Magnetic Properties of Igneous Rocks from Deep Sea Drilling Project Leg 63

Abstract

This study reports the magnetic properties of 30 samples of igneous rocks recovered from Sites 469 through 473 of Leg 63 of the Deep Sea Drilling Project. Such studies are relevant for determining the thickness and composition of the source layer of the marine magnetic anomalies. These studies are also relevant to the study of ophiolites, in which direct comparison of magnetic properties can lead to important inferences (e.g., Butler et al., 1976; Levi et al., 1978; Beske-Diehl and Banerjee, 1979). We made routine measurements of Paleomagnetism on all samples. The magnetic properties reported here include the following: 1) Intensity and stability of natural remanent magnetization (NRM). 2) Intensity and stability of anhysteretic remanent magnetization (ARM). 3) Intensity and stability of viscous remanent magnetization (VRM). 4) Saturation magnetization (M s). 5) Hysteresis parameters, i.e., saturation remanent magnetization (M r), the ratio (M r /M s) of saturation remanent magnetization to saturation magnetization, coercive force (H c), remanent coercive force (H cr), and initial and high-field susceptibilities (x, and x P) 6) Weak-field susceptibility (x). 7) Thermomagnetic properties, i.e., Curie temperature (7^.) and thermomagnetic curve characteristics. EXPERIMENTAL METHODS We measured the intensity and stability of NRM, ARM, and VRM on a Schonstedt spinner magnetometer coupled to a Hewlett-Packard computer and a Schonstedt single-axis alternating field demagnetizer. For the acquisition of ARM we applied a lOOQπoe alternating field (H) in the presence of a steady 1-oe direct field (H) and then reduced the alternating field (A.F.) slowly to zero oe. We then normalized the measured ARM intensity to 0.5 oe by dividing the measured value by 2; this was done so that the NRM and ARM intensities could be compared. We studied the acquisition of VRM in all samples by monitoring the magnetization acquired in 0.5 oe direct field for periods ranging from 15,000 to 28,800 min. For each sample we made 10 or more measurements of remanence and computed the VRM component.

Seawater alteration effects on REE, K, Rb, Cs, Sr, U, Th, Pb and Sr-Nd-Pb isotope systematics of Mid-Ocean Ridge Basalt.

“…The WR Cs concen tration increases during palagonitization but decreases during oxidation. Such a variation was also observed in an earlier study by Verma (1981a). The decrease of Cs during oxidation is consistently observed in all leach-fractions and residues as well (Table 4; Fig.…”

“…Dur ing oxidation of the crystalline MORB, this ratio increases only slightly from 0.70316 (for 487 LO) to 0.70325 (for 487 MO), but during palagonitization, much larger changes result. In an earlier study also, Verma (1981a) observed only a slight increase in 87Sr / 86Sr ratio from "fresh" (less -oxidized) basalt (-0 .70297) to the "oxidized" ( --0 .70312) one. The data on leach fractions (L1 and L2) also show similar trends.…”

“…2). The WR Sr results show a large decrease during palagonitiza tion (Verma, 1981a), but a very small rather in significant increase during oxidation. The Rb / Cs ratio of FrG WR is within the range of this ratio for "fresh" oceanic basalts (Hofmann and White, 1983), but is much lower for all other WR splits.…”

“…The exchange proc esses for Sr may include an addition of seawater Sr to basalt, removal of basaltic Sr into solution and incorporation of solution Sr in secondary phases (Staudigel et al, 1981;Verma, 1981b;Staudigel and Hart, 1983). The varying impor tance of these processes may determine whether Sr concentration decreases or increases (Verma, 1981a;Staudigel and Hart, 1983).…”

Data report: temporal variation in natural remanent magnetization observed for Pacific plate basement rocks: compilation from legacy data and new paleomagnetism and rock magnetism data from seafloor basalts cored during Expedition 320/321

“…Geochemical evidence of the tholeiites is compatible with their formation at a spreading center (Shibata et al, 1981). Verma and Banerjee (1981) investigated magnetic properties of the tholeiites and gave NRM data from two to four discrete samples for Holes 469, 470, 470A, and 472. Average NRM values are tabulated for individual holes in Table T1.…”