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Understanding Magma Behavior in Crustal Environments

Explore the rheology, volatile saturation, and deep processes of basaltic magma to understand caldera development and magma generation processes. Learn about links between mafic and silicic magmatism, from shallow to deep crustal levels. Acknowledgements to Guillaume Girard and Ben Kennedy.

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Understanding Magma Behavior in Crustal Environments

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  1. Behaviour of magmas in crustal environments

  2. This presentation attempts to link magma generation processes in the mantle and at various crustal levels to (a) shallow silicic magma systems and (b) caldera development on the surface.

  3. Acknowledgements • Guillaume Girard • Ben Kennedy

  4. 1. Rheology of basalt

  5. Effect of volatiles on magma rheology Haplogranite Note large changes at low H2O and small changes at high H2O Dingwell DB et al (1996) Contrib Mineral Petrol 124:19–28

  6. Controls on basalt rheology • DENSITY AND VISCOSITY • Dry vs. wet basalt • 100% liquid vs. 50% crystallized • Role of bubbles – shearing vs. non-shearing flow • Role of pressure: 75 kg m-3 / GPa (Annen et al. 2006) • Buoyancy of basalt in rift vs. arc environments (wet vs. dry?)

  7. 2. Volatile saturation of basalt • When and where does a hydrous arc basalt become volatile-saturated? • in the mantle wedge (“primary basaltic magma”) ? • at the base of the crust (30-50 km) ? • at deep-mid crustal levels (10-30 km) ? • at shallow crustal levels (< 10 km) ? • What are the roles of H2O and CO2 on volatile saturation? • Think about arc basalts (“wet”) as compared to rift basalts (“dry”)

  8. Water and carbon dioxide solubilities in magmasThe role of pressure is dominant

  9. Decompression degassing of a H20-CO2-bearing magma: note the strong depletion of CO2 but only small reduction of H2O during depressurization Saturation curves of H2O and CO2 solubility in rhyolite From: http://volcanoes.usgs.gov/staff/jlowenstern/Melt%20Inc%20Page/fluidsat.html

  10. CO2 degassing in a basaltic melt at very low H2O

  11. Decompression degassing of CO2-free magma: H2O strongly depleted during depressurization

  12. Isobaric degassing during crystallization of an H2O-CO2 bearing magma: note the CO2 depletion and H2O enrichment (with attendant rheological effects)

  13. Isobaric degassing during crystallization of a CO2-absent magma: here the water content of the magma is buffered at a constant value

  14. Role of CO2 • Limits buildup of H2O…implications for rheology • Allows early volatile saturation of the magma…again, rheological implications • Now think again of buoyancy of basalt in rift vs. arc environments (wet vs. dry?) based on these solubility relationships above

  15. 3. Behaviour of basaltic magma at deep, intermediate, and shallow levels • Deep crystallization of basalt in a hot regime (Annen et al), vs….. • …shallow “quenching” of basalt in a cold regime (Wiebe) • So the extent of basalt crystallization depends a lot on depth • Important question: how easy is it to get basalt to shallow levels in an extensional vs. subduction environment ? • What is the role of structural permeability of the crust here, e.g., structures such as faults, tears, crustal weaknesses, etc. ?

  16. 4. Deep processes Annen C et al. (2006) J Petrol 47:505-539 • Magma differentiation processes: • Partial melting of protolith (fertile vs. refractory) by basalt intrusions • H2O-poor, high viscosity • Presence/absence of hydrous minerals • Dehydration melting • Peraluminous liquids produced • Remelting of earlier intrusions by later basalt injections • Fractional crystallization of hydrous basalt (potentially lots of H2O, low viscosity) • Think about: • Combinations of the above • Differences in physical properties and rheology of the liquids produced • Chemical and isotopic signatures

  17. Some figures from Annen et al. 2006 Crystallization of hydrous basalt Ascent of derivative andesite magma

  18. 5. From deep to shallow Calc-alkaline, oxidized, cool, and evolved Sisson et al. 2005 • Annen et al. 2006 suggest that: • Arc magmas are wet and cool, promoting lots of residual melt from crystallization and little partial melting • Note links with Sisson et al. 2005 • Rift-related basalts are hot and dry, promoting melting of fertile crust with little residual melt generated • Note links with Wiebe for CMG • Wiebe: silicic magma chambers as traps for contemporaneous basaltic magmas (think about Δρ) Wiebe 1994 granite basalt

  19. (a) Analogue experiments(b) Cadillac Mountain Granite, Maine(c) Ossipee Ring Complex, New Hampshire 6. Links between mafic and silicic magmatism

  20. (a) Analogue experimentsFirst, let’s look at mafic magma being injected into a shallow silicic magma chamberThen let’s look at silicic magma replenishing silicic magma

  21. Dense intrusion Snyder D, Tait S (1995) Contrib Mineral Petrol 122:230-240

  22. Viscous fingering Snyder and Tait (1995)

  23. Buoyant intrusion Girard G, Stix J (2009) J Geophys Res 114:B08203

  24. Buoyant intrusion through mush Girard and Stix (2009)

  25. (b) Cadillac Mountain Granite, Maine Wiebe RA (1994) J Geol 102:423-437

  26. Cross-section through the Cadillac Mountain Granite Wiebe (1994) Snyder and Tait (1995)

  27. Base of Cadillac Mountain Granite

  28. Detail of Cadillac Mountain Granite

  29. Bob Wiebe’s cool work on the CMG • Wiebe’s model for the CMG works pretty well in terms of the model of Annen et al. 2006 at high crustal levels: • Heat from basalt drives convection in the granite • what is the rheological state of this granite ? • Basalt supplies heat to granite, rejuvenating it • Magma mixing and mingling between basalt differentiates (diorite?) and granite • Crystallizing basalt producing macrorhythmic units • Injection of hot basalt onto mushy floor of cool granite magma chamber enclaves Convecting granite Heat from basalt Differentiating basalt Granite mush

  30. Linking the CMG to caldera development • Widespread distribution of small (mm-cm) dioritic enclaves throughout the CMG: • Hybrid in composition….magma mixing….Steve Blake-style mixing at basalt-granite interface triggered by rapid evacuation of magma (start of super-eruption) (alternatives: basalt fountaining, quakes) • Are enclaves distributed from heating and convection of CMG by basalt injection? • Another idea – not mutually exclusive – is stirring of the magma by collapse of the roof into the chamber (Kennedy et al. 2008) • The breccia unit on the E and S margins of the CMG may be related….is it generated by rapid underpressure (-ΔP) during magma withdrawal…. • …or might the breccia develop as the result of (more gradual?) overpressure (+ΔP) prior to the super-eruption? • And/or by basalt being injected into the chamber?

  31. (c) Ossipee Ring Complex, White Mountains, New Hampshire Montreal Ossipee Kennedy B, Stix J (2007) Geol Soc Am Bull 119:3-17

  32. Geological map of Ossipee Ring Complex (redrawn from Kingsley, 1931 and Carr, 1980)

  33. 5cm 5cm 5cm 5cm 0 0 0 0 Textural variation from inclusion mingling in the syenite Anorthoclase Discrete inclusions Orthoclase Crystal transfer Anorthoclase rich patches evidence of old inclusion Diffuse inclusion

  34. Magma dynamics and caldera development at Ossipee

  35. Santorini Druitt T et al. (2012) Nature 482:77-82

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