Lithium Isotope Systematics of McMurdo Volcanic Group Lavas, and Other Intraplate Sites

Jeffrey G. Ryan (Dept. Geology, Univ. South Florida, 4202 East Fowler Ave, Tampa, FL 33620;

Philip R. Kyle (Dept. Geosciences, New Mexico Inst. Min. Tech, Socorro, NM)

To assess the geochemical cycle of Li isotopes, and the role of subduction in Li isotopic distributions in the Earth, it is necessary to understand the level of Li isotopic diversity in terrestrial mantle reservoirs. Accessing this information in via examining mantle-derived volcanic rocks may be complicated by the fact that unlike radiogenic isotope systems, Li isotopic ratios may suffer chemical fractionations during hydrothermal processes associated with magmatism, and may also be affected by isotopically variable Li reservoirs in the crust. To characterize the effects of open-system magmatic processes, as well as to constrain d7Li values for mantle sources, we have examined the Li abundance and isotopic systematics of well-characterized suites of lavas from the McMurdo Volcanic Group (Mt. Erebus region) of Antarctica, for comparison to other intraplate volcanic sites.

Both Li contents and d7Li for our samples were determined using the VG P54-30 MC ICP-MS at the Dept. Terrestrial Magnetism/CIW, following methods pioneered by P. Tomascek and coworkers. Recent improvements in Li sensitivity are reflected in improved short-term reproducibility : repeated measurements of reference samples, and replicate sample digestions indicate a precision on d7Li determinations of on the order of 0.7‰.

The McMurdo samples analyzed included basanites and moderately evolved lavas of the Erebus Lineage (EL) and evolved Enriched Iron series (EFS) lavas (phonolites and trachytes). d7Li in relatively primitive EL lavas (Li contents: 3-9 ppm) average +4.0‰ 1‰, while the more evolved rocks (Li contents: 11-19 ppm) range from +1.4‰ to +10.4‰. Basanites from the Crary Mtns, in the West Antarctic Rift System (Late Cenozoic) have d7Li of +4.1‰ to +5.0‰. Thus, while magmatic processes do impart variability on d7Li, the most primitive lavas erupted in this region over time preserve a consistent ratio, most likely reflecting that of the local mantle source. The McMurdo mantle source is isotopically similar to that of Iceland (+3.1‰, +4‰) and Reunion (+2.8‰, +4.4‰), but distinct from that of St. Helena (+7.0‰) and the Pribilof Islands (~+6.7‰). The Li isotopic variability of intraplate volcanic rocks thus appears to be somewhat larger than that of MORBs, and "normal" arc lavas, though it is still markedly less than what is seen in "old" arc settings such as Panama.