HR: 14:10h
AN: V53F-03    [Abstracts]
TI: Helium and Carbon Isotope and Relative Abundance Relationships in Lau Basin Basalts: Resolving Mantle Source Composition from Degassing and Contamination Effects
AU: * Vukajlovich, D J
EM: dvukajlo@ucsd.edu
AF: Scripps Institution of Oceanography, Geosciences Research Division UCSD, 9500 Gilman Drive, La Jolla, CA 92093-0244 United States
AU: Hilton, D R
EM: drhilton@ucsd.edu
AF: Scripps Institution of Oceanography, Geosciences Research Division UCSD, 9500 Gilman Drive, La Jolla, CA 92093-0244 United States
AU: Castillo, P R
EM: pcastillo@ucsd.edu
AF: Scripps Institution of Oceanography, Geosciences Research Division UCSD, 9500 Gilman Drive, La Jolla, CA 92093-0244 United States
AU: Hawkins, J W
EM: jhawkins@ucsd.edu
AF: Scripps Institution of Oceanography, Geosciences Research Division UCSD, 9500 Gilman Drive, La Jolla, CA 92093-0244 United States
AB: The Lau Basin has multiple mantle source components including contributions from the Indian and Pacific MORB sources, Tonga-Kermadec Arc and Samoan plume. In order to characterize the volatile systematics of these various sources and to map their spatial distribution, we have sampled basaltic glasses from over 50 dredge sites covering all known spreading centers in the basin as well as many off-axis seamounts. Here, we report He abundance and isotope results obtained by crushing, in addition to CO₂ released through stepped heating, from sites at the Mangatolu Triple Junction (MTJ), Rochambeau Bank (RB), Peggy Ridge, and the Northern, Eastern and Central Lau Spreading Centers. High ³He/⁴He ratios from RB (up to 23 R_A, where R_A = air ³He/⁴He) confirm the presence of a plume component in the northwestern Lau Basin (Poreda, EPSL, 1985). Central and Eastern Lau Spreading Center basalts have ³He/⁴He ratios between 8.3 and 9.4 R_A, consistent with a depleted, MORB-like mantle source with little influence from slab or crustal helium. In contrast, the large range in helium isotope ratios of MTJ samples (0.85 to 7.9 R_A) and the correlation between low He abundances (~2 - 3 × 10^-9 cm³/g) and low helium isotope ratios suggests the volatiles in this region have been severely affected by degassing and additions of radiogenic (crustal) He. CO₂ abundances and carbon isotopes for samples from RB vary from 70 to 119 ppm ([CO₂]_total) with δ¹³C_vesicle falling between -12.3 to -14.8 ‰ and δ¹³C_dissolved lying between -9.3 to -10.7 ‰. In the MTJ, low helium concentration samples have δ¹³C as low as -27.4 ‰ and [CO₂]_total as low as 7.6ppm; interestingly, this region also has samples with the highest measured values (up to -6.3 ‰ and 132ppm total C). Combining the carbon and helium data, CO₂/³He ratios in the MTJ range from arc-like values (~10¹⁰) to sediment or crustal values (~10¹³) showing the superimposition of degassing and/or contamination effects on a predominant slab-like signature. High ³He/⁴He samples from RB have CO₂ /³He ratios similar to the upper mantle value (2 × 10⁹), as at other back-arc localities exhibiting a high ³He/⁴He (plume) component (e.g. the Manus Basin; Shaw et al., GCA, 2004). Continuing analytical work will complete our He-C survey of the Lau Basin, identifying samples suitable for further effort involving Ne, Ar, H₂O plus other tracers of interest. Presently, we are modeling degassing/contamination effects to identify the nature and distribution of mantle source components throughout the Lau Basin. The volatile characteristics of the high ³He/⁴He samples are of particular interest as they provide insight into the different recycling and storage histories of volatiles between distinct mantle reservoirs.
DE: 1025 Composition of the mantle
DE: 1030 Geochemical cycles (0330)
DE: 1038 Mantle processes (3621)
DE: 3001 Back-arc basin processes
DE: 3037 Oceanic hotspots and intraplate volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005