Data Report: magnetic properties of sediments and basalts from the Costa Rica subduction margin (Expeditions 334 and 344)

Abstract

We measured the rock magnetic properties of 24 basalt, sediment, and tephra samples from Costa Rica Sites U1380, U1381, U1413, and U1414. Day plots indicate that most samples are dominated by populations of pseudosingle-domain (PSD) magnetic minerals, with significant contributions of single-domain (SD) grains in the tephra samples and multidomain (MD) grains in the basalts and coarser grained sediments. Coercivity distributions from first-order reversal curves (FORCs) from basalt samples generally peak at about 2-5 mT, rarely extend beyond 20 mT, and show interactions consistent with the presence of PSD and lesser amounts of MD particles. Distributions are consistent with titanomagnetite and/or titanomaghemite being the primary magnetic minerals, as is typical for oceanic basalts. FORCs for the tephras have broad coercivity distributions with peaks near 40-70 mT and with much less interaction than the basalts. The narrow interaction width reflects lower magnetic concentrations than the basalts, whereas the higher coercivity is consistent with the presence of significant SD magnetite, with possibly small amounts of iron sulfides. Coercivity distributions for sediments are typical of marine sediments with mixtures of mainly PSD and SD particles. These distributions have peaks between about 10 and 50 mT, with interactions that are dependent on magnetic concentrations. Demagnetization experiments of basalt samples show that, after the removal of a low-coercivity drilling overprint, the basalts have a primary magnetization component with relatively shallow inclination (∼20°), consistent with the tectonic history of the Cocos plate. Basalt samples from the top and flank of the Cocos Ridge have slightly different rock magnetic properties, possibly indicating varying components of hotspot volcanism and seafloor spreading.