Development of baddeleyite as a chronometer for recent silica-undersaturated lavas.

摘要

Baddeleyite is a frequently found accessory mineral in silica-undersaturated lavas. Because it is typically enriched in uranium, while having low initial lead, baddeleyite has long been a prime target for U-Pb geochronology for mafic rocks. The difficulties in retrieving small baddeleyite grains from volcanic samples and the lack of a detailed understanding of baddeleyite occurrence, however, have limited baddeleyite chronology largely to coarse-grained mafic intrusive rocks. In this thesis, the development of U-Th in-situ baddeleyite analysis using Secondary Ionization Mass Spectrometry (SIMS) is presented together with an assessment of baddeleyite occurrence in Quaternary silica-undersaturated lavas. Samples studied include the Cathedral Crag and Burroughs Mountain lava flows of Cascade arc volcanoes Mt. Baker and Mt. Rainier (Washington, USA), respectively, and Accademia, Punta Marmolite and Cuma lava domes of the Campi Flegrei caldera (Naples, Italy). Baddeleyite crystals were initially identified in cut and polished rock billets using scanning electron microscope (SEM) backscatter imaging and energy dispersive x-ray analysis, before they were separated by diamond-drilling for SIMS analysis. A total of 62 and 52 baddeleyite crystals were documented from Cathedral and Burroughs Mountain lava flows; 205, 42 and 15 baddeleyite crystals were documented from Accademia, Punta Marmolite and Cuma lava domes, respectively. U-Th baddeleyite isochron ages range between near secular equilibrium for Cathedral Crag lava flow: 404.5 -197.8+infinity ka (MSWD = 0.88; n = 3); and 4.36 -1.12+1.13 ka (MSWD = 2.9; n = 24) for the Accademia lava dome. The ages for Punta Marmolite (62.4 -3.8 +3.9 ka; MSWD = 1.2; n = 11) and Cuma (45.9 -3.5 +3.6 ka; MSWD = 2.2; n = 11) fall in between, and are in a favorable age range for U-Th dating approximately near one half-life of 230Th (∼75.69 ka). Burroughs Mountain lava was not dated because it is expected to be in secular equilibrium, but it was used to monitor the abundance of baddeleyite in basaltic andesite from an arc volcano. The U-Th baddeleyite ages generally agree with previously reported K-Ar ages, except for the Punta Marmolite lava flow, which has a significantly older U-Th baddeleyite age compared to the K-Ar age. The age comparisons suggest that baddeleyite overwhelmingly crystallized near the time of eruption, in contrast to the frequently observed protracted pre-eruptive residence of zircon in silicic volcanic rocks. Near-eruption crystallization is supported by observations that ∼50% of baddeleyite crystals occur at the walls of vesicles in most lavas studied here, including Punta Marmolite lava flow. The apparent hiatus between baddeleyite crystallization and eruption in Punta Marmolite thus could reflect a bias in the K-Ar age. U-Th baddeleyite dating is demonstrated to be a reliable chronometer for Quaternary silica-undersaturated volcanic rocks, and thus can be the primary dating technique for rocks that lack other datable minerals.