Thellier paleointensity theory and experiments for multidomain grains
Thellier paleointensity theory and experiments for multidomain grains
复制标题
DOI：
10.1029/2004jb003024
复制
发表时间：
2004-07
影响因子：
--
通讯作者：
Song Xu;D. Dunlop
中科院分区：
文献类型：
--
作者：
Song Xu;D. Dunlop
我们扩展了多畴（MD）晶粒的热消磁（TRM）和部分TRM （pTRM）理论，以模拟thlier古强度测定中的热退磁和pTRM获取步骤。由于零场和场内加热-冷却步骤相互交织以提高温度，因此任何步骤的初始状态都很复杂，并且理论建模比单独的pTRM产生或热退磁更为复杂。在低至中等温度T下，TRM损失超过了pTRM恢复，导致Arai图在理想的单畴（SD）线以下下垂。在中等至高的温度下，pTRM的获取大于TRM的损失。当T接近TCurie时，pTRM增益正好等于TRM损耗，Arai图变得理想。当ptrm垂直于原始TRM并直接测量时，而不是通过区分场开和场开结果进行测量，在低到中等t时，与理想情况的偏差较小。我们的理论与大MD磁铁矿（135 μm）的平行和垂直ptrm的结果半定量地一致。较小的磁铁矿（6 μm和20 μm）的Arai曲线较少，最小的磁铁矿（0.6 μm和1 μm）的Arai曲线几乎为线性。阳性pTRM检查表明，与Thellier实验中物理化学变化的岩石曲线图不同，MD颗粒的弯曲Arai图是可重复的，而阴性pTRM尾部检查表明未消磁的pTRM残差。低温退磁只能略微改善线性度。这项工作的实际应用包括使用预测阈值T，低于该阈值，平行Thellier实验中不会产生净pTRM，以筛选用于古强度拟合的数据。MD （135 μm）和PSD（0.6和1 μm）晶粒在Arai图中低t点和中t点处的拟合结果最高可达100%，而伪单畴（PSD）晶粒在Arai图上的拟合结果最高可达25%。然而，通过使用f值≥0.5的中高t数据的线性段，即使对于较大的PSD（6和20 μm）和MD颗粒，也可能获得合理的古强度估计。0.6 μm颗粒的中高t拟合给出的古强度在正确值的4%以内，基本上利用了整个数据集（f > 0.9）。垂直数据总是给出较好的线性拟合。因此，对于含有PSD和MD颗粒的岩石，建议使用垂直于实验室磁场的自然剩余磁化来定向样品。然而，双加热比单加热更可取，因为它们允许进行pTRM尾部检查。
[1] We extend theories of thermoremanent magnetization (TRM) and partial TRM (pTRM) in multidomain (MD) grains to model thermal demagnetization and pTRM acquisition steps in Thellier paleointensity determination. Because of the interleaving of zero-field and in-field heating-cooling steps to increasing temperatures the initial state for any step is complex, and theoretical modeling is more intricate than for pTRM production or thermal demagnetization separately. At low to moderate temperature T, TRM lost exceeds pTRM regained, causing convex down Arai plots sagging below the ideal single-domain (SD) line. At moderate to high T, pTRM acquisition outweighs TRM loss. As T approaches TCurie, pTRM gain exactly equals TRM loss, and the Arai plot becomes ideal. When pTRMs are produced perpendicular to the original TRM and measured directly rather than by differencing field-on and field-off results, there is less deviation from ideality at low to moderate T. Our theory agrees semiquantitatively with results for parallel and perpendicular pTRMs for large MD magnetites (135 μm). Smaller MD magnetites (6 and 20 μm) have less curved Arai plots, and the smallest magnetites (0.6 and 1 μm) have almost linear plots. Positive pTRM checks demonstrate that curved Arai plots of MD grains are reproducible, unlike curved plots for rocks that alter physicochemically in the Thellier experiment, while negative pTRM tail checks indicate undemagnetized pTRM residuals. Low-temperature demagnetization improves linearity only slightly. Practical applications of this work include using the predicted threshold T below which no net pTRM is produced in a parallel Thellier experiment to screen data used for paleointensity fits. Straight line fits through low- and medium-T points in Arai plots of MD (135 μm) grains overestimated the paleofield by as much as 100% and for small pseudo-single-domain (PSD) (0.6 and 1 μm) grains overestimated by about 25%. However, by using linear segments of medium- to high-T data with f values ≥0.5 it may be possible to obtain reasonable paleointensity estimates even for larger PSD (6 and 20 μm) and MD grains. Middle- to high-T fits for 0.6 μm grains gave paleointensities within 4% of the correct value, utilizing essentially the entire data set (f > 0.9). Perpendicular data always gave superior linear fits. Orienting samples with their natural remanent magnetizations perpendicular to the laboratory field is therefore recommended for rocks containing PSD and MD grains. However, double heatings are preferable to single heatings because they allow pTRM tail checks to be carried out.