Paleointensity record in zero-age submarine basalt glasses: testing a new dating technique for recent MORBs

Thellier-Thellier paleointensity experiments were conducted on a collection of glasses from three very recent submarine axial flows. Two were erupted along the Juan de Fuca ridge at around 46 degreesN and one along the East Pacific Rise South at around 18 degreesS. The within-sample dispersion of paleointensity results from the 'Animal Farm' flow (EPR south) is very low and leads to a well-defined mean value of 35.6 +/- 1 muT (95% error on the mean) based on 11 glass chips from four independent samples. Today's geomagnetic field intensity in the area is 31.2 muT. Comparing Animal Farm results with published field model reference curves developed for the past 400 yr suggests an eruptive date estimated between 1880 A.D. and 1950 A.D. (taking into account different sources of errors). This is consistent with qualitative evidence for the age of this flow and constitutes the first precise demonstration of using paleointensity as a dating tool for very recent mid-ocean ridge basalts (MORBs). However in the Juan de Fuca area results show a more erratic pattern with samples varying by up to 30% higher and lower from the expected value of about 55 muT The dispersion is attributed to the large local crustal magnetic anomalies in this area that can lead to inconsistent intensity values over the same unit. Local magnetic anomalies should thus always be checked when doing paleointensity on MORE samples which should also be distributed as widely as possible in a flow unit. When no significant magnetic anomalies are detected the paleointensity dating tool is anticipated to be especially efficient to investigate the volcanic cyclicity along the EPR axis during the last several hundred years. (C) 2000 Elsevier Science B.V. All rights reserved.