Magnetic and Mineralogical Investigations of Opaque Minerals: Preliminary Results

Abstract

Preliminary investigation of the non-crustal igneous material recovered at Site 462 suggests that it is fairly similar to crustal oceanic basalt and low-temperature-oxidized only to a relatively small degree. Curie points are between 150 and 350°C, somewhat lower than expected for Early Cretaceous material. The massive nature of much of the basaltic column is probably the reason for the low degree of oxidation and hence the lower Curie temperatures.Microscopic examination of these rocks in reflected light shows relatively little evidence of low-temperature oxidation: they have only minor staining and cracking of the Fe-Ti oxide grains. High-temperature oxidation is evident in the central portions of some of the thick sills. Preliminary electron-microprobe analyses suggest that the composition of the least oxidized titanomagnetites within these rocks is about the same as that of oceanic crustal material.

Petrology and Geochemistry of Nauru Basin Igneous Complex: Large-Volume, Off-Ridge Eruptions of MORB-Like Basalt during the Cretaceous

“…The stratigraphic trends shown in Figures 2 and 12 are consistent with those presented by Saunders (this volume) and except for the twofold subdivision of the upper sill complex. It should be noted that we did not base this subdivision on chemistry and volcanic stratigraphy alone, but also on the (1) different mean stable inclinations of Cores 14 to 32 (upper sills) and Cores 36 to 41 (lower sills) (-51.8 ±7.2°, N = 35, and -42.5 ±11.2°, N = 12, respectively), and (2) distinct and systematic differences in the Curie temperature, magnetic inclinations, and NRM paleomagnetic intensities of the opaque mineral contents of the upper and the lower sills (Steiner, 1981a and1981b). Sr values we present here are identical to those reported by Fujii et al (1981) for Leg 61 samples and the uniform 87 Sr/ 86 Sr ratio for these basalts is an excellent indication that the Nauru Basin igneous complex came from a common source.…”

“…This assumption is clearly violated for volatile elements and perhaps also certain other components that are sensitive to secondary alteration processes (like K, Na, Mn, FeO/Fe 2 O 3 ). Nevertheless, for most elements of interest, it is reasonable to assume that the composition of these basalts has not been greatly affected by phenocryst accumulation or by alteration processes, because Nauru Basin basalts are generally aphyric and only moderately altered; this is shown by the low water content (see Table 3; and , low oxidation state of opaques (Steiner, 1981b), and the small variation of major and trace elements of most of the samples analyzed (see Figs. 8-10; also Tokuyama and Batiza, 1981).…”