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 that
commonly occur in ignimbrites
Mechanisms to produce compositional
gaps and gradients
Gradients in ignimbrites (See Table 1 in text)
Type of
gradient
Abrupt
Linear
(monotonic)
Not measurable
Archtypal
Examples
Crater Lake,
Aniakchak, ToconaoAtana, Katmai
(Payne et al., V21C2122), Chaitén
(Lowenstern et al.,
V43D-2180)
Bishop Tuff,
Huckleberry
Ridge Tuff,
Bandelier Tuff
Monotonous
Dacites (Fish
Canyon Tuff, Lund
Tuff, Cerro Galan)
Rhyolites (Taupo)
Compositional Gap (“Daly Gap”)

Fig. 2 from paper
CF-induced Daly Gap
Same P-T,
isotopic ratios
Trace element
concentration =
crystal
fractionation
Interstitial melt in
mafic (crystalrich) endmember
compositionally
similar to silicic
end-member
(Crustal melting unlikely)
Interstitial melt expulsion from crystal-rich mushes
Bachmann and Bergantz, 2004
1. Crystal-melt separation time within longevity of
magma chambers
2. Melt expulsion enhancers (gas-driven filter-pressing,
earthquake fluidization)
Gradients in ignimbrites (See Table 1 in text)
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)
(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 sourcesome change in the environment
to produce kinetic energy
Well, What Dictates the Dynamic
Template?

The Reynolds
number:
uL
Re  
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., 1999
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
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
t
Da  advection
treaction

Damköler number:

If Da << 1, kinetics dominate

If Da >> 1, equilibrium assumption okay
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
Gradients in ignimbrites (See Table 1 in text)
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)
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???
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
Sluggish convection regime
1.
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
•
•
•


2.
Cooling slows down (at least by a factor of 2)
New magmas can not mix in => Heat plate
Unzipping
Thanks