Quantitative Assessment of Zircon Depth Profiling Plateau Ages
Published in China University of Geosciences (Beijing), 2025
Undergraduate thesis completed at China University of Geosciences (Beijing), June 2025.
Advisor: Ze Liu
Research Area: Zircon U-Pb Geochronology & Depth Profiling
Abstract
Over the past four decades, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and secondary ion mass spectrometry (SIMS) have been widely employed for in situ dating of zircon and other minerals. However, for minerals with extremely narrow mantles and rims (<5–15 μm), conventional microbeam analysis struggles to obtain accurate U-Pb ages for distinct zones due to the relatively large beam spot size (>15–50 μm). In situ depth profiling technique has emerged as a critical solution to this challenge, involving continuous ablation from the rim to the core of the mineral to acquire spatially resolved U-Pb isotopic data. Nevertheless, this approach suffers from two major limitations: (1) reliance on manual selection of age plateaus, which is inefficient, and (2) subjective human-defined plateau segmentation. This study develops a new quantitative method—Automated Depth Profiling Technique (ADEPT), including three steps: data optimization, age plateau segmentation, and age plateau screening, implemented as a fully automated workflow in R. We applied ADEPT to reprocess published depth profiling U-Pb data from Himalayan leucogranite zircon (N = 4615 grains) and compared the results with prior manual segmentation. Both methods yielded similar age peaks (~35 and ~9 Ma) for the leucogranite. However, the ADEPT method resolved additional reliable ages that were indiscernible visually.
The distinct age plateaus recorded from core to rim in zircon typically correspond to different episodes of magmatic activity. The results of this study reveal that 74% of single-zircon grains from Himalayan leucogranites exhibit 2–4 age plateaus. Notably, zircon age continuity (defined as the age difference between core/mantle and rim) exhibits a systematic eastward decrease across the Himalaya. Published whole-rock geochemical data further demonstrate a concomitant eastward increase in Pb content and decrease in Ti/Zn ratios for the leucogranite. These spatial trends align with the temporal-spatial patterns of the South Tibetan Detachment System initiation (STDS), ultrapotassic rock emplacement, and Indian lithospheric mantle tearing. We propose that zircon age continuity reflects regional tectonic regimes—compressional settings favor continuous age records, whereas extensional settings lead to discontinuous age distributions. Therefore, the ADEPT method, based on depth profiling data, not only enables rapid and efficient deconvolution of complex mineral age records but also provides a novel tool for understanding magmatic-geological processes.
Over the past four decades, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and secondary ion mass spectrometry (SIMS) have been widely employed for in situ dating of zircon and other minerals. However, for minerals with extremely narrow mantles and rims (<5–15 μm), conventional microbeam analysis struggles to obtain accurate U-Pb ages for distinct zones due to the relatively large beam spot size (>15–50 μm). In situ depth profiling technique has emerged as a critical solution to this challenge, involving continuous ablation from the rim to the core of the mineral to acquire spatially resolved U-Pb isotopic data. Nevertheless, this approach suffers from two major limitations: (1) reliance on manual selection of age plateaus, which is inefficient, and (2) subjective human-defined plateau segmentation. This study develops a new quantitative method—Automated Depth Profiling Technique (ADEPT), including three steps: data optimization, age plateau segmentation, and age plateau screening, implemented as a fully automated workflow in R. We applied ADEPT to reprocess published depth profiling U-Pb data from Himalayan leucogranite zircon (N = 4615 grains) and compared the results with prior manual segmentation. Both methods yielded similar age peaks (~35 and ~9 Ma) for the leucogranite. However, the ADEPT method resolved additional reliable ages that were indiscernible visually.
The distinct age plateaus recorded from core to rim in zircon typically correspond to different episodes of magmatic activity. The results of this study reveal that 74% of single-zircon grains from Himalayan leucogranites exhibit 2–4 age plateaus. Notably, zircon age continuity (defined as the age difference between core/mantle and rim) exhibits a systematic eastward decrease across the Himalaya. Published whole-rock geochemical data further demonstrate a concomitant eastward increase in Pb content and decrease in Ti/Zn ratios for the leucogranite. These spatial trends align with the temporal-spatial patterns of the South Tibetan Detachment System initiation (STDS), ultrapotassic rock emplacement, and Indian lithospheric mantle tearing. We propose that zircon age continuity reflects regional tectonic regimes—compressional settings favor continuous age records, whereas extensional settings lead to discontinuous age distributions. Therefore, the ADEPT method, based on depth profiling data, not only enables rapid and efficient deconvolution of complex mineral age records but also provides a novel tool for understanding magmatic-geological processes.
Key words: zircon; depth profiling; ADEPT; quantitative analysis; plateau segmentation; Himalaya; leucogranite
Recommended citation: Wang, Y. (2025). Quantitative assessment of zircon depth profiling plateau ages. Undergraduate Thesis, China University of Geosciences (Beijing).