Laser ablation U-Pb dating of fluorite
Gavin Piccione was an undergraduate student who worked on fluorite while at Stony Brook. Fluorite from Big Round Top (Sierra Blancas, Trans Pecos, TX) has very favorable 238U/206Pb ratios (approaching 1000) and we were able to date multiple generations of fluorite as well as associated nacrite and fluorite. We also recognized a REE carbonate that postdates fluorite formation demonstrating formation nearly 30 million years after the laccolith was emplaced. His work was published in Geosphere.
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REE carbonate
We found a REE carbonate that postdates the fluorite and comes before calcite. Some of the calcite has high U/Pb and gives an age of about 2 Ma. Thus REE remobilization is bracketed by the 6.2 Ma fluorite and the 2 Ma calcite tightly constraining the age of formation.
Multiple fluorite generations
The image to the left is fluorite from Big Round Top. The holes that can be seen in the Ca map (black) separate two distinct generations of fluorite. The top generation has elevated and zoned Sr with relatively low U and the bottom generation has the opposite pattern. Lining the holes that separate the two generations is a Y- bearing REE mineral that shows up superbly in the Y map. In SEM there is evidence for dissolution of fluorite and the REE mineral has a very close relationship to nacrite which also occurs along the dissolution surfaces.