Determination of initial leads of four terrestrial terrains, applying the Tulip methodology

Abstract

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: 206Pb/204Pb=11.088±0.024, 207Pb/204Pb=12.983±0.002, and 208Pb/204Pb=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.Projection of initial Pb by a database is caused by Th-U-Pb fractionation, in disturbing events superimposed on the rocks of a terrain. As demonstrated mathematically (in a companion paper), elemental fractionation can induce alignments of the data in linear trends of false ‘ages,’ which together with the rocks' isochron, converge (on various Pb plots) on a point that yields initial Pb. This may result in two counter effects: (1) precise determination of initial Pb, and (2) ambiguity in the exact meaning of an isochron (because of the possibility it was affected by fractionation, like the other lines). Consequently associating a Pb/Pb age with the determined initial may not always be meaningful. For SOI, the rocks' age is 3.84±0.05Ga.The ability to determine initial Pb opens the possibility for eventual unfolding of details of the evolutionary history of the Earth and other planets. However, that cannot be satisfactorily achieved without additionally developing a methodology for resolving initial Pb to its multi-stages. Because such methodology is still lacking, a procedure of Congruently Associated Profiles (CAPs), for resolving initial Pb to a maximum of three stages was developed and outlined in a companion paper (C, P, # 1, 2016). For SOI, only a two-stage CAP solution is possible. It indicates a U-Pb fractionation event at 67.5Ma after the Earth's accretion (that is a first stage lasting from 4.563 to 4.496Ga), with μ=238U/204Pb=0.45±0.25. For the second stage, extending from 4.496 to 3.8Ga, I obtain μ=9.25±0.02.The stated duration of 67.5Ma for the first stage is an upper limit, and in a multi- stage solution, if and when it becomes possible, the first stage duration would be shorter, and its meaning may be elucidated. For now it remains subject to qualitative speculations including the possibility of being ‘associated’ with core formation, or an early impact (resulting in formation of the moon?), or …or. This ambiguity underlines a crucial need for a methodology to resolve initial Pb to more of its multi stages.