Supplementary information to: “Processes and timescales of dacite magma
assembly and eruption at Tauhara volcano, Taupo Volcanic Zone, New Zealand”
by Marc-Alban Millet, Chelsea Tutt, Monica R. Handler and Joel A. Baker
Sample selection and preparation
All rock samples used in this study come from the Victoria University of Wellington
(VUW) collection (sample numbers: 19421–19527). This sample suite is that of
Worthington (1985) and was based on outcrop locations and accessibility. In total,
292 samples were collected from six domes (Western, Central, Hipaua, Trig M,
Breached and Main domes), as well as associated pyroclastic flows. From these
samples, 115 samples were selected by Worthington (1985) for whole-rock major and
(some) trace element analyses. Thirty-three samples were selected from these 115
samples for this study, based on major element variations, and analyzed for trace
elements. Amongst these, 21 samples were analysed for whole-rock Sr–Pb isotopes.
One representative sample from each dome was subjected to detailed in situ mineral
major and trace element analysis.
Weathered surfaces were removed using a diamond saw and the remaining unaltered
rock was cut into small rectangular slabs. One slab was used to make thin sections (30
μm thick) for petrography and a second slab was used to make thick sections (100 μm
thick) for in situ mineral major and trace element analysis. Approximately 50 g of
rock chips was crushed using a Fritsch Pulverisette in order to extract rock chips and
crystals ranging from 5 mm to <250 μm in size. The crushed rock was passed through
a 250 μm sieve to remove any powder and fine-grained crystal fragments. A hand
magnet was passed over the samples to remove magnetite‐bearing groundmass.
Groundmass chips that were 1–2 mm in size without obvious phenocrysts were then
handpicked from this separate for Sr–Pb isotope analysis. Specific crystal types were
handpicked under a binocular microscope from the crystal separate, including:
clinopyroxene, amphibole and small plagioclase (<1 mm) crystals for bulk Sr–Pb
isotope analyses; large (>2 mm) plagioclase phenocrysts for single crystal Sr–Pb
isotope analyses; whole quartz crystals for cathodoluminescence imaging. All crystal
and rock chip separates were rinsed with >18.2 MΩ H2O and ultrasonicated for 5 min.
Each separate was then rinsed three times with >18.2 MΩ H2O and the solution
decanted before drying in an oven at 50°C. Quartz crystals were mounted in Epo‐tek
301 1LB kit epoxy and hardened at 60°C, prior to carbon coating. The quartz grains
were orientated so that the c‐axis was flat.
Whole-rock trace element analysis
Approximately 60–75 mg of whole-rock powder for each sample was weighed into
Savillex® Teflon beakers. Acid digestions were conducted in ultraclean class 10
PicoTrace laminar flow hoods. The samples were first digested in 1 mL of 29 M HF
(Seastar®) and 0.3 mL of 16 M HNO3 (Seastar®) and left on a hotplate at 120°C for 5
d. After allowing the beakers to cool, the acid was evaporated to incipient dryness and
the sample was then nitrified by evaporation of 1 mL of 16 M HNO3 at 120°C. The
residues were then digested in 4 mL of 6 M HCl (Seastar®) for 24 h and inspected for
the presence of solids. If a digested sample was solid-free, then the acid was
evaporated and the sample nitrified by evaporation of 1 mL of 16 M HNO3 at 120°C
(twice), before addition of 10 mL of 1 M HNO3. At this point, the sample was left on
a hotplate for 2 d to allow the samples to fully dissolve prior to analysis. However, if
any undigested solids remained after 2 d, then the digestion procedure was repeated
again from the initial HF–HNO3 digestion step.
After diluting the digested sample, trace element analyses were carried out with an
Agilent 7500CS ICP-MS. Trace element analyses were carried out by external
normalization to bracketing analyses of the USGS standard BHVO‐2, and by using
43
Ca as an internal standard, based on the sample CaO content previously determined
by X-ray fluorescence spectrometry. Repeated digestions and dilutions of the USGS
standards BHVO‐2 and BCR‐2 were used to assess data accuracy and reproducibility.
Elemental analyses are typically accurate to ±5% for both these secondary standards,
with the exception of Pb and Mo. Mo is accurate to –17% in BHVO‐2, although the
Mo concentration for BHVO‐2 is not well constrained. Accuracy for Pb is +14% and
+10% for BHVO‐2 and BCR‐2, respectively (see supplementary data worksheet).
In situ major and trace element analysis
Prior to in situ analysis, crystals were imaged by either back-scattered electron (all
phases except for quartz) or cathodoluminescence (quartz) techniques using a JEOL
JXA-8230 electron microprobe at Victoria University of Wellington equipped with
five wavelength dispersive (WDS) spectrometers.
Mineral major element concentrations were determined by electron microprobe
analysis (JXA-8230) operated at an accelerating voltage of 15 kV and beam current of
12 nA, using a focused (~1 μm) electron beam. Standardization of element oxide
concentrations and count rates was achieved using the NMNH 115900 Plagioclase,
Engels Amphibole, Kakanui Augite, Johnson Meteorite Hypersthene, Lake County
Plagioclase and Ilmenite Mountains Ilmenite standards. Major element concentrations
of the Tauhara minerals were calculated using the ZAF correction method. Analyses
of standards were interspersed with the sample measurements to monitor and, if
necessary, correct for spectrometer drift. Major element data for mineral standards run
as unknowns are given in the supplementary data.
In situ trace element analyses of amphibole, clinopyroxene, plagioclase and quartzhosted melt inclusions were made with a New Wave 193 nm (deep UV) laser ablation
(LA) system coupled to an Agilent 7500CS ICP-MS at Victoria University of
Wellington. 43Ca was used as an internal standard for mineral analyses based on CaO
contents previously measured by electron microprobe analysis.
29
Si was used as an
internal standard for melt inclusion analyses. Calculation of trace element
concentrations by LA-ICP-MS analysis also requires bracketing analyses of a well
characterized and, preferably, matrix-matched reference material (i.e., a calibration
standard). Given that there are no widely available and well characterized mineral
trace element standards, we used the basaltic glass BHVO-2G as the calibration
standard for amphibole and clinopyroxene and the Si–Al-rich glass NIST612 as the
calibration standard for plagioclase and the melt inclusions. The BHVO-2G and
NIST612 reference values used were the “preferred values” from the online GeoReM
as of July 2010. Tuning and optimization of signal sensitivity and stability was
achieved whilst laser rastering across the BHVO-2G standard. LA-ICP-MS data were
acquired for 60 s during ablation of a 25–35 μm diameter pit at 5 Hz. Background
count rates were measured for 60 s prior to each analysis. Trace element
concentrations for standards run as unknowns are given in the supplementary data.
Sr–Pb isotope measurements
All samples for Sr–Pb isotopic analysis were weighed into pre-cleaned Savillex®
beakers and subsequently acid leached to remove any anthropogenic or secondary
contamination following procedures similar to that described by Millet et al. (2008).
Rock standards analysed for Sr–Pb isotopes were not leached. For whole-rock
samples, 500 mg of powder was leached in 6 M HCl at 120ºC for 2 h (once).
Groundmass chips (ca. 50 mg) were twice leached in 6 M HCl at 120ºC for 2 h.
Mineral separates were leached three times in 2 M HCl at 20ºC for 20 min and
washed with >18.2 MΩ H2O between each leaching step.
After leaching, all samples were digested in 1 mL of 29 M HF and 0.2 mL 16 M
HNO3 for 24 h at 120°C and then evaporated. The samples were then nitrified twice
by evaporation of 1 mL of 16 M HNO3. The samples were then brought into solution
in 3 mL of 0.8 M HBr at 120°C for 24 h to equilibrate the sample into bromide form.
Samples were then dried down and redissolved in 1.5 mL of 0.8 M HBr. Pb was
separated following the anion exchange procedures described by Baker et al. (2004).
Sr and matrix were collected in 0.8 M HBr and Pb was eluted in 6 M HCl. The Pb cut
was further purified by a second pass through the anion exchange chemistry. The Sr
and matrix cut was evaporated, converted to nitrate form, and dissolved in 3M HNO3
for Sr separation using the methods outlined in Waight et al. (2002).
Sr–Pb isotope measurements were made with a Nu Plasma MC-ICP-MS coupled to a
DSN-100 desolvating nebulizer at Victoria University of Wellington. Pb isotope
ratios were measured by sample–standard bracketing using SRM981 as the bracketing
standard (reference values taken from Baker et al., 2004). Pb isotope data accuracy
and precision were evaluated by repeated measurements of the JB2 standard (see
supplementary data). Precision reached over the course of this study is ±238, ±211
and ±270 ppm (2 sd for
206
Pb/204Pb,
207
Pb/204Pb and
208
Pb/204Pb) and analyses are
accurate within this precision. Sr isotope ratios were corrected for instrumental mass
bias by internal normalization to
86
Sr/88Sr = 0.1194 and any instrumental drift was
corrected by sample–standard bracketing using SRM987. Sr isotope ratios are
reported relative to an
87
Sr/86Sr ratio of 0.710248 for SRM987. Repeated
measurements of the BHVO-2 basalt standard were carried out to assess Sr isotope
data quality (see supplementary data). Measurements carried out at 8V of 88Sr provide
an average of 0.703463 ± 0.000019, in agreement with the reference value given by
Georem (0.703469 ± 0.000017). Measurements of small plagioclase crystals were
carried out with a smaller beam of 2V of
88
Sr. BHVO-2 measurements carried out at
this intensity gave an average of 0.703504 ± 0.000094 (2 sd; n = 3). This worse
precision is due to the stronger effect of variable 86Kr interference on 86Sr.
Amphibole–dacite Kd calculations
Amphibole–dacite partition coefficients were calculated by dividing the trace element
content of amphiboles (measured by LA-ICP-MS) by that of the groundmass. The
trace element concentrations in the groundmass were calculated using both wholerock and crystal specific data and modal proportions of groundmass and crystal
phases taken into account in this calculation (i.e., plagioclase, amphibole and
clinopyroxene). Although the modal proportions of other phases like quartz,
orthopyroxene or Fe-Ti oxides were taken into account, we did not subtract their
composition because we did not carry out LA-ICP-MS analysis on these phases.
Given that a large amount of transition metals are hosted in orthopyroxene and Fe–Ti
oxides, this prevents calculation of Kd for these elements. Another issue with this
calculation is the potential destabilization of plagioclase. Due to the high
concentration of Eu in plagioclase relative to the melt, this can artificially generate a
high amphibole–melt Eu partition coefficient. For this reason we did not include Eu in
our calculations.
Errors on final amphibole–melt partition coefficients take into account the 1 sd of
each population of amphibole investigated (Western Dome, Central Dome, Hipaua
Dome, Trig M Dome, Breached Dome, Main Dome deep and Main Dome shallow) as
well as the reproducibility of the whole rock trace element dataset (see Supplementary
Data).
Diffusion modeling
Modeling of Ti diffusion in quartz crystals was undertaken following the 1D strategy
of Morgan et al. (2004). We focused on the boundaries between high titanium bright
crystal rims and low titanium lower contrast cores, which are inferred to record the
mixing of a new mafic magma with the silicic magma originally hosting the quartz
crystals. Modeling is based on grey scale images obtained by cathodoluminescence.
Only straight boundaries were modeled and diffusion times were calculated by fitting
an error function on the grey scale profile (see Morgan et al., 2004 for equations). The
diffusion coefficient of Ti in quartz is highly dependent on temperature, but
dependence on crystal orientation is minimal (Cherniak, 2010). Since amphiboles
grew in equilibrium with the mixed dacites, we used apparent temperatures obtained
from amphibole thermometry (Ridolfi and Renzulli, 2012) to calculate diffusion
coefficients. Given that the core and rims of amphiboles record similar temperatures
(apart from Main Dome), we used the average of apparent temperatures for each
dome (and that of amphibole cores for Main Dome), and propagated the one standard
deviation of these averages onto calculated diffusion coefficients and diffusion
timescales. For samples where multiple diffusion times could be calculated, we then
calculated the probability density function for each sample. Times given for an
averaged sample are the peak probability times, and the errors associated with it are
the 95% confidence interval.
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207
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