HR: 0830h
AN: V51A-03    [Abstracts]
TI: A Geochemical Comparison of 2 Recent EPR Eruptions North and South of the Clipperton Fracture Zone
AU: * van der Zander, I
EM: izander@hawaii.edu
AF: SOEST, University of Hawai`i, Honolulu, HI 96822 United States
AU: Rubin, K H
EM: krubin@hawaii.edu
AF: SOEST, University of Hawai`i, Honolulu, HI 96822 United States
AU: Zierenberg, R A
EM: zierenberg@geology.ucdavies.edu
AF: Dept. of Geology, University of California-Davis, Davis, CA 95616 United States
AB: A fresh looking lava flow was discovered at 10°44'N north by Alvin divers during the FIELD (Nov 2003) cruise to the "magma starved" segment of the East Pacific Rise (EPR) north of Clipperton Fracture Zone (J. Voight, Chief Scientist). ²¹⁰Po-²¹⁰Pb dating demonstrates that an eruption occurred here within a few months of the cruise (van der Zander et al., Fall 2004 AGU abstract). The magmatically starved nature of this eruption locale provides for an interesting contrast with the 1991-92 lava flow at 9°50'N on a magmatically robust segment south of the Clipperton. Results of geochemical analyses reveal interesting contrasts in their petrogenesis. Compositional heterogeneity in the 10°44'N lava flow is much higher than in the 9°50'N flow (3.2 vs. 1.6 using the index in Rubin et al., EPSL, 2001), consistent with the much lower magma supply and great extent of crustal processing to the north. U-Series disequilibria signatures are very different between the two lava flows: (²¹⁰Pb/²²⁶Ra) from the area basalts all show (²¹⁰Pb)-excesses ranging from about 10 to 50 % excess, whereas 9°50'N sheet flow basalts show equilibrium up to 10 % deficits values. Within flow (²¹⁰Pb/²²⁶Ra) variations in the 10°44'N samples correlates with indices of fractionation such as SiO₂/MgO, FeO^*/MgO and trace element concentration, and is thus probably related to crustal processes. (²²⁶Ra/²³⁰Th) from the northern basalts are lower and more homogeneous than the south of Clipperton basalts, which may relate either to melting/melt transport conditions or decay during magma chamber residence. (²³⁰Th/²³⁸U) on the other hand is higher than for the 9°50'N samples, which can be interpreted as lower upwelling or melting rate at 10°44'N. However, the higher (²³⁰Th/²³⁸U) of the latter also correlates with the higher Th/U ratios, suggesting a source enrichment effect may also be playing a role on (²³⁰Th/²³⁸U). Ba/Th and ⁸⁷Sr/⁸⁶Sr of the 10°44'N are also elevated, whereas K/Ti ratios overlap in both flows. We discuss these differences and explore the possibility of crustal contamination as a contributor to the geochemical signatures as well as time constraints on crustal and melting processes at the NEPR.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1020 Composition of the crust
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly