Understanding Nonideal Paleointensity Recording in Igneous Rocks: Insights From Aging Experiments on Lava Samples and the Causes and Consequences of "Fragile" Curvature in Arai Plots. (22nd January 2021)

• Record Type: Journal Article
• Authors: Tauxe, L.; Santos, C.N.; Cych, B.; Zhao, X.; Roberts, A.P.; Nagy, L.; Williams, W.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 22:Number 1(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-01-22
• Subjects: paleointensity -- paleomagnetism; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2020GC009423
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: The theory for recording of thermally blocked remanences predicts a quasilinear relationship between low fields like the Earth's in which rocks cool and acquire a magnetization. This serves as the foundation for estimating ancient magnetic field strengths. Addressing long‐standing questions concerning Earth's magnetic field requires a global paleointensity data set, but recovering the ancient field strength is complicated because the theory only pertains to uniformly magnetized particles. A key requirement of a paleointensity experiment is that a magnetization blocked at a given temperature should be unblocked by zero‐field reheating to the same temperature. However, failure of this requirement occurs frequently and the causes and consequences of failure are understood incompletely. Recent experiments demonstrate that the remanence in many samples typical of those used in paleointensity experiments is unstable, exhibiting an "aging" effect in which the (un)blocking temperature spectra can change over only a few years resulting in nonideal experimental behavior. While a fresh remanence may conform to the requirement of equality of blocking and unblocking temperatures, aged remanences may not. Blocking temperature spectra can be unstable (fragile), which precludes reproduction of the conditions under which the original magnetization was acquired. This limits our ability to acquire accurate and precise ancient magnetic field strength estimates because differences between known and estimated fields can be significant for individual specimens, with a low field bias. Fragility of unblocking temperature spectra may be related to grain sizes with lower energy barriers and may be detected by features observed in first‐order reversal curves. Plain Language Summary: Earth's magnetic field acts as a shield against energetic solar storms and is thought to have been important in the evolution of life on Earth. The magnetic field is currently dropping rapidly in strength. Answering questions like "What is the average field?" and "What is the likelihood of a collapse associated with a reversal or excursion?" depends on our understanding of past field behavior. There are no human measurements of field strength prior to the nineteenth century, so we rely on igneous and archaeological records. A great deal of effort has been put into experimental protocols to develop reliable records of field strength and to assess data reliability. Yet, mysteries remain regarding the nature of these records. This paper focuses on expanding our understanding of magnetic recording in lava, which is one of the main archives used in paleointensity studies. In particular, we investigate the causes and consequences of failure of the principal assumptions in paleointensity experiments which appears to result in biased estimates. Key Points: Unequal blocking/unblocking temperature spectra can lead to biased paleointensity estimates Inequality grows through time in samples with coarser magnetic grain sizes Quantifying curvature in Arai plots and the use of iFORCs can identify possibly biased results