Detrital minerals done right

This afternoon, I got to go listen to a talk by Andy Morton. As anyone who hangs around the geochronology scene for any length of time knows, detrital provenance studies are done often, with a wide variety of quality and rigour. The basic idea is that you can learn about the source region of a sand by looking at the individual grains. How much you learn, and what sort of study is most informative, is something that isn’t always clear, as this is a field of study where the wheel is reinvented often and badly. The basic idea is so simple that people don’t often spend oodles of library time looking at how the field has evolved over the decades. So it was really cool to listen to the guy who has been at the forefront of doing detrital studies well for the last 30 years.
Like much great science, the talk was deceptively simple. He started out explaining the various sedimentary processes that give rise to sample bias, specifically diagenesis, transport, and weathering. He then went through the heavy mineral ratios, and the chemistry of each one- what the garnet chemistry tells you, what the tourmaline chemistry means, etc., through the entire mineral suite. Only after that did he get into geochronology. And each piece of the puzzle fell together- the tourmalines identified the granite types, the rutiles identified the high grade metamorphic conditions, the garnets identified the medium grade metapelites and deep crustal rocks, the zircons gave the ages. Occasionally spinels would make mafic igneous cameos. As it came together, the picture of the source would suddenly congeal- Of course that combination of X metamorphism and Y plutonism at time Z was from source region W- once all the pieces are assembled, it’s perfectly clear.
And once the provenance is determined, the alteration overprint on the source assemblage in various sands of similar derivation can be used to determine burial temperature, source weathering, flood plain behavior, and a host of other paleoprocesses that most detrital mineral amateurs don’t even begin to consider. It was awesome.