This is the fourth post in the “It’s Pronounced Fronkensteen!” series. This series of posts will follow our trials, travails, successes, and lack of successes in the lab.
This summer, Tiff is looking for Titanites. These minerals, also know as Sphene, could possibly be the next Zircon in the world of Uranium- Lead dating. Carl Spandler, Tiff’s PI at James Cook University, has explained to Tiff why Titanite could be a fantastic geochronomter. In a very watered down statement (translated by Tiff with Carl’s assistance), Titanite could give us a more detailed geologic history of the rock’s formation/any major events it went through, including metamorphism and hydrothermal events which may have formed Rare Earth Elements. This is because Titanite has a high closure temperature, giving us a clear age of its formation which makes these minerals ‘hard to kill.’ It’s also because this mineral, known as CaTiSiO5, can easily incorporate three different decay products of Uranium (lead isotopes 206, 207, and 208) into its crystal structure. This gives us three isotope clocks to look at in cross checking the mineral’s age. Decoding this chemistry within the Titanite sample can give us clues on any hydrothermal fluids that may have altered the mineral. In general, Titanite can tell us A LOT. Plus, it can be found in ore deposits, while Zircon cannot.
The only problem is that there is no reliable standard to measure the age of Titanite! A standard is a sample of the mineral which has a consistent age that we can depend on in comparing the ages of unknown samples. When using laser ablation to date minerals, the unknown minerals and the standards are ‘zapped’ to compare the ages. In a way, a standard seems to keep the whole process calibrated and correct for the best ages. A ‘dodgy’ standard can skew your data and make your research unreliable. If a solid Titanite standard is found, it will be sent out and used in labs around the world.
To find the best Titanite, we have to look at lots of tiny images with various high tech equipment- the polarizing microscope, the SEM (Scanning Electron Microscope), and the Microprobe. By analyzing these images, we can find the best Titanites to eventually zap with the laser, check their ages, and possibly use as a standard.
Up until this point, Tiff has been trying to figure out what a Titanite looks like. Then she is introduced to the Microprobe….a massive machine hooked up to three large computers. Three slides of sliced rock are put into the machine. These rock slices are mounted on glass and covered with a carbon coating to make it conductive. Electrons are shot at the slide, which reveals mineral images and x-ray angle measures give an estimate of what elements are inside the mineral. Crazy! So we are searching for the elements within Titanite- Calcium, Titanium, Silicon, and Oxygen. If the estimates look good, we can take a picture of that Titanite, get it’s element data, and put it on the list to be dated by the laser in the future.
After being taught how to use the Microprobe and find the Titanites (plus checking their elemental composition), Tiff is left to take pictures of as many that are visible. It’s like an audition for the ultimate role of the standard- “Do you have the correct percentage of elements? Do you look homogeneous? Do you have the Titanite crystal shape without any cracks or holes? If so, you may attend the callback with the laser.”
The second part of the Microprobe process is measuring the weight percentage of the elements in each Titanite. If the sample tallies up to 100%, it’s a good pick. With the assistance of Peng Sha, a post doctorate student, we preset spots along our selected Titanites to be measured. Compiling this data takes hours, but Mr Microprobe does it all.
It was a great deal to learn in one day. Tiff made a brief video trying to share the wonder of the Microprobe (and the fact that she was trusted alone with it). Please note- because she was delirious with excitement, Tiff often refers to looking at the ‘minerals’ instead of the ‘elements’ in the mineral. If you substitute those terms, this may possibly make sense:
Click here for the YouTube Link
At the callback, the laser will decide on who is Australia’s next top Titanite Standard.
May Wuebber 2013 