Multiple-specimen absolute paleointensity determination: An optimal protocol including pTRM normalization, domain-state correction, and alteration test

doi:10.1016/j.epsl.2010.06.006

Earth and Planetary Science Letters

Volume 297, Issues 1–2, 15 August 2010, Pages 84-94

https://doi.org/10.1016/j.epsl.2010.06.006 Get rights and content

Abstract

A recent proposal of a multiple-specimen technique promises to be a viable alternative to the classical Thellier–Thellier method of absolute paleointensity determination. However, to exploit the full potential of the multiple-specimen approach, a thorough understanding of its theoretical foundation, and a detailed experimental verification of its implicit assumptions is required. Here, the validity of the multiple-specimen technique is studied on a collection of synthetic samples covering grain sizes ranging from single domain (SD) over intermediate pseudo-single domain (PSD), to multidomain (MD). The experimental data indicate that the multiple-specimen method in its present form systematically overestimates paleointensity for intermediate PSD to MD particle sizes. This finding is investigated theoretically by a statistical theory of weak-field thermoremanence, and quantified by a phenomenological thermoremanence model. Based on this theoretical framework, and on the new experimental evidence, an extended version of the multiple-specimen technique is designed, which is more reliable in the critical domain-state range. The new measurement scheme improves normalization, and quantifies the PSD and MD overestimate, which then can even be corrected for. Furthermore, the proposed measurement scheme includes a thermal repeat measurement to assess the effect of alteration upon the accuracy of the final paleointensity result. The new technique is verified experimentally for the synthetic samples investigated.

Introduction

In the last years the progress in geodynamo modeling triggered increased efforts to study the history of the geomagnetic field on all time scales. Beyond the short period in which historic records are available lies the realm of archeomagnetism and paleomagnetism, where rockmagnetic reconstructions of paleofield direction and intensity are necessary. Absolute paleointensity determination from the thermoremanence (TRM) of baked clays and natural rocks is an essential prerequisite for studying secular variation, long term field evolution, geomagnetic reversals and excursions. Large uncertainties in paleointensity determinations also limit the possibilities to analyze the history of the geodynamo processes, which generate the Earth's magnetic field. The classical method to determine absolute paleointensity is the Thellier–Thellier technique (Thellier and Thellier, 1959) which is based on Néel's (1949) single-domain theory. This method over the last decades has been considerably extended and the versions used today carefully check many possible error sources. Due to these inherent tests, the Thellier–Thellier method requires many, and often repeated, heating steps to different temperatures, and finally completely demagnetizes the sample by heating it above its Curie-temperature. However, only few rocks are sufficiently stable to endure such repeated heat treatment. Commonly, high-temperature measurements lead to considerable chemical alteration. Also the domain structure of multidomain particles can change irreversibly during repeated heating which influences TRM capacity and inferred paleointensity (Fabian and Shcherbakov, 2004). Only few rocks have remanence carriers in the correct grain-size interval, and are sufficiently stable, to yield reliable paleointensity determinations in the Thellier–Thellier method. Therefore, in addition to being extremely time consuming, this method has a very low success rate. Due to these restrictions, multiple successful paleointensity determinations for individual cooling units are rare, resulting in limited statistical control of within-site variations. These shortcomings of the classical Thellier–Thellier method have become a nuisance for attempts to obtain a sufficient number of high-quality determinations of the paleofield strength. Improving the Thellier–Thellier method, or extending the toolbox for absolute paleointensity determination, consequently, is one of the foremost tasks in paleomagnetism.

Section snippets

Background and protocol of the multi-specimen technique

The new multiple-specimen method, recently proposed by Dekkers and Böhnel (2006), (here referred to as MSP-DB) has a large potential to improve and simplify absolute paleointensity determination.

The general idea behind MSP-DB is elegant and simple: The sample NRM is interpreted as a full TRM_H, acquired in a constant ambient magnetic field H. By heating this TRM_H in a laboratory field H_lab to some temperature T, and cooling back to room-temperature T₀, it is transformed in an overprinted partial

Experimental validation

While theoretical models produce widely different predictions concerning the validity of the MSP-DB approach, experimental studies on recent lava flows and comparison with Thellier–Thellier paleointensity (Michalk et al., 2008) indeed confirm a previous suggestion that MD samples may overestimate the paleofield (Fabian and Leonhardt, 2007). Here we use a set of thermally stabilized well-defined samples to systematically study the influence of different laboratory heating/cooling procedures and

Correcting the MSP-DB slope for NRM fraction f

In Fig. 6a the in-field heating data from Fig. 5 are collected in a single plot to simulate a MSP-DB result for a sampling site with specimens of significantly varying domain states. The artificial multiple-specimen data plots show significantly more scatter than any single-specimen plot in Fig. 5. The excess in scatter results from the different slopes of the best-fit lines for the individual specimens, and mainly is due to the different fractions f of their NRM remagnetized by the overprinted

Conclusion

Based on experimental and theoretical evidence it is demonstrated that the multi-specimen protocol MSP-DB, as proposed by Dekkers and Böhnel (2006), is domain-state dependent. A particularly strong effect, significantly biasing the result for large PSD and small MD grain sizes, is found for zero-field heating during pTRM^⁎(T) acquisition. Therefore, only in-field heating/cooling cycles should be used in the MSP-DB protocol, as requested by Dekkers and Böhnel (2006), but even then a substantial

Acknowledgments

We gratefully acknowledge the thoughtful reviews of M. Dekkers and L. Tauxe. Funding for this project was provided by R.L.: DFG grant Le1905/1-1 and FWF grant P21221-N14. K.F.: NGU crustal magnetism project 332400.

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Multiple-specimen absolute paleointensity determination: An optimal protocol including pTRM normalization, domain-state correction, and alteration test

Abstract

Introduction

Section snippets

Background and protocol of the multi-specimen technique

Experimental validation

Correcting the MSP-DB slope for NRM fraction f

Conclusion

Acknowledgments

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Phys. Earth Planet. Inter.

Earth Planet. Sci. Lett.

Phys. Chem. Earth

Phys. Earth Planet. Inter.

Transdomain thermoremanent magnetization

J. Geophys. Res.

Acquisition of thermoremanent magnetization in weak magnetic fields

Geophys. J. Int.

Statistical theory of weak field thermoremanent magnetization in multidomain particle ensembles

Geophys. J. Int.