In search of high-fidelity geomagnetic paleointensities: A comparison of single plagioclase crystal and whole rock Thellier-Thellier analyses

The strength of Earth's magnetic field, on timescales greater than a millio n years, is known primarily from Thellier-Thellier analyses of igneous rock s. In the Thellier-Thellier method, the natural remanent magnetization (NRM ) is compared with a thermoremanent magnetization (TRM) imparted during hea ting in a known laboratory field. Unfortunately, the multiple heating steps required by the method often result in alteration of magnetic minerals in clay-bearing groundmass of whole rock samples. This alteration may result i n the creation of new magnetic minerals that could affect TRM acquisition. Analyses of single plagioclase crystals, which contain magnetic inclusions that retain paleofield values, may offer a way to retrieve paleointensity d ata that is less affected by alteration. We test this hypothesis through a study of whole rocks and single plagioclase crystals from a mid-Cretaceous (113-115 Ma) basalt flow of the Rajmahal Traps (northeastern India). Magnet ic hysteresis and thermal demagnetization data from plagioclase crystals an d whole rock samples are similar, suggesting a common magnetic mineralogy. However, hysteresis data from heated whole rock samples indicate that a fin e-grained magnetic phase has formed. This effect was either absent, or much less pronounced in the single plagioclase crystals tested. Paleointensity data from these crystals suggest a paleofield value of 65.1 +/- 5.3 mu T(n= 15). Data from whole rock samples overlap these values but are on average l ower. We attribute the difference in this study to the preferential growth of new magnetic minerals in the whole rock samples, resulting in an enhance d acquisition of TRM and shallowing of the slope of NRM-lost versus TRM-gai ned curves. These data suggest that subtle alteration effects may bias Thel lier-Thellier results from some whole rock samples toward low paleofield va lues. In these cases the single plagioclase crystal approach may yield more reliable estimates of geomagnetic paleointensity.