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Processes and rates of magma
ascent, storage and
differentiation beneath arcs
Georg F. Zellmer
Institute of Earth Sciences,
Academia Sinica
[email protected]
Two main topics of this lecture
1.
How are arc magmas transferred from the mantle to
shallow level magma reservoirs?
We will discuss differences between porphyritic lava domes and
less viscous lava flows on a global basis, and compare transfer
processes and time scales using combined U-series data and
geospeedometric constraints.
2.
Where, how, and at what rate does magmatic differentiation
take place?
Various differentiation mechanisms will be outlined. We will
then focus on fractional crystallization, partial melting, and
magma mixing and assimilation, and discuss how geochemical
evidence, including time scale estimates on whole rocks and
crystals, can be used to gain insights into these processes.
Effusive eruptions make up 20% of all arc
volcanism. One in three produces a dome.
What are the systematics?
Soufriere Hills andesite
lava dome Montserrat
Lascar andesite lava flow, Northern Chile
Test how eruptive style links with surface heat flux
Heat flux based on shear-wave velocity model of crust and upper mantle, Shapiro & Ritzwoller 2004
Use this heat flux model to compare volcanic arcs, as resolution is low enough
to mask low l, high T anomalies (although too low to resolve oceanic arcs).
(calculated from GVP Holocene eruption database)
dimensionless average lava viscosity
Continental and transitional arcs:
Arcs with
ongoing crustal
deformation or
slab
discontinuities
are hotter.
Good correlation for regular arcs. But what is reason for range in
heat flux in general, and why are some arcs hotter?
What controls surface heat flux?
1. Background geothermal gradient? Maybe a little…
q H  T
•
Fourier’s law,
•
Thermal gradient depends on
•
(a) Moho temperature (~ constant +/- 10%)
•
(b) Crustal thickness (see relation in the next slide)
2. Thermal spikes through shallow level intrusions? For sure!
•
Depend on: (a) Geometry of magma plumbing system
•
(b) Dynamics of magma ascent
•
(c) Rate of melt generation = rate of slab dehydration
•
Depends on: (i) Water content of slab
•
(ii) Plate convergence rate (test influence in following slides)
(calculated from GVP Holocene eruption database)
dimentionless average lava viscosity
Testing effect of background geothermal gradient variations through
differences in crustal thickness (oceanic arcs in black):
No coherent variation, i.e. not a first-order effect.
Test correlation with convergence rate
Use Bird, 2003, plate boundary model:
dimensionless average lava viscosity
 Good correlation, including oceanic arcs (in black)


within-arc
mountain
building
(hinders transfer)
within-arc
transverse
faulting
(eases transfer)
Viscosity in most arcs is controlled by advection of heat via magma throughput,
which is greater at arcs with faster subduction.
Viscosity increases as magma transfer rate decreases? - will be tested later…
Slab detachment (MEX, Ferrari, 2004)
Slab tear (e.g. NCH, Barazangi
& Isacks, 1972)
Slab window (e.g. CJP, Mazzotti et al., 1999)
This and the following slide show
examples of irregular arcs with higher
surface heat flux.
The Mexican arc is one of them. One
may speculate why heat flux is higher.
Whatever the reason, good correlations
with convergence rate in all but CAS
and CJP suggest that in most arcs, these
anomalies do not have first order effects
on melt volume and transport dynamics,
although compositions may be affected,
cf. adakites.
Some arcs with deforming overriding plates…
Within-arc thrust faulting
(CJP, Seno, 1999;
Townend & Zoback, 2006)
Within-arc transverse faulting
(CAS, Miller et al., 2001)
cf. Mt. Bachelor
volcanic chain:
N-S alignment of
volcanic vents
Back to basics: what controls viscosity?
composition
dacite
67wt%
trachybasalt
47wt% SiO2
T
Lejeune and Richet, 1995
Giordano & Dingwell, 2003
 Up to intermediate compositions, at any given T, crystal content can have
a much greater and more abrupt effect on viscosity than composition.
Hypothesis: Viscous, porphyritic lava domes are remobilized plutons.
Test if this hypothesis is consistent with U-series data
Uranium series isotopes have half-lives are
similar to the timescales of fluid/volatile transfer
and magmatic processes at subduction zones.
They have a large range of incompatibility and
fluid mobility, and are therefore easily
fractionated by many geological processes.
In this talk we will use the 230Th-238U and the
226Ra-230Th systems, with a half-lifes of 75 and
1.6 kyrs, respectively.
This is one of the three
natural decay chains, in which
238U decays via a number of
intermediate daughters to
stable 206Pb.
Lava domes ( ) are close to U-Th equilibrium:
change of U/Th
activity ratios
over time
equilibrium
This is in support of long crustal residence times of lava dome rocks.
However, are the dome magmas stored as cool crystalline protoliths,
or as hot crystal-poor melts in thermally buffered deep magma chambers?
One may use geospeedometry to answer this question, i.e. employing the
diffusional modification of element profiles within or between crystals:
plag
 In simple crystals, a step starting
profile is assumed.
 Magmatic temperature is estimated.
 Modeling intracrystalline diffusion
of trace elements yields time.
 Problem: Crystals in arc magmas often exhibit
complex zoning. However, detailed case studies
still allow determination of crystal residence times
at magmatic temperatures.
@ 850oC
Comparing measured and modeled trends,
adapted from Costa et al. (2003)
Short (compared to U-series constraints)
crystal residence times of
~10 - ~103 yrs
at magmatic temperatures have been
derived for a number of volcanoes:
• Santorini, Greece (Zellmer et al., 1999)
• Soufriere, St. Vincent (Zellmer et al., 1999; Turner et al., 2003)
• Soufriere Hills, Montserrat (Zellmer et al., 2003)
• San Pedro, Chile (Costa et al., 2003)
• Vesuvius, Italy (Morgan et al., 2004)
• Taapaca, Chile (Zellmer & Clavero, 2006; Woerner, unpubl. data)
 At intermediate compositions, magma reservoirs are ephemeral and small.
 This indicates thermal buffering of dome-forming magmas is not operating.
 Remobilization of cool igneous protoliths is the best explanation for the
genesis of porphyritic dome lavas.
Some conclusions from the global data
1. Porphyritic lava domes commonly yield young diffusion
ages but old U-series ages, hence their protoliths are stored
in a cool environment prior to eruption by remobilization.
2. Globally, their genesis is dependent on plate convergence
rate, which determines the rate of magma generation in the
mantle wedge. Lower magma production rates lead to a
cooler crust and more frequent freezing of magmas.
3. Crustal thickness does not exert a first-order control,
although dome formation is rarer on thin crust.
4.
Mountain building hinders magma transfer through compressive
tectonic setting (CJP), transverse faulting eases magma transfer by
providing pathways (CAS). Within-arc extension does not appear to
have a first-order effect (NZL, cf. Zellmer 2008).
5. Implications for the
lower crustal hot zone
(cf. Annen et al., 2006):
The hot zone is in steady state,
i.e. the amount of magma influx
from the underlying mantle
wedge into the hot zone is
proportional to the magma
outflux from the hot zone into
shallow magma reservoirs,
because surface heat flux is
dominated by shallow level
intrusions.
Part II: Magmatic differentiation
How are felsic magmas generated? The answer is still very much debated, and may be different in
different systems and for different compositions.
• Fractional crystallization from basalts
Problem: Large amounts of crystallization required to make rhyolites, yet were are all the cumulates?
Possible solution: Cumulates are heavy and returned to mantle.
• Partial melting of mafic rocks
Problem: Too much heat required to partially melt mafic intrusions. Possible solution: Hydrothermal
alteration will decrease their melting point.
• Combination of the above, e.g. as proposed for the lower crustal hot zone.
• Mixing of mafic and felsic compositions to form intermediates
This is fine, but leaves the question how the felsic compositions were generated in the first place…
• Liquid immiscibility
This concept has long been established (cf. Daly, 1914) and there is experimental evidence (e.g.
Veksler et al., 2007, and references therein). However, there are not too many natural examples…
• Generation within the mantle wedge
Evolved melts may also be generated within the mantle wedge due to its metasomatic alteration
(including silica enrichment) by slab components prior to melting (e.g. Straub et al., Evidence from
high Ni olivines for a hybridized peridotite / pyroxenite source for orogenic andesites from the central
Mexican Volcanic Belt, G-cubed, in press and available from website).
How can time information contribute to understanding?
Marianas
Lesser Antilles
Vanuatu
Tonga-Kermadec
Phillipines
Indonesia
Antarctic Peninsula
Nicaragua
Kamchatka
Aleutians
New Britain/Bismarck
Aegean
8.0
7.0
0 ka
5.0
4.0
1.6 ka
3.0
(
226
Ra/
230
Th) eruptive
6.0
3.2 ka
2.0
1.0
0.0
45
basalts
basaltic
andesites
andesites
50
55
60
dacites
65
70
SiO2
Ra-Th data has been interpreted to reflect differentiation to andesitic compositions within a few
thousand years (cf. ka-scale), mainly based on the nice trend of some Tonga-Kermadec samples.
However…
…if rapid close system evolution was the dominant process everywhere, one would expect horizontal
trends of all arcs in terms of (238U/230Th), yet this is only observed for some Tongan samples.
Marianas
Lesser Antilles
Vanuatu
Tonga-Kermadec
Phillipines
Indonesia
Antarctic Peninsula
Nicaragua
Kamchatka
Aleutians
New Britain/Bismarck
Aegean
2.0
0 ka
1.8
(238U/230Th)eruptive
1.6
75 ka
1.4
1.2
150 ka
1.0
0.8
basalts
basaltic
andesites
andesites
dacites
0.6
45
50
55
60
65
70
SiO2
 To reconcile U-Th and Ra-Th information, mixing of young mafic melts
with older more evolved compositions must be operating in most systems.
There is plenty of field
and petrographic
evidence for magma
mingling between
mafic and intermediate
compositions, and
resulting mineral
disequilibrium textures:
here an example from
Nisyros in the Aegean
Volcanic arc:
However, hybridization across a wide
compositional range appears to be limited,
as seen for example at Santorini in the same
arc. Here, there is evidence for assimilation
of old crust (87Sr/86Sr > 0.71) operating
during the genesis of the most evolved
melts:
Thus, the following magma remobilization scenario as depicted here for
Montserrat might well work for many intermediate composition arc melts,
particularly porphyritic melts. Here, these melts are generated in the lower
crustal hot zone [“?” in the (a)], freeze in the upper crust, and eventually
partially remelt due to influx of hot mafic magma.
However, the generation of large volume rhyolites may instead be linked to
larger volume upper crustal melting, which takes time (see next slide).
Thermal modeling of sill intrusions shows that long incubation
times are required to melt significant quantities of old crust:
May this explain the long time periods between eruptions of very large volume ignimbrites?
The processes that produce felsic upper crustal compositions in the first place are still not fully
understood. In addition, the above diagram is just a model, it does not prove that this is the actual
and dominant mechanism of rhyolite generation.
Some additional insights may be gained from the crystals carried by variably
differentiated melts, particularly by the apparent age of the crystal assembleages:
The main observations and interpretations of the
previous slide are:
• There are large uncertainties (see the huge error bars)
This is because many of these ages are not yielded by tight mineral isochrons, but are from lines of
best fit (“errorchrons”) through scattered data. (This is elsewhere known as cheating.)
• Ra-Th and U-Th ages frequently give disparate results.
This probably suggests prolonged crystallization, with crystal cores dominating the U-Th age and
crystal rims dominating the Ra-Th age (also see Turner et al., 2003).
• Many mafic samples give old U-Th ages, comparable to cumulate ages.
Note that uptake of cumulate crystals in mafic melts is well documented. Old crystal ages in mafic
melts should thus not be misinterpreted as indicating long melt residence times. In fact, we have seen
above that mafic melts have the highest whole rock U-series disequilibria, and some make it to the
surface quite rapidly (i.e. within hours to days in some cases!)
• Many intermediate compositions give young U-Th ages, within error of eruption age.
A significant proportion of mineral phases may have thus have crystallized just prior to eruption.
However, we have seen above that the whole rock U-series disequilibria in most of the intermediate
melts are low, and that remobilization is a common phenomenon. Thus many crystals probably do
have old inherited cores, which again may be the reason for the large error bars.
• Crystal ages in felsic melts, based on zircon geochronology, scatter widely.
This is consistent with remobilization of different lithologies of variable age, either ancient crust or
previous intrusives (see next slide).
Detailed zircon chronology of rhyolites erupted from the Taupo Volcanic Zone in
New Zealand give the following interesting result (cf. Charlier et al., 2005):
100 ka
20ka eruption age range
The zircons from three
rhyolites erupted between
45ka and 25ka show a
common 100ka inherited
peak in addition to the
pre-/syn-eruptive peaks.
This suggests that each
rhyolite tapped the same
100ka old source, pointing
to remobilization of a
previous intrusion rather
than ancient crust.
So remobilization of previous intrusives appears to be operating even during
the petrogenesis of rhyolites. Differentiation by repeated fractional
crystallization and remobilization through partial melting is perhaps the key to
generating the most evolved melt compositions.
A model of the Taupo plumbing system
adapted from Charlier et al. (2005)
Some conclusions for differentiation
1. Differentiation processes remain controversial.
2. U-series isotope studies of whole-rocks show that magma
mixing and remobilization are important processes in the
generation of intermediate compositions.
3. U-series crystallization ages need to be critically assessed.
They point to complex processes involving uptake of
cumulate crystals, prolonged crystallization histories and
crystal inheritance, and suggest remobilization is also
operating during rhyolite petrogenesis.
4. Melting and assimilation of ancient crust does take place,
but how much of it contributes to rhyolite petrogenesis is
not easy to constrain, and may differ in different settings.
Concluding remarks
1. This lecture has purposely focused on global datasets, with
few examples from individual volcanic edifices.
2. Most studies in fact deal with individual volcanic edifices, and
some are able to provide tight constraints on the petrogenetic
processes operating at these individual sites.
3. Nevertheless, it is always important to critically assess
geochemical and other data. Too often, conclusions are drawn
prematurely, through making assumptions that may not be
valid. An example are the errorchron “ages”, which do give
insights but do not tell the complete story.
4. You should critically assess this lecture, too! Is the evidence
presented convincing? Are there any flaws in the logics of the
interpretation? What other topics may be important but were
not addressed?
And a final note:
Never lose sight of the big picture.
Most arc volcanism is explosive, not
effusive…
Tungurahua, Ecuador, 2006