| Electron, laser-ablation ICP-MS, Fourier transform infrared spectroscopy and Raman spectroscopy were used to measure the major and trace element concentrations and H2O contents of the minerals in the peridotite xenoliths hosted by the Tianchang Cenozoic basalts from eastern China. The H2O contents (weight in ppm) of the clinopyroxenes, orthopyroxenes and olivines are 70-280 ppm,35-140 ppm and ?0 ppm, respectively. Although hydrogen diffusion can be obviously found in olivine and orthopyroxenes, all the orthopyroxene grains showed a water diffusion platform in the core, where the water distribution were homogeneous and similar to others in the same sample. The H2O content and major element content of pyroxene grains also have a good relationship and the equilibrium H2O partitioning between clinopyroxene and orthopyroxene were 2, which indicate that the pyroxenes largely preserve the initial H2O content in the mantle source. Based on the mineral composition, with that the partition coefficient of H2O between clinopyroxene and olivine is 0.1, the calculated whole-rock H2O contents range from 14 to 93 ppm (average 52±25 ppm). These values fall in the range of the MORB source. The correlations among the major elements of the minerals define a melting trend, and the modeling of the Y and Yb contents in the clinopyroxenes indicates that the degree of partial melting ranges from 1% to 10%. Although no hydrous minerals are found, the enrichment of light rare earth elements and large ion lithophile elements (Th, U, etc.) indicates that some samples underwent cryptic mantle metasomatism. The lack of coherent variations between the H2O contents of the whole rocks and the metasomatic index (La/Yb)n ratio in clinopyroxene suggests that the mantle metasomatism did not modify the initial H2O contents after the melting event. The correlation of H2O content with the melting index (such as Yb content in clinopyroxene, Cr# in spinel) demonstrates that the extent of partial melting is the main factor controlling the variations in the H2O contents in these rocks. The variations in whole rock H2O contents can be modeled as a simple melting process from a MORB source using the available experimental partition coefficients of H2O between peridotite and basaltic melt (D=0.01-0.03). Based on the similarities in major and trace elements, and of H2O contents in the Tianchang peridotites with those inferred for the MORB source, we propose that the Cenozoic lithospheric mantle beneath Tianchang is accreted from an upwelled and cooled asthenospheric mantle.All the orthopyroxenes showed a hydrogen diffusion profile which were irrelevant to their location, this is the first observation of hydrogen diffusion in natural orthopyroxenes. The H2O distribution in clinopyroxenes, orthopyroxenes and olivines demonstrate that the involved H diffusion coefficients in ol, cpx and opx follow D ol>10 D opx>1000 D cpx (D is the effective diffusion rate of hydrogen). Using experimental diffusion coefficients we inferred that the xenoliths remains at least several years in contact with the magma below 900?. Cpx remains the most reliable recorder of mantle water, while opx should be used more cautiously but can be used to trace the slow transport or cooling history of magma. |
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