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Methods and materials

NRM data compilation from DSDP, ODP, and IODP sites on the Pacific plate

There are many reported igneous basement rock NRM data from DSDP, ODP, and IODP sites. Data are usually from discrete samples taken from igneous basement rocks or from split half-cores of basement rocks. We include data in this compilation if they satisfy all of the following criteria:

  • The sample was recovered from the Pacific plate.

  • The sample is categorized as extrusive basalt (for example, pillows or flows).

  • The sample originates from a mid-ocean-ridge spreading center and has mid-ocean-ridge basalt (MORB) geochemistry.

The first criterion was not adopted in the data compilation by Johnson and Pariso (1993b), as they compiled data regardless of the plate from which the sample was taken. The second and third criteria are the same as those adopted in Johnson and Pariso (1993b), who applied these criteria to limit the study to “normal” upper oceanic crust samples.

Rock magnetic and paleomagnetic measurements on Expedition 320/321 basement rocks

Expedition 320/321, “Pacific Equatorial Age Transect,” aimed at constructing a continuous Cenozoic record of the equatorial Pacific (see the “Expedition 320/321 summary” chapter [Pälike et al., 2010]). Coring was done at eight sites (U1331–U1338) located at the predicted paleoposition of the paleoequator at successive crustal ages on the Pacific plate. Although coring was designed primarily to recover pelagic sedimentary sections, short portions of igneous basement rocks were also recovered from seven sites in order to confirm that the coring reached to acoustic basement. We have completed rock magnetism and paleomagnetism studies on samples taken from five sites (Fig. F1), the crustal ages of which are estimated as follows: 49–50 Ma (Site U1332), 45–46 Ma (Site U1333), 38 Ma (Site U1334), 26 Ma (Site U1335), and 24 Ma (Site U1337) (see the “Expedition 320/321 summary” chapter [Pälike et al., 2010]).

Cylindrical minicores 2.5 cm in diameter were taken from split core catcher samples or working-half cores. The core catcher samples are all small cobbles, and they do not have either vertical or horizontal orientation information.

Minicores were taken from two cobbles from intervals 320-U1332B-18X-CC, 22–27 cm, and 18X-CC, 27–33 cm, and three cobbles each from intervals 320-U1333B-20X-CC, 18–24 cm, 20X-CC, 25–30 cm, and 20X-CC, 30–33 cm; 320-U1334A-32X-CC, 1–9 cm, 11–17 cm, and 32X-CC, 18–22 cm; and 320-U1335B-46X-CC, 30–35 cm, 46X-CC, 37–41 cm, and 46X-CC, 41–46 cm. We could put horizontal reference lines to the minicores only from the cobble from interval 320-U1333B-20X-CC, 18–24 cm, because this cobble is relatively long (~6 cm long) and thus unlikely to have overturned during coring.

The working-half cores consist of three pieces from intervals 321-U1337C-33X-4, 0–11 cm, 33X-4, 12–35 cm, and 33X-4, 35–47 cm, and one piece from interval 321-U1337D-49X-3, 18–39 cm. The cores were not azimuthally oriented because they were drilled with the extended core barrel. Each of four minicores was taken from Holes U1337C and U1337D from pieces with horizontal reference lines.

Remanence measurements were made on cubic specimens ~5 mm on a side cut from the minicores using one of three magnetometers: 2G Enterprises Model 755R and 760R superconducting quantum interference device magnetometers and a Natsuhara-Giken DSPIN-2 spinner magnetometer. Stepwise thermal demagnetization (ThD) was conducted at 20°–35°C steps up to 600°C using a Natsuhara-Giken TDS-1 thermal demagnetizer. Stepwise alternating field demagnetization (AFD) was performed at 1–30 mT steps up to 180 mT using a Natsuhara-Giken DEM-95 AF demagnetizer, or at 1–5 mT steps up to 80 mT using a 2G Enterprises Model 760R built-in AF demagnetizer. Before stepwise AF demagnetization, specimens underwent low-temperature demagnetization (Ozima et al., 1964; Heider et al., 1992) to try to isolate remanences carried by single domain (SD) particles. Laboratory anhysteretic remanent magnetization (ARM) was imparted by a 50.0 µT biasing field with a maximum AF of 180 mT using a Natsuhara-Giken DEM-95 AF demagnetizer.

Hysteresis parameters and first-order reversal curve (FORC) (Roberts et al., 2000) measurements were conducted on selected samples using a Princeton Measurement Corporation MicroMag 3900 vibrating sample magnetometer (VSM). The samples are tiny chips, and their weights are about a few tens of milligrams. A maximum field of ±1 T was used to saturate magnetization. Several chips per minicore were subjected to the hysteresis parameter measurements, whereas one chip per minicore was adopted for the FORC measurements. FORC diagrams were drawn using the UNIFORC code (Winklhofer and Zimanyi, 2006; Egli et al., 2010).

Thermomagnetic curve measurements were performed in both air and vacuum (~1–10 Pa) on chip samples using a Natsuhara-Giken NMB-89 magnetic balance. Weights of the chips were similar to those used in hysteresis measurements. Experiments were typically started from ~30°C, and a sample was gradually heated to 700°C. After reaching the highest temperature, the sample was then gently cooled to ~100°C. Throughout the temperature cycle, a field of 500 mT was kept applied to the sample. Heating and cooling rates (10°C/min) were the same between air and vacuum runs. To investigate low-temperature variation of saturation magnetization, we also used a Quantum Design magnetic property measurement system (MPMS-XL5). A chip sample was warmed from –263°C (10 K) to 27°C (300 K) with constant application of a 500 mT field.

Several specimens were observed by a JEOL JSM-6500F field emission scanning electron microscope (FE-SEM). For observations, sliced pieces from the cylindrical minicores were impregnated with epoxy and subsequently polished to mirror gloss.