Reduced sediment melting at 7.5-12 GPa: phase relations, geochemical signals and diamond nucleation
Reduced sediment melting at 7.5-12 GPa: phase relations, geochemical signals and diamond nucleation
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DOI：
10.1007/s00410-015-1166-z
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发表时间：
2015-08-01
影响因子：
3.5
通讯作者：
Woodland, A. B.
中科院分区：
文献类型：
--
作者：
Brey, G. P.;Girnis, A. V.;Woodland, A. B.
在7.5-12 GPa和800-1600 ℃的多砧装置中，实验研究了石墨或金刚石存在下碳酸盐沉积物的熔化。由氧化物、碳酸盐、氢氧化物和石墨制备类似于Plank和Langmuir的GLOSS（Chem Geol 145：325-394，1998）的两种起始材料。一种混合物（钠光泽）是相同的主要元素组成的GLOSS，和其他的是穷人在钠和丰富的K（K光泽）。两种起始混合物均含有类似于6重量%的CO2和7重量%的H2O，并以类似于100 ppm的水平掺杂有多种微量元素，包括REE、LILE和HFSE。近固相线矿物组合包含二氧化硅多晶型物（柯石英或stishovite），石榴石，蓝晶石，斜辉石，碳酸盐（文石和菱镁矿-菱铁矿固溶体），锆石，金红石，bearthite和含水相（多硅白云母和硬硅钙石在10 GPa）。含水相在接近900摄氏度时消失，碳酸盐在1000-1100摄氏度时仍然存在。在温度>1200摄氏度时，矿物组合由柯石英或斯石英、蓝晶石和石榴石组成。单斜辉石的稳定性强烈地依赖于起始混合物中的Na含量;在12 GPa下，它在高达1600 ℃的Na光泽组合物中保持不变，但在高于1200 ℃的K光泽实验中没有观察到。熔体或流体的组成随着温度的升高从富含水碳酸盐逐渐变化（1）。文石和铁镁碳酸盐的稀土元素分配系数相差很大（DMst-Sd/L近似为0.01，D-Arg/L近似为1）。Nb、Ta、Zr和Hf在这两种碳酸盐中是强不相容的。轻稀土的配分系数很高（> 10），重稀土的配分系数降低到接近1。所有HFSE在Bearthite中都是强不相容的。相比之下，Ta，Nb，Zr和Hf在ZrSiO 4和TiO 2相中是中等至强相容的。根据得到的分配系数，计算了深俯冲带沉积物熔融产生的移动的相的组成。该相强烈富集不相容元素，并显示出明显的负Ta-Nb异常，但没有Zr-Hf异常。虽然所有的实验都是在金刚石稳定性领域进行的，但在低温实验中只观察到石墨。在1200-1300 ℃以上的温度下观察到金刚石自发成核和石墨向金刚石的完全转变。我们推测，所观察到的石墨-金刚石转变的特征是由亚稳石墨溶解动力学和金刚石成核之间的关系在含水硅碳酸盐熔体中，是过饱和的C。
Melting of carbonated sediment in the presence of graphite or diamond was experimentally investigated at 7.5-12 GPa and 800-1600 degrees C in a multianvil apparatus. Two starting materials similar to GLOSS of Plank and Langmuir (Chem Geol 145: 325-394, 1998) were prepared from oxides, carbonates, hydroxides and graphite. One mixture (Na-gloss) was identical in major element composition to GLOSS, and the other was poorer in Na and richer in K (K-gloss). Both starting mixtures contained similar to 6 wt% CO2 and 7 wt% H2O and were doped at a similar to 100 ppm level with a number of trace elements, including REE, LILE and HFSE. The near-solidus mineral assemblage contained a silica polymorph (coesite or stishovite), garnet, kyanite, clino-pyroxene, carbonates (aragonite and magnesite-siderite solid solution), zircon, rutile, bearthite and hydrous phases (phengite and lawsonite at 10 GPa). Hydrous phases disappear at similar to 900 degrees C, and carbonates persist up to 1000-1100 degrees C. At temperatures >1200 degrees C, the mineral assemblage consists of coesite or stishovite, kyanite and garnet. Clinopyroxene stability depends strongly on the Na content in the starting mixture; it remains in the Na-gloss composition up to 1600 degrees C at 12 GPa, but was not observed in K-gloss experiments above 1200 degrees C. The composition of melt or fluid changes gradually with increasing temperature from hydrous carbonate-rich ( 1). Aragonite and Fe-Mg carbonate have very different REE partition coefficients (DMst-Sd/L similar to 0.01 and D-Arg/L similar to 1). Nb, Ta, Zr and Hf are strongly incompatible in both carbonates. The bearthite/melt partition coefficients are very high for LREE (> 10) and decrease to similar to 1 for HREE. All HFSE are strongly incompatible in bearthite. In contrast, Ta, Nb, Zr and Hf are moderately to strongly compatible in ZrSiO4 and TiO2 phases. Based on the obtained partition coefficients, the composition of a mobile phase derived by sediment melting in deep subduction zones was calculated. This phase is strongly enriched in incompatible elements and displays a pronounced negative Ta-Nb anomaly but no Zr-Hf anomaly. Although all experiments were conducted in the diamond stability field, only graphite was observed in low-temperature experiments. Spontaneous diamond nucleation and the complete transformation of graphite to diamond were observed at temperatures above 1200-1300 degrees C. We speculate that the observed character of graphite-diamond transformation is controlled by relationships between the kinetics of metastable graphite dissolution and diamond nucleation in a hydrous silicocarbonate melt that is oversaturated in C.