SUPPLEMENTARY MATERIAL
Here we present: (1) cathodoluminescence (CL) images, 89Y+/94Zr+ ion images, and line profiles
of Ti, P, Y, Ce, and Hf concentrations for the remaining zircons analyzed as part of this study
(YTT8, Fig. S1; Q3, Fig. S2; Q8, Fig. S3; SN11, Fig. S4; ADK29, Fig. S5; and ADK9, Fig. S6);
(2) a more detailed discussion of the pressure and temperature estimates assigned to each of the
samples; (3) our synthetic Ti line profiles that allow us to evaluate the potential effects of
smoothing on the natural images (e.g., decreasing peak heights and the broadening of peak
widths); (4) a discussion of titania activities (aTiO2) focusing on the differences between
rhyolite-MELTS
aTiO2
tmt-rhm
and a TiO2
values as calculated using coexisting glasses and titanomagnetite-
ilmenite compositions from low-pressure experiments on silica-rich compositions (Martel et al.
1999; Parat et al. 2008; Scaillet and Evans 1999; Tomiya et al. 2010), and how titania activities
evolve in experimental high-silica liquids with decreasing temperature (Costa et al. 2004; Martel
et al. 1999; Parat et al. 2008; Rutherford and Devine 2003; Scaillet and Evans 1999; Tomiya et
al. 2010); and (5) a discussion of the data set used in our fit of equation (4) in the main text: ln(Ti
ppm)zrc + ln(aSiO2) – ln(aTiO2) vs. A1 + A2/T(K) + A3(P – 1)/T(K).
Line profiles, CL images, and 89Y+/94Zr+ ion maps for the remaining samples
Data for the remaining samples analyzed as part of this study (YTT8, Q3, Q8, SN11, ADK29,
and ADK9) are presented in Figs. S1—S6. Since these samples are described in the main text,
we do not duplicate that discussion here.
Sample descriptions and petrologic estimates of P and T
Youngest Toba Tuff
The ~74 ka Youngest Toba Tuff (abbreviated YTT here) eruption produced ~2800 km3 of tuff
and ash-fall deposits; the quartz-bearing silicic pumices range from 68–77 wt% SiO2 (Chesner
1998). Major crystalline phases are quartz, plagioclase, sanidine, biotite, and amphibole, and
1
accessory minerals include titanomagnetite, allanite, zircon (zrc), ilmenite, apatite, and
orthopyroxene as well as rare occurrences of highly oxidized fayalite (Chesner 1998). Matrix
glass and quartz-hosted melt inclusions have silica contents (on a volatile-free basis) of 75.7 to
78.3 wt%, mean = 77.0 ± 0.6 wt%, 1, n = 30 (Chesner 1988; Chesner and Luhr 2010); Fourier
Transform Infrared spectrometry (FTIR) measurements of the melt inclusions indicate water
contents of 4.0–5.5 wt% (Chesner and Luhr 2010). The two zircons described in this study
(YTT11 and YTT8) are from sample 87, a welded tuff with ~72.4 wt% SiO2, anhydrous; the
matrix glass from this sample has 77.2 wt% silica (Chesner 1988) and its composition overlaps
with that of the average of all melt inclusions and matrix glasses at the 1 sigma level.
Experimental results of Gardner et al. (2002) suggest that the Youngest Toba Tuff
magma was nearly water-saturated at pressures of 1 to 1.5 kbar. For the high- and low-silica
rhyolites used as starting materials in that study, quartz appearance temperatures at 1.5 kbar vary
from ~720 to 770°C. If the quartz-hosted melt inclusions were vapor-saturated when entrapped,
their dissolved water contents suggest pressures of 1.1 to 1.4 kbar (Chesner and Luhr 2010).
Applying the thermobarometric expressions of Ridolfi et al. (2010) and Ridolfi and Renzulli
(2012) to amphiboles in the Youngest Toba Tuff non-welded and welded pumices yields
pressures of ~1.6 kbar [note, however, that Gardner et al. (2002) argue that amphibole is not an
equilibrium phase in the Youngest Toba Tuff magma chamber]. Temperature estimates for nonoxidized coexisting Fe-Ti oxides that pass the Mg-Mn partitioning “test” (Bacon and
Hirschmann 1988) range from 660–735°C [using the model of Sauerzape et al. (2008)] and 642–
753°C [using the model of Ghiorso and Evans (2008)]. Both models yield mean temperatures of
~681°C (± either 19 or 27°C, 1). It seems likely that the generally low Fe-Ti oxide
temperatures reflect varying extents of post-eruptive re-equilibration since at 1.5 kbar the watersaturated solidus in the system quartz-albite-orthoclase is close to ~695°C (Johannes and Holtz
1996) and, barring relatively high concentrations of Cl and F (not observed in the melt
inclusions), the water-saturated solidus temperature in this simple system establishes, for a given
pressure, a lower temperature limit for most granitic and rhyolitic melts. Post-eruptive re2
equilibration/alteration is further supported by the fact that over 76% of the 51 paired
titanomagnetite-ilmenite compositions reported in Chesner (1988) appear to be out of Mg-Mn
equilibrium. Zr contents (by SIMS) in the matrix glasses and in the quartz-hosted melt
inclusions (Chesner and Luhr 2010) yield zircon-saturation temperatures (Tzrc-sat, Boehnke et al.
2013) of 667–709°C with an average of 684 ± 15°C (1). Tzrc-sat values calculated for matrix
glasses (analyzed by XRF, Chesner 1988) range from 666–742°C (mean and 1 are 686 ± 18°C).
Bulk rock compositions with silica contents between 75.7 to 78.3 wt% (anhydrous, i.e., the same
range as the matrix glasses and melt inclusions) have zircon-saturation temperatures of 679 to
735 (mean and 1 are 707 ± 11°C). From these overlapping constraints, we infer that near-rim
zircon crystallization occurred at temperatures of 690–730°C and a pressure of ~1.5 kbar.
Bishop Tuff
The ~780 ka Long Valley eruption (Simon et al. 2014), which produced the Bishop Tuff (BT),
released >600 km3 of compositionally zoned rhyolitic magma (for an alternative view concerning
the compositional zoning, see Gualda and Ghiorso 2013). Our zircon sample (BT2-10) is from
the ignimbrite package Ig2E [referred to as “Tableland” in Hildreth (1979)]; rocks comprising
Ig2E contain quartz, sanidine, plagioclase, biotite, augite, and orthopyroxene and accessory
titanomagnetite, ilmenite, allanite, apatite, zircon, and pyrrhotite (Hildreth 1979; Hildreth and
Wilson 2007). Average silica content of the Ig2E ignimbrites is 76.8 wt% (Hildreth and Wilson
2007); including the compositions of the stratigraphically associated airfall deposits (F9) has
little effect on this average composition. The quenched glasses in quartz-hosted melt inclusions
from the Ig2E ignimbrites and F9 deposits contain, on a volatile-free basis, ~77.8 wt% SiO2
(Anderson et al. 2000; Dunbar and Hervig 1992; Wallace et al. 1999) and ~5.7 wt% H2O
(Wallace et al. 1999). The presence of vapor bubbles in the early-erupted Bishop Tuff air-fall
deposits indicate that at least the upper portions of the magma chamber were vapor-saturated
(Gualda and Anderson 2007). Volatile contents in quartz-hosted melt inclusions in coeval rocks
that comprise the Ig2E ignimbrites and F9 fall deposits suggest pressures of ~1.93 ± 0.13 kbar,
3
1 (Wallace et al. 1999). This pressure is consistent with estimates based on projected phase
equilibria and rhyolite-MELTS calculations (Gualda and Ghiorso 2013). Calculated
temperatures from coexisting Fe-Ti oxides in the Ig2E package (Hildreth 1977; Hildreth and
Wilson 2007), all satisfying Mg-Mn constraints (Bacon and Hirschmann 1988), range from 697–
803°C (Sauerzape et al. 2008) and 688–832°C (Ghiorso and Evans 2008); mean temperatures for
the two models are 739 ± 22°C and 749 ± 32°C (1), respectively. However, recent work by
Ghiorso and Gualda (2013) suggests that, on the basis of the positive correlation between
calculated titanomagnetite-ilmenite temperatures and TiO2 activities, the Bishop Tuff Fe-Ti
oxide pairs do not reflect pre-eruptive conditions—a question we return to below.
Although differing in detail, preliminary experimental results on several Bishop Tuff
samples at 2 kbar and varying PH2O (Johnson et al. 1994; Scaillet and Hildreth 2001) point to
crystallization temperatures of ~680–730°C for the major silicate and oxide phases. Some
fraction of this temperature variation must reflect differences in starting composition and PH2O;
unfortunately, detailed results of both studies are unpublished and thus it is impossible to
evaluate the significance of these temperature differences. The Zr contents in the quartz-hosted
melt inclusions from the Ig2E and the F9 samples (Anderson et al. 2000; Dunbar and Hervig
1992; Wallace et al. 1999) lead to calculated zircon-saturation temperatures (Boehnke et al.
2013) of 645 to 709°C; mean is 688°C (± 18°C, 1). Calculated mean Tzrc-sat for whole-rock
pumice and ash-fall deposits (Ig2E and F9) is 695 ± 16°C (range = 666–759°C; 97% of the Tzrc-sat
values are <730°C). Note that the two whole-rock samples that yield the highest zirconsaturation temperatures (741 and 759°C) do not have especially high Zr contents (107 and 112
ppm; range: 71–140 ppm). The high calculated temperatures reflect the high Al2O3 and low
Na2O contents of these two samples (relative to the remaining 120 Ig2E and F9 pumices and airfall deposits) that lead to low values for the compositional parameter in the zircon-saturation
thermometer (Boehnke et al. 2013). [The sample with 140 ppm has a calculated Tzrc-sat of 736°C.]
Given these various constraints, we assume that the near-rim regions of the zircons in the Ig2E
4
ignimbrites grew at a pressure of 1.93 kbar and at a temperature somewhere in the range of 685–
725°C.
Plutonic rocks
Three zircons from a quartz diorite in the ~65–56 Ma Quottoon Igneous Complex (also known as
the Quottoon pluton), British Columbia (e.g., Rusmore et al. 2005; Thomas and Sinha 1999)
were analyzed as part of this study. The Quottoon Igneous Complex intrudes the upper
amphibolite facies gneisses of the Central Gneiss Complex (e.g., Crawford et al. 1987) and, in
order of decreasing abundance, consists of plagioclase, biotite, hornblende, quartz, and Kfeldspar (Thomas and Sinha 1999). In addition to zircon, other minor/trace phases are apatite,
titanite, titanomagnetite, ilmenite, and epidote (which crystal morphology suggests is magmatic).
Based on amphibole rim compositions, crystallization pressures were 6–7 kbar (Rusmore et al.
2005). This pressure range is consistent with pressure estimates of 6–8 kbar for migmatized
gneisses 2 km from the diffuse southwestern boundary of the complex (Lappin and Hollister
1980). Further, if the epidote is in fact magmatic, this would place a lower limit of ~5 kbar on
the pressure of crystallization (Schmidt and Poli 2004).
Temperature constraints on the zircon crystallization interval are more problematic—
based on textural relationships, Thomas and Sinha (1999) and Thomas et al. (2002) suggest that
zircon began crystallizing before amphibole. Thus, for a quartz-diorite composition, the
appearance temperature of amphibole and 6–7 kbar solidus temperature would provide a
minimum crystallization interval for zircon. However, for water-rich bulk compositions this
interval can be ~100–300°C, [the temperature interval between the appearance of amphibole and
the solidus, Bogaerts et al. (2006); Conrad et al. (1988); Johnson and Rutherford (1989);
Lambert and Wyllie (1974); Scaillet and Evans (1999)]. For example, for tonalite/andesite
compositions (~60 wt% SiO2) at 6.5 kbar (PH2O = Ptotal), the temperature interval between the
appearance of amphibole and the solidus is ~290°C [~945 to 655°C, Lambert and Wyllie
(1974)]. However, zircons from the Quottoon contain melt inclusions, and the high silica
5
content of the experimentally reheated and homogenized melt inclusion glasses [75.9–79.2
volatile-free, Thomas et al. (2002)] suggests temperatures much closer to the solidus. While the
tonalite/andesite compositions used by Lambert and Wyllie (1974) to construct their P-T phase
diagram are similar to the average of the Quottoon Igneous Complex whole rock compositions
reported by Thomas and Sinha (1999), it is unlikely that the Quottoon magmas were watersaturated. An estimated water content of ~6–7 wt% can be inferred from the average oxide sum
of the zircon-hosted melt inclusions (Thomas et al. 2002). Based on the solubility model of Liu
et al. (2005), a water-saturated silica-rich melt would be expected to have between 9.9 and 10.5
wt% H2O at 6.5 kbar and temperatures of 655–706°C [i.e., from the solidus (Ts) to the
disappearance of quartz (Tqtz)]. If we assume that the high-silica zircon-hosted melt inclusions
reflect quartz-saturated residual liquids, then Tqtz provides a potential upper temperature limit on
zircon crystallization. We have used rhyolite-MELTS [code release 1.01; Gualda et al. (2012)]
to estimate the increase in quartz appearance temperature with decreasing water content for the
Lambert and Wyllie (1974) tonalite composition [coupling the calculations to the experimental
phase diagram allows us to “correct” for any temperature offsets in rhyolite-MELTS, e.g.,
Thomas and Watson (2012)]. These calculations suggest that during equilibrium crystallization
of the Lambert and Wyllie (1974) tonalite composition, residual liquids with ~6 wt% H2O
become quartz-saturated at ~745°C (compared to between 700–710°C at PH2O of 6.5 kbar). The
solidus in the quartz-albite-orthoclase system (Johannes and Holtz 1996) provides a lower
fl
temperature bound on zircon crystallization. At fluid mole fractions ( X H2O ) of 0.521 and 0.645
[corresponding to liquid water contents of 6 and 7 wt%, Liu et al. (2005)], solidus temperatures
in this simple system are 712 and 681°C, respectively. These constraints suggest a
crystallization interval of ~690–750°C for zircons with these silica-rich melt inclusions.
Although the zircons included in this study do not contain any obvious melt inclusions, we
assume that they crystallized under similar P-T conditions.
A zircon from a quartz diorite from the eastern Tehachapi Mountains, an expression of
the southernmost Sierra Nevada batholith, California, was also included in this study. The
6
Sierran zircon is from a hypersthene-hornblende quartz diorite with accessory alkali feldspar and
biotite and, although this particular sample has not been studied in detail, it is from a mapped
unit where other samples have yielded pressure estimates of ~8 kbar (J. Saleeby, personal
communication). Unfortunately temperature information is lacking and thus, although this
zircon provides another example of fine-scale minor- and trace-element zoning, we do not
discuss it in the context of the Ti-in-zircon geothermometer.
Adirondack Mountains
Two zircons were analyzed from a migmatitic metapelite unit from the southern Adirondack,
New York [zircons from this locality were also analyzed by Watson et al. (2006) and included in
the revised Ti-in-zircon geothermometer of Ferry and Watson (2007)]. In addition to quartz and
rutile, the migmatite contains two feldspars, garnet, biotite, and sillimanite. The bulk silica
content of the leucosome from which the zircons were extracted is ~76.3 wt% (anhydrous);
reaction textures suggest that melting involved the breakdown of biotite. Peak pressure and
temperature estimates for a number of migmatitic localities from the southern Adirondacks
(including the one that yielded the zircons studied here) range from ~7–9 kbar and 790–820°C
(Storm and Spear 2005); a lower temperature bound on zircon growth is provided by an
estimated solidus temperature of ~740°C (Storm and Spear 2005). The titanium content of
quartz from rutile-saturated southern Adirondack migmatites suggest peak temperatures of
≥~800°C (Storm and Spear 2009), consistent with those from garnet-biotite equilibria discussed
above [note however, that the calibration of the Ti-in-quartz thermometer is currently a topic of
active debate, Huang and Audétat (2012); Thomas and Watson (2012); Thomas et al. (2010);
Wilson et al. (2012)]. Taken together, these constraints suggest P-T crystallization conditions for
these two zircons of 7–9 kbar and 740–790°C [note that we have essentially followed Ferry and
Watson (2007) in our choice of pressures and temperatures].
Synthetic titanium maps and the effects of smoothing on Ti line profiles
7
As discussed in the main text, two smoothing procedures were applied to the raw 256256 pixel
NanoSIMS ion maps. Although we focus on the Ti maps here, all the ion images were smoothed
using the two steps described in the main text and below. First, after the ion counts (Ti, P, Y, Ce,
and Hf) in each pixel had been divided by the 94Zr+ counts in the same pixel, a 55 movingaverage window was applied to the ion image. Second, after selecting a location for a line
profile (perpendicular to oscillatory zoning as observed in the CL image) the line profile was
divided into n rectangular integration regions, 3–5 pixels wide by 20–250 pixels long with the
long axis of each region perpendicular to the line profile (e.g., see Fig. S1c), and the normalized
ion counts in each integration region were averaged and plotted at the mid-point of the
intersection of the integration rectangle and the line profile. To investigate the effects that these
two smoothing steps might have on peak widths and heights, we constructed a series of synthetic
256256 pixel maps in which the Ti and Zr counts in each pixel were selected so as to generate a
background of 2 ppm Ti (based on a typical NanoSIMS 49Ti+/94Zr+ vs. Ti ppm calibration line) as
well as oscillations of variable widths and heights perpendicular to the x-axis (the Ti oscillations
were designed as step functions with widths of ~0.6–6.2 m and amplitudes of 2–6 ppm). Figure
S7a shows an example of a 20-m line profile that represents the starting point of one of our
synthetic images: there are six peaks with widths and amplitudes between 0.62–2.0 m and 2–4
ppm, respectively (note that the widths and amplitudes of the Ti peaks in our synthetic line
profiles are comparable to those seen in the natural samples; see Table 2, main text). “Noise”
was then added to the Ti and Zr counts in each pixel using a Poisson distribution, and a new
49
Ti+/94Zr+ ratio was calculated and then converted to Ti ppm using the calibration curve
discussed above. One line of pixels perpendicular to the Ti oscillations is shown in Fig. S7b—
note the change in the y-axis scale between panels (a) and (b) and the fact that the peaks in (a)
are now largely obscured by noise. Figure S7c shows that applying the 55 moving-average
window substantially reduces the noise and that the six peaks in panel (a) are again clearly
discernable although peak heights have in three cases increased by up to ~30% (peak #3 has
increased from 6 to 8 ppm). However, following the integration (in this case using rectangles of
8
341 pixels), peak heights are within 8% of the starting values, and the widths (measured at halfmaximum) are also within 8% of the original values. Using a wider integration rectangle (541
pixels) yields smoothed peaks with heights and widths (at half maximum) within ~10% of the
starting values. We conclude that our two smoothing steps are unlikely to substantially modify
peak heights and widths in the natural samples.
Using rhyolite-MELTS to estimate silica and titania activities
Although all of the zircons imaged as part of this study co-exist with quartz and thus probably
crystallized from a quartz-saturated melt (i.e., aSiO2 was equal to 1), it is instructive to investigate
how rapidly aSiO2 decreases as temperature rises above that of quartz-saturation. Based on
rhyolite-MELTS calculations (Gualda et al. 2012) using whole rock compositions from the
Bishop and Youngest Toba tuffs and the Quottoon Igneous Complex and assuming 2–4 wt%
bulk H2O (leading to > 4 wt% H2O in the melt at quartz-saturation), aSiO2 values within 50°C of
the quartz-appearance temperature are always > 0.9. Given that calculated zircon saturation
temperatures for the average YTT and BT glass compositions are below the experimentally
determined appearance temperatures of quartz, it seems unlikely that zircon rim crystallization
temperatures were more than 50°C above quartz appearance temperatures for any of the YTT
and BT zircons and, by extension, for the Quottoon and Adirondack samples as well. These
calculations suggest that aSiO2 = 1 is a reasonable assumption for all of our samples.
As discussed in the main text, coexisting Fe-Ti oxides have been used to estimate
titanium activities (at magmatic temperatures) of the quenched melt compositions of the Bishop
Tuff (Reid et al. 2011; Wark et al. 2007). However, Ghiorso and Gualda (2013) argue that the
positive correlation between temperature and aTiO2 calculated using co-existing titanomagnetites
(tmt) and ilmenites (rhm) from the Bishop Tuff indicate that the compositions of these two
phases do not reflect conditions within the magma chamber [for an alternative view, see Evans
and Bachmann (2013)]. Following Ghiorso and Gualda (2011, 2013), Thomas and Watson
(2012) have suggested that rhyolite-MELTS (Gualda et al. 2012) may provide a better estimate
9
of aTiO2 for the silica-rich glasses that comprise the Bishop Tuff. These two approaches yield
very different estimates for aTiO2: 0.55 ± 0.04 (1) based on Fe-Ti oxide pairs in ignimbrites
from Ig2E (Hildreth 1977; Hildreth and Wilson 2007) calculated using version 1.1 of
FeTiOxideGeotherm (Ghiorso and Gualda 2013) vs. 0.24 based on a rhyolite-MELTS
calculation using the average of quartz-hosted melt inclusions (Table 1) from ignimbrites Ig2E
and fall deposit F9 (Wallace et al. 1999), a temperature of 726°C (Thomas and Watson 2012),
and a pressure of 2 kbar. Although the Fe-Ti oxides yield an average calculated temperature of
749°C, ~20°C higher than that used in the rhyolite-MELTS calculation, using the Bishop Tufftmt-rhm
based ln( a TiO2 ) vs. 1/T(K) expression discussed below [and presented in Table 1 (main text)]
and a temperature of 726°C yields a aTiO2 of 0.52—still substantially above aTiO2 calculated using
rhyolite-MELTS. Note that in the calculation of aTiO2 using rhyolite-MELTS, we “turn off” all
phases except melt and rutile, i.e., the phase that defines the activity of TiO2. This approach is
necessary because the program calculates that the average Ig2E and F9 Bishop Tuff melt
inclusion composition reported in Table 1 (main text) is > 90% crystalline at ~726°C and 2 kbar.
However, phase relations of Bishop Tuff whole rock compositions (Scaillet and Hildreth 2001)
strongly suggest that the average melt inclusion composition would be liquid at this T and P,
which in turn suggests a systematic temperature offset in rhyolite-MELTS [see also Thomas and
Watson (2012)]. Co-existing Fe-Ti oxides from the Youngest Toba Tuff also yield a positive
correlation between temperature and aTiO2, and any titanomagnetite-ilmenite pairs from the
Quottoon Igneous Complex would not reflect magmatic conditions (due to slow subsolidus
cooling in this plutonic environment), further suggesting that rhyolite-MELTS may well be the
required approach for estimating aTiO2 for the zircons in each of these suites of samples. This,
then, is the approach we have taken: the equations associated with option (A) in Table 1 (main
text) are based on rhyolite-MELTS calculations using the average melt inclusion compositions
and their associated range of petrologically constrained temperatures. As discussed above, these
calculations were done by “turning off” all phases in rhyolite-MELTS except melt and rutile and
thus the liquid compositions are held constant. Below we compare titania activities calculated
10
using experimental glass compositions and rhyolite-MELTS with those calculated from
coexisting Fe-Ti oxides using FeTiOxideGeotherm in order to evaluate any potential differences
between these two approaches.
We have compiled a suite of 15 hydrous silica-rich experimental liquids that co-exist
with titanomagnetite and ilmenite (Martel et al. 1999; Parat et al. 2008; Scaillet and Evans 1999;
Tomiya et al. 2010); run conditions were ~900–750°C and ~2–4 kbar and the silica and water
contents of the experimental glasses were 64–72 and 4.8–7.8 wt%, respectively [all experiments
were mass balanced and liquid alkali contents adjusted using the approach discussed in Hofmann
et al. (2013)]. Fe-Ti oxide temperatures (Ttmt-rhm) and oxygen fugacities (ƒO2tmt-rhm) calculated
using FeTiOxideGeotherm are in broad agreement with the experimental run temperatures and
tmt-rhm
ƒO2s, and calculated a TiO2
values range from 0.43 to 0.88. Using the calculated Ttmt-rhm and
ƒO2tmt-rhm values for each run and a fixed pressure of 2 kbar, we calculated titanium activities
using the experimental glass compositions and rhyolite-MELTS (as discussed above, the
calculations were done with all phases except rutile and liquid “turned off”). By using Ttmt-rhm
and ƒO2tmt-rhm values (and a fixed P of 2 kbar) instead of the experimental temperature, pressure,
tmt-rhm
and ƒO2 values in the rhyolite-MELTS calculations, both a TiO2
rhyolite-MELTS
same T, P, and ƒO2 conditions. The a TiO2
rhyolite-MELTS
and a TiO2
reflect the
values ranged from 0.21–0.78;
rhyolite-MELTS
tmt-rhm
values (RaTiO2) are plotted against wt% TiO2 in the experimental glasses in
aTiO2 / a TiO2
Fig. S8. For reference, we have also plotted the average TiO2 contents in the BT, YTT, and Q
melt inclusions. Note that RaTiO2 shows a weak but statistically significant (at the 95%
confidence level) inverse correlation with TiO2 content. Although RaTiO2 is almost certainly a
function of composition—and possibly of temperature and ƒO2 as well, given the scatter in Fig.
S8, we have chosen to simply average the RaTiO2 values to generate an approximate “correction”
rhyolite-MELTS
factor of 1.6 that may bring a TiO2
tmt-rhm
values more closely in sync with those of a TiO2 . In
Table 1 (main text), the option (A) equations describing aTiO2 in the YTT, BT, and Q average
melt inclusion compositions as functions of temperature are based on rhyolite-MELTS
calculations. We have applied the “correction factor” to these three sets of aTiO2 values to
11
generate a second set of titanium activities and have also fit them as a function of temperature.
This second set of equations is listed as option (B) in Table 1.
In option (C) in Table 1, we assumed that aTiO2 was independent of temperature, that it
had an uncertainty of ± 0.05, and that this uncertainty had a flat distribution. As we discuss in
the main text, for the Youngest Toba Tuff and the Bishop Tuff, the choice of the values in option
(C) are broadly based on those in option (B) and, if available, TiO2 activities calculated from FeTi oxides. For the Quottoon Igneous Complex, option (B) would have lead to an aTiO2 value for
(C) that was close to 1 (see Table 1 in the main text). Because we wanted to explore how a
lower, but still petrologically reasonable value for aTiO2 would affect calculated Ti-in-zircon
temperatures for the Quottoon, we selected a value of 0.65—one that is consistent with estimated
TiO2 activities for silica-rich magmas (Hayden and Watson 2007). Assuming that aTiO2 is
independent of temperature is equivalent to assuming that the activity of TiO2 remains relatively
constant in a high-silica melt saturated in one or both Fe-Ti oxides as the melt crystallizes at
rhyolite-MELTS
temperatures near its solidus. Figure S9 shows a TiO2
values calculated for silica-rich
experimental liquids at the T, P, and ƒO2 of each experiment (in these experiments Fe-Ti oxide
tmt-rhm
compositions were not reported and so a TiO2
rhyolite-MELTS
aTiO2
values could not be calculated. Although the
values calculated for the titanomagnetite-saturated melts of Costa et al. (2004) do
increase with decreasing temperature, in the remaining five experimental studies calculated
titanium activities for liquids that are saturated with either one or two Fe-Ti oxides appear to be
roughly independent of temperature—in the study of Tomiya et al. (2010), there is a suggestion
rhyolite-MELTS
that a TiO2
decreases when titanomagnetite joins ilmenite (Fig. S9d). Unfortunately, the
rhyolite-MELTS
uncertainties associated with the a TiO2
values can be large due to the relatively large
uncertainties reported for the liquid TiO2 contents (uncertainties associated with aTiO2 were
calculated by varying the TiO2 content of each experimental liquid by ± twice its reported
rhyolite-MELTS
uncertainty and recalculating a TiO2
while holding all other oxide concentrations constant).
A majority of the trends in Fig. S9 appear to support the assumption inherent in option (C) and
suggest that, if the zircons analyzed in this study grew from liquids that were evolving over the
12
stated ranges of temperatures in Table 1, then aTiO2 may well have remained approximately
constant.
For the Bishop Tuff, we constructed option (D) by taking the linear relationship between
tmt-rhm
ln( a TiO2
tmt-rhm
) and 1/T(K) at face value and generating an unweighted least-squares fit to the a TiO2
values and temperatures calculated using the coexisting titanomagnetites and ilmenites
compositions from the Bishop Tuff from Hildreth (1977) and Hildreth and Wilson (2007) using
FeTiOxideGeotherm (Ghiorso and Gualda 2013). Although the positive correlation between
tmt-rhm
a TiO2
and temperature may not have petrologic significance for the Bishop Tuff magma
chamber (as discussed above), it nevertheless seemed worthwhile to explore the effect that such
a positive relationship might have on calculated Ti-in-zircon temperatures.
Fitting the Ti-in-zircon thermometer with both T- and P-terms
Based on the uncertainty surrounding the appropriate titania activity for the Bishop Tuff,
we used only experiments and natural samples that are demonstrably rutile-saturated in our
calibration of equation (4) in the main text: runs 73, 71, 58, 65, 66, 63, and the two natural
samples ADK and SC all from Watson et al. (2006) and run 1300-SF-2.1 from Tailby et al.
(2011). The silica activities of all samples were referenced to -quartz at the temperature and
pressure of each experimental or natural sample using Berman (1988). Silica activities of the 10
kbar silica-undersaturated Watson et al. experiments (73, 71, 58, and 65) were calculated using
the Zr content of the coexisting rutiles and the 10 kbar Zr-in-rutile calibration line of Hofmann et
al. (2013); note that Watson et al. (2006) did not report a Zr concentration for rutile for run 101,
and run 57 did not contain rutile; therefore, we did not use these two experiments here. To
determine the silica activity of run 63 [1250°C and 12 kbar, Watson et al. (2006)], we adjusted
the Hofmann et al. Zr-in-rutile expression to 12 kbar using the 10 and 20 kbar experimental data
from Tomkins et al. (2007) to calculate the effect of pressure on the Zr-in-rutile line at 1250°C.
We initially included the Stillup Tal (ST) zircon composition and its estimated temperature and
pressure [all from Watson et al. (2006)] in the fit to equation (4) because the sample was reported
13
to be both rutile- and quartz-saturated. However, when included in the least-squares fit, the
deviation for this sample was extremely large (i.e., >40%). Zircons from this locality are from a
ductile shear zone that has experienced substantial fluid flow and an associated net loss of >40%
silica (Selverstone et al. 1991). As described by Selverstone et al. (1991) several of the samples
from the zone with abundant zircons “consist entirely of fine mats of mica with no free quartz.”
This observation raises the question as to whether the zircons in the Stillup Tal sample actually
grew under quartz-saturated conditions. In fact, using a silica activity of 0.66 (rather than 1)
moves the Stillup Tal datum directly onto the best-fit regression line. Given the uncertainty
surrounding the appropriate aSiO2 value associated with zircon growth in the ST, we have chosen
not to include the ST sample in the regression.
14
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18
Fig. S1. Toba zircon YTT8. (a) Trace-element distributions as a function of distance along the line profile
shown in panel (c). (b) CL image of region mapped (outlined in red). (c) 89Y+/94Zr+ ion map of the 20 ×
20 μm area outlined in the CL image; black line across bands indicates location of line profile. The
alternating gray rectangles indicate the integration areas (390 nm × 3.9 μm) for counts assigned to each
profile point.
19
Fig. S2. Quottoon zircon Q3. (a) Trace-element distributions as a function of distance along the line
profile shown in panel (c). (b) CL image of region mapped (outlined in red). (c) 89Y+/94Zr+ ion map of
the 20 × 20 μm area outlined in the CL image; black line across bands indicates location of line profile.
The alternating gray rectangles indicate the integration areas (310 nm × 9.4 μm) for counts assigned to
each profile point.
20
Fig. S3. Quottoon zircon Q8. (a) Trace-element distributions as a function of distance along the line
profile shown in panel (c); P line profile not shown due to mass-calibration errors during the NanoSIMS
analytical session. (b) CL image of region mapped (outlined in red). (c) 89Y+/94Zr+ ion map of the 20 ×
20 μm area outlined in the CL image; black line across bands indicates location of line profile. The
alternating gray rectangles indicate the integration areas (390 nm × 7.8 μm) for counts assigned to each
profile point.
21
Fig. S4. Sierran zircon 93TH234.11 (“SN11”). (a) Trace-element distributions as a function of distance
along the line profile shown in panel (c). (b) CL image of region mapped (outlined in red). (c) 89Y+/94Zr+
ion map of the 20 × 20 μm area outlined in the CL image; black line across bands indicates location of
line profile. The alternating gray rectangles indicate the integration areas (235 nm × 9.4 μm) for counts
assigned to each profile point.
22
Fig. S5. Adirondack zircon ADK29. (a) Trace-element distributions as a function of distance along the
line profile shown in panel (c). (b) CL image of region mapped (outlined in red). (c) 89Y+/94Zr+ ion map
of the 20 × 20 μm area outlined in the CL image; black line across bands indicates location of line profile.
The alternating gray rectangles indicate the integration areas (390 nm × 19.5 μm) for counts assigned to
each profile point.
23
Fig. S6. Adirondack zircon ADK9. (a) CL image; region mapped is outlined in red. Panels (b), (c), (d),
(e), and (f) show Y, Ti, P, Ce, and Hf intensity maps with concentrations given by color scales to the right
of each panel.
24
Fig. S7. Line profiles based on synthetic maps of Ti and Zr counts. (a) Represents one row of pixels from
the initial maps (ratios of Ti/Zr counts converted to Ti ppm using a representative calibration line); (b)
same row as in (a) but now after application of Poisson “noise”; (c) same row after application of the 55
moving average window; (d) same row but now with average values from the 3-pixel-wide and 41-pixellong integration regions projected onto the row (see text for more information).
25
Fig. S8. Ratio of titanium activities calculated using titanomagnetite (tmt)-ilmenite (rhm) pairs and
rhyolite-MELTS versus the TiO2 content of the coexisting experimental liquid (Martel, Pichavant, Holtz,
Scaillet, Bourdier and Traineau 1999; Parat, Holtz and Feig 2008; Scaillet and Evans 1999; Tomiya,
Takahashi, Furukawa and Suzuki 2010). The vertical gray bars denote the range of TiO2 contents in the
Bishop and Youngest Toba Tuff quartz-hosted melt inclusions and in the zircon-hosted melt inclusions
from the Quottoon Igneous Complex (Table 1, main text). See Supplementary text for further discussion.
26
rhyolite-MELTS
Fig. S9. a TiO2
values calculated at the T, P, and ƒO2 of each experimental liquid. Phase
assemblages are given in the legends and the range of liquid SiO2 contents (wt%, anhydrous) are reported
in each panel. In panels (a) and (b), solid lines show the effect of temperature on titanium activity in a
particular (noted) experimental liquid at constant composition. Error bars in panels (c–f) show how
rhyolite-MELTS
aTiO2
changes when liquid TiO2 contents are varied by ± two times the reported one sigma values;
for panels (a and b), uncertainties on the experimental liquids were not reported. Phase abbreviations: liq
= liquid, pl = plagioclase, cpx = clinopyroxene, opx = orthopyroxene, amp = amphibole, biot = biotite, qtz
= quartz, tmt = titanomagnetite, ilm = ilmenite.
27