40Ar/39Ar-ages of phlogopite in mantle xenoliths from South African kimberlites: Evidence for metasomatic mantle impregnation during the Kibaran orogenic cycle
40Ar/39Ar-ages of phlogopite in mantle xenoliths from South African kimberlites: Evidence for metasomatic mantle impregnation during the Kibaran orogenic cycle
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DOI：
10.1016/j.lithos.2008.09.001
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发表时间：
2008-12
期刊：
影响因子：
3.5
通讯作者：
J. Hopp;M. Trieloff;G. Brey;A. Woodland;N. Simon;J. Wijbrans;W. Siebel;E. Reitter
中科院分区：
文献类型：
--
作者：
J. Hopp;M. Trieloff;G. Brey;A. Woodland;N. Simon;J. Wijbrans;W. Siebel;E. Reitter
我们应用40 Ar/39 Ar定年方法对南非Bultfontein、莱索托Letseng-la-Terae和Liqhobong等矿区金伯利岩地幔捕虏体（主要含石榴石）中的金云母进行了测定。采用传统的高分辨率步进加热技术、激光增量加热技术和激光光斑分析技术进行了氩的提取。常规分析得到的所有年龄谱表明金伯利岩侵位过程中40 Ar有不同程度的损失，但从未导致Ar系统的完全重置。最有趣的是，样品特定的最大表观年龄集群之间的1.0和1.22 Ga金云母干扰最小的年龄谱。在激光加热分析过程中观察到的最大表观年龄为1.02 Ga。个别谷物往往产生较老的年龄在他们的核心，与连续年轻的年龄在他们的边缘。通过激光熔融技术和传统的分步加热技术获得的年龄范围相互一致，以及与文献数据。通常推断存在过剩的40 Ar在这些金云母不能解释连贯的年龄模式，在大套样品。因此，1.0-1.25 Ga的年龄限制被认为是地质意义，并分配到当地的地幔交代作用在基原造山作用的到来（1.00-1.25 Ga）。我们的研究结果的主要结果是：（一）金云母的氩系统可以保持封闭的长时间尺度，即使在800-1200 °C的环境温度在地幔内，最有可能是因为金云母和地幔中的其他主要阶段之间的Ar的固体/固体分配行为强烈有利于金云母，或者因为传统推断的金云母中的氩的扩散率被严重高估。因此，40 Ar/39 Ar金云母系统似乎是一个有价值的工具，破译古老的交代事件影响岩石圈地幔。(ii)在活动大陆边缘的洋壳俯冲过程中，非洲南部以下的岩石圈地幔可能经常受到不同时期流体或熔体迁移的影响。
We applied the40Ar/39Ar dating method to an extensive suite of phlogopites from kimberlite-hosted mantle xenoliths (dominantly garnet bearing) from the mines of Bultfontein (South Africa), Letseng-la-Terae and Liqhobong (Lesotho). Argon extraction was performed by conventional high resolution stepwise heating technique, laser incremental heating technique and laser spot analysis. All age spectra obtained by conventional analysis indicate various degrees of40Ar loss during kimberlite emplacement, but never resulted in a total reset of the argon system. Most intriguingly, the sample-specific maximum apparent ages cluster between 1.0 and 1.22 Ga for the phlogopites with the least disturbed age spectra. A maximum apparent age of 1.02 Ga was observed during laser heating analysis. Individual grains tend to yield older ages in their cores, with successively younger ages at their rims. The range in age obtained via the laser fusion technique and with conventional stepwise heating technique agrees with each other, as well as with literature data. The often inferred presence of excess40Ar in those phlogopites cannot explain the coherent age pattern in the large suite of samples. Hence, the age constraint of 1.0–1.25 Ga is regarded as geologically meaningful and assigned to metasomatism of the local cratonic mantle during the advent of Kibaran orogenesis (1.00–1.25 Ga). The major consequences of our findings are: (i) The argon system of phlogopite can remain closed for long time scales, even at ambient temperatures of 800–1200 °C within the mantle, most likely because the solid/solid partitioning behaviour of Ar between phlogopite and other major phases in the mantle strongly favours phlogopite, or because conventionally inferred diffusivity of argon in phlogopite is seriously overestimated. Thus, the40Ar/39Ar phlogopite system appears to be a valuable tool for deciphering ancient metasomatic events affecting the lithospheric mantle. (ii) The cratonic lithospheric mantle below southern Africa may have been frequently influenced by different episodes of fluid or melt migration during subduction of oceanic crust at active continental margins.