Magnetic properties of variably oxidized pillow basalt

Abstract

In our study of changes in magnetic properties during low temperature oxidation of oceanic pillows, we were able to essentially eliminate variations in grain size which plagued many previous studies. This was accomplished by keeping the cores parallel to the outer glassy margin of the pillow while drilling the variably oxidized portions of the basalt. Curie temperature (Tc) measurements indicated a large range in the degree of low temperature oxidation, although most were in the early stages of maghemitization (Tc<300°C). In samples with hysteresis parameters showing pseudo‐single domain behavior, the natural remanent magnetization (Jnrm) and susceptibility both decreased substantially with low temperature oxidation. Thus, the Koenigsberger ratio (Jnrm/Jinduced) did not change. The reduction in Jnrm was mirrored by comparable decreases in saturation magnetization and saturation remanence. Low temperature oxidation also increased the bulk coercivity and the coercivity of remanence in the oxidized basalt relative to the less oxidized. These later results are consistent with the theory that low temperature oxidation generally stabilizes the remanent magnetization of seafloor pillow basalts. The increase in coercivity with oxidation in these pillow basalts contrasts will synthetic samples which show an overall decrease. The behavior of natural samples reflects processes which occur during low temperature oxidation that do not normally occur in synthetic samples, specifically the loss of iron from the grain and perhaps grain cracking.

Magnetic Petrology of Basalts from Ninetyeast Ridge

“…This pattern is consistent with a slow process, with intensity of alteration primarily controlled simply by the age of the rock. Studies of this type of alteration in seafloor basalts suggest that although changes in NRM magnitude may take place, directional changes are probably negligible (Soroka and BeskeDiehl, 1984;Beske-Diehl and Soroka, 1984). The major difference here is that some of the samples may have been initially nonmagnetic.…”

Origin of apparent magnetic excursions in deep‐sea sediments from Mendeleev‐Alpha Ridge, Arctic Ocean

“…In nature, coercivity of titanomaghemites is higher than that of unoxidized titanomagnetites for high z values [ Dunlop and Özdemir , 1997]. Increases in coercivity observed in hysteresis loops [e.g., Marshall and Cox , 1972; Beske‐Diehl and Soroka , 1984] or in median destructive field (MDF) from AF demagnetization [e.g., Ryall et al , 1977; Petersen and Vali , 1987] are generally seen for natural titanomaghemite in ocean basalts with increasing oxidation sate. It is also likely that the oxidation of titanomagnetite to titanomaghemite preferentially occurs in finer grains that generally have larger surface area to volume ratio, leaving the larger grains less altered.…”

Temperature dependence of magnetic hysteresis

“…In addition, changes of domains structures at high/low temperatures can be recognized and interpreted. Although the low‐temperature hysteresis properties of titanomagnetites have been extensively studied [ Tucker , 1981; Schmidbauer and Schembera , 1987; Argyle and Dunlop , 1990; Hodych , 1990; Schmidbauer and Keller , 1996; Moskowitz et al , 1997; Hodych et al , 1998; Muxworthy , 1999; Özdemir , 2000; Kosterov , 2001, 2002; Smirnov and Tarduno , 2002; Özdemir et al , 2002], the high‐temperature hysteresis properties of (titano)magnetites have been relatively less studied [ Levi and Merrill , 1978; Özdemir and O'Reilly , 1981, 1982; Hartstra , 1982a, 1982b; Beske‐Diehl and Soroka , 1984; Dunlop , 1987; Heider et al , 1987; Bina and Prevot , 1989; Argyle and Dunlop , 1990; Schmidbauer and Keller , 1994; Keller and Schmidbauer , 1999].…”