Magnetic properties of oceanic pillow basalts: evidence from Macquarie Island

Abstract

A combined pdaeomagnetic, hysteresis, thermomagnetic, and electron microprobe study has been carried out on pillow basalts from Late Tertiary oceanic crust exposed on Macquarie Island. Oriented cores were collected from four sites in the North Head region where the basalts are unmetamorphosed but have suffered pervasive seafloor weathering and at four sites near Langdon Point where the pillow lavas have experienced metamorphism to the greenschist facies. Field geologic and petrologic evidence suggests that the North Head samples are representative of the weathered zone of Late Tertiary oceanic crust while the specimens from Langdon Point are representative of pillow basalts from > 5 0 0 m into the oceanic lithosphere. Geometric mean intensities of natural remanence (NRM) were 0.7 x C for North Head samples and 0.6 x 10-3C for specimens from Langdon Point. The geometric mean values of low field susceptibility (x)and Koenigsberger ratio (Q) for North Head samples (0.1 x 10-3GOe-1 and 9.8) are similar to those of DSDP basalts. The mean value of x and Q for Langdon Point samples (2.8 x 10-3G Oe-' and 0.3) is similar to those observed in metabasalt dredged from oceanic escarpments. Hysteresis, thermomagnetic and electron microprobe data indicate that the titanomaghemites in the North Head samples have suffered a high degree of low temperature oxidation. Langdon Point samples show reversible thermomagnetic curves with a magnetic Curie temperature (58OoC), indicating that metamorphism has altered the magnetic mineralogy. This metamorphism is thought to occur at or near the ridge during the seafloor spreading process. Thus, the NRM of the metamorphosed pillow basalts would still record geomagnetic reversals and contribute to magnetic lineations. The low NRM intensities are ascribed to prolonged seafloor weathering of North Head samples and to metamorphism of the basalts from Langdom Point. Since the 10-3G NRM intensities of pillow basalts are too low t o account for the 180 observed amplitude of marine magnetic anomalies, the underlying basaltic and doleritic sheeted dyke complex must also contribute to the anomalies.

Magnetic Properties of Basalts from Deep Sea Drilling Project Leg 83: The Origin of Remanence and Its Relation to Tectonic and Chemical Evolution

“…The principal objective of this work is to investigate the origin of remanence in these samples and the relationship of remanent and intrinsic magnetic properties to the alteration regime in this section of oceanic crust. This information is relevant to a variety of topics including the depth of the magnetic layer contributing to magnetic anomalies (e.g., Johnson, 1979) and the applicability of ophiolite magnetic data to actual marine crust (e.g., Butler et al, 1976;Levi et al, 1978;BeskeDiehl and Banerjee, 1979;Banerjee, 1980;Luyendyk et al, 1982;Swift and Johnson, in press). Some of this information should be relevant also to the more general question of the nature of the hydrothermal alteration that took place in much if not all of the section sampled by Leg 83.…”

Macquarie Island: Its geology, structural history, and the timing and tectonic setting of its N-MORB to E-MORB magmatism

“…Williamson (1979Williamson ( , 1988) estimated the age of the island in yet another way and arrived at a different result. Macquarie Island pillow lavas have magnetic properties that correspond well with those of ocean-floor basalts (Banerjee et al, 1974;Butler et al, 1976;Levi and Banerjee, 1977;Levi et al, 1978). Williamson (1979Williamson ( , 1988) measured a north-south magnetic profile on land along the island for 20 km within volcanic rocks.…”

Magnetic properties of Cenozoic ophiolites in the Hayama-Mineoka and Setogawa belts, South-Central Honshu, Japan.

“…The original NRM acquired at the time of solidification of basalts from magma seems to have been lost in these rocks. Magnetic properties of pillow and dyke basalts in ophiolite complexes from other localities have been reported by several investigators (VINE and MOORES, 1972;BUTLER et al, 1976;LEVI and BANERJEE, 1977;LEVI et al, 1978;TONOUCHI et al, 1981) and are summarized in Table 2 together with those of oceanic basalts. All the sheeted dikes of ophiolites in this table except from Mineoka and southern Setogawa exhibit reversible thermomagnetic curves with the Curie temperature of stoichiometric magnetite.…”