Double Heating Technique Research Articles

On the early postclassic collapse of Plazuelas (Northern Border of Mesoamerica): Archaeomagnetic evidence based on full geomagnetic vectors

We report a detailed rock-magnetic, archaeomagnetic and absolute intensity investigation on burned in situ remains of the Plazuelas archaeological site (Northern Border of Mesoamerica). Fifty-six specimens were subjected to step-wise alternating field demagnetizations to retrieve characteristic remanence directions. All samples yielded evidence of stable, single-component magnetization carried by magnetite or Ti-poor titanomagnetite. Sixty-four specimens were selected for paleointensity experiments following the Thellier double heating technique, including remanence anisotropy correction. The archaeomagnetic dating, based on full geomagnetic vectors, was performed using the global geomagnetic models and the regional Paleosecular Variation Curves available for Mesoamerica. The most probable age intervals obtained for the burned floor of Site 1 (Casas Tapadas North) are within the interval 972AD – 744 AD. In the case of the burned floor of Site 2 (Casas Tapadas West), the ages obtained fall within 971 AD - 752 AD. In the case of the burned floor of Site 3 (South Sector), the ages obtained fall within the interval 752 AD – 969 AD. The calculation of the most probable age intervals, carried out using the Bootstrap resampling method, provided the dates of 940 ± 6 AD, 896 ± 14 AD and 883 ± 14 AD, respectively. These ages agree well with various archaeological hypotheses on the sudden collapse of Plazuelas as a result of intentional destruction, mutilation and generalized firing due to the local war and/or social unrest.

Palaeomagnetic field intensity measurements from the 2.6 Ga Yandinilling dyke swarm (Western Australia)

SUMMARY Precambrian palaeointensity measurements provide fundamental constraints on the evolution of the deep Earth. Core evolution models predict trends in dipole moment on billion-year timescales that can be tested by palaeomagnetic records. Here, we report new palaeointensity results from the recently identified ∼2.62 Ga Yandinilling dyke swarm of the Yilgarn Craton, Western Australia, and consider them alongside published measurements spanning 500 Myr across the late Archaean to earliest Proterozoic. Rock magnetic and scanning electron microscopy analysis confirm that the magnetic mineralogy is fine-grained magnetite, appearing mostly as exsolved lamellae with ilmenite. Six sites produced acceptable palaeointensity estimates from thermal and microwave IZZI protocol Thellier experiments and from double-heating technique Shaw experiments. These site mean values of 9–26 µT translate to virtual dipole moments of 11–44 ZAm2 that are considerably lower than today's dipole moment of ∼80 ZAm2 and the value predicted for this time period by some thermal evolution models. Their average (median = 41 ZAm2) is, however, similar to the long-term average during both of the intervals 2300–2800 Ma (median = 44 ZAm2; N = 103) and 10–500 Ma (median 41 ZAm2; N = 997). While there is little evidence for a substantial net change in average dipole moment between the late Archaean and Phanerozoic, there is preliminary evidence that its variance has increased between the two intervals. This lower variance more than two billion years ago supports the idea that the geodynamo, even while not producing a stronger magnetic field, was more stable on average at the Archaean–Proterozoic transition than it is today.

New archeointensity data from South Brazil and the influence of the South Atlantic Anomaly in South America

We obtained six new high-quality archeointensity results for the Pelotas city region, in South Brazil with ages ranging from 1790 to 1943 CE. Archeointensity measurements were performed with the double heating technique, including partial thermoremanent magnetization (pTRM) checks and pTRM tail-checks. Measurements were corrected from anisotropy of TRM and cooling-rate effects. Archeological sites providing successful results comprised: Charqueada Santa Bárbara (1790–1838 CE; 36.4 ± 0.6 μT), Chácara da Brigada Militar (1830–1851 CE; 32.6 ± 2.9 μT), Casa Número 08 (1878–1879 CE; 32.1 ± 3.1 μT), Colégio da Família de Eliseu Maciel (1881–1883 CE; 28.7 ± 2.7 μT), Fundação Simon Bolívar (1884–1889 CE; 30.1 ± 2.0 μT), Campus Porto UFPel (1942–1943 CE; 27.8 ± 2.0 μT). Two other sites did not provide meaningful results. The new data complement previous results obtained in South and Southeast Brazil, Argentina and Chile and provide additional evidence for the rapid decay of the field in the region from about 1500 to 2000 CE. In addition, they show a gradual increase in the contribution of non-dipolar components in the geomagnetic field of South America since 1800 CE, likely indicating a strengthening of the South Atlantic Anomaly over the continent from this time onward.

Archeomagnetism of Jesuit Missions in South Brazil (1657–1706 AD) and assessment of the South American database

South America contributes only a small fraction of the global archeomagnetic data. Recent work in the region has expanded significantly the previous database with new data being generated from Chile, Argentina, Northeast and Southeast Brazil. We report here new results from Jesuit Missions in South Brazil, at the triple border with Argentina and Paraguay. Our archeological collection comprises a total of 24 fragments of baked clay construction materials from three Jesuit Missions, São Luiz Gonzaga 1657–1687 AD (3 fragments), São João Batista 1667–1697 AD (4 fragments) and Santo Ângelo 1676–1706 AD (17 fragments). Archeointensity determinations were performed with the double-heating technique in its modified form, with pTRM checks and pTRM tail checks. Measurements were complemented by anisotropy and cooling-rate corrections. A total of 24 specimens (11 fragments) passed strict quality selection, corresponding to a success rate of 45%. We also performed an experimental test for the 6-specimen average anisotropy correction technique and we show that it does not correct for the effects of TRM anisotropy. Results were similar within error for the three missions: São Luiz Gonzaga (40.2±2.4 μT), São João Batista (39.1±1.6 μT) and Santo Ângelo (41.1±2.0 μT). These data were then compared with the most reliable data from South America, after a critical assessment of the current database. According to our analysis, only 39 intensity data for the continent can be considered as high-quality, most within the last 700 years; only three data were retained for older periods (800–1100 AD). The filtered data match reasonably well the available models for the past five centuries. A combined curve for South and Southeast Brazil plus Argentina plot systematically below relocated data from NE Brazil and Chile. These differences are likely due to complexities in the geometry of the field in South America not appropriately accounted for by a simple axial dipole. Our analysis highlights the need for high-quality data for the continent.

Paleointensity variation across the Matuyama-Brunhes polarity transition: Observations from lavas at Punaruu Valley, Tahiti

[1] We have conducted a paleointensity study of the Matuyama-Brunhes (M-B) polarity transition recorded in 34 successive lava flows of Punaruu Valley on Tahiti. A reversed polarity is obtained from the lower part of the record, major directional changes are derived from the middle part of the record, and a normal polarity is recorded in the upper part of the record. These paleomagnetic directions and five 40Ar/39Ar ages yielding a weighted mean of 771 ± 8 (1σ) ka indicate that 30 lava flows recorded the geomagnetic field across the M-B transition. The 215 specimens from 32 flows were subjected to the double-heating technique of the Shaw method combined with low-temperature demagnetization (LTD-DHT Shaw method), yielding 73 successful results from 18 flows. For the reversed polarity period just prior to the major directional changes, paleointensity shows an oscillation-like variation between 3 and 38 μT corresponding to virtual dipole moments (VDMs) between 0.9 × 1022 and 9.6 × 1022 Am2. For the major directional changes, a weak paleointensity of 5 μT is obtained, which gives a VDM of 1.0 × 1022 Am2. For the normal polarity period, paleointensities are 14–21 μT, giving VDMs of 3.5–5.2 × 1022 Am2. For the reversed polarity period just prior to the major directional changes, a linear relationship with a correlation coefficient of 0.96 is recognized on the diagram of VDM versus virtual geomagnetic pole latitude. This linear relationship may be a precursory feature of the geodynamo at the onset of the M-B transition.

Paleointensity determination using anhysteretic remanence and saturation isothermal remanence

Determining the strength of ancient planetary magnetic field is pivotal to understanding the evolution of planets and asteroids in the solar system. While the Thellier‐type double heating technique provides the most faithful field strength information for rocks carrying a thermoremanent magnetization (TRM), many extraterrestrial rock samples respond unfavorably to heat treatment. The present study systematically examined the two well‐known normalization techniques that avoid any heating by comparing the ratios of TRM/anhysteretic remanent magnetization (ARM) and TRM/saturation isothermal remanent magnetization (SIRM). Both the ratios of TRM/ARM and TRM/SIRM are dependent on the grain size as well as the volume concentration of magnetite. The remanence ratios also were found to increase as the alternating field increased during demagnetization for fine‐grained magnetite. A new calibration relation of 2.60 ± 1.32 for the TRM/ARM and of (3.62 ± 1.28) × 10−2 for the TRM/SIRM was defined for an external field of 50 μT. The TRM/SIRM is superior to TRM/ARM because the former showed less dispersion in the grain size dependence, the volume concentration dependence, and the stability against AF demagnetization. In addition, the standard error of the mean for the TRM/SIRM ratio was smaller than that for the TRM/ARM ratio. Thus, whenever heating is inapplicable, the SIRM method seems to be a better choice than the ARM method. However, it should be emphasized that the uncertainty of TRM/ARM and TRM/SIRM is still nearly an order of magnitude larger than that of the high‐fidelity Thellier estimation and thus must be limited in use only when samples are irreversibly altered during heating. In practice, the best approach is to carry out both ARM and SIRM methods and check whether the two estimations agree within the uncertainties.

Synthesis of Silicon Carbide Platelets by Double-Heating Technique

A new method for producing silicon carbide platelets with low cost and high yield was introduced. The silicon carbide platelets were synthesized by double-heating technique with carbon black and SiO2 powder as raw materials without using any catalysts. The starting mixtures were heated at a temperature in the range of 1800-2000°C for the duration of about 2-4h to produce substantially completely unagglomerated silicon carbide platelets with the thickness of 5-15μm and the average diameter of 50-150μm. Compared to the conventional heating, double-heating technique has different heating mechanism and has advantages of less investment for equipment, easy to manufacture and convenient to operate. Furthermore, it is very suitable for realizing the scaled production because of the lower synthesizing temperature, shorter reaction time and greater output.

Archaeomagnetic dating of a High Middle Age likely iron working site in Corroy-le-Grand (Belgium)

Archaeological burnt materials and structures provide unique records of direction and intensity of the Earth’s magnetic field in the past, elements that can be absolutely determined applying the archaeomagnetic method. At present, such records within Europe are irregular in both space and time. Presented here is the archaeomagnetic investigation of three kilns that were discovered during a preventive excavation of an archaeological site considered of High Middle Age in Corroy-le-Grand (Belgium) and that are assumed to be related to iron working activities. Archaeological context dating points to kiln operation between the second half of the 10th century until the 12th century AD. As the site is not far from Paris, declination and inclination of the characteristic remanent magnetisation of the kilns were compared with the standard directional secular variation curve for France in order to propose archaeomagnetic dates for the cessation of kiln operation by using probability densities [Lanos, Ph., 2004. Bayesian inference of calibration curves, application to archaeomagnetism. In: Buck, C.E., Millard, A.R. (Eds.), Tools for Constructing Chronologies: Crossing Disciplinary Boundaries. Lecture Notes in Statistics. Springer Verlag, London, pp. 43–82; Lanos, Ph., Le Goff, M., Kovacheva, M., Schnepp, E., 2005. Hierarchical modelling of archaeomagnetic data and curve estimation by moving average technique. Geophysical Journal International 160 (2), 440–476]. This confirms the presumed archaeological age and resulted in more precise time constraints for the last kiln operation. Rock magnetic techniques, proposed by Spassov and Hus [Spassov, S., Hus, J., 2006. Estimating baking temperatures in a Roman pottery kiln by rock magnetic properties: implications of thermochemical alteration for archaeointensity determinations. Geophysical Journal International 167, 592–604], were applied to examine the suitability of the burnt materials from the kilns for archaeointensity determinations and to increase the success rate of the Thellier–Thellier double heating technique. An average value for the field intensity of 69.4 ± 2.5 μT was estimated from 10 specimens from a single kiln, which corresponds reasonably well with published data for Western Europe.

Silicon Carbide Whiskers Synthesized by Double-Heating Technique

A new technique of mass-producing silicon carbide whiskers at a low cost is introduced in this paper. Silicon carbide whiskers are synthesized by double-heating technique with the activated carbon and silica gel as raw material and CoCl2 as catalyst. The results indicate that the silicon carbide whiskers with the average diameter of 0.2μm, length of 10-50μm and high content of 81% can be obtained at a lower temperature of 1300°C and a shorter time of 1.5h. Compared with the conventional heating, the double-heating technique is suitable for realizing the scaled production because of the lower whiskerssynthesizing temperature, shorter reaction time and greater output.

Palaeointensities of the Auckland geomagnetic excursions by the LTD-DHT Shaw method

We report new palaeointensity results concerning the Auckland geomagnetic excursions using the double heating technique of the Shaw method with low temperature demagnetisation (LTD-DHT Shaw method). The excursional palaeodirections recorded in six volcanoes of the Auckland volcanic field, New Zealand, have been classified into three groups: north-down (ND), west-up (WU) and south-up (SU) directions. In the present study, five to six consistent palaeointensities have been obtained from each of five volcanoes recording the Auckland geomagnetic excursions. The Wiri (27 ka), Crater Hill and Puketutu volcanoes (ND group) yielded mean palaeointensities of 10.6 ± 1.2 (1 σ), 11.8 ± 2.8 and 11.1 ± 0.4 μT, respectively. The Hampton Park volcano (55 ka; WU group) gave 9.5 ± 1.2 μT while the McLennan Hills volcano (SU group) gave 2.5 ± 0.5 μT. It is notable that consistent palaeointensities have been obtained from the three different volcanoes which have almost the same palaeodirections (ND group), possibly supporting the reliability of the palaeointensity data. These five palaeointensities for the Auckland geomagnetic excursions correspond to virtual dipole moments (VDMs) of 0.6–2.1 × 10 22 A m 2, whereas three mean palaeointensities obtained from the Auckland volcanoes having non-excursional palaeodirections are 13.1–40.0 μT giving stronger VDMs of 2.1–6.9 × 10 22 A m 2. These results suggest that the dipole component of the geomagnetic field reduced to about 2 × 10 22 A m 2 or less during the Auckland geomagnetic excursions.

Geomagnetic field intensity during the last 5 Myr: LTD-DHT Shaw palaeointensities from volcanic rocks of the Society Islands, French Polynesia

SUMMARY On the basis of the Thellier palaeointensities reported so far it has been thought that the time-averaged virtual dipole moment (VDM) and virtual axial dipole moment (VADM) for the last few million years are almost the same as the present geomagnetic dipole moment (∼8 × 1022 A m2). This estimate has been called into question, however, because recent studies have revealed that the Thellier method occasionally overestimates palaeointensities by as much as twice the true values. In contrast, a recently developed palaeointensity technique, namely the double heating technique of the Shaw method combined with low-temperature demagnetization (the LTD-DHT Shaw method), can yield expected field intensities from samples which give unreliable palaeointensities using the Thellier method. Therefore, we have measured absolute palaeointensities from 0.5–4.6 Ma volcanic rocks from the Society Islands, French Polynesia, mainly using the LTD-DHT Shaw method. As a result, 195 out of 361 samples passed the selection criteria, some of which are compared with additional results obtained with Coe's version of the Thellier method. In the Thellier experiments, 18 out of 40 samples passed the criteria, giving palaeointensities both consistent and inconsistent with the LTD-DHT Shaw results. These samples are characterized mostly by two-segmented Arai diagrams. If we take the LTD-DHT Shaw palaeointensities, 24 reliable site means are available and give a mean VADM of (3.64 ± 2.10) × 1022 A m2. This is nearly half of the mean of the 0–5 Ma Thellier data selected from the latest palaeointensity database [(7.46 ± 3.10) × 1022 A m2, N= 458] as well as the present dipole moment. The LTD-DHT Shaw palaeointensities newly determined in this study suggest that the present-day field is so strong that it may not be typical of the past geomagnetic field.

地磁気逆転開始期の地球磁場変動

We report the paleomagnetic behaviors at the beginning of the latest geomagnetic polarity reversal (Brunhes-Matuyama reversal of 0.78 Ma age) recorded in Tahitian lava flows. We applied an improved paleointensity determination method (double heating technique of the Shaw method with low-temperature demagnetization, LTD-DHT Shaw method), and detected oscillation-like changes in virtual dipole moments (VDMs) with a large amplitude ranging from 1.5 × 1022 to 10.9 × 1022 AM2 just prior to the directional reversal. The VDM changes are almost linearly correlated with the virtual geomagnetic pole (VGP) latitudes (60-90°S). We call this VDM-VGP latitude trend “reversal line”. Although the Thellier data reported for the Brunhes-Matuyama reversal are more scattered than our data, they also show a weak trend in the VDM-VGP latitude diagram, supporting the reversal line from a different locality. The scatter in the Thellier data may be caused by overestimated paleointensities due to methodological problems which are indicated in the recent studies of historical lavas. Therefore it appears that the reversal line from the Tahitian lavas in this study does not show a local change but represents a global-scale phenomenon in the geomagnetic reversal process.

Validity of the LTD-DHT Shaw and Thellier palaeointensity methods: a case study of the Kilauea 1970 lava

The double heating technique of the Shaw method with low-temperature demagnetisation (LTD-DHT Shaw method) for determination of geomagnetic palaeointensity is applied to samples exhibiting high-temperature oxidation states from the Kilauea 1970 lava, Hawaii Island. Results are obtained for 11 of the 12 specimens prepared from five block samples, yielding an average palaeointensity of 38.2 ± 2.8 μT ( N = 11, ±1 σ). This value is consistent with the expected value determined from DGRF 1970 (35.8 μT), and does not appear to be significantly dependent on the high-temperature oxidation state. Coe's version of the Thellier method was also applied to nine specimens prepared from the same block samples, and successful results were obtained for seven specimens, giving an average palaeointensity of 43.2 ± 8.4 μT ( N = 7). Although this average is statistically consistent with the expected value, the results include erroneously high palaeointensities (52.1 and 55.4 μT) for specimens from one block sample of intermediate high-temperature oxidation state. The present results therefore reinforce the broader applicability of the LTD-DHT Shaw method for samples with high-temperature oxidation states compared with Coe's version of the Thellier method. It is also shown that the samples yielding overestimated Thellier palaeointensities tend to fall close to the mixing line between single-domain (and/or pseudo-single-domain) and multidomain components on the Day plot. This relationship may be useful as a pre-selection technique for application of the Thellier method.

Palaeointensity study of the Oshima 1986 lava in Japan: implications for the reliability of the Thellier and LTD-DHT Shaw methods

We have performed a palaeointensity study of the Oshima 1986 lava in order to examine the reliability of the Thellier and LTD-DHT Shaw methods with special reference to high-temperature oxidation states of magnetic grains. Coe’s version of the Thellier method was applied to fifteen specimens from five block samples. The palaeointensities determined by this Thellier method ranged from 44.8 to 59.0 μT with an average of 51.0 ± 4.1 μT ( N = 15). They are systematically higher than the expected intensity (45.5 μT from the DGRF 1985). Since the in situ magnetic survey at the sampling site indicates no strong local magnetic anomaly, these high palaeointensities are overestimation, probably due to some rock magnetic property and/or some thermal alteration during the experiments. Microscopic observations show that the samples, where some titanomagnetites surrounding vesicles were classified with intermediate oxidation indices of III–V, gave 10–30% higher palaeointensity values. Therefore, the erroneously high Thellier palaeointensities have a relation to the high-temperature oxidation state. For eight of the fifteen specimens used in the Thellier experiments, the applied DC field was perpendicular to the primary NRM component for quantitative evaluation of laboratory-induced chemical remanent magnetisation (CRM). Directional changes of NRM thermally demagnetised during the Thellier experiments indicate that most of the samples more or less acquired CRM due to laboratory heating suggesting that some magnetic phase with high blocking temperatures was produced. For the specimens of intermediate oxidation level, the CRM appears to have been gradually acquired at relatively low temperatures (≤400 °C). It would appear to be difficult to detect this gradual thermal alteration with the standard pTRM check, weak-field susceptibility measurements and hysteresis parameter changes. We also applied the double heating technique of the Shaw method combined with low temperature demagnetisation (LTD-DHT Shaw method) to seven specimens from the same block samples. The palaeointensities determined by the LTD-DHT Shaw method yielded an average of 46.4 ± 4.7 μT ( N = 6), which agrees with the expected intensity indicating no dependence on the high-temperature oxidation states.

Palaeointensity study of the Hawaiian 1960 lava: implications for possible causes of erroneously high intensities

It is known that the Hawaiian 1960 lava sometimes yields higher palaeointensities than the IGRF (36.2 μT). In order to clarify the causes, we have performed a comprehensive investigation of palaeointensity measurements on 19 cores from the lava. According to various rock magnetic analyses, they are classified into three groups with degrees of the deuteric oxidation. In Coe's version of the Thellier method, a significantly larger mean palaeointensity of 49.0 ± 9.6 μT (N= 17) was observed similar to previous studies. These palaeointensities showed a dependence on the oxidation indices, which can be explained by thermochemical remanent magnetization (TCRM) acquisition during the natural cooling stage of the lava. Although it is generally difficult to distinguish the TCRM from thermoremanent magnetization (TRM), a low-temperature demagnetization (LTD) treatment on NRM and TRM in the Thellier experiment seems to allow us to detect the influence of the TCRM on NRM. Another possible tool for TCRM detection is the double-heating technique (DHT) of the Shaw method combined with LTD (LTD–DHT Shaw method). It gave a better average of 39.4 ± 7.9 μT (N= 9), though the success rate was much lower. Most of the inappropriate results were screened by a combination of the double-heating test, the ARM correction and the LTD treatment. If two outliers are further excluded, the average is improved to be 35.7 ± 3.3 μT (N= 7), which agrees well with the expected intensity. The results of both methods emphasize the importance of taking and measuring samples from the parts of various oxidation degrees in a lava flow.

Improvement of the secular variation curve of the geomagnetic field in Egypt during the last 6000 years

A total of 115 ceramic specimens out of 41 samples from 9 archeological sites in Giza, Fayoum, Benisuef, El Minia, Malawy, and Sohag were collected. They represent 14 well-determined ages covering the last 6000 years. Rock magnetic properties such as Curie temperature and hysteresis loops have been measured for these samples to identify the magnetic carrier; it is found to be fine-grained magnetite. All specimens were investigated using the classic Thellier double heating technique (Thellier and Thellier, 1959) and the modification after Odah et al. (1995) to obtain the paleointensity data. These paleointensities and the previous results of Odah et al. (1995) were used to improve the secular variation curve of the geomagnetic field in Egypt for the last 6000 years. This curve shows a maximum of 72.8 µT at about 250 AD and a minimum of 33 µT at about 3500 BC. It also shows a general decrease of the magnetic moment during the last 2000 years.

K-Ar Ages and Paleosecular Variation of Direction and Intensity from Quaternary Lava Sequences in the Ruapehu Volcano, New Zealand.

Andesite lava sequences of the Quaternary Ruapehu Volcano, New Zealand were studied to obtain paleosecular variations, both direction and intensity. K-Ar dating was performed on 7 lavas of the younger Whakapapa Formation and 22 lavas of the older Te Herenga Formation. Initial experiments by the isotope dilution method using an 38 Ar spike were unsuccessful on the younger lavas, while an amplitude method in which the abundance of Ar is estimated from a peak height of mass spectrum gave consistent results of 32 ± 12 ka after the correction of the Ar isotopic ratios for mass dependent fractionation. The latter method also gave a more accurate result of 229 ± 35 ka for the older group. No significant relationship was observed in age versus stratigraphy for the 250 m thick older lava section. This suggests a short time span, less than the errors of dating, for the lava accumulation. Primary paleodirections were easily obtained, after cleaning by both AF and thermal demagnetization, from all lavas except two in which a possible effect of lightning was suspected. Paleointensity experiments, using mainly the Shaw method, were made on 15 lavas which showed good stability to thermal demagnetization. In the analysis of the Shaw results, the correction method of Kono (1978) was applied, since most samples showed changes in ARM capacity after heating. The double heating technique of Tsunakawa and Shaw (1994) was also applied to some lavas, and was useful to discriminate erroneous data when inconsistent results were obtained. Serial correlation was not detected in the paleodirections of any series of the lavas. In spite of the random appearance of the data, the dipole direction was not obtained from the older section which included 20 data.