Feb 10, 1981 - water will decrease the electrical resistivity by an order of magnitude. Several ... liquids and are also electrically conductive as liquids become.
JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 86, NO. B2, PAGES 931-936, FEBRUARY
10, 1981
ElectricalPropertiesof Granite With Implications for the Lower Crust GARY
R. OLHOEFT
Petrophysics and RemoteSensing,U.S. GeologicalSurvey,Denver,Colorado80225
The electricalpropertiesof granite appearto be dominantlycontrolledby the amount of free water in the graniteand by temperature.Minor contributionsto the electricalpropertiesare providedby hydrostaticand lithostaticpressure,structurallyboundwater,oxygenfugacity,and otherparameters.The effectof sulfurfugacitymay be importantbut is experimentally unconfirmed. In additionto changingthe magnitudeof electricalproperties,the amountand chemistryof water in granitesignificantlychangesthe temperaturedependenceof the electricalproperties.With increasingtemperature,changesin water content retain large, but lessened,effectson electricalproperties.Near room temperature,a monolayer of water will decreasethe electricalresistivityby an order of magnitude.Severalweight-percentwater may decreasethe electricalresistivityby asmuch as9 ordersof magnitudeand decreasethe thermal activation energyby a factorof 5. At elevatedtemperatures just belowgraniticmelting,a few weight-percentwater may still decreasethe resistivityby as much as 3 ordersof magnitudeand the activationenergyby a factor of 2. Above the melting temperature(650ø to 1100øCdependingupon water pressure),a few weightpercentwater will decreasethe resistivityby lessthan an order of magnitudeand will barely changethe activationenergy.Remarkably,the few weight-percentwater mustbe presentas free water. Experiments with hydratedhornblendeschist(with structuralwater) indicatean electricalresistivityvery similar to that for dry granite.The implicationsof theseresults,togetherwith the findingsof deepmagneticsounding and magnetotelluricsurveys,suggestmuch more free water than is commonlyassociatedwith the lower crust and possiblyinto the upper mantle.
INTRODUCTION
Electricalpropertiesare extremelyusefulin the studyof the interior of the earth, from near-surfacesurveysto find oil and minerals [Keller and Frischknecht,1966; Ward, 1973, 1976; Olhoefi and Scott, 1980] through the deeper crustal sounding techniquesused to study large geothermalsystemsand the roots of volcanoesor earthquakezones[Stanleyet al., 1977; Hermance and Pedersen,1980]. Invariably, recoursemust be made to laboratory-basedpetrophysicalmodelsin order to interpret the field electricalparametersin termsof more geological interestsuchas a temperatureprofile, ore depositsize,or depth of basement [Brace, 1971; Hermance, 1973; Olhoefi, 1976, 1980a].Unfortunately, althoughelectricalpropertiesare extremely useful in being highly sensitiveto subtle changes within the earth, they are also complicatedto interpret due to the great number of parameterswhich can influence equal changes.These attributeshave resultedin repeatedcalls for more laboratory experimentsto determine the factors that control variations in electrical properties[Ward, 1977; International Union of Geodesyand Geophysics (IUGG), 1979]. One of the key questionsconcernsthe distribution of a highly conductive material in an insulating material. Examplesare water in granite,clay in sandstone,dendritichematite or graphitein schist,disseminatedsulfidesin porphyritic granite, and melt in a partially molten silicate.Severalreviews have recently discussedthe various predictive and empirical models of such situations [Duhkin, 1971; Madden, 1976; Shankland and Waft, 1977; Garland and Tanner, 1978; Hermance, 1979;Olhoefi, 1980a].Also important are the chemical interactionsbetweenthe water and rock material at pore surfaces[Olhoefi, 1977, 1979a, 1980a;Drury and Hyndrnan,1979; Ucok, 1979]. Many electrical properties are strongly frequencydependentand sometimesdependentupon the amplitude of stimulus[Olhoefi, 1979b;Olhoefiand Scott, 1980],but This paperis not subjectto U.S. copyright.Publishedin 1981 by the American GeophysicalUnion. Paper number 80B1096.
theseand relatedfactorswill be neglectedin the following discussion.This paperwill focuson the effectsof environmental
parametersuponthe directcurrent(dc) electricalresistivity. VOLATILES
Mcintosh [1966] and Olhoefi [1975] have reviewedmuch of the relevant literature on the adsorptionof gasesinto materials and the resultant effectson electrical properties. Essentially, only substancesthat are both highly polar as gasesand liquids and are alsoelectricallyconductiveas liquidsbecome candidates for altering electrical properties by their introductioninto an insulatingmaterial through surfaceadsorption or volume addition. Carbon dioxide (a polar gas but insulatingliquid), nitrogen(nonpolargasand insulatingliquid), and related
substances have little
or no effect on electrical
properties.Water and several sulfur compoundsare both highly polar gasesand conductiveliquidswith very strongeffects on electrical properties;both also exist in the earth in substantialquantities[Burnham,1975;Fyfe et al., 1978; Gerlach and Nordlie, 1975a, b, c; Mysen and Boettcher, 1975; Mysen and Popp, 1980]. Olhoeft[1975] has shownthat water vapor adsorbedinto the pore structureof a low surfacearea,
insulating materiallike basaltShas no effecton electricalproperties until a sufficientamount is adsorbedto produce a connectedpath through the pore system(about 20% of a mono-
layer or about 0.002 wt % water in basalt).More than a full monolayer is required before dielectric properties begin to change,by which time the dc resistivityhasdroppedby an order of magnitude [Olhoeft, 1975, 1980a].A few weight percent more water decreasesthe resistivityby as many as 9 ordersof magnitude. Olhoefi et al. [1973, 1974] have shown that outgassingsulfur can drasticallyalter the electricalpropertiesof lunar samples,but experimental work is lacking to confirm the sulfur-related changesin electrical properties, although suchchangesare expectedfrom the highly similar and corrosive chemistriesof water and sulfur [Hauffe, 1965; Fontana and Greene, 1978; Foroulis, 1970; Turkdogan, 1980].
931
932
OLHOEFT: ELECTRICAL PROPERTIES OF GRANITE
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Fig. 1. Log electrical resistivity versus oxygen fugacity for aFig. 3. The effectof confiningpressureon water-saturated Westerly A1203[after Pappisand Kingcry,1961],MgO [afterParkin, 1972],and Granite [after Brace and Orange, 1968]. En93(010)pyroxene[after Duba et al., 1973]. At lower temperatures, the fO2 dependenceis even lesspronounced[Kofstad,1972],and in would be rare in the earth as most rocks are self-buffered in
basaltic melts there appears tobelittleornofO•_ dependence [Waft volatile fugacity [Greenwood, 1975; FyfeetaL,1978], butinter-
andWeill, 1975].
pretations fromlaboratory measurements require some eau-
tion, as it is possibleto make measurementsat oxygenfugaFigure1 illustrates a summaryof representative datafor the citiesoutsidethe stabilityfield of the sample,thus chemically oxygen fugacity dependenceof electricalresistivity.Essen- altering the sampleto a different material. tially, oxygenfugacity(and the relatedcarbondioxide/monWATER oxide fugacities)may be neglectedas an importanteffecton electricalpropertiesas a bestcasesituationrequires6 orders Figure 2 illustratesa summaryof the availabledata for the
of magnitude change in fO• to produce abouta halforderof electrical properties of graniteasa functionof temperature, magnitudechangein electricalresistivity.Even in the finest grained,mosthomogeneous granitespecimen, naturalstatistical variationsin electricalresistivitywouldproducea factorof 3 changeovera few centimeters displacement within the specimen. At lower temperaturesthan thoseshownin Figure 1, the effectsof oxygenfugacity are even less[Kofstad,1972], and in full silicatemeltsthereappearsto be little or no fO: dependence[Waft and Weill, 1975].Thus the oxygenfugacity may be safelyneglectedas an explanationfor major changes in electricalresistivitywithin the earth. Largechangesin f O: TEMPERATURE I
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water content,and pressure(a similar figure holdsfor basalt; see Olhoefi [1980a]).Lebedevand Khitarov[1964] and Hyndman and Hyndman [1968]have previouslydiscussed the importanceof water on electricalpropertiesdeep within the earth [seealsoBrace, 1971;Hermance,1973;Burnham,1975]. To incorporaterecentresults,the next few figuresquickly review the effectsof pressure,temperature,and salt concentration uponwaterin rocks[seealsoUcok, 1979;Olhoefi,1980a]. First, water has two striking effectsas shown in Figure 2. The amplitude of the resistivitychangesby many ordersof magnitudeas water is addedto granite,and the temperature
0
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Fig. 2. Summaryof the availabledata on electricalresistivityversustemperaturefor wet and dry granite.Open circlesare vacuumdry
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(10-li MPa) WesterlyGranite (this paper,usingthe experimental
I
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techniquesdescribedin Olhoefi[1979a,b, 1980b]).Closedcirclesare
(K)
ing the experimentaltechniques describedin Ucok[1979]).Solidlines (with waterpressureindicatedin MPa) are water-saturated El'dzurta Granite from the Northern Caucasus[after Lebedevand Khitarov,
Fig. 4. The effectof pressure and temperatureon the electricalresistivityof 0.1 molar NaC1 aqueoussolution.Below the critical temperatureand with at leastenoughpressureto maintaina liquid, the resistivityis independentof pressure[after data in Quistand Marshall,
1964].
1968].
0.4 molarNaClaqsolution-saturated WesterlyGranite(thispaper,us-
OLHOEFT.' ELECTRICAL
PROPERTIES OF GRANITE
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resistivityversustemperature, pressure, and water content.The
pcraturc, theactivation energy of theelectrical resistivity of dashed linesareforvarious water pressures in MPaforwater salin-
d• granite isaround 0.5cV,•crcas•gtoabout 1.5cVneara ities less than 0.1molar NaCI. Above asali•ty of0.1molar, thewet
fewhundred degrees Celsius. Theactivation energies • wet cu•e isessentially independent ofpressure through granitic melting. granitearcnear0.l cV andvc• constant with •crcas• s tern-
Figure 4 •ustratcs the effectof pressureand temperature
pcraturc (unt•themclt•s temperature ofthegranite orthe ona0.l molar NaCI solution. Unt•thecritical temperature of critical temperature ofthesolution • reached). thesolution isreached, theresistivity is•dcpcndcnt ofprcsFigure 3su•a•cs theeffect ofcon•s pressure uponsure aslong asthere exists atleast them•um pressure rethercs•tivit7 ofa water-saturated granite atroom temperqu•cd tokccp thewater asa •quid. Gaseous water (steam) aturc. Thecon•s pressure alters theamount ofpore spacehas about anorder ofmagnitude decrease • theresistivity of • which water may exist • the rock. With• the range of
a vacuumd• rock (asa• ncs•sibE on the scaEof Figure 2; c•stal pressures, the change• porosityducto •thostaticload scc Olhoefi [1975]). Above the critical temperature,the rcs•w• result • about an order of mas•tudc change• rcs•tivtivit7 is stronglyboth temperatureand pressuredependent. it7. Comparedto the 13 ordersof magnitudeof changeducto Figure 5 showsthat the criticalpo•t of NaC1 aqueoussolutemperature variationribplayed• Figure2, theeffectsof •th- tion is stronsl7 dependentupon salt concentrationabove 0. l ostaticload on resistivityarc asa• negligible.Pressureand molar. Below 0. l molar, the critical properties arc in-
other effects which alter pore st•cturc and s•cmay • •r-
rant • the laborato• duc to erosionand rcdcpositionof ma-
dist•suishablc from pure water. Above 0.l molar, both the
critical temperatureand the critical pressurerisc rapidly with
tcdal toenlarge orblock pores, and they may create t•c-dc- •crcas•g concentration. Atthetypical concentrations ofgcopendentphenomenaon laborato• t•c scales.However,at thc•al b•cs and fluid •clusions near 6 molar [Fyfe et •1., temperatures above5•øC • the lowerc•st, the chemical 1978], the critical tcmpcratucis above 6•øC and the critical ncticswould have cqu•bration t•cs of the order of hours pressureis above l• MPa. Ewang et •1. [1970] have conlcav•g temperatureand watercontentthe domeant parameters.
TEMPERATURE st so 8
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6 S •O •2 •4 W[I•HT P[EC[NT W•T[R Fig. 6. Log electdeal resistivityversusweight-per•nt water in El'dzu•a Gra•te [afterLebedev,1975]at 1273K with the data points labeledwith the hydrostaticpressure• MPa requiredto saturate•e
Fig. 8. Log electricalresistivityversusreciprocaltemperaturefor Westerly Granite and ho•blende schistusing expe•mental procedures as descdbedin Olhoefi [1979a, b, 1980b]. Note that the two cu•es are ve• similar eventhoughthe ho•blende has •iderably more st•ctural water than the gra•te (both measuredafter volatile
gr•tic melt•th that weight-per•ntwater.
outgass•gat eachtemperature in 10-li MPa vacuum).
934
OLHOEFT:
ELECTRICAL
PROPERTIES
salt of Duba [1976]). Resultsare very similar for basalt,sandstone, and other silicates[Ucok, 1979; Olhoeft, 1980a]. In the lower crust, most rocks will be wet with very low porosities, but the amount of water is subjectto some dispute.There is general agreementthat near melting, most crustal rocks are near water saturation. At lower temperatures,however, there may be extensivewater-deficientregions[Robertsonand Wyllie, 1971].Fyfe et al. [1978]have discussed the problemof water content in somedetail. It is important to note for electrical properties,that in additionto the distinctionbetweensurfaceconductingadsorbedwater and volume-conductingpore water, it is free water and not bound water that is significant. Figure 8 illustratesthe temperaturedependenceof the electrical resistivityof vacuum dry Westerly Granite as in Figure
•--•-----Z H2
4 8 12I( 40 48 Z,ATOMIc MASS UNITs HORNBLENDE
Fig. 9. Three dimensional plot of the amplitude of volatile outgassingfor hornblendeschistas it is heated in 10-• MPa vacuum while the electrical propertiesof the sample in Figure 8 were being measured.Pointsare plottedat 1 amu (atomicmassunit) and 50øC intervals.
firmed that the pressuredependenceof electrical resistivity also moves to higher temperaturesalong with the critical point parametersas brine concentrationincreases.Thus the electrical resistivityof a severalmolar solution will be independentof pressureto over 600øC. At that point, the water pressuremay havereducedthe meltingpoint of a silicatesuch as granite or basaltto below 700øC [Brown,1970],resultingin pressure-independent electricalresistivityuntil melting. Figure 6 illustratesthe dependenceof electricalresistivity upon weight-percentwater in granite at and above melting temperatures.The curve is in excellent agreementwith the measurementsof Shaw [1974] on the diffusivity of water in granitic liquids. Note that 0.3 weight-percentwater added to dry granite decreasesthe electricalresistivityby an order of magnitude,and a few weight percentmore water may further decreasethe resistivityby 2 or 3 additional ordersof magnitude. Pressureplays a secondaryrole here as it affects the amount of water that may be dissolvedin granitic melt.
2 and similar
data for an hornblende
schist. Both were mea-
sured by equipment and techniques described in Olhoeft [1979a, 1980b] while simultaneouslymonitoring the volatile outgassingwith a quadrupolemassspectrometer(Figure 9). The hornblendeschistis amphibolespecimen24 lB [Hunt and Salisbury,1976]composedof 85% hornblendeand 15%oligoclasefeldspar. Figures 10 and 11 showonly the water and carbon dioxide outgassingfor the granite and schist.In Figure 10, the schistis outgassingphysically adsorbed water up to 250øC. Near 650øC, the more strongly bound chemisorbedwater is being released.At 1000øC,the hornblendedecomposed,yielding all of its structurallybound water (the outgassingwas so strong and abrupt as to nearly overpowerthe vacuum pumpsand to saturate the massspectrometer).In the granite outgassingin Figure 11 (samescaleas Figure 10), no such!.•rgeamountsof water appear. The strongestoutgassingoccurredas the granite melted
near 1100øC.
The electricalresistivitymeasurementswere performedat a given temperature after thermal equilibrium had been reached and after all volatiles were outgassedat that temperature. Despite the large amount of structural water in the hornblende below 1000øC, the granite and hornblende have very similar dc electrical resistivities versus temperature. However, the dielectric relaxations in the hornblende were far
DISCUSSION
more pronouncedthan in the granite. Also, the granite was
Figure 7 summarizesthe electrical resistivityfor wet and dry granite versustemperature.The dashedcurve showsthe wet granite for 0.1 or less molar NaC1 saturatingsolutions where pressure-dependent electrical propertiesof water are important. Above 0.1 molar NaC1 concentrations,the solid line labeled'wet' is independentof pressureuntil higher than 700øC where pressureeffects water solubility and granitic meltingtemperatures.The 'dry' curve is representativeof totally vacuum dry granite. A few monolayersof adsorbedwater will displacethe dry curve to lower resistivitieswithout substantialchangein the temperaturedependence.As volume effectsof water-filled pore structuresbegin to dominate over the surfaceeffectsof adsorbedwater, then the temperaturedependenceof the electricalresistivitywill beginto be more like the wet granite.The olivine diffusionstudiesof Misener[1974] and the electrical measurementsof Duba et al. [1974] and related studies on other ceramics have demonstrated
OF GRANITE
A
that the
dry granite electrical propertiesare also relatively independent of pressure(with 1300 MPa requiredto producean order of magnitude change in resistivity). Betweenthe dry and wet curves,the electricalpropertiesare controlled by the amount of water in the granite (neglecting time dependenteffectsdue to somedisequilibriumas the ba-
Fig. 10. Three-dimensionalplot of hornblendeschistoutgassing with only H20 and CO2 shownfor greaterclarity. The first H:O peak near 250øC is physicallyadsorbedwater being driven off. The H:O peak near 650øCis the moretightly boundsurfacechemisorbed water being released.The massiveH:O peak at 1000øCis the hornblende decomposing(the peak amplitudeis clippeddue to massspectrometer overload).
OLHOEFT: ELECTRICAL
PROPERTIES OF GRANITE
935
entirely linear in electricalpropertiesover the entire temperature range,whereasthe hornblendeschistabove 700øC had a very distinctlynonlinearcomplexresistivitythat was characteristic of an unknown oxidation-reductionreaction [seeOlhoefi, 1979b;Olhoefi and Scott, 1980].
have shown that appreciableamountsof sulfur are likely in the upper mantle, and the estimatedamountsmay approach thosefor water [Fyfe et al., 1978]. In concert with the deep electromagnetic soundings of Stanleyet al. [1977], Hermanceand Pealersen[1980], and others,theseresultssuggesta requirementfor water saturationof SUMMARY a few weight percent in the deep crust, or much higher temAt 1000øC, in order to produce a 1 order of magnitude peraturesthan are commonly supportedby heat flow meachangein electricalresistivityof a dry granite, the oxygen surements,or massivevolumesof good conductingmaterials fugacity must be changedby more than 6 ordersof magni- (suchas graphite),or anothermechanismof as yet undetertude, or the dry hydrostaticpressureby 1300MPa, or the tem- mined origin, possiblyrelated to sulfur chemistry.These imperature by 55øC, or the water contentby 0.3 wt %. At lower plicationsare drawn here for granitic compositions,but very temperatures,the effectsof water are even more pronounced similar conclusionsare also supportedfor basaltic composiand dominant. tions by the data of Ucok [1979], Olhoefi [1975, 1977, 1980a], From room temperatureto melting, electrical resistivityof and Drury and Hyndrnan [1979]. granite is mostlycontrolledby water contentand temperature, Acknowledgments. This work was jointly supported by the U.S. relativelyindependentof either lithostaticor hydrostaticpressure. In low-salinity solutions(below 0.1 molar NaC1), pres- GeologicalSurvey Geothermal Programand the U.S. Department of Energy, Division of Geothermal Energy. sure dependencebecomesimportant above the critical temperature(approximately380øC), but pressureis of negligible REFERENCES importancefor more concentratedsolutions(above0.1 molar NaC1). At higher temperaturesand in melts,pressureis of sec- Brace, W. F., Resistivityof saturatedcrustalrocks to 40 km based uponlaboratorymeasurements, in TheStructureandPhysicalPropondary importancein controlling the melting temperatureof ertiesof the Earth's Crust,Geophys. Monogr. Ser., vol. 14, edited by the granite, and above melting, in controlling the amount of J. G. Heacock,pp. 243-255, AGU, Washington,D.C., 1971. water that may be in solutionin the melt. Finally, free water is Brace, W. F., and A. S. Orange, Further studiesof the effectsof presrequired;structuralwater appearsto have no significanteffect sure on electrical resistivity of rocks, J. Geophys.Res., 73, 54075420, 1968. on electrical properties(until it appearsas free water when Brown, G. C., A commenton the role of water in the partial fusionof minerals decompose). crustal rocks, Earth Planet. Sci. Lett., 9, 355-358, 1970. Last, there are large amountsof sulfur speciesobservedas Burnham, C. W., Water and magmas:A mixing model, Geochim.Cosminerals, volcanic gases,and in geothermalwaters and fluid mochim. Acta, 39, 1077-1084, 1975. inclusions.Sulfur fugacityis as important as oxygenfugacity Drury, M. J., and R. D. Hyndman,The electricalresistivityof oceanic basalts,J. Geophys.Res.,84, 4537-4546, 1979. in controlling the oxidation state of iron minerals and hence an important control of magneticproperties.The similarity Duba, A., Are laboratory electrical conductivitydata relevant to the earth?,Acta. Geod. Geophys.Montan. Acad. Sci. Hung., 11, 485may carry through to electricalproperties,but more impor495, 1976. tant will be the effectsof surfuricacid and other sulfurspecies Duba, A., J. N. Boland,and A. E. Ringwood,Electricalconductivity of pyroxene,J. Geol.,81, 727-735, 1973. in combinationwith water. Becausesulfur chemistryis very similar to water in being very highly polar, chemicallyreac- Duba, A., H. C. Heard, and R. N. Schock,Electrical conductivity of olivine at high pressureand under controlledoxygenfugacity, J. tive (corrosive),and a liquid electricalconductor,sulfur conGeophys.Res., 79, 1667-1673, 1974. tent, pressure,and chemistryare expectedto be nearly as im- Dukhin, S.S., Dielectricpropertiesof dispersesystems,in Surfaceand portant as water in determiningelectricalpropertiesdeep in Colloid Science,vol. 3, edited by E. Matijevic, pp. 83-166, John Wiley, New York, 1971. someregionsof the crustand upper mantle. The combination of sulfur and water togethermay be more significantthan wa- Fontana, M. G., and N. D. Greene, CorrosionEngineering,2nd ed., McGraw-Hill, New York, 1978. ter alone. Haughton et al. [1974], Shima and Naldrett [1975], Foroulis,Z. A. (Ed.), High TemperatureMetallic Corrosionof Sulfur Kuznetsovaand Krigrnan [1978], and Mysen and Popp [1980] and Its Compounds. 269 pp., CorrosionDivision, Electrochemical Society,New York, 1970. Fyfe, W. S., N.J. Price, and A. B. Thompson, Fluids in the Earth's Crust,383 pp., Elsevier, New York, 1978. Garland, J. C., and D. B. Tanner, (Eds.), ElectricalTransportand Optical Propertiesof Inhomogeneous Media, 416 pp., AmericanInstitute of Physics,New York, 1978. Gerlach, T. M., and B. E. Nordlie, The C-O-H-S gaseoussystem,part I, Compositionlimits and trendsin basalticglasses, Am. J. SCi.,275,
A
353-376, 1975a. Gerlach, T. M., and B. E. Nordlie, The C-O-H-S system,.part II,
Temperature,atomiccomposition, and molecularequilibriain Volcanicgases,Am. J. Sci., 275, 377-394, 197'5b• Gedach, T. M., and B. E. Nordlie, The C'O-H-S system,part III, Magmatic gasescompatiblewith oxidesand sulfidesin basaltic magmas,Am. J. Sci., 275, 395-410, 1975c. Greenwood,H. J., Bufferingof pore fluidsby metamorphicreactions, Am. J. Sci., 275, 573-593, 1975.
GRANITE
•/,/,_L•,3* 0 0'0 .u.,F. ' oC
Fig. l 1. Three-dimensional plot of WesterlyGraniteoutgassing with the samescalesand conditionsas Figures 8, 9, and 10.
Hauffe, K., Oxidationof Metals, 452 pp., Plenum, New York, 1965. Haughton,D. R., P. L. Roeder,and B. J. Skinner,Solubilityof sulfur in mafic magmas,Econ. Geol., 69, 451-467, 1974. Hermance, J. F., An electrical model for the sub-Icelandic crust, Geo-. physics,38, 3-13, 1973. Hermance,J. F., The electricalconductivityof materialscontaininga
936
OLHOEFT: ELECTRICAL PROPERTIES OF GRANITE
partial melt: A simple model from Archie's law, Geophys.Res. Lett., 6, 613-616, 1979.
Hermance, J. F., and J. Pedersen,Deep structureof the Rio Grande Rift: A magnetotelluricinterpretation,J. Geophys.Res.,85, 3899-
Judd, and R. F. Roy, McGraw-Hill, New York, 1980a. Olhoeft, G. R., Initial report of the PetrophysicsLaboratory: 19771979 Addendum, U.S. Geol. $urv. Open File Rep., 80-522, 1-9, 1980b.
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