GP52B-01 INVITED
Thellier's Betes Noires and how to Recognize Them
When Emile Thellier invented his classic paleointensity determination method, he made it quite clear that it applied only to NRMs which had originated as TRMs carried by non-interacting single-domain grains. Baked clays and bricks satisfied his stringent requirements but volcanic rocks were tested and rejected as unsuitable. While most paleomagnetists would regard Thellier's stance as unduly rigid, it certainly would help improve the usual dismal success rate in paleointensity studies if at the outset one could recognize and discard Thellier\'s betes noires: (1) NRMs of non-TRM origin, such as CRMs and TCRMs, which are particularly insidious because they may yield linear Arai plots with the wrong slope; (2) interacting grains, which compromise the independence of non-overlapping pTRMs; (3) grains larger than single-domain size with non-reciprocal blocking and unblocking. Pre-screening based on physical mechanisms would also be an improvement compared to relying solely on reliability parameters and checks. These are effective in detecting laboratory alteration of a rock but less so in filtering out samples which behave reversibly, yet are flawed recorders of the paleofield. FORC diagrams combine analysis of coercivity distribution, strength of interactions, and basic domain state. Their effectiveness in weeding out magnetic deviants has been demonstrated by Carvallo et al. (JGR 111, B12103, 2006). However, the CRM/TCRM problem remains severe. Almost all CRMs measured by Draeger et al. (GJI 166, 12-32, 2006) had ideal Arai plots but field intensity values were in error by as much as a factor 2. Unfortunately CRM/TCRM is not easily diagnosed in individual samples, and we are driven to reconsider Thellier's dictum: don\'t take risks by using rocks whose NRM could be other than TRM. Unfortunately this includes many attractive candidates, volcanic glasses, baked contacts and most metamorphic rocks among them. A crude approach might be to reject those samples in a collection whose NRM/SIRM ratios are conspicuously low compared to the average, although choosing the threshold for rejection would be highly subjective.
GP52B-02 INVITED
Saturation Isothermal Remanent Magnetization (IRMs) Normalization of Natural Remanent Magnetization (NRM): a Simple Observation with Interpretive Value.
A fundamental problem in paleomagnetism is the separation of magnetization acquired at the time interest from subsequent remagnetization. IRMs normalization of NRM can be useful because it is well known that igneous rock samples carrying NRM acquired as a Thermal Remanent Magnetization (TRM) in typical geomagnetic field intensities have a normalization, value of approximately 0.02. Strong departures from this value indicates the presence of secondary magnetizations, or anomalous field conditions. The method has also been used as a rough estimator of the intensity of field in which NRM of igneous rocks was acquired. The difficulty of this approach is the necessary demonstration that the magnetization is a TRM. Recently, the method has received some theoretical consideration (Kletetschka et al, 2003) in terms of TRM efficiency and its observed inverse relationship with saturation magnetization. We have been testng these ideas and focusing on the effects of different magnetic mineralogies upon the success, or failure of the normalization technique.
GP52B-03 INVITED
Evaluation of paleointensity experimental data: detection of non-ideal recording mechanisms