Plagioclase and epidote buffering of cation ratios in mid-ocean ridge hydrothermal fluids: Experimental results in and near the supercritical region

Journal Article · Fri Sep 01 04:00:00 EDT 1989 · Geochimica et Cosmochimica Acta; (USA)

DOI:https://doi.org/10.1016/0016-7037(89)90351-7· OSTI ID:7024633

Berndt, M E; Seyfried, Jr, W E ^[1]; Janecky, D R ^[2]

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1. Univ. of Minnesota, Minneapolis (USA)
2. Los Alamos National Laboratory, NM (USA)

Experiments have been performed with Na-Ca-K-Cl fluids of seawater chlorinity and diabase, basalt, and plagioclase bearing mineral mixtures at 350-425{degree}C and 250-400 bars to help constrain hydrothermal alteration processes at mid-ocean ridges. Dissolved Ca, Na, and pH for all experiments responded systematically to differences in dissolved SiO{sub 2} concentrations and the compositions of plagioclase reactants. Diabase alteration at low fluid/rock mass ratios (0.5 to 1) produces fluids undersaturated with respect to quartz during hydration of primary olivine and orthopyroxene, whereas basalt alteration under similar conditions yields fluids slightly supersaturated with respect to quartz during breakdown of glass to smectite and amphibole. Fluid chemistry in all experiments appears to approach a partial equilibrium state with the albite and anorthite components in plagioclase and approaches a pH consistent with plagioclase alteration to epidote. Trace element data from vent fluids, specifically B and Sr, together with major element chemistry, provides evidence that the reaction zone for black-smoker fluids at mid-ocean ridges is composed of only slightly altered diabase and is characterized by small amounts of epidote, nearly fresh plagioclase and clinopyroxene, and partially to completely hydrated olivine and orthopyroxene. Using equilibrium between plagioclase, the dominant reactant, and epidote, the dominant reaction product in experiments, the authors estimate that temperatures in reaction zones are in excess of 375{degree}C for most vent systems. These temperatures are higher than measured vent temperatures, suggesting that hot spring fluids commonly loose heat during ascent to the sea floor.

• OSTI ID: 7024633
• Journal Information: Geochimica et Cosmochimica Acta; (USA), Journal Name: Geochimica et Cosmochimica Acta; (USA) Vol. 53:9; ISSN GCACA; ISSN 0016-7037
• Country of Publication: United States
• Language: English

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15 GEOTHERMAL ENERGY
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58 GEOSCIENCES
580000 -- Geosciences
ALKALINE EARTH METAL COMPOUNDS
ALUMINIUM COMPOUNDS
ALUMINIUM SILICATES
ATLANTIC OCEAN
CALCIUM COMPOUNDS
CALCIUM SILICATES
CHEMICAL COMPOSITION
CHEMISTRY
DATA
ENERGY SYSTEMS
EPIDOTES
EQUILIBRIUM
EXPERIMENTAL DATA
FELDSPARS
FLUIDS
GEOCHEMISTRY
GEOTHERMAL FLUIDS
GEOTHERMAL SYSTEMS
GEOTHERMOMETRY
HYDROGEN COMPOUNDS
HYDROTHERMAL ALTERATION
HYDROTHERMAL SYSTEMS
IGNEOUS ROCKS
INFORMATION
IONIC COMPOSITION
IRON COMPOUNDS
IRON SILICATES
MID-ATLANTIC RIDGE
MINERALOGY
MINERALS
NUMERICAL DATA
OXYGEN COMPOUNDS
PACIFIC OCEAN
PH VALUE
ROCK-FLUID INTERACTIONS
ROCKS
SEAS
SEAWATER
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
SURFACE WATERS
TRANSITION ELEMENT COMPOUNDS
VOLCANIC ROCKS
WATER