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The Thellier-Thellier method in determination of the paleointensity of the geomagnetic field

The Thellier-Thellier method in determination of the paleointensity of the geomagnetic field. leiwang 08 23. 1 Introduction 2 Classic thellier-thellier method 3 pTRM tail check standard pTRM check Criteria Modification Conclusion. 1.introduction. N(TB) : 单位体积阻挡温度为 TB 的 单畴颗粒的数量

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The Thellier-Thellier method in determination of the paleointensity of the geomagnetic field

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  1. The Thellier-Thellier method in determination of the paleointensity of the geomagnetic field leiwang 08 23

  2. 1 Introduction 2 Classic thellier-thellier method 3 pTRM tail check • standard pTRM check • Criteria • Modification • Conclusion

  3. 1.introduction N(TB) :单位体积阻挡温度为TB的 单畴颗粒的数量 V :单畴颗粒的体积 Js(TB): 饱和磁化强度 :沿着磁场方向的bias of magnetic moments

  4. 当b<<1时

  5. 理论上上述方法可行，可是整个实验建立在比例常数A（粒径，饱和磁化强度，铁磁性颗粒的组分等）不变的假设上。实际中实验加热很难避免磁性矿物发生物理，化学性质的变化(Butler)。因此，要提取高温变化以前在低温时有用的古强度信息，通常通过一系列的逐步加热来实现(Coe 1967)。

  6. 2.Classic Thellier-thellier method Thellier夫妇于1959年正式提出了两步加热的Thellier-Thellier 方法。Coe 于1967年对此方法做了修改。

  7. Thellier 方法建立在以下假设之上（LEVI,1977）： • 1.不同温度段获得的pTRM相互独立并且可加。 • 2.NRM主要为primary TRM,且次生剩磁可以忽略 • 3.NRM与pTRM都与外加场成线形关系。 • 4.NRM与pTRM都与冷却速率无关。 • 5.样品退磁场在获得pTRM时的影响可以忽略。 • 6.实验室加热不会引起改变样品热剩磁性质的物理化学变化。

  8. 具体方法如下： • 1.测量出NRM, JN =Jn(Tc,Tr) • 2. 在零场中加热样品到 Ti(Ti>Tr),冷却到Tr，测量得 Jn(Tc，Ti)=JN-Jn(Ti,Tr)，加热过程中给加热炉充入氩气或放入少量活性炭，净化实验环境 • 3.在一个给定场Fa中加热样品到Ti,冷却到Tr,测量得Jn(Tc，Ti)+Ja(Ti,Tr) • 4.用3的测量值减去2的测量值，得到Ja

  9. 5.逐步升高温度，重复2— 4步，直到Ti=Tc,获得各个温点Ti处的Jn(Tc,Ti),Ja(Ti,Tr)，温度间隔从10到50度，因样品和温度段而定。 6.如果岩石的NRM为原生热剩磁，并且TRM谱遵循叠加原理，存在一个函数f(T,Tr)满足Jn(T,Tr)=Fe f(T,Tr)，Ja(T,Tr)= Fa f(T,Tr),将上式代入Jn,得到： Jn(Tc,Ti)=JN-(Fe/Fa)Ja(T,Tr) slope= -Fe/Fa=-Hpaleo/Hlab

  10. The original Thellier-thellier method

  11. Coe,JGR,1967

  12. Coe,JGR,1967

  13. Coe,JGR,1967

  14. Coe,JGR,1967

  15. 影响NRM-TRM的因素(Coe,1967)： • １．当地磁场在时空上的变化 • ２．样品产生的退磁场 • ３．获得TRM时的非线性 • ４．获得TRM的机制在某些方面不遵循thellier method • ５．实验室加热时引起样品的物理化学变化 • ６．获得TRM时的冷却速率 • ７．由次生剩磁或自发衰减引起的原生NRM的变化

  16. 3. pTRM tail check(Riisager and Riisager,2001) 在完成Thellier 方法的步骤(2) , (3) 后, 再重复步骤(2) ,即再将样品在零场中进行热退磁.

  17. Example of a NRM—TRM plot with pTRM-tail checks for baked sediments. (Riisager,PEPI,2001)

  18. Accepted paleointensity results that pass both standard pTRM checks and pTRM-tail checks(Rissager,PEPI,2001)

  19. Basalt Samples that pass the standard pTRM checks but failed pTRM-tail checks (Riisager,PEPI,2001)

  20. Basalt Samples that failed both the standard pTRM checks and pTRM-tail checks (Riisager,PEPI,2001)

  21. 4.pTRM check (Coe, 1978;Prvot,1985) 在一个零场加热后，跟着做一个或多个低温的有场加热冷却，来比较在这个低温时两次有场所获得的pTRM值。 当pTRM有明显差异时，我们可以确定TRM谱已经发生了变化，在这个温度以后获得的数据已不可信。

  22. Pan et al.,PEPI,2005

  23. 5.Criteria for determine the NRM-TRM curve: 1. artifical error criteria: • At least four points • Avoid segments contained by secondary components of NRM • Prefer low-temperature segment if there is no secondary NRM 2. parameter criteria: • NRM fraction f, • Gap factor g • Quality • Standard error

  24. Carlo Laj et al.,EPSL,2000

  25. 6.Modified approaches (J-P valet, 2000): (1) Kono and Ueno(1977) proposed applying a field perpendicular to the NRM direction. (2) Hoffman et al (1989)suggested split the sample into a number of sub-samples, each heated to a particular temperature, once in zero field and again in a known field.

  26. (3) Pick and Tauxe(1993) using submarine basaltic glasses(SBG) as an ideal material for absolute paleointensity determination. (4) Cottrell and Tarduno(1999) suggested the use of plagioclase crystals may provide a viable source of paleointensity data.

  27. 7.Conclusion: (1) though based on several hypothesis and so much disadvantage,also time-consuming, thellier-thellier method is still the most widely used one. (2) for so much uncertainties, there is still a lot of work to do .

  28. Thank you!

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