Zircon record of the plutonic-volcanic connection and protracted
rhyolite melt evolution
Chad D. Deering1, Brenhin Keller2, Blair Schoene2, Olivier Bachmann3, Rachel Beane4, and Maria Ovtcharova5
Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive,
Houghton, Michigan 49931, USA
2
Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA
3
Institute of Geochemistry and Petrology, ETH Zürich, Clausiusstrasse 25, 8092 Zurich, Switzerland
4
Department of Earth and Oceanographic Science, Bowdoin College, Brunswick, Maine 04011, USA
5
Department of Earth Sciences, University of Geneva, 13 Rue des Maraichers, 1205 Geneva, Switzerland
1
ABSTRACT
The potential petrogenetic link between a crystal-poor rhyolite (the
Rhyolite Canyon Tuff) and its associated subvolcanic intrusion and
crystal-rich post-caldera lavas from Turkey Creek, Arizona (USA),
is examined using zircon chemical abrasion–thermal ionization mass
spectrometry U-Pb geochronology and inductively coupled plasma
mass spectrometry trace element analyses. U-Pb ages indicate that
zircon growth within the rhyolite and the dacite-monzonite porphyry
magmas was coeval over ~300 k.y. prior to the large eruptive event.
Trends in zircon trace elements (Hf, Y/Dy, Sm/Yb, Eu/Eu*) through
time in the dacitic-monzonitic units and rhyolite reflect melt evolution
dominated by crystal fractionation. Importantly, the Y/Dy ratio in
zircons in both units remains mostly similar for the first ~150 k.y. of
the system’s evolution, but the dominant population in the rhyolitic
unit diverges from that of the dacite-monzonite porphyry ~150 k.y.
before eruption. We interpret this divergence in trace element composition to record the assembly time of the melt-rich cap within its
intermediate mush zone in the upper crustal reservoir. These results
are consistent with (1) a connection between plutonic and volcanic
realms in the upper crust, (2) a protracted time scale for constructing
an intermediate mush large enough to hold 500 km3 of rhyolite, and
(3) the prolonged extraction of that melt prior to eruption.
INTRODUCTION
Over the past few decades, constructing a general framework linking plutonic and volcanic realms has been key in trying to understand
magmatic processes (see recent reviews by Annen et al. [2015] and Bachmann et al. [2007]). However, fundamental questions regarding their
spatiotemporal relationship and the duration over which they evolve or
co-evolve remain unanswered. In large, silicic magmatic systems, the
origin of high-silica rhyolite by extraction from shallow, batholith-sized
intermediate mush magma was presented as one possible mechanism
(Bachmann and Bergantz, 2004; Hildreth, 2004). This model requires a
complementary intermediate cumulate residue to remain in the mid- to
upper-crustal environment and necessarily implies a direct relationship
between plutonic and volcanic systems in the shallow crust (Deering
and Bachmann, 2010; Gelman et al., 2014; Lee and Morton, 2015). The
mechanisms that lead to the accumulation of large volumes of evolved
magma in the upper crust have been suggested to occur over a time
scale on the order of hundreds of thousands of years (e.g., Schaltegger et
al., 2009). However, differentiating between the time scales of reservoir
growth and those of rhyolite extraction is problematic, with proposed time
scales for the latter varying from hundreds of years (e.g., Pappalardo and
Mastrolorenzo, 2012) to many thousands of years (e.g., Pamukcu et al.,
2015a; Wotzlaw et al., 2014).
GEOLOGY, April 2016; v. 44; no. 4; p. 267–270
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The optimal scenario for studying the relationship between plutonic
and volcanic rocks and the time scales of magma evolution is one where
remnants of an intrusive body are exposed within the caldera formed by
the eruption of a large rhyolitic body. At the Turkey Creek caldera (Arizona, USA), uplift and erosion have exposed the intra-caldera and outflow
facies of the Rhyolite Canyon Tuff, as well as a resurgent intermediate
intrusion and associated lava flows (Fig. 1). In all units, the ubiquity of
zircon provides us with the opportunity to investigate rates of processes
using a combination of high-precision geochronology and geochemistry
(U-Pb thermal ionization mass spectrometry with trace element analysis
[TIMS-TEA]). This paper presents a new data set for these eruptive products in an attempt to determine the (1) duration of magma reservoir growth
in the upper crust, (2) relationship between crystal-poor ignimbrite and
intermediate resurgent intrusion and lavas, and (3) time scale of melt-rich
rhyolite extraction and/or accumulation prior to eruption.
THE TURKEY CREEK CALDERA
The Turkey Creek caldera (~20 km in diameter) of southeastern
Arizona (Fig. 1) formed as the result of the catastrophic eruption of
109° 20’
109° 30’ W
109° 10’
AZ
N
Turkey Creek Caldera
32°
00’
Ring fault
Topographic rim
Undifferentiated older rocks
(+ Quaternary cover)
31°
50’
Tertiary extrusive rocks
Moat rhyolite lava
Monzonite/dacite porphyry
resurgent intrusion and lavas
Rhyolite Canyon
intracaldera tuff
31°
40’ N
0
10 km
Rhyolite Canyon
outflow tuff
Figure 1. Geologic map of Turkey Creek caldera in Chiricahua
Mountains, Arizona (AZ), southwestern USA (modified from du
Bray and Pallister, 1991). Black stars indicate sampling localities.
doi:10.1130/G37539.1
©
2016 Geological
Society
America.
permission to copy, contact [email protected].
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Published online 24 February 2016
267
Rhyolite Canyon Tuff
27.40
TC-RCM-060913-8b
TC-RCI-061113-1A
TC-RCM-050411-1
Dacite/Monzonite Porphyry Intrusion
TC-RCI-050711-1
TC-RCM-050611
TC-DPI-060713-2A
TC-DPI-050511
TC-DPI-060813-4
20
18
N = 56
14
12
10
8
6
4
2
0
27.0
27.20
27.1
27.2
Age (Ma)
27.3
27.4
206
Pb-238U age (Ma)
27.30
Frequency
16
27.10
End of zircon crystallization; Eruption Age?
27.00
Figure 2. U-Pb geochronology showing 206Pb/238U dates for individual zircon from Rhyolite Canyon Tuff (Arizona, USA) eruption and associated
dacite-monzonite intrusion of Turkey Creek caldera. Each bar is an individual analysis, and size of vertical bars are ±2s internal uncertainty.
Eruptive unit colors are coded as in geologic map in Figure 1. Zircons analyzed at University of Geneva (Switzerland) using ET2535 doublespike are shown with filled bars.
>500 km3 of high-silica rhyolite (Rhyolite Canyon Tuff). This event has
been previously dated to 26.97 ± 0.09 Ma (average Ar/Ar date on sanidine from tuff samples; du Bray and Pallister, 1991). The emplacement
of this ignimbrite was followed by a resurgent intrusion of monzonite
porphyry. Some of this magma reached the surface as crystal-rich dacite
lavas along ring fractures.
The Rhyolite Canyon Tuff outflow is primarily exposed to the north
of the caldera and is typically welded and devitrified. The outflow facies
deposits can be divided into three separate compound cooling units (upper:
Trcu; middle: Trcm; lower: Trcl) separated by air-fall and surge deposits.
The outflow facies high-silica rhyolite is crystal-poor to moderate (10–20
vol%), dominated by quartz and sanidine, with minor plagioclase and
accessory oxides, augite, apatite, zircon, and titanite. The intra-caldera
facies (Trci) contains the same minerals as the outflow, but with up to 30
vol% crystallinity (du Bray and Pallister, 1991).
The hypabyssal monzonite porphyry and the eruptive equivalent dacite
porphyry contain a similar mineralogy to the tuff with sanidine and plagioclase (up to several centimeters long). Glomerocrysts are ubiquitous
in both monzonite and dacite porphyry including plagioclase, sanidine,
quartz, biotite, clinopyroxene, hornblende, and magnetite with accessory
oxides, apatite, zircon, and titanite. The groundmass is texturally variable
in both the monzonite and the dacite porphyry from microcrystalline and
granophyric to glassy, respectively.
A representative sample suite of bulk-rock geochemistry was previously published by du Bray and Pallister (1991), which shows the range
of compositions extending from ~64 to 78 wt% SiO2. The rhyolite and
monzonite-dacite porphyry are separated by a compositional gap (~10
wt% SiO2); only a few samples fall within that gap and show textural
and compositional evidence of mixing and/or mingling (du Bray and
Pallister, 1991). The trace element characteristics of the magmas suggest
wet, oxidized conditions in the source region, consistent with subduction
zone magmatism. The pre-eruptive depth of the rhyolite was estimated
using the water-saturated eutectic point in the haplogranitic system with
the normative (quartz-albite-orthoclase) composition of the melt. This
method yields an average equilibration pressure of ~100 MPa indicating
a depth of at least 4 km (du Bray and Pallister, 1991), which is similar to
storage depths determined for many other high-SiO2 rhyolites of similar
size (e.g., Bégué et al., 2014; Pamukcu et al., 2015b).
268
METHODS
Zircon ages and trace element geochemistry were obtained by U-Pb
chemical abrasion–isotope dilution–thermal ionization mass spectrometry
with coupled trace element analysis (U-Pb CA-ID-TIMS-TEA). Individual
zircons were annealed, chemically abraded to remove metamict zones,
rinsed, and spiked with EARTHTIME (202Pb)-205Pb-233U-235U tracers prior
to dissolution and column chemistry. Eluted U and Pb were analyzed by
thermal ionization mass spectrometry at Princeton University (New Jersey,
USA) and UNIGE (University of Geneva, Switzerland), while eluted trace
elements were analyzed by inductively coupled plasma mass spectrometry
at Princeton University. All ages are reported with 2σ internal uncertainties calculated using U_Pb Redux (Bowring et al., 2011). Full analytical
methods, data tables, and zircon cathodoluminescence images are given
in the GSA Data Repository1.
ESTABLISHING THE PLUTONIC-VOLCANIC CONNECTION
We interpret zircon U-Pb dates to record crystallization from 27.359 ±
0.017 Ma to 27.029 ± 0.050 Ma for both the monzonite and dacite lavas
and the Rhyolite Canyon Tuff (Fig. 2; see the Data Repository). The
overlap in age distribution among samples implies that these two magmas
were crystallizing over the same period of time over ~300 k.y. in the upper
crust. This duration is similar to time scales determined in other studies
of large silicic magmatic systems (Bachmann et al., 2007; Guillong et
al., 2014; Schoene et al., 2012). The monzonite-dacite and rhyolite zircon
populations also include much older crystals not related to the magmatic
system that formed the Rhyolite Canyon Tuff (ca. 30–75 Ma) that are
distinct in composition (e.g., Hf, Y/Dy, Eu/Eu*) and overlap in age with
several shallowly exposed local plutons and ignimbrites (see the Data
Repository). The presence of these xenocrysts implies that a small amount
of assimilation occurred during the evolution of the magmatic system.
While U-Pb dates support zircon co-crystallization in the monzonitedacite and rhyolitic units in the magma plumbing system, trace element
chemistry in the whole rock provides us with an opportunity to further
1
GSA Data Repository item 2016081, U-Pb CA-ID-TIMS analytical methods,
zircon age and trace element geochemistry, and Figure DR1 showing the distinct
xenocrystic zircon population, is available online at www.geosociety.org/pubs
/ft2016.htm, or on request from [email protected] or Documents Secretary,
GSA, P.O. Box 9140, Boulder, CO 80301, USA.
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(A) bulk-rock
(B) zircon
Rhyolite Canyon Tuff
Intracaldera/upper unit
Outflow;
lower and middle units
65
15000
13000
chondrite
55
30
11000
45
200
300
400
500
600
Rb
700
35
0.00
9000
plagioclase-dominated
fractionation trend
Zr/Hf
20
100
apatite/titanite dominated
fractionation trend
Hf
Zr/Hf
Dacite/monzonite porphyry
40
(C) zircon
0.05
0.10
0.15
Eu/Eu*
0.20
0.25
0.30
7000
0.00
0.02
0.04
0.06
Sm/Yb
0.08
0.10
Figure 3. A: Zr/Hf versus Rb showing increase in incompatible element Rb with decreasing Zr/Hf ratio consistent with crystal fractionation. B:
Trace element diagram showing negative correlation between Eu/Eu* and Zr/Hf reflecting plagioclase fractionation and overlap between dacitemonzonite and rhyolite units. C: Decreasing Sm/Yb with increasing Hf (proxy for temperature) is likely governed by titanite/apatite fractionation.
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The size of the eruption (>500 km3) suggests that if the eruptible melt
was extracted from a mush, a significantly larger partly cumulative monzonitic body must exist beneath the Turkey Creek caldera (>1000 km3).
Partial remobilization of such bodies can occur during large volcanic
eruptions (in zoned ignimbrites; e.g., Deering et al., 2011, Bachmann et
al., 2014), and the intrusive equivalents have been imaged by geophysical
methods (e.g., gravity and seismic studies) in large magmatic provinces
around the world (see Lipman and Bachmann [2015] for a review). In
particular, the neighboring Southern Rocky Mountain volcanic field bears
the mark of a 15–20-km-thick batholith beneath the volcanic apron.
TIME SCALES OF RHYOLITE MELT EXTRACTION
In addition to providing evidence for rhyolite melt extraction from
a monzonite-dacite mush, our CA-ID-TIMS-TEA method also has the
potential to evaluate the duration of melt extraction. The monzonite-dacite
zircon population has a restricted range of Y/Dy ratios of <~15, with a
population of older rhyolite zircons that are similar in composition (Fig. 4).
Together, this group spans the entire time period of magma evolution
between ca. 27.3 and ca. 27.0 Ma (eruption age). However, the dominant
rhyolite zircon population has Y/Dy ratios >15 and cluster from ca. 27.15
Ma to ca. 27.05 Ma—a duration of ~150 k.y. when accounting for error in
age determinations. Variations in the Y/Dy ratio of zircon are heavily influenced by changes in melt composition due to the preferential incorporation
22
Extraction duration (~150 k.y.)
Eruption age
evaluate the potential connection between the high-silica rhyolite unit
and intermediate resurgent intrusion and lava flows emplaced along the
ring fault. Following zircon saturation, Zr/Hf decreases in both melt and
zircon such that crystal cumulates are expected to have higher Zr/Hf than
the residual melt (Claiborne et al., 2006; Samperton et al., 2015). Deering and Bachmann (2010) showed that the Zr/Hf ratio can thus be used
to determine the existence of zircon, and hence crystal, accumulation in
silicic magmas in a variety of tectonic settings. The bulk-rock Zr/Hf ratio
versus Rb of the Rhyolite Canyon magmas show a decrease from the monzonite and dacite to the rhyolite consistent with the intermediate magmas
representing the cumulate residual to the erupted rhyolite (Fig. 3A).
Zircons also reveal compositional trends parallel to those found in the
bulk rock, which are consistent with crystal fractionation. In particular,
the Eu anomaly (Eu/Eu*) can be used to trace the co-crystallization of
feldspars, the most abundant phases in the Turkey Creek system. Eu/Eu*
decreases steadily with decreasing Zr/Hf over a continuous compositional
range in zircons from the monzonite-dacite to the rhyolite units, indicating the strong influence of feldspar crystallization on zircon chemistry
(Fig. 3B). The combination of zircon geochronology and trace element
chemistry with bulk-rock trace element patterns provides evidence for the
derivation of the rhyolite from the monzonite-dacite magma.
Hafnium has been observed to increase in zircon with decreasing temperature (estimated using Ti-in-zircon) and increasing SiO2 of the melt
(Claiborne et al., 2006). Hafnium, therefore, can be used in conjunction
with other trace elements (e.g., rare earth elements [REEs]) that are incorporated into zircon to track the thermal and compositional evolution of
the melt from which it crystallized. For example, the Sm/Yb ratio in the
Turkey Creek units is negatively correlated with Hf (Fig. 3C). This relationship can be explained by the co-crystallization of titanite and apatite,
which preferentially incorporate Sm relative to Yb as the magma cools.
Our data also show that Hf in zircons from the rhyolite overlap with that
in the monzonite-dacite of similar composition (Fig. 3C), which is consistent with both magmas reaching the same thermal and melt evolution
state prior to eruption, albeit with different crystallinities.
The compositional gap between the monzonite-dacite and the rhyolite
(Fig. 3) is a common feature of volcanic series worldwide (e.g., Brophy,
1991) and has been explained by extraction of rhyolitic melt from a dacitic
mush once crystallinities are high enough to impede the stirring effect of
convection (>~50 vol%; Bowen, 1919; Brophy, 1991; Dufek and Bachmann, 2010). If such a model holds, minerals in the monzonite-dacite and
rhyolite would not show a clear compositional gap, as they would have
crystallized continuously, and would not have been limited to growth
at a fixed melt composition (Deering et al., 2011). Our zircon data are
consistent with this hypothesis; the compositional series displays trends
that overlap (Fig. 3).
18
Y/Dy
zircons formed in rhyolite
14
10
26.9
zircons formed in dacite mush
average 2σ
27.0
27.1
27.2
Age (Ma)
27.3
27.4
Figure 4. Zircon trace element variation with time determined by U-Pb
thermal ionization mass spectrometry with trace element analysis.
Dominant population of high-Y/Dy rhyolite zircons formed over ~150
k.y. prior to eruption in more-evolved magma, more heavily influenced
by titanite/apatite co-crystallization than dacite-monzonite zircons.
Older rhyolite zircons formed in similar conditions as those of dacitemonzonite or are interpreted to be antecrysts scavenged from mostly
crystalline portions of the reservoir. Symbols same as in Figure 3.
269
of middle REEs over Y in titanite (Colombini et al., 2011). We interpret this
change in Y/Dy as evidence for additional titanite (+ apatite) fractionation
in the rhyolite during and after extraction from the dacite mush. Therefore,
the recorded time interval in which the Y/Dy differs provides an estimate
for the duration of melt extraction and accumulation.
CONCLUSIONS
The U-Pb CA-ID-TIMS geochronology shows that the evolution of
high-silica rhyolite and that of associated dacite-monzonite porphyry of
the Turkey Creek system were coeval over ~300 k.y. The TIMS-TEA
results further suggest that the rhyolite and dacite-monzonite are related
by crystal fractionation. Using trace element ratios in the dated zircons
(in particular Y/Dy), we can also resolve changes that occur over relatively short durations and further suggest that the production, and possibly
extraction, of the dominant rhyolite melt body from the underlying mush
was protracted (up to 100–150 k.y.). These time scales are in agreement
with mechanical models suggesting sluggish extraction of highly viscous
rhyolitic melt from crystal mush (Bachmann and Bergantz, 2004; Deering
et al., 2011). While rapid—centuries to a few millennia—assembly of voluminous melt-rich rhyolite evacuated in large eruptions has been proposed
(e.g., Druitt et al., 2012; Pappalardo and Mastrolorenzo, 2012; Wotzlaw et
al., 2014), our data suggest that these may record shorter-lived processes
such as recharge or remobilization. These processes are not related to the
overall duration of assembly of the system, nor to a record of the time
scales for large volumes of silicic melt extraction. We do not rule out that
melt extraction may occur over relatively short time scales (hundreds to
thousands of years) for some systems (particularly smaller ones). However,
without direct sampling and dating of the progenitor mush, the interpretation will be severely hindered. The ability to combine the use of zircon
age and trace element data from coeval/co-genetic plutonic and volcanic
units is, therefore, crucial for deciphering the processes responsible for
the generation of the largest volumes of felsic magma to erupt on Earth.
ACKNOWLEDGMENTS
Deering and Beane acknowledge support from National Science Foundation grants
EAR-1249821 and EAR-1250259. Thanks to E. du Bray for his help in organizing
fieldwork. Andrew Barth, Jonathan Miller, and an anonymous reviewer provided
thoughtful and constructive suggestions that greatly improved the manuscript.
REFERENCES CITED
Annen, C., Blundy, J.D., Leuthold, J., and Sparks, R.S.J., 2015, Construction and
evolution of igneous bodies: Towards an integrated perspective of crustal
magmatism: Lithos, v. 230, p. 206–221, doi:10.1016/j.lithos.2015.05.008.
Bachmann, O., and Bergantz, G.W., 2004, On the origin of crystal-poor rhyolites: Extracted from batholithic crystal mushes: Journal of Petrology, v. 45,
p. 1565–1582, doi:10.1093/petrology/egh019.
Bachmann, O., Miller, C.F., and de Silva, S.L., 2007, The volcanic-plutonic
connection as a stage for understanding crustal magmatism: Journal of Volcanology and Geothermal Research, v. 167, p. 1–23, doi:10.1016/j.jvolgeores
.2007.08.002.
Bachmann, O., Deering, C.D., Lipman, P.W., and Plummer, C., 2014, Building
zoned ignimbrites by recycling silicic cumulates: Insight from the 1,000
km3 Carpenter Ridge Tuff, CO: Contributions to Mineralogy and Petrology,
v. 167, 1025, doi:10.1007/s00410-014-1025-3.
Bégué, F., Gualda, G.A., Ghiorso, M.S., Pamukcu, A.S., Kennedy, B.M., Gravley,
D.M., Deering, C.D., and Chambefort, I., 2014, Phase-equilibrium geobarometers for silicic rocks based on rhyolite-MELTS. Part 2: Application to
Taupo Volcanic Zone rhyolites: Contributions to Mineralogy and Petrology,
v. 168, p. 1082–1097, doi:10.1007/s00410-014-1082-7.
Bowen, N., 1919, Crystallization-differentiation in igneous magmas: The Journal
of Geology, v. 27, p. 393–430, doi:10.1086/622669.
Bowring, J., McLean, N.M., and Bowring, S., 2011, Engineering cyber infrastructure for U-Pb geochronology: Tripoli and U-Pb_Redux: Geochemistry
Geophysics Geosystems, v. 12, Q0AA19, doi:10.1029/2010GC003479.
Brophy, J.G., 1991, Composition gaps, critical crystallinity, and fractional crystallization in orogenic (calc-alkaline) magmatic systems: Contributions to
Mineralogy and Petrology, v. 109, p. 173–182, doi:10.1007/BF00306477.
Claiborne, L.L., Miller, C.F., Walker, B.A., Wooden, J.L., Mazdab, F.K., and
Bea, F., 2006, Tracking magmatic processes through Zr/Hf ratios in rocks
and Hf and Ti zoning in zircons: An example from the Spirit Mountain
270
batholith, Nevada: Mineralogical Magazine, v. 70, p. 517–543, doi:10.1180
/0026461067050348.
Colombini, L.L., Miller, C.F., Gualda, G.A.R., Wooden, J.L., and Miller, J.S.,
2011, Titanite and zircon in the Highland Range volcanic sequence (Miocene, southern Nevada, USA): Elemental partitioning, phase relations, and
influence on evolution of silicic magma: Mineralogy and Petrology, v. 102,
p. 29–50, doi:10.1007/s00710-011-0177-3.
Deering, C.D., and Bachmann, O., 2010, Trace element indicators of crystal accumulation in silicic igneous rocks: Earth and Planetary Science Letters, v. 297,
p. 324–331, doi:10.1016/j.epsl.2010.06.034.
Deering, C.D., Bachmann, O., Dufek, J., and Gravley, D.M., 2011, Rift-related
transition from andesite to rhyolite volcanism in the Taupo Volcanic Zone
(New Zealand) controlled by crystal–melt dynamics in mush zones with variable mineral assemblages: Journal of Petrology, v. 52, p. 2243–2263, doi:10
.1093/petrology/egr046.
Druitt, T., Costa, F., Deloule, E., Dungan, M., and Scaillet, B., 2012, Decadal to
monthly timescales of magma transfer and reservoir growth at a caldera volcano: Nature, v. 482, p. 77–80, doi:10.1038/nature10706.
du Bray, E.A., and Pallister, J.S., 1991, An ash flow caldera in cross section: Ongoing field and geochemical studies of the mid-Tertiary Turkey Creek Caldera,
Chiricahua Mountains, SE Arizona: Journal of Geophysical Research, v. 96,
p. 13,435–13,457, doi:10.1029/91JB00067.
Dufek, J., and Bachmann, O., 2010, Quantum magmatism: Magmatic compositional
gaps generated by melt-crystal dynamics: Geology, v. 38, p. 687–690, doi:10
.1130/G30831.1.
Gelman, S.E., Deering, C.D., Bachmann, O., Huber, C., and Gutiérrez, F.J., 2014,
Identifying the crystal graveyards remaining after large silicic eruptions:
Earth and Planetary Science Letters, v. 403, p. 299–306, doi:10.1016/j.epsl
.2014.07.005.
Guillong, M., von Quadt, A., Sakata, S., Peytcheva, I., and Bachmann, O., 2014,
LA-ICP-MS Pb-U dating of young zircons from the Kos-Nisyros volcanic
centre, SE Aegean arc: Journal of Analytical Atomic Spectrometry, v. 29,
p. 963–970, doi:10.1039/c4ja00009a.
Hildreth, W., 2004, Volcanological perspectives on Long Valley, Mammoth Mountain, and Mono Craters: Several contiguous but discrete systems: Journal of
Volcanology and Geothermal Research, v. 136, p. 169–198, doi:10.1016/j
.jvolgeores.2004.05.019.
Lee, C.-T.A., and Morton, D.M., 2015, High silica granites: Terminal porosity
and crystal settling in shallow magma chambers: Earth and Planetary Science
Letters, v. 409, p. 23–31, doi:10.1016/j.epsl.2014.10.040.
Lipman, P.W., and Bachmann, O., 2015, Ignimbrites to batholiths: Integrating
perspectives from geological, geophysical, and geochronological data: Geosphere, v. 11, p. 705–743, doi:10.1130/GES01091.1.
Pamukcu, A.S., Gualda, G.A., Bégué, F., and Gravley, D.M., 2015a, Melt inclusion shapes: Timekeepers of short-lived giant magma bodies: Geology, v. 43,
p. 947–950, doi:10.1130/G37021.1.
Pamukcu, A.S., Gualda, G.A., Ghiorso, M.S., Miller, C.F., and McCracken,
R.G., 2015b, Phase-equilibrium geobarometers for silicic rocks based on
rhyolite-MELTS—Part 3: Application to the Peach Spring Tuff (Arizona–
California–Nevada, USA): Contributions to Mineralogy and Petrology,
v. 169, p. 33–49, doi:10.1007/s00410-015-1122-y.
Pappalardo, L., and Mastrolorenzo, G., 2012, Rapid differentiation in a sill-like
magma reservoir: A case study from the Campi Flegrei caldera: Scientific
Reports, v. 2, 712, doi:10.1038/srep00712.
Samperton, K.M., Schoene, B., Cottle, J.M., Brenhin Keller, C., Crowley, J.L.,
and Schmitz, M.D., 2015, Magma emplacement, differentiation and cooling
in the middle crust: Integrated zircon geochronological–geochemical constraints from the Bergell Intrusion, Central Alps: Chemical Geology, v. 417,
p. 322–340, doi:10.1016/j.chemgeo.2015.10.024.
Schaltegger, U., Brack, P., Ovtcharova, M., Peytcheva, I., Schoene, B., Stracke,
A., Marocchi, M., and Bargossi, G.M., 2009, Zircon and titanite recording
1.5 million years of magma accretion, crystallization and initial cooling in
a composite pluton (southern Adamello batholith, northern Italy): Earth and
Planetary Science Letters, v. 286, p. 208–218, doi:10.1016/j.epsl.2009.06.028.
Schoene, B., Schaltegger, U., Brack, P., Latkoczy, C., Stracke, A., and Günther, D.,
2012, Rates of magma differentiation and emplacement in a ballooning pluton
recorded by U–Pb TIMS-TEA, Adamello batholith, Italy: Earth and Planetary
Science Letters, v. 355–356, p. 162–173, doi:10.1016/j.epsl.2012.08.019.
Wotzlaw, J.-F., Bindeman, I.N., Watts, K.E., Schmitt, A.K., Caricchi, L., and
Schaltegger, U., 2014, Linking rapid magma reservoir assembly and eruption
trigger mechanisms at evolved Yellowstone-type supervolcanoes: Geology,
v. 42, p. 807–810, doi:10.1130/G35979.1.
Manuscript received 19 November 2015
Revised manuscript received 3 February 2016
Manuscript accepted 4 February 2016
Printed in USA
www.gsapubs.org
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Volume 44
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Number 4
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GEOLOGY
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;JSDPOSFDPSEPGUIFQMVUPOJDWPMDBOJDDPOOFDUJPOBOEQSPUSBDUFESIZPMJUFNFMUFWPMVUJPO
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Zircon U-Pb CA-ID-TIMS analysis
!
Following standard zircon separation by density and magnetic susceptibility, individual zircon grains were rinsed in 3 ml teflon beakers with distilled acetone and MQ
H2O. Single crystals were loaded into 200 µl Savillex microcapsules with 100 µl 29 M HF
+ 15 µl 3 N HNO3 for a single leaching step in high-pressure Parr bombs at 195 °C for
12 h to remove crystal domains affected by Pb loss (Mattinson 2005; Mundil et al.
2004). Grains were rinsed post-leaching with 6 N HCl, MQ H2O, and 3 N HNO3 prior to
spiking with the EARTHTIME (202Pb-)205Pb-233U-235U tracer and addition of 100 µl 29
M HF + 15 µl 3 N HNO3. In particular, the 202Pb-contining “double spike” was used only
in samples analyzed at the University of Geneva (denoted with postscript “_G” in Fig. 2
and data tables).
!
Zircons were then dissolved to completion in Parr bombs at 210 °C for 48 h. Dissolved zircon solutions were subsequently dried down, redissolved in 100 µl 6 N HCl and
converted to chlorides in Parr bombs at 195 °C for 12 h, after which solutions were dried
again and brought up in 50 µl 3 N HCl. The U-Pb and trace element aliquots were then
separated by anion exchange column chromatography using 50 µl columns and AG-1 X8
resin (200-400 mesh, chloride form from Eichrom) (Krogh 1973) and dried down with a
microdrop of 0.05 M H3PO4. The dried U and Pb aliquot was loaded in a silica gel emitter (Gerstenberger & Haase 1997) to an outgassed, zone-refined Re filament. Isotopic
determinations were performed using an IsotopX Phoenix-62 TIMS at PU and a
Thermo-Finigan Triton TIMS at Geneva, with Pb analyses performed in peak-hopping
mode on a Daly-photomultiplier ion counting detector for the Phoenix and on a modified Masscom secondary electron multiplier (SEM) for the Triton. A correction for massdependent Pb fractionation was applied cycle-by-cycle, calculated from the deviation of
measured 202Pb/205Pb from the known tracer 202Pb/205Pb (0.99924 ± 0.00054 (2σ)). A
Daly-photomultiplier Pb dead time of 40.5 ns was used at PU, as determined by >100
measurements of NBS981 and NBS982 standards and using a least squares fit to measured data over the range of 100 kcps to 2.5 Mcps 208Pb; an SEM deadtime of 23.5 ns was
determined at Geneva by similar procedures.
!
Isobaric interferences under masses 202, 204, and 205 were monitored by measuring masses 201BaPO4 and 203Tl which can be correlated using natural isotopic abundances to 202BaPO4, 204BaPO4, 205BaPO4, and 205Tl, as well as by measuring masses 202
and 205 in unspiked samples. These interferences were not found to be a significant
source of error and no interference correction was performed. Typical ion yields for 205Pb
were 100-200 kcps for ~20 pg 205Pb, sustainable for 3-4 hours of analysis (160-220 ratios
collected) and typical 206Pb/205Pb ratios were 0.1-3.0.
!
Uranium measurements (as UO2+) at both PU and Geneva were performed in
static mode on Faraday cups with 1012 ohm resistors using an oxide composition of 18O/
16O of 0.00205 (Nier 1950; Wasserburg et al. 1981). Mass fractionation of U was determined cycle-by-cycle, calculated from the deviation of measured 233U/235U from the
known tracer 233U/235U (0.995062 ± 0.000108 (2σ)) and assuming a 238U/235U of 187.818
± 0.045 (2σ) for sample U (Hiess et al. 2012). Typical U ion beams on 1012 ohm resistors at PU were 500-1000 mV of 233U for ~1 ng 233U at PU, resulting in measurement
precision of 0.004-0.02% (2 SE) for 270U/265U.
!
Data reduction was performed using the programs Tripoli and U-Pb_Redux
(McLean & Bowring 2011; Bowring & McLean 2011) and the U decay constants of Jaffey et al. (1971). All Pbc was attributed to laboratory blank with a mean isotopic composition determined by total procedural blank measurements, carried out separately for
each analyst. Fractionation correction and tracer blank subtraction were conducted during data reduction using the ET2535 v3.0 tracer composition for double-spiked samples
and ET535 v3.0 for single-spiked samples, defined as follows:
ET2535 v3.0
Composition
Value
±1s (abs)
ET535 v3.0
Value
±1s (abs)
202Pb/205Pb
0.99924
0.00027
N/A
N/A
204Pb/205Pb
0.000105
0.000009
0.00009
0.000009
206Pb/205Pb
0.00048
0.00017
0.00038874 0.00016886
207Pb/205Pb
0.00043
0.00014
0.00029607 0.00014014
208Pb/205Pb
0.00104
0.00033
0.00074438 0.00034704
233U/235U
0.995062
0.000054
0.99506218 0.00005384
238U/235U
0.00307993 4.00E-07
0.00307993 3.9555E-07
[205Pb] (mol/g)
1.03E-11
2.60E-14
1.03E-11
2.58E-14
[235U] (mol/g)
1.03E-09
2.60E-12
1.03E-09
2.60E-12
!
The effects of initial daughter product disequilibrium were corrected using the
methods of McLean & Bowring (2011), using measured Turkey Creek magma Th/U ratios from du Bray and Pallister (1991). Uncertainties in reported U-Pb zircon dates are
reported at 2-sigma (the 95% confidence interval) and exclude uncertainties in tracer
calibration and decay constants unless otherwise noted.
!
In addition to semi-weekly measurements of Pb standards NBS981 and 982 to
monitor Pb fractionation and ion counter deadtime, laboratory reproducibility and interlaboratory bias were assessed using the natural zircon standard AUS-Z2 (Kennedy et al.
2014). Data from PU measured at the time of this study are presented in (Schoene et al.
2015) and yield a statistically significant cluster in concordia space with a weightedmean 206Pb/238U CA-ID-TIMS age of 38.905 ± 0.013/0.017/0.045 Ma (MSWD = 1.0, n
= 9/10; 2σ uncertainties stated as internal/+tracer calibration/+decay constant). These
values are in good agreement with the published age of 38.8963 ± 0.0044/0.012/0.043
Ma (MSWD = 1.0, n = 12/12) from Geneva (Kennedy et al. 2014). As seen in Figure 2,
Turkey Creek samples analyzed at Princeton University and the University of Geneva
agree well, with no obvious systematic bias.
Zircon trace element geochemistry
!
The trace element compositions of the same zircon fragments dated by ID-TIMS
were characterized following the analytical protocol of Schoene et al. (2010) at PU.
Trace element washes isolated during U-Pb column chemistry were dried down in precleaned 2.0 ml polypropylene vials (CETAC #SP5540) and redissolved in 1.0 ml 1.5 M
HF + 0.1 M HNO3 + 1 ppb Ir. Measurements were performed on a Thermo Fisher
ELEMENT2 sector field-inductively coupled plasma-mass spectrometer (SF-ICP-MS)
with a sample introduction system consisting of a CETAC Aridus II desolvating nebulizer + ASX-100 autosampler, with an uptake rate of 100 µl/min. Measured elements
included Zr, Hf, Y, Nb, Ta, REEs, Pb, U, Th and Ir, with iridium monitored as an internal standard during mass spectrometry. The instrument was tuned in low resolution
mode with an optimal signal intensity of 0.5–2 Mcps on the peak height (not the integrated signal) for 1 ppb Ir.
!
A matrix-matched, gravimetric external calibration solution was prepared with
the relative abundance of targeted elements representing that observed in natural zircon
(e.g., Zr/Hf = 50). A dilution series was generated using this solution to cover the range
of concentrations observed in unknowns (e.g., [Zr] = 101–104 ppb solution), which was
then used to generate a concentration-intensity calibration curve for each trace element
at the beginning of the analytical session. Samples and interspersed instrumental and
total procedural blanks were analyzed in sets of 24 over ~3 h, with a line washtime of
120 s and uptake time of 90 s. Following data acquisition, solution concentrations were
converted to stoichiometric concentrations in zircon by normalizing solution concentration data assuming all trace elements partition into the Zr4+ site in ZrSiO4, where Σ
Zr+Hf+…+Th = 497,646 ppm. Since the elements that partition into the Zr4+ site are
well-known, this normalization provides an more accurate estimate of trace element con-
centration in zircon than would be obtained using the ratio of zircon mass to column
wash solution volume, which is difficult to accurately measure.
!
Trace element uncertainties are reported at the 95% confidence level and include
subtraction of the mean and standard deviation of blank measurements. Uncertainties
in ratios were calculated using standard uncertainty propagation techniques, assuming
uncorrelated uncertainties in measured elemental abundances. Trace element data from
eight analyses of the AUS-Z2 zircon standard measured using the same techniques (including chemical abrasion and column chemistry) are presented in Schoene et al. (2015).
These single-zircon analyses show good reproducibility as well as concentrations of Zr,
Hf, and the REE that agree with published laser ablation ICPMS data (Kennedy et al.
2014) well within the range of total external 2-sigma uncertainty.
5BCMF%35VSLFZ$SFFL61CJTPUPQJDEBUB
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Methods References
Bowring, J.F. & McLean, N.M., 2011. Engineering cyber infrastructure for U-Pb geochronology: Tripoli and U-Pb_Redux. Geochemistry Geophysics Geosystems,
12(6).
Gerstenberger, H. & Haase, G., 1997. A highly effective emitter substance for mass spectrometric Pb isotope ratio determinations. Chemical Geology, 136(3-4), pp.309–
312.
Hiess, J. et al., 2012. 238U/235U Systematics in Terrestrial Uranium-Bearing Minerals.
Science, 335(6076), pp.1610–1614.
Jaffey, A.H. et al., 1971. Precision Measurement of Half-Lives and Specific Activities of
235U and 238U. Physical Review C, 4(5), pp.1889–1906.
Kennedy, A.K. et al., 2014. Eocene zircon reference material for microanalysis of U-ThPb isotopes and trace elements. The Canadian Mineralogist, 52, pp.409–421.
Krogh, T.E., 1973. A low-contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determinations. Geochimica et
Cosmochimica Acta, 37, pp.485–494.
Mattinson, J., 2005. Zircon U-Pb chemical abrasion. Chemical Geology.
McLean, N.M. & Bowring, J.F., 2011. An algorithm for U-Pb isotope dilution data reduction and uncertainty propagation - McLean - 2011 - Geochemistry, Geophysics, Geosystems - Wiley Online Library. Geochemistry Geophysics Geosystems.
Mundil, R. et al., 2004. Age and timing of the Permian mass extinctions: U/Pb dating
of closed-system zircons. Science.
Nier, A.O., 1950. A Redeterrnination of the Relative Abundances of the Isotopes of
Carbon, Nitrogen, Oxygen, Argon, and Potassium. Physical Review, 77(6),
pp.789–793.
Schoene, B. et al., 2010. A new method integrating high-precision U–Pb geochronology
with zircon trace element analysis (U–Pb TIMS-TEA). Geochimica et Cosmochimica Acta, 74(24), pp.7144–7159.
Schoene, B. et al., 2015. U-Pb geochronology of the Deccan Traps and relation to the
end-Cretaceous mass extinction. Science, 347(6218), pp.182–184.
Wasserburg, G.J., Jacousen, S.B. & DePaolo, D.J., 1981. Precise determination of Sm/
Nd ratios, Sm and Nd isotopic abundances in standard solutions. Geochimica et
Cosmochimica Acta, 45, pp.2311–2323.
15000
13000
Hf 11000
9000
7000
0.0
0.1
0.2
0.3
0.4
0.5
Eu/Eu*
15000
13000
Hf 11000
9000
7000
10
15
Y/Dy
20
25
Supplementary Figure.
Filled symbols repesent zircons from the Rhyolite Canyon Tuff considered xenocrystic because they are compositionally distinct. These grains also have a range in ages that overlap with pre-Turkey Creek caldera rocks (shown below)
that are from geographically local lithologies. Open symbols that are compositionally similar to the ‘xenocrystic’
zircons did not yield usable dates (e..g Pb loss) and are, therefore, not included in the discussion. Symbols are the same
as Figure 3.
Pre-Turkey Creek Caldera rocks (40Ar/39Ar)
Halfmoon Valley granodiorite
74.6±0.06 Ma
High Lonesome Canyon rhyolite tuff
Joe Glenn Ranch rhyolite
Rucker Canyon rhyolite lava
Rucker Canyon pyroclastic rocks
Mackey Canyon granodiorite
34.16±0.17 Ma
33.81±0.08 Ma
33.32±0.07 Ma
33.21±0.09 Ma
30.62±0.15 Ma
Xenocrystic zircon Rhyolite Canyon tuff ages (CA-TIMS)
Single zircon from intracaldera unit
85.16 Ma
Range of most zircons
30.78 to 34.2 Ma
All Ar-Ar age data are from: du Bray, E. A., Snee, L.W., and Pallister, J. S., 2004, Geochemistry and Geochronlogy of Middle Tertiary Volcanic
Rocks of the Central Chiricahua Mountains, SE Arizona: US Geological Survey Professional Paper 1684: 1-57.