Constraining the Age and Evolution of an Axial Volcanic Ridge on the Mid Atlantic Ridge


DOEI Project Funded: 2007


Axial volcanic ridges (AVRs) are a common feature found within the median valley of slow spreading ridges such as the Mid-Atlantic Ridge (MAR). They are sinuous volcanic ridges tens of kilometers long, a few kilometers wide and few hundreds of meters high and comprised of a variety of volcanic products varying from pillow lava benches to hummocky lava to isolated cones, lava terraces and lava lakes. Occurring in over 90% of MAR ridge segments surveyed to date, AVRs are widely recognized as the fundamental volcanic accretionary “building block” in slowly spread crust. Despite this morphological characterization, very little is known about the age and genetic relationships between the various volcanic products that make up an AVR or the age span and episodicity of the volcanic processes involved in its construction. In fact, there are no quantitative age dates of any volcanic products from the median valley of the Mid-Atlantic Ridge. Part of the reason for this has been the lack of dating techniques to address the age ranges involved (<300000 yrs), however, this has now been remedied by the development of U-series disequilibria which has been successfully applied in a variety of environments. Another reason has been the lack of precise geological context for the samples with maps of sufficient spatial resolution to document sample locations in relation to the various volcanic units. This too has been remedied with the use of near-bottom multibeam bathymetric and optical mapping that provide not only sub-meter resolution but also extend such coverage over several kilometers. High-resolution magnetic mapping has also benefited from this scale of mapping and we have found that rapid changes in geomagnetic field intensity (as opposed to polarity reversals) also generate magnetic anomalies that allow us to provide chronological markers as well as infer patterns of crustal accretion.

With these tools and expertise resident here at WHOI we feel the time is ripe to take an important step forward in our knowledge of volcanic emplacement at the slow spreading MAR. We therefore propose to provide the first quantitative age constraints on midocean ridge basalts from the MAR and thereby constrain the evolutionary history of an AVR. We seek funds to participate in a British research expedition that will investigate the history, genesis and evolution of an AVR on the MAR at 45°N using the remotely-operated vehicle (ROV) ISIS and deep towed sidescan sonar vehicle, TOBI. The British team is led by Prof. Roger Searle of DurhamUniversity with a primarily geochemical science party focused on issues related to the evolution of magmas and melting processes with respect to an AVR. Our contribution will be to provide age constraints using high-resolution magnetic chronology with sensors mounted to TOBI and ISIS and absolute age-dating of samples collected by ROV. The cruise is scheduled for 2008 on board RRS James Cook.These data will be important as they will not only provide quantitative age constraints necessary for understanding the evolution of a slow spreading midocean ridge but also how volcanic ages are related to hydrologic and biologic systems. While we will measure only 5-10 samples using these funds, these preliminary data will provide ‘proof-of-concept’ necessary to obtain NSF funding for a more complete study.