Abstract
Carbonate veins in the igneous basement and in the lithified sedimentary cover of the Cocos Ridge at International Ocean Discovery Program (IODP) Hole 344‐U1414A reveal the hydrologic system and fluid‐rock interactions. IODP Hole 344‐U1414A was drilled on the northern flank of the Cocos Ridge and is situated 1 km seaward from the Middle America Trench offshore Costa Rica. Isotopic and elemental compositions were analyzed to constrain the fluid source of the carbonate veins and to reveal the thermal history of Hole 344‐U1414A. The formation temperatures (oxygen isotope thermometer) of the carbonate veins in the lithified sedimentary rocks range from 70 to 92 °C and in the basalt from 32 to 82 °C. 87Sr/86Sr ratios of the veins in the altered Cocos Ridge basalt range between 0.707307 and 0.708729. The higher ratios are similar to seawater strontium ratios in the Neogene. 87Sr/86Sr ratios lower 0.7084 indicate exchange of Sr with the igneous host rock. The calcite veins hosted by the sedimentary rocks are showing more primitive 87Sr/86Sr ratios <0.706396. The isotopic compositions indicate seawater, modified into a hydrothermal fluid by subsequent heating, as the main fluid source. Low‐temperature alteration and the presence of a high‐temperature fluid resulted in different carbonate precipitates forming up to several cm thick veins. The geochemical data combined with age data of the sedimentary rocks suggest intraplate seamount volcanism in the area between the Galapagos hot spot and the Cocos Island as an additional heating source, after the formation of the Cocos Ridge at the Galapagos hot spot.
Highlights
At the erosive plate boundary offshore Costa Rica the Cocos Plate is being subducted under the Caribbean Plate and the highstanding aseismic Cocos Ridge (CCR) has lifted the seismogenic zone into the accessible reach of scientific oceanic drilling
We present analyses of the elemental and isotopic composition of calcite and aragonite veins (CCV) and of the surrounding host rocks to unravel the thermal and fluid history at International Ocean Discovery Program (IODP) Hole 344-U1414A and to clarify the temperature variations derived from fluid inclusions
The differences between the CCV-S and the CCV-B are most pronounced in their rare earth elements and Yttrium (REY) patterns, especially Y/Ho ratios, Mn, Fe, Sc, Na, Zr, Hf, Th, and U content, in their Mg/Ca ratios, oxygen isotopic composition and in their 87Sr/86Sr ratios
Summary
At the erosive plate boundary offshore Costa Rica the Cocos Plate is being subducted under the Caribbean Plate and the highstanding aseismic Cocos Ridge (CCR) has lifted the seismogenic zone into the accessible reach of scientific oceanic drilling. Drilling on the subducting and overriding plate was achieved as part of the Costa Rica Seismogenesis Project (CRISP) during International Ocean Discovery Program (IODP) Expedition 344. CRISP has the main objective to clarify processes that control nucleation and seismic rupture of large earthquakes at erosional subduction zones (Harris et al, 2013a). Vein-hosting sedimentary and igneous rock material from IODP Hole U1414A, located on the incoming Cocos Plate 1 km seaward from the Middle America Trench offshore the western margin of Costa Rica (Figure 1a), was investigated. The results from IODP Hole U1414A describe the hydrologic system and identify fluid pathways and fluid-rock interactions within the upper crustal sections of the oceanic Cocos Plate (Harris et al, 2013b)
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