Petrological Features and Magnetic Properties of Pillow Lavas from the Thetford Mines Ophiolite (Quebec)

1975 ◽  
Vol 12 (8) ◽  
pp. 1406-1420 ◽  
Author(s):  
M. K. Seguin ◽  
R. Laurent
Keyword(s):  
Remanent Magnetization ◽  
Outer Part ◽  
Detailed Examination ◽  
Intermediate Zone ◽  
Chemical Zoning ◽  
Fine Grained ◽  
Pillow Lavas ◽  
Paleomagnetic Study ◽  
Magnetic Signature ◽  
Ophiolitic Complex

A detailed examination was made to evaluate the variations of the remanent magnetization and magnetic susceptibility through four ophiolitic pillow lavas of Cambrian age. These pillowed metabasalts, which are covered by a thin layer of cherty argillite, derive from an olivine tholeiite of probable oceanic origin. They have been metamorphosed in the greenschist facies under a regime of low pressure, moderate temperature, and in the presence of water but absence of significant stress. They still display their primary structural zoning characterized by a thin outer shell, a much wider globulitic intermediate zone, and a porphyritic core. Morphologies of quenched microphenocrysts of olivine and plagioclase are well preserved.The magnetization resides in very fine-grained magnetite and, in spite of a very low remanent magnetization, the primary magnetic imprint appears to be still discernible. Remanent magnetization and Koenigsberger ratio vary from the pillow margin to its center in a fashion similar to the magnetic signature of some recent and fresh oceanic basalts. The magnetic zoning matches the textural, mineralogical, and chemical zoning characteristic of these pillow lavas. We found also that the N.R.M. vector is consistent and relatively stable within the pillow inner part of the intermediate zone and the outer part of the core and therefore that samples from these zones can be used for a regional paleomagnetic study of the ophiolitic complex.

Sediment-magnetic signature of land-use and drought as recorded in lake sediment from south-central Minnesota, USA

2004 ◽  
Vol 62 (2) ◽  
pp. 117-125 ◽  
Author(s):  
Christoph E. Geiss ◽  
Subir K. Banerjee ◽  
Phil Camill ◽  
Charles E. Umbanhowar
Keyword(s):  
Land Use ◽  
Remanent Magnetization ◽  
Recent Sediment ◽  
Fine Grained ◽  
South Central ◽  
Magnetic Signature ◽  
Recent Sediments ◽  
Bimodal Grain Size ◽  
Short Core ◽  
Organic Productivity

Sediment magnetic properties of a short core from Sharkey Lake, MN, record the effects of Euroamerican settlement and climate change over the last 150 yr. The onset of European-style farming led to increased erosion, reflected in high values of concentration-dependent parameters such as magnetic susceptibility (ĸ), Isothermal Remanent Magnetization (IRM), and Anhysteretic Remanent Magnetization (ARM). These high values are only partially due to increased supply of terrigenous material to the lake, and recent sediment contains an additional component of authigenic fine (single-domain) magnetite, most likely magnetosomes from magnetotactic bacteria. High organic productivity in the lake during the 1920s to 1940s drought increased this authigenic component resulting in highly magnetic fine-grained sediment. A comparison with older Holocene sediment from the same lake shows that, over time, most of the fine magnetic signal is lost after deposition, leading to decreases in magnetization and a bimodal grain size distribution of ultrafine, superparamagnetic grains and coarser multidomain particles, evident from measurements of ARM/IRM ratios, hysteresis measurements, and low-temperature analyses. The effects of dissolution and the superposition of climate and land-use signals complicate the use of recent sediments as modern analogs for sediment magnetic analyses.

scholarly journals Preferred orientation of ferromagnetic phases in rock-forming minerals: insights from magnetic anisotropy of single crystals

2019 ◽  
Vol 56 (9) ◽  
pp. 994-1001 ◽  
Author(s):  
Ann M. Hirt ◽  
Andrea R. Biedermann
Keyword(s):  
Magnetic Field ◽  
Magnetic Anisotropy ◽  
Single Crystals ◽  
Remanent Magnetization ◽  
Igneous Rocks ◽  
Flow Structures ◽  
Silicate Minerals ◽  
Fine Grained ◽  
Crystallographic Axes ◽  
Paleomagnetic Direction

In the early days of paleomagnetism, David Strangway was interested in understanding why igneous rocks are faithful recorders of the Earth’s magnetic field. He recognized that ferromagnetic (s.l.) grains that could be discerned by optical microscopy were too large to carry a stable remanent magnetization, and speculated whether fine-grained, ferromagnetic (s.l.) inclusions or exsolutions in silicate minerals are responsible. When these inclusions or exsolutions are randomly oriented, or the silicate hosts are randomly oriented in a rock, they can be a good recorder of the field. If these minerals, however, show an alignment within the silicate host, and the host is preferentially aligned due to flow structures or deformation, then the paleomagnetic direction and paleointensity could be biased. We examine the magnetic anisotropy arising from the ferromagnetic (s.l.) phases in silicate-host minerals. Single crystals of phyllosilicate, clinopyroxene, and calcite show most consistent ferrimagnetic fabric with relation to the minerals’ crystallographic axes, whereas olivine and feldspar display only a weak relationship. No discernable relationship is found between the ferrimagnetic anisotropy and crystallographic axes for amphibole minerals. Our results have implications when single crystals are being used for either studies of field direction or paleointensity or in cases where silicate minerals have a preferential orientation. Phyllosilicate minerals and pyroxene should be screened for significant magnetic anisotropy.

scholarly journals Further Observations on Stress-generated Ice in the Blue Glacier, Washington, U.S.A.

1970 ◽  
Vol 9 (55) ◽  
pp. 87-101 ◽  
Author(s):  
Charles A. Knight ◽  
E. LaChapelle
Keyword(s):  
Grain Boundaries ◽  
Thin Section ◽  
Liquid Water ◽  
Detailed Examination ◽  
Crystal Formation ◽  
Fine Grained ◽  
Water Filling ◽  
Flow Effects ◽  
Ice Crystallization

AbstractA more detailed examination is made of the stress-generated ice crystallization features already discussed by LaChapelle (1968), using mainly thin-section techniques. The crystallization features on the walls of a tunnel within the Blue Glacier are localized at fine-grained layers and are led by liquid water traveling along grain boundaries within the wall and within the deposits themselves. The water filling the crevasse encountered at the end of the tunnel was freezing uniformly to the crevasse walls as well as forming Thomson crystals within the water, and the evidence points to an important role for constitutional super-cooling in the Thomson crystal formation. The forms of most of the Thomson crystals are explainable qualitatively by beat flow effects.

Distribution of metals in sediments of the elbe estuary in 1994

1998 ◽  
Vol 37 (6-7) ◽  
pp. 109-116 ◽  
Author(s):  
Alexander Groengroeft ◽  
Ursula Jaehnig ◽  
Guenter Miehlich ◽  
Rolf Lueschow ◽  
Vera Maass ◽  
...  
Keyword(s):  
Grain Size ◽  
Size Fraction ◽  
Outer Part ◽  
Quality Criteria ◽  
Total Sample ◽  
Enrichment Factors ◽  
Contamination Level ◽  
Sediment Layer ◽  
Elbe Estuary ◽  
Fine Grained

In 1994 the quality of the uppermost sediment layer of the Elbe estuary was surveyed for various planning purposes. For example, metal contents were determined in the grain-size fraction <20 μm, and the concentrations of PAH were analyzed in the total sample, with correcting for grain-size effect being done numerically. Using the specific zinc contents in fine-grained particles, transport phenomena were recognized: particles of marine origin are transported >100 km upstream to the Hamburg area, causing contamination to decrease to about 50% of the fluvial level. The local metal contamination however varies strongly. Samples with low concentration could be detected as geogen; the samples with the highest level of contamination were assumed to have originated from periods of higher river pollution. Of the samples, 74% with medium enrichment factors (2 to 8.5 vs. background for Zn) were suspected to reflect the actual contamination level. Compared to previously reported quality criteria, sediment samples from the upper part of the estuary are still highly contaminated. In the outer part of the estuary the quality is better, but because the marine fine-grained particles still have contamination levels to class II-III in the ARGE Elbe classification system, a further reduction of sediment contamination in the whole estuary would be limited to this level.

Long-Term Settlement History and the Emergence of Towns in the Zuni Area

2004 ◽  
Vol 69 (3) ◽  
pp. 432-456 ◽  
Author(s):  
Keith W. Kintigh ◽  
Donna M. Glowacki ◽  
Deborah L. Huntley
Keyword(s):  
Detailed Examination ◽  
Social Processes ◽  
American Southwest ◽  
Settlement History ◽  
Fine Grained ◽  
The Social ◽  
Demographic Processes ◽  
History Of ◽  
Regional Perspective

From a regional perspective, the late thirteenth-century aggregation of village populations into large towns in the northern Southwest appears to be a brief and dramatic episode of social reorganization. That it is apparent across such a range of cultural and ecological circumstances suggests that a regional perspective will be needed to understand why it occurred. However, if we are to understand how it occurred—the social processes involved in the aggregation of populations into large towns—a high resolution, long-term view of the settlement history of particular localities provides a necessary complement to the regional view. We present a detailed examination of the development of one town in the American Southwest, the large, prehistoric Zuni town of Heshot uła. Without the long-term demographic reconstruction made possible by a fine-grained seriation and full-coverage survey, we would have seen this key transition as much more rapid and much less closely tied to the local situation than it now appears to be. Although this pueblo was built about A.D. 1275, we argue that its appearance was, at once, the culmination of demographic processes operating over at least 150 years, and the outcome of two more rapid, qualitative organizational transformations separated by a century.

‘Hard’ viscous remanent magnetization (VRM) in fine-grained hematite

1977 ◽  
Vol 4 (4) ◽  
pp. 163-166 ◽  
Author(s):  
David J. Dunlop ◽  
J. Mark Stirling
Keyword(s):  
Remanent Magnetization ◽  
Fine Grained

A paleomagnetic study of the layered mafic intrusion at Sept-Îles, Quebec

1987 ◽  
Vol 24 (7) ◽  
pp. 1431-1438 ◽  
Author(s):  
E. I. Tanczyk ◽  
P. Lapointe ◽  
W. A. Morris ◽  
P. W. Schmidt
Keyword(s):  
North America ◽  
Excellent Agreement ◽  
Host Rock ◽  
Remanent Magnetization ◽  
Mafic Intrusion ◽  
Paleomagnetic Study ◽  
Local Host ◽  
Layered Mafic Intrusion ◽  
The One

A paleomagnetic study of gabbroic and anorthositic members of the Sept-Îles layered mafic intrusion has isolated a remanence with direction D = 333°, I = −29 °(remanence A) and a corresponding pole of 141 °E, 20°N. The rocks are cut by numerous diabase dykes of unknown age. The remanence carried by the dykes has direction D = 188°, I = −85 °(remanence B), with a corresponding pole of 116°E, 59°S. Another remanence, statistically identical to B (D = 186°, I = −85°), is found at dyke contacts and in the local host rock; its associated pole, 115°E, 61°S, is identical to the one derived from the dykes. This overprint is significantly different from remanence A, and is obviously related to dyke emplacement. The location of the pole derived from remanence A is in excellent agreement with many other Cambrian poles from a variety of locations throughout cratonic North America. The rocks at Sept-Îles have been previously dated radiometrically at 540 Ma. Thus, all evidence indicates that remanence A is an original thermochemical remanent magnetization, acquired during initial cooling of the intrusion in the Cambrian and prior to the emplacement of the dykes.

The Mid-Atlantic ridge near 45° N. VII. Magnetic properties and opaque mineralogy of dredged samples

1970 ◽  
Vol 7 (2) ◽  
pp. 239-256 ◽  
Author(s):  
C. M. Carmichael
Keyword(s):  
Magnetic Properties ◽  
Remanent Magnetization ◽  
Complex Structure ◽  
Magnetic Anomalies ◽  
Natural Remanent Magnetization ◽  
Coarse Grained ◽  
X Ray Diffraction ◽  
Field Polarity ◽  
Fine Grained ◽  
Mid Atlantic Ridge

Measurements of the magnetic properties, paleomagnetic field intensity, and the inferred paleomagnetic field polarity have been made using fine grained basalt and coarser grained rock samples dredged from the mid-Atlantic ridge near 45° N and supplied by the Geological Survey of Canada. The opaque mineralogy of the samples was studied by microscope, Curie point, and X-ray diffraction techniques. The natural remanent magnetization of the basalt is of the order of 5 to 10 × 10−3 e.m.u./cm3 with some values from the center of the median valley reaching 10−1 e.m.u./cm3. Magnetic anomalies over the ridge can be accounted for by the remanent magnetization of a few hundred meters of this basalt. The coarse grained rocks were relatively weakly magnetized, and while they contribute little to the magnetic anomalies, their diverse character suggests that the major portion of the oceanic crust, below a thin veneer of fine grained basalt, has differentiated into a complex structure.

scholarly journals Remagnetization of Upper Jurassic limestones from the Danubian Unit (Southern Carpathians, Romania): tectonic implications

2012 ◽  
Vol 63 (6) ◽  
pp. 453-461 ◽  
Author(s):  
Panaiotu Cristian G. ◽  
Panaiotu Cristina E. ◽  
Lazăr Iuliana
Keyword(s):  
Remanent Magnetization ◽  
Upper Jurassic ◽  
Relative Rotation ◽  
Clockwise Rotation ◽  
Magnetization Component ◽  
Diagenetic Processes ◽  
Tectonic Implications ◽  
Paleomagnetic Study ◽  
Normal Polarity ◽  
Southern Carpathians

Abstract We present a pioneering paleomagnetic study on Upper Jurassic limestones from the Danubian Unit (Southern Carpathians, Romania). Thermal and alternating field demagnetizations were applied to define the characteristic remanent magnetization component in all six localities (81 samples). All samples have a normal polarity characteristic remanent magnetization. Negative regional and local fold tests suggest that this remanent magnetization is in fact a remagnetization produced by late diagenetic processes. The studied limestones were probably remagnetized during the collision of the Getic Unit and Danubian Unit which took place during the long normal polarity Chron C34 (82-118 Ma). The area mean direction (D = 75.5°, I = 50.0°, α95 = 10.2°, k = 44) implies about 75° clockwise rotation post remagnetization. Our paleomagnetic results further indicate the absence of significant relative rotation between the Getic Unit and the Danubian Unit during the Cenozoic.

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