Abstract
Early formed high-Mg-# olivine phenocrysts during evolution of MORB magmas usually host melt inclusions, which record importantw information about the early-stage evolution of magma. Five MORB samples from near East Pacific Rise (EPR) 13 degrees N vary little in K/Ti (0.07-0.12), Tb/Lu (1.72-1.84) and Sm/Nd (0.310-0.332) and have similar REEs patterns, indicating that depleted upper mantle has similar mineral composition. Sixty-five initial melt inclusions derived by correcting olivine fractionation and "FeO-Loss" show averagely higher MgO contents than their host rocks. Melt inclusions have higher CaO/Al2O3 ratios than their host rocks, and these CaO/Al2O3 ratios are positively and negatively correlated with MgO and Na2O respectively, suggesting that these magmas have experienced high pressure crystallization of clinopyroxene. Average crystallization pressure, which is calculated based on the pressure dependence of clinopyroxene crystallization, is 0.83 +/- 0.25 GPa, and implys that these melt inclusions are averagely trapped in mantle depth of similar to 24 km. These melt inclusions show negative correlations of Ca-8/Al-8 and Na-8 with Fe-8, and wider ranges of Ca-8/Al-8, Na-8, Fe-8 and K/Ti than their host rocks, suggesting that these melt inclusions formed by mixing magmas of different melting degrees and depths. According to the average value and ranges of Ca-8/Al-8, Na-8, Fe-8 and K/Ti, these magmas would necessitate other mixing ends in shallow crust except in upper mantle. The compositional diversity of melt inclusions in MORBs phenocrysts cannot always be used to indicate magma mixing and crystallization in shallow crust, and melt inclusions in high Mg-# olivine formed under mantle pressure must be excluded in study of the magma process at crustal level. This study shows that, in EPR, MORBs have experienced mixing of magmas formed by different melting degrees and depths in the mantle.
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