Lithospheric Processes Research
Above: Popple Hill gneiss outcrop in the Adirondack lowlands, NY, USA.
Understanding the processes that have influenced the development of Earth’s lithosphere requires the combination of field, geochronological, and geochemical techniques in a wide variety of temporal and spatial scales. Thus, a major objective of our group’s research is to combine our geochronologic expertise with both well-established and novel geochemical tracers, to develop a coupled temporal/petrologic understanding of lithospheric evolution (i.e., an approached that has come to be known as ‘petrochronology’).
Highlights
timescales of metamorphism and high-temperature thermochronology
Thermally-activated diffusion of slow diffusing elements involved in long-lived radioactive decay chains (e.g., U-Pb, Sm-Nd and Lu-Hf) provides a unique, yet still poorly exploited means, to quantify the timing and tempos of high-temperature lithospheric deformation. In collaboration with Elias Bloch (UNIL) we have, for the first time, combined all these chronometers with thermal history modeling and diffusion kinetics, to generate a quantitative temperature-time model of collisional orogenesis from a Proterozoic high-grade metasediment (Ibañez-Mejia et al., 2018). Our approach highlights the potential that combining these techniques presents for understanding lithospheric processes, as well as some of the challenges currently faced by high-temperature thermochronology.
studying lower orogenic crust through laser-ablation split stream (LASS) of complexly zoned minerals
Improvements in laser ablation (LA) and inductively coupled plasma mass spectrometers (ICP-MS) have significantly expanded the applicability of these instruments to research in the earth and planetary sciences. An emerging technique over the past decade is the ‘split-stream’ (or LASS) method, which involves splitting the aerosol generated by one laser into two separate streams that are analyzed in separate mass spectrometers. This technique allows for the simultaneous collection of age and geochemical (e.g., trace element concentration or isotope ratio) information from the same volume of ablated material, thus improving our ability to study complexly-zoned minerals. With collaborator Alex Pullen (Clemson U.) we developed methods for simultaneous U-Pb age and Lu-Hf isotopic analysis of zircon at the University of Arizona Laserchron Center using a Photon Machines laser system coupled to Element2 and Nu Plasma mass spectrometers. The methods are described in detail in Ibañez-Mejia et al. (2015). Since then, we have assisted other laboratories in the implementation of similar methods, and continue to pursue research that benefits from the collection of simultaneous age and geochemical information.
timescales and mechanisms of lithospheric breakup
The breakup of the continental lithosphere and subsequent establishment of oceanic spreading centers are central processes to the Wilson Cycle and are fundamental pieces of terrestrial plate tectonics. Nevertheless, understanding the temporal and geochemical evolution of hyperextended margins and how this evolution relates to the establishment of oceanic basins and spreading centers remains a topic of debate. In collaboration with Mike Eddy (Purdue U.) and Oliver Jagoutz (MIT) we studied the record of extensional magmatism associated with lithospheric break-up along the Newfoundland-Iberia margin, and proposed a timeline for the birth of seafloor spreading in one of the most important oceanic basins on Earth, the Atlantic Ocean.
Results of this study were published in Eddy et al. (2017) in the journal Geology.
