The Formation of K-Cymrite in Subduction Zones and Its Potential for Transport of Potassium, Water, and Nitrogen into the Mantle

IF 0.8 4区 地球科学 Q4 GEOCHEMISTRY & GEOPHYSICS Geochemistry International Pub Date : 2025-01-27 DOI:10.1134/S0016702924700745
A. G. Sokol, A. V. Korsakov, A. N. Kruk
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Abstract

The conditions of the formation of K-cymrite in volatile-rich pelite and partially devolatilized mica quartz–muscovite–chlorite schist were experimentally investigated at pressures of 5.5, 6.3, and 7.8 GPa and temperatures ranging from 900 to 1090°C corresponding to hot subduction geotherm. Experimental samples at these P–T conditions formed assemblage of solid phases (Grt + Coe + Phe + Cpx + Ky, with accessory Po + Ru + Zrn ± Mnz) and water-enriched supercritical fluid–melt. Analysis of the obtained data indicates that the stability of phengite and its potential replacement by K-cymrite depends on the P–T conditions and the amount of volatiles in the metasediment. In samples of volatile-rich pelite and mica schist at 5.5 GPa and 900°C, as well as at 6.3 GPa and 1000°C, phengite remains stable in equilibrium with 3–13 wt % of the fluid–melt. With increasing pressure up to 7.8 GPa and temperature up to 1090°C, the fraction of supercritical fluid–melt in pelite reaches 20 wt %, while phengite disappears. Only 5 wt % supercritical fluid–melt are formed in the schist at 7.8 GPa and 1070°C, while most part of phengite is preserved. For the first time, phase assemblage with phengite and K-cymrite (±kokchetavite) was obtained in the pelite and schist samples at 7.8 GPa and 1070°C. The assemblage was identified using Raman mapping. At stepwise devolatilization (with removal of fluid–melt portion forming in equilibrium with volatile-bearing minerals that are stable at P–T conditions of experiments), phengite has been preserved up to 7.8 GPa and 1090°C, but K-cymrite is not formed in the absence of fluid–melt. It was concluded that the most effective transport of volatiles (first of all, water) in the metasediment to depths over 240 km may occur during its partial and early (before the formation of supercritical fluid–melt) devolatilization. In this case, almost all phengite may reach depths of 240 km during metasediment subduction and then transform into water-bearing K-cymrite, or, in the presence of nitrogen in the metasediment, into nitrogen-bearing K-cymrite, thus facilitating the further transport of LILE (large-ion lithophile elements), water, and nitrogen. However, the formation of a significant portion of supercritical fluid–melt leads to the complete dissolution of phengite with increasing P–T conditions, making further transport of LILE, water, and nitrogen impossible. During deep multi-stage devolatilization, phengite remains stable up to depths of 240 km; however, during further subduction, it likely transforms into an anhydrous K-hollandite (KAlSi3O8).

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俯冲带k -硅长石的形成及其向地幔输送钾、水和氮的可能性
在热俯冲温度900 ~ 1090℃、压力5.5、6.3和7.8 GPa条件下,研究了富挥发性泥岩和部分脱挥发云母石英-白云母-绿泥石片岩中k -硅长石的形成条件。实验样品在这些P-T条件下形成固相组合(Grt + Coe + Phe + Cpx + Ky,外加Po + Ru + Zrn±Mnz)和富水超临界流体熔体。对所得数据的分析表明,辉云母的稳定性及其被k -硅长石取代的可能性取决于P-T条件和沉积物中挥发物的数量。在5.5 GPa和900°C以及6.3 GPa和1000°C时,富挥发性的云母片岩和云母片岩样品中,辉云母在3 - 13wt %的流体熔体中保持稳定的平衡。当压力升高到7.8 GPa,温度升高到1090℃时,泥铁矿中超临界流体熔体的比例达到20%,而云母矿消失。在7.8 GPa和1070℃条件下,片岩中仅形成5 wt %的超临界流体熔体,而大部分辉长石被保留。在7.8 GPa和1070℃条件下,首次在泥云母和片岩样品中获得了白云石和k -硅长石(±kokchetavite)的相组合。利用拉曼映射对该组合进行了识别。在逐步脱挥发过程中(去除与含挥发物平衡形成的流熔部分,这些含挥发物在实验的P-T条件下是稳定的),在7.8 GPa和1090°C的温度下,长云石被保存下来,但在没有流熔的情况下,k -长云石不会形成。结果表明,沉积中挥发性物质(首先是水)向240 km以上深度的最有效运输可能发生在其部分和早期(超临界流体熔体形成之前)脱挥发过程中。在这种情况下,几乎所有的云母岩都可能在沉积俯冲过程中到达240 km深度,然后转化为含水的k -硅灰岩,或者在沉积中存在氮的情况下转化为含氮的k -硅灰岩,从而促进LILE(大离子亲石元素)、水和氮的进一步输送。然而,随着P-T条件的增加,相当一部分超临界流体熔体的形成导致了辉长石的完全溶解,使得LILE、水和氮的进一步输送成为不可能。在深层多段脱挥发过程中,直达240 km深度的云母仍保持稳定;然而,在进一步的俯冲过程中,它可能转变为无水K-hollandite (KAlSi3O8)。
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来源期刊
Geochemistry International
Geochemistry International 地学-地球化学与地球物理
CiteScore
1.60
自引率
12.50%
发文量
89
审稿时长
1 months
期刊介绍: Geochemistry International is a peer reviewed journal that publishes articles on cosmochemistry; geochemistry of magmatic, metamorphic, hydrothermal, and sedimentary processes; isotope geochemistry; organic geochemistry; applied geochemistry; and chemistry of the environment. Geochemistry International provides readers with a unique opportunity to refine their understanding of the geology of the vast territory of the Eurasian continent. The journal welcomes manuscripts from all countries in the English or Russian language.
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