Fig. 1. Map of sites from the seven Ocean Drilling Program legs considered in this study. Drill sites are shown by black dots.

Fig. 2. Typical thermomagnetic curves for representative samples. (a) Aphyric basalt Sample 203-1143B-2R-1, 80–82 cm (11 Ma), from east Equatorial Pacific during weak-field measurement; (b) same sample during strong-field experiment, see text; (c) pillow basalt Sample 192-1185B-10R-2, 7–9 cm (121 Ma) from the Ontong Java Plateau; (d) dark-colored serpentinized peridotite Sample 210-1277A-9R-4, 19–21 cm from the Newfoundland basin; (e) massive basalt Sample 192-1185B-19R-1, 37–39 cm (121 Ma) from Ontong Java Plateau; (f) brown dunite Sample 210-1277A-9R-2, 51–53 cm from the Newfoundland basin. The directions of arrows indicate heating and cooling curves.

Fig. 3. Diagrams of room temperature hysteresis loops for representative rock samples in this study that exhibit SD, PSD, and MD behavior. (a) PSD Sample 203-1243B-10R-2, 35–37 cm from Equatorial Pacific; (b) PSD Sample 183-1136A-16R-1, 80–82 cm from the Kerguelen Plateau; (c) SD Sample 183-1140A-26R-2, 15–17 cm from the Kerguelen Plateau; (d) MD Sample 183-1142A-9R-3, 33–35 cm from the Broken Ridge; (e) MD Sample 210-1277A-9R-8, 43–45 cm; (f) MD Sample 210-1276A-88R-7, 39–41 cm. Horizontal axis is applied field up to 1 T. Vertical axis is normalized magnetization (not corrected for slope).

Fig. 4. Hysteresis ratios plotted on a Day et al. (1977)-type diagram for basement samples from drill sites of the seven ODP Legs for this study. J_s is saturation magnetization, J_r is saturation remanent magnetization, H_c is coercivity, and H_cr is remanent coercive force. The plot is usually divided into regions: single-domain (SD) for J_r/J_s > 0.5 and H_cr/H_c < 1.5, multidomain (MD) for J_r/J_s < 0.05 and H_cr/H_c > 4, and pseudo-single-domain in between (PSD). For samples from Site 1184, five subsamples were measured for each sample.

Fig. 5. Low-temperature variation of saturation isothermal remanence for several representative samples during zero field cooling from 300 to 19 K and zero field warming back to 300 K, which display behavior of different groups of low-temperature magnetometry, see text. (a) Pillowed basalt Sample 192-1187A-14R-4, 35–37 cm; ‘a’ group; (b) Dark-colored serpentinized peridotite Sample 210-1277A-9R-6, 35–37 cm; ‘a’ group; (c) Pillowed basalt Sample 192-1185A-10R-2, 7–9 cm, ‘b’ group; (d) Brown dunite Sample 210-1277A-9R-2, 51–53 cm, ‘c’ group; (e) Pillowed Sample 192-1183A-54R-5, 36–38 cm; ‘c’ group; (f) Aphyric basalt Sample 203-1243B-17R-1, 7–9 cm, ‘x’ group. Note this sample displays self-reversal behavior during the experiment. The directions of arrows indicate heating and cooling curves.

Fig. 6. Frequency and temperature dependence of the in-phase magnetic susceptibilities, k′ and temperature dependence of k_fd (defined as k^′_lf−k^′_hf, where k^′_lf−k^′_hf are the in-phase magnetic susceptibility measured at low- and high-frequency, respectively) for representative samples. (a), (c), (e), and (g) are k′ curves measured using MPMS with combined frequency (1, 10, 30, 50, 100 Hz) for samples 183-1140A-37R-4, 16–18 m, Sample 192-1187A-6R-3, 3–5 cm, Sample 203-1243B-11R-1, 83–85 cm, and Sample 203-1243B-17R-1, 7–9 cm, respectively; (b), (d), (f) and (h) are the corresponding temperature dependence of k_fd curves for these samples. (i) and (j) are k′(f,t) curves measured using Lakeshore magnetometer with combined frequency (40, 140, 400, 1000, 4000 Hz) for Sample 210-1277A-9R-8, 40–42 cm and (95 and 1000 Hz) for Sample 149-899B-29R-1, 118–120 cm, respectively. The ac field for all measurements were 100 A/m. Sample 203-1243B-17R-1, 7–9 cm displays self-reversal behavior during thermal demagnetization and low-temperature cycling experiments, see text; Sample 210-1277A-9R-8, 40–42 cm (from the Newfoundland margin) and Sample 149-899B-29R-1, 118–120 cm, from the Iberia margin are dark-colored peridotites. They exhibit consistent low-T behavior. Numbers in (b) and (f) are the estimated grain sizes of ferromagnetic minerals (magnetite and/or maghemite) based on the new model developed by one of us (QL). The room temperature k_fd is sensitive only to a narrow grain size about 20–22 nm for magnetite. With decreasing temperature, the grain size window gradually shifts to finer grain size region. Generally, for magnetite/maghemite grains with coercivities between 20 and 30 mT (for most of our samples considered in this study), maxima of k_fd at 200 and 300 K are sensitive to grain sizes of about 20 and 25 nm, respectively.

Fig. 7. Mössbauer spectra of: (a) Sample 183-1136A-16R-2, 140–142 cm (basalt sample from Southern Kerquelen Plateau) and (b) Sample 183-1141A-24R-2, 6–8 cm (subaerial basalt sample from the Broken Ridge).

Summary of rock magnetic properties of rock samples considered for this study

Notes: The rock type and degrees of alteration are from corresponding ODP initial result volumes, see text for references. Tc, Curie temperature (Tc1, Tc2: low and high Curie temperature, respectively); Tv, Verwey transition; b, c, x, groups of low-temperature magnetometry, see text.