GP53A-0953
Estimating Paleointensity Reliability Based on the Physical Mechanism of Natural Remanence
Data on the long-term evolution of Earth's magnetic field intensity are crucial for understanding the geodynamo and planetary evolution. However, paleointensity remains one of the most difficult quantities to determine. The conventional Thellier method is based on the assumption that the paleointensity signal is carried by non- interacting single-domain (SD) magnetic grains that hold a thermal remanent magnetization (TRM). Most bulk rock samples, however, deviate from this ideal case. This departure, coupled with the desire to tap the relatively plentiful potential record held by bulk rocks has led to the development of reliability criteria that largely rely on the observed NRM/TRM characteristics (Arai plots). While such methods may identify effects such as non-SD behavior and laboratory alteration, they assume that the paleointensity signal is a TRM. However, many paleointensity estimates in the current database are probably held by thermochemical remanent magnetizations (TCRMs) or crystallization remanent magnetizations (CRMs). Common processes that form such magnetizations include subsolidus reactions in magnetic grains during initial lava cooling (e.g., oxyexsolution), subsequent low- temperature oxidation (e.g., maghemitization), and the formation of secondary magnetic phases (e.g., hydrothermal magnetite). If unrecognized, such magnetizations can lead to large paleointensity underestimates or overestimates. In most cases, these processes cannot be identified using the Arai-based reliability controls. We suggest that additional criteria based on the physical mechanisms of recording and preserving the paleointensity signal should be utilized in order to assess the reliability of data. We introduce criteria based on whether the magnetization represents a TRM, TCRM or/and CRM based on rock magnetic and other analytical techniques. While such a categorization is needed to make further progress in understanding the nominal paleointensity signal of bulk rocks, we also recognize that much of the required supporting data is missing from the current database.
GP53A-0954
Thermal fluctuation fields in basalts and the Barbier plot
Néel's thermal fluctuation field (Hf) is central to thermoremanent acquisition models, which are key to our understanding of the reliability of palaeomagnetic data, however, Hf is poorly quantified for natural systems. We report Hf determinations for a range of basalts, made by measuring rate-dependent hysteresis. The results for the basalts were found to be generally consistent with the space of the Barbier plot, which is characterized by the empirically derived relationship; log Hf \propto 1.3 log HC (where HC is the coercive force), obtained from measurements on a wide range of different magnetic materials. Although the basalts appear to occupy the correct position within the space of the Barbier plot, the relationship within the sample set, log Hf \propto 0.54 log HC, is different to the Barbier relationship. This difference is attributed to the original Barbier relationship being derived from a wide range of different synthetic magnetic materials, and not for variations within one material type. We consider the relationship between HC and the activation volume, vact, i.e., HC \propto vact-0.78 for our sample set. This compares favourably with theoretical predictions, and with previous empirical estimates based on the Barbier plot, i.e., HC \propto vact-0.73.
GP53A-0955
Assessing the Effects of Mineral Alteration on Palaeointensity Determinations
We have analysed Madagascan basaltic samples of Cretaceous Normal Superchron age. Microwave palaeointensity experiments have been carried out on all samples. Rock magnetic, microscopy, X-ray diffraction and Mossbauer spectroscopy analyses have been carried out to determine the magnetic carriers in the samples. It is important to distinguish between the primary thermal remanent magnetisation, TRM and that of a secondary chemical remanent magnetisation, CRM, acquired at a later date. With palaeointensities only viable if the signature is of primary origin, differentiating these two components is paramount. Thermomagnetic plots have the potential to identify the presence of titanomaghemite (inversion on heating), indicative of a CRM, yet can this be accredited as a method of identifying all secondary magnetisation? Scanning electron microscope, SEM analysis allows us to identify both high temperature ductile alteration; the direct formation of ilmenite trellis, lamellae and zones within the titanomagnetite host, and low temperature brittle alteration; the cracking of maghemitised regions. X-ray diffraction and Mossbauer spectroscopy allow us to quantify this alteration. Should this alteration occur above the Curie temperature, Tc, of the identified sample then the sample is considered suitable for palaeointensity investigation. With the Tc almost linearly dependent on the titanium present in the sample however, this is another variable factor that must be clarified. Drawing from all data it is possible to create a cooling history for the basalts, allowing for remanence identification and the attempted clarification of this history and its relationship with the palaeointensity recorded in the samples.
GP53A-0956
Magnetic Record of Ordinary Chondrite Chondrules
The magnetic record of individual chondrules has been examined on two low-metamorphic grade ordinary chondrites – Bjurböle (L4) and Avanhandava (H4). The chondrule magnetic conglomerate study was done by removing oriented chondrules from the meteorite. The magnetic directions of the chondrules in both meteorites seems to be randomly oriented within both meteorites. The paleofield method based on the REM ratio (NRM/SIRM; Natural Remanent Magnetization / Saturation Isothermal Remanent Magnetization) calibrated for the TRM (Thermal Remanent Magnetization; Kletetschka et al., 2004) magnetization process reveals approximate paleofields between 5 μT and 20 μT (REM ~ 0.002) for Avanhandava chondrules and between 12 μT and 45 μT (REM ∼ 0.0015–0.0048) for Bjurböle chondrules. However, the timing of the magnetizing process remains unclear. The random pattern of the individual chondrule magnetic directions suggest that the chondrules were not remagnetized by recent terrestrial weathering nor by the exposure to the artificial magnetic fields. However the effect of post aggregation metamorphism (reaching 400 – 600°C) and impact related shock demagnetization on the parent body may be responsible for partial or complete loss of the magnetic information carried by the lower coercivity grains. In the case of Avanhandava meteorite this can be observed in the REM ratio. The values of REM ratio calculated in low AF (alternating Field) demagnetizing fields are progressively lower than in the higher AF fields. The Bjurböle meteorite contains significant fraction of the tetrataenite mineral. The origin of tetrataenite is related to the slow ordering of the FeNi lattice at temperatures below ~ 350°C). Thus the origin of tetrataenite must be post-metamorphic ruling out the high temperature TRM scenario. The origin of the magnetization in the Bjurböle chondrules are most likely associated to the post-metamorphic low temperature processes (i.e. electric discharges or impact related shock remanent magnetization on the parent body). The random magnetic directions in this meteorite can be partly controlled by the strong magnetic anisotropy of tetrataenite. Moreover, the paleofield method based on REM ratio (Kletetschka et al., 2004) is calibrated for the TRM magnetizing scenario and not for low temperature acquisition processes and thus of limited use in the Bjurböle case. The results of the paleofield studies of the Avanhandava meteorite shows that the paleofield carried out by its chondrules can be significantly underestimated due to post-formation metamorphic and impact events. Those processes can in the same way influence the paleointensity results previously published for other chondritic meteorites. The interpretation of Bjurböle data may be even more difficult due to the presence of tetrataenite. Acknowledgements: Authors would like to thank to Peter J. Wasilewski from NASA/GSFC for constructive consultations and access to the laboratory facilities. References: Kletetschka, G., Acuna, M. H., Kohout, T, Wasilewski, P. J., and Connerney, J. E. P. (2004), An empirical scaling law for acquisition of thermoremanent magnetization, Earth Planet. Sci. Lett., 226, 521– 528. http://www.volny.cz/tomkohout/meteo/