CHRONOLOGICAL CONSTRAINTS FOR THE PALAEOPROTEROZOIC LARGE, POSITIVE CARBON ISOTOPE EXCURSION (THE LOMAGUNDI-JATULI EVENT)

The Palaeoproterozoic Lomagundi-Jatuli event (LJE) is the largest known, positive, carbon isotope excursion (CIE) in Earth’s history. It is defined as δ¹³C > +5‰ recorded between ca. 2.2 Ga – 2.06 Ga and classically presented on an age versus δ¹³C diagram with data from all continents but Antarctica. Most workers consider the LJE to be globally synchronous and protracted, though radiometric constraints on individual LJE-bearing formations are normally too limited to be unequivocal.

New age data for the LJE termination from the Pechenga and Imandra-Varzuga segments of the Transfennoscandian Greenstone Belt help refine these constraints. Specifically, U-Pb data on zircon grains in volcanic tuff in the Kolosjoki Sedimentary Formation, Pechenga, give a ca. 2.052 Ga age, supported by new and published U-Pb detrital zircon data in the lower part of the same formation. The inferred equivalent unit in Imandra-Varzuga, the Il’Mozero Sedimentary Formation, is dated by U-Pb (detrital zircon) at ca. 2.052 Ga. In Pechenga, the Kuetsjärvi Volcanic Formation (underlying the Kolosjoki Sedimentary Formation) is dated by U-Pb (detrital zircon) dates at ca. 2.054 Ga. Thus all formations dated in the North Transfennoscandian Greenstone Belt overlie rocks recording δ¹³C > +5‰, dropping to < +5‰ up-section, thereby constraining the termination of the LJE CIE.

A review of age constraints on the LJE CIE from all cratons is synthesised by assigning radiometric dates into pre-, coeval-, or post-LJE categories, overcoming difficulties with assigning ages to specific formations and δ¹³C values. Termination of the LJE is apparently well constrained at ca. 2.06 Ga, the duration of the LJE is at least 140 Myr (2.2 – 2.06 Ga), proceeded by an absence of data between 2.3 Ga and 2.2 Ga, the former often being taken as the first pre-LJE time constraint. Importantly this global dataset has no conflicting constraints (e.g., coeval-LJE ages younger than post-LJE ages), supporting the hypothesis that the LJE is globally synchronous. Hence the termination of the LJE has the best radiometric age constraints and thus the best potential as a Palaeoproterozoic chronostratigraphic marker. The initiation of the LJE is yet to be well constrained.