Lead isotope planetary profiling (LIPP): Summation–depiction of Earth's many reservoirs

Abstract 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. The isochrons appear amenable to mutual organization so that they intersect each other (as subgroups) in six distinct and generally well-resolved foci. These potentially infer the present-day Pb isotopic composition of six different reservoirs, from which all terrestrial surface rocks were derived. If so, a fundamental global feature of Earth would have been discovered and summed up in a simple linear image, which constitutes an isotope-profile of the planet. As the profile is governed by the mathematics of the Pb isotope systematics, it is amenable to interpretive elaboration into events, processes and mechanisms. In this report, the six reservoirs were used to construct a model for Earth's Early Episodic Evolution ( EEEE ): The layout of the reservoirs suggests that they resulted from four, global differentiation events: The first which occurred at 4.42 Ga or earlier, involved the whole mantle and resulted in the formation of a homogeneous post accretion mantle, PAM (with a present-day 206 Pb /  204 Pb = 14.0 ± 1 and 207 Pb /  204 Pb = 14.4 ± 0.3), and a post accretion crust , PAC , with unknown average Pb composition, which was subsequently consumed and obliterated except rare altered vestiges. Operationally, PAM is represented by the first and most ancient reservoir #1, with a lower limit-age of 4.42 Ga. This is the parent-reservoir from which all other reservoirs were derived in three subsequent global differentiation events. Each time, the formed reservoir (i.e., isolated after it was homogenized in U and Pb), survived for > 1 Ga. The second event marking the insetting of global differentiation, mediated by subduction, occurred at 4.13 Ga. It was followed by two other events at 3.4 and 2.96 Ga. In each of the four global events, Pb was irreversibly lost (as a sulfide?) to an efficient hidden sink (the core?). Thus by default, the resulting reservoirs appear enriched in U. The familiar Earth's history, marked by continuous geologic processes, pertains to the last 2.96 Ga of Earth's existence. Inferring the encompassing global nature of the observation, the field of Ocean Island Basalt (OIB) shows affinity to some reservoirs. Also, the filtered isochrons of an ocean island and a group of Seamounts appear to be associated with two of the reservoirs. In addition, the untreated Pb / Pb data on surface rocks, collectively define a ‘flow-pattern’ to which OIB appears to belong. Because they were subducted, OIBs of ancient epochs appear to have left behind recognizable extinction gaps within the global pattern. This suggests episodic initiation of planet-wide hot-spot events, possibly in association with the formation of the reservoirs. Building on the model, the isotopic composition and temporal parameters for ‘the’ source of ocean island basalt ( SOIB ) are proposed.