Nephelinites in eastern China originating from the mantle transition zone
| Autor: | Xiao-Jun Wang, Lie-Wen Xie, Gang Zeng, Li-Hui Chen, Jian-Qiang Liu, Albrecht W. Hofmann, Xun Yu |
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| Rok vydání: | 2021 |
| Předmět: |
Basalt
Peridotite geography geography.geographical_feature_category 010504 meteorology & atmospheric sciences Partial melting Geochemistry Geology engineering.material 010502 geochemistry & geophysics 01 natural sciences Silicate Volcanic rock chemistry.chemical_compound chemistry Geochemistry and Petrology Transition zone engineering Phlogopite Amphibole 0105 earth and related environmental sciences |
| Zdroj: | Chemical Geology. 576:120276 |
| ISSN: | 0009-2541 |
| DOI: | 10.1016/j.chemgeo.2021.120276 |
| Popis: | Nephelinites, a highly alkaline volcanic rock type, have widely been thought to be formed in a shallow lithosphere previously impregnated with phlogopite and/or amphibole. Here we argue that nephelinites start melting at much greater depths than previously thought. Taking four Cenozoic nephelinitic volcanoes in eastern China as specific examples, we show that nephelinites have exceptionally low K/U and Ba/Th ratios, similar to most nephelinites worldwide, but their Nd-Sr-Hf isotopic compositions are inconsistent with a local lithospheric origin. They also have low, marine‑carbonate-like δ26Mg. We explain these features by partial melting of recycled, carbonate-bearing sediments in the mantle transition zone (MTZ; 410–660 km), where the high-pressure mineral liebermannite (KAlSi3O8, previously known as K-hollandite) is stable as a residual phase, preferentially retaining K, Ba, Rb and Pb relative to Th, U and light REE, thus releasing liquids with the characteristic trace-element patterns. Group-I kimberlites and oceanic HIMU basalts globally share the same peculiar geochemical anomalies characterizing the nephelinites, suggesting they may all have their roots in the MTZ. The nephelinites show correlations for Mg-Sr isotopes, 143Nd/144Nd-MgO, and δ26Mg-Ba/Th ratios, suggesting binary mixing between melts. We therefore invoke a two-stage process to generate these nephelinites. In the first stage, an initial carbonated silicate melt is formed by melting of carbonate-bearing sediments in the MTZ. Subsequently, during the ascent, the initial melt reacts with the surrounding peridotite, thereby generating a reacted melt, which ultimately mixes with the initial, unreacted carbonated silicate melt. Accordingly, Cenozoic nephelinites in eastern China can be regarded as evolved melts from the MTZ. |
| Databáze: | OpenAIRE |
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