Improved accuracy of U-Pb zircon ages by the creation of more concordant systems using an air abrasion technique
Improved accuracy of U-Pb zircon ages by the creation of more concordant systems using an air abrasion technique
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DOI：
10.1016/0016-7037(82)90165-x
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发表时间：
1982-04
影响因子：
5
通讯作者：
T.E Krogh
中科院分区：
文献类型：
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作者：
T.E Krogh
磨损与改进的顺磁分离技术相结合，消除了90%至100%的不一致性，从而使测年具有前所未有的准确性(±1至3英里)。几乎所有2700英里年都可以实现。来自深成岩或火山岩的古老的锆石群体。对于年轻的锆石来说，这种方法效果更好。除了去除可能已被淋洗的外层外，高U部件优先通过磨损去除，因为它们会因辐射损伤而软化。蚀变和破裂的锆石也往往被消除。在大多数情况下，新的一致数据沿着先前分析的顺磁部分建立的线向上移动，但在旧数据给出的上交点误差高达25英里的地方发现了一些异常。当存在两个时代的锆石时，减少或消除顺磁相关的铅损失极大地增强了我们确定火成岩或变质岩混合线的能力。磨损技术通过增强岩心材料中的样品来检测是否存在继承成分。在许多情况下，磨损去除了大约80%的常见铅，因此可以直接评估这种成分。当颗粒的外部被去除时，磁化率与铀含量之间的相关性保持不变，但铀水平与铅损失之间的通常相关性被降低或消除。因此，只有近地表的铀参与了银星(1963)的经典不一致性与铀水平的关联。
Abrasion combined with an improved paramagnetic separation technique eliminates 90 to 100 percent of discordance so that ages of unprecedented accuracy (±1 to 3 m.y.) can be achieved for virtually all 2700 m.y. old zircon populations from plutonic or volcanic rocks. The procedures work even better for younger zircons. Besides removing outer layers that may have been leached, high-U parts are preferentially removed by abrasion because they are softened by radiation damage. Altered and cracked zircons also tend to be eliminated.In most cases, the new concordant data move up the line established by previously analysed paramagnetic fractions but a number of anomalies have been found where old data give upper intersections that are in error by as much as 25 m.y. Reducing or eliminating paramagnetically correlated Pb loss greatly enhances our ability to define mixing lines for igneous or metamorphic rocks when two ages of zircon are present.The abrasion technique allows detection of an inherited component if it exists by enhancing the sample in core material. Abrasion in many cases removes about 80 percent of the common lead, thus allowing a direct evaluation of this component.When the outer parts of grains are removed, the correlation between magnetic susceptibility and uranium content is maintained but the usual correlation of uranium level with lead loss is reduced or eliminated. Therefore, only near surface uranium is involved in the classic discordance versus uranium level correlation of Silver (1963).