processes and rates of magma ascent, storage and differentiation beneath arcs georg f. zellmer...
TRANSCRIPT
Processes and rates of magma ascent, storage and
differentiation beneath arcs Georg F. Zellmer
Institute of Earth Sciences,Academia Sinica
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?
Lascar andesite lava flow, Northern Chile
Soufriere Hills andesite lava dome Montserrat
Test how eruptive style links with surface heat flux
Use this heat flux model to compare volcanic arcs, as resolution is low enough to mask low high T anomalies (although too low to resolve oceanic arcs).
Heat flux based on shear-wave velocity model of crust and upper mantle, Shapiro & Ritzwoller 2004
Continental and transitional arcs:
Good correlation for regular arcs. But what is reason for range in heat flux in general, and why are some arcs hotter?
Arcs with ongoing crustal deformation or
slab discontinuities
are hotter.
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What controls surface heat flux?
1. Background geothermal gradient? Maybe a little…
• 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)
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No coherent variation, i.e. not a first-order effect.
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Testing effect of background geothermal gradient variations through differences in crustal thickness (oceanic arcs in black):
Test correlation with convergence rate
Use Bird, 2003, plate boundary model:
Good correlation, including oceanic arcs (in black)
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…
within-arcmountainbuilding
(hinders transfer)
within-arctransverse
faulting(eases transfer)
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Slab detachment (MEX, Ferrari, 2004)
Slab window (e.g. CJP, Mazzotti et al., 1999)
Slab tear (e.g. NCH, Barazangi & Isacks, 1972)
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.
Within-arc transverse faulting(CAS, Miller et al., 2001)
Within-arc thrust faulting(CJP, Seno, 1999;Townend & Zoback, 2006)
cf. Mt. Bachelor volcanic chain:
N-S alignment of volcanic vents
Some arcs with deforming overriding plates…
Back to basics: what controls viscosity?
Lejeune and Richet, 1995
dacite trachybasalt67wt% 47wt% SiO2
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.
composition
T
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:
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?
equilibrium
change of U/Thactivity ratios
over time
One may use geospeedometry to answer this question, i.e. employing the diffusional modification of element profiles within or between crystals:
Comparing measured and modeled trends,adapted from Costa et al. (2003)
@ 850oC
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.
plag
Short (compared to U-series constraints) crystal residence times of
~10 - ~103 yrsat 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 basaltsProblem: 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 rocksProblem: 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 intermediatesThis is fine, but leaves the question how the felsic compositions were generated in the first place…
• Liquid immiscibilityThis 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 wedgeEvolved 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?
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…
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MarianasLesser AntillesVanuatuTonga-KermadecPhillipinesIndonesiaAntarctic PeninsulaNicaraguaKamchatkaAleutiansNew Britain/BismarckAegean
basaltsbasaltic
andesites andesites dacites
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1.6 ka
3.2 ka
…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.
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MarianasLesser AntillesVanuatuTonga-KermadecPhillipinesIndonesiaAntarctic PeninsulaNicaraguaKamchatkaAleutiansNew Britain/BismarckAegean
basaltsbasaltic
andesites andesites dacites
0 ka
75 ka
150 ka
To reconcile U-Th and Ra-Th information, mixing of young mafic melts with older more evolved compositions must be operating in most systems.
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:
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:
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):
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.
20ka eruption age range
100 ka
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 remarks1. 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?
Tungurahua, Ecuador, 2006
And a final note:
Never lose sight of the big picture.
Most arc volcanism is explosive, not effusive…