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首页> 外文期刊>Contributions to Mineralogy and Petrology >Reduced sediment melting at 7.5-12 GPa: phase relations, geochemical signals and diamond nucleation
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Reduced sediment melting at 7.5-12 GPa: phase relations, geochemical signals and diamond nucleation

机译：在7.5-12 GPa时降低沉积物融化：相关系，地球化学信号和金刚石成核

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摘要

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 <9 GPa and the hydrous aluminosilicates topaz-OH and phase egg 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 (<10 wt% SiO2) at 800-1000 degrees C to volatile-rich silicate liquids (up to 40 wt% SiO2) at high temperatures. Trace elements were analyzed in melts and crystalline phases by LA ICP MS. The garnet-melt and clinopyroxene-melt partition coefficients are in general consistent with results from the literature for volatile-free systems and silicocarbonate melts derived by melting carbonated peridotites. Most trace elements are strongly incompatible in kyanite and silica polymorphs (D < 0.01), except for V, Cr and Ni, which are slightly compatible in kyanite (D > 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.
机译：在多砧装置中，在7.5-12 GPa和800-1600摄氏度的温度下，对石墨或金刚石存在下的碳酸盐沉积物的熔融进行了实验研究。由氧化物，碳酸盐，氢氧化物和石墨制备了两种类似于Plank和Langmuir的GLOSS的起始原料（Chem Geol 145：325-394，1998）。一种混合物（Na光泽）的主要元素组成与GLOSS相同，另一种混合物的Na含量较差，K含量较高（K光泽）。两种起始混合物均包含相似于6 wt％的CO2和7 wt％的H2O，并以类似于100 ppm的水平掺杂了多种痕量元素，包括REE，LILE和HFSE。接近固相线的矿物组合包含二氧化硅多晶型物（堇青石或辉石），石榴石，蓝晶石，斜辉石-辉石，碳酸盐（文石和菱镁矿-菱铁矿固溶体），锆石，金红石，钙钛矿和水合相（小于9的辉石和钙钠石） GPa和含水铝硅酸盐黄玉-OH和相鸡蛋（> 10 GPa）。含水相在接近900摄氏度时消失，而碳酸盐则持续到1000-1100摄氏度。在温度大于1200摄氏度时，矿物组合由堇青石或辉石，蓝晶石和石榴石组成。 Clinopyroxene的稳定性很大程度上取决于起始混合物中的Na含量。它在12 GPa下高达1600摄氏度时仍保留在Na光泽成分中，但在1200摄氏度以上的K光泽实验中未观察到。熔体或流体的成分会随着温度的升高而逐渐变化（富含水的碳酸盐（<10）在高温下，将其在800-1000摄氏度下转化为富含挥发性的硅酸盐液体（最高可达40 wt％的SiO2）。通过LA ICP MS分析了熔体和结晶相中的痕量元素。石榴石熔体和斜茂铁熔体的分配系数通常与无挥发体系和熔融碳酸盐橄榄岩得到的硅碳酸盐熔体的文献结果一致。除V，Cr和Ni在蓝晶石中微溶（D> 1）外，大多数痕量元素在蓝晶石和二氧化硅多晶型物中均极不相容（D <0.01）。文石和Fe-Mg碳酸盐具有非常不同的REE分配系数（DMst-Sd / L与0.01相似，D-Arg / L与1相似）。 Nb，Ta，Zr和Hf在两种碳酸盐中都强烈不相容。对于LREE（> 10），其白云石/熔体分配系数非常高，对于HREE，其降到接近于1。所有HFSE在黑云母中都强烈不相容。相反，Ta，Nb，Zr和Hf在ZrSiO4和TiO2相中适度到强烈相容。根据获得的分配系数，计算出深俯冲带中沉积物融化所产生的流动相的组成。此阶段大量富含不相容元素，并显示出明显的负Ta-Nb异常，但没有Zr-Hf异常。尽管所有实验均在金刚石稳定性领域中进行，但在低温实验中仅观察到石墨。在高于1200-1300摄氏度的温度下观察到自发的金刚石成核和石墨向金刚石的完全转变。我们推测，观察到的石墨-金刚石转变特征是由亚稳态石墨溶解动力学和含水的金刚石成核动力学之间的关系控制的。在C中过饱和的硅碳酸盐熔体

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