Seagoing Cruise MV1203 to the Walvis Ridge seamount trail sailed in February-March 2012 and mapped 49 seamounts and placed in total 62 dredges, of which 47 contained relatively fresh basaltic rocks.
The Walvis Ridge is a hotspot-related seamount trail in the South Atlantic, and one of the most striking seafloor features on the African plate. The old end of the trail, beginning at the Etendeka flood basalt province of Namibia (130 Ma) is a continuous volcanic ridge, but at about 70 Ma this ridge fans out into a region of diffuse guyot volcanism. This transition may be related to the migration of the Mid-Atlantic Ridge away from the Tristan hotspot or to a natural decrease in magma supply of the Tristan and/or Gough plumes, although these hypotheses have yet to be confirmed. The presence of two spatially and geochemically distinct sub-tracks within the guyot province is also of great interest.
This project will use rock samples collected from the guyot province to better understand the evolution of volcanism at the young end of the Walvis Ridge. The scientific aims of the project are to verify the hotspot model for the young Walvis Ridge and to understand the role of the regional tectonic setting and state of the lithosphere on the formation of seamounts in this region. Additional goals are to trace the geochemical evolution of the Walvis Ridge and genetically link the older volcanism with the Tristan and Gough sub-tracks of the guyot province. The results will also hopefully allow us to construct an improved absolute plate motion model for the African Plate.
The samples for this project were collected during a seven-week dredging expedition to the young Walvis Ridge Guyot Province in February and March 2012 on the R/V Melville. Cruise MV1203 traveled a total of about 15’800 km, in a loop starting and ending in Cape Town, South Africa. During the cruise about 1280 kg of samples were collected, 75 seamounts were mapped, 78 seamounts were named, and 49 seamounts were dredged in a total of 62 dredge attempts.
Rock recovery was relatively low compared to other seamount dredging cruises, due to the large amount of sediment and manganese crust covering the seamounts. Rock types recovered include sediment, manganese crust, basalt cobble breccias, volcaniclastic breccias and both aphyric and porphyritic basalts in a wide range of alteration states. The dominant phenocrysts are clinopyroxene, plagioclase and in some cases biotite and alkali feldspar. Olivine is in all cases highly or completely altered. A variety of shells, corals and deep-sea organisms were also recovered.More Information about the Walvis Ridge Cruise can be found at EarthRef.org
Update #1 – January 2013
My name is Susan Schnur and I am a PhD student working on the Walvis Ridge project at OSU. I’ll be writing some updates on the work progress over the next few years as we analyze the samples from cruise MV1203. My current focus is on preparing the rocks for analysis in the OSU Argon Geochronology Lab. Once we have acquired reliable age spectra for many of the seamounts we can begin the task of piecing together the history of the Walvis Ridge. Figure 1 provides an overview of the ages already available and the locations (red circles) where we anticipate obtaining new ages.
Currently we are entering the processing stage of the project. The priority samples have been organized, sorted and catalogued. We have also described about 160 thin sections. Thin sections are an important way to screen rocks for 40Ar/39Ar geochronology as seawater alteration is a major problem in the analysis of seafloor basalts. Of the MV1203 thin sections more than half are highly or completely altered, which means that groundmass dating will likely not be successful for most of these rocks. However, we are fortunate enough to have abundant plagioclase and in some cases biotite and/or k-feldspar phenocrysts, which are likely to give us some high-quality ages.
The next step will be to process these samples in our lab by splitting and crushing the rocks then separating the mineral grains and finally acid leaching the groundmass and feldspar separates. This will be done with the help of our undergraduate student workers Julie Klath and Trevor Smith. Julie will also be doing an acid leaching study using some of the Walvis rocks, which will help us understand how our leaching procedure affects the quality of the age spectra for rocks with different degrees of alteration.