Development of stepwise laser spot demagnetization system and itsapplication to planetary geology

我一直在开发一种阶梯式激光光斑退磁系统，用于前寒武纪岩石、受冲击的陨石坑岩石和陨石的微古地磁研究。采用调Q 30W二次倍频Nd：YAG绿光激光器，数字双色高温计(300-1000℃)，石英光纤600微米，双向激光照射装置和ccd摄像机。这种激光退磁系统允许我们在定向薄片中逐步加热单个光斑区域。作为样品保持器，我开发了一个X-Y阶段控制真空室，将石英玻璃视窗嵌入水平放置的12折圆柱形硅钢中。目标样品被保持在25nT的非常弱的环境磁场中，真空条件为10^~(-2)；在激光退磁系统的基础上，利用超音速切割刀，对滕汉姆球粒陨石中的冲击熔融矿脉和车龙铺断裂、野岛断裂的断层泥样品进行了微区古地磁研究。通过这些研究，我发现震源熔岩脉是古代母体碰撞过程中环境磁场的可靠磁记录器，断层泥也是地震断层作用的可靠记录器。尽管这些激光退磁和显微古地磁研究的新技术是强大的，但前寒武纪岩石和陨石仍然经历了很长时间的变质作用。传统的Neel磁化理论告诉我们，单畴磁性粒子的剩余磁化强度在温度和时间的指数函数下松弛到能量稳定的状态。然而，自然磁化数据往往与理论不符，这是由于变质作用所致。为了解释自然数据，我将Neel理论推广到一个伸展的指数函数，并推导出了变质岩石和陨石的新的非线性时间-温度关系。

I have been developing a stepwise laser spot demagnetization system for the application to the micro-paleomagnetic study of pre-Cambrian rocks, shocked crater rocks and meteorites. I employed a Q-switched 30W second harmonic generation Nd : YAG green laser, a digital 2-color pyrometer (300-1000℃) with quartz fiber optics of 600 micrometer and a two-way laser irradiation unit with CCD camera. This laser demagnetization system allows us to stepwisely heat the individual spot area in oriented thin sections. As a sample holder, I developed a X-Y stage controlled vacuum chamber with quartz glass viewport embedded into horizontally-lying 12-folded cylindrical silicon steel. The target sample is hold in a very weak ambient magnetic field of 25nT with 10^<-2>Torr vacuum condition. In addition to laser demagnetization system, I have conducted a micro-paleomagnetic study of shock melt veins in Tenham chondrite and fault gouge samples of Chelungpu fault and Nojima fault, by using a supersonic cutter blade. Through these studies, I found that shock melt veins are a reliable magnetic recorder of ambient field during ancient parent-body collisions, and fault gouge are also a reliable recorder for the earthquake faulting. Although these new techniques of laser demagnetization and micro-paleomagnetic study are powerful, the Pre-Cambrian rocks and meteorites have still suffered metamorphism for a long time scale. Traditional Neel theory of magnetization tells us that single domain magnetic particles relax its remanent magnetization to energetically stable condition in an exponential function of temperature and time. However, it is often that natural magnetization data is not consistent with the theory, being due to metamorphism. To interpret the natural data, I have extended the Neel theory to a stretched exponential function, and derived a new non-linear time-temperature relation for metamorphosed rocks and meteorites.