Stable and Strontium Isotopic Profiles through Hydrothermally Altered Upper Oceanic Crust, Hole 504B

Abstract

Leg 148 penetrated 111m farther into Hole 504B, to a total depth of 2111 meters below seafloor (mbsf), and recovered diabase dikes similar to the immediately overlying rocks. Secondary mineralogy and whole-rock oxygen and strontium isotopic compositions were determined for samples from Leg 148. In addition, strontium isotopic analyses were performed on previously recovered samples of the lower dikes (1550-2000 mbsf). Trends observed in the lower dikes (1550-2000 mbsf) continue in the Leg 148 section (2000-2111 mbsf). These include the local presence of magnesiohornblende and secondary calcic plagioclase and high Ti contents of amphiboles. Cu, Zn, and S contents of the more altered rocks are low, and whole-rock δ 18 θ values of 33%<r-4.5%c continue the trend of decreasing δ' 8 O with depth. These trends reflect alteration at higher temperatures (>350°C) and greater extents of recrystallization than in the upper dikes. All of the data for the new Leg 148 section are consistent with the rocks being a continuation of a subsurface reaction zone, where metals and sulfur are leached from the crust by hydrothermal fluids and transported to form metal sulfide mineralizations on or within the crust or vent into seawater.Sr contents of the lower sheeted dikes (1550-2111 mbsf) are generally 48-62 ppm, and whole-rock 87 Sr/ 86 Sr ratios range from 0.70265 to 0.70304, only slightly elevated relative to fresh MORB values. The rocks were altered by seawater-derived hydrothermal fluids, with little change in the Sr concentrations of the rocks. Although amphibole is the most abundant secondary phase, the Sr isotopic composition of the rocks is probably dominated by the abundance of secondary plagioclase. The 87 Sr/ 86 Sr ratios of hydrothermal fluids were higher than measured in variably recrystallized (10%-80%) whole rocks. The seawater component of hydrothermal fluids increased through time, as documented by 87 Sr/ 86 Sr ratios of 0.7028-0.7034 for amphibole and chlorite, to 0.7035-0.7038 for vein epidotes. The latter values record the composition of upwelling black smoker-type endmember hydrothermal fluids at Site 504.The low 87 Sr/ 86 Sr ratios for the lower dikes differ from the uniformly high ratios of the thoroughly recrystallized sheeted dikes from the Troodos ophiolite. The oceanic dikes interacted with much smaller volumes of seawater than the ophiolitic rocks and apparently to a lesser extent. These differences are in some way related to the contrasting tectonic setting, mineralogy, and chemical composition of ophiolites compared to in situ ocean crust.

Evolution of fluid–rock interaction in the Reykjanes geothermal system, Iceland: Evidence from Iceland Deep Drilling Project core RN-17B

“…These results reflect changes over time in a very localized portion of the Reykjanes geothermal system. Our fluid inclusion and strontium isotope results for epidote in the RN-17B core are similar to fluid inclusions reported from Mid-Atlantic Ridge samples (Kelley and Delaney, 1987) and strontium isotope studies of core 504B from the East Pacific Rise (Alt et al, 1996;Teagle et al, 1998;Bach et al, 2003).…”

“…Strontium isotope fractionation is not temperature or mineral dependent, and 87 Sr/ 86 Sr ratios are significantly different for fresh volcanic rock and seawater (Alt et al, 1996). The low Rb concentration and the young age (b1 Ma) of Reykjanes basalts preclude significant radiogenic 87 Sr contamination in samples .…”

“…Alt et al, , 1986Alt et al, , 1996Vanko and Stakes, 1991;Gillis and Thompson, 1993;Alt, 1995). Amphibole compositions in seafloor hydrothermal systems commonly change with increasing depth and temperature from actinolite to Mg-hornblende with corresponding increases in Al and Ti contents (Gillis and Thompson, 1993;Alt, 1995).…”

“…For these reasons 87 Sr/ 86 Sr ratios are particularly useful for evaluating fluid-rock interaction in seawater dominated geothermal systems, and have been used previously to evaluate seawater influence on epidote precipitation in seafloor hydrothermal systems (i.e. Alt et al, 1996) and epidote from drill cuttings in the Reykjanes geothermal system . 87 Sr/ 86 Sr measurements of epidote from RN-17 and RN-17B drill cuttings, in addition to three powdered samples of RN-17B core whole-rock do not vary with depth or alteration grade, but do vary with increased loss on ignition (LOI) .…”

Strontium isotope constraints on fluid flow in the sheeted dike complex of fast spreading crust: Pervasive fluid flow at Pito Deep

“…Strontium was separated by standard cation exchange techniques [Weis et al, 2006] and measured using a Triton thermal ionization mass spectrometry at the Pacific Centre for Isotopic and Geochemical Research, University of British Columbia. The NBS 987 standard gave 87 Sr/ 86 Sr of 0.710251 ± 0.000017 (2 s.d., n = 26), the maximum difference between duplicates was ±0.000022 with an average reproducibility Sr of 0.70245 (n = 3 [Verma and Schilling, 1982;Schilling et al, 2003] Alt et al [1996aAlt et al [ , 1996b, Hékinian et al [1996], Hilgen et al [1995], Lonsdale [1988], Martinez et al [1991], Hey et al [1977].…”