THE GEOCHEMISTRY OF NATURAL WATERS

1. 1 THE GEOCHEMISTRY OF NATURAL WATERS MINERAL WEATHERING AND MINERAL SURFACE PROCESSES - II INCONGRUENT DISSOLUTION AND ACTIVITY DIAGRAMS CHAPTER 4 - Kehew (2001) Mass-balance calculations and weathering reactions

2. 2 LEARNING OBJECTIVES Learn about incongruent dissolution of silicates. Learn to calculate and use activity diagrams. Learn about the use of mass-balance calculations to infer weathering reactions. Apply the knowledge gained to rationalize compositions of natural waters in igneous and metamorphic rocks. Explore the implications of acid rain in igneous and metamorphic terrains. In Lectures 4 and 5, we dealt with congruent dissolution of oxides and carbonates, and incongruent dissolution of carbonates. In this lecture we concentrate on methods for dealing with incongruent dissolution of silicates. This will involve learning to construct and use a type of phase diagram called an activity diagram. We will also learn how to do mass-balance calculations to infer the weathering reaction that control the composition of waters during weathering of silicate minerals. In the process, we will apply what we learn to understanding how incongruent dissolution affects the compositions of natural waters in igneous and metamorphic rocks, and we will also briefly discuss the effect of acid rain on natural waters occurring in such rocks. In Lectures 4 and 5, we dealt with congruent dissolution of oxides and carbonates, and incongruent dissolution of carbonates. In this lecture we concentrate on methods for dealing with incongruent dissolution of silicates. This will involve learning to construct and use a type of phase diagram called an activity diagram. We will also learn how to do mass-balance calculations to infer the weathering reaction that control the composition of waters during weathering of silicate minerals. In the process, we will apply what we learn to understanding how incongruent dissolution affects the compositions of natural waters in igneous and metamorphic rocks, and we will also briefly discuss the effect of acid rain on natural waters occurring in such rocks.

3. 3 The diagram above presents, in the form of a bar graph, the compositions of ground waters from different igneous rock terrains. The data are from Hem (1989). This diagram demonstrates several things: Silica is present to a significant extent in each of these water analyses. This indicates that dissolution of silicate minerals play a significant role in controlling the compositions of these waters. The dominant anion is bicarbonate (HCO3-) even though silicate minerals do not generally contain any carbonate. This situation arises from the neutralization of carbonic acid via reaction with silicate minerals. Natural waters in rhyolites are dominated by Na and K. On the other hand, in waters from mafic and ultramafic rocks, Ca and Mg become important. This demonstrates that the composition of the rocks through which the waters flow exert an important control on the water compositions. The waters from the two basalts have almost the same ion proportions, even though the total concentrations of ions differ by about a factor of two. Mg dominates the cation composition of the spring in the olivine-tuff breccia. This is expected because of the strong susceptibility of olivine to weathering (recall that it is at the top of Goldich’s weathering series). The diagram above presents, in the form of a bar graph, the compositions of ground waters from different igneous rock terrains. The data are from Hem (1989). This diagram demonstrates several things: Silica is present to a significant extent in each of these water analyses. This indicates that dissolution of silicate minerals play a significant role in controlling the compositions of these waters. The dominant anion is bicarbonate (HCO3-) even though silicate minerals do not generally contain any carbonate. This situation arises from the neutralization of carbonic acid via reaction with silicate minerals. Natural waters in rhyolites are dominated by Na and K. On the other hand, in waters from mafic and ultramafic rocks, Ca and Mg become important. This demonstrates that the composition of the rocks through which the waters flow exert an important control on the water compositions. The waters from the two basalts have almost the same ion proportions, even though the total concentrations of ions differ by about a factor of two. Mg dominates the cation composition of the spring in the olivine-tuff breccia. This is expected because of the strong susceptibility of olivine to weathering (recall that it is at the top of Goldich’s weathering series).

4. 4 NATURAL WATERS IN IGNEOUS ROCK TERRAINS - I The previous diagram shows a number of points: Presence of dissolved silica demonstrates dissolution of silicate minerals. Natural waters in rhyolites are dominated by Na and K. In mafic and ultramafic rocks, Ca and Mg become important. The dominant anion is bicarbonate (HCO3-) resulting from interaction of carbonic acid with the silicates. The waters from the two basalts have similar ion proportions, in spite of different TDS.

5. 5 NATURAL WATERS IN IGNEOUS ROCK TERRAINS - II The composition of the rhyolite spring may be governed by a reaction such as: 2NaAlSi3O8 + 2H2CO30 + 9H2O(l) ? Al2Si2O5(OH)4 + 2HCO3- + 2Na+ + 4H4SiO40 This predicts a Si:Na ratio of 2:1. The actual ratio from the previous figure is ~1:1. Perhaps albite actually weathers to montmorillonite: 3NaAlSi3O8 + Mg2+ + 4H2O(l) ? 2Na0.5Al1.5Mg0.5Si4O10(OH)2 + 2Na+ + H4SiO40 This yields Si:Na = 1:2! If we assume that albite dissolves incongruently to form kaolinite during the weathering, then we would expect a Si:Na ratio of 2:1. However, the analysis of the spring water in the New Mexican rhyolite has a Si:Na ratio of 1:1. If we assume that the product of albite weathering is montmorillonite instead of kaolinite, the water should have a Si:Na ratio of 1:2. Thus, neither of these weathering reactions alone can explain the water composition. It is possible that both reactions might be operating together to yield the required 1:1 ratio. Calculations of this type, in which we attempt to reconstruct water analyses in terms of the weathering reactions, are called mass-balance calculations. If we assume that albite dissolves incongruently to form kaolinite during the weathering, then we would expect a Si:Na ratio of 2:1. However, the analysis of the spring water in the New Mexican rhyolite has a Si:Na ratio of 1:1. If we assume that the product of albite weathering is montmorillonite instead of kaolinite, the water should have a Si:Na ratio of 1:2. Thus, neither of these weathering reactions alone can explain the water composition. It is possible that both reactions might be operating together to yield the required 1:1 ratio. Calculations of this type, in which we attempt to reconstruct water analyses in terms of the weathering reactions, are called mass-balance calculations.

6. 6 Garrels (1967) recognized that kaolinite is perhaps the most common product of silicate weathering. He calculated the proportions of the various ions that would result from the weathering of certain primary aluminosilicate minerals. For example, the reaction governing K-feldspar is: 2KAlSi3O8 + 2H2CO30 + 9H2O(l) ? Al2Si2O5(OH)4 + 2K+ + 2HCO3- + 4H4SiO40 This reaction would result in a K:Si:HCO3- ratio of 1:2:1, which is indicated by the bar graph above. For Na-feldspar we have: 2NaAlSi3O8 + 2H2CO30 + 9H2O(l) ? Al2Si2O5(OH)4 + 2Na+ + 2HCO3- + 4H4SiO40 Which indicates a Na:Si:HCO3- ratio of 1:2:1. For Ca-feldspar we have: CaAl2Si2O8 + 2H2CO30 + H2O(l) ? Al2Si2O5(OH)4 + Ca2+ + 2HCO3- yielding a Ca:HCO3- ratio of 1:2 (note, no H4SiO40) is produced by this reaction. In fact, we can see that weathering of Ca-feldspar give the same fluid species ratios as weathering of calcite: CaCO3 + H2CO30 ? Ca2+ + 2HCO3- i.e., a Ca:HCO3- ratio of 1:2. For pyroxene we write: CaMgSi2O6 + 4H2CO30 + 2H2O(l) ? Ca2+ + Mg2+ + 4HCO3- + 2H4SiO40 which results in a Ca:Mg:Si:HCO3- ratio of 1:1:2:4. The above diagram implies ratios of 1:1:1:4. I believe this may represent an error in the original Garrels (1967) paper, or else Garrels may have assumed some substitution of Al for Si in the pyroxene. The more complicated silicates, biotite and hornblende, are treated similarly. Garrels (1967) recognized that kaolinite is perhaps the most common product of silicate weathering. He calculated the proportions of the various ions that would result from the weathering of certain primary aluminosilicate minerals. For example, the reaction governing K-feldspar is: 2KAlSi3O8 + 2H2CO30 + 9H2O(l) ? Al2Si2O5(OH)4 + 2K+ + 2HCO3- + 4H4SiO40 This reaction would result in a K:Si:HCO3- ratio of 1:2:1, which is indicated by the bar graph above. For Na-feldspar we have: 2NaAlSi3O8 + 2H2CO30 + 9H2O(l) ? Al2Si2O5(OH)4 + 2Na+ + 2HCO3- + 4H4SiO40 Which indicates a Na:Si:HCO3- ratio of 1:2:1. For Ca-feldspar we have: CaAl2Si2O8 + 2H2CO30 + H2O(l) ? Al2Si2O5(OH)4 + Ca2+ + 2HCO3- yielding a Ca:HCO3- ratio of 1:2 (note, no H4SiO40) is produced by this reaction. In fact, we can see that weathering of Ca-feldspar give the same fluid species ratios as weathering of calcite: CaCO3 + H2CO30 ? Ca2+ + 2HCO3- i.e., a Ca:HCO3- ratio of 1:2. For pyroxene we write: CaMgSi2O6 + 4H2CO30 + 2H2O(l) ? Ca2+ + Mg2+ + 4HCO3- + 2H4SiO40 which results in a Ca:Mg:Si:HCO3- ratio of 1:1:2:4. The above diagram implies ratios of 1:1:1:4. I believe this may represent an error in the original Garrels (1967) paper, or else Garrels may have assumed some substitution of Al for Si in the pyroxene. The more complicated silicates, biotite and hornblende, are treated similarly.

7. 7 These bar diagrams are based on the diagram shown in the previous slide. Garrels (1967) took two water analyses, one from a rhyolite and the other from a basalt, and figured out what proportion of each of the ions could be attributed to the incongruent dissolution of each of the silicate phases shown in the previous slide. For example, for the rhyolite analysis, he assumed that all the chloride present came from NaCl. To account for the Na contribution from NaCl, he subtracted an amount of Na equal to the Cl concentration from the total Na concentration. He then also assumed that all the sulfate came from gypsum, and corrected the total Ca concentration for the amount of Ca contributed by gypsum. Next, he assumed that the remainder of Na came from the incongruent dissolution of albite. According to the previous slide, for every mole of Na from albite, 1 mole of bicarbonate and 2 moles of Si are added to the water. It was then assumed that all the K in the water came from weathering of orthoclase, together with the stoichiometric amounts of bicarbonate and Si. The weathering of anorthite does not produce any Si, so the remaining Si in the water must be attributed to hornblende weathering. Once the amount of Si contributed by hornblende is known, its contribution of Ca, Mg and bicarbonate can be calculated. This leaves a quantity of Ca and twice as much bicarbonate, which can be accounted for by dissolution of anorthite. The proportions of minerals contributing to the water composition for the water in the basalt are calculated in a similar manner, but based on different primary minerals. These bar diagrams are based on the diagram shown in the previous slide. Garrels (1967) took two water analyses, one from a rhyolite and the other from a basalt, and figured out what proportion of each of the ions could be attributed to the incongruent dissolution of each of the silicate phases shown in the previous slide. For example, for the rhyolite analysis, he assumed that all the chloride present came from NaCl. To account for the Na contribution from NaCl, he subtracted an amount of Na equal to the Cl concentration from the total Na concentration. He then also assumed that all the sulfate came from gypsum, and corrected the total Ca concentration for the amount of Ca contributed by gypsum. Next, he assumed that the remainder of Na came from the incongruent dissolution of albite. According to the previous slide, for every mole of Na from albite, 1 mole of bicarbonate and 2 moles of Si are added to the water. It was then assumed that all the K in the water came from weathering of orthoclase, together with the stoichiometric amounts of bicarbonate and Si. The weathering of anorthite does not produce any Si, so the remaining Si in the water must be attributed to hornblende weathering. Once the amount of Si contributed by hornblende is known, its contribution of Ca, Mg and bicarbonate can be calculated. This leaves a quantity of Ca and twice as much bicarbonate, which can be accounted for by dissolution of anorthite. The proportions of minerals contributing to the water composition for the water in the basalt are calculated in a similar manner, but based on different primary minerals.

8. 8 MASS-BALANCE CALCULATIONS APPLIED TO SIERRA NEVADA SPRINGS Note that there are a number of errors in Table 4-4 in Kehew (2001). The errors are corrected in this lecture. The data shown in this table refer to ephemeral springs in the Sierra Nevada Mountains. Garrels and McKenzie (1967) used these data to infer reverse weathering reactions assuming kaolinite as a reactant. Their goals were to establish: 1) whether the reverse reactions would produce primary minerals similar to those actually observed in the rocks being weathered; and 2) whether they could account for all the dissolved components of the spring water with just a few of these types of reactions. Starting with the spring water composition, the composition of snow melt was first subtracted. This accounts for the ions in the recharge before it interacts with the rocks. The bicarbonate concentration was then adjusted upward a bit to maintain charge balance. The resulting composition is given in the second line in the above table, and this composition is taken to represent the contribution of rock weathering to the composition of the spring water. It was then assumed that all the Na and Ca resulted from weathering of a plagioclase feldspar, the Na:Ca ratio of which was identical to the Na:Ca ratio of the water. The proportions of Al and Si required to have a charge-balanced plagioclase were determined, and the amounts of Si and bicarbonate required to charge-balance the reaction were subtracted. This resulted in the formation of 0.177 mmol L-1 of plagioclase of a composition similar to that actually found in the rocks. The third line in the table above shows the water composition after subtraction of the required reactants. The discussion is continued in the notes to the next slide. Note that there are a number of errors in Table 4-4 in Kehew (2001). The errors are corrected in this lecture. The data shown in this table refer to ephemeral springs in the Sierra Nevada Mountains. Garrels and McKenzie (1967) used these data to infer reverse weathering reactions assuming kaolinite as a reactant. Their goals were to establish: 1) whether the reverse reactions would produce primary minerals similar to those actually observed in the rocks being weathered; and 2) whether they could account for all the dissolved components of the spring water with just a few of these types of reactions. Starting with the spring water composition, the composition of snow melt was first subtracted. This accounts for the ions in the recharge before it interacts with the rocks. The bicarbonate concentration was then adjusted upward a bit to maintain charge balance. The resulting composition is given in the second line in the above table, and this composition is taken to represent the contribution of rock weathering to the composition of the spring water. It was then assumed that all the Na and Ca resulted from weathering of a plagioclase feldspar, the Na:Ca ratio of which was identical to the Na:Ca ratio of the water. The proportions of Al and Si required to have a charge-balanced plagioclase were determined, and the amounts of Si and bicarbonate required to charge-balance the reaction were subtracted. This resulted in the formation of 0.177 mmol L-1 of plagioclase of a composition similar to that actually found in the rocks. The third line in the table above shows the water composition after subtraction of the required reactants. The discussion is continued in the notes to the next slide.

9. 9 INFERRED WEATHERING REACTIONS FOR SIERRA NEVADA SPRINGS Reaction 1: 0.123Al2Si2O5(OH)4 + 0.110Na+ + 0.068Ca2+ + 0.246HCO3- + 0.220SiO2 ? 0.177Na0.62Ca0.38Al1.38Si2.62O8 + 0.246CO2 + 0.367H2O Reaction 2: 0.0037Al2Si2O5(OH)4 + 0.0073K+ + 0.022Mg2+ + 0.051HCO3- + 0.015SiO2 ? 0.0073KMg3AlSi3O8(OH)2 + 0.051CO2 + 0.026H2O Reaction 3: 0.0065Al2Si2O5(OH)4 + 0.013K+ + 0.013HCO3- + 0.026SiO2 ? 0.013KAlSi3O8 + 0.013CO2 + 0.0195H2O The next step was to assume that all the Mg in solution resulted from biotite weathering, and the amounts of K, bicarbonate and silica required to convert kaolinite back to biotite were subtracted from the water. This resulted in the formation of 0.0073 mmol L-1 of biotite. Finally, the remaining K, bicarbonate and silica were employed to make 0.013 mmol L-1 of K-feldspar. All that is left is 0.009 mmol L-1; all the other major components were consumed in the process. The resulting reactions are given in this slide. The fact that almost all the water components are accounted for during this procedure implies that the three reactions shown above, running from right to left, are solely responsible for the observed water composition. Mass-balance calculations such as those illustrated here can be carried out using spreadsheets, or computer programs based on matrix algebra, such as BALANCE (Parkhurst et al., 1982), NETPATH (Plummer et al., 1994) and PHREEQC (Parkhurst, 1995). However, it must be remembered that there may well be more than one mass-balance solution that fit the data equally well. Just because a satisfactory mass-balance is obtained, does not mean that the reactions chosen are correct. Here is where professional judgment comes in. REFERENCES Parkhurst et al. (1982) BALANCE - A computer program for calculating mass transfer for geochemical reactions in groundwater. U.S. Geol. Surv. Water Res. Inv. Rept. 82-14. Plummer et al. (1994) An interactive code (NETPATH) for modeling net geochemical reactions along a flow path. Ver. 2.0. U.S. Geol. Surv. Water Res. Inv. Rept. 94-4169. Parkhurst (1995) Users guide to PHREEQC - A computer program for speciation, reaction-path, advective-transport, and inverse geochemical calculations. Geol. Surv. Water Res. Inv. Rept. 95-4227. The next step was to assume that all the Mg in solution resulted from biotite weathering, and the amounts of K, bicarbonate and silica required to convert kaolinite back to biotite were subtracted from the water. This resulted in the formation of 0.0073 mmol L-1 of biotite. Finally, the remaining K, bicarbonate and silica were employed to make 0.013 mmol L-1 of K-feldspar. All that is left is 0.009 mmol L-1; all the other major components were consumed in the process. The resulting reactions are given in this slide. The fact that almost all the water components are accounted for during this procedure implies that the three reactions shown above, running from right to left, are solely responsible for the observed water composition. Mass-balance calculations such as those illustrated here can be carried out using spreadsheets, or computer programs based on matrix algebra, such as BALANCE (Parkhurst et al., 1982), NETPATH (Plummer et al., 1994) and PHREEQC (Parkhurst, 1995). However, it must be remembered that there may well be more than one mass-balance solution that fit the data equally well. Just because a satisfactory mass-balance is obtained, does not mean that the reactions chosen are correct. Here is where professional judgment comes in. REFERENCES Parkhurst et al. (1982) BALANCE - A computer program for calculating mass transfer for geochemical reactions in groundwater. U.S. Geol. Surv. Water Res. Inv. Rept. 82-14. Plummer et al. (1994) An interactive code (NETPATH) for modeling net geochemical reactions along a flow path. Ver. 2.0. U.S. Geol. Surv. Water Res. Inv. Rept. 94-4169. Parkhurst (1995) Users guide to PHREEQC - A computer program for speciation, reaction-path, advective-transport, and inverse geochemical calculations. Geol. Surv. Water Res. Inv. Rept. 95-4227.