Diamond growth from oxidized carbon sources beneath the Northern Slave Craton, Canada: A δ13C–N study of eclogite-hosted diamonds from the Jericho kimberlite
Diamond growth from oxidized carbon sources beneath the Northern Slave Craton, Canada: A δ13C–N study of eclogite-hosted diamonds from the Jericho kimberlite
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加拿大北部奴隶克拉通下方氧化碳源中的钻石生长：对杰里科金伯利岩中榴辉岩托管钻石的 δ13C-N 研究
DOI：
10.1016/j.gca.2011.07.028
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发表时间：
2011
影响因子：
5
通讯作者：
L. Heaman
中科院分区：
文献类型：
--
作者：
K. Smart;T. Chacko;T. Stachel;K. Muehlenbachs;R. Stern;L. Heaman
本文对加拿大杰里科金伯利岩榴辉岩捕虏体的高MgO和低MgO组金刚石进行了碳同位素组成和氮含量分析。从两组中提取的金刚石显示出显著不同的氮丰度和δ 13 C值。高MgO榴辉岩中的金刚石含氮量低（5- 82 ppm），δ 13 C值极低，在-40 ‰，而低MgO榴辉岩中的金刚石含氮量高（> 1200 ppm），δ 13 C值在-3.5 ‰到-5.3 ‰。耦合阴极发光（CL）成像和西姆斯分析的杰里科钻石提供了深入了解钻石的生长过程。高MgO榴辉岩中的金刚石几乎没有CL结构，通常具有恒定的δ 13 C值和氮含量。其中一些金刚石具有次生边缘，δ 13 C值从−40‰增加到10 - 34‰，这表明金刚石的次生生长发生在氧化的生长介质中。高MgO榴辉岩金刚石的负δ 13 C值不能由平均幔源碳（-5 ‰）或典型有机质碳（-25 ‰）的瑞利同位素分馏产生。然而，δ 13 C值在约60‰的有机沉积记录中的偏移是已知的。2.7和2.0Ga，因此，高MgO榴辉岩中的金刚石可能是由俯冲作用带入Slave岩石圈地幔的类似有机质形成的。对低MgO榴辉岩中金刚石的西姆斯分析表明，外核δ 13 C值系统性地向边缘增加，同时氮含量减少，边缘有明显的交替生长带。耦合的δ 13 C-氮数据表明，金刚石在从氧化的流体/熔体分步结晶期间沉淀，其中氮在生长期间逐渐耗尽。δ 13 C-N的共变模型计算得出分配系数（KN）值为5，表明相对于生长介质，氮在金刚石中具有很强的相容性。金刚石核的δ 13 C值（−4‰）表明生长介质的δ 13 C值高于原生幔源碳。因此，低MgO榴辉岩金刚石的可能碳源包括在迁移过程中经历了一些分馏的氧化的地幔衍生（例如原金伯利岩或碳酸岩）流体/熔体或脱挥发分的俯冲碳酸盐。
Diamonds from high- and low-MgO groups of eclogite xenoliths from the Jericho kimberlite, Slave Craton, Canada were analyzed for carbon isotope compositions and nitrogen contents. Diamonds extracted from the two groups show remarkably different nitrogen abundances and δ13C values. While diamonds from high-MgO eclogites have low nitrogen contents (5–82ppm) and extremely low δ13C values clustering at ∼−40‰, diamonds from the low-MgO eclogites have high nitrogen contents (>1200ppm) and δ13C values from −3.5‰ to −5.3‰. Coupled cathodoluminescence (CL) imaging and SIMS analysis of the Jericho diamonds provides insight into diamond growth processes. Diamonds from the high-MgO eclogites display little CL structure and generally have constant δ13C values and nitrogen contents. Some of these diamonds have secondary rims with increasing δ13C values from −40‰ to ∼−34‰, which suggests secondary diamond growth occurred from an oxidized growth medium. The extreme negative δ13C values of the high-MgO eclogite diamonds cannot be produced by Rayleigh isotopic fractionation of average mantle-derived carbon (−5‰) or carbon derived from typical organic matter (∼−25‰). However, excursions in δ13C values to −60‰ are known in the organic sedimentary record at ca. 2.7 and 2.0Ga, such that diamonds from the high-MgO eclogites could have formed from similar organic matter brought into the Slave lithospheric mantle by subduction. SIMS analyses of a diamond from a low-MgO eclogite show an outer core with systematic rimwards increases in δ13C values coupled with decreases in nitrogen contents, and a rim with pronounced alternating growth zones. The coupled δ13C-nitrogen data suggest that the diamond precipitated during fractional crystallization from an oxidized fluid/melt from which nitrogen was progressively depleted during growth. Model calculations of the co-variation of δ13C–N yielded a partition coefficient (KN) value of 5, indicating that nitrogen is strongly compatible in diamond relative to the growth medium. δ13C values of diamond cores (−4‰) dictate the growth medium had higher δ13C values than primary mantle-derived carbon. Therefore, possible carbon sources for the low-MgO eclogite diamonds include oxidized mantle-derived (e.g. protokimberlite or carbonatite) fluids/melts that underwent some fractionation during migration or, devolatilized subducted carbonates.