Enriched MORB in the Northeastern Pacific, Petrological and Geochemical Features of igneous Basement at Site 1224, ODP Leg200

During Ocean Drilling Program (ODP) Leg 200, 45-Ma igneous basement was cored in the northeastern Pacific at Site 1224. The basement surface was assumed to be 28 m below seafloor (mbsf). Basement lithology down to 170 mbsf is divided into three units: Unit 1 massive flow, Unit 2 pillow breccia, and Unit 3 massive flow. The shallowest Unit 1 shows massive structure with some altered layers and vein deposits, and core recovery was 30%-50%. This unit is divided into two thick lava flows based on grain size stratigraphy and alteration layers. Intermediate-depth Unit 2 shows different characteristics than Unit 1. Core recovery was so low (<10%), and many of the small pieces retain almost circumferential alteration. Some calcite-cemented hyaloclastite layers were found. The difference in induration between the two units was so great that the drill bit broke at the Unit 1/Unit 2 boundary. The deepest Unit 3 shows features similar to Unit 1. Portions of at least two cooling units were recovered, and possibly more. Recovery in Unit 3 was lower than that in Unit 1 but higher than that in Unit 2 (20%~30%). Bulk compositions of Site 1224 rocks show interesting characteristics, the most important of which is high high-field-strength element (HFSE) content compared to typical normal and enriched mid-ocean-ridge basalts (MORB) (Sun and McDonough, 1989). Chemical stratigraphy (chemostratigraphy) differences among the three units at this site are clear. Unit 3 rocks are fractionated, and Unit 2 rocks have relatively primitive FeO/MgO ratios. Large-ion-lithophile element (LILE) content is higher Unit 2 than in Units 1 and 3. The high LILE content of Unit 2 is thought to be caused by hydrothermal alteration rather than a petrographic feature. HFSE content patterns are similar to those of FeO/MgO ratios. Unit 1 is separated into upper and lower subunits, which correspond to flow units, on the basis of HFSE content. Unit 2 has lower HFSE content and higher Y/Zr ratios, thought to result from a magma generation environment that was different from other units. The parent mantle of Unit 2 basalt was probably of a different, depleted composition compared to Units 1 and 3. Perhaps the most interesting result from this site is isotope characterization. The Sr and Nd isotope ratios are more enriched than typical Pacific MORB (Hickey-Vargas et al., 1995). These characteristics are attributed to mantle enrichment, and this enriched component is thought to still exist in the Pacific mantle. Many drill sites in the eastern Pacific show compositions similar to N-MORB (Sun and McDonough, 1989) (i.e. ODP Leg148; Brewer et al., 1996). Brewer et al. (1996) studied chemostratigraphy of the basement at from Holes 896A and 504B. They concluded that variations in chemostratigraohy were mainly caused by differentiation. When the Leg148 sites and Site 1224 results, are compared, Site 1224 basement shows more than twice the HFSE content than Sites 896 and 504, and the compositional changes is noted at Sites 896 and 504 differ from those of Site 1224. It is thought that the mid-ocean-ridge volcanism at Site 1224 was produced from more enriched mantle than that of the recent Eastern Pacific Rise and that these activities were associated with different mantle sources simultaneously. Chemostratigraphic and lithologic differences between the basement units correlate to differences in physical properties between the three units. Each lithologic unit displays different P-wave velocity, bulk density, and other physical properties. Physical properties are thought to be associated with petrological features.