SAMPLE SITES

Sites 169-171 were cored during DSDP Leg 17 in the Central Pacific. Site 169 (10.7°N, 173.6°E) is located on the abyssal seafloor ~300 km east of Mejit Island in the Marshall Islands (Shipboard Scientific Party, 1973a). Igneous rocks were recovered in four cores: diabase sills in Cores 17-169-5R and 6R (6.1 m recovered) and basalt in Cores 17-169-11R and 12R (4.0 m recovered). The igneous rocks from Site 169 have not been dated; however, the basalts from the bottom of the hole are probably crustal basalts (Bass et al., 1973), so the age can be inferred from magnetic lineations. Site 169 sits on Anomaly M21 (Nakanishi et al., 1992), which implies a date of 148 Ma using the timescale of Gradstein et al. (1994). The diabase sill is likely mid-Cretaceous in age, similar to other sills that are widespread in the western Pacific (Schlanger et al., 1981).

Site 170 was drilled on the abyssal seafloor in the Central Pacific Basin (11.8°N, 177.6°E). The only igneous rock recovered was 3.0 m of altered basalt in Core 17-170-16R (Shipboard Scientific Party, 1973b). Although the basalt has not been dated, an age can be inferred from magnetic lineations mapped in the Central Pacific Basin. Site 170 is located on Anomaly M16 (Nakanishi and Winterer, 1998), implying a date of 140 Ma (Gradstein et al., 1994).

Site 171 (19.1°N, 169.5°W) is located atop Horizon Guyot at the north end of the Line Islands chain (Shipboard Scientific Party, 1973c). Coring recovered a single basalt flow, ~3 m in thickness, from among Cenomanian limestones in Cores 17-171-26R and 27R. In addition, some basalt fragments were recovered at the bottom of the hole, in Core 17-171-33R, but no fragments were large enough for paleomagnetic sampling. Although basalt samples from Site 171 have not been dated, dredged basalt from the flanks of Horizon Guyot gave a ³⁹Ar/⁴⁰Ar total fusion date of 88.1 ± 0.4 Ma (Schlanger et al., 1984) (this and other age errors are given as 1).

Site 581 (43.9°N, 159.8°E) is located on the abyssal seafloor of the northwest Pacific plate, north of Shatsky Rise. The site was cored during Legs 86 and 88. Drilling during Leg 86, Hole 581 was cored as a pilot hole for a downhole seismometer experiment conducted during Leg 88. Hole 581 was terminated after basement was struck and 3.7 m of basalt in Core 86-581-19R (Shipboard Scientific Party, 1985) was recovered. Basalt cores were recovered during Leg 88 from Holes 581A, 581B, and 581C (Shipboard Scientific Party, 1987). According to the Leg 88 Initial Reports volume (Shipboard Scientific Party, 1987), Hole 581A was sited 293 m south of Hole 581, whereas Hole 581B was sited 305 m north of Hole 581A, which would put it within 12 m of Hole 581, if the positioning was accurate. As is seen in the results (below), differences in magnetic polarity suggest that the holes are not that closely spaced. No details of the siting of Hole 581C were given in the Leg 88 Initial Reports volume. In Hole 581A, one core (88-581A-3R) was drilled into basement, recovering 4.9 m of basalt. Coring in Hole 581B recovered 15.8 m of basalt from four cores (88-581B-1R through 4R). This section was divided into 13 flows on the basis of cooling boundaries (Shipboard Scientific Party, 1987). Hole 581C penetrated deepest into basement (20.6 m), recovering 16.3 m of basalt, divided into six flow units (Shipboard Scientific Party, 1987).

No accurate dating has been done for Site 581 basalts. However, an estimate of the age is possible from seafloor magnetic lineations. Site 581 is located between Anomaly M1 and another lineation that is un-identified (Nakanishi et al., 1999). From the pattern, it appears there was a ridge jump in the vicinity and the unidentified lineation is slightly older than Anomaly M1. Thus, we can estimate the age of the lineation at 124-127 Ma (Anomaly M1-M3) using the timescale of Gradstein et al. (1994).

During DSDP Leg 92, Site 597 was drilled on abyssal seafloor in the southeast Pacific (18.6°S, 129.8°W; Shipboard Scientific Party, 1986). The site is located on Anomaly C9, which indicates the seafloor is 27.5 Ma in age (Cande and Kent, 1995). Although basalt was cored in four holes, pieces large enough to be vertically oriented were only recovered in Holes 597B and 597C. Hole 597B penetrated 24.6 m of igneous section, recovering 5.4 m, whereas Hole 597C penetrated 91 m of basement, recovering 48.5 m. Although the section was lacking clear chilled margins that indicate flow boundaries, shipboard scientists divided the section into three units based on differences in olivine content (Shipboard Scientific Party, 1986). The boundary between the upper two units, I and II, is located between Sections 92-597C-7R-5 and 8R-1 (100.5 meters below seafloor [mbsf]), whereas the boundary between Units II and III is situated in Section 10R-2 (121.3 mbsf).

Paleomagnetic measurements were conducted on Site 597 samples because it appeared that the initial shipboard studies did not include demagnetization on basalt samples to properly remove overprints and determine the characteristic magnetization direction (Shipboard Scientific Party, 1986). However, alternating-field (AF) demagnetization was performed on 69 samples, reported in another section of the Leg 92 Initial Reports volume (Nishitani, 1986). Although the samples were demagnetized in fields up to 90 mT, Nishitani did not explain how the characteristic remanence inclination values were calculated. Having collected additional samples and made new analyses in this study, I report the new data here and combine them with the older data.

During Leg 129, three dolerite sill units that intruded into Berriasian sediments located on Jurassic-age abyssal seafloor in the western Pacific were cored in Hole 800A (21.9°N, 152.3°E; Shipboard Scientific Party, 1990). Approximately 7.1 m of igneous core was recovered from 46.4 m of section. These sills are probably part of a widespread mid-Cretaceous volcanic episode (Schlanger et al., 1981). A ³⁹Ar/⁴⁰Ar radiometric date of 126.1 ± 0.6 Ma was determined from Hole 800A dolerite samples (Pringle, 1992). Shipboard paleomagnetic studies used whole-core fragments and did not demagnetize above 12 mT (Shipboard Scientific Party, 1990), suggesting these preliminary results were not accurate owing to difficulties in measuring irregular core pieces and incompletely removed overprints.

A 22.5-m section of tholeiitic basalt flows and pillow lavas was cored in Hole 803A (2.4°N, 160.5°E) on the Ontong Java Plateau during Leg 130 (Shipboard Scientific Party, 1991). Approximately 12.9 m of igneous core was recovered, and shipboard scientists divided the section into nine igneous units (Shipboard Scientific Party, 1991). A weighted average of four ³⁹Ar/⁴⁰Ar radiometric dates given in Mahoney et al. (1993) yields an age of 90.2 ± 1.2 Ma for the igneous section. No magnetic measurements have been reported from these rocks.

Hole 865A (18.4°N, 179.6°W) was drilled into the summit of Allison Guyot in the Mid-Pacific Mountains during Leg 143 (Shipboard Scientific Party, 1993). Coring penetrated four basaltic sills, recovering 18.3 m of igneous core, but only three of the units yielded pieces suitably oriented for paleomagnetic study. A ³⁹Ar/⁴⁰Ar radiometric date of 110.7 ± 1.2 Ma was determined for these sills (Pringle and Duncan, 1995). Preliminary shipboard paleomagnetic measurements were made from large oriented core pieces (Shipboard Scientific Party, 1993) but are likely inaccurate for reasons previously stated.