Combined boron, radiogenic (Nd, Pb, Sr), stable (C, O) isotopic and geochemical investigations of carbonatites from the Blue River Region, British Columbia (Canada): Implications for mantle sources and recycling of crustal carbon
Combined boron, radiogenic (Nd, Pb, Sr), stable (C, O) isotopic and geochemical investigations of carbonatites from the Blue River Region, British Columbia (Canada): Implications for mantle sources and recycling of crustal carbon
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DOI：
10.1016/j.chemgeo.2019.07.015
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发表时间：
2019-07
期刊：
影响因子：
3.9
通讯作者：
Okay Çimen;C. Kuebler;S. Simonetti;Loretta Corcoran;R. Mitchell;A. Simonetti
中科院分区：
文献类型：
--
作者：
Okay Çimen;C. Kuebler;S. Simonetti;Loretta Corcoran;R. Mitchell;A. Simonetti
本文报道了加拿大不列颠哥伦比亚省蓝河地区冷杉、费利克斯、古姆和霍华德溪碳酸岩的主、次、微量元素组合、稳定（C、O）、放射成因（Nd、Pb和Sr）同位素组成和首次δ 11 B同位素数据。这些岩柱状产出物在晚寒武世和泥盆纪-密西西比纪的裂谷和伸展阶段侵入到晚元古代地层中，随后在碰撞型构造环境下变形变质为角闪岩级。Fir、Gum和Felix的碳酸岩含有方解石和白云石，而霍华德溪的碳酸岩仅含有方解石。本文报道的白云岩成分与交代橄榄岩地幔直接部分熔融的实验结果一致。所有样品的主量元素和微量元素组合以及δ 13 CPDB（−5.37 ~ −4.85‰）和δ 18 OSMOW（9.14 ~ 9.62‰）值与原生火成岩碳酸盐一致，支持其地幔成因。然而，这些特征不能归因于封闭系统熔体从单一母体熔体的分化。最初的Nd，Pb和Sr同位素比值是高度可变的，并建议从多个，小程度的父母来自异质地幔源熔体代。菲利克斯、古姆和霍华德溪碳酸盐的δ 11 B值在−8.67 ‰和−6.36‰之间，与软流圈地幔的δ 11 B值（−7.1 ± 0.9‰）重叠，而来自冷杉的两个样品的δ 11 B值更大，分别为−3.98 ‰和−2.47‰。后者表明地幔源区存在再循环的地壳碳，这与全球年轻（<300 Ma）碳酸岩的结果一致。放射性和B同位素结果的蓝河碳酸盐岩进行了比较，从对比，非造山构造环境在奇普曼湖，芬，Jaleiranga，并表明，类似的上地幔来源被挖掘的碳酸盐岩熔体生成。这里报告的蓝河碳酸岩原始的、类似地幔的δ 11 B值清楚地表明，这种同位素系统是稳健的，没有受到凝固后构造变质事件的干扰。这一观测结果表明，B同位素特征是一个有价值的工具，用于破译上地幔来源的岩浆成因的碳酸盐岩的性质。
This study reports the combined major, minor and trace element compositions, and stable (C, O), radiogenic (Nd, Pb, and Sr) isotopic compositions, and first δ11B isotopic data for the Fir, Felix, Gum, and Howard Creek carbonatites from the Blue River Region, British Columbia (Canada). These sill-like occurrences were intruded into Late Proterozoic strata during rifting and extensional episodes during the Late Cambrian and Devonian -Mississippian, and subsequently deformed and metamorphosed to amphibolite grade in relation to a collisional-type tectonic environment. The carbonatites at Fir, Gum, and Felix contain both calcite and dolomite, whereas the carbonatite at Howard Creek contains only calcite. The dolomite compositions reported here are consistent with those experimentally determined by direct partial melting of metasomatized peridotitic mantle. The combined major and trace element compositions and δ13CPDB(−5.37 to −4.85‰) and δ18OSMOW(9.14 to 9.62‰) values for all the samples investigated are consistent with those for primary igneous carbonate and support their mantle origin. However, these signatures cannot be attributed to closed system melt differentiation from a single parental melt. The initial Nd, Pb, and Sr isotopic ratios are highly variable and suggest generation from multiple, small degree parental melts derived from a heterogeneous mantle source. The δ11B values for carbonates from Felix, Gum, and Howard Creek vary between −8.67 and −6.36‰, and overlap the range for asthenospheric mantle (−7.1 ± 0.9‰), whereas two samples from Fir yield heavier values of −3.98 and −2.47‰. The latter indicate the presence of recycled crustal carbon in their mantle source region, which is consistent with those for young (<300 Ma) carbonatites worldwide. The radiogenic and B isotope results for the Blue River carbonatites are compared to those from contrasting, anorogenic tectonic settings at Chipman Lake, Fen, and Jacupiranga, and indicate that similar upper mantle sources are being tapped for carbonatite melt generation. The pristine, mantle-like δ11B values reported here for the Blue River carbonatites clearly demonstrate that this isotope system is robust and was not perturbed by post-solidification tectono-metamorphic events. This observation indicates that B isotope signatures are a valuable tool for deciphering the nature of the upper mantle sources for carbonates of igneous origin.