Isothermal Remanent Magnetization Acquisition Curves Research Articles

Palaeosecular variation in Northern Patagonia recorded by 0–5 Ma Caviahue–Copahue lava flows

SUMMARY Palaeosecular variation (PSV) determinations and studies of the geometry of the Earth’s main magnetic field provide important information about the field evolution, and to constrain numerical geodynamo models. Palaeomagnetic directional data from lava flows over the last few million years is of particular interest because the regional and global tectonic effects are minimal. However, the distribution of this type of palaeomagnetic data is uneven where the Southern Hemisphere is the destitute side. Therefore, the better knowledge of the geomagnetic field behaviour depends on the increased availability of high-quality data, especially in the Southern Hemisphere. A PSV and the time-averaged field (TAF) study was then performed in 0–5 Ma lava flows from the Caviahue–Copahue Volcanic Complex located in Northern Patagonia, Argentina (37°0′S, 71°10′W). The magnetic mineralogy of lava flows was investigated through thermomagnetic susceptibility curves, isothermal remanent magnetization (IRM) acquisition curves, hysteresis loops and first-order reversal curves (FORCs). Samples are essentially comprised of titanomagnetite with different Ti contents and magnetic domain structures typical of vortex state particles. A total of 50 volcanic sites were sampled, which provided 42 reliable palaeomagnetic site-mean directions after alternating field and thermal demagnetization. From these 42 sites, 36 are of normal and 6 are of reversed polarity. The mean direction from normal (reversed) sites is D = 356.2°, I = −50.1°, α95 = 4.0° and N = 36 (D = 176.5°, I = 59.5°, α95 = 14.1° and N = 6). Using only site-level data with the precision parameter k ≥ 100, we obtain 26 palaeomagnetic sites for PSV and TAF investigations in the study region. The filtered data set has a mean direction (D = 354.4°, I = −53.2° and α95 = 5.1°) close to the expected direction for a geocentric axial dipole (GAD) field (IGAD = −57.3°). The palaeopole (Plat = 84.4°, Plon = 229.1° and A95 = 5.7°) coincides with the Earth’s spin axis within the 95 per cent confidence interval. Virtual geomagnetic pole scatter ($S_{B} = {15.8^{18.9}_{11.8}}^{\circ }$) and the inclination anomaly ($\Delta I = 4.1_{-1.0}^{{9.2}^{\circ }}$) are both consistent at the 95 per cent confidence level with recent PSV and TAF models, respectively. Our results support the presence of small non-dipole field contributions (<3 per cent) superimposed on the GAD term, as reported by South American studies at mid southern latitudes.

Occurrence and Distribution Patterns of Magnetic Particles Within Stalagmite Growth Laminae

AbstractThe concentrations of magnetic particles in stalagmites depend greatly on the regional hydrologic condition, and thus can act as an important proxy for paleoclimate reconstruction. The central axis region of a stalagmite is usually selected for such magnetism study. However, little is known about the distribution and influence factors of exogenetic magnetic particles within the growth laminae. In this study, the occurrence and variations of magnetic particles along the growth laminae of three shape stalagmites were investigated by a set of magnetic measurements, including isothermal remanent magnetization (IRM) acquisition curves and low‐temperature magnetic analysis. The results show the main magnetic remanence carrier is detrital magnetite/maghemite, coexisting with small amounts of hematite and goethite. The distribution of soft magnetic particles along the growth laminae showed different patterns among the three shape stalagmites (dome, cone, and candle). For the dome shape stalagmite, the soft magnetic particle concentrations in all studied growth layers showed an increasing trend from center to edge. Similar patterns were also appeared in the bottom growth layers of the cone and candle shape stalagmites. This edge enrichment of magnetic particles is caused by drip water washing effect. However, such enrichment pattern became weaker or even inverse in the top growth layers of the cone and candle shape stalagmites, indicating that the shapes of stalagmites could also affect the surface distribution of soft magnetic minerals. These uneven distributions of magnetic particles imply that the central axis region of a stalagmite may not always be the optimal specimen for environmental magnetism study.

Integrated Magnetic Analyses for the Discrimination of Urban and Industrial Dusts

Industrial and urban dusts were characterized by investigating their magnetic properties. Topsoil composed of technogenic magnetic particles (TMP) originating from areas affected by three ironworks, street dust mainly composed of traffic-related pollution, and particulate matter (PM) from urban agglomeration in Warsaw, Poland were investigated. Several magnetic methods, namely magnetic susceptibility, thermomagnetic curves, hysteresis loops, decomposition of isothermal remanent magnetization acquisition curves, and first-order reversal curves, were performed to evaluate the magnetic fraction of dust. Magnetite was the main magnetic phase in all types of samples, with a small amount of high-coercive hematite within ironworks and street dust samples. Significant differences were observed in the domain structure (grain size) of industrial and traffic-related magnetic particles. The grain size of TMP obtained from steel production was in the range of 5–20 µm and was predominated by a mixture of single-domain (SD) and multidomain (MD) grains, with the prevalence of SD grains in the topsoil affected by Třinec ironwork. The traffic-related dust contained finer grains with a size of about 0.1 µm, which is characteristic of the pseudo-single-domain (PSD)/SD threshold. Street dusts were composed of a slightly higher proportion of MD grains, while PM also revealed the typical behavior of superparamagnetic particles.

Benchmarking Component Analysis of Remanent Magnetization Curves With a Synthetic Mixture Series: Insight Into the Reliability of Unmixing Natural Samples

AbstractGeological samples often contain several magnetic components associated with different geological processes. Component analysis of remanent magnetization curves has been widely applied to decompose convoluted information. However, the reliability of commonly used methods is poorly assessed as independent verification is rarely available. For this purpose, we designed an experiment using a series of mixtures of two endmembers to benchmark unmixing methods for isothermal remanent magnetization (IRM) acquisition curves. First‐order reversal curves (FORC) diagrams were analyzed for comparison. It is demonstrated that the parametric method, which unmixes samples using specific probability distributions, may result in biased estimates. In contrast, an endmember‐based IRM unmixing approach yielded better quantitative results, which are comparable to the results obtained by FORC analysis based on principle component analysis (FORC‐PCA). We demonstrate that endmember‐based methods are in principle more suitable for unmixing a collection of samples with common endmembers; however, the level of decomposition will vary depending on the difference between the true endmembers that are associated with distinctive processes and the empirical endmembers used for unmixing. When it is desired to further decompose endmembers, the parametric unmixing approach remains a valuable means of inferring their underlying components. We illustrate that the results obtained by endmember‐based and parametric methods can be quantitatively combined to provide improved unmixing results at the level of parametric model distributions.

Permian remagnetization of the Early Cambrian Guachos Formation, Eastern Cordillera, Argentina

The Permian remagnetization belt that affects the southwestern margin of Gondwana has an important geographic extension reaching several South American cratons and terranes. A paleomagnetic pole for the Early Cambrian Guachos Formation in the Eastern Cordillera of Argentina was computed (LG: Lat: 58.1°S, Long: 20.2°E, A95: 6.4°, n: 30), that likely corresponds to a Permian remagnetization and suggests that such belt might extend further north than previously envisaged. We performed several rock-magnetic studies on this succession of laminated fine grain sandstones and siltstones in order to determine the magnetic phases that carry the remanence. Thermomagnetic (k vs. T) curves, field and frequency dependence of bulk susceptibility, hysteresis loops, isothermal remanent magnetization acquisition and back-field curves, three-axis IRM thermal demagnetization, among others, permitted to establish magnetite as the main mineral carrying the remanence. Microscopic observations confirmed a secondary chemical origin for magnetite. Magnetic fabric studies indicate a tectonic fabric, consistent with cleavage developed on these rocks together with low-grade metamorphism. However, a Permian remagnetization, as suggested by the paleomagnetic pole position, cannot be related to Cambrian deformation and metamorphism and might be related to hydrothermal processes.

Last 3000 years of geomagnetic field intensity from India: New reference palaeointensity data from two east coast archaeological sites and archaeomagnetic dating insights

A comprehensive rock magnetic and absolute archaeointensity (AI) investigation was carried out on baked bricks from the east coast of India with an aim to obtain a set of well-defined palaeointensity (PI) data. The new PI data from India comprise a significant first step to improve the regional intensity reference curve, which can be used as reliable dating tool mainly for archaeomagnetic material. Standard Thellier-Coe experiments were applied on 12 independent brick samples collected from a Buddhist monument at Adurru in the Godavari delta and a Medieval Fort at Kolleti Kota site situated on an island in the Kolleru Lake. The samples thus collected represent two different archaeological periods, one around the 3rd century BCE and the other between the 4th and 7th centuries CE respectively. The data obtained from thermal demagnetization of Saturation Isothermal Remanent Magnetization (SIRM), temperature dependent magnetic susceptibility (χ–T), Isothermal Remanent Magnetization (IRM) acquisition curves, and hysteresis measurements showed that the primary carriers of Natural Remanent Magnetization (NRM) are fine-grained Pseudo-Single Domain (PSD) (Stable Single Domain (SSD) + Superparamagnetic (SP)) titanomagnetite with various titanium contents. This implies that the samples had significant stable Thermoremanent Magnetization (TRM), and, therefore, good candidates for PI analysis. Strict selection and quality-control criteria for detecting magnetomineralogical alteration and intensity data corrected for anisotropy and the cooling rate effects on TRM acquisition, offer high quality PI determinations with respect to the previous available data that missed such corrections.A total 33 specimen level PI determinations that correspond to linear NRM–TRM plots, were made using the Thellier GUI software. The corrected data yielded mean PI values of 44.6 ± 1.4 μT for the Buddhist Monument, and 36.6 ± 2.9 μT for the Fort site. By comparison and evaluation of new PI data with Global Geomagnetic Field Model predictions, the sampled bricks were archaeomagnetically dated using the Matlab tool developed by Pavón-Carrasco et al. (2011). Both ARCH3K.1 and SHA.DIF.14 K global models were used to evaluate the age estimates. The SHA.DIF.14 K model provided the most accepted age estimates that varied between 305 and 182 BCE for the early historic Buddhist Structure at Adurru, and 516–889 CE for the Medieval Fort at Kolleti Kota. These new reference archaeointensity data highlight the potential of archaeomagnetic dating to improve the archaeologically determined time periods of bricks, and also show the need for new high quality reference data to improve the geomagnetic field models from India.

Contribution of magnetic measurement methods to the analysis of iron sulfides in archaeological waterlogged wood‑iron assemblies

Pyrite and greigite were identified in the wood of two ancient shipwrecks using an original multi-technique analytical approach. Structural characterization methods such as environmental scanning electron microscopy, micro-Raman spectroscopy and X-ray diffraction were combined with magnetic measurement methods, such as magnetic susceptibility measurements and isothermal remanent magnetization acquisition curves. This is the first time that magnetic measurement methods are used in the field of cultural heritage to study wet organic archaeological materials. They proved to be particularly suitable to detect with a very high sensitivity ferromagnetic s.l. mineral phases inside waterlogged wooden samples, i.e. in the bulk. The occurrence of iron sulfides in archaeological shipwrecks extracted from waterlogged environments is usually attributed to microbiologically influenced corrosion of iron fasteners. This study demonstrates that the nature of the identified iron sulfides is consistent with a step-by-step in situ anoxic oxidation process of mackinawite.

Relative paleointensity (RPI) and age control in Quaternary sediment drifts off the Antarctic Peninsula

Lack of foraminiferal carbonate in marine sediments deposited at high latitudes results in traditional oxygen isotope stratigraphy not playing a central role in Quaternary age control for a large portion of the globe. This limitation has affected the interpretation of Quaternary sediment drifts off the Antarctic Peninsula in a region critical for documenting past instability of the West Antarctic Ice Sheet (WAIS) and Antarctic Peninsula Ice Sheet (APIS). Here we use piston cores recovered from these sediment drifts in 2015 during cruise JR298 of the RRS James Clark Ross to test the usefulness for age control of relative paleointensity (RPI) data augmented by scant δ18O data. Thermomagnetic and magnetic hysteresis data, as well as isothermal remanent magnetization (IRM) acquisition curves, indicate the presence of prevalent magnetite and subordinate oxidized magnetite (“maghemite”) in the cored sediments. The magnetite is likely detrital. Maghemite is an authigenic mineral, associated with surface oxidation of magnetite grains, which occurs preferentially in the oxic zone of the uppermost sediments, and buried oxic zones deposited during prior interglacial climate stages. Low concentrations of labile organic matter apparently led to arrested pore-water sulfate reduction explaining oxic zone burial and downcore survival of the reactive maghemite coatings. At some sites, maghemitization has a debilitating effect on RPI proxies whereas at other sites maghemite is less evident and RPI proxies can be adequately matched to the RPI reference template. Published RPI data at ODP Site 1101, located on Drift 4, can be adequately correlated to contemporary RPI templates, probably as a result of disappearance (dissolution) of maghemite at sediment depths >∼10 m.

Identifying pedogenic magnetic minerals in loess from China and Siberia using isothermal remanent magnetization acquisition curves

Aeolian loess is carried by wind and undergoes pedogenesis after deposition. Therefore, both detrital components from the source region and soil pedogenic components contribute to the magnetic properties of the loess. The pedogenic component can be identified by analyzing the coercivity spectra of loess with different degrees of pedogenesis. We used isothermal remanent magnetization acquisition curves to analyze the coercivity spectra of loess in China and Siberia and defined the low ( 100 mT) coercivity components, i.e., components 1, 2, and 3, respectively. In the arid region of Xinjiang, Northwest China, the Bole section, with negligible soil development has loess with only component 2 centered at ∼80 mT. In semiarid central China, the Xifeng section has both loess and paleosols with three coercivity components centered at ∼26 mT, ∼82 mT, and ∼960 mT. Component 1 has a pedogenic origin, and the remanence contribution increases in a positive linear relationship with the intensity of pedogenesis. In the humid region of Siberia, the Kurtak section has three coercivity components are centered at ∼23 mT, ∼78 mT, and ∼1014 mT. The remanence contribution of component 1 shows a low correlation with the intensity of pedogenesis. Component 3 is characterized by high-coercivity minerals and its remanence contribution is related to the intensity of pedogenesis. The soil development in the semi-arid Xifeng section tends to produce massive fine-grained ferromagnetic minerals, so the remanence contribution of component 1 is positively correlated with the intensity of pedogenesis. The humid pedogenic environment of the Kurtak section is prone to form high-coercivity minerals and destroys fine-grained ferromagnetic minerals, so the remanence contribution of component 3 increases with the degree of pedogenesis.

Evidence of small ferrimagnetic concentrations in mice (Mus musculus) livers and kidneys exposed to the urban dust : A reconnaissance study

Previous investigations carried out on Mexico City urban dust and soils revealed relatively high concentration of heavy metals due to the atmospheric pollution. Detailed magnetic analyzes demonstrated that the dust samples contain neo-formed magnetite phase coming from the vehicles combustion. Here, we report the results of magnetic measurements carried out on Mus musculus livers and kidneys in order to evaluate whether the magnetic particles may penetrate and accumulate in these organs. The systematic measurements of the magnetic susceptibility and isothermal remanent magnetization acquisition curves attest the existence of small concentrations of low/medium coercivity ferrimagnetic grains (most probably magnetite). This study confirms that fine magnetic minerals can penetrate and reach livers and kidneys of mice in relatively small concentrations.

Changing Abundance of Magnetofossil Morphologies in Pelagic Red Clay Around Minamitorishima, Western North Pacific

AbstractRecent investigations have discovered an unexpected abundance of magnetofossils in oxic pelagic red clays. These have potential to serve as paleoenvironmental tracers in otherwise nonfossiliferous sediments. Here, we report on variations in the abundance and morphology of magnetofossils in red clay from the western North Pacific. Magnetic measurements revealed that magnetofossils dominate the magnetic mineral assemblage of the sediments. An endmember analysis of isothermal remanent magnetization acquisition curves, supplemented by an analysis of S ratios, indicates that the magnetic assemblage can be unmixed into three endmembers, two corresponding to magnetofossils and one to terrigenous magnetic minerals. Direct counting of magnetofossil morphologies under a transmission electron microscope shows that the two magnetofossil endmembers differentiate equant magnetofossils and bullet‐shaped magnetofossils, respectively. The stratigraphic variation of the endmember contributions revealed that the equant magnetofossils are dominant for the most part, while an interval at around 7 m in core depth shows higher abundance of the bullet‐shaped magnetofossils. This may reflect enhanced organic carbon flux to the sediments. The organic carbon content is low throughout the sediments, and it does not show any change corresponding to the increase of bullet‐shaped magnetofossils, pointing at extensive remineralization of the organic carbon. On the basis of lithostratigraphic correlation to nearby drilling sites, we tentatively estimate the age of the bullet‐shaped magnetofossil increase as sometime between ∼75 and 25 Ma. These results suggest that environmental information can be obtained from magnetofossils in pelagic red clay.

Magnetic hysteresis properties and 57Fe Mössbauer spectroscopy of iron and stony-iron meteorites: Implications for mineralogy and thermal history

Since the solid matter in our solar system began to assemble 4.57 billion years ago, meteorites have recorded a large range of processes, including metamorphism, melting, irradiation and hypervelocity impacts. These processes as well as solar system magnetic fields can be accessed through the investigation of magnetic properties of meteorites. In this work, we present magnetic hysteresis properties, isothermal remanent magnetization acquisition curves and 57Fe Mössbauer spectra for nineteen iron and eleven stony-iron meteorites. These data will be the background for a discussion about the thermal and shock history of these meteorites. Although Mössbauer spectroscopy and hysteresis measurements are not able to provide cooling rates like the conventional metallographic method does, we show that the combination of the ordering degree of taenite phase measured by Mössbauer spectroscopy and hysteresis properties are useful for constraining the thermal and shock history of meteorites. In particular, strong shock and the associated thermal event that result in disordering of tetrataenite can be easily identified.

Can a primary remanence be retrieved from partially remagnetized Eocence volcanic rocks in the Nanmulin Basin (southern Tibet) to date the India‐Asia collision?

AbstractPaleomagnetic dating of the India‐Asia collision hinges on determining the Paleogene latitude of the Lhasa terrane (southern Tibet). Reported latitudes range from 5°N to 30°N, however, leading to contrasting paleogeographic interpretations. Here we report new data from the Eocene Linzizong volcanic rocks in the Nanmulin Basin, which previously yielded data suggesting a low paleolatitude (~10°N). New zircon U‐Pb dates indicate an age of ~52 Ma. Negative fold tests, however, demonstrate that the isolated characteristic remanent magnetizations, with notably varying inclinations, are not primary. Rock magnetic analyses, end‐member modeling of isothermal remanent magnetization acquisition curves, and petrographic observations are consistent with variable degrees of posttilting remagnetization due to low‐temperature alteration of primary magmatic titanomagnetite and the formation of secondary pigmentary hematite that unblock simultaneously. Previously reported paleomagnetic data from the Nanmulin Basin implying low paleolatitude should thus not be used to estimate the time and latitude of the India‐Asia collision. We show that the paleomagnetic inclinations vary linearly with the contribution of secondary hematite to saturation isothermal remanent magnetization. We tentatively propose a new method to recover a primary remanence with inclination of 38.1° (35.7°, 40.5°) (95% significance) and a secondary remanence with inclination of 42.9° (41.5°,44.4°) (95% significance). The paleolatitude defined by the modeled primary remanence—21°N (19.8°N, 23.1°N)—is consistent with the regional compilation of published results from pristine volcanic rocks and sedimentary rocks of the upper Linzizong Group corrected for inclination shallowing. The start of the Tibetan Himalaya‐Asia collision was situated at ~20°N and took place by ~50 Ma.

Untangling inconsistent magnetic polarity records through an integrated rock magnetic analysis: A case study on Neogene sections in East Timor

AbstractInconsistent polarity patterns in sediments are a common problem in magnetostratigraphic and paleomagnetic research. Multiple magnetic mineral generations result in such remanence “haystacks.” Here we test whether end‐member modeling of isothermal remanent magnetization acquisition curves as a basis for an integrated rock magnetic and microscopic analysis is capable of isolating original magnetic polarity patterns. Uppermost Miocene‐Pliocene deep‐marine siliciclastics and limestones in East Timor, originally sampled to constrain the uplift history of the young Timor orogeny, serve as case study. An apparently straightforward polarity record was obtained that, however, proved impossible to reconcile with the associated biostratigraphy. Our analysis distinguished two magnetic end‐members for each section, which result from various greigite suites and a detrital magnetite suite. The latter yields largely viscous remanence signals and is deemed unsuited. The greigite suites are late diagenetic in the Cailaco River section and early diagenetic, thus reliable, in the Viqueque Type section. By selecting reliable sample levels based on a quality index, a revised polarity pattern of the latter section is obtained: consistent with the biostratigraphy and unequivocally correlatable to the Geomagnetic Polarity Time Scale. Although the Cailaco River section lacks a reliable magnetostratigraphy, it does record a significant postremagnetization tectonic rotation. Our results shows that the application of well‐designed rock magnetic research, based on the end‐member model and integrated with microscopy and paleomagnetic data, can unravel complex and seemingly inconsistent polarity patterns. We recommend this approach to assess the veracity of the polarity of strata with complex magnetic mineralogy.

Magnetic and diffuse reflectance spectroscopic characterization of iron oxides in the tidal flat sequence from the coastal plain of Jiangsu Province, China

In this paper, we present the results of research on a coastal lowland Holocene tidal flat sequence in order to explore the variations in magnetic mineralogy during the process of tidal flat sedimentation and subsequent land formation. Three cores (∼6 m in length) were collected from the coastal plain in Jiangsu Province, China, and investigated with magnetic measurements, diffuse reflectance spectroscopy (DRS) and free iron oxide (Fed) analyses. The tidal flat sequence shows a fining-upward trend, and the top ∼2 m of each core with redoximorphic feature was interpreted to be a salt marsh facies in origin. Unmixing of isothermal remanent magnetization acquisition curves identify magnetite and maghemite as well as high-coercivity hematite and goethite, with the latter iron oxides confirmed by DRS analysis. Enrichment of goethite, hematite and maghemite occurs in the salt marsh deposits in comparison to the lower intertidal and subtidal deposits, with goethite being the dominant iron oxide. The changes in hard isothermal remanent magnetization acquired in field above 100 mT field (HIRM100) and the S–100 ratio largely reflect the presence of medium-coercivity maghemite in absolute content and relative proportions, respectively. The approach used for iron oxide characterization may offer an efficient diagnostic tool for recognizing sediments or soils subjected to redox condition oscillation elsewhere and contribute to studies of iron cycling in (palaeo-)environmental researches.

Micromagnetics and magnetomineralogy of ultrafine magnetite inclusions in the Modipe Gabbro

Iron oxide inclusions in pyroxene crystals from the Modipe Gabbro, Botswana, have been studied to determine their recording fidelity. Hysteresis parameters, first‐order reversal curves, isothermal remanent magnetization acquisition curves, and thermomagnetic data all indicate that the iron oxide occurs as stoichiometric magnetite in the form of single‐domain and/or small pseudo single‐domain particles. Using distributions for the grain shape and nearest‐neighbor spacings determined from optical micrographs as input, a first‐order reversal curve (FORC) diagram was simulated using a numerical micromagnetic model. The simulated FORC diagram was found to closely match the measured FORC distributions. Analysis of the interaction fields in the model found that the standard deviation of the interaction field distribution was 2.7 mT compared to a bulk coercivity of ~20 mT, suggesting that the majority of particles are unlikely to be affected by magnetostatic interactions. Thellier analysis of the samples induced with an initial laboratory thermoremanence yielded near‐perfect behavior. It is suggested that the Modipe Gabbro dated at 2784.0 ± 1.5 (2σ) Ma is potentially an ideal recorder of the geomagnetic field.

A method for unmixing magnetic hysteresis loops

Hysteresis loops provide essential information concerning both induced and remanent magnetizations and are an important tool for characterizing magnetic mineral assemblages. Although the hysteresis behavior of mixed natural magnetic assemblages has been a focal point of much recent work, little progress has been made in unmixing of hysteresis loops into characteristic components. Unmixing strategies can act as cornerstones for interpretation of rock magnetic data and have become popular for characterizing isothermal remanent magnetization acquisition curves. Unmixing of hysteresis loops is, however, a challenging task because the individual component loops in the mixture must meet stringent shape constraints. We present a new technique for decomposing an ensemble of hysteresis loops into a small number of end‐members based on linear mixing theory. The end‐members are not based on type curves but instead are derived directly from the hysteresis data. Particular attention is paid to the form of the end‐members, ensuring they meet the shape constraints expected for hysteresis loops of natural magnetic mineral assemblages. Marine sediments from the Southern Ocean and lake sediments from Butte Valley, northern California, provide case studies on which the proposed unmixing method is tested.

Rock magnetic properties of drill cutting from a hydrocarbon exploratory well and their relationship to hydrocarbon presence and petrophysical properties

AbstractThe Golfo San Jorge Basin is one of the most important hydrocarbon-producing basins in Argentina. A study of magnetic properties performed on drill cutting from an oil well drilled in this basin was carried out. The cutting samples, taken from an interval of about 400 m thickness, correspond to the upper units of the Pozo D-129 Formation, the main source rock of the basin. Magnetic susceptibility measurements were made and rock-magnetism studies were conducted. Concentration indices of the different magnetic species determined were calculated based on isothermal remanent magnetization acquisition curves. A correlation analysis among magnetic properties, hydrocarbon content and well logs (sonic, neutron, density, induction, resistivity and photoelectric factor) was performed. Several kinds of significant correlations were found: a positive correlation between susceptibility and relative hydrocarbon content; a positive correlation between magnetic properties and porosities (especially good with the neutron log porosity); and a negative correlation between the concentration indices of some magnetic species (magnetite and pyrrhotite) and resistivity. Pyrrhotite could be directly related to the presence or migration of hydrocarbons through the porous units.The qualitative correlations between magnetic data and key petrophysical parameters such as porosity, along with the association of magnetic mineralogy to hydrocarbon presence or migration, suggest the potential usefulness of these techniques for subsurface exploration.

End-member modelling as an aid to diagnose remagnetization: a brief review

AbstractRemagnetization of a palaeomagnetic signal is difficult to recognize independently of directional information. The situation becomes more complex when remagnetized rocks pass palaeomagnetic field tests, for example when the remagnetization of a rock sequence has occurred before folding. It is evident that palaeogeographic reconstructions are seriously flawed when actually remagnetized rocks are not identified as such. Here we discuss the merits and pitfalls of so-called end-member modelling of acquisition curves of the isothermal remanent magnetization (IRM) to recognize remagnetized strata. The technique requires no a priori information about the IRM acquisition curves. The algorithm unmixes a set of IRM acquisition curves into a number of invariant curves termed end members and calculates the mixing proportions of the end members for each sample. Since primary natural remanent magnetization (NRM) and remagnetized NRM are acquired by different processes, their signatures can be recognized from subtle differences in the magnetic properties. We illustrate the potential of the approach by three case studies, one from Spain and two from Turkey, in which the magnetic properties of remagnetized and non-remagnetized rocks are evaluated.

Rock magnetic characterization of early and late diagenesis in a stratigraphic well from the Llanos foreland basin (Eastern Colombia)

AbstractWe have carried out rock magnetic characterizations of different lithofacies along the stratigraphic well Saltarín 1A, in order to learn about the various diagenetic events that could have affected the Miocene sequence of the Llanos foreland basin (Colombia). Thermomagnetic and low-temperature susceptibility measurements performed on some selected samples were complemented with analyses of Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX) and isothermal remanent magnetization (IRM) acquisition curves. The identification of the magnetic mineral assemblages at each depth level analysed, as well as their relative concentrations, were determined from a direct signal analysis (DSA) of the IRM curves. Samples from the top of the Guayabo formation reveal the presence of hydrocarbons-related microscopic framboids of pyrite with partial replacement of magnetite. The bottom of the Guayabo formation shows hematite and goethite and appears to record a thoroughly documented Middle Miocene global regression. In samples from the León formation, pyrrhotite could have resulted from an early diagenesis that took place in a lacustrine environment via sulphate reduction. Traces of crude oil in samples from the Carbonera formation, and the additional occurrence of hematite and magnetite, suggest that a hydrocarbons-mediated late diagenesis could also have affected the lowermost levels of Saltarín 1A.