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  • 1
    Publication Date: 2016-04-07
    Description: Data on the evolution of Earth's magnetic field intensity are important for understanding the geodynamo and planetary evolution. However, the paleomagnetic record in rocks may be adversely affected by many physical processes, which must be taken into account when analysing the palaeointensity database. This is especially important in the light of an ongoing debate regarding core thermal conductivity values, and how these relate to the Precambrian geodynamo. Here, we demonstrate that several data sets in the Precambrian palaeointensity database overestimate the true paleofield strength due to the presence of non-ideal carriers of palaeointensity signals and/or viscous re-magnetizations. When the palaeointensity overestimates are removed, the Precambrian database does not indicate a robust change in geomagnetic field intensity during the Mesoproterozoic. These findings call into question the recent claim that the solid inner core formed in the Mesoproterozoic, hence constraining the thermal conductivity in the core to ‘moderate’ values. Instead, our analyses indicate that the presently available palaeointensity data are insufficient in number and quality to constrain the timing of solid inner core formation, or the outstanding problem of core thermal conductivity. Very young or very old inner core ages (and attendant high or low core thermal conductivity values) are consistent with the presently known history of Earth's field strength. More promising available data sets that reflect long-term core structure are geomagnetic reversal rate and field morphology. The latter suggests changes that may reflect differences in Archean to Proterozoic core stratification, whereas the former suggest an interval of geodynamo hyperactivity at ca. 550 Ma.
    Keywords: Geomagnetism, Rock Magnetism and Palaeomagnetism
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 2
    Publication Date: 2019
    Description: 〈span〉〈div〉Summary〈/div〉The age of the inner core nucleation is a first-order problem in the thermal evolution of the Earth that can be addressed with paleomagnetism. We conducted a paleointensity study on the 1.3 Ga Gardar basalts from southern Greenland to investigate previously reported high ancient geomagnetic field intensities. Biggin 〈span〉et al.〈/span〉 (2015) used the earlier result to identify nucleation of Earth's solid inner core at 1.3 Ga. We collected 106 samples from 39 flows from the lavas of the Eriksfjord Formation, sampling 17 of the lower flows, 8 of the middle flows and 14 of the upper flows. Rock magnetic analyses, including magnetic hysteresis, first order reversal curves (FORCs), and magnetic susceptibility versus temperature measurements, suggest that the predominate magnetic mineral in the lower basalts is low Ti titanomagnetite, whereas the middle and upper flows have varying amounts of hematite. The magnetic hysteresis data suggest magnetic grains range from multi-domain to single domain in character, with an apparent dominance of pseudo-single behavior. Thellier-Thellier double heating experiments using the IZZI methodology yielded vector endpoint diagrams and Arai plots showing two components of magnetization, one up to approximately 450˚C and the higher temperature component typically from 450˚C up to 580˚C, but sometimes to as high as 680˚C. We attribute the lower temperature component, to partial overprinting by the nearby Ilimaussaq intrusion, and acquisition of viscous remanent magnetization. We use the Thellier auto-interpreter (Shaar & Tauxe, 2013) assigning standard selection criteria vetted by cumulative distribution plots. This approach yields a paleointensity of 6.5 μT ± 5.9 μT (1 SD) based on 27 samples from 13 flows and a nominal virtual dipole moment (VDM) of 1.72 × 10〈sup〉22〈/sup〉 Am〈sup〉2〈/sup〉. However, we cannot exclude the possibility of bias in this value related to chemical remanent magnetization (CRM) and multi-domain effects. We isolate a conservative upper bound on paleointensity as the highest paleointensity result that is free of CRM effects. This yields a paleointensity of ∼18 μT, and a VDM of ∼4.5 × 10〈sup〉22〈/sup〉 Am〈sup〉2〈/sup〉, which is a field strength similar to many other Proterozoic values. Thus, our analysis of the Gardar basalts supports the conclusion of Smirnov 〈span〉et al.〈/span〉 (2016) that there is no paleointensity signature of inner core growth 1.3 billion years ago.〈/span〉
    Print ISSN: 2051-1965
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 3
    Publication Date: 2015-01-21
    Description: The archetypical example of inertial interchange true polar wander (IITPW), the rapid rotation of the entire solid Earth by 90°, is based on two nearly orthogonal directions seen in paleomagnetic studies of the Sept-Îles (ca. 565 Ma) intrusion (Quebec, Canada). This motion has also been proposed as the driving force for the early Cambrian explosion of life. Others have challenged these interpretations, linking the data instead to flips between an axial and equatorial dipole field configuration. We examine this enigma using single silicate crystal paleomagnetic analyses. We conclude that only one of the previously reported magnetic directions is carried by single domain magnetic grains and can be considered primary. Thus, the Ediacaran diversification occurred on a rotationally stable Earth driven by biotic and longer term abiotic forcing. Whether IITPW ever occurred in the past on Earth remains uncertain.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
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  • 4
    Publication Date: 2019-08-13
    Description: Diogenites are achondrites that are part of the HED (howardite, eucrite, diogenite) meteorite group thought to originate from asteroid Vesta. This suite of igneous rocks offers a glimpse of early planetary differentiation and subsequent igneous processes. While eucrites represent asteroidal basaltic crust and howardites the impact brecciated surface, diogenites are samples of the mantle and lower crust. Most of them are orthopyroxene (Opx) dominated cumulates, although harzburgites and rare dunites have also been found. The majority of diogenites are impact breccias. This study describes NWA 5480, a pristine, i.e. hardly altered and minimally shocked, harzburgitic diogenite.
    Keywords: Lunar and Planetary Science and Exploration
    Type: JSC-CN-32728 , Lunar and Planetary Science Conference; Mar 16, 2015 - Mar 20, 2015; The Woodlands, TX; United States
    Format: application/pdf
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