The lunar regolith is sensitive to the bombardment history of the Moon and contains a wealth of knowledge regarding the types of processes that have modified the lunar crust through time. Noble gases that are produced and trapped in the lunar regolith, as a result of this interaction with the space environment, can be used to determine the cosmic ray exposure age, maturity, shielding depth and antiquity age of lunar regolith samples. This thesis aims to probe this temporal archive to further understand the impact history of the Moon contained within the regolith. Initially, all the published noble gas literature data for the Apollo regolith breccias, drill cores and soils was compiled into a database where trapped and cosmogenic noble gas component were calculated. These data were used to summarise the history of the lunar regolith contained in the Apollo sample archive. A dichotomy between the "ancient" (determined by the antiquity indicator using the 40Ar/36Artr ratio) regolith samples and those formed in more recent times has been described previously (e.g., McKay et al., 1986).The ancient breccias and soils (&gt;~3.5 Ga) have typically experienced limited amounts of surface exposure (i.e., they are 'immature'). Whereas, regolith samples formed in more recent times ( < 3.5 Ga, << 2 Ga) show a range of maturities. It is likely that the difference in maturity between the ancient and younger breccias reflects the changing collisional conditions of the time i.e., impact flux and regolith turnover rates. Here, 12 lunar meteorite regolith breccias were analysed for their noble gas content (Ne, Ar, Xe isotopes) to determine if lunar meteorites show the same difference between (40Ar/36Ar)tr ratio and maturity. Lunar meteorites in this study and previously published data do show the same negative correlation between (40Ar/36Ar)tr ratio and maturity. Furthermore, many of the lunar meteorite samples have (40Ar/36Ar)tr ratio between 1 and 2.5 indicating antiquity ages of approximately 1-2 Ga. This potentially reflects a declining period of random intermediate impacts bracketing the period between the 'ancient' and 'recent' samples. The same techniques were applied to newly discovered lunar meteorite MIL 13317. This included a full petrology description, mineral chemistry, U-Pb and Pb-Pb ages, and analysis of noble gas content to decipher the regolith history of this new sample. The meteorite is a mixture of mare and highland components (including mare basalts, FAN, Mg-suite and KREEP) with ancient ages (~ 4.3Ga) and a complex regolith history (exposure age ~500 to 800 Ma, antiquity age ~1.92 Ga). MIL 13317 is an important addition to the lunar collection as it contains material from previously unsampled areas of the Moon which is interpreted here to be associated with the northern regions of the Procellarum KREEP Terrane. Work was also begun on Apollo 16 regolith breccias using the same analytical techniques. However, due to instrument issues and friable samples much of the work was not completed and will be continued after the PhD. Understanding the data collected here and the techniques used will feed forward to future missions to the Moon to understand noble gas concentrations in the lunar regolith.

abstract: Impact cratering has played a key role in the evolution of the solid surfaces of Solar System bodies. While much of Earth’s impact record has been erased, its Moon preserves an extensive history of bombardment. Quantifying the timing of lunar impact events is crucial to understanding how impacts have shaped the evolution of early Earth, and provides the basis for estimating the ages of other cratered surfaces in the Solar System. Many lunar impact melt rocks are complex mixtures of glassy and crystalline “melt” materials and inherited clasts of pre-impact minerals and rocks. If analyzed in bulk, these samples can yield complicated incremental release 40Ar/39Ar spectra, making it challenging to uniquely interpret impact ages. Here, I have used a combination of high-spatial resolution 40Ar/39Ar geochronology and thermal-kinetic modeling to gain new insights into the impact histories recorded by such lunar samples. To compare my data to those of previous studies, I developed a software tool to account for differences in the decay, isotopic, and monitor age parameters used for different published 40Ar/39Ar datasets. Using an ultraviolet laser ablation microprobe (UVLAMP) system I selectively dated melt and clast components of impact melt rocks collected during the Apollo 16 and 17 missions. UVLAMP 40Ar/39Ar data for samples 77135, 60315, 61015, and 63355 show evidence of open-system behavior, and provide new insights into how to interpret some complexities of published incremental heating 40Ar/39Ar spectra. Samples 77115, 63525, 63549, and 65015 have relatively simple thermal histories, and UVLAMP 40Ar/39Ar data for the melt components of these rocks indicate the timing of impact events—spanning hundreds of millions of years—that influenced the Apollo 16 and 17 sites. My modeling and UVLAMP 40Ar/39Ar data for sample 73217 indicate that some impact melt rocks can quantitatively retain evidence for multiple melt-producing impact events, and imply that such polygenetic rocks should be regarded as high-value sampling opportunities during future exploration missions to cratered planetary surfaces. Collectively, my results complement previous incremental heating 40Ar/39Ar studies, and support interpretations that the Moon experienced a prolonged period of heavy bombardment early in its history. / Dissertation/Thesis / Doctoral Dissertation Geological Sciences 2017