Low Sr isotopic ratios of alkali basalts from the Line Islands in the Pacific Ocean

Abstract

The initial (⁸⁷Sr/⁸⁶Sr) ratios of alkali rocks from the Line chain (Line Islands) in the Pacific Ocean are variable, but may be divided into three groups; (1) The low (⁸⁷Sr/⁸⁶Sr) ratios of the main chain (∼0.7031). (2) The high (⁸⁷Sr/⁸⁶Sr) ratios for the site located off the main chain (∼0.7043). (3) The low (⁸⁷Sr/⁸⁶Sr) ratios of the Line Cross chain (∼0.7034), which is not clearly discriminated from the main chain. The low (⁸⁷Sr/⁸⁶Sr) ratios of alkali basalts seem to suggest that the magma sources of hot spots are in the ‘depleted’ upper portion of the mantle, in spite of the fact that the hot spots should be situated in the deep position of the mantle.