Using Moderately Volatile Elements to Understand the Origin of the Earth-Moon System
Josh Wimpenny | 19-LW-027
Current heterogeneities in the isotope ratios of zinc and gallium (Zn and Ga) in lunar samples are inconsistent with straightforward loss of volatiles caused by the Moon-forming Giant Impact. The aim of this project was to make new Zn and Ga isotope ratio measurements in a range of lunar samples to better understand the processes that control their isotopic fractionation, which in turn would enable us to place better constraints on the timing and extent of volatile loss from the Moon. To this end we developed a new methodology to purify Ga and analyze its isotopic composition, showing that the isotopic systematics of Ga behave similarly to Zn during evaporative loss from silicate melts. Hence, if lunar Ga and Zn systematics are controlled only by evaporation, we would expect their respective isotopic compositions to co-vary. However, the two isotope systems are decoupled in lunar rocks. Zinc is highly fractionated in vapor condensate phases and late-stage lunar melts, which impart significant, but variable, isotopic effects to basaltic rocks and those from the lunar crust. In contrast, Ga is relatively insensitive to mobilization at the Moon's surface and its isotopic composition is far more homogenous in lunar rocks. Instead, small isotopic differences within the mare basalt suite may reflect fractionation during progressive solidification of the lunar magma ocean (LMO). Although most lunar rocks have heavier Zn and Ga isotope ratios than rocks from the Earth, heterogeneities in their isotopic compositions must have been caused by geological processing after the Moon had accreted. Thus, volatile loss could have occurred both prior to and after the Moon formed. Future work will need to better constrain the identity of these fractionating processes and their associated isotopic effects in order to estimate the composition of the Moon when it formed and ascertain the magnitude of volatile loss associated with the Giant Impact event.
This project has had several impacts on the Lawrence Livermore National Laboratory mission. The capability to measure the isotopic composition of moderately volatile elements to a high precision and development and testing of double-spike procedures is highly relevant to ongoing efforts to develop new geochemical proxies for nuclear forensics. For example, the fractionation of moderately volatile elements during rapid heating is relevant to understanding physical processes that occur in the fireball of a nuclear event. Development of procedures to measure isotope ratios by double-spike have been gaining new ground in both pre-detonation and post-detonation nuclear forensics. The work performed in this project has also helped to bring new work to the Laboratory. The principal investigator was successful in obtaining funding from NASA to continue this area of research based on these results outlined above. Furthermore, the Livermore group is directly involved with NASA's ANGSA (Apollo Next Generation Sample Analysis) program to analyze a previously unopened drive core sample from the Apollo 17 mission, for which moderately volatile element isotope data are a key requirement.
Publications, Presentations, and Patents
Sio, C., et al., 2019. "Iron isotope compositions of lunar highland rocks and mare basalts." Goldschmidt Geochemistry Conference 2019. LLNL-ABS-771027.
Sio, C., et al., submitted. "Iron isotope evidence of an impact origin for main-group pallasites." LLNL-JRNL-812755.
Wimpenny, J., et al., 2019. "Reassessing Gallium Isotopic Evidence for Volatile Loss from the Moon." Goldschmidt Geochemistry Conference 2019. LLNL-ABS-770761.
Wimpenny, J., et al., 2020. "Constraining the behavior of gallium isotopes during evaporation at extreme temperatures." Geochimica et Cosmochimica Acta 286: 54-71. LLNL-JRNL-798685.