Earth's Field Intensity Research Articles

Paleomagnetism, paleointensity and geochronology of a Proterozoic dolerite dyke from southern West Greenland

Archean to Paleoproterozoic rocks potentially record the evolution of the geodynamo and the tectonic mode of the early Earth. The paleomagnetic intensity and direction data provide important information on the Earth’s core–mantle revolution. Herein, we report the results of paleomagnetic and geochronological studies of a Proterozoic dolerite dyke from southern West Greenland. Clinopyroxene grains from the dyke yielded Ar–Ar plateau ages from 1808 to 1887 Ma (1816.0 ± 14.6 Ma; 2σ). The paleomagnetic direction of the dyke (D = 243.6°, I = 66.3°, α95 = 3.9°) yielded a virtual geomagnetic pole (VGP) of 33.5 °N and 96.4 °W. This 1.8 Ga pole falls in a limited area where Paleoproterozoic poles between 2.5 Ga and 1.7 Ga for southern West Greenland are distributed. Comparison of the Paleoproterozoic poles of southern West Greenland with those of North America suggests that the North Atlantic Craton of southern Greenland could have been an independent stagnant tectonic block, different from the drifting Superior and Slave Cratons in the Early Proterozoic. Thellier experiments on 13 specimens yielded a mean paleointensity value of 14.8 ± 2.3 μT, indicating a virtual dipole moment of 2.88 ± 0.46 × 1022 Am2. This value is approximately one-third of the present-day Earth's field intensity, and is consistent with the value for the period between 1400 Ma and 2400 Ma. This small paleointensity value of the Proterozoic rocks is due to a gradual dipole moment change over a long period (∼1 Gyrs) since 4000 Ma.

Paleomagneticists discover rock trait that may offer clues to seafloor evolution

Paleomagneticists at Columbia University have discovered a magnetic trait of seafloor rocks that may provide a new “clock” to map changes to the ocean floors on a timescale of several thousand years—which is a much more refined scale than that offered by prevailing methods. Such a tool may elucidate the evolution of mid‐ocean ridges and perhaps even the dynamics of the Earth's field intensity when combined with state‐of‐the‐art uranium calibration techniques.For decades, scientists have relied on reversals in the magnetic direction of rock found along the Earth's mid‐ocean ridges to reconstruct the history of the creation and movement of the Earth's crustal plates, whose boundaries lie along these ridges.

Intensity of the Earth's Magnetic Field in the Geological Past

RECENTLY, Krs1,2 has obtained two estimates of the Permo-Carboniferous geomagnetic field intensity from cassiterite, and one estimate of the Permian field intensity from ignimbrite, in central Europe, using the method developed by Thellier and Thellier3. Both the Permo-Carboniferous values (0.50 oersteds and 0.60 oersteds at lat. 50.12° N., long. 12.80° E.; maximum error ±10 per cent) and the Permian value (0.50±0.03 oersteds at lat. 50.63° N., long. 16.30° E.) were similar to the present field intensity in the same region (0.482 oersteds), the implication being that the Earth's field intensity has not changed appreciably since the Permo-Carboniferous. Furthermore, because the cassiterite was obtained from a greisen body. Krs considered that its thermoremaneiit magnetization was acquired over a period long enough to average out secular variations in intensity. The quoted Permo-Carboniferous field intensity estimates were thus thought to represent the mean geomagnetic field rather than the instantaneous field corresponding to a certain point on the secular variation curve.