For this study, we reexamined all core sections from Sites 1149 and 1179 containing discrete ash layers (i.e., Cores 185-1149A-1H through 14H, 191-1179B-1H through 6H, and 191-1179C-2H through 17H) and identified a total of 94 discrete layers of vitric ash in the upper 120 m of the sedimentary column from Site 1149 and 16 discrete ash layers in the upper 190 m of the sedimentary column from Site 1179. In addition, many layers of clay were recognized, particularly at Site 1179, which may correspond with altered ash layers. These altered clay beds were not considered in this study.
Samples were taken from the discrete ash layers for smear slide examination and 40Ar/39Ar analyses. Smear slides were made to assess the approximate percentage of volcanic glass in the samples in order to determine their usefulness for 40Ar/39Ar analyses. Smear slide analyses showed the discrete ash layers at both sites to be mainly composed of well-sorted glass shards with few lithic fragments. Samples were selected for 40Ar/39Ar analyses based on (1) composition of ash layers (>60% of vitric glass, but commonly >80%), (2) their position in the stratigraphic column with respect to other ash beds, and (3) their position in the stratigraphic column with respect to the available ages derived from shipboard magnetostratigraphic studies (Tables T1, T2).
Samples were prepared and irradiated at the Oregon State University (OSU) (USA) Noble Gas Mass Spectrometry Laboratory. Detailed explanation about sample preparation and the irradiation process can be obtained from the OSU Web site: www.coas.oregonstate.edu/research/mg/chronology.html. In general, samples were sieved starting with the smallest size to remove detrital clays. The procedure started with a sieve opening of 63 µm. Most samples from Sites 1149 and 1179 were very clean glass shards and did not contain detrital clay. Those samples containing clay (i.e., Samples 185-1149A-12H-6, 79–80 cm, and 13H-3, 115–116 cm) were sieved incrementally increasing the sieve size opening until most of the clay was removed and a glass shard concentrate of ~95% was obtained. The samples were then washed, dried, and passed through a Frantz magnetic separator, followed by mild acid cleaning (HNO3) for cleaning and final removal of detrital clay adhering to the glass shards. The concentrates were wrapped in copper foil, packaged in evacuated quartz vials, and irradiated in the OSU TRIGA reactor for 6 hr at 1 MW power. Reactor temperatures can reach 270°C. The neutron flux was measured using standard FCT-3 biotite (28.03 Ma) (Renne et al., 1994).
Values of the irradiation parameter J for individual sample packages were calculated by parabolic interpolation between the analyzed standards. After irradiation, milligram-sized fractions were spread on the bottom of 7-mm-diameter holes of a copper holder placed into an ultra high vacuum laser port and baked at 195°C for 48 hr during extraction line pump-down to ~10–9 torr.
The samples were incrementally heated. Depending on sample composition, the incremental heating experiment may start at 200°–600°C and typically is complete by 1400°C using a Heine low-blank resistance furnace with a Ta/Nb crucible and Mo liner. Each heating step is 20 min duration with an additional 5 min cooling and continued removal of active gases with St101 Zr-Al and St172 Zr-V-Fe getters.
A MAP 215-50 rare gas mass spectrometer, source at 3000 V, equipped with a Johnston MM1-1SG electron multiplier at 2050 V was used for the analysis. During the 15-min analysis time per heating step, data are collected for 10 cycles of m/z = 35–40 for baselines and peak tops. Data are reduced and age calculations completed using ArArCALC (version 2.2) for 40Ar/39Ar geochronology (Koppers, 2002), which for the age calculation includes the following decay constants: