite fission-track age determination should be supported by a confined track length distribution. (orig./RB)

[en] The Type B Ca-Al-rich inclusions in the Allende carbonaceous chondrite form a continuous range from the mineralogically concentrically zoned B1 subtype to the unzoned B2 subtype. These subtypes differ in (i) structure, texture, grain size and shape, (ii) mineralogical proportions and compositions, (iii) accessory mineralogy, (iv) relative abundance of spinel framboids, (v) rim layering, (vi) major element chemistry, and (vii) degree of secondary alteration. These differences, together with observations on the crystallization of synthetic melts, suggest that the B1 inclusions crystallised relatively rapidly from molten parental material while B2 types crystallised relatively slowly close to the solidus from material that had not been completely melted. The same data are used to construct an evaporative residue model for the origin of the parental Type B materials. In the model, dust in the protosolar nebula was heated with removal of more volatile elements, leaving completely melted (Type B1) residues at the highest temperatures and incompletely melted, less highly evaporated (Type B2) residues at lower temperatures. (author)

[en] Analytical techniques have been developed for using a secondary ion mass spectrometer, the ion microprobe mass analyzer (IMMA), to determine, in situ, ²⁰⁷Pb/²⁰⁶Pb and U/Pb ages on approximately 10-μm areas of individual mineral phases containing relatively abundant radiogenic Pb istopes. Standard samples of known age and U, Th and Pb contents, together with the Andersen-Hinthorne local thermal equilibrium (LTE) model for predicting ionization parameters are used to establish a semi-empirical relationship for correcting observed U/Pb intensities to atom ratios. Measurements of isotope standards show that mass fractionation corrections are not required and that the accuracy and precision of analysis are generally limited by Poisson counting statistics. Many U-rich accessory minerals yield spectra which consist only of Pb at masses 204, 206, 207 and 208; thus the measurement of ²⁰⁷Pb/²⁰⁶Pb ages is accomplished by simply measuring the intensities of these peaks and the background. (Auth.)

[en] Fission Track Dating has undergone something of a renaissance in recent years. What began as a rather unreliable method of geochronology has now become a rigorous and dependable means of measuring not only geological time but also paleotemperatures. Developments such as the zeta calibration technique, rigorous satistical data analysis, investigation of confined track lengths and detailed studies of annealing behaviour have all contributed to advances in the technique