U-Pb isotope systematics in josephinites and associated rocks
U-Pb isotope systematics in josephinites and associated rocks
复制标题
约瑟芬岩及伴生岩中的 U-Pb 同位素系统学
DOI：
10.1016/0012-821x(90)90095-f
复制
发表时间：
1990
影响因子：
5.3
通讯作者：
C. Allègre
中科院分区：
文献类型：
--
作者：
C. Göpel;G. Manhès;C. Allègre
约瑟芬矿结核是由金属镍铁合金与安德闪石石榴石共生而成。它们被发现于俄勒冈州西南部的一个倒伏蛇绿岩的橄榄岩剖面中。关于约瑟芬矿的起源存在广泛的争议：例如，先前的研究认为它是与岩脉侵入有关的低温同辉石化过程的副产品，并且它来自原始地幔，可以想象来自深至核心地幔边界。本文报道了由金红石和钛铁矿与硅酸盐共生组成的矽辉石（与矽辉石伴生的一种岩石）及其周围岩石（角闪石闪长岩和辉石闪长岩）的U-Pb数据。所有净化后浸出的josephite金属样品的Pb同位素组成均位于MORB场上方的Pb-Pb图中，而第一次浸出液的特征是较高的207pb /204Pb比率。在该菱铁矿渗滤液中测定的同位素Pb组成确定了一条斜率对应于年龄为159±8myr的线。哈茨堡土在铅同位素组成中分布广泛；所有样本都位于MORB场上方，三个样本位于4.55 Æ地质时线的左侧。角闪石闪长岩脉在MORB区显示出U、Pb含量最高的特征。这些不同的岩石都没有单一或均匀的铅组成特征。所有样品都受到次生蚀变过程的影响：热液流体的循环扰动了岩脉和超镁铁质岩石，蛇纹石化过程影响了哈尔茨伯尔岩、细纹石和菱铁矿。因此，今天测量的铅同位素组成代表了初始铅、放射性成因铅和遗传铅的混合，其比例是可变的。关于josephite的成因，这些结果表明josephite， wyrite和岩脉之间有密切的关系；例如：(1)菱铁矿的Pb-Pb年龄与岩脉相同；(2)净化后的josephite金属的Pb同位素组成与岩脉接近。
Josephinite nodules are composed of metallic nickel iron alloy intergrown with andradite garnet. They are found in the peridotitic section of an obducted ophiolite in SW Oregon. The origin of josephinite is widely debated: for example, previous investigation have proposed it as a byproduct of low temperature synserpentinization processes linked to the intrusion of dikes or and its derivation from primitive mantle, conceivably from as deep as the core mantle boundary.We report U-Pb data from josephinites, wyrdite (a rock associated with josephinite) consisting of rutile and ilmenite intergrown with silicates, and their surrounding rocks (hornblende diorites and harzburgites).The measured Pb isotopic composition of all decontaminated, leached josephinite metal samples plots in the Pb-Pb diagram just above/in the MORB field, while the first leachates are characterized by higher207Pb/204Pb ratios. The isotopic Pb composition measured in the leachates of the wyrdite defines a line whose slope corresponds to an age of 159 ± 8 Myr. The harzburgites show a wide spread in Pb isotopic compositions; all samples lie above the MORB field and three samples plot to the left side of the 4.55 Æ geochron. The hornblende diorite dikes, characterized by the highest U and Pb concentrations of all studied rocks, plot in the MORB field.None of these different rocks is characterized by a single or homogeneous Pb composition. All samples are affected by secondary alteration processes: the circulation of hydrothermal fluids disturbed the dikes and ultramafic rocks and serpentinization processes have affected harzburgites, josephinites, and wyrdites. Thus the Pb isotopic composition measured today represents a mixture of initial Pb, radiogenic Pb and inherited Pb in variable proportions.Concerning the origin of josephinite these results show a close relationship between josephinite, wyrdite, and the dikes; e.g.: (1) the Pb-Pb age of wyrdite is identical to that of the dikes; (2) the Pb isotopic composition of the decontaminated josephinite metal is close to that of the dikes.