Anthony F. Pivarunas

Does the Planetary Dynamo Go Cycling On? Re-examining the Evidence for Cycles in Magnetic Reversal Rate

The record of reversals of the geomagnetic field has played an integral role in the development of plate tectonic theory. Statistical analyses of the reversal record are aimed at detailing patterns and linking those patterns to core-mantle processes. The geomagnetic polarity timescale is a dynamic record and new paleomagnetic and geochronologic data provide additional detail. In this paper, we examine the periodicity revealed in the reversal record back to 375 million years ago (Ma) using Fourier analysis. Four significant peaks were found in the reversal power spectra within the 16-40-million-year range (Myr). Plotting the function constructed from the sum of the frequencies of the proximal peaks yield a transient 26 Myr periodicity, suggesting chaotic motion with a periodic attractor. The possible 16 Myr periodicity, a previously recognized result, may be correlated with pulsation of mantle plumes and perhaps; more tentatively, with core-mantle dynamics originating near the large low shear velocity layers in the Pacific and Africa. Planetary magnetic fields shield against charged particles which can give rise to radiation at the surface and ionize the atmosphere, which is a loss mechanism particularly relevant to M stars. Understanding the origin and development of planetary magnetic fields can shed light on the habitable zone.

Detrital Zircons Reveal Evidence of Hadean Crust in the Singhbhum Craton, India

The Singhbhum craton is one of five Archean cratons constituting the Indian subcontinent. It consists of four major lithotectonic units with broadly defined ages from Eoarchean to Neoarchean: the Older Metamorphic Group (3.7–3.2 Ga), Older Metamorphic Tonalite Gneisses (3.8–3.1 Ga), Singhbhum Granite (3.5–3.0 Ga), and Iron Ore Group (3.51–2.55 Ga). In this study, 270 zircons were separated from modern sediment of the Baitarani River, which is wholly contained within the craton. Zircons were analyzed with laser ablation ICP-MS for their U-Pb systematics; >50% were less than 5% discordant. Three primary age groupings account for ∼98% of analyses: 3.62–3.55 Ga (5%), 3.50–3.22 Ga (87%), and 3.10–3.06 Ga (6%). The preponderance of 3.50–3.22 Ga zircons is consistent with the local basement that includes a 3.47 Ga tonalite gneiss enclave within a 3.35–3.30 Ga outcrop of the Singhbhum Granite near Keonjhar. Lu-Hf systematics of zircons yielded 67% with positive initial εHf scattered above and below the mantle growth curve and 33% with negative initial εHf, indicating contributions from both depleted mantle and older crustal sources. Single-stage model ages range from 4.29 to 3.10 Ga. Of note is a single zircon with a 207Pb/206Pb age of 4015±9 Ma (1.3% discordant), which is the first Hadean zircon documented from any of the Indian cratons. This grain yielded an initial εHf of −5.30, which indicates an episode of Hadean felsic crust formation in the Singhbhum craton comparable to that proposed for the Jack Hills of the Yilgarn craton (Australia).