Magnetic susceptibility, petrofabrics and strain

Deep Sea Drilling Project hole 504B is located in 5.9 m.y. crust in the eastern Pacific Ocean about 200 km south of the Costa Rica Rift. At 1076 m subbasement, it is the deepest penetration of marine crust yet achieved. We present here magnetic data from this hole, especially from the recently cored leg 83 section (which constitutes the lowermost 500 m). These data, when combined with those of other studies, yield not only the deepest but also the most detailed and comprehensive picture of marine magnetic structure at a single site currently available. The basement rocks from hole 504B can be divided into three major magnetic units. The upper units (the top 500 m or so of basement) are essentially similar to other shallow marine basement sections, consisting of a mixture of various types of extrusive basalt. Low-temperature altered titanomagnetite is the dominant magnetic carrier, and the magnetic properties of these rocks are comparable to other shallow marine basalts. Below the upper units is a 200-m transition zone, consisting of a mixture of extrusives and dikes. High-temperature hydrothermal alteration (much of it greenschist facies grade) has caused oxidation-exsolution of primary titanomagnetite as well as its partial or total replacement by silicates. The dominant magnetic carrier is an Fe-rich titanomagnetite, magnetically indistinguishable from pure magnetite. The combination of oxidation-exsolution and silicate replacement results in a very low natural remanent magnetization (NRM). The remaining 300 m are in the upper portion of the sheeted dike complex. Primary titanomagnetite is exsolved, but alteration is less intense than in the transition zone. NRM values are substantially higher than those of the transition zone, due to less silicate replacement of primary titanomagnetite coupled with an uncertain contribution from secondary magnetite (which occurs as a silicate alteration product). The NRM magnitude is sufficiently high that the sheeted dike complex, in this location at least, may make a significant if not substantial contribution to the magnetic anomaly. Overall, the magnetic properties of this hole, especially the lower 500 m, are strongly influenced by postemplacement alteration and may bear little or no resemblance to their values upon initial cooling. As hydrothermal temperatures do not appear to have exceeded 400°C, the NRM in the lower two sections is apparently a chemical rather than a thermal remanence. As such, models of marine magnetic structure that combine cooling models of the crust with the assumption that magnetic remanence is purely of thermal origin do not appear sufficient to adequately predict the properties of the deeper crust. A better understanding of the nature of postemplacement alteration in the marine crust is required, especially its characteristics at high temperatures, and its effects on magnetic properties.

Magnetic structure of the upper kilometer of the marine crust at Deep Sea Drilling Project Hole 504B, eastern Pacific Ocean

Summary
The field and frequency dependences of the initial susceptibility of pyrrhotite have been analysed as a function of grain size, motivated by a strong field dependence recently observed for large (mm-sized) pyrrhotite crystals (Worm 1991) and smaller field dependences determined on smaller grain sizes by Clark (1984). In the present study, the frequency ranged from 30 Hz to 27 kHz. At 2 kHz, a field range from 0.05 to 1500 μT was investigated. Separate determinations of in-phase (K') and quadrature (k”) susceptibility components allow for the analysis of eddy current effects.

Up to 4 kHz the in-phase susceptibility of a pyrrhotite-ore specimen is practically independent of frequency whereafter it decreases while the quadrature component increases linearly with frequency to a value on the order of k' at 20kHz for large grains. k” is proportional to d2μ where d is grain diameter, μ the intrinsic permeability, σ the electrical conductivity and f the frequency. the frequency response of magnetite is essentially flat up to frequencies >20 kHz. Both frequency dependences agree well with calculations based on the theory by Wait (1951). the conductivity of the pyrrhotite ore has been determined to be σ= 1.40 (±0.05). 105Ω−1m−1.

The susceptibility of pyrrhotite and its field dependence increase strongly with grain size. While the susceptibility of grains smaller than 30 μm is field independent (up to 1.5 mT) it may increase as kα H0.25 for mm-sized crystals in fields >10μT. For most samples the Rayleigh law is inadequate to characterize induced magnetizations in weak alternating fields. When susceptibilities are measured for geomagnetic anomaly modelling, laboratory fields should be of similar intensity as the Earth's field and of frequency ≤1 kHz.

Magnetic Susceptibility of Pyrrhotite: Grain Size, Field and Frequency Dependence

Magnetics properties and oxide petrography results are presented from the most recent penetration at hole 504B during Ocean Drilling Program leg 111. Our results, combined with those from previous studies, show abrupt first-order changes in magnetic properties at alteration boundaries. Within the 504B crustal section, changes in style and degree of alteration fall near the boundaries of the three well-defined lithologic units: the extrusive basalts, the transition zone, and the sheeted dike complex. This postemplacement alteration heavily influences magnetic properties and is observed to change, in both style and degree, with depth within each lithologic unit. Our results indicate that, at 504B, the extrusive crustal section deeper than 600 m below seafloor has become magnetically less stable due to postemplacement reheating and alteration. The upper, more permeable basalts above this critical depth have not experienced this reheating. Within the sheeted dike complex, the subsolidus cooling rate and the degree of hydrothermal alteration of the opaque minerals both decrease with depth. The changes in alteration of the oxide minerals occur in parallel with the decrease in bulk permeability associated with increasing depth. Overall, these observations suggest that the effective penetration of water into layer 2C decreased with increasing depth and resulted in a lower rate of convective cooling. Magnetic properties of rock samples from the sheeted dike complex suggest that as a result of this gradient in hydrothermal alteration, the upper dikes have become magnetically more stable than the lower dikes. A review of all magnetic properties indicate that the dike section at 504B carries a lower remanent magnetization than its intrinsic rock magnetic properties and mineralogy would predict. We suggest that this lower remanent magnetization is a result of the long and complex thermal and alteration history which involves the acquisition of magnetic components in different directions. Despite a magnetization which is lower than expected, it appears that the sheeted dike complex at hole 504B is capable of making a substantial contribution to the overlying marine magnetic anomaly. Because of the systematic decrease in hydrothermal alteration of magnetic minerals, the ability of the dike section to contribute decreases as a function of depth.

Alteration processes at Deep Sea Drilling Project/Ocean Drilling Program Hole 504B at the Costa Rica Rift: Implications for magnetization of oceanic crust

SUMMARY The anomalous skewness of marine magnetic anomalies decreases with increasing spreading rate and becomes negligible above a spreading rate of about 50 km Myr-I. Previous magnetization models of the oceanic lithosphere fail to explain this observation. We propose a new model in which the magnetic structure of the oceanic lithosphere is dependent on spreading rate. Anomalous skewness measurements for anomalies 33 reversed and 25 reversed at various spreading rates are in agreement with a magnetization model in which layer 3 and the uppermost mantle bear a similar saturation magnetization, and the ratio of layer 3 over layer 2A saturation magnetizations varies linearly from about 0.4 at a spreading rate of 10kmMyr-' to about 0 at 55kmMyr-I. For spreading rates higher than 55 km Myr-', layer 3 and uppermost mantle magnetizations are negligible. Recent advances concerning accretionary processes at mid-ocean ridges suggest that the combined effect of variations with spreading rate of magma fractionation, alteration of layer 2A, and serpentinization of layer 3 and the uppermost mantle may account for the proposed variation of magnetization with spreading rate.

/pdf/spreading-rate-dependent-magnetization-of-the-oceanic-1vmtx56e3o.pdf

Spreading-rate-dependent magnetization of the oceanic lithosphere inferred from the anomalous skewness of marine magnetic anomalies

Leg 83 of the Deep Sea Drilling Project represents the deepest penetration of undisturbed marine crust yet achieved, providing samples from the lower part of the extrusive layer and the upper 300 m of the sheeted-dike complex. We present here measurements of the magnetic properties of rocks from Leg 83 and address the question of their suitability as sources for marine magnetic anomalies. We find that, due to extensive hydrothermal alteration, the extrusive part of the Leg 83 section is insufficiently magnetized to be part of the magnetic source layer. The sheeted-dike complex, on the other hand, does have sufficient magnetization and, contrary to recent models of marine magnetization based on ophiolite studies, may be a contributor to magnetic anomalies. The strong dependence of magnetic properties on the alteration history of the rocks, however, makes extension of these results to other regions problematical.

Source of marine magnetic anomalies: Some results from DSDP Leg 83

Ten samples of gabbro and peridotite, with varying degrees of serpentinization, were studied by magnetic techniques and reflected light microscopy. Evidence from these methods suggests that the natural remanent magnetization is primarily of chemical origin. It is generally weak for the gabbros and much stronger for the peridotites. This difference is offset by the fact that the peridotites have generally lower magnetic stability and Koenigsberger ratios. There is a considerable variation in both magnetic parameters and petrology even among closely spaced samples, which suggests that some combination of source heterogeneity and tectonic mixing was involved in the production of these rocks. However, the small number of samples makes this conclusion tentative. There may also have been significant postemplacement alteration involved. All samples show a significant anisotropy of weak field susceptibility that appears to be related to deformation. This anisotropy may be useful in defining petrofabrics.

http://www.deepseadrilling.org/82/volume/dsdp82_17.pdf

17. magnetic properties of plutonic rocks from the central north atlantic ocean

Selective destructive demagnetization of breccias from DSDP Leg 83: a microconglomerate test

The dependence of weak field susceptibility on applied magnetic field

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Source of marine magnetic anomalies: Some results from DSDP Leg 83

17. magnetic properties of plutonic rocks from the central north atlantic ocean

Selective destructive demagnetization of breccias from DSDP Leg 83: a microconglomerate test

The dependence of weak field susceptibility on applied magnetic field