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

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doi: 10.1029/JB091iB10p10337
Author(s): Smith, Guy M.; Banerjee, Subir K.
Author Affiliation(s): Primary:
Univ. Minn., Dep. Geol. and Geophys., Minneapolis, MN, United States
Volume Title: Journal of Geophysical Research
Source: Journal of Geophysical Research, 91(B10), p.10,337-10,354. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0148-0227
Note: In English. 56 refs.; illus. incl. 2 tables, sketch maps
Summary: 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.
Year of Publication: 1986
Research Program: DSDP Deep Sea Drilling Project
IPOD International Phase of Ocean Drilling
ODP Ocean Drilling Program
Key Words: 18 Geophysics, Solid-Earth; Alteration; Basalts; Crust; DSDP Site 504; Deep Sea Drilling Project; Dikes; East Pacific; Extrusive rocks; Facies; Greenschist facies; Hydrothermal alteration; IPOD; Igneous rocks; Intrusions; Leg 111; Leg 137; Leg 140; Leg 148; Leg 69; Leg 70; Leg 83; Leg 92; Magnetic anomalies; Magnetic minerals; Magnetic properties; Magnetization; Metaigneous rocks; Metamorphic rocks; Metasomatism; Natural remanent magnetization; Ocean Drilling Program; Oceanic crust; Pacific Ocean; Paleomagnetism; Remanent magnetization; Sea-floor spreading; South American Pacific; South Pacific; Southeast Pacific; Tectonophysics; Thermoremanent magnetization; Upper crust; Volcanic rocks
Coordinates: N011338 N011338 W0834349 W0834349
Record ID: 1987008561
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