JOURNAL OF PETROLOGY
VOLUME 55
NUMBER 11
PAGES 2217^2248
2014
doi:10.1093/petrology/egu055
C^O^H^Cl^S^F Volatile Solubilities,
Partitioning, and Mixing in Phonolitic^Trachytic
Melts and Aqueous^Carbonic Vapor Saline
Liquid at 200 MPa
J. D. WEBSTER1*, B. GOLDOFF1, M. F. SINTONI2, N. SHIMIZU3 AND
B. DE VIVO2
1
DEPARTMENT OF EARTH AND PLANETARY SCIENCES, AMNH, CENTRAL PARK WEST AT 79TH ST., NEW YORK,
NY 10024-5192, USA
2
DIPARTIMENTO DI SCIENZE DELLA TERRA, DELL’AMBIENTE E DELLE RISORSE, UNIVERSITA' DI NAPOLI FEDERICO
II, VIA MEZZOCANNONE 8, 80134 NAPOLI, ITALY
3
GEOLOGY AND GEOPHYSICS DEPARTMENT, WOODS HOLE OCEANOGRAPHIC INSTITUTION, 360 WOODS HOLE ROAD,
MS 23 WOODS HOLE, MA 02543-1541, USA
RECEIVED FEBRUARY 12, 2013; ACCEPTED SEPTEMBER 17, 2014
ADVANCE ACCESS PUBLICATION OCTOBER 29, 2014
Hydrothermal experiments were conducted at 200 MPa and
900^10188C to determine the solubilities, fluid(s)^melt partitioning, and mixing properties of H2O, CO2, S, Cl, and F in phonolitic^trachytic melts saturated in vapor, vapor plus saline liquid, or
saline liquid.The bulk compositions and S, Cl, and F concentrations
of the run-product glasses were determined by electron microprobe
and the H2O and CO2 contents by Fourier-transform infrared spectroscopy (FTIR). A new parameterization was developed to calculate molar absorption coefficients for FTIR analysis of carbonate in
glasses and applied to the run-product glasses. The concentrations of
volatiles in the fluid(s) were determined by mass-balance calculations and checked with chloridometer analysis and gravimetry. The
range in oxygen fugacity of these experiments is NNO to NNO þ 2
(where NNO is nickel^nickel oxide buffer). The phonolitic^trachytic melts dissolved up to 75 wt % H2O, 094 wt % Cl, 073 wt
% CO2, 075 wt % F, and 016 wt % S, and the integrated bulk
fluid(s) contained up to 99 mol % H2O, 34 mol % Cl, 82 mol %
CO2, 17 mol % F, and 37 mol % S. The mixing relationships of
H2O, CO2, and Cl in melt versus fluid(s) are complex and strongly
non-ideal at these pressure^temperature conditions, particularly
with two fluid phases stable. The concentrations of H2O and CO2
*Corresponding author. Telephone: 1 (212) 769-5401. Fax: 1 (212)
769-5339. E-mail: [email protected]
in melt change with the addition of Cl S to the system, and the
solubility of Cl in melt varies with S. The reductions in H2O and
CO2 solubility in melt exceed those resulting from simple dilution of
the coexisting fluid(s) owing to addition of other volatiles.The partitioning of H2O and CO2 between fluid(s) and melt varies as a
function of fluid(s) and melt composition. The experimental data
are applied to phonolitic and related magmas of Mt. Somma^
Vesuvius, Italy, Mt. Erebus, Antarctica, and Cripple Creek, USA,
to better interpret processes of fluid(s) exsolution in eruptive
and mineralizing systems. Application of the experimental results
also provides constraints on eruptive and mineralizing fluid(s)
compositions.
KEY WORDS: experimental petrology; chlorine; sulfur; water; carbon
dioxide; fluorine; magmatic degassing; volatile solubility
I N T RO D U C T I O N
Although they are uncommon eruptive products, phonolitic and trachytic magmas can erupt explosively in and
ß The Author 2014. Published by Oxford University Press. All
rights reserved. For Permissions, please e-mail: journals.permissions@
oup.com
JOURNAL OF PETROLOGY
VOLUME 55
around population centers. The greater metropolitan
region of Naples, Italy, includes villages located along the
phonolite-covered slopes of Mt. Somma^Vesuvius, and
some volcanic materials from the Campei Flegrei complex
located immediately west of Naples include trachytic^
phonolitic compositions. Mt. Erebus, the world’s southernmost subaerial volcano, erupts anorthoclase-bearing
phonolites (Oppenheimer et al., 2011), and other older noteworthy phonolitic volcanic complexes include Laacher
See, Germany (Harms et al., 2004); diatremes of Dunedin,
New Zealand (Price et al., 2003); Teide stratovolcano,
Tenerife (Ferguson, 1978; Ablay et al., 1998; Bryan et al.,
2002); Hoodoo Mtn., British Columbia (Edwards et al.,
2002); and the Gharyan volcanic field of northern Libya
(Lustrino et al., 2012).
Some phonolitic magmas and their plutonic counterparts, nepheline syenites, are of economic significance as
sources of ore metals. The mineralization of Cripple
Creek, Colorado, USA, is genetically linked to phonolitic
to tephritic magmas, and their hypabyssal nephelinesyenite equivalents have yielded more than 23 million
troy ounces of gold. In addition, some nepheline syenites
are associated with rare metal mineralization, including
Li, Cs, Be, Nb, Ta, Zr, Y, U, and the rare earth elements
(REE) (Chakhmouradian & Zaitzev, 2012; Mariano &
Mariano, 2012; Williams-Jones et al., 2012). Although volatile abundances in nepheline syenite whole-rock samples
(Table 1) are rarely reported and are of limited use in constraining magmatic volatile concentrations, previous studies (Kogarko, 1974; Konnerup-Madsen & Rose-Hansen,
1982; Larsen & Sorensen, 1987; Sorensen, 1997; Andersen
et al., 2010) suggest significant abundances of H2O, Cl, F,
CO2 and S in the parental magmas, given the observed
compositions of primary fluid inclusions, carbonate contents of whole-rock samples, and the variety of volatileenriched minerals unique to these alkaline rocks (Le Bas,
1987; Sorensen, 1997; Nivin et al., 2005; Fall et al., 2007;
Mariano & Mariano, 2012).
The crystallization conditions (Berndt et al., 2001;
Scaillet & Pichavant, 2004; Andu¤jar & Scaillet, 2012),
bubble nucleation behavior (Larsen & Gardner, 2004;
Shea et al., 2010), and viscosities of phonolitic melts
(Whittington et al., 2001) have been studied experimentally. Phonolitic eruptive rocks and plutonic nepheline
syenites are clearly derived from volatile-enriched
magmas, but published data on volatile concentrations
are limited. Hydrothermal experiments do not yet fully
address H2O, CO2, Cl, S, and F solubilities in such melts.
This investigation involves volatile solubility and partitioning experiments to determine how H2O, CO2, Cl, S,
and F dissolve in phonolitic to trachytic melts and coexisting fluid phases at c. 200 MPa and 900^10188C. We do not
know the number of fluid phases that were stable in
each experiment, but the observed volatile solubility
NUMBER 11
NOVEMBER 2014
relationships of our runs and results of similar, prior studies indicate that some runs included low-density aqueous
or aqueous^carbonic vapor, with or without a higherdensity saline liquid. In this study, the term liquid applies
to the more dense saline phase; melt refers only to the
molten silicate phase; and fluid is used generically to refer
to vapor, saline liquid, or coexisting vapor plus saline
liquid.
We begin with a description of the experimental and
analytical techniques used. The run products of the 29 experiments were studied petrographically, and the glasses
were analyzed by electron microprobe, transmission
Fourier-transform infrared spectroscopy (FTIR), and secondary ion mass spectrometry (SIMS). Because absorption
coefficients for determining accurate FTIR concentrations
of carbonate ion in phonolitic and trachytic glasses, with
these compositions, are not available, we applied published
data and determined a correlation that relates bulk glass
composition to the IR radiation absorption behavior of
carbonate in aluminosilicate glasses. This new relationship
is used to determine total CO2 in our run-product glasses
by FTIR. We also describe the various methods used to
calculate, through mass balance, the H2O, CO2, Cl, S,
and F concentrations of the fluids at run conditions, and
the methods used to test the computed fluid compositions.
We address the complexity and challenges in interpreting volatile and fluid behavior for systems involving multiple volatile components and, potentially, two coexisting
fluid phases (Fig. 1). Accurate determination of the compositions of these fluid(s) is complicated by the presence
of five volatiles and because of the presence of vapor plus
saline liquid in many of the experiments. This two-fluid
situation is associated with the strongly non-ideal mixing
behavior of H2O, CO2, and Cl. We describe how these experiments clearly demonstrate a fundamental change in
the intrinsic solubilities of H2O and CO2 as Cl S are
added to fluid(s)-saturated phonolitic and trachytic melts.
This study concludes by relating these observations to
magmatic^hydrothermal processes in natural eruptive
phonolitic and mineralizing plutonic systems.
E X P E R I M E N TA L T E C H N I Q U E S
Most of the experiments were conducted with a natural,
pumiceous, volatile-enriched phonolite sample that
erupted c. 8000 years ago at Mt. Somma^Vesuvius (Tables
1 and 2). Some other experiments were conducted using
glasses prepared from this sample by fusing the phonolite
in open precious metal capsules at 1060^13008C at 1 atm
to reduce the H, C, Cl, S, and F contents of the natural
phonolite (Table 1). This natural starting material is the
same as used in earlier experimental studies (Webster
et al., 2003, 2005, 2009).
The primary goals of this study were to determine the
dissolution behavior of five primary magmatic volatile
2218
2219
082
084
071
0065
012
10050
015
0037
023
030
564
898
192
084
027
538
2016
098
067
n.d.
310
10163
059
005
099
002
681
929
132
006
019
193
2405
009
5624
high-N/NK MI
Vesuvius
3
Mt. Somma–
092
042
b.d.
52
10011
062
b.d.
b.d.
b.d.
1132
547
198
014
016
187
2261
017
5577
low-N/NK MI
Vesuvius
4
Mt. Somma–
088
062
n.d.
n.d.
9864
084
n.d.
n.d.
004
629
673
176
029
029
353
1859
044
5915
WR
Dome, Italy
Cuma Lava
5
083
070
n.d.
40
9995
021
0089
017
002
673
1050
120
007
009
182
2175
032
5698
MI
Germany
6
Laacher See,
093
085
n.d.
n.d.
9296
023
n.d.
n.d.
n.d.
258
989
112
016
022
372
1929
017
5558
WR
NZ
7
Dunedin,
088
071
n.d.
n.d.
995
n.d.
n.d.
n.d.
006
560
890
145
026
017
253
2033
027
5993
WR
field8
Volcanic
Gharyan
071
069
n.d.
45
10015
035
n.d.
03
013
624
935
275
190
014
257
1916
058
5690
WR & MI
Tenerife9
075
073
02
25
9635
n.d.
003
n.d.
004
461
830
106
b.d.
021
815
153
025
5840
WR
Mtn., BC10
Hoodoo
088
069
n.d.
n.d.
9780
n.d.
n.d.
n.d.
013
558
823
231
077
015
408
2096
060
5499
WR
eline syenite11
Av. neph-
085
065
n.d.
n.d.
9843
035
011
052
011
612
733
088
065
001
23
9792
063
0035
069
002
666
807
017
174
290
018
195
2076
015
5687
WR
S(9)213
Vesuvius
Mt. Somma–
206
012
255
2040
072
5520
MI
CO12
Creek,
Cripple
MI, melt inclusion; MG, matrix glass; WR, whole-rock sample; n.d., not determined; b.d., below detection limit; totals do not include H2O or CO2. N/NK is molar
[Na2O/(Na2O þ K2O)]; A/CNK is molar [Al2O3/(CaO þ Na2O þ K2O)].
1
Representative phonolite sample 97006 of Oppenheimer et al. (2011).
2
Representative phonolite sample 97018 of Oppenheimer et al. (2011).
3
Average of 18 melt inclusions (Signorelli et al., 1999).
4
Average of four melt inclusions (Cioni, 2000).
5
Cuma lava dome sample DC3, Phlegrean Fields, Italy (Melluso et al., 2012).
6
Single melt inclusion composition representative of Laacher See phonolite magma; concentrations of H2O, S, Cl, and F are averages of 33 melt inclusion
compositions (Harms & Schminke, 2000; Harms et al., 2004).
7
Sample BC3 glass clast, Port Chalmers Breccia, Dunedin volcano, New Zealand (Price et al., 2003).
8
Representative type-1 phonolite of Lustrino et al. (2012).
9
Representative phonolitic pumice fall sample 36-Wavy of Bryan et al. (2002); H2O, F, and Cl values are maxima determined for melt inclusions by Ablay et al.
(1998).
10
Representative iron-enriched phonolite of Edwards et al. (2002).
11
Average, worldwide bulk composition of nepheline syenites (Barker, 1983).
12
Average composition of seven silicate melt inclusions in clinopyroxene hosts from biotite–clinopyroxene syenite (i.e. phonolite in bulk composition) of the
Vindicator Mine, Cripple Creek, CO, Au–Te deposit. Phenocryst host separates were picked and heated at 10758C and 1 atm in air for 4 h (J. D. Webster,
unpublished data).
13
Primary natural starting pumiceous Avellino phonolite rock powder of this study.
071
S
A/CNK
0197
0066
F
N/NK
049
P2O5
0117
536
K2O
008
857
Na2O
CO2
240
CaO
H2O
090
MgO
9960
021
MnO
Total
578
FeOT
0133
1968
Al2O3
Cl
135
TiO2
096
5564
5446
SiO2
Mt. Erebus2
low-CO2 MI
Mt. Erebus1
high-CO2 MI
(wt %)
Constituent
Table 1: Compositions of phonolitic eruptive units and related rocks
WEBSTER et al.
VOLATILES IN PHONOLITIC^TRACHYTIC MELTS
JOURNAL OF PETROLOGY
VOLUME 55
NUMBER 11
NOVEMBER 2014
Fig. 1. Flowchart showing how run-product vapor, saline aqueous liquid, silicate glass, and salt precipitates were investigated by chemical
analysis, gravimetry, and mass-balance computations to determine and confirm the compositions of melt and fluid(s) at run conditions.
components in phonolitic melts at fixed pressure, to cover
a wide range of melt as well as fluid compositions, and to
explore the effects of some change in temperature. The purpose of allowing composition to vary was to provide some
constraints on the applicability of these new results to natural phonolite- and nepheline syenite-forming magmatic
systems. To achieve this goal, we varied the source of
carbon, the relative Na to K contents, the alkalinity, and
the Fe contents of the starting charges, in addition to changing the starting volatile abundances. The final melt compositions differ from run to run because of the varied
starting charge compositions, variable melt^fluid(s) exchange of components, and because of minor crystallization of Fe-oxides and plagioclase in some experiments. In
particular, we note that runs involving elemental S in the
starting charges produced slightly less-alkaline melt compositions. We varied the source of added C to our runs by
using either PdC2O4 or oxalic acid dihydrate (C2H6O6)
although several other runs were conducted with Na2CO3
and K2CO3 in addition to PdC2O4 and/or C2H6O6. Five
runs conducted with added S included elemental S or anhydrite, and 10 runs were conducted with additional Fe
added as Fe3O4. We added 0^10 mg of C2H6O6, 0^05 mg
of elemental C, 0^1mg of Na2CO3, 0^2 mg of K2CO3,
0^36 mg of PdC2O4, 05^83 mg of distilled^deionized
H2O, 0^2 mg of Fe3O4, 0^20 mg of a 1:1 molar NaCl^KCl
mixture, 0^07 mg of elemental S, 0^2 mg of CaSO4, and
22^186 mg of rock powder to the starting charges.
The experiments were performed in 3^5 mm diameter
gold capsules with a wall thickness of 015^020 mm. Most
capsules were sealed with a C-tipped arc welder, and
others were sealed with a Puk-3 Spot Welder, which employs a W tip to avoid C contamination of the starting
charge. All capsules were weighed before and after welding
to test for volatile leakage during welding, and the capsules
were stored in a drying oven at 1158C for 1h to check for
2220
WEBSTER et al.
VOLATILES IN PHONOLITIC^TRACHYTIC MELTS
Table 2: Compositions (wt %, determined by EPMA and FTIR) of fused natural volatile component-enriched starting
materials used in volatile-solubility experiments and in other experiments required to generate relatively F-, Cl-, and
S-poor but H2O- CO2-bearing glasses
Start
Notes
material
T/duration Na2O K2O FeOT Al2O3 MgO SiO2 CaO TiO2 MnO P2O5 F
S
Cl
H2O
CO2
Total
(MPa) (8C/h)
Raw powder* Sealed capsule
1
P
150
1060/288
807
666 195
2076 017 5687 174 015 018 002 069 0035 063
Open 1 atm-05-05B 01
1025/3 &
805
654 223
2138 013 587
198 014 02
23
0012 10023
001 013 0028 0056 5001 n.d.
9960
1120/1
2
Open 1 atm-12-01
01
1175/24
82
68
3
1-07-07
200
920/264
725
603 149
194
2124 018 6004 183 014 02
002 05
0024 025 5001 n.d.
1969 022 5541 163 017 015 002 062 0012 020
75
001
1014
10041
*Natural rock powder from Mt. Somma–Vesuvius loaded into open gold capsule, heated at c. 1 bar and 1258C for several
hours to remove adsorbed H2O, and then welded shut; subsequently, the powder was completely fused in the sealed
capsule with an internally heated pressure vessel with no loss of volatile components.
The 1 atm experiments were conducted with unsealed silver–palladium and gold–palladium capsules in 1 atm
tube furnace to variably drive off volatile components. FeOT, total iron reported as ferrous iron oxide. n.d., not
determined.
weight loss (i.e. H2O loss from improperly welded
capsules).
The hydrothermal experiments were conducted in
an internally heated pressure vessel (IHPV) at the
American Museum of Natural History (AMNH) using
the methods of Webster et al. (2003, 2005, 2009).
Temperature ranged from 900 to 10188C for all experiments and was monitored with two chromel^alumel
thermocouples spanning the length of the capsules. The
recorded temperature variations as a function of time
were typically 5108C, and the temperature gradients
between the two thermocouples were 5158C for most experiments. Pressure was kept constant in the range of 197^
205 MPa during each experiment and was monitored
with factory-calibrated Bourdon-tube pressure gauges.
Run durations varied from 4 to 20 days; most were conducted for at least 10 days.
All experiments were quenched isobarically and were
cooled to c. 6008C in 30 s or less. During an isobaric
quench, the argon pumping system is brought to the pressure of the experiment, opened to the IHPV, and is activated to maintain the given pressure as soon as power to
the furnace is shut off and the furnace begins to cool. The
IHPV is rotated from its near-horizontal run orientation
to a vertical quench orientation to allow heat to rise away
from the capsules in the furnace and to hasten the temperature reduction. Pressure is held as constant as possible
with the pumping system until the furnace temperature is
at or below 708C. The volatile-component abundances of
the melts at run pressure and temperature are maintained
in the run-product glasses with this quench rate. In the
past, we conducted tests with H2O-saturated basalt, andesite, and phonolite melts at 200 MPa with this IHPV to determine the effects of the quench rate on the H2O
contents of run-product melts. Each melt was fused with
excess H2O for c. 5 days at superliquidus temperatures
and quenched with this method. The resulting run-product
glasses were analyzed for total H2O by FTIR, and they
contain 501 020, 505 006, and 633 022 wt %
H2O, respectively. For comparison, the H2O solubility
model of Moore et al. (1998)çwhich is based on experimentally determined H2O contents of melts synthesized
with a rapid-quench IHPV apparatusçpredicts 477, 499,
and 644 wt % H2O in melts of these same starting rock
powders, respectively. The agreement is good. It follows
that all volatiles measured in the run-product glasses represent those of the melts at run conditions because H2O
is the most rapidly diffusing volatile in silicate melts
(Baker et al., 2005) and because it is the most likely to
change potentially on quench.
At the conclusion of the runs, the Au capsules were
cleaned with ethanol and weighed, to check for leakage
during the run. Two of the experiments (1-05-06 and
1-05-11) leaked during the quench, as fluid droplets were
observed on the outer surface of the capsules after the
quench. Given that the droplets had not evaporated, the
leaking occurred at ambient to near-ambient temperatures.
Moreover, the measured H2O concentrations of these
two run-product glasses are consistent with the other
200 MPa experiments (with their added bulk volatile
contents), so the results of these runs are included in our
dataset.
2221
JOURNAL OF PETROLOGY
VOLUME 55
A N A LY T I C A L T E C H N I Q U E S
The run products include vapor, liquid, variably vesicular
glass (with or without included minerals), and crystalline
salts and other amorphous materials that precipitated
from the fluids during the quench. Several glasses contain
minor Ca-enriched plagioclase small crystals of Feoxides that were stable at run conditions (i.e. their textures
indicate that these minerals are not quench phases). No sulfides were observed in the run-product glasses and anhydrite is not present in the glass; consequently, no S-enriched
minerals were stable at run conditions. These phase relations are consistent with those of relatively oxidized,
phase equilibrium experiments for other phonolitic melts
(Scaillet et al., 2008).
Electron microprobe (EPMA)
The run-product glasses and silicate melt inclusions (MI)
in rocks from Cripple Creek, CO, were analyzed at 15 keV
in wavelength-dispersive mode with a CAMECA SX-100
electron microprobe at the AMNH. All glasses were analyzed for SiO2, Na2O, FeO, K2O, and F with a 2 nA beam
current (Table 3), whereas Al2O3, CaO, MgO, MnO,
TiO2, P2O5, Cl, and SO2 were analyzed with a 10 nA
beam current. Peak count times were 20^60 s. All glasses
were analyzed with a 5^10 mm diameter, defocused electron beam and the glasses were moved manually across
vesicle- and crystal-free areas of glass during analysis to
minimize Na, F, and K migration. The results reported
the average of 4^8 analyses of randomly selected spots on
the polished sample surface. Other glasses were analyzed
regularly to monitor accuracy and precision during the
investigation. Repeat analyses of an African obsidian,
known to contain 020 wt % Cl, returned a mean concentration of 020 001 (1s), whereas NBS 620 glass, known
to contain 0112 001wt % S, returned a mean S concentration of 013 0006 wt % (1s).
The valence state of S [i.e. oxidized (S6þ) versus reduced
2
(S )] was determined for those run-product glasses
(Table 4) containing sufficient S by measuring the wavelengths of the S Ka X-rays using EPMA wavelength-dispersive spectrometry and an LPET crystal (Carroll &
Rutherford, 1988; Matthews et al., 1999). We counted the S
X-rays with a spectrometer sin y range of 061158^061658
using sin y steps of 000002, a dwell time of 100 ms per
step, and a rastered and defocused 6 mm beam diameter
over a 30 mm 30 mm area to avoid glass oxidation resulting from heating owing to prolonged beam exposure
(Wallace & Carmichael, 1994; Me¤trich & Clocchiatti,
1996). The electron beam was set at 15 keV and 40 nA current, and we collected four accumulations on the reference
standards BaSO4 and FeS, and 30 scan accumulations
per analysis to improve precision. Details on data treatment used to derive the relative abundances of S6þ and
NUMBER 11
NOVEMBER 2014
S2^ as well as ranges in fO2 have been given by Webster
et al. (2011).
Fourier-transform IR spectroscopy
The concentrations of hydroxyl ion, molecular H2O, molecular CO2, and carbonate ion in the quenched glasses
were determined with FTIR at the AMNH. Doubly polished chips of run-product glass were prepared, and their
average thicknesses (55^460 mm) was determined with a
Mitutoyo digitometer (precision of 2 mm) taking c. 8^10
measurements per chip. Many of the glasses were analyzed
with two glass chip thicknesses; the thicker chips for
H-bearing species and the thinner chips for C-bearing species. Three to six FTIR analyses were collected in different
spots of each polished glass chip, and the beam diameter
was set to the largest possible size that allowed avoidance
of vesicles and the more rare crystals present in some
glasses (i.e. most beam diameters ranged from 30 to
50 mm). Measurements were conducted at room temperature in transmittance mode using a Nicolet Nexus 670
FTIR spectrometer with a Continuum IR microscope.
Dry N2 gas was passed through the instrument at
15 l min^1 during data collection. Infrared spectral data
were collected both in the mid-IR (450^4000 cm^1) and
the higher-energy near-IR (up to 8500 cm^1) regions using
a KBr beam splitter, an MCT/A detector, and a globar
source. For each spectrum, 400 scans were performed and
backgrounds were collected for each glass analyzed; the
analytical resolution was 4 cm^1. Absorbances were determined relative to curved baselines for these species, but
several glasses were best treated with linear baselines. The
height of each H2O and OH^ peak was measured relative
to the baseline using OMNIC software; determining baselines and backgrounds for the C peaks was more challenging and is described below. Glass densities were
calculated after Luhr (2001) using the Gladstone^Dale
rule (Gladstone & Dale, 1864; Silver et al., 1990) and
EPMA data; the associated precision of these density
values is 003 g cm^3 (Mandeville et al., 2002).
Secondary ion mass spectrometry
Total H2O concentrations of 21 run-product glasses and
raw C/Si ratios for 21 glasses (Table 5) were determined
by SIMS at the Northeast National Ion Microprobe
Facility (NENIMF) at the Woods Hole Oceanographic
Institution. Chips of these glasses and glass standard materials were pressed into indium mounts to reduce the H
and C backgrounds during analysis. The glasses were
gently repolished, coated with Au, and analyzed with a
Cameca 1280 ion microprobe. All glasses were ablated
under a Csþ ion beam in raster mode for 5 min to remove
that part of the sample surface damaged during polishing
and potentially influenced by residual H and/or C contamination. The samples were subsequently ablated for another
5 min in spot mode to collect analytical data. Charging of
2222
2223
202
200
202
202
204
204
204
203
203
203
1-07-10
1-07-11
1-07-12 A
1-07-12B
1-12-09 A
1-12-09B
1-12-09 C
1-12-14 A
1-12-14B
1-12-14 C
910/143
910/143
910/143
944/312
944/312
944/312
919/310
919/310
906/333
911/167
915/475
910/211
927/139
900/167
905/240
901/359
926/166
909/214
925/169
908/96
920/264
921/238
917/167
923/191
926/237
923/121
925/194
1018/141
1018/141
(8C/h)
T/duration
771
78
726
784
797
729
874
990
618
761
686
669
725
760
734
723
687
539
689
679
768
600
723
727
730
769
692
745
949
Na2O
687
663
636
635
641
612
816
913
695
644
629
628
603
640
625
887
710
517
871
597
678
53
781
750
616
76
556
799
816
K2O
178
186
187
191
193
200
179
183
169
171
160
144
149
166
179
180
202
071
190
115
197
119
192
181
193
156
148
171
182
FeOT
2050
2060
1980
2177
2134
2022
1924
1918
2007
2142
2020
2001
1969
1960
1966
2078
1896
1785
1892
1940
1934
1845
2011
2029
1942
2013
2094
1968
1961
Al2O3
018
018
018
017
5690
5650
5450
5528
5844
5537
016
016
5346
5266
5622
5529
5653
5676
5541
5480
5459
5686
5503
6376
5589
5615
5505
5955
5697
5742
5532
5530
5802
5592
5586
SiO2
019
018
018
017
012
011
022
014
016
014
014
001
013
015
019
013
013
024
063
016
016
016
019
MgO
158
173
172
188
177
169
168
165
279
142
102
100
163
162
172
100
156
102
113
163
166
124
140
133
169
140
154
299
175
CaO
019
017
014
018
018
02
015
016
017
012
014
018
017
011
013
013
006
010
010
010
015
012
013
014
017
015
012
011
014
TiO2
019
017
019
020
022
015
019
015
011
015
011
010
001
001
002
002
003
002
002
004
001
002
002
002
002
001
013
015
000
002
003
002
002
002
003
001
002
001
001
002
004
002
003
P2O5
017
004
001
001
006
010
015
003
010
009
016
015
006
014
014
MnO
002
011
011
040
015
028
062
066
049
070
066
052
062
066
062
074
060
060
056
052
072
065
061
062
067
05
075
057
07
F
002
002
0025
b.d.
0005
0004
0028
0028
0062
0096
0156
0152
0012
0028
0022
b.d.
0005
0006
0004
0016
0008
0004
b.d.
0004
0032
0004
0009
0073
0027
S
H2O
005
004
004
027
028
024
019
022
064
056
052
058
020
071
053
068
079
085
079
068
094
285
425
670
559
250
641
584
405
428
521
633
679
752
663
700
169
643
622
414
548
430
279
534
089
316
737
516
507
160
150
FTIR
092
084
067
081
088
028
062
Cl
041
036
021
031
046
017
028
050
027
021
015
015
001
013
013
035
018
014
025
012
022
019
019
024
015
024
016
069
073
FTIR
CO2
10071
10043
9913
10217
10185
10032
10058
10034
10011
10113
10071
10078
10042
10023
10011
10033
9979
10186
9949
9828
9919
9912
10030
10096
10168
10087
10171
9938
10077
Total
Molar
063
064
063
065
065
064
062
062
057
064
062
062
065
064
064
055
060
061
055
063
063
063
058
060
064
057
065
059
064
N/NK
089
089
090
094
092
093
073
066
088
097
101
102
092
088
089
089
087
109
083
094
084
103
088
090
089
089
104
075
071
A/CNK
Molar
1
1
1
2
2
2
3
3
Raw powder
Raw powder
Raw powder
Raw powder
Raw powder
Raw powder
Raw powder
Raw powder
Raw powder
Raw powder
Raw powder
Raw powder
Raw powder
Raw powder
Raw powder
Raw powder
Raw powder
Raw powder
Raw powder
Raw powder
Raw powder
material
Start
Molar N/NK is moles [Na2O/(Na2O þ K2O)]; molar A/CNK is moles [Al2O3/(CaO þ Na2O þ K2O)]. Totals include H2O and CO2. Detection limit for S is 0004 wt %;
b.d., below detection. The starting materials are identified in Table 1.
203
1-07-09
202
1-05-11
199
201
1-05-09
202
201
1-05-08
1-07-08
200
1-05-07
1-07-07
2027
1-05-06
199
202
1-05-02
1-07-03
201
1-04-16
197
199
1-04-15B
199
203
1-04-14
1-07-02
200
1-04-13
1-07-01
205
1-02-29D
200
205
1-02-29B
1-05-13
P (MPa)
Run no.
Table 3: Compositions (wt %) of run-product glasses determined by EPMA and FTIR
WEBSTER et al.
VOLATILES IN PHONOLITIC^TRACHYTIC MELTS
2224
025
nd
nd
nd
nd
nd
nd
nd
nd
1-07-11
1-07-12 A
1-07-12B
1-12-09 A
1-12-09B
1-12-09 C
1-12-14 A
1-12-14B
1-12-14 C
021
017
015
027
010
011
352
374
815
589
860
861
679
618
366
639
734
804
985
876
932
114
447
438
216
628
255
477
279
260
465
277
694
579
727
Mol % H2Ofls
645
616
173
404
815
89
183
265
221
266
140
760
001
740
290
680
740
179
410
134
266
172
161
117
211
201
138
346
262
Mol % CO2 fls
ee
01
01
ee
ee
01
56
49
158
15
29
30
06
22
16
341
210
150
338
92
161
141
209
248
122
178
67
09
01
Mol % Clfls
01
01
ee
01
03
03
ee
ee
24
31
32
24
ee
ee
ee
ee
ee
ee
ee
ee
ee
ee
ee
ee
ee
ee
ee
37
01
Mol % Sfls
02
07
09
06
ee
ee
ee
ee
17
ee
ee
06
ee
ee
ee
03
06
04
08
05
ee
02
04
07
ee
11
05
06
08
Mol % Ffls
12
10
16
14
03
18
15
18
10
16
16
17
19
19
19
07
10
10
06
15
07
12
11
09
09
07
18
34
47
Molar DH2O fls=mt
376
475
275
419
431
169
213
151
237
402
323
178
14
193
78
47
143
442
47
348
360
282
226
131
510
262
269
114
85
Molar DCO2 fls=mt
ee
06
06
ee
ee
01
77
52
58
07
16
14
08
09
09
97
74
49
99
34
41
39
50
65
54
60
19
06
00
Molar DC1 fls=mt
06
08
ee
215
107
250
ee
ee
81
75
50
40
ee
ee
ee
ee
ee
ee
ee
ee
ee
ee
ee
ee
ee
ee
ee
85
09
Molar DS fls=mt
02
05
06
02
ee
ee
ee
ee
04
ee
ee
02
ee
ee
ee
01
01
01
02
01
ee
01
01
01
ee
03
01
01
01
Molar DF fls=mt
NUMBER 11
026
033
023
025
022
030
039
027
028
037
025
039
043
043
045
032
027
017
018
019
015
061
005
(fluids/melts)
Mass ratio
VOLUME 55
n.d., apparent log oxygen fugacity of these runs is estimated to range from NNO to NNO þ 2. For other runs (1-02-29D, 1-07-08, 1-07-09, 1-07-10, and 1-07-11) the
fO2 was determined using S peak positions by EPMA. ee, excessive errors; these values of the fluid composition and/or molar partition coefficient were deleted
because they are anomalously high owing to extreme errors caused by large imprecision on glass analysis for S and F. Fluids (fls) refer to vapor, saline liquid, or bulk
integrated vapor plus saline liquid; mt refers to phonolitic and trachytic melts. (See text for details.)
067
nd
1-05-13
1-07-10
nd
1-05-11
077
nd
1-05-09
088
nd
1-05-08
1-07-09
nd
1-05-07
1-07-08
nd
1-05-06
nd
nd
1-05-02
1-07-07
nd
1-04-16
nd
nd
1-04-15B
1-07-03
nd
1-04-14
nd
nd
1-04-13
1-07-02
019
1-02-29D
nd
nd
1-02-29B
1-07-01
App. fO2
Run no.
Table 4: Fluid(s) compositions, partition coefficients, and other experimental parameters
JOURNAL OF PETROLOGY
NOVEMBER 2014
WEBSTER et al.
VOLATILES IN PHONOLITIC^TRACHYTIC MELTS
Table 5: Raw FTIR absorption data, glass chip characteristics, raw SIMS (X/30Si) ratios, and H2O concentration
by SIMS
Run
E4500 FTIR
E5200 FTIR
Glass width
E1425 FTIR
E2350 FTIR
Glass width
Glass
SIMS
Wt %
SIMS
number
absorption
absorption
(mm) for H2O
absorption
absorption
(mm) for
density
(16O1H/30Si)
H2O
(12C/30Si)
–1
–1
–1
–1
–1
–1
(l mol cm )
–1
–1
CO2
(l mol cm )
(l mol
1-02-29B
0021
0026
0198
0809
0009
009
2455
342
245
0043
1-02-29D
0041
0033
0268
0962
0012
0101
2633
n.d.
25
004
1-04-13
0067
0151
0289
0173
001
009
2347
512
n.d.
00025
1-04-14
0021
0049
0091
0252
0009
0091
2356
n.d.
53
00108
1-04-15B
0023
0079
0095
018
0012
0101
2321
504
70
0005
1-04-16
0019
00287
0098
0284
0009
0098
2398
223
32
0015
cm )
(l mol cm )
–1
(g l )
SIMS
1-05-02
0017
00211
0088
0195
0012
0088
2409
23
32
0009
1-05-06
0012
0031
0055
0123
0006
0055
2319
n.d.
53
00016
1-05-07
0038
007
0165
0228
0014
0087
2388
n.d.
37
0012
1-05-08
0033
0113
0185
0135
0096
2296
529
57
00032
1-05-09
0016
0029
0071
0209
1-05-11
0022
0055
0086
0135
1-05-13
0034
0101
0143
0183
1-07-01
0012
00092
0079
0327
bd
001
0071
2378
n.d.
39
000658
0086
2279
851
50
00039
0006
0089
2333
n.d.
65
00058
0013
0079
2429
17
245
0023
bd
1-07-02
0058
0204
0257
012
001
0079
2324
801
57
00043
1-07-03
0022
0071
0096
0147
0008
0096
2327
54
69
00043
1-07-07
0113
0391
0463
0011
0006
010
2309
995
n.d.
n.d.
1-07-08
0038
0118
0158
0152
001
009
2311
823
71
000426
1-07-09
0053
0148
0217
0176
0008
0104
2322
38
57
000576
1-07-10
0059
014
0256
0222
0006
009
2359
395
55
00128
1-07-11
0024
00449
0106
0356
0008
0106
2382
282
39
00122
1-07-12 A
0039
0104
0232
0543
002
009
2412
43
55
0018
1-07-12B
0041
0145
0215
036
0004
0109
2364
46
64
00056
1-12-09 A
0015
00457
0065
013
0006
0065
2317
n.d.
n.d.
n.d.
1-12-09B
0021
00229
0111
0565
0004
0101
2421
n.d.
n.d.
n.d.
1-12-09 C
0033
0053
0101
0381
0006
0104
2382
n.d.
n.d.
n.d.
1-12-14 A
0031
0074
0106
0223
001
0092
2335
n.d.
n.d.
n.d.
1-12-14B
0022
0037
0091
0392
0006
0091
2387
n.d.
n.d.
n.d.
1-12-14 C
0022
0039
014
0423
004
0086
2396
n.d.
n.d.
n.d.
Average glass chip thickness was determined from 8–10 measurements. FTIR absorptions for carbonate peak were
determined from peaks corrected for influence of H2O on baselines using CO2-free reference glasses (see text for
methods). n.d., not determined.
the sample surface was offset by application of a varying
electrical potential to the sample surface with an electron
gun. The Cameca electron gun is designed to decelerate to
zero kinetic energy as the electrons approach the sample
so that they are attracted wherever an excess charge
develops on the surface; this effect is achieved by applying
the same potential as the secondary ion acceleration of
10 kV. The mass resolving power during analyses was
c. 6000.
Secondary ions were measured for 16O1H and 30Si
that were counted for 10 and 5 s, respectively, and the
(16O1H/30Si) ratios of unknown glasses were compared
with those of phonolitic standard glasses containing 03
and 69 wt % H2O (determined by Karl Fischer titration;
KFT) to establish H2O concentrations. We used 16O1H as
a proxy mass for 1H. The ion yields for 16O1H show no
systematic variability with the differences in glass
composition.
Ion counts for 12C were collected for 10 s, and ratios
of (12C/30Si) were determined for run-product glasses.
Standard glasses with appropriate bulk compositions were
not available, so raw counts were not converted to CO2
2225
JOURNAL OF PETROLOGY
VOLUME 55
concentrations for comparison with FTIR data. The raw
(12C/30Si) are reported in Table 5, but these data are not
addressed further.
Gravimetry
We estimated, gravimetrically, the masses of CO2 other
gaseous compounds in the run-product vapor of those capsules that did not leak. The capsules were cleaned,
weighed, and cooled overnight to below 208C to freeze
the aqueous run-product liquid, and the capsules were
punctured subsequently in two or three locations with a
sharp probe and the capsule masses were recorded. The
mass difference between that of the capsule before and
after the freeze-and-puncture technique represents the
quantity of CO2 vapor and, potentially, minor other volatile species such as H2, SO2, H2S, and SO3 that were
released when the capsules were punctured. These vapor
constituents do not solidify at this freezing temperature
and, hence, escape when the capsule is opened to the
atmosphere.
The accuracy and precision of the balance were determined. The mass of a ‘standard’ run-product capsule
was measured eight times with this balance; the known
mass of this ‘standard’ capsule is 030850 g. This test returned a mean of 030861g and a 1s precision about the
mean of 000028 g.
Chloridometer
We measured, directly, the Cl concentrations of the liquid
phase plus salt precipitates in the run products of 12 experimental charges. The punctured capsules were soaked
in 05^1g of distilled and deionized H2O (DD H2O),
which dissolved all chloride salts that precipitated from
the fluids during the quench; the majority of capsules
were soaked for 2^4 days prior to analysis. The diluted
fluids were sampled with 10 ml capillary tubes and analyzed for chloride ion with a Buchler chloridometer
using the methods of Webster et al. (2009). The chloride
values were corrected for their individual dilution factors
(resulting from the post-run soaking of the capsules in
DD H2O). Replicate analyses of a standard aqueous NaCl
solution containing 35 wt % Cl, conducted throughout
this study, return a 1s precision of c. 45 rel. %.
After soaking and dissolution of the salt precipitates,
the punctured capsules were stored in a drying oven at
c. 1208C overnight. The capsules were re-weighed, and the
mass change (before and after soaking plus heating) represents the quantity of run-product liquid plus salt precipitates that dissolved in the DD H2O during soaking. This
mass is equivalent to that of the run-product fluid
phase(s) at run conditions, except that it does not account
for the masses of (1) CO2-dominated vapor, (2) fluid(s)
trapped in vesicles in glass, (3) amorphous silicates that
precipitated on quench, and (4) traces of salt in the capsules after heating. Trace coatings of white salts were
NUMBER 11
NOVEMBER 2014
observed on the surfaces of some run-product glasses
when the heated capsules were cut open and residual glass
was extracted for analysis. Study of these dried precipitates
shows they include Na-, K-, Ca- Fe-bearing chlorides;
rare CaF2 and CaSO4; and amorphous masses of mixed
aluminosilicate components that were dissolved in the
fluid(s) at run conditions.
A N A LY T I C A L R E S U LT S
Bulk compositions of starting and
run-product glasses
The glasses are phonolitic to trachytic in composition. The
molar A/CNK [i.e. Al2O3/(CaO þ Na2O þ K2O)] of the
starting rock powder and starting glasses is 088^092 and
the molar N/NK [Na2O/(Na2O þ K2O)] ranges from
063 to 065. In comparison, the molar A/CNK and
N/NK ratios of the quenched run-product glasses range
from 066 to 109 and from 054 to 065, respectively. It is
notable that three of the S-enriched runs exhibit A/CNK
ratios larger than those of most other runs owing to
enhanced alkali sequestration by the fluid(s). This experimental effect was addressed in detail by Webster &
Botcharnikov (2011). The non-volatile melt components in
glass that exhibit the largest ranges in concentration are
CaO, FeO, Na2O and K2O.
These experiments represent equilibrium conditions.
Previous time-series experiments involving phonolitic
melts at pressure^temperature conditions similar to ours
demonstrated achievement of melt^fluid(s) equilibration
in 55^6 days or longer (Signorelli & Carroll, 2000). Our
H2O^CO2-only added runs were conducted for either 59
or 13 days. The results described below show strong consistency in the dissolution of H2O and CO2 between the two
sets of experiments, even though the run durations varied
significantly. This consistency indicates that the 59 day
durations also represent equilibrium. All experiments but
two were conducted for more than 55 days, and the two
shortest-duration runs exhibit no detectable differences in
volatile solubility behavior that can be attributed to a
short run duration or lack of equilibrium.
Concentrations of volatile components in
run-product glasses
H2O by FTIR, SIMS, and EPMA
The concentration of total H2O determined by FTIR in
the glasses is based on the sum of molecular H2O plus
OH^. Owing to the high total H2O concentrations in
many of the glasses, the 3550^3570 cm^1 peak was typically
saturated so the IR absorbance values of the 5200 cm^1
(for molecular H2O) and 4500 cm1 (for OH^) peaks
were used. The molar absorption or extinction coefficients,
Ex, for phonolitic melts that were used for these peaks,
E5200 and E4500, are 110 012 L mol^1 cm^1 and
2226
WEBSTER et al.
VOLATILES IN PHONOLITIC^TRACHYTIC MELTS
with those of prior studies of H2O-saturated phonolitic
melts indicating a maximum of 79 03 wt % at 825^
9008C and 200 MPa (Larsen & Gardner, 2004; Schmidt
& Behrens, 2008).
CO2 by FTIR
Fig. 2. Comparison of the concentration of H2O (wt %) in phonolitic^trachytic run-product glasses determined by FTIR versus SIMS
and by FTIR versus indirect EPMA using the ‘by difference from
100 wt %’ technique. All determined H2O values exhibit good agreement, but the FTIR versus EPMA data exhibit greater dispersion.
The line plotted is 1:1line and not a fit to the data (see text for
discussion).
125 033 L mol^1 cm^1, respectively (Carroll & Blank,
1997). We tested the accuracy of our FTIR analyses for
total H2O using representative chips of haplophonolitic
glasses kindly provided by H. Behrens. One glass determined by KFT to contain 137 012 wt % H2O (Behrens
et al., 2009) returned 134 01wt % H2O by FTIR at the
AMNH, and a second glass containing 687 012 wt %
H2O determined by KFT (Behrens et al., 2009) returned
640 015 wt % H2O by FTIR.
The H2O concentrations of the glasses determined
by SIMS are consistent with those measured by FTIR
(Fig. 2). This agreement is meaningful, as the accuracy of
the SIMS data is based on bulk H2O contents of the standard glasses determined by KFT whereas the accuracy of
the FTIR data is anchored independently on the values
of E5200 and E4500 used.
We determined, indirectly, the H2O contents of all runproduct glasses by subtracting the measured EPMA totals
from 100 wt %, and these ‘by-difference-EPMA’ values are
also consistent with the FTIR data, even though the latter
comparison shows greater variability than the FTIR
versus SIMS data (Fig. 2). The larger dispersion in the
EPMA^FTIR comparison is largely due to the imprecision
in the EPMA totals, which are dominated by the error for
silica analysis and because it is the major component in
the glasses. The EPMA counting statistics show an imprecision of 075 rel %, which indicates that the measured
silica contents, totals, and apparent H2O concentrations
are reproducible to 05 wt %. The maximum H2O concentrations determined by FTIR in these melts at
200 MPa [752 04 wt % in this study and 770 018 wt
% in the study by Webster et al. (2003)] are consistent
The concentration of total CO2 in the glasses was determined by FTIR as the sum of molecular CO2 plus
CO3 2 . Carbon dissolves primarily as CO3 2; molecular
CO2 ranges from 5 to 120 ppm and most glasses contain
10^40 ppm. The ratios of (CO3 2 /CO2) increase with
increasing H2O in the melt, which is consistent with observations on C in andesitic melts (King & Holloway, 2002).
It has not been determined, however, if varying concentrations of H2O in melts may cause changes in carbon
speciation during the experimental quench of a hightemperature melt to a glass at ambient conditions. The
elevated (CO3 2 /CO2) of our glasses is consistent with
other intermediate-silica content glasses (Morizet et al.,
2002; King et al., 2004). The molar E2350 for the molecular
CO2 peak, determined by Morizet et al. (2002) for haplophonolitic glasses, that we used is 890 L mol^1 cm^1.
Interpreting absorptions of the carbonate bands in the
c. 1400^1600 cm^1 peak-doublet region of these phonolitic
glasses was challenging. Representative data for the IR
absorption intensity of CO3 2 could not be collected at
the c. 1525 cm^1 band because of its close proximity to the
large c. 1640 cm^1 H2O band. As a result, we collected absorption spectra for dissolved CO3 2 at the c. 1425 cm^1
band of the doublet as established by others (King et al.,
2004; Botcharnikov et al., 2006; Behrens et al., 2009; Vetere
et al., 2011). As noted by Behrens et al. (2004) for H2O- and
CO2-bearing dacitic glasses, establishing a relatively horizontal baseline at this band position is problematic with
elevated H2O in the glasses (owing to H2O and CO2
peak^band overlap). Similarly, the baselines for the c.
1425 cm^1 band in most of our glasses were also adversely
influenced by the variable H2O contents; baselines for
many H2O-enriched glasses were far from horizontal.
To resolve this overlap problem, we prepared and analyzed
C-, Cl-, and S-deficient phonolitic glasses with varying
H2O concentrations, and these glasses were matched with
the run-product glasses (for similar H2O contents) to provide appropriate spectral-subtraction baseline functions
for the c. 1425 cm^1 CO3 2 peak. These baselines were
slightly curved and horizontal as illustrated in Fig. 3a;
raw as well as baseline-subtracted FTIR spectra are
shown in this figure. This approach follows the methods
of King & Holloway (2002) and Behrens et al. (2004).
We also observed that the positions of and separation gap
between the two CO3 2 bands shift with composition
(Fig. 3b) as noted in prior work (Morizet et al., 2002).
The peak separation in our glasses increases with the
molar A/CNK, potentially reflecting the interaction of
CO3 2 with network-modifying Ca in the more alkaline
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Fig. 3. (a) Representative mid-IR absorption spectra highlighting bands of the c. 1415 and 1525 cm^1 carbonate doublet and the c. 1635 molecular
H2O peak. Dashed curve is a representative baseline applied to the corrected spectra. The spectra include raw absorbance data for a run-product glass and data for the same glass corrected for the influence of H2O by subtracting spectra for a CO2-free and H2O-poor phonolitic reference glass. Carbon as CO3 2 ion was analyzed at the c. 1415 cm^1 position for all run-product glasses using reference glass-subtracted spectra.
(b) Peak separation between the highest c. 1525 cm^1 and the lowest c. 1415 cm^1 carbonate peak versus the molar A/CNK ¼ [Al2O3/
(Na2O þ K2O þ CaO)] of the phonolitic^trachytic run-product glasses. Dissolved CO3 2 in glasses with high A/CNK bonds chemically with
alumina, and bonds with calcium in glasses with low A/CNK; NBO is non-bridging oxygens.
2228
WEBSTER et al.
VOLATILES IN PHONOLITIC^TRACHYTIC MELTS
melts [i.e. increasing non-bridging oxygens/tetrahedral,
network-forming cations ¼ (NBO/T)] and with networkforming Al in the more aluminous melts; this is consistent
with the spectral interpretations of Brooker et al. (2001a,
2001b) for phonolitic and other glasses.
We tested these analytical procedures with two relatively
anhydrous phonotephritic glasses; we used the published
molar c. E1525 of 308 L mol^1 cm^1 that was determined
experimentally for this specific composition (Behrens
et al., 2009). One glass containing 0265 wt % CO2 (measured by LECO, Behrens et al., 2009) returned 027 wt %
total CO2 by FTIR at the AMNH. A second glass containing 048 wt % CO2 returned 0475 wt % total CO2 by
FTIR.
A fundamental analytical issue of this study is establishing the accuracy of the total CO2 concentrations of the
run-product glasses. There are no published data for the
c. E1525 or c. E1425 for K2O-enriched phonolitic to trachytic glasses like these. Other studies (Cioni, 2000;
Oppenheimer et al., 2011) used values of E1525 (i.e. for this
band of the carbonate doublet) calculated via Dixon &
Pan (1995):
applied the H2O- and CO2-dissolution model of Papale
et al. (2006) and computed a maximum CO2 solubility
of c. 007 wt % in anhydrous tephriphonolitic melt at
200 MPa. However, phonolitic melts dissolve less CO2
than tephriphonolitic melts. In addition, extrapolation of
1^25 GPa solubility data (Morizet et al., 2002) indicates
maximum total CO2 concentrations of c. 009 and 007 wt
% dissolved, respectively, in anhydrous haplophonolitic
and Fe-bearing phonolitic melts at 13008C. Further extrapolation of their results from 13008 to the c. 9208C temperature of our runs would require an inordinately large
increase in CO2 solubility with decreasing temperature,
to confirm the presence of up to 16 wt % CO2 in our
melts. Consequently, we have analyzed published absorption coefficients for E1525 and E1425 (Table 6), and plotted
the available E1425 data as a function of bulk composition.
For missing E1425, we used available E1525 by assuming
equivalence between E1525 and E1425. This provides the relationship
E1525 ¼ 451 ½molar ðNa=Na þ CaÞ
E1425 ¼ 2113 þ 1179
ðSiO2 Þ ðCaO þ MgOÞ
molar
ðAl2 O3 þK2 O þ FeO þ TiO2 Þ
and subsequently assumed that E1425 is equivalent to E1525.
We calculated E1525 via Dixon & Pan (1995) and
assumed equivalence between E1525 and E1425 for each
run-product glass composition of this study (e.g. E1425
ranges from 133 to 177 L mol^1 cm^1), and determined maximum total CO2 concentrations of 156 wt % for glasses
containing the minimum H2O contents. These CO2
maxima, however, are inordinately high for phonolitic to
trachytic melts at 200 MPa. Oppenheimer et al. (2011)
(Fig. 4). This curve includes experimentally determined Ex
values for a variety of glasses and has an R2 coefficient
of fit of 088 (note that deletion of the outlier in the plot
increases R2 to 097). We have applied this relationship to
our run products and computed individual c. E1425 values
ranging from 218 to 229 L mol^1 cm^1 for all of our glasses.
These 60^70 rel. % larger values, compared with those of
Dixon & Pan (1995), return maximum total CO2 values
in melts that are 073 wt %.
Table 6: Absorption data for carbonate in silicate glasses
Glass composition
Absorption
coefficient
phonotephrite
Dixon & Pan
Absorption
E15251
coefficient
E14251
(1995) E1525
2
E14253 calculated
Reference
in this study
308
367
301
Behrens et al. (2009)
basanite
284
281
284
301
Dixon & Pan (1995)
tholeiite MORB
375
375
366
364
Fine & Stolper (1986)
leucitite
340
353
341
Thibault & Holloway (1994)
317
334
Shishkina et al. (2014)
Fine & Stolper (1986)
alkali basalt (B2507)
albite
311
200
dacite
basanite (A2549)
306
shoshonite
356
1
E1525
2
E1525
3
E1425
109
210
267
254
283
H. Behrens (personal communication)
322
328
344
Shishkina et al. (2014)
301
299
Vetere et al. (2011)
and E1425 values determined experimentally.
calculated with method of Dixon & Pan (1995).
calculated with method described in this study.
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Fig. 4. Published molar absorption coefficients (e) for the c. 1425 and 1525 cm^1 carbonate bands (Fine & Stolper, 1985, 1986; Thibault &
Holloway, 1994; Dixon & Pan, 1995; Behrens et al., 2009; Vetere et al., 2011; H. Behrens, personal communication) versus
molar {[(SiO2) (CaO þMgO)]/(Al2O3 þ K2O þ FeO þ TiO2)} for glasses ranging from albite to basaltic compositions. Outlying data
point indicated with question marks. The curve fitted to these data allows calculation of the absorption coefficients for each phonolitic^trachytic
run-product glass; the range in values used for this study is shown by the bold arrow. (See text for equation and discussion.) The 1s precision
is shown.
Summary: concentrations of volatile components in melts
of these experiments
We use the H2O and CO2 concentrations of the run-product glasses analyzed by FTIR as representative of these
volatiles in the phonolitic^trachytic melts at pressure and
temperature. The melts contained 15^75 wt % H2O,
004^094 wt % Cl, 0009^073 wt % CO2, 015^075 wt %
F, and detection limit (i.e. 0004 wt %)^016 wt % S.
Molar partition coefficients for the volatiles are reported
in Table 4. Using H2O as an example, the mole fractions
of volatile components in the melt, at run conditions, were
computed as
using the methods described previously. All reported fluid
compositions refer to vapor only, saline liquid only, or to
integrated-bulk fluid (i.e. the combined masses of vapor
plus liquid) for those experiments involving more than
one fluid phase at pressure and temperature. The mole
fractions of volatile components in these fluids were computed as
massfluidðsÞ
H O
XHfluidðsÞ
2O
2
18
¼8
fluidðsÞ
fluidðsÞ
fluidðsÞ
>
< massH O þ massCO þ massCl
2
18
>
:
massmelt
H O
2
44
35:5
9
>
=
>
massfluidðsÞ
massfluidðsÞ
massfluidðsÞ
F
S
cations ;
þ
þ
þ
32
19
257
2
melt
XH
2O
¼ massmelt
H2 O
18
þ
massmelt
CO2
44
18
massmelt
massmelt
massmelt
massmelt
anhydmelt
Cl
þ 355 þ 32S þ 19F þ
257
where anhydmelt is the molecular mass of anhydrous melt
calculated via Burnham (1981).
Fluid phase compositions and phase
relations in experiments and run products
The compositions of the fluids at run conditions were
determined by integrating the individual constraints on
CO2, H2O, Cl, F, and S in the run products established
where the rationale for the mass of cations used is explained below.
The quantity of CO2-dominated vapor determined gravimetrically after most experiments is closely representative
of the mass-balance determined quantity of CO2 in the
fluid(s) at run conditions. The total mass of CO2 in
the starting charge less the quantity of CO2 dissolved in
the melt is equivalent to the apparent mass of CO2 in the
bulk fluid or fluids. The latter mass is approximately
equivalent to the mass loss determined after freezing at
208C and puncturing the capsules (Fig. 5a), consistent
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WEBSTER et al.
VOLATILES IN PHONOLITIC^TRACHYTIC MELTS
Fig. 5. Constraints on the accuracy of the analytical, mass-balance,
and gravimetric methods used to determine the CO2, vapor plus
saline liquid plus salt precipitates, and the Cl abundances in the
run-product fluids at ambient or run conditions. (a) The post-run,
gravimetric mass loss (grams) of CO2-dominated vapor versus the
mass-balance computed mass of CO2 in the vapor (total mass of
CO2 in charge reduced by the mass of CO2 in run-product glass).
(b) The combined masses of run-product vapor, liquid, and salt precipitates determined gravimetrically versus the mass-balance constrained masses of vapor, saline liquid, or integrated vapor plus
saline liquid at run conditions. (c) Mass-balance concentration of Cl
in the bulk fluid(s) at run conditions (on a CO2-free basis) versus
the concentration of Cl in the CO2-free run-product fluid(s) measured with a chloridometer. (See Fig. 1 and text for details.) Straight
lines are fits to the data; 1s precision is shown.
with previous experimental work in the haplogranite^
H2O^CO2^S system (Webster et al., 2011). Both studies indicate that the quantities of compounds other than CO2
in the run-product vapor (e.g. H2, SO2, H2S, COS, and
others) are negligible at the levels of precision quoted;
otherwise the measured mass change would have exceeded
the mass of CO2 added to the starting charges of the runs.
Likewise, the mass of CO2, H2O, and dissolved salts in
the fluid(s) at run conditions was established gravimetrically by heating the previously frozen, punctured, and
soaked capsules above the boiling point of salt-bearing
H2O and determining the change in mass (Fig. 1). The
gravimetrically determined masses of fluid(s) are similar
to those computed by mass balance (Fig. 5b).
The mass-balance constrained concentrations of Cl in
the fluid(s), on a CO2-free basis, are similar to the chloridometer values (Fig. 5c) for 12 of the runs. The dilutioncorrected, chloridometer-determined Cl concentrations
bear on CO2-free aqueous run-product liquids because
the run capsules have lost CO2-rich vapor during the
freeze-and-puncture procedure.
The concentrations of S and F in the fluid(s) were also
calculated by mass balance (Fig. 1). This approach assumes
that no fluoride, sulfide, or sulfate phases were stable at
run conditions. It should be recalled that trace CaF2 and
CaSO4 were present in the heated and dried solid salts of
some run products, but neither phase was observed in the
glasses. Moreover, previous research demonstrates that
CaSO4 is highly soluble in alkali chloride-bearing fluids
at elevated pressure and temperature, and hence is unlikely
to have been stable with the melt or fluids of these experiments (Newton & Manning, 2005; Webster et al., 2009).
The reported fluid (Table 4) compositions were adjusted
for cations in the fluid(s). Most charged components are
chemically associated (i.e. anions with cations) in aqueous
fluids at these pressure^temperature conditions, because
of the strong influence of temperature in reducing the dielectric constant of H2O in aqueous solutions (Helgeson
& Kirkham, 1974; Holloway, 1981). We assume that all
chloride was associated with dissolved alkali and Ca ions;
the abundances of Hþ have been shown to be minor at
these conditions (Shinohara, 2009). We used a cation mass
of 23 (equivalent to that of Na) and note that the fluid
compositions are not sensitive to the relative abundances
of Hþ, Ca2þ, Naþ, and Kþ used. The cation ratios can be
modified significantly without strongly changing the final
fluid compositions.
At run conditions, the bulk integrated fluids as constrained by mass balance contained 11^98 mol % H2O,
001^81mol % CO2, c. 01^34 mol % Cl, below detection
limit (e.g. 01mol %)^37 mol % total S, and below detection limit (e.g. 01mol %)^17 mol % F. These fluids also
contained 40 mol % cations (treated as Naþ in this
study) and other aluminosilicate components. The
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calculated concentrations of H2O, CO2, and Cl in the
fluid(s) at run conditions are generally precise to 10^15
rel. %. These low errors are largely influenced by the variance for EPMA analysis of the run-product glasses and because of the large fluid:melt mass ratios used at run
conditions. Most runs had 2:10 to 45:10 mass ratios so the
associated weighing errors on any given component added
to the starting charges were small. The errors on S and F
in the fluid(s) are much larger primarily because of the
low concentrations of S and F in the starting charges and
in the run-product glasses. Consequently, we report and
discuss the S and F contents of fluid(s) for only those runs
with relatively low imprecision.
Oxygen fugacity and speciation of sulfur
during experiment
The equilibrium fO2 of these experiments is controlled by
the intrinsic fH2 of the steel-walled IHPV; the relatively
oxidizing or reducing nature of multivalent C, Fe, H, and
S in the starting charges and the activity of H2O. Previous
testing with oxygen sensors determined the ambient log
fO2 of the IHPV to be NNO þ 2 log units (where NNO is
nickel^nickel oxide buffer) for H2O-saturated runs at
2 kbar and 8008C. Half of these runs were conducted for
200^350 h, presumably allowing time for equilibration of
melts and fluids with the IHPV itself.
Most S in these melts was relatively oxidized. The speciation of S was determined, and the (S6þ/STOTAL) ranges
from 035 to 073 for the five most S-enriched run-product
glasses. Using methods of Carroll & Rutherford (1988)
and Wallace & Carmichael (1994), we calculated values of
log fO2 that ranged from NNO þ 02 to NNO þ 088 for
the most S-rich melts (Table 4).
The S concentrations of the other run-product glasses
are too low to determine apparent fO2 values. However,
prior experiments using this IHPV return a general range
in fO2 of NNO ^ 04 to NNO þ16 (Webster et al., 2009,
2011). It follows that these experiments were conducted at
oxidizing conditions in or near this range.
Behavior of individual C^O^H^Cl^S^F
volatile components at c. 200 MPa and
9208C
In the following discussion, we distinguish four categories
for the c. 9208C experiments: (1) previous H2O^Cl-only
200 MPa runs with no added CO2 or S from the study of
Webster et al. (2003); (2) new H2O^CO2-only added runs
(this study); (3) new H2O^CO2^Cl-only added runs (this
study); (4) new H2O^CO2^Cl^S-added runs (this study).
Experiments in which no Cl and/or S were added to the
starting charges still contained trace Cl and S in the starting rock powder or glasses, but for these runs the concentrations of Cl and S in the fluid(s) at run conditions were
503 mol % and should have had a minimal influence
on fluid phase relations. We include data for two 10188C
NUMBER 11
NOVEMBER 2014
experiments for comparison but do not discuss them
further; they are included only for general information.
Water
The concentrations of volatile components that dissolve in
these melts vary strongly with melt and fluid composition
(Figs 6^8). The H2O contents of phonolitic^trachytic
melts increase with increasing H2O in the coexisting
fluid(s) (Fig. 6a), but this relationship varies with the relative abundances of H2O, CO2, Cl, and S in the fluid(s).
Most melts of the S- and Cl-deficient, H2O^CO2-only
added runs, the H2O^Cl-only runs, and those of the
H2O^CO2^Cl^S-added runs show linearly decreasing
H2O in the melts with decreasing XH2O fl, and these data
fit closely to the curve in Fig. 6a. The H2O^CO2^Cl-only
added runs also follow this trend but exhibit greater dispersion. In particular, we note that six of the H2O^CO2^
Cl-only added melts and one of the H2O^Cl-only melts
lie above the curve and have elevated H2O concentrations,
and five of the H2O^CO2^Cl-only added melts lie below
the curve, reflecting lower H2O concentrations than those
of the curve for equivalent mole fractions of H2O in the
fluid(s).
The results also show that H2O solubility in these
fluid(s)-saturated melts increases rapidly to 30 mol % as
the XH2O fl increases from zero to 03. Similar H2O dissolution behavior was observed previously with S- and
Cl-free phonotephritic melts at 200 MPa and 12508C
(Behrens et al., 2009) and S- and Cl-free andesitic melts at
200 MPa and 12008C (Botcharnikov et al., 2007).
Carbon dioxide
Melts of the S- and Cl-deficient, H2O^CO2-only added
runs fit closely about a curve that projects towards the
origin with decreasing CO2 in the system (Fig. 6b). The
CO2 concentrations of these melts decrease with decreasing abundances of CO2 in the fluid(s). Conversely, the
variability of the data about this curve increases with
increasing Cl in the runs; the H2O^CO2^Cl-only added
runs and H2O^CO2^Cl^S-added runs show greater dispersion than the Cl-poor runs. If the two most alkaline
runs plotted in this figure are excluded, the melts that plot
farthest from the curve involve the most Cl-enriched fluid
compositions. The CO2 contents of these melts do not increase in a regular fashion with increasing CO2 in the
fluid(s) as those of the H2O^CO2-only added runs do;
seven of the H2O^CO2^Cl-only added runs lie above the
curve and two other such runs lie below the curve for
S- and Cl-deficient, H2O^CO2-only added runs.
Chlorine
The dissolution of Cl in phonolitic and trachytic melts increases with increasing Cl concentration in the fluid(s),
and the S-poor melts exhibit a Cl solubility maximum of
c. 35 mol % (i.e. 094 wt % Cl) (Fig. 6c) at which the Cl
2232
WEBSTER et al.
VOLATILES IN PHONOLITIC^TRACHYTIC MELTS
Fig. 6. Comparison of the concentrations of H2O (a), CO2 (b), and Cl (c) in fluid(s)-saturated, C^O^H^Cl^S^F-bearing phonolitic^trachytic melts
versus those in vapor, saline liquid, and bulk integrated vapor plus saline liquid at 200 MPa and 900^10188C. All melts except for the two 900^9278C
runs noted in the text and both 10188C runs have molarA/CNK ¼ [Al2O3/(Na2O þ K2O þ CaO)]408; all others range from 065 to 075. In (a), the
mole fraction of H2O in silicate melts increases with increasing mole fraction of H2O in coexisting fluid(s). Minimal dispersion and close fit to the curve
are shownby runs with COH-dominated vapors; in contrast, data for Cl- and S-enriched experiments exhibit greater dispersion. In (b), the COH-dominated vapors show minimal dispersion and a close fit to the curve, whereas fluids in Cl- and S-enriched experiments exhibit significant dispersion for
CO2. In (c), Cl in silicate melts increases with increasing Cl inthe fluid(s) untilthe melts achieve their solubility limit, whichvaries withbulkcomposition
(Webster & De Vivo, 2002) and with S content (Webster et al., 2009). Smaller upward-pointing filled triangles, H2O^CO2^Cl-only added runs at
900^9278C; larger upward-pointing filled triangles, H2O^CO2^Cl^S-added runs at 906^9158C; larger open triangles, H2O^CO2^Cl S-added runs
conducted at 10188C; filled squares, 910^9448C, H2O^CO2-only added runs with503 mol % S or Cl and51mol % F in fluids. Open crosses, H2O^Cl
only runs conducted at 924^9558C (Websteret al.,2003). Curves are applied and not fitted to data. (See text for discussion.)
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Fig. 7. Variation of the CO2 versus H2O concentrations in C^O^H^Cl^S^F-bearing phonolitic^trachytic melts saturated in vapor, saline
liquid, or bulk integrated vapor plus saline liquid at 200 MPa and 900^10188C. The continuous curve fits the melts of the 910^9448C, H2O^
CO2-only added runs with 503 mol % Cl or S and 51mol % F in the vapor. In (a), all melts except the four highlighted have molar A/
CNK ¼ [Al2O3/(Na2O þ K2O þ CaO)]408. The solubility of CO2 increases with melt alkalinity. The other 900^9278C, Cl- S-bearing
runs show lower H2O and CO2 solubilities in the melt relative to those of the curve. The four H2O^CO2^Cl^S-added runs are each labeled
with their oxygen fugacities. The area outlined with dotted lines has runs characterized by (XCl fl XCO2 fl )40004; all other runs have lower
(XCl fl XCO2 fl ). In (b), the dashed curve for H2O^CO2-only melts with molar A/CNK408 at c. 9008C and 120 MPa appears to fit some of the
Cl- and/or S-enriched melts; the curve is based on interpolated H2O solubility maxima for CO2- and Cl-free phonolitic melts at 101^151MPa
(Schmidt & Behrens, 2008). The suppressed H2O and CO2 concentrations of Cl- S-enriched phonolitic and trachytic melts at 200 MPa are
consistent with the 120 MPa, H2O and CO2 solubilities of C^O^H-only melts. Symbols are the same as in Fig. 6. Curves are schematic and
not fitted to the data. (See text for discussion.)
2234
WEBSTER et al.
VOLATILES IN PHONOLITIC^TRACHYTIC MELTS
Fig. 8. Variation of H2O versus Cl concentrations in phonolitic^trachytic melts saturated in C^O^H^Cl^S^F-bearing vapor, saline liquid, or
bulk integrated vapor plus saline liquid at 200 MPa and 900^9558C. Two runs with anomalously low, molar N/NK [i.e. Na2O/
(Na2O þ K2O)5075] in the melt are distinguished. The interpretative lines express the phase equilibria, solubilities of H2O and Cl in the
melt, and highly non-ideal mixing behavior of these volatiles when vapor and saline liquid coexist with silicate melt (filled circles). The square
on the y-axis indicates the approximate maximum H2O solubility in these melts when saturated in pure H2O vapor. The squares on the x-axis
and the continuous and dashed vertical lines reflect maximum Cl solubilities for (1) S-poor melts and for (2) S-enriched melts saturated in a
chloride liquid. Chlorine solubility in the melts decreases with increasing S, but does not appear to vary with the CO2 content of the system.
The data dispersed below the horizontal curve reflect the effect of varying melt composition on Cl solubility in the melts (see text for discussion).
Symbols are the same as in Fig. 6; 1s precision is shown.
contents of the melts remain fixed with additional increasing Cl in the fluid(s). The Cl solubility limit for S-enriched
melts is 25 mol %. This non-linear dissolution behavior
has been observed before with hydrous, CO2-free phonolitic melts (Signorelli & Carroll, 2000; Webster et al., 2003).
Previous work shows that the maximum solubility of Cl
in such melts varies with bulk composition (Webster &
De Vivo, 2002), pressure (Signorelli & Carroll, 2000), and
the concentration of oxidized S in the melts (Webster
et al., 2009). Thus, higher melt Cl contents are favored
under S-poor conditions relative to S-rich conditions; this
has been determined previously for CO2-poor experiments
involving phonolitic, basaltic, and rhyodacitic melts
(Webster & Botcharnikov, 2011). The two most alkaline
melts lie off the curves and, hence, show differing Cl solubilities from those of less alkaline melts. In addition, our
data show no clear relationship between the maximum Cl
solubility and the CO2 contents of the fluid(s), which
was also observed for fluid(s)-saturated COHCl-bearing
andesitic melts (Botcharnikov et al., 2007).
on volatile solubilities is less clear from this limited dataset.
Two runs involving c. 24 mol % S in the fluid(s) and the
other runs involving c. 31mol % S in fluid(s) show increasing solubility of S in melt with increasing fO2, which
is consistent with previous experimental observations at
fO24NNO (Wallace & Edmonds, 2011; Webster &
Botcharnikov, 2011; Wilke et al., 2011).
Sulfur
Experimental investigations of volatile component
solubility in aluminosilicate melts for H2O-, CO2-,
Cl-, H2S- and SO2-bearing systems are currently
The dissolution of S in melts of the H2O^CO2^Cl^Sadded runs varies strongly with fO2 but the influence of S
Fluorine
The dissolution of F in silicate melts shows no systematic
changes with melt composition, or the S or CO2 concentrations of the fluid(s) with increasing F in coexisting
fluid(s) from c. 01 to 17 mol %. We note here that the imprecision for F analysis by EPMA in the starting and runproduct glasses could mask such relationships.
D I S C U S S I O N A N D A P P L I C AT I O N
O F R E S U LT S
C^O^H^Cl^S^F volatile solubilities, mixing,
and phase relationships
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JOURNAL OF PETROLOGY
VOLUME 55
restricted to andesitic and basaltic melts (Botcharnikov
et al., 2007; Lesne et al., 2011). Relevant thermodynamic
models are similarly limited; Witham et al. (2012) recently
published a model bearing on C^O^H^S^Cl vapor coexisting with basaltic melts (i.e. at single fluid conditions).
Heterogeneous equilibria in the CO2-free, pseudoternary system H2O^NaCl^silicate melt indicate the presence of low-density aqueous vapor and higher-density
saline liquid with as little as 10 wt % NaCl in the system
at magmatic temperatures and shallow-crustal pressures
(Shinohara et al., 1989; Webster et al., 1999; Signorelli &
Carroll, 2000; Webster, 2004; Botcharnikov et al., 2007;
Webster & Mandeville, 2007; Stelling et al., 2008).
Modeling (Dreisner & Heinrich, 2007; see http://www.
geopetro.ethz.ch/people/td/sowat) indicates that the critical solution pressure is between 158 MPa at 8008C and
218 MPa at 10008C in the system NaCl^H2O. These conditions are similar to those of our c. 9208C and 200 MPa
runs and consistent with those runs involving two fluids.
Other research (Bischoff et al., 1996; Duan et al., 2006) has
shown that adding Ca and Mg to the system NaCl^H2O
increases the width of the stability field for vapor plus
saline liquid. This is relevant to our study because the
phonolitic^trachytic melts would have exchanged alkaline
earth ions with the fluid(s).
Observations on the ternary system NaCl^CO2^H2O
indicate that H2O-rich aqueous^carbonic vapor coexists
with saline liquid through much of the compositional
space at 200 MPa and 9208C, because addition of CO2 to
NaCl^H2O expands the two-fluid stability field (Bowers
& Helgeson, 1983a, 1983b; Joyce & Holloway, 1993; Duan
et al., 1995; Heinrich, 2007) and enhances the degree of
non-ideality of mixing in the fluid(s) (Duan et al., 1995;
Joyce & Holloway, 1993; Botcharnikov et al., 2004;
Shmulovich & Graham, 2004; Mao et al., 2010).
Extrapolation of the experimental data of Schmidt &
Bodnar (2000) for the compositionally simple NaCl^
CO2^H2O system to the conditions of our experiments
suggests that two fluids were stable with (XCl fl XCO2 fl )4
0011 and a single fluid was present with (XCl fl XCO2 fl )5
0007 in our runs.
The dissolution of H2O, CO2, S, and Cl in aluminosilicate melts reveals insights into volatile mixing behavior
(Botcharnikov et al., 2007). The mixing properties can be
interpreted through
Gmix ¼ Gideal mix þ Gexcess mix
where Gexcess mix expresses the non-ideal contribution to
mixing (Fletcher, 1993; Prausnitz et al., 1999). Spontaneous
mixing of components is associated with negative values
of Gmix (Anderko & Pitzer, 1993; Fletcher, 1993;
Prausnitz et al., 1999). In our experiments, H2O and CO2
in Cl-poor runs exhibit mixing properties and activity^
composition relationships (Fig. 7a and b) that more closely
NUMBER 11
NOVEMBER 2014
approach those of Raoultian, ideal-solution behavior (i.e.
Gexcess mix is zero), but this response contrasts with that
involving runs with added Cl (Fig. 7). Even though all experiments involved at least trace quantities of Cl^, H2O
and CO2 are the primary fluid components in Cl-poor
runs on a molar basis, and, hence, molecular interactions
involving H2O and CO2 dominate. Chemical interactions
between H2O^CO2 (Fig. 7a and b) exceed those between
like molecules (e.g. H2O^H2O and CO2^CO2) at these
pressure^temperature conditions. The interactions between H2O^CO2 lead to component activities that are
less than the respective mole fractions (e.g. aCO25XH2O
and aCO25XCO2), and in turn, the reduced aH2O and
aCO2 lead to negative values of Gexcess mix. As a result, a
single fluid phase is stable because of the strong chemical
drive toward volatile component mixing. Conversely, with
increasing Cl contents in the system (Fig. 8), interactions
between similar molecules or ions (e.g. Cl^^Cl^, H2O^
H2O, and CO2^CO2) become increasingly important,
and aCl XCl, aH2O XH2O, and aCO2 XCO2,
which causes Gexcess mix to become increasingly positive.
In turn, this leads to strongly non-ideal solubility behavior, which is expressed as relatively buffered volatile concentrations in melt as described below. Ultimately, with
continued increasing Cl in the system, strongly non-ideal
conditions cause unmixing of fluid in which a saline
liquid condenses from vapor (Webster et al., 1999;
Shmulovich & Graham, 2004).
The influences of S and F on fluid phase relations are undetermined. Nevertheless, Botcharnikov et al. (2004) noted
that the significant, permanent dipole moments for the
dominant S species H2S and SO2 support their chemical
interactions with alkali and alkaline earth ions, chloride,
and H2O dissolved in aqueous solutions. It follows that
with the addition of S to H2O^CO2^Cl fluids, the activities of the S species, Cl, H2O, CO2, and alkali ions dissolved in the fluid will be less than their mole fractions
and, hence, the Gexcess mix will be characterized by negative
values that support mixing. This volatile behavior reduces
the width of the vapor plus brine stability field. The role
of F in these processes and fluid(s) is unconstrained.
H2O vs CO2 Cl and S at 200 MPa
The H2O^CO2-only added runs exhibit increasing CO2
and decreasing H2O concentrations in phonolitic melt
with increasing CO2 abundance in the fluid(s) (Figs 6a, b
and 7a, b); this solubility behavior is generally characteristic of silicate melts free of Cl and/or S (Blank & Brooker,
1994; Behrens et al., 2004; Botcharnikov et al., 2005;
Moore, 2008). With the addition of Cl with or without S,
however, the volatile-component solubility behaviors
change, and volatile behavior correlates with the
(XCl fl XCO2 fl ) and melt alkalinity. With increasing Cl in
the systemçbut at relatively low (XCl fl XCO2 fl )50004ç
the H2O and CO2 solubilities in melts of the H2O^CO2^
2236
WEBSTER et al.
VOLATILES IN PHONOLITIC^TRACHYTIC MELTS
Cl-only added runs are similar to those of melts of
the H2O^CO2-only runs. Runs characterized by
(XCl fl XCO2 fl )40004 are contained within the dotted outline field in Fig. 7a. As the (XCl fl XCO2 fl ) exceeds 0004,
most of the melts dissolve less H2O and CO2 than those
values shown for the curve for H2O^CO2-only melts. It is
noteworthy, however, that several peralkaline melts
(molar A/CNK508) exhibit higher H2O and CO2 concentrations given that their (XCl fl XCO2 fl ) is40004; this influence of melt alkalinity on volatile solubility is consistent
with prior work (Vetere et al., 2011). The suppression of
volatile solubilities is significant for some runs. The H2O^
CO2^Cl-only added run with 28 wt % H2O and 019 wt
% CO2 in melt probably involved vapor plus saline liquid
given that the (XCl fl XCO2 fl ) for the bulk integrated fluids
is large (i.e. 0037). For comparison, a corresponding
H2O^CO2-only melt with 28 wt % H2O (located on the
bold curve) would dissolve 043 wt % CO2, which is more
than twice that dissolved in this H2O^CO2^Cl-only
added run, and an H2O^CO2-only melt with 019 wt %
CO2 (located on the curve) would dissolve 67 wt %
H2O, which is also more than twice as great.
The reduced H2O and CO2 solubilities in these melts
(Fig. 7) and the dispersion shown by the Cl- S-enriched
runs (Fig. 6a and b) reflect dilution of H2O and CO2 in
the fluids and melt by the addition of Cl and S, as well as
non-ideal volatile-solubility behavior associated with fluid
immiscibility. It is clear that the addition of Cl S will
reduce the activities of H2O and CO2 in the corresponding
melt and fluid(s) by dilution of H2O and CO2 in the bulk
system. The influence of dilution, owing to the addition of
Cl S, is less clear when comparing Fig. 6a with 7a.
Numerous Cl-enriched melts plot above the H2O^CO2only curve, potentially implying that increasing Cl and S
concentrations in the vapor liquid actually enhance
H2O solubility in melt. Likewise, comparison of Fig. 6b
with 7b could result in a similar misinterpretation that
CO2 solubility in melt increases with increasing Cl and S
concentrations in the vapor liquid. In this regard, it is
important to recall that the reported fluid compositions
for runs involving two fluids are the integrated fluid compositions and not the individual compositions of vapor
and saline liquid. We have not determined how the activities of H2O and CO2 in the vapor and in the saline
liquid, coexisting with these melts, actually changed with
addition of Cl S. Thermodynamically, it is the individual
activities of H2O and CO2 in the vapor and saline liquid
that influence the H2O and CO2 concentrations of the
melts.
The reduction in solubilities and dispersion shown by
CO2 may also reflect competition between Cl and CO3 2
dissolving in similar sites in the silicate melt. The solubility
of Cl at 200 MPa is dominated by the concentrations of
Ca and Mg in aluminosilicate melts; each mole of Ca and
Mg, for example, is associated with 024 moles of dissolved
Cl (Webster & De Vivo, 2002). This is also consistent with
NMR spectra (Sandland et al., 2004) implying that the
Cl-coordination environment involves network-modifying
Ca and Na in aluminosilicate glasses. Likewise, CO3 2
solubility in melt correlates with Ca and Mg concentrations (Brooker et al., 2001a; King et al., 2004), and in general
FTIR spectra indicate distinct chemical associations involving Ca and CO3 2 (Brooker et al., 2001b; Morizet et al.,
2002), which is consistent with interpretation of the peak
separations of the carbonate bands in these glasses
(Fig. 3b).
Hydrothermal experiments with C^O^H^Cl-bearing
andesitic melts (Botcharnikov et al., 2007) conducted at
12008C and 200 MPa exhibit similar reductions in CO2
and H2O solubility behavior. With the (XCl fl XCO2 fl ) 004, the CO2 concentrations in andesite melt are an
order of magnitude lower than those with less Cl in the
system and the H2O concentrations are reduced by 15^
3 wt % relative to that in Cl-poor melts. All other runs
with (XCl fl XCO2 fl )5001 exhibit CO2 and H2O concentrations in melt that are equivalent to those with no Cl.
With the addition of S as well as Cl, one of the melt
compositions is fitted reasonably well by the curve for
H2O^CO2-only added melts (Fig. 7a), but the other three
H2O^CO2^Cl^S-added runs show increasing separation
from the basic H2O^CO2 solubility curve. The volatile
solubilities also correlate with fO2, because the separation
from the curve increases as fO2 decreases toward NNO.
These reduced volatile solubilities of S-enriched,
phonolitic^trachytic melts are similar to that observed
when S was added to haplogranitic melts saturated in
C^O^H^S vapor with runs at fO2 NNO ^ 03 (Webster
et al., 2011). Addition of S led to lower CO2 contents of
their felsic melts. The CO2 and H2O contents of most of
our H2O^CO2^Cl-added runs and three of the H2O^
CO2^Cl^S-added runs show marginally to strongly
reduced CO2 and H2O solubilities and, as a consequence,
these 200 MPa volatile solubilities are more similar to
those of 120 MPa solubility curves for Cl- and S-free melts
(Fig. 7b).
H2O vs Cl CO2 at 200 MPa
The concentrations of H2O and Cl in the experimental
melts vary over a significant range, but their maximum
concentrations are fixed because of the strongly non-ideal
mixing behavior of volatiles at these conditions (Fig. 8),
consistent with previous observations (Fig. 6c). Five melts
exhibit relatively fixed H2O concentrations of c. 77 wt %
even though Cl abundances in the melt change from 02
to 09 wt %. Chlorine solubility maxima also vary with S
in phonolitic^trachytic melts. The variable Cl contents of
S-poor and S-enriched melts, respectively, are a function
of differences in bulk melt composition as well as differing
S contents. Conversely, the differing CO2 contents have
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JOURNAL OF PETROLOGY
VOLUME 55
no influence on Cl solubility in S-poor and S-enriched
melts. This discussion excludes the two runs with low
molar N/NK ratios; decreasing this ratio reduces H2O
solubility in the melt and also affects CO2 solubility.
The observation of buffered H2O concentrations in the
melt with increasing Cl contents is consistent with previous
experiments conducted at 200 MPa involving andesitic
melt and H2O^CO2^Cl fluid(s) (Botcharnikov et al.,
2007) and other runs at 25, 50, and 100 MPa involving basaltic melts and H2O^CO2^Cl vapor (Alletti et al., 2009).
In summary, the dissolution of H2O, CO2, S, and Cl in
silicate melts at shallow crustal pressures is strongly controlled by the fluid phase relations, which themselves are
a result of non-ideal solubility behavior. As described by
Botcharnikov et al. (2004), the tendency for unmixing for
alkali chloride-bearing aqueous fluids is strong, with activity coefficients of the volatile species 1 (Anderko &
Pitzer, 1993). The addition of CO2 enhances the degree of
non-ideality by further increasing the activity coefficients
(Duan et al., 1995; Joyce & Holloway, 1993; Shmulovich &
Graham, 2004).
Fluid^melt partitioning of H2O, CO2, Cl, S,
and F
The distribution of H2O, CO2, Cl, S, and F between
fluid(s) and melt is expressed as molar partition coefficients, Dxi, to account for the differing molecular masses
of each volatile (Fig. 9). Despite the dispersion in the data,
the partition coefficients clearly show that CO2, Cl, and S
dissolve strongly in the fluid(s) compared with the melt
and that H2O partitions preferentially in favor of fluid(s)
for most runs. Water, CO2, and S partition less strongly in
favor of fluid(s), by an order of magnitude (Fig. 9b), with
respect to phonolitic and trachytic melts compared with
systems involving rhyolitic to andesitic melts (Scaillet &
Pichavant, 2003; Webster et al., 2011) owing to greater solubilities of these volatiles in the former. All runs show that
F dissolves more strongly into phonolitic^trachytic melts
than fluid(s). The dissimilar partitioning behavior of F
versus Cl is important because it has been shown to lead
to halogen fractionation in fluid-saturated magmas
(Aiuppa et al., 2009; Le Voyer et al., 2010). Our experimental results, however, are not consistent with the results of
other studies of F partitioning between fluids and phonolitic melts (Chevychelov et al., 2008) and F partitioning
between fluids and basaltic melts (Alletti, 2008; see summary by Baker & Alletti, 2012) that indicate that F partitions more strongly into fluids relative to these melts.
The distribution of CO2 between fluid(s) and melt increases with the CO2 content of the fluid(s), but the other
volatile components show no discernible dependence on
CO2 in fluid(s) (Fig. 9a). This contradicts the observation
of Alletti et al. (2009) that increasing CO2 in vapor
causes more Cl to partition in favor of basaltic melts
at 50^100 MPa. Similarly, Botcharnikov et al. (2007)
NUMBER 11
NOVEMBER 2014
determined that Cl partitioning between andesitic melt
and fluid(s) shows no influence on the mol % CO2 in the
system. The partitioning of these volatiles shows no relationship with the molar H2O contents of phonolitic melts
(Fig. 9b). The partitioning of Cl, H2O, CO2, and F does
vary with the molar [Na2O/(Na2O þ K2O)] of the melts,
however (Fig. 9c).
Before interpretation of degassing and eruptive processes
for shallow, phonolitic volcanic systems with these 200 MPa
experimental data, we must first address a related issue.
Specifically, the total abundance of all chloride species measured in natural volcanic gases at near-surface pressure and
magmatic temperatures may have little bearing on the maximum concentrations of Cl dissolved in magmatic fluids at
depth (Fig. 10) because some Cl-bearing gases undergo
vapor^saline liquid immiscibility during ascent. As a result,
the compositions of these volcanic gases represent the chloride-poor, low-density vapor phase and reflect the consequences of chloride sequestration by a more dense saline
liquid (Giggenbach, 1997; Shmulovich & Churakov, 1998).
Experimental data for the system H2O^NaCl at 1MPa indicate that vapor coexists with saline liquid with 001wt %
NaCl at 8008C and with 02 wt % NaCl in the bulk integrated fluid at 10008C (Pitzer & Pabalan, 1986). The lower
the pressure (i.e. the more shallow the depth at which
liquid condenses from vapor), the lower the Cl content of
the vapor and the higher the Cl concentration of the saline
liquid. In addition, vapors from alkaline magmas may be
enriched in CO2 and CH4: these species expand the field
of vapor^liquid immiscibility (Heinrich, 2007). Shmulovich
& Churakov (1998) noted that the bulk of the Cl originally
dissolved in a melt may be retained by the magma as relatively anhydrous molten salt. In this regard, molten salt
was observed to flow from fissures in the Mt. Somma^
Vesuvius crater following its 1944 eruption (Chiodini et al.,
2001). It follows that the chloride concentrations of volcanic
gases will be less than those of deeply exsolved magmatic
fluids, and hence, equivalent to those of only the vapor in
equilibrium with coexisting saline liquid and H2O^CO2^
Cl S F-bearing, alkaline silicate melts at low pressures
(Webster & Mandeville, 2007; Aiuppa et al., 2009). It is noteworthy that fluids exsolved from metaluminous and peraluminous melts will include a significant HCl component and
that dissolved alkali choride species may be minor in comparison (Williams et al., 1997; Frank, 2003); thus, observations on the H2O^NaCl system may have less relevance to
fluid behavior for non-alkaline magmas.
Applications to degassing of phonolitic
eruptive systems and their mineralized
equivalents
These experimental results provide new constraints on the
depths of magma storage and processes of fluid^melt interaction during magma evolution for phonolitic volcanic
2238
WEBSTER et al.
VOLATILES IN PHONOLITIC^TRACHYTIC MELTS
Fig. 9. Variation of the molar fluid(s)/melt partition coefficient, Dvolatile fluidðsÞ=melt, for H2O, CO2, Cl, S, and F at 200 MPa and 900^9278C as a
function of: (a) the mol % CO2 in fluid(s), (b) the mole fraction of H2O in the melt, and (c) the molar [Na2O/(Na2O þ K2O)] of phonolitic^trachytic melts. In (a) the partitioning of H2O, Cl, S, and F does not vary, but CO2 partitioning increases slightly, with increasing CO2 concentration in the fluid(s). In (b) the partitioning of CO2, Cl, S, and F does not vary, but H2O partitioning does change with the H2O content
in the melt. In (c), data for the two trachytic runs with lowest alkali contents in the melt are distinguished with ellipses. The partitioning of
H2O and CO2 increase and the partitioning Cl decreases with increasing molar [Na2O/(Na2O þ K2O)] of these melts. Crossed squares, F;
half-filled squares, H2O; triangles, CO2; diamonds, Cl; circles, S partitioning. Arrows indicate volatile partitioning between fluids and rhyolitic
to andesitic melts (Scaillet & Pichavant, 2003). (See text for discussion.)
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JOURNAL OF PETROLOGY
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NUMBER 11
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Fig. 10. Variations of the compositions (mol %, cation free) of the vapor, saline liquid, or bulk integrated vapor plus saline liquid in equilibrium
with phonolitic^trachytic melts at 200 MPa and 900^9278C. (a) CO2^H2O^HCl; (b) CO2^H2O^(10 SO2); (c) CO2^H2O^(10 HF).
Fluids are dominated by H2O and CO2. Symbols are the same as in Fig. 6, except that all down-pointing triangles indicate CO2^H2O^Cl
added runs and pentagons represent the measured compositions of volcanic vapors emitted at the lava lake of Mt. Erebus, Antarctica,
December 2004 (Oppenheimer & Kyle, 2008). (See text for discussion.)
2240
WEBSTER et al.
VOLATILES IN PHONOLITIC^TRACHYTIC MELTS
feeding systems and their intrusive counterpartsçsome of
which are mineralized. Although these experimental data
are limited to one pressure condition, they are highly relevant because they involve all five of the primary volatile
components H2O, CO2, S, Cl, and F and they relate to a
pressure at which magmas pond, fractionate, and exsolve
fluid(s). The compositions of phonolitic and related
magmas and their melt inclusions (MI) and matrix glasses
provide constraints on the abundances of volatiles for eruptive systems (Table 1). Phonolitic volcanoes emit magmas
containing c. 01^5 wt % H2O, 001^012 wt % CO2, 01^
08 wt % Cl, 007^011wt % S, and 017^1wt % F in the
melt. Oppenheimer et al. (2011) noted that extensionrelated, alkaline magmas like these may be enriched in
CO2. Moreover, CO2-charged vapors may separate from
their source melt and flush through the overlying magma
in alkaline systems such as that of Campei Flegrei, Italy
(Cannatelli et al., 2007; Mangiacapra et al., 2008) and
affect the behavior of other volatile components dissolved
in the magma. Given the observation that complex,
mixed-volatile fluids alter the solubilities of CO2 and
H2O in melts significantly, it is instructive to apply these
relationships to relevant, volatile-enriched phonolitic
magmas.
Mt. Somma^Vesuvius, Italy
Melt (MI) and fluid inclusion (FI) studies indicate that
Mt. Somma^Vesuvius, Italy, has erupted trachytic, tephritic, and phonolitic magmas that are variably enriched in
H2O, CO2, S, Cl, and F (Table 1) for more than 25 kyr
(Belkin et al., 1985, 1998; Marianelli et al., 1995; Raia et al.,
2000; Melluso et al., 2012). Water concentrations in Mt.
Somma^Vesuvius MI range from 1 to 5 wt % on average
(Signorelli et al., 1999; Scaillet et al., 2008). Average abundances of Cl are 06^08 wt % (Table 1), whereas some MI
contain up to 115 wt % Cl (Fulignati & Marianelli, 2007;
Balcone-Boissard et al., 2008). Sulfur varies from hundreds
to thousands of ppm on average. Most studies of Mt.
Somma^Vesuvius MI do not include data for CO2, but several report total CO2 concentrations ranging from several
hundred to as much as 3000 ppm (Marianelli et al., 1995,
1999; Cioni, 2000). In addition, primary FI in ejected nodules from the plumbing system contain high CO2 concentrations (Belkin et al., 1985; Belkin & De Vivo, 1993).
Fluorine in MI is also variable, ranging from several thousand ppm to 1wt % (Signorelli et al., 1999; BalconeBoissard et al., 2008).
Four Mt. Somma^Vesuvius Plinian eruptions are interpreted to have undergone differentiation during magma
storage at pressures near 200 MPa, largely based on MI
compositions that were compared with volatile solubility
relations for H2O^CO2 only (Scaillet et al., 2008).
However, given the magmatic concentrations of CO2 and
Cl involved (based on MI data), the magmas are likely to
have passed through the two-fluid stability field during
ascent and the CO2 and H2O concentrations of the more
Cl- and CO2-enriched melts would have decreased owing
to the influence of strongly non-ideal mixing behavior.
In related studies, Gilg et al. (2001) and Fulignati et al.
(2001) observed FI and MI indicating that vapor and
saline (chloride^carbonate^sulfate fluoride) liquid coexisted with silicate melt when magma interacted with and
was contaminated by carbonate wall-rocks during skarn
formation at Mt. Somma^Vesuvius. It follows, given the
observed solubilities in Fig. 7b, that the depths of magma
storage for the more Cl- and CO2-enriched magmas must
have been deeper and possibly significantly deeper than
the 200 MPa depth equivalent. This revision of the MIconstrained pressures of magma evolution, based on our
new volatile solubility data, is consistent with FI-based
research that determined higher apparent pressures of
entrapment for magmatic CO2-enriched fluids of c. 300^
350 MPa (Belkin et al., 1985; Marianelli et al., 1999) and
with recent geophysical measurements in the Naples
region indicating apparent bodies of molten rock at pressures 4200 MPa (Nunziata et al., 2006; Nunziata, 2010).
Moreover, the study of 1944 MI (Marianelli et al., 1999)
determined that after input of the H2O-, CO2-, S-, and
Cl-enriched tephritic magmas at depth, the magmas may
have begun their fractionation to phonotephritic compositions at depths corresponding to pressures as great as
600 MPa. Study of FI and MI in cumulate nodules from
Mt. Somma^Vesuvius also indicates that a relatively large
magma chamber exists at pressures equivalent to 12 km
depth (Lima et al., 2003). The issue of magma storage
depth and pressure is crucial because the eruption dynamics of phonolitic magmas depends on magma storage
processes and pre-eruptive, magmatic volatile concentrations (Andu¤jar & Scaillet, 2012).
The experimental data allow estimation of the compositions of exsolved magmatic fluids. Comparison of the
compositions and volatile contents of MI having molar
N/NK 066 with volatiles in relevant experiments indicate that the bulk integrated magmatic fluid(s) in equilibrium with Somma^Vesuvius phonolitic melt at 200 MPa
would contain up to 25 mol % CO2, 3 mol % S, 2 mol %
Cl, and 02 mol % F.
Erebus volcano, Antarctica
Erebus is a well-characterized, active volcano in
Antarctica that erupts volatile-charged phonolitic magma
with vapors enriched in CO2 (Oppenheimer & Kyle,
2008). The most-evolved phonolitic melts are the products
of fractional crystallization of a basanitic parental melt
containing c. 15 wt % H2O and 06 wt % CO2
(Oppenheimer & Kyle, 2008). Phonolitic MI, on average,
and matrix glass compositions indicate magmatic S concentrations of 0037^0066 wt % and 013^015 wt % Cl
(Kelly et al., 2008; Oppenheimer et al., 2011). Water
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VOLUME 55
concentrations in the MI are low (c. 01wt %)
(Oppenheimer et al., 2011).
Oppenheimer & Kyle (2008) determined that complete
exsolution and accumulation of all the CO2 and H2O
(contained in the phonolitic MI) under closed-system conditions could not generate gas compositions equivalent to
those with elevated CO2/H2O ratios emitted directly at
Erebus. This discrepancy is significant because the exsolution and emission of CO2-enriched gases from phonolitic
magma are directly responsible for explosive Strombolian
eruptive activity (Oppenheimer et al., 2011). In contrast to
those associated with passive activity, volcanic gases
released during explosive eruptions contain higher CO2
relative to H2O and these gases accumulate in the volcanic
conduit under closed-system conditions (Oppenheimer
et al., 2011). Oppenheimer & Kyle (2008) concluded that a
pre-eruptive vapor enriched in CO2 was emitted from
basanitic magma deep within the system and as it
NUMBER 11
NOVEMBER 2014
ascended through the magma conduit. They explained the
observed CO2/H2O ratios of emitted gases to be the result
of the strong influence of reduced pressure in decreasing
CO2 solubility in silicate melt, over the pressure range
of 800 MPa to surface conditions, and subsequent CO2
flushing of shallowly emplaced magma during magmatic
fluid ascent. Our determined solubility relations for phonolitic melts indicate strongly non-ideal aH2O, aCO2, and
aCl in the presence of coexisting vapor and saline liquid, and resulting reductions in CO2 solubilities in
phonolitic melt. The parameter [(mol % CO2)1/2/(mol %
H2O) in fluid(s)] correlates positively with the Cl content
of the fluid(s), and the square root function for CO2 relative to H2O expresses the comparative influence of Cl on
H2O versus CO2 degassing to magmatic fluids at
200 MPa. Thus, the presence of Cl in Erebus magmas
may have played a partial role in the observed degassing
behavior there (Fig. 11). Moreover, Erebus gases contain
Fig. 11. Variation of the square root of the mol % CO2 normalized to the mol % H2O in the fluid(s) versus mol % Cl in the C^O^H^Cl^S^Fbearing fluid(s) at 200 MPa (symbols the same as in Fig. 6). As demonstrated in Fig. 7, increasing Cl in the fluid can reduce the CO2 concentration
of coexisting phonolitic^trachytic melts and increase the CO2 content of the fluid(s), causing the [(mol % CO2)1/2/(mol % H2O)] ratio to increase.
For comparison, the compositions of gases emitted from Mt. Erebus, Antarctica, in December 2004 (Oppenheimer & Kyle, 2008) are represented
by a double-headed arrow. During this period, the averaged Erebus vapor contained 387 mol % CO2, 58 mol % H2O, 15 mol % SO2, 07 mol %
HCl, and 127 mol % HF. This broad compositional range in CO2 relative to H2O for the Erebus gases reflects reduced H2O and CO2 solubilities
in phonolitic melts (Oppenheimer & Kyle, 2008) owing to decompression accompanying magma ascent, but varying Cl in the fluid may also
have influenced Erebus gas geochemistry. (See text for discussion.) Curve is fitted to data; representative 1s precision is shown.
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WEBSTER et al.
VOLATILES IN PHONOLITIC^TRACHYTIC MELTS
1mol % HCl, and these low contents probably reflect the
influence of H2O^NaCl^KCl-dominated vapor^saline
liquid separation in the presence of HCl-poor alkaline
magmas.
Cripple Creek Au^Te deposit, Colorado
The Cripple Creek^Victor mining district in Colorado has
produced more than 23 million troy ounces of Au since
1890, making it one of the leading five Au mining districts
in the USA and the leading Au producer in Colorado at
present. The Au is hosted in hydrothermal veins and brecciated rock. The most common igneous rocks include
lamprophyre dikes and more evolved phonolites, phonotephrites, tephriphonolites, and their sub-volcanic equivalents (Mu«ller & Groves, 2000). Kelley et al. (1998)
recognized a genetic relationship between the phonolitic
rocks and the mineralized Au-bearing veins, based on
close similarities in Pb isotopic compositions of the
phonotephrite^phonolite rocks and those of Pb-bearing
K-feldspar and galena in hydrothermal veins. Kelley et al.
(1998) also suggested that hydrothermal fluids enriched
in H2O, CO2, Cl, F, and SO2 accumulated in the apices
of the magma body, and these fluids played key roles
in the processes of brecciation, hydrothermal alteration,
and mineralization. Primary FI from these rocks are
enriched in alkali chlorides and CO2 (Thompson et al.,
1985), and MI show significant S, Cl, and F contents
(Table 1).
Our experimental data allow estimation of the compositions of exsolved magmatic fluids (i.e. potentially mineralizing magmatic^hydrothermal fluids) from Cripple
Creek magmas; this comparison is justified as the mean
molar N/NK and A/CNK of the experimental glasses
are 064 and 09 whereas those for the Cripple Creek
MI are 065 and 085, respectively. Comparison of the
compositions and volatile contents of MI with molar
N/NK 06 with volatiles in relevant experiments indicates that the bulk integrated magmatic fluid(s) in equilibrium with Cripple Creek phonolitic melt would
contain up to 5 wt % S (4 mol %), 3 wt % Cl (3 mol %),
and 02 wt % F (02 mol %) at 200 MPa. The experiments also show that the partitioning of S, Cl, H2O, and
F between such fluids and phonolitic melts does not vary
with the CO2 abundances in the magmatic systems;
these volatile components are the primary ligands
involved in the dissolution and complexation of numerous
ore metals (Simon et al., 2005). Thus, CO2 should not
reduce the ore metal contents of fluids and, hence, ‘early’
and deeper exsolution of relatively CO2-rich magmatic
fluid(s) can be effective in generating Cl- and S-enriched
fluids that are capable of dissolving and transporting
gold. The issue of timing is important because the more
time available for magmatic fluids to interact with melt
and phenocrysts, the better the approach to equilibrium
partitioning of ore metals as well as volatiles. Moreover,
the precipitation of Au is a strong function of vapor^
saline immiscibility (Heinrich, 2007), and these experiments provide constraints on fluid phase equilibria for
phonolitic magmas at 200 MPa.
AC K N O W L E D G E M E N T S
We thank Y. Morizet and H. Behrens for sharing samples
of their CO2-bearing run-product glasses that were used
as standard materials for FTIR and SIMS analysis.
Gordon Moore kindly provided rock samples used for the
testing of the rate of water diffusion during the quenching
of runs versus the IHPV quench rate. Phonolite and syenite
samples were collected in Cripple Creek, CO, with assistance of Karen Kelley. Some work was accomplished while
visiting the Institute for Mineralogy, University of
Hannover, and J.D.W. wishes to express sincere thanks
for travel support and discussions with Harald Behrens,
Francois Holtz, Roman Botcharnikov, and Francesco
Vetere while in Hannover. We thank Editor W. A. Bohrson
and reviewers M. Carroll, J. Hammer, and J. Lowenstern
for their thoughtful and detailed comments. Appreciation
is also expressed to Rick Hervig, Charles Mandeville,
Edmond Mathez, Gordon Moore, and Roberto Moretti
for informative discussions, but remaining errors are
our own.
FU N DI NG
This work was supported by the National Science
Foundation awards EAR-0308866 and EAR-0836741 to
J.D.W. Support for operation of the NENIMF by the
National Science Foundation is gratefully acknowledged
by N.S.
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