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Recycled oceanic crust and marine sediment in the source of alkali basalts in Shandong, eastern China: Evidence from magma water content and oxygen isotopes
Author(s) -
Liu Jia,
Xia QunKe,
Deloule Etienne,
Chen Huan,
Feng Min
Publication year - 2015
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1002/2015jb012476
Subject(s) - phenocryst , geology , geochemistry , basalt , mantle (geology) , olivine , lithosphere , crust , incompatible element , mineralogy , partial melting , volcanic rock , volcano , tectonics , paleontology
The magma water contents and cpx δ 18 O values in alkali basalts from the Fuyanyshan (FYS) volcano in Shandong, eastern China, were investigated by an inverse calculation based on the water content of clinopyroxene (cpx) phenocrysts, the iv Al cpx ‐dependent water partitioning coefficient D water c p x / melt , and secondary ion mass spectrometer, respectively. The calculated water content (H 2 O wt.) of magma ranges from 0.58% to 3.89%. It positively correlates with heavy rare earth element concentrations and bulk rock 87 Sr/ 86 Sr ratios, and it negatively correlates with Nb/U ratios. However, it is not correlated with bulk Mg# (Mg# = 100 × Mg / (Mg + Fe)) and (La/Yb)n (n represents primitive mantle normalization). Combined with the rather homogenous distribution of water content within cpx grains, these correlations indicate that the water variations among different samples represent the original magma signature, rather than results of a shallow process, such as degassing and diffusion. The δ 18 O of cpx phenocrysts varies from 3.6‰ to 6.3‰ (±0.5‰, 2SD), which may be best explained by the involvement of components from the lower and upper oceanic crust with marine sediments within the mantle source. The H 2 O/Ce ratios of the calculated melts range from 113 to 696 and form a positive trend with bulk rock 87 Sr/ 86 Sr, which cannot be explained by the recycled Sulu eclogite or by the metasomatized lithospheric mantle. Our modeling calculation shows that the decoupling of εHf and εNd could be caused by the involvement of marine sediments. Combing the high Ba/Th ratios, positive Sr spikes, and low Ce/Pb ratios for the Fuyanshan basalts, we suggest that the hydrous nature of the FYS basalts was derived from the hydrous mantle transition zone with ancient sediments.