Water content of the Xiaogulihe ultrapotassic volcanic rocks, NE China: implications for the source of the potassium-rich component
2015
Chen, Huan | Xia, Qun-ke | Ingrin, Jannick
Water has become an effective means to trace the mantle source of basaltic magmas recently. To investigate the source of the potassium-rich component in the Xiaogulihe ultrapotassic volcanic rocks of NE China, we measured the water content of clinopyroxene (cpx) phenocrysts by Fourier transform infrared spectrometry and calculated the H2O content of the equilibrated melts according to the partition coefficients of H2O between cpx phenocrysts and basaltic melts. The estimated H2O content of the “primary” magmas is low (0.36wt%–0.50wt%), within the range of mid-ocean ridge basalts and ocean island basalts, while it is obviously lower than that of back-arc basin basalts and island arc basalts. The calculated H2O/Ce ratio of the “primary” magmas is about 15, which might be similar to that of the dehydrated sediments (<100), but observably lower than that of the normal depleted mantle (DMM, H2O/Ce = 150–210). The low water content and especially low H2O/Ce ratio of the “primary” magmas demonstrate that the K-rich component of these ultrapotassic volcanic rocks seems unlikely to originate from K-bearing hydrous minerals (such as phlogopite) in metasomatic subcontinental lithospheric mantle. Combined with the low 206Pb/204Pb ratios, moderately high 87Sr/86Sr ratios of the bulk rocks and the high δ18 O values of olivine phenocrysts, we suggest that the K-rich component in the mantle source of the Xiaogulihe ultrapotassic volcanic rocks may come from ancient continental-derived sediments which dehydrated significantly during subduction (e.g., dry K-hollandite).
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