Photomicrographs in backscatter electron image mode of tholeiitic basalt samples
Photomicrographs in backscatter electron image mode of tholeiitic basalt samples. (a) An untreated sample of basalt from São Tomé island (Gulf of Guinea); titanomagnetite grains are light grey. (b) After heating to 570 degrees Celsius and cooling back to room conditions, a grain has altered into domains of exsolved ilmenite lamellae and Ti-poor magnetite, with consequently higher Curie temperature than the original titanomagnetite shown in (a). The intensity of thermochemical remanence resulting from cooling the sample in a known laboratory field can be accurately recovered with standard paleointensity techniques. Credit: Shcherbakov et al. [2019], adapted from Figure 2
Source: Journal of Geophysical Research: Solid Earth

An important aspect in studies of the Earth’s past magnetic field strength is how chemical changes in rocks, arising for example from metamorphic overprinting, may have affected magnetic remanence and hence paleointensity estimates.

A new laboratory study by Shcherbakov et al. [2019] combining petrological and magnetic methods scrutinizes the acquisition of thermochemical remanence within titanomagnetite, one of the most abundant iron oxides present in igneous rocks. Samples from São Tomé island and the Red Sea Rift, heated to 570 °C and subsequently cooled back to room conditions in a controlled magnetic field, reveal the formation of ilmenite lamellae within titanomagnetite grains. This chemical alteration process leads to the creation of Ti-poor intragrain regions, of composition and Curie temperature representative of magnetite, with the interesting property that the obtained paleointensity estimates are like those predicted from a strictly thermal remanence.

The results contribute to a long-standing debate among paleomagnetists about how to treat exsolution lamellae in iron oxides, which can give experimentally superior results in a paleointensity experiment, but whose thermochemical remanence nature has subjected them to suspicion. This opens for a new set of potential targets of paleointensity studies and may have application to older rocks that have undergone thermochemical alteration and metamorphism, such as in the Precambrian eon.

Citation: Shcherbakov, V. P., Gribov, S. K., Lhuillier, F., Aphinogenova, N. A., & Tsel’movich, V. A. [2019]. On the reliability of absolute palaeointensity determinations on basaltic rocks bearing a thermochemical remanence. Journal of Geophysical Research: Solid Earth, 124.

—Bjarne S. G. Almqvist, Associate Editor, JGR: Solid Earth

Text © 2019. The authors. CC BY-NC-ND 3.0
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