A FORWARD LOOK IN APPLICATIONS OF HIGH-SPATIAL RESOLUTIONLA-ICP-MS U-Th-Pb GEOCHRONOLOGY

摘要

It is now easier than ever to begin dating materials using U-Th-Pb geochronology. A basicquadrupole ICP-MS and laser ablation system is all that are needed and with recent improvementsin efficiency of both instruments and analytical set-up, our ability to target lower U materials hasincreased significantly. As a result, younger zircons and lower U minerals such as apatite, rutileand carbonates are now possible to date with the U-Pb system. Sub-million year ages can now bedetermined and laser ablation ICPMS sensitivities even enable determination of U-series ages.This gives the potential to date materials in the 100 - 500 ka range by both U-Pb and U-series usingLA-ICP-MS, allowing cross-calibration of these different dating methods applied to more recentvolcanic activity.Improvements in transmission efficiency of the laser ablated material from the sample cell to themass spectrometer have had significant impact on the speed and throughput of data acquisition,allowing high speed elemental and isotopic mapping of materials to be a practical proposition. Theinsights these maps bring provide context to the interpretation of conventional static spot analyses,hugely increasing the strength of interpretation. This efficiency can be turned to the verticaldimension, increasing spatial resolution to the 10 - 100 nm scale whilst depth profiling mineralgrains. Analysis of petrographic thin sections, again providing important context for interpretation,is therefore easier with this increased control on depth resolution which also lends itself to diffusionstudies and within-grain U-Pb Discordia interpretation. This petrochronology, interpreting theelemental, isotopic and age information with petrographic context, usually using a ‘split-stream’two mass spectrometer arrangement, represents a significant growth area for the geochronologycommunity. This approach and ideology is then applied to a range of non-zircon minerals such asmonazite, titanite, apatite, xenotime and rutile, to enhance understanding of thermal histories(magmatism, metamorphism and cooling) related to tectonics and the understanding of uplift anderosion rates.The diversity of applications for laser ablation geochronology is increasing rapidly. The flexibilityof this analytical tool and improvements in data quality are encouraging targeting of more variedmineral types and greater numbers of samples. Volumes of data generated are vast and tools forhandling and visualising the data appropriately are required to ensure that data are validated andthat appropriate interpretations are made. This brings together geoscientists with data scientistsand mathematicians, broadening the skill base in U-Th-Pb geochronology and the requiredunderstanding and skills of the user. The future for LA-ICP-MS U-Th-Pb geochronology istherefore, bright, growing rapidly, with seemingly endless possibilities for application. Some ofthese future possibilities will be highlighted here along with the current applications that arepushing the boundaries of the methodology and enabling new scientific directions.