Geomagnetic field intensities from Tertiary and late Carboniferous igneous rocks of the British Isles and Australia using modified Thellier and microwave palaeointensity techniques
Thermal Coe-modified-Thellier (CMT) and microwave (MW) palaeointensity (PI) investigations have been conducted on the Permo-Carboniferous Great Whin Sill (GWS) of northeast England, the Tertiary dyke swarm from the Isle of Skye and rocks from the Eastern Australia Cenozoic igneous province. These results provide important new palaeointensity data for the global palaeointensity (PINT) database in sparsely covered time windows. The GWS complex comprises at least three temporally distinct phases of intrusive activity. Fifty-six samples split between two of these phases, dated at ca. 269Ma and ca. 294Ma, have been investigated. Rock-magnetic investigations indicate pseudo-single- domain (PSD) grain assemblage of magnetite, with Curie temperatures between 550[sup]°C and 560[sup]°C in most samples. On comparison, data from CMT and MW investigations could not be distinguished in terms of quality or their respective mean PI value. Combined CMT and MW results yield Virtual Dipole Moments (VDM) of 1.2±O.1x10[sup]22 Am[sup]2 and 1.6± O.2x10[sup]22 Am[sup]2 for the younger and older phases respectively. These results, when coupled with previous estimates within the PINT database, support the view that the geomagnetic field intensity was dominantly lower during the Permo-Carboniferous Reverse Superchron in comparison with the present day value. The Tertiary dyke swarm of Skye comprises basaltic and doleritic intrusions. Thermomagnetic results suggest that the dominant magnetic mineral is magnetite or low-Ti titanomagnetite, with Curie temperatures between 490[sup]°C and 570[sup]°C. Hysteresis data suggest that samples comprise a PSD grain size assemblage. Many samples carry two anti-parallel remanence components, but the secondary low temperature (100- 300[sup]°C) is unstable. A mean 2.2± 0.91x 10[sup]22 Am[sup]2 VDM is calculated for the ChRM between 300-550[sup]°C, from forty-three samples. Several basaltic lavas and intrusions were sampled and investigated from the Australian Cenozoic igneous rocks. Rock magnetic analysis indicates Ti-enriched titanomagnetite (TM[sub]70- TM[sub]50) as the dominant magnetic phase in all samples with pseudo-single domain characteristics. The primary titanomagnetite carries a stable single component magnetization. Comparable MW and CMT data combine to produce three mean VDMs; 4.7± 0.7x 10[sup]22 Am[sup]2, 2.9± 1.4x 10[sup]22 Am[sup]2 and 3.1± 1.7x 10[sup]22 Am[sup]2 for volcanic rocks dated at c.48Ma, 46Ma and 22Ma respectively. The low mean PI from both the British and Australian igneous provinces is comparable with results from contemporaneous rocks, documented in the PINT database, suggesting the early Tertiary field was weaker than that of the present day. Results from each of the sampled igneous provinces demonstrates comparability between the microwave and thermal CMT palaeointensity techniques. However, while the microwave method significantly shortens experimental time, no significant enhancements in data quality or success rates were achieved on the rocks investigated.