Unraveling the magnetic carriers of igneous cores from the Atlantic, Pacific, and the southern Indian oceans with rock magnetic characterization

Previous rock magnetic investigations of oceanic basement samples have been mainly restricted by single drill holes of dredge collections. In this study, we present new and published data on the rock magnetic properties of igneous rock samples recovered during seven Ocean Drilling Program (ODP) legs in the Atlantic, Pacific, and the southern Indian oceans. The recovered igneous rocks from these legs are mainly basaltic flows (both submarine and subaerial), diabase sills, and serpentinized peridotites, with approximate ages of 140,000 years, 10-12, 34, 69, 100-108, and 110-121 Ma, which offers an opportunity to investigate magnetic properties and mineral changes attending alteration of basement rocks over a wide range of ages from variable tectonic settings. Titanomagnetite and titanomaghemite are commonly present in igneous rock samples of the Ontong Java Plateau, the Kerguelen Plateau/Broken Ridge, the eastern Equatorial Pacific, the middle Atlantic ridge (Trans-Atlantic Geotraverse area), and the Newfoundland-Iberia rifted margins. For basaltic rock samples, three general groups (A, B, and C, respectively) can be divided in terms of rock magnetic properties. Samples from group A have a single phase of Ti-poor titanomagnetite with Curie temperatures ranging between 480 and 580 degrees C and exhibit a strong Verwey transition in the vicinity of 110 K. Basalts from this group are most likely good paleomagnetic recorders and probably have preserved original and stable magnetic remanences. Group B is mainly observed in pillow lavas and is characterized by a Curie temperature of 260-280 degrees C, which is typical of low-temperature oxidized titanomaghemite or titanium rich titanomagnetite. The low-temperature curves for group B do not show the Verwey transition. Group C has more than one Curie temperature, which suggests the presence of multiple magnetic phases. The thermomagnetic signature indicates the inversion of titanomaghemite to a strongly magnetized magnetite, as shown by the irreversible cooling curves. For the serpentinized peridotites recovered from both sides of the Newfoundland-Iberia rifted margins, results from low-temperature measurements show that (titano)magnetites are present in the dark-colored peridotites, with a strong Verwey transition in the vicinity of 110 K, and with field- and frequency-dependent susceptibility curves that resemble those of synthetic TMO. In contrast to the magnetic properties observed from the dark-colored peridotites, the low-temperature curves for the yellow-brown-colored peridotites did not show any Verwey transition. Thermomagnetic analysis also failed to show evidence for titanomagnetites. The remanent magnetization is carried by a thermally unstable mineral that breaks down at about 420 degrees C, probably maghemite. The magnetic signatures of the serpentinized peridotites recovered from both sides of the Newfoundland-Iberia rift appear not in conflict with the notion that conjugate margins will have generally similar crustal structure and evolution history. Our rock magnetic data indicate that the differences in the rock magnetic properties of basaltic rocks are mainly a function of mineralogy and alteration. There is no apparent coincidence between the age of the rocks and the degree of low-temperature alteration, as suggested by the presence of nearly unoxidized titanomagnetite in the Cretaceous aged basalts and by an examination of Curie temperature versus sample age that does not unambiguously show a positive relationship between the degree of low-temperature oxidation and crust ages. (c) 206 Elsevier B.V. All rights reserved