Introduction

The Bent Hill area, which is situated in the Middle Valley, northern Juan de Fuca Ridge, was the subject of detailed investigation during Ocean Drilling Program (ODP) Legs 139 and 169 (Fig. F1). A massive sulfide mound is located ~100 m south of the southern edge of Bent Hill (Bent Hill Massive Sulfide deposit, BHMS) (Fig. F2). A second mound of massive sulfide is present ~300 m further south (Ore Drilling Program Mound) (Fig. F2). Drilling during Leg 139 established that the BHMS deposit was formed by the discharge of hydrothermal fluids (350°-400°C) at the ocean floor; it also established that the metals forming the massive sulfides were derived from leaching of a basaltic source (Davis, Mottl, Fisher, et al., 1992; Goodfellow and Peter, 1994; Peter et al., 1994). The lack of intercalations of sediments suggested a rapid formation of the sulfides. The BHMS was revisited during Leg 169 mainly for the purpose of assessing the thickness and the lateral extent of the mineralization. Various holes were drilled on north-south (Holes 1035A, 1035C, 1035F, 1035E, 1035H) and east-west (Holes 1035A, 1035D, 856H, 1035G) transects (Fig. F2). The sulfides recovered were subdivided into three sulfide zones consisting of clastic sulfides at the top, massive sulfides, and a sulfide feeder zone at the base (Fouquet, Zierenberg, Miller, et al., 1998). The clastic sulfides consist of sulfide breccia and rubble that derive from the top of the massive sulfide unit. The massive sulfide unit is characterized by a sulfide content >50%. Krasnov et al. (1994) distinguished five zones within the massive sulfide unit. The first zone is mainly composed of pyrite with minor amounts of sphalerite and magnetite. Pyrrhotite is the dominant mineral in the second zone, whereas the third zone is mainly composed of pyrite. The fourth zone consists of pyrrhotite with minor amounts of chalcopyrite. Based on geochemical data and mineralogical observations, Krasnov et al. (1994) suggested that secondary alteration and recrystallization significantly influenced the second and third zones.

Age dating of the massive sulfide deposits from the Bent Hill area on the basis of biostratigraphy proved to be difficult. Few investigations have been done on the Quaternary subarctic biostratigraphy in the Middle Valley area (Brunner, 1994), and calcareous material is poorly preserved in upper Pleistocene sediments because of the shallow calcite compensation depth during this time. Furthermore, fossils like diatoms, radiolarians, and calcareous nannofossils are found only rarely in hydrothermal deposits (Stakes and Franklin, 1994).

In recent years many successful investigations have been conducted to date hydrothermal deposits by using natural radionuclides of the U-Th decay chain as chronometers (Lalou et al., 1993; Lalou et al., 1985; Lalou and Brichet, 1987; Lalou et al., 1998). The disequilibria between ²³⁰Th/²³⁴U and ²³¹Pa/²³⁵U can be utilized to date hydrothermal deposits covering time intervals of between 1,000-300,000 yr and 1,000-120,000 yr, respectively. There are, however, several assumptions to be made if ²³⁰Th/²³⁴U and ²³¹Pa/²³⁵U disequilibria are to be used for dating purposes (Lalou and Brichet, 1987):

1. No exogenous ²³⁰Th and ²³¹Pa should be incorporated into the deposits during and after their formation. Therefore, all the ²³⁰Th and ²³¹Pa measured in the sample ought to come from the decay of authigenic ²³⁴U and ²³⁵U, respectively. An indication of the validity of this assumption would be the absence of ²³²Th, which is generally of exogenous origin.

2. All the U measured in the sample must be coprecipitated with the deposit and no addition or loss of U may occur thereafter (closed system).

We have studied the ²³⁰Th/²³⁴U and ²³¹Pa/²³⁵U disequilibria in massive sulfides obtained during Legs 139 and 169 from the BHMS and the Ore Drilling Program Mound to estimate the ages and history of massive sulfide formation in these areas.