Авторы: Baker E., Muxworthy A.

2023 г.

JGR Solid Earth

The behavior of grain-growth chemical remanent magnetizations (gCRM) are investigated for different coercivity and magnetostatic-interaction-field distributions and acquisition conditions using a thermally activated Preisach model for assemblages of interacting single-domain grains. A new growth-rate dependent equation was derived, from which it was found that gCRM intensity is over 10% more sensitive to growth rate than previously modeled. We compare the gCRM results to the behavior of thermoremanences (TRM). gCRMs are two times more sensitive to changes in coercivity distribution, whereas TRMs are four times more sensitive to changes in magnetostatic interactions. The Thellier-Thellier-Coe paleointensity protocol was simulated in Preisach space, and gCRMs were found to produce concave-up Arai plots with pTRM checks which plot to the left of the Arai plot and positive partial-TRM tail checks that increase with magnetostatic interactions. This often leads to the failure of selection criteria, but high-temperature segments can pass the criteria for weakly interacting gCRMs; these estimates can underestimate the field by up to 66%. Our model predicted gCRM/TRM ratios in the range 0.28–1.2 for various grain distributions, acquisition times (1,000 s –10,000 years) and temperatures (27°C–527°C). gCRM(527°C)/TRM ratios of 0.85 and 0.89 for PN2 and PW2 were calculated from the Aria plots and a maximum gCRM/TRM of 1.2 was found for gCRMs acquired over 10,000 years relative to TRMs acquired on laboratory timescales (Figures 4, 5 and 10). A large variation in gCRM/TRM ratios has also been reported experimentally: Stokking and Tauxe (1990) found gCRM/TRM ≈ 0.15 acquired in hematite at 97°C, whereas Hoye and Evans (1975) measured gCRM/TRM ≈ 0.6 for magnetite formation from oxidation of olivine at 500°C, by measuring anhysteretic remanent magnetzation (ARM) and using TRM = 1.2ARM. Our model, by comparison, predicted gCRM(527°C)/TRM of 0.84 and 0.90 for PN2 and PW2. Gendler et al. (2005) measured a gCRM(350°C)/TRM of 0.9 for maghemite formed from lepidocrocite, we calculated gCRM(350°C)/TRMs of 0.65 and 1.08 for PN2 and PW2 using the maghemite parameters. These experimental ratios are generally lower than our model predicts. This may be attributed to our Preisach model modeling magnetite and maghemite particles using synthetic coercivity distributions or alteration during gCRM or TRM acquisition in experimental studies. All previous theoretical gCRM studies have modeled lower ratios than seen experimentally (McClelland, 1996; Shcherbakov et al., 1996), which could be caused by non-SD behavior or the models not capturing the experimental cases accurately. In our case, we have estimated the input Preisach distribution. We modeled gCRM acquisition using a Preisach model for assemblages of interacting SD particles. We showed gCRM intensity is sensitive to magnetostatic interactions, coercivity distributions and acquisition conditions (Figures 4, 5 and 10). Increasing interactions decreases gCRM intensity, in agreement with the literature (Shcherbakov et al., 2017). gCRMs carried by lower coercivity distributions, acquired at higher temperatures or over longer time scales all have stronger intensities. These factors all increase the blocking volume of a particle during gCRM acquisition, which increases the probability of field alignment. By deriving and incorporating a growth-rate dependent effective time into the model, we found gCRMs display a stronger dependency on growth rate (Figure 4) (McClelland, 1996). We show that gCRMs are approximately four times less sensitive to magnetostatic interactions than TRMs, because particles block at ∼4% of their terminal volume during gCRM acquisition (Figure 5). gCRMs have a lower thermal stability than TRMs (Figure 6) because the majority of a gCRM is carried by lower coercivity particles which unblock at lower temperatures.

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