Components Of Natural Remanent Magnetization Research Articles

Magnetic characterization of the Daule chondrite (Ecuador's first meteorite fall): The case of elusive tetrataenite?

We investigated the magnetic properties of Ecuador's first reported meteorite fall (March 23, 2008), the Daule ordinary chondrite (L5, S4, W0) using thermomagnetic analyses at high and cryogenic temperatures, analyses of magnetic hysteresis and first-order reversal curves, and thermal and alternating field (AF) demagnetization of natural remanent magnetization (NRM). The mineralogical and chemical composition of Daule was examined using scanning electron microscopy with energy-dispersive x-ray spectroscopy. Most methods indicate that the magnetic properties of Daule are dominated by multidomain FeNi alloys (kamacite) with Ni content varying between ∼4% and ∼17%. However, backfield demagnetization (BFD) analyses revealed the presence of high-coercivity tetrataenite that survived shock metamorphism. The differential survival of tetrataenite at the millimeter scale indicates heterogeneity of the impact-related temperature and pressure fields within the Daule meteorite. BFD curves may serve as an efficient tool for identifying minor amounts of tetrataenite that otherwise cannot be discerned from the signal from magnetically-soft FeNi mineral phases by methods based on induced magnetization. Thermal demagnetization experiments unveiled the presence of a well-defined characteristic component of NRM, which remains resistant to AF demagnetization. We interpret this component as a pre-impact thermochemical remanence carried by tetrataenite and acquired during the thermal metamorphism of the parent body. At cryogenic temperatures, the magnetic properties of Daule are dominated by low-Mg magnesiochromite with the Curie temperature at 60–70 K.

Paleomagnetism of Gedemsa magmatic segment, Main Ethiopian Rift: Implication for clockwise rotation of the segment in the Early Pleistocene

One-hundred-seventy oriented paleomagnetic core samples from 23 sites were collected from volcanic lava flows belonging to the Nazareth Group (1.7–1.9 Ma) and the Wonji group (≤0.3 Ma) along the Gedemsa magmatic segment (MS) in the, Main Ethiopian Rift (MER). Samples were subjected to progressive thermal (Th) and Alternating Field (AF) demagnetization for directional analyses, and representative samples were used for rock-magnetic experiments. The rock-magnetic study indicated that titano-magnetite, titano-hematite and magnetite are the primary carrier of remanent magnetization. Two components of Natural Remanent Magnetization (NRM) were revealed: the first, low stability component is removed by AF of 20-30mT or heating up to 200 °C. The second, high stability component isolated above these steps is directed toward the origin and considered Characteristic Remanent Magnetization (ChRM). All specimens we studied show normal geomagnetic polarity. The overall mean direction for the Wonji group is Decs = 0.60, Incs = 5.10 (α95 = 6.50, K = 35.8, N = 14) while for the Nazareth group is Decs = 18.90, Incs = 13.40 (α95 = 10.30, K = 35.6, N = 7). When these mean directions from the two groups are compared with their respective reference geomagnetic dipole fields (Besse and Courtillot, 1993, 2002), the former shows no significant block rotation and a clockwise rotation R = 170 ± 3.40 were detected in the latter. These results indicate the Gedemsa magmatic segment is characterized by complex deformation due to the opening of DebreZeit-Butajira (DzB) and Yerer – Tullu Wellel Volcano-tectonic Lineament (YTVL segments. Our study indicates that detailed paleomagnetic observations in the Rift are important to identify quantitative deformation history of the region.

Remagnetization of Permian Emeishan basalts: Constraints on the timing of native copper mineralization in northeast Yunnan Province, China

New paleomagnetic results from the Permian Emeishan basalts in the Zhaotong area, NE Yunnan province, China show four natural remanent magnetization components. Detailed stepwise thermal demagnetization of basaltic samples from 16 flows from the Dadi section, which represent basalt units III and IV, isolated two groups of characteristic remanent magnetizations. Samples in unit IV (five flows) record a southwest declination and a moderate downward inclination that is considered to be a partial remagnetized remanence. The bottom flows from unit III (11 flows) record a normal polarity direction, interpreted as a remagnetization, which yields a tilt-corrected mean direction of Ds/Is= 8.8°/31.6° (N = 9, ks= 39.7,α95= 8.3°), with a corresponding paleomagnetic pole at 77.1°N, 240.0°E (K = 49.2, A95= 7.4°). The secondary directions have steeper inclinations than primary ones that have been successfully recovered from other studies in this area of the Emeishan basalts. By comparison with the Phanerozoic paleomagnetic poles of the South China Block, the preferred timing of remagnetization is the Lower-Middle Jurassic. Field relationships suggest that the remagnetization of the Emeishan basalts is coeval with the spatially related, but localized, copper mineralization. Thus the timing of the main copper mineralization hosted in the Emeishan basalts is hypothesized to occur in the Early-Middle Jurassic.

Loess deposits in southern Tajikistan (Central Asia): Magnetic properties and paleoclimate

Abstract The continental accumulation of dust during the Quaternary led to the formation of widespread loess deposits in southern Tajikistan. In this area, the accumulation of loess is commonly associated with the occurrence of dust storms and the widespread distribution of loess provides evidence of dust storms becoming more frequent in arid Central Asia at least since the Early Pleistocene. Southern Tajikistan represents one of the largest loess deposits in Central Asia. We conducted a thorough study on the magnetostratigraphy, grain size, and magnetic susceptibility of the Chashmanigar section to reconstruct the stratigraphy of loess deposits and paleoclimate of Tajikistan. Based on our new data, the lower boundary of the basal ages of the Olduvai and Reunion subchrons were established for the studied Chashmanigar section. Rock magnetic analyses showed that the predominant ferrimagnetic minerals are large pseudo-single domain grains of magnetite, including limited maghemite. Standard demagnetization techniques yielded a characteristic component of natural remanent magnetization, which was used to obtain a well-defined magnetostratigraphy. In southern Tajikistan, paleosols consistently exhibit finer grain size distribution and higher magnetic susceptibility than loess horizons, suggesting that the environment of the Chashmanigar section was colder, drier, and dustier during glacial periods than during interglacial periods. Through correlation with astronomically tuned oxygen isotope records, sophisticated dating of the loess-paleosol sequence at Chashmanigar could be achieved, and the global significance of the recorded paleoclimatic variations could be revealed. The resulting grain size, magnetic susceptibility, and correlation with astronomically tuned oxygen isotope clearly provide information about the climatic pattern during the Early Pleistocene.

Верхнедевонские отложения (франский ярус) северо-запада Русской платформы: палеомагнитные данные

Paleomagnetic determinations for the Devonian strata of the Main Devonian Field, available in the international paleomagnetic data base, do not make it possible to construct both detailed magnetostratigraphic scales and reliable trajectories of the apparent polar wander path of the paleomagnetic pole. This is primarily due to the insufficient amount of data that determine modern reliability criteria. Obtaining more complete series of reliable paleomagnetic determinations is one of the important tasks of paleomagnetic studies. The paper presents new paleomagnetic determinations from a collection of rocks of the Frasnian stage of the Upper Devonian (Ilmen and Bureg Beds of the Semiluky Formation), sampled on the southern shore of Lake Ilmen, east of the village. Korostyn in 2009. Magnetomineralogical studies were carried out to determine the magnetic minerals of carriers of natural remanent magnetization. Three characteristic components of natural remanent magnetization have been identified. Two components correspond to the Late Paleozoic magnetization reversal (in the Carboniferous and Permian times). The third, bipolar component of the Frasnian age passes the reverse polarity test of the geomagnetic field.. The obtained position of the paleomagnetic pole by the bipolar component of the natural remanent magnetization is consistent with the available data on the section of the Late Upper Devonian of the Main Devonian Field.

Paleomagnetic evidence for counterclockwise rotation of the Dofan magmatic segment, Main Ethiopian Rift

Twenty-six paleomagnetic sites in basalt and trachyte flows and ignimbrite deposits sampled in the Dofan magmatic segment, Main Ethiopian Rift (MER). From each site, 6 to 8 core samples were collected. The samples were then cut into 200 standard specimens and their Natural Remanent Magnetization (NRM) directions were measured using a JR6A spinner magnetometer. Most specimens were subjected to stepwise alternating field (AF) and at least one specimen per site to thermal (TH) demagnetization. The directional analysis of these individual specimens revealed either one or two components of NRM. Where two components are present, the first is isolated below a temperature of 300 °C or AF field below 20 mT; the second is isolated above those steps and mostly defined straight lines directed towards the origin and are interpreted as the Characteristic Remanent Magnetization (ChRM) acquired during cooling. Rock magnetic experiments on representative specimens indicate that the dominant magnetic minerals are titanium poor titanomagnetite and in few cases titanohematites. The overall mean directions calculated for the 23 sites of Dofan is Dec = 354.1°, Inc. = +11.6° (N = 23, K = 35.1, α95 = 5.2°). When these values are compared with the 1.5 Ma expected mean geomagnetic dipole reference field directions Dec = 1.0°, Inc. = +16.4° (N = 32, K = 105.6, α95 = 2.3°), obtained from African Apparent Polar Wander Path Curve; a difference in declination ΔD = −6.9° ± 4.7° and inclination ΔI = +4.8° ± 5.5° are determined. The declination difference is interpreted as a very slight counterclockwise rotation about vertical axis of the Dofan magmatic segment and the result is consistent with previous paleomagnetic reports and analogue modeling in Fentale magmatic segment.

Paleomagnetism of Precambrian dikes in the Kola part of northeastern Fennoscandia and its relation to the Svecofennian orogeny

Based on the results of the preliminary paleomagnetic investigation of 57 Precambrian dikes of the Kola Peninsula, in 31 of them a stable monopolar component of natural remanent magnetization (NRM) is revealed (D = 353.2°, I = 53.0°, K = 58, and α95 = 3.4°). The peculiarities of the distribution of this magnetization component within the Kola Peninsula and the rock magnetic characteristics of the dikes in which this component is isolated suggest its secondary nature and relate the mechanism and formation time to the remagnetization processes which took place in the northwest of Fennoscandia about 1.8 billion years ago during the Svecofennian orogeny. The corresponding geomagnetic pole of Fennoscandia has the coordinates Plat = 54.5°, Plong = 224.0°, and A95 = 3.9° and is located in the immediate vicinity of the known Paleoproterozoic (1.9–1.7 Ga) poles of Baltica (Khramov et al., 1997; Veikkolainen et al., 2014).

Paleomagnetic and rock magnetic study of Lower Devonian red beds from Podolia: remagnetization problems

Results of paleomagnetic studies of red bed Early Devonian deposits of Podolia, which make the Dniester series (southwestern Ukraine, the Dniester basin) have been given. As a result two components of natural remanent magnetization have been subdivided in addition to viscous one. The first one has got south-southwestern declination and negative inclination, it is identified in almost all samples within the limits of deblocking temperatures from 150—200°С to 530—630°С. Its corresponding paleomagnetic pole (47°S, 351,5°E) is close to the Permian fragment of APWP for the Baltic/Stabile Europe. Its bearers are newly-formed magnetic minerals (secondary authigenous hematite), responsible for production of “late” chemical magnetization. The second component is subdivided in some samples within the limits from 590—610°С to 680—690°С, it has got southwestern declination and positive inclination. Its corresponding paleomagnetic pole (2.3°S, 338.4°E) is drawn towards Early Devonian fragment of APWP This component is caused by presence of detritic grains of hematite. There are solid grounds for consideration of this magnetization to be primary one, which reflects the direction of the Devonian field. New results are in good agreement with paleomagnetic directions obtained earlier in this area for red beds and grey beds of Early Devonian.

Remagnetization mechanisms in Triassic red beds from South China

Paleogeographic reconstructions based on paleomagnetic data rely on the reliability of the natural remanent magnetization (NRM) as a primary geomagnetic signal. Remagnetizations, however, can be common in many rock types, including late Paleozoic and Mesozoic red beds, and they complicate paleogeographic interpretations. Extracting the primary NRM from partially remagnetized rocks, and understanding the remagnetization mechanism are important in these contexts. We carried out a systematic paleomagnetic study of red bed samples from the Triassic Huangmaqing Formation, Nanjing (32.0°N, 118.9°E), South China. Two NRM components carried by secondary and primary hematite are isolated in 47 of the 94 samples studied, where the latter component has a direction in stratigraphic coordinates of D=29.2°, I=34.6° (α95=10.9°, 47 samples from 6 sites) that yields a paleopole of λ=60.8°N, ϕ=228.1°E, dp/dm=12.5/7.2, which is consistent with Triassic pole positions for the South China Block. A secondary chemical remanent magnetization (CRM) (D=227.1°, I=80.8°, α95=7.3°) is documented in all 94 samples from 10 sites and is carried by pigmentary hematite that is inferred to have been generated by magnetite oxidation during orogenic activity. This secondary component has steep inclinations and is interpreted to have been influenced by a combination of the remanence carried by original parent magnetite, the orogenic stress field, and the prevailing geomagnetic field direction during deformation. This CRM direction is recorded commonly by red beds from the South China Block, and is significant for regional tectonic studies in the area.

Virtual Directions in Paleomagnetism: A Global and Rapid Approach to Evaluate the NRM Components

We introduce a method and software to process demagnetization data for a rapid and integrative estimation of characteristic remanent magnetization (ChRM) components. The virtual directions (VIDI) of a paleomagnetic site are “all” possible directions that can be calculated from a given demagnetization routine of “n” steps (being m the number of specimens in the site). If the ChRM can be defined for a site, it will be represented in the VIDI set. Directions can be calculated for successive steps using principal component analysis, both anchored to the origin (resultant virtual directions RVD; m * (n2+n)/2) and not anchored (difference virtual directions DVD; m * (n2-n)/2). The number of directions per specimen (n2) is very large and will enhance all ChRM components with noisy regions where two components were fitted together (mixing their unblocking intervals). In the same way, resultant and difference virtual circles (RVC, DVC) are calculated. Virtual directions and circles are a global and objective approach to unravel different natural remanent magnetization (NRM) components for a paleomagnetic site without any assumption. To better constrain the stable components, some filters can be applied, such as establishing an upper boundary to the MAD, removing samples with anomalous intensities, or stating a minimum number of demagnetization steps (objective filters) or selecting a given unblocking interval (subjective but based on the expertise). On the other hand, the VPD program also allows the application of standard approaches (classic PCA fitting of directions a circles) and other ancillary methods (stacking routine, linearity spectrum analysis) giving an objective, global and robust idea of the demagnetization structure with minimal assumptions. Application of the VIDI method to natural cases (outcrops in the Pyrenees and u-channel data from a Roman dam infill in northern Spain) and their comparison to other approaches (classic end-point, demagnetization circle analysis, stacking routine and linearity spectrum analysis) allows validation of this technique. The VIDI is a global approach and it is especially useful for large data sets and rapid estimation of the NRM components.

Magnetostratigraphic constraints on the age of the lower Beaufort Group, western Karoo basin, South Africa, and a critical analysis of existing U‐Pb geochronological data

AbstractWe studied three partially overlapping sections with a composite thickness of ∼600 m in the upper Permian fluvial siltstones and fine‐grained sandstones of the Abrahamskraal Formation, the basal unit of the Beaufort Group, in the Karoo Basin of Western Cape Province, South Africa. Paleomagnetic analysis reveals three components of Natural Remanent Magnetization (NRM). Heating to ∼180°C removes a remanent magnetization parallel to the present‐day field, which is interpreted as a viscous overprint. An intermediate unblocking temperature component is removed by heating to 450°C; this direction is always of normal polarity and is identical to a regional overprint imparted during the Early Jurassic emplacement of the Karoo Large Igneous Province. A high‐temperature component isolated above 450°C is of dual polarity and is interpreted as primary on the basis of a positive reversals test. The virtual geomagnetic pole position for the Abrahamskraal Formation computed from the average high‐temperature characteristic remanent magnetization direction is in agreement with the late Permian directions for stable Gondwana and with previous results from the lowermost Abrahamskraal Formation and Waterford Formation at the Ouberg Pass section. The predominantly normal polarity of this magnetization is in agreement with either a middle‐late Lopingian age (ca. 254–256 Ma) or a late Guadalupian age (ca. 262 Ma) according to the global geomagnetic polarity time scale. We integrate these new results with existing magnetostratigraphic, biostratigraphic, and geochronological results from the Karoo Basin, with particular emphasis on the controversy over zircon age data reported from the underlying Ecca Group.

Magnetic properties of Triassic rocks of Svalbard Archipelago - preliminary results

Studies were carried out in three rock formations: the Vardebukta Formation (Early Triassic) and the Bravaisberget Formation (Middle Triassic) from Spitsbergen and the Botneheia Formation (Middle Triassic) from Edgeoya (Eastern Svalbard). Collected samples are diversified and represent various lithologies with different thermal and tectonic histories. Magnetic properties of the specimens were studied using rock-magnetic methods including: thermal demagnetization of saturated isothermal remanent magnetization (SIRM), hysteresis loops and analysis of magnetic susceptibility variations at high temperatures. Subsequently anisotropy of magnetic susceptibility (AMS) and analysis of the structure of natural remanent magnetization (NRM) using thermal and alternating field (AF) demagnetization were investigated. The obtained results show that magnetite represents the main magnetic mineral in all studied rocks, whereas pyrrothite, as a second magnetic information carrier, was noted in some of specimens only. The samples from Edgeoya possess the highest magnetic susceptibility (ca. 240∙10−6 SI) which tends to decrease in sites located further to the west and reaches the minimum value for samples from western Spitsbergen (ca. 30–40∙10−6 SI). AMS studies revealed that the Middle Triassic samples are characterized by inverse magnetic fabric and prolate AMS ellipsoids, whereas the Early Triassic specimens represent normal sedimentary fabric and oblate AMS ellipsoids. Inverse magnetic fabric can be carried by many minerals including: tourmaline, cordierite or goethite (Chadima et al., 2006). For investigated samples two models proposed by Rochette (1988) are highly probable: (1) presence of single-domain elongated magnetite grains or (2) presence of ferroan carbonates whose maximum susceptibility is parallel to the c-axis. Paleomagnetic results of the Botneheia Formation revealed only one characteristic NRM component characterized by steep inclination and maximal unblocking temperatures in range of 425–450°C. In turn, the Early Triassic rocks record several NRM components with partially overlapping unblocking temperatures. We have concluded that different magnetic properties observed in studied rocks might be related to various lithologies but also to distinct tectonic and thermal histories associated with complex tectonic evolution of the Barents Sea Shelf.

NEW DATA ABOUT MATUYAMA-BRUNHES BOUNDARY IN ROXOLANY SECTION

From new palaeomagnetic data of 55-meter key section of Quaternary loess-soil sediments on the Dniester estuary near Roxsolany village the position of Matuyama-Brunhes boundary have been determined. Earlier this section had been studied by different research groups [Третяк, 1983; Третяк и др., 1987; Третяк и др., 1989; Tsatskin et al., 1998; Tsatskin et al., 2001; Dodonov et al., 2006; Gendler et al., 2006] and the position and ages of individual soil horizons caused much discussion. The primary component of natural remanent magnetization and their polarity was identifying both in loess and soil horizons due to detail measurements on the modern equipments. Position of Matuyama-Brunhes boundary has been identified at a depth of 46.6 m between Martonosha and Lubny soil horizons. This is agree with modern knowledge of Ukrainian scientists about Quaternary records in south Ukraine and is not consistent with the results which identified this boundary at a depth of 34 m in loesses above the level of РК7.

AMS–NRM interferences in the Deccan basalts: Toward an improved understanding of magnetic fabrics in flood basalts

AbstractThe evaluation of flow direction in volcanic rocks is among the most important applications of magnetic fabrics studies. A statistically significant sample set of titanomagnetite‐bearing lava flows from the Malwa Plateau, the northern part of the Deccan traps in India, has been investigated for a possible interference of induced and natural remanent magnetization (NRM). The NRM alters the scalar anisotropy of magnetic susceptibility (AMS) parameter and the orientations of the AMS principal magnetic axes, which are crucial for the evaluation of the flow direction. For cleaning of the NRM component, the lava samples have been demagnetized by use of an alternating field (AF) tumbling demagnetizer (peak fields of 100 mT) as previous studies have shown that static AF demagnetization can bias the results. Samples with normal magnetic fabrics demonstrate a redistribution of their principal axes after the demagnetization. The evaluated flow directions show a more differentiated flow pattern of the Malwa area, which seems to fit better into the regional geological setting. In samples with inverse magnetic fabrics, carrying a higher portion of single‐domain particles, AMS principal axes remain unchanged after the demagnetization, indicating that these samples with high coercivity of magnetic carriers are not suitable for geological interpretations. According to these results, we propose that the AMS measurements after tumbling demagnetization give a better reflection of the intrinsic anisotropy of magnetic carriers (at least for samples with normal magnetic fabrics) and therefore a more precise and better reflection of the “actual” mineral fabric.

Paleomagnetism of Mesoproterozoic Satakunta sandstone, Western Finland

A paleomagnetic study of the Mesoproterozoic Satakunta sandstone formation, Western Finland is presented here. Two components of natural remanent magnetization (NRM) were isolated in the sandstone with alternating field (AF) and thermal demagnetization treatments. The first is a high coercivity/unblocking temperature component which, after tilt and inclination shallowing corrections, yields a remanent magnetization of D=25.2°, I=3.9° with α95=9.1° (10 sites), corresponding to a paleomagnetic pole of Plat=27.8°N, Plon=173.2°E, with A95=6.5°. Positive reversal and tilt tests, an inverse baked contact test, as well as a stratigraphic order of the site mean poles, indicates a primary remanence. The relative age of the sandstone is estimated at ca. 1600 based on the proximity to other well defined poles of 1540–1770Ma from Baltica. However, because the pole also overlaps with poles with the ages of 1452–1630Ma, the age of the sandstone cannot be accurately defined. The second component is a lower coercivity/unblocking temperature component with a remanent magnetization direction D=34.0°, I=−40.7° with α95=8.1° (13 sites) similar to that of the Postjotnian Satakunta diabase intrusions, which are widespread in the area. Comparing APWPs of Baltica and Laurentia during 1.77–1.27Ga gives strong support to the Mesoproterozoic NENA configuration at 1.6Ga.

Magnetic study of large Apollo samples: Possible evidence for an ancient centered dipolar field on the Moon

We present new rock magnetic and paleomagnetic results obtained from seventeen Apollo samples. Measurements of the magnetic properties of the samples confirm that the magnetic properties of lunar rocks are dominated by about 0.1wt.% of multidomain FeNi grains. The ferromagnetic mineral assemblage is to the first order the same in all lunar lithologies, with higher concentration in basalts than in anorthosites (some of which are diamagnetic). Impact processing leads to an increase of the ferromagnetic content. Out of the seventeen samples, eleven show at least one stable component of natural remanent magnetization. In some cases, this magnetization may be a magnetic contamination. However, five basalt samples have a component of magnetization that may have recorded a lunar paleofield acquired on the Moon. For these samples (plus two similar samples from a previous study), using the rock magnetic fabric as a paleohorizontal proxy, paleoinclinations can be estimated from the paleomagnetic data. These paleoinclinations are best explained with a dipolar field and a magnetic paleopole located at 75 N. Therefore our results suggest the existence on the Moon, at least between 3.8 and 3.3Ga, of a dipolar field whose axis is close to the present-day rotation axis of the Moon. The estimated surface field intensities of several tens of μT support previous result that the Moon had a dynamo field in the past, and the proposed paleofield geometry suggests that this dynamo was centered on the lunar present-day rotation axis.

Composite detrital and thermal remanent magnetization in tuffs from Aeolian Islands (southern Tyrrhenian Sea) revealed by magnetic anisotropy

This paper reports on the complex relation between rock emplacement and remanence acquisition in tuffs deposited by pyroclastic density currents, disclosed by systematic measurements of the anisotropy of magnetic susceptibility and natural remanent magnetization (NRM). Thermal demagnetization shows that the NRM consists of two components with different blocking-temperature spectra. The direction of the low-temperature component is consistent with the geocentric axial dipole value, whereas the high-temperature component has dispersed directions. The magnetic fabric is oblate, the magnetic foliation is close to the bedding and the lineations are generally dispersed along a girdle within the foliation plane. The directions of the magnetic lineation and the high-temperature remanence component of individual specimens are close to each other. This correspondence suggests that the high blocking-temperature grains acquired a remanence aligned to their long dimension before deposition, while cooling within the explosive cloud and the moving pyroclastic current. Thereafter, during deposition, the traction processes at the base of the current oriented the grains along the flow direction and affected both fabric and high-temperature remanence. This NRM component results from mechanical orientation of previously magnetized grains and is thus detrital in origin. A second, thermal component was then acquired during the cooling of the low blocking-temperature grains after deposition. These results show that NRM in fine-grained pyroclastic rocks is affected by the Earth’s magnetic field as well as the emplacement processes and that magnetic fabric data are essential to unravel its complex nature.

Rock-magnetic analyses as a tool to investigate archaeological fired sediments: a case study of Mirador cave (Sierra de Atapuerca, Spain)

SUMMARY Here we report a detailed mineral magnetic study of Neolithic burnt levels in the Mirador Cave (Sierra de Atapuerca, Burgos, Spain) to reconstruct the burning history and to investigate their suitability for archaeomagnetic purposes. As a consequence of the ancient burning, a characteristic facies sequence was developed along the Holocene stratigraphy. From top to bottom it includes: (i) 2–10 cm ash layers, (ii) ∼2 cm underlying rubefied layers and (iii) clay, mainly unburnt and of variable thickness. In some cases a thermally altered facies (2–6 cm) with a heterogeneous texture was identified (facies TF), usually between rubefactions and the unburnt levels. 126 oriented samples from 4 units (MIR12, 15, 18 and 21) and a 2 m section, all comprised between units MIR21 (6380 ± 40 14C BP) and MIR9 (5090 ± 40 14C BP) were analysed with rock magnetic methods. In addition, bulk sediment from each facies that comprise the Neolithic sequence was investigated. Measurements included: stepwise alternating field and thermal demagnetization of natural remanent magnetization (NRM), viscosity experiments, determination of the anisotropy of the magnetic susceptibility (AMS), the susceptibility frequency dependence at room temperature and determination of the temperature dependence of the susceptibility. Additional experiments consisted in the determination of the behaviour of anhysteretic and isothermal (IRM) remanences, magnetic hysteresis loops, first-order-reversal-curve diagrams, and thermal demagnetization of three-axial IRM. It appeared that the facies all show a fairly similar magnetic mineralogy and grain size dominated by low-titanium magnetite that is often partially maghaemitized. Main differences constitute the amount of superparamagnetic particles that is higher in unburnt strata concurring with a less well-defined NRM behaviour. The magnetic mineral concentration is notably higher in ashes. This homogeneity strongly suggests that similar sources and burial conditions prevailed during Neolithic times. Agreeing with archaeological observations and favoured by rapid burial conditions, very limited alterations have been deduced. AMS data revealed the absence of fluid flow in the ash lenses sampled. Demagnetization revealed a stable single NRM component in ashes, a single or two-component NRM in rubefactions and less stable multicomponent behaviour in clays. In ashes, magnetic minerals are likely secondary in origin formed by low-temperature oxidization soon after burning. Although this thermochemical nature of the NRM invalidates the use of these sediments for palaeointensity studies, archaeomagnetic (directional) data can be successfully obtained because the burning and oxidation are closely confined in time.

Paleomagnetism of the Lower Ordovician and Cambrian sedimentary rocks in the section of the Narva River right bank: For the construction of the Baltic Kinematic model in the Early Paleozoic

The paleomagnetic study of the Lower Ordovician and Cambrian sedimentary rocks exposed on the Narva River’s right bank revealed a multicomponent composition of natural remanent magnetization. Among four distinguished medium- and high-temperature magnetization components, the bipolar component, which carries the reversal test, is probably the primary component and reflects the geomagnetic field direction and variations during the Late Cambrian and Early Ordovician. The pole positions corresponding to this component have coordinates 22°N, 87°E (dp/dm = 5°/6°) for the Late Cambrian, and 18°N, 55°E (dp/dm = 5°/7°) for the Early Ordovician (Tremadocian and Arenigian). Together with the recently published paleomagnetic poles for the sections of the Early Ordovician in the Leningrad Region and the series of poles obtained when the Ordovician limestones were studied in Sweden, these poles form new key frameworks for the Upper Cambrian-Middle Ordovician segment of the apparent polar-wander path (APWP) for the Baltica. Based on these data, we propose a renewed version of the APWP segment: the model of the Baltica motion as its clockwise turn by 68° around the remote Euler pole. This motion around the great circle describes (with an error of A95 = 10°) both variations in the Baltic position from 500 to 456 Ma ago in paleolatitude and its turn relative to paleomeridians. According to the monopolar components of natural remanent magnetization detected in the Narva rocks, the South Pole positions are 2°S, 351°E (dp/dm = 5°/9°), 39°S, 327°E, (dp/dm = 4°/7°), and 42°S and 311°E (dp/dm = 9°/13°). It is assumed that these components reflect regional remagnetization events in the Silurian, Late Permian, and Triassic.

Paleomagnetism of the Fentale-magmatic segment, main Ethiopian Rift: New evidence for counterclockwise block rotation linked to transtensional deformation

We carried out the first ever paleomagnetic investigations in the area around the Fentale volcanoes, Main Ethiopian Rift. A total of 28 paleomagnetic sites were sampled around the two Fentale (Tinish and Tilik) volcanic centers. Lava and pyroclastic flows ranging in age from 2 Ma to 0.1 Ma was sampled for this study. After routine paleomagnetic collection and sample preparation procedures specimens were subjected to progressive alternating fields and thermal demagnetizations. The directional analysis shows either one or two components of Natural Remanent Magnetization to characterize the entire measurements. The low stability components are removed either by alternating fields up to 15 mT or by heating up to 250 °C. The high stability and primary components are isolated above those steps and vector is directed towards the origin, and it represents the Characteristic Remanent Magnetization. The principal component analysis and site means directions calculations yielded normal and reversed polarities together with two anomalous directions. When an overall mean direction is calculated from the individual site mean directions for the Fentale region, D s = 354.3°, I s = 19.3°, N = 27, K = 33, α 95 = 4.9° are obtained. When this result is compared with the mean expected reference dipole geomagnetic field direction ( D = 1.0°, I = 16.4°, α 95 = 2.3°, K = 105.6, N = 32) for an average age of 1.5 Ma, the following differences of Δ D = −6.7° ± 4.3°, Δ I = −2.9° ± 4.3° can be obtained. These results from the region indicate ∼7° counterclockwise block rotations about vertical axis consistent with the transtentional deformations that characterize the Fentale magmatic segment.