Decoupling of Hf‐Nd Isotopes in Challenger Deep Sediments, Mariana Trench: Implications for Sedimentary Hf and Nd Recycling in Subduction Zones

• Published in: Journal of geophysical research. Solid earth 2021-06, Vol.126 (6), p.n/a
• Main Authors: Pang, Chong‐Jin, Tian, Liyan, Wang, Xuan‐Ce, Zhang, Hanyu, Wen, Shu‐Nv, Krapež, Bryan, Liu, Wen‐Jing, Liu, Zhi‐Xing, Wu, Tao
• Format: Article
• Language: English
• Subjects: Challenger Deep; Chemical analysis; Clay; Components; Composition; Decoupling; Density; Density currents; density‐current deposits; Detritus; Diatoms; Fluid dynamics; Fluid flow; Grain size; Hafnium; Heterogeneity; Hf‐Nd isotopic decoupling; Hydrodynamics; Intervals; Isotope composition; Isotopes; Laminates; Lamination; Manganese; oceanic hydrodynamic mineral sorting; Oceanic trenches; Particle size; Recycling; Seawater; Sediment; Sediment samplers; Sediment samples; Sediment transport; sedimentary Hf‐Nd recycling; Sediments; Slopes; Stratigraphy; Subduction; Subduction (geology); subduction factory; Subduction zones; Variation; Water analysis; Water depth; Weathering
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2021JB021641

Decoupling between Hf and Nd isotopes in trench sediments is an important way to produce, via subduction‐related recycling, the mantle chemical heterogeneity that is evident in many arc‐related magmas. Sediments collected from >7,000 m water depths on the southern and northern slopes of the Challenger Deep were analyzed to understand the factors controlling the variation in Hf and Nd isotopic compositions. These sediments were transported to both flanks of the Challenger Deep by synchronous density currents. All sediment samples show positive ɛHf values and negative ɛNd values, mostly plotting above the seawater and terrestrial arrays in Hf‐Nd isotope space. Hafnium isotopic compositions in each depositional unit vary (high up to 9.2 ɛHf units) from lower, coarse‐grained laminae to upper, fine‐grained graded intervals, showing a good correspondence between ɛHf values and lamination within depositional units. Such variations are explained by the concentration of Fe‐Mn oxyhydroxide‐bearing clays in the upper graded intervals, whereas diatom fragments and arc‐volcanic detritus are preferentially sorted into lower laminae. These findings demonstrate that the decoupling of Hf‐Nd isotopes in trench sediments is not only linked to the nature of provenance but also to hydrodynamic sorting processes during sediment transport. Our results suggest that geographical and stratigraphic variations in sedimentary components (resedimented density‐current deposits vs. oceanic pelagites), as well as overall grain size, of the subduction wedge can complicate Hf‐Nd isotopic decoupling in resulting arc magmas. Plain Language Summary Sedimentary processes (e.g., mineralogical and grain size sorting) substantially control the decoupling of Hf‐Nd isotopes in sediments during continental weathering and delivery to the ocean. However, what is less well known is whether marine processes and hydrodynamics further fractionate Hf‐Nd isotopes in sediments transported to trenches, where sediments are recycled into the mantle and contribute to the mantle heterogeneity that is evident in many arc‐related magmas. This information is crucial for understanding sedimentary Hf‐Nd isotope recycling in the subduction factory. Our paper presents sedimentological features, and geochemical and Hf‐Nd isotopic compositions of sediments from the slopes of the Challenger Deep in the southern Mariana Trench, aiming to evaluate the role of provenance and hydrodynamic sorting in determining Hf‐Nd isotopic v