The Occurrence and Origin of Andesites and Dacites From the Southern Juan de Fuca Ridge

We have studied the geology of the Cleft segment of the Southern Juan de Fuca Ridge and the ridge-transform intersection (RTI) during three cruises of MBARI's R/V Western Flyer beginning in 2000. A total of 53 rock cores and 276 precisely located rock or glass samples were collected during sixteen dives with the ROV Tiburon. These ROV dive samples and observations along with regional (EM300) and local-scale (side scan and video imaging) mapping, allow us to test models regarding the magmatic evolution of this segment and the relationships between specific tectonic and morphologic features and magmatic processes. An extremely wide range of N-type lavas were recovered which are, on average, more evolved (lower MgO) off-axis, away from the present neovolcanic zone (the cleft), and towards the RTI. During the fall of 2004, a ROV dive was specifically planned to investigate a region of unfaulted, curved volcanic ridges that overshoot the Blanco Transform. During the dive 39 samples were recovered that range from ferrobasalt to andesite and dacite (SiO2 = 50.3 to 66.4 wt.% ; Mg# = 41 to 7). The highly evolved lavas were recovered from two large constructional domes comprised of unusually large pillow flows, and extremely blocky, vesicular flows, similar to some terrestrial silicic domes. Some of the andesite-dacite hand samples are extremely vesicular with elongate vesicles (1-10 cm) in a glassy matrix. The andesitic and dacitic lavas also have elevated Cl- levels that range from 4000-6000 ppm. Mineral assemblages are dominated by microphenocrysts of ferroaugite and ferropigeonite, with lesser amounts of sodic plagioclase and FeTi oxides, and rare fayalite and zircon. A few of the more magnesian phenocrysts (xenocrysts?) exhibit normal zoning whereas more Fe-rich crystals are reversely zoned. In addition, there appear to be inclusions of quenched basaltic material within some of the evolved lavas. These samples represent an extensive and unique set of some of the most highly fractionated ocean floor rocks that have ever been recovered; particularly from such a well-documented setting. Fractional crystallization models which predict over 80% crystallization do not adequately explain the major element chemistry of the silicic lavas and most incompatible trace elements exhibit significant enrichments relative to predicted concentrations. The highly evolved nature of the dacites, crystal zoning patterns and the presence of basaltic inclusions suggest the lavas are the result of magma mixing between more typical basalts and rhyolites likely generated from partial melting of hydrated basaltic. Non-steady state propagation of mantle-derived melts into the static RTI environment may lead to the generation of such silicic oceanic lavas.