Presentation on theme: "Authors: Robert M. Garrels Fred T. Mackenzie 1/20/10."— Presentation transcript:
Authors: Robert M. Garrels Fred T. Mackenzie 1/20/10
Introduction This paper explores the origin and composition of springs in the Sierras. -Sierra Nevada setting was chosen b/c of availability of careful water and soil analyses by Feth, et al. -Granitic rocks high in Feldspars, and quartz Two parts to this paper: -Weathering process/analysis, and reconstruction of original rock from observed dissolved solids -Evaporative concentrations of Sierra waters
-Sierra Nevada setting was chosen b/c of availability of careful water and soil analyses by Feth, et al. -Granitic rocks high in Feldspars, and quartz
Igneous rocks + soil waters This weather system is assumed to be closed, so there is little loss or gain of C02 or H20. Soil minerals + spring water
Composition of Ephemeral and Perennial Springs
Reconstruction of original materials Back react spring water w/ kaolinite (chief weathering product) and see if original rock minerals can be formed Cations and anions in snow water are removed from spring H2O solutes. ALL Na and Ca, and some HCO3 and Si02 react w/ kaolinite to form Plag. ALL Mg, and some K, HCO3, and Si02 react with kaolinite to form biotite, and k-spar.
Qualitative Aspects of weathering process -More silica in H20 than from only the breakdown of plag. Comes also from quartz, and the silica represents CO2 changed into HCO3 from weathering. -SiO2 and Aluminum concentrations and pH are reasonable for an alumino-silicate of the appx. stability of kaolinite. -80% of the dissolved solids in the ephemeral springs can be accounted for by the breakdown of plag.
“Pick up” of constituents by continued, deeper circulation A better idea of weathering occurring after initial “strong attack”, can be derived from subtracting out the ephemeral analyses from the perennial
- Next, Mg2+ and K+ are “used up” with kaolonite to from biotite mica and K-spar. - The remaining SiO2 was apportioned b/t reconstruction of both kaolinite and montmorillonite to plagioclase. -Considerable amounts of Ca2+ and HCO3- remain. It is postulated that during deep circulation the waters have lost their original aggressiveness and start to pick up Ca2+
Paths of water evolution
This was a great a analyses, but Garrels et al. want to take into account what occurs when springs are isolated from the parent rock and allowed to evaporate isothermally in equil. w/ the CO2 of the present atmosphere.
Evaporation changes water from a neutral Na-Ca-HCO3 to highly alkaline Na-HCO3-CO3
-Dissolved silica derived from incongruent solution of silicate minerals -Waters in equil. w/ atmosphere will eventually become highly alkaline if concentrated greatly
References Feth, J. H., Roberson, C. E., Polzer, W. L., U.S. Geol. Surv. Water Supply Paper , 170 (1964) Garrels, R. M., Mackenzie, F. T. Origin of the Chemical Compostion of Some Springs and Lakes