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°30'-18°30'S on the East Pacific Rise. Absolute cooling rates determined by relaxation geospeedometry at five of these sites range from ~10 to ~330°C min⁻¹ at the glass transition (~650°C). Over the dominant range of remanence blocking temperatures (~200-400°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 ~29 to 59μT, with an average standard error of ~1μT. 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.
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- Geochemistry, Geophysics, Geosystems