ALBERT

Description: During Ocean Drilling Program Leg 200, ~45 Ma igneous basement, was cored in the northeastern Pacific at Site 1224. The basement surface was inferred to be 28 meters below seafloor (mbsf). Basement lithology down to 170 mbsf is divided into three major units: Unit 1 = massive flow, Unit 2 = pillow breccia, and Unit 3 = intermixed pillows and thin flows. The bulk compositions of Site 1224 rocks have high abundances of the high-field strength elements (HFSE) Y, Zr, and Nb relative to normal and even enriched mid-ocean-ridge basalts. Chemical stratigraphic differences among the three units at this site are clear. Unit 3 rocks have the highest FeO/MgO ratio and HFSE concentration, and Unit 2 lavas have the lowest. Unit 2 lavas also have the highest Y/Zr ratios. Unit 1 is separated into lower and upper flow units based on differences in HFSE content. Clearly, there were significant differences in the petrogenetic processes that created these units. Compositional and lithologic differences among the basement units correlate with differences in physical properties among the units described by the Leg 200 Shipboard Scientific Party. Physical properties are therefore associated with petrological features.

Description: The ~46-m.y.-old igneous basement cored during Leg 200 in the North Pacific represents one of the few cross sections of Pacific oceanic crust with a total penetration into basalt of 〉100 m. The rocks, emplaced during the Eocene at a fast-spreading rate (~14 cm/yr; full rate) are strongly differentiated tholeiitic basalts (ferrobasalts) with 7-4.5 wt% MgO, relatively high TiO2 (2-3.5 wt%), and total iron as Fe2O3 (9.1-16.8 wt%). The differentiated character of these lavas is related to unusually large amounts of crystallization differentiation of plagioclase, clinopyroxene, and olivine. The lithostratigraphy of the basement (cored to ~170 meters below seafloor) is divided into three units. The deepest unit (lithologic Unit 3), is a succession of lava flows of no more that a few meters thickness each. The intermediate unit (lithologic Unit 2) is represented by intermixed thin flows and pillows, whereas the shallowest unit (lithologic Unit 1), comprises two massive flows. The rocks range from aphyric to sparsely clinopyroxene-plagioclase-phyric (phenocryst content = 〈3 vol%) and from holocrystalline to hypohyaline. Chilled margins of pillow fragments show holohyaline to sparsely vitrophyric textures. Site 1224 oxide minerals present a type of alteration not previously seen, where titanomagnetite is only partially destroyed and the pure magnetite component is partially removed from the mineral, leaving, in the most extreme case, a nearly pure ulvöspinel residuum. As a result of this dissolution, iron, mainly in the oxidized state, is added to the circulating solvent fluids. This means that a considerable metal source can result from low-temperature reactions throughout the upper ocean crust. The coarsest-grained lithologic Unit 1 rocks have interstitial myrmekitic intergrowths of quartz and sodic plagioclase (~An12), roughly similar in mineralogy and bulk composition to tonalite/trondhjemite veinlets in abyssal gabbros from the southwest Indian Ocean and Hess Deep, eastern equatorial Pacific. Based on idiomorphic relationships and projections into the simplified Q-Ab-Or-H2O granite ternary system, the myrmekitic intergrowths formed at the same time as, or just after, the oxide minerals coprecipitated and at low water vapor pressure (~0.5 kbar). Their compositions correspond to SiO2-oligoclase intergrowths that are considerably less potassic than dacitic glasses that erupt, although rarely, along the East Pacific Rise or that have been produced experimentally by partial melting of gabbro. Based on the crystallization history and comparison to experimental data, the original interstitial siliceous liquids resulted from late-stage immiscible separation of siliceous and iron-rich liquids. The rare andesitic lavas found along the East Pacific Rise may be hybrid rocks formed by mixing of these immiscible siliceous melts with basaltic magma.