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 P acific Rise South at around 18 degreesS. The within-sample dispersion of pa leointensity results from the 'Animal Farm' flow (EPR south) is very low an d leads to a well-defined mean value of 35.6 +/- 1 muT (95% error on the me an) based on 11 glass chips from four independent samples. Today's geomagne tic 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 s uggests an eruptive date estimated between 1880 A.D. and 1950 A.D. (taking into account different sources of errors). This is consistent with qualitat ive evidence for the age of this flow and constitutes the first precise dem onstration of using paleointensity as a dating tool for very recent mid-oce an ridge basalts (MORBs). However in the Juan de Fuca area results show a m ore 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 lar ge local crustal magnetic anomalies in this area that can lead to inconsist ent intensity values over the same unit. Local magnetic anomalies should th us always be checked when doing paleointensity on MORE samples which should also be distributed as widely as possible in a flow unit. When no signific ant magnetic anomalies are detected the paleointensity dating tool is antic ipated to be especially efficient to investigate the volcanic cyclicity alo ng the EPR axis during the last several hundred years. (C) 2000 Elsevier Sc ience B.V. All rights reserved.