Fluidi in ambiente vulcanico e geotermico: sources and processes

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1 Fluidi in ambiente vulcanico e geotermico: sources and processes

2 L’interpretazione della composizione dei fluidi, i processi primari e secondari che la determinano sono causa di tremendi mal di testa per la comunità scientifica internazionale

3 Cause del mal di testa mondiale ?
Mancanza di un sistema di campionamento ed analisi codificato… Modelli interpretativi vecchi o non ancora verificati Complicatezza intrinseca della materia Mancanza di conferme dirette per via sperimentale

4 Geothermal prospection Per cosa si utilizzano i modelli geochimici basati sulla composizione chimica ed isotopica di fluidi ? volcano monitoring

5 Iinterpretazione e modelli avanzati

6 Valutazione di contributi profondi (magmatici) e superficiali (idrotermali) sono lo scopo principale del monitoraggio vulcanico Studi vulcanologici e in ambiente geotermico implicano l’utilizzo degli stessi parametri e strumenti geochimici

7 What is a “volcanic gas” ?
Volcanic gas is… “a multi-component and multi-source hot gaseous fluids naturally emitted in a closed spatial and temporal association with a volcanic system” What is a “volcanic gas” ? Due to the very large spectrum of gas exhalations and volcanic systems, this definition seems to be quite ambiguous…..

8 the status of a “volcanic system”:
A first problem: the status of a “volcanic system”: “erupting” (Stromboli volcano) “active” but presently dormant (Vesuvius; Copahue; Cordon Caulle before 2011 etc.) “recently active” but still degassing (Panarea volcano, southern Italy) “extinct”, sometimes still degassing (quaternary volcanoes… many examples…)

9 A second problem: where should volcanic gas exhalations be located with respect to the volcanic edifice ? Within the “active” or “recently active” crater ? Along the crater rim ? Along the flanks ? At the base of the volcanic edifice ? At a certain distance from the base ?

10 A third problem: volcanic gases can be described by many independent parameters such as: Measurable temperature (low, medium, high) Chemical equilibria temperatures Emission “styles” (diffuse steaming ground, fumaroles, bubbling gases, dry vents etc.) Chemical composition (Steam content, acids, redox species) Flux (diffuse vs concentrated emissions) Origin

11 Therefore, a general, comprehensive classification scheme for volcanic gases is difficult to attain….. and a certain degree of confusion exists even for the definition of “volcanic gas”…

12 Examples of widely used descriptive terms
“Boiling pools or bubbling gases”: gases bubbling through water pools at the surface. Temperature at the boiling point or lower. Low to very high fluxes. Steam condensation and enrichment in dry gases. “Mud volcanoes”: superficial emissions of a complex mixture of CH4-dominated gas, liquid and solid material. Usually, but not only, associated with sedimentary environments “Geysers”: icelandic term referring to transient and violent emission of boiling waters and steam. Temperature at the boiling point. Examples of widely used descriptive terms “Steaming ground” indicates a diffuse steam emission with temperatures at the boiling point or lower. Low to very low fluxes. “Fumarole”: the italian term “Fumarola” means a smoke-like emission. This descriptive term is used to indicate an emission appearing as a withish plume. It is usually hot with medium to high concentrated fluxes. “Dry vents”: cold emission of dry gases, usually CO2-dominated.

13 Mixture of Gas + vapors Volcanic gas
Sub-critical gas phase at standard conditions (25°C, 1 bar) (equilibrated with liquid or solid phases) at “standard conditions”) Whatever the “style” of emission, any possible classification scheme should proceed from the chemical composition of the gas mixture Vapors Iper-critical gas phase at standard conditions (25°C, 1 bar) Gas

14 “Operative” classification scheme of the gas species
H2O NH4Cl, S, MenCln etc. CO2 H2S SO2 HCl HF H2 CO O2 N2 CH4 Ar, He, noble gases Idroc. VOC Steam Vapors Sublimates Acid, water soluble Condensables/water soluble Very acid, water solubles Uncondensables or residuals “Operative” classification scheme of the gas species

15 “Behavioural” classification of the gas species
H2O H2S SO2 HCl HF H2 CO O2 CH4 Idroc. N2 Ar He Ne Stable isotopes Chemically reactive (P,T geoindicators) Inerts (tracers)

16 “Genetic” classification of the gas species
A main difficulty to provide a genetic classification scheme for volcanic gases derives from their complex, multi-component nature. The idea that tens of chemical components could have been originated from the same source appears very unlikely. A “genetic classification scheme” for volcanic gases appears as the most meaningful criterion for investigation of active volcanoes and particularly useful for volcanic surveillance. The multi-source origins of gas and vapors

17 Possible sources of gas and vapors: Magma generation and degassing
Boiling of water at depth Metamorphism (thermal decomposition of minerals and equilibria) Decay of radioactive isotopes Thermal or Bio-degradation of organic matter

18 Possible sources of gas and vapors: Magma generation and degassing
Boiling of water at depth Metamorphism (thermal decomposition of minerals and equilibria) Decay of radioactive isotopes Thermal or Bio-degradation of organic matter

19 The story of a volcanic gas starts with magma generation…….
Gases directly related to magma degassing are defined “magmatic gases”. Therefore, magmatic gases represent a specific type of “volcanic gases”. Magmas are produced due to partial melting of different type of sources in different geological settings: Divergent plate boundaries (Mean Ocean Ridges) Convergent plate boundaries (Andesitic volcanoes) Hot spots Magmas are produced due to partial melting of different type of sources in different geological settings: Divergent plate boundaries (Mean Ocean Ridges) Convergent plate boundaries (Andesitic volcanoes) Hot spots Magmatic gases can be investigated in different ways: sampling and analysing “volcanic gases” analysing melt and/or fluid inclusions in crystals

20 But magma degassing is often only the beginning of the “story”.
Cooling, chemical re-equilibration, steam condensation and scrubbing of volatile compounds according to their relative solubility Addition of fluids (steam and gases) due to boiling Direct uprising without significant modifications (“magmatic gases”) But magma degassing is often only the beginning of the “story”. Continuous aquifers To investigate “magmatic gases”, to minimize secondary modifications during uprising, acid-bearing high temperature “fumaroles” (>400°C) and “chemical or isotopic tracers” must be considered. Magma generation, uprising, cooling and degassing

21 The use of chemical tracers: N2, He, Ar, Ne
A case study: Vulcano Island (Southern Italy) Inert chemical species (“tracers”) and stable isotopes as genetic markers

22 Low temperature gas exhalations at the eastern beach (bubbling gases)
High temperature gas exhalations along the crater rim (fumaroles) End member air air saturated water A large amount of non-atmospheric N2 is present in the gas mixture. We define it as: N2-excess

23 The “atmospheric” chemical tracers: The comparison with He
Azacualpa geoth. field (Honduras) Vulcano island (Italy) Tacora (Chile) Lascar (Chile) The “atmospheric” chemical tracers: The comparison with He air Air saturated water Crust and/or mantle A first chemical characteristic of volcanic gases at convergent plate boundaries: N2-excess; He-excess Neither He nor a large fraction of N2 can be referred to atmospheric contribution.

24 Geothermal Systems Geothermal Systems
Gas geochemistry He enrichment Geothermal Systems * N2 enrichment, magmatic (andesitic) contributions and/or fluid-rock interaction with organic sedimentary rocks * He enrichment by crustal contamination, long residence time of fluids in the crust (fluid-rock interactions), 4He enrichment Geothermal Systems * Principally meteoric contributions (air and/or ASW input) * N2 enrichment, contribution of N2 from no atmospheric source * He enrichment

25 1) He input from magma (input of 3He, mantle signature)
Gas geochemistry He enrichment Volcanic Systems * High N2, magmatic (andesitic) contributions, related to sedimentary material incorporated in the subduction process, typical for arc volcanism, like CAVZ * Limited meteoric contributions (air and/or ASW input) * High He/N2, typical andesitic contributions (N2) with high He contributions Volcanic Systems CAVZ: He enrichment by crustal contamination, long residence time of fluids in the crust, fluid-rock interactions with granite-type, which typically constitute the upper crust of the CAVZ, produce indeed severe 4He enrichment Volcanic Systems High He contents: 1) He input from magma (input of 3He, mantle signature) 2) He input from crust (input of 4He, crustal signature)

26 interpretation Main gases

27 interpretation tracers

28 Horizonatal ad vertical zonation
interpretation Horizonatal ad vertical zonation

29 interpretation Inert gases

30 interpretation water vapor source

31 interpretation mantle and subduction

32 vs. Sistemi geotermici vulcani Lascar Lastarria El Tatio Puchuldiza Tacora Putana Surire C. Aguicho Irrupucuntu

33 H2O, CO2, H2S (HCl), CH4, N2, H2, CO, air
Caratteristiche composizionali di fluidi naturali da vulcani attivi e campi geotermici Vulcani attivi H2O, CO2, H2S, HCl, SO2, HF up to 700 °C Campi geotermici H2O, CO2, H2S (HCl), CH4, N2, H2, CO, air up to 160 °C

34 Composition and characteristics of hydrothermal fluids
- Temperature: up to 400°C - Pressure: depends on water depth (mostly bar) - pH value: mostly acidic (pH 2-6) - Redox potential: reducing - Salinity: 1/10 to >2-fold seawater salinity (--> boiling) - Gas content: high concentrations of methane, hydrogen sulfide, carbon dioxide, hydrogen, helium - Ion content: some ions are depleted compared to seawater (such as Mg, sulfate, partly alkali metals) most metals are strongly enriched (Mn, Fe up to 106-fold)

35 Composition and characteristics of hydrothermal fluids
Variables for chemical control of hydrothermals fluids: 1. p-T conditions Important: p and T in the subseafloor reaction cell and at the seaflorr 2. Boiling and Phase separation: Separation of gases and salts + metals, and phase segregation (spatial separation of vapor and brine) 3. Chemical composition and mineralogy of the rock, alteration state 4. Ratio water/rock Degassing of magma (important for gases CO2, 3He) 6. Time; largely unknown, how long fluids remain on the respective T-p paths

36 Sources of Geothermal Fluids
Sources of Natural Waters: Meteoric Water (rain, snow) Sea Water Fossil Waters (trapped in sediments in sedimanary basins) Magmatic Waters Metamorphic Waters

37 Principle of a hydrothermal circulation cell
Hydrothermal habitats White ( °C) and Black (up to 400°C) Smoker Plume Pillow lava Diffuse fluids (<100°C) Cold seawater Cooling by mixing Mineral precipita-tion Sheetflow lava Conductive cooling Hot endmember fluid (up to 400°C) Magma chamber

38 Magmatic degassing e reazioni di alta temperatura
SO2 3He magmatic degassing HF HCl CO2 H2 CO Magmatic degassing e reazioni di alta temperatura H2O ? CH4 H2S N2 Hydro 4He Processi secondari

39 Principali componenti delle fasi gassose fumaroliche

40 Low T High T H M

41 Periphery of volcanic edifices
* Meteoric-related (related to input of air and/or meteoric water) compounds tend to increase at increasing distance from the volcano summit Periphery of volcanic edifices Periphery of volcanic edifices * Dominated by magmatic-related end-member mixed, at variable degrees, with hydrothermal fluids especially at the periphery of the volcanic edifices * Origin of gas in volcanic systems: a) Magmatic (SO2), b) hydrothermal (H2S + CH4) and c) meteoric (Ar)

42 Las especies gaseosas mayores

43 traccianti

44 Zonazione composizionale con profondità ed in senso spaziale

45 Composti atmosferici

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47 The use of stable isotopes
H2O; CH4 He CO2; CH4 H2O; CO2 The use of stable isotopes The isotopic composition is expressed with the notation d: d13C/12C ‰ = {[(13C/12C)m - (13C/12C)std]/(13C/12C)std)} * 1000

48 L’Elio ha due isotopi Radiogenico Primordiale He He 4 3
Alvarez e Cornig (1939) scoprirono 3He e notarono chei alcuni pozzi il rapporto 3He/4He era superiore di 10 volte rispetto a quello nell’atmosfera Scoperto da Rutherford come prodotto di decadimento di U e Th Radiogenico Primordiale

49 Elio nell’atmosfera ca. 5.2 ppm
E rapporto 3He/4He nell’atmosfera: 1.39x10-6 In materiale extra-terrestre ricco di gas il rapporto 3He/4He misurato era di 2-4x10-4 Nucleogenic 3 He 6 Li + n H + a H He + b 7 Li + 8 Be + g 4 Trizio con T1/2=12.5y

50 3He/4He nell’atmosfera: 1.39x10-6
Mamyrin & Tolstikhin (1984) I rapporti 3He/4He prodotti da queste reazioni sono comprese fra e 10-9, ed inoltre, dimostrarono che nessun processo nucleare può produrre rapporti superiori a 10-8 e quindi, trascurabili rispetto ai rapporti come quelli misurati nell’atmosfera. 3He/4He nell’atmosfera: 1.39x10-6 Anomalie: depositi di Li (produzione di alti rapporti 3He/4He) - depositi di U (produzione di bassi rapporti 3He/4He) - centrali nucleari (produzione di alti rapporti 3He/4He)

51 GAS DISCIOLTI NELLE ACQUE SOTTERRANEE E TERMALI
Questo ha portato alla conclusione che i rapporti elevati di 3He/4He nelle inclusioni fluide di minerali magmatici, nei gas vulcanici e nelle acque fossero dovuti alla presenza di 3He originario (primordiale) presente nelle porzioni profonde della Terra e degassante verso la superficie INCLUSIONI FLUIDE FUMAROLE GAS DISCIOLTI NELLE ACQUE SOTTERRANEE E TERMALI

52 1.39x10-6 Quindi, parleremo di R/Ra dove:
Il rapporto isotopico misurato è riferito a quello dell’ARIA. Questo evita di trascinarsi dietro numeri molto piccoli vista la differenza di abbondanza tra 3He e 4He. Quindi, parleremo di R/Ra dove: R è il rapporto 3He/4He misurato: Ra è il rapporto 3He/4He dell’ARIA: 1.39x10-6

53 3He/4He aumenta con la profondità
Uranio e Torio sono elementi incompatibili e tendono quindi ad arricchirsi nei fusi residuali crustali con, dunque, una maggior produzione di 4He. Degassamento 3He/4He aumenta con la profondità

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55 Isotopi dell’elio

56 European subcontinental mantle
Crosta 0.02 R/Ra MORB 8±1 Hot-spot fino a 30 European subcontinental mantle 6.5±1

57 Geochimica dell’He 3He non è prodotto da nessun processo naturale terrestre. Tutto 3He nella Terra è o primordiale o portato dalla polvere cosmica 4He è radiogenico, prodotto dalla alpha-decays 8 atomi di 4He per 238U, 7 per ogni 235U, 6 per ogni 232Th He è il solo gas nobile in grado di sfuggire dall’atmosfera. Tempo di residenza di He in atmosfera: Massa 3He = 1.3 x 10–6 * 5.24 ppm * 5.2 x 1021 g Flusso di 3He = 1000 moli/anno = 3000 g/anno Tempo di residenza = 10 Ma

58 Geochimica isotopica dell’elio
Gli isotopi dell’elio sono una misura del rapporto (time-integrated ratio) (U+Th)/3He: L’elio si comporta come un elemento incompatibile durante la fusione del mantello (i.e. meglio i fusi dei minerali) He dovrebbe essere più incompatibile di U e Th durante la fusione del mantello He non è riciclato nel mantello Se così, alti 3He/4He riflettono un materiale del mantello meno degassato

59 Helium has important advantages as genetic marker:
The isotopic composition of He is expressed as Rc/Ra where Rc is the air-corrected Rm value Rm = 3He/4He Ra (air) = 1.4 x 10-6 Helium has important advantages as genetic marker: As noble gas is a chemically inert compound with very low water solubility It has two stable isotopes: one primordial (3He) and an other radiogenic (4He). Due to the crustal enrichment of U and Th (radioactive parents), 4He is strongly enriched in crustal gases. Rc/Ra=((Rm/Ra)(He/Ne)m-(He/Ne)a) ((He/Ne)m-(He/Ne)a) c: air-corrected; m=measured; a=air MORB 8 Rc/Ra 0.05 CRUST Etna Southern Volc. Zone of Chile Aeolian Is. 100% Mantle contribution 0%

60 Istogramma degli isotopi dell’He in MORB
Nessuna relazione fra composizione isotopica e “spreading rate” ma la varianza è inversamente proporzionale allo “spreading rate” Questo riflette - efficienza del mixing nel mantello superiore - differenze nel grado di omogeneizzaione del magma Graham 2002

61 Confronto fra rapporti isotopici di He per MORB, OIB, e hot-spot continentale
Il valore medio di 3He/4He dai differenti segmenti di ridge è quasi identico sebbene la varianza sia differente OIB sono molto più variabili I rapporti 3He/4He inferiori rispetto ai MORB sono frequentemente associati con Pb (HIMU) radiogenico e riflettono componenti riciclati nel mantello After Barford, 1999

62 Rapporti isotopici di He OIB
OIB hanno un ampio intervallo composizionale La distribuzione degli isotopi dell’He ha un doppio picco: massimo a 8 RA e 13 RA Il primo è identico allla media MORB che indica il coinvolgimento di mantello depleto nel vulcanismo OIB Il secondo è poco chiaro Farley and Neroda 1998 MORB: well-mixed degassed mantle e bassi 3He/(U+Th) OIB: eterogeneo, less degassed mantle con alto 3He/(U+Th)

63 Il paradosso elio-calore
O’Nions and Oxburgh, 1983 ~75% di He che entra in atmosfera è prodotto da crosta continentale ~25% dal mantello ~10% di He dal mantello è primordiale, il resto è radiogenico

64 IL paradosso elio-calore
4He è prodotto da decadimento radiattivo di U e Th 10-12 J è l’energia liberata dal decadimento alfa Il flusso di 4He dal mantello corrisponde a 2.4 TW di produzione di calore Il flusso di calore terrestre è 44 TW (Pollack et al., 1993) TW dalla crosta (e.g., Rudnick and Fountain, 1995) , 3-7 dal nucleo (Buffett et al., 1996) ; e TW dal mantello Di TW dal mantello, 18–22 è raffreddamento secolare; il calore radiogenico è 9-14 TW, più alto di un fattore 4-6 di 2.4 TW che è il calore calcolato in base al flusso di 4He Implica una barriera nel mantello che rilascia calore ma blocca 4He (O’Nions and Oxburgh, 1983)

65 Combinazione isotopi Elio e Carbonio (CO2) in Italia
R/Ra 10 MORB Circum-Pacific Italy Volcanic Arc 8 Mt. Etna 6 Crustal influence Aeolian Arc 4 Neapolitan Magmatic Province 2 13 C-CO Tuscan-Roman d 2 Magmatic Province -8 -6 -4 -2

66 Crustal He Limestone CO2 Mantle He Mantle CO2

67 CO2 provenance Different origins of CO2 :
Sediments (carbonate vs organic) Mantle degassing How can the origin of CO2 be recognized ?? Tacora (Chile) organic matter limestone Lascar (Chile) mixing lines Lastarria (Chile) Two combined parameters CO2 provenance d13C ‰ Organic sediments → -30 ‰ Carbonates → ≈ 0 ‰ Mantle (MORB) → - 6.5±2.5 ‰ Vulcano (Italy) Azacualpa geoth. Field (Honduras) mantle CO2/3He Organic sediments → 1 x 1013 Carbonates → 1 x 1013 Mantle (MORB) → 1.5 x 109 Global average arc value → 1.2 x 1010

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69 M (mantle) + L (Limestone) + S (Organic sed.) Mass balance equations
The distribution of data suggest that in the subduction zone gas samples can be explained by three-component mixing. M (mantle) + L (Limestone) + S (Organic sed.) Mass balance equations Aguas Calientes (Puyeue, SVZ) L:S:M = 51:27:22 Where L/S = 1.9 Sediments at ODP site 1232 (Chile basin) L/S = 11 (13C/12C)obs = (13C/12C)MORBM + (13C/12C)LimL + (13C/12C)SedS 1/(12C/3He)obs = M/(12C/3He)MORB + L/(12C/3He)Lim + S/(12C/3He)Sed M + S + L = 1

70 Boiling and steam separation at shallow depth
Addition of fluids (steam and gases) due to boiling of lateral aquifers Boiling and steam separation at shallow depth Cooling, chemical re-equilibration, steam condensation and scrubbing of volatile compounds according to their relative solubility Continuous aquifers It’s unlikely that “reactive” chemical species (such as CO2) do not suffer for interactions with hosting rocks and groundwaters during uprising. Lateral aquifers But again, magma generation, uprising and degassing is often just the beginning of the “story”

71 Possible effects of groundwaters:
CO2/He fractionation due to different water solubilities in hydrothermal systems: preferential He loss associated with gas-phase separation Calcite precipitation into the hydrothermal system Addition of steam from groundwaters Additions of volatile hydrocarbons and more complex organics from thermal decomposition of organic matter from waters and sediments Addition of dissolved air Crustal contamination

72 Boiling of water

73 Single step vapor separation (isoenthalpic)
Relationships between the fraction of separated steam (y) and the enthalpies of steam and waters Single step vapor separation (isoenthalpic) Additions of gas species due to boiling H0 = HL (1-y) + Hv y y = (H0 – HL) / (HV – HL) The additions of gas species due to boiling depends of: type of boiling temperature of water and separated steam water solubility of the gas species The specific entalphy of steam is is much greater than the water in equilibrium with. At boiling conditions (saturated steam) temperature only depends on pressure. Any increase of heat supply results into an increase of steam fraction y. Therefore until the last drop of liquid waters evaporates, temperature remains absolutely constant.

74 low solubility medium solubility high solubility A multistep steam separation gives rise to an evolution of the separated gas mixture towards a relative increase of the high solubility compounds

75 Removal of gas species due to dissolution
Waters in liquid phase give rise to partial or total removal of gas species due to their dissolution. Removal of gas species due to dissolution Acids are dissolved according to their molecular solubilities, dissociation constant and alkalinity of water solutions SCRUBBING

76 Interazioni secondarie
“Magmatic gas scrubbing” “qualsiasi processo relago a reazioni acqua-gas-roccia (dissolution, formation of precipitates, gas-water chemical reactions etc.) in grado di ridurre le cocnentrazioni di gas primari” Crustal contamination Interazione magma-materiale sedimentario Composti organici; composti radiogenici Air addition Contaminazioni di fluid profondi con composti dell’atmosfera (aria o ASW)

77 Hydrothermal Cold Hot Magmatic

78 Acid water/rock interactions
By dripping an acid water solution over a carbonate rock a sparkling phenomenon can be obseved. CaCO3 + 2H+ = Ca2+ + CO2 + H2O

79 Thermomethamorphic reactions:
CaCO3 = CaO + CO2 ( °C) Ca-Al2-silicates + K-feldspar + CO2 = K-mica + calcite 3 K-feldspar + Ca2+ + CO2 = K-mica + CaCO3 + 2K+

80 Direct uprising without significant interactions with hosting rocks and groundwaters. Magmatic gases
Direct uprising without significant interactions with hosting rocks and groundwaters but affected by lateral inputs of hydrothermal fluids. Magmatic/hydrothermal gases Passing through (percolation) a single aquifer. Water is boiled off. Temperature is buffered but acids are only partially scrubbed. Hydrothermal/magmatic gases Passing through (percolation) a single aquifer. Water is boiled off. Temperature is buffered and acids are completely scrubbed. Redox is oxidant (gas buffered) Volcanic/Hydrothermal gases. Passing through (percolation) multilayered aquifers. Gases are mostly supplied by the more superficial aquifer. Temperature is buffered and acids are completely scrubbed. Redox is reducent (rock buffered). (R/Ra » 0.05) Hydrothermal/Volcanic gases. Magma at depth is completely degassed and only supply heat. More superficial groundwaters are heated up by hot rocks. (R/Ra ~ 0.05) Geothermal gases. In conclusion……… on chemical and isotopic bases volcanic gases can be distinguished as follows:

81 Ciclo idrologico dell'acqua
Impatto antropico Geochimica de las aguas Contaminazione naturale Contaminazione Naturale Ciclo idrologico dell'acqua Composti organici e metalli pesanti Combustione benzine al Pb (61%) Produzione di metalli ed acciai (23%) Industria min. e fusione (8%) Combustione carboni (5%) Solo il 4% del Pb atmosferico é naturale Pb antropogenico: origine ed effetti Cadmio Mercurio Arsenico Bario Berillio Zinco Rame Cromo Pesticidi Erbicidi Benzene Atrazina Diossina Cloruro di vinile Interazioni acqua-roccia

82 L’aria secca permette una lenta alterazione delle rocce (iscrizioni storiche). E’ l’acqua l’agente “destabilizzante”, in quanto dissolve e mantiene in soluzione le sostanze disciolte. Altre sostanze sono O2, CO2, acidi organici e acidi azotati. Ciclo dell’Acqua

83 La composizione chimica di un’acqua naturale riflette l’azione di alterazione operata dalle acque meteoriche nei confronti delle rocce con le quali viene a contatto e quindi, è funzione del grado di alterabilità di una roccia e dei suoi minerali

84 mg/L Il pH dell’acqua di pioggia è generalmente acido e tale acidità é attribuita a: CO2 + H2O = H2CO3 H2CO3 = HCO3- + H+ HCO3- = CO32- + H+ Altre reazioni che conferiscono acidità: SO2 + 1/2O2 --> SO3 SO3 + H2O --> 2H+ + SO42- 2NO2 + 1/2O2 + H2O --> 2H+ + 2NO3- + H2O 2NO + 3/2O2 + H2O --> 2H+ + 2NO3-

85 Nel processo magmatico si parla di
Rimangono nel cristalli Elementi compatibili Rimangono nel fuso Elementi incompatibili Nei processi superficiali si parla di: Tendono a liberarsi dal mezzo di origine durante i processi chimico-fisici Elementi mobili Tendono a rimanere nel mezzo di origine durante i processi chimico-fisici Elementi immobili Sono termini relativi al materiale geologico che si studia

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87 Processi di alterazione chimica
Include tra l’altro: Dissoluzione congruente Dissoluzione incongruente Processi di ossidazione Dissoluzione completa di un certo minerale, e.g. NaCl 4FeS2 +15O2+8H2O --> 2Fe2O3+8H2SO4 Dissoluzione parziale con formazione di un minerale secondario, e.g. K-Feld + H2O --> Kaol. + K+ + H4SiO4

88 temperatura composizione bassa entalpia 10-50 °C sistema geotermici
HCO3, Ca, SO4, Na, sistema geotermici °C Cl, Na, SO4, HCO3, Ca, volcani attivi °C Cl, SO4, Mg, Na, F

89 mg/L il pH dell’acqua di pioggia es generalmente acido a causa di:
CO2 + H2O = H2CO3 H2CO3 = HCO3- + H+ HCO3- = CO32- + H+ Altre reazioni che producono acidità: SO2 + 1/2O2 --> SO3 SO3 + H2O --> 2H+ + SO42- 2NO2 + 1/2O2 + H2O --> 2H+ + 2NO3- + H2O 2NO + 3/2O2 + H2O --> 2H+ + 2NO3-

90 Denominazione delle acque
Acque dolci: TDS<1000ppm Acque salmastre: 1000ppm<TDS<20000ppm Acque salate: ≈35000ppm Acque di salamoia: >35000ppm

91 Composizioni tipiche delle acque

92 H2O proveniance Chilean volcanoes
Whatever their location, steam condensates from high temperature volcanic gases (>400°C) differ from “primary” magmatic waters (dD = -60 ± 10 ‰) and converge to a common “isotopic type” named by Taran (1989) “andesitic water”. dD = -20 ± 10 ‰ d18O = + 10 ± 3 ‰ 18O-shift of seawater Volcanic gases Liquid after vapor separation incorporation of deep waters (connate, metamorphic) mixing seawater/primary magmatic water mixing 18O-shifted seawater/meteoric waters incorporation of seawater-derived fluids from subducted slabs H2O proveniance The isotopic compositions of steam condensates. Separated vapor How can this recurrent convergence of meteoric-steam alignements be interpreted ?? 18O shift Local meteoric water After Giggenbach, 1992) N.Zealand Indonesia Philippines Japan Kamchatka Americas Italy Greece Chile With few exceptions, the distributions of waters from volcanic exhalations in andesitic volcanoes reflect mixing processes between meteoric and “andesitic” waters. Volcanoes at convergent plate boundary

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96

97 Water is carried along the subducted slab in different forms:
Porosity water dD‰ = 0 Structural water (clays) dD‰ = -35‰ Structural water (chlorite, amphibole, serpentine) dD‰ = -40 ÷ -80‰ Porosity water Amphibole breakdown The observed dD‰ is consistent with waters released from the subducted crust, followed by fractionation during magma degassing (vapor-melt fractionation factor = 20‰) Micas breakdown Chlorite breakdown For water the contribution of “primary” or “juvenile” waters from mantle melting appear negligible. Lawsonite breakdown

98 Subduction-related volatile compounds
Water and other types of volatiles are carried at depth by the subducted slab Subduction-related volatile compounds The mantle wedge above the subducted slab is metasomatized by fluids coming from the subducted marine carbonate and organic sediments The subducted slab releases H2O and CO2 with minor amounts of B and less volatile LILE (mobilized into the hydrous fluids), which are later find as trace elements in the erupted magmas Partial melting of the mantle wedge Hydrous fluids Melts Sediments Hydrated Mg-silicates from mafic and ultramafic complexes (Chlorite, Amphibole, Antigorite etc.) H2O Hydrated silicates from marine sediments (smectites) H2O Carbonates from marine sediments CO2 Organic matter from marine sediments CO2, N2 Mafic complex Ultramafic complex N2-excess