Lu-Hf and Sm-Nd Systematics of Apollo 17 Sample 78236: Ages, Evolution and Investigations into the Neutron Fluence Correction on the Lu-Hf System
Simmons, Samuel 1988-
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This thesis presents Lu-Hf, Sm-Nd, and Gd isotope and trace element concentration data of Apollo sample 78236,28. Apollo 17 sample 78236 was selected because it represents an early stage of lunar evolution. Sample 78236,28 yielded a 3-point neutron fluence bulk scale corrected Lu-Hf isochron age of 4.418 ± 0.022 Ga, 176Hf/177Hf initial of 0.279706 ± 0.000017 (2σ; MSWD = 1.7), a 3-point neutron fluence mineral scale corrected Lu-Hf isochron age of 4.446 ± 0.023 Ga, 176Hf/177Hf initial of 0.279692 ± 0.000017 (2σ; MSWD = 1.8), and a 5-point Sm-Nd isochron age of 4.448 ± 0.032 Ga (2σ; MSWD = 2.3) with an initial 143Nd/144Nd ratio of 0.507024 ± 0.000038. Relative to chondritic uniform reservoir (CHUR) parameters of Bouvier et al. (2008), the ε143Nd(i) and ε176Hf(i) are +2.76 ± 0.75 and -6.71 ± 0.69, respectively. Based on the measured data and model parameters, the uncorrected initial ε176Hf(i), using Lu-Hf CHUR parameters of Bizzarro et al. (2012), is -2.13 ± 0.69. The apparent decoupling of the Nd and Hf initial isotope compositions may be indicative of system disturbance due to metamorphism, neutron capture effects, or perhaps related to uncertainties in applicable CHUR parameters. The Hf isotope data contain 180Hf isotope anomalies that are observed after mass bias correction using the exponential law. The measured isotope anomalies and modeling of neutron fluences indicate that neutron flux effects are a likely explanation for the observed Hf isotope data. The neutron fluences calculated by Edmunson et al. (2009) to correct their Sm-Nd data do not fully correct the Hf isotope data to natural values. In addition, the neutron fluences calculated from Sm and Gd isotope data measured in this study do not fully correct the Hf isotope data to natural values either. Based on neutron fluence modeling in this study, the fluences calculated from Sm and Gd data are likely a product of a dwindling free neutron budget due to other neutron absorbers in the system. It is hypothesized that 174Hf, 176Hf, 177Hf, 178Hf, and 180Hf demonstrated a greater sensitivity to free neutrons in the system than Sm or Gd, possibly due to absorption frequency overlap issues related to Fe and Ti in the sample.