The hydrothermal addition of excess 40Ar at the lava flows from the early Jurassic in the Hartford Basin (Northeastern U.S.A.): Implications for the time scale

Abstract

Conventional KAr dates were obtained for specimens of basalt flows from 15 locations throughout the Hartford Basin of Connecticut. This basin contains sedimentary rocks that, on the basis of paleontologic information, accumulated over a period of time from the Late Triassic to Early Jurassic. The lava flows were extruded in the Early Jurassic. The KAr dates for the Hampden Flow exhibit considerable scatter and increase to anomalously high values with increasing degree of hydrothermal alteration and decreasing K content. The average dates for the Hampden Flow samples with a K content >0.3% and >0.4% (i.e. those most suitable for dating) are 206±13 and 198±7 Ma (2σ), respectively. Dates for the Talcott Flow are invariant over a range of K concentrations from 0.3–1.4% and average 187 ± 3 Ma (2σ, standard error on the mean). A comparison of. (1) 40Ar/g vs. % K; (2) dates vs. % K; (3) degree of scatter of dates; (4) 40 Ar 39 Ar age spectra; and (5) degree of concordancy of dates for mineral separate-whole rock pairs for the two flows, strongly suggests the presence of small amounts of excess radiogenic 40Ar in some samples of the Hampden Flow and its absence from the Talcott Flow. The mean dates for flows and sills of the Hartford Basin cited by others in attempts to define the numerical age of the Triassic-Jurassic boundary are equivalent to that for the Hampden Flow and thus are likely to reflect the presence of excess radiogenic 40Ar The 187±3 Ma (2σ) date obtained in this study for the Talcott Flow probably represents a reliable estimate of the age of the igneous rocks of the Hartford Basin and should replace older dates previously cited for these rocks in attempts to establish the numerical age of the Triassic-Jurassic boundary.