Long-term cosmogenic 3 He production rates (152 ka^1.35 Ma) from 40 Ar/ 39 Ar dated basalt £ows at 29³N latitude Tibor J. Dunai *, Jan R. Wijbrans Isotopengeologie, Faculteit der Aardwetenschappen, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands Received 3 June 1999; received in revised form 1 December 1999; accepted 1 December 1999 Abstract A set of time integrated cosmogenic 3 He 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 þ 26 ka, 281 þ 19 ka and 1.35 þ 0.01 Ma ( þ 2c). Three or four different olivine phenocryst samples have been analyzed from each flow for their cosmogenic 3 He abundances. The resulting 3 He production rates in olivine at sea level at 29³ latitude are 82 þ 14 and 82 þ 8 atoms g 31 a 31 , 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 þ 8 atoms g 31 a 31 is consistent with the cosmogenic 3 He 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 3 He in olivine at sea level and high latitudes ( s 60³) of 118 þ 11 atoms g 31 a 31 (þ2c). The correspondingly revised value of Cerling and Craig, and Ackert et al.) is 123 þ 6 atoms g 31 a 31 (þ2c). The mean value of these two calibrations is 121 þ 6 atoms g 31 a 31 (þ2c). We suggest that the production rate of 121 þ 6 atoms g 31 a 31 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. ß 2000 Elsevier Science B.V. All rights reserved. Keywords: cosmogenic elements; helium; production; rates; magnetic ¢eld; exposure age 1. Introduction The reliability of application of in-situ pro- duced cosmogenic nuclides to dating of exposed surfaces critically depends on the accurate knowl- edge of production rates [6^8]. However, the pro- duction rates vary spatially, and through time. The spatial dependence is the combined e¡ect of atmospheric depth, through absorption of pri- mary and secondary cosmic rays (e.g. [9]), and the in£uence of the geomagnetic ¢eld which de- £ects the charged primary cosmic rays as a func- tion of its horizontal ¢eld strength and inclination 0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII:S0012-821X(99)00308-8 * Corresponding author. Tel.: +31-20-4447398; Fax: +31-20-6462457; E-mail: dunt@geo.vu.nl Earth and Planetary Science Letters 176 (2000) 147^156 www.elsevier.com/locate/epsl