Chemical zoning and homogenization of olivines in ordinary chondrites
Chemical zoning and homogenization of olivines in ordinary chondrites
复制标题
普通球粒陨石中橄榄石的化学分带和均质化
DOI：
10.1029/jb091ib12p12804
复制
发表时间：
1986
期刊：
影响因子：
--
通讯作者：
M. Duke
中科院分区：
文献类型：
--
作者：
M. Miyamoto;D. Mckay;G. Mckay;M. Duke
研究了第三类普通橄榄岩中橄榄石化学分带的均一化程度和范围，以获得橄榄岩冷却历史的制约因素。根据镁铁分带和CaO分带，将第3类辉长岩中的橄榄石分为4种类型。单个球粒通常包含具有相同分带类型的橄榄石。微斑状橄榄岩显示了所有四种分带类型。条纹橄榄石通常显示出几乎均匀的化学分带。我们已经计算出冷却速率或埋藏深度均匀化的化学分区通过求解扩散方程，使用分区配置文件作为初始条件。在初始冷却过程中，橄榄石的Mg-Fe分带可能发生变化，而CaO分带几乎不发生变化。棒状橄榄石在初始冷却过程中可以被均质化，因为它们的尺寸相对较小。对于模拟的微斑状橄榄石球粒，从液相线以下以适度高的速率冷却比从液相线以上以低的速率冷却更可取。对于3型辉长岩的堆积后变质作用，如果我们根据Fe-Ni数据假定冷却速率，那么为了保持Mg-Fe分带不变，最高变质温度必须小于400°C。计算的冷却速率的Fa和CaO均匀化是一致的Fe-Ni数据的4型辉长岩。在母岩表面形成几十米厚的热喷出物覆盖层，如果覆盖层温度为600-700°C，就足以使Mg-Fe分带均匀化。26 Al加热的母体中的岩石学类型的普通方解石的埋藏深度与先前提出的大致一致。
We studied the extent and degree of homogenization of chemical zoning of olivines in type 3 ordinary chondrites in order to obtain some constraints on cooling histories of chondrites. Based on Mg-Fe and CaO zoning, olivines in type 3 chondrites are classified into four types. A single chondrule usually contains olivines with the same type of zoning. Microporphyritic olivines show all four zoning types. Barred olivines usually show almost homogenized chemical zoning. We have calculated cooling rates or burial depths needed to homogenize the chemical zoning by solving the diffusion equation, using the zoning profiles as an initial condition. Mg-Fe zoning of olivine may be altered during initial cooling, whereas CaO zoning is hardly changed. Barred olivines may be homogenized during initial cooling because their size is relatively small. To simulated microporphyritic olivine chondrules, cooling from just below the liquidus at moderately high rates is preferable to cooling from above the liquidus at low rates. For postaccumulation metamorphism of type 3 chondrites in order to keep Mg-Fe zoning unaltered, the maximum metamorphic temperature must be less than about 400°C if we assume cooling rates based on Fe-Ni data. Calculated cooling rates for both Fa and CaO homogenization are consistent with those by Fe-Ni data for type 4 chondrites. A hot ejecta blanket several tens of meters thick on the surface of a parent body is sufficient to homogenize Mg-Fe zoning if the temperature of the blanket is 600–700°C. Burial depths for petrologic types of ordinary chondrites in a parent body heated by 26Al are broadly consistent with those previously proposed.