Fouad

TeraHe/His

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.

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.