Evidence for 182Hf in the early Solar System and constraints on the timescale of terrestrial accretion and core formation
Evidence for 182Hf in the early Solar System and constraints on the timescale of terrestrial accretion and core formation
复制标题
早期太阳系中存在 182Hf 的证据以及对陆地吸积和核心形成时间尺度的限制
DOI：
10.1016/0016-7037(96)00027-0
复制
发表时间：
1996
影响因子：
5
通讯作者：
S. Jacobsen
中科院分区：
文献类型：
--
作者：
C. L. Harper;S. Jacobsen
我们提出的证据钨（W）同位素测量与live 182 Hf（T1/2= 9 Ma）在早期太阳系的存在相一致。这是基于观察到从托卢卡铁陨石中分离出的182 W/183 W比值中有大约万分之四的缺陷。这种不足被解释为地球标准成分中由于182 W的放射性生长而过量。如果太阳系中182 Hf/180 Hf的初始比值小于2 × 10−4，那么其中的一些生长一定发生在地球的硅酸盐部分，其Hf/W比约为10- 20倍。这种分馏是亲铁元素分离形成地核的结果，很可能与吸积同时发生。因此，钨同位素可以提供有用的约束地球核心的形成和增生的年代学。观测到的效应的大小大致与以下两点一致：（1）根据II型超新星源模型预测的太阳系中182 Hf的初始丰度，其中大多数放射性核素的半衰期小于17 Ma 129 I（哈珀，1995，1996 b）;（2）Wetherill（1986）的吸积计算（预测吸积平均年龄约为10 Ma）。同位素不均匀性和186 Os的α衰变的替代解释不能解释观察到的效应的大小。Masarik和Reedy（1994年）的中子输运代码计算表明，观测到的异常值超过6倍，不可能是由于空间照射期间W上的中子俘获造成的。然而，由于W核输入数据中的未知不确定性，这种可能性不能明确排除，必须通过直接实验校准进行检查。原则上，182 Hf-182 W系统提供了最直接的地球化学方法来确定地球的吸积和核心形成的时间尺度，具有出色的时间分辨率，但需要精确的校准的基础上internal 182 Hf-182 W等时线研究陨石的独立已知的年龄。我们探索方面的同位素演化在连续分馏模型的边界假设描述地幔W和吸积W之间的平衡程度在亲铁分区，并发现这些是非常重要的测量效果的解释。我们的初步结果暗示了一个更快的时间尺度比Safronov-Wetherill型模型的可能性。快速吸积时间尺度将与气体阻力和密度波加速吸积模型（例如，Ward，1986，1989，1993）。然而，其他解释是可能的。
We present evidence from tungsten (W) isotopic measurements consistent with the presence of live182Hf (T1/2= 9 Ma) in the early solar system. This is based on the observation of a well-resolved deficit of about four parts in ten thousand in the ratio182W/183W in W separated from the Toluca iron meteorite. This deficit is interpreted as an excess in the terrestrial standard composition due to radiogenic growth of182W. If the initial182Hf/180Hf ratio in the solar system is <2 × 10−4, then some of this growth must have taken place in the silicate portion of the Earth which has Hf/W ~ 10–20x the chondritic ratio. This fractionation is a consequence of siderophile element segregation to form the Earth's core, which most likely occurred contemporaneously with accretion. Thus W isotopes may provide useful constraints on the chronology of terrestrial core formation and accretion. The magnitude of the observed effect is broadly consistent with (1) the predicted initial abundance of182Hf in the solar system based on a Type II supernova source model for most of the radionuclides with half-lives less than 17 Ma129I (Harper, 1995, 1996b) and (2) the Wetherill (1986) accretion calculations (which predict an ~10 Ma mean age of accretion). Alternate explanations involving isotopic heterogeneity and α-decay of186Os cannot explain the magnitude of the observed effect. Neutron transport code calculations by Masarik and Reedy (1994) indicate the observed anomaly is too large by more than a factor of six to be due to neutron capture on W during space exposure. However, due to unknown uncertainties in the W nuclear input data, this possibility is not definitively excluded and must be checked by direct experimental calibration. In principle, the182Hf-182W system offers the most geochemically direct way to determine the time-scale for the Earth's accretion and core formation with excellent time resolution, but requires precise calibration based on internal182Hf-182W isochron studies of meteorites of independently known age. We explore aspects of isotopic evolution in continuous fractionation models under boundary assumptions describing the degree of equilibration between mantle W and accreted W during siderophile partitioning, and find these to be highly significant for interpretation of measured effects. Our initial results hint towards the possibility of a more rapid timescale than obtained in Safronov-Wetherill-type models. A fast accretion timescale would be in agreement with gas drag and density-wave-accelerated accretion models (e.g., Ward, 1986, 1989, 1993). However, other interpretations are possible.