Low-temperature alteration and magnetic changes of variably altered pillow basalts

Pillow basalt fragments from the East Pacific Rise, dredged during the Phoenix expedition, often show discoloured rims due to alteration. A suite of nine pillow basalts with such discoloured rims and ranging in age between 200 and 820 ka has been characterized in terms of their Fe–Ti-oxide mineralogy and rock magnetic properties. These large pillow fragments show relatively unaltered grey interiors, surrounded by darker, concentric halos, which vary in thickness as measured from glassy pillow rims and surfaces caused by large cracks penetrating into the original pillow interior. The discoloured zones are characterized by precipitation of abundant secondary minerals, such as Fe 3+ -rich clays that filled vesicle spaces. Fe–Ti oxides in subsamples from discoloured rims and grey interiors have been investigated with electron microscopy and rock magnetic techniques. The subsamples come from traverses that are parallel to the outer glassy pillow rims, allowing us to study the low-temperature alteration effects and rock magnetic properties without having to take variable grain size into account. Not surprisingly, titanomaghemites in discoloured rims are, in a general sense, oxidized to a higher degree ( z typically >0.55) than those in the relatively unaltered grey interior ( z typically 0.6 within 200 000 yr. The difference between the oxidation states of titanomaghemite within the grey pillow interior and the discoloured rims gradually diminishes with increasing age, so that for samples with ages of 800 ka the oxidation state of titanomaghemites in the grey interior approaches that of the discoloured rim. Our study demonstrates that visible discolouration of pillow basalts is not a suitable proxy for z . Because average Ti content can vary from sample to sample, Curie temperatures are also inaccurate proxies for z . If one wants to study possible correlations between z and rock magnetic parameters, the best technique is to determine z for each subsample by using transmission electron microscopy (TEM), electron microprobe, MÖssbauer or similar techniques. In agreement with many (but not all) previous observations on natural samples, we find that bulk coercivity ( H c ) , and high-field susceptibility (Χ hf ) increase, whereas low-field susceptibility (Χ l f ) , natural remanent magnetization intensity and saturation magnetization ( M s ) generally decrease with increasing oxidation state.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73443/1/j.1365-246X.2005.02819.x.pd

Source of oceanic magnetic anomalies and the geomagnetic polarity time scale

Marine magnetic anomalies provide the framework for the geomagnetic polarity timescale for the Late Jurassic to Recent (since 160 Ma). Magnetostratigraphic records confirm that the polarity reversal sequence interpreted from magnetic anomalies is complete to a resolution of better than 30 ky. In addition to this record of polarity reversals, magnetic anomalies also appear to preserve information on geomagnetic intensity fluctuations. The correspondence of coherent near-bottom anomaly variations with independent estimates of field intensity provides strong evidence that geomagnetic intensity modulates the magnetization of the ocean crust. Indeed, many short wavelength anomaly variations in sea-surface magnetic profiles over fast-spreading ridges are likely attributable to geomagnetic intensity variations. Although longer-term geomagnetic field behavior may also be reflected in anomaly amplitudes, documenting such a signal requires a better understanding of time-dependent changes in the magnetic source (e.g., from low-temperature alteration) that may also affect magnetic anomalies. The extrusive layer, with an average remanence of ∼ 5 A m−1, is the largest contributor to magnetic anomalies. However, enhanced sampling of oceanic gabbros (average remanence ∼ 1 A m−1) and, to a lesser extent, dikes (average remanence ∼ 2 A m−1) reveals that these deeper (and thicker) layers likely generate anomalies comparable to those from the lavas. Lava accumulation at intermediate- and fast-spreading ridges typically occurs over a narrow (1–3 km) region and dike emplacement is even more narrowly confined, resulting in a relatively high fidelity record of geomagnetic field behavior. The slow cooling of the gabbroic layer, however, results in gently dipping polarity boundaries that significantly affect the skewness of the resulting anomalies, which is also a sensitive measure of net rotations of the source layer(s). The magnetizations of the dikes and gabbros are characterized by high stability and are not expected to significantly change with time, although there are insufficient data to confirm this. The lavas, however, typically show evidence of low-temperature alteration, which has been long regarded as a process that progressively reduces the magnetization (and degrades the geomagnetic signal) in the extrusive layer and reduces the amplitude of magnetic anomalies. Sufficient data have become available to examine this conventional wisdom. There is a substantial (∼ 4x) reduction in magnetization from on-axis samples to immediately off-axis drillsites (∼ 0.5 My), but little further change in half-dozen or so deep crustal sites to ∼ 160 Ma. High paleointensity that characterizes the last few thousand years may contribute significantly to the high on-axis magnetization. The task of evaluating changes in remanence of the extrusive layer is made more difficult by substantial cooling-rate-dependent changes in magnetic properties and the systematic variation in remanence with iron content (magnetic telechemistry). The commonly cited magnetic anomaly amplitude envelope is in fact not systematically observed – the Central Anomaly is elevated at slow-spreading ridges but is not as prominent at faster spreading rates. Nonetheless, magnetic anomaly amplitudes are consistent with magnetization change is poorly constrained. Direct determinations of the degree of low-temperature oxidation reveal the presence of highly oxidized titanomagnetite in samples less than 1 My old, suggesting a short (∼ 105 years) time constant though the effects of low-temperature oxidation are quite heterogeneous. While low-temperature oxidation does have some affect on lava magnetization and anomaly amplitudes, there is increasing evidence that marine magnetic anomalies are capable of recording a broad spectrum of geomagnetic field behavior, ranging from millennial-scale paleointensity variations to polarity reversals to apparent polar wander to, more speculatively, long-term changes in average field strength. Several emerging tools and approaches – autonomous vehicles, oriented samples, absolute paleointensity of near-ridge lavas, and measurements of the vector anomalous field – are therefore likely to significantly advance our understanding of the geomagnetic signal recorded in the oceanic crust, as well as our ability to utilize this information in addressing outstanding problems in crustal accretion processes

Ilmenite lamellae and stability of magnetization

I have analyzed magnetic data from four deep‐sea basalts which have undergone the “exsolution” stage of high‐temperature oxidation, but have not begun the “replacement” stage. Even when ilmenite lamellae development was extensive, coercivities were low with a coercive force (Hc) of 11.7 mT (1 mT=10 Oe) and medium destructive field (MDF) of 9.1 mT. The remanence vector exhibited a directional swing of 30° upon alternating field (AF) demagnetization. Thus, few of the titanomagnetite intergrowths behaved like single‐domain grains. This tentative conclusion is supported by those previous studies of subaerial basalts and synthetic materials which optically identified the degree of oxidation and in which no titanomagnetite granulation was observed. In these studies, MDF\u27s generally did not increase above 15 mT during ilmenite lamellae formation. Those natural and synthetic samples which exhibit features of the “replacement” stage have large natural remanent magnetization (NRM) intensities and high coercivities suggesting that despite intense hematization, single‐domain magnetite may still be present. Copyright 1988 by the American Geophysical Union

Variation of magnetic directions within pillow basalts

Relative directions of magnetization have been measured within individual pillow basalts collected from the Atlantic Ocean and Caribbean Sea. The angle between the magnetic directions was determined and is referred to as the directional difference. Although one pillow contained a directional difference of 44°, the remaining ten pillows had differences less than 14°. The maximum orientation and measurement error was 7°. Dispersion on the scale found in these fine-grained pillow basalts would not appreciably affect the magnetic anomaly pattern on the sea floor. We detected no reversals of magnetization despite the sometimes large and variable low-temperature oxidation. Comparison of directions within homogeneous segments of the pillow, viscous remanent magnetization (VRM) acquisition experiments, and alternating field (AF) demagnetization indicate a large portion of the dispersion was due to the acquisition of a viscous component in the larger grained, less oxidized portion of the pillows. Evidence from one variably weathered pillow suggests that extreme low-temperature oxidation may lead to the acquisition of a secondary component with high coercivities (20-80 mT). We could not determine whether this was a chemical remanent magnetization (CRM) or a VRM acquired by single domain grains near the superparamagnetic threshold. Hysteresis properties confirmed by microscopic examination indicated that the magnetic grain size in all the pillows was at least as small as pseudo-single domain. © 1984

Metamorphism in the Troodos ophiolite: Implications for marine magnetic anomalies

The magnetic mineralogy and Koenigsberger ratio (the ratio of the remanent to induced magnetization) of basaltic rocks from the Troodos ophiolite undergo critical transformations at the zeolite-greenschist facies boundary. At this boundary the magnetic mineralogy changes abruptly from titanomaghaemite to magnetite and the Koenigsberger ratio decreases to less than 1. If similar metamorphism is occurring near the active oceanic ridges today, then the application of the two-layer model to marine magnetic anomalies may be primarily due to the degree of metamorphism and not to the mode of emplacement, intrusive versus extrusive. We show here that apparently in the Troodos ophiolite and, therefore, probably in oceanic rocks formed at spreading centres, the presence of magnetite at depth is a function of the pressure and temperature regime seen at the spreading centre. © 1980 Nature Publishing Group

Use of magnetic hysteresis properties and electron spin resonance spectroscopy for the identification of volcanic ash: A preliminary study

This initial study investigates the possible use of hysteresis parameters and electron spin resonance (ESR) spectroscopy to identify and correlate volcanic ash. ESR and hysteresis properties are sensitive to characteristics such as the chemical composition, mineralogy, and grain size and shape. These characteristics are determined by the tectonic setting of the volcano and by the magmatic and eruptive history of the volcanic ash. Hysteresis properties and ESR spectra, therefore, should be distinct for each ash eruption and may help to identify the eruptive source of the ash and to correlate ash from unknown sources. We conducted ESR spectroscopy at room temperature and magnetic hysteresis measurements on 19 samples of a single ash, the 1974 October 14 eruption of the Fuego volcano, Guatemala, and on single samples of ash obtained from eight different volcanoes. The Fuego ash samples were obtained at increasing distances from the volcano. For the single Fuego ash, ESR spectra and hysteresis parameters become increasingly similar as the distance from the volcano increases. At distances greater than 30 km, ESR spectra and hysteresis properties are uniform. The variability of magnetic and ESR properties with distance from Fuego is due to the preferential fall-out of pbenocrysts doser to the volcano. At large distances, the ash is more uniform, containing more glass and microcrystals. All eight ash samples from the different volcanoes can be distinguished from the distal Fuego 1974 October 14 ash using ESR spectra and hysteresis parameters. These results suggest that ESR and hysteresis measurements have a potential to be used as tools to identify distal ash when used in conjunction with geochemical, mineralogical and or other types of data

Morava River floodplain development during the last millennium, Strážnické Pomoraví, Czech Republic

Floodplain sediments deposited along the lower course of the Morava River (eastern Czech Republic), were studied in the Strážnické Pomoraví region to describe the alluvial history of the river over the last millennium. The sediments exposed in up to 5 m high erosional river banks were analysed using mineral magnetic, geochemical and chemical approaches. The age model of the sedimentary sequences was constructed from radiocarbon dates in association with 206Pb/207Pb and POP (DDT, PCB) analysis and 137Cs activity data. The Cu-trien method was used for stratigraphically correlating these deposits based on the variation of expandable clay minerals in the sediments. The resulting stratigraphic pattern reveals the alluvial history of the currently active river channel system since the end of the first millennium AD. Fine overbank clayey sediments deposited during the \u27Mediaeval Warm Period\u27 were eroded from cultivated fields newly formed during Mediaeval colonization between 1250 and 1450. These fine deposits are overlain by coarser floodplain sediments of the \u27Little Ice Age\u27, indicating a change in the sediment source since the sixteenth century AD, and a substantial increase in the sediment load in the second half of twentieth century. The Strážnické Pomoraví floodplain deposits represent a valuable palaeoenvironmental archive of the last millennium, containing records of fluvial processes considerably altered by human activities. © 2009 SAGE Publications