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SUMMARY Accurate absolute palaeointensity is essential for understanding dynamo processes on the Earth and other planetary bodies. Although great efforts have been made to propose techniques to obtain magnetic field strength from rock samples, such as Thellier-series methods, the amount of high-fidelity palaeointensities remains limited. One primary reason for this is the thermal alteration of samples that pervasively occurred during palaeointensity experiments. In this study, we developed a comprehensive rock magnetic experiment, termed thermal rock magnetic cycling (TRMC), that can utilize measurements of critical rock magnetic properties at elevated temperatures during multiple heating-cooling cycles to track thermal changes in bulk samples and individual magnetic components with different Curie temperatures in samples for palaeointensity interpretations. We demonstrate this method on a Galapagos lava sample, GA 84.6. The results for this specimen revealed that GA 84.6v underwent thermophysical alteration throughout the TRMC experiment, resulting in changes in its remanence carrying capacity. These findings were then used to interpret the palaeointensity results of specimen GA 84.6c, which revealed that the two-slope Arai plot yielded two linear segments with distinct palaeointensity values that were both biased by thermophysical alteration. To further test the TRMC method, we selected another historical lava sample (HS 2) from Mt Lassen, detecting slight thermal-physical changes after heating the specimen HS 2–8C to a target temperature of 400 °C. We also isolated a stable magnetic component with a Curie temperature below 400 °C using the TRMC method, which may provide a more reliable palaeointensity estimate of 51 μT. By providing a method for tracking thermal alteration independent of palaeointensity experiments, the TRMC method can explore subtle, unrecognizable thermal alteration processes in less detailed palaeointensity measurements, which can help to assess the thermal stability of the measured samples and interpret the changes in the TRM unblocking spectrum and palaeointensity estimates, facilitating the acquisition of more reliable records for constrain the formation of the inner core and the evolution of Earth's magnetic field.
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Absolute palaeointensity estimates from Precambrian India and the long-term thermal evolution of the Earth
SUMMARY Despite significant progress in palaeomagnetic research over the last century, the origin, evolution and long-term behaviour of the geomagnetic field remains poorly understood. One significant open question is when and how the inner core nucleated. Since geomagnetic field behaviour is intrinsically linked to the thermal evolution of the core, scientists have turned to the global palaeointensity record to search for proxies for inner core nucleation. From this record, two signals have been identified as possible indicators of inner core nucleation: (1) a spike in magnetic field strength between 1.5–1.0 Ga, and (2) an initially strong, but gradually decreasing field strength that resulted in a weak dynamo in the Ediacaran. Although both these hypotheses are vastly different, they do have one common challenge hindering rigorous testing: a paucity of palaeointensity data. This is especially true for the Precambrian time period for which well-preserved outcrops are scarce and weathering/alteration is nearly inescapable. Despite making up almost 90 per cent of Earth's history, data from this super eon comprise < 10 per cent of the global palaeointensity database. This lack of data for most of Earth's history represents a considerable gap in our knowledge and greatly impedes our ability to understand the origin and evolution of our planet and its magnetic field. In an effort to fill in this gap, we performed palaeointensity experiments on Precambrian-aged mafic dykes from India (Malani Igneous Suite and Bastar, Dharwar and Bundelkhand Cratons) with ages ranging from ∼740 Ma to ∼2.36 Ga. To monitor thermal alteration and minimize the effects of non-ideal grain sizes, the Thellier method following the IZZI protocol was used. Successful results were obtained for samples from the Bundelkhand (∼740 Ma) and Bastar (∼1.89 Ga) cratons. The Bastar results fall in a ∼40 Myr gap in the database and corroborate field trends predicted by the Monte Carlo axial dipole moment model, which suggests that intensity values were moderately low (2–4 × 1022 A m2) in the middle Palaeoproterozoic. The Bundelkhand result suggests that the field may have been rapidly decaying in the late Tonian to early Cryogenian.
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- Award ID(s):
- 2016763
- PAR ID:
- 10673678
- Publisher / Repository:
- Oxford Academic
- Date Published:
- Journal Name:
- Geophysical Journal International
- Volume:
- 241
- Issue:
- 1
- ISSN:
- 0956-540X
- Page Range / eLocation ID:
- 291 to 307
- Format(s):
- Medium: X
- Associated Dataset(s):
- View Associated Dataset(s) >>
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1093/gji/ggaf038
