On the origin of silicate-bearing diamondites
On the origin of silicate-bearing diamondites
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DOI：
10.1007/s00710-009-0091-0
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发表时间：
2010-05
影响因子：
1.8
通讯作者：
G. Dobosi;G. Kurat
中科院分区：
文献类型：
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作者：
G. Dobosi;G. Kurat
用LA-ICP-MS分析了可能产自非洲南部的37种菱铁矿（“bort”、“polycrystalline diamond aggregates”、“polycrystalline diamond”、“framesite”）中与钻石共生的石榴石和斜辉石的微量元素含量。金刚石-硅酸盐的紧密共生表明两者是在相同的结晶过程中从相同的流体中析出的。本研究区分了5个化学石榴石群：“橄榄岩”(P)、“中间”(I)和3个“榴辉岩”（E1、E2和E3）。石榴石群的球粒陨石标准化微量元素模式与主要元素丰度大致相关。大多数P石榴石呈现复杂的、温和的正弦型reenpattern，具有相对平坦的hreen - mgreen，在Sm-Nd处有一个小驼峰，LREEN减少，且具有较高的Nb、Ta、U和Th含量。E型石榴石的稀土丰度模式不同于P型石榴石，表现为从轻稀土到重稀土的持续增加，而轻稀土和高不相容元素相对于P型石榴石的减少。在所有石榴石组中，E3石榴石的高不相容微量元素和镁含量最低。P石榴石的模型平衡流体表明，含镁碳酸盐流体/熔体结晶，其中非常富含不相容的微量元素，与金伯利岩类似。E1和E2石榴石的假设平衡熔体也是镁质的，相对于典型的金伯利岩或碳酸盐岩熔体，低稀土元素和高不相容元素含量较低。使P石榴石和大部分E石榴石结晶的流体具有相似的数字，表明两者都有橄榄岩来源。Cr和高不相容元素含量的差异可能是流体形成和/或演化的差异，而不是不同烃源岩的结果。cr2o3和mg2与石榴石中高不相容元素丰度的正相关表明，形成菱铁矿的流体主要是流体-岩石分馏过程而非火成岩分馏结晶过程。
Garnets and clinopyroxenes, intergrown with diamonds in 37 diamondites (“bort”, “polycrystalline diamond aggregates”, “polycrystalline diamond”, “framesite”), presumably from southern Africa, were analyzed for trace element contents by LA-ICP-MS. The intimate diamond-silicate intergrowths suggest that both precipitated from the same fluids during the same crystallization events. In this study we distinguish 5 chemical garnet groups: “peridotitic” (P), intermediate (I) and 3 “eclogitic” groups (E1, E2 and E3). Chondrite-normalized trace element patterns for the garnet groups roughly correlate with major element abundances. Most of P garnets show complex, mildly sinusoidal REENpatterns with relatively flat HREEN-MREEN, a small hump at Sm-Nd and depleted LREEN, and have relatively high contents of Nb, Ta, U, and Th. The REENabundance patterns of E garnets differ by showing a continuous increase from LREE to HREE and depletion in LREE and highly incompatible elements relative to the P garnets. Of all garnet groups, E3 garnets are the poorest in highly incompatible trace elements and in Mg. Model equilibrium fluids for P garnets suggest crystallization from magnesian carbonate-bearing fluids/melts, which were very rich in incompatible trace elements — similar to kimberlites. Hypothetical equilibrium melts for E1 and E2 garnets are also magnesian and poorer in LREE and highly incompatible elements relative to typical kimberlitic or carbonatitic melts. Fluids that crystallized the P and most of the E garnets have similarmgnumbers indicating a peridotitic source for both. The differences in Cr and highly incompatible element contents can be the result of differences in fluid formation and/or evolution rather than different source rock. The positive correlation of Cr2O3andmgwith the abundances of highly incompatible elements in garnets indicate fluid-rock fractionation processes rather than igneous fractional crystallization processes being responsible for the evolution of the diamondite-forming fluids.