Paleomagnetic and environmental magnetic studies are commonly conducted on samples containing mixtures of magnetic minerals and/or grain sizes. Major hysteresis loops are routinely used to provide information about variations in magnetic mineralogy and grain size. Standard hysteresis parameters, however, provide a measure of the bulk magnetic properties, rather than enabling discrimination between the magnetic components that contribute to the magnetization of a sample. By contrast, first-order reversal curve (FORC) diagrams, which we describe here, can be used to identify and discriminate between the different components in a mixed magnetic mineral assemblage. We use magnetization data from a class of partial hysteresis curves known as first-order reversal curves (FORCs) and transform the data into contour plots (FORC diagrams) of a two-dimensional distribution function. The FORC distribution provides information about particle switching fields and local interaction fields for the assemblage of magnetic particles within a sample. Superparamagnetic, single-domain, and multidomain grains, as well as magnetostatic interactions, all produce characteristic and distinct manifestations on a FORC diagram. Our results indicate that FORC diagrams can be used to characterize a wide range of natural samples and that they provide more detailed information about the magnetic particles in a sample than standard interpretational schemes which employ hysteresis data. It will be necessary to further develop the technique to enable a more quantitative interpretation of magnetic assemblages; however, even qualitative interpretation of FORC diagrams removes many of the ambiguities that are inherent to hysteresis data.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JB900326

First-order reversal curve diagrams: A new tool for characterizing the magnetic properties of natural samples

Our research on FORC diagrams has
been supported over the years by the
U.S. National Science Foundation, the UK
Natural Environment Research Council,
the European Union, the Royal Society of
London, and the Australian Research
Council (grant DP120103952).

/pdf/understanding-fine-magnetic-particle-systems-through-use-of-2qzqfmjvfs.pdf

Understanding fine magnetic particle systems through use of first???order reversal curve diagrams

http://web.gps.caltech.edu/~jkirschvink/pdfs/magnetofossils.pdf

Elongated prismatic magnetite crystals in ALH84001 carbonate globules: Potential Martian magnetofossils

SUMMARY Sediments and soils often contain superparamagnetic (SP) magnetite or maghemite grains that cause a frequency dependence of low-field susceptibility X fd which does not exceed 15 per cent/decade of frequency. Present models predict very diVerent volume distributions for samples with the largest observed frequency dependence of susceptibility. While Stephensons’ (1971) power-law model predicts most grains to be smaller than the stable single domain (SSD) threshold, the phenomenological model of Dearing et al. (1996) suggests that most grains are between 10 and 25 nm in diameter. Finally, the recent calculations of Eyre (1997) indicate very broad volume distributions. This study reviews the nature of the superparamagnetic‐stable single domain (SP‐SSD) transition. The change of AC susceptibilities with grain size (or temperature) at the SP‐SSD boundary is more gradual than commonly assumed. When distributions of particle coercivities and volumes are also considered, X fd values are much smaller than those calculated by Eyre (1997). Nonetheless, X fd can be larger than 15 per cent, and a larger frequency dependence has indeed been measured for some samples. The question whether the observed limited X fd of soils and sediments is a result of a broad distribution or of a bimodal distribution, where SP and SSD grains are restricted to a certain relative abundance, can potentially be answered by susceptibility determinations at more than two frequencies and by measurements of the temperature dependence of susceptibility.

On the superparamagnetic—stable single domain transition for magnetite, and frequency dependence of susceptibility

Estimates of the depth to the Curie temperature isotherm in Nevada are in accordance with other regional geologic and geophysical information and together can be explained in the context of present-day tectonism. A method to estimate the depth extent of magnetic sources from the statistical properties of magnetic anomalies was applied to a statewide compilation of aeromagnetic data from Nevada. Basal depths of magnetic sources show no apparent correlation with the so-called magnetic quiet zone, which trends northerly through the eastern part of the state, or with basin-and-range topography. However, certain correlations with published heat flow measurements are apparent and suggest that undulations in basal depth of magnetic sources are related in part to undulations in the Curie temperature isotherm. For example, an area of shallow basal depth (<10 km) near Battle Mountain corresponds to an area of exceptionally high conductive heat flow and indicates a shallow depth to the Curie temperature isotherm in this region. A narrow zone of shallow basal depth extends south from the Battle Mountain area along the 118°W meridian to at least latitude 38°N, which also is a zone of historic surface offsets and high-magnitude earthquakes. The correspondence along the 118° meridian of shallow basal depth, high heat flow, high lower crustal seismic velocities, attenuated P and S wave arrivals, historic faulting, and large earthquakes suggests that they each are related to an active north trending spreading zone in this part of the Basin and Range province.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/JB093iB10p11817%4010.1002/%28ISSN%292169-9356.COX1

Curie temperature isotherm analysis and tectonic implications of aeromagnetic data from Nevada

During the last 20 years, studies have been conducted regarding the regional, or long-wavelength, geomagnetic anomalies of the lower crust of the earth. The present paper is mainly concerned with the petrologic and geophysical interpretation of the observed features. Attention is given to magnetic mineralogy and magnetic properties of high-grade rocks from Lofoten and Vesteralen, regional geology and geophysics, field measurements and sampling, magnetic mineralogy of the lower crust, the effects of metamorphism upon magnetic mineralogy and magnetic properties of high-grade rocks, susceptibility-temperature relations and the Hopkinson effect, and petrologic sources of regional crustal magnetic anomalies.

Magnetization of lower crust and interpretation of regional magnetic anomalies: Example from Lofoten and Vesterålen, Norway

Although it is widely recognized that remanent magnetism in ash-flow tuffs is carried by fine-grained Fe oxides, the origin, mineralogy, and significance of such magnetic carriers are not well understood. The authors have obtained transmission electron microscope images of distinctive Fe-oxide microcrystals in rhyolitic samples located 3.8, 7.6, and 18.5 m above the base of a 110-m-thick section of the Miocene Tiva Canyon Member of the Paintbrush Tuff. The Fe-oxide microcrystals are lath shaped and increase in size from 20 x 140 nm in the lowermost sample (near base of the member) to 120 x 800 nm in the uppermost sample (within the flow interior). Microcrystals in this size range are within or close to the range of single-domain grain size for magnetite. Electron diffraction and analytical X-ray data indicate that the microcrystals in the lower two samples are cubic Fe-oxides (magnetite/maghemite), with less than 10 mol% Ti end member, and that those in the uppermost sample are manganiferous hematite. Systematic variations in magnetic properties are consistent with the observed variations in size and mineralogy of the microcrystals. These microcrystals are morphologically distinct from grains that the authors interpret to be fragments of phenocrysts. The morphology and spatial distribution ofmore » the microcrystals as well as their increase in grain size, from the rapidly cooled base of the ash-flow sheet into the flow interior, are consistent with an origin by nucleation and growth from volcanic glass at elevated temperature, subsequent to emplacement.« less

Fe-oxide microcrystals in welded tuff from southern Nevada: origin of remanence carriers by precipitation in volcanic glass

The magnetic susceptibility (χ) and remanent magnetization of an ash flow sheet are profoundly influenced by cooling history after emplacement. Spatial variations in these properties, therefore, provide a potentially useful tool for geological studies. Maxima and minima in χ measured along profiles at Yucca Mountain, Nevada, identify persistent magnetic marker horizons within vitric portions of the Tiva Canyon and underlying Topopah Spring Members of the Paintbrush Tuff. The correlatable variations of χ with vertical position in these units exist over the 8 km of strike examined. The observed stratigraphic changes in magnetic properties reflect variations in amounts and mineralogy of Fe-Ti oxide phenocrysts, and the presence, shape, size, and mineralogy of magnetic Fe-oxide microcrystals that precipitated at high temperature after emplacement of each sheet. Vitrophyre near the top of the Topopah Spring Member has a distinctive maximum in χ, due to titanomagnetite phenocrysts. Susceptibility in the lower portion of the overlying Tiva Canyon Member reaches maximum values of about 1.3×10−2 (SI) near the base of the member, beneath the columnarly jointed vitrophyre, due to the presence of abundant superparamagnetic cubic Fe-oxide. These elongate microcrystals are several tens of nm long, and are typically deformed and tapered. Up section in the Tiva Canyon Member, the susceptibility decreases, going to a minimum within the columnar vitrophyre. At this stratigraphic level the precipitated Fe-oxides are larger and carry intense NRM of up to 30 A/m; χ is only 3×10−3. The whisker-like microcrystals of Fe-oxide, which typically nucleated on what appears to be chain silicate, grew as needles up to 25 μm in length. The size variations of the precipitated Fe-oxides, which were established using transmission electron microscopy (TEM) and petrographic observation, are consistent both with variations in magnetic susceptibility measured at the outcrop and with variations in the intensity of remanent magnetization. Several interpretations of the shape anisotropy of the precipitated Feoxide are possible, including growth by a dislocation mechanism. Additionally, the observed elongation of precipitated microcrystals is consistent with theoretical predictions for growth in a uniaxial stress field. Susceptibility variations as established at the outcrop, as well as in the borehole, offer a potentially useful tool for stratigraphic correlation of ash flow sheets.

Magnetism and magnetic mineralogy of ash flow tuffs from Yucca Mountain, Nevada

Magnetic and optical variations in volcanic glasses of the East African Rift have been investigated by means of experimental and analytical study. The results add to the formal understanding of magnetism in volcanic glasses and give us new perspectives on the nature, origin, and significance of fine-grained magnetic minerals and on the magnetic petrology of glassy rocks. The samples that we have studied in detail, glass shards from the KBS Tuff of northern Kenya, contain a superparamagnetic precipitate of microcrystals, ∼2 to ∼10 nm in size. The precipitate is likely to have formed at elevated temperature, during or subsequent to eruption, about 1.9 m.y. ago. We have studied these microcrystals by means of transmission electron microscopy (TEM), ac and dc magnetic methods, electron spin resonance (ESR), Mossbauer spectroscopy, and optical spectroscopy. Our efforts have concentrated on determining the magnetic mineralogy and understanding the mechanism(s) responsible for the optical and magnetic behavior of these glasses. The particle size determination for the larger of these microcrystals has been achieved using TEM imaging and Mossbauer spectroscopy. The mineralogy is nonstoichiometric magnetite, based on Mossbauer, X ray diffraction, and electron diffraction results. The ratio of saturation remanent magnetization to saturation magnetization for the larger microcrystals is very nearly that predicted by single-domain theory for a uniaxial anisotropy, the origin of which may lie with stress or surface effects. We interpret magnetic and ESR results to infer the possible existence of microcrystals or amorphous clusters of Fe atoms, ∼1 nm or smaller, in KBS glasses that appear homogeneous in high-resolution TEM images. Results from optical absorption experiments show that the color variations in KBS shards arise due to optical absorption by the precipitate, a result consistent with classical electromagnetic theory.

The nature of magnetic single domains in volcanic glasses of the KBS Tuff

Granulites from southwest Lofoten, Norway, often carry intense and stable components of natural remanent magnetization (NRM). Coercivities in excess of 100 mT and blocking temperatures up to 580°–645°C are common. Several phases in these rocks can carry stable components of NRM. Clinopyroxenes (augite with exsolved pigeonite and orthopyroxene) usually contain abundant Fe-Ti oxides, the majority of which are too small to be studied conclusively with visible light. Additionally, these rocks typically contain coarse-grained (∼100–1000 μm) exsolution intergrowths of rhombohedral Fe-Ti oxides and coarse-grained magnetite. We have studied the magnetism and mineralogy of the titanohematite and the inclusion-bearing augites in one granulite sample using transmission electron microscopy (TEM) and magnetic experiments to understand better the remanence properties of naturally occurring Fe-Ti oxides and their relationships to silicate minerals. The coarse-grained titanohematite has a coercivity of 350 mT and a laboratory saturation remanence that unblocks at 570°–600°C. The hysteresis characteristics of this titanohematite are typical of much smaller ∼10μm-sized single domains, which apparently reflects the presence of exsolved ilmenite lamellae, as small as <10 nm in thickness. These lamellae presumably inhibit (1) domain wall nucleation and (2) movement of walls that manage to nucleate. The inclusions in the augites serve as carriers of lower coercivity remanence with blocking temperatures less than about 560°C. Magnetite, hematite, and ilmenite are present in the augite, and intergrowth of the rhombohedral phases, revealed by TEM, persists down to the 20-nm scale. The distribution of the magnetite inclusions in the augite appears to be unrelated to any silicate microstructure, while that of the rhombohedral Fe-Ti oxides is strongly controlled by silicate microstructure, with intergrowths of these oxides commonly decorating lamellae of pigeonite, orthopyroxene, and clinoamphibole within the augite. The NRM of this sample possibly dates from the amphibolite facies metamorphism that the region experienced about 1000 m.y. ago. Paleomagnetic studies of these rocks could readily recover virtual geomagnetic poles dating from this event. In addition to the included oxides, the augites display a number of unusual features that probably result from the protracted polymetamorphic history of these granulites: pigeonite lamellae are not parallel to the b-crystallographic axis, pigeonite lamellae cut across those of orthopyroxene and numerous included minerals occur. Rare earth elements are concentrated within inclusions that reside in the augite.

Charles M. Schlinger

Papers

Magnetization of lower crust and interpretation of regional magnetic anomalies: Example from Lofoten and Vesterålen, Norway

Fe-oxide microcrystals in welded tuff from southern Nevada: origin of remanence carriers by precipitation in volcanic glass

Magnetism and magnetic mineralogy of ash flow tuffs from Yucca Mountain, Nevada

The nature of magnetic single domains in volcanic glasses of the KBS Tuff

Magnetism and transmission electron microscopy of Fe-Ti oxides and pyroxenes in a granulite from Lofoten, Norway