(ULTRA)HIGH-PRESSURE TECTONISM: INSIGHTS FROM LASER ABLATION SPLIT-STREAM (LASS) PETROCHRONOLOGY

One of the biggest challenges in the determination of the timing and rates of continental subduction is tying the age of a particular mineral to the conditions (i.e., pressure, temperature, fluid composition) at which that phase grew. Recent advances in microbeam techniques have greatly increased our understanding of crustal evolution by enabling this linkage. The most common target for U-Th-Pb petrochonology is zircon: its REE pattern can reveal the presence/absence of garnet (depleted HREE) and plagioclase (positive Eu/Eu*), and cathodoluminescence imaging can be used to link the ages and trace-element concentrations of spot analyses. The age of monazite can also be linked to certain conditions: most commonly, yttrium zoning is used a proxy for (re)crystallization in the presence or absence of garnet.

Here we present a more accurate, comprehensive, rapid, and simplified procedure to obtain petrochronologic data and thus assess the P–T–t conditions of spot analyses. The LASS—laser ablation split-stream—technique consists of concurrent analyses of single laser ablation spots on both a multi-collector (U-Th-Pb age) and single-collector (trace-element data) ICP-MS. LASS allows rapid (<1 minute/spot), high-precision (<1%/age population), and high spatial precision (down to 8 um) measurements and an unambiguous link between mineral age and (re)crystallization conditions.

The Western Gneiss Region of western Norway provides the perfect natural laboratory to showcase the advantages of LASS petrochronology. Zircons from eclogites across the WGR indicate garnet-stable conditions as early as 440 Ma and as late as 395, with most of the zircon growth between 420 and 400 Ma. Monazite yields similar periods of garnet-stable and plag-unstable crystallization as well as late, low-pressure growth during exhumation.