1. Chemistry of Igneous Rocks Characterization of different types (having different chemistries): –Ultramafic  Mafic  Intermediate  Felsic Composition commonly presented in weight % of the oxides –40-78% SiO 2 –12-18% Al 2 O 3

2. Melts Liquid composed of predominantly silica and oxygen. Like water, other ions impart greater conductivity to the solution Si and O is polymerized in the liquid to differing degrees – how ‘rigid’ this network may be is uncertain… Viscosity of the liquid  increases with increased silica content, i.e. it has less resistance to flow with more SiO 2 … related to polymerization?? There is H 2 O is magma  2-6% typically – H 2 O decreases the overall melting T of a magma, what does that mean for mineral crystallization?

3. Thermodynamic definitions G i(solid) = G i(melt) Ultimately the relationships between these is related to the entropy of fusion (  S 0 fus ), which is the entropy change associated with the change in state from liquid to crystal These entropies are the basis for the order associated with Bowen’s reaction series  greater bonding changes in networks, greater entropy change  lower T equilibrium

5. Minerals which form are thus a function of melt composition and how fast it cools (re-equilibration?)  governed by the stability of those minerals and how quickly they may or may not react with the melt during crystallization Liquid hot MAGMA Ca 2+ Na + Mg 2+ Fe 2+ Si 4+ O 2- rock cooling Mg 2+ Fe 2+ Mg 2+

6. Processes of chemical differentiation Partial Melting: Melting of a different solid material into a hotter liquid Fractional Crystallization: Separation of initial precipitates which selectively differentiate certain elements… Equilibrium is KEY --? Hotter temperatures mean kinetics is fast…

7. Melting First bit to melt from a solid rock is generally more silica-rich At depth in the crust or mantle, melting/precipitation is a P-T process, governed by the Clausius-Clapeyron Equation – Slope is a function of entropy and volume changes! But with water… when minerals precipitate they typicaly do not pull in the water, melt left is ‘diluted’  develop a negative P-T slope

8. Melt-crystal equilibrium 1 Magma at composition X (30% Ca, 70% Na) cools  first crystal bytownite (73% Ca, 27% Na) This shifts the composition of the remaining melt such that it is more Na-rich (Y) What would be the next crystal to precipitate? Finally, the last bit would crystallize from Z X Y Z

9. Melt-crystal equilibrium 1b Precipitated crystals react with cooling liquid, eventually will re-equilibrate back, totally cooled magma xstals show same composition UNLESS it cools so quickly the xstal becomes zoned or the early precipitates are segregated and removed from contact with the bulk of the melt

10. Why aren’t all feldspars zoned? Kinetics, segregation IF there is sufficient time, the crystals will re-equilibrate with the magma they are in – and reflect the total Na-Ca content of the magma IF not, then different minerals of different composition will be present in zoned plagioclase or segregated from each other physically

11. What about minerals that do not coexist well – do not form a solid solution – are immiscible??

12. More than 1 crystal can precipitate from a melt – different crystals, different stabilities… –2+ minerals that do not share equilibrium in a melt are immiscible (opposite of a solid solution) –Liquidus  Line describing equilibrium between melt and one mineral at equilibrium –Solidus  Line describing equilibrium with melt and solid –Eutectic  point of composition where melt and solid can coexist at equilibrium Diopside is a pyroxene Anorthite is a feldspar Solidus Liquidus Eutectic

13. Melt at composition X cools to point Y where anorthite (NOT diopside at all) crystallizes, the melt becomes more diopside rich to point C, precipitating more anorthite with the melt becoming more diopside-rich This continues and the melt continues to cool and shift composition until it reaches the eutectic when diopside can start forming A B S1S1 Z C S2S2 At eutectic, diopside AND anorhtite crystals precipitate Lever Rule  diopside/anorthite (42%/58%) crystallize until last of melt precipitates and the rock composition is Z

14. Melting  when heated to eutectic, the rock would melt such that all the heat goes towards heat of fusion of diopside and anorthite, melts so that 42% diopside / 58% anorthite… When diopside gone, temperature can increase and rest of anorthite can melt (along liquidus)

15. Melt-crystal equilibrium 2 - miscibility 2 component mixing and separation  chicken soup analogy, cools and separates Fat and liquid can crystallize separately if cooled slowly Miscibility Gap – no single phase is stable SOUP of X composition cooled in fridge Y vs freezer Z Miscibility Gap Fat Water % fat in soup Temperature (ºC) -20 50 0 100 10 90 70 50 30 SOUP X fats ice Y Z

16. Melt-crystal equilibrium 2 - miscibility 2 component mixing and separation  chicken soup analogy, cools and separates Fat and liquid can crystallize separately if cooled slowly Miscibility Gap – no single mineral is stable in a composition range for x temperature Miscibility Gap microcline orthoclase sanidine anorthoclase monalbite high albite low albite intermediate albite Orthoclase KAlSi 3 O 8 Albite NaAlSi 3 O 8 % NaAlSi 3 O 8 Temperature (ºC) 300 900 700 500 1100 10 90 70 50 30

17. Combining phase and composition diagrams for mineral groups Mica ternary Biotite series Annite KFe 3 (AlSi 3 O 10 )(OH) 2 Phlogopite KMg 3 (AlSi 3 O 10 )(OH) 2 Muscovite KAl 2 (AlSi 3 O 10 )(OH) 2 No micas Miscibility Gap

18. SOLID SOLUTION Occurs when, in a crystalline solid, one element substitutes for another. For example, a garnet may have the composition: (Mg 1.7 Fe 0.9 Mn 0.2 Ca 0.2 )Al 2 Si 3 O 12. The garnet is a solid solution of the following end member components: Pyrope - Mg 3 Al 2 Si 3 O 12 ; Spessartine - Mn 3 Al 2 Si 3 O 12 ; Almandine - Fe 3 Al 2 Si 3 O 12 ; and Grossular - Ca 3 Al 2 Si 3 O 12.

19. GOLDSCHMIDT’S RULES 1. The ions of one element can extensively replace those of another in ionic crystals if their radii differ by less than approximately 15%. 2. Ions whose charges differ by one unit substitute readily for one another provided electrical neutrality of the crystal is maintained. If the charges differ by more than one unit, substitution is generally slight. 3. When two different ions can occupy a particular position in a crystal lattice, the ion with the higher ionic potential forms a stronger bond with the anions surrounding the site.

20. RINGWOOD’S MODIFICATION OF GOLDSCHMIDT’S RULES 4. Substitutions may be limited, even when the size and charge criteria are satisfied, when the competing ions have different electronegativities and form bonds of different ionic character. This rule was proposed in 1955 to explain discrepancies with respect to the first three Goldschmidt rules. For example, Na + and Cu + have the same radius and charge, but do not substitute for one another.

21. INCOMPATIBLE VS. COMPATIBLE TRACE ELEMENTS Incompatible elements: Elements that are too large and/or too highly charged to fit easily into common rock-forming minerals that crystallize from melts. These elements become concentrated in melts. Large-ion lithophile elements (LIL’s): Incompatible owing to large size, e.g., Rb +, Cs +, Sr 2+, Ba 2+, (K + ). High-field strength elements (HFSE’s): Incompatible owing to high charge, e.g., Zr 4+, Hf 4+, Ta 4+, Nb 5+, Th 4+, U 4+, Mo 6+, W 6+, etc. Compatible elements: Elements that fit easily into rock- forming minerals, and may in fact be preferred, e.g., Cr, V, Ni, Co, Ti, etc.

22. Changes in element concentration in the magma during crystal fractionation of the Skaergaard intrusion: Divalent cations

23. Changes in element concentration in the magma during crystal fractionation of the Skaergaard intrusion: Trivalent cations

24. THREE TYPES OF TRACE- ELEMENT SUBSTITUTION 1) CAMOUFLAGE 2) CAPTURE 3) ADMISSION

25. CAMOUFLAGE Occurs when the minor element has the same charge and similar ionic radius as the major element (same ionic potential; no preference. Zr 4+ (0.80 Å); Hf 4+ (0.79 Å) Hf usually does not form its own mineral; it is camouflaged in zircon (ZrSiO 4 )

26. CAPTURE Occurs when a minor element enters a crystal preferentially to the major element because it has a higher ionic potential than the major element. For example, K-feldspar captures Ba 2+ (1.44 Å; Z/r = 1.39) or Sr 2+ (1.21 Å; Z/r = 1.65) in place of K + (1.46 Å, Z/r = 0.68). Requires coupled substitution to balance charge: K + + Si 4+  Sr 2+ (Ba 2+ ) + Al 3+

27. ADMISSION Involves entry of a foreign ion with an ionic potential less than that of the major ion. Example Rb + (1.57 Å; Z/r = 0.637) for K + (1.46 Å, Z/r = 0.68) in K-feldspar. The major ion is preferred.

28. Partition Coefficients How can we quantify the distribution of trace elements into minerals/rocks? Henry’s Law describes equilibrium distribution of a component (we usedit for thinking about gases dissolved in water recently): –a i min = k i min X i min –a i melt = k i melt X i melt –All simplifies to: Often termed K D, values tabulated… http://www.earthref.org/databases/index.html?main.htm

29. Limitations of K D What factors affect how well any element gets into a particular rock???

30. Melting and Crystallization Considering how trace elements incorporate the melt or solid: Where K D (rock)=  K D (j minerals)X j For consideration of trace elements into a solid, use Rayleigh fractionation equation: Where F is the fraction of melt remaining

31. However, most K D values reported close to equilibrium T-P values commonly encountered (?) and are reasonable, at least in terms of relative values between different elements…

32. Homework Chapter 8 Problems 2, 6