Magnetic Studies of Rocks and Sediments Obtained by Deep Drilling 1 * 
John C Belshe 
The magnetic properties of the crustal ma- 
terials which may be drilled in Project Mohole 
should have many points of geochemical inter- 
est. For example, the magnetic susceptibility, 
the Curie temperature, the magnetic hysteresis, 
and the magnetic anisotropy may reflect pres- 
sure, temperature, and chemical relationships in 
the crust and upper mantle. In addition, the 
rocks and sediments may be expected to have 
a remanent magnetism which has been ac- 
quired in the earth’s field and may have paleo- 
magnetic significance. 
Inasmuch as the average heat flow value in 
the area of the proposed drilling site is normal, 
an average conductivity figure of 2X 10" 3 
Cal/°C sec yields a thermal gradient value of 
70°C/km. Thus, the depth to the magnetic 
Curie isotherm typical for oceanic basalts can 
be expected to lie more than 8 km below the 
surface of the sediments — too deep to be 
reached by the drilling operation. For most of 
the material encountered during drilling, we 
can therefore expect a remanent magnetism 
which is controlled by the cooling of the mate- 
rial through the Curie point as it formed, that 
is to say, a thermo-remanent magnetization. In 
the sediment overlying the crustal rock there 
may be such magnetizations produced by heat- 
ing from below, but, in addition, there will be 
some sedimentary acquired magnetism. Previous 
work of the writer on exactly such material has 
shown that it may be of great importance to 
our understanding of past changes in the geo- 
magnetic field. 
Many workers have contributed to studies of 
ancient magnetizations in igneous rocks. In Ha- 
waii, Doell and Cox (1961^) and Tarling and 
McDougall (1963) have made measurements 
of the magnetism remanent in several lava 
flows. The Hawaii Institute of Geophysics is 
preparing to extend such measurements during 
the coming year. To illustrate how studies such 
1 Hawaii Institute of Geophysics Contribution No. 
101 . 
as this may give support to Project Mohole, we 
will discuss the studies made on material ob- 
tained by the trial drilling off Guadalupe in 
1961 on the drilling barge "Cuss II.” 
In that operation drilling extended to 181 m 
beneath the sea floor. The final 11 m were 
drilled in a basaltic rock which was overlain by 
pelagic clays. The natural magnetic remanence 
of this basalt showed a reversed polarity and 
no detectable anisotropy was found in the mag- 
netic susceptibility. Doell and Cox (1961 b) 
have reported studies made on 23 specimens of 
this material. The writer worked on 5 samples 
taken from material between 100 and 110 cm 
into the basalt. These samples showed a high 
uniformity in direction of the natural mag- 
netic remanence with only a Vi° scatter in decli- 
nation values and a IV 2 0 scatter in the inclina- 
tion. Three Curie temperature measurements 
were made on the material. All indicated a 
Curie point of 325°C ±10° (Fig. 1). There 
was no indication of a multiple Curie point 
pattern in any of the curves. One measure- 
ment was conducted under an astatic mag- 
netometer so that the direction of the natural 
remanence could be observed as heating pro- 
ceeded. This measurement showed no appre- 
ciable change in direction of the remanence as 
the temperature approached the Curie point. 
The Curie point agreed well with petrologi- 
cal examinations of the sample. The rock was 
a fresh, medium grained, olivine basalt, quite 
rich in pyroxene. The color of the pyroxene 
suggested that the mineral was low in titan- 
ium, and that this element could therefore 
account for the low Curie temperature in the 
titano-magnetite mineral fraction. The petro- 
logical description of this rock given by Engel 
and Engel ( 1964) fitted well the samples which 
we examined. 
Structural and petrological examination sug- 
gested that this basalt had been intruded under 
some cover of sediment. The amount of cover 
could not be readily ascertained. This suggested 
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