Strontium- and Lead-Isotope Composition of Some Basalts from Deep Sea Drilling Project Hole 504B, Costa Rica Rift, Legs 69 and 70

Barrett, T J

doi:10.2973/dsdp.proc.69.138.1983

Cited by 14 publications

(10 citation statements)

References 34 publications

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“…Trans = transition zone. Strontium data from Table 1 (whole rocks = solid circles; secondary minerals are labeled) and various sources Friedrichsen, 1982, 1987;Barrett, 1983;Hart and Mottl, 1983;Friedrichsen, 1985;Staudigel and Hart, 1985;Kawahata et al, 1987;Kusakabe et al, 1989;Shimizu et al, 1989). Oxygen data from Table 1 (solid circles), Alt et al (1986bAlt et al ( , 1989bAlt et al ( , 1995, and Nesbitt et al (1987).…”

Section: Methodsmentioning

confidence: 99%

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

Alt¹,

Teagle²,

Bach³

et al. 1996

Proceedings of the Ocean Drilling Program, 148 Scientific Results

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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%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.

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Section: Methodsmentioning

confidence: 99%

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

Alt¹,

Teagle²,

Bach³

et al. 1996

Proceedings of the Ocean Drilling Program, 148 Scientific Results

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“…5, 6;Barrett, 1983;Barrett and Friedrichsen, 1982;Honnorez et al, 1983;Noack et al, 1983;Alt et al, 1986aAlt et al, , 1986bAlt et al, , 1989aIshikawa and Nakamura, 1992). The red and black halos generally exhibit the greatest chemical and isotopic changes.…”

Section: Volcanic Section (2745-846 Mbsf)mentioning

confidence: 99%

“…Errors are smaller than plotted points, except for δ"B, where bars indicate where errors are larger. Data from Barrett (1983), Barrett and Friedrichsen (1983), Friedrichsen (1985), Alt et al (1986bAlt et al ( , 1989aAlt et al ( , 1989bAlt et al ( , 1995, Kawahata et al (1987), Nesbitt et al (1987), Shimizu et al (1989), Mengel and Hoefs (1990), Ishikawa and Nakamura (1992), Hart et al (1994), and Agrinier et al (1995). Vertical lines and shaded bars indicate MORB and seawater compositions.…”

Section: Transition Zone and Upper Dike Sectionmentioning

confidence: 99%

Hydrothermal Alteration of a Section of Upper Oceanic Crust in the Eastern Equatorial Pacific: A Synthesis of Results from Site 504 (DSDP Legs 69, 70, and 83, and ODP Legs 111, 137, 140, and 148)

Alt¹,

Laverne²,

Vanko³

et al. 1996

Proceedings of the Ocean Drilling Program

152

255

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Mineralogical, chemical, and isotopic results from seven drilling legs that visited DSDP/ODP Hole 504B over 14 years are compiled here to present an integrated view of hydrothermal alteration of oceanic crust at Site 504. Hole 504B reaches to 2111 mbsf, through 274.5 m sediment, 571.5 m of volcanic rocks, a 209 m transition zone, and 1050 m into a sheeted dike complex. The volcanic section was altered through a series of processes involving interaction with seawater at low temperatures, with the effects of cold, oxidizing seawater decreasing downward. These processes and their effects on the volcanic section are generally similar to those in other oceanic upper crustal sections.The transition zone and upper dikes were altered in a subsurface mixing zone, where hydrothermal fluids upwelling through the dikes mixed with cooler seawater circulating in the overlying more permeable volcanic rocks. Alteration of the transition zone and upper dikes (down to 1500 mbsf) occurred in a series of stages, reflecting the thermal and chemical evolution of the hydrothermal system from (1) early chlorite, actinolite, albite-oligoclase, and titanite, to (2) quartz, epidote and sulfides, to (3) anhydrite, and finally to (4) zeolites and local calcite. The maximum temperature estimated for the first two stages is 350°-380°C, and the inferred mineral assemblages for these early stages are typical of the greenschist facies.The lower dikes (1500-2111 mbsf) underwent an early, high-temperature (>400°C) alteration stage, resulting in the formation of hornblende and calcic secondary plagioclase, consistent with reactions inferred to occur in deep subsurface reaction zones, where hydrothermal vent fluids acquire their final compositions. Much of the subsequent reactions produced greenschist assemblages at ~300°-400°C. The lower dikes have lost metals and sulfur and are a source of these elements to hydrothermal vent fluids and seafloor sulfide deposits. The lower dikes underwent subsequent alteration stages similar to the upper dikes, with rare epidote + quartz veins recording the presence of upwelling hydrothermal fluids, and limited late off-axis effects (zeolites and prehnite). Anhydrites in the lower dikes indicate more reacted fluid compositions than in the upper dikes.Alteration of the sheeted dikes from Hole 504B is heterogeneous, with recrystallization controlled by fracturing and access of fluids. Defining the position of the seismic Layer 2/3 transition depends upon the scale of observation, but the change at Site 504 occurs within the sheeted dikes and is correlated with progressive changes in porosity and hydrothermal alteration. However, we still do not know the nature of the transition from sheeted dikes to gabbros in in situ ocean crust, or the nature of the inferred fault at the base of Hole 504B and its role in fluid flow and alteration.

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“…5; see Alt et al, this volume, figs. 5, 6;Honnorez et al, 1983;Noack et al, 1983;Barrett, 1983;Barrett and Friedrichsen, 1982;Alt et al, 1986aAlt et al, , 1986bIshikawa and Nakamura, 1992). Red and black halos generally exhibit the greatest chemical and isotopic changes.…”

Section: Whole-rock Chemistrymentioning

confidence: 99%

Ridge-Flank Alteration of Upper Ocean Crust in the Eastern Pacific: Synthesis of Results for Volcanic Rocks of Holes 504B and 896A

Alt¹,

Teagle²,

Laverne³

et al. 1996

Proceedings of the Ocean Drilling Program, 148 Scientific Results

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The lithostratigraphy and alteration of volcanic basement from Holes 504B and 896A, located in different parts of a ridge flank circulation cell, are summarized and compared. The 290-m-thick volcanic section of Hole 896A is located on a basement high that coincides with high heat flow and upwelling fluids. The 571.5-m-volcanic section of Hole 504B is located ~l km away in an area of ambient heat flow. Subtle differences in lithostratigraphy include slightly greater proportions of massive units and fewer pillow basalts in Hole 896A than in Hole 504B. The volcanic sections are geochemically similar, but there is no direct correlation of lithologic or geochemical units between the two sites. Veins are comparable in abundance in the two sections (-30 veins/m, mean vein width < 1 mm), but carbonate veins and thick (>2 mm) saponite and carbonate veins are more abundant in Hole 896A than in Hole 504B. Permeabilty values of the upper basement sections in Holes 896A and 504B are similar (~IO~1 3 to 10~1 4 m 2 ), suggesting that the upper -200 m of basement is sufficiently permeable on a regional scale to support circulation of seawater through basement.Alteration effects in basement from Hole 896A are similar to those in the upper 320 m of volcanic rocks in Hole 504B. These include celadonitic phyllosilicates in fractures and alteration halos and reddish, Fe-oxyhydroxide-rich alteration halos along fractures. Dark gray rocks, characterized by the presence of saponite ± carbonate ± pyrite occur throughout Hole 896A and the entire Hole 504B volcanic section. Alteration reflects evolution from open circulation of cold, oxidizing seawater, to more restricted circulation of seawater caused by burial of the crust by sediments and sealing of fractures with saponite. Latestage carbonates and minor zeolites formed in veins throughout both holes from reacted seawater fluids (decreased fluid Mg/ Ca). Oxygen and strontium isotopic evidence indicate an early generation of carbonates in both holes that formed at relatively low temperatures (~25°-35°C) during open circulation, whereas later carbonates in Hole 896A formed at slightly higher temperatures (~50°-70°C) during more restricted circulation, possibly similar to the present ridge flank hydrothermal upflow conditions (~50°-80°C).Chemical changes in altered upper crust include oxidation, increased alkalis, Mg, CO 2 , and H 2 O; local uptake of P; elevated δ l8 θ, δD, δ"B, and 87 Sr/ 86 Sr; and lower S contents and δ 34 S. The greatest chemical changes occur in alteration halos and breccias, and the smallest chemical changes occur in the lower volcanic section of Hole 504B. Secondary minerals filling fractures and cementing breccias are sites of uptake of Mg, CO 2 , and H 2 O, in addition to changes occurring in altered rocks.

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Strontium- and Lead-Isotope Composition of Some Basalts from Deep Sea Drilling Project Hole 504B, Costa Rica Rift, Legs 69 and 70

Cited by 14 publications

References 34 publications

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

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

Hydrothermal Alteration of a Section of Upper Oceanic Crust in the Eastern Equatorial Pacific: A Synthesis of Results from Site 504 (DSDP Legs 69, 70, and 83, and ODP Legs 111, 137, 140, and 148)

Ridge-Flank Alteration of Upper Ocean Crust in the Eastern Pacific: Synthesis of Results for Volcanic Rocks of Holes 504B and 896A

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