HR: 1330h
AN: B12A-0746    [PDF]
TI: MORB Petrogenesis and Crustal Development of the East Pacific Rise (EPR) at the R2K ISS (9$\deg$-10$\deg$ N)
AU: * Perfit, M
EM: perfit@geology.ufl.edu
AF: Dept. of Geological Sciences, University of Florida Box 112120, Gainesville, FL 32611-2120 United States
AU: Maclennan, J
EM: maclenna@ipgp.jussieu.fr
AF: Laboratoire de Geosciences Marines, Institut de Physique du Globe de Paris, Paris, 75005 France
AU: Fornari, D
EM: dfornari@whoi.edu
AF: Woods Hole Oceanographic Inst., Geology and Geophysics, Woods Hole, MA 02543 United States
AU: Ridley, I
EM: iridley@usgs.gov
AF: Mineral Resources Team, U.S. Geological Survey, Denver, CO 80225 United States
AU: Sims, K
EM: ksims@whoi.edu
AF: Woods Hole Oceanographic Inst., Geology and Geophysics, Woods Hole, MA 02543 United States
AU: Smith, M
EM: mcsmith@geology.ufl.edu
AF: Dept. of Geological Sciences, University of Florida Box 112120, Gainesville, FL 32611-2120 United States
AB: During the past decade (1991-2002) we collected over 1200 basalt samples from the 9$\deg$ 17' N to 10$\deg$ N section of the EPR; the focus area for the 8$\deg$-11$\deg$ N EPR R2K Integrated Study Site (ISS). Many of the samples were recovered from ~1 km-wide area around the AST with ALVIN, but more than a dozen off-axis traverses documented the geology up to 5 km away from the axis. All of the basalts have been analyzed for major elements and many have complete sets of trace elements. A representative set has been analyzed for Sr, Nd, Pb, Hf and U-series isotopes and volatiles. The data set now available from this area represents the most concentrated and extensive for any portion of the MOR and provides us with the ability to investigate the spatial and temporal variability of magmatism at this ISS and how magmatism is related to crustal structure and melt distribution. Our spatial analyses show that the distribution of basalt types is not symmetric across the crestal plateau and there are distinct chemical discontinuities that correspond to 3d-order ridge segmentation. Chemical variation observed over scales as small as a few 100 meters suggests major changes in magmatic systems on time scales of less than 5 kA. Relatively evolved ferrobasalts and T- and E-type MORB only exist in small areas ($<$ 1 km2) on the crestal plateau and commonly are spatially related to prominent scarps or fissures. Evolved and/or high-K/Ti basalts have not been recovered proximal to the axis (excepting near the 9$\deg$ 37' OSC). Off-axis lavas are statistically different from the more homogenous and mafic axial basalts. All of the samples collected above the axial magma chamber (AMC) contain more than 7.2 wt% MgO while approximately one third of the basalts away from the AMC contain less than 7.2 wt% MgO. Similarly, none of the samples lying above the AMC have K/Ti values of over 12, while about 25% of the samples away from the AMC have more elevated values. Although most of the chemical variability in the N-type MORB can be explained by shallow-level fractional crystallization of similar parental melts, Ra and Th isotopic systematics suggest some of the variation is due to differences in the depths and extents of mantle melting. The combined information obtained from our field studies, chemical analyses and U-series dating confirm that a significant terrain up to ~2.5 km away from the ASC on the crestal plateau has been affected by more recent volcanism than predicted by spreading rate-distance estimates predict. Some of this volcanism is a consequence of relatively voluminous lobate and sheet flows that extend away from their axial eruptive source and some is related to small-volume eruptions related to the in-situ development of off-axis faults and fissures. We suggest a petrogenetic model whereby the differences between the compositions of lavas /magmas erupted on-axis and those erupted off-axis are related to their location relative to crustal magma chambers. Axial eruptions are derived from the region of high melt fractions, and highest temperatures where it is expected that convection will be more vigorous and the AMC appears to be relatively efficient at homogenizing the melt whereas more evolved and enriched lavas are derived from the flanks of the magma chambers/lens. The off-axis eruptions may be due to sills that propagate toward the surface along off-axis faults.
DE: 1020 Composition of the crust
DE: 1749 Volcanology, geochemistry, and petrology
DE: 3035 Midocean ridge processes
DE: 3640 Igneous petrology
SC: Biogeosciences [B]
MN: 2003 Fall Meeting