Cluster analysis on a sphere: Application to magnetizations from metasediments of the Jack Hills, Western Australia
Cluster analysis on a sphere: Application to magnetizations from metasediments of the Jack Hills, Western Australia
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DOI：
10.1016/j.epsl.2017.12.007
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发表时间：
2018-02-15
影响因子：
5.3
通讯作者：
Cottrell, Rory D.
中科院分区：
文献类型：
--
作者：
Bono, Richard K.;Tarduno, John A.;Cottrell, Rory D.
杰克山的变质沉积物含有已知最古老的陆地矿物，其形式为锆石，年龄近44亿年。这些锆石的古强度数据为冥古宙的地球发电机提供了42亿年的证据。考虑到这些锆石对限制地核最早历史的重要性，了解锆石记录的真实性至关重要。一个基本方面提供的背景下保存的主要磁信号是叠印的性质，预计已被赋予岩石的杰克山，由于太古代元古代变质事件。要成为冥古宙地球发电机的有效磁性记录，锆石的磁化方向应不同于这些次级磁化。为了评估这些二次磁化，我们报告的68个石英岩鹅卵石大小的碎屑岩从杰克山变质沉积物类似0.5至1.0公里的发现网站（已产生最古老的锆石和paleofield估计）的全面采样古地磁分析。虽然应用标准的古地磁测试表明，合奏的鹅卵石方向不能区分从随机分布，一个新的聚类分析的方向上的一个球体和非参数reservation方法揭示了显着的方向之间的子集的数据。一个，在最低温度下分离分析[200至300摄氏度，12月（12月）= 316.8度，倾角（Inc.）= -51.1度]似乎是由现在的领域占主导地位。另一个，在较高的分离（但仍然相对较低的解封温度，我们称之为“中间”，类似于-350-500摄氏度，12月= 243.8度，公司。= 9.5度）与从Jack Hills Discovery现场分离的次生铬铁云母紫红色矿分离的磁性叠印一致，在80%置信水平下通过了现场测试。没有证据表明，在我们的数据中，或在其他人收集的类似杰克山岩性的数据，广泛的1 Ga重磁化事件。相反，我们解释了石英岩最普遍的二次磁化（即，中解块）和紫红石特征剩磁的年龄与2.65Ga相似，与峰期变质作用（高达约2.65Ga）相一致。475摄氏度）的杰克山。这种独特的二次磁化的存在，它的差异，从杰克山锆石记录在高解除阻断温度，并在高解除阻断温度的鹅卵石数据（预期的结果为一个主磁化）缺乏一个占主导地位的再磁化方向，借给进一步支持的Hadean地磁记录的保真度。二次磁化的存在也支持这样的结论，即大多数的杰克山沉积变质层沉积在太古代，只有轻微的改造和潜在的构造交错的元古代组件。总的来说，本文提出的新方向聚类分析的应用有可能揭示高度分散数据集中的磁性方向，这是弱磁化粗粒沉积岩的典型特征（C）2017作者。出版社：Elsevier B. V.
Metasediments of the Jack Hills contain the oldest known terrestrial minerals in the form of zircons nearly 4.4 billion years old. Paleointensity data from these zircons provide evidence for a Hadean geodynamo as old as 4.2 billion years old. Given the importance of these zircons for constraining the earliest history of the core, it is vital to understand the fidelity of the zircon record. A fundamental aspect providing context for the preservation of primary magnetic signals is the nature of overprints predicted to have been imparted on rocks of the Jack Hills due to Archean to Proterozoic metamorphic events. To be viable magnetic records of a Hadean geodynamo, zircon magnetization directions should differ from these secondary magnetizations. To evaluate these secondary magnetizations, we report paleomagnetic analyses of a comprehensive sampling of 68 quartzite cobble-sized clasts from the Jack Hills metasediments similar to 0.5 to 1.0 km from the Discovery Site (which has yielded the oldest zircons and paleofield estimates). While application of standard paleomagnetic tests suggests that the ensemble of cobble directions cannot be distinguished from those drawn from a random distribution, a new cluster analysis of directions on a sphere and non-parametric resampling approaches reveal significant directions amongst subsets of the data. One, isolated at the lowest temperature analyzed [200 to 300 degrees C, Declination (Dec.) = 316.8 degrees, Inclination (Inc.) = -51.1 degrees] appears to be dominated by the present day field. Another, isolated at higher (but still relatively low unblocking temperatures that we call "intermediate", of similar to-350-500 degrees C, Dec. = 243.8 degrees, Inc. = 9.5 degrees) agrees with a magnetic overprint isolated from the secondary Cr-Fe mica fuchsite isolated from the Jack Hills Discovery site, passing a field test at the 80% confidence level. No evidence is found in our data, or in the data of others collected on similar Jack Hills lithologies, for a widespread 1 Ga remagnetization event. Instead, we interpret the most prevalent secondary magnetization of the quartzite (i.e., intermediate unblocking) and the fuchsite characteristic remanent magnetization to be similar to 2.65 Ga in age, coincident with peak metamorphism (as high as ca. 475 degrees C) of the Jack Hills. The presence of this distinct secondary magnetization, its difference from that recorded by Jack Hills zircons at high unblocking temperatures, and the lack of a dominant remagnetization direction at high unblocking temperatures in the cobble data (the expected result for a primary magnetization), lends further support to the fidelity of the Hadean geomagnetic record. The presence of the secondary magnetization also lends support to the conclusion that most of the Jack Hills metasediments were deposited in the Archean, with only minor reworking and potential tectonic interleaving of Proterozoic components. Overall, the application of the new directional cluster analysis presented here has the potential to reveal magnetic directions in highly scattered data sets, typical of weakly magnetized coarse-grained sedimentary rocks (C) 2017 The Authors. Published by Elsevier B.V.