Multivectorial paleointensity determination from the Cordova Gabbro, southern Ontario

Abstract

We report geomagnetic field intensities for the A (V1000 Ma) and B (V850 Ma) natural remanent magnetizations of the Cordova Gabbro of southern Ontario. We interpret A and B to be secondary thermal overprints produced during regional uplift and cooling following the Grenvillian Orogeny. Multivectorial A+B paleointensity data were analyzed using pairs of Arai diagrams for the separated A and B vectors. From 18 reliable A results and 15 reliable B results, we find paleofield values of 18.1 þ 2.1 WT and 7.1 þ 1.5 WT, respectively. The corresponding virtual axial dipole moments (VADMs) are (3.12 þ 0.36) and (1.82 þ 0.38)U10 22 A m 2 , about 70% and 40% of the average Phanerozoic dipole moment based on data from 0.3^300 Ma. The Cordova data and other Precambrian paleointensities are generally within a range of approximately 0.5^1.5 times the Phanerozoic mean VADM. Three sets of paleofield values as a function of paleolatitude, including the Cordova data, support a dipole configuration for the Earth's field in 251 50 Ma time windows around 2700 Ma, 1100 Ma, and 850 Ma. ß

Paleomagnetism and petrophysics of the Jänisjärvi impact structure, Russian Karelia

“…Additionally, the data consist of one single measurement only. At the same time, the data is in obvious contrast with reliable results recently obtained for the Cordova Gabbro (850 Myr) by Yu et al (2002), showing a VDM of 1.8 ± 0.4 10 22 Am 2 . The comparison with the other PINT03 data suggests that the intensities determined from Jänisjärvi are considerably biased towards higher values.…”

Partial anhysteretic remanent magnetization in magnetite 2. Reciprocity

“…The lake sediments and gabbros used in this study had intermediate AF behavior, with about 50% of pARM demagnetizing over the field blocking range (Figure 4). They nevertheless have previously yielded successful paleointensity results by the pseudo‐Thellier and Thellier methods, respectively [ Brachfeld and Banerjee , 2000; Yu and Dunlop , 2001, 2002]. This success is rather surprising in view of their ambivalent behavior in pARM reciprocity experiments.…”

“…Relative intensities are plotted in Figure 7. For SD and PSD samples 1 (0.065 μm), 4 (0.24 μm), 5 (0.34 μm), 6 (1.06 μm), 456 A, 578 A, T 19, and C 12 (Cordova Gabbro, Ontario [ Yu and Dunlop , 2002]), the intensities of pARM n * and pARM n are almost identical. However, for large PSD and MD samples 9 (16.9 μm), 10 (18.3 μm), Bu 5, Bu 8, and S 50, pARM n * is 6–14% more intense than pARM n .…”

Effect of low‐temperature treatments on pseudo‐Thellier paleointensity determination

“…Twenty‐eight natural samples were also studied: two andesites (An 1, An 3), one pumice (Na 2), one red scoria (Km 3) [ Yu , 1998], 14 gabbros [ Yu and Dunlop , 2001, 2002], three granites [ Dunlop , 1984; Dunlop et al , 1984], and seven freeze‐dried lake sediments [ Brachfeld and Banerjee , 2000]. The natural samples selected have magnetic and paleomagnetic properties that are well documented.…”

“…They were chosen from a much larger collection of several hundred cores on the basis of their low magnetic fabric anisotropy, their reproducible ARM and TRM intensities (ARM only for sediments), and minimal viscous magnetic changes. The gabbros [ Yu and Dunlop , 2001, 2002] and lake sediments [ Brachfeld and Banerjee , 2000] had yielded reliable paleointensities by the Thellier and pseudo‐Thellier methods, respectively.…”