Sm-Nd chronology and petrogenesis of mesosiderites
Sm-Nd chronology and petrogenesis of mesosiderites
复制标题
中菱铁矿的 Sm-Nd 年代学和岩石成因
DOI：
10.1016/0016-7037(94)90100-7
复制
发表时间：
1994
影响因子：
5
通讯作者：
G. Wasserburg
中科院分区：
文献类型：
--
作者：
B. Stewart;D. Papanastassiou;G. Wasserburg
摘要 我们获得了四种中菱铁矿硅酸盐碎屑的 Sm-Nd 数据，其中包括来自 Vaca Muerta 1A 型中菱铁矿的三种具有多种结构的碎屑和来自 Mt. Padbury 中菱铁矿的一种辉长岩碎屑。辉长岩 Vaca Muerta Pebble 12 和玄武岩 Pebble 16 产生相同的 147 Sm-143 Nd 年龄，分别为 4.48±0.19 AE 和 4.48±0.09 AE，而高度重结晶 Pebble 5 的年龄为 4.42±0.02 AE； Mt. Padbury 的年龄为 4.52±0.04 AE。所有碎屑都显示出 142 Nd 144 Nd 与 147 Sm 144 Nd 的相关性，并为它们形成时存在活的 146 Sm 提供了明确的证据。计算出的初始 146 Sm 144 Sm 值范围为 0.004（Pebble 5）至 0.006（Pebbles 12、16 和 Mt. Padbury），并且与 147 Sm-143 Nd 年龄基本一致。然而，全岩浸出和残留数据的不一致以及 146 Sm-142 Nd 相对年龄和 147 Sm-143 Nd 绝对年龄之间的一些分歧表明对 Sm-Nd 系统的干扰虽小但显着。年龄范围和初始 146 Sm 144 Sm 和 143 Nd 144 Nd 值表明，这些硅酸盐碎屑中的每一种在与金属混合之前都在其母体上经历了单独的、长期的演化。结构和微量元素标准表明，来自同一中菱铁矿的各个碎屑在并入陨石之前通常具有非常不同的火成岩来源和热历史。 Pebble 12 的 LREE 极度贫乏，可能是几次熔融萃取事件的结果，而 Pebbles 5 和 16 以及 Mt. Padbury 几乎具有球粒状 Sm/Nd，其大量 REE 浓度比球粒陨石高 5 至 15 倍。一般来说，中菱铁矿硅酸盐的 Sm-Nd 系统学需要在经历相对早期和极端分化的母行星上形成硅酸盐。多样化的古老年代的保存和 146 Sm 的存在意味着金属硅酸盐的混合并没有严重改变这些碎屑的 Sm-Nd 同位素记忆。我们提出了一种金属硅酸盐混合模型，它将冷却历史与热覆盖情况下的同位素再平衡结合起来。我们表明，样品中同位素再平衡的总量可能与初始温度、埋藏深度、晶粒尺寸和扩散参数有关。将该模型应用于本研究中测量的硅酸盐碎屑表明，如果金属和硅酸盐在混合过程中在金属固相线温度以上达到热平衡，那么碎屑在最初的高温冷却阶段必须埋藏在风化层中不超过1-10 m，以防止Sm-Nd系统广泛重置。为了使这些结果与金属相出溶研究确定的较低温度下的缓慢冷却速率相一致，我们推断，在最初的金属-硅酸盐混合过程中，热量从热金属快速传递到冷硅酸盐材料，并且混合物的深埋部分在~600-700°C达到热平衡后缓慢冷却。这项研究的数据指出了中铁矿母体的以下历史：1.(1)硅酸盐母体在第一~太阳系 50 多年的历史形成了不同的母岩浆； 2.(2)原始和分异镁铁质岩浆在行星表面附近就位，并在近地表环境中广泛分异； 3.(3)通过近地表的冲击园艺形成和改造风化角砾岩； 4.（4）熔融的铁镍金属与风化层混合，然后在太阳系起源后快速冷却100-150米； 5.（5）缓慢冷却……
Abstract We have obtained Sm-Nd data from four mesosiderite silicate clasts, including three clasts with a variety of textures from the Vaca Muerta type 1A mesosiderite and one gabbroic clast from the Mt. Padbury mesosiderite. The gabbroic Vaca Muerta Pebble 12 and basaltic Pebble 16 yield identical 147 Sm-143 Nd ages of 4.48±0.19 AE and 4.48±0.09 AE, respectively, while the highly recrystallized Pebble 5 gives an age of 4.42±0.02 AE; Mt. Padbury yields an age of 4.52±0.04 AE. All clasts show a correlation of 142 Nd 144 Nd with 147 Sm 144 Nd, and provide unequivocal evidence for the presence of live 146 Sm at the time of their formation. Calculated initial 146 Sm 144 Sm values range from 0.004 (Pebble 5) to 0.006 (Pebbles 12, 16, and Mt. Padbury) and are generally consistent with the 147 Sm-143 Nd ages. However, discordance of whole-rock leach and residue data and some disagreement between 146 Sm-142 Nd relative ages and 147 Sm-143 Nd absolute ages indicate small but significant disturbances to the Sm-Nd systematics. The ranges of ages and initial 146 Sm 144 Sm and 143 Nd 144 Nd values suggest that each of these silicate clasts underwent a separate, protracted evolution on its parent body prior to mixing with metal. Textural and trace element criteria indicate that individual clasts from the same mesosiderite often had very different igneous sources and thermal histories prior to their incorporation in the meteorite. Pebble 12 is extremely LREE depleted, probably a result of several melt extraction events, whereas Pebbles 5 and 16 and Mt. Padbury have nearly chondritic Sm/Nd with bulk REE concentrations higher than chondrites by factors of 5 to 15. In general, the Sm-Nd systematics of mesosiderite silicates require formation of the silicates on a parent planet which underwent relatively early and extreme differentiation. Preservation of diverse, old ages and the presence of 146 Sm imply that metal-silicate mixing did not seriously alter the Sm-Nd isotopic memories of these clasts. We present a model for metal-silicate mixing which combines the cooling history with isotopic reequilibration for the case of thermal blanketing. We show that the total amount of isotopic reequilibration in a sample can be related to the initial temperature, depth of burial, grain size, and diffusion parameters. Application of this model to the silicate clasts measured in this study indicates that if the metal and silicate were thermally equilibrated above the metal solidus temperature during mixing, then the clasts must have been buried no deeper than 1–10 m in regolith during the initial high-temperature cooling phase in order to prevent the Sm-Nd systems from being extensively reset. In order to reconcile these results with the slow cooling rates at lower temperatures determined from studies of exsolution in the metal phase, we infer that heat was transferred rapidly from hot metal to cold silicate material during initial metal-silicate mixing, and that the deeply buried portions of the mixture then cooled slowly after reaching thermal equilibrium at~ 600–700° C. The data from this study point to the following history for the mesosiderite parent body: 1.(1) differentiation of a silicate parent body within the first~ 50 my of solar system history to form diverse parent magmas; 2.(2) emplacement of primitive and differentiated mafic magmas near the planetary surface, and extensive differentiation in the near surface environment; 3.(3) formation and reworking of regolith breccias through impact gardening at the near surface of the body; 4.(4) mixing of molten Fe-Ni metal with the regolith followed by rapid cooling 100–150 my after the origin of the solar system; 5.(5) slow cooling …