Mechanisms of metamorphism and metasomatism on the local mineral scale : The role of dissolution-reprecipitation during mineral re-equilibration

Transcription

1 Chapter 5 Mechanisms of metamorphism and metasomatism on the local mineral scale : The role of dissolution-reprecipitation during mineral re-equilibration Andrew Putnis & Håkon Austrheim

2 Equilibration at a crystal - fluid interface µ fluid µ cryst crystal fluid The fluid may be an aqueous solution (or a melt), out of equilibrium with the crystal.

3 Equilibration at a crystal - fluid interface diffusion profile µ fluid µ cryst crystal fluid Equilibration by volume diffusion from the fluid into the crystal?

4 Equilibration at a crystal - fluid interface crystal recrystallisation front fluid Equilibration by interface-coupled dissolution-precipitation?

5 a b c d Reaction interfaces on an outcrop scale

6 The core of the garnet (granulitic) has lower Fe/Mg ratio than the lighter (eclogitic) rim granulite granulite eclogite

7 The core of the garnet has lower Fe/Mg ratio than the lighter rim recrystallisation front crystal fluid Pollok et al., (2008) Equilibration by interface-coupled dissolution-precipitation.

8 In all the examples we have studied, and examples taken from the literature, equilibration involves recrystallisation to form a more stable phase. See: Putnis (2009) Mineral Replacement Reactions In: Reviews in Mineralogy and Geochemistry Vol. 70 (Thermodynamics and Kinetics of Water-rock interaction) The free energy drive for mineral replacement reactions can be very small: e.g. Uptake of 133 Ba by BaSO 4 from a saturated solution at 23 0 C involves a dissolution-precipitation mechanism. Curti et al., (2010) Geochim. Cosmochim. Acta 74,

9 BaSO 4 recrystallisation front BaSO 4 enriched with 133 Ba Fluid saturated with respect to BaSO 4 but doped with 133 Ba Uptake of 133 Ba by BaSO 4 from a saturated solution at 23 0 C involves a dissolutionprecipitation mechanism. Curti et al., (2010) Geochim. Cosmochim. Acta 74, Suggests that small free energy differences may be sufficient to drive interface-coupled dissolution-precipitation reactions.

10 What is interface-coupled dissolution-precipitation? crystal a b c d e (a) Dissolution of even a few monolayers of the parent crystal may result in an interfacial fluid layer which is supersaturated with respect to another phase (b) This product phase may nucleate on the surface of the parent (c) The porosity generated depends on both the solid molar volume change and the relative solubilities of parent and product in the fluid (d-e) A reaction/replacement interface moves through the parent crystal

11 Pseudomorphic Mineral Replacement Examples

12 Example 1: Experiment Single crystal aragonite replaced by calcite

13 BSE of cross sections of an aragonite crystal partially replaced by calcite in a hydrothermal experiment

14 The dissolution of aragonite and precipitation of calcite results in porosity in the calcite allowing fluid to access the dissolution-precipitation front Even though the molar volume of calcite is greater than that of aragonite

15 Raman map of the 18 O distribution when the fluid is enriched in 18 O The calcite is enriched in 18 O showing that oxygen is also replaced in the transformation i.e. coupled dissolutionprecipitation

16 The aragonite calcite transition can also take place by a solid state mechanism*. * solid state mechanism - one that involves the reorganisation of the structure without the need for a fluid phase (i.e. no dissolution and reprecipitation) The activation energy for the wet process is about half that of the dry process, so that at a temperature of 200 o C the transformation is faster by a factor of about

17 Example 2: Experiment Calcium carbonate (Carrara marble) replaced by apatite

18 Cross section of cube of Carrara marble reacted with phosphate solution at C for 1 week Calcite is partially replaced by apatite Ca 5 (PO 4 ) 3 (OH) BSE image: L.Jonas

19 calcite is being replaced by apatite, Ca 5 (PO 4 ) 3 (OH) Experiments with Carrara marble with a hydrothermal fluid containing phosphate ions. The dissolution of the calcite results in a Ca-P bearing fluid, from which apatite nucleates. The dissolution and precipitation are coupled so that the parent phase is replaced by the product.

20 porous apatite replacing calcite

21 How relevant is this to metamorphism?

22 The conversion of gabbro to eclogite Image: Timm John Gabbro Eclogite Igneous texture of the gabbro preserved: garnet replaces plagioclase, omphacite replaces augite from Putnis & John, Elements 6, 2010

23 from Putnis & John, Elements 6, 2010

24 Albitisation one of the most common metasomatic reactions

25 Fluid - induced chemical reequilibration of feldspars Albitisation of granitoid in the Bamble sector, Norway Plagioclase (Na,Ca) feldspar replaced by albite (NaAlSi 3 O 8 )

26 Fluid - induced chemical reequilibration of feldspars Albitisation of granitoid in the Bamble sector, Norway Plagioclase (Na,Ca) feldspar replaced by albite (NaAlSi 3 O 8 )

27 Feldspar replacement microstructure An 22 Ab 77 Note the porosity developed in the product phase An 2 Ab 95 The crystal structure is preserved across the sharp interface Engvik et al. (2008) Can. Mineral

28 What happens to the charged fluid which has albitised the country rock? - Ore deposits after Korneliussen et al., 2000

29 In Australia, < 100 kms south west of Broken Hill Albitisation in the Curnamona Province, South Australia

30 Albitisation in the Curnamona Province, South Australia

31 How much saline fluid is needed to albitise such large volumes? How does the fluid move through the rock? Albitisation in the Curnamona Province, South Australia

32 Ultimately albitisation can replace the whole rock Albite quarry in Turkey David Ettner

33 Albite (dark) growing within phlogopite (pale)during albitisation Ab Image: Håkon Austrheim

34 Albite (dark) growing within phlogopite (pale) during albitisation How is this extra volume created? Ab Image: Håkon Austrheim

35 Metasomatism and Metamorphism Both involve dissolution-transport-precipitation Is it just a question of the spatial scale? Can we define any equilibrium reactions?

36 Metasomatic rocks often are not recognised as such... but how do we recognise a metamorphic event? Vernon RH, White RW, Clarke GL (2008) False metamorphic events inferred from misinterpretation of microstructural evidence and P-T data. Journal of Metamorphic Geology, 26, In Vernon s context a metamorphic event constitutes a series of reactions through P,T space, and he addresses the problem of recognising textures which indicate crossing a particular P,T reaction line., i.e. recognising phases which define an equilibrium assemblage at some point in time.

37 Vernon identifies two reliable microstructural criteria for recognising a metamorphic event: partial replacement textures reaction coronas at grain boundaries between two reacting phases

38 Metasomatic rocks often are not recognised as such... but how do we recognise a metamorphic event? Two typical partial replacement textures: An 22 An 2 Apatite-OH replacing Apatite-Cl Albite (An 2 ) replacing plagioclase (An 22 )

39 Metasomatic rocks often are not recognised as such... but how do we recognise a metamorphic event? Two typical partial replacement textures: In these examples the equilibrium (if any) is An 22 between the product phase and the interfacial fluid and An not between the parent and the product 2 solid phases Apatite-OH replacing Apatite-Cl Albite (An 2 ) replacing plagioclase (An 22 )

40 A sharp interface between two apparently coexisting phases does not mean they are in equilibrium. Partial replacement textures cannot be used as a criterion for a metamorphic event in the sense of crossing a reaction line in P,T space.

41 Vernon identifies two reliable microstructural criteria for recognising a metamorphic event: partial replacement textures reaction coronas at grain boundaries between two reacting phases

42 Corona textures A concentric arrangement of one or several rims of minerals around a core mineral Dimensions: µm to dm across Easy to interpret and frequently used to unravel the history of rocks

43 Granulite from the Bergen Arcs, Norway

44 Interpretation A reaction texture formed due to changing P and T. Assumptions: Core is older than the rim The system is closed Solid-solid reaction Reaction deduced: A + B = C + D + E Converting a gabbro to a granulite facies rock B C D E A

45 Application in petrology P-T diagram Provided we know the stability fields of the mineral assemblages, we can infer the P - T path. Pressure (P) Temperature (T)

46 Mechanism of Corona formation 1. Nucleation at the contact between the reacting minerals 2. Diffusion of material through the product mineral 3. The growth must slow down with time 4. New rims may nucleate as P and T continue to change 5. Regarded as solid solid reaction Ol Opx Cpx Grt Plag

47 Corona formation and fracturing Double opx rims around olivine Discontinuous rims. How can we explain this? oliv plag

48 Are coronas a reliable indicator of a metamorphic event? A typical reaction corona: spr + plag Straume and Austrheim, 1999 Reaction between garnet + kyanite to plagioclase + sapphirine along grain boundaries (but not all grain boundaries)

49 Are coronas a reliable indicator of a metamorphic event? spr + plag (i) The grt + ky reaction only takes place on some interfaces (ii) The plag contains >10% albite component Straume and Austrheim, 1999 Conclusion: Reaction has only taken place where an Nabearing fluid has infiltrated

50 Partial reactions on some grain boundaries and not others is typical of such corona reaction textures and is more consistent with a metasomatic reaction limited by fluid availability than with a solid state volume diffusion controlled reaction. Both of the Vernon et al. reliable criteria for a metamorphic event are indicators of metasomatism. The equilibrium (if any) is not likely to be that between the parent and product phases but between the fluid and the product.

51 The Journal of Geology, 26, (1918) Metamorphic reactions should be balanced on volume

52 The conversion of gabbro to eclogite Image: Timm John Gabbro Eclogite Igneous texture of the gabbro preserved: garnet replaces plagioclase, omphacite replaces augite

53 The problem of identifying metasomatism when there is no chemical or physical reference frame. Small zircon crystals are often associated with ilmenite grain boundaries in a wide range of rock types. During metasomatism where the ilmenite and hornblende are replaced by phlogopite, the zircon may remain unreacted and outlines the position of the former ilmenite grain boundary. Note: no deformation has been involved in this replacement. Austrheim et al., 2008

54 Serpentinisation of olivine : Volume increase?

55 Molar volumes: Olivine ~ 46.5 ccs Antigorite ~ 110 ccs 2Mg 2 SiO 4 + 2H + + 2H 2 O Mg 3 [Si 2 O 5 ](OH) 4 + Mg 2+ 3Mg 2 SiO 4 + H 4 SiO 4 (aq) + 2H 2 O 2Mg 3 [Si 2 O 5 ](OH) 4 5Mg 2 SiO 4 + 2H 2 O +8H + 2Mg 3 [Si 2 O 5 ](OH) 4 + 4Mg 2+ + H 4 SiO 4(aq)

56 Pressure (kbars) Jadeite + Quartz Albite Temperature 0 C NaAlSi 3 O 8 NaAlSi 2 O 6 + SiO 2 Does this reaction represent the actual mechanism?

57 Albite Albite being replaced isovolumetrically by jadeite a Jadeite 0.2 mm NaAlSi 3 O 8 NaAlSi 2 O 6 + SiO 2 (aq)??? Molar volumes: Albite~ ccs Jadeite~ 60.4 ccs Albite To balance the reaction on volume: 0.6 Albite + Fluid 1.0 Albite Jadeite To mass balance the reaction the Fluid phase must add 0.4 moles of Na and Al, and 0.2 moles of Si b Image: R. Rysza

58 Jadeite being replaced isovolumetrically by albite NaAlSi 2 O 6 + SiO 2 (aq) NaAlSi 3 O 8??? a Molar volumes: Albite~ ccs Jadeite~ 60.4 ccs To balance the reaction on volume: 1.6 Jadeite 1.0 Albite + Fluid b To mass balance the reaction the Fluid must remove 0.6 moles of Na and Al, and 0.2 moles of Si Image: M.Shigeno

59 Some problems this raises How do we determine the thermodynamics of these open system reactions? The textures imply a very significant element exchange in the fluid phase? How does the fluid pass through the minerals?

60 Partial transformation of single crystal coesite (cubic, isotropic) to polycrystalline low quartz (trigonal, anisotropic) Image: J.R.Smyth 0.2 mm 0.5 mm Image: J.R.Smyth

61 Element maps of metasomatic tournaline. The Ca and Ti distribution preserve chemical features of the precursor phases. Bast R. MSc 2010

62 Pervasive metamorphism requires pervasive fluid infiltration and diffusion of elements through the fluid. How do fluids move through low permeability rocks? 1.Reaction generated porosity. Plenty of experimental evidence for interface-coupled dissolution-precipitation, which involves fluids accessing internal reaction interfaces within crystals. Fluid infiltration through nanopores which are generated within the product phase due to differences in the relative molar volumes and solubilities of parent and product phases.

63 Pervasive metamorphism requires pervasive fluid infiltration and diffusion of elements through the fluid. How do fluids move through low permeability rocks? 2. Reaction generated fractures.. or dissolution pathways? The product phase nucleates within these fractures.

64 Implications of interface-coupled dissolution-precipitation as a mechanism of mineral replacement Fluid control of metamorphic reactions Austrheim et al., (1987), Pollok et al., (2008)

65 Implications of interface-coupled dissolution-precipitation as a mechanism of mineral replacement Is the fluid more than just a catalyst in a metamorphic reaction? A + B + F 1 C + D + F 2? or Pressure C + D A + B Temperature

66 Acknowledgements This work is supported by: The EU ITN Network Delta-Min (Mechanisms of Mineral Replacement Reactions) The Humboldt Foundation