8. HEAT-FLOW MEASUREMENTS DSDP LEG 371

R.D. Hyndman, Victoria Geophysical Observatory, Earth Physics Branch,Department of Energy, Mines, and Resources, Victoria, B.C., Canada

R.P. Von Herzen, Woods Hole Oceanographic Institution, Woods Hole, MassachusettsA.J. Erickson, Geology Department, University of Georgia, Athens, Georgia,

andJ. Jolivet, Institute de Physique du Globe, Université de Paris VI, Paris, France

ABSTRACTHeat-flow measurements have been made in five deep crustal

holes drilled into the Mid-Atlantic Ridge by Glomar Challenger thatprovide new information on the hydrothermal circulation thatprobably controls the crustal temperature distribution near oceanridge spreading centers. Nearly constant very low heat flow of 0.6±0.1 µcal cm"2 sec"1 (25 ±4 mW m~2) was found for three holes andthree ocean probe measurements across one 3-km-wide sedimentpond on 3.5 m.y. old sea floor and to a depth of 400 meters into theunderlying basaltic basement. The heat flow predicted fortheoretical models of plate accretion is much higher, about 6.4(269). Heat must be carried to the surface by extensive hydrothermalcirculation that may extend to the lower part of the crust.Temperature measurements in the basement part of one boreholeinto 13 m.y. old sea floor indicate that seawater was flowing rapidlydown the hole into a permeable horizon near the bottom. The heatflow of 0.5 (21) measured in the overlying sediments suggests thatbefore the hole was drilled, water may have been flowing horizontal-ly in this horizon at about 10°C.

INTRODUCTION

There is a serious discrepancy between the geo-thermal heat flux measured and that predicted bytheoretical models for spreading ocean ridges. Over athousand heat-flow measurements have been made onocean ridges using the standard ocean probe tech-nique. They show a very large scatter with some highvalues, but the smoothed means are little above theaverages for older ocean basins (e.g., Langseth and VonHerzen, 1971). In contrast, all theoretical conductivecooling models for the formation of the oceaniclithosphere by upwelling under ocean ridges predictvery high heat flow near the ridge crests, decreasingsmoothly with increasing age of the sea floor. Thispattern is obvious in the simple cooling plate models(Langseth et al., 1966; McKenzie, 1967; Sleep, 1969;Sclater and Fracheteau, 1970; Parker and Oldenburg,1973; Davis and Lister, 1974). The pattern is similar,although the ridge crest values predicted are somewhatlower, if the detailed geometry of the ridge crestvolcanism is considered (e.g., Palmason, 1973) and ifthe effects of the melt crystallization and phase changesare included (e.g., Bottinga, 1974). There is strong sup-port for the general form of the theoretical models fromthe elevations of ridges and from their gravity field,

Contribution of the Earth Physics Branch No. 596.

which are readily explained by thermal expansion.Thus, the major source of the discrepancy must besought in the heat-flow data.

The normal ocean probe measurements involvepenetration of only 2 to 5 meters into the sea floorsediments, and these may be subject to the disturbingeffects of short-term variations in bottom watertemperatures, small-scale bottom topography, andlocal sea floor processes. However, there are now over20 well-determined heat-flow measurements in the deepholes drilled several hundred meters into the sea floorsediments by Glomar Challenger (Erickson et al., 1975)which agree well with nearby ocean probe measure-ments, although the deep holes have less scatter ofvalues.

Confirmation of the validity of ocean probe values ofheat flow also comes from a comparison with the heatflow found in the preliminary Mohole site (Von Herzenand Maxwell, 1964) and with the heat flow found in adeep hole drilled into the island of Bermuda (Hynd-man et al., 1974b). Thus the source of the disturbanceto the thermal field must be deeper than several hun-dred meters.

It is now clear that the low and variable heat-flowvalues from ridge crests arise because of hydrothermalcirculation in the oceanic crust. Heat is lost in theregion of the ridge crests by water convection in thefractured and porous crustal rocks rather than by con-duction and thus is not measured by the normal sedi-

347

R. D. HYNDMAN, R. P. VON HERZEN, A. J. ERICKSON, J. JOLIVET

ment heat probe (Palmason, 1967; Langseth and VonHerzen, 1971; Talwani et al., 1971; Lister, 1972; Hynd-man and Rankin, 1972; Anderson, 1972; Williams etal., 1974; Sclater et al., 1974). A recent compilation ofall heat-flow values taken where there is a uniformblanket of sediment to seal off the water circulation, sothat all heat must be conducted to the surface, althoughcirculation may continue in the crustal rocks (J.G.Sclater, personal communication), shows good agree-ment with theoretical models. The hydrothermal cir-culation is of vital importance not only to thermalmodels, but also to our understanding of the genesis ofthe oceanic crust and of metalliferous sediments.Hydrothermal circulation is needed to explain theobserved alteration in crustal rocks (e.g., Aumento etal., 1971; Muehlenbachs and Clayton, 1971; Hekinian,1971; Hart, 1973; Spooner and Fyfe, 1973) and is un-doubtedly the source of the metal-bearing submarinehydrothermal solutions required to produce many ofthe metallic sediments observed (e.g., Corliss, 1971;Cronan et al., 1971; Dymond et al., 1973; Andersonand Halunen, 1974).

Leg 37 provided an opportunity to obtain additionalheat-flow measurements in deep sediment holes nearthe crest of a spreading ridge and for the first time tomake measurements in deep crustal holes that mightoutline the nature and extent of the hydrothermal cir-culation. Heat-flow values were obtained in thesedimentary sections of five holes and in the deeperbasaltic basement in two holes, with crustal ages from3.5 to 13 m.y. One sediment pond was studied in detailwith three holes and three standard ocean probemeasurements.

In the following discussion the conductivity and heatflux values are given first in units of meal cm~1 sec~1

°C~λ and µcal cm~2 sec~1, respectively, and secondly, inbrackets, in units of Wm~1 K 1 and mW m~2. Gradi-ents are given in units of °C kmr1 or equivalently ofmK>m-1.

HEAT-FLOW MEASUREMENTSIN BASEMENT HOLES

Near spreading ridge crests, temperature and thermalconductivity measurements in deep crustal holes shouldpermit a check on the hydrothermal circulationhypothesis. The best method for confirming thehypothesis is to measure temperature in the rock wherecirculation may be occurring since zones of significantcirculation penetrated should be defined by thetemperature-depth profile. A uniform gradient resultalso may be used for comparison with models ofgeneral deep circulation and conduction.

Heat-flow measurements near spreading ridges varyby more than two orders of magnitude, so that evenquite low accuracy results can give very useful informa-tion. A gradient with an error of 20% is a useful result.Higher accuracy is needed for useful interpretationaway from ridges, for example, to define variationswith sea floor age and depth. The temperature measure-ment technique used in sediment holes cannot be usedin crystalline basement so that temperatures can bemeasured only in the hole itself where there is a large

disturbance from the drilling fluid. Thus measure-ments must be made some time after drilling circulationhas stopped. A large part of the disturbance must havedecayed for an accurate extrapolation to equilibrium. Agradient accuracy of ±20% may be obtained by ex-trapolation using a single measurement if about 50% ofthe disturbance has decayed (see below). With three ormore temperature measurement repetitions to definethe decay of the disturbance, a gradient accuracy of±20% may be obtained if about 30% of the disturbancehas decayed. If less than 30% of the disturbance haddecayed even repeated measurements will not permit agradient of useful accuracy to be obtained, because ofthe limit of accuracy of temperature measurement andbecause of the necessary approximations in the modelsused for extrapolation to equilibrium.

TEMPERATURE MEASUREMENT TECHNIQUES

There are three possible approaches to obtaining es-timates of equilibrium temperatures in deep crustalholes.

1) The bottom-hole temperature may be recorded atseveral depths during the drilling. The duration of thedisturbance at the bottom is short so that the return toequilibrium is rapid. However, at least several hours ofrecording after the drilling circulation has stopped arerequired for sufficient decay of the disturbance. Therecording can be done with the instrument used forsediment temperatures (Erickson et al., 1975).Temperatures must be recorded as close as possible tothe bottom where the disturbance time is shortest.Ideally, the temperature sensor would rest on the bot-tom and be connected to the instrument in the corebarrel by thin wires so that it is not moved by the ver-tical motion of the ship and drill pipe. Reasonableresults might be obtained with the sensor inside the bit,and the bit resting on the bottom, but vertical ship'smotion then may pump water through the drill bit andthere may be a problem of the bit becoming stuck in"sticky" holes if it rests on the bottom without circula-tion or rotation. Near hole bottom temperaturemeasurements were attempted at Sites 334 and 335.

2) A completed hole, or a hole temporarily suspend-ed for bit replacement, may be logged over its wholedepth. At least 1 day will be required after terminationof drilling for sufficient approach to equilibriumtemperatures and then only temperatures from near thebottom of the hole will be useful. Repeated logging willhelp define the decay curve. A bit change and re-entryinto the hole requires 1 to 2 days, so immediately after are-entry before circulation is commenced is the obvioustime for temperature measurement. The present down-hole temperature recorders are quite suitable for log-ging temperature in the drill pipe but not in the openhole. Fortunately, the thermal mass of the pipe isrelatively small compared to that of the fluid in the holeso the pipe will equilibrate sufficiently with the fluidand the surrounding rock in about 1 hr. Slightly longertimes may be required for the drill collar assembly,which makes up the bottom 80 meters or so of the drillstring. Thus, the drill pipe may simply be lowered to thebottom and a temperature log made inside it. Lowering

348

HEAT FLOW MEASUREMENTS

the temperature recorder in the pipe attached to a corebarrel disturbs the temperatures in the pipe slightly so itis better to hold the instrument for about 2 min at depthintervals, say 20 meters, rather than to move it con-tinuously. Measuring during instrument lowering alsowill give less disturbance than during raising the instru-ment. Temperatures were measured after a bit changeand re-entry in Hole 332B.

3) A string of temperature sensors may be placed inthe hole through the re-entry cone after completion,leaving a recoverable recorder or an acoustic telemeterto transmit the data to the surface. This method wouldgive the best results, but is the most difficult and has ahigh chance of complete failure. It has not yet beenattempted.

Decay of Drilling Disturbance in Glomar ChallengerBoreholes

As described above, it usually will not be possible towait for temperatures to reach equilibrium in GlomarChallenger deep crustal holes. Thus, the measurementsmust be extrapolated to equilibrium using the geometryof the hole, the thermal properties of the rock, anddrilling fluid and the disturbance history. The ex-trapolation is of critical importance to the accuracy ofthe heat-flow result so it is considered in some detailbelow.

Description of the Problem

The process of drilling causes disturbance oftemperature because of the heat exchange between thedrilling water and the rock surrounding the hole. Whendrilling is terminated, if there is no water flow in thehole, the temperature in the hole gradually approachesthe in situ, equilibrium temperature of the rock. Therate of return to equilibrium depends on the durationand intensity of the disturbance, on the diameter of thehole, and on the thermal properties of the rock and ofthe material in the hole. Because boreholes on landfrequently cannot be kept open sufficiently long forequilibrium temperatures to be reached, this problemhas been studied extensively (Bullard, 1947; Lachen-bruch and Brewer, 1959; Cooper and Jones, 1959;Cheremenski, 1960; Jaeger, 1961; Oxburgh et al., 1972).

Temperature measurements in the drill pipe showthat during drilling by Glomar Challenger, the hole ismaintained at a temperature very close to that of thedeep ocean. There is sufficient heat exchange throughthe drill pipe in the seawater column to bring the drill-ing water to the external deep-sea temperature, andflow rates are sufficiently high that this temperature isnot significantly changed in the borehole; Jaeger (1961)shows this theoretically. The heat generated at the bitmay be important in diamond drilling of small holes,but it can be neglected in Glomar Challenger holesbecause of the high drilling water flow rates.

When the drilling fluid circulation ceases, thetemperature disturbance gradually dissipates by radialconduction, the fluid in the hole warming toward theoriginal rock temperature. Two simple idealizations arepossible for the subsequent behavior of the fluid in theborehole: (1) There is sufficient small-scale convection

or turbulence for the water column to have uniformtemperature across its cross-section. The water can beconsidered to be a perfect conductor with known heatcapacity. The slow settling of cuttings in the hole mayfacilitate such local mixing, as will high lateraltemperature gradients; (2) The fluid in the hole is com-pletely stable with no mixing or convection. In thiscase, the fluid in the hole can be considered as a solid ofknown thermal properties that may be different fromthose of the rock. This approximation is appropriate ifthere is any viscous drilling mud or a slurry of settledfine drill cuttings, particularly from the sedimentarypart of the hole. The difference between the two ap-proximations is negligible for "long" disturbance timesof more than several hours, for which most of the heatof disturbance is contained in the rock, but is importantfor "short" disturbance times for which most of the dis-turbance heat is contained in the fluid in the hole. Thelimit of the latter case is the instantaneous emplacementof the fluid of a different temperature to that of therock, the fluid then being allowed to warm undis-turbed.

All of the temperature measurements in the GlomarChallenger holes were sufficiently far from the bottomof the hole for end effects to be neglected. Also, thedrilling rates generally are much faster than the rate ofheat conduction in the rock so cylindrical symmetrycan be assumed. Some error is involved in assuming aconstant hole diameter equal to that of the drill bitbecause the holes probably frequently are enlarged inzones of fractured basalt.

Mathematical Solutions

A) The simplest solution of the drilling disturbanceproblem represents the drilling operation by a linesource of heat in an infinite medium (Bullard, 1947;Lachenbruch and Brewer, 1959; Cheremenski, 1960).This solution neglects the effect of the presence of theborehole filled with fluid of different thermal propertiesto that of the rock, during the decay of the dis-turbance. The magnitude of the line source of heat isnot known, but is taken to be constant over the distur-bance period with intensity equal to that required togive the observed, or estimated temperatures in the holeat the finish of drilling. As discussed above, in GlomarChallenger holes, constant internal hole temperatureduring the drilling is a better approximation than a con-stant heat supply. However, for long times after the dis-turbance period, the form of the decay of the distur-bance is very similar for both approximations, constanthole temperature, or constant disturbance heat. Forthis model the temperature disturbance decays ap-proximately as (e.g., Jaeger, 1965):

T 1Λ (1 + to/0

To \n(4ktja2- 0.577)

where:To is the temperature on the inner surface of the holeat the end of the time of disturbance,to is the duration of the disturbance,

349

R. D. HYNDMAN

/ is the time since the disturbance ceased,k is the diffusivity,a is the radius of the hole.

The thermal properties of the fluid in the hole and thesurrounding rock are assumed to be the same and it isassumed that temperatures are measured at the surfaceof the hole, i.e., radius a. No mixing of the fluid in thehole is implied (see comments above).

ß) The minimum disturbance decay time for the drillhole temperatures is given by the instantaneousemplacement of the cold, deep-sea temperature, drillingfluid into the hole with no circulation. Bullard (1954)discussed the theory involved, in connection with thedissipation of the frictional heating of ocean bottomsediment gradient probes, and Cooper and Jones (1959)use the theory for the extrapolation of the temperaturesmeasured near the bottom of boreholes where the dis-turbance times were very short. The same theory is ap-propriate for the extrapolation of transient tempera-tures measured with the Glomar Challenger sedimenttemperature probes (e.g., Erickson et al., 1975). Thesolution assumes that the fluid in the borehole is aperfect conductor, i.e., well-mixed laterally (see com-ments above). The temperature of the fluid after a timet is given by (Carslaw and Jaeger, 1959):

— 4 fT 9 J

du

0u u

where:To is the temperature of the fluid at emplacement

rock

uidi.e. twice the ratio of rock to fluid

heat capacity

u = [uJo («) -oc/j («)] 2 +[uYo (II) -cc Yx (M

and/o,

are Bessel functions.

This approach permits the incorporation of differentheat capacities for the fluid and the surrounding rock,but is valid only for short disturbance times, times forwhich at the termination of the disturbance, the dis-turbance heat in the rock is much less than in the fluidin the hole. In the Glomar Challenger holes, this theoryis a good approximation for disturbance times up toabout 30 min. Von Herzen et al. (1971) discuss thismodel for Glomar Challenger holes drilled rapidly intosoft sediments. Carslaw and Jaeger (1959) provide plotsgiving the decay of the disturbance.

C) The most complete theory for the decay of drill-ing disturbance has been given by Jaeger (1956, 1961).It is valid both for short and for long drilling distur-bance times. Its only serious failure is that it assumesthe thermal properties of the fluid to be the same asthose of the rock during the decay of the disturbance,however, the theory probably could be extended to in-clude different thermal properties. Thus, the theorymay be a poorer approximation than the instantaneous

emplacement model above for very short disturbancetimes. The theory assumes that the hole is held at con-stant temperature To for a time t0, the drilling dis-turbance time, then allowed to warm undisturbed. Thefluid in the hole is assumed to be stable, i.e., has finitethermal conductivity (see comments above). The tem-perature at a time t after a disturbance of duration *o atthe center of the hole r=O, is (Jaeger, 1956):

l-R2/4nro\f(R)dR

o l

where

_TCQ(u,Ru)du

0

and C0(u, Ru) = J0(ú) Yo (Ru) - Y0(u) J0(Ru) are Besselfunctions

kt r tT = — R=— n = —

a2 a fo

Physically, this solution represents the maintenance ofthe cylindrical surface of the hole at constanttemperature for a time t0, producing a radialtemperature distribution f(R), then with the hole filledwith material all initially at temperature t0, allowing thedisturbance to decay. Jaeger (1961) has provided plotsof TjTo as a function of n, the number of disturbancetimes since the end of the disturbance, for variousvalues of r0 = ktja

2. The first term in the expressionfor T corresponds to the effect of the heat stored in thefluid in the hole. It can be neglected for long times sincethe disturbance ceased, e.g., nro>25. The second termcorresponds to the effect of the heat stored in the rock.It can be neglected for very short times, e.g., «ro

HEAT FLOW MEASUREMENTS

before the latter measurement. The regular sedimenttemperature instrumentation was used to log tem-peratures in the drill pipe. Temperature first was re-corded for 15 min with the sensor extending throughthe bit and the bit 10 meters above the hole bottom.Then recordings were made inside the drill pipe for 2min each, at 20-meter intervals from the bottom(Figure 1).

Hole 332B Temperature Log in Drill Pipe(in basalt)

Depth 541 m. sub-bottom(approx. IOm. off holebottom) I4.I5°C

I Iε £o

r~i n

72

Bottom Water 3.04C

20 36 44 52Time (min.)

68

Figure 1. Records of temperature versus time for basementparts of holes. The instrument was kept stationary for 2min at each depth. The interpreted temperatures also aregiven.

The temperatures in the drill pipe form a nearlylinear gradient, but one that is considerably greaterthan the in situ gradient. The drilling disturbance timeand thus temperature disturbance increase roughlylinearly up the hole. Thus, these temperatures may beextrapolated to the bottom of the hole, giving an ap-proximate temperature of 14.6°C. The theory of Jaeger(1956, 1961) (see above section for discussion) has beenused to extrapolate the measured temperatures to insitu equilibrium values. The disturbances remaining foreach measurement were taken from the plot of Jaeger(1961), which gives limited but sufficient accuracy. Theparameters used in the computation and the results aresummarized in Tables 1 and 2. The temperature duringthe disturbance period was taken to be that of the deepocean water. The duration of the disturbance and thetime since the disturbance were taken from the drillinglog. The duration of the disturbance was taken to equalthe time interval between when the hole reached themeasurement depth and the termination of drilling,minus the times during the interval, when there was nocirculation e.g., the period when core was beingbrought to the surface. Again, this is a rough ap-proximation sufficient for the data, that could be im-proved. The results are not greatly different if the non-

circulation times are not subtracted. The diffusivity ofthe rock was taken to be 6.0 × 10~7 m2 sec"1. Thetheory requires that the diffusity of the material thatfills the hole be the same as surrounding rock. For-tunately, the combination of low diffusivity drillingwater and cuttings and high diffusivity drill pipeprobably gives an effective diffusivity not too differentfrom the value taken for the rock. The thermal effect ofthe hole also becomes negligible several hours after thedisturbance. The diameter of the hole was taken to be40 cm. The drill bit has a diameter of about 25 cm butthis hole probably is significantly larger, because of thefractured porous formation, particularly in rubblezones. The hole diameter is a major uncertainty in thecomputation, and variations in hole diameter may bethe major source of the scatter in computed equilibriumtemperatures. Repeated temperature measurementswould permit removal of most of this uncertainty. Theestimates of accuracy are subjective. The computedequilibrium temperatures extrapolate well to the es-timated temperature at the sediment-basalt contactfrom the sediment temperature measurements. Theresulting gradient in the basement and an estimatederror are 18.3 ±0.7. The temperature at the bottom ofthe hole at 551 meters is 14.9°C, very close to the14.6°C estimated from the uncorrected temperatures.

At Site 334, temperatures were measured when thehole depth was 348 meters during a brief stop in drill-ing. The first measurement was to 250 meters depth inthe hole, where the core barrel containing the instru-ment jammed in the pipe, 1 hr after drilling circulationhad stopped and the second at 20-meter intervals to 342meters depth, 3 hr after circulation (Figure 1). Themeasurement on the first lowering was 100 metersabove the bottom and still in sediment. It showed onlya very small decay of the drilling disturbance, thetemperature being about 0.3°C above the ocean bottomwater temperature, or less than a 5% decay of the drill-ing disturbance. The second lowering reached to 5meters from the hole bottom. The temperatures arealmost identical to those of the first lowering at 250meters depth, but the return to equilibrium increasesrapidly toward the bottom of the hole. The more rapiddecay of the disturbance below 250 meters comes main-ly from the shorter disturbance time and partly becauseof the higher diffusivity of the basement rocks com-pared to the sediments. At 5 meters from the bottom,about 30% of the disturbance had decayed. Thisamount of disturbance decay, with only one measure-ment, permits only a very rough estimation ofequilibrium temperatures, and that only for near thebottom of the hole. The extrapolation was carried outas for Hole 332B except that a hole diameter of 35rather than 40 cm was used since the rocks are less frac-tured. The resulting temperatures agree well with thegradient defined in the sediment, but the accuracy is notsufficient to estimate a gradient in the basement rocksalone.

The results from this hole show that useful hole bot-tom temperatures can be obtained after only 2 or 3 hrfrom last circulation if temperatures are recorded con-tinuously or at 2 or 3 times during that period, and if

351

R. D. HYNDMAN

TABLE 1Histories of Borehold Drilling Disturbance for Holes 332B and Site 334 Prior to Basement Temperature Measurements

Depth(m)

Time BitReached Depth

Hole 332B

401421441461481501521541

Site 334

302332342

16:00 June 2622:00 June 2610:00 June 2817:30 June 2801:00 June 2905:30 June 2910:00 June 2915:00 June 29

08:15 July 1611:45 July 1617:00 July 16

Time of Endof Circulation

16:00 June 29

18:00 July 16

MeasurementTime

14:10 July 214:1014:1014:0514:0514:0014:0013:55

21:35 July 1621:3521:30

Disturbance.Interval

(h)

726630.022.615.010.56.01.0

9.76.21.0

NonCirc.Time(h)

4140

654320

2.51.50

DisturbanceDuration (f_)

(h)

31262417.511.0

7.54.01.0

7.24.71.0

Time SinceDisturbance

(h)

7070707070707070

3.63.63.6

Ktoa2

1.671.401.300.950.590.410.220.054

0.510.350.071

n

2.262.692.924.006.369.33

17.570.0

0.500.773.50

Fraction ofDisturbanceRemaining

0.290.270.250.240.180.140.100.06

0.870.780.64

TABLE 2Correction to Basement Borehole Temperature for Drilling Disturbance, Hole 332B and Site 334

Depth inhole (m)

Duration ofdisturbance (h)

Time since endof disturbance (h)

Disturbancedecay (%)

Disturbancetemperature

Measuredtemperature (°C)

Computedequilibriumtemperature (°C)

Estimatedaccuracy

401

33

70

71

4.45

9.65

11.8

0.5

421

26

70

73

4.45

•10.20

13.2

0.5

441

24

67

76

4.45

10.75

12.9

0.4

Hole 3 3 2B

461

17.5

67

76

4.45

11.37

13.6

0.4

481

11.0

67

82

4.45

12.15

13.8

0.3

501

7.5

67

86

4.45

12.80

14.2

0.3

521

4.0

67

90

4.45

13.25

14.2

0.2

541

1.0

67

94

4.45

14.15

14.8

0.2

302

7.0

3.0

12.5

3.0

4.5

15.0

4.0

Site 334

332

5.0

3.0

21.5

3.0

5.75

15.8

3.0

342

1.0

3.0

36

3.0

7.55

15.6

2.0

Note: Diffusivity (k), 0.006 cm2 sec"1; hole diameter (2a); 332B = 40 cm; 334 = 35 cm.

the sensor is held as close as possible to the bottom.Ideally, the hole should be cleared thoroughly whendrilling is stopped, then temperatures measured withthe drill bit very near or on the bottom and the sensorextending through or just inside the bit.

At Site 335, three successive temperature logs at 20meters depth intervals were measured in an attempt todefine the decay of the drilling disturbance after shorttimes. They were approximately 1, 2, and 3 hr afterdrilling circulation was terminated. In each case thecomplete hole, even in the sediments, was at virtuallyconstant temperature, slightly above, but within 0.5°Cof the temperature of the deep ocean water (Figure 1).In fact there was a small but significant decrease intemperature with time, the successive measurementseach more closely approaching the bottom ocean watertemperature. The only way that this surprising result

can be explained is by ocean water flowing down thehole and escaping into a permeable horizon at the bot-tom. This explanation is supported by the drillingrecord, which indicates a very soft easily drilled zonenear the bottom of the hole. At the very bottom of thehole, drilling was terminated in a very hard formationfrom which no core was recovered. It is difficult toguess what would drive such a downward flow, even ifthe water is able to escape at the bottom of the hole.The flow rates must be quite high, probably at least 1m/min downhole, to maintain constant temperature.One interesting possibility is that a pressure deficiencyis produced by a thermally driven hydrothermal up-welling in the surrounding topographic highs (e.g.,Lister, 1972). The recharge then would be in the valleyswherever the water flow is not blocked by lowpermeability sediments.

352

HEAT FLOW MEASUREMENTS

Thermal Conductivity of Basement Samples

The thermal conductivity of 21 basement samples, 19basalts, 1 gabbro, and 1 serpentinite, were measured us-ing a divided bar apparatus using solid brass bars withconstant temperature ends (e.g., Beck, 1957). SeeJolivet (1966), for a detailed description of the instru-ment used. Calibration was with quartz and fused silicausing the values of Ratcliffe (1959). The mean sampletemperature was 21°C, slightly higher than the in situtemperatures.

Three discs 1, 3, and 6 mm thick were measured fromeach sample to compute the contact resistance and toprovide a better average conductivity. A total of 63 discsthus was measured. The faces of the discs were silver-plated and polished and a pressure of 75 kg cm~2 ap-plied to minimize the contact resistance between thebrass bars and the samples. A correction was appliedfor a difference between the diameter of the dividedbars and the discs (Jaeger and Beck, 1955). The samplesall were water saturated before measurement. Correc-tions to in situ temperature and pressure should benegligible. The accuracy of the measurements is betterthan ±5%. The values are given in Table 3.

The mean thermal conductivity of 19 basalt samples,comprising 57 discs, from the holes, mainly 332B, is

3.97 ±0.04 (1.66 ±0.02) with no obvious systematicvariation with depth (see Hole 332B), or between holes.This compares with the mean of 3.70 (1.55) for sixbasalt samples from a DSDP Leg 26 hole into 100-m.y.old sea floor in the eastern Indian Ocean (Hyndman etal., 1974a), a mean of 3.9 (1.63) for 16 basalts from veryshallow drill holes, and a mean of 4.2 (1.76) for 11dredged basalts all from the Mid-Atlantic Ridge near45°N (Hyndman and Jessop, 1971). The Leg 26 rockswere subjected to surface weathering and the conduc-tivity increases with depth. An average value of 4.0(1.67) is thus reasonable for young sea floor layer 2basalts. It should be pointed out, however, that onlysolid basalts are recovered by DSDP drilling. Theremainder of the upper layer 2 section frequently hasvery extensive voids, rubble, and some sediment. Theamount of solid basalt, such as that measured, is es-timated to average 50% in the upper 500 meters of Hole332B (see Hyndman, this volume). The remainder willhave a conductivity of about 3.0 (1.3), e.g., a mix ofwater, 1.5 (0.63); consolidated carbonate sediment, 3.0(1.3); and basalt, 4.0 (1.7). Thus a reasonable estimatefor the mean conductivity and error of the upper 500meters of Hole 332B is 3.5 ±0.3 (1.5 ±0.1) approaching4.0 (1.7) at greater depths. In regions of the sea floorwhere layer 2a is not present, e.g., Site 335, the mean

TABLE 3Thermal Conductivity of Basement Rocks

Sample(Interval in cm)

Hole 332A21-1, 131-13422-1, 37-4029-1, 74-7733-2, 92-9537-1, 116-119Mean

Hole 332B

2-2, 86-892-6, 129-13234, 10-139-2, 104-10614-2, 81-8325-4,47-4926-2, 7-1028-2, 23-2633-1,62-6536-5,40-4243-2, 94-96Mean

Site 334

21-1,39-4126-2, 19-22

Site 335

5-2, 36-389-2, 74-7610-5, 101-103Mean

DepthInto

Basement(m)

143.8152.4219.2258.9295.7

11.918.362.6

192.6249.3356.5362.6381.7426.9462.4525.0

50.999.7

1.033.048.5

Mean all 19 basalts

Rock Type

BasaltBasaltBasaltBasaltBasalt

BasaltBasaltBasaltBasaltBasaltBasaltBasaltBasaltBasaltBasaltBasalt

GabbroSerpentinite

BasaltBasaltBasalt

Thermal Conductivitymeal cm~^ec~^C"1 ( W m ^ r 1 )

4.04 (1.69)3.92(1.64)4.04 (1.69)4.11 (1.72)3.96(1.66)

4.01 ±0.03 (1.68 ±0.01)

4.04 (1.69)3.99(1.67)4.42(1.85)3.96 (1.66)3.70 (1.55)3.99 (1.67)4.06 (1.70)3.92(1.64)4.08(1.71)4.01 (1.68)3.58(1.50)

3.98 ±0.06 (1.67 ±0.03)

4.71 (2.39)7.61 (3.19)

4.06 (1.70)3.65 (1.53)3.94 (1.65)

3.88 ±0.12 (1.63 ±0.05)

3.97 ±0.04 (1.66 ±0.02)

353

R. D. HYNDMAN

conductivity should be close to 4.0 (1.7), throughoutbasaltic layer 2. One gabbro from Site 334 has a con-ductivity of 5.7 (2.4). This is somewhat higher thansome previously measured gabbros, e.g., 4.60 (1.9)(Birch and Clarke, 1940). One serpentinized peridotitehas a conductivity of 7.6 (3.2). The conductivity of thelatter is higher than most serpentinites, e.g., twodredged samples from 45°N have a mean 4.3 (1.8)(Hyndman and Jessop, 1971), reflecting the low degreeof serpentinization of the Site 334 sample.

MEASUREMENTS IN SEDIMENTSSuccessful heat-flow measurements have been made

in over 20 deep sediment holes on previous Deep SeaDrilling Project legs, arid the technique has beendescribed in detail (Erickson, 1974; Erickson et al.,1975). Temperature-depth profiles are obtained byforcing a sensor that is attached to and extends below thedrill bit into the undisturbed sediments below the bot-tom of the hole, at intervals during drilling. Thetemperature of the probe thermistor is recorded in aself-contained instrument in the core barrel. Forcingthe probe into the sediment produces frictional heatingwhich gradually decays (e.g., Figures 2-5). Thetemperatures can be extrapolated to equilibrium usingthe theory outlined by Bullard (1954) or Jaeger (1956).If the probe is undisturbed for at least several minutes,the "long time" asymptote may be used. The distur-bance then decays with I/time (Figure 6). Thetemperature resolution and accuracy of relativetemperatures are about 0.02°C, and the absolutetemperatures are accurate to 0.1 °C. Temperatures canbe obtained only at intermediate depths in the holes. Atshallow depths the sediments frequently are too soft tosupport the drill bit assembly so the sensor cannot bekept stationary in the sediments. At great depths thesediments are too consolidated to permit penetration ofthe sensor probe. A maximum of four temperaturepoints have been obtained in any one previous DSDPhole.

On Leg 37, all measurements in the sediments weremade with the temperature sensor locked into the corebarrel. Temperatures were obtained in all five of theholes drilled. The temperature-time records are given inFigures 2-5, the extrapolation to equilibrium in Figure6 and the temperature-depth profiles in Figures 7-10and Table 4.

The thermal conductivity of the sediments penetratedby the holes is measured in the laboratory onboard shipusing the transient needle probe technique (Von Herzenand Maxwell, 1959) on the cores recovered. The coresare allowed to equilibrate at laboratory temperature forat least 4 hr before measurement. Any residualtemperature drift is assumed to be linear over themeasurement period of 5 min and is removed by least-square analysis. The results are corrected to in situtemperature and pressure using the factors given byRatcliffe (1960) (see also Erickson, 1974). The accuracyof measurement is about 5%. The measured thermalconductivities generally increase systematically withdepth in the holes because of compaction. Carbonate

I 6β 4

3 3 2 A

83 m

/'Lowered I. m.

Equilibrium 6.74 + 0.04βC *

Bottom Water 4.5O±O.IOβC

20 24 28 32 36Time (min.)

4 0 44 48

10

8

6

4

2

£ 0

332B

69 m

•

i

/'Lower

^ j ^ / Rough estimate of equilib.j F ~ ^ Bit on Bottom 6.0t0.20°C

^Bottom Water 4.451O.IO°C

[̂Stop at mud linei i i i i

temp. \

ε ~1oo

CD

o

CO

0 109a.

1 8

6

4

16 2 0

94.5m

24 28 32 36 40 4 4

N

•Equilibrium 6.52±O.O3βC

•Bit on Bottom

Bottom Water 4.45lO.IO°C

^Held at mud line

20 24 28 32Time (min.)

36 40 44

Figure 2. Records of sediment temperature versus time, Site332.

10

_ 8o

a. 4

2|-

-

156 m

-

-

U-ΦT^.\

TD

J-

t lo

w

CD

Bottom

^Equilibrium 7 . 4 5 * 0 4 0 °C

f" '" "" |vCorebarrel locked in

and bitWater 4.5*0.05°C

lowered

1—I1\

•*

20 24 28 32 36Time (min.)

40 44 48

Figure 3. Records of sediment temperature versus time, Site333.

sediments have significantly higher conductivity thanterrigenous sediments, with average conductivity 2.65+0.002 per meter of hole depth (1.11 +0.0008) (e.g.,Hyndman et al., 1974a). The values measured on Leg37 are in general agreement with this relation. Themeasured thermal conductivities are given in Figure 11and in Table 5.

354

HEAT FLOW MEASUREMENTS

139m

• « -

^

; ^Equilibrium 8.92iO.O2βC

Penetration

"Bottom Water 3.I2±O.O5°C

1,0

20 24

( 7 7 m

28 32 36 40 4 4 48

? ™ p l0 4 < > c

Bit on bottom

^Bottom Water 3,l2tO.O5°C

20 24 28 32 36 4 0 44 48Time (min.)

Figure 4. Records of sediment temperature versus time, Site334.

, . A Equilibrium 4.59i0.02°C

» -̂On bottom

"^Bottom water 2.60i0.05 β C

8

6

D

. 23

So

20 24

229.5m.

- .-

28 32

^Stop

3 6 4 0 4 4 48

f Equilibrium 6.60i0.20 β C

^Bit on bottom

^Bottom water 2.60i0.05°C

at mudline

12

10

8

6

4

2

0

16 20

324.5 m.

-

-

- _ . _

-.=•σ

σ

1

2 4

<

/••

Bottom

2 8

I

;

water

32

Equilibrium

2.6010.05

3 6

789tO.O2'

4 0 4 4

24 28 32 36 40 44 48Time (min)

Figure 5. Records of sediment temperature versus time, Site335.

LEG 37 HEAT-FLOW RESULTS

Hole 332A

One good sediment temperature of 6.74 ±0.04°C wasmeasured at 83 meters depth in Hole 332A (Figure 2).The ocean bottom water temperature also was well

0.01 0.02 0.03I/time (recorder cycles)

0.04

Figure 6. Extrapolation to equilibrium of recorded sedi-ment temperatures.

Temperature (°C)6 8 10 12

200

-£300

ao400

500

600

Conductivity

(meal cm"'sec"' "C"1)

20 30 4.0

0.70*0.04^332 A Cond. Sed. 2.60

Avg. H. F. 0.63

Top of Basalt

Gradient 18.3Cond. Basalt Section

βstim. 3.5H.F. 0.64+0.06

Temp. Log inside *"-.drill pipe after re-entry

Corr. Temps.

Bottom of Hole at time,,of measurement'

-

-

11

j

i

>

Figure 7. Temperature and conductivity with depth at Site332. (Units: gradient, °C km~*; conductivity, mcalcm~^sec~* °C•; heat flux, µcal cm~^ s~* ).

recorded at 4.50 ±0.05°C giving a gradient of 27.0 ±0.8(Figure 7). The harmonic mean and standard error ofthe mean for 15 conductivity measurements at Hole332A is 2.57 ±0.04 (1.08 ±0.02). The heat flow and es-timated uncertainty are then 0.69 ±0.03 (29 ±1).

Hole 332B

Two sediment temperatures were measured in Hole332B (Figure 2) one at 69 meters giving a rough es-timate of 6.0 ±0.2°C and the second at 94.5 meters giv-ing a well-determined value of 6.52 ±0.03. The bottomwater temperature is 4.50 ±0.05°C. The three values

355

R. D. HYNDMAN

Temperature ( C)

4 ^ 6

300 L

Figure 8. Temperature with depth at Site 333.

0 2 4 6 8 10 12 14 16 18

^QQ[_ Plutonic Complex

Figure 9. Temperature with depth at Site 334.

Temperature (°C)0 2 4 6 8

100 _

2 0 0 .

10

300 _a.a>a

1 '

_

-

-

Sediment

/ / / ii/Htntrttf

Basalt

i\111

1i

111

1111111111

\

\

\

Temperaturelogging

I i

\

\

from

400-

500 _

600LFigure 10. Temperature with depth at Site 335.

form a very linear gradient of 21.5 ±0.3 (Figure 7). Thethermal conductivity estimate of 2.57 ±0.04 (1.08±0.02) gives a heat flux of 0.55 ±0.02 (23 ±1).

Reliable temperatures were obtained in the basementpart of the hole between 401 and 541 meters depth (seesection above) (Figure 1). This depth range is too smallfor an accurate gradient estimate but least-squaresvalue of 20.6 ± 1.8 agrees well with the more precisevalue of 18.3 ±0.7 obtained also using the extrapolatedtemperature at the sediment-basalt interface of 7.56°Cat 142 meters from the sediment measurements (Figure7). The previously estimated basement (see above) con-ductivity of 3.5 ±0.3 (1.5 ±0.1) gives a heat flux anderror estimate for the basalt part of the hole of 0.64±0.06 (27 ±3). This value is within the error limits ofless than ±10% of the heat fluxes for the sedimentarypart of this hole and of the adjacent Hole 332A. It is ac-tually equal to the mean of the heat-flows in sedimentsfor the two holes. Thus, there is a uniform heat fluxwithin ±10% to a depth of 541 meters, 400 meters intobasement, at this site.

Hole 333AOne temperature measurement was made in the sedi-

ment at 156 meters depth in Hole 333A (Figure 3). Therecord is very disturbed probably because there wassome difficulty in getting the core barrel and instrumentlocked into the bit. We have taken the maximumtemperature reached several times on the later part ofthe record of 7.45 ±0.40°C as the best estimate at 156meters and 4.50 ±0.05°C as the bottom watertemperature giving a gradient of 18.9 ±3.0 (Figure 8).The mean conductivity is 2.90 ±0.04 (1.21 ±0.01) fornine measurements. The heat flux is then 0.55 ±0.09 (23±4).

Site 334Two sediment temperatures were attempted at Site

334 (Figure 4), one at 139 meters giving a reliable valueof 8.92 ±0.02°C and a second at 177 meters that ap-parently did not penetrate undisturbed sediment. Thegood value with a bottom-water temperature estimatefrom two lowerings of the instrument in the drill pipe of3.12 ±0.05°C gives a gradient 41.7 ±0.4 (Figure 9). Thethermal conductivity mean for 28 measurements is 2.60±0.06 (1.08 ±0.03). The heat flux is then 1.08 ±0.04 (45±2).

The temperatures measured in the basement part ofthe hole (Figure 1) are discussed above. Very ap-proximate equilibrium temperatures, to ±2°C, wereobtained for the bottom part of the hole. They are ingood agreement with the gradient defined in the sedi-ment, but are of too low accuracy and are over toosmall a depth range to define an independent gradientin the basement part of the hole. The mean conductivityfor the upper 60 meters basalt portion of basement isestimated to be about 4.0 (1.7) and for the underlyingplutonic complex about 6.0 (2.5). Thus, the illustratedgradients of 29 and 19 in Figure 9.

356

HEAT FLOW MEASUREMENTS

Site

332A

332B

333

334

335

LocationN

36°52.7'

36°52.8'

36°50.5'

37°02.1'

37° 17.7'

33C

33C

33C

34C

35C

W

'38

'38

'40

'24

'11

.5'

.6'

.1'

.9'

.9'

CrustalAge (m.y.)

3.5

3.5

3.5

8.9

- 1 3

TABLE 4Summary of Leg 37 Heat-Flow Data

WaterDepth (m)

1818

1806

1666

2632

3198

SubbottomDepth (m)

0830

6994.5

401-541

0156

0139177

302-342

095

229.5324

Temperature

4.50 ±0.056.74 ±0.04

4.50 ±0.056.00 ±0.206.52 ±0.03

(Basement)

4.50 ±0.057.45 ±0.40

3.12 ±0.058.92 ±0.02

>7.5(Basement)

2.60 ±0.054.59 ±0.026.60 ±0.207.89 +0.02

Gradient(°Ckm-l)

27.0 ±0.8

21.5 ±0.3

18.3 ±0.7

18.9 ±3.0

41.7 ±0.4

-25-25

16.1 ±0.01

Conductivitymeal cm"l sec "1 °C"1

(W m-1 K"1)

2.57 ± 0.04(1.08 ± 0.02)

2.57 ± 0.04(1.08 ± 0.02)

3.5 ± 0.3(1.5 ± 0.1)

2.90 ± 0.04(1.21 ±0.01)

2.60 ± 0.06(1.08 ± 0.04)

-5.0(2.1)

3.03 ± 0.08(1.26 ± 0.03)

Heat Fluxµcal cm~2 sec"!

(mW m-2)

0.69 ± 0.03(29 ± 1)

0.55 ± 0.02(23 ± 1)

0.64 ± 0.06(27 ± 3)

0.55 ± 0.09(23 ± 4)

1.08 ± 0.04(45 ± 2)

(50)

0.49 ± 0.02(21 ± 1)

Site 335

Three sediment temperature measurements wereattempted at Site 335. (Figure 5). The results are: 4.59±0.02 at 95 meters; 6.60 ±0.20 at 229.5 meters; 7.89±0.02 at 324 meters depth. The record at 229.5 metershas a warming toward equilibrium so the temperatureis uncertain. The two other values are well determined(Figure 10). The bottom water temperature was wellrecorded in the drill pipe at 2.60 ±0.05. Thetemperatures define a gradient that increases smoothlywith depth. The least-squares gradient is 16.1 ±0.1.Taking the mean conductivity of 3.03 ±0.08 (1.26±0.03) for eight measurements gives a heat flow of 0.49±0.02 (21 ±1). Constant heat flux with depth impliesthat conductivity increases at a rate of +0.003 (0.0013)per meter depth, which is not apparent in the measuredvalues. The rate also is slightly more rapid than the0.002 (0.008) suggested by Hyndman et al. (1974).

As described above, temperatures were measured inthe basement part of the hole three successive timesafter the completion of drilling (Figure 1). All recordednearly constant temperature in the hole which is inter-preted as resulting from seawater flowing down thehole into a permeable horizon at the bottom.Equilibrium temperatures in the rock cannot bepredicted.

The heat flow results from Leg 37 are shown withnearby shallow oceanographic probe values in Figure12.

DISCUSSIONThe most important geothermal result of DSDP Leg

37 is the constancy and low value of heat flow 0.6 ±0.1(25 ±4) for three holes across one sediment pond on3.5-m.y. old sea floor and to a depth of 400 meters intothe basaltic basement beneath the pond. The results forHole 332B represent the first heat-flow measurement

into normal oceanic basement, layer 2. The data for thepond are summarized in Figure 13. Also shown arethree measurements of heat flux in the pond using astandard 2.5-meter long oceanographic probe (Lewis,1975; Table 6), which gave almost the same heat flow asthe drill holes. Thus, our data show that the heat flowmeasured at the surface represents not only that deep inthe sediment, but also that at least 400 meters into theunderlying basement. This pond is only one example,but it is very near a spreading ridge crest where thereare very large variations in the values measured by thestandard oceanographic probe (e.g., Williams, 1974)and where also all measured values are much less thanthose predicted by theoretical models.

To show that the low flow into the sediment pondcannot arise through thermal refraction by low conduc-tivity sediments or by the effect of topography, the ther-mal field of the region has been computed using a two-dimensional finite difference solution for steady-stateheat conduction (Figure 13). The deviation from anassumed regional flux of 0.7 (29) is shown for the sur-face, e.g., oceanographic probe, and at 500 metersdepth, e.g., deep borehole. The variation in flux issmall, less than 0.1 (4) except at the surface near thesteep west side of the pond. The simple approximationof a hemi-ellipsoidal body of sediment (Von Herzenand Uyeda, 1963) indicates that the flux in the pondmay be less than the regional value, but the reduction isnot more than 10%. The numerical solution shows thatthe effect of the topography is of similar magnitude andof opposite sign.

The heat flow of 0.6 (25) is very low compared to theworld average of 1.5 (63) and especially compared tothe values of about 6.4 (269) predicted by theoreticalmodels for 3.5-m.y. old ocean floor. However, as dis-cussed in the introduction, such low values are verycommon near ridge crests and are ascribed to heattransport by hydrothermal circulation. Importantquestions now are: over what time period or distance

357

R. D. HYNDMAN

Conductivity (mcαl cm-2.0 2.4 2,8 3.2 3.6

50

.100

Q

I 1 1 1 1

Hyndmαn, Erickson

and Von Herzen, 1974

(carbonates)

150

200

250

3 0 0

Figure 11. Thermal conductivity of Leg 37 sediments.

from the axis of spreading does the circulation persist,and to what depth does it extend. Lister (1971, 1972),Davis and Lister (1974) and Williams et al. (1974)suggested that circulation and cooling may extend tothe base of the crust although it seemed reasonable toexpect most of the circulation to be concentrated in thefractured and porous upper part of layer 2. Water cir-culation probably is excluded from the mantle by theexpansion associated with serpentinization that will filland close any cracks and fissures. However, Hart(1973) suggests that the low temperature alterationproducts in the crust will have high permeability andwill facilitate further circulation.

Having at the same time such a low and such a uni-form heat flow is a puzzle. The low values require cir-culation to have continued at least to within 0.5 m.y. ofthe present since the thermal time constant of the crustis only about 0.5 m.y. But the nearly constant heat fluxfrom six measurements over a horizontal distance of 3km and to a depth of 540 meters requires that, eitherthe circulation stopped a long time ago so that small-scale temperature variations have been smoothed outor that the circulation has a very deep and regular

character with large horizontal scale. We suggest thatthe circulation is concentrated in the lower part of thecrust, perhaps in layer 3, but at least in the lower part oflayer 2. This possibility, contrary to first expectation,can be explained readily. The upper part of layer 2primarily consists of many thin successive flows, eachone cooled before the subsequent one arrives. Fracturesand cooling cracks in a flow tend to be sealed off by thefollowing flows. In contrast, the deeper crust is pro-duced in a region that is relatively hot and that coolsslowly. Seawater penetrating to this hot region throughfractures some time after its formation will producerapid cooling and cracking from thermal contraction.The process of a cracking front in such a hot regionpenetrated by seawater has been discussed in detail byLister (1974). The schematic diagram, using the ob-served bathymetry, of Figure 14 illustrates the pro-posed type of water flow in this region. The flow direc-tions undoubtedly are highly variable but there may bea systematic pattern of sources to the west and dis-charge to the east toward the axis of accretion wheretemperatures and elevations are higher. It is importantto note that for the observed heat flow the crust must bemaintained by water flow at very low temperatures,e.g., 15°C at 1 km and as low as 75°C at 5 km depth.

Lister (1972), Williams et al. (1974), and Williams(1974) showed that high heat flows on ridges generallyoccur on topographic highs and frequently but notalways near steep scarps. However, for the GalapagosRidge the horizontal wavelength between heat-flowhighs or lows is about 6 km and does not seem to becontrolled by the topography which has shortercharacteristic wavelengths. The regular heat-flow pat-tern found by Williams et al. (1974) on the Galapagosspreading center suggests a penetration of circulation to3 to 4 km depth if the permeability is uniform with onlythe detailed location of the upwelling and downwellingbeing controlled by topography and perhaps fractures.

Bodvarsson and Lowell (1972) however suggest,based on studies in Iceland, that the permeability ofridge crest crustal rocks is highly variable. The horizon-tal permeability in the upper crust is furnished mainlyby tubular and irregular openings at the contacts of thelava flows, while the vertical permeability is providedprimarily by open spaces between and within dikes, andby deep vertical fractures. Extensive fractures must oc-cur in the crust from thermal contraction as thelithosphere cools moving away from the spreadingcenter. They show that low heat flows such as weobserved can be produced by a horizontal crack orlayer only a few millimeters thick connected to the sur-face by vertical fractures.

The heat flow of 1.08 (45) at Site 334 is higher thanthe values near the ridge crest but still below thetheoretical value of about 4.0 (167) for 9-m.y. old oceanfloor. Thus, hydrothermal circulation must have beenimportant in this region although it may not be still ac-tive at present.

The heat flow of 0.49 (21) measured at Site 335 also ismuch below the theoretical value for 11 to 13 m.y. oldocean floor. Hydrothermal circulation must be still ac-tive in the region of this hole. The flow of seawaterdown the hole into a horizon 100 meters into the base-

358

HEAT FLOW MEASUREMENTS

TABLE 5Theπnal Conductivity of Leg 37 Sediments

Sample(Interval in cm)

Hole 332A

1-1, 1151-2, 882-2, 852-3, 852-4,843-2, 753-3, 753-4,754-3, 754-4, 754-5, 755-2, 755-3, 755-4, 756-2, 75

Hole 332B

1-1, 752-1,754-1, 75

SubbottomDepth (m)

8.19.4

66.467.869.375.777.278.786.788.289.794.796.297.7

104.2

142.7152.2228.2

Thermal Conductivity

Measured

2.542.742.882.792.842.592.622.972.432.832.632.522.582.682.64

2.652.803.05

(meal cm"1 sec ^C~ 1)

Corrected

2.412.592.742.662.532.472.492.832.322.702.512.402.462.562.52

2.532.682.93

Harmonic mean = 2.57 ±0.04

Site 333

2-2, 752-6, 753-2, 753-5, 754-3, 754-5, 755-4, 756-2, 957-2, 75

Harmonic mean =

Site 334

2-1, 752-2, 752-3, 752-4,752-5, 752-6, 755-2, 755-3, 755-4, 756-2, 757-4,757-5, 757-6, 758- , 758-2, 759-2, 759-3, 759-4, 7510-3, 7511-1,7511-2,7511-3,75114,75124,7513-2,75134,7513-6, 7514-1, 100

147.7153.7166.7171.2187.2190.2198.2204.9214.2

2.90 ±0.04

130.2131.7133.2134.7136.2137.7160.2161.7163.2169.7182.2183.7185.2187.2188.7198.2199.7201.2209.2215.7217.2218.7220.2229.7236.2239.2242.2244.5

2.913.052.893.022.892.983.213.213.16

3.232.952.763.062.752.782.952.392.192.272.732.322.632.042.302.162.502.622.992.733.053.012.732.692.593.192.913.40

2.782.912.762.892.772.853.073.083.03

3.112.842.662.952.652.682.852.312.122.192.642.252.551.982.232.092.432.542.902.652.962.932.652.622.523.112.843.32

Harmonic mean = 2.60 ±0.06

359

R. D. HYNDMAN

TABLE 5 - Continued

Sample(Interval in cm)

SubbottomDepth (m)

Thermal Conductivity

Measured

Site 335

1-1, 1001-2, 751-3, 751-4, 753-1,303-1, 1204-3, 304-3, 120

Harmonic mean =

88.089.290.792.9

220.3221.2318.7319.2

3.03 ±0.08

2.973.023.292.893.573.252.993.41

(meal cm •̂ sec ^

Corrected

2.832.883.192.763.433.122.873.29

36°55'N

36°50'N -

33°40'W 33°35'W

Figure 12. Comparison of the Leg 37 heat flow values withnearby shallow ocean probe data from Lewis (1975). Thehatched area is approximate extent of the sediment pond.

ment that was deduced from the temperature measure-ments supports such a circulation. The observed sur-face heat flow indicates that there may be water flowingin this horizon at about 10°C.

ACKNOWLEDGMENTSWe wish to express our appreciation to Mr. A. Porter, Heat

Flow and Electronics Technician onboard D.V. GlotnarChallenger for his very able efforts in obtaining downholetemperatures. This work was done while one of us (R.D.Hyndman) was at Department of Oceanography, DalhousieUniversity, Halifax, N.S., Canada.

REFERENCES

Anderson, R.N., 1972. Petrologic significance of low heatflow on the flanks of slow-spreading mid-ocean ridges:Geol. Soc. Am. Bull., v. 83, p. 2947-2956.

Anderson, R.N. and Halunen, J., Jr., 1974. Implications ofheat flow for metallogenesis in the Bauer Deep: Nature,v. 251, p. 473-475.

Aumento, F., Loncarevic, B.D., and Ross, D.I., 1971. Hud-son geotraverse: Geology of the Mid-Atlantic Ridge at

-

-

4.5< >C /*- m - /

β.o

11,5°

15,0* - - ~

. E

.surface

~500m depttr

0.62

-*•*- 0.911 10.66 1 . . .

^~*~^Λ 2±^^^^r‰rrfr!i7777\^332 A\f////0.70^=5^

^~^^djf/, K * 2.6 /vZ

- - " " " K•332 B (basalt)

v 0 . 6 4 _ _ _ - - - - - -

Vertical Exaggeration

33 //T/t/l

1

3.7

X4

\

b

/ _

W

I.I"o 1.0

× V3c? 0,9

So 0.7x 30 .6

0.5

1600 "

1600

.2000

2200

2400 -

0 1 2 3 4 5 6 7Distance (km)

Figure 13. Cross-section of sediment pond penetrated byHoles 332A, 332B, and 333 on 3.5-m.y. old sea floor.The heat flows measured in the holes along with threeshallow probe values from Lewis (1975) are shown. Thecomputed thermal refraction effect of the sediment pondand topography on heat flow is shown in the upper graphand the effect on deep temperature by the isotherms onthe cross-section.

Figure 14. Schematic diagram of possible hydrothermal cir-culation in the region of Sites 332 and 333.

360

HEAT FLOW MEASUREMENTS

45°N: Phil. Trans. Roy. Soc. London, Sci. A. v. 268,p. 623r65(λ

Beck, A.E., 1957. A steady state method for the rapidmeasurement of the thermal conductivity of rocks: J. Sci.Instrum., v. 34, p. 186-189.

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