Chemical abrasion applied to SHRIMP zircon geochronology: An example from the Variscan Karkonosze Granite (Sudetes, SW Poland)
Chemical abrasion applied to SHRIMP zircon geochronology: An example from the Variscan Karkonosze Granite (Sudetes, SW Poland)
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DOI：
10.1016/j.gr.2011.07.007
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发表时间：
2012-05
影响因子：
6.1
通讯作者：
R. Kryza;Q. Crowley;A. Larionov;C. Pin;T. Oberc-Dziedzic;K. Mochnacka
中科院分区：
文献类型：
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作者：
R. Kryza;Q. Crowley;A. Larionov;C. Pin;T. Oberc-Dziedzic;K. Mochnacka
在SIMS锆石U-Pb定年中，可以成功地利用热退火后的锆石酸蚀(化学磨损或CA)来最大限度地减少或消除主要和隐蔽的铅损失的影响。通过对波兰西南部苏台特斯Karkonosze花岗岩中未经处理和化学磨损的锆石进行U-Pb SIMS技术演示了这一过程。对未经处理的锆石进行常规U-PbSIMS测年，得到了一个明显一致的年龄群(n=9)，其加权平均206Pb/238U年龄为306±4 Ma。一些未经处理的锆石显示出异常年轻的206Pb/238U年龄(约225 Ma和238 Ma)，可能遭受了大量的铅损失。同一样品中的一组锆石被化学磨损。在物理上，以这种方式处理的锆石显示出不同程度的蚀刻和部分溶解。极端的例子形成了亚μm通道的3D网络，该网络沿着高U(暗CL)区或线性缺陷，例如微裂缝或模糊的解理面。处理过的锆石(n=11)的U-PbSIMS年龄为322±3 Ma，平均206Pb/238U年龄。两个处理过的锆石的分析仍然显示出较年轻的206Pb/238U年龄(约297 Ma和301 Ma)，归因于铅损失的影响。对于所分析的样品，由化学磨损的锆石测得的U-Pb年龄比未处理的锆石测得的年龄老约5%。这归因于有效地去除了易受铅损失影响的变质区。CA技术还去除了微量夹杂物，从而降低了常见的铅，减少了基质效应。未经处理的锆石中神秘的铅损失只有在与化学磨损的对应物或使用其他同位素技术确定的年龄相比较时才能被识别出来。这清楚地表明了CA对高空间分辨率方法的实用性，并强调无论使用何种分析技术，都可以在一定范围内检测到铅的损失。
Thermal annealing followed by acid etching of zircon (chemical abrasion or CA) can be successfully utilised to minimize or eliminate the effects of major and cryptic Pb-loss for SIMS U–Pb zircon dating. The procedure is demonstrated by applying the U–Pb SIMS technique to both untreated and chemically abraded zircons from the Karkonosze Granite, Sudetes, SW Poland. Conventional U–Pb SIMS dating of untreated zircons yields an apparently coherent age population (n=9) with a weighted mean206Pb/238U age of 306±4Ma. Some untreated zircons display anomalously young206Pb/238U ages (c. 225 and 238Ma) and are likely to have suffered substantial Pb-loss. A sub-set of zircons from the same sample was chemically abraded. Physically, zircons treated in this manner display a range in the degree of etching and partial dissolution. Extreme examples developed a 3D network of sub-μm channels which follow high-U (dark CL) zones or linear defects, such as micro fractures or indistinct cleavage planes. U–Pb SIMS dating of treated zircons (n=11) yields a mean206Pb/238U age of 322±3Ma. Two analyses of treated zircons still display younger206Pb/238U ages (c. 297 and 301Ma) ascribed to the effects of Pb-loss. For the analysed sample, U–Pb ages determined from chemically abraded zircons are c. 5% older than those from untreated zircons. This is attributed to effective removal of metamict domains susceptible to Pb-loss. The CA technique also removes micro-inclusions thus lowering common Pb and reducing matrix effects. A cryptic Pb-loss in untreated zircons is only recognised when compared with chemically abraded counterparts or ages determined using other isotope techniques. This clearly demonstrates the utility of CA to high-spatial resolution methods and stresses that Pb-loss is detectable at a range of scales, regardless of the analytical technique used.