DISTINGUISHING ERUPTIVE UNITS ON THE SOUTHERN EAST PACIFIC RISE USING GEOMAGNETIC PALEOINTENSITY AND GEOCHEMISTRY

Abstract

The size and frequency of mid-ocean ridge eruptions are important variables in understanding ocean crustal accretion, but are poorly understood. Young flows are difficult to date because recurrence intervals are typically shorter than the resolution of most radiometric dating systems. We attempt to differentiate eruptive units at 17°-19°S on the superfast spreading southern East Pacific Rise (SEPR), by combining geochemical data with geomagnetic paleointensity of submarine basaltic glass (SBG) specimens to assess inter- and intra-flow variability through a joint cluster analysis. At the SEPR, geochemistry alone is not necessarily indicative of distinct eruptive events. Paleointensity allows us to add a temporal constraint, as field intensity is recorded at the time of eruption. Samples were collected during Alvin and Nautile dives where direct observations enabled relative age assessments (Sinton et al., 2002). Specimens were chosen from known flow fields to contrast with data across contacts and within the identified flow. These known flow classifications allow us to evaluate the natural scatter in a single eruptive event and to test the cluster approach. Each flow was represented by 2 to 15 samples, but a statistical analysis indicates these numbers are insufficient to differentiate most flows based on paleointensity alone. The cluster analysis models four data clusters interpreted to represent distinct flow units in the 17°26’S region and seven clusters around 18°38’S. Clusters generally align with prior flow observations, and incongruities could indicate a newly identified flow or a lack of samples needed to provide statistical distinction. The addition of geochemistry can differentiate flows with non-statistically distinct paleointensity means. However, if both geochemistry and paleointensity are similar, a larger sample size would be required to differentiate flows. In addition to identifying the number and size of eruptive clusters, global magnetic field models evaluated at the SEPR allow us to estimate absolute ages for defined flows based on paleointensity. Paleointensity estimates range from 28-43 μT at flows surrounding 17°26’S and 30-48 μT at flows surrounding 18°38’S. Estimated ages for 17°26’S flows place eruptions between 1887-2016. Estimated dates for 18°38’S eruptions are between 1890-1920. Once flows were defined, absolute ages from paleointensity determined recurrence intervals at 17°26’S to be ~100 years and intervals at 18°38’S to be a few decades. Ref: Sinton et al. (2002), J. Geophys. Res., 107, 1-21, doi: 0.1029/2000JB000090.