An active low-temperature fluid flow system operates within ~24Ma EPR-generated oceanic crust of the Cocos plate near the Middle American trench, as indicated by anomalously low surface and borehole heat flow values (Langseth and Sliver,...
moreAn active low-temperature fluid flow system operates within ~24Ma EPR-generated oceanic crust of the Cocos plate near the Middle American trench, as indicated by anomalously low surface and borehole heat flow values (Langseth and Sliver, GRL, v23, 1996). ODP Legs 170 and 205 examined fluid flow pathways at the Costa Rica margin through coring, downhole logging, and subseafloor monitoring (Kimura et al., Init. Rept. 170, 1997; Morris et al., Init. Rept. 205, 2003). Lateral seawater flow in the upper igneous basement is inferred from basal sediment pore water profiles, which show a return toward seawater chemistry above basement (Silver et al., Geology, 2000). Interpretation of initial results from in situ subseafloor monitors (CORK-IIs) suggests that the upper basement is highly permeable and is hydrologically connected to points of distant recharge (IODP Leg 301T Prel. Rept., 2005). Thermal modeling indicates advective heat extraction in the uppermost 100-600m of basement (Fisher et al., GRL, v30, 2003), an area where lateral permeability may be affected by the abundance of sills identified through correlation of TicoFlux seismic profiles with ODP cores (Silver et al., GRL, v 31, 2004). Legs 170 and 205 cored 180m of low-K tholeiitic gabbros and basalts, consisting of an upper 31m gabbroic sill separated from >148m of a lower igneous unit by ~30m of post-18.2Ma sediment. These units are derived from a homogeneous mantle source enriched relative to EPR MORB; differences in degree of partial melting and crystallization control limited geochemical variation (Dreyer et al., submitted). Low-temperature alteration in the sill is characterized by palagonitization, replacement of primary minerals by clay, zeolitization, and void and vein filling by clays +/- calcite. The lower unit has experienced higher degrees of alteration, though discrete alteration remains low (generally 1-5%, but locally up to 50%). Sparse celadonite and Fe-oxyhydroxides in the recovered rocks suggests reducing conditions prevailed Greater fracture density is observed in cores and borehole logging at depths >510mbsf. Trace elements were measured for 34 igneous rocks from Leg 205 and are within the expected range for variably altered basement. Ratios of fluid-mobile to fluid-immobile incompatible trace elements extend up to ~2x mantle values (Li/Yb, Cs/Rb, Pb/Ce, Ba/La in decreasing order of enrichment). Maximum values generally occur near 513mbsf (Site 1253, Core 24R), coincident with the highest 87Sr/86Sr (0.70516), first downhole appearance of a basalt rather than gabbro, and change in secondary zeolite mineralogy and physical properties. Above this depth, alteration-sensitive trace element ratios increase without changing Sr isotopic ratios and may indicate hydrothermal alteration. Deeper, elevated trace element and Sr isotopic ratios correlate with each other and also with measures of increased fracture density (Pfender and Villinger, submitted) and likely reflect alkali uptake from modified seawater by secondary zeolites. Using seawater compositions, models of open system alteration suggest minimum fluid:rock ratios of ~6 for the most altered sample. These results suggest that Cocos Plate basement cored during Leg 205 records interactions with two distinct fluids over a range in temperatures; CORK-II OsmoSamplers installed in this section of the basement directly sample these fluids.
