INTERNAL CHEMICAL VARIATION OF MASSIVE BASALT LAVA FLOWS, WALVIS RIDGE HOTSPOT

International Oceanic Discovery Program (IODP) Expedition 391 recovered core from basalts from four sites along the Walvis Ridge hotspot track, a chain of ridges and seamounts extending southwest off the coast of Namibia. The purpose of the expedition is to document the evolution of the Walvis Ridge hotspot, as well as understand the volcanology of ridges and seamounts along the hotspot track as an aid to piecing together the evolution of hotspots around the world.

Shipboard portable X-ray fluorescence (pXRF) analyses of cores from these sites (U1575, U1576, U1577, and U1578) identified geochemical zonation internal to several massive lava flows. These massive lava flows range from 5 m to 15 m thick, similar to many continental flood basalt lavas. High recovery rates for these flows (typically 98-100%, with some around 90-93%) allow us to show that they are single flow units bounded by igneous flow contacts or sediment. Detailed analyses (3 cm step size) using the X-Ray Fluorescence (XRF) core scanner at IODP (College Station TX), document changes in major and trace element concentrations within many of these flows that are independent of flow contacts, as well as others that reflect internal crystal fractionation and accumulation. In U1578A Unit 6 (242-257 mbsf) an olivine cumulate layer, visible in hand sample, shows a progressive increase in MgO and Ni below 251 mbsf. In the same interval, magnetic susceptibility increases from 742.7 SI x 10^-5 at 218.0 mbsf to a high of 2221.5 SI x 10^-5 at 219.4 mbsf before returning to 1015 SI x 10^-5 by 219.8 mbsf.

Other flows show internal variations that do not reflect internal crystal fractionation or accumulation. These compositional variations include up to 6% in FeO*, 2% CaO, 1% TiO2, and 70 ppm Sr, among others. In some flows, variations in FeO and CaO/Sr are antithetic, consistent with variable fraction of mafic phases and plagioclase. We propose that these variations may result from multiple instances of flow lobe inflation during emplacement. This interpretation is supported by observations of textural variations in core, e.g., zones of finer-grained basalt separating a dominant coarse grain texture. Multiple episodes of flow lobe inflation may explain the occurrence of these unusually thick massive flows on the seafloor, rather than thin sheet flows or pillow lava.