Magnetic properties of submarine basalts and the implications on the structure of the oceanic crust.

Abstract

X-ray, chemical and thermomagnetic analyses were made on ferromagnetic minerals separated from six submarine basalts of various ages, which are dredged from seamounts and ridge or drilled from sea floor. Lattice parameters, Curie temperatures and oxidation parameters are determined for these samples. From a comparison between K-Ar or Ar39-Ar40 ages and the magnetic properties, it is evident that major ferromagnetic minerals in young submarine basalts are titanomagnetites, whereas older submarine basalts contain essentially titanomaghemites. We conclude that the primary titanomagnetites have undergone maghemitization in suboceanic environment. The maghemitization process is estimated to have a time constant of a few tens of millions of years and maghemitized depth may extend to a few tens of meters in older (say older than 5X10y) oceanic crust.NRM of several maghemitized submarine basalts are subjected to a-f, pressure and low temperature demagnetization, hoping to see if any of the demagnetization techniques could discriminate a secondary magnetization which might be acquired during the maghemitization. Although the demagnetization techniques erased considerable portion of the NRMs, none of them was successful to demagnetize preferentially a remanent magnetization carried by titanomaghemites.

Rock- and Paleomagnetism of Deep Sea Drilling Project Leg 54 Basalts—East Pacific Rise and Galapagos Rift

“…Data from ocean-floor basalts from the Atlantic Ocean and the Mediterranean Sea have been included in this graph. The result clearly differs from the relationship given by Ozima et al (1974) and also from the logarithmic relationship of Petersen et al (1979). Instead, we observe two linear regions with a distinct change of slope at about 3.5 m.y.…”

“…It can be seen that none of the Hole 428 A samples has retained this original value. The increase of Curie temperature is most likely caused by submarine low-temperature oxidation (Ozima et al, 1974;Prévot and Lacaille, 1976;Bleil and Petersen, 1977;Johnson and Hall 1978;Petersen et al, in press) and will be subsequently discussed in more detail.…”

“…c) Variation of Curie Temperature with Age Given a certain grain size of the titanomagnetites in submarine basalt, the degree of low-temperature oxidation increases with the age of the rock. Ozima et al (1974) assumed a first-order rate process of the following form…”

Rock- and Paleomagnetism of Basalt from Site 396B, Leg 46

“…Curie temperatures between 200° and 400°C are typical for low-temperature oxidized titanomagnetites in oceanic basalts (Johnson and Merrill, 1973;Ozima et al, 1974;Ade-Hall et al, 1976;Bleil and Petersen, 1977). The Curie temperature of the original unaltered titanomagnetite must have been much lower.…”

Rock Magnetic Properties of Igneous Rock Samples—Leg 45

“…One of the characteristics of titanomagnetite oxidized at low temperature is that the Curie temperature increases and the J S (T) curve becomes irreversible. This irreversibility is a result of the breakdown of titanomaghemite into a low-titanium magnetite and an ilmenite phase plus one of the polymorphs of rutile (Ozima and Larson, 1970;Readman and O'Reilly, 1970;Marshall and Cox, 1972;Ozima et al, 1974;Grommé and Mankinen, 1976;Hall and Ryall, 1977). Several studies have been made to determine the degree of oxidation necessary for the J s ( T) curves of submarine basalts to become irreversible.…”