1. Experiments were conducted using a multi-anvil press to constrain the melting curve of calcite (CaCO3) at 6GPa by performing falling-sphere experiments. 2. In one experiment at 6GPa and 1830°C, the calcite fully melted as evidenced by the platinum sphere sinking to the bottom. In another at 6GPa and 1780°C, no melting occurred. 3. The results provide some support for a previous study's melting curve over another study, but more data is still needed to conclusively define the melting point of calcite at 6GPa.

1. 1
Experiments on the Melting Curve of CaCO3 at 6GPa
By Matt Karl
Submitted as a Senior Project
Department of Earth and Environmental Sciences
University of Michigan
December 2013
Abstract:
The meltingcurve of calcite (CaCO3) ispoorlyconstrainedat highpressures.Previousexperiments
conductedat the Universityof Michiganandelsewhere have collecteddatathatsuggestdifferent
meltingcurvesforCaCO3 at highpressure.Inorderto betterconstrainthe meltingcurve of CaCO3 at
6GPa several sinkingsphereexperimentswere conductedusingthe 6-8multi-anvil apparatusatthe
Universityof Michigan.The resultsof these experimentswere analyzed inthe Mineral Physics
laboratory usingoptical microscopyinadditiontoRamanspectroscopy.These experimentsoffersome
supportto the resistance methodbaseddatacollectedbyZeyuLi,whilecontradictingthe resultsof the
studyby Suitoetal (2001). However,more dataisrequiredtosupportthese resultsandconclusively
constrainwhere the meltingpointof CaCO3 isat 6GPa. Inorder to achieve this,several revisionsneedto
be made to the methodologyof these experimentstofacilitatereproducibility.
Introduction:
Carbonatessuchas calcite (CaCO3) have extraordinarypropertiesasmelts. CaCO3 accessesthe
interiorof the Earth throughthe subductionof oceaniclithosphere. The resultof the subductionof
carbonatesis to induce meltingandmobilize structurallyboundmineral water(H2O).The additionof
H2O to the lowermantle actsto lowerthe mantle solidusormeltingcurve. The resultantmeltingand
alterationthatoccurs isknownas metasomatism. The resultof metasomatizingmantle istoenrichthe
mantle material inrare Earth elements.Thiscanbe usedto helpexplainthe observedoccurrencesof
carbonatite magmasand the compositionof alkaline magmas,inadditiontoexplainingseismiclow
velocityzoneswithinthe mantle. Furthermore,understandingthe meltingcurve of CaCO3 canhelp
facilitate understandingof the depthsatwhichcarbonate meltscanexistinthe mantle,whichcould
helpaidthe understandingof the deepcarboncycle andthe billionyear-scaleevolutionof terrestrial
carbon.
The pressure andtemperature goalsof thisexperimentwere chosentoaid inthe clarificationof a
portionof the meltingcurve of CaCO3 thathas notbeenrigorouslydefined throughexperiment. The
pressure thatthe experimentswere conductedatwas6GPa or ~95Bars. Several fallingsphere
experimentswerethenconducted toconstrainthe melting pointof calcite atthispressure. Previous
experimentsconductedbyIrvingandWyllie focusedonmuchlowerpressure goalsforthe meltingcurve
of CaCO3,anddid notexceedpressuresof approximately3GPa,as shownin figure 1.A previousstudyby

2. 2
Suitoetal. carriedout experimentsthroughagreater pressure range,andplaceda meltingpoint for
CaCO3 at approximately1710°C at 6GPa, see figure 1. However,recentresearchconductedbyZeyuLi,a
graduate studentinthe Earth and Environmental SciencesDepartmentatthe University of Michigan,
suggeststhatthe meltingcurve mayactuallylie athighertemperaturesatthispressure,seefigure 1.
Background:
Thisexperimentwasachievedusinga6-8 multi-anvil apparatus.Thisassemblyutilizes anestof 8
tungstencarbide anvilswhichfitinto6machine tooledsteelwedges.The sampleandsample assembly
are containedwithinaceramicoctahedron,whichisinturnplacedwithinthe nestof tungstencarbide
anvils. The desiredpressure isthenachievedbythe pneumaticpressactingonthe 6 wedges,whichact
on the 8 anvils,whichcompressthe octahedron.The amountof pressure thata multi-anvil apparatus
can generate isdependentuponthe truncationedge length(TEL) of the carbidesbeingused, the quality
of the carbide used,andthe type of gasketsystemusedtoprotect the carbide.
The TEL of the carbide usedinthisexperimentwas12 mm, and waspairedwitha 12 mm ceramic
octahedron.The gasketsusedinthisexperimentwere the cast-onfinsonthe octahedron,whichwere
furtherreinforcedwithW10tape. To furtherprotectthe carbides,2x2” G10 squareswere gluedtothe
assembledcarbide nest,andwere sprayedwithTeflonspraytodecrease the frictionbetweenthe anvils
and the wedges.
The desiredtemperature isachievedthroughutilizingaresistance furnace.Anexternal voltage (V)is
appliedtothe furnace throughthe carbide anvilsandwedges.The power(P) appliedtothe furnace can
be calculatedbythe equation:
𝑃 = I*V
Where Iis the currentthrough the furnace,andV isthe external voltage.The temperature of the
furnace ismeasuredbyusinga thermocouple.A thermocoupleoperatesbyconnectingtwowiresmade
of differentalloys,whichfollowaconsistentrelationshipbetweentemperatureandvoltage.Whena
temperature difference isgeneratedacrossthe wires,ameasurable voltage isthenproduced.
The thermocouplesusedinthisexperimentwere .6mm, Type C,W26Re. Thistype of thermocouple
was chosenbecause itisratedto temperaturesashighas~2300° C. The thermocoupleswere installedin
a radial off-centerconfiguration,asshowninfigure 2.
Methods:Assembly:
The assemblyusedforthisexperiment remainsindevelopment,andunderwentanew iterationfrom
the firstexperimenttothe followingtwoexperiments,see figure 2.Althoughthe assembliesdisplayed
infigure 2 are ratedfor experimentsattemptingtoachieve lessthan1500°C, it wasdeterminedtobe
withinthe tolerancesof the partsinvolvedtogoa few hundreddegreesCoverthatrating. The reason
that the assemblyunderwentrevisionbetweenthe firstandsucceedingexperimentswasbecausethe
firstassemblydidnotplace the thermocouple inasymmetricpositionwiththe sample bottom.As
showninfigure 2, the thermocouple isactuallyclosertoone endof the assemblythanthe other.The

3. 3
reasonthat thisisundesirable isbecausethe thermocouple canactuallyreadatemperature thatis
lowerthanactuallyexperiencedbythe sample. The seconditerationof the assembly,showninfigure 2,
has revisedthe dimensionsof the componentsinordertoallow forsymmetricplacementof the
thermocouple withthe sample bottom.
For the purposes of thispaper,onlythe methodologyof the revisedassemblywill be discussedin
detail,butthe processremainedbasicallyunchangedfromthe firsttoseconditerations,withonlythe
dimensionsof the componentsinvolvedbeingadjusted.The assemblyused aRhenium(Re) heater,of
height18.3 mmand width9.74 mm that wasrolledintoatube of diameter3.10 mm. The heightof 18.3
mm allowedthe Re heatertohouse the entire 15.3 mmsample assemblyandallowedfor1.5 mm excess
on eachend.Thisexcesswas cut intotabs that wouldlaterbe foldedontothe octahedrontoholdthe
heaterandsample assemblyinplace.
The octahedrausedinthisexperimentwere 12mm ceramicswith3.18 mm gasketsthatwere cast-on
fins.The sample assemblybore holewas made inthe centerof one of the octahedronfaceswitha1/8”
(3.17 mm) drill bitanda mill tool.The thermocouple bore hole wasmade withthe use of avise anda
drill blank.The thermocouple bore hole wasmade witha1/32” (.79 mm) drill bitand a drill press.The
placementof the thermocouplebore hole was10.3 mmfrom the corner of one of the octahedron’s
faces,andwas drilledoff-centeronthe fin.A needle wasusedtocheckthe placementandalignmentof
the thermocouple bore hole.Ideally,the thermocouple bore holewouldpassthroughthe centerof the
sample assemblybore holeandalignsoneitherside,6.6mm fromone end,and11.7 mm fromthe
otherendof the sample assemblybore hole.
Once the appropriate bore holeswere made inthe octahedral,the Re heaterwasrolledintoatube
usinga 3.10 mm gauge and thenpositionedin the assemblybore hole of the octahedron. A seriesof
precisionhypodermicneedleswereusedtopuncture the Re heaterbypassingthe needlesthroughthe
thermocouple bore hole,until the holeinthe Re heaterwasapproximately.8mm.Analternative
methodinvolvedusingablankmade of G10 and a centerpunchto make the hole inthe Re heater,and
thenliningthe holesinthe Re heaterupwiththe thermocouple bore holes.However,thismethodwas
difficulttoachieve because itrequiredveryhighprecisionof the locationof the holesinboththe heater
and the octahedron.Also,evenwiththe use of anilluminatedmicroscope,visibilityislimitedwithinthe
sample assemblybore hole,whichaddstothe difficultyof aligningthe holesinthe heaterand
octahedron.
Afterthe Re heaterwasemplacedandpunchedforthe thermocouple,the thermocouple itself was
passedthroughthe thermocouple bore holeandaligned sothatthe junctionof wireswaslocatedinthe
centerof the sample assemblybore hole. 0.6mmsingle bore mullitetubingwasplacedoneitherwireof
the thermocouple andpushedthrougheitherthermocouple bore hole sothattheymetsnugglywiththe
junctionof the thermocouple,see figure4.
Once the thermocouple wasinplace withinthe sampleassemblybore hole,the Al2O3 spacerof
length5.7 mm and diameter3.1mm was markedwitha groove byuse of a diamondfile,sothatthe
thermocouple junctionandmullite tubingcasingwouldfitsnugglywithinthe groove,see figure 4.

4. 4
Ideally,the bottomof the Al2O3 spacerwouldsitflushwiththe edge of the sampleassemblybore hole.
Both the Al2O3 spacerof 5.7 mm and 5.2 mmlengthwere cutfrom a machinedlengthwiththe use of a
mill tool.Next,ahardAl2O3 casingof length3.75 mm anddiameter3.1 mm wasinsertedthroughthe
top of the sample assemblybore hole andpusheddownuntil itsaton topof the thermocouple junction
and mullite tubingwiththe use of agauge.The hard Al2O3 casingwascut downfroma machinedlength
to the appropriate lengthwiththe use of aprecisionsaw.
The graphite capsule of length3.75 mm, diameter2.44 mm, innerdiameter1.58mm and bore depth
3.25 mm was insertedintothe hardAl2O3 casing.A graphite lidtothe graphite capsule of length.5mm
and diameter2.44 mm wasplacedon topof the graphite capsule once loadedwiththe experiment.
Both the graphite capsule andthe graphite lidwere machinedwiththe use of amill tool,inadditiontoa
1.5 mm drill bit.Once the graphite capsule wasloadedand the graphite lidinplace,the final 5.2mm
Al2O3 spacerwasloaded.Ideally,the topof thisAl2O3 spacerwouldsitflushwiththe topof the sample
assemblybore hole. Once the entireassemblywasloaded,the finsof the Re heatercouldbe folded
downeitherontothe face of the octahedronorover the endsof the Al2O3 spacers,sothat both the
heaterandassemblywasheldinplace.
FallingSphere:
The meltingtemperature of CaCO3 at6GPa was determinedby performing“falling-sphere”or“sink
float”experiments.A “falling-sphere”experimentisperformedbyplacingaPlatinum(Pt) sphere (1-200
micronsindiameter) withinasample capsule filledwith CaCO3 (99.997% metalsbasis). Since the melting
pointof Pt is muchhigherthanthe meltingpointof CaCO3 at6Gpa (~1980°C, see figure 5),the Pt sphere
remainssolidattemperaturesthat CaCO3 meltsat.Inaddition,Ptismuch denserthan anyof the phase
of CaCO3.Therefore,the initial andfinal positionsof the Ptsphere canbe usedas evidence forwhether
or not CaCO3 has melted,andtowhat degree the CaCO3 hasmelted. Forinstance,if the Ptsphere
remainsunmovedfromitsinitialpositionatthe endof the experiment,the CaCO3 didnotachieve
melting.If the Ptsphere sankdownwardsintothe sample capsuleof calcite asthe resultof gravityand
itsgreaterdensity,thenthe CaCO3 achievedatleastpartial melting.If the Ptsphere sanktothe bottom
of the sample capsule,thentotal meltingof the CaCO3 wasachieved.
The experimentswereconductedbybringingthe samplesfromapproximately3Barsto95Bars over
18 hours.The experimentswere thenheatedfromroomtemperature tothe targettemperature ata
rate of 100°C/min. Once the target temperature wasachieved,the sample washeldatthe target
temperature forbetween5-15minutes.The experimentwasthenmanuallyquenchedbyturningoff the
powersource.Decompressionoccurredtypicallyoverthe next18hours.
Retrieval of the sample wasaccomplishedbyfirstgrindingdownone of the edgesof the octahedron
to getcloserto the sample assembly,andtoflattenone side of the octahedron.Thiswasaccomplished
witha grindingwheel and 100-180 grit grindingpads.The sample wasthenplacedinanepoxy,which
made the sample botheasiertogrindwhile addingtothe overall structural stabilityof the sample
duringthe grindingprocess.The epoxytypicallytookbetween4-5hoursto set upwhile beingheatedat

5. 5
approximately90°F.Afterthe epoxyset,the sample wasprogressivelyandcarefullygrounddownuntil
the inside of the sample capsule wasexposedrevealingthe CaCO3 andthe Ptsphere.
Results:
The resultsof these experimentswereanalyzedusingaLeciaoptical microscope anda Renishaw
inViaRamanmicroscope fromthe Mineral PhysicslaboratoryinC.C.Little. The Ramanspectrumsfor
these experimentswere collectedwiththe assistance of ZeyuLi. The resultsof experiment1,10/30/13
were thatat 6GPa and 1830°C, there wascomplete meltingof the CaCO3.This wasevidencedbythe
observationthatthe Ptsphere sankfromits initial positiontothe bottomof the sample capsule,as
showninfigure 6. Inaddition,the Ramanspectrumtakenof experiment1showedthatthe CaCO3 inthe
sample wasuniformlyof the aragonite phase,asshowninfigures8and9. The resultsof experiment2,
11/13/13 were that at 6Gpa and1780°C, there wasno meltingof the CaCO3.Thiswasevidencedbythe
observationthatthe Ptsphere didnotsinkfromits initial position,asshowninfigure 7.In addition,the
Raman spectrumtakenof experiment2showedthatthe CaCO3 in the sample wasa heterogeneousmix
of boththe calcite and aragonite phases,asshowninfigures10 and11. The resultsof experiment3,
11/27/13 were unfortunatelylostduringthe grindingprocess,sonoresultswere collectedfromthat
experiment. The resultsof the experimentsconductedconstrainthe meltingcurve of CaCO3 between
1780°C and 1830°C at 6GPa, as showninfigure 3.
Discussion:
The resultsof these experimentsseemtosupportthe resistance methodmeltingcurve forCaCO3
proposedbyZeyuLi over the proposedmeltingcurve inthe studybySuitoetal. (2001). The resultsof
experiment2seemtocontradictthe data of meltingoccurringat~1710°C at 6GPa fromSuitoetal.,see
figure 1. However,the resultsfromtheseexperimentsappeartosupportthe extrapolatedmeltingcurve
for the 09/11/13 resistance methoddata. However,more muchmore datais requiredtosupportthese
results,andconclusivelyconstrainwhere the meltingpointof CaCO3 isa 6GPa.
The resultsof the Raman spectracollectedfromexperiments1 alsoseemstocontradictthe phase
diagramproposedbySuitoetal. (2001), see figure 12. The resultsof the Ramanspectrumfrom
experiment1indicate thatthe quenchedsample wasuniformlycomposedof aragonite,showninfigure
8 and 9. However,the phase diagramproposedbySuitoetal.(2001) seemstosuggestthat the
aragonite phase isonlystable from~400-1100°C at 6GPa. Followingthe phase diagramproposedby
Suitoetal. itwouldhave beenexpectedtoobserve aRamanspectrumfordisorderedcalcite atthe
temperature achievedinexperiment1(1830°C). Inaddition,the Ramanspectraresultsof experiment2
alsodo not fitthisphase diagram.The mixingof calcite andaragonite phasesaccordingshouldoccur
onlyat eitherapproximately400 or 1100°C, notnear the 1780°C goal of experiment2.These
contradictionsseemtosuggestthateitherthe phase diagramproposedbySuitoetal.needsrevision,or
that the temperature wasnotaccuratelyrepresentedbythe thermocouple inthese experiments.

6. 6
Inadditiontothe necessityof more data, these experimentscouldbe improveduponbyfindinga
wayto facilitate the reproducibilityof the experiments.A majorobstacle inthisresearchprojectwasthe
abilitytoaccuratelyreproduce the precisionpartsrequiredinthe assemblyof thisexperiment.Margins
of errorof fractionsof a millimeterinindividual componentscompoundedthemselvesinthe overall
assembly.Itemsof particularimportance are the circumference of the Re heaterandthe accuracy of the
thermocouple bore holes.If the dimensionsof the Re heaterare not as specifiedinthe sample
assembly,itwill be impossibletoachieve the temperature goal of the experiment.Thisisbecause the
resistance of the sample will be toohighif the Re heaterdoesnotcompletelycoverthe insideof the
sample assemblybore hole. Inordertoensure the accuracy of the Re heater,aguide or blankshouldbe
cut to the precise dimensions,andoverlainoverthe Re sheetmaterial.Inaddition,aprecisionshear
shouldbe usedforall of the cuttingof the Re sheetmaterial.
The accuracy of the thermocouple bore hole alsoneedssignificant improvement.A more complicated
formof drill blankisrequiredinordertoensure thatthe thermocouple bore holepassesaccurately
throughthe octahedronon bothsidesandthroughthe centerof the sample assemblybore hole. The
currentmodel doesnotfix the errorinvolvedinslightmisalignmentof the octahedrononthe blank,
whichcan stronglyaffectthe positioningof the bore hole. Inordertoremedythis,the blankshouldnot
onlycenterthe octahedronovera hole thatthe bit isallowedtopassthrough,butshouldalsoholdand
supportthe octahedronitself.
Conclusion:
The resultsof these experimentsshow some supportforthe resistance methodmeltingcurve
proposedbyZeyuLi,while contradictingboththe meltingcurve dataandthe phase diagramproposed
by Suitoetal (2001). There is definitelypotential forfuture researchtobe conductedonthistopic,with
improvedmethodsaimedatfacilitatingthe reproducibilityof the experiments. Throughsimilar
experimentsandwithmore data,the meltingcurve of CaCO3 at highpressure couldbe more closely
constrained.
Acknowledgements:
I wouldlike to thankProf.Jie (Jackie)Li andZeyuLi fortheirinvaluable supportandexpertise in
assistingme inthisresearchproject.

7. 7
Figures
Figure 1 Compilation of data regarding the melting curve of calcite
a. b.
Figure 2 a. First iteration of the 12mm sample assembly for the calcite melting experiments
b. Second iteration of the 12mm sample assembly for the calcite melting experiments

8. 8
Figure 3 Results of falling sphere experiments conducted at 6Gpa
Figure 4 the placement of the thermocouple within the sample assembly. From experiment 1, 10/30/13
1700
1720
1740
1760
1780
1800
1820
1840
1860
1880
1900
4 6 8
T,°C
GPa
Sink
Float

9. 9
Figure 5 Comparison of low-pressure measurements of platinum melting with an extrapolation of the current data to low
pressures using the Kraut-Kennedy melting equation (bold dotted and solid lines: constrained and unconstrained,
respectively, by the P =0 melting temperature of Pt). Laser-heated diamond-cell data are shown as squares. Piston-cylinder
results shown as open circles, solid, dotted and dash-dotted lines represent other studies conducted. From (Kavner&
Jeanloz, 1998)
Figure 6 Result of falling sphere experiment 1 10/30/13, 1830°C at 6Gpa. Note that the Pt sphere has sunk to the bottom of
the sample capsule, indicating complete melting of the CaCO3.
Up
Direction
Pt sphere

10. 10
Figure 7 Result of falling sphere experiment 2 11/13/13, ~1500°C at 6Gpa. Note that the Pt sphere has not moved from its
initial position at the top of the sample capsule, indicating no melting of the CaCO3.
Up
Direction
Pt sphere
Figure 8 Comparison of Ramanspectrum of experiment 1 with aragonite Raman spectrum from rruff.info shows that the
resultant phase of calcite of experiment 1 was aragonite.

11. 11
Figure 9 A blended image of experiment 1, 10/30/13 of the optical image and aragonite map (red). This shows that the
sample was composed of a uniform composition of aragonite.
Figure 10 Comparison of Raman spectrum of experiment 2 with aragonite and calcite Raman spectrum from rruff.info shows
that the resultant phase of calcite of experiment 1 was a blend of both calcite and aragonite.

12. 12
Figure 11 Ablended image of experiment 2, 11/13/13 of the optical image with both aragonite map (red) and calcite map
(green). This shows that the sample was composed of a non-uniform composition of both aragonite and calcite.
Figure 12 Phase relations of CaCO3 obtained from the present experiments. Key: filleddiamond = calcite I, open diamond =
calcite II, filled circle = calcite III, open box = aragonite, open triangle = disordered calcite, open circle = liquid, ■★, ❍★, ▲
★ = retrieval experiment, Br = After Bridgman (1938), H.G.& E = After Hess et al. (1991), I & W = After Irving and Wyllie
(1975). From Suito, et al. (2001)

13. 13
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