Hematite-bearing Rocks Research Articles

Magnetism in cold subducting slabs at mantle transition zone depths.

The Earth's crust-mantle boundary, the Mohorovičić discontinuity, has been traditionally considered to be the interface between the magnetic crust and the non-magnetic mantle1. However, this assumption has been questioned by geophysical observations2,3 and by the identification of magnetic remanence in mantle xenoliths4, which suggest mantle magnetic sources. Owing to their high critical temperatures, iron oxides are the only potential sources of magnetic anomalies at mantle depths5. Haematite (α-Fe2O3) is the dominant iron oxide in subducted lithologies at depths of 300 to 600 kilometres, delineated by the thermal decomposition of magnetite and the crystallization of a high-pressure magnetite phase deeper than about 600 kilometres6. The lack of data on the magnetic properties of haematite at relevant pressure-temperature conditions, however, hinders the identification of magnetic boundaries within the mantle and their contribution to observed magnetic anomalies. Here we apply synchrotron Mössbauer source spectroscopy in laser-heated diamond anvil cells to investigate the magnetic transitions and critical temperatures in Fe2O3 polymorphs7 at pressures and temperatures of up to 90 gigapascals and 1,300 kelvin, respectively. Our results show that haematite remains magnetic at the depth of the transition zone in the Earth's mantle in cold or very cold subduction geotherms, forming a frame of deep magnetized rocks in the West Pacific region. The deep magnetic sources spatially correlate with preferred paths of the Earth's virtual geomagnetic poles during reversals8 that might not reflect the geometry of the transitional field. Rather, the paths might be an artefact caused by magnetized haematite-bearing rocks in cold subducting slabs at mid-transition zone depths. Such deep sources should be taken into account when carrying out inversions of the Earth's geomagnetic data9, and especially in studies of planetary bodies thatno longer have a dynamo10, such as Mars.

Haematite natural crystals: non-linear initial susceptibility at low temperature

Several works have reported that haematite has non-linear initial susceptibility at room temperature, like pyrrhotite or titanomagnetite, but there is no explanation for the observed behaviours yet. This study sets out to determine which physical property (grain size, foreign cations content and domain walls displacements) controls the initial susceptibility. The performed measurements include microprobe analysis to determine magnetic phases different to haematite; initial susceptibility (300 K); hysteresis loops, SIRM and backfield curves at 77 and 300 K to calculate magnetic parameters and minor loops at 77 K, to analyse initial susceptibility and magnetization behaviours below Morin transition. The magnetic moment study at low temperature is completed with measurements of zero field cooled-field cooled and AC susceptibility in a range from 5 to 300 K. The minor loops show that the non-linearity of initial susceptibility is closely related to Barkhausen jumps. Because of initial magnetic susceptibility is controlled by domain structure it is difficult to establish a mathematical model to separate magnetic subfabrics in haematite-bearing rocks.

Evidence for the Permo-Triassic transtensional rifting in the Iberian Range (NE Spain) according to magnetic fabrics results

Anisotropy of magnetic susceptibility (AMS) techniques are applied to Permo-Triassic red beds from the Castilian Branch (Iberian Range, NE Spain) that were deposited in an extensional basin inverted during Cenozoic times. The main goal of this work is to characterize the tectonic evolution of the basinal stage by differentiating synsedimentary to early diagenetic magnetic fabrics from the secondary tectonic fabrics related to compression, which are scarcely developed because no penetrative structures related to compression have been recognized. Oblate magnetic fabrics, with kmin axes perpendicular to bedding ,are observed in most cases. Magnetic lineations are variable, showing a dominant ENE-WSW maximum, which fits with a dextral transtensional regime acting on NW-SE master faults during the Triassic. We propose that variations in the orientation of the magnetic lineation are associated with transfer faults which fragment the basin and trigger strain partitioning in different areas. Magnetic fabrics are locally modified by Cenozoic compression, with intermediate and minimum axes distributed along girdles perpendicular to fold axes. Comparing all these results with macrostructures and mesostructural kinematic indicators, we conclude that the fine-grained hematite-bearing rocks carry a consistent magnetic fabric which can be used to reconstruct the basin history.

Paleomagnetic analysis of the Marwar Supergroup, Rajasthan, India and proposed interbasinal correlations

The Marwar Supergroup refers to a 1000–2000m thick marine and coastal sequence that covers a vast area of Rajasthan in NW–India. The Marwar Basin uncomformably overlies the ∼750–770Ma rocks of the Malani Igneous Suite and is therefore considered Late Neoproterozoic to Early Cambrian in age. Upper Vindhyan basinal sediments (Bhander and Rewa Groups), exposed in the east and separated by the Aravalli–Delhi Fold Belt, have long been assumed to coeval with the Marwar Supergroup. Recent studies based on detrital zircon populations of the Marwar and Upper Vindhyan sequences show some similarity in the older populations, but the Vindhyan sequence shows no zircons younger than 1000Ma whereas samples taken from the Marwar Basin show distinctly younger zircons. This observation led to speculation that the Upper Vindhyan and Marwar sequences did not develop coevally.While there are alternative explanations for why the two basins may differ in their detrital zircon populations, paleomagnetic studies may provide independent evidence for differences/similarities between the assumed coeval basins. We have collected samples in the Marwar Basin and present the paleomagnetic results. Previous paleomagnetic studies of Marwar basinal sediments were misinterpreted as being indistinguishable from the Upper Vindhyan sequence. The vast majority of our samples show directional characteristics similar to the previously published studies. We interpret these results to be a recent overprint. A small subset of hematite-bearing rocks from the Jodhpur Formation (basal Marwar) exhibit directional data (Dec=89° Inc=−1° α95=9°) that are distinct from the Upper Vindhyan pole and may offer additional support for temporally distinct episodes of sedimentation in these proximal regions. A VGP based upon our directional data is reported at 1°S 344°E (dp=5°, dm=9°). We conclude that the Marwar Supergroup developed near the close of the Ediacaran Period and is part of a larger group of sedimentary basins that include the Huqf Supergroup (Oman), the Salt-Range (Pakistan), the Krol–Tal belt (Himalayas) and perhaps the Molo Supergroup (Madagascar).

Propagating and incorporating the error in anisotropy-based inclination corrections

SUMMARY Sedimentary rock palaeomagnetic inclinations that are too shallow with respect to the ambient field inclination may be restored using anisotropy-based inclination corrections or techniques that rely on models of the past geomagnetic field. One advantage of the anisotropy technique is that it relies on measured parameters (declinations, inclinations, bulk rock magnetic fabrics and particle magnetic anisotropy) that have measurement errors associated with them, rather than relying on a geomagnetic field model and statistical treatment of the data. Sofar,however,theerrorassociatedwiththemeasurementshasnotbeenpropagatedthrough the corrections and the reported uncertainties are simply the α95 95 per cent confidence circles of the corrected directions. In this paper we outline different methodologies of propagating the error using bootstrap statistics and analytic approximations using the case example of the Shepody Formation inclinationcorrection.Bothtechniquesareingoodagreementandindicateamoderate, ∼15per cent, uncertainty in the determination of the flattening factor (f) used in the correction. Such uncertainty corresponds to an ∼0.31 ◦ increase of the confidence cone and a bias that steepens the mean inclination by 0.32 ◦ . For other haematite-bearing formations realistic uncertainties for f ranging from 0 and 30 per cent were used (together with an intermediate value of 15 per cent) yielding a maximum expected increase in the confidence cones and steepening of the inclinationsof ∼1 ◦ .Suchresultsindicatethatformoderateerrorsoff theinclinationcorrection itself does not substantially alter the uncertainty of a typical palaeomagnetic study. We also compare the uncertainties resulting from anisotropy-based corrections to those resulting from the elongation/inclination (E/I) technique. Uncertainties are comparable for studies with a large sample number (>100), otherwise the anisotropy-based technique gives smaller uncertainties. Whenanisotropydataarenotavailable,itispossibletoestimateacorrectionusingflattening factors (f) obtained from the literature. A range of flattening factors has been observed for both magnetite and haematite-bearing rocks (0.4 ≤ f ≤ 1 for haematite and 0.54 ≤ f ≤ 1f or magnetite), but the exact value is specific to the anisotropy of the formation. To evaluate the maximum effects of inclination shallowing, the smallest f (for magnetite or haematite) should be used.

Demagnetization of terrestrial and extraterrestrial rocks under hydrostatic pressure up to 1.2 GPa

We carried out hydrostatic pressure demagnetization experiments up to 1.24 GPa on samples of terrestrial and extraterrestrial rocks and minerals of different lithologies as well as on synthetic samples. The magnetic remanence of samples was measured directly under pressure using a non-magnetic high-pressure cell of piston-cylinder type that was inserted into a high sensitivity SQUID magnetometer. In order to bring light on the pressure demagnetization effect, we investigated 50 samples with different magnetic mineralogies, remanent coercivities ( B cr) and hysteresis parameters. The samples consisted of pyrrhotite-, magnetite- and titanomagnetite-bearing Martian meteorites, taenite-, tetrataenite- and kamacite-bearing ordinary chondrites and pyrrhotite-bearing Rumuruti chondrite; magnetite- and titanomagnetite-bearing basalts, andesites, ignimbrites, obsidians and granites; a variety of pyrrhotite- and hematite-bearing rocks and minerals (jasper, schist, rhyolite, radiolarite); samples of goethite and greigite as well as synthetic samples of dispersed powders of magnetite, hematite, pyrrhotite and native iron set into epoxy resin. Under hydrostatic pressure of 1.24 GPa, applied in a low magnetic field (<5 μT), the samples lost up to 84% of their initial saturation isothermal remanent magnetization (SIRM) without any changes in their intrinsic magnetic properties. We found that the efficiency of the pressure demagnetization is not exclusively controlled by the magnetic hardness of the samples ( B cr), but that it is strongly dependent on their magnetic mineralogy. For a given magnetic mineralogy the resistance to hydrostatic pressure is roughly proportional to ln( B cr). It was shown that there is no simple equivalence between pressure demagnetization and alternating field demagnetization effects. The pressure demagnetization was shown to be time-independent but repeated application of the same pressure level resulted in further demagnetization.

Simplification of the anisotropy-based inclination correction technique for magnetite- and haematite-bearing rocks: a case study for the Carboniferous Glenshaw and Mauch Chunk Formations, North America

SUMMARY An anisotropy-based inclination correction was applied to the Carboniferous Glenshaw Formation from southwestern Pennsylvania. A combination of low-temperature thermal demagnetization followed by alternating field demagnetization isolated a palaeomagnetic remanence direction similar to that previously reported for these rocks. The inclination correction was conducted by fitting the samples’ anisotropy of anhysteretic remanence (AAR) to the theoretical correction curves for a magnetite remanence that yielded a remanence individual magnetic particle anisotropy, the aγ factor, equal to 1.86. Direct measurement of individual particle anisotropy in magnetic grain extracts, yielded an aγ value of 2 in good agreement with the curve fitting technique. The inclination-corrected formation mean directions for these two a factors were statistically indistinguishable. This result shows that curve fitting is an easier, but accurate, method of applying an anisotropy-based inclination correction than direct measurement of the individual grain anisotropy in magnetic grain extracts. The corrected Glendale Formation magnetite-based palaeopole is very similar to haematite-based palaeopoles from the Carboniferous Canadian Maritimes. A new technique is described that extracts the detrital haematite grains from red sedimentary rocks. The measured haematite aγ factors from the Carboniferous Mauch Chunk and Cretaceous Kapusaliang Formations yield values consistent with susceptibility individual particle anisotropies, aχ , determined previously by curve fitting techniques, extending the use of curve fitting for individual particle anisotropy determination to haematite-bearing rocks. However, the corrected Mauch Chunk Formation palaeopole is significantly different from the corrected Glendale Formation palaeopole, calling the accuracy of the Mauch Chunk palaeopole into question. Tectonic strain during folding may have added to the compaction strain in the rocks, leading to an overcorrection of the inclination. A new, inclination-corrected palaeopole for the North American Carboniferous is reported for the Glenshaw Formation at 28.6uN, 119.9uE.

Low-temperature behaviour of haematite: susceptibility and magnetization increase on cycling through the Morin transition

SUMMARY It has been realized previously (e.g. Borradaile 1994) that cycling through the Morin transition (Tm, occurring in ideal a-Fe2O3 at x10 uC) may have implications for the NRM of some haematite-bearing rocks. We investigate the behaviour of the low-field susceptibility (xlf), several magnetizations (in fields of 5, 25, 100 and 1600 mT) and SIRM on cycling through Tm of several well-characterized haematite types of varying crystallinity and particle shape. Before low-temperature treatment, xlf of the haematites varied between y40 and y235r10 x8 m 3 kg x1 . Below Tm, where only haematite’s defect moment resides, xlf was much more uniform at y19 to y28r10 x8 m 3 kg x1 . After return to room temperature, increases in xlf of up to y50 per cent were observed (when cycling in the Earth’s magnetic field as well as in a field-free space), inferred to be a function of the domain state of the haematite. This was shown for one of the haematites (LH2 which occurs as small platelets and is particularly well crystalline) where a relation y=(8.60t1.01) ln(x)x2.98 was obtained, where x is the grain size (mm) and y is the percentage susceptibility increase. We suggest that transdomain changes induce the change in xlf. The nucleation of (additional) domain walls in ‘metastable’ single-domain (SD) to pseudo-single-domain (PSD) grains is made possible by the low anisotropy at the Morin transition. In view of this mechanism, small stable SD haematite particles would not be affected and the grain size corresponding to y= 0( y1.5 m mf or LH2) would represent the ‘real’ SD threshold size. Thermal cycling to over the Curie temperature (680 uC) is needed to return to the original domain state before the LT treatment, as expressed by a return to the original xlf values. Measuring xlf between alternating field (AF) demagnetization steps shows that AF demagnetization gradually removes the xlf increase, which appears to be soft; 30 mT is sufficient to erase 90 per cent. Thermal cycling in a 5 mT field between temperatures above Tm showed that irreversible changes in domain structure are noticeable before the isotropic point is passed. After cycling, magnetization is added to PSD and multidomain (MD) grains that intriguingly appears to be remanence, probably induced by the broadening and subsequent irreversible displacement of loosely pinned domain walls. Complete cycling through the isotropic point considerably enhances the new remanence component in ‘metastable’ SD to MD particles by an increase in the number of domains. If this behaviour can be extrapolated to the intensity of the Earth’s magnetic field, this would imply that large ‘metastable’ SD to MD specularite crystals with a well-developed Morin transition are susceptible to acquiring geologically irrelevant remanence components when subjected to low ambient temperatures. Fine-grained haematite pigment, on the other hand, would not be affected. Thermal demagnetization alone would not be able to separate these two remanences as the new domain structure persists up to close to the Curie temperature. Our findings indicate that a cleaning procedure consisting of an initial AF step followed by stepwise thermal demagnetization is preferable in order to isolate the original remanence component properly in haematite-bearing rocks.

Deformation of remanent magnetism in a synthetic-aggregate with hematite

A hematite-calcite aggregate, bonded with Portland cement is deformed triaxially at 200 MPa confining pressure and at a constant strain-rate of 10 −5 s −1, in some cases with a pore fluid pressure of 120 MPa. This aggregate deforms in a ductile fashion at room temperature and is a good textural analog of naturally, penetratively strained hematite-bearing rocks. The changes in inclination of an isothermal remanent magnetisation are indistinguishable from the behaviour expected of a homogeneously strained non-material line rotating according to Wettstein's equation. This is encouraging for paleomagnetists who might wish to restore remanence in tectonically strained hematite-bearing rocks. An unknown combination of rigid-grain rotation and intracrystalline stress must be responsible for rotation of the grains that carry remanence and thus for the changes in inclination of remanence. The reason that Wettstein's equation is approximately valid may be because the large aspect ratios of rigid hematite grains cause them to rotate nearly as quickly as a passive line. p ]Hydrostatic compaction, wet deformation and dry deformation in that order, are progressively more effective in reducing the intensity of remanence. However, wet deformation is more effective than dry deformation in reducing the intensity of the most coercive components, presumably by spinning the finest grains in the cement matrix and scattering their individual moments. p ]Where multiple-direction components of remanence are present, deformation reduces the components with higher coercivities (> 70 mT) more than those with lower coercivities. In previous experiments on magnetite-bearing samples the lower-coercivity components were most reduced. Thus the natural tectonic deformation of an NRM may be expected to preserve selectively low-coercivity components in hematite and high-coercivity components in magnetite. This may be used to predict the success of preservation of a characteristic NRM and conversely the success of overprinting by syntectonic remagnetisation.

房総半島，鴨川産の嶺岡変成岩に見られる多様な鉱物組合せの起源 特に，高い酸素フュガシティの効果について

Various mineral assemblages are observed in the Mineoka metamorphic rocks of basic and siliceous compositions from Byobu-jima, Kamogawa fishing port. Major basic schists are divided into two groups, hematite-bearing and ilmenite-pyrrhotite-bearing. These rocks often contain a rutile-sphene-quartz-calcite sub-assemblage in common, which may have been a buffer of CO2 and thereby H2O fugacities in these graphite-free rocks during metamorphism (estimated XCO2 is around 0.3). The hematite-bearing rocks have a mineral assemblage defining the epidote-amphibolite facies whereas the ilmenite-pyrrhotite-bearing rocks the amphibolite facies, showing the great effect of oxygen fugacity on the silicate mineral mineral assemblages in basic schists. The aegirinaugite-bearing sillceous schists always contain a small amount of K-feldspar in addition to albite, while the epidote-bearing rocks are free of K-feldspar. Thus the occurrence of aegirinaugite is restricted to the rocks deficient in alumina. Some of the aegirinaugite-bearing rocks contain piedmontie and biotie rich in copper and manganese, indicating extremely high oxygen and very low sulfur fugacities in these rocks during metamorphism. Minor psammitic schist occasionally includes pebbles of (alkali) granite, offering a new insight into the sedimentary environment of the original rocks of the Mineoka metamorphi rocks. Estimated metamorphic pressure and temperature are 5 kb and 500-550°C, respectively.

Peaked thermomagnetic curves for hematite-bearing rocks and concentrates

Hematite-bearing rocks commonly have thermomagnetic curves with pronounced peaks in magnetization at temperatures which vary between samples. These peaks and their variability were reproduced for a sample of nearly-pure hematite in a series of different applied fields. With increasing applied field the peak sharpness became less marked and the “peak temperature” decreased. These results are similar to those obtained by Day (1975) for synthetic titanomagnetite powders, and a similar interpretation is proposed. Most laboratory fields are insufficient to saturate hematite, and the unsaturated magnetization increases and is unblocked and aligned with the field at temperatures critically dependent on the hysteretic properties of the sample. The natural tendency for magnetization to decay with increasing temperature then produces the observed peak.

Determination d'un nouveau pole paleomagnetique africain sur des formations cambriennes du Maroc

Thermal and alternating field cleaning treatments of Cambrian sedimentary and volcanic rocks from the Moroccan Anti-Atlas yield three directions of magnetization. The oldest, of high thermal stability, gives a new Cambrian pole 47°N, 42°E with an associated radiometric age of 530 m.y. A secondary magnetization, of lower thermal stability, has been superimposed during a thermal event which occurred in Permo-Triassic time, according to the palaeomagnetic evidence (pole 47°S, 88°E). A third component was acquired viscously in the present-day magnetic field direction. This study emphasises the possibility of thermal demagnetization measurements, particularly on hematite-bearing rocks, and of comparing superimposed magnetizations with radiometric resetting ages.