Thermal Conductivity of Pyrite

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Mottl, M.J., Davis, E.E., Fisher, A.T., and Slack, J.F. (Eds.), 1994Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 139

45 . DATA REPORT:PHYSICAL PROPERTIES OF MASSIVE SULFIDE FROM SITE 856,MIDDLE VALLEY, NORTHERN JUAN DE FUCA RIDGE1

Henrike M. Grschel-Becker,2 Earl E. Davis,3 and James M. Franklin4

A B S T R A C TThe results ofeight high-pressure velocity experiments andeight divided-bar thermal-conductivity m easurements onmassivesulfide samples recovered from Ocean Drill ing Program Holes 856G and856H arepresented. Densities, porosities, velocities, andthermal conductivities arehighly variable owing to textural andmineralogical variations and thehighly porous nature ofmost ofthe five types of sulfide reported onhere. Porosity occurs asopen topartially filled intergranular spaces, vugs, veins, cracks, andfractures. Sulfide sam ples are not ideal foreither high-pressure velocity experiments ordivided-bar thermal-conductivity measure-ments; limitations and sources ofpotential error are discussed inthe text. Sample geometry and textural makeup particularly causedproblems in thevelocity experiments. Thelack of a suitable shipboard high-conductivity standard also contributed toerrors, as isapparent from a comparison of the shore-based divided-bar data with shipboard half-space measurements conducted on the samesamples. These velocity and thermal-conductivity data, are, to the best of our knowledge, the first to be reported for massivesulfides in the open literature.

INTRODUCTIONThe penetration of more than 159 m of massive sulfide at Bent

Hill, Middle Valley, at Ocean Drilling Program (ODP) Holes 856Gand 856H provided an unusual opportunity for the study of the physi-cal properties of a deposit that probably precipitated at and above theseafloor. Compressional (V p)- and shear (\{)-wave velocities of eightoriented massive sulfide minicore cylinders were measured at vary-ing confining pressures using a pulse-transmission technique similarto that of Birch (1960,1961). Two samples were from Hole 856G andsix were from Hole 856H (Table 1). Bulk densities (>b) and porositieswere also determined. Results are presented for a Hole 856G/Hole856H composite section since the two boreholes were drilled no morethan 20 m apart (Shipboard Scientific Party, 1992b). A shore-baseddivided-bar apparatus was used to obtain thermal conductivities ofeight 2-cm3 cubes of massive sulfide cut for shipboard physical prop-erties measurements (Table 2).

SAMPLE DESCRIPTIONSamples are described according to the sulfide classification scheme

developed aboard the ship (Figs. 87 and 88, Shipboard Scientific Party,1992b). Velocity measurements were conducted on two type 1 samples(homogenous, massive, fine-grained pyrrhotite), two type 4 samples(heterogenous and veined, coarse-grained pyrite-pyrrhotite), and fourtype 5 samples (massive colloform and vuggy pyrite). Thermal conduc-tivities were determined for two cubes of type 4, two of type 5, two oftype 2 (homogeneous, massive, fine-grained pyrite-pyrrhotite), and twoof type 3 (homogeneous, medium- to coarse-grained pyrite-pyrrhotite).

The average grain density (pg) of our samples is high (4.52 g/cm3).Variable proportions of pyrite (pg _4.92 g/cm3), pyrrhotite (pg = 4.55g/cm3), and magnetite ( p = 5.15 g/cm3) (H ora i , 1971), 1 0 % - 2 5 % ofwhich is present in type 4 and 5 rocks, result in high p g and p b values.Successive replacement of "primitive" type 1 sulfide to "m a ture" type

1 M ott l, M.J., Davis, E.E., Fisher, A.T., and Slack, J .F . (Eds.) , 1994. Proc. ODP, Sci.Results, 139: College Station, TX (Ocean Drilling Program).

2 Division of Marine Geology and Geoph ysics, Rosenstiel School of Marine andAtmospheric Science, Un iversity of Miam i, 4600 Rickenbacker Causeway, Miami, FL33149-1098, U.S.A.

3 Geological Survey of Canad a , Pacific Geoscience Centre, P.O. Box 6000, Sidney,British Columbia V8L4B2,Canada.

4 Geological Survey of Canad a , 601 Booth Street, Ottawa, OntarioK l A0E8, Canada.

4 and type 5 sulfide occurs through progressive dissolution, replace-ment , and reprecipi ta tion of primary pyrrhotite with pyrite, chalcopy-rite, sphalerite, and magnetite (Shipboard Scientific Party, 1992b).Chalcopyri te and sphalerite have considerably lower grain densities(4.09 and 4.10 g/cm3, respectively), than the other sulfide minerals(H ora i , 1971), are c o m m o n as late- stage, coarse- grained crystals intype 4 sulfide and, with abundant veining, cause its heterogenous,com m only porous , texture. In contrast , the typical reticulate boxworktexture of type 5 sulfide, with prismatic pores of 0.1- 2.0 mm diame-ter, results from pyrite precipitated after dissolution of precursor sul-fide. Open vugs 2- 4 mm in diameter commonly comprise up to 10%of this sulfide type. Pyrite dom inates, and other sulfides are minor.Open porosity of up to 50% by volume occurs in type 1 sulfide as aconsequence of interpenetra ting fine-grained blades ofdom ina nt pyr-rhotite . Chalcopyrite and sphalerite are ubiquitous and occur intersti-tially to the pyrrhotite and m inor (usually less than 10%) pyrite. Per-centages of n o n o p a q u e minerals, including green smectite, chlorite,a m orphous silica, and carbonate, occur interstitially most commonlyin types 1-3 and also are present as vein and vug infill in types 4 and5, where white amorphous silica and chlorite typically form 15% ofth e rock.

METHODSVelocity experiments were performed on oriented minicores in heRosenstiel School of Marine and Atmospheric Science ( R S M A S )Petrophysics Laboratory at hydrostatic pressures from 3 to 100 MPa.

Wet-bulk densities and porosities were also obtained. Sample prepa-ration and experimental methods were generally the same as thosedescribed in Grschel-Becker et al. (this volume). Preparation of thesamples was difficult owing to textural variations and surface irregu-larities, especially for type 5 (massive colloform and vuggy pyrite)sulfides. The polishing required before the samples were loaded intothe velocimeter varied from minimal for relatively soft pyrrhotite-rich samples to extensive for the hard, dense, massive pyrite.

Thermal conductivities of oriented cube samples were measuredon a divided-bar apparatus at the Pacific Geoscience Centre (PGC)following the general procedures outlined in Davis and Seemann (thisvolume). Cross-sectional areas and thicknesses of all samples werederived from caliper measurements of the cubes. A minor amount ofpolishing was done on sample faces to ensure good thermal contact.Samples with broken edges or corners were not included in the study.

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D A TA R E P O R T .

Tab le 1 . Var ia t ions in densi t ies and com pressiona l- and shear -w ave veloci t ies fo r massive su lf ide m in icore samp les f rom Holes8 5 6 G a n d 8 5 6 H.Core, section,interval (cm)

139-856G-7R-4, 15-177R-4, 18-20

139-856H-3R-1, 18-21

3R-1,24-27

1 OR-1,52-55

1 OR-1,56-58

15R-1, 16-18

15R-1,20-22

Direction

hV

V

h

h

V

h

V

R o c ktype

5:Py

5:Py

4:Py/Po

4:Py/Po

5:Py

5:Py

i:Po

!:Po

Density(g/cm3)

3.864.00

4.01

3.90

3.85

3.49

3.63

3.60

M od e

PSIPSIS2PSIS2PSIS2PSIS2PSIPSIS2PSIS2

10

49902710695035103650

586032703180589032803220619035003330643032203210317017901580335017101780

Velocity (m/s)20

50202750710035403670

590033103200597033003230635035303510653032603230322018401660340017701820

30

50502790715036103720

594033203240600033203240648035703590660033103250331018501740346018101850

40

50702860718036503750

59703340325060303320342066003680365066603350326033501780350018401880

50

50802960720037103780

599033503260606033303250667036403660673033703290342018801860354018701900

60

51003050723037803820

60303360328060903330326067603690684033903300345019001890358019001920

Pressure (MPa)70

51503120725038303860

60603380330061203360328068903750691034303400345019201890360019301950

80

51503120728038403870

60803390330061403410328069403790695034403460346019201900363019401960

90

51303120730038003840

609033903310615033503290696036603840698034503460349019301910365019601980

100

51203120730037703830

612033903310615033503290699036803870699033903370350019301920367019701990

Note: h = hor izontal minicore or iented perpendicular to core axis; v = ver tical m inicore or iented parallel to core axis; Py = pyr ite; Py/Po = pyrite/pyrrhotite;Po = pyrrhotite; P = compressional w ave; SI = shear wave propagating perpendicular to m inicore alignment d irection; S2 = shear wave propagatingparallel to m inicore alignment d irection .

Table 2 . Thermal conduct iv i t ies , porosi t ies , and major const i tuen t minera ls o f su lf ide cubes samples f rom Holes 856G and8 5 6 H .

Core, section,interval (c m)

Pieceno.

De pt h(mbsf) Type/1 ithology

Shipboardporosity(%)

Shipboardthermal

conductivity(W/m-K)

P G C thermalconductivity(a + b + c)( W / m - K )

139-856G-3R-1,394R-1, 346R-1, 1407R-4, 9

139-856H-3R-1, 163R-3, 1079R-1,5913R-1.40

65235

298

17.927.247.760.6

22.325.853.171.3

Type 2: homogeneous massive f ine-gr . pyrite-pyrrhotiteType 3: homogeneous m edium- to coarse-gr . pyr ite-pyrrhotiteType 3: homogene ous medium - to coarse-gr . pyr ite-pyrrhotiteTy p e 5: massive colloform and vuggy pyriteType 4: heterogenous and veined coarse-gr . pyr ite-pyrrhotiteTy p e 4: heterogenous and veined coarse-gr . pyr ite-pyrrhotiteTy p e 5: massive colloform and vuggy pyr iteTy p e 2: homogenous massive f ine-gr . p yr ite-pyrrhotite

11.917.817.614.0

6.916.822.216.8

6.23.67.23.84.16.34.6

6.85.97.813.4

7.35.112.16.5

E X P E R I M E N T A L L I M I T A T I O N SMassive sulfides are, in gen eral, not ideal samples for high-p ressureveloci ty exper iments. Surface i r regular i t ies owing to vugs, micro-cracks, and par t ial ly to ful ly f i l led veins, prevented opt imum contactof the t ransducers with the cyl inder faces and reduc ed the accuracy ofthe m easurement s , especia l ly Vs. The saturat ing f luid may have par-t ial ly drained from these porous samples pr ior to measurement , re-sul t ing in lower Vp than expected at saturated condi t ions for the rela-

t ively low pressu res used in this study; the effect of sa turat ion on Vs isnegl igible (Christensen and Sal isbury, 1975, and references therein;Bonner and Schock, 1981). These two problems were most apparentin t ype 5 su l f ides . Repeat m easurement s w ere not poss ib le becau sefour of eight samples fai led at pressures between 70 and 90 MPa. Ingeneral , sulf ide Vp an d Vs resul ts are good to 1% to 2% up to 80 MPa;veloci t ies of intact igneous rock s are good to 1%.The greatest source of er ror in the measurem ent of conduc t ivi ty ofthese samples probably ar ises f rom thermal contact resistances. Aviscous wet t ing agent was used on the cube faces to reduce this er rorbecause the smal l temperature difference across the highly conduc-t ive massive sulf ide cubes (on the order of 0 .5 K) relat ive to that

across the standard samples of the bar ( typical ly 3-4 K), makes anytemperature drop across the imperfect contacts signif icant . Orientedcubes w ere cut f rom spl i t core sect ions w ith a twin-blade cut-off saw(Shipboard Scient i fic Par ty, 1992a); hen ce, the two pairs of oppo singfaces norm al to the long core (a) axis and norma l to the spl i t core face(b axis) were ge neral ly qui te paral lel (Fig. 1). The grea t est nonparal-lelism occurred between the spl i t face and i ts opposi te side (normalto the c ax i s); imper fec tions of most samples could be accomm odatedby the g imbaled moun t of t he upper par t of the d iv ided-bar appara tus .Rep eat measurem ents w ere made in several cases where the a-, b- , andc-axis values differed by m ore than 10% to 20%. No large or system-at ic differences were observed in the measurements along any of thethree axes , and the va lues g iven in Table 2 ar e average va lues .

RESULTSDensity, Porosity, and Velocity

Wet-bulk densi t ies of the eight veloci ty samples range from 3.49g/cm3 to 4.01 g/cm3; the greatest var iat ions occur in type 5 sulf ides(Table 1). Porosi t ies are high, f rom 10.6% in type 4 Sample 139-856H-3R-1,18-21 cm, to 28 .3% in type 5 Sam ple 139-856H -10R-1 ,

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DATA REPO RT

t o p o fsection split face ofh a l f - r o u n d c o r e

Figure 1. Shipboard convention fororientation of cube samples taken from theworking halves of consolidated sediment or hard-rock cores. The a axis isparallel to the vertical axis of the core section, the b axis is across the coreparallel to the split core face, and the c axis is through the core perpendicularto the split core face.

56- 58 cm (Table 2). The average porosity of type 5 pyrite rocks is24.7%. Differences in these two properties between adjacent horizon-tally and vertically oriented sulfide m inicores are apparent in Figure2, which plots wet-bulk density and porosity vs. depth in a Site 856composite section. Shipboard cube values are plotted for comparison;these and porosity data were recalculated using dry volumes only tobe consistent with shore- based values. D ensity variations of up to 9.9%between nearby samples and observed velocity variations reflect dif-ferences in textures, porosity resulting from the prevalence of openan d partially filled intergranular pore spaces, vugs, veins, cracks, andfractures, and infilling by nonopaque minerals, as well as variablepercentages of sulfide minerals.

ThermalConductivityT he sulfide conductivities span a large range, from 5.9 to 13.4

W/m - K. The two highest values were measured on samples consist-ing primarily of pyrite, a mineral that has a particularly high thermalconductivity of 19.2 W/ m- K (Horai, 1971). Others contain signifi-cant amounts of pyrrhotite, carbonate, silica, and other thermally lessconductive minerals, which appears to be the primary reason for theirrelatively lower conductivities. Detailed petrographic analyses of oursamples are needed to analyze the effects of intergranular structure interms of grain contacts and its relation to conductivity and the sys-tematics of pyrite and other constituent minerals.

Porosities range from 7% to 22%, and although the contrast inthermal conductivity between any of the constituent minerals andwater is large, variations in porosity alone cannot account for thevariation in conductivity. This is seen clearly in F igure 3, in which themeasured thermal conductivities of the samples are plotted againsttheir porosities, as are the relationships predicted by a simple geomet-ric mean mixing law (e.g., Brigaud and Vasseur, 1989) with variousconstituent mineral conductivities and the conductivity of water of 0.6W/m- K. The conductivity of 19.2W/ m- K for pure pyrite produces theupper curve; use of a value of 9.0 W/m- K results in a reference curvethat provides a crude lower bound for the sulfide data, assuming thatth e mean harmonic model can be applied. A value of 2.6 W/ m- Kprovides the best fit for the sediment and igneous data. Despite thewide range of implied constituent conductivity values, the minimumvalue is still high, three times the value inferred from the conductivitiesan d porosities measured on sediment and basalt samples (Davis andSeemann, this volume; shown also in Fig. 3 for comparison).

As is discussed previously, the set- up of the divided-bar apparatusused in this study was not ideal for the measurement of metallicsulfide rocks having high thermal conductivity. The largest source ofsystematic error is expected to be associated with thermal contactresistance, which would cause values to be erroneously low. An upperlimit for the magnitude of such an error can be estimated by calculat-ing the effect of a 0.1-mm gap between the sample and the brasscontact plate filled with contact gel having a thermal conductivity of1 W/ m- K. Th is would int roduce an error of 5% into the determination

0102030

s=* 40COQ

>Q 60

708090100

3.0Wet-Bulk Density (g /crrT)

3.5 4.0 4.5 5.01 1

-

o-

0

_ 1

o

10

1 '

ooo

0

t o0

1 1

1 '

-

o----

1 1

Porosity (%)10 15 20 25 30

i I ' I ' i ' I

oo

-I I I I I I I I I I

Figure 2. Wet-bulk density vs. depth and porosity vs. depth for a Site 856 composite sulfide section. Filled circles represent data from this study; open circlesrepresent shipboard data.

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DATA REP ORT.

i [ i ' i r i i ' i i > i ' i ' i < r

= 1 9 ^ W / m - K

10 20 30 40 50Sh ipboard Poros i ty (%)

O 70

Figure 3. Divided-bar thermal conductivity vs. shipboard porosity for sulfide(filled squares), igneous (open squares), and sedimentary (circles) rocks fromMiddle Valley. Geometric mean mixing relationships, using three differentconstituent mineral conductivities (19.2, 9.0, and 2.6 W/m- K) plus water (0.6W/m- K), are shown assolid lines and are explained in detail in the text. D a t afor igneous and sedimentary rocks are taken from Davis and Seemann (thisvolume). D a t a for sulfide rocks are given in Table2.

4 6 8 10 12PGC Thermal Conductivity (W/m-K)

14

of the conductivity of a2-cm 3 rock sample with aconductivity of 10W/m-K. Acomparison of thedivided-bar measurements reportedhere with the line- source shipboard measurem ents reveals adiscrep-ancy of the opposite sense and of amuch larger magnitude (F ig. 4).This result isnot surprising, asthe method ofmeasuring c onductivitywith a line- source imbedded in ahalf-space involves anempiricallydetermined calibration factor determined over arange ofconductivi-ties that waslimited relative tothe values for the sulfide rocks atSite856 (0.96 to 2.05W/m - K; Shipbo ard Scientific Party, 1992a). Con -cern about this potent ial source oferror was also expressed at the timethe shipboard measurements were made (seeShipboard ScientificParty, 1992b). The error associated with this approximat ion increaseswith increasing conductivity; shipboard estimates (table 37, Ship-board Scientific Party, 1992b) are roughly half those determ ined withthe divided bar atthe highest conductivities. Asignificant systematicerror appears topersist down tovalues in theregion of 3 W/m-K;shipboard values are about 10% lower. No systematic difference existsbetween m easured values below 2W/m- K, th e range over which cali-bration standards are available.

The velocity and therm al con ductivity data, are, tothe best ofourknowledge, thefirst to be reported for massive sulfides inthe openliterature. Electrical resistivity measurements on cube samples areplanned.

ACKNOWLEDGMENTSHG B thanks R. Zierenberg for his tireless assistance in sam-

ple selection and enthusiasm for sulfide physical pro perties, and JOI/USSAC for providing post- cruise funding; Gregor Eberli and JimNatland graciously mad e their labs available. T.Lewis kindly pro-vided EE D with th e divided bar apparatus.

REFERENCES*Birch, R, 1960. The velocity ofcompressional waves in rocks to10kilobars,

1.J. Geophys. Res., 65:1083-1102., 1961. The velocity of compressional waves in rocks to 10kilobars,

2. J. Geophys. Res., 66:2199- 2224.Bonner, B.P., and Schock, R.N., 1981. Seismic wave velocity. InTouloukian,

Y.S., and Ho, C.Y. (Eds . ) , Physical Properties ofRocks and Minerals:McGraw-H ill/CINDAS D a t a Series on Material Properties, 11-2:221-256.

Brigaud, F ., and Vasseur, G ., 1989.Mineralogy, porosity and fluid control onthermal conductivity ofsedimentary rocks. Geophys.J., 98:525-542.

Christensen, N . I . , and Salisbury, M.H ., 1975. Structure and constitution ofthelower oceanic crust. Rev. Geophys. Space Phys., 13:57- 86.

H o r a i , K., 1971. Thermal conductivity ofrock- forming minerals. J. Geophys.Res., 76:1278-1308.

Shipboard Scientific Party, 1992a. Explanatory notes. In Davis, E.E., M ott l,M.J. , Fisher, A.T., et al., Proc. ODP, Init. Repts., 139: College Stat ion, TX(Ocean Drilling Program), 55- 97.

, 1992b. Site 856. In Davis, E.E., M ottl, M.J., Fisher, A.T.,etal.,Proc. ODP, Init. Repts., 139: College Station, TX(Ocean DrillingPro-gram), 161-281.

Figure 4. Shipboard thermal conductivity determined with a line sourceimbedded in a half-space vs. shore-based conductivity determined with adivided bar. D a t a sources and symbol conventions are the same as in Figure 3.

* Abbreviations for namesof organizations and publications in O D P reference lists followth e style given in Chemical AbstractsService Source Index (published by AmericanCh e m ica l Society).

Date ofinitial receipt: 13 January 1993Date ofacceptance: 28April1993M s 139SR-249

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Thermal Conductivity of Pyrite