Recycled oceanic crust and marine sediment in the source of alkali basalts in Shandong, eastern China: Evidence from magma water content and oxygen isotopes

Abstract

The magma water contents and cpx δ18O 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 ivAlcpx‐dependent water partitioning coefficient Dwaternormalcnormalpnormalxtrue/melt, and secondary ion mass spectrometer, respectively. The calculated water content (H2O wt.) of magma ranges from 0.58% to 3.89%. It positively correlates with heavy rare earth element concentrations and bulk rock 87Sr/86Sr 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 δ18O 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 H2O/Ce ratios of the calculated melts range from 113 to 696 and form a positive trend with bulk rock 87Sr/86Sr, 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.

Boron isotopes reveal multiple metasomatic events in the mantle beneath the eastern North China Craton

“…This signature cannot be derived from recently subducted Pacific slab (Kuritani et al, 2011). Hf-Nd isotopic modeling of the origins for this FME rich mantle source leads to two possibilities: that this mantle source includes contributions from ancient marine sediments and oceanic crust (Supplemental Figures;Li et al, 2014;Liu et al, 2015), or that delaminated and evolved lower continental crust is responsible for its signature (e.g., Chen et al, 2009;Zeng et al, 2010). By contrast, the FME-depleted mantle source has HIMU-like trace element signatures but MORB-like Sr-Pb isotopic systematics (Fig.…”

Light Mg Isotopic Composition in the Mantle Beyond the Big Mantle Wedge Beneath eastern Asia

“…Low Ca and high Ni contents of their olivine phenocrysts (e.g., Hong et al, ; Li, Xu, et al, ; Liu, Ren, et al, ; Qian et al, ; Wang et al, ; Xu et al, ), high whole‐rock FC3MS values (here FC3MS = FeO/CaO – 3 * MgO/SiO 2 ; >0.65; Yang & Zhou, ; Yang et al, ), and high Fe/Mn (>60) and high 10,000 * Zn/Fe (>14) ratios (Liu et al, ; Xu et al, ; Zhang et al, ) suggest that these basalts (sensu lato) have pyroxenitic/eclogitic mantle sources. The distinct δ 18 O values of phenocrysts relative to typical mantle indicate that such pyroxenitic/eclogitic components may derive from subducted oceanic crusts and sediments (e.g., Liu, Xia, Deloule, et al, , Liu, Xia, Ingrin, et al, ; Liu, Wang, et al, ; Xu et al, ). Heavy δ 66 Zn signatures (δ 66 Zn = 0.30‰ to 0.63‰; Liu et al, ) of the basalts, compared to peridotite mantle values (δ 66 Zn = 0.16 ± 0.06‰; Sossi et al, ), reflect contributions from isotopically heavy subducted carbonated material (average δ 66 Zn = 0.91 ± 0.47‰; Pichat et al, ).…”

Evolution of the mantle beneath the eastern North China Craton during the Cenozoic: Linking geochemical and geophysical observations

“…The postrifting basalts in the NCC have been the subject of several recent studies [e.g., Zeng et al ., , ; Xu et al ., ; Sakuyama et al ., ; Liu et al ., ; Li et al ., , ]. The bulk chemical compositions of these basalts vary systematically with their SiO 2 concentrations.…”

“…(3) The rifting‐stage basalts in the BBB also preserve an EMI signature (Figure ), and this signature shows mixing relationships with an isotopically depleted signature from shallower, fertile peridotite mantle sources, similar to that of MORB but clearly distinct from that of the Dashan nephelinite (Figures , , and ). (4) Recent work on the water contents of Shandong basalts reveal that the shallower EMI mantle‐derived basalts have higher H 2 O/Ce than the deeper‐derived basalts [ Liu et al ., ]. As such, the EMI signature evident in NCC basalts is probably not related to the wet upwelling of the MTZ.…”

“…Numerous studies have been conducted exploring the possible links between geophysical and geochemical observations in eastern Eurasia, with some advocating that the low P wave velocity of shallower upper mantle is resulted from wet upwelling from the MTZ, triggered by dehydration of the stagnant Pacific slab. They also propose the EMI (isotopically enriched component) signatures in basaltic lavas erupted in northeast China are derived from the MTZ through the advocated wet upwelling [e.g., Zhao et al ., ; Kuritani et al ., ; Liu et al ., ]. Major arguments in this regard include (1) basalts of the central Changbaishan region that are more enriched in fluid mobile elements, e.g., Ba, Sr, and Pb, than basalts from other nearby regions [ Kuritani et al ., ] and (2) the presence of a clear P wave low‐velocity anomaly beneath the active Changbaishan volcano in northeast China (see location in Figure a), which is evident in the upper mantle down to about 410 km [ Zhao et al ., ; Huang and Zhao , ].…”