NRM intensity of altered oceanic basalts across the MAR (21°N, 0-1.5 Ma): a record of geomagnetic palaeointensity variations?

Abstract

Summary During cruise Tammar (May 1996) of R/V Nadir and submersible Nautile, 66 basaltic rock samples were collected along two long cross-sections encompassing the oceanic crust between 0 and 1.5 Myr at the centre of the mid-Atlantic ridge segment located at 21°N. This dense sampling provides a means to investigate natural remanent magnetization (NRM) variability with age using rock magnetic studies (high-field and k–T experiments, palaeofield intensity determinations by the original Thellier method) together with the good geophysical and geological knowledge of this peculiar segment. NRM intensities range from 1.3 to 25.4 A m−1 but do not display any rapid exponential decay with age, as expected. Despite the scatter, they seem to present short-wavelength variations consistent on both flanks of the two lines. Because of the relative uniformity in grain size (i.e. single domain, SD), ulvospinel content (i.e. x = 0.6) and amount of magnetic minerals in Tammar samples, the observed across-axis NRM intensity variations may be due either to oxidation degree variations or to geomagnetic field intensity changes. Curie temperatures display no increase towards the flanks nor a clear relationship with NRM intensities, suggesting that oxidation degree is not the major process controlling the NRM variations. Half of the collected samples, either fresh or highly altered, provide good-quality palaeointensity determinations. Depending on the alteration degree of samples, two major types of NRM/TRM diagrams are observed. For relatively fresh materials, only one line segment and the major part of NRM is used for field intensity calculations. For moderately or highly altered specimens, two line segments are observed with the first most probably corresponding to thermal demagnetization of the original remanence, and the second to the product resulting from chemical modification (i.e. inversion). Despite the difference in the amount of NRM that can be thermally demagnetized, samples of similar age but of different non-stoichiometric degree give coincident palaeofield strengths. Palaeofield values, from 15 to 62 µT (i.e. VADM from 3.4 to 13.6 × 1022 A m2), are in good agreement with the global palaeointensity database, and display coherent short-wavelength undulations with age, consistent (assuming a 1-km-wide neo-volcanic zone and steady, symmetrical spreading) with the relative palaeointensity record deduced from sediment core analysis (Guyodo & Valet 1999). Comparison between NRM and field intensity variations with age reveals a remarkable coincidence between both signals, suggesting that the NRM intensities are more sensitive to the field intensity than to the alteration degree. These observations suggest that magnetic particles are titanomaghemites, generated during the initial magma cooling and storing a durable record of the geomagnetic field intensity. The type of remanence is either (1) a chemical remanent magnetization that closely mimics the properties of the initial NRM or (2) a thermoremanent magnetization (the maghemitization precedes the TRM acquisition) or (3) a mixture of both.