Abstract:
During Leg 168 of the Ocean Drilling Program a transect was drilled across the eastern flank of the Juan de Fuca Ridge in an area where the volcanic basement is covered by sediments of variable thickness. Samples of the basement volcanic rocks were recovered from nine locations along the transect, where the basement sediment interface is presently heated to temperatures of 15 degrees to 64 degrees C. Altered rocks with secondary calcium carbonate were common at four of the sites, where present day temperatures range from 38 degrees to 64 degrees C. Thin sections containing carbonates were analyzed using cathodoluminescence (CL) to determine compositional zoning patterns of the carbonate minerals and to determine mineral species. The secondary alteration minerals, calcite and aragonite, occur as veins, lining or filling vesicles or as a replacement for phenocrysts or groundmass. These calcium carbonate minerals occur alone or with saponite clay. Crosscutting relationships between the clay veins and the carbonate veins indicate that the carbonate veins occur late in the alteration sequence. Monominerallic veins of calcite or aragonite and veins containing intergrowths of the two minerals were also observed. Most of the carbonate veins exhibited compositional zoning to some degree, visible during cathodoluminescence (CL) analysis. The chemical compositions of representative samples of secondary calcite and aragonite were determined with both an electron microprobe and a laser-ablation ICPMS microprobe. These two techniques yielded consistent analyses of the same minor elements (Mg and Sr) in the same specimens. The combined results show that secondary aragonites contain very little Mg, Mn, Fe, Co, Ni, Cu, Zn, Rb, La, Ce, Pb, or U. However, they do contain significant amounts of Sr. In contrast, secondary calcites contain significant Mg, Mn, Fe, Cu, Ni, Zn, and Pb, yet very little Co, Rb, Sr, La, Ce, or U. Secondary calcium carbonates provide sub-seafloor reservoirs for some minor and trace elements. Replacement of aragonite by calcite should result in the release of Sr, Rb and Zn to solution, and it provides a sink for Mg, Mn, Ni, Cu, Zn, and Pb. The use of known equilibrium distribution coefficients made it possible to calculate equilibrium fluid parameters based on analysis of the carbonate samples. These calculations show that the calcite-forming fluids are depleted in Mg and Sr relative to seawater. In contrast, the aragonite forming fluids were barely modified from seawater in their Sr concentration. Crosscutting relationships with respect to clay veins indicate that the carbonates generally occur late in the alteration sequence. This study indicates that the aragonite formed early in the alteration sequence from fluid only slightly altered from seawater. In contrast, the calcite formed later in the alteration sequence from fluids significantly altered from seawater.