Evolution of magmas 2 Evolution of liquids Descent

2 -C Eutectic Systems Example: Diopside - Anorthite No solid solution 1600 1553 idus

Gravity settling – Cool point a olivine layer at base of pluton if first

1713 Liquid Thermal Divide Tr + L Ne + L 1070 Ab + L

Nepheline-Fayalite-Si. O 2 Not a very good system, as it is a poor equivalent

Thermal divide separates the silica-saturated (subalkaline) from the silica-undersaturated (alkaline) fields at low pressure

22 10 Bivariate (x-y) diagrams Al 2 O 3 Mg. O 17 5 0

B Al 2 O 3 BA A D RD R 16 14 • Extrapolate

Use the green line and extrapolate the other curves back BA B green line

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Evolution of magmas 2 - Evolution of liquids Descent lines

2 -C Eutectic Systems Example: Diopside - Anorthite No solid solution 1600 1553 idus Liquid Liqu 1500 T o. C 1400 Anorthite + Liquid 1392 1300 Diopside + Liquid 1200 1274 Diopside + Anorthite Di 20 40 60 Wt. % Anorthite 80 An Isobaric T-X phase diagram at atmospheric pressure (After Bowen (1915), Amer. J. Sci. 40, 161 -185.

Gravity settling – Cool point a olivine layer at base of pluton if first olivine sinks – Next get ol+cpx layer – finally get ol+cpx+plag Cumulate texture: Mutually touching phenocrysts with interstitial crystallized residual melt

1713 Liquid Thermal Divide Tr + L Ne + L 1070 Ab + L Ne + Ab Ne 1060 Ab + Tr Ab Q

Nepheline Leucite

Nepheline-Fayalite-Si. O 2 Not a very good system, as it is a poor equivalent of magmatic rocks –but allows to see nice fetaures.

Thermal divide separates the silica-saturated (subalkaline) from the silica-undersaturated (alkaline) fields at low pressure Cannot cross this divide by FX, so can’t derive one series from the other (at least via low-P FX) Ol Opx 1713 e lin ka Ab + L Ne + Ab Ne Al Ne + L Tr + L 1060 Ab + Tr Ab Q Ne ld fie 1070 fie Thermal Divide e lin lka ba Su ld Liquid Ab Dividing line Q

22 10 Bivariate (x-y) diagrams Al 2 O 3 Mg. O 17 5 0 12 15 Fe. O* 10 10 5 Harker diagram for Crater Lake Ca. O 5 0 0 4 6 3 Na 2 O 4 2 2 0 45 1 0 50 55 60 Si. O 2 65 70 75 45 50 55 60 Si. O 2 65 70 75 K 2 O

B Al 2 O 3 BA A D RD R 16 14 • Extrapolate BA B and further to low Si. O 2 • K 2 O is first element to 0 8 (at Si. O 2 = 46. 5 red line) Mg. O 4 10 6 Fe 2 O 3 2 0 8 Thus the green line will the concentration of all other oxides 4 4 Na 2 O Ca. O 0 3 2 4 2 0 45 55 65 Wt. % Si. O 2 75 K 2 O

Use the green line and extrapolate the other curves back BA B green line and read off X B Al 2 O 3 BA A D RD R 16 14 10 6 Results: Remove plagioclase, olivine, pyroxene and Fe-Ti oxide 8 Mg. O Fe 2 O 3 2 4 0 8 Oxide Wt% 4 Cation Norm 4 Si. O 2 Ti. O 2 Al 2 O 3 Fe 2 O 3* Mg. O Ca. O Na 2 O K 2 O Total 46. 5 1. 4 14. 2 11. 5 10. 8 11. 5 2. 1 0 98. 1 ab an di hy ol mt il 18. 3 30. 1 23. 2 4. 7 19. 3 1. 7 2. 7 100 Na 2 O Ca. O 0 3 2 4 2 0 45 55 Then repeat for each increment BA A etc. 65 Wt. % Si. O 2 75 K 2 O