The depth of sub-lithospheric diamond formation and the redistribution of carbon in the deep mantle
The depth of sub-lithospheric diamond formation and the redistribution of carbon in the deep mantle
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DOI：
10.1016/j.epsl.2016.12.017
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发表时间：
2017-03-01
影响因子：
5.3
通讯作者：
Frost, Daniel J.
中科院分区：
文献类型：
--
作者：
Beyer, Christopher;Frost, Daniel J.
大多数钻石形成于地球岩石圈，但一小部分含有富含硅的多晶石榴石内含物，表明形成于更深的地幔。同生石榴石包裹体的成分可以提供金刚石形成的深度和地幔岩性信息。已经使用在多砧装置中在6和16 GPa之间的压力和1000至1400摄氏度的温度下进行的实验的结果来校准石榴石组合物中的压力依赖性变化。使用这些实验的结果，基于两种主要的多晶取代的经验参数化来配制气压计，所述多晶取代被称为多晶取代（Maj; Al 3 + = Mg 2 + + Si 4+）和Na-多晶取代（Na-Maj; Mg 2 + + Al 3 + = Na+ + Si 4+）。此外，先前公布的实验石榴石组合物的玄武岩，金伯利岩，科马提岩和橄榄岩散装组成包括在校准中，因此涵盖压力从6至20 GPa和温度从900至2100摄氏度。实验压力在这些条件下重现，标准偏差为0.86 GPa.The气压计是用来确定平衡压力的约500个报告石榴石夹杂物的钻石从一系列的地方。由于这些包裹体中的大多数被认为是同生的，因此可以使用包裹体化学来建立钻石形成深度和相关源岩的详细图像。金刚石源岩的地理差异被映射到岩石圈下地幔超过500公里的深度。连续的金刚石形成发生在这个深度范围内的岩性与榴辉岩的亲和力，但也在岩性之间出现过渡榴辉岩和橄榄岩的散装组合物，与辉石的亲和力。榴辉岩和辉石岩金刚石源岩之间的地理差异是合理的金刚石形成的下降流和上升流制度分别。与周围地幔相比，具有辉石岩成分的岩石中的宏观金刚石形成可能通过较高的平均氧化态和较低的矿物H2O溶解度在深部地幔中促进，这有助于C-O-H挥发性物质的流动性。具有橄榄岩亲和力的包裹体的明显缺乏可能是由于这种岩性中通常较低的氧逸度（其降低了碳的流动性）以及缺乏合适的氧化剂以允许金刚石从CH 4形成。这种对深部碳循环过程的一瞥意味着地幔中碳含量、氧化还原状态和化学成分的不均匀性可能是强烈耦合的。(C)2016爱思唯尔B. V.保留所有权利。
Most diamonds form in the Earth's lithosphere but a small proportion contain Si-rich majoritic garnet inclusions that indicate formation in the deeper mantle. The compositions of syngenetic garnet inclusions can potential yield information on both the depth and mantle lithology in which the diamonds formed. Pressure dependent changes in garnet compositions have been calibrated using the results of experiments conducted in a multi-anvil apparatus at pressures between 6 and 16 GPa and temperatures of 1000 to 1400 degrees C. Using the results of these experiments a barometer was formulated based on an empirical parameterisation of the two major majoritic substitutions, referred to as majorite (Maj; Al3+ = Mg2+ + Si4+), and Na-majorite (Na-Maj; Mg2+ + Al3+ = Na+ + Si4+). Moreover, previously published experimental garnet compositions from basaltic, kimberlite, komatiite and peridotite bulk compositions were included in the calibration, which consequently covers pressures from 6 to 20 GPa and temperatures from 900 to 2100 degrees C. Experimental pressures are reproduced over these conditions with a standard deviation of 0.86 GPa.The barometer is used to determine equilibration pressures of approximately 500 reported garnet inclusions in diamonds from a range of localities. As the majority of these inclusions are proposed to be syngenetic this allows a detailed picture of diamond formation depths and associated source rocks to be established using inclusion chemistry. Geographic differences in diamond source rocks are mapped within the sub-lithospheric mantle to over 500 km depth. Continuous diamond formation occurs over this depth range within lithologies with eclogitic affinities but also in lithologies that appear transitional between eclogitic and peridotitic bulk compositions, with an affinity to pyroxenites. The geographic differences between eclogitic and pyroxenitic diamond source rocks are rationalised in terms of diamond formation within downwelling and upwelling regimes respectively. Macroscopic diamond formation in rocks with pyroxenite compositions are likely facilitated in the deep mantle by higher average oxidation states and low mineral H2O solubility compared to the surrounding mantle, which aid the mobility of C-O-H volatile species. The apparent lack of inclusions with a peridotite affinity may result from generally low oxygen fugacities in such lithologies, which reduces carbon mobility, and the lack of a suitable oxidising agent to allow diamonds to form from CH4. This glimpse of deep carbon cycle processes implies that heterogeneities in the carbon content, redox state and chemical composition of the mantle may be strongly coupled. (C) 2016 Elsevier B.V. All rights reserved.