Lithium isotope fractionation during magma degassing: Constraints from silicic differentiates and natural gas condensates from Piton de la Fournaise volcano (Réunion Island)

Recent volcanic products from the Piton de la Fournaise Volcano, Reunion, show pronounced depletion or enrichment in lithium and significant isotopic fractionation related to degassing. (1) trachytic pumices from the April 2007 eruption show extreme Li depletion (90%) and isotopic fractionation (δ7Li of − 21‰). The depletion of water and volatiles (Cl, F, B, Cs) in these samples suggests that Li loss occurred in response to degassing, which most likely occurred as the small, isolated volume of magma underwent extensive differentiation near the surface. Because the pre-degassing composition is relatively well known, the composition of the degassed pumice constrains the partition coefficient to 60 < DV–M < 135 and the isotopic fractionation factor, αV–M, to 1.010 at magmatic temperatures. Unlike DV–M, αV–M does not depend on whether crystallization and degassing occurred successively or concomitantly. (2) basaltic samples from the interior wall of the long-lived 1998 Piton Kapor were extensively altered by acidic gas. They also show extreme Li depletion, but barely significant isotopic fractionation (δ7Li = + 4.5‰), suggesting that high-temperature leaching of Li by volcanic gas does not significantly fractionate Li isotopes. (3) high-temperature (400–325 °C) gas condensates formed during degassing of the thick lava flow of April 2007 display high Li contents (50–100 ppm), which are consistent with Li being as volatile as Zn and Sn. Their isotopically light Li signature (average of − 1.7‰) is consistent with their derivation from isotopically heavy vapor (+ 13.5‰) if the factor of isotopic fractionation between condensate and vapor is less than 0.985. A degassing-crystallization model accounts for the evolution of trace species, which, like lithium, are volatile but also moderately incompatible.

Research Highlights
► Lithium strongly partitions into vapor during low-pressure, vapor-saturated crystallization.
► The enrichment of 7Li in vapor is consistent with equilibrium isotopic fractionation.
► Conversely, cooling of volcanic gas produces condensates enriched in 6Li.
► A degassing-crystallization model accounts for the behavior of Li isotopes.