pTRM Tail Research Articles

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 determinations from the Etendeka province, Namibia, support a low‐magnetic field strength leading up to the Cretaceous normal superchron

Paleointensity estimates provide much needed information on field generation within Earth's core and upon the convective processes at work within the mantle. We present new paleointensity estimates from the early Cretaceous Etendeka large igneous province in Namibia (∼135Ma), which add to the sparse Southern Hemisphere data set. The Early Cretaceous marks an important change in the Earth's magnetic field from a state of rapid polarity reversals, to one of long-term stability associated with the onset of the Cretaceous Normal Superchron at ∼121 Ma. Paleointensity determinations, using the IZZI protocol, were carried out on a total of 172 specimens from 14 sites encompassing the exposed stratigraphy of the Etendeka province. Numerous checks of data reliability were considered before results were accepted, including partial thermoremanent magnetization (pTRM) checks and pTRM tail checks, hysteresis properties, thermomagnetic analyses, observations under reflected light, and changes to room-temperature susceptibility during the experiments. Following these checks, a total of 64 individual samples from five sites were considered to provide reliable paleointensity determinations. These results were combined to provide site mean data with an overall average virtual dipole moment (VDM) for the study of 2.5 ± 1.0 × 1022 Am2. This value equates to approximately 30% of present Earth's field and, when considered alongside existing studies, suggests that Earth's field strength was low in the time leading up to the Cretaceous normal superchron.

Towards the robust selection of Thellier‐type paleointensity data: The influence of experimental noise

The process of data selection in paleointensity studies is an essential step to ensure data fidelity. There is, however, no consensus as to the best approach to consistently select data with most studies using arbitrarily defined thresholds for selection. We present a new numerical model that simulates the variability of paleointensity data from hypothetical ideal samples acquiring a thermoremanent magnetization (TRM) by incorporating experimental noise, which has been constrained using over 75,000 data measurements. Using Monte Carlo analyses, we investigate the behavior of simulated data and characterize the distributions of parameters typically used to select paleointensity data. We use the 95th percentiles of the distributions to define thresholds for the maximum likely parameter values that can result from experimental noise. These represent values below which we cannot distinguish non‐ideal behavior from noise. We find that a number of parameters are highly sensitive to noise and laboratory field strength (e.g., partial TRM, pTRM, checkCDRAT and pTRM tail check δt*); this sensitivity may diminish their ability to identify non‐ideal behavior. The fractional (f) dependence of some parameters and the proportion of inaccurate results provide justification for f≥ 0.35 when selecting data from both Thellier‐Thellier and Coe protocol experiments. The manifestation of noise in the original Thellier method, however, is different to that of methods that use zero‐field heating steps. This suggests that the data selection procedure for the Thellier method should be different, but it also suggests that, contrary to previous analyses, the accuracy and scatter of results from this method are more sensitive to noise than methods that use zero‐field heating steps. The general approach taken here is shown to be a powerful means of understanding the behavior of selection parameters and has the potential to be extended to models incorporating non‐ideal behavior resulting from alteration and multidomain grains.

Archaeomagnetic intensity data from the Saadien Tombs (Marrakech, Morocco), late 16th century

Several archaeomagnetic intensities have been obtained on pieces of enamelled tiles that were part of the decoration (zellij) of the royal necropolis named Saadien Tombs (Marrakech, Morocco). Thermoluminescence dating confirms that the tombs were built in the late 16th C AD, during the reign of sultan Ahmed al-Mansur. Archaeointensities were determined using the 14.4 GHz Microwave system at the University of Liverpool, pTRM and pTRM tail checks are positive and the intensity values show low dispersion grouping around 42 μT.

The behaviour and detection of partial thermoremanent magnetisation (PTRM) tails in Thellier palaeointensity experiments

Abstract We analyse the results of Thellier palaeointensity experiments modified so that the measured magnetisation at any stage may be decomposed into components of natural remanent magnetisation (NRM) and laboratory thermal remanent magnetisation (TRM). We demonstrate that the very long high temperature tails of pTRM acquired by multidomain (MD) grains may be detected in Thellier experiments without recourse to explicit pTRM tail check measurements. This can save time in the experimental process and provides a more sensitive indication of how MD behaviour may affect that particular experiment than either pTRM tail checks or the observation of zigzagging in an IZZI experiment. We observe that the action of imparting a pTRM tail also involves some removal of NRM. However, the pTRM tails are not the analogues of classical overprints as may be intuitively expected but instead appear to favour remagnetisation over demagnetisation. This and other observed behaviour is not consistent with any phenomenological model of MD TRM but is consistent with predictions made by kinematic theory.

Effect of multi-cycle heat treatment and pre-history dependence on partial thermoremanence (pTRM) and pTRM tails

We test two fundamental assumptions embedded in Thellier experiments, the initial state dependence and the effect of multi-cycle heat treatment. We observe that the magnitude of partial thermoremanent magnetizations (pTRMs) imparted on an initial state of thermal demagnetization is larger than those of pTRMs in the presence of a TRM when the field used to impart pTRM is equal in magnitude and parallel to that used to produce TRM. A multi-cycle Thellier analysis on coarse-grained magnetites progressively produces more intense pTRMs and progressively erases more of the pTRM tails. Both pre-history and multi-cycle dependence will likely enhance the non-linear features of the Arai plot for coarse-grained magnetites.

Testing the IZZI protocol of geomagnetic field intensity determination

A new paleointensity determination protocol (the IZZI method) was recently proposed. The IZZI technique combines the Aitken (in‐field, zero‐field; IZ) and Coe (zero‐field, in‐field; ZI) methods. The IZZI protocol of paleointensity method was experimentally tested, showing a strong angular dependence resulting from the undemagnetized portions of partial thermoremanent magnetization (pTRM) tails. The IZZI method is better than the conventional techniques (Aitken, Coe, and Thellier) in three respects: (1) it can easily detect the angular dependence; (2) it provides a quantitative estimate for the consistency of the outcome between IZ and ZI step; and (3) it is quicker because the extra pTRM tail check step is unnecessary.

High-quality microwave archaeointensity determinations from an early 18th century ad English brick kiln

SUMMARY Samples of bricks from the Dogmersfield Park post-medieval brick kiln (Hampshire, UK), have been used to determine the intensity of the geomagnetic field during the early 18th century ad. Extensive magnetic mineralogical analyses have been carried out revealing weathered zones and multidomain dominated areas within the bricks. Likewise, statistical data treatment has been used to identify mineralogical heterogeneities. Once the most suitable parts for the study were selected, archaeointensity determinations were carried out applying Coe's double-step variant of the Thellier method. The 14 GHz microwave system used in this study was developed at Liverpool University and the thermal demagnetizer is a commercial instrument. The former uses microwaves instead of heat to demagnetize the sample, the latter uses heat and was used for comparison. Data from the microwave system were of very high quality and uncertainties assigned to single measurements are smaller than the intrinsic variability of the archaeointensity stored within the bricks. Both pTRM and pTRM tail checks rarely fail, and even pass when the sample is almost fully demagnetized. Microwave intensities group around a central value (∼53 μT), slightly higher than the present-day field intensity, and agree well with previous studies and geomagnetic field models. Thermal results exhibit higher and more scattered values; some samples show evidence of laboratory thermal alteration. The different behaviour of samples when subjected to microwave and thermal experiments appears to be linked with the magnetic mineralogy (high-Ti titanomagnetite): easy microwave demagnetization without alteration is achieved whereas sample alteration is almost unavoidable during thermal demagnetization. High-Ti titanomagnetite seems to be the ideal mineralogy for microwave experiments instead of pure magnetite, preferred in conventional Thellier experiments. The last part of this paper discusses the usefulness of conventional alteration tests applied to microwave experiments. It appears that for this technique negative alteration checks can originate from poor reproducibility of the absorbed microwave power.

Linear and nonlinear Thellier paleointensity behavior of natural minerals

We tested single‐domain (SD) and multidomain (MD) theories of the acquisition and thermal demagnetization of thermoremanent magnetization (TRM) and partial TRM (pTRM) in Thellier paleointensity determination, using as samples plagioclase and mafic minerals extracted from Matachewan diabase dikes. Susceptibility, hysteresis, and coercivity data indicate that the plagioclase contains mainly SD grains of magnetite while the mafic minerals (clinopyroxene, biotite, and actinolite) contain mainly MD magnetite. The extracted minerals were used in simulated Thellier experiments with laboratory TRM as a proxy for natural remanent magnetization (NRM). The plagioclase behaved ideally, as predicted by Néel SD theory, giving a linear NRM‐pTRM (Arai) plot with positive pTRM checks. The mafic minerals behaved nonideally, with strongly curved Arai plots sagging below the ideal SD line, in accord with observations for MD magnetites and with predictions that in heating to moderate temperatures T, more TRM is lost than can be regained as pTRM, causing convex‐down Arai plots. In experiments with pTRM parallel to NRM, all pTRM tail checks were zero, but pTRM checks increasingly deviated from the Arai curve toward the SD line as T increased. However, in repeat Thellier experiments, pTRMs at all T agreed with the original pTRMs. The negative pTRM checks were therefore not due to physicochemical alteration but rather to departure from initial states with different magnetic histories than those of the original pTRMs. The inability of pTRM checks to reliably distinguish between MD behavior and irreversible alteration could lead to overestimates of paleointensity from straight‐line fits to the low‐T “unaltered” part of the Arai curve or underestimates from fits to high‐T data. Alternating field cleaning to 15 mT before each Thellier step made the Arai plot only marginally less curved. Low‐temperature demagnetization (LTD, zero‐field cycling through the 120 K Verwey transition) hardly changed the curvature of the Arai plot for pTRM parallel to NRM. For pTRM perpendicular to NRM, LTD led to an inflected Arai curve straddling the ideal SD line. Data of either form would not give reliable paleointensity values. No presently available pretreatment renders the paleointensity data of MD grains usable. SD and nearly SD grains which behave ideally in the Thellier experiment remain the only useful recorders of absolute paleointensity.

Thellier paleointensity theory and experiments for multidomain grains

We extend theories of thermoremanent magnetization (TRM) and partial TRM (pTRM) in multidomain (MD) grains to model thermal demagnetization and pTRM acquisition steps in Thellier paleointensity determination. Because of the interleaving of zero‐field and in‐field heating‐cooling steps to increasing temperatures the initial state for any step is complex, and theoretical modeling is more intricate than for pTRM production or thermal demagnetization separately. At low to moderate temperature T, TRM lost exceeds pTRM regained, causing convex down Arai plots sagging below the ideal single‐domain (SD) line. At moderate to high T, pTRM acquisition outweighs TRM loss. As T approaches TCurie, pTRM gain exactly equals TRM loss, and the Arai plot becomes ideal. When pTRMs are produced perpendicular to the original TRM and measured directly rather than by differencing field‐on and field‐off results, there is less deviation from ideality at low to moderate T. Our theory agrees semiquantitatively with results for parallel and perpendicular pTRMs for large MD magnetites (135 μm). Smaller MD magnetites (6 and 20 μm) have less curved Arai plots, and the smallest magnetites (0.6 and 1 μm) have almost linear plots. Positive pTRM checks demonstrate that curved Arai plots of MD grains are reproducible, unlike curved plots for rocks that alter physicochemically in the Thellier experiment, while negative pTRM tail checks indicate undemagnetized pTRM residuals. Low‐temperature demagnetization improves linearity only slightly. Practical applications of this work include using the predicted threshold T below which no net pTRM is produced in a parallel Thellier experiment to screen data used for paleointensity fits. Straight line fits through low‐ and medium‐T points in Arai plots of MD (135 μm) grains overestimated the paleofield by as much as 100% and for small pseudo‐single‐domain (PSD) (0.6 and 1 μm) grains overestimated by about 25%. However, by using linear segments of medium‐ to high‐T data with f values ≥0.5 it may be possible to obtain reasonable paleointensity estimates even for larger PSD (6 and 20 μm) and MD grains. Middle‐ to high‐T fits for 0.6 μm grains gave paleointensities within 4% of the correct value, utilizing essentially the entire data set (f > 0.9). Perpendicular data always gave superior linear fits. Orienting samples with their natural remanent magnetizations perpendicular to the laboratory field is therefore recommended for rocks containing PSD and MD grains. However, double heatings are preferable to single heatings because they allow pTRM tail checks to be carried out.

On partial thermoremanent magnetization tail checks in Thellier paleointensity determination

Coarse magnetite grains, of multidomain (MD) or large pseudo‐single‐domain (PSD) size (≥10 μm approximately), cause nonlinearity and other problems in paleomagnetic field intensity determination and can lead to spurious paleointensity values. One suggested method of detecting their presence is to carry out an additional zero‐field heating step following the acquisition of partial thermoremanent magnetization (pTRM) to check for complete demagnetization of the pTRM. Residual undemagnetized pTRMs (“pTRM tails”) are characteristic of MD and large PSD grains and cause nonideal behavior in Thellier‐type paleointensity experiments. We have measured pTRM tails in synthetic and natural magnetites of many sizes and domain states (including PSD and MD), starting from two different initial states. First, a thermally demagnetized sample was given a pTRM by cooling in a field H from the blocking temperature TB. The pTRM tail is the remanence ΔMptr remaining after zero‐field reheating to TB. ΔMptr is most prominent for large grain sizes and when TB ≥ 500°C. We next measured pTRM tails ΔMptr* produced in a Thellier paleointensity experiment. The initial state in this case is a total thermoremanent magnetization gradually reduced by double heatings (zero‐field followed by in‐field) to a set of increasing TB. For all synthetic and natural samples containing large PSD and MD magnetites, regardless of grain size or lithologic differences, ΔMptr is significant but ΔMptr* is negligible. That is, the pTRM tail checks are always zero in our model Thellier experiments. However, in practical paleointensity studies, the pTRM tail check method has been shown to be successful in detecting MD grains. The reason for these contrasting results is that our experiments were carried out with the field Hlab that produced pTRM equal in intensity and parallel to the field H that produced natural remanent magnetization (NRM). In this special situation the pTRM tail is completely masked. For the pTRM tail check method to be most effective, either Hlab should be applied at a large angle to the NRM direction or Hlab should be larger than (twice or more) the paleofield H. This will highlight the pTRM tail; in the first case by deflecting the apparent NRM direction after the tail‐check step, and in the second case by increasing the apparent NRM intensity.

The application of acceptance criteria to results of Thellier palaeointensity experiments performed on samples with pseudo-single-domain-like characteristics

The results of a simulated Thellier palaeointensity experiment show that the low temperature portion of Arai plots produced by samples with pseudo-single-domain (PSD)-like hysteresis properties can produce significant inaccuracies despite passing conventional acceptance criteria. Furthermore, full Arai plots which fail these criteria can still produce reasonably accurate results. The evidence presented suggests that during a palaeointensity experiment: pTRM checks can fail despite no alteration occurring, pTRM tail checks are not reliable as indicators of MD-like behaviour, room temperature susceptibility monitoring is unreliable for detecting alteration, and that the minimum requirement for the f (fraction)-value should be increased from 0.15 to at least 0.5.

Paleomagnetism, paleointensity and geochronology of Miocene basalts and baked sediments from Velay Oriental, French Massif Central

Paleomagnetic study of Miocene basalts and baked sediments from Velay Oriental, France, yields a paleomagnetic pole with coordinates 84.1°N, 171.2°E (A95 = 8.6°, K = 29, N = 11). The pole is supported by a positive reversal test, and it corresponds well with the apparent polar wander path for Eurasia for 10 Ma. Paleosecular variation, estimated as the angular standard deviation of the VGP distribution 14.9° +6.5°/−3.5°, is close to expected, suggesting that the paleomagnetic pole represents a time‐averaged field. Moreover, five new 40Ar/39Ar plateau ages, falling between 9.20 and 13.55 Ma, were obtained. Thellier paleointensity experiments were carried out on both basalts and baked sediments. Almost all basalts are chemically unstable during the laboratory heatings and of the 36 preselected samples only three yield usable results. In contrast, the baked sediments are ideal for Thellier experiments with 11 out of 13 samples giving reliable paleointensity estimates. Rock magnetic experiments show that the main ferromagnetic minerals in the baked sediments are fine‐grained magnetite and hematite with a large proportion of superparamagnetic grains. In addition to the standard partial thermoremanent magnetization (pTRM) check, used in Thellier experiments to detect chemical/crystalline alteration, we introduce a second “pTRM tail” check designed to test the basic prerequisite of the method, namely, the equality of unblocking and blocking temperatures. For the baked sediments, unblocking and blocking temperatures are found to be identical, which indicates that the remanence is carried by single‐domain grains.