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Isotopic and chemical compositions of rocks and minerals from the TAG hydrothermal mound, ODP Site 957

Publication Date
DOI: 10.1594/pangaea.704808
  • 158-957B
  • 158-957C
  • 158-957E
  • 158-957H
  • 158-957I
  • 158-957M
  • 158-957P
  • 87Sr/86Sr
  • 87Sr/86Sr E
  • Calculated
  • Calculated From Measured 87Sr/86Sr Assuming Mixing Between Seawater And Tag End-Member Black Smoker
  • D18O
  • Delta 18O
  • Drilling
  • For So4 Vs
  • Smow
  • Icp-Ms
  • Inductively Coupled Plasma - Mass Spectrometry
  • Joides Resolution
  • Kd
  • K-Distr
  • K-Distribution Parameter
  • Label
  • Leg158
  • Magnesium/Calcium Ratio
  • Mass Spectrometer Multicollector Vg Sector Ii
  • Mass Spectrometer Vg Prism
  • Mg/Ca
  • Mixed
  • Mmol/Mol
  • North Atlantic Ocean
  • Ocean Drilling Program
  • Odp
  • Odp Sample Designation
  • Of Seawater
  • Calculated From Measured 87Sr/86Sr Assuming Mixing Between Seawater And Tag End-Member Black Smoker
  • Prop
  • Proportion
  • Rock
  • Rock Type
  • Sample Code/Label
  • Sample Type
  • Samp Type
  • Sr
  • Sr/Ca
  • Strontium
  • Strontium/Calcium Ratio
  • Strontium 87/Strontium 86
  • Strontium 87/Strontium 86
  • Error
  • Temp Cal
  • Temperature
  • Calculated
  • Vs
  • Smow
  • Quartz
  • Vs
  • Smow
  • Whole Rock
  • Zirconium
  • Zr
  • Earth Science
  • Geography


Strontium- and oxygen-isotopic measurements of samples recovered from the Trans-Atlantic Geotraverse (TAG) hydrothermal mound during Leg 158 of the Ocean Drilling Program provide important constraints on the nature of fluid-rock interactions during basalt alteration and mineralization within an active hydrothermal deposit. Fresh Mid-Ocean Ridge Basalt (MORB), with a 87Sr/86Sr of 0.7026, from the basement beneath the TAG mound was altered at both low and high temperatures by seawater and altered at high temperature by near end-member black smoker fluids. Pillow breccias occurring beneath the margins of the mound are locally recrystallized to chlorite by interaction with large volumes of conductively heated seawater (>200°C). The development of a silicified, sulfide-mineralized stockwork within the basaltic basement follows a simple paragenetic sequence of chloritization followed by mineralization and the development of a quartz+pyrite+paragonite stockwork cut by quartz-pyrite veins. Initial alteration involved the development of chloritic alteration halos around basalt clasts by reaction with a Mg-bearing mixture of upwelling, high-temperature (>300°C), black smoker-type fluid with a minor (<10%) proportion of seawater. Continued high-temperature (>300°C) interaction between the wallrock and these Mg-bearing fluids results in the complete recrystallization of the wallrock to chlorite+quartz+pyrite. The quartz+pyrite+paragonite assemblage replaces the chloritized basalts, and developed by reaction at 250-360°C with end-member hydrothermal fluids having 87Sr/86Sr ~0.7038, similar to present-day vent fluids. The uniformity of the 87Sr/86Sr ratios of hydrothermal assemblages throughout the mound and stockwork requires that the 87Sr/86Sr ratio of end-member hydrothermal fluids has remained relatively constant for a time period longer than that required to change the interior thermal structure and plumbing network of the mound and underlying stockwork. Precipitation of anhydrite in breccias and as late-stage veins throughout most of the mound and stockwork, down to at least 125 mbsf, records extensive entrainment of seawater into the hydrothermal deposit. 87Sr/86Sr ratios indicate that most of the anhydrite formed from ~2:1 mixture of seawater and black smoker fluids (65%±15% seawater). Oxygen-isotopic compositions imply that anhydrite precipitated at temperatures between 147°C and 270°C and require that seawater was conductively heated to between 100°C and 180°C before mixing and precipitation occurred. Anhydrite from the TAG mound has a Sr-Ca partition coefficient Kd ~0.60±0.28 (2 sigma). This value is in agreement with the range of experimentally determined partition coefficients (Kd ~0.27-0.73) and is similar to those calculated for anhydrite from active black smoker chimneys from 21°N on the East Pacific Rise. The d18O (for SO4) of TAG anhydrite brackets the value of seawater sulfate oxygen (~9.5?). Dissolution of anhydrite back into the oceans during episodes of hydrothermal quiescence provides a mechanism of buffering seawater sulfate oxygen to an isotopically light composition, in addition to the precipitation and dissolution of anhydrite within the oceanic basement during hydrothermal recharge at the mid-ocean ridges.

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