Can a igneous rock be attracted to a magnet?
The measured magnetic properties of submarine igneous rocks,
comprising data from approximately 300 specimens, are summarized.
Basaltic rocks dominate the collection numerically, and are
distinguished by their high Q (ratio of remanent to induced
magnetic intensities). Limited numbers of altered samples indicate
that spilitization, chloritization, and serpentinization can
drastically reduce the intensity of magnetization. The available
thermomagnetic data suggest that low Curie points may be typical of
quenched basalts. The limited range of submarine igneous rock types
examined, and the strong bias towards quenched samples necessitates
a supplement to this summary in the form of a discussion of studies
of magnetic properties from selected igneous rocks outcropping
above sea level. In these studies, serpentinization of ultrabasic
rocks has been observed in one case to increase the intensity of
magnetization; chloritization and spilitization are confirmed as
being magnetically destructive; maghaemitization may have
destructive effects; titanomagnetite oxidation variation dominates
in magnetic change of basaltic lavas (and some corresponding
chemical changes are likely to occur); basaltic intrusives have a
much more limited titanomagnetic oxidation range than is generally
observed in lavas; and spontaneous demagnetization with time
probably exists, at least in basalts. New data are presented. These
include the magnetic properties of harzburgites dredged from the
Macquarie Ridge, and eight pillow basalts from the South Pacific
and Scotia Sea. The former suggest that harzburgite is capable of
creating strong magnetic anomalies. Samples for the latter study
were sufficiently large for study of the variation of magnetic and
petrological properties with depth beneath the cooling surface.
Systematic texturual changes from glassy exterior, through a
variolitic zone to aphanitic interior characterize the silicates in
most samples. Chloritization is present in some aphanitic parts.
Serpentinization is present in some aphanitic zones and also next
to joints. The opaque minerals were studied in detail in one
pillow. The titanomagnetites are all fine and of low oxidation
state. Very fine sulphides are common. The intensity of
magnetization and suceptibility variation are closely related to
the changes in titanomagnetite grain size. Although optically
undetectable in the titanomagnetites, a zone of slightly higher
oxidation is inferred to exist towards the centre of the pillow by
the presence of higher Curie points and magnetic stability, and
lower sulphide content. New data are also presented from traverses
of Icelandic lavas and dykes, and from spilites of St Thomas,
Virgin Islands. It is concluded that the submarine basalt magnetic
properties which have so far been determined are largely a function
of quenching, in contrast with the data from lavas outcropping
above sea level which have generally experienced longer cooling
periods, and which therefore include a greater range of
titano-magnetite grain size and oxidation states. The quenching
process can apparently proceed faster than the oxidizing process in
basalts. Magnetic properties of the surface of submarine basalts
are therefore largely a function of cooling history, rather than
any upper mantle phenomenon. The new data confirm that deuteric or
post-cooling alteration of basalts and ultrabasic rockscan be
magnetically destructive: chloritization is always associated with
a decreasing intensity of magnetization and Q ratio. Spilitization
is similarly destructive. The magnetic effect of serpentinization,
however, is not uniquely predictable. The magnetic data for
submarine ultrabasic rocks show much variation, but are too limited
for further generalization.