Dr Chris Mark awarded SFI Starting Investigator Research Grant

Title

Advanced Geochronology of Earth-System Processes (Age-Pro): Constraining rates and dates of geological processes by novel U-Pb analysis

Figure caption

SEM image of an apatite crystal, acquired by Dr Chris Mark at TCD

Summary

This project applies a novel mineral-dating approach to key geological questions. Uranium decays over time to lead. Importantly, lead becomes mobile when minerals are heated above a characteristic temperature by geological processes (e.g., granite intrusion), allowing the lead to diffuse towards the outside. Diffusion results in a decreasing amount of lead from the mineral core to the rim. Because the speed of lead loss is temperature-dependent, we can reconstruct the time-temperature history with diverse applications, e.g. solving the mystery of how continent-scale regions can be exhumed from 100km-depths; fingerprinting sediment from melting ice-caps; and dating the formation of gold deposits.

Thermochronology is a critical tool to reconstruct past geological processes, but effective coverage of the conventional fission track and 40Ar/39Ar systems is restricted to upper crustal levels. This project will use U-Pb thermochronometry of apatite, coupled with titanite, rutile, and monazite U-Pb geochronometry (by means of quadrupole and multi-collector laser ablation inductively-coupled plasma-mass spectrometry, LA-ICPMS) to resolve key geological questions: (1) How are major subducted continental terranes exhumed from mantle depths (>100 km)? (2) Did past climate change analogous to the Anthropocene trigger collapse of the West Antarctic ice sheet? (3) When and how did Ireland’s gold occurrences form? Thermal history recovery for (1) and (3) will be maximised using a novel multicollector-quadrupole laser-ablation split-stream (LASS) ICPMS approach to measure the spatial distribution of Pb within single apatite grains, which will be inverted to yield high-resolution temperature-time paths at lower crustal temperatures (ca. 375-550 °C). LASS analysis also permits simultaneous analysis of trace-element abundances, to assess the (re)crystallisation history of the apatite grain. Question (2) will be addressed using a multi-proxy provenance approach, employing U-Pb analysis of detrital apatite, titanite, rutile, and monazite, plus Pb-isotope characterisation of K-feldspar, on sediment recovered by recent marine drilling expeditions offshore West Antarctica.