Generation of magma

1. Melting results in P,T and X changes: • Temperature increase: • Mass movement of rock or magma • Descending lithosphere • Underplating of magma • Shearing: shear heating • Radioactive decay • Tectonic thickening Decompression: positive slope of solidus on P-T diagram results in decompression melting Generation of magma

2. Composition changes Changing water concentration Changes in modal composition

3. Mantle source rock oxide wt%, volatile free Earth BSE UM SiO2 33.42 45.96 44.9 TiO2 0.107 0.181 0.13 Al2O3 2.39 4.4 4.28 FeO 35.7 7.54 8.07 MgO 23.95 37.78 38.22 CaO 1.89 3.21 3.5 Na2O 0.196 0.332 0.29 K2O 0.019 0.032 0.007 Mantle inclusions: xenoliths Cr-diopside lherzolite Al-augite pyroxenites Metasomatism: Cryptic and modal Mineral phases: Olivine (Mg,Fe2+)2SiO4 Orthopyroxene (Mg,Fe2+)SiO3 Clinopyroxene Ca(Mg,Fe2+)Si2O6 Spinel (Mg,Fe2+)(Cr,Al,Fe3+)2O4 Garnet)Mg,Fe2+,Ca)3(Al,Cr)2Si3O12

4. Composition continued

5. Migration of liquids • Liquids move through aggregates by hydraulic fracturing: porous flow • Movement of melt along grain boundaries. • Movement controlled by minimizing surface energy • Intersecting crystal liquid interfaces form a dihedral angle, . • is related to the relative magnitude XX-L and XX--XX interfacial energy, CL and CC such that • =0o grains are complete wetted, • 0o<Q<600 no film exists but pipes exists along three grain edges, connectivity • Q>60o, melt exists as individual pockets, no connectvity

6. Metasomatism

7. Magma generation from peridotite -Partial melting of peridotite -Equilibrium or fractional

8. Cont’d

9. Melting the mantle

10. Trace elements and Partition coefficient F is fraction of melt, (1-F) fraction of solid, co is initial concentration. Resulting in and Note: perfectly incompatible element (D=0), maximum enrichment 1/F

11. Trace elements cont’d

12. Trace elements cont’d Instantaneous removal of the liquid. Develop symmetrical to fractional crystallization. Which can be rewritten using mass balance as: , or Log differential of the concentration: resulting in: With F being the melt fraction: Intergrate to give the solid: , and instantaneous liquid: And the aggregated liquid concentration:

13. Size and charge First order control on partitioning is size and not charge Goldschnidt rules for substitution: 1. If 2 ions have the same radius and the same charge they will enter a given lattice with equal facility 2. If two ions have similar radius and same charge the smaller ions will enter a site more readily 3. If two ions have similar radii, the ion with the higher charge will enter a given site more readily. Later (Ringwood) added electronegativity: 4. The element with the lower electronegativity will be preferentially incorporated (stronger bond)

14. Size and charge cont'd Trace element partitioning predictions based on size and charge: NiOl+Mg2SiO4 = MgOl + NiMgSiO4. Assume that the ratio of activity coefficients is 1, than So Ni partition coefficient can be predicted based on knowledge of the Mg partitioning and free energy of exchange reaction. According to strain theory the strain energy is: whereby r0 is radius of the unstrained site, K is bulk modulus whereby s is Poisson's ratio and Olivine/melt

15. Size an charge cont'd With a slightly different apporach equation can be rewritten to provide an intuitive understanding of partitioning. For partitioning into the M2 site for clinopyroxene: Whereby r0 is size of site, D0 is D at r0, EM2 is Young's modulus for M2 and NA is Avogadro's Number

16. Equilibrium condition for the exchange reaction: -lnKeq =-lnKeq,id-lnKeq,xs=-nlnn =-nlnn -nlnn = H°/RT –S°/R + PV°/RT + nlnn where –lnKeq,id=nlnn Define complex d which contains P, T and X dependence: lndi = A + B/T + CP/T + Dnlnn) Composition dependence of D di: example

17. Compositional dependence on D

18. Mineral melt partitioning • Selection of partition coefficients for mafic-ultramafic systems: • Errors in measurement and experimental limitations are such that measured partition coefficients will be too large more often than too low. Thus low values are generally preferred. • In situ microanalytical techniques (e.g., ion probe) of experiments on natural basaltic liquids were preferred over bulk analysis of natural samples. Undoped experiments were preferred over doped experiments. • Knowing relative relationship of the partition coefficient of one element to that of another is more important than knowing absolute values. Accuracy of relative values is maximized when partition coefficients for all elements are measured in a single experiment or series of experiments. Thus studies where partition coefficients for a large number of elements were determined were preferred over more limited set of elements. • Partition coefficients measured experimentally using natural basaltic liquids with temperatures in the range of 1200-1400°C were preferred. • Elements known to have partition coefficients that depend strongly on composition or temperature (e.g., Ni) have not been included.

19. Starting magma dry conditions Primary: in equilibrium with mantle material Primitive: least modified high Mg# mafic magma Parental: more evolved magmas can be related to it through crystallization Primary magma: Mg#=68-75, >8wt% MgO, 400ppm Ni, 1000ppm Cr, mantle xenoliths, xenocrysts Subarc mantle

20. Fluid in the subduction zone Exact mass balance of fluid depends on: Age of slab How much subducted Composition sediments Shear heating Slab dip Vigor of convecting wedge

21. Melting mantle wedge

22. Alkaline magmas • Silica undersaturated • Enrichment of incompatible elements cannot be explained by melting • Need source enrichment and low degree of melting: metasomatism • Silica-undersaturated magmas can be generated at high CO2 abundances and high pressure. • Metasomatism seen as addition of: phlogopite, rutile, Al-Fe-Ti-rich cpx

23. Melting of crust In the continent During subduction Slab melting: two possibilities: Sediment melting Sediment and oceanic crust melting How to recognize: Enrichment in fluid immobile elements for sediments Sediment and crust: formation of adakites Adakite: dacite with high Al2O3, Na, Sr, Eu content, LREE enrichment, low Mg, Ti, Yb. Melting is dehydration melting In continental crust: Continental arcs, magma ascent from mantle is halted (buoyancy), heats crust: crustal melting Continental rifts: thinning of crust by rising (hotter) mantle or underplating of basalts Crustal thickening: mountain building

24. Muscovite melts first: • Musc+plag+qtz=hydrous melt+Kfs+sill+gt • Biotite first (at higher T): • bio+qtz=hydrous melt+gt and bio+plag+qtz=hydrous melt+opx • After musc: hbl+qtz=hydrous melt+cpx+opx • Migmatite: Dehydration melting 1 GPa 0.7GPa

25. Melt composition • Melting through melt reaction: incongruent melting • Felsic partial melts Si, Na, K, H2O-rich, Ti, Mg, Fe Ca poor: Peraluminous, metaluminous or peralkaline. • Source rock with two feldspars and Qtzmelt granite (thermal minimum of granitic systems) • Basaltic sourcegranitic melts <5%; Trondhjemites ~10%; tonalites>10%

26. S- and I-type granites S-type: sedimentary source Melting of shales results in peraluminous magmas: Al-rich mineral (andalusite, sillimanite, garnet, tourmaline, cordierite, muscovite) as well as two fsp (and qtz). I-type: igneous source: amphibole, biotite, fsp, qtz A-type granites: anorogenic (not clear S or I-type.