Meteorite cloudy zone formation as a quantitative indicator of paleomagnetic field intensities and cooling rates on planetesimals
Meteorite cloudy zone formation as a quantitative indicator of paleomagnetic field intensities and cooling rates on planetesimals
复制标题
陨石云带的形成作为古磁场强度和星子冷却速率的定量指标
DOI：
10.1016/j.epsl.2019.02.027
复制
发表时间：
2019
影响因子：
5.3
通讯作者：
J. Bryson
中科院分区：
文献类型：
--
作者：
C. Maurel;B. Weiss;J. Bryson
缓慢冷却的富铁陨石中的金属微观结构反映了其母星子的热和磁历史。特别令人感兴趣的是多云区，它是贫镍基体中富镍岛的纳米级共生体，通过旋节线分解在~350°C以下形成。长期以来，岛屿的大小一直被认为反映了陨石母体的低温冷却速度。然而，一直缺乏能够根据岛屿大小提供定量冷却速率估计的模型。此外，这些岛屿还能够在其生长过程中保存环境磁场的记录，但从这些岛屿的磁测量中恢复可靠的古强度估计所需的一些关键物理参数受到的限制很少。为了解决这两个问题，我们提出了一个云区结构和成分演化的数值模型，作为冷却速率和局部成分的函数。我们的模型产生的岛屿尺寸与当今测量的尺寸一致。该模型能够显着改进古强度估计和相关不确定性的校准。特别是，我们现在可以准确地量化与获取磁化强度的有限数量的岛屿相关的统计不确定性以及记录时岛屿大小的不确定性。我们利用这一新的认识来重新审视之前对多云区域的开创性古地磁研究的古强度。我们表明，这些可能被高估了一个数量级，但仍然需要在其母体上存在大量磁场。我们的模型还允许我们估计冷却速度低于 <10,000 °C My−1 的陨石的绝对冷却速率。我们展示了这些冷却速率估计如何独特地限制陨石母体的低温热历史。以主族橄榄石为例，我们表明我们的结果与之前提出的半径约 200 公里的主族橄榄石母体在低温下不受扰动的传导冷却是一致的。
Metallic microstructures in slowly-cooled iron-rich meteorites reflect the thermal and magnetic histories of their parent planetesimals. Of particular interest is the cloudy zone, a nanoscale intergrowth of Ni-rich islands within a Ni-poor matrix that forms below ∼350 °C by spinodal decomposition. The sizes of the islands have long been recognized as reflecting the low-temperature cooling rates of meteorite parent bodies. However, a model capable of providing quantitative cooling rate estimates from island sizes has been lacking. Moreover, these islands are also capable of preserving a record of the ambient magnetic field as they grew, but some of the key physical parameters required for recovering reliable paleointensity estimates from magnetic measurements of these islands have been poorly constrained. To address both of these issues, we present a numerical model of the structural and compositional evolution of the cloudy zone as a function of cooling rate and local composition. Our model produces island sizes that are consistent with present-day measured sizes. This model enables a substantial improvement in the calibration of paleointensity estimates and associated uncertainties. In particular, we can now accurately quantify the statistical uncertainty associated with the finite number of islands acquiring the magnetization and the uncertainty on their size at the time of the record. We use this new understanding to revisit paleointensities from previous pioneering paleomagnetic studies of cloudy zones. We show that these could have been overestimated by up to one order of magnitude but nevertheless still require substantial magnetic fields to have been present on their parent bodies. Our model also allows us to estimate absolute cooling rates for meteorites that cooled slower than <10,000 °C My−1. We demonstrate how these cooling rate estimates can uniquely constrain the low-temperature thermal history of meteorite parent bodies. Using the main-group pallasites as an example, we show that our results are consistent with the previously-proposed unperturbed, conductive cooling at low temperature of a ∼200-km radius main-group pallasite parent body.