DUAL MULTICOLLECTOR LASS-ICP-MS FOR LU-HF AND U-PB ISOTOPES: APPLICATION TO ZIRCON IN RHYOLITE FROM THE YELLOWSTONE HOTSPOT

The laser-ablation split-stream technique allows for simultaneous measurement of Lu-Hf and U-Pb isotopes in zircon while consuming one sample volume, ensuring a direct relationship between crystallization age and Hf-isotope ratios. In this study, we describe a method for simultaneously measuring Lu-Hf and U-Pb isotopes on zircons using two multicollector mass spectrometers, a Thermo Scientific Neptune and a Neptune Plus, termed Dual Multicollector Laser-Ablation Split-Stream ICP-MS (DM-LASS-ICP-MS). Heretofore, the Hf–U-Pb split-stream technique has utilized either a quadrupole or single-collector ICP-MS to measure the U-Pb isotopic ratios; the use of two multicollectors is thus far unique and static, rather than dynamic, counting increases the precision on simultaneous measurements of U-Pb and Hf isotopes. DM-LASS-ICP-MS has a ~2% and 1.1 ± 0.3% 2 S.D. reproducibility for U-Pb and εHf, respectively, demonstrated by long-term analysis of five natural zircons (91500, Plešovice, Mudtank, Temora-2, R33) and three synthetic zircons. A 53 µm diameter laser spot was used after many trials with several different beam sizes. The larger beam size sacrifices some spatial resolution for improved precision in the isotopic measurements.

This technique was applied to zircons in rhyolites of the Bruneau-Jarbidge volcanic center. Located in the central Snake River Plain of Idaho, the Bruneau-Jarbidge center is an expression of the Yellowstone hotspot (YHS), and was active from 12.7–8.0 Ma. Rhyolite magmas of the YHS erupted over its 16 million-year history contain zircons exhibiting a wide range of εHf values (+10 to -40) that span the entire terrestrial array. Zircons from the Bruneau Jarbidge volcanic center exhibit a more narrow range in εHf from -1.5 to -15, recording a period interpreted as increased basaltic input and suppression of a strong Archean crustal signature that is associated with increased melt production. Abundant antecrysts, comprising ~75% of analyzed spots and dating back to 15 Ma, offer insight into a two-million-year regime of partial melting of the crust prior to the generation of magma volumes of sufficient size to produce the 10 large-volume silicic ignimbrites of the Cougar Point Tuff and subsequent voluminous rhyolite lava flows.