A new model for the evolution of diamond-forming fluids
A new model for the evolution of diamond-forming fluids
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DOI：
10.1016/j.lithos.2009.05.038
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发表时间：
2009-04
期刊：
影响因子：
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通讯作者：
Y. Weiss;R. Kessel;W. Griffin;I. Kiflawi;O. Klein;D. Bell;J. W. Harris;O. Navon
中科院分区：
文献类型：
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作者：
Y. Weiss;R. Kessel;W. Griffin;I. Kiflawi;O. Klein;D. Bell;J. W. Harris;O. Navon
我们报告了产自几内亚坎坎的七颗涂层钻石的主量、微量和挥发性元素成分。四颗钻石用高镁碳酸盐岩高密度流体 (HDF) 捕获微包裹体，三颗钻石将硅质携带到低镁碳酸盐岩 HDF。岩心中的矿物包裹体和氮聚集表明生长温度为 ∼5GPa、1100–1200°C。氮聚集状态的显着差异（三颗钻石的核心、内层和边缘中的 A+B、A 和 A+C 中心）表明在多个事件期间发生生长，且时间间隔很好。所有钻石均表现出放射状化学演化。高镁碳酸盐钻石中的 HDF 向边缘生长出更多的碳酸盐，硅质钻石则生长出更多的硅质，分区钻石在涂层的不同部分显示出对比的趋势。 HDF 高度富集 Ba、Th、U 和轻稀土元素； K、Rb 和 Cs 的富集度稍低。归一化的 Sr、Ti 和 Y 浓度相对于中重 REE 较低。低镁碳酸岩到硅质组中的微量元素随着 HDF 硅质特征的增加而系统地变化。高镁 HDF 通常偏离此类阵列。提出了一种新的金刚石形成流体演化模型，表明高镁碳酸盐岩HDF是通过含盐HDF与橄榄岩相互作用形成的，而低镁硅质到碳酸盐岩HDF是富钾含水流体渗透到榴辉岩中的结果。 HDF 高钾含量的另一个可能来源是岩石矿物组合中的含钾相。当将具有陡峭 REE 模式的 HDF 引入 LREE 贫化方辉橄榄岩中时，可能会形成单晶金刚石石榴石包裹体中常见的正弦 REE 模式。除非石榴石和钻石是在两个单独的事件中形成的，否则这种相似性表明单晶钻石是从类似于 HDF 的流体中生长的。
We report major, trace and volatile element compositions for seven coated diamonds from Kankan, Guinea. Four diamonds trapped microinclusions with high-Mg carbonatitic high-density fluids (HDFs) and three carry silicic to low-Mg carbonatitic HDFs. Mineral inclusions and nitrogen aggregation in the cores indicate growth at ∼5GPa, 1100–1200°C. Significant differences in the nitrogen aggregation state (A+B, A and A+C centres in cores, inner coats and rims of three diamonds) indicate growth during multiple events, well separated in time. All diamonds show radial chemical evolution. The HDFs in high-Mg carbonatitic diamonds grow more carbonatitic towards the rim, the silicic one grows more silicic, and the zoned diamonds show contrasting trends in the different parts of the coats. The HDFs are highly enriched in Ba, Th, U and the light REE; enrichment in K, Rb and Cs is somewhat lower. Normalized Sr, Ti and Y concentrations are low relative to the middle and heavy REE. Trace elements in the low-Mg carbonatitic to silicic suite vary systematically with increasing silicic character of the HDFs. The high-Mg HDFs commonly deviate from such arrays. A new model is proposed for the evolution of diamond-forming fluids, suggesting that high-Mg carbonatitic HDFs are formed through interaction of saline HDF with peridotitic rock, while low-Mg silicic to carbonatitic HDFs are the result of penetration of K-rich hydrous fluid into eclogitic rock. Another possible source for the high potassium content of the HDFs is K-bearing phases in the mineral assemblage of the rock. Sinusoidal REE patterns that are common in garnet inclusions in monocrystalline diamonds may form when HDFs with steep REE patterns are introduced into a LREE-depleted harzburgite. Unless garnet and diamond were formed in two separate events, this similarity suggests growth of monocrystalline diamonds from fluids similar to the HDFs.