JOURNAL OF PETROLOGY
VOLUME 38
NUMBER 12
PAGES 1793–1807
1997
Chloride Solubility in Felsic Melts and the
Role of Chloride in Magmatic Degassing
JAMES D. WEBSTER∗
DEPARTMENT OF EARTH AND PLANETARY SCIENCES, AMERICAN MUSEUM OF NATURAL HISTORY,
CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024-5192, USA
RECEIVED JANUARY 1997; ACCEPTED AUGUST 1997
The degassing of Cl- and H2O-bearing magmas has been investigated
experimentally at ~1 bar and 2000 bars and 800–1075°C by
determining the solubility of H2O and Cl– in felsic to intermediate
liquids variably enriched in F, P, B, C, and excess alkalis or
aluminum. Chloride solubility in H2O-undersaturated silicate liquids is low and increases with increasing values of the molar
[(Al+Na+Ca+Mg)/Si] of silicate liquids, F content, the abundance of network-modifying Al, and pressure. The presence of B or
P in peraluminous felsic liquids has no discernible influence on Cl–
solubility. The experimental solubility data are compared with H2O
and Cl– in silicate melt inclusions from seven high-silica rhyolites
and tin and topaz rhyolites of western North America. Small ranges
in Cl– concentration, minimum Cl– contents of 600–800 ppm,
large ranges in H2O content, and steeply negative to near-infinite
slopes in plots of H2O vs Cl– for many of the melt inclusions are
similar to abundances and trends of H2O vs Cl– for volatile phasesaturated felsic liquids, suggesting that some portions of each of
these magmas might have been saturated in volatile phases before
melt inclusion entrapment and eruption.
Chlorine has a fundamental influence on magmatic and
post-magmatic processes even though it is a minor to
trace constituent in most magmas and igneous rocks.
The abundance of chloride ions (Cl–) that are dissolved
in magmatic–hydrothermal fluids, for example, can
control the number of fluid phases that are stable. Experiments demonstrate that NaCl affects the occurrence
of boiling and the critical point in NaCl- and H2Obearing hydrothermal fluids (Keevil, 1942; Sourirajan &
Kennedy, 1962) and small quantities of Cl– may lead to
magmatic volatile phase (MVP) exsolution from silicate
liquids characterized by low water activities (Webster,
1997). Chloride is also a highly effective complexing
agent for many metals (Helgeson, 1964; Burnham, 1967;
Candela & Piccoli, 1995), and it aids in the transport of
ore constituents in mineralizing, magmatic–hydrothermal
fluids. In addition, Cl– species are important constituents
of gaseous volcanic emissions (Anderson, 1974; Gerlach,
1980; Symonds et al., 1992).
This study describes the results of an experimental
investigation of Cl– solubility in a variety of aluminosilicate liquids of felsic to intermediate composition and
the comparison of these solubility data with volatile
abundances of silicate melt inclusions to interpret processes of MVP exsolution from magmas characterized
by low to high water activities. Intermediate-composition
liquids were studied because these magmas have the
potential to degas significant quantities of Cl to the
atmosphere (Anderson, 1974; Symonds et al., 1988), and
F-, P-, Al-, ±B-enriched liquids were included because
evolved peraluminous magmas are genetically associated
with a large variety of granitophile mineral deposits.
These new results are coupled with previously published
results for subaluminous haplogranites and F-bearing
haplogranites to provide broad constraints on the influences of pressure, temperature, and composition on
MVP exsolution.
In the following discussion, the term liquid refers to a
molten aluminosilicate phase and chloride solubility is
∗e-mail: [email protected]
 Oxford University Press 1997
KEY WORDS:
chloride; volcanic degassing; experimental petrology; melt
inclusions
INTRODUCTION
JOURNAL OF PETROLOGY
VOLUME 38
used, loosely, to describe the summed solubilities of the
dominant chloride species in silicate liquids. At
supercritical pressure–temperature conditions, silicate
liquid coexists with a single H2O-, NaCl-, and KClbearing MVP. At subcritical conditions, silicate liquids
coexist with either a vapor (i.e. a low-density, H2Odominated volatile phase) or a hydrosaline melt (i.e. a
higher-density, alkali chloride dominated volatile phase),
or both phases.
Degassing of chloride-enriched magmatic
volatile phases
Experimental investigations of volatile solubility and studies of fluid and melt inclusions can determine the influence
of volatile-phase geochemistry on MVP exsolution and
the timing of volatile exsolution in Cl-rich magmatic
systems. The summary studies of Roedder (1984, 1992)
indicate that many hypersaline fluid inclusions, some of
which occur with melt inclusions, are trapped in granitic
phenocrysts at magmatic temperatures. The alkali chloride contents of hypersaline fluid inclusions from Cu and
Sn porphyries, skarns, and Sn–W greisens, for instance,
range from 60 to 80 wt % (on an NaCl-equivalent basis).
Experiments demonstrate that volatiles of comparatively low solubility, such as Cl– or CO2, cause
magma to exsolve a separate volatile phase at pressures
and depths much greater than those at which it would
do so without Cl– or CO2 (Holloway, 1976; Webster,
1997). The solubility of Cl– in silicate liquids is of particular importance because it exhibits a strong, inverse
dependence on the abundance of silica (Webster, 1997).
If crystal fractionation increases the abundance of SiO2
in water-undersaturated magmas, it will also reduce the
solubility of Cl– in residual silicate liquids, which increases
the tendency to generate a Cl-enriched MVP.
EXPERIMENTAL METHOD
Chloride solubility was determined for three natural
samples including a latite, the Peruvian Macusani obsidian, and a topaz rhyolite doped with excess F. Chloride
solubility was determined also for six synthetic aluminosilicate-gel starting materials including subaluminous,
peraluminous, and peralkaline haplogranites; a peraluminous F-rich haplogranite; a peraluminous P-rich
haplogranite; and a peraluminous F- and P-rich haplogranite (Table 1). Fluorine was added as reagent-grade
NaF and AlF3 and P as reagent-grade AlPO4. Excess
Al2O3 was added as a synthetic Al2Si6O18 gel. Excess
Na2O and K2O were added as Na2CO3 and K2CO3, and
CO2 was driven off by heating in an unsealed preciousmetal capsule at 1 atm and temperatures less than or
equal to the melting point.
NUMBER 12
DECEMBER 1997
Each Cl-bearing experiment began with 15–20 mg of
silicate starting material, 0·5–2 mg of alkali chloride(s),
and 0·3–3 mg of H2O. Most silicate starting materials
were mixed with a 1:1 molar mixture of reagent-grade
NaCl and KCl. Several experiments were also conducted
with silver oxalate as a source of CO2 (and minor H2O).
The starting materials were loaded into Ag70Pd30,
Au90Pd10, Pt, or Au capsules having a wall thickness of
0·005 inch. The capsules were crimped shut, sealed with
an arc welder, and stored in a drying oven at 115°C for
at least 1 h to test for leaks. For several experiments, open
capsules were stored in a muffle furnace at temperatures
Ζ600°C for at least 10 h to drive off excess H2O from
the silicate starting materials and from fluid inclusions
within the alkali chlorides before the capsules were sealed.
All experiments were conducted in an internally heated
pressure vessel (IHPV) at the American Museum of
Natural History using experimental techniques of Webster (1992a, 1992b). Temperature was monitored with
chromel–alumel thermocouples, and the temperature
variation of most runs was typically Ζ10°C. Pressure was
monitored with factory-calibrated bourdon tube gauges.
The experiments were quenched isobarically. Most
quenches were fairly rapid. A 1050°C experiment was
quenched to 600°C in <40 s.
The O2 buffering capacity of the IHPV was determined
with an H2 sensor; the vessel imposes an f(H2) that is
approximately equal to that of the Mn1–xO–Mn3O4 solid
oxygen buffer at 820°C and 2 kbar for runs with a water
activity of unity. Computations suggest that a liquid
containing 1 wt % H2O at 820°C and 2 kbar has a water
activity near 0·1, and hence, it should have been exposed
to a comparatively reduced oxygen fugacity [i.e. an f(O2)
near the Ni–NiO oxygen buffer].
Several Cl– solubility experiments were conducted in
air, i.e. at 1 atm confining pressure, using sealed precious
metal capsules that were heated in a muffle furnace.
Temperature was monitored with chromel–alumel thermocouples, and temperature variations are estimated at
Ζ10°C. These experiments were quenched in seconds
by dropping the hot metal capsules into water. These
capsules were puffed up after quenching, and consequently, the pressures are not well constrained because
the capsules could have contained pressures greater than
1 atm without bursting. Run pressures are believed to
be near 1 bar.
ANALYTICAL METHOD
Electron microprobe
The run product glasses were analyzed at 15 keV and 4
nA beam current for Si, Al, Na, K, Ca, Fe, Mn, Ti, Mg,
F, P, and Cl with a Cameca SX-100 microprobe using
wavelength-dispersive techniques (see Webster &
1794
WEBSTER
CHLORIDE SOLUBILITY IN FELSIC MELTS
Table 1: Composition of starting materials 1
Peralum.
2
Peralk.
3
hpg.
hpg.
Peralum.
Peralum.
4
Peralum.
5
F-hpg.
P- & F-hpg.
6
P-hpg.
Macusani
TR8009C8
7
obsidian
SiO2
75·10
74·50
71·20
63·30
74·2
72·26
63·9
Al2O3
15·50
11·30
16·10
17·20
15·8
15·83
14·8
CaO
0·00
0·00
0·00
0·00
0·00
0·22
Na2O
4·60
6·35
3·30
3·60
3·25
4·14
5·20
K 2O
3·90
6·90
3·80
3·85
3·60
3·66
5·25
FeO9
0·00
0·00
0·00
0·00
0·00
0·60
0·38
MgO
0·00
0·00
0·00
0·00
0·00
0·02
0·06
TiO2
0·00
0·00
0·00
0·00
0·00
0·04
0·05
MnO
0·00
0·00
0·00
0·00
0·00
0·02
0·06
F
0·00
0·00
5·20
5·81
0·00
1·33
8·46
Cl
0·00
0·00
0·00
0·00
0·00
0·04
0·05
P 2 O5
0·00
0·00
0·00
5·86
2·71
0·53
n.d.10
0·12
B 2 O3
n.d.
n.d.
n.d.
n.d.
0·00
0·60
n.d.
Li2O
n.d.
n.d.
n.d.
n.d.
0·00
0·74
n.d.
Total
99·10
99·05
99·60
99·62
99·56
100·03
98·33
A/CNK11
1·32
0·63
1·69
1·70
1·70
1·42
1·02
N/NK12
0·64
0·58
0·57
0·59
0·58
0·63
0·60
ANCM/S13
0·36
0·34
0·36
0·43
0·34
0·43
0·43
1
Average compositions of most starting materials used in this study; starting compositions of subaluminous haplogranite
and GW latite have been reported by Webster (1997). Analysis of glasses by electron microprobe; totals do not account for
F in exchange for O.
2
Average composition of peraluminous haplogranite (hpg.) glass prepared from silicate gels.
3
Average composition of peralkaline haplogranite glass prepared from silicate gels and NaCO3 and KCO3.
4
Average composition of F-bearing peraluminous haplogranite glass prepared from silicate gels and NaF and AlF3.
5
Average composition of F- and P-bearing peraluminous haplogranite glass prepared from silicate gels and NaF, AlF3, and
AlPO4.
6
Average composition of P-bearing peraluminous haplogranite glass prepared from silicate gels and AlPO4.
7
Composition of Macusani obsidian pebble from Peru; sample JV-1 of Pichavant et al. (1987).
8
Composition of F-doped topaz rhyolite from Spor Mtn, Utah.
9
Total iron as FeO.
10
Constituent not determined.
11
Molar ratio of [Al2O3/(CaO+Na2O+K2O)] in starting material.
12
Molar ratio of [Na2O/(Na2O+K2O)] in starting material.
13
Molar ratio of [(Al+Na+Ca+Mg)/Si] in starting material.
Duffield, 1991). One set of analyses (4–8 per glass) was
conducted with a defocused electron beam (10–20 lm
diameter) and peak count times of 10–30 s to determine
the bulk glass composition. The samples were moved
relative to the defocused beam during analyses to minimize Na and F migration. With this technique, the
electron microprobe determined 4·4 wt % Na2O for an
obsidian standard containing 4·3 wt % Na2O. The Cl–
concentrations were determined in a second set of analyses (10–50 analyses per glass with 30 s count times)
conducted on vesicle-free areas of glass with a beam of
1 lm diameter. Replicate analyses on the same spot of
glass show that Cl counts are stable at these conditions.
Obsidians of known composition were also analyzed
to determine analytical accuracy and precision. The
microprobe determined 0·19–0·21 wt % Cl– in an obsidian containing 0·20 wt % Cl– as determined by wet
chemical analysis. The predicted precision for Cl– analysis, based on counting statistics, ranges from 3% to
<1% relative for glasses containing 0·2–1 wt % Cl–,
respectively, and the measured analytical precision for
Cl– in an obsidian containing 0·2 wt % Cl– is 5% relative.
Ion microprobe
The H contents of the glasses were determined by secondary ion mass spectrometry (SIMS) using a Cameca
IMS-3f ion microprobe at the Woods Hole Oceanographic Institution. Five to ten replicate analyses were
1795
JOURNAL OF PETROLOGY
VOLUME 38
conducted on each glass. The primary O– ion beam
was typically 10–15 lm in width. All constituents were
analyzed as high-energy ions to minimize the effects of
mass interferences and reduce matrix effects, and only
positively charged, secondary ions with excess kinetic
energies in the 75±25 eV range were analyzed. The ion
yields for Na, K, and Cl were determined for all glasses
analyzed by SIMS, and comparison of the ion yields
with the electron microprobe results indicates that the
SIMS data were free of matrix effects. Consequently, the
reported H2O concentrations are accurate within quoted
analytical precision (Table 2).
PRESENTATION AND DISCUSSION
OF EXPERIMENTAL RESULTS
All run-product glasses were vesicular, and vesicle diameters ranged from >100 to <1 lm. Most glasses were
free of crystals, but some latite glasses contained <10
vol. % phenocrysts. It is noteworthy that although several
experiments were conducted with essentially Cl-free,
H2O-saturated silicate liquids, the bulk of the experiments
measured Cl solubility in H2O-undersaturated liquids
(i.e. liquids containing Ζ2 wt % H2O that were saturated
in one or two MVPs). Glass compositions are reported
in Table 2.
Effects of pressure and temperature on
chloride solubility in aluminosilicate
liquids
Changes in temperature from 800° to 1075°C have
a negligible influence on alkali chloride solubility in
haplogranite liquids (Webster, 1997). Conversely, the
experiments of Iwasaki & Katsura (1967) suggest that
HCl solubility in plagioliparite liquids shows a weak,
inverse relationship with temperature.
The influence of pressure on Cl– and H2O solubilities at
800–1075°C was determined by saturating a haplogranite
liquid in molten NaCl and KCl at ~1 bar and comparing
the Cl– abundances of this liquid with extant solubility
data (Webster, 1997) for haplogranite liquids at 2000
and 500 bar (Fig. 1). Three observations are apparent:
(1) as pressure varies between 1 and 2000 bar, the change
in Cl– solubility for an H2O-undersaturated liquid is ~30
times less than the change in the solubility of H2O in an
H2O-saturated liquid; (2) Cl– solubility at 2000 bar is
marginally greater than that at 500 bar; (3) Cl– solubility
in the liquid at 500 bar cannot be distinguished from
that at ~1 bar within analytical precision (i.e. 200 ppm).
There are few published solubility data for chloride
compounds at low pressure. Iwasaki & Katsura (1967)
determined the solubility of HCl for a plagioliparite
NUMBER 12
DECEMBER 1997
liquid with an initial molar ANCM/S of 0·3. This liquid
dissolved about 0·05 wt % HCl at 1200°C and 0·04 wt
% HCl at 1290°C when exposed to a constant flow of
HCl±N2 gases at 1 atm, and these values are significantly
less than the solubilities determined in the present study.
Effect of composition on chloride solubility
The molar (ANCM)/S and water concentration
Chloride solubility has been determined at 2 kbar for
mildly peralkaline to metaluminous haplogranite and
latite liquids (Webster, 1997). Chloride solubility increases
with decreasing H2O abundance and increases linearly
(Figs 1 and 2) with the molar (ANCM)/S ratio (i.e. the
moles of [(Al + Na + Ca + Mg)/Si]) of these liquids.
Prior study shows that Cl– solubility does not vary significantly with changes in the abundances of K, Ti, Mn,
or Fe3+ in anhydrous, felsic silicate liquids (Webster,
1997).
The molar A/CNK
The solubility of Cl– in strongly peralkaline liquids, like
that in mildly peralkaline to metaluminous liquids, is a
linear function of the molar (ANCM)/S ratio at 2 kbar
(Fig. 2a). Chloride concentrations of peraluminous liquids
(i.e. liquids containing more Al than that required to
charge balance alkalis) plot above this line, however, so
Cl– solubility cannot be predicted by the linear relationship involving the (ANCM)/S ratio for peraluminous liquids. This behavior suggests that each mole
of triply charged, network-modifying Al in peraluminous
liquids is associated with more moles of Cl– than each
mole of network-modifying Na (±K) in peralkaline
liquids and each mole of network-forming Al and Na
(±K) in all liquids.
The abundance of F, P, B, Li
Fluorine increases Cl– solubility in felsic liquids (Webster,
1997; Webster & Rebbert, in press), but the influence of
F varies with the A/CNK and the F abundance of the
liquid. The Cl– contents of metaluminous and peralkaline
liquids with Ζ1·2 wt % F plot on or near the molar
(ANCM)/S vs Cl– line for haplogranite to latite liquids
(Fig. 2b), and hence, for these F abundances the solubility
of Cl– in these liquids does not vary with F content.
Conversely, the Cl– contents of metaluminous and peralkaline liquids containing 6–8 wt % F plot well above
the molar (ANCM)/S vs Cl– line, indicating that F
concentrations >1·2 wt % increase Cl– solubility considerably. Moreover, Cl– solubility in peraluminous
liquids containing 5 wt % F is extremely high; run
1796
WEBSTER
CHLORIDE SOLUBILITY IN FELSIC MELTS
Table 2: Experimental run conditions and results
Experiment
Pressure
Temperature Duration
Starting
Cl in glass
Molar
Molar
Molar
H2O
no.
(kbar)
(°C)
(h)
material1
(wt %)2
A/CNK3
N/NK4
ANCM/S5
(wt %)6
1atm-95-3H
0·001
1055
141
Hpg.
0·26±0·04
0·98
0·53
0·28
<0·2
1atm-95-3F
0·001
1055
141
Hpg.
0·22±0·04
0·85
0·60
0·36
<0·2
1atm-95-4
0·001
1050
301
Hpg.
0·24±0·02
0·99
0·61
0·30
0·3
1atm-96-1C
0·001
990
291
Hpg.
0·23±0·02
0·88
0·64
0·39
0·6
1atm-96-1D
0·001
990
291
Hpg.
0·18±0·04
0·99
0·44
0·36
0·5
96-9F
2·00
1052
184
Hpg.+CO2
0·27±0·01
0·98
0·60
0·32
0·6
96-13E
2·03
1051
264
Hpg.+CO2
0·35±0·04
0·94
0·54
0·33
n.d.7
94-17G
2·00
972
143
GW latite
1·24±0·03
0·83
0·68
0·54
2·9
96-8E
2·03
1051
264
GW latite
1·47±0·04
0·89
0·62
0·60
0·6
96-11A
1·99
1172
173
GW latite
1·26±0·04
0·84
0·60
0·58
0·4
97-5G
1·99
1118
156
GW latite
1·1±0·02
0·89
0·66
0·56
4·0
94-19B
2·02
1075
301
Prlum. hpg.
0·69±0·02
1·30
0·66
0·35
1·7
96-13C
2·03
1051
264
Prlum. hpg.
0·89±0·04
1·20
0·43
0·34
n.d.7
8
94-16E
1·99
807
136
Prlk. hpg.
0·44±0·02
0·56
0·54
0·36
4·6
94-16D
1·99
8078
136
Prlk. hpg.
0·42±0·02
0·60
0·58
0·33
4·3
94-21A
2·03
8258
400
Prlk hpg.
0·50±0·02
0·48
0·62
0·35
1·3
96-14A
2·01
1073
216
Prlk. hpg.
0·33±0·02
0·72
0·55
0·32
0·6
96-14B
2·01
1073
216
Prlk. hpg.
0·36±0·03
0·63
0·54
0·34
0·2
94-13D
1·96
786
137
Macusani
0·53±0·02
1·44
0·71
0·38
0·6
94-14C
2·00
992
161
Macusani
0·53±0·06
1·39
0·66
0·38
0·9
96-8B
2·03
1051
264
Macusani
0·52±0·01
1·17
0·50
0·40
0·2
94-17A
2·00
972
143
TR8009C
1·27±0·03
1·03
0·60
0·43
0·8
96-8A
2·02
1050
262
TR8009C
0·84±0·13
0·88
0·61
0·37
0·3
96-6E
2·03
1043
183
P-hpg.
0·84±0·10
1·47
0·50
0·35
0·2
96-6A
2·03
1043
183
F–P-hpg.
1·17±0·04
1·22
0·66
0·52
0·5
96-8G
2·03
1051
264
F-hpg.
2·20±0·07
1·17
0·43
0·38
0·4
1
Starting materials include: subaluminous haplogranite (hpg.), peraluminous haplogranite (prlum. hpg.), peralkaline haplogranite (prlk. hpg.), natural latite (GW latite), natural Macusani obsidian from Peru (also contains 0·62 wt % B2O3, 0·74 wt %
Li2O, and 1·3 wt % F), F-doped topaz rhyolite (TR8009C), P-enriched haplogranite (P-hpg.), F-enriched haplogranite (F-hpg.),
and F- and P-enriched haplogranite (F–P-hpg.).
2
Cl concentration (±1r precision) of run product glass determined by electron microprobe (see text for method).
3
Molar [Al2O3/(CaO+Na2O+K2O)] of run product glass.
4
Molar [Na2O/(Na2O+K2O)] of run product glass.
5
Molar [(Al+Na+Ca+Mg)/Si] of run product glass.
6
Water concentration of run product glass determined by SIMS; ±1r precision ranges from 0·2 to 0·4 wt % (see text for
method).
7
Water concentration of run product glass not determined, but no water was added to experiment and apparent water
content should be <1 wt %.
8
Run preconditioned at 950°C for 73 h before coming to final run T.
product glass 96-6A contains more than three times the
Cl– of that in F-deficient, metaluminous glasses with
similar [ANCM]/S ratios. It appears that F disrupts the
structure of silicate liquids in a manner that enhances
Cl– solubility by making additional Al, Na, Ca, or Mg
ions available for complexing with Cl– (Webster, 1997).
Chloride solubility has been determined for three phosphorus-bearing, peraluminous liquids (Fig. 2b), but the
influence of P on Cl– solubility is not well constrained at
present. For instance, the Cl– contents of the Macusani
liquid (with 1·3 wt % F and 0·6 wt % P2O5) and one of
the synthetic liquids (with nearly 6 wt % each of F and
P2O5) are well defined by the linear Cl– vs (ANCM)/S
relationship, but the Cl– content of a F-free peraluminous
liquid containing 2·7 wt % P2O5 plots above the line and
is, in fact, similar to the enhanced Cl– contents of the
1797
JOURNAL OF PETROLOGY
VOLUME 38
Fig. 1. Plots showing effects of pressure (a) and liquid composition (b)
on H2O and Cl– concentrations of aluminosilicate liquids in equilibrium
with H2O-, NaCl-, and KCl-bearing volatile phases. In (a), new experimental data (Χ; data in Table 2) for ~0·001 kbar are combined
with 2 kbar (Φ) and 0·5 kbar data (Α) from Webster (1997) for liquids
with molar [(Al + Na + Ca + Mg)/Si] ratios [i.e. the (ANCM)/S] of
0·28–0·39 and molar [Na2O/(Na2O + K2O)] ratios of 0·44–0·64 at
800–1075°C. Water content of ~0·001 kbar datum (filled and lined
circle) was computed. Error bars represent 1r precision; dashed curves
interpolate through large gaps in data. The curves delimit isobaric
solubilities of MVP-saturated felsic silicate liquids and show that small
quantities of Cl– have a dramatic effect on volatile phase exsolution.
In (b), the curves designate the H2O and Cl– solubilities for volatile
phase-saturated latite liquid (filled crosses, data in Table 2; open crosses,
data in Webster, 1997) and haplogranite liquid (Φ; data from Webster,
1997) at 2 kbar and 825–1170°C. The molar (ANCM)/S ratios and
molar [Na2O/(Na2O + K2O)] ratios of latite range from 0·4 to 0·6
and from 0·54 to 0·69, respectively; these differences in liquid compositions have a small effect on H2O solubility and a large influence
on Cl– solubility.
peraluminous, P-free liquids of Fig. 2a. Thus, it appears
that P does not have a strong influence on Cl– solubility
in peraluminous silicate liquids.
The Macusani experiments are of additional interest
because this peraluminous, F- and P-bearing starting
material is also enriched in B and Li. The Cl– abundances
of this liquid follow the linear Cl– vs (ANCM)/S
NUMBER 12
DECEMBER 1997
Fig. 2. (a) Effects of the molar (ANCM)/S ratio and the A/CNK ratio
on Cl– dissolution in alkali-chloride-saturated (H2O-undersaturated)
silicate liquids containing <0·1 wt % of [F + P2O5 + B2O3] at 2
kbar. Chloride solubility in mildly peralkaline, metaluminous, and
subaluminous liquids (Η; data from Webster, 1997) and in strongly
peralkaline liquids (Ν; data in Table 2) is a linear function of the molar
(ANCM)/S ratio; whereas Cl– solubility in peraluminous liquids (×;
data in Table 2) is distinctly greater. (b) Effects of fluxing components
on Cl– dissolution in alkali-chloride-saturated (H2O-undersaturated)
silicate liquids at 2 kbar; Cl– solubility is a complex function of the F
and P2O5 contents and the molar A/CNK ratios of silicate liquids. Η,
same data set as in (a); Β, Cl– solubility in topaz rhyolite liquid. For
data located in Table 2: Ε, Cl– solubility in topaz rhyolite and synthetic
haplogranite liquids doped with excess F, P, or Al; filled crosses, Cl–
contents of molten Macusani obsidian. Numeric labels specify the molar
A/CNK ratio, wt % F, and wt % P2O5 of the liquids. Peraluminous
or metaluminous liquids containing Ζ1·2 wt % F or P2O5 dissolve Cl–
in amounts roughly similar to those of mildly peralkaline, metaluminous,
and subaluminous liquids containing little or no F or P. Peraluminous
or peralkaline silicate liquids containing >1·3 wt % F or P2O5 dissolve
more Cl– than mildly peralkaline, metaluminous, and subaluminous
liquids containing little or no F or P and peraluminous liquids containing
elevated levels of both F and P2O5 (see text for discussion).
relationship, which demonstrates that the presence of
Ζ0·8 wt % of B2O3 and Li2O does not have a detectable
effect on Cl– solubility in F- and P-bearing, peraluminous
silicate liquids.
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Carbon dioxide
Initial experimental work suggests that the effect of CO2
on Cl– dissolution in subaluminous liquids at 2 kbar is
insignificant (Table 2). The solubility of Cl– in haplogranite liquids equilibrated with CO2-rich MVPs is the
same, within precision, as that in CO2-free liquids at
equivalent pressure and temperature. Under certain conditions, however, the effect of CO2 on Cl– solubility in
silicate magmas may be different. Prior research has
clearly shown that the phase relations of CO2- and alkalichloride-bearing MVPs vary as a function of the CO2
content of the system (Bowers & Helgeson, 1983). In
addition, the Cl– activity of an MVP-saturated silicate
liquid is a function of heterogeneous phase relations; for
example, the activity of Cl– in all phases of a system is
fixed if two volatile phases are stable (Shinohara et al.,
1989). This means that for two magmas at identical
pressures and temperatures, the phase relations and the
Cl– solubility in the silicate liquid of a CO2-bearing
magma may differ significantly from those of a CO2-free
magma because of the effect of CO2 on phase equilibria.
APPLICATION OF RESULTS: THE
ROLE OF CHLORINE IN MAGMATIC
DEGASSING
Before applying these data, it is useful to constrain the
number of volatile phases that were present during the
experiments. Observations in the system NaCl–H2O
(Sourirajan & Kennedy, 1962; Bodnar et al., 1985; Chou,
1987) suggest that the 0·5 kbar runs, with roughly 2 wt
% H2O and 0·24 wt % Cl in the liquid, involved two
volatile phases, whereas all other 0·5 kbar runs as well
as the 2 kbar experiments were conducted with a single
volatile phase. The ~0·001 kbar haplogranite experiments were conducted with a single volatile phase
because they were conducted under extremely dry conditions and not within a field involving two volatile
phases. The number of volatile phases involved in the
experiments conducted near 2 kbar and temperatures of
1050–1170°C is not so well constrained because extrapolation of published experimental data to these conditions gives conflicting results. Extrapolation of one data
set for the system NaCl–H2O suggests that two volatile
phases may be stable at temperatures [925°C with
roughly 40 wt % NaCl at 2 kbar (Chou, 1987); whereas,
extrapolations involving another set of experimental data
for the same conditions suggest that two volatile phases
are not stable until temperatures are well above 1000°C
(Bodnar et al., 1985).
Predicted behavior of water and chloride in
silicate magmas and consequent effects on
compositions of silicate liquids and silicate
melt inclusions
The crystallization of quartz and feldspar in felsic magma
is a primary process that governs the H2O and Cl–
contents of residual fractions of silicate liquid. The concentrations of H2O and Cl in liquids crystallizing at MVPabsent conditions can be modeled with the Rayleigh
equation:
C L/C o=F (D – 1).
Computations show that the abundances of H2O and
Cl– in residual liquids increase progressively with crystallization and that the volatile enrichments are fitted by
lines projecting away from the origin in plots of H2O vs
Cl– in silicate liquid for magmas that are free of Clbearing or hydrous minerals (Fig. 3a). Crystallization of
hydrous and/or Cl-bearing minerals will cause these lines
to deviate from linearity, albeit slightly, so that the overall
influence of crystallization is to increase the concentrations of H2O and Cl– in residual liquids.
The behavior of H2O and Cl– is different during
crystallization of MVP-saturated silicate liquids at isobaric
conditions, because the H2O and Cl– concentrations of
residual liquids should remain on the isobaric volatile
solubility curves (Fig. 3b). Experiments show that volatiles
may display at least two types of behavior depending on
the number of phases in the system. In one case, plots
of H2O vs Cl– in the liquid exhibit negative correlations
and the Cl– concentrations of the liquid approach maximum values that are limited by the solubility of alkali
chlorides in that liquid at that pressure if a silicate liquid
coexists with a single Cl–-bearing volatile phase. This
behavior can be seen in the 2 kbar curves for haplogranite
and latite in Fig. 1; the point of intersection between the
solubility curve and the abscissa determines the Cl–
saturation value for the liquid. It is interesting that many
experimental investigations of felsic liquids at 2 kbar
show similar Cl– saturation values ranging from 0·26 to
0·3 wt % (Webster & Holloway, 1988; Malinin et al.,
1989; Metrich & Rutherford, 1992; Webster, 1992a,
1992b, 1997). In the second case, where silicate liquid
coexists with a hydrosaline melt and a low-Cl– vapor,
the behavior of H2O and Cl– is more complex. Even
though crystallization tends to increase the abundances
of H2O and Cl– in the residual silicate liquid, the activities
of H2O and Cl– in the liquid are fixed and so the relative
quantities of vapor and hydrosaline melt vary but not
their compositions (Shinohara et al., 1989; Webster,
1992a, 1992b; Shinohara, 1994). This behavior is apparent in the data cluster located at the break in slope
for the 0·5 kbar curve of Fig. 1a; the poorly clustered
nature of this group of data is largely a result of the
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Fig. 3. Predicted changes in H2O and Cl– concentrations of residual haplogranite liquids (bold arrows) as a function of magmatic process
(assuming closed magmatic system and no change in molar [ANCM]/S of silicate liquid during crystallization). Background curves show H2O
and Cl– concentrations of volatile phase-saturated liquids at 2, 0·5, and ~0·001 kbar. (a) Volatile-phase-absent crystallization of quartz and
feldspar; it should be noted that trends of H2O vs Cl– increase linearly and point away from the origin. (b) Isobaric crystallization of haplogranite
liquid in equilibrium with H2O- and Cl-bearing volatile phase(s): [1] H2O and Cl– are constrained to isobaric solubility curves; [2] H2O
concentrations either increase or remain roughly constant as Cl– concentrations of liquid decrease. (c) Polybaric decompression-driven degassing
with volatile phase(s) stable from 2 to ~0·001 kbar. In (c) the presence of volatile phase(s) constrains concentrations of H2O and Cl– in silicate
liquid to values on individual isobaric solubility curves within the ruled area between bold arrows.
influence of analytical imprecision on the fixed or buffered
H2O and Cl– activities of the liquids. In summary, excess
quantities of H2O and Cl– that are incompatible with
the silicate liquid and coexisting minerals during
crystallization will be sequestered by vapor, supercritical
fluid, or hydrosaline melt. Thus, even though the abundances of other incompatible elements such as Cs, U,
and Th in residual liquids will increase in response to
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CHLORIDE SOLUBILITY IN FELSIC MELTS
crystallization, MVP-saturated silicate liquids will show
constant H2O and Cl– concentrations if two volatile
phases are stable, or alternatively will show limits in their
maximum Cl– contents and steeply negative correlations
between Cl– and H2O concentrations if a single MVP is
present.
The behavior of H2O and Cl– in MVP-saturated liquids
is a strong function of other fluxing components that are
dissolved in the liquid. The strong influence of F and
network-modifying Al on Cl solubility in aluminosilicate
liquids (Fig. 2) means that MVP exsolution from compositionally evolved, F-enriched peraluminous magmas
may be controlled by changes in melt composition that
are a result of fractional crystallization. For example, the
crystallization of F-free minerals increases the F content
of residual liquids, which, in turn, acts to forestall MVP
exsolution, because the accumulation of F in silicate
liquid enhances Cl– solubility in the liquid even though
the Cl– and H2O concentrations of residual liquids are
also increasing. If an F-rich, peraluminous liquid begins
to crystallize F- and/or Al-rich phases such as fluorite
or topaz, however, the abundances of F and Al may be
held roughly constant or they may decrease. If a reduction
in F and/or Al activity in the residual liquid occurs as a
result of crystallization, it may force the liquid to exsolve
a Cl-bearing volatile phase. Furthermore, the exsolution
of a volatile phase may cause a composition-driven
quench of the magma, because volatile phase exsolution
may remove significant quantities of fluxing components
such as F or B from silicate liquids and consequently the
solidus temperature of the residual silicate liquid may
rise dramatically. In contrast, preliminary experimental
results suggest that P2O5 has a minimal influence on Cl–
solubility in silicate liquids. Thus, changes in the P2O5
activity of P-rich, peraluminous felsic liquids, brought
about by magma crystallization, should not exert a strong
direct influence on the exsolution of Cl-rich MVPs.
The H2O and Cl– concentrations of MVP-saturated
silicate liquids will also change during polybaric, decompression-driven degassing. If we assume that a given
magma behaves as a closed system during polybaric
degassing, then the MVPs will be retained within the
magma and degassing will force the H2O and Cl– contents
of residual liquids to decrease from any point along a
solubility curve at some higher, initial pressure to some
point on a solubility curve at a lower, final pressure
(Fig. 3c). For example, at a final pressure of roughly 1
bar, the silicate liquid will contain H2O and Cl– contents
similar to those of the 1 bar solubility curve.
Although it has not been formally computed for this
study, it follows that polybaric, MVP-present crystallization of a Cl-enriched felsic liquid that contains only
a few weight percent H2O and coexists with a vapor
and hydrosaline melt can result in comparatively large
changes in H2O content of the residual liquid and little
change in its Cl– content even though two MVPs are
stable. The Cl– concentrations of this liquid will undergo
little change even though magma degassing is removing
volatiles from the liquid and, simultaneously, crystallization is attempting to increase the Cl– content of
the liquid, because of the small effect of pressure on Cl–
solubility and the large effect of pressure on H2O solubility
in felsic liquids. In fact, under certain conditions (for a
liquid with very low H2O and very high Cl– contents)
plots of H2O vs Cl– in the polybarically degassing liquid
will appear as near-vertical lines showing widely varying
H2O contents and relatively fixed Cl– contents (see the
short bold arrow in Fig. 3c).
In summary, MVP-present crystallization of felsic
liquids gives rise to (1) negative or near-infinite slopes for
H2O and Cl–, and (2) highly variable H2O contents and
comparatively small ranges in Cl– abundance.
Water and chloride in silicate melt
inclusions from felsic volcanic rocks
We can investigate processes of magmatic degassing by
comparing experimentally determined volatile solubilities
for MVP-saturated aluminosilicate liquids with the
abundances of volatile components in silicate melt inclusions. The H2O and Cl– contents of the MVP-saturated
liquids in this investigation and in that of Webster (1997)
are used to interpret the abundances of H2O and Cl– in
melt inclusions from a variety of rhyolites to constrain
MVP exsolution in these magmas. The reported H2O
concentrations of some melt inclusions, however, may
have decreased relative to that of the silicate liquid at
the time of entrapment because H2 and/or H2O may
have been lost from the inclusions as a result of volatile
diffusion through the host phenocrysts (Qin et al., 1992)
or by volatile leakage along cracks in host phenocrysts
(Lowenstern, 1995).
What are typical pre-eruptive abundances of Cl– and H2O
in felsic liquids?
Most silicate melt inclusions (containing <0·4 wt %
F) from non-mineralized, high-silica rhyolites exhibit
distinctly greater H2O/Cl– ratios than melt inclusions
from F-enriched, tin and topaz rhyolites that are genetically associated with F, Sn, U, Be, Li, Rb, and Cs
mineralization (Fig. 4a). For plots of H2O vs Cl– in
melt inclusions, <10% of the inclusions from high-silica
rhyolites lie below the line with a slope of +10, and less
than 30% of the inclusions from tin and topaz rhyolites lie
above the line. Assigning the Pine Grove melt inclusions to
either group is problematic, because their host volcanic
units are unmineralized but they are genetically associated
with an intrusive Mo-mineralized porphyry. Technically,
these melt inclusions from Plinian airfall belong to the
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unmineralized category, which is consistent with their
lower H2O/Cl– ratios. Thus, lithophile-element mineralized and F-enriched rhyolites are derived from liquids
exhibiting H2O/Cl– <10 whereas barren, high-silica rhyolites are associated with liquids characterized by H2O/
Cl– [10. This relationship is consistent with experiments
and modeling showing that lithophile and chalcophile
ore metal solubility in aqueous fluids is a strong function
of their Cl– content (Candela, 1989; Candela & Piccoli,
1995). Thus, not only can Cl-rich MVPs exsolve under
nominally H2O-undersaturated conditions, they can be
highly efficient in extracting and transporting a variety
of ore metals. This relationship may prove useful for
discriminating between barren and mineralized felsic
magmas, but use of the H2O/Cl– ratio assumes that it
has not been diminished by loss of H2 and/or H2O from
the melt inclusions.
The compositions of silicate melt inclusions from seven
rhyolite complexes of western North America suggest
that most of the rhyolite magmas contained variable H2O
contents and restricted abundances of Cl– (Figs 5 and 6).
The inclusions occur in rock samples from: Plinian and
ignimbrite deposits of the Bishop Tuff, California (Dunbar & Hervig, 1992a); Plinian and ignimbrite deposits of
the Bandelier Tuff (Dunbar & Hervig, 1992b; Stix &
Layne, 1996); the Plinian fallout of the Pine Grove
rhyolite tuff (Lowenstern, 1994; Lowenstern et al., 1994);
and tin and topaz rhyolites from Taylor Creek, New
Mexico (Webster & Duffield, 1991, 1994), Cerro el Lobo
and Cerro el Pajaro, Mexico (Webster et al., 1996), and
the Honeycomb Hills, Utah (Gavigan et al., 1991; Webster
et al., 1991). The abundances of Cl– in the Bishop Tuff
liquids, for example, show little variability (i.e. the range
in Cl– is <0·06 wt %; Fig. 5). This contrasts with Cl– in
liquid that erupted to form volcanic units IDP and RDM
of the Taylor Creek Rhyolite; the Cl– contents of these
liquid fractions varied by as much as 0·35 wt % (Fig. 6).
In summary, the melt inclusions indicate that H2O in
most of the liquids ranged from 0·5 to 8 wt %, nearly
all of the rhyolite liquids contained at least 0·08 wt %
Cl–, many of the high-silica rhyolite liquids contained
Ζ0·3 wt % Cl– (Lowenstern, 1995), the most F-enriched
liquids typically contained higher Cl– contents, and none
of the liquids contained >0·6 wt % Cl (Figs 5 and 6).
What do melt inclusion compositions indicate about magma
evolution and degassing?
Silicate melt inclusions from two F-poor eruptive units
of the Taylor Creek Rhyolite show relationships between
Cl– and H2O that are consistent with fractional crystallization of rhyolite magma without the presence of an
MVP (Fig. 5a). Lines with positive slopes that intersect
the origin fit the Cl– and H2O concentrations of these
melt inclusions; the lines are similar to that predicted by
NUMBER 12
DECEMBER 1997
Fig. 4. Volatile contents of rhyolite-forming silicate liquids as determined from compositions of melt inclusions from North American
rhyolites. (a) Concentrations of Cl– and H2O in >370 silicate melt
inclusions entrapped in quartz phenocrysts from non-mineralized, Fpoor, high-silica rhyolites (filled crosses), mineralized tin and topaz
rhyolites (Φ), and the Plinian fallout of the Pine Grove rhyolite tuff
that is itself unmineralized but is genetically associated with the Momineralized Pine Grove porphyry (Β). Melt inclusions from nonmineralized rhyolites are distinguished from melt inclusions from mineralized rhyolites by a line with slope of 10; the latter have pre-eruptive
H2O/Cl– <10. Data are from Webster et al. (1991), Webster & Duffield
(1991, 1994), Dunbar & Hervig (1992a, 1992b), Lowenstern (1994),
Lowenstern et al. (1994), Stix & Layne (1996), and Webster et al. (1996).
(b) Histogram showing approximate average Cl– contents of silicate
melt inclusions from 36 individual rhyolite eruptive units; some contain
<0·4 wt % F (diagonally ruled columns) whereas others contain 0·4–4
wt % F (stippled columns). On average, all rhyolite liquids contain
<0·6 wt % Cl–, F-poor melts generally contain Ζ0·25 wt % Cl–, and
F-enriched liquids contain 0·2–0·4 wt % Cl–. Sources of data have
been summarized by Lowenstern (1995); additional data are from
Payette & Martin (1990) and Webster et al. (1996). The data are
centered on the maximum value for that individual range in Cl content;
e.g. for the column at 0·05 wt % Cl—these data represent the average
Cl– content of seven felsic units containing 0·05–0·099 wt % Cl.
Rayleigh fractionation of crystals from an MVP-free
magma (Fig. 3a).
Most melt inclusions from the other rhyolites exhibit
minimum Cl– contents of 600–800 ppm, small ranges in
Cl– concentration, large ranges in H2O content, and
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CHLORIDE SOLUBILITY IN FELSIC MELTS
Fig. 5. Concentrations of Cl– and H2O in silicate melt inclusions containing <0·4 wt % F that show limited ranges in Cl– content and nonnegative correlations between Cl– and H2O. The melt inclusions are from eruptive units: (a) lavas IDC and CBT of Taylor Creek, NM; (b)
Cerro Toledo rhyolite of Bandelier Tuff, NM (Ε, Β); (c) Plinian and ignimbrite deposits of the Bandelier Tuff, NM (Ο, Plinian deposits; Β,
ignimbrites; three data points labeled U.P. represent melt inclusions from Upper Plinian deposits); (d) the Bishop Tuff, CA [symbols same as in
(c)]; (e) Plinian fallout of the Pine Grove, UT, rhyolite tuff. Arrows labeled C.S.V. represent the Cl– saturation value for an anhydrous silicate
liquid at 2 kbar with a molar (ANCM)/S ratio equivalent to the average (ANCM)/S of eruptive unit shown. As discussed by Dunbar & Hervig
(1992b) and Stix & Layne (1996), the Cl– contents of some melt inclusions from the Bandelier Tuff are consistent with volatile phase exsolution
before melt inclusion entrapment. Melt inclusions in (a) imply liquid evolution by fractional crystallization under volatile-phase-absent conditions.
All melt inclusions in (d) and (e) and open circles of (b) are problematic because the comparatively low and fixed Cl– contents are inconsistent
with minimum experimentally determined Cl– solubilities of volatile-phase-saturated silicate liquids (see text for discussion). Data are from (a)
Webster & Duffield (1991), (b) Stix & Layne (1996), (c) Dunbar & Hervig (1992b), (d) Dunbar & Hervig (1992a), and Lowenstern et al. (1994).
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Fig. 6. Concentrations of Cl– and H2O in silicate melt inclusions from tin and topaz rhyolites containing 0·4 to >5 wt % F. These inclusions
show negative correlations between Cl– and H2O, and (a)–(c) show limited ranges in Cl– content. The melt inclusions are from eruptive units
of: (a) the Honeycomb Hills, UT; (b) Cerro el Lobo, San Luis Potosi, Mexico; (c) Cerro el Pajaro, Guanajuato, Mexico; (d) lavas IDP and RDM
of Taylor Creek, NM. Arrows labeled C.S.V. represent the Cl– saturation value for a water-undersaturated silicate liquid containing Ζ1 wt %
F at 2 kbar with a molar (ANCM)/S ratio equivalent to the average (ANCM)/S of eruptive unit shown. Similarity of Cl– and H2O patterns in
melt inclusions with experimental solubility curves implies that portions of each magma were volatile phase saturated during crystallization (see
text for discussion). Data are from (a) Webster et al. (1991), (b) and (c) Webster et al. (1996), and (d) Webster & Duffield (1994).
steeply negative to near-infinite slopes in plots of Cl– vs
H2O (Figs 4, 5 and 6). Previous investigations of melt
inclusions have cited the large ranges in H2O content,
near-infinite slopes in plots of Cl– vs H2O, and limits in
maximum Cl contents of ~0·28 wt % (Fig. 5b and c) as
evidence that either an H2O-rich vapor phase (Dunbar
& Hervig, 1992b) or two immiscible fluid phases (Stix &
Layne, 1996; Webster & Rebbert, in press) exsolved
from magma before eruption [see review by Lowenstern
(1995)]. The following discussion addresses MVP exsolution through use of melt inclusion compositions and
volatile solubility data for a variety of liquid compositions
at ~1–2000 bar, but given the strong influence of F on
Cl– solubility, the F-rich rhyolites containing [0·4 wt %
F (Fig. 6) are addressed separately from the rhyolites that
contain less F (Fig. 5). Also, Cl– saturation values are
included in several of the data plots (Figs 5b–e and 6b
and c). These saturation values represent the maximum
Cl– solubility for an anhydrous liquid at 2 kbar with
a molar (ANCM)/S ratio equivalent to the average
(ANCM)/S ratio of that group of melt inclusions. It
should be noted that these Cl– saturation values may not
apply for MVP saturation at pressures other than 2 kbar.
Interpretation of Cl– and H2O contents of melt inclusions from the other F-poor, high-silica rhyolites is
problematic. Melt inclusions from the Cerro Toledo
Rhyolite, Bishop Tuff, and the tuff of Pine Grove exhibit
linear trends in Cl– and H2O showing near-infinite slopes
and maximum Cl– concentrations that are much lower
than 0·2 wt %, and some of the Bandelier inclusions
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CHLORIDE SOLUBILITY IN FELSIC MELTS
studied by Stix & Layne (1996) show limited chloride
abundances that are well below 0·2 wt %. These patterns
are similar to those shown for MVP-saturated silicate
liquids (Fig. 3) and imply that Cl– in the silicate liquids
was buffered, but the anomalously low Cl– maxima
are inconsistent with all experimental data. Experiments
show that the minimum Cl– saturation value for an
anhydrous silicate liquid with the smallest geologically
likely (ANCM)/S (i.e. ~0·28) at the lowest geologically
relevant pressure (i.e. ~1 bar) is near 0·22 wt %; all other
compositional variables studied either increase or have
no effect on Cl– solubility. Based on extant solubility
data, there is no combination of pressure, temperature
and composition that is known to generate an MVPsaturated silicate liquid containing fixed Cl– values <0·2
wt % at equilibrium conditions. Thus, it appears that
these liquids were saturated with one or two MVPs, but
that some other component, such as CO2, may have
been present in sufficient abundance to reduce the activity
of Cl– in the silicate liquid and coexisting MVP(s). This
theory is clearly speculative and requires systematic examination.
Fluorine-enriched silicate melt inclusions exhibit negative linear correlations in plots of Cl– vs H2O, comparatively wide ranges in H2O abundance, and somewhat
restricted ranges in Cl– abundance (Fig. 6). The trends
imply that Cl– and H2O abundances of some fractions
of the F-rich magmas were controlled by the presence
of a single MVP before eruption, because the negative
slopes are consistent with those shown by haplogranite
liquids coexisting with a single volatile phase at 2 kbar
(Fig. 3b). It should be noted that Congdon & Nash (1988)
and Webster & Rebbert (in press) described textural
evidence (i.e. pegmatoidal voids) and other geochemical
evidence (i.e. melt inclusions showing increasing abundance of H2O and decreasing Cl, B, and Be contents with
increasing stratigraphic level within the magma chamber)
suggesting that the Honeycomb Hills and Cerro el Lobo
rhyolite magmas, respectively, were MVP saturated before eruption. The 2 kbar Cl– saturation values for Ffree liquids are shown for the average Cerro el Lobo and
Cerro el Pajaro liquid compositions (Fig. 6) because these
melt inclusions contain <1 wt % F, on average, and
these F levels do not have a significant influence on the
Cl– saturation value. The distinct difference between the
2 kbar saturation values and the intercepts of the inclusion
data with the abscissa suggests that MVP exsolution
in these two systems may have occurred at pressures
considerably <2 kbar. In contrast, many Honeycomb
Hills and Taylor Creek Rhyolite inclusions contain highly
variable F and Cl– contents and >3 wt % F. This
dispersion in F precludes estimation of the pressures at
which the volatile phase exsolved because of the strong
effect of F on Cl– solubility.
The comparison of experimental data with melt inclusion compositions, in a more broad sense, provides
additional insights into processes of magmatic degassing,
because the Cl– and H2O contents of melt inclusions
from an MVP-saturated magma can be interpreted in
two profoundly different ways. Assuming that a melt
inclusion represents an individual aliquot of silicate liquid
that equilibrated with an MVP, then each suite of melt
inclusions which shows large variability in H2O content
and comparatively small changes in Cl– abundance may
be a result of and thus an indication of either: (1)
successive entrapment of aliquots of an evolving silicate
liquid during an extended period of time, or (2) simultaneous entrapment of aliquots of a heterogeneous
liquid. In the first case, the silicate liquid may be homogeneous initially, and the composition of the liquid
changes progressively because of crystal fractionation and
because the liquid equilibrates with the coexisting MVP
during ascent and depressurization. The melt inclusions
track the compositional evolution of the liquid through
time. Geochemical modeling and fluid inclusion studies
indicate that an MVP is most enriched in Cl– when
it first exsolves and that it evolves to more H2O-rich
compositions (Roedder, 1984; Candela, 1989). Consequently, coexisting silicate liquids should evolve to
larger H2O/Cl– ratios (i.e. move up the H2O–Cl– solubility curves) at constant pressure. In the second case,
the melt inclusions may represent aliquots of an MVPsaturated liquid that were entrapped in quartz at roughly
the same time. This situation occurs if (a) a silicate liquid
is heterogeneously enriched in Cl– and H2O and is
saturated with respect to multiple (i.e. compositionally
distinct) MVPs that are not in communication with one
another, and (b) if the liquid contains concentrations of
Cl– and H2O that are fitted by the volatile saturation
curve for that liquid at that pressure and temperature.
The volatile solubility curves suggest that, theoretically,
alkali-chloride-rich MVPs and H2O-rich MVPs can exsolve simultaneously from a single magma containing a
heterogeneous distribution of Cl– and H2O. Thus, the
‘initial’ exsolution of volatiles may take place as individual
saturation events occurring simultaneously throughout a
magma, and many or all of the individual MVPs may
eventually come in contact with one another to form
larger volumes of ‘fluid’ as a result of progressive magmatic evolution and degassing.
ACKNOWLEDGEMENTS
This paper received thoughtful and detailed reviews by
R. Kinzler, J. Lowenstern, H. Shinohara, and P. Wallace.
Assistance with the experiments was provided by C.
Rebbert, and assistance with electron microprobe and
ion microprobe analyses was provided by C. Rebbert
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and A. Leger, and N. Shimizu and G. Layne, respectively.
This material is based upon work supported by the
National Science Foundation under Grant EAR
9315683.
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