Time series analysis of mantle cycles, Part III: The 90 Myr cycle in the frequency of igneous zircon ages

A 90 Myr quasi-periodic cycle in zircon ages from igneous rocks has persisted for at least 3500 Myr. Although some age peaks from the 90 Myr cycle may correspond to the supercontinent cycle, there is no obvious relationship with the assembly or breakup phases. Our time-series of the globality of igneous activity shows that even the largest igneous events comprise less than 36% of the continental crust by volume. This implies that igneous events are regional rather than global in extent. The globality estimates of large igneous provinces are also rather small, in the range of 5%-15%, having a far smaller sample size than that from the igneous database. Another time-series shows that peaks in surface area of orogenic activity fluctuate proportionally to variation in igneous activity. This correlation indicates that the tectonic processes that trigger orogenic activity are closely associated with global variation in igneous activity. A dominant mantle event at 2700 Ma resulted in a rapid increase in the orogen/terrane area ratio, perhaps reflecting initiation of widespread plate tectonics. Considering the collective results from all time-series, the tectonic activity associated with the 90 Myr cycle appears to be regional and in any case not global.Both collisional and accretionary phases of orogens are characterized by mixed ε_Hf(t) and ε_Nd(t) values, positive in some instances and negative in others, with no discernable pattern. The large troughs in ε_Nd(t) values at 2000-1800 Ma and 600-350 Ma correlate with the assembly of supercontinents Columbia and Gondwana-Pangea, respectively, and peaks in ε_Nd(t) with positive signatures (1400, 1200, 800, and 700 Ma) correspond to the breakup of these two supercontinents. Although some of the peaks from the 90 Myr model have positive ε_Hf(t) and ε_Nd(t) and negative δ¹⁸O suggesting juvenile crustal additions, they do not require net growth of juvenile crust because other peaks that align with this cycle have crustal reworking signatures.We speculate that the 90 Myr cycle is linked to periodic mantle magma generation caused by thermal instabilities in the deep mantle. This process could be reinforced by a related cycle in mantle plumes. It is also possible that some episodes of subduction, slab avalanches, and other upstairs-driven mantle events are at least partly controlled by an already existing 90 Myr cycle produced by these thermal instabilities in the deep mantle.