Magnetic effects of maghemitization of oceanic crust

Abstract

Both theoretical considerations and available experimental results indicate that magnetic effects of maghemitization are strongly dependent on the grain size of the originally unoxidized titanomagnetite. Maghemitization of single‐domain titanomagnetite results in a decrease in coercivity, an increase in susceptibility, and a large decrease in Q ratio. Maghemitization of multidomain titanomagnetite results in an increase in coercivity, a decrease in susceptibility, and no large changes in Q ratio. Single‐domain titanomagnetite is probably resistant to the development of a chemical remanent magnetization (CRM), whereas multidomain titanomagnetite can acquire a CRM during maghemitization. The behavior of pseudo‐single‐domain titanomagnetite, which is the main carrier of remanence in submarine extrusive rocks, is investigated by comparing the magnetic properties of the French‐American Mid‐Ocean Undersea Study (FAMOUS) (less than 0.1 m.y. old) and the Leg 37 (3.5 m.y. old) pillow basalts recovered from the Mid‐Atlantic Ridge near 37°N. Combining electron microprobe analyses, Curie temperature measurements, and cell edge determinations, we find that the FAMOUS rocks are already oxidized (z = 0.38), possibly as a result of some high‐temperature maghemitization during cooling of the magma. Comparison with the more highly oxidized (z = 0.7) Leg 37 pillow basalts indicates that low‐temperature maghemitization of such rocks does not result in appreciable changes of coercivity and susceptibility, although the Q ratio does decrease and CRM seems to be acquired. Such a CRM could account for the anomalously low magnetic inclinations observed at most of the Leg 37 sites.

Magnetic Properties and Paleomagnetism of Basalts from Leg 107 (Holes 651A and 655B)

“…However, Prevot et al (1981) and Smith (1983) found an increase of the susceptibility for SD grains while a decrease was found for multidomain (MD) grains. The Koenigsberger ratio (Q) is related to the intensity of the NRM and to the susceptibility (K).…”

“…It seems well accepted that during maghemitization, in SD grains, Q decreases because the intensity of the remanence decreases more rapidly than the susceptibility. In MD grains, the measured Q value suggests that this parameter remains more or less constant during low temperature oxidation (Ade-Hall et al, 1976;Marshall, 1978;Prevot et al, 1981).…”

Correlation among the Changes with Alteration in the Mineralogical, Chemical, and Magnetic Properties of Upper Ocean Crust, Hole 896A

“…after basalt emplacement. Other investigations on very young ocean-ridge basalts (Prévot et al, 1981;Kent and Gee, 1994) indicate an even shorter time constant by an order of magnitude for the maghemitization process.…”

The behaviour and detection of partial thermoremanent magnetisation (PTRM) tails in Thellier palaeointensity experiments

“…Measurements of the laboratory Königsberger ratio (Q L ) and the viscosity index (VI), as described by e.g. Prévot et al (1981), were made for specimens from all core samples. These, together with other rock magnetic parameters measured for the samples used in this study are also given in Table 1.…”