Cooling rate effects on paleointensity estimates in submarine basaltic glass and implications for dating young flows

Cooling rate effects on the intensity of thermoremanent magnetization (TRM) have been well documented in ceramics. In that case, laboratory cooling is generally more rapid than the initial cooling, leading to an overestimate of the paleofield by 5-10% in Thellier-type paleointensity experiments. The reverse scenario, however, has never been tested. We examine the effects of cooling rate on paleointensity estimates from rapidly quenched submarine basaltic glass (SBG) samples from 13 sites at 17 degrees 30'-18 degrees 30'S on the East Pacific Rise. Absolute cooling rates determined by relaxation geospeedometry at five of these sites range from similar to 10 to similar to 330 degrees C min(-1) at the glass transition (similar to 650 degrees C). Over the dominant range of remanence blocking temperatures (similar to 200-400 degrees C), the natural cooling rates are approximately equal to or slightly slower than the laboratory cooling rates during the Thellier experiment. These results suggest that while the cooling rate effect might introduce some within-site scatter, it should not result in a systematic bias in paleointensity from SBG. Paleointensity estimates from the 15 sites range from similar to 29 to 59 mu T, with an average standard error of similar to 1 mu T. Comparison with models of geomagnetic field intensity variations at the site indicate the youngest group of samples is very recent (indistinguishable from present-day) and the oldest is at least 500, and probably several thousand, years old. These age estimates are consistent with available radiometric ages and geologic observations.