Event Name : Togiak volcanics 263000 yBP
|Start: 263000 (± 22000 Years) || Years BP Ar/Ar || |
|Lava flow: ||
|Eruption Product: || basalt ||
From Kaufman and others (2001): "To more closely date the tuya eruption, we performed single-step, laser-fusion 40 Ar/39Ar analyses on a capping basalt flow. The dated sample (DK95-17; Fig. 2 [in original text]) was collected from near the base of the subaerial flows, about 17 m above the well-exposed pillow lava described above. We choose to date basalt that erupted subaerially to avoid potential complications of incomplete degassing of the chilled pillows. In selecting subsamples for laser fusion, phenocrysts (another potential source of excess Ar) were avoided and groundmass-rich subsamples were dated (cf. McDougall and Harrison, 1988). Six sub-samples yielded a weighted average age of 263+/-22 ka (Table 4 [in original text]). They averaged 85% atmospheric Ar content, which, together with visual inspection, indicates that the basalt is not signicantly altered. The isotopic compositions of the subsamples varied only slightly, indicating anisotopically homogeneous sample, but precluding precise isochron determination of the age. Because we found no evidence for deposition or erosion separating the pillow lava from the capping subaerial flow, we believe that 40Ar/39Ar age closely limits the age of the glacial lake. The age also overlaps with the amino acid age estimate of the youngest aminozone on Hagemeister Island, indicating that the eruption of the tuya may have partially overlapped with the deposition of glacial-marine sediment in Togiak Bay."