SIMS Microanalysis of the Strelley Pool Formation Cherts and the Implications for the Secular-Temporal Oxygen-isotope Trend of Cherts
SIMS Microanalysis of the Strelley Pool Formation Cherts and the Implications for the Secular-Temporal Oxygen-isotope Trend of Cherts
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DOI：
10.1016/j.precamres.2017.11.005
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发表时间：
2018
影响因子：
3.8
通讯作者：
J. Cammack;M. Spicuzza;A. Cavosie;M. V. Kranendonk;A. Hickman;R. Kozdon;I. Orland;K. Kitajima;J. Valley
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文献类型：
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作者：
J. Cammack;M. Spicuzza;A. Cavosie;M. V. Kranendonk;A. Hickman;R. Kozdon;I. Orland;K. Kitajima;J. Valley
太古宙燧石中氧同位素比的意义长期以来一直存在争议。来自c的樱桃。 3.4Ga 斯特雷池地层 (SPF)（皮尔巴拉克拉通，西澳大利亚）拥有一些最古老的叠层石和微化石，证明生命存在，但硅胶结物的成因和形成时间尚不清楚。现场关系、岩相学以及激光氟化和石英中 δ18O 的原位 SIMS 测量相结合表明，SPF 的层状燧石最初以碳酸盐形式沉淀，后来广泛被石英取代。对三个地点的燧石中 δ18O(Qz) 进行了研究和分析： 1) Camel Creek：叶状燧石、变质燧石、层状燧石和米级黑色燧石脉； 2）不整合面脊及ABDP8钻芯：叠层质、层状燧石，上覆基底碎屑石英砂岩； 3) Trendall 地区：“层状”叠层燧石取代了原始白云石、低温热液石英和毫米级至分米级燧石石英脉。 δ18O(Qz) 的激光氟化（毫米级）值范围为：Camel Creek 的 14.2 至 18.2‰ VSMOW；不整合脊处为 9.3 至 18.9‰； Trendall 为 13.7 至 25.7‰。 Trendall 的 δ18O(Qz) 值（以厘米为单位）到分米级热液燧石脉切割层状碳酸盐的范围从约15至16‰，而“层状燧石”为17至26‰。这些激光数据包括古太古代沉积物中燧石报告的最高 δ18O 值，比其他研究报告的上限 ∼22‰ 高出 4‰，这与长期存在的长期趋势形成鲜明对比，后者仅在较年轻的燧石中显示如此高的 δ18O。然而，1 毫米尺度的激光氟化分析无法解析岩相学上看到的微观结构。相比之下，原位 SIMS 分析可以解析岩相显微纹理，并显示在 Trendall 地点的“层状”燧石中 10 μm 尺度的 δ18O(Qz) 范围更大，为 7–31 ‰，比长期-时间趋势高出 9 ‰。在 Trendall 地点光学观察到的纹理分为：微石英、中石英、玉髓、巨型石英脉和空腔兆石英。 SEM-CL 成像显示了两代中观石英和兆石英；明亮的CL具有发达的生长分区，深色的CL具有块状或斑驳的纹理。微石英是最早的结构代石英，根据 SIMS 检测，其 δ18O(Qz) 最大值为 ∼22‰。 Dark-CL 中观石英与微石英具有相似的 δ18O，并且被解释为也是早期石英。亮区-CL介观石英形成于太古代之后，具有更高的δ18O，高达29‰。脉状巨型石英横切了大多数石英世代，并且 δ18O 范围有限，大多为 16 至 19 ‰。玉髓假形菱形空腔和裂缝，排列在脉的边缘，并且具有与脉相似的 δ18O (16–19‰)。晚腔巨石英是明亮的，并被 CL 分带，生长成晚开腔，并且具有皮尔巴拉报告的最高 δ18O(Qz) 值，高达 31.3‰。因此，Trendall 地区δ18O 最高的石英胶结物是最年轻的，可能与风化有关。早期硅化以及微石英、玉髓和低 δ18O 中石英的形成发生在太古代低温热液活动期间。所检查的 SPF 石英均不被解释为是由古太古代海水直接沉淀物形成的。因此，δ18O(Qz) 值不记录太古宙海洋的水化学或温度。原位 SIMS 分析表明，高于 22‰ 的高 δ18O(Qz) 值仅在 Trendall 地区的晚形成腔兆石英和亮区 CL 中石英中发现。我们样品套件的 SPF 结果表明能够……
The significance of oxygen isotope ratios in Archean chert has long been debated. Cherts from the c. 3.4 Ga Strelley Pool Formation (SPF) (Pilbara Craton, Western Australia) host some of the oldest stromatolite and microfossil evidence for life, but the genesis and timing of silica cements has been unclear. Field relations, petrography and a combination of laser fluorination and in-situ SIMS measurements of δ18O in quartz show that bedded cherts of the SPF were originally precipitated as carbonates and were later widely replaced by quartz. Three localities were studied and analyzed for δ18O(Qz) in chert: 1) Camel Creek: foliated, metamorphosed, bedded cherts and meter-scale black chert veins; 2) Unconformity Ridge and ABDP8 drill core: stromatolitic and bedded chert overlying basal detrital quartz sandstone; and 3) the Trendall locality: “bedded” stromatolitic chert replacing original dolomite, low temperature hydrothermal quartz, and mm- to decimeter-scale chert-quartz veins. Laser fluorination (mm-scale) values of δ18O(Qz) range from: 14.2 to 18.2‰ VSMOW at Camel Creek; 9.3 to 18.9‰ at Unconformity Ridge; and 13.7 to 25.7‰ at Trendall. Values of δ18O(Qz) in cm to decimeter-scale hydrothermal chert veins cutting bedded carbonates at Trendall range from ca. 15 to 16‰, whereas “bedded cherts” are 17 to 26‰. These laser data include the highest δ18O values reported for cherts in Paleoarchean sediments and are up to 4‰ higher than the upper limit of ∼22‰ reported in other studies, in apparent contrast to the long-standing secular-temporal trend which shows such high δ18O only in younger chert. However, analysis by laser fluorination at the 1-mm scale cannot resolve microtextures seen petrographically. In contrast, in-situ SIMS analyses can resolve petrographic microtextures and show δ18O(Qz) at 10-μm scale have an even greater range of 7–31‰ in “bedded” cherts at the Trendall locality, up to 9‰ above the secular-temporal trend.Textures observed optically at the Trendall locality were classified as: microquartz, mesoquartz, chalcedony, megaquartz veins, and cavity megaquartz. SEM-CL imaging shows two generations of meso- and megaquartz; bright CL with well-developed growth zoning, and dark CL with massive or mottled texture. Microquartz is the earliest textural generation of quartz and has a maximum δ18O(Qz) of ∼22‰ by SIMS. Dark-CL mesoquartz has similar δ18O to microquartz and is interpreted to also be early. Bright-zoned-CL mesoquartz, which formed post-Archean, has even higher δ18O, up to 29‰. Vein megaquartz crosscuts most quartz generations and has a restricted range of δ18O, mostly from 16 to 19‰. Chalcedony pseudomorphs rhombic cavities and fractures, lines the edges of veins, and has similar δ18O to veins (16–19‰). Late cavity megaquartz is bright and zoned by CL, grows into late open cavities, and has the highest δ18O(Qz) values reported from the Pilbara, up to 31.3‰. Thus, the highest-δ18O quartz cements at the Trendall locality are the youngest and may be related to weathering. Early silicification and the formation of microquartz, chalcedony and low δ18O mesoquartz occurred during low temperature hydrothermal activity in the Archean.None of the SPF quartz examined is interpreted to have formed as a direct precipitate from Paleoarchean seawater. Thus, values of δ18O(Qz) do not record either water chemistry or temperature of Archean oceans. In-situ SIMS analysis shows that high-δ18O(Qz) values above 22‰ are only found in late-forming cavity megaquartz and bright-zoned-CL mesoquartz at the Trendall locality.The SPF results from our sample suite demonstrate the ability to …