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这项工作分析了祭坛石的两个30 µm抛光的薄片(MS3和2010K.240)和来自苏格兰东北部的ORS(补充信息4)。CQ1来自Orkney的Cruaday(59°04'34.2'n,N,3°18'54.6'W),AQ1来自Spittal,Caithness(58°28'13.8''N,N,3°27'33.6英寸W)。传统的光学显微镜(传输和反射光)和通过Tescan综合矿物分析仪自动化的矿物学,在LA-ICP-MS分析过程中洞悉了纹理和矿物学和指导点位置。Clara场发射扫描电子显微镜用于通过高分辨率微米尺度成像在后刻表电子和阴极发光下通过高分辨率微米尺度成像的单个矿物(锆石,磷灰石和金红石)的质地表征。祭坛石是一种细粒度良好的砂岩,平均晶粒尺寸直径为≤300µm。石英颗粒是亚行和单晶。长石的变化变为细粒白云母。MS3和2010K.240具有较弱的平面织物和非平面重型矿物质薄片约100–200 µm厚。电阻重型矿物带以锆石,金红石和磷灰石为主,晶粒通常宽10-40 µm。该岩石主要由碳酸盐水泥,碳酸盐和石英水泥的局部区域。参考文献中提供了祭坛石岩石学的详细说明。1,59。
在澳大利亚科廷大学的约翰·德·莱特中心(JDLC)的地球历史设施完成了两个锆石U – PB分析课程。使用Laurin Technic S155细胞使用准分子激光分辨率LE193 nm ARF创建锆石内的消融。同位素数据是用Agilent 8900三倍四极杆质谱仪收集的,具有高纯度AR为等离子体载体气体(流速1.l min-1)。〜2.3–2.7 J CM-2的样本能量为5–7 Hz重复速率,用于烧蚀30-40 s(背景捕获25-60 s)。使用了两个清洁脉冲进行分析,并使用超高纯度He(0.68 ml min-1)和N2(2.8 mL min-1)冲洗样品单元。在15-20个未知数之后,分析了参考矿物块。祭坛石的小,高度圆形的目标颗粒(通常 <30 µm in width) necessitated using a spot size diameter of ~24 µm for all ablations. Isotopic data was reduced using Iolite 460 with the U-Pb Geochronology data reduction scheme, followed by additional calculation and plotting via IsoplotR61. The primary matrix-matched reference zircon62 used to correct instrumental drift and mass fractionation was GJ-1, 601.95 ± 0.40 Ma. Secondary reference zircon included Plešovice63, 337.13 ± 0.37 Ma, 9150064, 1,063.78 ± 0.65 Ma, OG165 3,465.4 ± 0.6 Ma and Maniitsoq66 3,008.7 ± 0.6 Ma. Weighted mean U–Pb ages for secondary reference materials were within 2σ uncertainty of reported values (Supplementary Information 5).
Across two LA-ICP–MS sessions, 83 U–Pb measurements were obtained on as many zircon grains; 41 were concordant (≤10% discordant), where discordance is defined using the concordia log distance (%) approach67. We report single-spot (grain) concordia ages, which have numerous benefits over conventional U–Pb/Pb–Pb ages, including providing an objective measure of discordance that is directly coupled to age and avoids the arbitrary switch between 206Pb/238U and 207Pb/206Pb. Furthermore, given the spread in ages (Early Palaeozoic to Archaean), concordia ages provide optimum use of both U–Pb/Pb–Pb ratios, offering greater precision over 206Pb/238U or 207Pb/206Pb ages alone.
Given that no direct sampling of the Altar Stone is permitted, we are limited in the amount of material available for destructive analysis, such as LA-ICP–MS. We collate our zircon age data with the U–Pb analyses1 of FN593 (another fragment of the Altar Stone), filtered using the same concordia log distance (%) discordance filter67. The total concordant analyses used in this work is thus 56 over 3 thin sections, each showing no discernible provenance differences. Zircon concordia ages span from 498 to 2,812 Ma. Age maxima (peak) were calculated after Gehrels68, and peak ages defined by ≥4 grains include 1,047, 1,091, 1,577, 1,663 and 1,790 Ma.
For 56 concordant ages from 56 grains at >95%的确定性,最大的无资助的分数计算出整个均匀碎屑种群的9%。在任何情况下,对于任何数量的分析,都将对最普遍的重要组成部分进行采样。69。我们分析了薄节中该技术空间极限内的所有锆石70。我们使用了原位薄片分析,可以减轻碎屑研究中的污染和采样偏见71。添加磷灰石(U – Pb和Lu – Hf)和金红石(U – PB)分析了我们对出处解释的信心,因为这些矿物在运输过程中会做出不同的反应。
英国和爱尔兰地下室地下室的锆石U – PB汇编源自裁判。20,26。用于比较的ORS碎屑锆石数据集包括丁格尔半岛盆地20,盎格鲁 - 韦尔什盆地72,米德兰山谷盆地35,svalbard ors37和orcadian basin25的同位素数据。NRS锆石U – PB年龄来自Wessex Basin33。根据我们的定义高于20,26,对比较数据集进行了不一致的过滤。使用内核和直方图带宽为50 mA,在Isoplotr61中创建了年龄群体的内核密度估计。
实施了两次样本的Kolmogorov – Smirnov统计测试,以将编译的锆石年龄数据集与祭坛石进行比较(补充信息6)。这项双面测试比较了两个累积密度年龄函数之间的最大概率差异,评估了零假设,即两个年龄光谱都是根据分析数量和所选置信度水平的临界值来从同一分布中得出的。
所使用的比较数据集中的锆石年龄数从祭坛石(n = 56)到laurentia(n = 2,469)不同。因此,为了解决对样品N的依赖程度,我们还针对Kolmogorov – Smirnov检验实施了蒙特卡洛重新采样(1,000次)程序,包括根据每个样品分布的分布分布的每个年龄确定P值的不确定性,以重新计算P值和标准信息7)。使用Monte Carlo重采样(和多维分析)的Kolmogorov – Smirnov测试的结果,考虑到样本n引起的不确定性,还支持以下解释:以> 95%的确定性,在出处中没有区别,可以在Altar Stone Zircon Zircon Age数据集(n = 56)之间(n = 56)进行区分(n = 212)(n = 619)和劳伦(Laurentian)地下室(补充信息7)。
使用参考文献的MATLAB脚本创建了锆石数据集的MDS图。58。在这里,我们采用了一个引导再采样(> 1,000次),并用procrustes旋转Kolmogorov – Smirnov值,该值在95%的置信度下输出不确定性椭圆(图3A)。在MDS图中,应力是数据集中的差异和MDS图上距离之间的拟合指标的优点。低于0.15的应力值是理想的58。对于图3A中的MDS图,该值为0.043,表示“出色”拟合58。
在澳大利亚Curtin University的JDLC的地理历史设施完成了一个金红石U – PB分析课程。使用COMPEX 102 IMPIMER激光器使用谐振分辨率M-50A-LR采样系统烧蚀金红石晶粒(24 µM),并使用Agilent 8900三倍四倍体质量分析仪测量。分析参数包括2.7 j cm-2的样本能量,总分析时间为45 s的重复率为7 Hz,背景数据捕获60 s。用超高纯度以0.68 l min -1和N2的流速在2.8 ml min -1中清除样品室。
对于R-10金红石主要参考材料材料73(1,091±4 MA),降低了金红石分析的U – PB数据。用于监测U – PB比率准确性的二级参考材料为R-19金红石。R-19的平均加权238U/206pb年龄为491±10(平均平方加权偏差(MSWD)= 0.87,p(χ2)= 0.57)在不确定性的489.5±0.9 mA的不确定性中。
可以使用208pb校正来校正具有可忽略的TH浓度的金红石晶粒74。以前使用的TH内容阈值包括75,76 th/u< 0.1 or a Th concentration >5%U.但是,MS3金红石的Th/U比通常> 1。因此,不适用208pb校正。取而代之的是,我们使用基于207的公共PB校正31来解释普通PB的存在。使用U – PB地球体学还原方案和Isoplotr61,在Iolite 460中减少了金红石同位素数据。
在U – PB单节中获得了92个金红石U – PB分析,该会议定义了TERA – Wasserburg图上的两个连贯的年龄组。
第1组构成了83个U – PB金红石分析,在常见和放射性PB组件之间在TERA-WASSERBURG图上形成了明确的混合阵列。该阵列的上部截距为207pb/ 206pbi = 0.8563±0.0014。较低的截距意味着年龄为451±8 mA。围绕该线的散射(MSWD = 2.7)被解释为反映了在岩浆结晶期间和之后在〜600°C下通过放射性PB闭合温度的不同晶粒大小的金红石通过的可变传递77。
第2组包括9种晶粒,207pb校正了238U/206pb年龄,范围为591–1,724 MA。第2组的三种晶粒定义了一个年龄峰值68在1,607 MA。鉴于U – PB年龄的扩散,我们将这些元古代晶粒解释为代表来自各种来源的碎屑金红石。
在澳大利亚Curtin University的JDLC的地球历史设施进行了两次磷灰石U – PB LA-ICP-MS分析课程。对于这两个课程,使用分辨率193 nm准分子激光消融系统连接到具有分辨率LE193 nm ARF和Laurin Technic S155 Cell ICP -MS的分辨率193 nm激光消融系统进行消融。其他分析细节包括2 J CM2的通量和5 Hz的重复率。对于祭坛石部分(MS3)和Orcadian盆地样品(补充信息4),分别使用了24 µm的斑点尺寸。
用于磷灰石U – PB分析的基质匹配的主要参考材料是马达加斯加磷灰石(MAD-1)78。分析了一系列二级参考磷灰石,包括FC-179(Duluth Complex),年龄为1,099.1±0.6 Ma,Mount McClure80,81 526±2.1 MA,Otter Lake82 913±7 MA和Durango 31.44±0.1883 MA。二次参考材料的锚定回归(通过报告的207pb/206pbi值)在报告值的2σ不确定度中产生了较低的截距年龄(补充信息8)。
祭坛石的第一届MS3的磷灰石U – Pb会产生117个分析。在TERA -WASSERBURG图上,这些分析形成了具有不同下截距的常见和放射性PB组件之间的两个不一致的混合阵列。
来自第2组磷灰石的阵列由9个分析组成,产生的截距相当于年龄为1,018±24 mA(MSWD = 1.4)的年龄,上截距为207pb/ 206pbi = 0.8910±0.0251。第2组的磷灰石分析的F207%(使用207pb方法估计的普通PB百分比为16.66–88.8%,平均为55.76%。
第1组磷灰石由108个分析定义,较低的截距为462±4 mA(MSWD = 2.4),上部截距为207pb/ 206pbi = 0.8603±0.0033。第1组中F207%的磷灰石分析范围为10.14–99.91%,平均为78.65%。磷灰石回归线的轻微分散可能反映了这些晶体中PB闭合温度的某些变化84。
第二个磷灰石U – PB会话从CQ1和AQ1进行了138个分析。这些数据构成了Tera – Wasserburg图上放射性PB组件和普通PB组件之间的三个不一致的混合阵列。
通过Cruaday样品(CQ1)通过第1组Apatite(n = 14)进行的未经常规回归得出的较低截距为473±25 Ma(MSWD = 1.8),上部截距为207pb/ 206pbi = 0.8497±0.0128。F207%跨度为38-99%,平均值为85%。
来自Spittal样品(AQ1)的第1组,由109个分析组成,得出等于466±6 mA的较低截距(MSWD = 1.2)。207pb/ 206pbi上方等于0.8745±0.0038。该组的F207%值范围为6-99%,平均值为83%。SPITTAL样品中通过第2组分析的回归(n = 17)的截距较低的截距为1,013±35 Ma(MSWD = 1)和上截距207pb/ 206pbi,为0.9038±0.0101。F207%的值占25-99%,平均为76%。从CQ1和AQ1和AQ1(n = 123)的第1组的U – PB分析得出的较低截距等于466±6 Ma(MSWD = 1.4)和上截距207pb/ 206pbi,为0.8726±0.0036,在Orcadian Basin basin Kernel -dsimpter in n.4bastimate in in n.4bastimate in n eftemimate in。
使用分辨率-LR 193 nm准分子激光消融系统,通过原位LU – HF方法在薄片的薄片中呈磷灰石晶粒,并与Agilent 8900 ICP – MS/MS85,86耦合。NH3中的气体混合物在质谱仪反应仪中用于促进高阶HF反应产物,而等效的LU和YB反应产物可以忽略不计。176+82小时和178+82小时的质量偏移(+82 AMU)反应产物达到了可测量范围的最高敏感性,并被分析不含同质干扰。假设自然丰度,从178小时计算177小时。175lu以176LU为代理85测量。激光消融用激光束为43 µm,以7.5 Hz的重复速率,流利度约为3.5 j cm -2。The analysed isotopes (with dwell times in ms between brackets) are 27Al (2), 43Ca (2), 57Fe (2), 88Sr (2), 89+85Y (2), 90+83Zr (2), 140+15Ce (2), 146Nd (2), 147Sm (2), 172Yb (5), 175Lu (10), 175+82Lu(50),176+82hf(200)和178+82hf(150)。同位素短时间的同位素(<10 ms) were measured to confirm apatite chemistry and to monitor for inclusions. 175+82Lu was monitored for interferences on 176+82Hf.
Relevant isotope ratios were calculated in LADR87 using NIST 610 as the primary reference material88. Subsequently, reference apatite OD-30678 (1,597 ± 7 Ma) was used to correct the Lu–Hf isotope ratios for matrix-induced fractionation86,89. Reference apatites Bamble-1 (1,597 ± 5 Ma), HR-1 (344 ± 2 Ma) and Wallaroo (1,574 ± 6 Ma) were monitored for accuracy verification85,86,90. Measured Lu–Hf dates of 1,098 ± 7 Ma, 346.0 ± 3.7 Ma and 1,575 ± 12 Ma, respectively, are in agreement with published values. All reference materials have negligible initial Hf, and weighted mean Lu–Hf dates were calculated in IsoplotR61 directly from the (matrix-corrected) 176Hf/176Lu ratios.
For the Altar Stone apatites, which have variable 177Hf/176Hf compositions, single-grain Lu–Hf dates were calculated by anchoring isochrons to an initial 177Hf/176Hf composition90 of 3.55 ± 0.05, which spans the entire range of initial 177Hf/176Hf ratios of the terrestrial reservoir (for example, ref. 91). The reported uncertainties for the single-grain Lu–Hf dates are presented as 95% confidence intervals, and dates are displayed on a kernel density estimate plot.
Forty-five apatite Lu–Hf analyses were obtained from 2010K.240. Those with radiogenic Lu ingrowth or lacking common Hf gave Lu–Hf ages, defining four coherent isochrons and age groups.
Group 1, defined by 16 grains, yields a Lu–Hf isochron with a lower intercept of 470 ± 28 Ma (MSWD = 0.16, p(χ2) = 1). A second isochron through 5 analyses (Group 2) constitutes a lower intercept equivalent to 604 ± 38 Ma (MSWD = 0.14, p(χ2) = 0.94). Twelve apatite Lu–Hf analyses define Group 3 with a lower intercept of 1,123 ± 42 Ma (MSWD = 0.75, p(χ2) = 0.68). Three grains constitute the oldest grouping, Group 4 at 1,526 ± 186 Ma (MSWD = 0.014, p(χ2) = 0.91).
A separate session of apatite trace element analysis was undertaken. Instrumentation and analytical set-up were identical to that described in 4.1. NIST 610 glass was the primary reference material for apatite trace element analyses. 43Ca was used as the internal reference isotope, assuming an apatite Ca concentration of 40 wt%. Secondary reference materials included NIST 612 and the BHVO−2g glasses92. Elemental abundances for secondary reference material were generally within 5–10% of accepted values. Apatite trace element data was examined using the Geochemical Data Toolkit93.
One hundred and thirty-six apatite trace element analyses were obtained from as many grains. Geochemical classification schemes for apatite were used29, and three compositional groupings (felsic, mafic-intermediate, and alkaline) were defined.
Felsic-classified apatite grains (n = 83 (61% of analyses)) are defined by La/Nd of <0.6 and (La + Ce + Pr)/ΣREE (rare earth elements) of <0.5. The median values of felsic grains show a flat to slightly negative gradient on the chondrite-normalized REE plot from light to heavy REEs94. Felsic apatite’s median europium anomaly (Eu/Eu*) is 0.59, a moderately negative signature.
Mafic-intermediate apatite29 (n = 48 (35% of grains)) are defined by (La + Ce + Pr)/ΣREE of 0.5–0.7 and a La/Nd of 0.5–1.5. In addition, apatite grains of this group typically exhibit a chondrite-normalized Ce/Yb of >5 and ΣREEs up to 1.25 wt%. Apatite grains classified as mafic-intermediate show a negative gradient on a chondrite-normalized REE plot from light to heavy REEs. The apatite grains of this group generally show the most enrichment in REEs compared to chondrite94. The median europium (Eu/Eu*) of mafic-intermediate apatite is 0.62, a moderately negative anomaly.
Lastly, alkaline apatite grains29 (n = 5 (4% of analyses)) are characterized by La/Nd > 1.5和A(LA+CE+PR)/σRee> 0.8。该组的欧洲异常中位数为0.45。该分组还显示了σree的升高的软骨量标准CE/Yb> 10,> 0.5 wt%。
有关研究设计的更多信息可在与本文有关的自然投资组合报告摘要中获得。
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本文概览: 这项工作分析了祭坛石的两个30 µm抛光的薄片(MS3和2010K.240)和来自苏格兰东北部的ORS(补充信息4)。CQ1来自Orkney的Cruaday(59°04'3...
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