Water Content and Oxygen Isotopic Composition of Alkali Basalts from the Taihang Mountains, China: Recycled Oceanic Components in the Mantle Source

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Abstract

The genesis of Cenozoic continental basalts in Eastern China is highly debated. Subducted oceanic crust (most probably the Pacific oceanic slab) has been increasingly suggested to be involved in their mantle source. The Taihang Mountains are located at a surface position corresponding to the western edge of a stagnant subducted Pacific slab revealed by geophysical investigations, and thus provide an opportunity to study the spatial extent of the effect of Pacific subduction on the mantle characteristics of Eastern China. In previous studies, alkali basalts from the Taihang Mountains were considered to be the products of interaction between asthenosphere-derived melts and the old, enriched lithospheric mantle; their magnesium isotope signature suggested the contribution of oceanic carbonate. To explore the possible contribution from recycled oceanic crust in the genesis of the Taihang basalts we estimated the water contents of the magmas by a phenocryst-based approach (

JOURNAL/jpetr/04.02/00009961-201504000-00002/math_2MM1/v/2017-10-16T173704Z/r/image-png

) and measured the oxygen isotope compositions of clinopyroxene phenocrysts by secondary ion mass spectrometry for the same sample suite as used in previous studies. The calculated water contents (H2O by weight) of the parental magmas range from 0·20 to 1·07 wt %, and the corresponding H2O/Ce ratios correlate well with (Nb/La)n (where n represents primitive mantle normalization), (Ba/Th)n and εNd. The oxygen isotope ratios (δ18OSMOW) range from 5·8 to 7·4‰, and the average value for each sample ranges from 6·6 to 7·0‰. These new data indicate a large contribution from extremely dehydrated recycled oceanic crust together with entrained sediments. The minor element compositions (Ca, Mn and Ni contents) of olivine phenocrysts in the Taihang basalts suggest a pyroxenite source that was probably formed by the interaction between oceanic crust-derived melt and ambient mantle peridotite.

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