کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
6429086 | 1634753 | 2014 | 12 صفحه PDF | دانلود رایگان |
- Nitrogen isotope compositions of metamorphic rocks subducted to >120-200 km.
- Anomalously low N isotope composition as low as â15.8â°.
- A possible record of abiotic N reduction process in fossil continental hydrothermal system.
Modern nitrogen (N) fixation is primarily mediated by biological processes. However, in the early Earth where biological activity was absent or limited, abiotic N reduction in hydrothermal systems is thought to be a key process to transform atmospheric N2 and NOx to ammonium, an essential nutrient to support the emergence of life and also an N form that can be incorporated into rocks. Surprisingly, evidence for abiotic N reduction in the rock record has not been clearly identified. In this study, we reported anomalously low N isotope compositions (δN15 values as low as â15.8â°) of mica samples in ultrahigh-pressure metamorphic rocks from the Donghai area in the Sulu orogenic belt, eastern China. Compared with mica samples with typical crustal δN15 values (3-9â°) in similar metamorphic rocks from the western Dabie orogen, the 15N-depleted mica samples from the Sulu orogen are characterized by significant N enrichment (10 times higher) and extreme 18O depletion (δO18 values as low as â9â°). These features can be best explained by assimilation of N from a source characterized by extremely low δN15 values (less than â¼â16â°). The extremely low δN15 value would be produced by abiotic N reduction during reaction of a meteoric-hydrothermal fluid with crustal rocks before subduction. This observation provides a clue to the occurrence of abiotic N reduction in continental supracrustal rocks and infer that abiotic N reduction process could be a fundamental process driving the geological N cycling in early Earth.
Journal: Earth and Planetary Science Letters - Volume 403, 1 October 2014, Pages 67-78