Noble gases in corundum megacrysts from the basalts in Changle, Shandong Province, eastern China

摘要

This study presents noble gaseous data of the corundum megacrysts from the Cenozoic basalts in Changle, Shandong Province, eastern China. It is known that no noble gaseous data of corundum megacryst have been documented before. The ~3He/~4He ratios (1.13—7.37 Ra) of the corundums from Changle vary from atmosphere to MORB values; the ~(20)Ne/~(22)Ne (9.67-10.75) and ~(21)Ne/~(22)Ne (0.0280-0.0372) data define two linear trends on Ne three-isotope diagram, respectively, along the MFL and the correlation line between atmosphere and MORB; the ~(38)Ar/~(36)Ar (0.177-0.194) ratios, the ~(40)Ar/~(36)Ar (280.9 —404.2) ratios and the ~(128-136)Xe/~(132)Xe ration with obvious ~(129)Xe excess are generally higher than atmospheric component, but the ~(40)Ar/~(36)Ar ratios are much closer to atomospheric ratio. The isotopic compositions of noble gases (particularly for He and Ar) of the corundums are similar to those of pyroxene, anorthoclase megacrysts, and mantle-derived xenoliths from this area, and those of mantle-derived xenoliths from several areas in eastern China. Therefore, the noble gases trapped in the corundums probably are from mantle source, representing a 'mixed fluid' produced by the interaction between the lithospheric mantle and fluids releasing from the convective plate. Both the noble gas isotopic compositions and the oxygen isotopic compositions of the solid corundums are not the characteristics of crustal source. These suggest that the corundums crystallized from mantle-derived magmas with minimal crustal contamination.