-
33. PHYSICAL PROPERTIES OF BASALTS FROM DSDP LEG 55
Masaru Kono and Yozo Hamano, Geophysical Institute, University
of Tokyo, Tokyo 113, Japanand
W. Jason Morgan, Department of Geological Sciences, Princeton
University, Princeton, New Jersey
INTRODUCTION
Four seamounts in the Emperor Seamount chainwere drilled during
DSDP Leg 55. Holes 430A (öjinSeamount), 432A (Nintoku), and 433A,
433B, and 433C(Suiko) reached volcanic basement; in particular,
Hole433C penetrated 390 meters of basaltic layers, of which250
meters was recovered. The basalts from these holesconstitute almost
the only available samples from sea-mount interiors, although many
samples from sea-mount surfaces had previously been obtained by
dredg-ing. Below a thin sediment cover, the seamounts areprobably
comprised mostly of extrusive and some in-trusive volcanic rocks.
Geophysical anomalies (gravity,magnetism, etc.) caused by some
seamounts have beenstudied intensively and hypotheses advanced to
explainthem. Measurements of various physical and
magneticproperties of these basalts are therefore of
considerableinterest in interpreting such anomalies.
Petrological studies (Kirkpatrick et al., this volume)
andmagnetic studies (Kono, this volume) confirm that thesebasalts
erupted subaerially and that they are quite simi-lar in composition
to the basalts found on the HawaiianIslands. Moreover, the ages of
Ojin, Nintoku, andSuiko seamounts (55, 56, and 65 m.y.,
respectively:Dalrymple et al., this volume) correlate well with an
8cm/year rate of movement of the Pacific plate over theHawaiian hot
spot. It seems likely, therefore, that thesebasalts have physical
properties very similar to those ofHawaiian basalts.
In this chapter, we report the results of physical prop-erty
measurements and discuss their implications. Inparticular, we will
compare these data with measure-ments on normal oceanic basalts
(Hyndman and Drury,1976; Hamano, in press) and on Hawaiian basalts
(Rob-ertson and Peck, 1974). The physical properties mea-sured are
bulk density (ρ), porosity (Φ), compressionalvelocity (Vp), shear
velocity (Vs), thermal conductivity(x), and electrical resistivity
(σ~'). Other quantities suchas grain density, Poisson's ratio
(i>), and other elasticconstants can be calculated from these
data. All mea-surements except that of porosity were carried out
onsalt-water-saturated samples, since the in-situ propertiesof
these rocks are of interest from the geophysical pointof view.
EXPERIMENTAL METHODS
Measurements were made on minicore samples2.5 cm in diameter and
about 2.3 cm long, both on the
Glomar Challenger and in our Tokyo laboratory.Samples were kept
in salt water between measurementsto preserve the in-situ
condition.
On board the Glomar Challenger, bulk density andcompressional
wave velocity were measured. Bulk den-sity was determined by a
two-minute count on the Gam-ma Ray Attenuation Porosity Evaluator
(GRAPE),with an estimated accuracy of ±2 per cent. Compres-sional
wave velocity was measured using the HamiltonFrame velocimeter. For
some of the samples, velocitywas measured in both the horizontal
(axis of the mini-core) and vertical (across minicore diameter)
directions.The differences between horizontal and vertical
velocitywere mostly within a few per cent and always smallerthan
about 5 per cent. This suggests that these basaltshave isotropic
elastic properties. Velocity measurementon board was reproducible
to within about 2 per cent.
In our Tokyo laboratory, we measured porosity, bulkdensity,
compressional velocity, shear velocity, thermalconductivity, and
electrical resistivity. Bulk density wasmeasured by the
conventional immersion technique,with an estimated accuracy of 0.5
per cent. Porosity wasdetermined for each sample after all the
other measure-ments were finished. Samples were heated to 120°C
andkept at that temperature for 24 hours. Porosity was cal-culated
from the difference between the weight of theoriginal,
salt-water-saturated sample and the weightafter drying. The
porosity thus determined may be aminimum estimate (Hyndman and
Drury, 1977). Thatgrain density is independent of porosity (Figure
1)shows, however, that the porosity estimates are ade-quate.
To determine elastic-wave velocities, we used a stan-dard pulse
transmission method with 2-MHz (for Vp)and 1-MHz (for Vs)
transducers. The higher frequency(compared with the Hamilton Frame
[400 kHz]), as wellas the better shape of shore samples, gave
better accu-racy, about ± 1 per cent.
We measured electrical resistivity using a commercialAC bridge.
Because the frequency dependence of a salt-water-saturated sample
is small in a frequency range of100 Hz to 1 MHz (Katsube et al.,
1977), we used a singlefrequency of 1 kHz, and our measurements are
accurateto within about ±5 per cent. Thermal conductivity
wasmeasured on a QTM-1 thermal conductivity meter (Sho-wa Denko
Ltd.) which uses a modified needle method.Accuracy of measurement
is difficult to estimate, butthe nominal precision of this
apparatus is ±5 per cent,and the reproducibility was better than a
few per cent.
715
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M. KONO, Y. HAMANO, W. J. MORGAN
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Figure 1. Porosity dependence of grain density.
RESULTS
Results of the shipboard measurements are shown inthe Site
Reports (this volume). Shore-based experimen-tal data are
summarized in Table 1 of this chapter.Figure 2 shows histograms of
measured physical proper-ties. The distribution of Vp values
obtained on board(dashed line in Figure 2) disagrees substantially
withthat of values determined in our laboratory, althoughthe
distributions of densities are similar. TheVp measured on board may
be systematically smallerthan the real value. Accordingly, the
discussions tofollow are based solely on the shore-based
experiments.
Table 2 summarizes the differences between physicalproperties of
tholeiites and alkalic basalts (includinghawaiites). Tholeiites
have smaller means for all themeasured physical properties except
thermal conductiv-ity, but the differences are not significant, and
arecaused mainly by differences in porosity; tholeiitic
rockscontain more pores than do alkalic basalts.
Porosity Dependence of the MeasuredPhysical Properties
The physical properties measured cover wide rangesof values, as
shown in Figure 2. To make clear the originof this wide dispersion,
we have plotted the followingproperties as functions of porosity:
bulk density (Figure3), grain density (Figure 1), compressional-
and shear-wave velocities (Figures 4 and 5), thermal
conductivity(Figure 6), and electrical resistivity (Figure 7).
Thesephysical properties, except grain density, correlatestrongly
with the porosity measured on the same speci-men. This suggests
that the volume of pores or fracturessaturated with salt water is
mainly responsible for thevariation of these physical properties.
The similarchemical and mineral compositions of the seamount
ba-salts (Kirkpatrick et al., this volume) confirm this
inter-pretation; to examine further its validity, we shall
nowdiscuss separately the relationship between porosity andeach
measured physical property.
Bulk Density and Grain DensityFigure 3 indicates a very strong
correlation between
bulk density and porosity for a wide range of porosity (2
to 35%). A linear decrease in density with increasingporosity is
theoretically expected if the variation iscaused by the difference
in pore volume. A least-squaresfit of the data gives a linear
relation of ρ (bulk density,g/cm3) = 2.99 - 2.20 0 (porosity), with
a high correla-tion factor of R = 0.974. The high correlation
factorand the constancy of the grain density (Figure 1) con-firm
the above interpretation.
The slope of the density-versus-porosity plot is slight-ly
higher than the theoretical estimate, as has also beenobserved for
normal sea-floor basalts (Hamano, inpress). Underestimation of the
porosity, or some low-density secondary minerals filling some of
the pores,may cause such a difference (Hamano, in press).
In Figures 1 and 3, tholeiitic and alkalic basalts areidentified
by different symbols. The porosity dependen-cies of bulk density
and grain density for each basalttype are not significantly
different. Data for one vol-caniclastic sandstone suggest that the
sandstone consistsof material derived from these basaltic
rocks.
Compressional- and Shear-Wave Velocities
Porosity dependencies of compressional- and shear-wave
velocities are shown in Figures 4 and 5, respective-ly. The
velocities also tend to decrease with increasingporosity.
Theoretical estimates of elastic properties ofcomposite materials
have been studied extensively. Wattet al. (1976) reviewed the
previous studies. In the presentcase we can assume that the samples
consist of twophases: solid rock and salt water. Among the
theoriestreating the two-phase material, bounding theories,such as
Voigt and Reuss bounds (Hill, 1963) or Hashinand Strikman bounds
(Hashin and Strikman, 1963),give a broad constraint if one of the
phases has a vanish-mgly small elastic constant. The bounds are so
broad asto be practically useless to us here. In contrast, a
deter-ministic theory, such as that of Walsh (1973) or ofKuster and
Toksoz (1974), will give some informationon the configuration of
pores and cracks in the samples,as well as the intrinsic
(pore-free) property of the rocks.Analysis along these lines is
still in progress in ourlaboratory.
The relation between compressional- and shear-wavevelocities is
shown in Figure 8. The correlated variationof both velocities
indicates the same mechanism for thevelocity change. Poisson's
ratio increases from about0.29 (Vp = 6.3 km/s) to 0.36 (Vp = 3.3
km/s) withdecrease of the velocities. The variation is
consistentwith the assumption that the increase of the water
con-tent causes the velocity decrease.
Thermal Conductivity
The relation between thermal conductivity and po-rosity is given
in Figure 6. Although the distribution ofdata points is not so
systematic as for the other physicalproperties, correlation is
apparent. Robertson and Peck(1974) measured the thermal
conductivity of Hawaiianvesicular basalt with porosity ranging from
2 to 98 percent. Their data for water-saturated rocks within
thepresent porosity range are also plotted in Figure 6. Thetwo sets
of data reveal no systematic disagreement; this
716
-
PHYSICAL PROPERTIES OF BASALTS
TABLE 1Physical Properties of Basalts from DSDP Leg 55
Sample Piece(Interval in cm) No.
Compressional ShearRock Density Velocity VelocityType Porosity
(g/cirß) (km/s) (km/s)
Thermal ElectricalConductivity Resistivity
(W/m°C) (ohmm)
Hole 430A
5-2, 30-325-3, 107-1095-5, 44-465-5, 103-1056-1,67-696-1,
88-906-2, 54-566-2, 122-1246-4,15-17
Hole 43 2A
2-1, 33-352-1, 74-762-1, 107-1092-2, 72-742-3,12-142-3,
21-233-2, 67-694-1, 64-664-1, 73-754-1, 120-1224-2, 80-825-2,
69-71
Hole 433A
20-2, 2-420-2, 14-1620-2, 24-26
Hole 433C10-3, 92-9410-3,145-14710-4,69-7110-6,
38-4011-1,139-14111-3,84-8611-3,112-11411-5,36-3812-1,109-11112-3,5-713-1,125-12713-2,103-10514-2,
37-3915-1,17-1915-4,48-5015-4, 84-8615-5, 72-7416-1, 80-8219-2,
9-1119-2,40-4219-2,64-6619-3,74-7620-1,103-10520-2, 20-2220-2,
35-3721-1, 128-13021-3, 7-921-4, 64-6621-4, 101-10322-1,
104-10623-4, 99-10125-2, 7-9
4B3E6A
10B5B67
12IB
3B8A8E
11AIB1C74C4D6C4A3E
1AIB2A
9D11
3E3C9C2E2F49C1A6CIN2A2A474A6BIB2D2G1M
1034B9B17
1011
7A1A
AAAAAAAAA
SAAAAAAAAAAA
AAA
TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT
0.0540.0400.1800.0860.0660.0660.1870.0670.052
0.2410.0460.1020.1480.1800.1520.0390.0960.0860.1260.0960.048
0.0830.0960.105
0.2580.2020.2310.1180.1560.0960.1060.2070.3580.1300.0690.0490.3480.1650.2300.2370.2340.0150.1250.0980.1170.2040.1800.1300.1220.1750.1430.0980.1020.1300.3000.098
2.7642.7792.4742.8212.7592.7612.5502.7402.760
2.3632.8362.7182.5
812.4882.5582.9332.7292.7602.7382.7542.888
2.8212.7822.794
2.4092.4992.3982.7162.6112.7612.7282.5
832.2922.6942.8212.8752.1922.5942.5452.5
222.5723.0172.7382.7902.7462.5562.5772.6542.7022.6392.6592.7632.7522.6712.3352.764
5.355.404.955.095.345.164.725.215.37
3.785.745.124.754.204.376.114.995.154.995.155.93
5.355.175.13
3.883.894.145.354.925.385.254.743.425.325.585.893.554.434.114.144.246.455.335.375.434.984.994.944.904.624.865.175.285.074.095.25
2.832.882.652.712.832.702.312.772.82
1.722.982.832.562.082.243.162.502.722.602.773.13
2.842.712.74
1.881.891.952.822.492.732.732.341.592.662.973.021.782.162.052.192.213.602.912.902.912.532.602.612.582.462.552.752.892.612.032.80
1.6271.7931.2101.6521.7601.7331.3411.6921.482
1.1911.8481.7231.5801.3451.1981.8611.5251.5041.6031.6281.767
1.6741.7491.695
1.4541.5
261.5961.5961.6821.6251.7361.5361.1401.3281.7681.7941.1011.4171.5191.6401.6512.0181.6421.6341.6081.1691.3411.6201.5461.6311.7061.6491.7371.5461.5121.699
116185
2179867343
140117
9.8438
61281819
64839544348
447
524443
1124
9.049306651174.0
1866
1755.6
21171516
537489873324343463043634855
7.973
717
-
M. KONO, Y. HAMANO, W. J. MORGAN
TABLE 1 - Continued
Sample(Interval in cm)
Hole 43 3C25-2, 27-2925-2, 54-5625-5, 103-10526-5, 94-9626-6,
129-13127-5, 41-4328-4,121-12329-2,115-11731-4,
83-8532-1,38-4035-1, 112-11435-6, 48-5035-6,121-12335-7,
116-11836-1,45-4736-1,105-10736-3, 60-6236-4, 56-5836-5, 29-3137-3,
74-7638-1,57-5938-5, 119-12139-5,96-9839-6, 78-8041-1,51-5342-1,
68-7042-2,136-13842-5, 134-13643-1,33-3544-1,41-4344-4,
108-11045-2, 45-4745-5, 51-5345-6, 43-4546-3, 54-5647-1, 26-2847-5,
25-2748-3, 30-3249-1, 142-14449-2, 50-52
PieceNo.
IBIE1H117AID8A
163B3B1MIE
105AIE3A1H1GIDUIF1QIDIDIE1G6EINIBIE4IDID1C2B2A3A1C8B1H
RockType
TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTAATT
Porosity
0.1040.1510.1020.0630.1360.1770.2030.1130.2270.2260.1360.0330.2170.1410.1260.2770.1140.1230.1400.0910.0250.0640.0200.0570.1030.1270.1300.0920.0910.0660.1280.0880.0530.1080.1180.0870.2200.0280.1470.138
Density(g/cm3)
2.7432.6582.7892.8272.6732.5792.5392.7712.4412.3832.7392.9222.5
202.7402.7392.3542.7062.7292.6672.8072.8872.8022.9372.8462.7832.7342.7002.8152.8352.8812.7522.8272.9112.7362.7332.8342.4492.9172.6572.693
CompressionalVelocity(km/s)
5.155.035.465.294.954.324.985.184.153.485.006.064.635.085.214.364.814.784.785.316.075.476.275.775.225.085.185.255.235.484.895.195.764.945.025.274.435.985.135.04
ShearVelocity(km/s)
2.662.622.922.792.522.202.832.591.861.642.663.162.332.692.692.122.352.312.472.853.162.893.343.032.842.692.722.952.802.912.712.863.182.672.692.812.373.202.652.51
ThermalConductivity
(W/m °C)
1.6351.4551.8611.8341.6811.5291.4601.6831.6101.4751.8621.9501.6791.6521.8291.4211.7671.7231.7511.8951.9981.8732.0131.9001.8281.8211.7591.7701.8381.8891.7321.8661.9211.7051.6621.8291.5601.9401.6721.831
ElectricalResistivity(ohm m)
554373
12737152947
7.91061
662193432
9.527262852
1241104989200
45404338458233549527594330
4972624
is compatible with petrologic studies on seamount sam-ples under
discussion (Kirkpatrick et al., this volume).As pointed out by
Robertson and Peck (1974), thermalconductivity of their
high-porosity (> 10%) samples canbe explained reasonably well by
a theoretical estimate ofthe pore effect. The observed thermal
conductivity ofthe low-porosity basalts, however, is
appreciablysmaller than the conductivity determined from fullysolid
rock. They attributed the discrepancy to the insu-lating effect of
micropores and thin microfractures, andobtained a prediction
formula for these low-porosityrocks by applying an empirical
correction factor. Thesimilarity between their data and our thermal
conductiv-ity data here suggests not only that conductivity is
deter-mined mainly by the water content of the samples, butalso
that the intrinsic conductivity of the fully solid rockof our
seamount samples is quite similar to that of theHawaiian
basalts.
Figure 6 indicates that the conductivity of alkalicbasalts is
somewhat lower than that of tholeiitic basalts.The difference may
be attributed to their different min-eral compositions. The effect
of mineral composition isappreciable, probably because thermal
conductivity ismuch more sensitive to mineral composition than
areother physical properties, and because the differencebetween the
conductivity of the solid rock (2.55W/m°C) and that of water (0.59
W/m°C) is relativelysmall. These reasons also account for the
observedbroad distribution of the conductivity data.
Electrical Resistivity
Electrical resistivity and porosity are closely corre-lated, as
shown in Figure 7. The correlation and the lowresistivity of these
samples compared with the resistivityof dry rocks (Nafe and Drake,
1968; Parkhomenko,1967) indicate that resistivity and its variation
are much
718
-
PHYSICAL PROPERTIES OF BASALTS
20(40)
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Porosity
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\Jy
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0.1 0.2 0.3 0.4 1.5 2.0 2.5 3.0 3.5 1.2 1.4 1.6 1.8 2.0
Figure 2. Histograms of the observed physical properties of DSDP
Leg 55 basalts: (a) density, (b) porosity,(c) compressional-wave
and (d) shear-wave velocities, (e) electrical resistivity, and (f)
thermal conductivity. Dottedlines in density and compressional-wave
velocity show distribution of the onboard measurement.
TABLE 2Average Physical Properties of Leg 55 Tholeiites
and Alkalic Basalts
Bulk density (g/cm^)PorosityGrain density (g/cm ̂ )Compressional
velocity (km/s)Shear velocity (km/s)Thermal conductivity
(W/m°C)Electrical resistivitya (ohm m)
Tholeiites(70 samples)
Mean
2.6840.140
4.992.601.6691.61
Std.Dev.
0.1620.072
0.620.400.1940.46
Alkalic Basalts(25 samples)
Mean
2.7260.098
5.172.721.6201.87
Std.Dev.
0.1340.053
0.470.270.1920.45
1 1 1
(§> ' O .
1 1
1 1
1
aMean and standard deviation of log (σ~l).
affected by the salt water (resistivity = 0.23 ohm m) inthe
samples.
The variation of electrical resistivity with water con-tent has
been studied extensively for various rock types(cf. Parkhomenko,
1967). The variation is usually ex-pressed by a semi-empirical
formula called Archie's law(cf. Keller, 1967, for review). In its
general form, Ar-chie's law is expressed as
POROSITY
Figure 3. Porosity dependence of bulk density.
σ-i/σw-i = AΦ~"
where σ~1 and σ w- 1 are the electrical resistivity of the
water-saturated rocks and pore fluid, respectively; Φ isthe
porosity; and A and n are empirical parameters de-termined for each
rock type. A least-squares approxi-mation to the present data gives
A = 5.09 and n = 1.67,where σ^~1 of 0.23 ohm m was applied. These
values are
719
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M. KONO, Y. HAMANO, W. J. MORGAN
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Figure 4. Porosity dependence of compressionalvelocity.
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Figure 7. Porosity dependence of electrical resistivity.
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Figure 5. Porosity dependence of shear velocity.
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comparable to those obtained for highly porous volcan-ic rocks
(Keller, 1967). The parameter n depends on theconfiguration of the
pores. If the electrical conductionis through mutually connected
cracks with equal thick-nesses, n = 1. And for conduction through
pores thatare nearly equidimensional, n = 2. The present value
of1.67 suggests that the effects of vesicles in the
observedresistivity is dominant.
• THOLEIITE
O ALKALIC BASALT
» SANDSTONE
5.0(km/s)
The values of electrical resistivity for our Seamountsamples are
comparable to those for sea-floor basalt(Hyndman and Drury, 1976).
This does not necessarilymean that their intrinsic resistivity
values are the same,because the contribution of rock resistivity to
the ob-served resistivity is negligible. For the same reason, it
isnot possible to obtain the intrinsic resistivity from
ourdata.
Average Physical Properties of Seamount Basalts
Hole 433C (Suiko) penetrated nearly 400 meters ofbasaltic layer.
Because of the high recovery (-65%)and deep penetration, the
samples from this hole can beconsidered representative of the upper
part of a typicalSeamount. Since this is the first opportunity to
obtainand examine an appreciable portion of Seamount basalt,it is
pertinent to discuss the average physical propertiesof the samples.
Comparison with normal sea-floor ba-salts is convenient because
extensive data are available(cf. Christensen and Salisbury,
1975).
720
Figure 6. Porosity dependence of thermal conductivity.
Figure 8. Relation between compressional- and shear-wave
velocity. Equi-Poisson's ratio (v) lines for v =0.25, 0.30, 0.35,
and 0.40 in the diagram are alsoshown.
-
PHYSICAL PROPERTIES OF BASALTS
As discussed in the foregoing, the intrinsic propertiesand the
porosity dependence of the bulk properties ofthe seamount basalt
samples are comparable to those ofnormal sea-floor basalts. Average
properties, however,show some differences. The average properties
of theseamount basalts are compared in Table 3 with those
ofocean-floor basalts from DSDP Leg 37 (Hyndman andDrury, 1976) and
from Legs 51-53 (Hamano, in press).The average porosity of the
seamount basalts is largerthan that of the oceanic tholeiite, and
all the otheraverage properties of the seamount basalt have
smallervalues than those of the tholeiite. This tendency is
rea-sonable because the bulk physical properties are con-trolled
mainly by the salt water content filling the porespace, and the
corresponding physical properties valuesof salt water are smaller
than those of the solid basalts.The larger average porosity of the
seamount basalts canbe attributed to subaerial eruption, deduced
from thepetrologic study (Kirkpatrick et al., this volume) and
themagnetic study (Kono, this volume). The subaerialeruption may
also be responsible for the wider disper-sion in values of bulk
properties.
SUMMARY AND CONCLUSIONSPhysical properties were measured on 98
basalt
samples obtained during Leg 55. These basalts can betaken as
typically representative of the upper parts ofthe seamounts in the
Emperor Seamount chain. Varia-tions in the measured physical
properties among thesamples result from their water content or
porosity. Thevariations and the intrinsic (pore-free) properties of
theseamount basalts are comparable to those of normalsea-floor
basalts. The higher average porosity value andthe wider dispersion
of porosity data for the seamountbasalts, reflecting their
subaerially erupted origin, result
TABLE 3Average Physical Properties of the Whole Leg 55
Basalts
Compared with Those of Oceanic Tholeiite
Leg 55 Leg 37a Legs 51-53b
Bulk density(g/cm3)
Porosity (vol.fraction)
Compressionalvelocity (km/s)
Shear velocity(km/s)
Thermal conduc-tivity (W/m°C)
Electrical resis-tivity0 (ohm m)
2.695 ±0.154 2.795 ±0.082 2.785 ±0.128
0.129 ±0.067 0.078 ±0.041 0.084 ± 0.055
5.037 ± 0.582 5.94 ±0.34 5.48 ±0.48
2.632 ±0.369 3.27 ±0.15 3.10 ±0.21
1.656 ±0.192 1.66 ±0.07 1.80 ±0.07
48 220 120
in smaller average physical properties values for thesebasalts
compared with sea-floor basalts.
ACKNOWLEDGMENTS
We thank Seiya Uyeda for use of the thermal
conductivitymeter.
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