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Valerie Justice

U-Pb Isotope Dating of Monazite from the Sierra Estrella Mountains of Arizona: Evidence for ca. 1.4 Billion Year Old Metamorphism and Deformation


Valerie Justice ’21


Faculty Mentor(s):

Christopher Daniel, Department of Geology and Environmental Geosciences

Funding Source:

Bucknell Department of Geology and Environmental Geosciences


U-Pb isotope dating of monazite from the Sierra Estrella mountains near Phoenix, Arizona, yield metamorphic ages between about 1.42 and 1.40 billion years old (b.y.o.). These ages suggest that the high temperature and high pressure metamorphism (T = 800 °C, P = 9 kbar) experienced by these rocks was associated with the 1.50 to 1.35 b.y.o. Picuris Orogeny. About 20 samples were collected from the Sierra Estrella mountains. Four samples were selected for U-Pb analyses of monazite, a rare earth element phosphate mineral (REE(PO4)). Monazite gains were identified and imaged using the scanning electron microscope in the Department of Geology, Bucknell University. U-Pb analyses of monazite were conducted at the University of New Brunswick using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS). Two samples from the north end of the mountain range (CD20-03Ba and CD20-07Bb) yield concordia ages of 1.400 ± .005 b.y.o (n= 8, MSWD = 0.13) and 1.410 ± 0.004 b.y.o. (n = 8, MSWD = 0.25), respectively. Two samples from about 20 km to the south (CD20-09 and CD20-14b) yield concordia ages of 1.410 ± 0.01 b.y.o. (n=8, MSWD = 0.74) and 1.420 +/- 0.006 b.y.o. (n = 8, MSWD = 0.18), respectively. Our new U-Pb ages are similar to 1.420-1.380 b.y.o. monazite previously reported in New Mexico that are also attributed to the Picuris Orogeny.

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