The Origin of Carbonate Veins Within the Sedimentary Cover and Igneous Rocks of the Cocos Ridge: Results From IODP Hole U1414A

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.

Broader Impacts of the Metasomatic Underplating Hypothesis

“…Dissolved Ca in the infiltrating fluid would thereby be extracted before it reaches the mantle, and replacement of calcium silicates with calcium carbonates is possible if the Ca 2+ cation is scarce. This scenario is consistent with Brandstätter et al (), who found carbonate veins within upper‐basement drill cores on the Cocos Ridge, which we interpret as a relict track of the Galapagos hot spot. These chemical alterations within the plume track offer a pathway for large‐scale volatile transport into the deep mantle within the slab's cool interior, transported much later via subduction.…”

“…With scarce olivine to serpentinize, and serpentinization reactions between pyroxene and seawater that are sluggish at lower temperatures (Bach et al, ), the oceanic crust undergoes negligible volume expansion and develops throughgoing crack networks that allow seawater access to the sub‐Moho peridotite. This crack scenario is a prediction for the oceanic crust, not an observation, but is nevertheless consistent with the crack topology described by Brandstätter et al () within the shallow crustal basement basalts of IODP Hole U1414A on the Cocos Ridge.…”

“…Chemical and strontium‐isotope analyses of these calcite and aragonite veins reveal precipitation under reducing conditions from circulating seawater. Brandstätter et al () report that the 87 Sr/ 86 Sr values of basalt‐hosted carbonate veins at IODP Hole U1414A are consistent with precipitation from Miocene seawater as old as 20 Ma, as old or older than the onset of the Cocos‐Nazca spreading center.…”

“…However, there is evidence that oceanic crust of the Cocos Ridge, at least, is carbonated. Brandstätter et al () report carbonate veins in ~100 m of basaltic igneous basement drilled at Integrated Ocean Drilling Program (IODP) Hole U1414A on the Cocos Ridge, just seaward of the Middle‐America Trench. Chemical and strontium‐isotope analyses of these calcite and aragonite veins reveal precipitation under reducing conditions from circulating seawater.…”

Geochemistry and Microtextures of Vein Calcites Pervading the Izu‐Bonin Forearc and Rear Arc Crust: New Insights From IODP Expeditions 352 and 351

“…The volcanic units of the Izu-Bonin forearc and rear arc are substantially and similarly pervaded by veins that document extensive fluid flow through the oceanic crust and subsequent secondary mineral precipitation (Arculus, Ishizuka, Bogus, & the Expedition 351 Scientists, 2015;Christeson et al, 2016;Reagan et al, 2015). Numerous previous studies on calcite veins pervading the oceanic crust used δ 18 O, δ 13 C, and 87 Sr/ 86 Sr isotopic compositions, rare earth element and yttrium (REE+Y) concentrations, and fluid inclusions to investigate the geochemical composition of circulating fluids, and the age and temperature of secondary mineral precipitation (e.g., Alt & Teagle, 2003;Alt et al, 1998;Brandstätter et al, 2016;Brandstätter et al, 2018;Hart & Staudigel, 1978;Quandt et al, 2018;Quandt, Micheuz, Kurz, Kluge, et al, 2019;Schroeder et al, 2015). However, studies on calcite veins from the Izu-Bonin-Mariana trench in particular are rare.…”