HR: 1340h
AN: T33B-1348    [Abstracts]
TI: Petrography of Lava Flows from the 9 North Overlapping Spreading Center, East Pacific Rise
AU: * Zaino, A J
EM: ajz20@duke.edu
AF: Duke University, Nicholas School of the Environment and Earth Sciences, Box 90227 103 Old Chemistry, Durham, NC 27708, United States
AU: Perfit, M
EM: perfit@geology.ufl.edu
AF: University of Florida, Dept of Geological Sciences, Box 112120 University of Florida, Gainesville, FL 32611, United States
AU: Klein, E
EM: ek4@duke.edu
AF: Duke University, Nicholas School of the Environment and Earth Sciences, Box 90227 103 Old Chemistry, Durham, NC 27708, United States
AU: Wanless, D
EM: dorseyw@ufl.edu
AF: University of Florida, Dept of Geological Sciences, Box 112120 University of Florida, Gainesville, FL 32611, United States
AU: Ridley, W I
EM: iridley@usgs.gov
AF: United States Geological Survey, Denver Federal Center Box 25046, MS 973, Denver, CO 80225, United States
AB: The region of the 9 deg 03" N overlapping spreading center (OSC) along the East Pacific Rise was sampled extensively (~300 lavas) by the ROV Jason II in spring 2007 (AT 15-17). Sampling locations included both the east and west limbs of the OSC, as well as off-axis areas of interest, and within the overlap basin. Diverse lava compositions were recovered, ranging from basalts to dacites. Here we report initial petrographic results. Approximately 85% of samples are either aphyric or sparsely phyric, but the remainder (42 samples) contain more than 3% phenocrysts by volume. The most porphyritic rocks (11 samples) are comprised of up to 20% phenocrysts. The highest silica rocks contain few phenocrysts, of which most are plagioclase (with minor clinopyroxene) <1 mm in length. The dominant crystal phases in evolved basaltic rocks are plagioclase and clinopyroxene, up to 4 mm in length. The groundmass of most samples exhibit a variolitic texture. Microphenocrysts are predominantly plagioclase and clinopyroxene. Cotectic intergrowth of these two phases is common in rocks retrieved from the east limb, and may either aggregate into microlitic dendrites, or surround larger xenocrysts. Equilibrium-phase skeletal olivines can only be identified in samples from the west limb, though highly resorbed olivines exist in samples from both limbs. Phenocrysts of plagioclase, clinopyroxene, olivine, and rarely orthopyroxene are seen in samples from the east limb. Many of these exhibit oscillatory (plag, cpx) or sector zoning (cpx), or are highly resorbed, suggesting an extended history of multistage residence in the magma chamber. The complex phase chemistry coupled with macroscopic flow banding common in some higher silica samples suggest mixing between basaltic and high silica magmas. These observations of phenocryst populations and crystal growth patterns can be used to elucidate the physical and chemical processes that occurred in the magma reservoir system prior to eruption, and which lead to the great diversity of magma compositions sampled. Particularly when combined with the major and trace element analyses of the quenched glasses, the study of these crystals allows us to gain a more complete picture of the recent history of magmatism at this OSC, including processes such as fractional crystallization, magma mixing, and influxes of new melt into the magma reservoirs.
DE: 3614 Mid-oceanic ridge processes (1032, 8416)
DE: 3640 Igneous petrology
SC: Tectonophysics [T]
MN: 2007 Fall Meeting