JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 100, NO. El0, PAGES 21,265-21,270, OCTOBER 25, 1995
Competition betweenNaaSO4and Na sulfide
in the upper crust of Io
D. S. Burnett
Divisionof GeologicalandPlanetarySciences,
CaliforniaInstituteof Technology,
Pasadena
Abstract. The Na atmosphere
of I0 requiresa Na-S-O phasein the outersurfacelayers.
Consideringthe variousmechanisms
for extractionof Na to the surface,the possibleprimary
phasesareNa20, Na2S•,andNa2SO4.However,regardless
of theprimaryphases
broughtto the
surface,the shallowcruslalrecyclingof materialimpliedby theongoingvolcanismwill tendto
producethermochemical
equilibriumandcauseall Na to endup asNa2SOn
or Na2Sx.This
hypothesis
is investigated
by relativelymodel-independent
thermodynamic
calculations.The
majorassumption
is that materialis statistically
circulatedto sufficientlyhigh temperatures
by
burialthat thermochemical
equilibriumcanbe attainexl.For a widerangeof asstuned
crustal
(PT) conditions,
Na20 will beconverted
to Na2SOn.Duringresidence
in theshallowest
crustal
regionsdominatedby liquidSO2,e.g., SO2geysers
or fumaroles,
or for anycrustalregimeswhere
SO2and S are in comparable
abundances,
Na-st•des will beconverted
to Na2SO4.However,in
high-temperature,
low-pressure
regimeswith a low relativeabundance
of SO2relativeto S (e.g.,
dueto outgasingof SO2),Na2SOnis converted
to Na sulfides.Suchregimescouldbe relatively
commonon Io, e.g., associated
with flows,lava lakes,or shallowintrusions.Consequently,
because
of thermochemical
equilibration
in differentcrustalenvironments,
bothNa2SOnandNa
sulfides will coexist on the Io surface.
Introduction
and Burnett, 1993; (3. R. VanHecke and D. B. Nash,
unpublished
manuscript,
1984].
The questioninvestigated
in this studyis, regardless
which
planetary
Na atmospheres.
In thecaseof Io it is widelyaccepted of the aboveprimary sourcemechanisms
(1-4) predominates,
that Na is removedfrom the outersurfacelayersby sputtering what would be the stableNa-bearingphasesif thermochemical
with magnetospheric
ions [e.g., Johnsonand Matson, 1989; equilibriumis establishedduringrecyclingwithin the upper
Chengand Johnson,1989]. The Io Na cloudis mosteasily crustallayerson Io? Takingrapid c•
recyclingon Io as
understoodif sputteringoccurs from surface materials as established[e.g., Veederet al., 1994], threebasicasstunptions
opposedto atmosphericconstituents. The spectraof the are made.
magnetospheric
ionsshowonly O, S, and Na, whichhasbeen
1. SO• andelementalS are the dominantsurfacecompounds,
interpreted
by mostworkers[e.g.,JohnsonandBurnett,1990, in excesscompared
with anyNa-bearingphase.I am essentially
1993] as indicatingthat atmospheric
Na arisesfrom Na-S-O adoptingthe Io modelof Smithet al. [1979] or Kieffer [1982]
surfacephasesand that negligibleNa is containedin silicate with an uppercrustwhichis composed
almostentirelyof S-SO2
phases.Relativeto Na, significantupperlimitsfor Si in the Io and throughwhichvolatilesare rapidlyrecycled.The regions
torus have been reported for the most probable states of beneaththe uppercrustare predominantly
silicate,but I assume
ionization[McGrath,1993].
that the uppercrustcontainsonly minoramountsof silicatein
Four plausiblehypotheses
for the natureand originof the the form of occasionalmagmatic intrusions. SO2 is a
Na-bearingphaseshave been suggested. (1) The Na is documentedsurfaceconstituent[e.g., Lellouch et al, 1990];
indicativeof evaporitedepositswhichare residuesof ancient elementalS is not but is asstuned
to be presentby mostworkers
Ionian oceans[Fanale et al., 1974; Nash and Fanale, 1977; [e.g.,MosesandNash, 1991].
Kargel, 1993]. Terrestrialanalogieswouldsuggest
sulfateand
2. Crustalrecyclingburiesthe phasesproduced
by all of the
halide salts, but the high chloridedsulfate
ratio for terrestrial four primary mechanisms(1-4) to sufficientlyhigh crustal
oceam is primarily a consequence
of biologicalreductionof temperatures
that thermochemical
equilibriumcanbe obtained.
sulfate[Schlesinger,1991]. Thus, giventhe high chondritic This assumptionis crucial because,for-125-140 K surface
S/CI, a predominance
of sulfateswouldbe expectedfor Ionian temperatures
[e.g.,JohnsonandMatson, 1989],thermochemical
oceans. (2) The Na was themrallyevaporatedfrom molten reactiontimes are likely to be long,evenrelativeto geologic
silicateduringvolcaniceruptionsas Na atoms[Fanaleet al., time. A detailedmodelof the recyclinghasbeenworkedoutby
1982]. (3) TheNa wasextracted
fromsilicatematerials
asNa- Kieffer [1982] in whichbothS and SO• are carriedto sufficient
sulfidesby elementalS [Lunineand Stevenson,
1985;Johnson crustaldepthto be the sourceof the observed
fumaroles(SO•?)
and Burnett, 1990; also G. R. VanI-Iecke and D. B. Nash, and volcanoes (S?) [McEwen and Soderblom, 1983].
unpublishedmanuscript,1984]. (4) Na-rich sulfateswere Thermochemicalequilibriumneed not be achievedon every
produc• by interactionof SO2with silicatematerials[Johnson cycle; it is sufficientthat it be achievedstatisticallyover
geologictime. (3iven the essentiallyinexhaustiblenature of
Copyright1995by •heAmericanGeophysical
Unioo_
tidal heating,it is reasonable
to assume
that the presentcrustal
Relatively little is known about formationmechanismsfor
Papernumber95 JE01165.
conditions
onIo havebeenin placeforat least109years,far
0148-0227/95/9 SJE-01165505.00
longer than for any specificgeothem•alregion in the outer
21,265
21,266
BURNE•:
Na2SO4AND Na-SULFIDE COMPEITIION IN UPPER CRUST OF IO
kilometerof the Earth. However,becauseit cannotbe proved This temperaturerangeextendsfrom the minimmntemperature
that thermochemical
equilibriumis acquiredrapidly, it is (200 K) at whichthermochemical
reactions
might:beeffectiveon
necessmyto assume:
ge01o•c timescalesto silicate magmatictemperatures,
the
3. During
residence
in theouter
micron
Ofthesurface,
maximumplausiblefor the Io crust. The 200 K limit also
radiation
chemical
pr•,•sses
do not significantly
alterthe approximately
corresponds
to the meltingpointof SOl. Liquid
productsof thermoch'emical
equilibrium. Becauseof the SCh providesa fluid medium to facilitate thermochemical
recyclingall upper crustal materialswill periodicallybe reactions. Although arbitrary, the adoptedpressurerange
transientlyexposedto surface irradiation. Here radiation corresponds
to the outer2 kin, and by analogywith terrestrial
chemistry
wouldincludethe effectsof solarultravioletphotons, hydrology,it is probablyunreasonable
to considera compound
as volatileas SO: to be routinelyburiedto greaterdepthsthan
nmgnetospheric
electrons,
andmagnetosphe[i,'c
ions.
For clarityall calculations
beloware discussed
in termsof this. As illustratedin Figure1, it is usefulto dividethis (PT)
the most importantsingle reactionleadingto the dominant space into three regions defined by the gas-liquidphase
products. Alternativereactionshave been systematicallyboundariesof SCh and elemental S.
considered
to determinethatthisapproach
is valid. Moreover,
RegionI is characterized
by the presence
of liquid SOl, and
theresultsof severalof thecalculations
havebeenverifiedusing evenfor Iothermsashighas 1000K/km, a significant
portionof
a sophisticated
freeenergy 'minimization
program
incorporatingtheuppercrustwouldbe in the SO• liquidfield [Kieffer,1982].
a largethennodynmnic
databaseof Na-S-Ocompounds
[Sharp, S is mainlysolidin I, althoughin the high-P,high-T(>388 K)
992].
comerof I, liquid S and liquid SO: coexist. For condensed
Theimportance
of cruslalrecycling
waspreviously
discussed phases,as in regionI, thedirectionof reaction(1) is determined
by Nashand VanHecke
[1992]with emphasis
on the enthalpy by the Gibbsfree energychange(AG), whichcanbe calculated
changesduringrecycling. The approachand resultsof these fromtabulated
standard-state
free•energies
andthe SChphase
authorsappearconsistent
with mine.
diagram.Valuesin therangeof-"900'
kJoulesareobtained
for
(1),indicating
that,in thepresence
of liquidSChandcondensed
S, Na20 would be convertedto Na•SO4in regionI. There is
little uncertainty
in thisresult.
Calculations
RegionII is definedby SChgas(strictlyspeaking,
fluid,as
Oxide conversion
muchof region1I is above
theSChcriticaltemperature)
and
Fanaleet ai. [1982]discuss
volcanic
outgasing
of Na atoms. condensedS (mainly liquid, but solid S in the low-T, 1ow-P
In thepresence
ofeventm• amounts
ofH, theg• phase
species portions
of region
II). Anymi•ibiiityof liquidS andSO•is
wouldbeNaOH;howeverit is quitepossible
thatH is muchless neglected.In regionII the equilibriumconstant
for reaction(1)
thanNa in the uppercrustresultingin a negligible
fractionof canberelated
to theequilibrium
SChfugacity:
K = 1/J(SCh)
•,
volatileNa beingtiedupasNaOH. Regardless
of whether
Na or
NaOHis thevolatilespecies,
it is likelythatrapidoxidation
to
NaO by radiation-produced
O •ecies wouldoccurevenat the
low surf• temperatures.
It is notpossible
to evaluatethefate
of chemisorbed
NaO on surfaces
precisely,
but eventually
NaO
wheretheequilibriumSO2fugacityis the valuewhereNa20 and
Na2SO4 can coexist.
K can be calculated from tabulated
standardstateGibbs free energiesof formation[Barin, 1989],
yieldingequilibrium802 fugacities,as shownin Figure2. The
will be converted to Na20 or to sulfide if adsorbedonto an
elementalS grain. Even if photochemically
mediatedgrain
MP MP CP
surfacereactionsare not effective,thermochemical
conversion
of
so2 s so2
theNaO radicaluponburialshouldbe rapid,with the products
beingeitherNa20, Na•SO4,or Na sulfides.The fateof Na•SO4
and Na sulfides is discussed below.
CP
1
Here we consider the
destruction
of Na20 on burialto produce
Na•SO4.Althougha
varietyof reactionmechanisms
are possible,
all pathways
that
we haveconsidered
(e.g., throughNa•SO3)canbe summarized
schematically
by thereaction
2Na20+ 3SO2= S + 2Na2SO4
logP
i
(1)
Na20 is solidthroughout
therelevant(PT) regions.Themelting
pointof Na2SO4is 1157K. Liquidimmiscibility
with S is
assumed;
thusNa2SO4hasbeentakenas a purephasefor all
calculations.
As discussed
below,thermodynamic
calculations
showthat
for the wholerangeof plausible(PT) conditions
for the outer
few kilometersof Io, reaction(1) proceeds
in the directionas
written.Thealternative
Na20destruction
reaction
leading
toNa
sulfideformationis alsoeffective. TI• competition
between
/
/
TIT
-1
2OO
/I
/600
I
I
1000
!
1400
T,K
Figure 1. The rangeof pressureand temperature
consid•edis
conveniently
divided
intothreeregions
based
onthevapor
piessure
curvesof SO2(solid curve)and elementalS (dashedcurve). In
regionlboth• ands arecondensed
ptmes. In regionlI, SO•isa
gas,
but
S
remaim
cendens•
Region
II is •
dividedinto
Na2804 and Na sulfides is discussedbelow.
regionsIIA and 1I• aboveand below 525 K, the transition
Realistically,
the crestof Io is a verycomplex,thermally tempemtu•of solidNa•S•to an immiscible
Na•S•liquidcoexi•
heterogeneous
region. Thus,ratherthanadoptan assumed withliquidS. In regionlll bothSO2andS aregases.Forreference,
temperature
gradient(Iotherm),we consider
a broadrangeof themeltingpoints(lVlP)andcriticalpoints(CP)areshown.Pressure
(PT) conditions
rangingfrom0.1 to 100attoand200 to 1500K. variations
of themeltingpointscanbeneglecte&
BURNE•:
Na2SO4AND Na-SULFIDE COMPE•ON
-10
_
0
-30
Na2SO4
Na:Ss(s)+ 2SO•1) = 6S(c) + Na:SO•(s)
--
•.400
-•0
-50
21,267
4. ExperimentsindicateNa:S: as the productof elementalS,
silicatereactions[Johnson
and Burnett,1990]. However,in the
presence
of elementalS theNa-S phasediagram[e.g.,Rosenand
Tegman,1972] showsthat Na:Ss is the stablephaseat low
(<525 K) temperature.Thus for regionI (Figure 1) with all
condensed
phases,sulfate-sulfide
competition
is represented
by
RegionaII andIII. 2Na20 +3SO2=S+2Na2SO4
-2O
IN UPPER CRUST OF IO
wheres, 1,c referto solid,liquid,andcondensed,
respectively
(c
- Na20 •
is usedforS beca• a smallportion
of I is above,thes melting
-
-60
0.5
I
I
I
I
1.5
2
I
Z5
--a- RegionIII p2=0.1 ._,_ RegionIII p2=100 •
Numberson pointsaredegrees
K
(2)
298
I
I
3
3.5
4
RegionII
point.) Gibbs free energiesof formationare not availablefor
Na:S•; thesehavebeenestimatedbasedon datafor Na:Sxwith
x=l-4. The adoptedvaluesare shownin Table 1. The errors
introduced
in the calculatedAG for (2) fromtheseestimates
(+510 kJ) arenotsignificant.
For regionI (Figure 1) with all condensed
phasesthe DG
valuesfor(2) are-220to -320kJ showing
that,withS andliquid
SO• present,Na2S•is converted
to Na2SO•in thisregion.
FigureZ. Thisdiagram
shows
therelativestability
fieldsof NaO
andNa2SOn,
coexisting
withSO•andelemental
S. Thecurveshows
In region II above 525 K an immiscibleNa2S-S liquid
thecalculated
equilibrium
SO••_•acity
(arm)at whichNa•' and coexistswith liquid S andSO• gas;thusregionII is dividedinto
Na•SO4cancoexist.Na•SO4is thestablephasein thefieldabove regionsHA and lIB, above and below 525 K, respectively
the line,andNa• is the stablephasein the fieldbelowtheline. (Figure1).
Althoughcalculated
for region1I, the rehtienshipis a good
In regionIIA, sulfate-sulfide
competition
canbe described
by
SO••hedc
•½h
indi•
(2) except that gaseousSO• is present;thus the equilibrium
constant
for reaction (2) can be related to SO• fugacity:
high,
pressure
[Ldlouch
etal.,1990]
is10'•atn•Inthe
uppermastofIo, SO•fugacities
should
beatleast•
conversion
ofNa•Oto
curvedefinesa phaseboundarybetweenthe stabilityfieldsof
Na:SOn (lower temperaturesand higher SO2 fugacities)and
Na•O (higherT and low SChfugacities).Estimated
fugaci•
coefficients[Walas, 1985] indicate that SO• fugacity and
pressurediffer by muchlessthan a factorof 2 over the (PT)
regiondefinedby Figure 1. Thus, Figure2 showsthat the
equilibriumSChpressures
arequitelow overthe wholerangeof
temperature
in regionII, favoringNa2SO4at theexpense
ofNa•.•
for a planetsuchasIo with SO• asthe dominantvolatile.
K=I/./(SO•)
:. GiventhefactthatSO•is a majoruppercrustal
constituent,
the resultingequilibriumSO• fugacitiesshownin
Figure3 are low, indicatingthat Na•SOnis stronglyfavoredin
regionIIA. For referencethe daytimeatmospheric
SO• surface
pressure
is roughly
10'9 atto,andSO•pressures
in theupper
crustshouldbe muchhigherthanthis.
Calculationof sulfate-sulfidecompetitionin region
requiresknowledgeof the activitiesof Na2S•(I)components
in
the immiscibleNa•S-S liquid. These activitiescannotbe
calculatedexactly, and this representsthe major sourceof
unity
in theresultsof thisstudy.However,basedonvapor
pressure
andcell EMF data,modelsof the relativeproportions
Inregion
Ill bothSO•andSaregases.
eras
phase
spec!9tionof Na•S•t species(i.e., chain length distributions)have been
forelemental
S is complex,
containing
a distribution
of species formulated[Cleaver and Sime, 1983; Tegrnan,1976] as a
Sx,withx ranging
from2 to 8;however,
thennodynami•
'9 dataare functionof temperatureand bulk composition
(Na•S) of the
available
fromwhichthe gasphasecompositions
• be Na•Sxliquid. The presentcalculationsare basedon the Cleaver
calculatedas a functionof temperature
and pressure[Chaseet and Sime modelCz•. RegionII is definedby the presence
of
al., 1985]. At highertemperatures,
S2 is dominant;thusit is liquid S, thus the S activityis 1, fixing the Na2S, liquid bulk
logicalto write chemicalreactions
usingS: astherepresentative composition
as a function of T. The Cleaver-Sime
modelthen
• phaseelementalS speciesat all temperatures.Thus the gives the mole fractionsof all Na•S•t components.Sulfateequilibriumconstantfor reaction(1) in regionlII canbe written sulfidecompetitioncan be evaluatedby asstuningideal solid
as J(se)ln/j(SO•)
a. To calculate
equilibrium
J(SO•) [or solutionof the Na•S•t components,
equatingthe Na•S• mole
J•SO•)/J•S:)], ./(S:) is taken as an independentvariablewith fractionto its activity. Any Na•S•tcomponent
can be usedto
valuesofj•S:) rangingfrom0.1 to 100 atmospheres.
However, describesulfate-sulfidecompetition;Na2Sa has been chosen
sincethe equilibriumJ(SO•)<<•S2),the changesin logJ(SO•) becauseit is usuallya majorspecies
andbecause
thermodynamic
dueto variationsin./(S: are relativelysmall,andthe calculated
SO• fugacitiesfor regionlII are closeto thosefor regionII, as
shownonFigure2.
Table1. Esfixnated
Standard
Free
In smmna•, except for crustal regionsalmost totally
of Formafi .onof Na,2Ss
depletedin SO•, Na• will be thermochemically
converted
to
Na:SO, The possibilityof extremeSO• depletionrequires
fiu•er discussion,but this is deferreduntil the role of Na
T,K
AG*/KJ•I
sulfidesis discussed
in thefollowingsection.
Suifide-SuifateCompetition
Thereare a varietyof stableNa sulfidesof the typeNa2Sx,
with x=l, 2, 3, 4, 5. Thermodynamic
dataareavailableforx=l -
•
300
-415
400
500
-405
-389
ondataforNa.• withx = 1-4fromBar/n[1989]
,
21,268
BURNETT: Na2SO4AND Na-SULFIDE COMPETITION IN LIPPERCRUST OF IO
Regionslib andIlL (Na2S4)I+2SO2g=SS+Na2SO4
RegionIIA: Na2S5s
+2SO2õ=6Se+Na2SO4s
-5
-10
5
Na2SO 4
-15
-20 300
_o'•
-25 -30
-
-35
-
600
200
-40
1.5
I
I
I
I
I
I
I
2
Z$
3
3.5
4
4.5
$
0.6
i
i
i
i
i
I
I
I
I
I
i
0.7
0.8
0.9
1
1.!
1.2
1.3
1.4
1.5
1.6
1.7
1.8
5.5
1000/T
_•_ 532=0.1
labelson pointsaredeE K
._,_ t5'2=1
_•_ 532=10
-o- rS2=100
_._ Regionlib
Figure4. In regionlIB theequilibrium
SO2fugaeity
whereNa2S•(1)
Figure3. Analogous
to Figure2 forNa20,thisdiagram
shows
the
and Na•SO4coexistcan be represented
by a singlecurve.
relativestability
fieldsforNa2Ss
andNa•SO4forregion
l• of Figure
Calculations
weredoneforNa2S4
asa represen•ve•t
of
1. Theetnveshows
thecalmlaled
equilibrium
fugaeity
whereNa2S•
the
NA•S•
liquid.
For
region
117
the
equilibrium
SO•
fugacities
andNa2504cancoexist.Lowertemperatm-e
regions
(belowmugldy
dependon S• fugacity
as well as temperature.
Calculations
are
shownfor differentvaluesof J•S•). At higherteatlXaamres
and
highexj(S•)
thestability
fieldofNa•(1) •
relative
toNa•O,
andASO9varyindepehntlyin uppecrust
data for pure Na2S4(necessm•to calculatethe equilibrium
700 K) favorthef rnmtionof Na•SO,
of Io, andmostof theregions
in thisfigurearcplausible
forIo. For
constant)are available. Thusin regionlib
(Na2S4X1)
+ 2SCh(g)= 5S(1)+ Na2SO4(s)
(3)
example,
in volcanic
plumcsor S lavalakcswhereSOzventing
Iauduccs
a S-rich,hightenkocratum
environment,
Na2S•will bc
fomled.
wheretheparentheses
emphasize
thatNa2S•(1)is a component
in
a Na2Sxliquid. For reaction(3) in regionlib the equilibrium so in general,the high-T regionsof the Io uppercrustfavor
constant
K = l/[X4J•SO2)2],
where
X4isthemolefraction
ofthe
Na2S•o
Na2S4(1)
component
in the immiscibleNa-S liquid. With liquid
The implicationsof the calculations
for Io in region111are
S present,
X4 is a knownfunctionof T only,K canbe calculated somewhat
easierto evaluateusingisothemud
plotsof/(S2) rs.
fromthe Gibbsfree energiesof formationof the purephases, J(SCh). An examplefor 1100 K is shownin Figure5. As
andthusthe equilibrium
J(SO•)for regionlib canbe calculated expectS, higherJtS2) favorsNa•S• and lowerJ•S•) favors
as shownin Figure4.
Na2504. In generalunless there is essentiallycomplete
Region11Iis definedby the presenceof bothelementalS and depletion of SO2 relative to S, Na•SO4 is favored for
SChin the gasphase. AdoptingS2as the gasphasespecies,
as temperatures
less than about 800 K. However,at higher
discussed
in the previoussection,the equilibriumconstant
for temperatures
the stabilityfield of Na2Sxincreases
to higherSO2
reaction(3) is givenby log K= (5/2) logJ(82)-2logJ(SO•)-log fugacities.
(X0.
On Io, J(s•) and•SCh) vary ind•dently, thus the
equilibriumJ•802) has beencalculatedallowingJ•S2)to range
Region III, T= 1lOOK
from0.1 to 100arm(Figure4).
(Na2S4)!+ 2 SO2g=5/2S2g+ Na2SO4e
-!
The calculations
for region11Iwere doneusingequilibrium
82 vapor pressuredata for Na-S liquids as summarizedby
Tegman[1976]. Takingp(S2) and T as independent
variables,
-1.5
Figure4 of Tegman[1976] was usedto obtainthe bulk Na2S/S
of the Na2Sxliquid in equilibriumwith the chosenp(S2).
Na2 SO4
Significantextrapolationof the availabledata is requiredto
calculatethe highestT points shownon Figure 4, and the
calculations above 1100 K are much less reliable. Given a T and
bulk liquid Na2S/S,the Cleaverand Simesmodelwas usedto
calculate
X•. WhenthisX•, basedon a givenp(S2),is combined
with the equilibriumconstantfor (3), it is being somewhat
indirectlyassumed
thatJtS2)=p(S2.This assumption
introduces
someerror for the higher (10-100 atm) p(S2). Theseerrors
cannotbe assessed,
but it is unlikely that the qualitative
conclusions of the calculations are affected. '
For a constant
JtS2), Figure4 showsthat the equilibrium
J•SO•) decreases
with decreasing
T until the condensation
T for
liquidS is reachedandtheregion117
andlIB curvesintersect.In
the higher T portionsof both regionslIB and Ill the
requiredto stabilizeNa•SO•relativeto Na2S•arerelativelyhigh,
• -2.5 -
.9.
--3
I
-3.5
-1.5
-1
I
-0.5
0
I
i
0.5
1
1.5
!og(fS2),eq
Figure8. A high•
isothermal
•abmtyfielddiagram
for
Na•SO•andNa2S•(1).
For•
ragions
whereventing
hasdepleted
SO• relativeto S, Na•SO•canbe conver• to Na•(1). If SO•can
Na•SO4
will bethestable
Na-I•
pha•.
BURNETr: Na2SO4AND Na-SULFIDE COMPE•ON
IN UPPER CRUST OF IO
21,269
represents
a rathercommonuppercrustaltemperature.In many
cases,giventhe rapidtimescales
duringventing,the priorNa
To put the resultsin Figures2-5 in perspective,
it is phaseswill be quenched
in, sothat,e.g.,if Na•SO4werepresent
importantto notethat availabledataindicatean outermost
crust or produced
(regionH) priorto venting,it wouldbe preserved,
of Io (at least 0.1-1 kin) composedprimarily of SO2 and independent
of the detailsof the (PT) trajectories
andof the
elementalS with the Na-S phasesand silicatematerialsbeing effectsof SO2-Sfractionation
which is very likely to occur
relativelyminorconstituents
[Smithet al., 1979;Kieffer,1982]. duringeruption.
Further,S and/orSO: mustbe buriedby recyclingto sufficiently
However,situationswhen SO2 is preferentiallydegasseA
high(PT) conditions
to becomethepressurized
volatilesdriving relativeto S are also plausible;thesefavorNa2S=relativeto
theextensivevolcanicactivity. LiquidSO: hasa broadstability Na2SO•. If a region is heatedby an approaching
silicate
field, and it is inevitablethat shallowcrustallayerscontain magninticintrusionbut initially remainsunsealedrelativeto
liquidSO:aquifers[Kieffer,1982]andthatSO: is a majorphase venting,SO: will preferentially
be degasseA
first. Then,if the
to depthsdetenuinedprimarily by the strengthof overlying regionis sealed,e.g., by beingcappedby a surfacelava flow,
crustal layers. These depths and the corresponding
(PT) temperaturescan rise, and elementalS contained. To use a
conditions
will be subjectto local geologyand are likely to be specificexamplebasedon Figure5, if the regionis heatedto
highlyvariable. For purposes
of illustration
consider
a rangeof 1100 K after previous SO: degassingto produce
localIothermsrangingfrom 70 to 1000 K/kin [Kieffer,1982; •SCh)/•Sx)xe0.001,then the regionwill lie within the Na2Sx
Matsonet al., 1993] and a hydrostatic
pressuregradientof 50 stabilityfield. This specificexampleis arbitrary,but the
areOkra. For Iothermsless than 200 K/kin the hydrostatic conclusion
is not highlysensitiveto the chosen
parameters,
and
pressure
is sufficientto containSCh(1)downto the triple point eventsof thistypeseemplausible,e.g.,convection
in a liquidS
(431 K). Even at 1000 K/kin, the liquid SO2vaporpressure lava lake would provideeffectiveventingof SO:, leadingto
doesnot exceedthe IotherlUuntil temperatures
greaterthan325 Na:S• formation.
K. Thus 325 K is a reasonable estimate for the minimum
The plume sourceregionsare the Io environments
most
temperature
that SO2will reachduringrecycling.McEwanand closelycorresponding
to regionIiI, for whichbothmajorupper
Soderblom[1983] distinguishlarge ("plumes")and small crustalconstituents
are in the vaporphase.A long-lived
plume
("fumaroles")
activevolcaniccenterson Io andpropose
thatthe will developa localrecyclingsystemof crustalvolatiles,which
fumarolesare SO2-•ven whereasthe plumescorrespond
to serves
to transport
theheatfroma near-surface
silicatemagmatic
elementalS volcanoesøThe fizmarolesprobablyrepresent intrusion.Giventhe largedifference
in volatilitybetweenSO:
regionsof lowerthanaverageheatflow locatedbetweenmajor and elementalS, the evolutionof this recyclingsystemwill
•
silicatemagmaticintrusions.The fumarolicenvironment relativelyquicklyproduce
a pureelemental
S plume.Thishighis thusa SO2-rich
uppercrust,corresponding
to regions
I andlEA temperature,continuallyventing environmentwill produce
(Figure 1), and in this regionthe equilibriumNa phaseis Na:S,. A complication
with thisanalysis
is that,exceptpossibly
Na2SO4for eitherNa•O or Na sulfidesas inputphasesfromthe for transientexplosiveevents,any Io crustalregionwhich
surface. The kineticsof reactions(1) and (2) in fizmarolic containselementalS vapor,e.g., plumethroatsand source
regionsmightbe slo•, howeverit is likelythat today's
crustal regions,musthavea solidsilicate"container.'Thusthe region
recycling
hasbeenessentially
thesameforat least10• years, IiI calculations
whichneglectany silicateinteractions
maybe
makingthermochemical
equilibriumin regionI on Io plausible oversimplifications.
It may be betterto -thinkof the plume
evento temperaturesas low as 300 K.
sourceregionsas a majorsite of primaryNa extraction
from
Forregionswith Iothermslessthan200 K/kin or, in general, silicate. ff the gas phasein the plumesourceregionsis
forregionsof highcrustalstrength,
it will be possible
to contain predominantly
elementalS, thenthe source
regionsmaycontain
SChvaporin portionsof the Io crustcorresponding
to regionH relatively large pools, mini magma chambers,of Na:S:l
(Figure1). In thesesituations
the SO2fluid pressure
would Recyclingof S in plumesis the steadystate situation,but
greatlyexc_•d_the minimumrequiredto be in the Na2SO, occasio•ly anintensepulseof silicatemagnmtism
will produce
stabilityfield according
to Figures2-4. Regionsof highcrustal surfacesilicatelava flowswhichprovidesan oppo••
for
strengthwouldmostprobablyoccurasa consequence
of surface transferof Na to the uppercn• by Na volatilization
or by
silicatelava flowsor shallowintrusions
[Johnson
et al., 1988] interactionwith S or SO:.
whichburysurfaceSChandS deposits.Figure4 showsthatif
GiventhatNa:SO• is a verystablecompound,
my origirad
SOzpressures
of 10-100atmospheres
canbe contained,
Na2SO, expectation
was that crustalrecyclingwould produceonly
will be the equilibriumphaseall the way to silicatemagmatic Na:SO4, yielding a clean, model-independent
conclusion.
teluperatures.
Thusa largevarietyof crustalenvironments
favor Althoughin major regionsof the Io crust,thermochemistry
the conversionof Na2S=to Na2SO,.
favorsNa2SOsformation,other regionswhich have been
However, silicate magmaticintrusionsalso create high- degassed
of SO• relativeto S will be locations
whereNa sulfides
temperature
regions,whichasFigures3-5 indicate,canfavorthe will be formed.Thus,evenif Na sulfides
arenottheprimary
conversion
of Na2SO,to Na2S=,depending
on whetherSChis phasein whichNa is initiallybroughtto theIo surface
(e.g.,if
contained
or vented. The consequences
of suddenheatingof mechanisms
1 and4, discussed
in the introduction,
dominate),
SOz-Smixturesare model-dependent,
but in general,if the Na sulfidesmay be importantphaseson the present-day
Io
heatingis fast and accompanied
by eruptiveventingof bothS surface.Giventhe realisticviewthata widevarietyof •
andSO•, it is possiblethat the fugacities
of bothspecies
would regimeswill be presentand that the thermochemical
reaction
remainin theNa2SO4stabilityfield. For example,as illustrated products
fromall theseregimes
will betransported
tothesurface
inFigure
$ for1100K, roughly./(S2)/./(SO2)
> 102-10
• isrequiredduringeruptions,I concludethat the Io Na atmosphere
has
to convertpreexisting
Na2SO,to Na2S•.Althougharbitrary,the contributionsfrom both Na•SOs and Na-sulfides. This is not as
1100K temperature
illustratedin Figure5 is plausiblefor large tight a constraintas originallyhoped,but a two-component
volumessurrounding
silicateintrusions
andflowsandprobably mixture is still a relativelysimpleresult. Althoughthe
Discussion
21,270
BURNET:
Na2SO4AND Na-SULFIDE COMPE•ON
IN UPPER CRUST OF IO
M.L, andD.S.Bum• Igneous
originfortheNa in •e cloudof Io,
discussion
of lqa•) destruction
waspresented,
for simplicity,in •
Geophys.
Res.LetL 17, 981-984,1990.
terms of Na2SO4production,in regionsof low ./•SO•)/J(S2),
Jolmscag
M.L, andD.S. Burm• •
intetactk•on Io: Reaction
under
lqa2Sx
will be formedinstead. If thermochemistry
is dominant,
highly
o 'xglizing
coMifions,
J. Geo•s. R•,s.,98, 1223-1230,
1993.
only transientconcentrations
of Na20 are possibleon the Io Jolmson
T.V.,andD.L Matson,
1o%
tenuous
am•phere,inOriginandEvolution
surface,even if Na volatilizationis the primary meansof
ofPlanetary
andSatellite
Atmospheres,
edited
byS.Alteya,
J.Pollack,
and
M. Mallhews,
pp.666-681,Univ.ofAriz.Press,
Tuscon,
1989.
extractingNa fromsilicatemagmas.
The proportionsof Na2SO4and sulfidesdependson Io Jdmson,T.V., G.J. Veeder,D.L Matson,ICI4_Btow• ION. Nelson,andD.
Monison,Io: Evidenee
for 'salicate
volcanism
in 1986,,%n•nce•
242, 1280geology,but in turn,observational
knowledge
of theproportions
1283, 1988.
canbe a geologicalconstraint.Suchobservations
are possible; Kargd,J.S.,Cnmml
smuaure
andigneous
processes
ina •
Io(abs•a•
Lunar P/aneLSc•:,•
751-752, 1993.
for example,it is likely that fast Na atomsin the Io torusare
produced by dissociationof accelera• molecular ions
plummonIo, in Satellites
of'Jupiter,
edited
byD. Monisogpp.647-723,
[Schneideret aL, 1991]. The nature of these Na-bearing
Univ. of Aria Press,Tucson,1982.
moleculesis unknownat present,but the proportion
of NaO to •
E., M. Belto• L &Pater,S. Gull&, andT. •
Io%am•phere
NaS moleculesis a measureof the sulfate/sulfide
surfacemixing
frommicrowave
detection
of SO• Nature,346, 639-641,1990.
Physics
and•
of sulfurlakeson Io,
ratio. For example,if vented,hightemperature
(e.g., lava lake) Lunine,J.L,andD.J.Stevemon,
environmentsare dominant, NaS moleculeswill be dominant
Na-bearingspeciesin the Io atmosphere.
A resultof this studyis that,contraryto generale•tations,
the Na mineralogy
of the Io surfacemayhavelostall recordof
the primarymechanisms
(1-4, etc.) by whichNa was extracted
fromsilicatestartingmaterials.Nevertheless,
thesemechanisms
are importantscienceissuesbeca• they are an important
cons•t on Io geologicprocesses
and history,as well as a
means,in principle,of identifyingthe typesof silicatemagmas
involved.Evenif theprimaryNa mineralogy
hasbeendestroyed
by crustal recycling,the overall chemistryof the primary
processeshas not. Thus data, even good upper limits, on
"minor"elements(K, Ca, Mg, Fe, etc.)in theIo atomiccloudor
toruswouldbe of greatimportancein advancing
knowledgeof
boththe geologyand chemistryof Io [cf. Johnsonand Burnett,
1991.
Icarus,64, 345-367, 1985.
MatsonD.L, G.J. Veeder,T.V. Jolmscag
D.L Blaney,and J.D. Gogtung
A
decade%
overview
of lo%vdmic activity
(abstract),
Lunar P/anet
XI7E, 939-940,1993.
McEwen,A.8., andLA. 8odegblm•Two classes
of volcanic
plmmson Io,
Icarus,55, 191-217, 1983.
Mo•, J.L,a•l D.B. Na•h,•
l•ar•'om•atio•a•d Jh••
re•l•an• of
304, 1991.
Na•h,D.R, a•l F.P.Fanale,
1o%
•
•
•
on•
19T7.
Na•h,D.B.ariaO.•, V•
•
work• nu• •
f•
1992.
•tium monc•ulf• •Jium •
sulfur,½•
•.,
2, 221-225,
1972.
Acknowledgmen• I gratefullyacknowledge
•t
thennodynamicdiscussions
with JohnBeckett,and manuscript
reviewsby Fnmer
1991.
Fanaleand DouglasNash. This researchwas su•
by Planetary 8ch•4•, N.M., J.T.Tra•ge•,J.I• Wilso• D.L Brown,R.W.Eva• aratD.E.
Materialsand Geochemistry
grant NAGW 3534. Caltechcontribution
8h•,
Mole•ula•o•ginof lo%fa• sodium,
•½ac• 253, 1394-1397,
5523.
1991.
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