The geodynamo as recorded in Archean and Hadean zircons

English Abstract:

Isotopic studies of nitrogen (Marty et al., Science, 2013) suggest a lack of the fractionation expected if the early atmosphere was eroded by the early, intense solar wind in the absence of magnetic shielding. Recent estimates of core thermal conductivity and mantle evolution modeling, however, question the presence of a geomagnetic field prior to ~3.5 Ga. To investigate this paradox, we are developing techniques to test the presence/absence of Hadean-Paleoarchean magnetic fields through measurement of zircons from the Jack Hills metaconglomerate (Yilgarn Craton, Western Australia). Magnetic force microscopy shows the presence of single-domain magnetic inclusions within the zircons. An ultra-sensitive 3-component DC SQUID magnetometer (noise floor, 10^-11 emu) that has been optimized for single silicate crystal studies is used for these experiments. We choose zircons with initial natural remanent magnetizations between 1-3 x 10^-9 emu for analyses. These select samples have been handpicked from a sample crush, and examined with transmitted and reflected light microscopes to exclude specimens with visible cracks or large opaque inclusions. Thellier-Coe and total thermal remanent magnetization experiments using a CO₂ laser system have been designed to obtain paleointensity estimates while limiting potential laboratory-induced alteration. We employ a routine that stacks multiple orthogonal component magnetizations at each demagnetization step to reduce measurement noise. After paleointensity analysis, these samples are then analyzed with a Sensitive High Resolution Ion Microprobe (SHRIMP) to obtain age information. We will present data from our ongoing analyses, and discuss the potential of measuring even weaker samples using a SERF magnetometer, currently under development by the University of Rochester and TwinLeaf LCC.