Ion microprobe U-Pb dating and REE analysis of apatite from kerogen-rich silica dike from North Pole area, Pilbara Craton, Western Australia
Ion microprobe U-Pb dating and REE analysis of apatite from kerogen-rich silica dike from North Pole area, Pilbara Craton, Western Australia
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DOI：
10.2343/geochemj.38.243
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发表时间：
2003-12
影响因子：
0.8
通讯作者：
M. Nishizawa;K. Terada;Y. Ueno;Y. Sano
中科院分区：
文献类型：
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作者：
M. Nishizawa;K. Terada;Y. Ueno;Y. Sano
为了提供一个时间限制的13 C-贫化干酪根形成的硅岩脉，侵入3.5 Ga绿岩从皮尔巴拉基隆在西澳大利亚州，我们进行了离子微探针U-Pb测年和稀土元素（REE）分析的磷灰石从岩脉之一。根据岩脉中磷灰石的产状，确定了两种类型的磷灰石。一个是棒状磷灰石（1型磷灰石）在次生硅微脉切割硅岩脉。另一种是出现在硅岩脉基质中的粒状磷灰石（2型磷灰石）。产在次生微脉（1型），非火成的稀土元素模式（1型和2型），化学分区（1型和2型的一些），铁硫化物内磷灰石（2型的一些），和共生的磷灰石和硫化物（2型的一些）表明，这两种类型的磷灰石结晶在硅岩脉。1型磷灰石离子探针U-Pb测年未给出严格的年龄，而2型磷灰石的Tera-WasserburgConcordia截距年龄为3214 ± 140 Ma（95%置信水平，MSWD = 0.6）在三维238 U/206 Pb-207 Pb/206 Pb-204 Pb/206 Pb图中，204 Pb/206 Pb-207 Pb/206 Pb等时线年龄为3191 ± 150 Ma（95%置信水平，MSWD = 0.5）。2型磷灰石的U-Pb和Pb-Pb年龄是结晶年龄还是变质年龄，其U-Pb系闭合温度的估算范围与变质温度的估算范围有部分重叠。无论哪种情况，都可以得出岩墙和干酪根的最小年龄为3.2Ga的结论。这些年龄可能允许解释干酪根至少在3.2Ga之前由生物固碳和/或非生物反应产生。
In order to provide a time constraint on the 13C-depleted kerogen formation in silica dikes that intruded 3.5 Ga greenstone from Pilbara Craton in Western Australia, we have carried out an ion microprobe U-Pb dating and rare earth element (REE) analysis of apatite from one of the dikes. Two types of apatite were identified in the dike, based on their occurrences. One is stick-shape apatites (Type 1 apatite) in secondary silica micro-veins that cut the silica dike. The other is granular apatites (Type 2 apatite) that occur in matrix of the silica dike. Occurrence in the secondary micro-veins (Type 1), non-igneous REE patterns (Type 1 and 2), chemical zoning (some of Type 1 and 2), iron sulfide within apatite (some of Type 2), and intergrowth of apatite and sulfide (some of Type 2) suggest that both types of apatite were crystallized in the silica dike. Ion microprobe U-Pb dating of Type 1 apatite did not give a well-constrained age, while Type 2 apatite yields a Tera-Wasserburg concordia intercept age of 3214 ± 140 Ma (95% confidence level, MSWD = 0.6) in a three-dimensional 238U/206Pb-207Pb/206Pb-204Pb/206Pb diagram, and a 204Pb/206Pb-207Pb/206Pb isochron age of 3191 ± 150 Ma (95% confidence level, MSWD = 0.5). It is difficult to judge whether the U-Pb and Pb-Pb age of Type 2 apatite is crystallization age or metamorphic age, since the estimated range of closure temperature of U-Pb system in the apatite and that of metamorphic temperature is partly overlapped. In either case, it can be safely concluded that the minimum age of the dike and kerogen is 3.2 Ga. These ages might allow the interpretation that the kerogen was produced by biological carbon fixation and/or abiological reaction at least before 3.2 Ga.