Geochemistry: water-rock interaction, water and gas geochemistry, isotopes, geothermometry O. Vaselli (CNR-IGG, Florence) & Dept. Earth Sciences, Univ. Florence orlando.vaselli@unifi.it
Georg Pawer (Agricola), 1556: Non reagent nisi soluti The presence of fluids triggers most geological processes, which at their turn are regulated by geochemical processes: WATER-ROCK INTERACTION (WRI)
Differently from other geological disciplines operating in Geothermics, (Fluid) Geochemistry allows a direct contact with what it is commonly discharged from a geothermal reservoir Thermal and mineral waters Soil diffuse gas Gas discharges: bubbling & boiling pools and fumaroles
Different fuid (gas and water) emissions
The chemical composition of natural waters reflects the chemical weathering processes operated by the meteoric waters to the minerals they are interacting with (WRI). They are depending on the alterability degree (solubility) of the minerals: the higher it is the higher the ions getting into the solution. A solution may get saturated in certain ion pairs, which may originate precipitating salts.
Main sealing minerals in geothermal applications: 1) Calcite (almost always oversatutared) 2) Silica (strictly temperature-dependent) 3) Fe-hydoxides (strongly p H and Eh dependent) 4) Hg-Sb-As-sulphides (in volcanic areas)
Chemical weatheirng Congruent dissolution CaCO 3 + H 2 O = Ca 2+ + HCO 3 - + OH - Incongruent dissolution NaAlSi 3 O 8 + 11/2H 2 O = Na + + OH - + 1/2Al 2 Si 2 O 5 (OH) 4 CaAl 2 Si 2 O 8 + 11/2H 2 O = Ca 2+ + OH - + 1/2Al 2 Si 2 O 5 (OH) 4 Redox reactions FeS 2(s) + 7/2O 2 + H 2 O Fe 2+ + 2SO 4 2- + 2H +
Each single chemical and isotopic composition we obtain is of paramount importance since it reflects a direct information from underground. There is a (big) problem Gas and isotopic composition Water and isotopic composition Which is the meaning of the chemical and isotopic compositions we measure at the surface? ?
Physico-chemical Analyses Components in liquid phase: Ca, Mg, Na, K, HCO 3 , CO 3 , SO 4 , Cl, SiO 2 , NH 4 , NO 3 , F, Br, Li, B, S II , CO 2 , Al, As, Ba, Be, Cd, Co, Cr, Cs, Cu, Fe, Hg, Mn, Mo, Ni, P, Pb, Rb, Sb, Se, Sn, Sr, Th, Tl, U, V, W, Zn, Zr Physical parameters: T, p H, Eh, p H 2 S, p NH 4 , p O 2 , p CO 2 Gas: H 2 O, CO 2 , N 2 , HCl, HF, Ar, CH 4 +hydrocarbons, O 2 , Ne, H 2 , He, H 2 S Dissolved gas: CO 2 , N 2 , Ar, CH 4 , O 2 , Ne, H 2 , He Isotopes: 18 O/ 16 O; D/H; 13 C/ 12 C in DIC (Dissolved Inorganci Carbon) ; 13 C/ 12 C in CO 2 , 3 He/ 4 He, 87 Sr/ 86 Sr
1 CO 2 3 2 2a “ Magmatic gas scrubbing ” “ any process able to reduce emissions during reactions between gas, water and rocks (dissolution, formation of precipitates, gas-water chemical reactions etc.)”
Main volcanic gases H 2 O SO 2 Typical magmatic gases H 2 S HF Acidic gases HCl CO 2 Isotopes CO - Essential: 18 O and 2 H in H 2 O, 3 He/ 4 He, 13 C in CO 2 ; CH 4 + hydrocarbons - Very Useful: 3 H in H 2 O, 34 S in S-bearing Noble gases (He, Ar, Ne, Kr, Xe, Rn) species, H 2 - Useful: 13 C and 2 H in CH 4 , 2 H in H 2 , 15 N in N 2 , 40 Ar/ 36 Ar, etc. NH 3 N 2 CFC, COS, S 2 , heavy metals Irrutupuncu, N. Chile
Interaction between magmatic…
…and hydrothermal fluids
N 2 , H 2 O, O 2 , Ar, CO 2 CO 2 H 2 O H 2 H 2 S Noble gases + N 2 CO boiling H 2 O H + Cl - F - CO 2 H 2 S SO 2 Hydrothermal system C > S > Cl > H 2 O > F H 2 O CO 2 SO 2 H 2 S H 2 HCl HF CO Mass and hear transfer M A G M A
Gas species directly deriving by magma degassing are defined Gas species derived by as “ juveniles ”, i.e. they boiling processes at see the sunlight for the depth first time in their history Gas species derived by mobilization processes (e.g. CO 2 from carbonatic rocks) due to thermometamorphism Gas species such as CO 2 , hydrocarbons and N-bearing specie by thermal or bacterial decompoposition of organic matter Recycling of atmospheric gases or by degassing processes of air-saturated waters.
‘Wet’ Degassing H 2 O CO 2 H 2 S tracce di SO 2 , HCl, HF … Shallow Boiling Scrubbing Dry Zone SO 2 H 2 S CO H 2 O H 2 CO 2 HF HCl Bubbly Magma
Secondary interactions “ Magmatic gas scrubbing ” “ any process able to reduce emissions during reactions between gas, water and rocks (dissolution, formation of precipitates, gas-water chemical reactions etc.)” Crustal contamination Inputs derived by biological activity and/or radioacrive decay Addition of air ASW (Air Saturated Water) and/or direct contamination of air
Generally speaking, it can be said that the magmatic systems are are dominated by oxidizing conditions, whereas in the hydrothermal systems reducing conditions prevail. Consequently, the former will have SO 2 and other magmatic gases, whereas the latter show CO 2 , H 2 S, H 2 , CO and CH 4 , which are also favored by scrubbing processes.
Volcanic/Geothemal Gases mixture with mixing with biogenic gases air (soil CO 2 ,…) mixing with mixing with meteoric waters groundwaters thermal degradation condensation of organic matter precipitation of less fluid-rock soluble components interaction decarbonation mixing with processes hydrothermal system Magma Volatiles
Cl-SO 4 acidic waters 1 Typical of crater lakes such as El Chichón, Kawah Ijen, Poás, Sirung, Yugama and Yakeyama. Inflow of magmatic gases rich in HCl, SO 2 and H 2 S, whose dissolution forms acidic solutions that are strongly aggressive to the rocks. Paucity of Cl-SO 4 acidic waters in geothermal reservoirs associated with volcanic systems SO 2 tends to be disproportionated: 4SO 2 + 4H 2 O ⇒ H 2 S + 3H 2 SO 4 These (oxidized) solutions are chemically reactive and remove cations from the hosting rocks, depositing in most cases alunite, anhydrite, pyrite and kaolinite. At depth the magmatic gases interact with higher contents of waters and rocks with respect to what is occurring at the surface. It is at depth the Na-Cl-rich waters (almost neutral) likely form.
Water types Circulating waters in deep-seated 2 geothermal reservoirs and high enthalpy: Na-Cl with Cl up to thousands of mg/L (155.000 mg/L; Salton Sea, California) and acidic to neutral pH values with high SiO 2 , K, Li, B and F, whereas Mg is low. The main dissolved gases are CO 2 and H 2 S. These waters are usually fed by meteoric waters, although connate or 2a magmatic waters can be present. The waters at depth are initially acidic and turn to be neutral Na-Cl waters due to WRI processes and removal of magmatic sulfur species by transformation to sulfate/sulfide. The deep Na-Cl waters can get to the surface or mix with shallow aquifers to produce Cl-diluted waters. Often they can be found at several kms from the volcanic edifice.
SO 4 -acid waters 3 They are normally located above the geothermal system where the vapor phase separation occurs. As a boiling process occurs gas species (CO 2 and H 2 S) go into the vapor phase. This vapor may react with shallow and/or surface waters. The steam may partly condense to produce “ STEAM-HEATED WATERS ”. Here, H 2 S oxidises to sulfuric acid, producing SO 4 -acid waters. At their turn, the STEAM-HEATED WATERS may be boiling, separating a secondary vapor that gets to the surface to produce low-pressure steaming grounds.
Solutes: main anions Chloride seawater Cl Bicarbonate 19,350 mg/kg ~50 to ~20,000 mg/kg <1 to several 1000 mg/kg (to ~200,000 mg/kg in (for most purposes, effectively the same as hypersaline brines) “alkalinity”) Sources: traces of Na-K-Cl in volcanic Sources: reactions of rocks (seawater origins), connate dissolved CO 2 from seawater in sedimentary rocks, halite atmosphere and/or in deposits geothermal/volcanic steam, Sulfate with silicate minerals in rocks, with carbonate ~10 to ~1500 mg/kg minerals (limestone) (to ~100,000 mg/kg in acid volcanic steam condensates Sources: oxidized sulfide minerals and H 2 S, sulfate mineral deposits (gypsum, anhydrite) Approximate range among non-volcanic Extremes of volcanic and steam heated are acidic (no HCO 3 ) geothermal systems (higher SO 4 exist)
2- Cl + SO 4 50 0 Na-Cl Na-HCO 3 25 50 0 Na-Cl waters ? Mare Meteo III II Na + + K + Ca 2+ + Mg 2+ 25 25 IV I HCO 3 /SO 4 HCO 3 /SO 4 HCO 3 /SO 4 Sup 0 50 25 Ca-SO 4 Ca-HCO 3 0 50 - + CO 3 2- HCO 3
CaCO 3(s) + CO 2(g) +H 2 O <--> Ca 2+ (acq) + 2HCO 3 (acq) - Ion-exchange reactions If a mineral is able to adsorb ions onto its surface when in an electrolytic solution, some ions can be “captured” by the mineral from the solution while others may “leave” the mineral. Na-clay+ Ca 2+ <---> Ca-clay+ 2Na +
Source of water solutes All samples are close to SO 4 /Ca+Mg = 1: stoichiometric dissolution of sulfate minerals (gypsum, anhydrite) (Ca-Mg)SO 4 + H 2 O Ca ++ (+Mg ++ ) + SO 4 -- + H 2 O All samples are close to Na/Cl = 1: stoichiometric dissolution of evaporitic minerals (halite) or Na-Cl waters as seen before NaCl + H 2 O Na + + Cl - + H 2 O
18 O/ 16 O sample - 18 O/ 16 O V-SMOW 18 O ‰ = x 1000 18 O/ 16 O V-SMOW 2 H/ 1 H sample – 2 H/ 1 H V-SMOW 2 H ‰ = x 1000 2 H/ 1 H V-SMOW 13 C/ 12 C sample – 13 C/ 12 C V-PDB 13 C ‰ = x 1000 13 C/ 12 C V-PDB Helium isotopes R/Ra: R is the measured 3 He/ 4 He ratio and Ra is the 3 He/ 4 He ratio in the AIR : 1.39x10 -6
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