Long-term cosmogenic ~3He production rates (152 ka-1.35 Ma) from ~(40)Ar/~(39)Ar dated basalt flows at 29°N latitude

摘要

A set of time integrated cosmogenic ~3He production rates in olivines for the last 1.35 Ma are presented. We investigated three basaltic lava flow tops from Lanzarote, Canary Islands, Spain, The ~(40)Ar/~(39)Ar ages determined for those basalt flows by incremental laser heating of leached groundmass samples are 152 ± 26 ka, 281 ± 19 ka and 1.35 ± 0.01 Ma (±2σ). Three or four different olivine phenocryst samples have been analyzed from each flow for their cosmogenic ~3He abundances. The resulting ~3He production rates in olivine at sea level at 29°latitude are 82±14 and 82±8 atoms g~(-1) a~(-1), as obtained from the 152 ka and 281 ka old flows, respectively. Considering effects of erosion on the 1.35 Ma old flow we find that the production rate of 82±8 atoms g~(-1) a~(-1) is consistent with the cossmogenic ~3He production during the last 1.35 Ma. There appears to be a 14% discrepancy between previously published production rates derived at higher latitudes and altitudes if the scaling factors of Lal are used to compare results. This discrepancy is greatly reduced, however, if the revised scaling factors of Dunai (this issue) are applied. Using the new scaling factors we derive a production rate for cosmogenic ~3He in olivine at sea level and high latitudes (＞60°) of 118±11 atoms g~(-1) a~(-1) (±2σ). The correspondingly revised value of Cerling and Craig, and Ackert et al.) is 123 ± 6 atoms g~(-1) a~(-1) (±2σ). The mean value of these two calibrations is 121 ± 6 atoms g~(-1) a~(-1) (±2σ). We suggest that the production rate of 121 ± 6 atoms g~(-1) a~(-1) at sea level and high latitudes may be applied to the complete time range where paleomagnetic data indicate that there was no long-term averaged intensity variation in the Earth's magnetic field, i.e. over the last 10 Ma.