Abstract
Abstract Natural rock samples may not always be the ideal material for the Thellier-type method as they occasionally result in high paleointensities. The Kilauea 1960 lava, Hawaii, is one such example. Several previous studies have suggested that one of the possible causes for this undesirable behavior is an acquisition of thermochemical remanent magnetization (TCRM) during lava formation. In order to examine this possibility quantitatively, equilibrium temperatures of titanomagnetite grains, which are associated with samples previously subjected to Thellier experiments, are estimated by a Fe−Ti oxide geothermometer. The results show that two specimens from the rock magnetic group giving relatively ideal Thellier paleointensities have clustered equilibrium temperatures of about 800–900 and 700–800°C. In contrast, two swarmed temperatures around 300 and 700°C were observed for the specimen from a group yielding high paleointensities. Although these are semi-quantitative estimates, when the time scales of Fe−Ti interdiffusion and lava cooling are taken into consideration, the last specimen could have acquired the TCRM during its formation. For such specimens, simple calculation predicts that TCRM/TRM (thermoremanent magnetization) ratios could be 1.19–1.72 for the blocking temperature range of 400–480°C, assuming a grain-growth model. The extent of this overestimation (20–70%) is comparable to the magnitude of the observations. It is therefore suggested that attention be paid to titanomagnetite grains with well-developed ilmenite lamellae, as these could be potential sources of overestimations of Thellier paleointensities of up to a few tenths of percentage points.
Highlights
The ability to make precise determinations of absolute paleointensities is an essential criterion for investigating the evolution of the geodynamo, as this in turn provides information on the Earth’s core
One possible cause which has been proposed is the acquisition of thermochemical remanent magnetization (TCRM), which is the magnetization acquired when slow cooling is accompanied by volume growth of a magnetic phase below its Curie temperature (Dunlop and Ozdemir, 1997)
Grommeet al. (1969) first pointed out that in some basaltic lavas the magnetic minerals may form through subsolidus reactions at temperatures well below their final Curie temperatures, resulting in a mixture of thermoremanent magnetization (TRM) and TCRM
Summary
The ability to make precise determinations of absolute paleointensities is an essential criterion for investigating the evolution of the geodynamo, as this in turn provides information on the Earth’s core. I discuss the relationship between the equilibrium and Curie temperatures of the samples These estimations require that the chemical compositions of both TM hosts and coexisting ilmenite lamellae be determined and are undertaken by a combined approach of analyses by an electron probe micro-analyzer (EPMA) and measurements of magnetic susceptibilities at both low and high temperatures. Samples The samples subjected to the present analyses had been collected for the study of Yamamoto et al (2003) They were drilled from an outcrop of the Kilauea 1960 lava, Hawaii, and were classified into three groups (A, B, and C) according to increasing high temperature oxidation states (C1–C3, C2–C4, and C4–C7; Haggerty (1991)). All of the TM grains observed showed a sandwich-type high-temperature oxidation: large TM hosts associated with broad ilmenite lamellae (Fig. 3A).
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