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doi:10.2204/iodp.proc.324.201.2013

Results

Scanning electron microscope observations

Hole U1347A

Four samples were analyzed. The Fe-Ti oxides show various morphologies even within one sample. The presence of skeletal grains in some samples (e.g., Sample 324-U1347A- 27R-5, 12–14 cm; Fig. F1A), is an indication of rapid cooling. On the other hand, some samples only show very small grains that seem to be fresher (e.g., Sample 324-U1347A-28R-1, 5–7 cm; Fig. F1B). In one of the samples, patterns that resemble oxy-exsolution patterns were observed (Sample 324-U1347A-27R-6, 6–8 cm; Fig. F1C). However, these patterns could also be caused by topography effects. Energy-dispersive X-ray measurements indicate a large variation in titanium content even within a sample. For instance, the Ti/Fe ratio for the grains analyzed in Sample 324-U1347A-28R-1, 5–7 cm, range from 0 to 0.9.

Hole U1349A

The five observed samples show little alteration, and no skeletal grains were observed in any of the samples (Fig. F1D–F1F). Iron oxide grains have sizes that range between a few micrometers and a few tens of micrometers. Again, within one sample, Ti/Fe ratios vary from 0 to 0.8.

Hole U1350A

Seven samples were analyzed. In most of them, the iron oxide grains have a skeletal shape. (e.g., Sample 324-U1350A-25R-7, 5–7 cm; Fig. F1G). In the most extreme cases of alteration, the remaining iron oxide grains are very small (~1 µm) as a result of the larger grains breaking up (e.g., Sample 324-U1350A-26R-7, 35–37 cm; Fig. F1H). One sample seems to show patterns that are characteristic of oxy-exsolution (Sample 324-U1350A-24R-1, 111–113 cm; Fig. F1I).

Hysteresis and first-order reversal curve diagram measurements

The FWHM as well as coercive fields for which the FORC distribution is maximum are shown in Table T1 for all the measured samples, together with hysteresis parameters measured from classical hysteresis loops. Some results and representative FORC diagrams are given in Carvallo et al. (in press).

Hole U1347A

Most samples from Hole U1347A have a single-domain (most contours are closed; Fig. F2A, F2B, F2E) to pseudosingle-domain (PSD; some contours intersect the H_c = 0 axis) (Fig. F2C, F2D, F2F, F2G) behavior, with moderate magnetostatic interactions. Only one coercivity peak is observed on all the FORC diagrams. The squareness ratios range between 0.096 and 0.324, with seven out of nine values <0.2. Bulk coercivity fields range between 3.25 and 8.37 mT, and the sample with the highest M_r/M_s ratio has a bulk coercivity field >13 mT. The H_cr/H_c ratios are between 1.40 and 5.13, with eight values out of nine <3 mT. The FWHM values are between 5 and 15 mT, which indicates small to moderate magnetostatic interactions. Finally, it is interesting to note that the variations of coercivity values that correspond to the maximum of the FORC distribution are consistent with the variations of bulk coercivities obtained from hysteresis loops, but are consistently lower.

Hole U1349A

For the samples from Hole U1349A, one sample per lava flow was measured. Sample 324-U1350A-12R-1, 87–89 cm, has clear PSD-like characteristics (Fig. F3C), but the other three are single-domain-like, with most contours that are closed (Fig. F3A, F3B, F3D). This is in agreement with hysteresis parameters: M_r/M_s ratios are between 0.26 and 0.42 for these five samples and bulk coercivities are between 15 and 25 mT. H_cr/H_c ratios are <2 for the single-domain-like samples and <4 for the PSD-like sample. Magnetostatic interactions are moderate, as suggested by the FWHM values, which are <20 mT in four out of five samples.

Hole U1350A

All 12 samples measured from Hole U1350A are very similar (Fig. F4). FORC diagrams are single-domain-like, with only two or three contours that do not close. M_r/M_s ratios all range between 0.173 and 0.276, and bulk coercivities range between 6 and 13 mT, with 13 values out of 14 <10 mT. The H_cr/H_c ratios range between 1.53 and 2.17. The FWHM values are all <10 mT, which shows that interactions are low. The variability of these parameters is much lower than that found in Holes U1347A and U1349A.

When plotted on a Day diagram, the hysteresis parameters of most of the samples fall on the single-domain–multidomain mixing line of Dunlop (2002), with a higher concentration of >50% single-domain grains (Fig. F5).

Susceptibility versus temperature

Hole U1347A

The three measured samples from Hole U1347A all show different behaviors, but they have in common that the Curie temperatures upon heating are all quite similar and between 160° and 200°C. These are values that are typical of titanomagnetite Fe_3–xTixO₄ with x = 0.6. However, the degrees of reversibility of these three samples are all different. In the three samples, susceptibility increases strongly from low temperature to the Curie temperature and then decreases very sharply. Sample 324-U1347A-28R-1, 5–7 cm (Fig. F6A), is almost perfectly reversible. This indicates the presence of homogeneous Ti-rich titanomagnetites (Kontny et al., 2003; Camps et al., 2011). According to these previous studies, this type of curve also corresponds to samples that are predominantly multidomain at room temperature, but the FORC measurements for these samples are rather characteristic of PSD grain size. Sample 324-U1347A-27R-5, 12–14 cm (Fig. F6B), seems to transform to titanomagnetite with a higher Curie temperature (i.e., lower titanium content) when heated. Finally, Sample 324-U1347A-27R-2, 89–91 cm (Fig. F6C), shows a complex mineralogy: two components are visible upon cooling after heating to 450°C and the susceptibility is strongly irreversible after subsequent heating to 550°C.

Hole U1349A

Four samples from Hole U1349A were measured. Two different types can be identified in the k-T curves. The first type, in the samples from the uppermost part of the flow, is characterized by a fairly reversible susceptibility with heating and a high Curie temperature, close to that of pure magnetite (Fig. F6D, F6F, F6G). Similar curves were measured by Kontny et al. (2003) and Camps et al. (2011); the presence of magnetite can be interpreted as resulting from high-temperature oxidation. The second type, in the lowermost part of the flow, shows the presence of two Curie temperatures (Fig. F6E): one around 380°C and more or less pronounced, indicative of Ti-rich titanomagnetite(-maghemite), and one around 550°C, indicative of Ti-poor titanomagnetite. Upon cooling, only the highest Curie temperature remains and the component with the lowest Curie temperature completely disappears. This behavior could be caused by the inversion of small grains of the low-temperature titanomaghemite phase into hematite.

Hole U1350A

Ten samples from Hole U1350A were measured. Because of their complex behavior, two or three partial k-T curves were also measured for some samples: the heating phase was stopped at an intermediate temperature, the sample was cooled down, and the susceptibility was measured again to a higher temperature and cooled down again. The k-T curves display a range of different behaviors, but the main features are similar to Type 1a/2 from Kontny et al. (2003): two Curie temperatures are present upon heating, indicating the presence of two magnetic phases. After heating, only one Curie temperature remains. The curves are quite reversible upon heating to ~300°C, indicating that the mineralogical changes take place mainly after that temperature. The three Curie temperatures can all be very close (Fig. F6N, F6O, for instance), and the irreversibility can be more or less marked. Samples 324-U1350A-25R-2, 64–66 cm, and 26R-7, 35–37 cm (Fig. F6I, F6Q), have a particularly irreversible behavior, with a Curie temperature upon cooling much lower (by ~100°C) than the lowest Curie temperature of the starting material, as well as a lower bulk susceptibility.

Low-temperature measurements

Hole U1347A

Three samples from Hole U1347A were measured and show two different behaviors. The first behavior is observed in two flows from Hole U1347A, in Samples 324-U1347A-26R-2, 89–91 cm, and 27R-5,12–14 cm (Fig. F7A, F7B). The room-temperature SIRM stays constant until cooled to 150 K and then decreases through a broad transition to 60% of its original value. The low-temperature SIRM increases with heating to 120 K and then decreases almost linearly to room temperature, reaching half its original value. This behavior could indicate the presence of a very broad Verwey transition indicative of low–Ti content titanomagnetite. Sample 324-U1347A-28R-1, 74–76 cm, is characterized by a different behavior (Fig. F7C): although the room-temperature SIRM varies very little with cooling, the shape of the SIRM demagnetization curve is very similar to that of synthetic titanomagnetite of composition x = 0.6 (Moskowitz et al., 1998).

Hole U1349A

Samples from Hole U1349A also have two different behaviors. The first behavior was observed for two samples (Fig. F7D, F7E): (1) the room-temperature SIRM cooling curve shows a very small Verwey transition, more or less broad depending on the sample, which decreases the magnetization by no more than 10%, and (2) the low-temperature SIRM decreases sharply at low temperature and then almost linearly, down to about two-thirds of the SIRM at 10 K. This could indicate the presence of a superparamagnetic fraction whose magnetization is blocked at low temperature and unblocks with heating. A slightly different behavior is observed in the other five measured samples (e.g., Sample 324-U1349A-10R-1, 116–118 cm Fig. F7G): the room-temperature SIRM is almost constant with cooling and no transition is observed, whereas the low-temperature SIRM decreases almost exponentially to about half its original value, showing again the presence of superparamagnetic grains.

Hole U1350A

All the samples from Hole U1350A have a similar behavior (Fig. F7K, F7N): the room-temperature SIRM decreases by two-thirds through a broad transition centered around 120 K. The low-temperature SIRM decreases through a small transition at 45 K and then drops almost linearly between 100 and 300 K to reach a value about half the original low-temperature SIRM.

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