GP52A-01 INVITED
A re-examination of directional and paleointensity results from the Kiaman Reversed Polarity Superchron at Kiama: A single crystal approach
Long-term paleointensity trends are largely based on comparisons of values from the Cretaceous Normal Polarity Superchron and the preceding and succeeding mixed polarity intervals. Numerous data sets including single plagioclase crystals, submarine basaltic glass and the natural remanent magnetization of submarine basalt suggest an inverse relationship between reversal frequency and field strength over the last approximately 180 million years. Here we extend this examination further into the geologic record by examining the Kiaman Reversed Polarity Superchron. Irving and Parry (1963) recognized that a long period of reversed magnetic polarity existed during the Permian-Late Carboniferous time, named after study of the lavas at Kiama, Australia. A new examination of detailed stepwise demagnetization data from whole rock samples demonstrates complex magnetizations in some of the flows. This complexity probably reflects subsequent geologic events, and the propensity to record such secondary signals in non-ideal magnetic carriers present in whole rock samples. Alternatively, single plagioclase crystals may yield data that can see through the later geologic history as they can contain minute magnetic inclusions. Preliminary paleointensity studies of unoriented plagioclase crystals from the Bombo flow and Dapto latite demonstrate that the single crystal approach is a viable technique to see beyond laboratory and geologic alteration. Results using several variations of the technique will be presented.
GP52A-02 INVITED
Introduction of the LTD-DHT Shaw method and its application to historical and older volcanic rocks
Since the 1970s, Thellier and Shaw methods have been most commonly used to determine absolute paleointensity of the geomagnetic field. The principles behind these methods are the same in using thermal remanent magnetization but very different in demagnetization techniques and reliability checks. For ideal samples (e.g. assemblage of non-interactive single-domain particles without alteration due to laboratory heating), the Thellier method is regarded as valid and robust. However, recent studies have revealed that natural rock samples are not always ideal material for the Thellier method: i.e. some historical lava flows give systematically high paleointensities (e.g. Yamamoto et al. 2003; Mochizuki et al., 2004). Though some improvements have been proposed, methods other than the Thellier-type are useful for crosschecking the reliability of paleointensity. We have worked on improvement of the Shaw-type method originally proposed by Shaw (1974). At present, the most advanced version is the LTD-DHT Shaw method (Tsunakawa and Shaw, 1994; Yamamoto et al., 2003): it utilizes the anhysteretic remanent magnetization (ARM) correction, the double heating test (DHT) for checking the ARM correction applicability, low temperature demagnetization (LTD) and thermal remanence acquired by heating in vacuum. During the past few years, we have successfully applied the LTD-DHT Shaw method to several historical lava flows in Japan and Hawaii. In recent years, the application has been extended to older volcanic rocks in order to obtain datasets independent of the Thellier type. From the preliminary analysis of the new dataset, mean virtual dipole moments (VDMs) for the last five million years are calculated to be 3.70 [10 22 Am2] (N = 34) for stable geomagnetic filed and 1.17 [10 22 Am2] (N = 9) for transitional field. These values are lower than those obtained from the Thellier data set.
GP52A-03 INVITED
An Alternative View on Absolute Paleointensity Determination
Determination of the so-called absolute paleointensity (absPI) is tedious because many rocks do not satisfy the criteria for Thellier style determination protocols. Recently an entirely new approach was proposed that involves exploring the field dependence of pTRMs rather then its temperature dependence, the ‘multispecimen parallel differential pTRM method' (Dekkers and Böhnel, 2006, EPSL 248, 507-516). The method involves imparting a laboratory pTRM parallel to the original NRM of the rock sample and scaling the intensity difference to the NRM. When the laboratory field is stronger than the original field a stronger composite remanence results, when the laboratory field is weaker the reverse is true. The absPI is the field value where no change occurs. In multidomain grains, previous thermal-magnetic history determines in part the outcome and each treatment is done with a fresh specimen which undergoes only one in-field thermal cycle. Therefore, the specimens must have the same thermomagnetic properties. All magnetic grain sizes can be processed enabling absPI determination on many more flows than otherwise would be possible. We show examples of the approach illustrating its merit and discuss how incipient alteration can be recognized.
GP52A-04
Absolute Paleointensities for the Last 10Kyrs From the Hana Volcanics (East Maui, Hawaii, USA) Determined With the Multispecimen Parallel Differential pTRM Method
We have determined the paleointensity of eight lavas flows that recorded the last 10kyrs of geomagnetic field behavior from the youngest and largest of the two edifices of the island of Maui (i.e. Hana Volcanics, East Maui) with the multispecimen parallel differential pTRM method (Dekkers and Bohnel, EPSL, 248, 508-517, 2006). The flows are characterized by irreversible Curie curves indicating two kinds of magnetic carriers, one almost pure magnetite and the second one Ti-rich magnetite with a possibility of traces of titanomaghemite. The coercivity of remanence (Hcr) suggests that low-coercivity grains carry the NRM. Magnetic minerals from all of these flows are scattered within the PSD range with exception of site HKAM (age 4.07±0.09 ka) that lie in the SD range. The multispecimen method involves giving a laboratory pTRM to pristine specimens in different field strengths parallel to the original TRM; note that all pTRM are given from the same temperature. From an existing sample collection for paleosecular variation studies (Herrero-Bervera and Valet, PEPI, 161, 267-280, 2007) we could process 9 flows for paleointensity determination with this protocol ranging in age from 0.83±0.06 ka to 8.19±0.06 ka. pTRM were given by cooling from 175oC to avoid alteration, low-field susceptibility variation appeared to be less than 10 percent. Eight flows yielded good quality data; one flow appeared to show erratic and non- interpretable behavior (that flow had a larger susceptibility increase). The paleointensity increases to ~ 46 μTesla at ~ 2.2 ka whereas it drops to ~ 22 μTesla at ~ 3.5 ka, and at ~ 8.2 ka ~ 39 μTesla is obtained, i.e. slightly higher than present-day value (36 μTesla). Our paleointensity results (at least 7 flows) correlate well with the absolute paleointensity global determinations