The Processes and Timescales That Produce Zoning and

Homogeneity in Magmatic Systems

George Bergantz, Olivier Bachmann and Philipp Ruprecht

University of Washington

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?

Three types of zoning patterns thatcommonly occur in ignimbrites

Mechanisms to produce compositional gaps and gradients

Type of gradient

Abrupt Linear (monotonic)

Not measurable

Archtypal Examples

Crater Lake, Aniakchak, Toconao-Atana, Katmai (Payne et al., V21C-2122), Chaitén (Lowenstern et al., V43D-2180)

Bishop Tuff, Huckleberry Ridge Tuff,Bandelier Tuff

Monotonous Dacites (Fish Canyon Tuff, Lund Tuff, Cerro Galan)Rhyolites (Taupo)

Gradients in ignimbrites (See Table 1 in text)

Compositional Gap (“Daly Gap”)

Fig. 2 from paper

CF-induced Daly GapSame 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)

Interstitial melt expulsion from crystal-rich mushesBachmann and Bergantz, 2004

1. Crystal-melt separation time within longevity of magma chambers

2. Melt expulsion enhancers (gas-driven filter-pressing, earthquake fluidization)

Type of gradient

Abrupt Linear (monotonic)

Not measurable

Archtypal Examples

Crater Lake, Aniakchak, Toconao-Atana, Katmai (Payne et al., V21C-2122), Chaitén (Lowenstern et al., V43D-2180)

Bishop Tuff, Huckleberry Ridge Tuff,Bandelier Tuff

Monotonous Dacites (Fish Canyon Tuff, Lund Tuff, Cerro Galan)Rhyolites (Taupo)

Gradients in ignimbrites (See Table 1 in text)

(Hildreth and Wilson, 2007)

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

Well, What Dictates the Dynamic Template?

The Reynolds number:

Re uL

Most of us know that this number delimts three regimes:

1) Re << 1, laminar flow, neglect inertia

2) Re > 104, fully turbulent, self-similar flow MIXING TRANSITION

3) 104 > Re >1 chaotic advection, both inertia and viscosity important

Demonstrate dripping crystal plumes

See paper by Bergantz and Ni, 1999 cited in chapter

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

Jellinek et al., 1999Jellinek et al., 1999

Formation of a cap by escape from sill-like mush (instead of from the walls)

Unzoned cap

Bringing together types of zoning into a common framework

What happens in the cap?Top: cooling and assimilationBottom: T-buffered mush belowConvection in cap but weak, low-Reynolds number

Gaps and zoning- no big deal after all!

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?

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

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.

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

advectionreaction

tDa t

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

Type of gradient

Abrupt Linear (monotonic)

Not measurable

Archtypal Examples

Crater Lake, Aniakchak, Toconao-Atana, Katmai (Payne et al., V21C-2122), Chaitén (Lowenstern et al., V43D-2180)

Bishop Tuff, Huckleberry Ridge Tuff,Bandelier Tuff

Monotonous Dacites (Fish Canyon Tuff, Lund Tuff, Cerro Galan)Rhyolites (Taupo)

Gradients in ignimbrites (See Table 1 in text)

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???

Homogeneity

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…

Spectacular small-scale disequilibrium in FCT, a “homogeneous intermediate”

Reflects a long history of overturn

(Charlier et al., 2007)

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.)

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

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

Unzipping

1. 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

2. Unzipping

Thanks