Rock magnetism of tiny exsolved magnetite in plagioclase from a Paleoarchean granitoid in the Pilbara craton
Rock magnetism of tiny exsolved magnetite in plagioclase from a Paleoarchean granitoid in the Pilbara craton
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DOI：
10.1002/2014gc005508
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发表时间：
2015-01
期刊：
影响因子：
3.7
通讯作者：
Y. Usui;T. Shibuya;Y. Sawaki;T. Komiya
中科院分区：
文献类型：
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作者：
Y. Usui;T. Shibuya;Y. Sawaki;T. Komiya
花岗岩类广泛分布于前寒武纪岩石和中生代造山带中，但在古地磁研究中很少受到关注，因为花岗岩类通常含有丰富的粗粒度，磁性不稳定的氧化物。在这项研究中，第一个例子的微小，针形，出溶氧化物斜长石在古太古代花岗岩报道。本文研究了含有出溶氧化物包裹体的斜长石单晶的磁性，以确定其古地磁记录保真度。退磁实验和磁滞参数表明，氧化物夹杂物是近化学计量比的磁铁矿，磁性非常稳定。一阶反转曲线（FORC）图显示可忽略的静磁相互作用。最小的相互作用也反映了非常有效的收购无滞后resistance磁化。单斜长石晶体表现出很强的剩磁各向异性，这需要校正其古方向和古强度数据。尽管如此，单斜长石晶体的各向异性张量的定量考虑表明，可以通过适当地平均从几十个单晶的数据来减轻偏差。通过对斜长石晶体非线性热剩磁的测量，我们估计斜长石晶体可以重建高达50 μ T的古强度。局部变质条件表明这些磁铁矿可能携带有3.2~3.3Ga的剩磁。我们认为，出溶磁铁矿在花岗岩类是一个潜在的合适的目标，为研究的早期历史的地磁场，并提示详细的显微镜调查以及古地磁测试，以限制剩磁的年龄。
Granitoids are widespread in Precambrian terranes as well as the Phanerozoic orogenic belts, but they have garnered little attention in paleomagnetic studies, because granitoids often contain abundant coarse‐grained, magnetically unstable oxides. In this study, the first example of tiny, needle‐shaped, exsolved oxides in plagioclase in a Paleoarchean granitoid is reported. The magnetic properties of single plagioclase crystals with the exsolved oxide inclusions have been studied to determine their paleomagnetic recording fidelity. Demagnetization experiments and hysteresis parameters indicate that the oxide inclusions are near stoichiometric magnetite and magnetically very stable. First‐order reversal curve (FORC) diagrams reveal negligible magnetostatic interactions. Minimal interactions are also reflected by very efficient acquisition of anhysteretic remanent magnetization. Single plagioclase crystals exhibit strong magnetic remanence anisotropies, which require corrections to their paleodirectional and paleointensity data. Nonetheless, quantitative consideration of anisotropy tensors of the single plagioclase crystals indicates that the bias can be mitigated by properly averaging data from a few tens of single crystals. From the nonlinear thermoremanence acquisition of the plagioclase crystals, we estimate that the plagioclase crystals can reconstruct paleointensity up to 50 μT. Local metamorphic condition suggests that those magnetite may carry remanence of ∼3.2 to 3.3 Ga. We suggest that exsolved magnetite in granitoids is potentially a suitable target for the study of the early history of the geomagnetic field, and prompt detailed microscopic investigations as well as paleomagnetic tests to constrain the age of remanence.