PB PARADOXES SHINE LIGHT ON THE DARK AGES OF CRATONS AND KEELS

The “Dark Ages” of Earth’s geochemical past fall largely within the informal Hadean eon, from 4.56 to 4.00 Ga (or 3.8 Ga in some models). Other than the Hadean parts of the Acasta gneiss and a few detrital zircons, the physical record of BSE evolution during this time is rare to non-existent. While Eoarchean-Hadean detrital zircons provide important insight into the dark ages, it is difficult to relate their Lu-Hf and O-isotope systematics to Earth’s major geochemical reservoirs (crust and mantle). In contrast, the U-Pb isotopic systematics of some extant Earth reservoirs contain clear evidence of geochemical processes, and to a lesser extent physical processes, that operated on the early Earth. Because the half-life of ²³⁸U (4.47 Ga) is long compared to the half-life of ²³⁵U (0.70 Ga), this combination distinguishes the U-Pb system from other commonly applied systems with much longer half-lives (¹⁴⁷Sm: 106 Ga; ¹⁷⁶Lu: 35.6 Ga) and, therefore, less temporal resolution. It is, however, the U-Pb systematics of Archean to recent magmatic rocks that provide unique insights into the evolution of major geochemical reservoirs, such as the continental crust and the depleted mantle. It is the comparison of the open system behavior of the U-Pb system in the mantle to the closed system evolution of some Archean crust that leads to the connection between Earth’s reservoir-scale “Pb paradoxes”. Pb-paradox refers to systems in which the observed uranogenic Pb isotopic composition cannot be the result of closed system decay of the observed U/Pb ratio. In the mantle (MORB source), Pb isotopic compositions cannot be produced by the mantle’s inferred U/Pb ratio (based on partition coefficients) in the age of the earth (aka 1^stPb paradox). Ancient gneisses in some Archean cratons exhibit similar systematics (aka the crustal or 2^ndPb paradox). There are many important implications of these paradoxes. Foremost is that both require interaction with a high U/Pb reservoir early in Earth history because both are distinguished by ²⁰⁷Pb/²⁰⁴Pb ratios higher than average crust or any evolved primordial mantle. Although age-dependent, a 4.0 Ga reservoir with the U/Pb ratio of MORB would need to exist for only ~50 Ma to produce the ²⁰⁷Pb-rich reservoir needed, which may or may not still exist physically.