Remanence Intensity Research Articles

On low-latitude virtual geomagnetic poles in Icelandic basalt lava sequences

In two paleomagnetic studies from the Neogene lava sequences of Iceland published in 1977, of the order of 10% of the reliably determined remanence directions corresponded to virtual pole positions below 40° latitude. This was a larger proportion than found in most other volcanic locations. To explain it, Harrison [Harrison, C.G.A., 1980. Secular variation and excursions of the Earth's magnetic field. J. Geophys. Res. 85, 3511–3522] suggested that some 10% of the observed virtual geomagnetic poles (VGP) from Iceland might be distributed at random over the globe. These “random poles” can be envisaged as dating from periods when the geomagnetic field is weak and dominated by variable non-dipole terms. Much additional data on remanence directions and intensities has been collected since then, and the present study analyses results from over 3500 lava flows in Iceland. The relative frequency and strength of virtual dipoles in this collection vary with the pole latitude in a similar way as in the collection studied by Harrison. Normalizing of remanence intensity data by the use of anhysteretic remanence (ARM) may improve the resolution of this type of analysis.

Calibration of magnetic granulometric trends in oceanic basalts

The validity of magnetic granulometric estimates relies heavily on the ability to distinguish ultrafine particles from coarser grains. For example, populations with dominantly superparamagnetic (SP) or multidomain (MD) grains both are characterized by low remanence and coercivity, and distinguishing these endmembers may provide valuable clues to the origin of magnetization in the intervening stable single domain (SD) size range. The natural grain size variations associated with variable cooling rates in submarine lavas provide a rare opportunity for examining progressive changes in average magnetic grain size, from SP–SD mixtures in submarine basaltic glass to SD–MD mixtures in flow interiors. Based on microanalysis and rock magnetic measurements on pillow basalt samples dredged from the flanks of the Mid-Atlantic Ridge (ages <1 Ma to 70 Ma), a model of preferential dissolution with time of the finest-grained titanomagnetites has recently been suggested as the major process contributing to long-term temporal changes in remanent intensity of mid-ocean ridge basalts. We evaluated the local and long-term temporal trends in effective magnetic grain size predicted by this model using hysteresis data from a large number of submarine basalt samples which span a range of ages from ∼0 to ∼122 Ma. Specimens were systematically taken along transects perpendicular to the chilled margin of each sample. The large number of data (∼750 loops) and the inferred progressive change in grain size approaching the chilled margin allow recognition of mixing trends between MD and SD grains and between SD and SP grains on a Day-plot. These trends in hysteresis parameters are crucial to resolving the inherent, but frequently overlooked, ambiguity in inferring grain size from hysteresis parameters. We illustrate that two additional rock magnetic tests (warming of a low-temperature isothermal remanence and hysteresis loop shapes) often used to address these ambiguities are inconclusive, requiring some independent knowledge of whether SP or MD grains are likely to be present. Even with a considerably larger data set the substantial intrasample variability in oceanic basalts precludes recognition of any systematic trend in magnetic grain size with age.

Geomagnetic long-term secular variations in Italian Lower Cretaceous shallow-water carbonates

SUMMARY The remanent magnetic properties of an 88 m bore core are unrelated to either the dolomite content or the sedimentological textures and are considered to be carried primarily by biogenetic magnetite that was cemented in during very early diagenesis. Individual readings represent time intervals of c. 720±32 yr and, after 40 mT partial demagnetization, they provide an almost continuous record of averaged geomagnetic secular variations over a period of some 3.17 Myr. The magnitude of directional secular variation is twice that of the present day, despite being smoothed, and the secular variations appear to grade into polarity transitions, suggesting no diVerence in their mechanisms. The rates of change in direction between subjacent levels in the core have a log-normal distribution which extends smoothly beyond 90° and has a median value of 13°/700 yr, the same as for unsmoothed European secular variation during the last 2000 yr. The intensity of remanence, after 40 mT partial demagnetization, appears to provide a reasonable approximation to geomagnetic field intensity. This tends to be weaker when the direction is moving faster, reflecting averaging, but is unrelated to the distance of the vector from the mean direction; that is, it depends on the rate of change and not on the virtual pole latitude. The virtual poles, after correction for tectonic rotations about horizontal and vertical axes, have latitudes that form a strongly platykurtic Fisherian distribution, while their longitudes have a circular distribution on which are superimposed two Gaussian peaks, 180° apart. This bore core thus provides detailed information of smoothed geomagnetic secular variation in the Lower Cretaceous (127±3 Ma) which shows clear regularities in behaviour, some related to changes in the Earth’s orbital parameters.

Secular variation and reversals in a composite 2.5 km thick lava section in central Western Iceland

The direction and intensity of primary remanence has been measured in oriented specimens from 367 lava flows of Late Miocene age in Western Iceland. The lavas which were sampled in 8 overlapping profiles, were generally good material for paleomagnetic measurements. In a 2500-m composite section, at least 15 reversals of polarity and several excursions are recorded. The mean remanence direction and other overall paleomagnetic parameters for the present collection of lavas are similar to those found elsewhere in Iceland. The average rate of buildup of this lava pile was rather low and possibly episodic. Hence, correlations to the geomagnetic polarity time scale and to polarity patterns in other composite sections mapped in Iceland are not straightforward.

Low-temperature properties of a single crystal of magnetite oriented along principal magnetic axes

We have measured saturation induced and remanent magnetizations and induced magnetization as a function of field at low temperatures, between 300 K and 10 K, on an oriented 1.5-mm single crystal of magnetite. The induced magnetization curves along the cubic [001], [1 1 ̄ 0] , and [110] axes at 10 K have very different approaches to saturation. The crystal is easy to magnetize along [001] but difficult along [1 1 ̄ 0] and [110], the hard directions of magnetization for monoclinic magnetite. The temperature dependence of saturation magnetization between the Verwey transition temperature, T v = 119 K, and 10 K is also different along the three axes, indicating that below T v the crystal has uniaxial symmetry. The room-temperature saturation remanence (SIRM) produced along [001] decreases continuously in the course of zero-field cooling, levelling out at the isotropic temperature, T i = 130 K, where the first magnetocrystalline anisotropy constant becomes zero. At T i, 86% of the initial SIRM was demagnetized. The domain wall pinning responsible for this soft remanence fraction must be magnetocrystalline controlled. The remaining 14% of the SIRM is temperature independent between T i and T v and must be magnetoelastically pinned. This surviving hard remanence is the core of the stable magnetic memory. The Verwey transition at 119 K, where the crystal structure changes from cubic to monoclinic, is marked by a discontinuous increase in remanence, indicating that the cubic [001] direction suddenly becomes an easy direction of magnetization. The formation of monoclinic twins may also affect the intensity of remanence below T v. Reheating from 10 K retraces the cooling curve, with a decrease at T v back to the original remanence level, which is maintained to 300 K. When SIRM is not along [001], the initial SIRM is larger but the reversible changes across the Verwey transition are much smaller. The SIRM produced at 20 K is an order of magnitude larger than the 300 K SIRM, but the only change during warming is a discontinuous and irreversible drop to zero at T v.

Remanent Magnetization in Portland-Cement-Based Materials

The purpose of this research is to demonstrate the possibility of applying one geophysical method to studying untraditional systems as is the case with Portland-cement-based materials. The research demonstrates how conventional paleomagnetic methodology can be employed in studying the mode of magnetic recording in present-day industrial materials. Portland-cement admixtures such as fly ashes and furnace slags should be discriminated, because those particles interact in soils and sediments in nature. Moreover, a better undertanding of magnetic remanent acquisition in model materials can serve to improve the interpretation of magnetic remanent acquisition in natural rocks formed a long time ago. The magnetic constituents of Portland-cement paste and mortar acquire a magnetic remanence due to their alignment with the earth's magnetic field at the casting place. This magnetization can be measured using ordinary paleomagnetic techniques. The alignment of the individual magnetic particles accounts for the intensity of the magnetic remanence, which can be increased by adding water and by vibration before setting and hardening. Blast furnace slag admixtures also add to the enhancement of the intensity of remanence. The magnetization of Portland-cement-based materials shows a near linear relationship with the water /cement (w/c) ratios employed in the experimental work; the w/c ratios range between 0.2 – 0.6 in pastes and 0.3 – 0.6 in mortar. Stable remanent magnetization was obtained during the first seven days of setting and hardening, a period necessary for magnetic particles to become locked parallel to the earth's magnetic field. The stability of magnetic remanence predicts the usefulness of the methodology in studying the properties of Portland cement and particularly in the control of iron-bearing admixtures.

Diagenetic magnetic enhancement of sapropels from the eastern Mediterranean Sea

Diagenetic dissolution of magnetic minerals has been widely observed in organic-rich sediments from many environments. Organic-rich sediments from the eastern Mediterranean Sea (sapropels), recovered during Leg 160 of the Ocean Drilling Program, reveal a surprising catalogue of magnetic properties. Sapropels, from all sites studied across the eastern Mediterranean Sea, are strongly magnetic and the magnetization is directly proportional to the organic carbon content. The magnetization of the sapropels is dominated by a low-coercivity, probably single domain magnetic mineral (with an inverse magnetic fabric) that exhibits a clear decay in magnetic properties when exposed to air. During heating, the magnetic particles irreversibly break down between 360 and 400°C. The contrast between the magnetic properties of sapropels and surrounding sediments is marked, with remanence intensities of sapropels often being more than three orders of magnitude higher than those of underlying sediments. The contrast between the magnetic properties of sapropels and the surrounding sediments is apparently controlled by non-steady-state diagenesis: sulphate-reducing conditions dominated during sapropel deposition, while overlying sediments were deposited under oxic conditions. The mineral responsible for the magnetic properties of sapropels is most likely to have formed under sulphate-reducing conditions that existed during times of sapropel formation. Attempts to identify this mineral have been unsuccessful, but several lines of evidence point toward an unknown ferrimagnetic iron sulphide phase. The influence of diagenesis on the magnetic properties of cyclically-deposited eastern Mediterranean sedimentary sequences suggests that magnetic parameters may be a useful proxy for diagenesis in these sediments.

Late Quaternary Vegetation and Climate Change in the Amazon Basin Based on a 50,000 Year Pollen Record from the Amazon Fan, ODP Site 932

Hemipelagic sediments from the Amazon deep-sea fan, ODP Site 932 (5° 12.7′N, 47° 1.8′W), and continental shelf provide a 50,000-yr-long pollen record of Amazon Basin vegetation. The age model for Hole 932A is constrained by eight magnetic remanence intensity features, one paleomagnetic excursion, and three AMS14C dates.Alchornea,Melastomataceae, Myrtaceae, and Moraceae/Urticaceae are dominant taxa in the pollen record between 40,200 and 19,800 cal yr B.P. Andean taxa, such asPodocarpusandHedyosmum,increase in abundance between 19,800 and 11,000 cal yr B.P. and prior to 40,200 cal yr B.P. The Holocene pollen assemblage, derived from Amazon River and continental shelf sediments, is dominated by secondary growth taxa, such asCecropia.Climatic factors influencing the development of glacial and interglacial tropical vegetation are considered by comparing marine with terrestrial records of vegetation change. This comparison shows that the Amazon Basin forests were not extensively replaced by savanna vegetation during the glacial period, contradicting the refugia hypothesis.

Magnetic surveys south and southeast of Iceland

Aeromagnetic surveys were carried out in 1990–1992 over parts of the insular shelf south and southeast of Iceland, to extend the coverage provided by a 1973 marine survey. Prominent magnetic anomalies associated with the shelf edge over a distance of some 400 km, are interpreted in terms of a continuous basement edge in this region. This edge lies several kilometres landward of the shelf edge as defined from bathymetry, indicating the presence of a sediment lens of the order of 1 km in thickness. The formations making up the shelf seem to form thicker polarity zones and have a higher intensity of remanence than Neogene basalt lava flows and dikes in Iceland, but their age is unknown. Other geophysical information from the shelf area is too limited to constrain modelling of the anomaly sources.

Rock magnetic properties and ore microscopy of the iron ore deposit of Las Truchas, Michoacan, Mexico

Iron ore and host rocks have been sampled (90 oriented samples from 19 sites) from the Las Truchas mine, western Mexico. A broad range of magnetic parameters have been studied to characterize the samples: saturation magnetization, Curie temperature, density, susceptibility, remanence intensity, Koenigsberger ratio, and hysteresis parameters. Magnetic properties are controlled by variations in titanomagnetite content, deuteric oxidation, and hydrothermal alteration. Las Truchas deposit formed by contact metasomatism in a Mesozoic volcano-sedimentary sequence intruded by a batholith, and titanomagnetites underwent intermediate degrees of deuteric oxidation. Post-mineralization hydrothermal alteration, evidenced by pyrite, epidote, sericite, and kaolin, seems to be the major event that affected the minerals and magnetic properties. Magnetite grain sizes in iron ores range from 5 to >200 μm, which suggest dominance of multidomain (MD) states. Curie temperatures are 580±5°C, characteristic of magnetite. Hysteresis parameters indicate that most samples have MD magnetite, some samples pseudo-single domain (PSD), and just a few single domain (SD) particles. AF demagnetization and IRM acquisition indicate that NRM and laboratory remanences are carried by MD magnetite in iron ores and PSD–SD magnetite in host rocks. The Koenigsberger ratio falls in a narrow range between 0.1 and 10, indicating the significance of MD and PSD magnetites.

Widespread chemical remagnetization: Orogenic fluids or burial diagenesis of clays?

Research Article| July 01, 1998 Widespread chemical remagnetization: Orogenic fluids or burial diagenesis of clays? B. Katz; B. Katz 1School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73019 Search for other works by this author on: GSW Google Scholar R. D. Elmore; R. D. Elmore 1School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73019 Search for other works by this author on: GSW Google Scholar M. Cogoini; M. Cogoini 1School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73019 Search for other works by this author on: GSW Google Scholar S. Ferry S. Ferry 2Université Claude Bernard, Centre des Sciences de la Terre, Lyon, F-69622 Villeurbanne Cédéx, France Search for other works by this author on: GSW Google Scholar Geology (1998) 26 (7): 603–606. https://doi.org/10.1130/0091-7613(1998)026<0603:WCROFO>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation B. Katz, R. D. Elmore, M. Cogoini, S. Ferry; Widespread chemical remagnetization: Orogenic fluids or burial diagenesis of clays?. Geology 1998;; 26 (7): 603–606. doi: https://doi.org/10.1130/0091-7613(1998)026<0603:WCROFO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Paleomagnetic, rock magnetic, and geochemical results from Mesozoic carbonates in the Vocontian trough in southeast France support a hypothesized acquisition of a chemical remanent magnetization (CRM) during burial diagenesis of smectite and are inconsistent with an orogenic fluid remagnetization mechanism. The geographic and vertical distribution of a widespread CRM, which resides in magnetite, is parallel to the degree of burial diagenesis of clays. The CRM is absent only where there is no evidence for clay diagenesis. The CRM is poorly developed where significant smectite is still present and is well developed with higher natural and anhysteretic remanent intensities where smectite has completely altered to illite. Strontium isotope results do not support alteration by orogenic-type fluids, a commonly invoked mechanism for similar CRMs. The results suggest that the burial diagenesis of clays is a viable remagnetization mechanism for limestones in the Vocontian trough. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Equatorial western Atlantic Ocean circulation changes linked to the Heinrich events: deep-sea sediment evidence from the Amazon Fan

Abstract The Amazon Fan is an excellent area to obtain climatic records of the last glacial-interglacial cycle. Glacial sedimentation rates ranging from 1 to over 50 m ka −1 provide an opportunity to obtain marine records approaching the resolution of the Greenland ice cores. Ocean Drilling Program Leg 155 Sites 932 and 933 from the Eastern Middle Amazon Fan complex have been studied at a resolution of 50 cm (100–400 years). The identification of palaeomagnetic excursions, distinctive patterns in the palaeomagnetic remanence intensity records and AMS 14 C dates have provided an age framework for both Sites 932 and 933. However, due to extensive reworking in the Site 933 records, it was only possible to construct a detailed age model for Site 932. Planktonic foraminiferal stable oxygen and carbon isotope records of six species were obtained for both sites. The planktonic foraminiferal carbon isotope records of both Sites 932 and 933 show a distinct negative deviation during Termination I (13–15 calendar ka). We suggest this could have been caused by an increase in the sediment discharge of the Amazon River because of the deglaciation of the Andes and/or the release of significant quantities of gas hydrates as large parts of the Amazon Fan sediment column failed because of increased sea level. Positive δO deviations are also observed in the Site 932 records which seem to be coeval with the Heinrich events. It is suggested that the enhanced ice-rafting in the North Atlantic during the Heinrich events increased the latitudinal thermal gradient and thus the zonal component of the wind system. This reduced the northward penetration of the Inter-Tropical Convergence Zone, curtailing the cross-equatorial export of the North Brazilian Coastal Current (NBCC) and resulting in a permanent NBCC-reroflection. This increased the surface water salinity of the Amazon Fan and thus led to the observed positive δO deviations.

Magnetization of axial lavas from the southern East Pacific Rise (14°–23°S): Geochemical controls on magnetic properties

Although the spatial association of iron‐rich lavas and high‐amplitude magnetic anomalies is well documented, a causal link between enhanced iron content and high remanent magnetization has been difficult to establish. Here we report magnetic data from approximately 250 samples, with 8–16% FeO* (total iron as FeO), from the southern East Pacific Rise (EPR) that provide strong support for the presumed geochemical dependence of remanent intensity. The limited age range (0–6 ka) of axial lavas from this ultrafast spreading ridge (∼150 mm/yr full rate) effectively minimizes variations resulting from time dependent changes in geomagnetic intensity or low‐temperature alteration. Systematic sampling relative to the chilled margin illustrates that substantial grain size‐related variations in magnetic properties occur on a centimeter scale. Both microprobe data and Curie temperatures suggest that the average groundmass titanomagnetite composition in the southern EPR samples is approximately constant (modal modified ulvöspinel content = 0.67) over a wide range of lava compositions. Saturation magnetization and saturation remanence are highly correlated with FeO* (R = 0.73 and 0.83, respectively), indicating that more iron‐rich lavas have higher abundances of otherwise similar titanomagnetite. We show that there is a good correlation between natural remanent magnetization (NRM) and FeO*, provided that sufficient specimens are used to determine the average NRM of a sample (R=0.63). Because the range of iron contents in mid‐ocean ridge basalts is limited, the best fit slope (4.44 A/m per %FeO* in an ambient field of 0.030 mT) should provide reasonable bounds on the equatorial magnetization of submarine lavas (∼10 A/m at 8.5% FeO* and ∼50 A/m at ∼16% FeO*). Finally, we demonstrate that along‐axis variations in NRM closely parallel geochemical changes along the southern EPR. Where magnetization values deviate significantly from those predicted from the range of measured FeO* contents, these discrepancies may reflect additional unrecognized geochemical variability.

High-resolution rock-magnetic variability in shallow marine sediment: a sensitive paleoclimatic metronome

An outer shelf deposit in central Japan centered on the Olduvai normal polarity event in the reversed Matuyama chron reveals a close correlation of both the magnetic susceptibility and remanent intensity with the sedimentary cyclicities apparent in lithologies and molluscan assemblages. Two sedimentary cycles are characterized by distinctly similar, but double-peaked magnetic cyclicities. The rock-magnetic variability is primarily attributed to the relative abundance of terrigenous magnetic minerals, and the double peak of the variability is characterized by the concentration of finer-grained magnetic minerals. The concentration is suspected to be controlled by both climatic change and shifting proximity of the shoreline as a function of rise and fall of the sea level due to glacio-eustasy. Rock-magnetic study reveals the record of a 21 ka period of orbital precession cycles within the sedimentary cyclicity attributable to a 41 ka period of orbital obliquity forcing.

Variation of Rock Magnetic Parameters and Paleointensities over a Single Holocene Lava Flow.

A recent (2000 BP) lava flow from central Mexico has been sampled along a vertical profile with 55 cores covering a total flow thickness of 6.6 m. A wide range of physical and magnetic parameters have been studied to characterise the samples: Curie temperature and saturation magnetisation as intrinsic properties; density, magnetic susceptibility and remanence intensity as bulk properties; Konigsberger Q-factor and hysteresis parameters as coercivity parameters. All parameters vary smoothly over the profile, most probably due to grain size variation of the magnetic minerals present in the samples. Optical observations indicate that the main opaque minerals are deuterically oxidised titanomagnetites (C3-C5) and ilmenites (R2-R3), which increase in size away from the edges of the flow. Paleointensity (PI) was determined using the double heating Thellier-Thellier method with pTRM checks. According to reliability parameters (f-, g-, and q-factor) the obtained PIs are of reasonable to good quality. PI shows marked variation with vertical position in the flow, across a range of about 25 to 125 μT, with most samples having a PI between 50 and 100 μT. The flow-mean PI of 72 μT is higher than the present day field, consistent with global data for this time-period. No obvious correlation could be found between PI and any other measured parameter. The variation of PI with vertical position in the flow may show some systematic behaviour. It is important, therefore, to sample a flow both horizontally and vertically in order to obtain a reliable paleointensity.

Magnetic properties of the stable fraction of remanence in large multidomain (MD) magnetite grains: Single‐domain or MD?

It has been recognized since the early work of Verhoogen (1959) that a considerable proportion of remanence in multidomain (MD) magnetite grains is resistant to low‐field a.f. or low‐temperature demagnetization. The source of this high stability is still a matter of debate. A number of workers have suggested that MD grains of all sizes contain a remanence fraction with truly single domain (SD) character. We suggest that the critical diagnostic features which should be investigated to determine whether the high stability fraction is SD or MD in character are whether blocking (Tb) and unblocking (Tub) temperatures are equivalent, and whether the intensity of remanence is affected by the thermal pre‐history of the sample. We have carried out such experiments on samples containing crushed natural magnetites in 7 grain sizes from 5–10 µm, to 100–150 µm. We show that Tb and Tub are equivalent for pTRM40020 for grain sizes up to 15–20 µm, but that Tub extends up to the Curie temperature for larger grain sizes. We also show that the stable fraction of MD TRM and pTRM has the same dependence on pre‐history as the total TRM. Our experiments demonstrate that the stable fraction has magnetic properties which are truly MD in character for magnetite grains larger than 20 µm.

A Statistical Model of the Fluctuations in the Geomagnetic Field from Paleosecular Variation to Reversal

The statistical characteristics of the local magnetic field of Earth during paleosecular variation, excursions, and reversals are described on the basis of a database that gathers the cleaned mean direction and average remanent intensity of 2741 lava flows that have erupted over the last 20 million years. A model consisting of a normally distributed axial dipole component plus an independent isotropic set of vectors with a Maxwellian distribution that simulates secular variation fits the range of geomagnetic fluctuations, in terms of both direction and intensity. This result suggests that the magnitude of secular variation vectors is independent of the magnitude of Earth's axial dipole moment and that the amplitude of secular variation is unchanged during reversals.

A Phenomenological Model of Orientational Magnetic Anisotropy of Sediments

SUMMARY A phenomenological model has been proposed to investigate a magnetic anisotropy induced in sediments by a magnetic field applied during sedimentation. It is assumed that the aligning effect of the sedimentation field produces a Fisher distribution of grain magnetic moments. Thus, anhysteretic remanence (ARM) acquisition and ARM anisotropy were investigated for assemblages of identical grains with known intrinsic (micro) anisotropy. We have considered uniaxial grains, grains with two and three mutually orthogonal axes of easiest magnetization, and grains with four easiest axes directed along spatial diagonals of a cube. We have derived explicit formulae for the intensity and direction of anhysteretic remanence as a function of alignment degree and the orientation of the anhysteretic magnetization field with respect to the sedimentation field. Computation shows that for weakly aligned magnetic moments the ARM anisotropy of an assemblage of uniaxial grains is ellipsoidal, while for multiaxial grains under the same conditions it can be ignored. For assemblages with highly aligned grains, the ARM anisotropy is dependent on the intrinsic anisotropy of the grains.

Relative paleointensity of the geomagnetic field during the Brunhes Chron

We conducted a paleomagnetic study of four sediment cores taken from different regions in the low latitudes of the Pacific Ocean in order to recover relative paleointensity variations during the Brunhes chron. Three cores have good age control from oxygen isotope stratigraphy. We extensively examined rock magnetic uniformity by the iso-thermal remanent magnetization (IRM) acquisition and the alternating-field demagnetization of the anhysteretic remanent magnetization (ARM) and the IRM. Only rock-magnetically homogeneous sediments were used. Biogenic magnetites were found in all but one core and their dissolution in early diagenesis was not significant. The four cores showed coherent variations of the remanent intensity normalized by the ARM, implying that these variations represent the behavior of the geomagnetic field. Long-term paleointensity variations during the Brunhes chron are characterized by an asymmetric pattern. Rapid increases in the intensity occurred just after the Brunhes-Matuyama transition, in the mid-Brunhes from about 500 to 400 ka, and during the last 100 ka, and gradual decreases between them. The first one supports the idea of rapid regeneration of the geomagnetic field after polarity reversals [1]. The other two may have been triggered by very short polarity reversals.

Rock magnetism and magnetic surveys in the iron ore deposit of El Encino, Mexico

In this study, we report results of petrophysical properties such as bulk (low field) magnetic susceptibility, remanent magnetization, Köenigsberger ratios, density, unblocking temperature spectra, isothermal remanent magnetization acquisition, and thermomagnetic analysis from surface and borehole samples of rock units and iron ore bodies in the iron mining district of El Encino, Jalisco, southwestern Mexico. Iron mineralization consists mainly of magnetite (from 65% to 90% volume) and hematite. It is associated with granodioritic intrusions and volcanic units in a mid-late Cretaceous volcano-sedimentary sequence. The magnetite in the iron ore has generally large grain size and a multidomain behavior. Stable remanent magnetization components associated with pseudo- or single domain behaviors are also observed. Iron ore bodies and associated intrusives show large dipolar magnetic anomalies. For the iron ore bodies examined, either the induced or the remanent component is responsible for the magnetic anomaly response. Data on bulk susceptibility, remanent intensity, and direction provide important constraints for modeling the magnetometric anomalies i.e., by quantifying the relative contributions of induced and remanent magnetization components; and thus permitting a better control of the geometry of source bodies.