Erik J. Schwarz

Geomagnetic intensity between 100 million and 2500 million years ago

Abstract The decay of the NRM of 146 specimens (including the 41 specimens reported on earlier) of igneous rocks during heating in steps of 25 or 50 °C between 200 or 300 °C and 550 °C was compared to the acquisition of TRM (H = 0.35 Oe) during subsequent cooling to 20 °C. The K-Ar ages of the specimens are distributed between 100 m.y. and 2500 m.y. The selection of results suitable for intensity determination was based on constancy of the rate of decay of the NRM and acquisition of TRM determined from at least 3 successive determinations over a temperature interval of at least 100 °C, an appreciable decay of the supposed original NRM in that temperature interval, and constancy in the NRM direction in that temperature interval. This selection yielded a total of 41 equatorial paleointensities. The intensity results obtained for several specimens collected from the same rock unit show a reasonable degree of internal consistency. The results indicate that the mean equatorial geomagnetic intensity during most of the Phanerozoic and the Precambrian eras as far back as 2.5 billion years was 0.25 Oe with a standard deviation of 0.13 Oe. The large scatter in the equatorial intensities may be due to various possible errors and/or fluctuations in geomagnetic moment of periods shorter than the experimental error (5 to 10%) of the radiometric ages of the specimens, possibly including a period as short as that suggested by the archeomagnetic results for the last five millenniums. The dipole representation of the paleomagnetic field as far back as 2500 m.y. is not contradicted by the variation of the paleointensity values with paleomagnetic latitude. Thus, several characteristics of the present geomagnetic field seem to have been present as far back as 2500 m.y. ago.

Identifying types of remanent magnetization in igneous contact zones

Abstract The magnetization of host rock adjacent to an igneous intrusion (e.g., dyke) will be completely reset near the contact, partially reset in a zone of hybrid directions farther away, and unaffected at even greater distances. If the intrusion-related direction and the host direction are known, the hybrid zone magnetization can be analysed in terms of pTRM, VRM and CRM overprints of the host magnetization using orthogonal component plots of thermal demagnetization results. Straight-line segments representing the host and the intrusion-induced magnetization show a sharp intersection in cases of (1) a pure pTRM overprint, (2) a VpTRM overprint of one magnetic carrier, (3) two magnetic minerals with a VpTRM partial overprint of the high blocking temperature mineral and a complete VpTRM overprint of the low blocking temperature mineral, and (4) a CRM overprint where its blocking temperature range is entirely distinct from that of the host component. A gradual transition between both components during thermal demagnetization is expected for (a) two magnetic minerals both of which have a VpTRM partial overprint, (b) a VpTRM partial overprint of a continuous range of magnetic mineral compositions and (c) a CRM overprint in which the blocking temperature range partially overlaps that of the host component. Swings from one direction to the other and back are observed if the blocking temperature of one of the components completely overlaps that of the other, one of them being due to CRM.

On the intensity of the paleomagnetic field between 100 million and 2500 million years ago

Abstract The thermal decay of the natural remanent magnetization was compared with the acquisition of thermoremanent magnetization in a field of 0.49 Oe for 41 samples of igneous rocks. The radiometric ages of these rocks are between 100 m.y. and 2500 m.y. Both the low and high stability components of the natural remanence of most of the samples are complex. However for 25 samples, at least the component of intermediate stability against heating is interpreted to be of the thermoremanent type. The data obtained after heating to at least four successive temperatures in the intermediate temperature range were used to calculate the field strength at the contemporary magnetic equator. The results of these calculations tentatively suggest that the equatorial paleomagnetic intensity 1) decreased between 200 m.y. and 100 m.y. ago, 2) increased between 600 m.y. and 200 m.y. ago, and 3) ranged between limits of 0.2 and 0.9 Oe with an average of approximately 0.45 Oe for the rest of the Precambrian era investigated in this preliminary study. The Precambrian results tend to suggest a high and low equatorial field strength respectively 1100 m.y. and 1400 m.y. ago.

Magnetic expressions of intrusions including magnetic aureoles

Abstract The aeromagnetic expressions of two diabase dykes in the Abitibi greenstone belt are compared with each other as are those of three granitic plutons in the Eastern Townships in Canada with the benefit of ground magnetic survey data, known magnetic properties and known geology. Similar aeromagnetic expressions for these examples i.e. positive aeromagnetic anomalies for the diabase dykes and a ring-shaped structure associated with two of the granite plutons, are shown to be due to different geological sources in each case. In the case of the two diabase dykes, the concomitant aeromagnetic anomalies are due to a strong downward directed total magnetization in one of the dykes and to strongly magnetized contact zones along the other dyke. In the case of the granitic plutons, the ring-shaped feature of one pluton is due to a more basic, more strongly magnetized ring dyke, whereas for the other pluton it is due to a magnetic halo within the surrounding host rock. Quantitative interpretation of aeromagnetic anomalies without ground follow-up is therefore hazardous. The formation of a magnetic halo in the host rock is an important accompanying feature and is considered in some detail using heat conduction theory.

Magnetic anomalies due to pyrrhotite in Paleozoic metasediments in Nova Scotia, eastern Canada☆

Abstract The metasedimentary Meguma Group consists of Goldenville Formation meta-greywackes and quartzites with minor occurrences of green slates conformably overlain by Halifax Formation black slates. Elongate positive magnetic anomalies parallel to bedding strike occur over the Halifax Formation. Analysis of rock samples of Halifax Formation with a recording thermomagnetic balance indicates that pyrrhotite is the dominant magnetic constituent and therefore the probable cause of the anomalies. The pyrrhotite phase is Fe7S8, lacks Fe9S10 intergrowths, and occurs as lenses up to 2 cm long along the cleavage planes. It was determined that 1 wt% Fe7S8 corresponds to a bulk magnetic susceptibility of 2.2 × 10−3 SI. The pyrrhotite concentrations within the Halifax Formation were modelled from aeromagnetic profiles using Magrav 2, a2.5-dimensional interactive computer program. The model geometry was constrained by bedding strike/dip measurements. Remanent magnetization data were inadequate so modelling was based on the induced component only. The models indicate that pyrrhotite concentrations of 6–12 wt% are required to match the observed anomalies. These values are higher than the 2 wt% concentration determined from surface sample measurements. The combined use of rock properties measurements, aeromagnetic image analysis, and modelling can be used to define subsurface fold geometry which is of interest to gold explorationists, because the gold occurrences in the area are structurally controlled.

Detecting Buried Placers By Magnetic Survey

Concentrations of heavy minerals in alluvial deposits and laterites may include those of economic interest such as gold and also magnetite. Computer assisted modelling shows that if the magnetite content is at least 0.5% higher in a long plate of thickness 1 meter, width 10 meters, and depth 10 meters with respect to that of the sediments surrounding the buried plate, the latter is detectable by a detailed ground vertical gradiometerltotal field survey using a sensitive magnetometer (at least 0.1 nT). Testing over post-glacial river deposits in the Eastern Townships of Quebec shows well developed total field and vertical gradient trends with a range of spatial frequency components related to the valley directions rather than the NE-SW trend of the underlying bedrock, indicating that they are due to magnetite concentrations within the river sediments and/or buried channels in the bedrock. High intensity anomalies are observed in areas of ultramafic intrusions where magnetite is more abundant. Power spectra show well developed linear relation ships between the logarithm of the energy and wave number for the sorted river sediments but show a smooth relation over unsorted facial till, (homogeneous distribution of magnetite). These observations reinforce the conclusion that the anomalies observed are due to sources within the river sediments. Contour maps of total field and vertical gradient indicate the areas of primary interest in placer exploration.