Some rock magnetic properties of mid-Cretaceous basalts from Israel and India (Rajmahal traps), and their bearing on palaeointensity experiments

A wide range of rock magnetic properties have been determined from two collections of mid-Cretaceous basalts; one from Israel, the other from the Rajmahal traps in northeast India. Deuteric oxidation is rare in both collections, with titanium-rich titanomagnetite being the principal remanence carrier in most cases. There are a number of differences in rock magnetic properties between the two groups. Some of these seem to be primary, whereas others appear to be caused by hydrothermal alteration and weathering, which are more prevalent in the Indian rocks. These rocks are being used in palaeointensity experiments, from which it is hoped to determine the strength of the Earth's magnetic field during the long period of normal polarity in the mid-Cretaceous. Thellier palaeointensity experiments have been performed on two samples from each site. The degree of agreement between the two results is highly variable. The low blocking temperatures and the presence of secondary viscous components in many samples make Thellier palaeointensity experiments very difficult. A further problem is that of thermal alteration, two main types of which are observed. The first manifests itself as a large and sudden increase in partial thermoremanent magnetization (pTRM) capacity, and the second as a steady decrease in the size of pTRM with increasing temperature.     

Palaeointensity record through the Lower Mammoth reversal from the Waianae volcano, Hawaii

The normal to reverse Lower Mammoth reversal (3.33 Ma) has been recorded in several sequences of lava from the Waianae Volcano, the Island of Oahu, Hawaii. 137 samples from 29 flows from the Pu'u Paheehee section have been the subject of a palaeointensity study using the microwave technique. Duplicate sister samples from the directional study of Herrero-Bervera and Valet (Earth Planet. Sci. Lett. 171 (1999) 139–148) were used. Microwave demagnetisation was carried out on all samples and the directions compared to the published flow mean directions. Microwave palaeointensity experiments were carried out on all accepted samples using the 8.2 GHz and 14 GHz microwave systems. The perpendicular applied field palaeointensity method and a Coe analogue method were used. Eighty-four samples from 24 flows gave acceptable palaeointensity results. The results indicate that the geomagnetic field was low (mean 5.9±1.3 μT (N=7)) prior to the transitional directions. During the first stage of the reversal the field remains low. Results however could only be obtained from three transitional flows. The field then strongly recovers with very high intensity (70 μT) and reversed direction. After this the intensity decreases before the field becomes transitional again for a cluster of four flows. The field does not reduce as much as previously, rather it is about twice the pre reversal intensity. For the final section of reversed flows the intensity is more than twice the pre reversal mean value, 15.1±5.9 μT (N=7). Whilst some similarities are seen between this reversal and other reversals of different ages and locations there is not enough data at present to say whether there is any systematic behaviour.     

The method of high harmonics for determining reheating palaeotemperatures

Summary When reheated to temperatures below the Curie temperature (T_c) and subsequently cooled in a constant magnetic field it has been shown that rocks which contain magnetic minerals will acquire an induced magnetic anisotropy (IMA). The IMA is capable of retaining information on the palaeointensity of the geomagnetic field and the palaeotemperature of this most recent heating. A novel technique for determining the palaeotemperature has been developed, employing the high-harmonics method.     

地球磁场古强度测定方法综述

地球磁场的演化历史对研究地核与地幔边界的动力学和地核内的磁流体力学过程提供了重要的信息，古地磁的研究为大陆漂移和海底扩张提供了定量的信息，然而到目前为止，对地球磁场古强度的研究还相当薄弱，由于地球磁场古强度数据的匮乏，使得人们对于地质历史时期地球磁场强度缺乏，本文对测定古强度的方法作了较详细的总结，并对一些实验方法作了评述，在这些方法中，认为最可信的古强度测定的方法是Coe校正后的Thellier法。     

New Cretaceous palaeointensity data and the constraints on geodynamics

A combined geochronologic (K-Ar) and palaeomagnetic study has been conducted on a basalt lava sequence at Yixian Formation in Liaoning Province, northeastern China. The new K-Ar age obtained from thirteen lava flows is 120.93±0.88 Ma. Detailed rock-magnetic investigations were conducted on each lava flow to determine their remanence carriers. The modified version of the Thellier-Thellier palaeointensity method with systematic partial thermoremanent magnetization (pTRM) checks was used for the palaeointensity determination. Virtual dipole moment (VDM) value is (3.66±0.10)x10²² Am². This low dipole-field intensity value is approximately forty-five percent of the today field VDM. Combined with all of the other published palaeointensity data, possible links between the earth’s interior process and its control on the variation of the earth’s magnetic field during the geological time were tentatively discussed.     

火星磁场和行星发电机理论

火星磁场是火星主要观测物理场之一,火星磁场研究对火星探索具有重要的科学意义.本文侧重介绍火星磁场探测所取得的主要成果,在介绍火星行星磁场起源的行星发电机理论的基础上,重点讨论了动力学机制、起始时间、停止的原因等关键性问题,并指出了研究中存在的一些问题.     

Thermomagnetic monitoring of lithic clasts burned under controlled temperature and field conditions. Implications for archaeomagnetism

We carried out a combined thermal and magnetic evaluation on experimentally knapped clasts of different lithologies (chert, quartzite, limestone, sandstone and obsidian) heated under controlled field and temperature conditions. The main aim of this study is to estimate the feasibility of use of these raw materials, which are commonly found in prehistoric archaeological sites for archaeomagnetic purposes. Rock magnetic analysis included measurements of low-field magnetic susceptibility, isothermal remanent magnetisation (IRM) acquisition curves, hysteresis loops and thermomagnetic curves of lithic clasts both before and after experimental heating. All lithologies, except the obsidian, recorded an increase of up two orders of magnitude in their magnetic concentration-dependent parameters revealing the formation of new ferrimagnetic minerals. Obsidian and sandstone are the most reliable magnetic carriers, followed by limestone, chert and quartzite. Magnetic susceptibility values show significant differences among lithologies. Isothermal remanent magnetisation proved also to be highly discriminatory as well as the room temperature hysteresis parameters. The main macroscopic alterations resulted in colour changes, rubefactions, potlids in cherts and the massive formation of internal fissures in obsidian specimens. The multispecimen palaeointensity technique was applied on selected samples yielding satisfactory results for heated obsidian and sandstone samples. The archaeological applicability of the results is discussed as well as their geomagnetic significance.     

地球磁场相对强度研究现状与展望

对利用沉积物确定地球磁场相对强度的实验方法进行了综述评述，在些基础上本文总结了目前常用的恢复沉复沉积物记录古强度的方法，重点介绍了全新世、晚更新世和布容时地球磁场相对强度变化特征，评价了环境因素对沉积物记录地球磁场相对强度的影响。对未来有关地球磁场相对强度研究发展趋势提出了借鉴。     

火星感应磁场模型及其磁力线分布

本文利用火星具有电离层而无内禀磁场的特点以及它与太阳风相互作用的性质,通过适当的假设,建立了火星感应磁场模型.此模型建立如下,利用电流连续的特性: Δ·j=0 (j为感应电流)以及对火星磁层中的电流体系分布的合理假设给出电流,并由毕奥-萨伐尔定理得到火星周围的磁场强度的表达式;利用我们自编的磁力线跟踪程序由求得的磁场强度得到火星周围的磁力线分布.我们发现:利用此火星磁场模型得到的火星周围的磁力线分布与卫星观测的结果以及其他方法得到的结果符合的很好.     

Thellier palaeointensity experiments on Faroes flood basalts: technical aspects and geomagnetic implications

Based on palaeomagnetic and rock magnetic results, 108 samples from 27 flows from Palaeogene flood basalts of Faroe Islands were chosen for whole-rock Thellier palaeointensity experiments. Altogether 90 samples were rejected due to either chemical alterations or typical multidomain (MD) behaviour evidenced by pTRM-tails. AF pre-treatment was used to reduce the effect of MD grains on Thellier experiments. Only five flows (18 samples) yielded acceptable palaeointensity estimates, with flow mean VDMs ranging from 3.5 to 7.4×10²² A m². Modest selection criteria imposed on all published 5-160 Ma palaeointensity data left only 15 palaeomagnetic dipole moments: eight from whole-rock samples, six from submarine basaltic glass and one from single plagioclase crystals. More data are needed before the intriguing differences between results from different materials can be put into a geomagnetic context.     

Effect of Stress on the Magnetic Memory of Induced Magnetic Anisotropy of Rocks and Its Mathematical Model

When re-heated to temperatures below the Curie temperature and subsequently cooled in a constant magnetic field (H _T), rock samples which contain magnetic minerals can acquire an induced magnetic anisotropy (IMA). As the result of acquiring the IMA, a constriction develops in the hysteresis loop of the magnetization of these rocks at the values of the magnetizing field close or equal to the HT. Thus the IMA is capable of retaining the information on the palaeointensity of the geomagnetic field, i.e., if IMA was created in a rock in the geomagnetic field in a past geological epoch, it preserves the information on the intensity of that field. Investigations have shown, that when IMA is created in a rock under external stress, the stress has an impact on the magnetic memory. Here we also deal with the issue of how stress affects the magnetic memory of IMA. A mathematical model for the effect of stress on magnetic memory phenomena related to induced magnetic anisotropy in rocks containing multidomain magnetite and titanomagnetite grains is proposed herewith. The effect of temperature on the magnetic memory of rocks is discussed also.     

New magnetic mineralogy and archaeointensity results from greek kilns and baked clays

Seven archaeological sites in Northern Greece and a pottery assemblage from Ithaki Island (Ionian Sea) have provided material from kilns and a collection of baked clays. The whole dataset consists of 69 samples and covers a period of almost 4000 years. Although the majority of the samples was oriented, only few directions could be obtained, mostly due to small sample size and fragility. Detailed rock magnetic experiments identified magnetite/titano-magnetite and substituted magnetite as the main magnetic minerals. Variable amounts of haematite were detected in some cases. In three out of eight sites, multi-domain grains prevail, whereas single-, pseudo-single domain or mixtures are detected in the remaining five. The classical Thellier-Thellier palaeointensity method was applied to 94 specimens. Following stringent acceptance criteria, 66 results from the 8 sites were considered successful and retained. These new results show a convergence with regional and global compilations and improve the knowledge of the past geomagnetic field behaviour in Greece.     

Rock Magnetic Investigation of Historical Lavas Used in Palaeointensity Studies

Five samples from historical lava flows on Mt. Etna, which had previously been used in a palaeointensity study, were examined using a combination of rock magnetic and microscopic techniques to elucidate the causes of failure of palaeointensity determination. The samples were characterised using a combination of low-temperature susceptibility (LT-), Bitter pattern imaging and scanning electron microscope (SEM). High-temperature susceptibility curves and hysteresis loops had been previously measured by Calvo et al. (2001). Of the five samples only one gave an accurate palaeointensity. This sample was deuterically oxidised and consisted of large exsolved ore grains. It was only possible to distinguish this sample either optically or magnetically from a similar sample by the LT- warming curves; the unsuccessful sample displayed alteration in the LT- warming curves, which was not readily observed in the high temperature susceptibility curves. It is proposed that the measurement of LT- curves before and after heating could be a more sensitive method of determining suitability for palaeointensity determination than previous rock magnetic pre-selection techniques.     

An absolute palaeointensity record from SOH1 lava core,Hawaii using the microwave technique

Absolute palaeointensity results were obtained from 392 samples (166 of 241 flows) from the upper 779 m of the Scientific Observation Hole 1 (SOH1) basalt core, Kilauea volcano, Hawaii, using the microwave palaeointensity technique. Based on K–Ar dates and a previously published age model, this covers the last 45 ka thus giving a high-resolution record of geomagnetic intensity in Hawaii. Comparisons with other Hawaiian absolute palaeointensity data show similar trend of higher intensity than present day for the last 3 ka and then a general decrease in intensity over the last 45 ka, though this study reports consistently lower palaeointensity values. Direct comparison with previous Thellier data from the SOH1 core shows some discrepancies between the two methods, with the Thellier data yielding generally higher palaeointensity estimates than the microwave data. In view of these discrepancies between the two methods, the introduction of raw palaeointensity data into the public domain is suggested.     

A detailed palaeointensity and inclination record from drill core SOH1 on Hawaii

A new record of absolute palaeointensity was obtained from drill core Scientific Observation Hole 1 (SOH1) on Kilauea volcano, Hawaii. Kilauea’s high eruption rate resulted in a relatively continuous record and stratigraphic constraints preserved the chronological order. Three hundred and sixty samples were studied with the Thellier-Thellier technique, which gave 195 successful palaeointensity and 271 successful inclination determinations. Three geomagnetic excursions were observed, which exhibited intensity reductions of about 50%. Initial age control from K-Ar and Ar/Ar dating only constrained the total age between 20 and 120 ka. The final age model was obtained by stretching the SOH1 record relative to other Hawaiian palaeomagnetic data. This gave an age range of 0-45 ka for the flows and identified the excursions as the Hilina Pali, Mono Lake and Laschamp events. The SOH1 record of the Hilina Pali event is the most detailed ever, incorporating data from around 40 flows. This age model suggests that Kilauea had a burst of activity at the SOH1 site around 20 ka. All available data was combined to form a composite record of palaeointensity and inclination on Hawaii for 0-45 ka.     

Geomagnetic Field Variations as Determined from Bulgarian Archaeomagnetic Data. Part II: The Last 8000 Years

The knowledge about past secular variations of the geomagnetic field is achieved on the basis of archaeomagnetic researches of which the Bulgarian studies form an extended data set. In Part I (Kovacheva and Toshkov, 1994), the methodology used in the Sofia palaeomagnetic laboratory was described and the secular variation curves for the last 2000 years were shown. In Part II (this paper), the basic characteristics of the prehistoric materials used in the archaeomagnetic studies are emphasised, particularly in the context of the rock magnetic studies used in connection with palaeointensity determinations. The results of magnetic anisotropy studies of the prehistoric ovens and other fired structures are summarised, including the anisotropy correction of the palaeointensity results for prehistoric materials, different from bricks and pottery. Curves of the direction and intensity of the geomagnetic field during the last 8000 years in Bulgaria are given. The available directional and intensity values have been used to calculate the variation curve of the virtual dipole moment (VDM) for the last 8000 years based on different time interval averages. The path of virtual geomagnetic pole (VGP) positions is discussed.     

Nature of magnetic grains and their effect on the remanent magnetization of basalts

The nature of magnetic grains in basalts obtained from different parts of the world has been investigated. Results indicate that magnetic behaviour attributable to cation-deficient magnetite is common in basalts younger than Cretaceous, while that due to multidomain magnetite is widespread in much older rocks. Superparamagnetic grains occur in basalts more abundantly than originally presumed, which seem to be mainly responsible for the viscous remanent magnetization of such samples. Basalts which are inferred to contain predominantly optimum single-domain grains are found to be most suitable for palaeomagnetic work. However, many samples generally contain a wide range of grain sizes and this can account for the observed variation in their magnetic stability. One possible mechanism for the formation of such magnetic grains in basalts and its implications to palaeomagnetism is presented. Basalts whose magnetic behaviour is completely reversible on heating and cooling are very rare and because of this fact the reliability of palaeointensity determinations, involving heating of the samples even for one time, may be reduced to a considerable extent.     

白垩纪地球物理场异常与地球深部动力学

通过对白垩纪超静磁带（CNS）期间发生的重要地质事件，如大洋缺氧事件，大量火山活动以及温度升高等的综合分析，并结合地球磁场古强度研究结果，探讨了白垩纪地球物理场异常与地球深部动力学的可能相关性。     

Magnetic properties of Antarctic shergottite meteorites EETA 79001 and ALHA 77005: possible relevance to a Martian magnetic field

The possibility that the parent body of the SNC meteorites is Mars implies that the magnetic properties of these meteorites may provide evidence concerning ancient Martian magnetic fields. EETA 79001 possesses a weak, very stable primary magnetization, the properties of which are consistent with its acquisition in an ambient magnetic field either during the meteorite's formation or during the severe shock event later in its history. The samples of ALHA 77005 studied possessed no measurable primary magnetization: the observed remanence appears to be a viscous magnetization acquired in local laboratory fields. The magnetic carriers in the meteorites are fine-grained magnetite and a lower Curie point mineral, probably titanomagnetite or pyrrhotite, present to the extent of less than 0.1% by weight. Estimates of the strength of the magnetizing field for EETA 79001 are in the range 1–10 μT.     

Changes in Magnetic Properties of Baked Archaeological Samples. Implications for Palaeointensity Determination

Rock magnetic investigations of archaeological materials of burnt clay from Eneolithic ovens (4500 years BC) showed particular changes with time in the magnetic mineralogy of samples, stored under normal conditions. Our results indicate that well-burnt clay from the archaeological materials contains a significant amount of very fine magnetic grains, which could notably influence the rock magnetic properties and behavior at room temperature. The main observations after 4 years of storage under laboratory conditions are as follows: 1) decrease in the final unblocking temperature of NRM from 600–620°C to 580°C and 2) increase in the capacity of laboratory TRM acquisition. The most probable mechanism responsible for the observed changes is supposed to be fast low-temperature oxidation of the finest (superparamagnetic) grains and the development of the maghemite shell in coarser single-domain grains. The Thellier palaeointensity experiments, carried out at the beginning of the study, showed very good results, which satisfy all acceptance criteria, applied to evaluation of the results, quite well. Palaeointensity determinations repeated 4 years later on samples from the same material showed the experimental results to be of significantly inferior quality. The main difference is the presence of the significant deviation (change in the slope) on the Arai diagram after T>350–400°C. The calculated palaeointensity is either higher than the one obtained before, or similar, but evaluated with large uncertainty. Therefore, we conclude that the possibility to obtain biased palaeointensity values increases during short-time storage (i.e. several years) due to the low-temperature changes of the material.