The 40Ar/39Ar and U/Pb dating of young rhyolites in the Kos‐Nisyros volcanic complex, Eastern Aegean Arc, Greece: Age discordance due to excess 40Ar in biotite
The 40Ar/39Ar and U/Pb dating of young rhyolites in the Kos‐Nisyros volcanic complex, Eastern Aegean Arc, Greece: Age discordance due to excess 40Ar in biotite
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希腊爱琴海东部科斯尼西罗斯火山群年轻流纹岩的 40Ar/39Ar 和 U/Pb 测年：黑云母中 40Ar 过量导致的年龄不一致
DOI：
10.1029/2010gc003073
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发表时间：
2010
期刊：
影响因子：
3.7
通讯作者：
R. Spikings
中科院分区：
文献类型：
--
作者：
O. Bachmann;B. Schoene;C. Schnyder;R. Spikings
通过黑云母上的40 Ar/39 Ar和锆石上的U/Pb对活跃的科斯-尼西罗斯火山中心（希腊爱琴海弧）的第四纪岩浆进行高精度定年，揭示了一个复杂的地质年代学故事。来自3个不同单元（Agios Mammas和Zini穹丘，Kefalos Serie火山碎屑岩）的U/Pb ID-TIMS多颗粒和单颗粒锆石分析的年龄范围为0.3至0.5至10-20 Ma。最年轻的年龄提供了最大的喷发年龄，而最古老的锆石表明继承当地大陆地壳（中新世和更老）。对新鲜黑云母的1-3个晶体进行的逐步加热40 Ar/39 Ar实验产生了高度干扰的Ar释放模式，其平台年龄通常比大多数U/Pb年龄更老。这些古老的高原年龄可能不是继承捕虏晶黑云母的结果，因为Ar在岩浆温度下扩散非常快，并且比率在几天内重置。根据（1）升高和/或不精确的40 Ar/36 Ar比值，（2）Ar释放光谱的形状，以及（3）科斯-尼西罗斯地区的高地幔3 He通量，我们认为黑云母晶体保留了一些地幔40 Ar，导致了观察到的异常古老的年龄。相比之下，来自科斯-尼西罗斯火山中心（形成破火山口的科斯高原凝灰岩）的唯一含透长石单元的透长石晶体似乎没有储存任何相对于大气成分过量的40 Ar。爱琴海弧的东部边缘构造复杂，正在经历快速的伸展，靠近一个主要的构造边界。在地幔挥发分通量高的地区，黑云母40 Ar/39 Ar年代学可能因40 Ar过量而导致错误结果，应与40 Ar/39 Ar透长石或U/Pb锆石年龄进行核对。
High‐precision dating of Quaternary silicic magmas in the active Kos‐Nisyros volcanic center (Aegean Arc, Greece) by both 40Ar/39Ar on biotite and U/Pb on zircon reveals a complex geochronological story. U/Pb ID‐TIMS multi and single‐grain zircon analyses from 3 different units (Agios Mammas and Zini domes, Kefalos Serie pyroclasts) range in age from 0.3 to 0.5 to 10–20 Ma. The youngest dates provide the maximum eruption age, while the oldest zircons indicate inheritance from local continental crust (Miocene and older). Step‐heating 40Ar/39Ar experiments on 1–3 crystals of fresh biotite yielded highly disturbed Ar‐release patterns with plateau ages typically older than most U/Pb ages. These old plateau ages are probably not a consequence of inheritance from xenocrystic biotites because Ar diffuses extremely fast at magmatic temperatures and ratios are reset within a few days. On the basis of (1) elevated and/or imprecise 40Ar/36Ar ratios, (2) shapes of the Ar release spectra, and (3) a high mantle 3He flux in the Kos‐Nisyros area, we suggest that biotite crystals retained some mantle 40Ar that led to the observed, anomalously old ages. In contrast, sanidine crystals from the only sanidine‐bearing unit in the Kos‐Nisyros volcanic center (the caldera‐forming Kos Plateau Tuff) do not appear to store any excess 40Ar relative to atmospheric composition. The eastern edge of the Aegean Arc is tectonically complex, undergoing rapid extension and located close to a major structural boundary. In such regions, which are characterized by high fluxes of mantle volatiles, 40Ar/39Ar geochronology on biotite can lead to erroneous results due to the presence of excess 40Ar and should be checked either against 40Ar/39Ar sanidine or U/Pb zircon ages.