HR: 1330h
AN: V32A-1002    [PDF]
TI: Cobb Hotspot Volcanism Prior to 7 Million Years ago
AU: * Keller, R
EM: rkeller@coas.oregonstate.edu
AF: Oregon State University, College of Oceanic and Atmospheric Sciences, Corvallis, OR 97331 United States
AU: Fisk, M
EM: mfisk@coas.oregonstate.edu
AF: Oregon State University, College of Oceanic and Atmospheric Sciences, Corvallis, OR 97331 United States
AU: Duncan, R
EM: rduncan@coas.oregonstate.edu
AF: Oregon State University, College of Oceanic and Atmospheric Sciences, Corvallis, OR 97331 United States
AU: Rowe, M
EM: rowem@geo.orst.edu
AF: Oregons State University, Department of Geosciences, Corvallis, OR 97331 United States
AU: Russo, C
EM: crusso@coas.oregonstate.edu
AF: Oregon State University, College of Oceanic and Atmospheric Sciences, Corvallis, OR 97331 United States
AU: Dziak, R
EM: dziak@pmel.noaa.gov
AF: OSU/CIMRS, Hatfield Marine Science Center, Newport, OR 97365 United States
AB: From where the Cobb hotspot currently resides beneath Axial Seamount on the Juan de Fuca Ridge, a discontinuous trail of seamounts of increasing age extends 1800 km to the northwest, all the way to the Alaskan Trench off of the southern tip of Kodiak Island. These seamounts record the evolution of mantle melting and volcanism at the Cobb hotspot over the past 30+ million years, including how the approach of the Juan de Fuca Ridge from the east affected the hotspot. We conducted multibeam mapping and stratigraphically-controlled rock sampling of several of the seamounts created by the Cobb hotspot up until 7 Ma. Using the Alvin submersible to do depth transects for geological observations and rock sampling allowed us to establish the volcanic style and setting represented by each sample, and to avoid the thick ferro-manganese oxide coatings and abundant ice-rafted debris common in the Gulf of Alaska. Our goal is to understand the volcanic histories and morphologies of these seamounts with an eye to how volcanism at the hotspot was affected by the approaching ridge. Our targeted seamounts included, from SE to NW, Warwick ($\sim$7 Ma on 9 Ma crust), Murray ($\sim$28 Ma on 39 Ma crust), Patton ($\sim$30 Ma on 42 Ma crust), and Marchand (30+? Ma on 43 Ma crust). Marchand Seamount, though small compared to the others, appears to be the oldest unsubducted volcanic product of the Cobb hotspot. So far, we have XRF data for our samples, and argon dating and trace element analyses are underway. Warwick Seamount yielded only tholeiitic basalts, while most of the samples from the other seamounts are evolved alkalic rocks. Murray samples are entirely alkalic, being dominantly trachytes and trachydacites, with a few mugearites. Rocks from Patton are mainly hawaiites and mugearites, with rare tholeiitic to transitional basalts and a single trachyte. Marchand samples are trachydacites and trachytes similar to the differentiated Patton and Murray samples. Basement drilling at ODP Hole 887D in a saddle near Murray Seamount did recover tholeiites beneath about 40m of alkalic basalt flows interbedded with sediments, so apparently tholeiites, while poorly exposed today, did play some role in the construction of the 28+ Ma seamounts. The lack of alkalic rocks on Warwick may be related to the fact that it formed near the Juan de Fuca Ridge (on 2 m.y. old crust), while the other seamounts formed on 11+ m.y. old crust.
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 9355 Pacific Ocean
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting