Download Measuring Timescales of Magmatic Evolution

Measuring Timescales
of Magmatic Evolution
Simon Turner1 and Fidel Costa2
A
dvances in analytical methods have provided new insights into the
timescales of magmatic processes. Data on the abundances of U-series
isotopes in bulk rocks and crystal separates indicate magma differentiation over thousands of years. Residence and differentiation times of silicic
magmas based on single-crystal, in situ age data vary from 10,000 to 100,000
years, with abundant evidence for crystal recycling from previous intrusive
episodes. Chemical zoning patterns in single crystals indicate that processes
such as mixing and mingling of magmas and crustal assimilation may occur
over much shorter timescales of months to decades. Quantifying the rates of
magma generation, emplacement and differentiation constrains the processes
involved and may contribute to the evaluation of volcanic hazards.
complementary contributions are
improving our understanding of
magmatic processes. Reid (2003) and
Hawkesworth et al. (2004) recently
reviewed timescale information.
METHODOLOGY
Short-Lived Radioactive
Isotopes and In Situ
Determinations
Advances in mass spectrometry
techniques, including SIMS (secondary ion mass spectrometry),
have enabled increasingly precise
KEYWORDS: timescales, magmatic processes, diffusion, U-series, phenocrysts measurements of the nuclides of
the U-series decay chains in bulk
rocks and mineral separates and
INTRODUCTION
now permit in situ dating of (mainly) zircon and allanite
The timescales and rates of magmatic processes are key (e.g. Bourdon et al. 2003). U-series disequilibria in parental
pieces of information for understanding and modeling magmas are typically produced during mantle melting; sysmany aspects of igneous activity. For example, the rates at tems of particular interest here are 230Th–238U (half-life
which magmas are transferred from the mantle to the crust 75,000 years), 226Ra–230Th (half-life 1600 years) and 210Pb–226Ra
determine the types of physical and chemical processes that (half-life 22 years) (see FIG. 1). Return to secular equilibrium
can occur during magma transport. Similarly, the mass and occurs over several (~5) half-lives of the daughter nuclide (see
longevity of magmas below calderas or active volcanoes page 273). Lava suites from a number of different volcanoes
affect both the time-integrated thermal fluxes available for show a decrease in disequilibria with increasing differentiageothermal energy and the likelihood of hazardous erup- tion (e.g. FIG. 2). Thus, U-series disequilibria can, in principle,
tions. The number and quality of timescale determinations date melting and crystallization events up to several
of igneous processes have increased dramatically in the last 100,000 years old and, in the case of 210Pb, which has a
15 years, mainly due to advances in experimental and ana- gaseous progenitor (222Rn), degassing up to 100 years old.
lytical techniques. Elemental concentrations and isotopic At the same time, technical improvements have also
ratios are now determined at unprecedented levels of preci- allowed the acquisition of in situ age data from very young
sion and spatial resolution (e.g. Ginibre et al. 2007 this
issue; Davidson et al. 2007 this issue). Technical advances
include (1) higher-precision measurements of isotopic
clocks, including U-series isotopes, on smaller amounts of
material and in situ isotope determinations on very young
crystals, and (2) modeling of the diffusive re-equilibration
of isotopes and elements in a variety of minerals using more
precise diffusion coefficients and better spatial resolution.
The former allow dating and provide ages whereas the latter gives relative time, which can sometimes be transformed
into an age if the age of another event is known (e.g. eruption year). By combining the two techniques, geological
processes that span a few hours to millions of years can be
determined (FIG. 1). Here we briefly describe the basis and
the findings of the two approaches and discuss how their
1
GEMOC, Department of Earth and Planetary Sciences,
Macquarie University, Sydney NSW 2109, Australia
E-mail: [email protected]
2
CSIC, Institut de Ciències de la Terra 'Jaume Almera',
c/ Lluís Solé i Sabarís s/n, 08028 Barcelona, Spain
E-mail: [email protected]
ELEMENTS, VOL. 3,
PP.
267–272
FIGURE 1
267
Ranges of timescales that can be determined using radiometric
dating and diffusion modeling of chemical zoning in crystals
A UGUST 2007
A
B
(226Ra/230Th) versus Th (used as an index of differentiation)
plot comparing lavas from Akutan volcano (filled squares)
with lavas from Aniakchak volcano (filled circles) and those from the Asal
rift and Sangeang Api in the Sunda arc. After George et al. 2004
FIGURE 2
minerals; typically the U–Th system in zircon is studied
using a secondary ion microprobe (SIMS). These new age
determinations have provided new insights and raised new
questions. The relationship between the ages of “datable”
accessory minerals and main differentiation events driven
by the crystallization of major rock-forming minerals is not
straightforward. The distinction between entrainment of
old crystals and the age of differentiation is important
because it bears on the longevity of the magmatic systems,
which relates in turn to the rates and types of magmatic
inputs into the crust and the associated mode and frequency of volcanic activity.
C
Diffusion Modeling of Zoning Patterns
in Minerals and Glasses
Diffusion modeling exploits the presence in many minerals
of elemental or isotopic zoning. The chemical zoning patterns
in crystals and glasses provide a record of magmatic
processes. These patterns dissipate or evolve toward equilibrium profiles at a rate that can be experimentally calibrated in terms of diffusion coefficients. Thus, if we know
the diffusion coefficient and if we are able to measure the
compositions with sufficient spatial resolution and precision, we can obtain elapsed time using Fick’s second law of
diffusion (FIG. 3). The approach has been applied for some
time to determine the thermal histories of metamorphic
rocks. Renewed interest in igneous systems (e.g. Zellmer et
al. 1999; Costa et al. 2003; Morgan et al. 2004) has resulted
from the many new and precise determinations of diffusion
coefficients made possible by analytical and experimental
techniques. One advantage of the method is that the time
information can be linked to the textural and chemical features of the rocks at the scale of a thin section (FIG. 4). The
method can provide enough data to be treated statistically,
because large numbers of crystals can be analysed using
routine analytical techniques (e.g. electron microprobe)
and because multiple elements can be determined on one
or more minerals in the same rock (FIG. 4).
TIMESCALES OF MAGMATIC EVOLUTION
Magma Residence and Differentiation Times
The simplest approach to constraining differentiation
timescales is based on the observation that U-series disequilibrium decreases with increasing differentiation in cogenetic
ELEMENTS
Schematic illustration of the principles of obtaining
timescales from modeling chemical or isotopic gradients
in crystals. (A) Crystal is growing from a liquid; in this case it is assumed
to be unzoned. (B) A change in environmental conditions (temperature,
pressure, composition) occurs in response to a process like magma mixing, and this is recorded in the zoning of the crystal. In the case shown,
the system responds with the growth of a rim of a different composition
(e.g. Davidson et al. 2007). (C) The measured concentration will be a
combination of growth or dissolution plus diffusion. The first task is to
determine the initial profile prior to diffusion (initial profile = Cini), which
can be accomplished by considering elements that have different diffusion rates or by using other geological arguments about the evolution
of the sample (e.g. Costa et al. 2003; Morgan et al. 2004). The zoning
of the crystal is analyzed in terms of Fick’s second law; this requires
knowing the diffusion coefficient (D) and the boundary conditions (e.g.
if the crystal exchanges mass with the liquid, as shown in the example).
The timescale of the process depends on how far the measured profile
is from the initial and the equilibrium concentrations (Ceq).
FIGURE 3
suites of volcanic rocks. FIGURE 2 illustrates that, for many
small- to moderate-size magmatic systems, this decrease is
observed for (226Ra/230Th) (where brackets indicate activity
ratios, and secular equilibrium is defined by an activity ratio
of 1), from which it is inferred that the timescale of differentiation is on the order of thousands of years. The approach
assumes that the time elapsed during differentiation is the
only cause of a decrease in (226Ra/230Th). However, in cases
where partial melts of crustal materials are in secular equilibrium (e.g. Berlo et al. 2004), the effects of assimilation
will also lead to a decrease in (226Ra/230Th) and a corresponding shift in (230Th/238U) towards 1. Conversely, the
common trend of decreasing disequilibrium with increasing
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A UGUST 2007
A
the results can be more complex to interpret, not least
because the crystals contained in a rock are not the ones
that separated from the parental liquid to produce the bulkrock composition (Hawkesworth et al. 2004). For example,
Cooper and Reid (2003) and Turner et al. (2003) analysed
U–Th–Ra disequilibria in plagioclase separates from a number of volcanoes and found discrepancy between the U–Th
and Ra–Th systems (e.g. FIG. 5). This was interpreted to indicate that the crystals contain recycled cumulate materials,
and it is likely that individual crystals are also zoned in age.
In some cases, textural information from crystal size distributions supports the notion of entrainment of cumulates
(FIG. 5; Turner et al. 2003).
B
D
C
E
Example of zoning patterns and diffusion modeling of
multiple elements in olivine from a basalt of the
Tatara–San Pedro volcanic complex. (A) Photomicrograph (partially
crossed polars) of olivine crystals in a plagioclase-rich matrix. The positions of two electron microprobe traverses are shown as arrows labeled
T6 and T7. The rectangle delimits the area of the X-ray map. (B) X-ray
map of Mg concentration. Red is high concentration and blue is low. (C)
Crystallographic axis orientations for the same crystal and electron
microprobe traverses projected onto the lower hemisphere. Note that
T6 is almost parallel to the a-axis and T7 to the c-axis. The orientation
of the olivine was obtained in situ by electron backscatter diffraction.
(D) and (E) Profiles of traverses T6 and T7 showing the measured concentrations, initial conditions, model profiles, and calculated times for
1125°C and oxygen fugacity at the Ni–NiO buffer. The diffusion model
includes a compositional dependence for Fe–Mg and Ni, the effect of
oxygen fugacity, and the effect of anisotropy. The results of the two
traverses for all elements overlap at 5.4 ± 2.5 years. This suggests that
the zoning patterns are due to diffusive exchange, that the assumption
of initial profile is appropriate, and that the diffusion coefficients are correct. In this case the times reflect the duration between assimilation of
gabbroic plutons by basaltic magma and eruption. Modified after Costa
and Dungan (2005).
FIGURE 4
differentiation can be taken as evidence that crustal partial
melts are typically close to secular equilibrium.
Nevertheless, because the effects of assimilation are likely to
reduce the disequilibria inherited from parental magmas,
the timescales inferred should be viewed as maxima, and in
the extreme case of mixing of two end-members, differentiation could occur instantaneously with respect to the halflife of 226Ra.
A complementary approach is to obtain ages from crystals
in volcanic rocks. In some cases good mineral isochrons
appear to provide robust information on the timescales of
crystallization (e.g. Heumann and Davies 2002). However,
ELEMENTS
The previous section highlights some of the limitations of
analyses of bulk mineral separates but, unfortunately, measurement of U–Th disequilibria in individual crystals is not
yet possible for most mineral species. High-U–Th accessory
minerals such as zircon are exceptions to this generalization, and these have provided insight into the evolution of
several large-volume rhyolite systems. Studies of large silicic
deposits (>100 km3) have provided evidence that some
magma residence times, including the time taken for differentiation, are one to two orders of magnitude longer than
the ~1000 years inferred above (FIG. 6). Some of the most
spectacular results arise from the in situ age determination
of zircon and allanite. Brown and Fletcher (1999) dated
zircon crystals from Whakamaru Group ignimbrites (Taupo
Volcanic Zone) and found crystal cores 250,000 years older
than the rims. Similarly, Vazquez and Reid (2004) reported
U-series age data from allanite from the Toba eruption;
there, crystal cores are 160,000 years older than the rims. In
both examples the range of ages between core and rim suggests that the crystals might have grown uninterrupted for
long periods of time. Charlier and Zellmer (2000) reported
data from different size fractions of bulk zircon separates
from the Taupo Oruanui eruption and showed that a correlation exists between crystal size and age. These ages range
from 6000 to 12,000 years, which is broadly consistent with
differentiation timescales inferred from island arc wholerock 226Ra data. However, models allowing for continuous
zircon growth imply much older ages of ~90,000 years
(Charlier and Zellmer 2000), similar to in situ zircon data
(see below). In other examples it has not been possible to
spatially resolve more than ‘cores and rims’, and in most
SIMS studies only a single point on each crystal has been
analysed. Ages from the Bishop Tuff (Reid and Coath 2000;
Simon and Reid 2005) and the Rotoiti and Oruani tuffs in
Taupo (Charlier et al. 2003, 2005) indicate residence times
of a few thousand to several hundred thousand years
(FIG. 6). However, it is not always clear whether the dated
crystals grew from the host magma or were recycled from
pre-existing intrusions (e.g. Charlier et al. 2005). In a few
studies, diffusion has been used to obtain timescales for
magmatic differentiation. Morgan and Blake (2006) used Sr
and Ba concentrations in sanidine from the Bishop Tuff and
obtained timescales of ca. 100,000 years for magma differentiation. This result is in agreement with the higher end of
the age range obtained using SIMS (FIG. 6). In contrast,
Zellmer et al. (1999) used Sr zoning in plagioclase crystals
from St. Vincent (Lesser Antilles) and Kameni (Aegean) volcanoes and determined that the time elapsed between shallow-level crystallization and eruption was of the order of
100 to 450 years. This range is shorter than those inferred
from U–Th–Ra studies. These age differences may be due to
the recycling of crystals which were cooled before they
could equilibrate by diffusion (see Turner et al. 2003 for further discussion).
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A UGUST 2007
A
Magma Assimilation, Magma Mixing and
Pluton Remobilisation
B
Diffusion studies have provided significant constraints on
the nature of open-system processes. The time required for
magma mixing or mingling has been investigated using
Fe–Ti gradients in oxides and major- plus trace-element
zoning in olivine and plagioclase (see data compilation in
Costa and Chakraborty 2004). The pattern that has emerged
from these studies is that mixing between end-members
that are compositionally similar (e.g. two mafic magmas)
requires less time—only a few months—than the years to
decades estimated for mixing between dacite and basaltic
andesite. Comparison of these data with measured timescales
for deformation of volcanic edifices (Costa and Chakraborty
2004) suggests that it may be possible to correlate the
behaviour of the igneous system at surface with its behaviour
at depth. Similarly, constraints have been placed on the
amount of time elapsed between assimilation of crustal wall
rocks and eruption in mafic and silicic systems (FIG. 6; e.g.
Wolff et al. 2002; Costa and Dungan 2005). Costa et al. (2003)
determined timescales on the order of 100 years for melt
and fluid percolation and metasomatism of gabbroic xenoliths using Ca–Na, Mg, Fe, K and La zoning in plagioclase.
The time required for remobilisation of completely or partially crystallized rocks to yield silicic magmas has also been
explored in ways other than those used for obtaining the
SIMS zircon ages discussed above. Based on oxygen isotope
disequilibrium between different minerals, Bindeman and
Valley (2001) calculated that between 500 and 5000 years
elapsed between melting and eruption of the post-caldera
lavas in Yellowstone. Similar time periods (10 to 1200 years)
were obtained by Zellmer et al. (2003) using Sr and Ba zoning in plagioclase from the Soufrière Hills volcano (FIG. 6).
These periods are, however, much longer than the days to
weeks determined for the same rocks by Devine et al. (2003)
using Fe–Ti zoning in magnetite. The different results
obtained by the two approaches probably reflect the much
faster Fe–Ti diffusion rates in oxides compared to those for
Sr and Ba in plagioclase, such that the former record only
the last reheating event.
C
Magma Transport Rates
D
(A) Photomicrograph of a Tongan andesite (plag = plagioclase, cpx = clinopyroxene, mt = magnetite). (B) Crystal size distribution with crystal growth times for linear sections of the
plots based on a plagioclase growth rate of 10-11 cm·s-1 (n = the number of crystals measured). (C) U–Th equiline diagram. (D) 226Ra/Ba versus time evolution diagram for late groundmass and calculated liquid in
equilibrium with the plagioclase separate (after Turner et al. 2003).
FIGURE 5
ELEMENTS
Magma transport rates can be constrained by U-series disequilibria if the site of origin and/or the magnitude of initial
disequilibria are known. For example, the positive correlation between 226Ra excess and slab fluid indices in arc lavas
(e.g. Ba/Th or Sr/Th ratios) suggests that these magmas
ascend through the mantle wedge at ≥100–1000 m/yr
(Turner et al. 2001). Similarly, data from ocean island lavas
have been interpreted to require melt ascent at ≥10–100
m/yr (Stracke et al. 2006), and recent 210Pb data suggest that
melt formed beneath mid-ocean ridges may rise at more
than 1000 m/yr (Rubin et al. 2005). These results have led
to the consensus that melt is extracted from the mantle via
high-porosity channels. Magma transfer times and histories
have also been obtained using a diffusion approach. Kelley
and Wartho (2000) used 40Ar/39Ar age data from phlogopite
to infer transport times of hours to days from the mantle to
the crust. Several hours to days were also obtained by
Demouchy et al. (2006) and Peslier and Luhr (2006) using
zoning in H content in olivine from mantle xenoliths.
Notwithstanding these data, Klügel (2001) and Shaw et al.
(2006) used Fe–Mg zoning in olivine to constrain the duration of two different events: fast transport of magma
between reservoirs and the surface in hours to days, and
storage of xenoliths in different crustal reservoirs for years
to decades (up to a hundred years or more) before finally
reaching the surface. These longer timescales are in agreement
with the U-series data discussed above (FIG. 6).
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A UGUST 2007
A
B
Summary of the timescales determined from radioactive
isotopes and from diffusion modeling. See text and references for details.
FIGURE 6
In several recent studies, (210Pb/226Ra) measurements have
been used to constrain magma degassing rates. Gauthier
and Condomines (1999) presented data from Stromboli and
Merapi volcanoes and a model to determine the time required
to produce 210Pb deficits, i.e. [(210Pb/226Ra) < 1], by degassing.
A survey of island arc volcanic rocks (Turner et al. 2004)
showed that many have 210Pb deficits consistent with
degassing over a few decades prior to eruption. However,
many also contain 210Pb excesses, and this would appear to
require gas fluxing from fresh magma at depth (Berlo et al.
2004; Turner et al. 2004). Such signals could potentially be
monitored to assess the relative amounts of fresh magma
beneath a volcano, and thus they may contribute to eruption forecasting models (Berlo et al. 2006; Reagan et al. 2006).
Interestingly, these degassing times are longer than those
obtained from diffusion studies. Castro et al. (2005) determined
times for bubble nucleation, degassing and quenching using
compositional profiles of H2O and found that the processes
required between 0.4 and 15 days. In this case, the difference
in time between the two approaches may highlight their
complementary nature (FIG. 6). The diffusion data refer to
the last degassing event and thus are relevant to bubble formation and magma fragmentation processes that happen during eruption, whereas the isotope data relate to degassing of
magma at depth and are thus applicable to understanding
the longer-term behavior of shallow magma reservoirs.
DISCUSSION
The timescales determined by radiometric clocks can complement those deduced from measured diffusion profiles
(FIG. 6). In some cases the radiometric clocks yield significantly longer times, but this difference can sometimes be
attributed to the inferred temperature–time path of the crystals.
This is schematically illustrated in FIGURE 7, in which the
U–Th-series ages for some crystals are on the order of 50,000
years whereas the time obtained from diffusion is about 100
years. Crystal ages could record the total time since crystallization, cumulate formation and remobilization by intrusions,
whereas the diffusion approach may record only the time
since the last replenishment event. This can be visualized
by considering the studied crystals as complexly zoned and
made of an old core and a much younger rim. The cores
ELEMENTS
Interpretation of difference in time information obtained
from radioactive isotope and diffusion clocks in the same
mineral. (A) The system crystallizes and accumulates minerals for 50,000
years and the radiometric clock is started if the mineral and system are
close to diffusive exchange. (B) A large input of new magma disrupts
cumulates and partially reacts with them creating a new rim much
younger than the core (e.g. Fig. 2). During this event the diffusion clock
starts, but shortly after, magma reaches the surface (time = t2 = less than a
hundred years). Then, the time obtained from the radiometric clock
(50,000 years + t2, but depends on the mass proportions of the old cores
and young rims) is probably much longer than that of the diffusion time t2.
FIGURE 7
Magma Degassing
might be from cumulates that were stored for a long time
before they were disaggregated by the powerful intrusion of
a batch of magma that also triggered the eruption. The time
information obtained from the radiometric clock will be a
mixed record from the old cores and the rims of crystals
formed from this last batch of magma. In contrast, the diffusion clock will record only the time elapsed since formation
of the rims and, thus, the time since the last replenishment
event that created the driving force for diffusion. Therefore,
it is expected that the time obtained from the diffusion
clock will be shorter than that derived using the radiogenic
approach. Another aspect to note is that the error associated
with the U-series ages can be on the order of 1000 years,
which is less than the time range for processes that happen
just prior to eruption (e.g. a replenishment event). In this
case, a U-series clock might indicate the presence of cumulates
as old as 50,000 years and allow inferences about the overall
longevity of the system, whereas the diffusion data record
information related to the processes that lead to eruption.
Future progress will rely on further improvements in analytical
techniques, better diffusion coefficient data and detailed
case studies.
ACKNOWLEDGMENTS
FC acknowledges many discussions with Sumit Chakraborty
about diffusion in igneous and metamorphic rocks. FC is
funded by a Ramon y Cajal Fellowship from the Ministerio
de Educación y Ciencia de España and by the DFG (SFB 526,
project B7). ST acknowledges a Federation Fellowship from
the Australian Research Council. We thank Kari Cooper and
Georg Zellmer for helpful reviews and Jon Davidson and
Dougal Jerram for their suggestions and their efforts in
bringing together this issue. .
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A UGUST 2007
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A UGUST 2007