Further details on the applicability of Thellier paleointensity method: The effect of magnitude of laboratory field

Twenty years after Tanaka and Kono's pioneering contribution (Tanaka and Kono, 1984), we give some new details on the effect of applied field strength during Thellier paleointensity experiments. Special attention is paid to the relation of magnitude of laboratory field and Coe's quality factors (Coe et al., 1978). Full thermoremanent magnetizations were imparted on natural samples containing low-Ti titanomagnetites of pseudo-single domain structure in a 40-μT magnetic field from 600?°C to room temperature. The samples were subjected to the routine Thellier procedure using a wide range of applied laboratory fields. Results indicate that values of laboratory fields may be accurately reproduced within 2% of standard error. The quality factors, however, decrease when the magnitude of ‘ancient’ field does not match to applied laboratory fields. To cite this article: J. Morales et al., C. R. Geoscience 338 (2006).     

On correction of loss of mass during Thellier experiments

Determination of geomagnetic paleointensity by the Thellier method compares the decay of natural remanent magnetisation with the gain of a laboratory induced thermoremanent magnetisation. If fragile samples lose some amount of their material after each heating step, the Thellier experiment will be systematically disturbed and paleointensity will be over-estimated. For a lost of 5% of the sample’s mass an over-estimate of 10% in paleointensity is observed. This can easily be corrected by a normalisation to the initial mass of the unheated sample. This is necessary for any fragile materials such as baked clays or when a specimen breaks into pieces during the Thellier experiment.     

An integrated paleomagnetic study of Rio Grande de Santiago volcanic succession (trans-Mexican volcanic belt): revisited

We carried out an integrated paleomagnetic, rock-magnetic and paleointensity study of Miocene volcanic succession from the trans-Mexican volcanic belt (TMVB) north of Guadalajara. A total of 37 consecutive basaltic lava flows (326 oriented standard paleomagnetic cores) were collected at Lazo locality. Continuous susceptibility measurements with temperature and hysteresis experiments yield in most cases reasonably reversible curves with Curie points close to that of pseudo-single-domain magnetite. Two geomagnetic reversals were observed in the 300 m thick composite section. Paleosecular variation was lower than the one observed in general during Miocene. It appears that the volcanic units have been emplaced during a relatively short time span of about 1 Ma. The mean paleomagnetic directions obtained from this study do not differ significantly from that expected for the middle Miocene. The mean paleomagnetic direction calculated from all data is I=31.1°, D=354.6°, k=124 and ₉₅=2.1°, N=37. Seventy-two samples with apparently preserved primary magnetic mineralogy and without secondary magnetization, mostly belonging to reverse polarity chron were pre-selected for Thellier paleointensity determination. The flow-mean paleointensity values are ranging from 22.4±3.4 to 53.8±6.0 μT and the corresponding virtual dipole moments (VDMs) are ranging from (5.4±0.8) to (12.0±1.4)×10²² A m². This corresponds to mean value of (7.7±2.2)×10²² A m², which is close to present day geomagnetic field strength. Altogether, our data suggest the existence of relatively high geomagnetic field strength undergoing low fluctuations.     

TSD-1仪的改进及其在古地磁学的应用

美国TSD-1型热退磁仪本身不能用于地磁场古强度的研究,为了克服这一缺点,作者改进了该仪器.利用改进后的TSD-1仪可获得可靠的古强度,而且这一改进具有显著的经济效益。     

TSD-1仪的改进及其在古地磁学的应用

美国TSD-1型热退磁仪本身不能用于地磁场古强度的研究,为了克服这一缺点,作者改进了该仪器.利用改进后的TSD-1仪可获得可靠的古强度,而且这一改进具有显著的经济效益。     

Reliability of geomagnetic paleointensity data: the effects of the NRM fraction and concave-up behavior on paleointensity determinations by the Thellier method

To test the reliability of the Thellier method for paleointensity determinations, we studied six historic lavas from Hawaii and two Gauss-age lava flows from Raiatea Island (French Polynesia). Our aim is to investigate the effects of the NRM fraction and concave-up behavior of NRM–thermal remanent magnetization (TRM) diagrams on paleointensity determinations. For the Hawaiian samples, the paleointensity results were investigated at both sample and site levels. For consistency and confidence in the paleointensity results, it is important to measure multiple samples from each cooling unit. The results from the Raiatea Island samples confirm that reliable paleointensities can be obtained from NRM–TRM diagrams with concave-up curvature, provided the data are accompanied by successful partial TRM (pTRM) checks and no significant chemical remanent magnetization (CRM) production. We conclude that reliable determinations of the paleofield strength require analyses of linear segments representing at least 40–50% of the total NRM. This new criterion has to be considered for future studies and for evaluating published paleointensities for calculating average geomagnetic field models. Using this condition together with other commonly employed selection criteria, the observed mean site paleointensities are typically within 10% of the Definitive Geomagnetic Reference Field (DGRF). Our new results for the Hawaii 1960 lava flow are in excellent agreement with the expected value, in contrast to significant discrepancies observed in some earlier studies.

Overestimates of paleointensity determinations can arise from cooling-rate dependence of TRM acquisition, viscous remanent magnetization (VRM) at elevated temperatures, and TRM properties of multidomain (MD) particles. These outcomes are exaggerated at lower temperature ranges. Therefore, we suggest that, provided the pTRM checks are successful and there is no significant CRM production, it is better to increase the NRM fraction used in paleointensity analyses rather than to maximize correlation coefficients of line segments on the NRM–TRM diagrams.

We introduce the factor, Q = Nq, to assess the quality of the weighted mean paleointensity, H_w, for each cooling unit.     

An experimental evaluation of Shaw’s paleointensity method and its modifications using Late Quaternary basalts

Absolute paleointensity experiments were carried out using Shaw’s method [Geophys. J. Res. Astr. Soc. 39 (1974) 133] and its modifications [Geophys. J. Res. Astr. Soc. 54 (1978) 241; Geophys. J. Int. 80 (1985) 773; Geophys. J. Int. 118 (1994) 781] on 49 samples belonging to six Late Quaternary basaltic flows from central Mexico. Samples were selected from a large collection because of their low viscosity index, stable remanent magnetization and close to reversible continuous thermomagnetic curves. Moreover, they previously yielded high quality Thellier paleointensity results, which makes them good candidates to assess the reliability of Shaw’s paleointensity method. Only 13 samples yielded acceptable results using Shaw’s original method (ARM₂/ARM₁ ratio varies from 0.95 to 1.05 for accepted determinations) although 6 samples do not pass the validity test proposed by Tsunakawa and Shaw [Geophys. J. Int. 118 (1994) 781] and thus should be rejected for paleointensity analyses. Rolph and Shaw’s [Geophys. J. Int. 80 (1985) 773] method gives reliable determination only in one case and no single determination was obtained by Kono’s [Geophys. J. Res. Astr. Soc. 54 (1978) 241] modification. Our results indicate an extremely low success rate of Shaw’s paleointensity method, which may be due to magneto-chemical changes that occurred during heating of samples above their Curie temperatures.