Halogen Composition (Cl, Br, I) of the Ancient Terrestrial and Lunar Mantles: Developing an Understanding of Volatiles in the Earth-Moon System

Abstract

The Earth and Moon are widely believed to have a common origin. Studying the Earth and the Moon in parallel hence provides a more complete picture of their heavy halogen history, and adds to developing an understanding of volatiles in the Earth-Moon system. The first study in the thesis uses neutron irradiated noble gas mass spectrometry (NI-NGMS) to constrain Cl, Br, and I compositions of Archaean (~3.3 to 2.5 Ga) komatiites, taken as representative of the ancient terrestrial mantle. Little is currently known about the composition and behaviour of ancient mantle halogens, and how they compare with those of present day processed mantle. Ancient komatiite heavy halogen compositions were found to not vary between 3.3 Ga to 2.5 Ga. Moreover, the komatiites have compositions within the range of mid-ocean ridge/ocean island basalts (MORB/OIB), which are representative of present-day processed mantle. Subduction of relatively halogen-rich serpentinites, therefore, does not appear to have significantly affected mantle halogen compositions over the past ca. 3 Ga. This study is consistent with the notion of potential long-term storage site(s) for mantle halogens, previously suggested to be possibly within subcontinental lithospheric mantle beneath subduction zones, or in the transition zone. A single halogen-enriched komatiite with respect to other Archaean komatiites and present day MORB/OIB is potentially from a volatile-rich primitive mantle source, or contains a component altered by ancient seawater that had a different heavy halogen composition to present day seawater. The second study involved developing the NI-NGMS technique to measure heavy halogen abundances within Apollo lunar mare basalts, which were selected as representative of partial melts of the nearside lunar mantle. This study is the first to extend the range of heavy halogen abundances previously measured for mare basalts to include significantly lower values for Br and I (up to one and two orders of magnitude, respectively). This indicates a heterogeneous nearside lunar mantle with respect to Br and I, which includes highly halogen depleted regions with respect to Earth and CI chondrites. This comparative heavy halogen study supports a common CI/ordinary chondritic origin for lunar and terrestrial mantle halogens. The heavy halogen results are consistent with K, Zn, and Ga isotope records that favour a relatively depleted lunar interior, as the result of vapour condensation from a volatile poor Moon after formation in a giant impact event. A late addition of chondritic material is not necessarily required to account for the heavy halogen mantle compositions of the Earth and Moon. The NI-NGMS techniques developed for this PhD are ideal for analysing halogens in mm-sized samples from upcoming robotic sample return missions to the Moon (e.g. Chang'e 5). Such analyses will be vital for expanding our understanding of the complex history of halogens and other volatiles of the Earth-Moon system.

Date of Award	31 Dec 2018
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Raymond Burgess (Supervisor) & Katherine Joy (Supervisor)