⁴⁰Ar/³⁹Ar ages of a diabase sill and a basalt in the Central-Pacific Basin

Abstract

THE deep sea drilling project (DSDP) Leg 17 has covered the area of the Central-Pacific Basin south of the Mid-Pacific Mountains and east of the Marshall Islands. Since this region belongs to the magnetic quiet zone, no definite ages of oceanic crust could be assigned. Results of drilling at Site 166 (3°45.7′N, 175°4.8′W) and at site 167 on Magellan Rise (7°4.1′N, 176°49.5′W) have given estimates of crustal ages of about 120 and 135 Myr respectively from the basement fossils, which suggests that ages increase northward on this set¹. Furthermore, the parallelism of linear bathymetric features around site 167 to the magnetic and bathymetric trends around site 166 has been interpreted by Winterer¹ to mean that they belong to the same spreading ridge system with an average spreading rate of 2–4 cm yr⁻¹ for the crust between the two sites. Since sites 169 and 170 are located to the north-west of the sites 166 and 167 in the magnetic quiet zone, they were expected to give older ages, probably Jurassic, from the extrapolation of the trend stated above and by referring to the time scale of magnetic reversals². It has been revealed, however, that at site 169 (10°40.2′N, 173°3.0′E) the oldest sediment recovered yields Albian age (100–104 Myr), which overlies basalt. At this site, about 8 m of diabase sill was drilled, which intruded Turonian (87–95 Myr) or Cenomanian (95–100 Myr) sediments³. At site 170 (11°48.0′N, 177°37.0′E), the oldest sediment overlied the basalt is also of Albian age⁴. These ages are much younger than those expected from ages obtained at sites 166 and 167, which may reflect the different history of the ocean floor from those at sites 166 and 167. To clarify this point, two igneous rocks were dated by ⁴⁰Ar/³⁹Ar method. Details of experimental procedures are described elsewhere⁵.