Isotopic Constraints on the Origin and Evolution of Geothermal Fluids, Long Valley, CA

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Isotopic Constraints on the Origin and Evolution
of Geothermal Fluids,Long Valley, CA

• Shaun T. Brown1, B. Mack Kennedy1, Donald J.
  DePaolo1,2, William C. Evans3
• 1Earth Sciences Division, Lawrence Berkeley
  National Laboratory, Berkeley, CA 94720
• 2Earth and Planetary Science, University of
  California, Berkeley, Berkeley, CA 94720
• 3U.S. Geological Survey, Menlo Park, CA 94025

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Acknowledgements

• Larry Nickerson, Ormat Technologies Inc.
• U. S. Department of Energy, Office of Basic
  Energy Sciences and Office of Geothermal
  Technologies under contract DE-AC02-05CH11231.

Casa Diablo power plant
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Statement of the problem

• It is difficult to predict the physical and
  chemical processes that effect reservoir
  characteristics such as permeability and heat
  exchange in geothermal systems.
• Goal to use isotope geochemistry as an
  additional dimension for reactive transport
  models of geothermal systems.

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Isotopes In Geothermal systems

• Changes in chemical and isotopic composition of
  fluid along a flow path reflect
• Mixing
• Mineral dissolution
• Precipitation
• Role of reactive transport models is to quantify
  these competing processes.
• Isotopic changes are often unidirectional
  preserving important chemical information and
  adding additional constraints on fluid evolution.

Precipitation Fractionating
Initial Fluid
Precipitation Non-fractionating
Isotopic composition
Mixing
2nd Fluid
Dissolution
Reservoir Rock/Minerals
Distance along fluid flow path
Reservoir Rock/Minerals
Dissolution
2nd Fluid
Concentration
Mixing
Initial Fluid
Precipitation
Distance along fluid flow path
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Water Isotopes in Geothermal Systems

• Little information regarding water-rock
  interaction
• Excellent indicator of more than one fluid and
  fluid mixing along a flow path
• Changes often correlate with conservative solutes
  (e.g.Cl)

Initial Fluid
Mixing
2nd Fluid
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Strontium Isotopes in Geothermal Systems
Initial Fluid
Dissolution
In rocks/minerals 87Sr/86Sr F(87Rb/86Sr, t)
Precipitation
Isotopic composition
Dissolution

• Large variations in the Rb/Sr ratio in geologic
  materials results in significant variations in Sr
  isotopic compositions in reservoir
  rocks/minerals.
• By analytic convention Sr isotopes are
  non-fractionating
• The Sr isotopic composition records fundamentally
  different information than the Sr concentration

Reservoir Rock/Minerals
Distance along fluid flow path
Reservoir Rock/Minerals
Concentration
Precipitation
Dissolution
Dissolution
Initial Fluid
Distance along fluid flow path
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Calcium Isotopes in Geothermal Systems
Initial Fluid

• Ca isotopes small, but significant, variations
  among rocks/minerals related to conditions of
  mineralization.
• By analytic convention Ca isotopes are
  fractionating.
• Ca isotopes not very sensitive to dissolution but
  can track precipitation.

Precipitation
Dissolution
Dissolution
Isotopic composition
Reservoir Rock/Minerals
Distance along fluid flow path
Reservoir Rock/Minerals
Concentration
Precipitation
Dissolution
Dissolution
Initial Fluid
Distance along fluid flow path
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Generalized map of Long Valley Caldera and
Geothermal System
Flow path based on the model of Sorey et al, 1991
JVGR
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Long Valley Water Isotopes

• Evolved waters enriched in 18O are located near
  the proposed upwelling zone (Well 44-16).
• Mixing with local meteoric water from
  west-to-east along flow path
• Casa Diablo samples are consistent with the
  addition of 20 meteoric water.

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Sr isotopic variation in Long Valley Geothermal
Fluids

• Sr data are broadly consistent with water-rock
  reaction
• Not consistent with one fluid flow path based on
  low Sr isotopes in the western wells
• Mixing between meteoric water and geothermal
  waters may complicate the Sr data

Pcp ?
Diss ?
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Ca Isotopes

• Ca and CO2 concentrations decrease west to east
• Ca isotope ratios increase west to east.
• Ca isotopes could be consistent with either
  calcite precipitation or mixing

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He-CO2
He and Ar are inert and have low solubility
sensitive to mixing and boiling Conclude that
changes in CO2 concentration are consistent with
vapor loss
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Ca Isotopes
Correlated CO2 and Ca concentrations in LV fluids
are consistent with decreasing carbonate
solubility eastward. Ca isotopes in LV fluids
record the precipitation of calcite Future
modeling will attempt to quantify the amount of
calcite precipitation and its effect on
permeability.
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Conclusions

• Isotopes can be used to quantify the roles of
  physical and chemical processes in geothermal
  systems
• Water isotopes sensitive to mixing and record
  dilution with meteoric water
• Sr isotopes consistent with dissolution of the
  reservoir rock and dissolution reaction rates
• First observation of Ca isotope variation in
  geothermal fluids
• Suggests that calcite precipitation can be
  monitored and potentially quantified using Ca
  isotopes
• These data will substantially improve our ability
  to model important chemical reactions occurring
  along flow paths in geothermal systems