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Potassium-Argon Ages and Magnetic Properties of Some Dredged Submarine Basalts and Their Geophysical Implications
Abstract
susceptibility of about 10 -4 emu/cc. Three of the seven dredged basalts studied here showed self-reversal of thermoremanent magnetization upon heating to 300øC in air. Q ratios range from 5 to 57. Judging from the comparison between the thermal decay of NRM and the production of TRM, the NRM of most samples seems to be of TRM origin. These experiments also suggest that the geomagnetic field intensity when these rocks were formed was not much diffrent from the present one. Both Curie temperature 0 and saturation magnetization I, of the submarine basalts increase markedly when they are heated in air or in vacuum. The I, generally increases by a factor of more than 3. Such irreversible increase of I, and 0 is also studied on the ferromagnetic separates from the experimental Mohole basalt. From thermomagnetic, chemical, and X-ray analyses and microscopic observation, it is suggested that these abnormal increases of 0 and I, are caused by rearrangements of ionic configuration in titanomaghemite. On the basis of these magnetic characteristics and I-Ar ages of the submarine basalts, an interpretation of magnetic anomaly linearion pattern of the ocean floor is presented.
... Analytical uncertainty in K and Ar analyses is about 2 percent for each. More details about the K and Ar analysis procedures are described elesewhere (OzIMA et al., 1968). ...
The ages of, submarine rocks dredged from seamounts along the Japan Trench were determined by K-Ar method. In order to obtain more reliable age, the effect of acid leaching on an altered submarine rock was examined. Some samples treated by acid solution gave K-Ar ages up to 20% older than the original untreated sample. This result suggests a possibility that, if suitable acid teratment conditions are found, they would result in more reliable ages for submarine rocks. Some submarine rocks dredged from seamounts along the Japan Trench on the oceanic side gave K-Ar ages of about 70-80 m.y, whereas a sample dredged from a seamount on the Japanese Islands side gave a substantially younger age. Considering the present result as well as previous data, the last volcanic activity along the Japan Trench seems to have occurred about 70-80 m.y ago or older.
... The latter is important both for what it has to say about the alteration history of the basalt and in providing physical explanations for the observed wide ranges in magnetic properties. Ozima et al. (1968) show that cation-deficient titanomagnetites do not show reversible behavior during thermal cycling to 600°C in a saturating magnetic field, even in vacuum conditions. This criterion is used in 9 to identify samples containing stoichiometric titanomagnetite. ...
... In particular, more of the reversible type curves were obtained at Wyoming, and more frequently Tokyo curves had three intensity drops before a completely paramagnetic state is reached than did Wyoming curves. As already seen in previous work (Ozima et al., 1968), two types of samples can be distinguished from their thermomagnetic behavior: a thermally reversible type (Figure la) and an irreversible one (Figure lb). With irreversible samples, magnetization generally decreases with increasing temperature, to reach nearly zero or a minimum value at a certain temperature (Tci), and increases with further heating until it finally becomes paramagnetic above a higher Curie temperature. ...
Comprehensive investigation of ferromagnetic minerals contained in basalt samples collected on DSDP Leg 49 (Reykkjanes Ridge and northern Mid-Atlantic Ridge traverse), using combined techniques of thermomagnetic measurement, electron microprobe analysis, and reflected light microscopy, suggests the following conclusions: 1) the degree of low-temperature oxidation of titanomagnetite is not necessarily related to the age of the rocks; 2) the degree of low-temperature oxidation does not directly depend upon the sub-bottom depth; 3) the degree of oxidation is surprisingly low at all of the Leg 49 sites except at Site 410, where the rocks are of breccia and closely related origins; 4) the Curie temperature of the basaltic layer was certainly higher than room temperature and possibly higher than 150oC when the rocks initially erupted at the mid-oceanic ridge. The lack of relationships between oxidation and age or depth suggests tha oxidation is probably more influenced by hydrothermal circulation in the basement.-Authors
... The initial Curie temperatures are in the region 250"-350 "C, but a characteristic ' hump ' appears in the heating curves around 450"-500 "C, and the cooling curves correspond with material having Curie temperatures from 550"-580 "C and much larger room-temperature magnetizations. This type of thermomagnetic curve has often been observed (e.g., Ozima, Ozima & Kaneoka 1968); it is interpreted as signifying oxidation during heating of initially homogeneous titanomagnetite to a mixture of Ti-poor titanomagnetite and ilmenite (GrommC, Wright & Peck 1969). Type l a is illustrated by lava 13 (Fig. 4), in which the process of oxidation appears to have been partially completed in nature and more nearly completed during the heating experiment. ...
Natural remanent magnetization (NRM) has been measured in 21 lava flows and 1 tuff in the south-west wall of Ngorongoro caldera, Tanzania. The lowest three lavas are normally magnetized, the next two have intermediate directions, and the remainder are reversed; potassium-argon dating places the reversal at 2.45 My, corresponding to the Gauss Matuyama epoch boundary. Alternating-field demagnetization was effective in removing the ubiquitous lightning-produced secondary magnetizations, but partial thermal demagnetization was not. Curie temperatures of the lavas fell into two groups: 250°-350°C and 570°-585°C, probably corresponding to unoxidized and oxidized titanomagnetites respectively. NRM directions in the lavas occurred in several distinct and stratigraphically continuous groups, suggesting intermittent eruption of groups of lavas. No correspondence was found between NRM groups and other parameters, such as Curie temperatures. The dispersion of mean NRM directions is large in comparison with late Quaternary lavas from low latitudes in the Pacific basin, but probably is not large relative to the proportion of the geomagnetic field in Africa that is non-dipolar. The mean palaeomagnetic pole is offset 10.5° from the geographic pole, but because of the large dispersion, the difference is not considered significant.
... Other authors have suggested in situ formation of the magnetic lineations. Ozima, Ozima, and Kaneoka (1968) concluded that the ages of submarine rocks taken from seamounts along magnetic lineations in the western Pacific do not seem to resemble the values expected from the ocean floor spreading hypothesis. However, because of the ambiguity in assigning K-Ar ages of seamounts to the ocean floor and some experimental uncertainties due to the alteration of samples, they do not rule out the possibility of ocean floor spreading. ...
The direct determination of source rock magnetization on the basis of observed magnetic field values involves the numerical solution of a linear integral equation. This solution is obtained for a two-dimensional body of specified geometry and uniformly magnetized volume elements by the Seidel iteration technique. Consideration of magnetic anomaly profiles associated with ocean floor spreading indicates that the magnetization values at the ridge crest are only slightly greater than those away from the crest. This, together with the evidence for narrow transition zones between sections of normal and reverse polarity, lends support to the dike injection mechanism for bringing mantle material to the surface at the ridge crest.
... The low observed Curie temperatures (150ø-350øC, in Hall et al. [1973] tested the method using a historically erupted Icelandic basalt, and Carmichael [1970] compared the results of this method for one specimen with the results of the Thelliers' double-heating method; in both cases the comparisons were satisfactory. Ozima et al. [1968] have performed paleointensity determinations using the Thelliers' method on five dredged fragments of basalt from the Pacific Ocean, and obtained values of 0.2-1.7 times the present field. The ages of these rocks ranged from 40 to 90 m.y., and ,4de-Hall et al. [1973] pointed out that all of the samples but one contained titanomaghemite and that the reliability of the results cannot be judged from the published data. ...
Measurements of geomagnetic paleointensity using the Thelliers' double-heating method in vacuum have been made on 10 specimens of submarine pillow basalt obtained from seven fragments dredged from localities 700 000 yrs old or younger. In the magnetic minerals, the titanium/iron ratio parameter x and the cation deficiency (oxidation) parameter z were determined by X-ray diffraction and Curie temperature measurement. Fresh material (z=0) provided excellent results: most of the natural remanent magnetization (NRM) could be thermally demagnetized before the magnetic minerals became altered, and the NRM-TRM lines were straight and well constrained, and geologically reasonable paleointensities were obtained. Somewhat oxidized material (z=0.2) also provided apparently valid paleointensities: values were similar to those from fresh specimens cut from the same fragments, although only half or less of the NRM could be thermally demagnetized before alteration of the magnetic minerals. More highly oxidized material (z=0.6) gave a result seriously in error: the paleointensity value is much too low, because of continuous disproportionation of titanomaghemite during the heating experiments and because seafloor weathering had decreased the NRM intensity. From limited published data, the extent of oxidation of titanomagnetite to cation deficient titanomaghemite in pillow basalt exposed on the seafloor appears to be approximately z=0.3 at 0.2-0.5 My, z=0.6 at 1 My, and z=0.8-1.0 at 10-100 My. This implies that valid paleointensities can be obtained from exposed submarine basalt but only if the basalt is younger than a few hundred thousand years. Equally good paleointensities were obtained from strongly magnetized (H-type) basalt and moderately magnetized (L-type) basalt.-from Authors
... 2. Titanomaghemite of remarkably uniform composition is the magnetic phase in a number of Pacific ocean basalts dredged from widely separated localities (Ozima et al. 1968), and in some samples it exhibits self-reversal. Caution may therefore be necessary when comparing palaeomagnetic results from continental and oceanic basalt suites. ...
The correlation between reversed polarity in rocks and strong oxidation of magnetic minerals is briefly re-examined, with especial reference to a well-documented Columbia Plateau basalt suite, recently described in this journal. The correlation appears to vary in quality from place to place and some reasons for this are suggested. Attention is particularly drawn to recent studies which suggest that for palaeomagnetic purposes there may be two groups of lavas, divisible on the basis of whether their magnetic minerals became oxidized under high or low temperature conditions.
A sizable body of opinion now holds (not necessarily correctly) that self-reversals are a negligible factor in the correlation between oxidation and reversed polarity, and that some compositional effect is most probably involved. Virtually the only known petrographic criterion which can (statistically) distinguish reversely from normally magnetized rocks is their (statistically) higher oxidation states. An obvious implication is that the water content of magmas, and perhaps of high-grade metamorphic rocks, is somehow increased during episodes of reversed polarity, so that a slightly larger proportion of rocks is more strongly oxidized.
Since motions of the Earth's core, upper mantle processes, and field reversals are apparently all related, compositional changes originating in the upper mantle could well be also related to field reversals. Alternating linear zones of more and less altered baslat recently reported from one place near the mid-Atlantic ridge might have arisen in such a way.
The importance of relating palaeomagnetic measurements to detailed mineralogical studies of magnetic minerals is again stressed.
... These rocks are typically basalts and contain titanomagnetites partly oxidized ta titanomaghemites containing 40-60 per cent ulvospinel (Carmichael 1969). The N6el points lie between about 100 and 400°C (Ade-Hall 1965, Ozima, Ozima & Kaneoka 1968, Wasilewski 1968). This magnetic material is thus not very different from that which we have produced by oxidizing samples of the Rauher Kulm basalt in the laboratory in air at 400°C. ...
The Rauher Kulm basalt from Oberpfalz, West Germany contains homogeneous titanomagnetites as its only ore mineral. Samples of this basalt have been progressively oxidized by heating in air at 400°C for prolonged times in the laboratory. During the heatings a CRM was built up in the oxidized (daughter) phase in a field of 0-45 Oe and the samples were then cooled in zero field to room temperature. A TRM was nevertheless acquired by the mother phase due to an internal magnetic field originating in the CRM of the daughter phase. The strength of this interaction has been calculated to be 0.15 Oe and is sufficient to influence the growth and decay of viscous magnetization at room temperature. The magnetic viscosity of the heat treated rock is noticeably stronger than that of the natural rock and is attributed to the existence of very small nuclei of the daughter phase. With progressive heat treatment at temperatures up to about 400 °C in the laboratory, and in nature with the passage of geological time, when temperatures of up to 200 °C may have been attained, these nuclei grow and in so doing acquire a stable remanence the strength of which is influenced by the interaction field from the TRM of the mother phase. This phenomenon is thought to be of importance in the interpretation of marine magnetic anomalies because basalts from the Ocean floor are in fact slightly oxidized. In particular, the relatively large size of the central anomaly may be explained in this way.
The minerals carrying the magnetic remanence in geological samples are commonly a solid solution series of iron‐titanium spinels known as titanomagnetites. Despite the range of possible compositions within this series, micromagnetic studies that characterize the magnetic domain structures present in these minerals have typically focused on magnetite. No studies systematically comparing the domain‐states present in titanomagnetites have been undertaken since the discovery of the single vortex (SV) structure and the advent of modern micromagnetism. The magnetic properties of the titanomagnetite series are known to vary strongly with composition, which may influence the domain states present in these minerals, and therefore the magnetic stability of the samples bearing them. We present results from micromagnetic simulations of titanomagnetite ellipsoids of varying shape and composition to find the size ranges of the single domain (SD) and SV structures. These size ranges overlap, allowing for regions where the SD and SV structures are both available. These regions are of interest as they may lead to magnetic instability and “partial thermal remanent magnetization (pTRM) tails” in paleointensity experiments. We find that although this SD + SV zone occupies a narrow range of sizes for equidimensional magnetite, it is widest for intermediate (TM30‐40) titanomagnetite compositions, and increases for both oblate and prolate particles, with some compositions and sizes having an SD + SV zone up to 100s of nm wide. Our results help to explain the prevalence of pTRM tail‐like behavior in paleointensity experiments. They also highlight regions of particles with unusual domain states to target for further investigation into the definitive mechanism behind paleointensity failure.
Palaeomagnetic field intensity measurements, derived from rocks with ages that span geological time, provide a crucial constraint on the evolution of Earth’s deep interior and its magnetic environment. The palaeointensity database PINT has been updated to version v.8.0.0 and includes palaeointensity site-mean records spanning an interval from 50 ka to 4.2 Ga, compiling efforts from the palaeomagnetic community spanning from 1959 to the end of 2019. Nearly all site-mean palaeointensity records have been assessed using the qualitative reliability of palaeointensity (Quality of Palaeointensity, QPI) framework. This updated database brings together and harmonizes prior QPI and PINT compilation efforts into a unified database referred to as the PINT database, incorporating recent efforts since 2014 to assess QPI. The spatio-temporal distribution of the PINT database is analyzed, revealing substantial biases towards young records (from the Brunhes chron) in the Northern hemisphere, and intervals with little to no palaeointensity data with a duration of 10s to 100s of millions of years in the Paleozoic and Precambrian. General QPI compliance is characterized for the PINT database, which shows that the median QPI scores range from 2 to 3 (out of a total possible score of 10), with a positive trend towards increasing QPI scores in studies published after the year 2000. This illustrates an increasing community awareness of what is required to establish confidence in palaeointensity data and an increasing robustness of the large scale interpretations that can be made with these data. We additionally present a description of the long-term average dipole field strength with descriptive statistics for distinct intervals of Earth history.
The Cretaceous Normal Superchron is a period of great interest to investigate global scale variations of the geomagnetic field. Long periods of single polarity are still a matter of debate: up to now there are two contradicting theories, which try to relate geomagnetic field intensity and reversal rate. We aim to shed light on the geomagnetic field strength during the Cretaceous Normal Superchron because data are still scarce and of dissimilar quality. To obtain reliable, absolute paleointensity determinations we investigate volcanic rocks from the Western Cordillera of Colombia. Several age determinations allow relating the samples to an age of about 92.5 Ma. To characterize the samples, we investigate rock magnetic properties and determine the characteristic remanent magnetization behavior. To determine paleointensities, we use a multimethod approach: first, we apply the classic Thellier-Coe protocol, and then, the relatively new multispecimen method. Rock magnetic measurements indicate magnetite as the main ferrimagnetic mineral, a stable magnetization revealed by reversible and nearly reversible thermomagnetic curves, and grain sizes that are either in the pseudosingle domain range or a mixture of single and multidomain grains. Alternating field and thermal demagnetization are rather complex, although we observe a few vector diagrams with a single, essentially uni-vectorial component with a small viscous overprint. Paleointensity determination with the Thellier-Coe protocol was unsuccessful, while with the multispecimen protocol we obtained four successful determinations out of 20. The failure of the Thellier-Coe protocol can be attributed to multidomain grains, which were observed during demagnetization and in rock magnetic experiments, and to the inhomogeneity of the volcanic rocks. Our multispecimen paleointensity determinations support low field strength at around 90 Ma during the Cretaceous Normal Superchron.
Work in rock magnetism during the last 4 years in the United States has
been concentrated on several important intrinsic problems, the solutions
of which are needed to answer some of the larger questions now being
raised in paleomagnetism. The questions include whether it is feasible
to measure variations in geomagnetic field intensity in rocks as old as
Precambrian, what the exact frequency of geomagnetic reversals
throughout the earth's history is, and whether the natural remanence of
rocks is thermal or chemical in origin and, if it is chemical, when
during the rocks' history it was acquired. A summary of some of this
research is contained in the following paragraphs. Additional references
to significant research in rock magnetism and related instrumentation
are contained in the bibliography.
K/Rb and ( 87Sr/ 86Sr) 0 ratios were
measured for 14 submarine basalts dredged from seamounts in the Pacific
Ocean. The K/Rb ranges from 200 to 700, which is significantly lower
than that of oceanic ridge tholeiites. Petrographic examination and the
low value of K/Rb indicate that seamount basalts are alkaline. Submarine
basalts from the east side of the Japan trench have significantly lower
( 87Sr/ 86Sr) 0 than basalts from the
Izu-Mariana islands located on the west (continental) side of the
trench. This may be explained on the basis of difference in the depth of
their magma source region. Samples from Shatsky Rise have comparatively
high ( 87Sr/ 86Sr) 0, which may be in
accordance with a recent suggestion that Shatsky Rise was a fossil
boundary between rigid oceanic plates.
This catalogue contains all ancient geomagnetic field intensity data known to the author at 1968 June 1 and not previously appearing in these lists (Smith 1967c, d). In addition, a short list of publications relevant to the study of ancient field intensities, but containing no new data, is included.
The alteration state of basalt lavas is shown to be largely the result of the combined action of two alteration processes, deuteric oxidation and regional hydrothermal alteration. The former process is well described in the literature while the latter is shown here to be another widespread source of lava alteration. The environment of regional hydrothermal alteration is that of the zeolite metamorphic facies, with temperatures of up to 300 °C, and abundant groundwater being the major elements. Independent and rapid spatial variation of the two alteration processes results in the wide range of observed lava properties. Microscopically undetectable titanomagnetite alteration, indicated by steady rise in strong field Curie point, is the first response of a non-deuterically oxidized basalt to regional hydrothermal alteration. With more extreme alteration, titanohematite etc. is seen to pseudomorph titanomagnetite grains. Ilmenite also shows very distinctive mineralogical changes. Initially, high deuteric oxidation specimens appear to be immune magnetically to moderate alteration but these too eventually succumb when conditions are extreme enough for the formation of prehnite in the rock.
The geophysical implications of this widespread post eruption lava alteration process is discussed. We conclude that it is probably possible to obtain information on the original TRM of basalts at least in many cases but that the implications for potassium-argon dating need serious investigation.
Curie point and lattice parameter measurements were made on 57 titanomaghemite samples produced from sintered titanomagnetites with x (mole fraction ulvospinel) values of 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0. Oxidation was carried out in various atmospheres at temperatures as low as 113oC for durations of about 24 hours. The oxidation process was successful in producing homogeneous titanomaghemites with z (oxidation parameter) values between 0.0 and 1.0. Spinels with z less than 0.6 are generally single phase, but the necessity of using temperatures greater than 300oC to produce strongly oxidized phases resulted in spinel oxidation in the presence of a rhombohedral phase. Curie temperature and lattice constant data are significantly different from results presented by Ozima and Sakamoto (1971), Readman & O'Reilly (1972), and Nishitani (1979). Although some of these differences may be due to unmixing (inversion), most of them apparently result from differences in techniques of titanomaghemite preparation and oxidation.-Authors
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 rocks can 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.
The direction and intensity of magnetization of a uniformly magnetized
seamount can be computed if its shape and associated magnetic anomaly
are known. This technique has been applied to nine seamounts close to
Hawaii, twelve seamounts close to the tip of Baja California, and
twenty-eight seamounts surveyed during the Pioneer survey. All available
paleomagnetic results from the Pacific basin have been used to establish
a preliminary polar curve relative to the northeastern Pacific. This
polar curve is weighted in favor of the Cretaceous point. This point at
61°N, 16°E is the mean pole position of seventeen seamounts and
has an α95 of only 8°. The curve confirms earlier
results indicating a 30° northward movement of the Northeastern
Pacific since the Cretaceous. Paleomagnetic results obtained from
seamounts near Japan suggest that, since the Cretaceous, the
northwestern and northeastern Pacific have not always been rigidly
connected. The pattern of long linear fracture zones in the South
Pacific can be used to determine the motion of the Pacific plate
relative to Antarctica since the Upper Cretaceous. Paleomagnetic data
show that Antarctica has remained stationary in latitude over the same
time span. A comparison of the paleomagnetic data obtained on the
northeastern Pacific and on Antarctica yields a relative motion in
conflict with that predicted by the South Pacific fracture zones. This
conflict can be resolved if these two areas of the Pacific were
decoupled some time between the Cretaceous and the present. Thus, it
appears likely that the present Pacific plate is a composite of at least
three distinct plates. Volcanic aseismic ridges within the Pacific Ocean
may be the fossil boundaries of these plates now coalesced into a single
plate.
K-Ar conventional and 40Ar/39Ar ages on
plagioclase from altered basalt samples show that Khatchaturian and
Rachmaninoff Seamounts in the Musicians Seamounts, north-central
Pacific, are 65.2 ± 2.6 and 86.6 ± 5.2 m.y. old,
respectively. The minimum age of Wentworth Seamount, located on the
Hawaiian Ridge near Midway, is 71 ± 5 m.y. Wentworth appears to
be a Cretaceous volcano that was incorporated into the Hawaiian volcanic
chain. A single boulder of rhyolite dredged from the northern slope of
the seamount underlying Necker Island has an age of 77.6 ± 1.7
m.y. Apparently Necker is a composite seamount constructed of both
Cretaceous and late Tertiary volcanoes. With one possible exception, the
ages of these and other Cretaceous seamounts in the north-central
Pacific are less than or equal to the age of the adjacent sea floor
indicating that the seamounts formed at or near the crest of the East
Pacific Rise.
The paleomagnetism of Cretaceous Pacific seamounts is reexamined. Herein
techniques for nonuniform magnetic modeling are applied to determine
paleomagnetic pole positions and their associated confidence limits.
Modeling techniques are presented for reconstruction of both uniform and
nonuniform components of the seamount magnetization. The uniform
component yields an estimate of the paleomagnetic pole position, and the
nonuniform component accounts for irregularities in the seamount
magnetization. A seminorm minimization approach constructs maximally
uniform magnetizations and is used to represent seamount interiors. A
statistical modeling approach constructs random nonuniform
magnetizations and is used to determine the confidence limits associated
with each pole position. Mean paleopoles are calculated for groups of
seamounts, including their associated error bounds. The mean paleopole
for seven reliably dated Upper Cretaceous seamounts is located close to
the position predicted by Pacific-hotspot relative motion. The paleopole
for five seamounts with Cretaceous minimum dates is located west of the
hotspotpredicted apparent polar wander path and may represent a Lower
Cretaceous or Upper Jurassic pole.
Nineteen pillow basalts dredged within the rift valley of the Mid-Atlantic Ridge at 36.8oN were studied by the Thellier stepwise heating method in order to determine the paleointensity of the geomagnetic field when they erupted on to the sea floor. Previously reported fission track ages are 2000-6000 yr for the youngest rocks (mainly olivine basalts) and 10 000-100 000 yr for the others (mainly plagioclase basalts and pyroxene basalts). All but three pillow basalts meet the conditions commonly considered as indicative of quite reliable paleointensity estimates; stability of the direction of NRM during its thermal demagnetization, constant ratio of NRM/TRM (natural remanent magnetization to thermoremanent magnetization) over 50% or more of the original NRM intensity (80 to 94% for 11 specimens), and reproducibility of low-temperature partial TRM(PTRM). However, strong field thermomagnetic measurements indicate that 11 of these 16 samples display a significant increase in Curie temperature (15 to 80oC) during the paleointensity experiments below 250oC, notwithstanding the linearity of the NRM-TRM plot in this temperature interval. This alteration, probably due to low-temperature oxidation of the specimens, seems typical of young pillow basalts and may result in paleointensity estimates which are too high.-from Authors
Theoretical considerations appear to exclude viscous demagnetization as the cauof the gradual decay of the linear magnetic anomalies away from oceanic ridges. Oxidation of titano- magnetite to titanomaghemite during the weathering of submarine basalt seems to be the main cause of the lateral decrease in magnetization. In a detailed study of a partially weathered fragment of pillow basalt from the Juan de Fuca ridge, the remanent and saturation magnetizations of the weathered rim were found to be 40% less than those of the unweathered interior. The Curie temperature increases from 170øC in the interior to 235øC in the weathered rim, and the magnetic coercive force increases from 145 to 270 oe. The cubic cell edge shrinks from 8.460 to 8.434 A during oxidation, and the titanomagnetite grains become less brown. The direction of remanence is the only magnetic parameter unaffected by weathering. The magnetic properties of the basalt fragment vary with weathering in the same way that the average magnetic properties of dredged basalts vary with distance from the mid-Atlantic ridge. This study confirms the conclusion that these lateral magnetic variations are indeed due to the oxidation of titanomagnetite to titanomaghemite. A decrease in the amplitude of marine mag- netic anomalies with distance from mid-oceanic
The Thelliers' method was applied to 28 samples from 14 magnetically stable lavas of the Deccan traps, India. They correspond to a time interval longer than 104 years but shorter than 3 m.y. about 60 m.y. ago. Sixteen samples from nine lavas were successful in experiments and gave estimates of palcointensities of the geomagnetic field. Unsuccessful samples suggest that changes in their blocking temperature spectra were induced by the heating. Those samples with reversed polarity gave palcomoments of the geomagnetic field not much different from present or very recent values (mean, 8.40 x 10 '5 G cm). On the other hand, those corresponding to a reversed normal transition period gave substantially smaller values (mean, 2.85 X 10 '5 G cmS). Palcointensity data obtained so far by the The!!iers' method suggest a binary structure of the geomagnetic field for the Cenozoic and Late Cretaceous: i.e., the strength of the geomagnetic field, except in the Brunhes normal epoch and short transition periods, has been similar regardless of the polarity, normal or reversed. It is suggested that with the use of a precisely controlled magnetic field and magnetically stable samples it should be possible to perform the Thelliers' experiments by single heatings, instead of the conventional paired heatings, to individual temperatures. The modification will greatly reduce the labor in palcointensity studies without losing the merits of the The!!iers' method. Because of the difficulties and complexities in experimentation and in interpretation, much less is known about the change in intensity (palcointensity) than about the change in direction of the geomagnetic field in the remote past. Although changes in direction of the palcomagnetic field supply information about possible landmass movement in the ;t ty data lies in the fact that sucfi hazardous alterations occur quite commonly in heatings of natural rocks [Ozima and Larson, 1970: Kono and Nagata, 1968: Kono, 1968] and that it cannot be guaranteed that TRM characteristics are unchanged before and after heating even if no significant changes are observed in other properties of ferromagnetic minerals such as initial susceptibility, Curie temperature, or coercive force [McElhinny and Evans, 1968]. s of it has been observed that such changes during heating in-air the geomagnetic field. They impose, together with other palcomagnetic findings such as polarity reversals and secular variation, constraints on any theory of the origin of the geomagnetic field. For palcointensity determination from volcanic rocks, several methods have been proposed based on comparisons of the natural remanent magnetization (NRM) and 'the thermoremanent magnetization (TRM) produced in the same rock in a laboratory. These methods can be classified into three categories: (1) comparison of thermal decays of NR M and artificial TRM [Wilson, 1-961], (2) comparison of af demagnetization spectra of NRM and TRM [van Zijl et al., 1962], and (3) comparison of demagnetized parts of NRM and partial TRM's acquired in paired heatings to the same temperature, heatings being carried out to successively higher temperatures [Thellier and Thellier, 1959]. The first two methods require heating to or above the Curie temperature before any NRM-TRM ratios can be determined and therefore give erroneous results when the ferromagnetic minerals alter upo n heating to high temperatures [Coe and Groinre& 1972]. In the Thelliers' method such alterations are usually detectable in the experiment [Coe, 1967a; Kono, 1968], and errors can .be minimized either by discarding such samples or by using only the lower-temperature
Re-examination of the sintering process of Akimoto, Katsura, and Yoshida
indicates that titanomagnetite made according to this process does not
undergo low-temperature oxidation to a single-phase cation-deficient
spinel (γ-titanomaghemite) but instead oxidizes and unmixes to
Mtss and Ilss at about 350-400°C, and
Mtss, Ht, rutile, and pseudobrookite at about 600°C.
Since this unmixing (high-temperature oxidation) reduces the Ti content
in the spinel phase, the saturation magnetization Js and
Curie point increase up to 4 μB (Bohr magnetons at 0°K) and
580°C, respectively, and the lattice parameter decreases down to
8.39 A. From experimental results on synthesized titanomagnetite and
titanomaghemite, we conclude that the irreversible increase in
Js and Curie point of some submarine basalts during heating
in air or in vacuo is the result of unmixing of Ti-rich titanomagnetite
and/or breakdown of γ-titanomaghemite into Mtss, Ht,
pseudobrookite, and rutile. This conclusion is supported by microscopic
obervation and chemical and X-ray analysis.
Thermomagnetic analyses at H -- 3,000 oe were carried out for 24 submarine rocks dredged from 10 sites mostly from seamounts in the Pacific Ocean. Most of the samples, which are believed to be in situ, show an irreversible increase of both Jo and Curie temperature on heating both in air and in vacuum. This increase is explained as unmixing of the original titanomaghemite to magnetite and Ti-rich phases such as hemo-ilmenite and/or pseudo-brookite. The characteristic irreversible change in the thermomagnetic curve is almost sufficient to discriminate uniquely submarine basalts, erupted and preserved in the sea, from those erupted in air. It has been reported that many submarine basalts show an irreversible thermomagnetic curve, in which both the magnetization and Curie point increase markedly on heating lAdeHall, 1964; Ozima et al., 1968; Wasilewski, 1969]. Figure I shows such a typical thermomagnetic curve of submarine basalt measured at H -- 3,000 oe in vacuum of 10 -8 mm Hg. A similar thermomagnetic curve is also obtained when the experiment is carried out in air. In a few cases, however, a reversible thermomagnetic curve that is very similar to that of magnetite is observed (Figure 2). Ozima and Larson [1970] examined several dredged submarine basalts under a microscope and found that an essential part of the opaque minerals is highly oxidized titanomaghemite. On the basis of microscopic observation and thermomagnetic analyses, they suggested that the irreversible change is due to unmixing of original titanomaghemite to magnetite and Ti-rich phases such as hemo-ilmenite and/or pseudo-brookite. The purpose of the present study is (1) to investigate further the origin of the irreversible change in magnetization, and (2) to examine if the irreversible change is an essential characteristic of ferromagnetic minerals in submarine rocks or, rather, an accidental feature.
Tertiary lava (53 Ma) from Barrington Tops, New South Wales, Australia has been studied using conventional thermal and microwave techniques. Twenty-seven flows in two sections were sampled and their rock magnetic characteristics determined. On heating the majority of samples exhibited a single Curie temperature at around 200°C, indicating a titanium rich titanomagnetite. Some samples exhibited two magnetic phases and a few a single low titanium titanomagnetite phase. Thermal demagnetisation yielded a mean direction of D=189.5, I=63.6, α95=4.3, which corroborates the previous findings of Wellman et al. [Geophys. J. R. Astron. Soc. 18 (1969) 371–395]. A pilot conventional Thellier palaeointensity analysis was unsuccessful mainly due to the samples being highly susceptible to thermo-chemical alteration. Far greater success was achieved (58%) using the microwave palaeointensity technique, where heating of the bulk sample and hence alteration is vastly reduced. Palaeointensity estimates range from 3 to 28 μT (mean 11±5 μT), which, assuming that the remanence is a primary thermal remanent magnetisation, indicates a low field intensity (a fifth of the present day value) in the early Tertiary. This study demonstrates the applicability of the microwave palaeointensity technique to ancient lava.
A detailed study of a 35m thick Palaeogene tholeiite from Northern Ireland is described. Deuteric oxidation of titanomagnetite is found to control many magnetic properties (including natural remanence intensity and stability, saturation magnetisation and Curie temperature) as has been described for other basaltic lavas. The lava is unusual in the widespread occurrence of pyrite (FeS2) in its lowest 24m, and the implication of the presence of this mineral for the development of deuteric oxidation in the flow is discussed. The opaque mineralogy of this lowest 24m (skeletal titanomagnetite in a very low deuteric oxidation state, pyrite and scarce ilmenite) is similar to that found in many submarine pillow lavas.
A single 2.5 cm diameter core of 30 cm length has been taken normal to the cooling surface of a single submarine alkalic pillow basalt, recovered from a seamount in the Scotia Sea. The core traverses from a glassy rim, through a variolitic zone, to an aphanitic interior, and back into a mixed glassy/variolitic segment. The silicate petrology and textures, and the magnetic and opaque mineralogical parameters have been determined at intervals of 2.2 cm along the core.Considerable systematic variations in the magnetic properties are due to quenching rate differences, except towards the lower-center of the pillow, where an optically-undetectable slight increase in oxidation is expressed by change in magnetic stability, Curie point, and sulfide content. It is suggested that very low oxidation state titanomagnetites, abundant fine sulfides, and low Curie points may characterize quenched submarine basalts, probably because cooling rates exceed oxidation rates.Because of the considerable variation in magnetic parameters (which has also been detected in seven tholeiitic pillow basalts) it is suspected that representative data may not be readily obtained from submarine extrusives. Averaging of results from several specimens per sample, or establishment of a specimen rejection criterion is required. It is suggested that restriction of analyses to the variolitic segments of pillow basalts may minimize the effect of within-sample variations, when between-sample comparisons are made.
The desirability of accurate delineation of areas of active crustal spreading is the motive for presentation of arguments against unrestricted application of the crustal spreading hypothesis to analyses of all linear magnetic anomalies associated with mid-oceanic rises. It is suggested that no clear delineation will result if spreading rate changes are invoked to force a fit of local observation to hypothetical geomagnetic polarity changes; if major crustal tectonic histories are proposed by the absence rather than the presence of the classic magnetic anomaly patterns; if minor characteristics of magnetic anomalies and available local submarine geological data are ignored; and if for the period prior to 4 m.y. ago a geomagnetic polarity time scale which is either hypothetical or insufficiently well defined is utilized. A summary is made of relevant rock properties and igneous thermal and structural histories before examining a published magnetic traverse and other data from the mid-Atlantic ridge at 22.5° North. It is concluded that a series of flat finite prisms of alternating polarity, as required by the simple crustal spreading model, is inconsistent with the observations.
Thermomagnetic, paleomagnetic and opaque petrology studies were carried out on 53 specimens from 16 samples of oceanic basalt obtained from Deep Sea Drilling Project holes in the Atlantic Ocean. The specimens exhibited irreversible thermomagnetic curves and contained homogeneous titanomagnetites that were slightly maghemitized. They were categorized as deuteric oxidation class I basalts that had undergone slight to moderate regional hydrothermal alteration.
The natural remanent magnetization intensities (geometric mean value 1.0 × 10−3 Gauss) and the initial susceptibilities (geometric mean value 1.7 × 10−4 G/Oe) were considerably lower than those reported for oceanic basalts obtained from dredge hauls, but the ratios of the two parameters (geometric mean value 6) were comparable to values reported for other oceanic basalts. The remanent magnetizations were very stable against alternating field demagnetization with median destructive fields around 300 Oe. The site paleolatitudes deduced from the remanent inclination agreed well with a model reconstruction of the opening of the Atlantic Ocean, except at two sites where the basalts may have been drilled from sills.
The weaker magnetizations and susceptibilities of these specimens were attributed to a lower titanomagnetite concentration than found in dredge-hauled specimens. The amplitudes of model profiles computed with the observed NRM intensities agreed well with the amplitudes of marine magnetic anomalies measured near two of the DSDP holes. The model required an effective thickness of 2.5 km for the magnetized layer.
Normal and reversed remanent magnetization has been found in Plio-Pleistocene basalts from Bulgaria. Ore microscope observations show that the magnetic mineral is a homogeneous ‘titanomagnetite’: electron microprobe studies show that it contains a high proportion of Ti. Comparison of thermomagnetic measurements with the Curie points expected for stoichiometric titanomagnetites with the analysed chemical composition suggest that the titanomagnetites must be slightly oxidized, i.e. be slightly cation deficient.
This paper is an account of the Bakerian Lecture given to the Royal Society on 15 June 1967. Reversals of the Earth's magnetic field can be studied in the magnetization of lavas and sediments on land, in the magnetization of deep sea cores and in the magnetic pattern on the ocean floor. The lavas give radiometric dates but not a continuous sequence; the cores give continuity, great detail and a resolution as fine as 1000 years; the magnetic pattern gives information all through the Tertiary and connects the spreading of the ocean floor with the radiometric time scale. The dynamo theory of the Earth's magnetic field may be able to account for reversals as an instability in the dynamo, but only models with a finite number of degrees of freedom have been investigated. Spreading of the ocean floor is believed to be associated with convective motions in the upper mantle, although there are difficulties connected with the equality of the oceanic and continental heat flows. There is some evidence for the extinction of radiolaria at times of reversal of the magnetic field; it has been suggested that this is due to the effect of the field on cosmic rays but this appears impossible. If the extinctions are due to the reversals, the mechanism is unknown. Reversely magnetized rocks are more highly oxidized than normally magnetized ones. The cause of this is unknown and is one of the outstanding problems of Earth science.
Hotspot theory was first proposed on the basis of the observation of linear volcanic chains on the Pacific plate and assumed age progression within these chains. Knowledge of the ages of islands and seamounts is therefore of primary importance to analyzing intraplate volcanism and deciphering the history of hotspot tracks. In this paper we review published radiometric ages of islands and seamounts on the Pacific plate to help further reconstruction. We present a compilation of 1645 radiometric ages sorted by chain and further by island or seamount, along with a brief overview of each chain. Paleomagnetic ages obtained from seamount magnetism have not been considered, except for some oceanic plateaus (e.g., Shatsky rise). We do not consider foraminifer ages, which only give minimum ages of seamounts. Reliability problems intrinsic to the samples and to the radiometric dating methods must be considered. Dating of whole rocks must generally be disregarded unless they have been subject to special treatment, Ar/Ar incremental heating dating should be preferred over other methods, and data that do not pass the reliability criteria discussed by Baksi (this volume) should be disregarded. Thus use of the ages compiled in our database must be done in the light of filtering, and we encourage the user to check critically the initial papers in which the dates were published.
The average intensity of magnetization of a layer of pillow basalt depends on (l) variations in the intensity of the earth's field as the lava is formed, (2) variations in the direction of magnetization within the layer, (3) the percent of nonmagnetic pore space between pillows, and (4) variations in the thermoremanence of the basalt due to variations in composition and in the degree of crystallization. To evaluate (4) we made a centimeter-by-centimeter examination of submarine pillow fragments and found that the remanence increases from almost zero in the glassy crust to high values of .04 emu/cc in the interior. The main control on remanence is the degree of crystallization. In very large pillows the remanence reaches a peak and decreases toward the center, the decrease apparently being due to an increase in grain size. The magnetization of the basalts resides in titanomagnetite grains possessing natural remanences in the range 1 to 2 emu/cc. The variation of natural remanence with grain size in the range 3 to 6 microns suggests pseudo-single domain behavior. Our best estimate of the average magnetization of a layer of submarine pillow basalt is .014 emu/cc. A layer less than 1 km thick with a value of remanence this high is adequate to account for most marine magnetic anomalies.
A diverse suite of rocks has been sampled from ocean basin escarpments in the North Atlantic and the Caribbean: fresh and weathered basalts, metabasalts (zeolite and greenschist facies), gabbros, met, agabbros (greenschist and amphiolite facies), serpentinized peridotites, and actinolite rocks. One hundred and three representative specimens were chosen from this diverse suite of rocks, and the natural remanent magnetization (NRM) intensity and susceptibility for each specimen were measured in the laboratory. Unaltered and unweathered basalts had the highest measured NRM intensities (geometric mean, 10.69 X 10 -8 emu/cmS). The weathered basalts, metamorphosed basalts, gabbros, metagabbros, and actinolite rocks all had NRM intensities much lower than those of the fresh basalts (geometric means, 0.111.60 X 10 - emufcmS). The serpentinized peridotites, however, are strongly magnetic, the geometric mean intensity being 7.86 X 10 emu/cm s. The serpentinized peridotires also had high susceptibilities (geometric mean, 3.298 X 10 -a emu/cmS), whereas all the other rock types had low susceptibilities (geometric means, 0.047-0.480 X 10 -8 emu/ cmS). The measured magnetic parameters for the several rock types are compared with published models of the oceanic crust based on measurements of the magnetic field over the oceans. These geophysical models suggest that the upper part of the oceanic crust is a strongly magnetized layer of basalt and the majority of the crust below is weakly magnetized. The fresh basalts measured in this study have intensities and susceptibilities compatible with those in the geophysical models. The metabasalts, gabbros, metagabbros, and actinolite rocks have weak intensities and therefore could be, on the basis solely of their magnetic character, volumetrically important components of the weakly magnetized part of the oceanic crust. Serpentinized peridotite has intensities and susceptibilities too high to be an important component of the oceanic crust. Sampling of the Mid-Oceanic ridge, fracture zones, seamounts, abyssal hills, and trenches has shown that basalts, metabasalts, gabbros, metagabbros, and serpentinites characterize the oceanic escarpments [Wiseman, 1937; Shand, 1949; Quon and Ehlers, 1959; Engel and Engel,
The natural remanent magnetization of titanomagnetite is not destroyed by oxidation if the oxidation occurs below the original Curie temperature of the titanomagnetite.
The concept of plate tectonics has developed during the past four years from the hypotheses of continental drift and sea-floor spreading, supported by a variety of evidence from paleomagnetism, geochronology, and marine geology and geophysics. A series of four contiguous papers in the March 1968 issue of the Journal of Geophysical Research correlated on a global scale the linear magnetic anomalies, which are parallel to and bilaterally symmetrical about the oceanic ridge system, with the polarity reversals of the earth's magnetic field imprinted on new oceanic crust as it was generated at the oceanic ridge crests [Pitman et al., 1968; Dickson et al., 1968; Le Pichon and Heirtzler, 1968; Heirtzler et al., 1968]. In 1967 and 1968, four major papers introduced plate tectonics: the earth's surface is considered to be made up of a few rigid crustal plates or blocks in motion relative to each other [McKenzie and Parker, 1967; Morgan, 1968; Le Pichon, 1968; Isacks et al., 1968].
We report magnetic properties of submarine basalts 3.5 to 16 Ma in age recovered from depths as great as 530 m in layer 2 near the Mid-Atlantic Ridge at 37° N during Leg 37 of the Deep Sea Drilling Project. The rocks are classified as type-I if they have reversible in-vacuum thermomagnetic curves and as type-Il if they are thermomagnetically irreversible and develop a high-Curie-point phase (believed to be magnetite) when heated. Initial Curie points are low: 140–200 °C in type-I rocks, 250–300 °C in type-II rocks. The phases responsible are thought to be stoichiometric and cation-deficient (oxidized) titanomagnetite, Fe2.4Ti0.6O4, respectively. Only the 3.5 Ma basalts contain any type-I material; the older basalts are completely oxidized.Viscous magnetization is uniformly strong in type-I rocks, weaker and variable in type-II rocks. Hysteresis properties explain this difference. It is not due primarily to the chemical difference between stoichiometric and oxidized titanomagnetites, but to a difference in grain size. Type-I rocks are magnetically very soft: the coercive force (Hc) is 15–90 Oe (1194–7162 A/m), the median demagnetizing field of natural remanent magnetization (NRM) is 35–135 Oe (2785–10743 A/m), the ratio between saturation remanence Jrs and saturation induced magnetization Js is generally and the ratio of remanent coercive force, HR, to Hc is . These results all indicate multidomain grains of titanomagnetite ≥ 40 μm in size. Opaques of this size are seen in polished thin sections. Type-II rocks have Hc > 150 Oe (11937 A/m), (27 853 A/m), and HR/Hc generally < 2, indicating single-domain or pseudo-single-domain behaviour in micron- or submicron-size grains. The small magnetic grain size in type-II rocks could result from preferential oxidation of fine grains and/or subdivision of larger grains by inhomogeneous oxidation. The pronounced viscous magnetization of type-I rocks is therefore thought to be due to coarse, unoxidized multidomain grains of titanomagnetite.Long-term viscous magnetization is simulated by measuring viscous decay curves at temperatures up to 200 °C. Relaxation times are strongly temperature dependent: relaxation times as long as 106 yr can be activated in laboratory experiments at 75 °C if a low-Curie-point phase like Fe2.4Ti0.6O4 carries the viscous magnetization, or at 200 °C if Fe3O4 is the carrier. Viscous remanent magnetization (VRM) over 106 years seems to be no more than a factor 2 or 3 times the VRM estimated by extrapolating room-temperature data determined over a laboratory time scale. Even in type-I rocks, long term VRM is insufficient to completely erase the NRM.
Natural remanence inclinations and paleointensities have been obtained from six drill cores of oceanic tholeiite pillow basalt taken from within a 4 km by 3 km area of the median valley of the Mid-Atlantic Ridge near 45 °N. Only two of the remanence inclinations agree with the predicted value for the area of +64°. Two others are of normal polarity but at +37 °and +47 °are significantly shallower than the predicted inclination. The remaining two cores have reverse inclinations of −14 °and −47°. Bottom photographs suggest that at least the +37 °and −47 °drill cores were obtained from in situ material.The paleointensities are uniformly high, ranging from 0.63 Oe to 1.17 Oe.The combined remanence inclination and paleointensity results are consistent with the presence on the floor of the median valley of basalts erupted during the Laschamps event, a conclusion in line with the fission-track ages for the basalts.It is suggested that the general lack of agreement between measured and predicted remanence inclinations, a feature that may be widespread in North Atlantic submarine basalts, is a side effect of the tectonic process responsible for the transport of new crust from the floor of the Median Valley to the adjacent crestal mountains.
Palaeomagnetically observed reversals so far provide the strongest evidence1 that reversals originate from processes of core magnetohydrodynamics (MHD), thus furnishing a crucial constraint for geomagnetic dynamo modelling. The long-term variations in reversal frequency, favouring2 independent mechanisms for the dynamo and its reversals, provide an added constraint. Here we present evidence for a correlation between polarity superchrons and palaeointensity data, and also marine-magnetic and MAGS AT anomalies. Our results indicate that the dipole moment is correlated to the reversal frequency, being appreciably greater during the fixed polarity superchrons. This new information derived from the reversal record provides fresh insight into the workings of the geodynamo, and also explains why the oceanic crust associated with the Cretaceous quiet zone carries such a strong thermoremanence.
Some basalts from the Azores Islands, containing homogeneous titanomagnetites, exhibit a rapid increase in magnetization when heated in air to about 400 °C. A new magnetic phase with a Curie point of about 540 °C is produced. By measuring the rate of increase in magnetization at different temperatures the activation energy of the chemical change involved has been calculated to be 0–5 eV per molecule, and it is suggested that this corresponds to migration of cations in the spinel lattice.
The palaeomagnetism of Cretaceous Pacific seamounts, as deduced from their magnetic anomalies, is reviewed. Results from 35 seamounts are reported here for the first time. Of these 35 results, 15 are of high enough quality to be useful. This has resulted in a considerable increase in the available information concerning the palaeomagnetism of the Pacific plate. Using only the most reliable results, we calculate a palaeomagnetic pole for the upper Cretaceous at 58° North, 350° East, with a 95 per cent cone of confidence of only 5°. This pole is considerably more accurate than the one previously reported and is farther to the west. The previously reported difference between the mean pole position from seamounts close to Japan and the mean pole position from seamounts close to Hawaii is not real, although one group of seamounts close to Japan gives an anomalous pole position.
Several mechanisms have been suggested whereby errors might be introduced in the determinations of pole positions from seamounts. No error is introduced in the pole position because of the restricted extent of surveys and the consequent inaccuracy in the determination of the regional field. The demagnetization effect, which can produce differences between the observed direction of magnetization and the direction of the magnetizing field in an elongated body, is unimportant for seamounts.
The new pole position is sufficiently accurate for use in many types of calculations in which it is desired to locate the Pacific plate with respect to the Earth's spin axis.
KAr ages of 16 submarine basalts dredged from 5 seamounts in the Western Pacific area are presented. The KAr ages range from 30 my to 80 my. Because of large amounts of radiogenic argon in these samples (more than 10−10 moles/g), excess argon would not affect the KAr ages significantly and the discordant KAr ages can be best explained by diffusion loss of radiogenic argon. We conclude that the KAr ages obtained here can give at least a good estimate of minimum age of the seamounts.In the Western Pacific area, the KAr ages of the seamounts lie between the Cretaceous and the Early Tertiary in marked contrast to the relatively quiet volcanic activity in the Western Pacific Island arcs. Likelihood of the long time span for the growth of seamounts (more than 10 my) sets a minimum depth of 50 km to the supposed moving oceanic layer.
The radiogenic argon and helium contents of three basalts erupted into the deep ocean from an active volcano (Kilauea) have been measured. Ages calculated from these measurements increase with sample depth up to 22 million years for lavas deduced to be recent. Caution is urged in applying dates from deep-ocean basalts in studies on ocean-floor spreading.
Successive lava flows exposed in the Rio Grande gorge, near Taos, New Mexico, can be divided into three groups, each of different magnetic polarity.: the upper and lower groups have a normal polarity, whereas the middle one has a reversed polarity. K-Ar radiometric ages indicate that the base of the upper normal polarity sequence is about 3.7 m.y. old and that the reversed flows range from 3.7 m.y. to 4.4 m.y. The lowermost normal flow has the K-Ar age of 4.5 m.y. The age difference between the base of the upper normal and the top of the adjacent reversed flow group is about 0.05 m.y., indicating rapid polarity switching.
A total force geomagnetic survey was made in the northwestern Pacific during the US-Japan Scientific Cooperation Cruise of the R/V Argo of the Scripps Institution of Oceanography. In this survey, eleven seamounts were surveyed in detail both magnetically and bathymetrically. Combined with the existing data taken previously by both Japanese and US researchers, revised charts of the total geomagnetic field and its anomaly have been drawn for the area west of 172°E between 24°N and 46°N. The existence of magnetic lineations trending SWW-NEE which were noted previously have been confirmed, and their extension has been noted. They generally disappear as they get closer to the coast, but some seem to penetrate into the Japan Trench area. In the Shikoku Basin, the linear anomalies do not seem to exist. Some magnetic anomaly profiles have been investigated to see if there is any correlation with the anomalies in the Eastern and Central Pacific area. For a definite conclusion, however, good profiles in NNW-SSE direction in the northern Pacific area between 160°E and 180°E would be needed. 日米協同観測計画による上記の地磁気全磁力測量結果及び,現存する日米両国での北西太平洋海域の地磁気データにもとづいて,北緯24°～46°の間,東経172°E以西での地磁気全磁力及びその異常の分布図が得られた.この結果,従来しられたSWW-NEE方向にのびる縞状磁気異常が確認された.このような縞状構造のパターンを,東部太平洋でのN-S方向のそれ関係づけることは,太平洋の生成を知る上に重要であることが示される.
The direction and magnitude of the magnetization of four seamounts in the Pacific near the Japanese Islands are computed, using the total magnetic force and topographic survey data reported previously (UYEDA et al., Bull. Earthq. Res. Inst., 42, 555-570, 1964). Computation is made following Vacquier's method in which uniform magnetization is assumed. The results indicate that these seamounts are magnetized normally in declination but much shallower or reversed in inclination compared with the present geomagnetic field. Depending on the relative intensity of induced magnetization and remanent magnetization, the estimated palaeomagnetic pole positions fall in the central to north Atlantic Ocean. Some Cretaceous index fossils were reported from one of the seamounts. Some speculations are made regarding the possible significance of the obtained pole positions on the movements of ocean floors. 本州東北部および北海道沖の太平洋に存在する4個の海山について,その測深結果および,磁気測量(プロトン磁力計による全磁力)結果にもとづき,磁化方向,強度を推定した.推定にはVacquierの方法を用い,計算はスタンフォード大学のIBM 7090によつて行なわれた.海山はいづれも現在の地球磁場に比べて順方向に帯磁しているが,伏角はいづれも小さく,上向きの場合もあることが結論された.磁化は残留磁化が主成分と考えられるので,この結果にもとずき,古地磁気学的磁極を計算すると,大西洋北部に比較的よく集中する.これらの海山の生成年代は中生代と考えられるので,ここに得られた結果は,太平洋底の移動によるものかもしれない.但し,その可能性を確めるには,更に多くの海山について同様の研究を行うことが必要である.
Geomagnetic total force was measured by a towed proton precession magnetometer in the north-western Pacific(142°E-160°E,40°N-44°N) during the cruises of M/S "Ryofu Maru" and M/S "Takuyo" in JEDS(Japanese Expedion of Deep Seas) VI, VII(1963) and VIII(1964). Together with the data obtained in a previous cruise in JEDS IV(1961) and the data by "Spencer F. Baird" of the Scripps Institution of Oceanography during the Japanyon Expedition(1961), a magnetic total force distribution chart in the area is tentatively obtained. Subtracting the regional trend, an anomaly chart is also drawn. The charts indicate that there are anomaly lineations in the NEE-SWW direction with an amplitude of several hundred gammas. The lineations are of a width of about 30km, and are not apparently associated with the bottom topography. Four sea mounts were surveyed somewhat closely and the anomalies associated with them mapped. 表記航海において,気象庁観測船"凌風丸",および水路部観測船"拓洋"に便乗し,プロトン磁力計によつて地磁気全磁力の測量を行なつた．海域は主として,142°E-160゜E,40°N-44°Nであり,従来の結果,および,Scripps海洋研究所観測船"Baird"号による結果をまとめて,全磁力分布図を得た.また,地域全休の分布を観測値からさし引くことによって,局地異常図もつくられた.
The deeper holes drilled at the Guadalupe site bottomed in basalt. Most of the basalt has a subophitic texture. It consists of subradiating laths of plagioclase and augite, with rare phenocrysts of olivine, fine-grained magnetite, ilmenite, calcite, serpentine, chlorite, palagonite, hematite, and zeolites. Amygdules occur locally. The chemical analysis and a norm of the Mohole basalt are listed. Average analyses and norms of somewhat similar basalts from oceanic and plateau areas are included for comparison. It is apparent that compared with the Hawaiian and plateau basalts, the Mohole basalt is relatively high in CaO and in the ratio Fe 2 O 3 /FeO. It contains less TiO 2 , total Fe, and K 2 O. The high ratio of Fe 2 O 3 /FeO and the abundant total water may reflect the emplacement of the basalt in watery muds at or near the interface with ocean water.
This chapter highlights the ways in which rocks become magnetized. It also interprets paleomagnetic results in terms of the theories of polar wandering, continental drift, and an expanding Earth. The chapter describes the salient characteristics and trends of the geomagnetic field during the period of direct observation. Several questions are posed concerning the extension of these properties back to earlier times. Some of these questions can now be answered from the results of paleomagnetic research. The intensity of the main dipole component of the geomagnetic field appears to have been decreasing since well before the period of direct observation. Furthermore, two important properties of the geomagnetic field have been established by spherical harmonic analysis. First, the field is derivable from a potential and, hence, contributions to the total field from currents flowing across the Earth–air interface are negligibly small. Secondly, 2–5% of the field is of external origin and is because of movements of charged particles in the space around the Earth.
A ship-towed magnetometer survey of an area 250-300 miles wide off the foot of the continental slope along the coast of California has revealed a narrow pattern of anomalies of about 400 gammas magnitude trending north-south for more than 500 miles. The pattern is interrupted at the known faults and elsewhere; offset of the pattern at the Murray fault suggests a right-lateral displacement of about 84 nautical miles. The anomalies are such as might be expected of slablike structures underlying the ocean floor; geological possibilities include basic lava flows, topography of the main crustal layer, and intrusion of ultrabasic material from the mantle.
Phase-equilibrium studies in the system FeO-Fe2O3—TiO2 permit determination of the temperature and oxygen fugacity of formation of coexisting pairs of titaniferous magnetite and
ilmenite in many rocks. Temperatures thus obtained are probably accurate to ±50°C. Temperatures indicated for most igneous
and metamorphic rocks for which data are available are generally consistent with temperatures inferred by other methods. Temperatures
for certain gabbroic rocks are too low for magmatic crystallization and probably reflect the migration of ilmenite from titaniferous
magnetite to form separate granules upon cooling.
The experimentally determined solubility of ilmenite in magnetite is much too small to account for most ilmeno-magnetites
by simple exsolution. Subsolidus oxidation of magnetite-ulvöspinelgSS to yield ilmenite-magnetite intergrowths has been experimentally verified and probably takes place during cooling of many
igneous and perhaps some metamorphic rocks. Oxidation at surface or hypabyssal conditions may produce metastable titanomaghemites.
In order of increasing intensity of oxidation, the following Fe—Ti oxide pairs occur in plutonic rocks: ulvöspinel-rich magnetiteSS+ilmeniteSS, ulvöspinel-poor magnetiteSS+ilmeniteSS, ulvöspinel-poor magnetiteSS+hematiteSS, hematiteSS+rutile.
Physical mechanism causing the scattered polarization or the unstable magnetization of some dolerite sheets was investigated through various magnetic measurements and microscopic observation. It was found from X-ray and chemical analyses that titanomaghemite exclusively occurs in the rocks showing unstable magnetization. This implies that the scattered polarization or the unstable magnetization of the rocks may be attributable to the following mechanism: the thermoremanent magnetization is destroyed by natural oxidation from original titanomagneite to titanomaghemite with the result that the remanent magnetization measured in the laboratory is only the isothermal remanent magnetization produced by the magnetic after effect under the geomagnetic field. The situations that the magnetic instability of rocks varies systematically according to the degree of the alteration or weathering were also elucidated.
A series of solid solution xTiFe2O4.·(1-x) Fe3O4 was synthesized by ceramic method over a whole range of the composition, 1≥x≥0. Changes of the Curie point, saturation moment and the lattice parameter with the composition were examined. Generalized titanomagnetite having some vacant site in the structure normally occupied in a spinel was also prepared by oxidizing the TiFe2O4-Fe3O4 solid solution series. The region within which the spinel structure can be in existence as a single phase was settled in a FeO-Fe2O3-TiO2 system. An equal lattice parameter line, equal Curie point line and equal saturation moment line for the generalized titanomagnetite were drawn on the FeO-Fe2O3-TiO2 system.
Magnetic minerals separated from some acidic rocks undergo an irreversible change in Curie point and saturation magnetization when they are heated in vacuum: both Curie point and saturation magnetization decrease. Microscopic observation of polished sections reveal that all rocks which show such irreversible changes contain, in addition to titanomagnetite, some amount of titanomaghemite. All the other experimental resules such as chemical, X-ray and low-temperature thermomagnetic analyses indicate that the irreversible change reflects a break down of the γ-phase titanomaghemite to the α-phase ilmenite-hematite series during heating.
A detailed bathymetric-magnetic survey was conducted over the crest of the mid-Atlantic ridge between 42°N and 47°N (about 300 to 800 km northwest of the Azores). These survey data were analyzed to yield depth to source, intensity, and, to a limited extent, direction of magnetization. In addition, ten unweathered basaltic rock samples were dredged at two sites on the inner slopes of the median rift valley. Eight specimens of dense basalt from one site showed remanent magnetization of 0.002 to 0.011 cgs (average 0.007), and two samples of glass-sheathed basalt dredged from the second site yielded remanent intensities of 0.00055 and 0.00075 cgs. Q ratios ranged from 5 to 120 in the ten samples, with a median of 60. The high Q suggests that induced magnetization plays only a minor role in the large magnetic anomalies observed. The magnetic properties measured here are similar to those previously reported for oceanic basalts. The magnetic field structure in this area has two principal elements: (1) Anomalies of short wavelength (3 to 7 km) and (2) a long-wavelength anomaly (about 15 km) centered over the median rift valley. The short-wavelength anomalies are shown to be of shallow origin, being largely caused by topographic highs. A number of topographic features are either devoid of magnetic expression, however, or are magnetized only in part. Intensities of magnetization computed from the larger anomalies vary from 0.005 to 0.025 cgs and are thus comparable to intensities measured from dredge samples of the first site. Directions of magnetization are roughly ambient; in some cases the remanent declination may be somewhat to the west, and the inclination may be as much as 30° shallower than ambient. The long-wavelength magnetic high is approximated by a computed anomaly due to a vertical north-striking dike about 13 km wide and underlying the central rift valley at a depth of about 7 km below sea level (average deepest sounding over valley was 3 km). A magnetization contrast of at least 0.002 cgs is directed roughly parallel to the ambient field.
An extensive high-resolution magnetic survey of total field at sea level reveals unusual north-south lineations and much crustal faulting. Computations indicate that the linear pattern is due to strongly magnetized mafic rocks beneath the sediments.
A potassium-argon date from a basalt boulder dredged from a seamount near the crest of the northern extension of the East Pacific rise is 27 ñ 6 m.y. This note presents evidence for the association of Cobb seamount and its geologic age with the axial zone of the East Pacific rise. The theory of sea floor spreading by Hess and Dietz and the associated hypotheses of rise formation and magnetic anomalies [Vine and Matthews, 1963; Wilson, 1963a, b; and Vine and Wilson, 1965] would lead one to expect a date of I m.y. [Hess, 1965] or, according to the most recent analysis, 3.5 m.y. [Vine, 1966] for this portion of the East Pacific rise. Vacquier et al. [1961] lean toward a pre-Mesozoic age for the region immediately south of Figure 1, and Menard [1964] suggests pre-Tertiary volcanoes and the development of the East Pacific rise at the beginning of Tertiary time. The age of Cobb seamount is in conflict with these speculations. The area of controversy (Figure 1), part of the ridge and trough province of Menard and Dietz [1951], is now. thought to be the northern extension of the East Pacific rise [Menard, 1960], which emerges from under the continent off northern California. The north-south trending slight ridge delineated by shadLug in Figure 1, named variously Juan de Fuca ridge [Wilson, 1965] or Cobb rise [McManus, 1967] is probably the crest of the rise. This ridge is thought to be interrupted north of Heck seamount by a seismically active fault zone
The average direction of the remanent magnetization of the basalt layer at the bottom of hole EM 7 is inclined 36 ø above tile horizontal, indicating that the layer is reversely magnetized. Detailed demagnetization experiments with alternating magnetic fields to 800 oer- steds peak intensity indicate that the drilling operations did not remagnetize the basalts. The natural magnetization is extremely stable and is probably thermoremanent magnetization ac- quired as the basalt cooled through its Curie temperature, which is 355øC. The average inten- sity of the natural remanent magnetization is 0.0054 emu/cm 8, and the intensity of the induced magnetization is less than 5 per cent that of the reinanent. The ratio of remanent to induced magnetization, which is of importance in magnetic anomaly interpretations, decreases mark- edly with depth below the upper flow surface and emphasizes the danger of determining this ratio from material recovered only from the upper surfaces of submarine layers. Description ospecimens. Twenty-three specimen cylinders 2.49 cm in diameter by 2.29 cm long were collected from the basalt in core runs 5 and 7 of hole EM 7 of the experimental drilling phase of the Mohole pro,ject (Guadalupe site) for measurement of magnetic properties. These specimen cylinders were drilled with a water-coo.led diamond core drill and were o.ri- ented so that their axes were normal to the axis of the 9.53-cm-diameter large core. Since the large cores are unoriented azimuth- ally, the direction of the remanent magnetic vector can be determined only with respect to the axis of the large co,re. Errors in o.rienting the vertical axis of pieces o.f the large core, estimated to be --6 ø (standard deviatio.n), arise from the small size and irregular shape of so.me of the pieces. Additional errors due to deviations of the drill hole from the vertical are probably no.t greater than 5 ø, as is indicated by multishot surveys at a depth of 500 feet (Horton, 1961). A detailed description of these core runs has been given by Riedel, Ladd, Tracey, and Bram- lette (1961). The petrology and chemical com- position of the basalt has been described by Engel and Engel (1961). Depth osamples. Hole EM 7 was drilled 125 miles. from the coast of Baja California in 3568 meters of water. The basalt layer was encountered after the drill penetrated about 170
SECULAR variation of the intensity of the geomagnetic field in the past is of considerable interest as it may provide a new approach to the problem of the origin of the geomagnetic field.
A tectonic interpretation of the magnetic anomalies off the coast of California, Oregon and Washington between 40 and 52 north latitude shows that in the area surveyed the oceanic crust is cut by seven major dislocation zones which divide the region in eight areas. For five of these areas the original connection can be reconstructed. The existence of a window of young crust surrounded by older crust and of a short, isolated length of active oceanic ridge southwest of Vancouver Island as proposed recently by different authors is not confirmed.
The potassium-argon method of geochronometry may be applied quantitatively to biotite, hornblende and sanidine if these minerals
have been kept at low temperature since their formation. Fine-grained whole rocks, particularly of mafic varieties, may also
have 100 % retentivity under suitable conditions. Other whole-rock samples may give 75 – 85 % of the true age. Young volcanic
glasses may also be used for dating if they have not been devitrified but probably give somewhat low ages even in the best
cases.
Available samples of oceanic basalt have all proved to be young. In those cases where carbonate sedimentary rock overlies
the basalt, the carbonate deposition occurred shortly after the formation of the volcanic mountain or basaltic floor.
Introduction. Five dredge haauls were obtained during the Atlantis mid-Atlantic ridge expedition of 1948–1949 between latitude 30°01′N, longitude 42°25′W and latitude 31°49′N, longitude 43°25′W (Figure 1). Some of the specimens from these dredges were previously described by Shand [1949], Quon and Ehlers [1963], and Muir et al. [1966]. Olivine basalts, spilitic basalts, gabbros, and ultramafics are represented in the collection.
Vine and Matthews1 suggest that the pattern of local magnetic anomalies on the flanks of a mid-oceanic ridge is strongly lineated parallel to the ridge, and that these magnetic `stripes' represent strips of material in the upper mantle the directions of permanent magnetization of which are alternately parallel and anti-parallel to the present local geomagnetic field. Vine and Matthews suggest that mantle material cools as it rises convectively under a ridge and then spreads2 horizontally outward. As the material cools through its Curie point it is magnetized parallel to the contemporary local geomagnetic field. Because this field reverses quasi-periodically3,4 with a period 2T, T being of the order of 0.5-1.0 million years, stripes of alternate permanent magnetization are produced the width of which is vT, v being the local horizontal velocity with which material at the surface of the mantle spreads away from the centre of the ridge. The stripes are observed1 to have widths of the order of 20 km. If T is 0.5 million years, v is 4 cm/yr. Convective velocities of this order are also indicated by palæomagnetic data5.
K-Ar ages of rocks dredged southwest of the Hawaiian Islands (abstract)
- Dymond
Titanomaghemite in igneous rocks
- Katsura
Quoted in a paper by J. A. Cooper, and J. R. Richards
- McDougall
- Hess