The intensity of the geomagnetic field in the late-Archaean: new measurements and an analysis of the updated IAGA palaeointensity database
The intensity of the geomagnetic field in the late-Archaean: new measurements and an analysis of the updated IAGA palaeointensity database
复制标题
DOI：
10.1186/bf03352881
复制
发表时间：
2009-01-01
影响因子：
3
通讯作者：
Langereis, Cor G.
中科院分区：
文献类型：
--
作者：
Biggin, Andrew J.;Strik, Geert H. M. A.;Langereis, Cor G.
我们首先展示了对来自西澳大利亚皮尔巴拉克拉通的9个晚太古代（2724 - 2772百万年）火山单元的54个样本进行的详细古强度研究的结果。这些结果受到实验室加热过程中发生的磁矿物学变化的严重影响，因此需要应用一种校正程序。该校正使得来自三个熔岩的结果通过了严格的筛选标准，但我们认为其中只有一个表现出足够的内部一致性，可被认为是中等可靠的。它得出的虚拟偶极矩为47 ± 6 ZAm²，是现今值的60%。我们将这一测定结果与来自更新的国际地磁与高空大气物理学协会（IAGA）古强度数据库PINT08的经过筛选的数据集相结合。最近已表明，自太古代以来，方向长期变化已发生根本性改变，这可能是自那时以来内核生长的结果。然而，在此我们认为，尚不清楚这种演化是否伴随着平均极向场强度的单一长时间尺度变化。虽然前寒武纪古强度测定结果的整体分布明显低于过去300百万年的分布，但我们表明这一发现在很大程度上反映的是来自元古代的数据。来自晚太古代 - 最早元古代的更古老测量值的分布与过去300百万年的分布无法区分，这可能表明在两个较高场强时期之间存在一个“元古代偶极子低值”时期。如果这种长期地磁强度变化模式在未来因新数据的增加而得到支持，那么它可能表明地核动力学存在一个相关的三阶段演化，即：在地球历史早期由高地核 - 地幔热通量引起的强烈热对流，随着热通量下降后来出现的较弱热对流，以及自内核成核以来的强烈成分对流。
We firstly present the results of a detailed palaeointensity study performed on 54 samples from 9 volcanic units of late Archaean age (2724-2772 Ma) from the Pilbara Craton, Western Australia. These results were severely affected by magnetomineralogical alteration occurring during the laboratory heating process necessitating the application of a correction procedure. The correction allowed results from three lavas to pass strict selection criteria but we deem that only one of these exhibits sufficient internal consistency to be considered moderately reliable. It yields a virtual dipole moment of 47 +/- 6 ZAm(2) which is 60% of the present-day value. We combine this determination with a filtered dataset from the updated IAGA (international Association of Geomagnetism and Aeronomy) palaeointensity database, PINT08. Directional secular variation has recently been shown to have changed fundamentally since the Archaean, probably as a consequence of inner core growth since that time. However, here we argue that it is Still Unclear whether this evolution was accompanied by a single long timescale change in average poloidal field intensity. While the distribution of Precambrian palaeointensity determinations as a whole is significantly lower than that for the last 300 Myr, we show that this finding largely reflects data from the Proterozoic aeon. The distribution of more ancient measurements from the late Archaean-earliest Proterozoic is indistinguishable from that of the last 300 Myr which might suggest that a 'Proterozoic dipole low' period existed between two periods of higher Held intensity. Were this pattern of long-term geomagnetic intensity variation to be supported by the addition of new data in the future, then it could indicate a related three-stage evolution ill core dynamics, namely: vigorous thermal convection caused by high core-mantle heat flux early in the Earth's history. weaker thermal convection later as the heat flux fell, and finally, strong compositional convection since the inner core nucleated.