HR: 1340h
AN: T33D-0587    [Abstracts]
TI: Primitive cumulates in the upper-plutonics from the East Pacific Rise
AU: * Perk, N
EM: neilperk@hotmail.com
AF: University of Victoria, SEOS, Victoria, BC V8W 3P6 Canada
AU: Coogan, L A
EM: lacoogan@uvic.ca
AF: University of Victoria, SEOS, Victoria, BC V8W 3P6 Canada
AB: A suite of olivine rich primitive gabbroic rocks that formed at the East Pacific Rise (EPR) were recovered from the Pito Deep near the north-east corner of the Easter microplate. The samples were collected along a greater than 600 m vertical section of the lower oceanic crust that is less than 250 m below the base of the sheeted dike complex. Twenty samples were collected with the Jason II ROV. The samples are mainly troctolites and olivine gabbros with lesser gabbros and a single amphibole-apatite-oxide gabbro with no clear variation in modal mineralogy with depth. Mineral proportions range from 35 to 96 percent plagioclase, 0 to 30 percent olivine, 0 to 44 percent clinopyroxene and minor orthopyroxene and oxides. Plagioclase crystals range in size from 0.1 to 4.0 mm, are euhedral to anhedral, and show development of sub-grain boundaries and deformation twins in some samples. Olivine crystals range in size from 0.5 to 4.0 mm, are euhedral-anhedral and also show crystal-plastic deformation microstructures. Clinopyroxene generally form interstitial crystals and range in size from 0.1 to less than 10 mm. Preliminary electron microprobe analysis reveals a narrow range of primitive mineral compositions (spinel Cr number 45-55, olivine Fo 82-88, plagioclase An 80-84, and clinopyroxene Mg number 87-89). Clinopyroxene Cr2O3 contents range from 0.3 to 1.1 and are highest at the shallowest levels but most other elements show little variation with sampling depth. These results imply that primitive melts can be transported to shallow levels beneath the East Pacific Rise (EPR) where they crystallize primitive cumulates. This contrasts with plutonics recovered from a similar level in the crust at Hess Deep which are much more evolved suggesting that spatial-temporal variations in magma chamber processes are important along the EPR. The evidence for crystal plastic deformation suggests that the upper level gabbros undergo high-temperature strain consistent with crystal subsidence models or an element of tectonic extension even at this ultra-fast spreading portion of the EPR.
DE: 1021 Composition of the oceanic crust
SC: Tectonophysics [T]
MN: Fall Meeting 2005