Magnetism of basalt cores from the Nazca Plate and implications for magnetic anomaly interpretation

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doi: 10.1029/JB081i023p04188
Author(s): Deutsch, E. R.; Paetzold, R. R.
Volume Title: Journal of Geophysical Research
Source: Journal of Geophysical Research, 81(23), p.4188-4198. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0148-0227 CODEN: JGREA2
Note: In English; illus. incl. tables, sketch map
Summary: Measurements of magnetic and thermomagnetic properties of basalt from leg 34 of the Deep-Sea Drilling Project indicate that our 46 samples contained stable single-domain (SD) or pseudo-SD magnetite or titanomagnetite. The mean inclination after af and thermal demagnetization is Ī= -15°, close to the present dipole field, at site 321 but is anomalous (Ī = +53°) at site 319. In 31 samples of massive basalt from both sites the stable remanence at 20°C was masked by a low-coercivity component. High-field hysteresis loops are narrow at 20°C and broad below the -155°C magnetite transition. These samples, which we denote type 1, exhibited wide or constricted Rayleigh loops in 10 Oe. Although the magnetite in type 1 samples tends to be coarse (>100 µm), our findings demonstrate that the effective source of type 1 magnetite properties is not multidomain structures but fine predominantly superparamagnetic interacting particles that become stable SD at low temperatures. The other 15 samples (type 2), containing finer-grained magnetite or titanomagnetite, were stable at 20°C, did not show Rayleigh loops, and produced highly irreversible thermomagnetic curves consistent with SD behavior and oxidation upon heating. Between 20° and 105°C all type 1 k-T curves rose spectacularly towards a Hopkinson-type peak, while the natural remanence and the Koenigsberger ratios (∼1-10 at 20°) decreased sharply. Therefore in areas underlain by such rocks, magnetic anomalies must be interpreted with care.
Year of Publication: 1976
Research Program: DSDP Deep Sea Drilling Project
Key Words: 17 Geophysics, General; Applications; Basalts; DSDP Site 319; DSDP Site 320; DSDP Site 321; Deep Sea Drilling Project; Domain structure; East; Effects; Experimental studies; Geophysical methods; Geophysical surveys; Hysteresis; Igneous rocks; Interpretation; Leg 34; Magnetic anomalies; Magnetic methods; Magnetic properties; Magnetization; Marine; Natural remanent magnetization; Nazca Plate; Pacific Ocean; Paleomagnetism; Properties; Remanent magnetization; Stability; Surveys; Susceptibility; Volcanic rocks
Coordinates: S130102 S130102 W1013128 W1013128
S090024 S090024 W0833148 W0833148
S120117 S120117 W0815414 W0815414
Record ID: 1977005231
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