2002 Articles

Grain-size-dependent paleointensity results from very recent mid-oceanic ridge basalts

Carlut, Julie; Kent, Dennis V.

We report paleointensity analyses on a suite of four samples from two axial zero-age mid-oceanic ridge basalt flows from the East Pacific Rise. Paleointensity experiments have been performed on several profiles, each consisting of a batch of small (millimeter scale) subsamples going from the rapidly cooled glassy margin to the interior. The Coe version of the Thellier double-heating procedure was used with back checks performed after every other heating step. Most of the samples show very good behavior, i.e., constant ratio between the lost and acquired magnetization through a large temperature range (quality factor usually above 10) and positive checks, which lead to unambiguous paleointensity determinations. Paleointensities obtained on glasses reported here and in a previously published study are very consistent and reproducible and in agreement with expected in situ values of Earth's magnetic field intensity. However, results found within the crystalline part of the samples show values that are considerably (up to 50%) higher than expected. These variations seem to be correlated with the cooling history of the samples and appear to be universal since all samples exhibit the same intriguing pattern. An extensive study of magnetic properties allows us to link this incongruent behavior to (1) the presence of multidomain effects and (2) the cooling rate difference between laboratory experiments and in situ cooling.