Previous   |   Next

doi:10.2204/iodp.proc.301.204.2009

Introduction

During Integrated Ocean Drilling Program (IODP) Expedition 301, 3.5 Ma age crust was drilled on the flank of the Juan de Fuca Ridge at Site U1301, located at 47°45.2′N, 127°45.8′W (see the “Expedition 301 summary” chapter). The site targeted an area of hydrothermal fluid flow for an experiment examining the hydrology of the ocean crust. Although the ridge flank is buried by unusually thick sediments because of the proximity of land, it is otherwise the site of normal oceanic crust. More than 200 m of upper oceanic crust basalt was cored in Hole U1301B, making it one of the relatively few sites where such a large section of crustal rock has been sampled. For scientists who study paleomagnetism, this represents an excellent opportunity to learn more about the magnetization of the upper crust.

We sampled the basalt section from Hole U1301B with the goal of learning about the magnetization properties of the recovered basalt samples. Because the crust appears normal, we expected to find that the basalts were magnetized in the direction of the magnetic field 3.5 m.y. ago, when the crust at Site U1301 was formed. Our working hypothesis was that the samples should behave as typical ocean crust basalts, should be normally polarized (because the site is on a normal polarity magnetic anomaly), and should give magnetization inclinations that are similar to the geocentric axial dipole inclination for the site (65.6°). We also expected that some of the samples would display evidence of hydrothermal alteration, owing to the location of the site in an area of hydrothermal circulation.

Background

Hole U1301B was one of four holes drilled at Site U1301. The drill string encountered the seafloor at a depth of 2656 meters below sea level (mbsl) and penetrated to 582.8 meters below seafloor (mbsf). Drilling in Hole U1301B showed that the upper 265.2 m of the section is sedimentary. Thus, the lower 317.6 m of the drilled section penetrated upper crustal igneous rocks.

Because of problems coring the fractured upper part of the igneous section at nearby Site 1026 during Ocean Drilling Program (ODP) Leg 168 (Shipboard Scientific Party, 1997), the upper 85 m of the igneous section in Hole U1301B was not cored. Only ~5% recovery was obtained throughout this section during Leg 168, so the upper igneous section is not well known. After casing for hole stability during Expedition 301, igneous rocks below 85 m beneath the sediment/basalt contact were continuously cored. A total of 69.1 m of igneous core was recovered from 235 m of cored section for a recovery percentage of ~30%.

The cored igneous section was divided into eight units defined by changes in lava morphology, rock texture, and grain size (see the “Site U1301” chapter). Units 1, 3, 5, 7, and 8 are characterized by pillow basalts, whereas Units 2, 4, and 6 are massive units (Fig. F20 in the “Site U1301” chapter). The massive units are generally thin (<10 m thick), so most of the igneous section (and samples recovered) consists of pillow basalts.

Pillows from the cores are generally sparsely vesicular and micro- or cryptocrystalline in texture. Massive basalts are similar but have few vesicles. Both types of rock are normal depleted mid-ocean-ridge basalts (MORB) as indicated by geochemical analyses (see the “Site U1301” chapter). Alteration in the core samples ranges from slight to moderate (5%–25%), with the slightly altered rocks generally dark gray in appearance. The most unaltered parts of the samples are in flow interiors, and the alteration has proceeded to saponite grade (see the “Site U1301” chapter). More intense alteration has proceeded generally along cracks and veins, often forming alteration halos around such features. The halos have varied colors (black, brown, and green) depending on the mix of secondary minerals. Potentially magnetic iron oxyhydroxides were found in ~44% of the veins examined, pyrite was found in ~3%, and goethite was found in one vein.

Top of page   |   Previous   |   Next