Date of Award
Master of Science (MS)
Scott D. Samson
Physical Sciences and Mathematics
Minerals chosen for provenance studies are typically those that are resistant to resetting during low-moderate temperature metamorphic events and thus preserve information about their source region. However, minerals that are extremely resistant to weathering (e.g. quartz, zircon) can be recycled, often multiple times, and thus can cause confusion about the amount of material derived from immediate sources versus those from the original source region. Resistant minerals, such as zircon, that generally only form under very high temperature events (i.e. magmatic events) often miss recording orogenic events that are dominated by moderate temperature, magma-poor, metamorphic events. To overcome some of these difficulties, detrital monazite from Modern alluvium, potential bedrock sources, and Pennsylvanian-Permian sandstones from the southern Appalachian Mountains have been analyzed via Laser Ablation Split Stream Inductively Coupled Mass Spectrometry (LASS-ICP-MS) for U-Th-Pb age and Nd isotopic composition. Previous studies have investigated these units, and related ones, using U-Pb dating of detrital zircon. Because monazite can form at temperatures much lower than that of zircon crystallization, it is able to record lower grade metamorphic events in addition to original source signals, thus a more complete provenance analysis can be acquired. The results from our modern alluvium detrital monazite U-Th-Pb analysis show a bimodal signature of Paleozoic ages and Grenville ages (~1250-950 Ma), with the majority of the ages coinciding with the Taconic Orogeny ~450 Ma. Age peaks matching the multiple distinct Paleozoic orogenies are much more clearly captured in the detrital monazite record than the zircon record that sees almost no ages overlapping the Acadian (~420-380 Ma) and Alleghanian (~330 – 280 Ma) orogenies. The Grenville-aged peak in the detrital monazite record is additionally more representative of the area of Grenville-aged bedrock actually eroding in the watershed compared to the overpowering Grenville signal recorded by detrital zircon. The Nd data offer composition information about the original source of the monazite (i.e. predominantly metamorphic rocks) but also sedimentary characteristics for diagenetic monazite. The ranges in ϵNdi values from Neoproterozoic low-grade metasediments (~ -7 – +4), Paleozoic bedrocks (~ -9.5 – +0.5), and modern alluvium (~ -14.5 – +3.5) support an argument for crustal recycling of Grenvillian crust as the primary juvenile component during the Taconic Orogeny. Utilizing combined U-Th-Pb and Nd isotopes as a dual detrital monazite characterization method provides a high throughput workflow for detailed provenance analysis. Information gathered with this method is critical to a more comprehensive understanding of sedimentary systems.
Kittross, Sarah Wade, "INSIGHTS FROM COMBINED DETRITAL MONAZITE GEOCHRONOLOGY AND ND ISOTOPIC COMPOSITION ON SEDIMENTARY RECYCLING AND PROVENANCE RECONSTRUCTION" (2019). Theses - ALL. 346.