Magnetic Properties and Alteration in Basalt, Hole 504B, Deep Sea Drilling Project Leg 83

Kinoshita, Hajimu; Furuta, Toshio; Kawahata, Hodaka

doi:10.2973/dsdp.proc.83.116.1985

Cited by 13 publications

(18 citation statements)

References 6 publications

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“…To place the crustal section drilled during leg 111 within the context of 504B as a whole, we have compiled oxide petrography results and rock magnetic data from previous DSDP paleomagnetic studies; these include those of Furuta and Levi [1983], Pechersky et al [1983], Facey et al [1985], Kinoshita et al [1985], and Smith and Banerjee [1985b]. The basic magnetics data through leg 83 are presented by Smith and Banerjee [1986], and we include these previously results primarily as (1) a review, (2) for comparison with the new results from the sheeted dike complex, and (3) as a means of developing a model of crustal evolution for the full 504B crustal section.…”

Section: Resultsmentioning

confidence: 99%

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

Pariso¹,

Johnson²

1991

J. Geophys. Res.

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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 posternplacement 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 posternplacement 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 propellies of rock saniples 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. INTRODUCTION The recognition of polarity reversals of the geomagnetic field [Cox et al., 1964] and the identification of the record of this behavior in the magnetic anomalies of oceanic crust [Vine and Matthews, 1963; Morley and Larochelle, 1964] were fundamental in placing the process of ocean crust formation within the context of plate tectonics. Over time, however, our concept of oceanic crust as a magnetic tape recorder has evolved from the original, uniformly magnetized block model of Vine and Matthews, into a much more complex picture. Although progress has been made on understanding ocean crustal magnetization, we still do not have a complete model of the processes associated w...

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Section: Resultsmentioning

confidence: 99%

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

Pariso¹,

Johnson²

1991

J. Geophys. Res.

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“…Maxine magnetic anomalies are integral to the study of the marine crust. While we have been aware of the existence of these anomalies for some time, the question of t, heir origin, i.e., the magnetic structure of the marine crust, is still a matter of some dispute [e.g., Harri,son, 1976, In order to provide as complete and detailed a picture as possible, we have also included previously published data from legs 69 and 70 [Furuta, 1983;Furuta and Levi, 1983;Pechersky et al, 1983] and leg 83 [Kinoshita et al, 1985;Facey et al, 1985]. For some parameters (e.g., natural remanence and initial susceptibility) this roughly triples the amount of data available to us, especially in the upper part of the hole, where our sampling was relatively sparse.…”

Section: Introductionmentioning

confidence: 99%

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

Smith¹,

Banerjee²

1986

J. Geophys. Res.

107

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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.

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“…To place the crustal section drilled during Leg 111 within the context of Hole 504B as a whole, we have compiled magnetic data from previous DSDP paleomagnetic studies, including those of Furuta and Levi (1983), Pechersky et al (1983), Facey et al (1985), Kinoshita et al (1985), and Smith and Banerjee (1985). These data are plotted vs. depth in Figure 3.…”

Section: Discussionmentioning

confidence: 99%

Magnetic Properties and Oxide Petrography of the Sheeted Dike Complex in Hole 504B

Pariso¹,

Johnson²

1989

Proceedings of the Ocean Drilling Program

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Magnetic properties measurements were performed on 47 samples drilled during Leg 111 of the Ocean Drilling Pro gram and oxide petrography was studied in 32 samples taken at depths throughout the sheeted dike complex in Hole 504B. Integration of these data with results from previous DSDP legs shows that while natural remanent magnetization is constant with depth, magnetic susceptibility increases and median demagnetizing field and the Q ratio decrease with depth in the section. These trends appear to be a result of an increase in deuteric oxidation and a decrease in hydrother mal alteration of primary titanomagnetite with depth. A distinct change in stable magnetic inclination occurs between the extrusive basalts and the sheeted dikes and may be a result of tectonic rotation of the upper extrusive basalts.

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Magnetic Properties and Alteration in Basalt, Hole 504B, Deep Sea Drilling Project Leg 83

Cited by 13 publications

References 6 publications

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

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

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

Magnetic Properties and Oxide Petrography of the Sheeted Dike Complex in Hole 504B

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