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Carbon isotope systematics of a mantle "hotspot": A comparison of Loihi Seamount and MORB glasses

R.A. Exley·D.P. Mattey·D.A. Clague·C.T. Pillinger
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Abstract

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. δ 13 C = −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 δ 13 C = −6.6‰ , consistent with previous studies (overall MORD average δ 13 C = −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 δ 13 C 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 δ 13 C values significantly (avg. −5.8‰). Loihi tholeiites have higher δ 13 C (avg. −5.6‰) than the alkali basalts (avg. −7.1‰). Carbon abundances correlate well with He concentration data. Comparison of the δ 13 C values with trace element and He, Sr, Nd, and Pb isotope data from the literature suggests that the Loihi samples with highest δ 13 C have high 3 He/ 4 He and possibly the least depleted 143 Nd/ 144 Nd and 87 Sr/ 86 Sr. The carbon isotope data are consistent with previous models for Loihi involving several mantle sources, lithospheric contamination, and mixing. The slightly higher δ 13 C of Loihi tholeiites suggests that the undegassed “plume” component manifested by high 3 He/ 4 He values might have δ 13 C about 1‰ higher than the MORB average.

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R.A. Exley, D.P. Mattey, D.A. Clague, C.T. Pillinger. 1986. Carbon isotope systematics of a mantle "hotspot": A comparison of Loihi Seamount and MORB glasses. https://doi.org/10.1016/0012-821x(86)90060-9

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