The effect of weathering on U-Th-Pb and oxygen isotope systems of ancient zircons from the Jack Hills, Western Australia
The effect of weathering on U-Th-Pb and oxygen isotope systems of ancient zircons from the Jack Hills, Western Australia
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DOI：
10.1016/j.gca.2016.10.005
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发表时间：
2017-01-15
影响因子：
5
通讯作者：
Whitehouse, M. J.
中科院分区：
文献类型：
--
作者：
Pidgeon, R. T.;Nemchin, A. A.;Whitehouse, M. J.
本文报道了对澳大利亚西部杰克山44个年龄在4.3 ~ 3.3Ga的古老锆石进行的西姆斯U-Th-Pb和O-OH研究结果。我们已经调查的行为，氧同位素和水的颗粒，通过确定δ O-18和OH值在一些位置上的抛光表面上的每个颗粒。根据锆石的U-Th-Pb和OH-氧同位素特征，将其分为五组。第一组具有一致的U-Th-Pb年龄，最小的共同铅，δ O-18值与来自地幔源岩的锆石一致，没有检测到OH含量。第2、3、4组锆石中的U-Th-Pb系统受裂隙的影响，从一致到极不一致。在大约零Ma年龄的不一致性与一致性的拦截被解释为干扰的锆石U-Th-Pb系统的风化溶液在广泛的，深风化，影响了太古代Yilgarn的西澳大利亚州，因为至少在二叠纪。风化溶液进入裂缝导致Th和U的流入。第2组颗粒的δ O-18值大约落在“地幔”范围内，OH在背景水平内或略微升高。第3组晶粒的δ O-18值的特征在于δ O-18随OH含量增加而降低的初始趋势。随着OH的进一步增加，这种趋势逆转，并且δ O-18随着OH的增加而变得更重。第4组晶粒具有随OH增加而增加Δ O-18的明显趋势。这些趋势的解释方面的反应，软化水与变质锆石结构，似乎是独立的分析重叠裂缝。第五组锆石的特点是U-Pb系统，似乎包括一个以上的年龄，但只有轻微的U-Pb不一致。尽管如此，该组颗粒中δ O-18相对于OH的趋势与其他组中观察到的趋势相似。观察到的趋势三角洲O-18与OH在杰克山锆石是类似的锆石从澳大利亚西南部的太古代花岗岩在以前的研究报告。(C)2016爱思唯尔有限公司版权所有
We report the result of a SIMS U-Th-Pb and O-OH study of 44 ancient zircons from the Jack Hills in Western Australia with ages ranging from 4.3 Ga to 3.3 Ga. We have investigated the behaviour of oxygen isotopes and water in the grains by determining delta O-18 and OH values at a number of locations on the polished surfaces of each grain. We have divided the zircons into five groups on the basis of their U-Th-Pb and OH-oxygen isotopic behaviour. The first group has concordant U-Th-Pb ages, minimal common Pb, delta O-18 values consistent with zircons derived from mantle source rocks and no detectable OH content. U-Th-Pb systems in zircons from Groups 2, 3 and 4 vary from concordant to extremely discordant where influenced by cracks. Discordia intercepts with concordia at approximately zero Ma age are interpreted as disturbance of the zircon U-Th-Pb systems by weathering solutions during the extensive, deep weathering that has affected the Archean Yilgarn Craton of Western Australia since at least the Permian. Weathering solutions entering cracks have resulted in an influx of Th and U. delta O-18 values of Group 2 grains fall approximately within the "mantle" range and OH is within background levels or slightly elevated. delta O-18 values of Group 3 grains are characterised by an initial trend of decreasing delta O-18 with increasing OH content. With further increase in OH this trend reverses and delta O-18 becomes heavier with increasing OH. Group 4 grains have a distinct trend of increasing delta O-18 with increasing OH. These trends are explained in terms of the reaction of percolating water with the metamict zircon structure and appear to be independent of analytical overlap with cracks. Group five zircons are characterised by U-Pb systems that appear to consist of more than one age but show only minor U-Pb discordance. Nevertheless trends in delta O-18 versus OH in this group of grains resemble trends seen in the other groups. The observed trends of delta O-18 with OH in the Jack Hills zircons are similar to those reported in a previous study of zircons from an Archean granite from southwestern Australia. (C) 2016 Elsevier Ltd. All rights reserved.