He,Ne and Ar isotopic composition of Fe-Mn crusts from the western and central Pacific Ocean and implications for their genesis

The noble gas nuclide abundances and isotopic ratios of the upmost layer of Fe-Mn crusts from the western and central Pacific Ocean have been determined. The results indicate that the He and Ar nu- clide abundances and isotopic ratios can be classified into two types: low 3He/4He type and high 3He/4He type. The low 3He/4He type is characterized by high 4He abundances of 191×10-9 cm3·STP·g-1 on average, with variable 4He, 20Ne and 40Ar abundances in the range (42.8―421)×10-9 cm3·STP·g-1, (5.40―141)×10-9 cm3·STP·g-1, and (773―10976)×10-9 cm3·STP·g-1, respectively. The high 3He/4He samples are characterized by low 4He abundances of 11.7×10-9 cm3·STP·g-1 on average, with 4He, 20Ne and 40Ar abundances in the range of (7.57―17.4)×10-9 cm3·STP·g-1, (10.4―25.5)×10-9 cm3·STP·g-1 and (5354―9050)×10-9 cm3·STP·g-1, respectively. The low 3He/4He samples have 3He/4He ratios (with R/RA ratios of 2.04―2.92) which are lower than those of MORB (R/RA=8±1) and 40Ar/36Ar ratios (447―543) which are higher than those of air (295.5). The high 3He/4He samples have 3He/4He ratios (with R/RA ratios of 10.4―12.0) slightly higher than those of MORB (R/RA=8±1) and 40Ar/36Ar ratios (293―299) very similar to those of air (295.5). The Ne isotopic ratios (20Ne/22Ne and 21Ne/22Ne ratios of 10.3―10.9 and 0.02774―0.03039, respectively) and the 38Ar/36Ar ratios (0.1886―0.1963) have narrow ranges which are very similar to those of air (the 20Ne/22Ne, 21Ne/22Ne, 38Ar/36Ar ratios of 9.80, 0.029 and 0.187, respectively), and cannot be differentiated into different groups. The noble gas nuclide abundances and isotopic ratios, together with their regional variability, suggest that the noble gases in the Fe-Mn crusts originate primarily from the lower mantle. The low 3He/4He type and high 3He/4He type samples have noble gas characteristics similar to those of HIMU (High U/Pb Mantle)- and EM (Enriched Mantle)-type mantle material, respectively. The low 3He/4He type samples with HIMU-type noble gas isotopic ratios occur in the Magellan Seamounts, Marcus-Wake Seamounts, Marshall Island Chain and the Mid-Pacific Sea- mounts whereas the high 3He/4He type samples with EM-type noble gas isotopic ratios occur in the Line Island Chain. This difference in noble gas characteristics of these crust types implies that the MagellanSeamounts, Marcus-Wake Seamounts, Marshall Is- land Chain, and the Mid-Pacific Seamounts originated from HIMU-type lower mantle material whereas the Line Island Chain originated from EM-type lower mantle material. This finding is consistent with varia- tions in the Pb-isotope and trace element signatures in the seamount lavas. Differences in the mantlesource may therefore be responsible for variations in the noble gas abundances and isotopic ratios in the Fe-Mn crusts. Mantle degassing appears to be the principal factor controlling noble gas isotopic abundances in Fe-Mn crusts. Decay of radioactive isotopes has a negligible influence on the nuclide abundances and isotopic ratios of noble gases in these crusts on the timescale of their formation.

He,Ne and Ar isotopic composition of Fe-Mn crusts from the western and central Pacific Ocean and implications for their genesis

The noble gas nuclide abundances and isotopic ratios of the upmost layer of Fe-Mn crusts from the western and central Pacific Ocean have been determined. The results indicate that the He and Ar nuclide abundances and isotopic ratios can be classified into two types: low ³He/⁴He type and high ³He/⁴He type. The low ³He/⁴He type is characterized by high ⁴He abundances of 191×10⁻⁹ cm³·STP·g⁻¹ on average, with variable ⁴He, ²⁰Ne and ⁴⁰Ar abundances in the range (42.8−421)×10⁻⁹ cm³·STP·g⁻¹, (5.40−141)×10⁻⁹ cm³·STP·g⁻¹, and (773−10976)×10⁻⁹ cm³·STP·g⁻¹, respectively. The high ³He/⁴He samples are characterized by low ⁴He abundances of 11.7×10⁻⁹ cm³·STP·g⁻¹ on average, with ⁴He, ²⁰Ne and ⁴⁰Ar abundances in the range of (7.57−17.4)×10⁻⁹ cm³·STP·g⁻¹, (10.4−25.5)×10⁻¹ cm³·STP·g⁻¹ and (5354−9050)×10⁻⁹ cm³·STP·g⁻¹, respectively. The low ³He/⁴He samples have ³He/⁴He ratios (with R/RA ratios of 2.04−2.92) which are lower than those of MORB (R/R _A=8±1) and ⁴⁰Ar/³⁶Ar ratios (447−543) which are higher than those of air (295.5). The high ³He/⁴He samples have ³He/⁴He ratios (with R/R _A ratios of 10.4−12.0) slightly higher than those of MORB (R/R _A=8±1) and ⁴⁰Ar/³⁶Ar ratios (293−299) very similar to those of air (295.5). The Ne isotopic ratios (²⁰Ne/²²Ne and ²¹Ne/²²Ne ratios of 10.3−10.9 and 0.02774−0.03039, respectively) and the ³⁸Ar/³⁶Ar ratios (0.1886−0.1963) have narrow ranges which are very similar to those of air (the ²⁰Ne/²²Ne, ²¹Ne/²²Ne, ³⁸Ar/³⁶Ar ratios of 9.80, 0.029 and 0.187, respectively), and cannot be differentiated into different groups. The noble gas nuclide abundances and isotopic ratios, together with their regional variability, suggest that the noble gases in the Fe-Mn crusts originate primarily from the lower mantle. The low ³He/⁴He type and high ³He/⁴He type samples have noble gas characteristics similar to those of HIMU (High U/Pb Mantle)-and EM (Enriched Mantle)-type mantle material, respectively. The low ³He/⁴He type samples with HIMU-type noble gas isotopic ratios occur in the Magellan Seamounts, Marcus-Wake Seamounts, Marshall Island Chain and the Mid-Pacific Seamounts whereas the high ³He/⁴He type samples with EM-type noble gas isotopic ratios occur in the Line Island Chain. This difference in noble gas characteristics of these crust types implies that the Magellan Seamounts, Marcus-Wake Seamounts, Marshall Island Chain, and the Mid-Pacific Seamounts originated from HIMU-type lower mantle material whereas the Line Island Chain originated from EM-type lower mantle material. This finding is consistent with variations in the Pb-isotope and trace element signatures in the seamount lavas. Differences in the mantle source may therefore be responsible for variations in the noble gas abundances and isotopic ratios in the Fe-Mn crusts. Mantle degassing appears to be the principal factor controlling noble gas isotopic abundances in Fe-Mn crusts. Decay of radioactive isotopes has a negligible influence on the nuclide abundances and isotopic ratios of noble gases in these crusts on the timescale of their formation. Supported by the China Oceanic Mineral Resources R & D Association (COMRA) (Grant No. DY105-01-04-02) and the National Natural Science Foundation of China (Grant No. 40376016)