HR: 0800h
AN: V51C-1493    [Abstracts]
TI: Sr, Nd and Pb Isotope Geochemistry of Near-ridge Seamounts in Eastern Pacific: Implications for Upper Mantle Composition and EPR Magmatic Segmentation
AU: * Castillo, P R
EM: pcastillo@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, La Jolla, CA 92093-0212 United States
AU: White, W M
EM: white@geology.cornell.edu
AF: Cornell University, Geological Sciences Dept., Ithaca, NY 14853 United States
AU: Batiza, R
EM: rbatiza@nsf.gov
AF: The National Science Foundation, GEO/OCE, 4201 Wilson Blvd., Arlington, VA 22230 United States
AB: Near-ridge seamount lavas tend to reflect the true composition of the upper mantle source of MORB because these are generated by relatively smaller degrees of melting of smaller volumes of the mantle compared to nearby axial lavas; they also by-pass the axial chamber mixing and fractionation processes that are responsible for the relatively more uniform chemical and isotopic composition of normal-MORB. New Sr, Nd and Pb isotope data combined with published data for lavas from near-ridge seamounts on either side of the EPR segment between the 11^o45' OSC and Orozco Transform at 15^o00' show latitudinal isotopic variation very similar to that shown by the rise axial lavas (Castillo et al., G³ 1, 1999). Seamount and axial lavas at both ends of the rise segment have on average slightly higher and more limited range of ¹⁴³Nd/¹⁴⁴Nd, but slightly lower ²⁰⁶Pb/²⁰⁴Pb and ⁸⁷Sr/⁸⁶Sr ratios than lavas at the center of the segment. Some of the seamounts are located on ~8 Ma rise flank crust although most of the seamount lavas are fairly young (e.g., lavas from Seamount 6 on ~3 Ma crust are only 3 to 900 kyr - Graham et al., Nature 326, 1987). Thus near-ridge seamount isotope data provide the first documentation for a large-scale (~350 km long x ~720 km wide), systematic compositional variation of the upper mantle source of EPR MORB. Such a scale of variation is larger and longer than the size and <1 myr life span of the majority of non-transform offsets, which are supposed to be responsible for the along-axis compositional variations of EPR MORB according to the "bottoms up" model of magmatic segmentation.
DE: 3035 Midocean ridge processes
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8416 Mid-oceanic ridge processes (1032, 3614)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005