Across-arc variations in Mo isotopes and implications for subducted oceanic crust in the source of back-arc basin volcanic rocks

• Published in: Geology (Boulder) 2021-10, Vol.49 (10), p.1165-1170
• Main Authors: Li Xiaohui, Li Xiaohui, Yan Quanshu, Yan Quanshu, Zeng Zhigang, Zeng Zhigang, Fan Jingjing, Fan Jingjing, Li Sanzhong, Li Sanzhong, Li Jie, Li Jie, Yang Huixin, Yang Huixin, Wang Xiaoyuan, Wang Xiaoyuan
• Format: Article
• Language: English
• Subjects: alkaline earth metals; back-arc basins; Basins; Benioff zone; chemical composition; crust; Dehydration; Eclogite; Geochemistry; Geology; igneous and metamorphic rocks; igneous rocks; Isotope ratios; Isotopes; Lava; Lead; Lithosphere; Magma; Mariana Islands; Mariana Trough; metals; Micronesia; Mo-98/Mo-95; Molybdenum; Nd-144/Nd-143; neodymium; Neodymium isotopes; North Pacific; Northwest Pacific; Oceania; Oceanic crust; Pacific Ocean; Pb-207/Pb-204; Petrogenesis; Petrology; plate tectonics; radioactive isotopes; rare earths; Ratios; Recycling; Rocks; Sr-87/Sr-86; stable isotopes; strontium; Strontium 87; Strontium isotopes; Subduction; Subduction (geology); Subduction zones; Volcanic rocks; West Pacific
• ISSN: 0091-7613; 1943-2682
• DOI: 10.1130/G48754.1

Molybdenum (Mo) isotope ratios provide a potential means of tracing material recycling involved in subduction zone processes. However, the geochemical behavior of Mo in subducted oceanic crust remains enigmatic. We analyzed Mo isotope ratios of arc and back-arc basin lavas from the Mariana subduction zone (western Pacific Ocean), combining newly obtained element and Sr-Nd-Pb-Li isotope data to investigate subduction zone geochemical processes involving Mo. The Mo isotope ratios (δ98/95MoNIST3134; U.S. National Institute of Standards and Technology [NIST] Mo standard) of the volcanic rocks showed clear across-arc variations, decreasing with increasing depth to the Wadati-Benioff zone. The high δ98/95Mo values in the Mariana Islands (-0.18 ppm to +0.38 ppm) correspond to high 87Sr/86Sr, low 143Nd/144Nd, and radiogenic Pb isotope ratios, suggesting that altered upper oceanic crust played an important role in the magma source. The low δ98/95Mo values in the Central Mariana Trough (-0.65 ppm to -0.17 ppm) with mantle-like Sr-Nd-Pb but slightly low δ7Li values provide direct evidence for the contribution of deep recycled oceanic crust to the magma source of the back-arc basin lavas. The isotopically light Mo magmas originated by partial melting of a residual subducted slab (eclogite) after high degrees of dehydration and then penetrated into the back-arc mantle. This interpretation provides a new perspective with which to investigate the deep recycling of subducted oceanic lithosphere and associated magma petrogenesis.