Lead isotopes are compared in two equivalent diagrams. With a range in U/Pb, a closed system that evolved its radiogenic Pb in a single stage yields data that define a line of exactly the same age in the two presentations. This strict reproducibility (within +/- a few 10(6) yr, at most) is the crux of the concept of Pb-isotope synchronism. In contrast, an open system produces data that disperse variably in the two diagrams, yielding ages which are often different by much greater than 107 yr. However, because an age-calculation reconciles uncertainty in the age with the degree of dispersion in the data, a highly disturbed system yields false ages (from both diagrams), which nevertheless overlap within the calculated errors.

By using accelerator mass spectrometry we have measured the Be-10 concentrations of 86 Australasian tektites. Corrected to the time of tektite production similar to0.8 My ago, the Be-10 concentrations (10(6) atom/g) range from 59 for a layered tektite from Huai Sai, Thailand, to 280 for an australite from New South Wales, Australia. The average value is 143 +/- 50. When tektites are sorted by country, their average measured Be-10, concentrations increase slowly with increasing distance from Southeast Asia, the probable location of the tektite producing event, from 59 +/- 9 for 6 layered tektites from Laos to 136 +/- 20 for 20 splash-form tektites from Australia. The lowest Be-10 concentrations for tektites fall on or within a contour centered off the shore of Vietnam, south of the Gulf of Tonkin (107degreesE; 17degreesN), but also encompassing two other locations in the area of northeastern Thailand previously proposed for the site of a single tektite-producing impact. The Be-10 concentrations of layered tektites show only a weak anticorrelation (R similar to -0.3) with the numbers of relief crystalline inclusions.

An approach aiming at a summation-depiction of the isotopic characteristics of a planet is attempted. The approach, Lead Isotope Planetary Profiling (LIPP), is a thematic derivation from the general field of planetary geochemistry. Specifically, the recently developed methodology of Pb isotope synchronism, which often allows the accurate dating of disturbed geologic systems, has been applied to a large body of terrestrial Pb isotopic data in the literature. The exercise resulted in 26 Pb/Pb lines most of which appear to be highly precise isochrons.

Serpentinite: clasts and muds erupted from Conical Seamount, Mariana forearc, show substantial enrichment in boron (B) and B-11 (delta B-11 up to +15 parts per thousand) relative to mantle values. These elevated B isotope signatures result from chemical exchange with B-rich pore fluids that are upwelling through the seamount. If the trends of decreasing delta B-11 with slab depth shown by cross-are magmatic suites in the Izu and Kurile arcs of the western Pacific are extended to shallow depths (similar to 25 km), they intersect the inferred delta B-11 of the slab-derived fluids (+13 parts per thousand) at Conical Seamount. Simple mixtures of a B-rich fluid with a high delta B-11 and B-poor mantle with a low delta B-11 are insufficient to explain the combined forearc and are data sets. The B isotope systematics of subduction-related rocks thus indicate that the fluids evolved from downgoing slabs are more enriched in B-11 than the slab materials from which they originate. Progressively lower delta B-11 in are lavas erupted above deep slabs reflects both the progressive depletion of B-11 from the slab and progressively greater inputs of mantle-derived B. This suggests that the slab releases B-11-enriched fluids from the shallowest levels to depths greater than 200 km. (C) 2001 Elsevier Science B.V. All rights reserved.

A method for determining the initial Pb of a terrain, on the basis of the measured isotopic compositions of its rocks, is put forward in this report. The method was inspired by the premise that the initial Pb inherited by the rocks from a reservoir from which they were extracted, is immutable and inerasable, irrespective ofthe multitude of disturbances that may have subsequently been superimposed on the terrain. This is because while these disturbances may have altered the isotopic composition of some or all the rocks, they lacked the vehemence to re-melt the entire terrain or at least a very large portion of it, which is a pre-requisite for altering the isotopic composition of initial Pb. If this rational is valid, then a large Pb isotope database (including data on mineral separates with low affinities for U and Th) that is representative of a terrain, when plotted on any Pb isotope correlation diagram (e.g., the conventional Pb/Pb plot), may define a dispersion field that tapers toward a single spot. That single spot (once unambiguously determined) is the initial Pb isotopic composition.

The recently developed TULIP methodology for determining Initial lead, based on the measured Pb isotopic compositions of rocks, was applied to four terrestrial terrains, and the results are shown and discussed in this report. Particular emphasis was given to the determination of initial Pb of the South Of Isua (SOI) terrain, because of the availability of a large high-quality database on its rocks and feldspar separates. The initial Pb results for SOI, are: Pb-206/Pb-204 = 11.088 +/- 0.024, Pb-207/Pb-204 = 12.983 +/- 0.002, and Pb-208/Pb-204 = 31.196 +/- 0.014. Initial Pb was also precisely determined for the Beartooth Mountains, and the results are: 206/204 = 13.571 +/- 0.071, 207/204 = 14.891 +/- 0.003, and 208/204 = 32.41 +/- 0.08. These results demonstrate the feasibility of routine determination of initial Pb by the developed methodology, once large databases for the terrains are established. Extending the methodology to terrains of other planets should be possible.