American Mineralogist,
Volume 63, pages 516-519, 1978
Experimentalextensionof thesphaleritegeobarometer
to 10kbar
JoHNLusr
Schoolof Earth Sciences,MacquarieUniuersity
North Ryde,NewSouthWales2l13, Australia
ANDC. E. FoNo
GrantInstitute of Geology,Uniuersityof Edinburgh
WestMqins Road,Edinburgh,Scotland
Abstract
Sphalerite
coexistingwith pyrrhotiteand pyrite has a constantFeScontentof 10.3I0.5
mole percentat a confiningpressureof - l0 kbar and temperatures
between420" and700'C.
Experimentsthereforeshow that the sphaleritegeobarometeris applicableup to at least 10
kbar.
Introduction
Bartonand Toulmin(1966)and Scottand Barnes
(1971)pointedto the importanceof the FeScontent
of sphaleritecoexistingwith pyrite and hexagonal
pyrrhotiteas a potentialgeobarometer.
Scott(1973)
subsequentlycalibratedthe pressure-dependence
of
sphalerite
solviiat pressures'of
2.5,5, and 7.5-8kbar
andtemperatures
between325"and710'C. For pressuresof 2.5 and 5 kbar, he demonstrateda temperature-independence
below 600'C, and determined
FeScontents
of 17.9*0.4and 14.5*0.Imolepercent,
respectively.
Howeverhe interpretedhis 7.5-8 kbar
results as indicating both a temperature-and pressure-dependence.
Lusk el al. (1975)suggested
that Scott's(1973)
inferred7.5 kbar solvusmay be in error, primarily
due to his relianceon a suspectresult(i.e. Run 12,
Table2,p. a68).Scott(1976)hassinceconceded
that
this resultis unreliable,and now prefersa 7.5 kbar
isobarwhichis temperature-independent,
at leastbetween500oand 700'C.
Experimentsand analyses
5 M NH.CI were used as fluxesin the majority of
(Scott,1973).However
recrystallization
experiments
for runs I and2, and for run 9, eutecticmeltsof KCILiCl and NHrCI-LiCl mixtureswereusedas fluxes,
respectively,
accordingto the method of Boorman
(1967).
pressure
vesRunsweremadein internally-heated
sels with chargetemperaturescontrolled to better
than t3oC, althoughtemperaturegradientsalong
the lengthsof the capsulesare estimatedto have
produceda mean total uncertaintyof 16oC. Pressuresweremeasured
daily usinga calibratedmanganin pressurecell.Leakageof the argonpressure
medium was a constant problem of varying severity
from run to run, and was aggravatedby the long
durations(14to 38 days)of the runs.Howeverprecisionswerebetterthan i0.25 kbar for most runs.
weredeterminedusingan
Sphaleritecompositions
Ernc microprobeoperatedat an accelerating
voltage
of 15 kV and a specimen
currentof 0.5 microamps.
SyntheticFe-freesphaleriteand pyrrhotitewereused
asprimary standardsfor Zn and for Fe and S, respectively. A sphaleritestandardof similarcomposition
(12.1+0.1molepercentFeS)to the experimental-run
sphaleriteswas analyzed several times with each
group of analyses
to checkfor machinedrift.
At leastfifteen 30-secondspot analyseswereperformedon eachsample,and not morethan3 analyses
waspossible
madeon anyonegrain.A singleanalysis
products
only on coarsergrainsof very fine-grained
Sulfidenutrientsusedin the presentexperiments
comprisedchemically-precipitated
iron-freesphalerite (andgalenain runs 5 and ll, Table l), together
with pyrrhotite and pyrite synthesizedfrom moderately pure Fe and S. Theseproducts were finely
ground and loadedinto heavy-walled
gold capsules
20 mm in length.Aqueoussolutionsof 4.5M KCI or
0003-N4x/78/0506-0516$02.00
516
LUSK AND FORD: SPHALERITE GEOBAROMETER
Table 1. SphaleriteFeScontentsand conditionsof l0 kbar exDeriments
Run No.
duratlon
Pressure. kb
mole
( d a v s)
1*
2*
4
-2
f
Analyses
s.9: 0.2
+
9.9 : 0.2
15
27
s . 6! 0 . 2 5
1 o . oI 0 . 2
2L
r o . 4 I o .2
L4
r o . ot o . t 5
ll
1 o . ot o . l s
2L
703
9.9 f 0.3
22
652
e . eJ o . 1 5
g,g: o.z
653
+
9.7 : 0.10
ro.z ! 0.2
t7
L7
24
o
Number of
sphalerlte
in
l9
% FeS
lo
7
Iq
ouz
9.9 I 0.15
+
10.5 : 0.5
8
3Z
552
s.s! 0,2
10.5t 0.5
28
9*
32
552
s . sT o . z
LO.2+ 0.4
16
s00
s . s! 0 , 2
LO.2! 0.4
20
420
9.7 : 0.6
ro.s j r.ozr3
18
10
34
nz
lestimated
2"..p1""
J6
mean er?or
contained
galena
of lower temperature
runs(e.g.l l). A smallproportion of analyses
werefound to be appreciablymore
Fe-richthan commonvaluesfor thesesamples,and
the former wererejectedon the groundsthat pyrrhotite contaminationwas probablyresponsible.
None
of the lower Fe-containinganalyseswere rejected.
Resuttsand discussion
Sphaleritecompositionstogetherwith detailsof
respectiveexperimentalrun conditionsare presented
in Table I and are plottedin Figurel. Theseresults
show that the mean mole percentFeS contentsof
sphaleritesfrom all runs agreewithin the precision
limits(+ lo) of the analyses.
However,a slightlowering of FeS content with increasingtemperatureis
implied for the two highesttemperatureruns (i.e.
runs I and2 [776'C] and run 3 1725'Cl),suggesting
that the solvuscompositionis constantfrom at least
420"to 700"C.Theweightedmeansphaleritecompositionoverthistemperature
intervalis 10.3*0.5mole
percentFeS,corresponding
to a meanpressure
of 9.9
kbar.
Runs5 and I I wereunusualin that theycontained
galenain addition to sphalerite,pyrrhotite, and pyrite. The resultsshow, as expected,that galenahas
3
.......
rerJ-aDlrlcy
:*eutectic
sal!
uncerEarn
mixture
used as flux
not noticeablyaffectedZn-Fe-S equilibria. Nevertheless,
the reliabilityof Run I I is regardedasuncertain because
of development
of seriousvessel
leakage
whenthe pressurefell as low as7 kbar duringthe first
l0 daysof a 38-dayrun. Thiswascorrected,
and minimal leakageoccurredduring the 28 daysfollowing.
No reversalruns were attemptedin this study,
because
all of the experiments
wererun for durations
similar to Scott's(1973)5 kbar experiments,
which
demonstrated
the attainmentof equilibriumdown to
400"C. The presentresultsare thereforeinferred to
be equilibriumresults,or closeapproximations.
The pressure-dependence
of the temperature-independentportionsofsphaleritesolviicorresponding
to
pressuresof 0-1, - 2.5, 5, 7.5-8, and l0 kbar is
shown in Figures I and 2. Theseexperimentaldata
includethoseof Boorman(1967),Scottand Barnes
(1971),and Scott(1973)for pressures
of0, 0.25-1.0,
2.5-8.0kbar, respectively,
in additionto the present
- l0 kbar plots.Someof thesedatahavebeenoniitin computinga least-squares
ted from consideration
regressionequation which takes mole percentFeS
(+ lo) errors'into account.Only sphalerite
composi1 Pressure errors wer'e not considered because of their small
contribution to the total uncertainty in the regressionanalysis.
LUSK AND FORD: SPHALERITE GEOBAROMETER
usefulequation for readily computing pressureemploys P(kbar) as the dependentvariable.The equation
P(kbar):26.18 - 1.903(molepercentFeS)
+ 0.0309(mole percentFeS)z
i/|OLE% ftS IN SPHALERITE
(2)
allows this, but it is lessreliable becauseerrors in
werenot considered.
molepercentFeSmeasurement
The geobarometriccurveplotted in Figure2 is for
equation(l), which has an uncertainty(lo) of mole
percentFeS predictionof < +0.18. Pressures
estimatedfrom equation(2) deviateby < 0.06kbar from
pressures
predictedfrom equation(l). Extrapolation
of the calibration curve to 10.0 kbar suggestsa
sphaleritecomposition of
temperature-independent
l0.ll+0.18 mole percentFeS.It is noteworthythat
the 2.5 kbar interceptequateswith a predictedmole
percentFeS value of 17.3110.18,which is appreciably lower than Scott's(1973)estimateof 17.92+.
0.37. This correspondsto a pressuredifferenceof
0.61+0.41kbar. A pressuredifferenceof approximately0.3 kbar is alsoindicatedwhenScott's(1976)
tentativeextrapolationto l0 kbar is comparedwith
the 10.0kbar interceptfor the calibrationcurvepresentedhere.
A seriousdiscrepancyexists betweenexperiment
and theoreticalanalysesfor the sphaleritegeobaof attainment
rometer.Scott's(1973)demonstration
Fig. l Mole percentFeSin sphalerites
coexistingwith pyriteand
hexagonalpyrrhotiteas a function of temperatureand confining
pressure.Samplenumberscorrespondwith thosein the respective
sourcesreferredto.
tions from experimentsrun at temperaturesbetween
300'-500"Cat 0 kbar,390"-500"Cat2.5kbar,325"600"C at 5 kbar, 4l l"-605oc at 7.5 to 8 kbar. and
420"-703"Cat -10 kbar wereaccepted,becausethey
fall well within the temperature-independent
portions
of respectivesphaleritesolvii. A small number of
other data pointswereexcludedalsobecause
of too
(e.9.l7R [1 grain];28R [2 grains]),apfew analyses
parent uncertainty(e.g. ll), or acknowledged
unreliability (e.g. 12).
Severaltypes of curve were fitted to the accepted
data,but varianceanalysisshoweda quadraticmodel
to yield an excellentfit. The equation of the computedcurveis:
mole percentFeS : 20.53- 1.313P(kbar) + 0.0271
(I)
P(kbar)'
which yieldsa correlationcoefficientof 0.997.A more
F
z
2
7
22
21
20
15
H
13
t2
|
t8
t7
6
MOtr % ftS IN SHALERIB
id.Fbt
d
dui
ldn hFEruDtsd3
CEkiE
t!'
n
I
of mole percentFeS contentsof
Fig. 2, Pressuredependence
portions of sphalerite
sphaleritefrom temperature-independent
solvii.
LUSK AND FORD, SPHALERITE GEOBAROMETER
of equilibriumfor temperature-independent
portions
of sphaleritesolviito 5 kbar, togetherwith the present extensionof temperature-independent
relations
to l0 kbar, stronglyimply that the experimental
data
arereliable.On the otherhand,theoreticaltreatments
by Scott and Barnes(1971)and Scott (1973)predictedthat sphaleritesolvii are both pressureand
temperature-dependent
over the sameP-7 range.It
is thereforeapparentthat the discrepancy
stemsfrom
usageof unreliablevaluesfor at leastsomeof the
parametersin the relevant thermochemicalcalculations(seeScott, 1973,p. 471).
Acknowledgments
We thank C. Begg of the Grant Institute of Geology, University
of Edinburgh, and G. D. Pooley of the School of Earth Sciences,
Macquarie University, for their willing help in surmounting experimental and analytical problems. Drs. H. M. Hudson and E. A.
Eyland of the School of Economic and Financial Studies, Macquarie University, are thanked for their assistancein treating the
experimental data. Drs. R. G. Arnold of the Saskatchewan Res e a r c h C o u n c i l , S a s k a t o o n , a n d S . R . R . S a n g a m e s h w a ro f t h e
519
N.S.W. Institute of Technology,Sydney,kindly suppliedsulfide
sampleswhich wereusedas probe standards.
Finally, J. L. extendssincerethanksto the Crant Instituteof
Geologyand N.E.R.C. for providing experimentalfacilities,and
MacquarieUniversity for a study leave grant which made the
presentwork possible.
References
Barton,P. B. and P. Toulmin, III (1966)Phaserelationsinvolving
sphaleritein the Fe-Zn-S system.Econ.Geol.,6l,815-849.
studiesin the ZnS-FeS-FeSz
Boorman,R. S. (1967)Subsolidus
systemEcon.Geol.,62, 614-631.
Lusk,J., F. A. Campbelland H. R. Krouse(1975)Applicationof
to
sphaleritegeobarometryand sulfur isotopegeothermometry
oresof the QuemontMine, Noranda,Quebec.Econ.Geol.,70,
1070-r083.
Scott, S. D. (1973) Experimentalcalibration of the sphalerite
geobarometer.
Econ.Geol.,68, 466-474.
(1976)Application of the sphaleritegeobafometerto regionally metamorphosed
terrains.Am. Mineral.,61, 661-6'10.
geothermometry
and
and H. L. Barnes(1971)Sphalerite
geobarometryEcon.Geol.,66, 653-669.
Manuscriptreceiued,August9, 1977;accepted
for publication,December12, 1977.