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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

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


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 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


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


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


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,.,,

chlorine, fluid immiscibility, and degassing in ... › ammin › am79 › am79_353.pdfin the two...

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