Carbon isotope systematics of a mantle ``hotspot'': a comparison of Loihi Seamount and MORB glasses

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The carbon isotope geochemistry of glasses from Loihi Seamount has been compared with that of MORB glasses. Stepped heating shows two carbon components in both sample suites: (1) isotopically light carbon (avg. δ13C = -26.3‰) released < 600°C, ascribed to surficial contamination, and (2) isotopically heavy carbon released > 600°C, regarded as indigenous. The high-temperature component in MORB samples varied from 52 to 169 ppm C, average δ13C = -6.6‰, consistent with previous studies (overall MORD average δ13C = -6.4 +/- 0.9‰), and new results for Indian Ocean glasses are similar to Atlantic and Pacific Ocean samples. Carbon release profiles produced by stepped heating may be typical of locality, but there are no significant differences in δ13C values between MORB samples from different areas. Lower yields (17-110 ppm C) correlated with depth in the Loihi samples suggest that they are partially degassed. This degassing has not affected δ13C values significantly (avg. -5.8‰). Loihi tholeiites have higher δ13C (avg. -5.6‰) than the alkali basalts (avg. -7.1‰). Carbon abundances correlate well with He concentration data. Comparison of the δ13C values with trace element and He, Sr, Nd, and Pb isotope data from the literature suggests that the Loihi samples with highest δ13C have high 3He/4He and possibly the least depleted 143Nd/144Nd and 87Sr/86Sr. The carbon isotope data are consistent with previous models for Loihi involving several mantle sources, lithospheric contamination, and mixing. The slightly higher δ13C of Loihi tholeiites suggests that the undegassed ``plume'' component manifested by high 3He/4He values might have δ13C about 1‰ higher than the MORB average.

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