The geochronologic record of detrital minerals in ancient and modern sediments is used to study Earth history and tectonometamorphic processes. In many cases, detrital minerals in sediments represent the only preserved record of long since eroded mountains and continental fragments. However, the available detrital mineral geochronologic archive has important limitations including, importantly, the ability for these minerals to directly constrain tectonic (i.e. mountain building and subduction) processes. This project pioneers the use of detrital garnet Samarium-Neodymium (Sm-Nd) geochronology as a brand new window into Earth?s tectonic past. Garnets grow in response to prograde metamorphism and record the formative portions of mountain building events (typically related to continental collision) that can be completely missed in the currently dateable detrital mineral archive. Furthermore, by analyzing the well-understood chemistry of a garnet grain, it is possible to place its age into a specific pressure-temperature range and tectonic context wherein it grew (e.g. subduction zone, regional, or contact metamorphism). The goal of this project is to explore, define, and push the limits on sample size and precision attainable by streamlining existing analytical and sample preparation methods developed over the past several years for application to single grain detrital garnet age dating. The method will be tested using samples collected from New England as well as well-studied detrital mineral suites from three diverse field settings, each with its own set of potential future research applications: 1. Archean sediments such as the Jack Hills conglomerate; 2. Paleozoic orogenic sandstones eroding the classic Barrovian type-locality in Scotland; 3. modern stream sediments draining the Appalachian Mountains in North Carolina.
By developing and utilizing an archive of ancient detrital garnet ages (and their mineral chemistry) there is the potential to tap the very oldest direct record of tectonometamorphic processes on the planet, explore global age clusters of mountain building events to test inferences about the pace of global continental growth drawn from other detrital mineral age populations, and address regional scale questions involving the onset, progression, and episodicity of prograde metamorphism throughout a mountain building event.
We have established a brand new method to date tiny detrital (i.e. sedimentary) grains of garnet. Detrital minerals have the potential to record their growth age and conditions that go back before the sedimentary processes that deposited them. Other detrital minerals have been dated and well studied previously (such as zircon, apatite, and monazite) but detrital garnet has never been dated due to numerous analytical challenges that we have been able to overcome. Garnet provides a different and complementary history as compared to zircon, apatite, monazite or other detrital minerals in that garnet typically grows and retains information about the early to peak stages of metamorphism. By studying detrital garnet we have a new window in to the Earth’s tectonic and metamorphic history. The new detrital garnet geochronometer is based on the radioactive decay of samarium to neodymium. While this dating system is not new, our innovation is in precisely and accurately making these measurements on tiny garnet grains and their micromineral inclusions which together constrain the growth age of the garnet. We have developed a pre-characterization scheme using an SEM to acquire semi-quantitative chemical data on individual garnet grains before they are consumed in the geochronologic analysis. This chemical data is useful in both identifying possibly distinct chemical (and age) populations, and in placing broader tectonic context on each garnet age. In general, a garnet’s specific chemical composition can be related back to the pressure, temperature, and rock composition from which it originally formed. This information in turn can tell us about the broader tectonic context tat its growth records. We have demonstrated the success and utility of the method in several rock types including: 1. The Jack Hills metasedimentary rocks where the planets oldest zircons have previously been found (garnets were dated at ~2.6 Ga), 2. Ancient sedimentary rocks from Scotland (garnets were dated at ~460 Ma), 3. Modern river sediments from the North Carolina in the southern Appalachians (garnet were dated at ~420-450 Ma), 4. Stream and beach sands from New England (garnets were dated at ~380 Ma). Each of these samples yielded garnet ages that are consistent with predictions based on broader geologic context, though the Jack Hills garnets appear to be younger than best estimates of sedimentary deposition indicating that theses garnets might in fact reflect more recent metamorphism of the sediment itself. At the North Carolina site the detrital garnet ages show good agreement with existing detrital monazite ages. And in New England the detrial garnets show good agreement with the range of garnets exposed in the bedrock catchments that are being eroded. The work has involved a collaborative team of researchers from Boston University, Syracuse University, University of Colorado, University of Massachusetts, and the Woods Hole Oceanographic Institute. Two PhD students and one undergraduate from Boston University contributed significantly to this study. The methods we have established have been presented a national and international conferences and will be published in peer-reviewed journals.
