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Solid solutions Example: Olivine: (Mg,Fe) 2 SiO 4 two endmembers of similar crystal form and structure: Forsterite: Mg 2 SiO 4 and Fayalite: Fe 2 SiO 4 isomorphism complete mixing range exists between forsterite and fayalite ( isomorphous mixture )
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Solid solutions • Example: Olivine: (Mg,Fe)2SiO4 • two endmembers of similar crystal form and structure: • Forsterite: Mg2SiO4 and Fayalite: Fe2SiO4 • isomorphism complete mixing range exists between forsterite and fayalite (isomorphous mixture) • Mg/Fe ratio is variable, but (Mg+Fe)/Si ratio is constant! Olivine is a solid solution between these two endmembers! (like a solution, olivine is “homogeneous” over the entire range of Mg/Fe ratios) Mg2+ and Fe2+substitute for each other in the olivine crystal structure
Solid solutions pure albite melts at 1118 oC, pure anorthite at 1557 oC, mixtures are intermediate Melt with 60% anorthite (and 40% albite) cools to 1470 oC (X) before the first crystals with composition Y appear. These crystals are higher in anorthite than the melt and hence deplete the melt in anorthite (the melt follows X -> K). The older crystals continuously re-equilibrate with the melt until crystallization stops at K. At K, all melt is crystallized and these crystals have composition L (which is the starting composition of the melt).
D C B A Solid solutions pure albite melts at 1118 oC, pure anorthite at 1557 oC, mixtures are intermediate Melting plagioclase with 40% anorthite (and 60% albite) starts at ca 1245 oC (A) and produces melt of composition B. This melt is higher in albite than the solid and hence depletes the solid in albite as melting proceeds (the solid and melt follow A -> C and B -> D, resp.). At D all solid is melted and turned into a liquid with 40% anorthite and 60% albite (which is the starting composition of the plagioclase).
Eutectic melts D -> E and A -> E are the melting points of the possible diopside + anorthite mixtures For example: Melt with 20% anorthite and 80% diopside cools (M -> N) the first diopside crystallizes at 1350 oC composition of melt changes (N -> E) at E the melt has 58% diopside and 42% anorthite, anorthite begins to crystallize temperature remains constant until crystallization is complete E = eutectic (temp.) = lowest temperature at which melt exists in the system E = eutectic composition of a diopside + anorthite melt
Partial melting: a solid (in the mantle a peridotite or in the continental crust a granite, for example) moves above the solidus and starts to (partially) melt. batch melting(equilibrium fusion, equilibrium partial melting): melt and residual solid continually react with each other and re-equilibrate at the site of melting until the melt escapes as a single batch of magma (probably applicable to production of felsic melts in the upper and lower continental crust). fractional melting(Rayleigh melting): small amount of melt forms and is instantly removed from the source (probably applicable to production of basaltic melts in the mantle).
Geochemistry-Quiz 2, 11/04/04: 1. What is the difference between LILEs and HFSEs? (give an example of each) 2. How do KD values depend on temperature, pressure, and SiO2 content? 3. What is the difference between batch melting and Rayleigh melting? 4. Why is Dr important for magma genesis?
Homework Assignment 2 Partial Batch Melting: CL/C0 = 1 / [DRS + F(1-DRS)] CL = C0 / [DRS + F(1-DRS)] CL = concentration in liquid (partial melt) C0 = concentration in unmelted solid (mantle peridotite) F = fraction of melt produced during partial melting DRS = bulk distribution coefficient for the residual solid (peridotite after melt extraction) Calculate the Sr and Yb concentrations in a melt after 3% and 30% partial melting, respectively, of a mantle with 50 ppm Sr and 3 ppm Yb leaving a residuum with a) 40% ol + 20% opx + 20% cpx + 20% plag b) 40% ol + 20% opx + 20% cpx + 20% gt Display the results in a Sr vs. Sr/Yb plot and discuss them !
