Magnetic Petrology of Basalts from Ninetyeast Ridge

Abstract

Given the importance of the inversion of seamount magnetic anomalies, particularly to the motion of the Pacific plate, it is important to gain a better understanding of the nature of the magnetic source of these features. Although different in detail, Ninetyeast Ridge is composed of submarine and subaerial igneous rocks that are similar to those found at many seamounts, making it a suitable proxy. We report here on the magnetic petrology of a collection of samples from Ninetyeast Ridge in the Indian Ocean. Our purpose is to determine the relationship between primary petrology, subsequent alteration, and magnetic properties of the recovered rocks. Such information will eventually lead to a more complete understanding of the magnetization of seamounts and presumably improvements in the accuracy of anomaly inversions. Three basement sites were drilled on Ninetyeast Ridge, with recovery of subaerial basalt flows at the first two (Sites 756 and 757) and submarine massive and pillow flows at the final one (Site 758). The three sites were distinctly different. Site 756 was dominated by ilmenite. What titanomagnetite was present had undergone deuteric alteration and secondary hematite was present in many samples. The magnetization was moderate and stable although it yielded a paleolatitude somewhat lower than expected. Site 757 was highly oxidized, presumably while above sea level. It was dominated by primary titanomagnetite, which was deuterically altered. Secondary hematite was common. Magnetization was relatively weak but quite stable. The paleolatitude for all but the lowermost flows was approximately 40° lower than expected. Site 758 was also dominated by primary titanomagnetite. There was relatively little oxidation with most primary titanomagnetite showing no evidence of high-temperature alteration. No secondary hematite was in evidence. This site had the highest magnetization of the three (although somewhat low relative to other seamounts) but was relatively unstable with significant viscous remanence in many samples. Paleolatitude was close to the expected value. It is not possible, at present, to confidently associate these rocks with specific locations in a seamount structure. A possible and highly speculative model would place rocks similar to Site 757 near the top of the edifice, Site 756 lower down but still erupted above sea level, and Site 758 underlying these units, erupted while the seamount was still below sea level.

Magnetic Petrology and Magnetic Properties of Western Pacific Guyots: Implications for Seamount Paleopoles

“…The addition of aluminum, magnesium, and chromium results in even lower Curie temperatures (e.g., O'Reilly, 1984;özdemir and Moskowitz, 1992). As noted previously by Smith et al (1991), the low inferred Curie temperatures for many of these titanomagnetites suggest that much of the remanence may be a chemical remanence acquired as the Curie temperature is elevated during low-temperature oxidation. The modified ulvöspinel content (x') should provide a reasonable estimate of the initial Curie temperature for samples in which Al + Cr > Mg + Mn (e.g., Site 871).…”

“…Characterization of the magnetic mineralogy and alteration history of seamount rocks may further aid in evaluating the significance of the remanent magnetization. For example, the high ulvöspinel contents (and inferred low original Curie temperatures) of titanomagnetites in hotspot-related volcanic rocks from the Ninetyeast Ridge have been interpreted as circumstantial evidence for a delayed acquisition of a chemical remanence acquired as Curie temperatures were elevated during oxidation (Smith et al, 1991). Rice et al (1980) have documented remagnetization of rocks from the Bermuda Seamount related to hydrothermal alteration associated with intrusive activity that substantially postdates seamount construction.…”

Data Report: Magnetic Properties and Magnetic Oxide Mineralogy of Upper Crustal Rocks from Holes 504B and 896A

“…Secondary phases, such as titanomaghemite, ilmenite, magnetite, hematite, and titanite, are produced by thermal and chemical alteration of titanomagnetite. In some cases, secondary phases and alteration textures, that is, the exsolution of primary titanomagnetite into low-titanium titanomagnetite and high-titanium hemoilmenite (Haggerty, 1976;Smith et al, 1991), can be observed using reflected light microscopy (Ade- Hall et al, 1971;Johnson and Hall, 1978) or backscattered electron imaging in conjunction with electron microprobe analysis. In other cases, however, the exsolution lamellae are too fine to be observed optically but may .…”

“…be detected by the disagreement between the bulk composition of a grain (determined by electron microprobe analysis) and the Curie temperature (the temperature above which a mineral behaves paramagnetically and cannot retain a magnetic remanence) predicted on the basis of that bulk composition (Evans and Wayman, 1974;Rice et al, 1980;Smith and Banerjee, 1986;Smith et al, 1991). Deuteric oxidation of titanomagnetites has been classified by Ade-Hall et al (1968) and modified by Haggerty (1991).…”

Data Report: Magnetic Mineralogy, Major- and Trace-Element Geochemistry, and Rock Magnetic Properties of Hole 504B Upper Crustal Rocks

“…Secondary phases, such as titanomaghemite, ilmenite, magnetite, hematite, and titanite, are produced by thermal and chemical alteration of titanomagnetite. In some cases, secondary phases and alteration textures (i.e., the ex solution of primary titanomagnetite into low-titanium titanomagnetite and high-titanium hemoilmenite; Haggerty, 1976;Smith et al, 1991) can be observed using reflected light microscopy (Ade-Hall et al, 1971;Johnson and Hall, 1978). In other cases, however, the exsolution lamellae are too fine to be observed optically but can be detected by the disagreement between the bulk composition of a grain (determined by electron microprobe analysis) and the Curie temperature (the temperature above which a mineral behaves paramagnetically and cannot retain a magnetic remanence) predicted on the basis of that bulk composition (Evans and Wayman, 1974;Smith and Banerjee, 1986;Smith etal., 1991).…”