Effects of internal mineral structures on the magnetic remanence of silicate-hosted titanomagnetite inclusions: An electron holography study
Effects of internal mineral structures on the magnetic remanence of silicate-hosted titanomagnetite inclusions: An electron holography study
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DOI：
10.1029/2006jb004498
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发表时间：
2006-12-30
影响因子：
3.9
通讯作者：
Renne, Paul R.
中科院分区：
文献类型：
--
作者：
Feinberg, Joshua M.;Harrison, Richard J.;Renne, Paul R.
[1]辉石和斜长岩中的钛磁铁矿包裹体是辉长岩、斜长岩、麻粒岩和闪长岩等缓慢冷却岩石中稳定剩磁的载体。针状包裹体的平均尺寸为1×1×25微米，由寄主硅酸盐出溶外延形成。对单斜辉石包裹体的仔细研究揭示了包裹体的内部微结构，它由磁铁矿(Fe3O4)棱柱和超尖晶石(Fe2TiO4)片层组成，这些片层是在初始冷却过程中相分离形成的。这种内部结构对每个包裹体的剩磁性质产生了深远的影响，主要是通过将其从多磁区颗粒转变为一组静磁相互作用的单磁区棱柱。在这里，我们使用离轴电子全息术来成像各个棱镜的磁化态以及它们之间的静磁相互作用。结果表明，夹杂物呈现出单磁区和集体磁态，这主要取决于单个磁铁矿棱柱的形状各向异性、紧密排列的棱柱堆之间的静磁相互作用以及针尖本身的形状各向异性。被厚的蓝尖晶石薄片隔开的棱镜显示出均匀的磁化和/或涡旋状态。相反，紧密排列的磁铁矿棱柱表现为多部分涡旋或长复合柱，其强大的净磁化强度可能与针或组成棱柱的取向和形状没有直接关系。含有微溶包裹体的单斜辉石晶体记录的总体剩磁方向既反映了包裹体的伸长方向，又反映了包裹体内的棱柱排列。
[ 1] Titanomagnetite inclusions in pyroxene and plagioclase are carriers of stable magnetic remanence in some slowly cooled rocks such as gabbros, anorthosites, granulites, and diorites. Needle-shaped inclusions with average dimensions of 1 x 1 x > 25 mu m form epitaxially by exsolution from their host silicate. Close examination of clinopyroxene-hosted inclusions reveals an internal microstructure, which consists of magnetite (Fe3O4) prisms and ulvospinel (Fe2TiO4) lamellae that formed as a result of phase unmixing during initial cooling. This internal structure exerts a profound influence on the magnetic remanence properties of each inclusion, primarily by transforming it from a multidomain grain into an assemblage of magnetostatically interacting single-domain prisms. Here we use off-axis electron holography to image the magnetization states of individual prisms and the magnetostatic interactions between them. We show that the inclusions exhibit both single-domain and collective magnetic states that depend primarily on the shape anisotropies of individual magnetite prisms, magnetostatic interactions between closely spaced prism stacks, and the shape anisotropy of the needle itself. Prisms that are separated by thick ulvospinel lamellae show uniformly magnetized and/or vortex states. In contrast, closely spaced magnetite prisms behave as multipart vortices or as long composite columns, whose strong net magnetization may not be related directly to the orientation and shape of either the needle or the constituent prisms. The overall remanence direction recorded by clinopyroxene crystals containing finely exsolved inclusions is a reflection of both the inclusions' elongation directions and the prism arrangements within them.