Methodology

The fossil teeth being utilized in this study come from Dinosaur National Monument (DNM, loan L.2021.004) have previously been analyzed for δ¹⁸O and δ¹³C ratios (Fricke et al., 2011), as well as δ⁴⁴Ca, but extensive work has not been done to assess the diagenetic conditions in these and other Jurassic dinosaur teeth. Use of the LA-ICP-MS will be particularly useful in this case as it can explore the entirety of a sectioned tooth. Utilizing the UT LA-ICP-MS, we will analyze the teeth in high resolution along the growth axis to see how elemental abundances vary along the tooth, with a particular focus on differences between dentine and enamel. The targeted analytes include species which “belong” in enamel, as well as rare earth elements and other diagenetic species such as U, Na, Fe, and Mg. The full lists of analytes is as follows:

²³Na ²⁴Mg ²⁵Mg ²⁷Al ³¹P ⁴²Ca ⁴⁴Ca ⁵⁷Fe ⁸⁸Sr ⁸⁹Y ¹³⁷Ba ¹³⁸Ba ¹³⁹La ¹⁴⁰Ce ¹⁴¹Pr ¹⁴⁶Nd ¹⁴⁷Sm ¹⁵³Eu ¹⁵⁷Gd ¹⁵⁹Tb ¹⁶³Dy ¹⁶⁵Ho ¹⁶⁶Er ¹⁶⁹Tm ¹⁷²Yb ¹⁷⁵Lu ²³²Th ²³⁸U

To calibrate the ICP-MS, we used three standards: NIST 610, NIST 612, and MAPS4, with NIST 610 being used as the primary calibration standard. Despite this, MAPS4 is a particularly useful standard when working with bone and biomineral samples as it’s composition mimics that of bone. Thin sections and calibration standards were mounted in the laser ablation chamber using double sided tape. We performed pre-ablation using the following settings:

• Spot size = 125 µm
• Scan rate = 75 µm/second
• Laser energy = 75% (~3.4 J/cm²)
• Repetition rate = 10 Hz
• Helium & Argon flows = 850 ml/minute
• Sample depth = 6.5

Because the ICP-MS had been set up for solution-mode, we could not perform calibration or analysis immediately. After the previous experiment was completed, we tuned the ICP-MS in solution mode then over to laser ablation mode to prepare for analysis. Our tuning settings for the laser were as follows:

• Spot size = 75 µm
• Scan rate = 5 µm/second
• Repetition rate = 10 Hz
• Helium & Argon flows = 800 ml/minute
• Sample depth = 6.5

We tested laser energy in increasing increments (55%, 65%, 75%, 85%) with increasing fluence (2 J/cm², 2.45 J/cm², 2.95 J/cm², 3.61 J/cm²) to determine which would provide the best signal for sampling. 55% provided 50,116 cps for 141Pr as well as the greatest precision of all energy level. We required no changes after testing our initial method on sample material, so our final settings for sampling were as follows:

• Spot size = 100 µm
• Scan rate = 15 µm/second
• Laser energy = 75% (~3.4 J/cm²)
• Repetition rate = 10 Hz
• Helium & Argon flows = 850 ml/minute
• Sample depth = 6.5

We sampled three thin sections of a Camarasaurus tooth from Dinosaur National Monument (971.1, 971.2, 971.3). On each tooth we set up sampling transects from the core to the exterior of the tooth, as well as along the growth axis of the enamel and dentine to determine if location in the tooth changed isotope measurements significantly. Lastly, we reduced the data using IOLITE to allow for easier processing of the results.