A measurement method for U-Th-Sm/He dating of apatite by laser ablation
Date
2018-05-14
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Abstract
Thermochronology is a useful tool for dating thermal evolution of rocks in the upper crust. Thermochronometers have the ability to record the cooling of a rock through varying temperature windows. The apatite U-Th/He system is one of the low-temperature thermochronological systems that records cooling temperatures from 45-80°C. Apatite U-Th/He dating is conventionally done through whole grain analysis, where grains are picked and measured and then packed into Nb capsules which are heated in a vacuum to extract and measure the 4He content. Grains are subsequently dissolved and their U, Th, and Sm concentrations are measured by solution ICP-MS. This study addresses a new method of U-Th/He dating using apatite by laser ablation. In the laser ablation method grains are mounted in a vacuum stable medium and then ablated using a laser in a vacuum cell. The laser ablation method has several advantages over whole-grain measurements that limit bias from the grain selection process and improve analytical efficiency. An independently characterised megacryst of Durango apatite is used as a calibration material for the new laser ablation method. The megacryst was characterised at the University of Calgary and the University of Colorado Boulder by conventional whole-grain dating. The analysis yields an age of 32.56 ± 0.34 Ma. The measured age compares well to published ages for Durango apatite. I present here a new method of laser ablation U-Th/He on apatite. The method generally follows previously published laser ablation methodologies for zircon, monazite and titanite with modifications suited for apatite. Grains were mounted using a heated platen press, which pressed samples into FEP bonded to an aluminum backing plate. Samples were loaded into an ultra high vacuum chamber and ablated using a excimer laser and the liberated 4He is measured using an quadrupole mass spectrometer system. The ablated sites were imaged using an optical profilometer and ablated pit volume measured using PitVol, a custom MatLabTM algorithm. After the pit volume measurement, U, Th and Sm concentrations were measured by laser ablation ICP-MS and the U-Th-Sm/He date calculated. The method presented was validated using the characterized Durango apatite megacryst and Fish Canyon Tuff (FCT) apatite reference materials. Weighted average of Durango dates (32.50±0.32 Ma) compare well with conventional whole grain methods for chips of the same Durango crystal. Systematic errors in the method are expected, particularly in the pit measurement and U, Th, Sm measurement to which correction (κ-value) was applied. These Durango dates were used to produce a ҡ-value to correct the secondary reference material and unknown samples. After correction, FCT apatite yields a weighted average age of 28.37 ± 0.96 Ma, which agrees well with published data.
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Keywords
Apatite, U-Th/He
Citation
Pickering, J. (2018). A measurement method for U-Th-Sm/He dating of apatite by laser ablation (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/31938