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doi:10.2204/iodp.proc.324.104.2010 PaleomagnetismThe major goals in paleomagnetism on the recovered cores were to characterize the paleomagnetic remanence and resolve paleolatitude from the magnetization components recorded in the igneous rocks. At Site U1347, we focused only on discrete sample measurements from the working halves of the igneous cores using the Molspin Minispin magnetometer and alternating-field (AF) demagnetizations using the DTech degausser. We also carried out thermal demagnetizations on discrete samples using the Schonstedt oven. Our use of discrete samples only was dictated by ongoing problems with the passthrough 2G cryogenic magnetometer (see "Paleomagnetism" in the "Methods" chapter). Working-half discrete sample measurementsWe measured and analyzed 61 discrete basalt samples (7 cm3) from various lithologies downhole (Table T11). Samples from this site are less altered compared to Hole U1346A. We preferred carrying out thermal demagnetization because this method is thought to be better for isolating magnetization components in oceanic basalt samples that may contain self-reversed chemical remanent magnetization (e.g., Doubrovine and Tarduno, 2005). Twenty-three samples were AF demagnetized up to 140 mT in steps of 2, 5, or 10 mT, and thirty-nine were thermally demagnetized up to 600°C. Ten samples from Sections 324-U1347A-24R-1 through 29R-4, which were thermally demagnetized, were treated with a 5 mT AF demagnetization step prior to the 100°C heating step in an attempt to erase the drilling overprint. Magnetic susceptibility was measured after each heating step in order to detect possible mineralogical changes (for more details, see "Paleomagnetism" in the "Methods" chapter). Room temperature susceptibilities for basalts range between 2.0 x 10–2 and 9.2 x 10–2 SI. Most of the samples have a steep vertical component at natural remanent magnetization (NRM) and first demagnetization steps, typical of drilling overprint (e.g., Audunsson and Levi, 1989; Wilson, Teagle, Acton, et al., 2003), but this is usually erased at ~10 mT or 200°C (Fig. F58). Alternating-field demagnetizationsBefore the removal of the drilling overprint, samples carry a very strong NRM. However, the median destructive fields (MDF) are very low, from 1.2 to 13.9 mT with an arithmetic mean of 4.6 ± 3.0 mT (Fig. F58; Table T11). This low MDF often makes it more difficult to separate overprints from the characteristic remanent magnetization compared to samples from Site U1346. Nevertheless, it is usually possible to isolate a stable component that decays toward the origin of orthogonal vector plots for field steps between ~15 and 80–100 mT. To further investigate the origin of the low-coercivity components in igneous rocks recovered from Hole U1347A, we carried out a simple anhysteretic remanent magnetization (ARM)-AF demagnetization test on a few samples after the completion of the AF demagnetization of the NRM. This test allows us to understand the coercivity of those samples and possible domain state of magnetic carriers. Three samples from massive flows (Samples 324-U1347A-27R-2, 23–25 cm; 28R-5, 16–18 cm; and 28R-8, 80–82 cm) were selected for this test. We first applied an ARM (with a peak AF of 100 mT superimposed on a direct current field of 0.1 mT) on the samples and then demagnetized them at 0, 5, 10, 20, 30, 40, and 50 mT using the DTech demagnetizer. Resulting MDF values from the ARM-AF demagnetization are 7.61 mT for Sample 324-U1347A-27R-2, 23–25 cm; 2.99 mT for Sample 28R-5, 16–18 cm; and 2.89 mT for Sample 28R-8, 80–82 cm (Fig. F59). These low values confirm that multidomain grains are most likely the magnetization carriers. Inclinations and declinations were calculated using principal component analyses (Kirshvink, 1980) anchored to the origin for AF demagnetization steps between ~15 and 80–100 mT. Low values of maximum angular deviation show that these directional results are of high quality (Table T11). Three AF demagnetization results from Section 324-U1347A-12R-1 show very shallow negative inclinations, whereas all samples taken from Section 324-U1347A-12R-2 and deeper show positive inclinations. Thermal demagnetizationsCompared to AF demagnetization results, most thermal demagnetizations poorly show the characterization of the magnetic remanence. The variation of the magnetization with temperature reveals that two unblocking temperatures are generally present, one at ~300°–400°C, corresponding to titanomagnetite (-maghemite) with an ulvospinel fraction x of ~0.4–0.5 (Hunt et al., 1995) and the other at ~575°C, indicating the presence of almost pure magnetite. Many thermal demagnetizations also show a large drop in magnetization after heating to 100°C, suggesting the presence of an overprint with a very low unblocking temperature. Bulk magnetic susceptibilities measured at room temperature after each heating step stay more or less constant for heating steps up to ~300°C. At higher temperature, susceptibilities increase to 2–3 times the room temperature values and then decrease again. This could indicate that the primary magnetic carrier could be titanomaghemite that inverts to strongly magnetic magnetite as a result of heating (Özdemir and O'Reilly, 1982). Two main behaviors were observed in the thermal demagnetizations. In the samples from Sections 324-U1347A-12R-1 to 26R-1 (stratigraphic igneous Units IV–XIV), the low-temperature part of the demagnetization (from room temperature to ~450°–500°C) behaves so erratically that it is impossible to define a stable direction. However, the high-temperature part (500°–600°C) often enables us to obtain a fairly stable direction with maximum angular deviation <8°. However, in this temperature range the carrier of magnetization is probably no longer the primary carrier, which has been oxidized by the heating, as indicated by large changes in bulk susceptibility. Nevertheless, it is likely that the alteration product, which might be the component with the high unblocking temperature, inherits its remanence direction from the original titanomaghemite (Marshall and Cox, 1971; Matzka and Krása, 2007). Furthermore, this high-temperature direction does point toward the origin on orthogonal vector plots indicating a stable, primary remanence. The second behavior was observed in the lowermost part of the cored section (Sections 324-U1347A-26R-2 through 29R-4; stratigraphic igneous Unit XV). The low-temperature part of the demagnetization (up to ~300°C) is stable enough in some cases to give a direction pointing toward the origin of orthogonal vector plots and with maximum angular deviation <8°, whereas the high-temperature part is too weak to give a consistent direction. As a result, we could not extract a stable remanence direction from about half of these samples. The susceptibility variation with temperature is also different; no large increase in susceptibility with heating exists. Instead, the susceptibility decreases at ~300°C, stays more or less constant, and then drops after 500°C. This suggests that the magnetic mineralogy is different from that observed in the samples measured from shallower cores. Also, it must be noted that the very last sample that we studied, which belongs to a different flow (Unit XVI; see "Igneous petrology") shows a behavior similar to that from the shallow part of the section.
Overall, samples that exhibit simple univectorial decay are rare in thermal demagnetization results from the igneous section in Hole U1347A. Only eight samples show a consistent behavior with maximum angular deviation <5°. We can classify the rest of the samples into three groups:
For the stratigraphic plot of inclination, we kept only samples for which maximum angular deviation is <8°. Downhole inclinations and tectonic implications
Slight changes in inclination correlating with the different stratigraphic units exist in Hole U1347A (Fig. F60). The averaged inclinations show four inclination groups:
The top negative polarity found in the upper massive basalt flow (Section 324-U1347A-12R-1) may suggest the existence of polarity reversals in Hole U1347A, indicative of a late-stage eruption with respect to underlying massive flow sections. The massive basalt stratigraphic Units IV–VII show steeper inclinations than those of upper and middle pillow lava Units IX–XIV. A few explanations are possible for this discrepancy. First, geomagnetic secular variation may not be averaged throughout Hole U1347A because the series of eruptions happened over a short period of time. Second, the drilling overprint may be not completely erased because of abundant multidomain titanomagnetites formed by the relatively slow cooling of the massive flows. Third, steeper than expected inclinations may have resulted from block rotations of a stack of flows caused by lava loading and regional tectonics. The averaged inclination for the lowermost section (lower pillow lava to lower massive basalt flow) should be interpreted with caution because most of the samples from this section gave unreliable results. Of the four AF demagnetized samples used, two have a steep inclination, whereas the other two have a shallow inclination. Magnetic susceptibility values suggest an abundance of magnetic minerals in the rocks from these sections. However, the slow cooling of the thick lava units probably formed multidomain titanomagnetite, which was chemically altered during low-temperature oxidation as this part of Shatsky Rise aged. As a result, the magnetic properties in this section might have become unstable for retaining NRM. We found that several samples are plagued with a large viscous remanent magnetization, sometimes on the timescale of the laboratory measurements, which is consistent with the suspected presence of large multidomain grains as carriers of the magnetization. Detailed rock magnetic measurements will be necessary on shore before we can interpret these directional results with certainty. | |||
