Lee Hyunwoo, Fischer Tobias P, de Moor J Maarten, Sharp Zachary D, Takahata Naoto, Sano Yuji
Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM, 87131, USA.
Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba, 277-8564, Japan.
Sci Rep. 2017 Oct 24;7(1):13933. doi: 10.1038/s41598-017-14287-y.
Efficient recycling of subducted sedimentary nitrogen (N) back to the atmosphere through arc volcanism has been advocated for the Central America margin while at other locations mass balance considerations and N contents of high pressure metamorphic rocks imply massive addition of subducted N to the mantle and past the zones of arc magma generation. Here, we report new results of N isotope compositions with gas chemistry and noble gas compositions of forearc and arc front springs in Costa Rica to show that the structure of the incoming plate has a profound effect on the extent of N subduction into the mantle. N isotope compositions of emitted arc gases (9-11 N°) imply less subducted pelagic sediment contribution compared to farther north. The N isotope compositions (δN = -4.4 to 1.6‰) of forearc springs at 9-11 N° are consistent with previously reported values in volcanic centers (δN = -3.0 to 1.9‰). We advocate that subduction erosion enhanced by abundant seamount subduction at 9-11 N° introduces overlying forearc crustal materials into the Costa Rican subduction zone, releasing fluids with lighter N isotope signatures. This process supports the recycling of heavier N into the deep mantle in this section of the Central America margin.
对于中美洲边缘地区，有人主张通过弧火山作用将俯冲的沉积氮（N）有效地循环回大气中；而在其他地区，质量平衡的考虑以及高压变质岩中的氮含量表明，大量俯冲的氮被添加到地幔中，并越过了弧岩浆生成带。在此，我们报告了哥斯达黎加弧前和弧前泉的氮同位素组成、气体化学和稀有气体组成的新结果，以表明进入板块的结构对氮俯冲到地幔的程度有深远影响。与更靠北的地区相比，喷出的弧气体的氮同位素组成（9 - 11 N°）意味着俯冲的远洋沉积物贡献较少。9 - 11 N°处弧前泉的氮同位素组成（δN = - 4.4至1.6‰）与火山中心先前报道的值（δN = - 3.0至1.9‰）一致。我们主张，9 - 11 N°处大量海山俯冲增强的俯冲侵蚀作用将上覆的弧前地壳物质引入哥斯达黎加俯冲带，释放出氮同位素特征较轻的流体。这一过程支持了中美洲边缘这一区域较重的氮向深部地幔的循环。
