Petrogenesis and Alteration of Young Pacific Ocean Crust Formed at a Fast-Spreading Ridge: Results From ODP Leg 203, Hole 1243B
Résumé
ODP Leg 203 established a new legacy hole for a long-term Dynamics of Earth and Ocean Systems/Ocean Seismic Network observatory and sampled basement in a young fast-spreading environment at Site 1243. The age of basement at this site, based on a full spreading rate of 141 mm/yr, the East Pacific Rise subsidence curve, and the lower most sediments is $\sim$11 Ma. Basement was drilled and cored to a total depth of 195.3 mbsf, sampling 87.1 m of oceanic crust with an average recovery of 25.5\%. Although Leg 203 did not quite achieve the proposed crustal penetration of 100 m, the core returned from Hole 1243B is significant given the sparse sampling of deep basement rocks from young Pacific seafloor. All the recovered volcanic rocks are aphyric or plagioclase$\pm$olivine-phyric basalts. On a SiO2 vs total alkali diagram, two alkaline units are distinguished from the five remaining tholeiitic units. Samples from Units 1 and 3 display the most primitive compositions while the alkaline units are more evolved. The tholeiitic units display REE patterns characteristics of N-MORB and the alkaline units are typical of E-MORB. The basement sampled at Hole 1243B is relatively fresh with Loss On Ignition (LOI) varying between 0.10\% and 3.95\%. The alteration paragenesis is dominated by iron-oxyhydroxide (Fe(O,OH)x), brown clay minerals, Ca-carbonate and zeolites and restricted to narrow rims of pillows. The Sr isotope compositions of the basement samples are close to unaltered basalt isotope compositions. Most 87Sr/86Sr ratios vary from 0.7029 to 0.7035 although three samples yielded higher 87Sr/86Sr ratios (from 0.7042 to 0.7047). All measured O isotope compositions are above mantle values ranging between 6.5 and 8.5 within the tholeiitic units and from 6.6 to 9.1$\permil$ within the alkaline units. There is a clear relationship between both the O and the Sr isotope compositions of the basalts and their LOI. These geochemical data are indicative of low temperature interactions between rocks and seawater-derived fluids and typical of the low temperature seafloor weathering of the ocean crust. The characteristics of basement recovered at Site 1243 are distinctive from those previously sampled in young Pacific oceanic crust in the well studied Holes 504B or 896A. The main differences are (a) the occurrence of more enriched units composed of alkali basalts and typical of E-MORB and (b) the overall low degree of alteration of the oceanic crust.