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doi:10.2204/iodp.proc.324.201.2013

Methods and materials

Scanning electron microscope observations

SEM observations are useful for determining the size and morphology of the iron oxides as well as identifying the chemical elements present in the sample using X-ray diffraction. The observations were performed at the Institut de Minéralogie et de Physique des Milieux Condensés on selected samples prepared as rock powder mounted on carbon stubs using a SEM/FRG ZEISS Ultra 55 scanning electron microscope, and the concentrations of heavy metals were determined with the help of a Noran System 7 energy dispersive spectrometer (EDS) detector. The spatial resolution is ~1 µm.

Hysteresis and first-order reversal curve diagram measurements

First-order reversal curve (FORC) diagrams are an extension of hysteresis measurements that allow qualitative characterization of the magnetic domain structure and magnetostatic interactions even for materials containing a mixed grain-size assemblage (Pike et al., 1999; Roberts et al., 2000). When measuring a FORC diagram, the inside of the hysteresis loop is explored, which gives access to the distribution of microcoercivity rather than bulk coercivity, as when measuring simple hysteresis loops. FORC diagrams were measured at the Laboratoire des Sciences du Climat et de l’Environnement (LSCE), Gif-sur-Yvette, using an alternating gradient magnetometer. One hundred FORCs were measured with an averaging time of 0.1 s. The FORC diagrams were drawn using the method described in Pike et al. (1999). At least one sample per lava flow or unit was measured.

In simple cases, as is the case here, one way of quantifying FORC diagrams is by measuring the coercivity field that corresponds to the maximum of the FORC distribution and the full-width at half-maximum (FWHM) of the distribution profile parallel to the coercive field H_c = 0 mT axis that goes through the maximum of the distribution. This gives a measurement of the interaction field.

Susceptibility versus temperature

Low-field susceptibility versus temperature curves (k-T curves) were measured in order to test the stability of the Fe-Ti oxides upon heating and to determine the Curie temperature. Measurements were performed at low and high temperatures with the CS-L cryostat apparatus and the CS-3 furnace under argon atmosphere coupled to the KLY 3 Kappabridge (Agico, Czech Republic) at the University of Montpellier. For this experiment, bulk rock samples were reduced to powder in an agate mortar and sieved to obtain 0.4–0.8 mm size fractions. At least one sample per flow was heated first from liquid nitrogen temperature (78 K) to 900 K and cooled to room temperature. In some cases, the low-temperature measurements were repeated by heating again from 78 K to room temperature. Raw susceptibility data were corrected for the empty sample holder and normalized to the maximum susceptibility. At least one sample per lava flow was measured.

Low-temperature measurements

Low-temperature magnetization curves were measured with a magnetic properties measurement system from Quantum Design at the Institut de Physique du Globe de Paris. A 2.5 T magnetic field was first applied in order for the samples to acquire a saturation isothermal remanent magnetization (SIRM), the samples were cooled to 10 K in zero-field, and their magnetization was measured during the cooling run. Subsequently, the samples were given another SIRM in a 2.5 T field at 10 K and then warmed in zero-field to 300 K, and the magnetization was measured during the heating run. At least one sample per lava flow was measured.

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