2.6 BASALT SAMPLING

2.6a NeMO 2000 Cruise Basalt Sampling Program - John Chadwick

The basalt sampling program obtained 13 wax core samples (11 at Axial, 2 on the southern Cleft segment of the Juan de Fuca ridge) between ROPOS dives, and 35 samples from ROPOS itself. The total for the three-year NeMO program is 124 obtained by wax corer and 105 collected by the submersible, for a total of 229. These basalts are being used to 1) characterize the geochemical gradients in the Axial seamount and rift zone areas, to identify the effects of fractionation and magma mixing in these areas, and to understand how Axial interacts with the Juan de Fuca ridge; 2) to understand the age relationships in the rift zones; and 3) to characterize the geochemical variations in the 1998 lava flow.

Major element, trace element, and isotopic data for the basaltic glass samples are being collected using X-ray fluorescence, electron microprobe, thermal ionization mass spectrometer, and laser ablation techniques. Combined with morphologic and tectonic information from sonar maps, the geochemical data has been used to identify spatial and temporal patterns in geochemistry in the region, as well as different magma sources for the Axial rift zone eruptives.

The intensive basalt sampling program at Axial has revealed geochemical gradients that peak in the caldera region and decline into the northern and southern rift zones, trends that are similar to other hotspot/ridge interactions. The rift zones are composed of parallel-trending sets of dike-fed ridges that emanate from the flanks of Axial seamount and have been suggested to be acting as a segment of the Juan de Fuca ridge, albeit a magmatically oversupplied one. However, age relations discovered by the NeMO sampling suggest that the basalts are increasingly younger from west to east (indicated by the amount of weathering of the samples, amount of sediments recovered with them, and bathymetry), observations that are inconsistent with a normal symmetrical spreading center. The recent interaction of the ridge and hotspot (only 200,000 to 500,000 years) suggests that the upper mantle portion of the magma plume that feeds Axial may be adjusting to the new tectonic setting, and may be temporarily migrating to the east.