100 research outputs found
A method for the separation of paramagnetic, ferrimagnetic and haematite magnetic subfabrics using high-field torque magnetometry
In this study, the contribution of the paramagnetic, ferrimagnetic and haematite components to the magnetic anisotropy is separated by means of high-field torque magnetometry. Torque measurements at different fields, which are high enough to saturate the ferrimagnetic minerals, however, still low enough that the torque resulting from the haematite is linear with field, allow for the separation of the three magnetic anisotropy components. The method has been applied to haematite single crystals in which no paramagnetic or ferrimagnetic components have been found contribute to the torque signal. The mean direction of the poles to the crystallographic basal plane in the haematite single crystals is subparallel to the minimum-susceptibility direction measured in low-field. The separation analysis has also been applied to highly deformed red beds from the Lower Glarus nappe complex (Switzerland). No ferrimagnetic phases are present in the rocks and, therefore, they cannot contribute to the anisotropy of magnetic susceptibility. The magnetic fabric arises from a paramagnetic subfabric carried by the phyllosilicate minerals and haematite, in which the basal planes of both phases are in the cleavage plane. The measured magnetic lineation seen in low-field anisotropy of magnetic susceptibility appears to be an apparent lineation that arises from a weak girdling of haematite and the paramagnetic minerals conforming the roc
A refined biomonitoring study of airborne particulate matter pollution in Rome, with magnetic measurements on Quercus Ilex tree leaves
Elevated levels of airborne particulate matter (PM) are a current problem for air quality in many major metropolitan areas. Many European cities have tightened the PM limits in the air, due to advances in monitoring PM levels. In order to establish guidelines for monitoring and curbing anthropogenic PM output, a better understanding of its origin, composition and diffusion is required. Biomonitoring of magnetic properties of tree leaves has been suggested previously to be a good approach to measure pollution levels in cities both in space and time. We report on a magnetic biomonitoring study of PM in the city of Rome, conducted from 2005 October to December. We collected approximately 180 different sample sets of tree leaves of Quercus ilex, an evergreen oak widely distributed in Rome, at 112 different locations. Specific magnetic susceptibility χ of the leaf is used as a fast, easy and cost-effective proxy to assess levels of primary anthropogenic airborne PM pollution. Highly polluted areas correlate with high traffic areas, with an average susceptibility value of χ = 3.2 × 10−7 m3 kg−1. Low traffic zones are characterized by values more than an order of magnitude lower at χ = 1.4 × 10−8 m3 kg−1, and the background magnetic susceptibility is around χ = 2.6 × 10−9 m3 kg−1. The data show that distance dependence from the source is the most significant factor for the concentration of magnetic PM, and that pollution levels and sources can be reliably delineated by measuring magnetic susceptibility values on tree leaf samples of Q. ilex. A new protocol for magnetic susceptibility measurements is proposed, in order to account for changes due to water evaporation in the leaves as a function of time after collection of the samples. Additional magnetic analyses, such as acquisition of artificial remanences and hysteresis properties, were used to characterize the mineralogy and grain size of the magnetic PM. The results indicate that the population of ferrimagnetic phases have a homogenous composition and grain size throughout the investigated are
Separation of magnetic subfabrics by high-field, lowtemperature torque measurements
The anisotropy of magnetic susceptibility
(AMS) can serve as a good indicator
of strain in deformed carbonate rocks
with diamagnetic susceptibility (Owens
and Rutter 1978; de Wall 2000). However,
the magnetic fabric due to the
diamagnetic carbonate minerals is usually
very weak and interpretation of the
AMS in these rocks is often complicated
by the presence of paramagnetic
and ferromagnetic phases which overprint
the diamagnetic subfabric. For
this reason contributions from ferromagnetic
and paramagnetic minerals to the
AMS should be separated for a reliable
interpretation of the AMS. Ferromagnetic
contributions to the AMS can be
separated by high-field measurements,
using a torque magnetometer (Martin-
Hernandez and Hirt 2001). The remaining
paramagnetic and diamagnetic
contributions can be discriminated by
their different temperature dependencies.
The paramagnetic susceptibility
increases as an inverse function of temperature,
whereas the diamagnetic part
remains constant. Altogether, AMS
measurements at high fields and low
temperatures allow for the discrimination
of all three subfabrics.
Test measurements with the high-field
torque magnetometer at liquid nitrogen
temperature were performed. It is possible
to keep the specimens at low temperature
over the measurement period using a cryostat. The main problem is the
suppression of mechanical disturbances
during the measurement so that the sensitivity
of the instrument is retained.
The torque of paramagnetic minerals
increases strongly at low temperature
which results in an amplification of the
paramagnetic subfabric. The quantitative
separation of diamagnetic and paramagnetic
subfabric is under investigation.
The result is promising when there
is a significant diamagnetic signal.conferenc
Separation of diamagnetic and paramagnetic anisotropy by high-field, low-temperature torque measurements
The anisotropy of magnetic susceptibility (AMS) of rocks can be composed of contributions from ferromagnetic, paramagnetic and diamagnetic minerals. However, in general the AMS of only one fraction is of interest. While there are several approaches to isolate the ferromagnetic contribution to the AMS, the separation of the diamagnetic from the paramagnetic contribution is still problematic. A new method for the separation of these two contributions based on high-field torque measurements at room and low-temperature is presented. The paramagnetic anisotropy increases at low temperature according to the Curie-Weiss law, whereas the diamagnetic contribution is temperature independent. If the paramagnetic AMS is due to perfectly oblate or prolate minerals and the ratio of the susceptibility differences at two temperatures is known, paramagnetic and diamagnetic AMS can be separated. When measuring in fields high enough to saturate the ferromagnetic phases all three contributions to the AMS can be separated. The separation of paramagnetic and diamagnetic AMS is demonstrated on natural crystals and synthetic calcite-muscovite aggregates. A high-field torque magnetometer, equipped with a cryostat for measurements at 77 K, allows sensitive measurements at two different temperatures. The sensitivity at 77 K is 3 × 10−7 J and standard-sized (palaeomagnetic) samples of 11.4 cm3 can be measured. This new method is especially suited for the investigation of diamagnetic fabrics of impure carbonate rock
Decoupling of paramagnetic and ferrimagnetic AMS development during the experimental chemical compaction of illite shale powder
Inclination shallowing of detrital remanent magnetization in sedimentary strata has solely been constrained for the mechanical processes associated with mud deposition and shallow compaction of clay-rich sediment, even though a significant part of mud diagenesis involves chemical compaction. Here we report, for the first time, on the laboratory simulation of magnetic assemblage development in a chemically compacting illite shale powder of natural origin. The experimental procedure comprised three compaction stages that, when combined, simulate the diagenesis and low-grade metamorphism of illite mud. First, the full extent of load-sensitive mechanical compaction is simulated by room temperature dry axial compression. Subsequently, temperature controlled chemical compaction is initiated by exposing the sample in two stages to amphibolite or granulite facies conditions (temperature is 490 to 750°C and confining pressure is 170 or 300 MPa) both in the absence (confining pressure only) and presence of a deformation stress field (axial compression or confined torsion). Thermodynamic equilibrium in the last two compaction stages was not reached, but illite and mica dehydroxylation initiated, thus providing a wet environment. Magnetic properties were characterized by magnetic susceptibility and its anisotropy (AMS) in both high- and low-applied field. Acquisition of isothermal remanent magnetization (IRM), stepwise three-component thermal de-magnetization of IRM and first-order reversal curves were used to characterize the remanence-bearing minerals. During the chemical compaction experiments ferrimagnetic iron-sulphides formed after reduction of magnetite and detrital pyrite in a low sulphur fugacity environment. The degree of low-field AMS is unaffected by porosity reduction from 15 to ∼1 per cent, regardless of operating conditions and compaction history. High-field paramagnetic AMS increases with compaction for all employed stress regimes and conditions, and is attributed to illite transformation to iron-bearing mica. AMS of authigenic iron-sulphide minerals remained constant during compaction indicating an independence of ferrimagnetic fabric development to chemical compaction in illite shale powder. The decoupling of paramagnetic and ferrimagnetic AMS development during chemical compaction of pelite contrasts with findings from mechanical compaction studie
Measurement of calcite crystallographic-preferred orientations by magnetic anisotropy and comparison to diffraction methods
The anisotropy of magnetic susceptibility
(AMS) of rocks reflects the preferred
orientations of minerals. Therefore
AMS is a quick and easy way to
characterize rock fabrics (Hrouda 1982,Borradaile 1988); the obtained result is also called the magnetic fabric of the
rock. The method has been often used
to measure the orientation of ferromagnetic
minerals, mainly magnetite, but in
recent studies it has been increasingly
used to measure textures of paramagnetic
minerals as phyllosilicates (Lüneburg
et al. 1999, Cifelli et al. 2004).
A further application is the measurement of
diamagnetic textures, especially calcite
textures. Calcite is suitable for the AMS
method, because it has a high magnetic
anisotropy with the minimum susceptibility
along the crystallographic c-axis.
Therefore a preferred orientation of the
c-axes, which can be induced by deformation,
generates a magnetic fabric...conferenc
Correlation of magnetic fabric and crystallographic preferred orientations of naturally deformed carbonate — mica rocks from the Alpi Apuane in Italy and the Damara Orogen in Namibia
The anisotropy of magnetic susceptibility
(AMS) is a time-efficient method to
describe crystallographic preferred orientations
of rocks and has been applied
in a wide field of sedimentary, metamorphic
and magmatic geology. The method, however, suffers from limitations
which mainly result from the
interference of diamagnetic, paramagnetic
and ferromagnetic fabrics (de Wall
2005) — the term ferromagnetism is
used in a wider sense here, including e.g.
ferrimagnetism. The AMS is an integral
parameter which describes a crystallographic
preferred orientation as an ellipsoid.
The quantitative correlation of
the AMS with the crystallographic preferred
orientations should help to allow
a closer view at the applicability and the
limitations of the AMS analysis (see also
Schmidt et al. 2006 a, b)...
The results of this study are based
on a large variety of fabric types of
carbonate-mica marbles and mylonites,
i.e. varying mica content, grain sizes,
grain shapes, types and intensities of
the crystallographic preferred orientation.
The presented first correlations of
the AMS and CPO for the single mineral
phases in general demonstrate a good
matching. Regarding the comparison of
texture types and the AMS, limitations
are possible. While single c-axis maxima
and girdle-like c-axis distributions
can be also distinguished by the AMS,
it is obvious that distinguishing between
these types and the double c-axis type
is not possible at the present stage.conferenc
Magnetic fabric in ilmeniterich norites of the Bjerkreimer-Sokndal Layered Intrusion, Norway
The Bjerkreim-Sokndal (BKS) is a layered
intrusion, located in the Mid-
Proterozoic Egersund anorthosite-norite
province within the Sveconorwegian
province of the Baltic Shield, south
Norway. The layered intrusion formed by influxes of more primitive magma
into more evolved magma to produce
six Megacyclic units (MCU), each of
which can be divided into up to six
subunits. From bottom to top in
each megacycle the rocks consist of
early plagioclase-rich norites, intermediate
hemo-ilmenite-rich norites and later
magnetite-rich norites. Aeromagnetic
maps over the intrusion show large
negative and positive anomalies. A
negative anomaly with amplitude to -
13000 nT at 60m above ground is associated
with hemo-ilmenite-rich norite
layer MCU Ive. This layer IVe contains
plagioclase, orthopyroxene, hemoilmenite,
magnetite, and minor clinopyroxene,
biotite, apatite and sulfides.
Multi-domain (MD) magnetite makes
up 2–3% of the rock.
The negative magnetic anomaly associated
with MCU IVe reaches its most
negative value on the east limb of the
Bjerkreim Lobe near Heskestad. The
anomaly at Heskestad is part of a longer
negative anomaly, which follows MCU
IVe for more than 20 km around a large
syncline. The average NRM intensity
decreases from 25AM−1 along the east
fold limb to 10AM−1 towards the hinge
area to 7AM−1 at the hinge. The
BKS has a penetrative deformation fabric
within the syncline with the weakest
deformation found in the hinge area
and the strongest on the east limb. Electron
backscatter diffraction (EBSD) was
used to determine the lattice-preferred
orientation (LPO) of orthopyroxene and
ilmenite. The (100)-planes of the orthopyroxenes
are found to lie parallel
to a foliation in the rock, which is subparallel
to the cumulate layering. Orthopyroxene
c-axes form the steep lineation
within the foliation plane.
The anisotropy of magnetic susceptibility
(AMS) was measured for samples
that were taken at five locations from
the eastern limb to the hinge area of the
syncline to investigate if the change in
NRM intensity could be related to magnetic
fabric.conferenc
Beyond the second order magnetic anisotropy tensor: Higher-order components due to oriented magnetite exsolutions in pyroxenes, and implications for paleomagnetic and structural interpretations
Exsolved iron oxides in silicate minerals can be nearly ideal paleomagnetic recorders, due to their single-domain-like behaviour and the protection from chemical alteration by their surrounding silicate host. Because their geometry is crystallographically controlled by the host silicate, these exsolutions possess a shape preferred orientation that is ultimately controlled by the mineral fabric of the silicates. This leads to potentially significant anisotropic acquisition of remanence, which necessitates correction to make accurate interpretations in paleodirectional and paleointensity studies. Here, we investigate the magnetic shape anisotropy carried by magnetite exsolutions in pyroxene single crystals, and in pyroxene-bearing rocks based on torque measurements and rotational hysteresis data. Image analysis is used to characterize the orientation distribution of oxides, from which the observed anisotropy can be modelled. Both the high-field torque signal and corresponding models contain components of higher order, which cannot be accurately described by second order tensors usually employed to describe magnetic fabrics. Conversely, low-field anisotropy data do not show this complexity and can be adequately described with second-order tensors. Hence, magnetic anisotropy of silicate-hosted exsolutions is field-dependent and this should be taken into account when interpreting isolated ferromagnetic fabrics, and in anisotropy corrections
Evidence of Early Cretaceous remagnetization in the Crimean Peninsula: a palaeomagnetic study from Mesozoic rocks in the Crimean and Western Pontides, conjugate margins of the Western Black Sea
We report on a palaeomagnetic study from Mesozoic sedimentary and volcanic rocks from the conjugate areas of the Western Black Sea Basin; that is, the Crimean Peninsula in the north and the Western and Central Pontides in the south, to better constrain their palaeogeographic relationships within the southern margin of Eurasia. From the study of 87 sites in Crimea, we found that Triassic to Lower Jurassic sandstones and siltstones from the Tavric series, and Middle-Upper Jurassic sandstones, siltstones and limestones exhibit remagnetization. Both fold and conglomerate tests confirm a widespread remagnetization in Crimea. Comparison of palaeopoles with the expected reference apparent polar wander path (APWP) of Eurasia and results from conglomerate tests suggest that the remagnetization occurred in the Early Cretaceous. In the Central Pontides, no reliable palaeomagnetic results can be obtained from Triassic-Upper Jurassic rocks, however, a negative fold test in Upper Jurassic-Lower Cretaceous rocks from the Western Pontides shows that the palaeolatitude agrees with Lower Cretaceous data from Crimea. Our new palaeomagnetic results indicate a pervasive remagnetization in Crimea and the Western Pontides that could be attributed to the rifting phase of the Black Sea Basin during Lower Cretaceou
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