地质记录中的微球粒

MICROSPHERULES IN THE GEOLOGIC RECORD

地质记录中的微球粒根据成因可以分为宇宙尘、地外物体撞击成因微球粒、火山成因微球粒、生物成因微球粒、沉积作用微球粒和现代微球形飞灰等。宇宙尘含有丰富的宇宙物质,出现Fe、Ni核心和方铁矿等,具有宇宙尖晶石和宇宙不丰富的元素的亏损是判断宇宙尘的关键特征。撞击事件可以产生近源区的撞击熔融玻璃球和远源区的撞击汽云凝结球两类微球粒,常与其他撞击成因矿物伴生。火山微球粒是在低粘稠度的岩浆岩喷发形成的玻璃质火山灰中,有球形、液滴形和哑铃形的微颗粒。这三种成因的微球粒在各种地层中的赋存对于地层对比和地史事件研究中有着非常重要的意义,为判断和研究不同的地质事件提供了一个很好的媒介,因此长期以来一直是微球粒研究的主要对象。另外,在现代工业生产过程中也可以产生大量微球形飞灰,这些飞灰经常包含非常复杂的表面纹饰和独特的化学组成,比如高的ZnO含量等。

Microspherules in the geologic record include cosmic spherules, impact microspherules, volcanic microspherules, bio-microspherule, sedimentary microspherule, and industrial fly ash. Cosmic spherules are usually between 25μm to 2 mm in size and are subspherical to spherical objects produced by melting of incoming interplanetary dust and large objects during atmospheric entry. A large extraterrestrial object striking Earth at cosmic velocity can melt and vaporize silicate materials, which can condense into highly spheroidal, sand-size particles. These particles, known as impact spherules, have been occasionally recorded in great abundance in a relatively small number of thin, discrete layers （e. g. , Cretaceous-Eogene boundary bed）. Unaltered impact spherules consist entirely of glass （microtektites） or a combination of glass and crystals grown in flight （microkrystites）. Volcanic eruptions, especially eruption of low-viscosity lavas, can produce microspherules. Volcanic microspherules range from 2μm to several mm in size and generally have smooth surface and high content of igneous elements （e. g. , Ti）. These microspherules are good markers for recognizing impact events and isochronous stratigraphical correlation. Most features present in cosmic dust, impact spherules and volcanic microspherules are also present in industrial fly ashes. To some extent, the surface and internal structures of industrial fly ashes could be more complex. Industrial fly ashes are mainly spheroidal and rarely exhibit splash forms. The surficial or internal structures can not provide sufficient information to differentiate among various kinds of microspherules. Thus, chemical compositions and mineralogical characters are very important in distinguishing the origins, natures, or definitive characteristics of different microspherules. For instance, Ni- and Cr-rich spinels in microspherules are important evidences for an extraterrestrial origin, including cosmic dusts and type 2 impact microspherules. On the other hand, high contents of BaO in glass spherules and ZnO in crystals of glass spherules are distinct characters of industrial fly ashes.