Paleomagnetic studies on single crystals separated from the middle Cretaceous Iritono granite
Paleomagnetic studies on single crystals separated from the middle Cretaceous Iritono granite
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DOI：
10.1186/s40623-018-0945-y
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发表时间：
2018-11-13
影响因子：
3
通讯作者：
Kirschvink, Joseph L.
中科院分区：
文献类型：
--
作者：
Kato, Chie;Sato, Masahiko;Kirschvink, Joseph L.
超时研究是理解地球发电机长期变化和地幔控制作用的关键。花岗岩可以很好地记录深时地磁场行为，但全岩花岗岩样品的古地磁测量往往受到风化等蚀变的干扰，以及多域磁铁矿的存在。为了避免这样的困难和测试的有用性的硅酸盐单晶古地磁，在这里，我们报告岩石磁性和古地磁性能的单晶和比较的主机花岗岩。我们研究了从中白垩世Iritono花岗岩中分离出来的单个锆石、石英和斜长石晶体，过去的研究为它们提供了严格的古地磁和古强度约束。磁铁矿在锆石和石英中的产出率很低。另一方面，斜长石晶体中含有大量的细粒单畴至伪单畴磁铁矿。斜长石晶体的显微特征和独特的磁性行为表明磁铁矿包裹体是由出溶作用形成的。因此，我们进行了古强度实验Tsunakawa-Shaw方法对17个斜长石晶体。9个样品通过了可靠的古强度测定的标准选择标准，获得的平均值与以前报道的全岩古强度值一致。虚拟偶极矩估计高于8.9 +/- 1.8x10（22）Am（2），表明白垩纪正常超时中期的时间平均场强是非超时期间的数倍。含有出溶磁铁矿包裹体的斜长石单晶比常规的全岩样品或其他硅酸盐颗粒更适合于磁信号的识别和古地磁记录的解释。
Investigations of superchrons are the key to understanding long-term changes of the geodynamo and the mantle's controlling role. Granitic rocks could be good recorders of deep-time geomagnetic field behavior, but paleomagnetic measurements on whole-rock granitic samples are often disturbed by alterations like weathering, and the presence of multi-domain magnetite. To avoid such difficulties and test the usefulness of single silicate crystal paleomagnetism, here we report rock-magnetic and paleomagnetic properties of single crystals and compare those to the host granitic rock. We studied individual zircon, quartz and plagioclase crystals separated from the middle Cretaceous Iritono granite, for which past studies have provided tight constraints on the paleomagnetism and paleointensity. The occurrence of magnetite was very low in zircon and quartz. On the other hand, the plagioclase crystals contained substantial amounts of fine-grained single-domain to pseudo-single-domain magnetite. Microscopic features and distinctive magnetic behavior of plagioclase crystals indicate that the magnetite inclusions were generated by exsolution. We therefore performed paleointensity experiments by the Tsunakawa-Shaw method on 17 plagioclase crystals. Nine samples passed the standard selection criteria for reliable paleointensity determinations, and the mean value obtained was consistent with the previously reported whole-rock paleointensity value. The virtual dipole moment was estimated to be higher than 8.9 +/- 1.8x10(22)Am(2), suggesting that the time-averaged field strength during middle of the Cretaceous normal superchron was several times as large as compared to that of non-superchron periods. Single plagioclase crystals which have exsolved magnetite inclusions can be more suitable for identification of magnetic signals and interpretation of paleomagnetic records than the conventional whole-rock samples or other silicate grains.