Processes and Timescales for Zoning and Homogeneity in Magmatic Systems

1. The Processes and Timescales That Produce Zoning and Homogeneity in Magmatic Systems George Bergantz, Olivier Bachmann and Philipp Ruprecht University of Washington

2. How to Link Observations Across Scales? • How to expand our toolbox for magma forensics? • What are the dynamic templates that produce large scales? • How are they reflected at the crystal scale?

3. Three types of zoning patterns thatcommonly occur in ignimbrites

4. Mechanisms to produce compositional gaps and gradients

5. Gradients in ignimbrites (See Table 1 in text)

6. Compositional Gap (“Daly Gap”) • Fig. 2 from paper

7. CF-induced Daly Gap Same P-T, isotopic ratios Trace element concentration = crystal fractionation Interstitial melt in mafic (crystal-rich) end-member compositionally similar to silicic end-member (Crustal melting unlikely)

8. Bachmann and Bergantz, 2004 Interstitial melt expulsion from crystal-rich mushes • Crystal-melt separation time within longevity of magma chambers • Melt expulsion enhancers (gas-driven filter-pressing, earthquake fluidization)

9. Gradients in ignimbrites (See Table 1 in text)

10. (Hildreth and Wilson, 2007)

11. Gradients require mixing- what do we need? Stretching + Folding: Circulation (many scales of strain) Mixing requires a: 1)a magma chamber 2) paddle, thermal plumes, crystal plumes, bubble plumes, compositional effects 3) an energy source- some change in the environment to produce kinetic energy

12. Well, What Dictates the Dynamic Template? • The Reynolds number: • Most of us know that this number delimts three regimes: • Re << 1, laminar flow, neglect inertia • Re > 104, fully turbulent, self-similar flow MIXING TRANSITION • 104 > Re >1 chaotic advection, both inertia and viscosity important

13. Demonstrate dripping crystal plumesSee paper by Bergantz and Ni, 1999 cited in chapter

14. Jellinek et al., 1999 Mixing “Efficiency” • For ‘system-wide’ mixing caused by vertical transport, e.g. some flavor of plume, Jellinek and others proposed the concept of “mixing efficiency.” • BUT be very careful about this concept- it is really a measure of STRATIFICATION

15. Bringing together types of zoning into a common framework • Formation of a cap by escape from sill-like mush (instead of from the walls) • Unzoned cap What happens in the cap? Top: cooling and assimilation Bottom: T-buffered mush below Convection in cap but weak, low-Reynolds number

16. Gaps and zoning- no big deal after all!

17. Processes that Produce Complexity in a Crystal Cargo • Mixing • In-situ hyper-solidus recycling: dynamic mush • Concurrent melting, assimilation and deformation What are links to the dynamic templates?

18. Simulations of gas driven overturn with “smart” crystals • Movies from: “Modeling of gas-driven magmatic overturn: Tracking of phenocryst dispersal and gathering during magma mixing” Ruprecht, Bergantz and Dufek, G3, v. 9, no. 7, 2008

19. Conclusions from simulations: • For 2x105 crystals report back: • A single overturn is sufficient to gather crystals onto a thin-scale from as much as a 100 m initial separation. Continued choatic stirring can increase these distances, in accord with natural examples.

20. But what do crystals really remember? • Depends on rate of travel through regions of distinct chemical potential vs. rate at which crystals can record to changes • Damköler number: • If Da << 1, kinetics dominate • If Da >> 1, equilibrium assumption okay

21. Crystals as recorders of events in real-time • For rapid, e.g., gas driven overturn, crystal growth will lag and only record an “echo” of the process (Da << 1), but dissolution may reach Da ~ 1 • For slower processes rate-limited by heat transfer, both growth and dissolution will have Da ~1 or more

22. Gradients in ignimbrites (See Table 1 in text)

23. Homogeneity • Mostly in large, crystal-rich magmas with intermediate (dacitic) composition (Monotonous Intermediates) • Also true for large granodioritic batholiths (main upper crustal building block) • How to reach homogeneity on large volumes of viscous crystal-rich magmas? • Low Re convection inevitably leads to gradients???? • How to retain homogeneity on large volumes? • New magma recharge will inevitably occur???

24. New mass injections limited to similar compositions? • Once a critical crystallinity is reached, silicic mushes act as density filter, buffer for T, C • But crystals often very strongly zoned…

25. Spectacular small-scale disequilibrium in FCT, a “homogeneous intermediate” • Reflects a long history of overturn (Charlier et al., 2007)

26. Time scales have dual nature: homogeneity at the large scale, heterogeneity at the small scale • Toba: chem oscillations in allanites > .4 M.y. before eruption; cycling of crystals through hyper-solidus domains (Reid et al.) • Bandelier Tuff: reheating prior to eruption (Wolff et al.) • Fish Canyon: reverse mineral zoning, complex crystal compositions (Bachmann, Charlier et al.) • Tuolumne Intrusive Suite: complexly zoned zircons, • Spirit Mtn., Mojave system: complex rejuvenation of intrusive sheets, zoned zircon (Miller et al.)

27. Lengthscale-dependent mixing • Some bulk mixing must occur • Crystals record a changing environment- not just change in intensive variables • Zoning patterns different in juxtaposed crystals • Homogeneous at hand sample scale

28. Large silicic system are NOT just “strips” of rhyolite- geophysical evidence: Long Valley Caldera. Very different from Mt. St. Helens. New injections of basalt or intermediate magma common

29. Unzipping • Sluggish convection regime • Gradients induced by crystal plumes, assimilation, mixing • As system grows, assimilation and mixing become more transparent • Lock-up from floor as crystal accumulation reaches ~50 %vol • Cooling slows down (at least by a factor of 2) • New magmas can not mix in => Heat plate • Unzipping

30. Thanks