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doi:10.2204/iodp.proc.324.202.2013

Results

Site U1346

A total of 31 samples were measured from Site U1346 cores, but only 29 samples produced good results (Table T1). All samples are aphyric basalts from pillow flows. Both AF and thermal demagnetization techniques were used to isolate the primary magnetization. WWU measurements were divided into seven samples measured with AF demagnetization and eight examined with thermal demagnetization. Most samples have a drilling overprint that is removed at ~5–10 mT during AF demagnetization and at ~200°–300°C during thermal demagnetization. The AF demagnetized rocks show two responses, one type with low median destructive field (MDF; <10 mT) and the other with a higher MDF (>10 mT) (Figs. F3, F4). After the drilling overprint is removed, the samples usually display univectorial decay to the origin, with a few samples showing erratic behavior at high demagnetizing steps. The samples that show a MDF of <10 mT are >90% demagnetized by 20–30 mT (Fig. F4), whereas the samples with a MDF of >10 mT are >90% demagnetized by 40–50 mT (Fig. F3). Some thermally demagnetized samples show a small range of temperatures with partial self-reversal at ~300°C (e.g., Doubrovine and Tarduno, 2004, 2005) and/or a large overprint that is removed at ~200°C (Fig. F5). After removal of the overprint or at temperatures above the self-reversal portion of demagnetization, most of the samples display univectorial decay to the origin. All thermally demagnetized samples were >90% demagnetized at 450°–500°C. Of the 15 samples measured at WWU, 13 samples produced good results and were judged as samples giving a consistent principal direction that points toward the origin with a maximum angular deviation (MAD) of <10°. The other 16 samples measured during Expedition 324 showed similar behaviors in both AF and thermal demagnetizations (see the “Expedition 324 summary” chapter [Expedition 324 Scientists, 2010]). Four samples were dunked in liquid nitrogen as a test to determine its effect on sample overprints. Very little change was apparent in the magnetic intensity of the rocks, at most 0.5 A/m (Table T4). There were no noticeable effects of the low-temperature treatment on sample behavior during demagnetization.

Sample inclinations calculated using anchored and unanchored PCA solutions showed negligibly different values. The inclinations vary little throughout the length of the cored section, except for one outlier sample with an inclination of 27° (Fig. F6). Ignoring the single outlier, the average inclination is –21°, with a standard deviation of 5.7°. The lack of downhole variation and the low standard deviation imply that the entire section is recording essentially the same magnetic direction, so very little paleosecular variation was recorded, and the section was likely erupted in a short period of time.

Site U1347

From Site U1347 cores, 126 samples were measured, 61 during Expedition 324 and 65 at WWU. Only 119 samples produced good results (Table T2). The samples are aphyric and plagioclase phyric basalts from massive flows and pillow flow units. To isolate primary magnetization, both AF and thermal demagnetization techniques were applied to different subsets of samples. At WWU, 32 of the 65 samples were demagnetized using AF demagnetization and 33 were demagnetized using thermal demagnetization. The demagnetized samples have varying amounts of drilling overprint. In the AF demagnetized samples, the drilling overprint is typically directed vertically downward (a sign of drill pipe remagnetization; e.g., Fuller et al., 2006) and is removed after 10–15 mT. For thermally demagnetized samples, the overprint is usually removed progressively up to 310°–360°C. For both demagnetization types, the overprint was apparently never removed entirely from a small number of samples. Two responses observed with AF demagnetization are samples with low MDF and samples with higher MDF (Figs. F7, F8). The low-MDF samples are >90% demagnetized at 30 mT and the higher MDF samples are >90% demagnetized at 40–60 mT. Once the overprint is removed, most samples display univectoral decay toward the Zijderveld plot origin. In the lower cores (324-U1347A-25R through 29R), which sampled thick massive flows, sample directions become erratic at AF steps above 50 mT.

The thermally demagnetized samples displayed two different demagnetization behaviors. For one group, the intensity versus temperature curve shows a sharp decline around 300°C. In the other group, a nearly linear decrease in magnetic intensity is seen throughout the measurements (Figs. F9, F10). Some samples showed erratic directions in steps above 520°C, especially samples from Cores 324-U1347A-25R through 29R. For a few samples, small partial self-reversal sections occurred in demagnetization steps around 300°C (e.g., Doubrovine and Tarduno 2004, 2005) (Fig. F11). Typically, the sample increased in intensity for only 1–3 demagnetization steps, and the intensity increase was only 10%–15% of the NRM.

Of the samples measured at WWU, 59 of the 65 produced good PCA solutions. Once again, there was little difference between solutions calculated with and without being anchored to the origin. The good results have a MAD of <10° and display a consistent direction after overprint removal. Shipboard AF demagnetization results were similar, but the thermally demagnetized samples displayed much more erratic behavior. Of the shipboard measurements, characteristic remanence directions for 48 of the 60 with MAD <10° were judged to have produced reliable results (Table T2).

Low-temperature treatment was applied to 39 samples at WWU (Table T5). The massive flow units had the greatest change in magnetic intensity. Samples that showed a large drop of intensity were dunked more than once to assure full removal of multidomain magnetite and hematite overprints. Because this treatment was used to reduce the effect of overprint acquired by multidomain magnetic grains, this result implies that multidomain grains contribute significantly to the magnetization of these units. The thin inflation units displayed little change in intensity with low-temperature dunking, implying that they have few multidomain grains contributing to their magnetization.

Site U1347 characteristic remanence inclinations are mostly low and positive (Fig. F12). Samples from the uppermost ~15 m of the basement (~160–175 mbsf) and lowermost ~40 m, below ~270 mbsf, show higher scatter than elsewhere, mostly because of greater scatter in thermal demagnetization results. Samples appear to show at least three groups of inclinations. Between ~175 and 210 mbsf, the average inclination is 20°–30°, and the same is true for the section between ~240 and 270 mbsf. In between, the average inclination appears shallower, ~10°–15°. Sample inclination scatter is high in the uppermost ~15 m and lowermost ~40 m, so we cannot tell whether those samples give a significantly different inclination than the middle section without further analysis.

Site U1348

Only five volcaniclastic samples from Site U1348 were measured as a test. Only three samples were strong enough to measure NRM. Of the three samples, two were measured using thermal demagnetization and one was measured using AF demagnetization. Demagnetization did not produce consistent magnetization directions, so the Site U1348 section was considered unsuitable for further study.

Site U1350

A total of 109 samples were measured from Site U1350 cores, 42 samples measured during Expedition 324 and 67 measured at WWU. Good results were produced from 102 samples (Table T3). At WWU, 33 samples were demagnetized using AF demagnetization and 34 using thermal demagnetization. The AF demagnetized samples showed two responses similar to the previous sites, low MDF (Fig. F13) and higher MDF (Fig. F14). Both types of samples display univectorial decay after the overprint is removed. The amount of overprint correlates with the MDF. A low-MDF sample typically displays a large overprint, whereas a higher MDF sample displays smaller overprint. The thermally demagnetized samples displayed two behaviors, one with a small partial self-reversal at 300°C (e.g., Doubrovine and Tarduno, 2004, 2005) and one without (Figs. F15, F16). Some thermally demagnetized samples showed erratic behavior above 500°C. Varying degrees of overprint were evident, but for most samples the overprint did not have a significant effect on the measured direction. After the overprint was removed, samples with both thermal demagnetization behaviors showed univectorial decay to the origin. Of the 67 samples, 63 yielded a consistent primary magnetization with a MAD <10°. As with samples from other sites, no significant difference existed between PCA solutions using the origin as an anchor and those without an anchor. The 42 samples that were measured during Expedition 324 gave similar AF results, but the thermally demagnetized samples measured on the ship show a much more erratic behavior than those measured at WWU. Results from samples with consistent directional behavior were similar to those measured at WWU. A total of 39 of the 42 samples had a MAD <10° and are considered to have produced reliable results (Table T3). In all, 32 samples were treated by dunking in liquid nitrogen to remove the effects of multidomain grains, but the effects were negligible (Table T6).

A plot of inclination versus depth shows that inclinations are all close to zero with slightly negative inclinations being the norm (Fig. F17). AF and thermal demagnetized sample inclinations give similar values, but results from thermal demagnetization are more erratic between ~195 and 235 mbsf. Inclinations at the top and bottom of the hole appear indistinguishable, but the section between ~195 and 235 mbsf may have a more positive average inclination. Because this is also the section with higher scatter, this inference cannot be confirmed without further analysis; it may be that all samples from Site U1350 record the same inclination.

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