Title: Secondary magnetite in ancient zircon precludes analysis of a Hadean geodynamo
Zircon crystals from the Jack Hills, Western Australia, are one of the few surviving mineralogical records of Earth’s first 500 million years and have been proposed to contain a paleomagnetic record of the Hadean geodynamo. A prerequisite for the preservation of Hadean magnetization is the presence of primary magnetic inclusions within pristine igneous zircon. To date no images of the magnetic recorders within ancient zircon have been presented. Here we use high-resolution transmission electron microscopy to demonstrate that all observed inclusions are secondary features formed via two distinct mechanisms. Magnetite is produced via a pipe-diffusion mechanism whereby iron diffuses into radiation-damaged zircon along the cores of dislocations and is precipitated inside nanopores and also during low-temperature recrystallization of radiation-damaged zircon in the presence of an aqueous fluid. Although these magnetites can be recognized as secondary using transmission electron microscopy, they otherwise occur in regions that are indistinguishable from pristine igneous zircon and carry remanent magnetization that postdates the crystallization age by at least several hundred million years. Without microscopic evidence ruling out secondary magnetite, the paleomagnetic case for a Hadean–Eoarchean geodynamo cannot yet been made. more »« less
Borlina, Cauê S.; Weiss, Benjamin P.; Lima, Eduardo A.; Tang, Fengzai; Taylor, Richard J.; Einsle, Joshua F.; Harrison, Richard J.; Fu, Roger R.; Bell, Elizabeth A.; Alexander, Ellen W.; et al
(, Science Advances)
null
(Ed.)
The time of origin of the geodynamo has important implications for the thermal evolution of the planetary interior and the habitability of early Earth. It has been proposed that detrital zircon grains from Jack Hills, Western Australia, provide evidence for an active geodynamo as early as 4.2 billion years (Ga) ago. However, our combined paleomagnetic, geochemical, and mineralogical studies on Jack Hills zircons indicate that most have poor magnetic recording properties and secondary magnetization carriers that postdate the formation of the zircons. Therefore, the existence of the geodynamo before 3.5 Ga ago remains unknown.
Taylor, Richard J. M.; Reddy, Steven M.; Saxey, David W.; Rickard, William D. A.; Tang, Fengzai; Borlina, Cauê S.; Fu, Roger R.; Weiss, Benjamin P.; Bagot, Paul; Williams, Helen M.; et al
(, Science Advances)
A potential record of Earth’s magnetic field going back 4.2 billion years (Ga) ago is carried by magnetite inclusions in zircon grains from the Jack Hills. This magnetite may be secondary in nature, however, meaning that the magnetic record is much younger than the zircon crystallization age. Here, we use atom probe tomography to show that Pb-bearing nanoclusters in magnetite-bearing Jack Hills zircons formed during two discrete events at 3.4 and <2 Ga. The older population of clusters contains no detectable Fe, whereas roughly half of the younger population of clusters is Fe bearing. This result shows that the Fe required to form secondary magnetite entered the zircon sometime after 3.4 Ga and that remobilization of Pb and Fe during an annealing event occurred more than 1 Ga after deposition of the Jack Hills sediment at 3 Ga. The ability to date Fe mobility linked to secondary magnetite formation provides new possibilities to improve our knowledge of the Archean geodynamo.
Tarduno, John A.; Cottrell, Rory D.; Bono, Richard K.; Oda, Hirokuni; Davis, William J.; Fayek, Mostafa; Erve, Olaf van; Nimmo, Francis; Huang, Wentao; Thern, Eric R.; et al
(, Proceedings of the National Academy of Sciences)
Determining the age of the geomagnetic field is of paramount importance for understanding the evolution of the planet because the field shields the atmosphere from erosion by the solar wind. The absence or presence of the geomagnetic field also provides a unique gauge of early core conditions. Evidence for a geomagnetic field 4.2 billion-year (Gy) old, just a few hundred million years after the lunar-forming giant impact, has come from paleomagnetic analyses of zircons of the Jack Hills (Western Australia). Herein, we provide new paleomagnetic and electron microscope analyses that attest to the presence of a primary magnetic remanence carried by magnetite in these zircons and new geochemical data indicating that select Hadean zircons have escaped magnetic resetting since their formation. New paleointensity and Pb-Pb radiometric age data from additional zircons meeting robust selection criteria provide further evidence for the fidelity of the magnetic record and suggest a period of high geomagnetic field strength at 4.1 to 4.0 billion years ago (Ga) that may represent efficient convection related to chemical precipitation in Earth’s Hadean liquid iron core.
Cameron, Emilia M.; Blum, Tyler B.; Cavosie, Aaron J.; Kitajima, Kouki; Nasdala, Lutz; Orland, Ian J.; Bonamici, Chloe E.; Valley, John W.
(, American Mineralogist)
Abstract Detrital zircons from the Jack Hills are the dominant source of Hadean (pre-4000 Ma) terrestrial material available for study today. Values of δ18O in many of these zircons (6.0 to 7.5‰ are above the mantle-equilibrated value. For two decades, these mildly elevated values have been the primary evidence that protoliths of the zircon-forming magmas interacted at low temperature with liquid water before burial and melting, implying that the surface of Earth cooled quickly after core and moon formation, and that habitable conditions for life existed within 250 Myr of the formation of Earth, over 800 Myr before the oldest generally accepted microfossils. These conclusions are based on oxygen isotope analyses of zircon domains with well-defined growth zoning and nearly concordant U-Pb ages within zircon grains with low magnetic susceptibility, which are further inferred to be unaltered by various tests. However, no studies of Jack Hills zircons have directly correlated oxygen isotope ratios and radiation damage, which facilitates alteration in zircon. Several previous studies have selected zircons that show radiation damaged, discordant and/or hydrous domains, and have shown that such altered material is not reliable as a record of igneous composition. In contrast, this study targeted zircons that are interpreted to pristine and not altered, and demonstrates the importance of testing zircons for radiation damage and alteration as part of any geochemical study, regardless of age. This study expands on existing data, and presents the first comprehensive evaluation of δ18O, OH/O, CL imaging, U-Pb concordance and radiation-damage state within Jack Hills zircons. A total of 115 Hadean zircon grains in this study have water contents similar to nominally anhydrous standard reference zircons and are interpreted as pristine. In situ Raman data for band broadening correlated with δ18O analyses document low levels of radiation damage, indicating significant annealing. The present-day effective doses (Deff) are uniformly less than the first percolation point (dose where damage domains, that are isolated at lower damage state, overlap to form a continuous pathway through the crystal, ~2×1015 α-decays/mg, Ewing et al., 2003) and most zircons have Deff<1×1015 α-decays/mg. Modeling of representative alpha-recoil damage and annealing histories indicates that most zircons in this study have remained below the Deff of the first percolation point throughout their history. The δ18O values for these primary zircons include many that are higher than would be equilibrated with the mantle at magmatic temperatures and average 6.32 ± 1.3‰ in the Hadean and 6.26 ± 1.6‰ in the Archean. There is no correlation in our suite of pristine Hadean zircons between δ18O and OH/O, Deff, age, or U-Pb age-concordance. These carefully documented Hadean-age zircons possess low amounts of radiation damage in domains sampled by δ18O analysis, are water-poor. The mildly elevated δ18O values are a primary-magmatic geochemical signature. These results strengthen the conclusion that mildly elevated-δ18O magmas existed during the Hadean, supporting the hypothesis that oceans and a habitable Earth existed before 4300 Ma.
Bristol, Katie E; Borlina, Caue S; Feinberg, Joshua M
(, American Geophysical Union Fall Meeting Abstracts)
Magnetic fields in the protoplanetary disk likely play a central role in mass and angular momentum transport, protosolar accretion, and planetary system evolution. Because direct observations are limited, paleomagnetic records from meteorites and their constituents can provide key constraints on this early magnetic environment. Refractory inclusions, including calcium-aluminum-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs), are the oldest solar system solids and are strong candidates for preserving such records. Prior measurements of CAIs in the CO chondrite DOM 08006 demonstrated that they can retain high-fidelity magnetization. To build on this, we performed comprehensive rock magnetic characterization to assess whether inclusions in other chondrite groups are also capable of preserving reliable magnetic records. We analyzed CO (ALHA 77307), CR (GRA 95229), and L (QUE 97008) chondrites, performing fusion crust baked contact tests, ARM paleointensity experiments, fidelity tests, hysteresis analyses, FORCs, and low-temperature magnetic measurements. All samples carry a low-coercivity (10-20 mT) overprint and a distinct higher coercivity component. FORC diagrams reveal tri-lobed patterns characteristic of isolated vortex-state grains. Coercivity unmixing suggests two to three components, consistent with magnetite, kamacite, and minor sulfide or tetrataenite phases. In all three samples, magnetite carries the majority (71-86%) of the signal. ARM paleointensity experiments show that the high-coercivity components were imparted by very low fields, implying weak or absent magnetic fields during parent-body aqueous alteration. These results indicate that the higher coercivity components preserve primary remanence, but the samples contain multiple carriers, representing a mixture of ideal and non-ideal recorders. To isolate the paleomagnetic record from primary kamacite, we are now conducting microscopy and elemental analyses to locate and classify refractory inclusions by type, mineralogy, and size while documenting any textural evidence of melting, alteration, or replacement. These data will guide the selection of inclusions most likely to retain high-fidelity primary magnetization for future high-resolution paleomagnetic study.
Tang, Fengzai, Taylor, Richard J., Einsle, Josh F., Borlina, Cauê S., Fu, Roger R., Weiss, Benjamin P., Williams, Helen M., Williams, Wyn, Nagy, Lesleis, Midgley, Paul A., Lima, Eduardo A., Bell, Elizabeth A., Harrison, T. Mark, Alexander, Ellen W., and Harrison, Richard J. Secondary magnetite in ancient zircon precludes analysis of a Hadean geodynamo. Retrieved from https://par.nsf.gov/biblio/10098173. Proceedings of the National Academy of Sciences 116.2 Web. doi:10.1073/pnas.1811074116.
Tang, Fengzai, Taylor, Richard J., Einsle, Josh F., Borlina, Cauê S., Fu, Roger R., Weiss, Benjamin P., Williams, Helen M., Williams, Wyn, Nagy, Lesleis, Midgley, Paul A., Lima, Eduardo A., Bell, Elizabeth A., Harrison, T. Mark, Alexander, Ellen W., & Harrison, Richard J. Secondary magnetite in ancient zircon precludes analysis of a Hadean geodynamo. Proceedings of the National Academy of Sciences, 116 (2). Retrieved from https://par.nsf.gov/biblio/10098173. https://doi.org/10.1073/pnas.1811074116
Tang, Fengzai, Taylor, Richard J., Einsle, Josh F., Borlina, Cauê S., Fu, Roger R., Weiss, Benjamin P., Williams, Helen M., Williams, Wyn, Nagy, Lesleis, Midgley, Paul A., Lima, Eduardo A., Bell, Elizabeth A., Harrison, T. Mark, Alexander, Ellen W., and Harrison, Richard J.
"Secondary magnetite in ancient zircon precludes analysis of a Hadean geodynamo". Proceedings of the National Academy of Sciences 116 (2). Country unknown/Code not available. https://doi.org/10.1073/pnas.1811074116.https://par.nsf.gov/biblio/10098173.
@article{osti_10098173,
place = {Country unknown/Code not available},
title = {Secondary magnetite in ancient zircon precludes analysis of a Hadean geodynamo},
url = {https://par.nsf.gov/biblio/10098173},
DOI = {10.1073/pnas.1811074116},
abstractNote = {Zircon crystals from the Jack Hills, Western Australia, are one of the few surviving mineralogical records of Earth’s first 500 million years and have been proposed to contain a paleomagnetic record of the Hadean geodynamo. A prerequisite for the preservation of Hadean magnetization is the presence of primary magnetic inclusions within pristine igneous zircon. To date no images of the magnetic recorders within ancient zircon have been presented. Here we use high-resolution transmission electron microscopy to demonstrate that all observed inclusions are secondary features formed via two distinct mechanisms. Magnetite is produced via a pipe-diffusion mechanism whereby iron diffuses into radiation-damaged zircon along the cores of dislocations and is precipitated inside nanopores and also during low-temperature recrystallization of radiation-damaged zircon in the presence of an aqueous fluid. Although these magnetites can be recognized as secondary using transmission electron microscopy, they otherwise occur in regions that are indistinguishable from pristine igneous zircon and carry remanent magnetization that postdates the crystallization age by at least several hundred million years. Without microscopic evidence ruling out secondary magnetite, the paleomagnetic case for a Hadean–Eoarchean geodynamo cannot yet been made.},
journal = {Proceedings of the National Academy of Sciences},
volume = {116},
number = {2},
author = {Tang, Fengzai and Taylor, Richard J. and Einsle, Josh F. and Borlina, Cauê S. and Fu, Roger R. and Weiss, Benjamin P. and Williams, Helen M. and Williams, Wyn and Nagy, Lesleis and Midgley, Paul A. and Lima, Eduardo A. and Bell, Elizabeth A. and Harrison, T. Mark and Alexander, Ellen W. and Harrison, Richard J.},
}
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