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Eutectic and Peritectic Systems

Eutectic and Peritectic Systems. Winter, Chapter 6. Eutectic Systems. Example: Diopside - Anorthite No solid solution. Isobaric T-X phase diagram at atmospheric pressure. After Bowen (1915) , Amer. J. Sci. 40, 161-185 . Cooling from composition a:. bulk composition = An 70.

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Eutectic and Peritectic Systems

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  1. Eutectic and Peritectic Systems Winter, Chapter 6

  2. Eutectic Systems Example: Diopside - Anorthite No solid solution Isobaric T-X phase diagram at atmospheric pressure. After Bowen (1915), Amer. J. Sci. 40, 161-185.

  3. Cooling from composition a: bulk composition = An70

  4. First crystal forms at 1455oC (point b) with a compositon of pure An

  5. Cooling continues as Xliq varies along the liquidus • Continuous reaction: liqA® anorthite + liqB

  6. At 1274oC f = 3 (three phases co-exist) Therefore, F = 2 - 3 + 1 = 0 This is an invariant point • (P) T and the composition of all phases is fixed • Must remain at 1274oC as a discontinuous reaction proceeds until a phase is lost

  7. A discontinuous reaction occurs at d Temperature remains constant at 1274 • Use geometry to determine liquid crystal ratios

  8. Left of the eutectic there is a similar relationship

  9. The melt crystallizes over a T range up to ~280oC • A sequence of minerals forms over this interval • - And the number of minerals increases as T drops • The minerals that crystallize depend upon T • - The sequence changes with the bulk composition

  10. Augite Forms Before Plagioclase Gabbro of the Stillwater Complex, Montana This forms on the left side of the eutectic

  11. Plagioclase Forms Before Augite Ophitic texture Diabase dike This forms on the right side of the eutectic

  12. The last melt to crystallize in any binary eutectic mixture is the eutectic composition • Equilibrium melting is the opposite of equilibrium crystallization • Thus the first melt of any mixture of Di and An must be the eutectic composition as well

  13. Fractional Crystallization Isobaric T-X phase diagram at atmospheric pressure. After Bowen (1915), Amer. J. Sci. 40, 161-185.

  14. Partial Melting:

  15. Binary Peritectic Systems Three phases involved: enstatite = forsterite + SiO2 Figure 6-12. Isobaric T-X phase diagram of the system Fo-Silica at 0.1 MPa. After Bowen and Anderson (1914) and Grieg (1927). Amer. J. Sci.

  16. Binary Peritectic Systems Figure 6-12. Isobaric T-X phase diagram of the system Fo-Silica at 0.1 MPa. After Bowen and Anderson (1914) and Grieg (1927). Amer. J. Sci.

  17. Figure 6-12. Isobaric T-X phase diagram of the system Fo-Silica at 0.1 MPa. After Bowen and Anderson (1914) and Grieg (1927). Amer. J. Sci.

  18. i m k 1557 d c Fo En • i = “peritectic” point • At 1557oC there is colinear equilibrium of Fo-En-liq • geometry indicates a reaction: Fo + liq = En • consumes olivine (and liquid) ® resorbed textures When is the reaction finished? Bulk X

  19. x i k y m 1543 1557 d Cr c bulk X Fo En

  20. i 1543 1557 d Cr c bulk X Fo En • Incongruent Melting of Enstatite • Melt of En does not produce a melt of same composition • Rather En goes to Fo + Liq i at the peritectic • Partial Melting of Fo + En (harzburgite) mantle • En + Fo also ® firsl liq = i • Remove i and cool • Result = ?

  21. Consequence For Basalt Magma The Fo-En-Q system causes compositions to migrate from alkali basalt toward tholeiite as the degree of crystallication or melting progresses

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