Experimental and theoretical constraints on the origin of mid-ocean ridge geothermal fluids

Thesis/Dissertation · Thu Jan 01 04:00:00 EST 1987

OSTI ID:7047999

Berndt, M E

Hydrothermal experiments were performed using basalt, diabase, and two synthetic plagioclase bearing assemblages and Na-Ca-K-Cl fluids of seawater chlorinity at conditions from 350 to 425/sup 0/C and 250 to 400 bars. Dissolved Ca, Na, SiO/sub 2/, and pH appear to be controlled by equilibrium with plagioclase and epidote. Fluids reacting with diabase at low fluid/rock ratios (0.5-1) remain undersaturated with respect to quartz due to formation of olivine hydration products, whereas fluids reacting with basalt become supersaturated with respect to quartz due to breakdown of fractionated glass and formation of amphibole. High SiO/sub 2/ activities during basalt alteration, leads to high Ca and base metal concentrations and low pH compared to diabase alteration at the same conditions. Dissolved Li, K, Rb, and Ba concentrations reach higher levels during basalt alteration than during diabase alteration. Since these elements avoid incorporation into crystalline phases during solidification of magmas they are concentrated in the glass which is easily altered by fluids and explains their increased mobility during basalt alteration. Na-Ca-pH-SiO/sub 2/ relationships in vent fluids can be used to constrain reaction zone conditions assuming the fluids are equilibrated with plagioclase and epidote. The temperatures predicted by such models are higher than measured vent fluid temperatures. Dissolved Sr/Ca ratios for ridge crest fluids are similar to those produced during diabase alteration and higher than those produced during basalt alteration. This observation supports deep-seated reaction of the hydrothermal fluids with diabase dikes and/or gabbro for vent fluid origin. Only 4% of the Sr initially present in basalt is mobilized during hydrothermal alteration even after 800 hours of reaction.

• Research Organization: Minnesota Univ., Minneapolis (USA)
• OSTI ID: 7047999
• Country of Publication: United States
• Language: English

Related Subjects

15 GEOTHERMAL ENERGY
152003* -- Geothermal Data & Theory-- Rock-Water-Gas Interactions
58 GEOSCIENCES
580100 -- Geology & Hydrology-- (-1989)
ALKALI METALS
ALKALINE EARTH METALS
ANORTHOSITES
BARIUM
BASALT
BENCH-SCALE EXPERIMENTS
CALCIUM
CHALCOGENIDES
DIABASES
ELEMENTS
FLUIDS
GABBROS
GEOLOGIC MODELS
GEOTHERMAL FLUIDS
GLASS
HYDROTHERMAL ALTERATION
IGNEOUS ROCKS
LITHIUM
MAGMA
METALS
MID-ATLANTIC RIDGE
MINERALS
ORIGIN
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
PH VALUE
PHASE TRANSFORMATIONS
PLUTONIC ROCKS
POTASSIUM
RESERVOIR TEMPERATURE
ROCK-FLUID INTERACTIONS
ROCKS
RUBIDIUM
SILICA
SILICON COMPOUNDS
SILICON OXIDES
SODIUM
SOLIDIFICATION
STRONTIUM
VOLCANIC ROCKS