Magma Types Based on Chemistry

1. Magma Types Based on Chemistry 50% SiO2 60% SiO2 70% SiO2 gabbro/basalt diorite/andesite granite/rhyolite

2. Processes That Change Magma Compositions • Primary Process  • Partial Melting of Different Materials (e.g. mantle, oceanic crust, continental crust, etc.) • Secondary Processes - “Magmatic Differentiation” • Fractional Crystallization • Assimilation/Magma Mixing • Combined Process (AFC)

3. Partial Melting of Different Starting Materials Peridotite (mantle rock)  Basaltic Magma Basaltic (oceanic crust)  Andesitic Magma Andesitic (continental crust)  Rhyolitic Magma

4. Mantle Melting and the Origin of Basaltic Magma

5. Primary Basaltic Magmas • Mantle melts with no subsequent modification by secondary processes (fractional crystallization, assimilation, mixing). • Criteria: • MgO = 11 – 17 wt% • Cr > 1000 ppm • Ni > 400-500 ppm • Rarely observed, “Holy Grail” of basalts, very important for petrologists and geochemists, fingerprint of mantle source!

6. Processes That Change Magma Compositions • Primary Process  • Partial Melting of Different Materials (e.g. mantle, oceanic crust, continental crust, etc.) • Secondary Processes - “Magmatic Differentiation” • Fractional Crystallization • Assimilation/Magma Mixing • Combined Process (AFC = Assimilation + Fractional Crystallization)

7. Fractional Crystallization magma composition progressively changes as crystals are physically “removed” from the magma.

8. Bowen’s Reaction Series • Theoretical order in which common igneous crystallize, and generally speaking correct! • Used in early 1900’s (by N.L. Bowen) to explain the origin of granite from primary basaltic magmas. True only in rare cases.

9. Fractional Crystallization Remaining Magma is Andesitic Basaltic Magma

10. Palisades Sill – (western shore of Hudson River, NJ)

11. Palisades Sill – (western shore of Hudson River, NJ) Sandstone Basalt Diorite Sodium-rich plagioclase feldspar; Fe-rich pyrox, hbl Basaltic intrusion 245–275 m (800–900 ft) Gabbro (or “diabase”) Calcium-rich plagioclase feldspar and pyroxene; Olivine gabbro Olivine, plag, pyrox Basalt Sandstone Basalt cooled quickly at the edges of the intrusion.

12. Palisades intrusion ~1200°C Olivine crystals Magma with composition A Olivine crystallizes first.

13. Palisades intrusion ~1200°C ~1100°C Plagioclase feldspar Olivine crystals Pyroxene Olivine Magma with composition A Magma with composition B Olivine crystallizes first. Pyroxene and plagioclase feldspar crystallize.

14. Palisades intrusion ~1200°C ~1100°C ~1050°C Plagioclase feldspar Plagioclase feldspar Olivine crystals Pyroxene Pyroxene Olivine Olivine Magma with composition A Magma with composition C Magma with composition B Olivine crystallizes first. Pyroxene and plagioclase feldspar crystallize. Pyroxene and plagioclase gradually change in composition.

15. Palisades intrusion ~1200°C ~1000°C ~1100°C ~1050°C Plagioclase feldspar Plagioclase feldspar Plagioclase feldspar Olivine crystals Pyroxene Pyroxene Pyx+Plag Olivine Olivine Olivine Magma with composition A Magma with composition C Magma with composition B Magma with composition D Olivine crystallizes first. Pyroxene and plagioclase feldspar crystallize. Pyroxene and plagioclase gradually change in composition. Plagioclase feldspar continues to crystallize.

16. Processes That Change Magma Compositions • Primary Process  • Partial Melting of Different Materials (e.g. mantle, oceanic crust, continental crust, etc.) • Secondary Processes - “Magmatic Differentiation” • Fractional Crystallization • Assimilation/Magma Mixing • Combined Process (AFC = Assimilation + Fractional Crystallization)

17. Assimilation Magma Mixing Basaltic magma Rhyolitic magma Continental Crust Basaltic + Rhyolitic Blocks of continental crust fall into basaltic magma and dissolve. basaltic magma + assimilated blocks = andesitic magma Andesitic magma Heat transfer from hot, basaltic magma melts wall rock HOT (1200°C) basaltic magma

18. Assimilation + Fractional Crystallization (AFC) Continental Crust Blocks of continental crust fall into basaltic magma and dissolve. basaltic magma + assimilated blocks = andesitic magma magma composition progressively changes as crystal are physically “removed” from the magma Heat transfer from hot, basaltic magma melts wall rock HOT (1200°C) basaltic magma

19. Harker Diagrams Trends usually indicate that rocks are “related” by some petrogenetic process (ie. crystal fractionation, mixing, or AFC) basalt andesite rhyolite basalt basalt andesite andesite rhyolite rhyolite basalt andesite rhyolite = Parental magma = differentiation trend Data for Crater Lake Volcanics, Cascade Range, Oregon

20. AFM Diagram basalt andesite rhyolite basalt basalt andesite andesite rhyolite rhyolite basalt andesite rhyolite basalt andesite rhyolite = Parental magma = differentiation trend Data for Crater Lake Volcanics, Cascade Range, Oregon

21. Processes That Change Magma Compositions • Primary Process  • Partial Melting of Different Materials (e.g. mantle, oceanic crust, continental crust, etc.) • Secondary Processes - “Magmatic Differentiation” • Fractional Crystallization • Assimilation/Magma Mixing • Combined Process (AFC = Assimilation + Fractional Crystallization)

22. Origin of Basaltic Magmas Decompression melting of mantle (peridotite) to produce primary basaltic magmas Fractional crystallization in shallow crustal magma chambers

23. Origin of Andesitic Magmas Modification of a Basaltic “Parent” 1. Crystallization in Magma Chambers 2. Assimilation of Continental Crust 3. Magma Mixing: (basalt + rhyolite = andesite) Origin of Granitic/Rhyolitic Magmas Partial Melting of Continental Crust (± AFC processes)