-
American Mineralogist, Volume 79, pages 353-369, 1994
Chlorine, fluid immiscibility, and degassing in peralkaline
magmasfrom Pantelleria, Italy
Jlcon B. LownNsrnRN*Mineral Resources Department, Geological
Survey ofJapan, l-l-3 Higashi, Tsukuba, Ibaraki 305, Japan
Ansrnlcr
This paper documents immiscibility among vapor, highly saline
liquid, and silicate meltduring the crystallization of peralkaline
rhyolites from Pantelleia, Italy, prior to theireruption.
Experiments conducted in a mufre furnace and with a
high-temperature heatingstage revealed three major types of
silicate melt inclusions trapped in quartz phenocrysts.After
entrapment in the host phenocryst, type I inclusions contained
silicate melt. TypeII inclusions contained silicate melt +
hydrosaline melt (-60-80 wto/o NaCl equivalent),and type III
inclusions contained silicate melt + HrO-CO, vapor. Two inclusions
con-tained all three immiscible fluids: vapor, hydrosaline melt,
and silicate melt. Fluid inclu-sions within outgassed matrix glass,
viewed at room temperature, are interpreted as thecrystallized
equivalents of the hydrosaline melts within type II inclusions.
These inclu-sions, 2-10 pm in size, consist ofa bubble typically
surrounded by a spherical shell ofhalite.
The presence of both vapor and hydrosaline melt in the magma
indicates that thepantellerite was saturated with subcritical
NaCl-HrO fluids. At a given temperature andpressure, the fixed
activity of Cl in these two fluids delermines the activity and
concen-tration of Cl in the silicate melt. The high concentrations
of Cl in these pantellerites(-9000 ppm) are thus a function of the
low activity coefficient for NaCl in pantelleriterelative to
metaluminous silicate liquids. The Cl contents of Pantellerian
rhyolites indicateequilibration at pressures between 50 and 100
MPa. The high Cl contents of outgassedpantellerites may be due to
minimal loss of HCI (not NaCl) during eruption, as comparedwith
metaluminous rhyolites, which exsolve more HCI-rich vapors.
Discrepancies between the results of heating-stage experiments
and longer muffie-fur-nace experiments indicate that measurements
of melting and homogenization tempera-tures of melt inclusions may
not be accurate unless sufficient time (> I h) is allowed
forequilibration at magmatic temperatures.
fNtnoouctloN
Experimental studies show that the NaCl-HrO systemis
characterized by immiscibility under a wide range ofpressures and
temperatures in the shallow crust (Souri-rajan and Kennedy, 1962;
Bodnar et al., 1985; Chou,1987). Furtherrnore, research on the
silicate melt-HrO-alkali chloride ternary indicates that the Cl and
HrO con-tents of many magmas are sufficient to saturate the
meltwith immiscible vapor and liquid (hydrosaline melt)phases
(Shinohara et al., 1989; Malinin et al., 1989; Me-trich and
Rutherford, 1992; Webster, 1992a). Evidencefor immiscibility
between silicate and HrO-NaCl fluids iswidespread in fluid
inclusions found in phenocrysts ofintrusive igneous bodies such as
granites, syenites, andporphyry ore deposits (Roedder, 1972, 1984,
1992;Roedder and Coombs, 1967; Frost and Touret, 1989:
tPresent address: U.S. Geological Survey, M.S. 910,
345Middlefield Road, Menlo Park, California 94025, U.S.A.
0003-o04x/94l0304-035 3$02.00
Hansteen, 1989; Frezzotti, 1992). Some silicate meltsshow
evidence for saturation with both vapor and hydro-saline melt
(e.g., Frost and Touret, 1989). Because NaCl-HrO fluids are
precursors to ore-forming hydrothermalsolutions, it is important to
determine the factors that con-trol their evolution and
composition. Volcanic rocks areideal for such studies because they
contain quenched ma-trix and glass inclusions that can preserve the
concentra-tions of magmatic volatiles during preeruptive
degassing.
Studies of fluid inclusions in phenocryst-poor volcanicrocks
have only rarely been undertaken. This stems, inpart, from the
scarcity offluid inclusions in volcanic rocks(Tuttle, 1952),
despite the oft-repeated conclusion thatmany igneous systems are
fluid-saturated during crystal-lization and prior to eruption
(Newman et al., 1988; An-derson et al., 1989; Luhr, 1990;
Lowenstern et al., l99l;Lowenstern, 1993). Ofthe handful of studies
of coexistingfluid and melt inclusions in volcanic systems, several
havefocused on rhyolites from Pantelleia, ltaly. Abstracts
byClocchiatti et al. (1990) and Solovova et al. (1991) re-
353
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354 LOWENSTERN: CHLORINE IN PERALKALINE MAGMA
320
20
Thorium (ppm)
Fig. l. Trace-element trends for glassy, unaltered Panteller-ian
rhyolites with an agpaitic index >1.75. (A) Relatively
in-compatible elements Th and La (distribution coefficient
betweenbulk crystal and melt
-
I End-member lmmiscible FluidsData trom Metrich and Rutherford
(1 992)o 50MPaA 100 MPa _ SO Mpa
LOWENSTERN: CHLORINE IN PERALKALINE MAGMA 355
q)
.9 r.oo
c
. 9 0 5os>
I Paffelleribs lromJ Pantslleria
1 10 100Wt% NaCl in Fluid
Fig. 2. Cl content of pantellerite melt vs. bulk compositionof
NaCl-HrO fluid at 830 "C. The Cl contents of evolved pan-tellerites
from Pantelleria (7500-9000 ppm) are consistent withtheir having
equilibrated at 50- 100 MPa with subcritical NaCl-HrO fluids. At 50
and 100 MPa, Cl concentrarions in the meltare fixed as long as the
coexisting bulk fluid lies within the fieldof immiscibility for the
system NaCl-HrO (data from Metrichand Rutherford, 1992). As long as
both non-sihcate fluids (vaporand hydrosaline melt) are present,
their fixed compositions (at agiven temperature and pressure)
require that the actiwity, andthus concentration, of Cl in the melt
remains constant. At higherpressures, the fluid is supercritical,
and any increase in Cl con-tent of the system results in increasing
Cl concentration in themelt and fluid (for all fluid compositions).
The interpretive curvesare based on Shinohara et al. (1989). The
compositions ofthevapor and hydrosaline melt at 50 and 100 MPa (at
825 oC; fromBodnar et al., 1985) define the pivot points of the
curves. Thelarge solid circle represents the Cl content of
NaCl-saturated,anhydrous, pantellerite (1.11 + 0.03 wto/o; see
Appendix l).
and Hildreth (1986) and Lowenstern and Mahood (1991).Lowenstern
and Mahood (1991) identified two groups ofsilicate melt inclusions
in P32. P104. and other units.Glassy inclusions had degassed prior
to or during erup-tion, usually along narrow capillaries that
connect theinclusions to the outside of the host phenocryst, but
alsoalong cracks (see also Anderson, l99l). This populationof melt
inclusions had < I Mo/o HrO, ranging down to
-
3s6 LOWENSTERN: CHLORINE IN PERALKALINE MAGMA
U
U>u>U(ro-
utt'r=ura"orrra
Fig. 3. Schematic pressure-temperature trajectories for
threemelt inclusions. Inclusion l, trapping vapor-undersaturated
sil-icate melt at fr,, cools along Isochore | (A to B) until
reachingvapor saturation at 7b, and a bubble nucleates. The
inclusioncools along the melt-vapor curve (from B to C), and the
size ofthe bubble increases until 573 "C or 7'u," Q. At C, the
quartzhost undergoes lolo volumetric contraction, and the
inclusionincreases in pressure (C to D). The inclusion then cools
along anew melt-vapor curve (from D to E) until I" (E), when
thesilicate melt passes through the glass transition, and the
bubbleceases to grow. Upon hearing, the inclusion retraces the
samepath (E to D to C lo B\ until bubble and melt homogenize at
B(?n",), and the inclusion joins Isochore l. An alternative
meta-stable cooling path for Inclusion I occurs if a bubble fails
tonucleate (so that To < Tn). The inclusion then cools along
itsmetastable isochore and becomes underpressured, or
vapor-su-persaturated (B to F), until it reaches ?noi, when the
bubble nu-cleates and equilibrates with the silicate melt (-F to
G).
Inclusion 2, which trapped two phases, silicate melt + a
pri-mary vapor bubble, atT,,(B), has ?"n > Z, because the
inclusionmust be overpressured to dissolve the extra vapor. In the
labo-ratory, such an inclusion must be heated (from B to H) until
thevapor is dissolved at Zn,. Upon further heating, the
inclusionfollows Isochore 2 (shown as the dotted curve).
room temperature, the internal pressure in the bubblemay change
as gases condense to their liquid state. At 25'C, bubbles composed
of pure HrO should have internalpressures equivalent to the vapor
pressure ofHro (0.026atm). Bubbles with relatively noncondensable
gases (e.g.,COr) retain higher internal pressures at room
tempera-ture (up to -60 atm if liquid CO, is absent; Angus et
al.,r97 6).
If a melt inclusion is heated along the melt-vapor curve,its
bubble homogenizes into the melt at In. Ideally, Z"should be = 7,.
However, some inclusions may contain avapor bubble that was trapped
along with silicate melt(i.e., two phases were trapped). Such
inclusions must bebrought to higher pressure, by heating above 71,
to dis-solve the extra increment of vapor and should have high-er
homogenization temperatures than inclusions that
TABLE 2. Notation for describing characteristics of melt
inclu-srons
Description
temperature of entrapment of silicate melt inclusion in host
phe-nocrysr
temperature at which bubble nucleates during cooling of
silicatemelt inclusion
temperature during cooling at which silicate melt
undergoestransition to glassy state
temperature at which quartz undergoes phase transformation( - 5
7 3 r c a t l a t m )
temperature, during heating, at which a microcrystalline
silicatemelt inclusion is converted to silicate melt t vapor
temperature, during heating, ot homogenization of silicatemelt +
vapor to a single phase
trapped only silicate melt. In such cases, Tn ) T, (Inclu-sion 2
in Fig. 3).
AN.lLYrrcAl, TECHNTeuES
Heating stage experiments
Quartz phenocrysts bearing melt inclusions were dou-bly polished
to provide optimal viewing conditions dur-ing high-temperature
experiments. Quartz grains werepreferred over feldspar because they
had significantly lesstendency to break during sample preparation
and inclu-sion homogenization. Also, because quartz is the last
ma-jor phase to crystallize in pantellerite magmas, its inclu-sions
are representative of melt compositions shortlybefore eruption.
Once doubly polished, crystals were typ-ically soaked in acetone to
remove mounting resin andimpurities. All experiments were done at I
atm in anHrO-cooled l-nltz 1350 microscope heating stage at-tached
on an Ortholux II Pol-MK microscope with pho-tographic capabilities
at the Geological Survey ofJapan.Temperature was measured with a
Pts?Rhr3 thermocou-ple and recorded on a chart plotter. The system
was cal-ibrated at the melting temperatures of KrCrrO? (398
'C),
Ag (961 "C), Au (1063 "C), and NaCl (800 "C). Temper-ature
gradients within the sample were negligible becauseof the small
size of individual quartz grains (
-
LOWENSTERN: CHLORINE IN PERALKALINE MAGMA
Trsle 3. Results of heating-stage experiments on Pantellerian
melt inclusions
35'r
Inc size0rm)
I n l h
fC) fc) rypeI n
Sample' fC)4 Opaque Inc size
fC) Type.. crystals? (pm)t SampleOpaquecrystals?
P32-26 800P32-28 850P32-29 840P32-32.1 1000900900
>1080850850850
NoYesYesYesYesNoNoNoNONoNoYesYesNoNoNONONoYesYesYesNoYesYesYesNoNoNoNoNONoYesYesNoNoNoNoYesYesYes
l l + l l l
850850
>980885890890820N.R.880880880880850850880850880870870
>900850850850850850850850
>850>850
850
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358 LOWENSTERN: CHLORINE IN PERALKALINE MAGMA
TABLE 4. Summary of melt inclusion types and their
characteristics
rype L fC)No. of Phases present at
4 fC). examples.* 900 "CtPhases at 25'C aftermelting
experiment
Phases present at']i (inferred)
t l
i l lIV
775-850 825-90077s-850 825-900
775-850 >950glassy at 25 not studied
43-44 silicate melt24 silicate melt + hydrosa-
line melt3-10 silicate melt + vapor
not studied
4-8 microcrystalline silicatemass + vapor
silicate glass + shrinkage bubblesilicate glass + (halite +
small bub-
ble) + shrinkage bubblesilicate glass + large vapor
bubbledegassed silicate glass + bubble
microcrystalline silicate mass + vapor
silicate meltsilicate melt + hydrosa-
line meltsilicate melt + vaporsilicate melt t vapor:
degassed during erup-tion
silicate melt a vapor+ hydrosaline melt:degassed during
erup-tion
>1000 >1000
'These homogenization temperatures were measured with the
heating stage. 4 for group I and ll inclusions thus may be 25-75"
higher than I.Because type lll inclusions represent heterogeneous
entrapment of magmatic vapor + silicate melt, 4 for them has no
real geological significance.
" Some inclusions are classified as more than one type (e.9., ll
+ lll) and others are uncertain (e.9., lll or V)t All type l, ll,
lll, and V inclusions contained quartz blebs before, during, and
after high-temperature experiments. The small blebs ( 700'C, they
contained small colorless spherical globules I -4pm in size (Fig.
5) of a substance with high optical reliefcompared with the
silicate melt. During heating experi-ments on microcrystalline
inclusions, the globules be-came visible as the inclusion began to
melt, between 650and 750 "C. During reheating of glassy, previously
meltedinclusions (more transparent than the microcrystalline
in-clusions), the features were visible at room temperatureas one
or more small (
-
was heated above 600 oC, the cube and bubbles homog-enized to a
single phase: high-relief, spherical globules.Homogenization was
complete at temperatures below 700"C. The globules did not change
size during subsequentheating above 700 "C and did not dissolve
into the silicatemelt even after 30 min at 1000'C in the heating
stage or48 h at 900 "C in the muffie furnace. There was no
cor-relation between the size of an inclusion and the numberof
colorless globules within it. If there was more than oneinclusion
in a phenocryst, a globule might be located inone inclusion, but
none would be present in the others.During heating, most globules
were not located near va-por bubbles; however, some bubbles
apparently con-tained these small globules within them. At fr,
these bub-bles would be resorbed into the silicate melt, leaving
onlythe globule remaining.
As in some type I inclusions, most type II inclusionscontained
micrometer-sized opaque minerals that meltedat temperatures above
900'C (Table 3 and Fig. 58) or atlower temperatures (e.g., 800 'C)
during longer experi-ments.
Cooling of type II inclusions. As with type I inclusions,when
type II inclusions were cooled below In, vapor bub-bles would
nucleate between 700 and 600 "C, dependingon cooling rate. Some
vapor bubbles nucleated on spher-ical globules, though most bubbles
formed independentlyof these features (Fig. 5A).
All globules in type II inclusions crystallized to one ormore
cubes and an approximately equal volume of bub-ble at 490 + l5 "C.
This temperature is essentially iden-tical to the 500 + l0'C
reported by Clocchiatti et al.(1990) for hydrosaline melts within
melt inclusions frompantellerites of Montagna Grande on
Pantelleria. Bubblesformed by this process never grew >l pm in
size (i.e.,they were much smaller than bubbles formed by shrink-age
of the silicate melt), presumably because the silicatemelt went
through the glass transition close to 490 'C
(Bacon, 1977), preventing these bubbles from
growing.Crystallization of colorless globules was rapid (< I s).
Thetemperature of crystallization was not affected by thecooling
rate ofthe inclusion, and colorless globules couldnot be metastably
quenched without crystallization, evenat cooling rates of 400
'Clmin. At room temperature,remnants of the colorless globules
consisted of a smallbubble and a subequal volume of crystal. The
two phaseseither constituted a sphere (Fig. 5E), or the spherical
bub-ble touched the cubic crystal at one of its corners (Fig.5D).
During the months following the heating-stage ex-periments, these
features changed shape, indicating theywere able to equilibrate or
recrystallize at room temper-ature. As discussed in a later
section, their composition,thermometric behavior, and
crystallization is consistentwith that of hydrosaline melts.
Type III: Yapor-rich silicate melt inclusions
This group of inclusions melted to silicate melt + bub-bles at
similar temperatures as inclusions of types I andII but contained
more or larger bubbles than the othertypes (Fig. 6). Some of these
inclusions did not reach Zn,
359
even at ll00'C. Instead, they consisted of silicate melt+
bubbles. Only one bubble was large enough Io analyzeby FTIR (36
pm), and the analysis showed that the in-clusion contained
considerable COr. Aines et al. (1990)also found CO, in several
large, Cu-rich bubbles withinpantellerite inclusions and
interpreted them to be COr-bearing vapors present along with
silicate melt in the in-clusions (i.e., two phases were trapped).
After quenchingfrom 900 "C to room temperature, bubbles in these
in-clusions were sufrciently large that they made up >3 vol0/oof
their host inclusions. Shrinkage bubbles in type I in-clusions,
even when allowed to equilibrate at 600-700'Cfor 20 min, never made
lp >2 volo/o of the inclusion. Iinterpret type III inclusions as
containing one or moremagmatic vapor bubbles, as well as silicate
melt. Bothphases were trapped together in the inclusion at the
timeof quartz crystallization (Lowenstern et al., l99l). Sucha
conclusion is consistent with their high
homogenizationtemperatures, the presence of COr, and their similar
I*to type I and II inclusions.
Mixed II + III inclusions
Two inclusions contained both hydrosaline melts andlarge vapor
bubbles that homogenized with silicate meltabove 950 'C. Both of
these inclusions contained morethan l5 globules. The globules in
P32-81 crystallized tocolorless cubes + small (- I pm) bubbles at
600 + l0 "C,about 100 "C higher than globules in type II
inclusions.P32-41.1 decrepitated at I 100 'C and therefore could
notbe observed during cooling.
Type IY: Glassy melt inclusions
The group of glassy inclusions was studied by Lowen-stern and
Mahood (1991: Fig. la of that paper) and wasshown to have degassed
through cracks and narrow cap-illaries. No additional heating
experiments were per-formed on this population of inclusions.
Type V: Leaked microcrystalline inclusions
Some inclusions could not be melted at temperaturesbelow 1000'C.
Some of these were located near obviouscracks, though no crack was
visible near others. Theseinclusions are interpreted as having
partially degassedduring or after eruption. Presumably, enough HrO
wasleft within the inclusion (or cooling was slow enough) topromote
devitrification of inclusion glass, and so this classof inclusions
may be differentiated from type IV inclu-sions. Some partially
devitrified inclusions, with capillar-ies visible, appear to be an
intermediate class of inclu-sions between types IV and V.
Murrr,n-TURNACE ExPERTMENTS
Several experiments were done in a mufle furnace toassess the
effect of time on melting and the homogeni-zation of melt
inclusions (Table 5). In one experiment,microcrystalline inclusions
were heated for 30 h at 750'C. Three inclusions (out of 16) melted
completely andhomogenized to a single melt phase. The cooling rate
wasevidently fast enough to prevent shrinkage bubbles from
LOWENSTERN: CHLORINE IN PERALKALINE MAGMA
-
LOWENSTERN: CHLORINE IN PERALKALINE MAGMA
-
LOWENSTERN: CHLORINE IN PERALKALINE MAGMA 3 6 1
e-
Fig. 4. Transmitted light photographs of type I melt inclu-sion
(P32-52; 105 pm in maximum diameter, trapped in quartz)during
heating-stage experiment. (A) During heating, the inclu-sion
remained microcrystalline until temperatures above 700 "C,when (B)
melting began around the inclusion periphery. (C) At800 qC, the
inclusion had reached I. and consisted of pantel-lerite melt (m),
refractory quartz (q), and vapor bubbles (v). (D)At 850'C, the
inclusion reached Zn, when the bubble was ho-mogenized into the
silicate melt. (E) During cooling of the in-clusion, a vapor bubble
nucleated at 70, which, in this example,was - 140 qC below Z'.
nucleating. Because these inclusions had reached Zn,
theexperiment apparently indicates that at least some of
theinclusions were trapped at temperatures as low as 750 "C,100 "C
lower than the temperatures recorded in mostheating-stage
experiments. Other inclusions, though, re-mained partially
crystalline. Interestingly, a greater pro-portion of large
inclusions (>50 pm) than small inclu-sions had reached Z-.
Another experiment showed that the spherical globulesin type II
inclusions were not resorbed into the silicatemelt even after as
much as 48 h at 900'C. Two experi-ments on type III inclusions
showed that the number ofbubbles in these inclusions decreased
significantly afterfive or more hours at 850'C. The remaining
bubbles grewlarger, though the total volume of bubbles stayed
ap-proximately the same. Because the bubbles did not ap-pear to
move during these experiments, the growth oflarge bubbles is likely
due to Ostwald ripening rather thanthe actual coalescence
ofbubbles.
An experiment on inclusion P32-8 I showed that thequartz blebs,
present within all inclusions, shrank in size(by about 500/o) after
48 h at 900 'C. Additionally, thewalls of this inclusion had become
more faceted and lessrounded. Skirius et al. (1990) discussed
faceting in meltinclusions from the Bishop Tuffand concluded that,
giv-en sufficient time at high temperature, the walls of
meltinclusions will recrystallize to form inclusions with neg-
(-
Fig. 5. Transmined lighr photographs of rype II melr
(m)inclusions trapped in quartz. (A) Inclusion P32-29 (60 pn
inmaximum diameter) reached To after cooling from Zn. Threebubbles
simultaneously formed at 700 "C; none of them nucle-ated on the
white globule (hydrosaline melt dropler; h), thoughtwo bubbles
nucleated on a refractory quartz bleb (q). (B) Twohydrosaline melt
droplets (4 pm diameter each) were presentwithin P32-49.1 (105 pm
in maximum diameter). During heat-ing, at 875 'C, some opaque
crystals (o) remained unmelted butwere dissolved above 900 'C. (C)
During cooling, below 490 +15 "C, the hydrosaline melts
crystallized and could not be clearlyviewed except at 1250x
magnification (D and E, for left andright hydrosaline melts,
respectively), which showed them toconsist ofhost gJass (m), a
vapor (+ liquid?) bubble, and a whitecrystal with cubic habit
(presumably halite). The host crystal wasflipped and rotated before
photographing D and E.
25 pm
l)::
Fig. 6. Transmitted light photograph of type III melt
inclu-sron,P32-49.2 (135 pm in maximum diameter) at room
tem-perature. The inclusion consists of pantellerite glass (g),
refrac-tory quartz (q), and hve vapor bubbles, two ofwhich are in
focus.The inclusion had In >930'C, and the bubbles did not
homog-enize after 6 h in the mufle furnace at 850 'C. The largest
bubble(-39 pm in diameter) contained COr, as detected by
infraredspectroscopy.
ative crystal shapes (Clocchiatti, 1975). I interpret thequartz
blebs in pantellerite melt inclusions in quartz tobe daughter
products that form during crystallization ofthe silicate melt to
the blue microcrystalline mass. Duringhigh-temperature experiments,
melting initiates at the in-clusion-host border. Partial
dissolution of the host mightcause the inclusion to become
saturated with respect toSiO, before all the inclusion contents are
melted; as such,no further quartz can be dissolved, and quartz
daughtercrystals (blebs) remain. However, given sufficient time,the
quartz blebs may dissolve and be reprecipitated onthe inclusion
wall because ofthe favorable energetics ofinclusions with negative
crystal shapes.
AssnssrvrnNT oF EeurLrBRruM IN THELABORATORY AND NATURE
The reliability of f, measurements
Data from this study can be used to constrain the tem-perature
of entrapment of silicate melt inclusions to be-tween 750 and 875
"C. Much of this spread appears to bedue to real diferences in the
temperature of entrapmentof inclusions. However, several
experiments done in themufle furnace indicated lower Zn for the
melt inclusionsthan experiments using the heating stage. The
primarydifference between these types of experiments was thetime
allowed for equilibration. This means that temper-atures measured
during heating-stage experiments maynot reflect the actual Zn
because they did not allow suf-ficient time for diffusion of HrO
between the vapor bub-ble and silicate melt. The Zn values in Table
3 appear tobe between 25 and 75 'C too high, as compared
withresults shown in Table 5. Similar heating-stage experi-ments
(J. B. Lowenstern, unpublished results) on bubble-bearing melt
inclusions from the Valley of Ten ThousandSmokes showed In between
25 and 75 'C higher than
-
5 0 2
TABLE 5. Results of experiments in mufile furnace
LOWENSTERN: CHLORINE IN PERALKALINE MAGMA
T tSample Type fC). (hf'
Descriotion of inclusionbefore exoerimentt
Descriotion of inclusionafter exoeriment Interpreted result+
P32-26P32-49.1, 3,5
P32-49 2
P32-51.1P32-54
P32-59.1
P32-59.2P32-63
P32-64
P32-68
P32-69
P32-70
P32-71
P32-81
I
i l, il, t|
il l
l l l or V7
I
t , t l
t , l
t , l
t , t , t , l
t , l
l l + l l l
825 4850 6
8s0 6
825 4850 6
750 30
30
30
30
30
48
-70 pm inc: g + large v90 x 60,30, and 20 pm incs:
a l l w i t h g + h m r135 x 90 rrm inc: g + -30 v
75 x 70 pmt g + 1 large v150 x 150 pm inc: g * -30 v
105 x 80 pm inc: g
3 5 r r m i n c : g + x + v60 pm inc, 20 pm inc, 30 pm
inc, 20 pm inc, 15 pm inc.A l l g + x
2 partially melted incs(110 x 35 and 20 x 50 rrm)
1 large (80 x 110 pm) + 1small (30 rrm) micro inc
1 large (140 pm) + 1 small (35pm) micro inc
100 pm inc + 50 pm inc + 40rm inc + 25 pm inc. All mi-cro
rncs
120 x 60 and 50 pm microIncs.
270 x 60 rrm inc: g + >20 vbubb les+hmr+qua r t zblebs$
g + 2 v
g + l l a r g e vhost crystal cracked and inc
vesiculateds
g + v + c o a r s e r xall incs had g + coarser x
2 incs with g + x + v: largerinc has hmr
large inc: g; smallI n c : g + v + x
large inc: g; smalli n c : v + g + x
100 pm inc: g + v; oth-e r s : g + v + x
g + v + x
g + 3 v + h m r + s m a l l e r ( b y-50%) quartz blebs
T 825 or inc leakedInc leaked
T^ < 750 (?) or silicatemelt metastable; v bub-ble did not
nucleate at750
L > 7 5 0L > 750; v bubbles do
not nucleate at 750
hydrosaline melt stable at8 5 0 : L a n d 4 > 8 5 0(tor both
incs)
largeinc: Land 4 < 750;small inc: I. > 750
large inc: 7- and 7i < 750;small inc: I- > 750
100 pm inc I. < 750; oth-ers: I. > 750
L > 7 5 0
hydrosaline melts stable at900 and did not coa-lesce; large v
bubblesgrew: small bubbleswereresorbed. T" > 900;quartz blebs
dissolve ifgiven sufficient time
sg + n m r
750 30750 30
850
750
/cu
750
750
900
' T: experiment temperature in degrees Celsius.'* f: length of
experiment. Time was apparently sufficient to ensure accurate I.
and 4 values.t Dimensions indicate longest and shortest sides of
cylindrical and parallelepiped inclusions or average diameter of
spherical inclusions. Values
rounded off to nearest multiple of five. All observations were
made at room temperature. Abbreviations used: micro:
microcrystalline; g : glass;hmr: the products of crysrallization of
the hydrosaline melts (i.e., micrometer-sized cube + subequal
bubble); inc: inclusion; v : vapor bubble;x : silicate or oxide
crystals (not quartz blebs).
+ Unless otherwise stated, ?i was not reached during the
experiment (i.e., f < 4). Tin degrees Celsius.$ All inclusions
>30 pm in diameter contained small quartz blebs. Those in P32-81
were observed in greater detail.
preeruptive temperatures indicated by iron titanium ox-ide
geothermometry (Hildreth, I 983).
Using solutions provided by Qin et al. (1992) for dif-fusional
exchange between a sphere ofradius a (bubble)located within a
sphere of radius b (inclusion), one cancalculate the time necessary
for the attainment of equi-librium. If a/b : 0.01, b : 50 pm, and
the diffusioncoemcient for HrO (or other diffusing species) is
10-7cm2ls, the system reaches >950/o equilibrium in 1.6 min(if
the melt-vapor partition coefficient for diffusing spe-cies is
>0.1). Larger bubbles equilibrate faster than thisestimate. A
decrease of I log unit in the diffusion coefr-cient increases the
time necessary for equilibration by afactor of ten. Above 800 'C,
HrO probably diffuses fastenough to attain equilibrium within the
time frame ofheating-stage experiments (Karsten et al., 1982).
How-ever, because some inclusions appeared to homogenizeat lower
temperatures during the mufle furnace experi-ments than in the
heating stage, > I h may be necessaryfor full equilibration at
temperatures below 800 "C. Cl
and COr, slower diffusing species (Watson, l99l), wouldreach
equilibrium with the bubbles in several tens of min-utes to several
hours, within the time allotted for themuffie furnace experiments
and many of the heating-stageexperiments at temperatures above 800
"C.
The major- and trace-element compositions of Pantel-lerian melt
inclusions should become homogeneous with-in the time scale of most
heating-stage experiments. Be-cause the phases within
microcrystalline inclusions arevery small (< I pm except for
quartz blebs) and appearto have homogeneous distribution, diffusion
paths areshort, and remelted inclusions should become homoge-neous
within several tens of minutes.
The control of cooling rate on shrinkage bubblevolumes
Besides its strong control on Zo, the cooling rate alsoaffects
bubble size. Comparison of the sizes of bubbles insilicate melt
inclusions from volcanic rocks may thereforebe misleading, unless
inclusions with a similar host and
-
LOWENSTERN: CHLORINE IN PERALKALINE MAGMA t63
similar size, cooling history, and composition are com-pared.
Comparison of bubble volumes in quartz and pla-gioclase may be of
little value because of the strong effectof the quartz 0 to a
transition on the size of bubblesmeasured at room temperature. A
more reproduciblemethod for comparing sizes of shrinkage or primary
bub-bles in melt inclusions would be to measure them at near-ly
magmatic temperatures. Even then, care should be takento allow
sufficient time to eliminate any compositionalgradients in the
inclusion and to allow the bubble to reachits equilibrium
volume.
When cooling rates are very rapid, homogenized inclu-sions may
not nucleate a bubble. Many authors have not-ed that melt
inclusions from crystals in Plinian eruptiveproducts tend not to
contain bubbles (e.g., Anderson,1991; Dunbar and Hervig, 1992;
Lowenstern, 1993),whereas those from ignimbrites almost always
containthem. Clocchiatti(1972) concluded that crystals from thel9l2
ignimbrite of the Valley of Ten Thousand Smokesmust have had a
relatively slow cooling history becausethey all contained bubbles.
Data from the present studyindicate that melt inclusions in hydrous
peralkaline rhy-olites should not contain shrinkage bubbles
ifcooled from7"n at rates faster than -300"/min.
IonNtmIc.luoN oF corroRlEss GLoBULES ASHYDROSALINE MELTS
The behavior ofthe colorless spherical globules duringcooling,
including their crystallization to a small cube *bubble around
500'C, indicates that these features areneither silicate nor oxide
phases. Instead, their behavioris consistent with that of
hydrosaline melts. They did nothomogenize with the silicate melt,
even during a 48-h-long experiment at 900 "C (which is above the
liquidus;see Appendix 1) and other experiments at lower
temper-atures, indicating that they represent a separate
phase.Their presence in many, though not all, inclusions makesit
likely that they were trapped by the quartz along withthe silicate
melt (i.e., two phases were trapped, which wastermed mixed type
I-II inclusions by Roedder andCoombs, 1967). The obvious difference
in behavior ofvapor (shrinkage) bubbles and hydrosaline melts, the
factthat these bubbles did not always nucleate on the hydro-saline
melts, and the observation that the two phases couldtouch each
other without coalescing indicate that theywere not miscible.
Furthermore, when coexisting primary(trapped) bubbles and
hydrosaline melts were observed,as in P32-81 (a mixed II-III
inclusion), the two phasestouched each other at temperatures
>800 "C and yet didnot mix.
The salinity of the hydrosaline melt may be estimatedby the
temperature at which this phase crystallizes duringcooling. If the
fluid were an NaCl-HrO mixture, the crys-tallization temperature of
490'C would correspond to theliquidus for a solution with -60 wt0/o
NaCl (Gunter etal., 1983). However, 490'C could represent a
metastablecrystallization temperature if a phase more saline than
60wto/o were supercooled. Therefore, the homogenization
Fig. 7 . (A and B) Transmitted light photographs of fluid
in-clusions in outgassed pantellerite matrix glass (g) from
sampleP32. These features (both 9 pm across) consist ofa
parallelepi-ped-shaped bubble (v) inside a spherical crystalline
shell (s, pre-sumably halite). Several unidentified opaque crystals
line theinclusion walls. The inclusions are interpreted to be the
crystal-lized remains of hydrosaline melts. (C) Synthetic fluid
inclusion(3'l rrm in diameter), similar to natural inclusions (A
and B),produced by saturating pantellerite melt with a solution of
800/oNaCl and 2lo/o H2O at 200 MPa and 900 'C (see Appendix l).
temperature of the globules may be more useful towarddetermining
the composition of this phase. During heat-ing experiments, the
cube * bubble homogenized to thehydrosaline liquid at temperatures
between 600 and 700"C, liquidus temperatures for solutions with
75-85 wto/oNaCl. Therefore, if this phase was an NaCl-HrO
solution,it contained between 60 and 85 wto/o NaCl. Though
thehydrosaline melt could have contained KCl, FeClr, orother salts,
the high Na and Cl contents of pantelleritemake it probable that
the phase was mostly NaCl andHrO. Furthermore, features within
outgassed pantelleritematrix, discussed below, appear to
corroborate the hy-pothesis that the cubes within melt inclusions
were pre-dominantly halite.
Fr,urn INCLUSToNS AND HALTTE CUBES rNOUTGASSED MATRIX GLASS
Small (l-10 pm), spherical fluid inclusions were iden-tified in
matrix glass of samples P32 (Fig. 7) and P104.In transmitted light,
the fluid inclusions of P32 usuallyappear as spherical droplets
with two main phases ar-ranged concentrically. The outside ofthe
sphere consistsof a transparent crystalline material that often
displayscubic cleavage or habit. Inside the crystalline phase
re-sides a bubble with a spherical to rectangular shape.Sometimes,
though, the bubble touches the host glass. In
-
J O 4 LOWENSTERN: CHLORINE IN PERALKALINE MAGMA
sample PI04, fluid inclusions are ellipsoidal and
elongateparallel to flow lineations in the glass, so that the
bubblecommonly touches the sides of the inclusion (the hostglass).
Presumably, the bubbles contain liquid as well asvapor, although
that could not be verified optically. In allinclusions, the bubble
has lower relief than both the crys-talline host and silicate glass
(r : 1.516), whereas thecrystalline material has higher relief than
silicate glass(consistent with halite: n: 1.544\. Small.
submicrome-ter, opaque crystals could be seen within the
inclusionsbut could not be identified. The inclusions are
virtuallyidentical in appearance to synthetic fluid inclusions
pro-duced by saturating pantellerite melt with hydrosaline melt(80
wto/o NaCl) at high temperature and pressure (Fig. 7C;see Appendix
l). The inclusions could not be homoge-nized at high temperature
because heating above 500'Ccaused cracking and further degassing of
the host matrixglass. Similar fluid inclusions were not found in
pheno-crysts from the pantellerites from this study;
however,Solovova et al. (1991) reported finding highly saline
fluidinclusions (>90 wto/o NaCl) in anorthoclase from
felsicvolcanic rocks ofPantelleria (locality not specified).
In this study, the identification of crystalline materialin
fluid inclusions was aided by use of the scanning elec-tron
microscope (SEM). Small clusters of halite, with andwithout
associated bubbles, were observed in SEM im-ages of crushed matrix
glass from sample P32. For ex-ample, Figure 8 shows examples of the
- 100 halite cubesfound in three SEM mounts. Halite was typically
foundas one to ten small cubes embedded in glass. Commonly,these
cubes would be next to a small cavity or bubble (C,D, and F of Fig.
8). Occasionally, no bubble would bevisible, as in A, B, G, and H.
Other times, groups of cubeswould be found in a circular region,
with an associatedbubble (e.g., E). All features labeled in Figure
8 were ver-ified to contain NaCl by energy-dispersive analysis.
Noother salts (e.g., KCI) were found in the pantellerite ma-trix,
though not all of the very smallest cubes were ana-lyzed. The
abundance of halite-bearing inclusions is es-timated at 0.0 I -0. I
volo/o of the rock, meaning that thesefeatures contain only 0.6-6.0
wto/o of the total Cl in themagma.
Bprrl,vron oF HyDRosALTNE MELTS DURTNGDEGASSING AND ERUPTION
I interpret most of the halite cubes viewed in SEMimages (Fig.
8) as corresponding to fragments (or crosssections) of spherical
fluid inclusions observed in trans-mitted light images of matrix
glass (Fig. 7). The hemi-spherical cavities in many SEM images also
may corre-spond to bubbles observed within fluid inclusions suchas
those shown in Figure 7A and 78. Because these fea-tures are
reminiscent of cubes and bubbles formed duringcrystallization of
hydrosaline melts in type II silicate meltinclusions (Fig. 5D), I
interpret them to be a related phe-nomenon. They are the cooled and
dehydrated remainsof hydrosaline melts present during magma storage
in ashallow reservoir. The hydrosaline melt would crystallize
to halite + vapor before eruption and extrusion (at pres-sures
of 300-400 bars for a fluid with 50-85 wto/o NaCl:Chou, 1987). As
long as the magma temperature was
-
LOWENSTERN: CHLORINE IN PERALKALINE MAGMA 365
Fig. 8. (A-H) Secondary electron images (with SEM) of halite
embedded in naturally outgassed, glassy matrix of pantelleriteP32
(an obsidian flow). Many of these crystals are interpreted to be
crystallized remnants of hydrosaline melts (Figs. 5 and 7). Seetext
for details.
Iprvrrscrnr,B FLUTDS rN pANTErr.ERrrES ANDOTHER MAGMATIC
SYSTEMS
Equilibration depth vs. Cl content of pantellerites
Geological constraints indicate that pantellerite magmachambers
may reside at relatively shallow depths (2-6km) beneath the surface
(Mahood, 1984). Informationavailable from pantellerite melt
inclusions is consistentwith this assertion. Because the glass in
pantellerite in-clusions contains little CO, (Lowenstern and
Mahood,I 99 I ), true shrinkage bubbles should contain mostly
HrO.Immiscibility between these HrO-rich shrinkage bubblesand
hydrosaline melts at 800'C requires that the pressurein the
inclusion be lower than 160 MPa, or the two NaCl-HrO fluids would
mix (Chou, 1987). Moreover, the data
of Metrich and Rutherford (1992) show that
pantelleritesequilibrated with vapor and hydrosaline melt at
pressures> 100 MPa should have lower Cl contents than the
unitsconsidered here (compare Figs. I and 2). This should holdas
long as the CO, in the system does not strongly affectthe
solubility of Cl in silicate melt.
The HrO contents of the pantellerite melt inclusionsare also
consistent with fractionation at relatively lowpressures. Given
that the solubility of HrO in peralkalinemelts is about l5olo
greater than that in metaluminousrhyolites (Webster, 1992b), the
1.8-2.10/0 H,O measuredin melt inclusions from the pantellerites of
this study(Lowenstern and Mahood, l99l) would be consistent withH,O
saturation at 30-40 MPa (Silver et al.. 1990). How-
-
366
ever, the presence ofCOr-bearing bubbles in type III in-clusions
indicates that the magma was saturated with amixed H'O-CO, vapor.
As such, the HrO contents of themelt inclusions would be consistent
with vapor saturationat higher pressures (e.g., at 80 MPa, if the
vapor con-tained - 50 molo/o H,O). Lowenstern and Mahood ( I 99 I
)argued that P32 and Pl04 were not HrO saturated be-cause HrO
contents continued to increase with differen-tiation. It thus
appears that these pantellerites equilibrat-ed with a COr-bearing
vapor and hydrosaline melt atpressures between 50 and 100 MPa. If
the solubility ofCO, in pantellerites is similar to that in
metaluminousrhyolites (Fogel and Rutherford, 1990), then the CO,
con-tents (
-
pantellerites have moderate to high HrO contents (Ko-valenko et
al., 19881 Lowenstern and Mahood. l99l:Webster et al., 1993), but
the observation of Nicholls andCarmichael ( I 969) is still valid;
Cl appears to be retainedin the silicate melt during pantellerite
eruptions. The rel-ative nonvolatility of Cl was demonstrated by
Webster etal. (1993), who showed that Cl contents of
pantelleritemelt inclusions from Fantale, Ethiopia, are very
similarto those of outgassed matrix. Similar relationships holdat
Pantelleria, where Cl appears to be held in the meltdunng eruption
(Kovalenko et al., 1988, 1993). Unpub-lished data of Lowenstern
show an average of 8700 +1000 ppm Cl in 12 melt inclusions vs. 9150
+ 770 ppmCl in matrix glass. This contrasts with metaluminous
rhy-olites, where matrix glass commonly has lower Cl con-tents than
silicate melt inclusions (e.9., Dunbar et al.,I 989; Westrich et
a1., 199 l; Bacon et al., 1992). Becauseof the low solubility of
NaCl in high-temperature HrO-vapor at low pressure (Pitzer and
Pabalan, 1986), nomagma is likely to lose significant amounts of
NaCl dur-ing eruptive degassing. However, HCl, a minor compo-nent
of magmatic vapors at pressures >50 MPa, increas-ingly
partitions into the vapor (not hydrosaline melt) atlow pressure
(Shinohara et al., 1984; Shinohara, l99l).Besides pressure, the
major factor controlling HCI parti-tioning between silicate melt
and vapor is melt compo-sition. Urabe (1985), in experiments done
at 350 MPa,showed that the HCI concentration of magmatic fluid
isinversely proportional to the peralkalinity ofthe coexist-ing
silicate melt. Evidently, metaluminous magma tendsto buffer the
vapor toward more acidic compositions. Thismay account for the
greater loss of Cl during degassingof metaluminous magmas (as HCI)
and the associationof H* metasomatism (argillic alteration) with
shallow calc-alkaline intrusions.
AcxNowr-nocMENTS
Support for this research was provided by the Japanese Agency
forIndustrial Science and Technology. The data were gathered at the
Geo-logical Survey of Japan (G.S J.); I thank A. Sawaki for
offering use of theI-eitz 1350 homogenization stage, H. Shinohara
for help with the inter-nally heated pressure vessel, and Y.
Okuyama for aid with the JEOL 6400SEM. The manuscript was completed
at the U.S. Geological Survey, withsupport from the National
Research Council. Photographic equipmentwas made available by C.R.
Bacon, and R. Oscarson operated the SEM.G.A. Mahood of Stanford
University allowed me to use some of herunpublished data, shown in
Figure I and Table l, and provided the sam-ples used in this study.
Initial study of these samples began while I wassupponed by N.S.F.
gant EAR-8805074 to Mahood. I am grateful forreviews by C.R. Bacon,
H. Belkin, G.A Mahood, E. Roedder, H. Shi-nohara, and J. Webster
Finally, I am indebted to H. Shinohara for hisinsi&tful
comments, friendship, and generosity during my stay at G.S.J.
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M,c.NuscnrFr RECEIVED Juxe 14, 1993M,',m-rscnrrr ACCEPTED
NowbrseR 22, 1993
AppBNorx 1.
Powdered matrix glass from unit P32 was equilibrated at 200MPa
and 900 "C for 95 h. The /o, was kept close to the Co-CoObuffer.
The samples were quenched by turning offpower to theinternally
heated pressure vessel, after which the sample tem-perature dropped
to
-
LOWENSTERN: CHLORINE IN PERALKALINE MAGMA 369
and Mahood, l99l)l was crystal free after the experiment,
in-dicating that 900 "C is above the liquidus for this sample.
Theexperiment shown in Figure 7C was equilibrated with a solutionof
80 wto/o NaCl and 20 vlto/o HrO (added to the charge as
NaClcrystals and deionized HrO). The glassy product contained
ap-proximately 9.5 wt9o NarO, 0.40lo K2O,4.4o/o FeO,.,, -40lo
HrO,6500 ppm Cl, and amounts of other elements similar to thoseof
the starting composition. The loss of K and Fe from the sam-
ples was due to the lack of those elements in the added fluid
(assalts) and the high fluid to glass ratio (2.5). The
NaCl-saturatedpantellerite (large solid circle in Fig. 2) was
synthesized undersimilar conditions, and the product contained
approximately 9.6wtVo NarO, 2.0o/o KrO, 7.8o/o FeO,.,,